WO2024023625A1

WO2024023625A1 - Battery

Info

Publication number: WO2024023625A1
Application number: PCT/IB2023/057210
Authority: WO
Inventors: 高橋辰義; 種村和幸; 三上真弓; 村椿将太郎; 栗城和貴
Original assignee: 株式会社半導体エネルギー研究所
Priority date: 2022-07-29
Filing date: 2023-07-14
Publication date: 2024-02-01

Abstract

The present invention provides: a positive electrode active material for which a decrease in discharge capacity during charge and discharge cycles is suppressed; and a battery which uses said positive electrode active material. Alternatively, the present invention provides a battery having high safety. This battery comprises a positive electrode. The positive electrode has a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector has a metal foil, such as a stainless foil, and a covering layer for at least partially covering the surface of the metal foil. The covering layer contains aluminum. The positive electrode active material included in the positive electrode active material layer contains a lithium cobalt oxide containing nickel and magnesium. The detected amount of nickel in a surface layer part of the positive electrode active material is greater than the detected amount of nickel in the inside of the positive electrode active material. The detected amount of magnesium in the surface layer part of the positive electrode active material is greater than the detected amount of magnesium in the inside of the positive electrode active material. The surface layer part of the positive electrode active material has a region where the distribution of nickel and the distribution of magnesium overlap each other.

Description

battery

One embodiment of the present invention relates to a power storage device (also referred to as a battery or a secondary battery). Furthermore, the present invention is not limited to the above-mentioned fields, but relates to semiconductor devices, display devices, light emitting devices, lighting devices, electronic equipment, vehicles, and manufacturing methods thereof. The battery of the present invention can be applied to the semiconductor device, display device, light emitting device, lighting device, electronic device, and vehicle described above as a necessary power source. Batteries include secondary batteries such as lithium ion secondary batteries and sodium ion secondary batteries. For example, the above-mentioned electronic devices include information terminal devices equipped with lithium ion secondary batteries. Furthermore, the above-mentioned power storage device includes a stationary power storage device and the like.

In recent years, various storage batteries, such as lithium ion secondary batteries, lithium ion capacitors, air batteries, and all-solid-state batteries, have been actively developed. In particular, demand for high-output, high-capacity lithium-ion secondary batteries is rapidly expanding along with the development of the semiconductor industry, and they have become indispensable in today's information society as a source of rechargeable energy. .

It is said that it is difficult to achieve both high capacity and safety in lithium ion secondary batteries. For example, a positive electrode active material having a layered rock salt crystal structure is expected to have a high capacity because a lithium ion diffusion path exists two-dimensionally within the crystal structure. However, positive electrode active materials with a layered rock-salt crystal structure are said to be susceptible to thermal runaway if too many lithium ions are desorbed during charging, causing the crystal structure to break, leading to safety issues. Safety tests include nail penetration tests, and in order to suppress the rise in battery temperature during abnormal situations such as nail penetration, for example, in Patent Document 1, a protective layer is installed between the positive electrode composite layer and the positive electrode current collector. A configuration has been proposed.

Lithium cobalt oxide (LiCoO ₂ ) and the like are known as positive electrode active materials with a layered rock salt crystal structure. Lithium cobalt oxide has a layered rock salt type crystal structure, and lithium ions can move two-dimensionally between layers made of CoO ₆ octahedrons, so it also has good cycle characteristics. However, lithium cobalt oxide has had the problem of phase changes during charging and discharging. For example, when lithium ions are released to some extent during charging, lithium cobalt oxide undergoes a phase change from hexagonal to monoclinic. Therefore, in order to use lithium cobalt oxide with good cycle characteristics, the amount of lithium ions released has been limited. In order to solve these problems, for example, Patent Documents 2 to 4 propose a structure in which additive elements are added to lithium cobalt oxide. When considering additive elements, the content described in Non-Patent Document 1, known as Shannon's ionic radius, may be referred to. Research on the crystal structure of positive electrode active materials has also been conducted (Non-Patent Documents 2 to 5).

Furthermore, X-ray diffraction (XRD) is one of the methods used to analyze secondary batteries using positive electrode active materials. XRD data can be analyzed by using ICSD (Inorganic Crystal Structure Database) introduced in Non-Patent Document 6. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 7 can be referred to from ICSD. Furthermore, for example, the analysis program RIETAN-FP (Non-Patent Document 8) can be used for the Rietveld method analysis.

Further, as image processing software, for example, ImageJ (Non-Patent Documents 9 to 11) is known. By using this software, for example, the shape of the positive electrode active material can be analyzed.

Ultrafine electron diffraction is also effective in identifying secondary batteries of positive electrode active materials, especially secondary batteries in the surface layer. For example, the analysis program ReciPro (Non-Patent Document 12) can be used to analyze the electron beam diffraction pattern.

Furthermore, fluorides such as fluorite (calcium fluoride) have been used as fluxes in iron and steel manufacturing for a long time, and their physical properties have been studied (Non-Patent Document 13).

It is known that when the temperature of a lithium ion secondary battery increases during charging, it goes through several states and reaches thermal runaway (Non-Patent Document 14 and Non-Patent Document 15).

JP 2019-129009 Publication JP2019-179758A WO2020/026078 pamphlet JP2020-140954A

In a charged state, lithium cobalt oxide (LiCoO ₂ , sometimes referred to as LCO) is said to have low thermal stability. Also, a nail penetration test is one of the safety tests for lithium ion secondary batteries. When an internal short circuit occurs during a nail penetration test, Joule heat is generated, so when using lithium cobalt oxide as described above, the oxygen released from the lithium cobalt oxide reacts with the electrolyte, etc., leading to thermal runaway. There is. Furthermore, it is known that lithium cobalt oxide causes a thermite reaction with aluminum, which is commonly used as a positive electrode current collector, when the temperature becomes high (Non-Patent Document 15).

When the thermite reaction occurs, the temperature becomes as high as 1000°C or more. Therefore, it is important to suppress the thermite reaction between the positive electrode active material and the aluminum foil.

In view of the above description, an object of one embodiment of the present invention is to provide a battery with high safety. Furthermore, an object of one embodiment of the present invention is to provide a battery with high capacity and high safety.

Alternatively, an object of one embodiment of the present invention is to provide a novel material, active material, power storage device, or method for manufacturing the same.

Note that the description of these issues does not preclude the existence of other issues. Note that one embodiment of the present invention does not need to solve all of these problems. Note that problems other than these can be extracted from the description, drawings, and claims.

One embodiment of the present invention has a positive electrode, the positive electrode has a positive electrode current collector, and a positive electrode active material layer, the positive electrode active material layer is provided on the positive electrode current collector, and the positive electrode has a positive electrode current collector. is a battery that includes a metal foil and a coating layer that covers at least a portion of the surface of the metal foil.

In the above, it is preferable that the metal foil is a stainless steel foil and that the coating layer contains aluminum.

Furthermore, in the above, the thickness of the metal foil is preferably 1 μm or more and 30 μm or less, and the thickness of the coating layer is preferably 1 nm or more and 1 μm or less.

Furthermore, in the above, the battery preferably has an exterior body that encloses the positive electrode, and the exterior body is preferably a stainless steel laminate film.

In the battery according to any one of the above, the positive electrode active material layer has a positive electrode active material, the positive electrode active material has lithium cobalt oxide containing nickel and magnesium, and the positive electrode active material layer has a surface layer of the positive electrode active material. The detected amount of nickel in the positive electrode active material is larger than the detected amount of nickel inside the positive electrode active material, and the detected amount of magnesium in the surface layer of the positive electrode active material is larger than the detected amount of magnesium inside the positive electrode active material. It is preferable that the distribution of nickel and the distribution of magnesium have an overlapping region.

In the above, nickel is preferably detected on a surface other than the (001) surface of lithium cobalt oxide in the surface layer of the positive electrode active material.

Furthermore, in the above, in the EDX-ray analysis, the difference between the depth of the peak of the detected amount of nickel and the depth of the peak of the detected amount of magnesium in the surface layer of the positive electrode active material is preferably within 3 nm.

In addition, in the above, the positive electrode active material contains aluminum, and in the EDX-ray analysis of nickel, magnesium, and aluminum contained in the positive electrode active material, the maximum value of the detected amount of aluminum is the maximum value of the detected amount of nickel and the maximum value of the detected amount of magnesium. When the peak width at a height of 1/5 of the maximum value of the detected amount of aluminum is divided into two by a perpendicular line drawn from the maximum value to the horizontal axis, it is smaller than the peak width Ws on the surface side. , it is preferable that the peak width Wc on the inner side is large.

In the battery described in any one of the above, when the battery using lithium for the positive electrode and the counter electrode is charged to 4.6 V and the positive electrode is analyzed by powder X-ray diffraction using CuKα1 ray, the diffraction of the positive electrode active material It is preferable that the pattern has at least a first peak whose 2θ is 19.13 or more and less than 19.37, and a second peak whose 2θ is 45.37° or more and less than 45.57°.

In the battery described in any one of the above, the positive electrode active material preferably contains fluorine, and the amount of fluorine detected in the surface layer of the positive electrode active material is preferably larger than the amount of fluorine detected inside the positive electrode active material.

According to one embodiment of the present invention, a highly safe battery can be provided. According to one embodiment of the present invention, a battery with high capacity and high safety can be provided.

Alternatively, according to one embodiment of the present invention, a novel material, active material, power storage device, or method for manufacturing them can be provided.

Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily need to have all of these effects. Note that effects other than these will become obvious from the description, drawings, claims, etc., and effects other than these can be extracted from the description, drawings, claims, etc. It is.

FIG. 1A is a perspective view of the battery, FIG. 1B is a cross-sectional view of the battery, and FIG. 1C is a cross-sectional view of the positive electrode.
2A and 2B are graphs showing the internal temperature of the battery.
FIGS. 3A and 3B are diagrams illustrating a nail penetration test.
4A to 4D are diagrams illustrating configuration examples of the positive electrode.
5A to 5E are diagrams illustrating configuration examples of secondary batteries.
6A to 6C are diagrams showing configuration examples of secondary batteries.
FIGS. 7A to 7C are diagrams showing configuration examples of a secondary battery.
FIGS. 8A to 8C are diagrams illustrating an example of the structure of a laminate.
9A to 9C are cross-sectional views of the positive electrode active material.
FIGS. 10A to 10C are examples of distributions of additive elements included in the positive electrode active material.
FIG. 11A is an example of the distribution of additive elements included in the positive electrode active material. FIG. 11B is a diagram illustrating the distribution of additive elements.
FIG. 12 is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and magnesium fluoride.
FIG. 13 is a diagram illustrating the results of DSC measurement.
FIG. 14 is an example of a TEM image in which the crystal orientations are approximately the same.
FIG. 15A is an example of a STEM image in which the crystal orientations are approximately the same. FIG. 15B is an FFT pattern of a region of rock salt crystal RS, and FIG. 15C is an FFT pattern of a region of layered rock salt crystal LRS.
FIG. 16 is a diagram illustrating the crystal structure of the positive electrode active material.
FIG. 17 is a diagram illustrating the crystal structure of a conventional positive electrode active material.
FIG. 18 is a diagram illustrating the charging depth and lattice constant of the positive electrode active material.
FIG. 19 is a diagram showing an XRD pattern calculated from the crystal structure.
FIG. 20 is a diagram showing an XRD pattern calculated from the crystal structure.
FIGS. 21A and 21B are diagrams showing XRD patterns calculated from the crystal structure.
22A to 22C show lattice constants calculated from XRD.
23A to 23C show lattice constants calculated from XRD.
24A and 24B are cross-sectional views of the positive electrode active material.
25A to 25C are diagrams illustrating a method for manufacturing a positive electrode active material.
FIG. 26 is a diagram illustrating a method for producing a positive electrode active material.
FIGS. 27A to 27C are diagrams illustrating a method for manufacturing a positive electrode active material.
FIG. 28A is a perspective view of a battery pack showing one embodiment of the present invention, FIG. 28B is a block diagram of the battery pack, and FIG. 28C is a block diagram of a vehicle having the battery pack.
29A to 29D are diagrams illustrating an example of a transportation vehicle. FIG. 29E is a diagram illustrating an example of an artificial satellite.
FIG. 30A is a diagram showing an electric bicycle, FIG. 30B is a diagram showing a secondary battery of the electric bicycle, and FIG. 30C is a diagram explaining a scooter.
31A to 31E are diagrams illustrating an example of an electronic device.

Hereinafter, embodiments for implementing the present invention will be described using drawings and the like. However, the present invention is not interpreted as being limited to the following embodiments. It is possible to change the mode of carrying out the invention without departing from the spirit of the invention.

In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin symbol). In addition, crystal planes and crystal directions are expressed using Miller indices. Space groups, crystal planes, and crystal directions are expressed in terms of crystallography by adding a superscript bar to the number, but in this specification, etc., due to formatting constraints, instead of adding a bar above the number, they are written in front of the number. It is sometimes expressed by adding a - (minus sign) to it. Also, the individual orientation that indicates the direction within the crystal is [　], the collective orientation that indicates all equivalent directions is <　>, the individual plane that indicates the crystal plane is (　), and the collective plane that has equivalent symmetry is {　}. Express each. In addition, the trigonal crystal represented by the space group R-3m is generally represented by a complex hexagonal lattice of hexagonal crystals for ease of understanding the structure, and unless otherwise mentioned in this specification, the space group R-3m is It is expressed as a complex hexagonal lattice. In addition, not only (hkl) but also (hkil) may be used as the Miller index. Here, i is -(h+k). In this specification and the like, with respect to space group R-3m, unless otherwise specified, crystal planes and the like are expressed in a complex hexagonal lattice.

In this specification, etc., the term "particles" is not limited to only spherical shapes (circular cross-sectional shapes), but also includes particles whose cross-sectional shapes are elliptical, rectangular, trapezoidal, triangular, square with rounded corners, and asymmetrical. Examples include shape, and further, individual particles may be amorphous.

Further, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the lithium that can be intercalated and desorbed from the positive electrode active material is desorbed. For example, the theoretical capacity of LiCoO ₂ is 274 mAh/g, the theoretical capacity of LiNiO ₂ is 275 mAh/g, and the theoretical capacity of LiMn ₂ O ₄ is 148 mAh/g.

Further, the amount of lithium that can be intercalated and desorbed remaining in the positive electrode active material is indicated by x in the composition formula, for example, x in Li _x CoO ₂ . In the case of a positive electrode active material in a lithium ion secondary battery, x=(theoretical capacity−charge capacity)/theoretical capacity. For example, when a lithium ion secondary battery using LiCoO ₂ as the positive electrode active material is charged at 219.2 mAh/g, it can be said that Li _0.2 CoO ₂ or x=0.2. When x in Li _x CoO ₂ is small, it means, for example, 0.1<x≦0.24.

When properly synthesized lithium cobalt oxide before being used in the positive electrode approximately satisfies the stoichiometric ratio, it is LiCoO ₂ and x=1. Furthermore, the lithium cobalt oxide contained in the lithium ion secondary battery that has finished discharging is also LiCoO ₂ and can be said to be x=1. Here, the term "discharge completed" refers to a state where the voltage is 3.0 V or 2.5 V or less at a current of 100 mA/g or less, for example.

The charging capacity and/or discharging capacity used to calculate x in Li _x CoO ₂ is preferably measured under conditions where there is no or little influence of short circuits and/or decomposition of the electrolytic solution. For example, data from a lithium ion secondary battery that has undergone a sudden change in capacity that appears to be a short circuit must not be used to calculate x.

Additionally, the space group of a lithium ion secondary battery is identified by XRD, electron beam diffraction, neutron beam diffraction, etc. Therefore, in this specification and the like, the terms belonging to a certain space group, belonging to a certain space group, or being a certain space group can be rephrased as identifying with a certain space group.

In addition, if the anion has a structure in which three layers are shifted from each other and stacked like ABCABC, it is called a cubic close-packed structure. Therefore, the anion does not have to be strictly in a cubic lattice. At the same time, since real crystals always have defects, the analysis results do not necessarily have to match the theory. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM image, a spot may appear at a position slightly different from a theoretical position. For example, if the orientation with respect to the theoretical position is 5 degrees or less, or 2.5 degrees or less, it can be said that the structure has a cubic close-packed structure.

Furthermore, the distribution of a certain element refers to a region in which the element is continuously detected in a non-noise range using a certain continuous analysis method.

In addition, a positive electrode active material to which additive elements are added may be expressed as a composite oxide, a positive electrode material, a positive electrode material, a positive electrode material for lithium ion secondary batteries, etc. Further, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably contains a compound. Further, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably has a composition. Further, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably has a composite.

Furthermore, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, etc., all particles do not necessarily have to have the characteristics. For example, if 50% or more, preferably 70% or more, more preferably 90% or more of three or more randomly selected positive electrode active material particles have the characteristic, it is sufficient to have the positive electrode active material and the same. It can be said that this has the effect of improving the characteristics of lithium ion secondary batteries.

As the charging voltage of a lithium ion secondary battery increases, the voltage applied to the positive electrode generally increases. Since the positive electrode active material of one embodiment of the present invention is stable in a charged state, a lithium ion secondary battery can be obtained in which a decrease in charge and discharge capacity due to repeated charging and discharging is suppressed.

Moreover, an internal short circuit or an external short circuit of a lithium ion secondary battery not only causes problems in the charging operation and/or discharging operation of the lithium ion secondary battery, but also may cause heat generation and ignition. In order to realize a safe lithium ion secondary battery, it is preferable that internal short circuits or external short circuits be suppressed even at high charging voltages. In the positive electrode active material of one embodiment of the present invention, internal short circuits or external short circuits are suppressed even at high charging voltages. Therefore, a lithium ion secondary battery that has both high discharge capacity and safety can be obtained. Note that an internal short circuit in a lithium ion secondary battery refers to contact between a positive electrode and a negative electrode inside the battery. Furthermore, an external short circuit of a lithium ion secondary battery is assumed to occur due to misuse, and refers to contact between the positive electrode and the negative electrode outside the battery.

Unless otherwise specified, the materials included in the lithium ion secondary battery (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) will be described in terms of their state before deterioration. Note that a decrease in discharge capacity due to aging treatment and burn-in treatment during the manufacturing stage of a lithium ion secondary battery is not called deterioration. For example, when a lithium ion secondary battery consisting of a single cell or an assembled battery has a discharge capacity of 97% or more of the rated capacity, it can be said to be in a state before deterioration. The rated capacity is based on JIS C 8711:2019 for lithium ion secondary batteries for portable devices. In the case of other lithium ion secondary batteries, they comply with not only the JIS standards mentioned above but also JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

In this specification, etc., the state of the material of a lithium ion secondary battery before deterioration is referred to as the initial product or initial state, and the state after deterioration (discharge of less than 97% of the rated capacity of the lithium ion secondary battery) The state in which the product has a capacity) is sometimes referred to as a used product or in-use state, or a used product or used state.

In this specification and the like, a lithium ion secondary battery refers to a battery using lithium ions as carrier ions, but the carrier ions of the present invention are not limited to lithium ions. For example, an alkali metal ion or an alkaline earth metal ion can be used as a carrier ion in the present invention, and specifically, a sodium ion or the like can be used. In this case, the present invention can be understood by reading lithium ions as sodium ions, etc. Furthermore, if there are no limitations on carrier ions, the battery may be referred to as a secondary battery.

(Embodiment 1)
In this embodiment, a configuration example of a battery according to one embodiment of the present invention will be described.

[battery]
FIGS. 1A and 1B are diagrams illustrating an example of a battery according to one embodiment of the present invention. FIG. 1C is a diagram illustrating an example of a positive electrode included in a battery according to one embodiment of the present invention.

A battery 10 is shown in FIG. 1A. The battery 10 includes an exterior body 50 and a positive electrode lead 21 and a negative electrode lead 31 extending from the inside of the exterior body 50 to the outside. The exterior body 50 is sealed with the sealing part 24, the sealing part 34, and the sealing part 51.

FIG. 1B is a schematic cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 1A. The battery 10 includes a positive electrode 20, a negative electrode 30, a separator 40, and an exterior body 50. The positive electrode 20, the negative electrode 30, and the separator 40 are enclosed in an exterior body 50. The positive electrode 20 includes a positive electrode current collector 22 and a positive electrode active material layer 23 provided on the positive electrode current collector 22. The negative electrode 30 includes a negative electrode current collector 32 and a negative electrode active material layer 33 provided on the negative electrode current collector 32. Separator 40 has at least a region located between positive electrode active material layer 23 and negative electrode active material layer 33.

The positive electrode current collector 22 and the positive electrode lead 21 are electrically connected, and the positive electrode lead 21 extends from the inside of the exterior body 50 to the outside. Although not shown, the negative electrode current collector 32 and the negative electrode lead 31 are electrically connected in the same manner as the positive electrode current collector 22 and the positive electrode lead 21, and the negative electrode lead 31 extends from the inside of the exterior body 50 to the outside. .

[Positive electrode]
An example of a positive electrode included in a battery of one embodiment of the present invention will be described with reference to FIG. 1C. FIG. 1C is an example of an enlarged view of area A surrounded by a broken line in FIG. 1B.

In a partially enlarged cross-sectional schematic diagram of the positive electrode 20 shown in FIG. 1C, the positive electrode active material layer 23 provided on the positive electrode current collector 22 includes the positive electrode active material 100 and the binder 13. Note that the positive electrode active material layer 23 may have a conductive material 14 in addition to the positive electrode active material 100 and the binder 13, but if the conductivity of the positive electrode active material 100 is sufficiently high, it may not have the conductive material 14. Good too. The positive electrode active material 100 is preferably a composite oxide having a layered rock salt type crystal structure belonging to space group R-3m, such as lithium cobalt oxide. Note that details of the positive electrode active material 100 of one embodiment of the present invention will be described in Embodiments 2 to 4.

It is preferable that the positive electrode current collector 22 has a low content of a metal such as aluminum that has a high ionization tendency and has a low melting point. For example, if the positive electrode current collector contains a large amount of aluminum, a large amount of heat may be generated when a thermite reaction occurs between the composite oxide that is the positive electrode active material and aluminum. On the other hand, it is preferable that the positive electrode current collector 22 has aluminum, which is a metal with a low melting point, on its surface. The reason is that since aluminum is a metal (sometimes called valve metal) that forms a passive film in the electrolyte, the stability of the positive electrode current collector 22 can be increased against high oxidation potentials. It is from. Therefore, it is preferable that the positive electrode current collector 22 has a minimum mass of aluminum in the region in contact with the electrolyte. The mass of aluminum contained in the positive electrode current collector 22 is, for example, preferably 0.002% or more and 40% or less, more preferably 0.03% or more and 6% or less of the mass of the positive electrode current collector 22. It is preferably 0.2% or more and 2.5% or less.

Therefore, in the battery of one embodiment of the present invention, a laminated metal sheet is preferably used as the positive electrode current collector 22. In this specification, the laminated metal sheet refers to a metal foil with a low ionization tendency, such as a stainless steel foil, or a metal foil with a high melting point, coated thinly with aluminum.

A case where a laminated metal sheet is used as the positive electrode current collector 22 is shown in FIG. 1C. The laminated metal sheet has a metal foil 22a and a covering layer 22b. It is preferable that the coating layer 22b covers at least a portion of the surface of the metal foil 22a. More preferably, the coating layer 22b is provided on one or both surfaces of the metal foil 22a. Furthermore, it is more preferable that the coating layer 22b covers the entire metal foil 22a, including the side edges of the metal foil 22a.

The coating layer 22b is preferably made of aluminum, a fluoride film, or a metal material capable of forming a film containing fluoride. In addition to aluminum, valve metals (also referred to as valve metals) such as titanium, tantalum, and chromium may be used. Note that the valve metal refers to a metal that can form a passive state on its surface.

When the coating layer 22b is provided on the surface of the metal foil 22a, even if the metal foil 22a is made of a material that cannot form a fluoride coating or a coating containing fluoride, the coating layer 22b can prevent the electrolysis of the components of the metal foil 22a. Elution into the liquid can be suppressed.

The causes of the components of the metal foil 22a eluting into the electrolyte are LiPF ₆ used as the electrolyte of lithium ion batteries, moisture contained as an impurity in the electrolyte, or moisture entering from outside the battery 10. It is thought that the metal foil 22a, which cannot form a fluoride film, is eluted by the hydrogen fluoride.

As the metal foil 22a, for example, one or more of stainless steel foil, chrome foil, nickel foil, molybdenum foil, tantalum foil, tungsten foil, gold foil, platinum foil, iridium foil, etc. is preferable. The metal foil 22a can be produced using a rolling method.

The thickness of the metal foil 22a is preferably 1 μm or more and 30 μm or less, more preferably 5 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less.

The coating layer 22b is formed using the metal foil 22a as a base material using a sputtering method, a vapor deposition method, a CVD (Chemical Vapor Deposition) method, an MBE (Molecular Beam Epitaxy) method, or a PLD (Pulsed Laser Deposition) method. ion) method or ALD (Atomic Layer Deposition) ) method.

The thickness of the coating layer 22b is preferably 1 nm or more and 1 μm or less, more preferably 10 nm or more and 500 nm or less, and more preferably 50 nm or more and 200 nm or less.

As described above, in a battery according to one embodiment of the present invention that uses a laminated metal sheet as the positive electrode current collector 22, when the battery reaches a high temperature, the composite oxide that is the positive electrode active material and the positive electrode current collector 22, There is less risk of a thermite reaction occurring, and it is possible to prevent the battery from catching fire and spreading the fire to the surrounding area. Alternatively, even if a thermite reaction occurs between the composite oxide, which is the positive electrode active material, and the positive electrode current collector 22, the amount of heat generated by the thermite reaction can be reduced, thereby preventing battery ignition and the surrounding area of the battery. Fire spread can be prevented. In other words, the battery of one embodiment of the present invention can suppress thermal runaway. The thermal runaway of a battery will be explained below.

[Battery thermal runaway]
Regarding the principle of thermal runaway in a general lithium ion battery, the graph shown on page 69 [FIG. 2-11] of Non-Patent Document 14 is quoted, and a partially revised diagram is shown in FIG. 2A. For example, when a battery as described above is charged, when the temperature (specifically, the internal temperature) rises, the battery goes through several states and reaches thermal runaway. FIG. 2A is a graph of battery temperature versus time. For example, when the battery temperature reaches or near 100° C., (1) SEI (Solid Electrolyte Interphase) of the negative electrode collapses and generates heat. Furthermore, when the temperature of the battery exceeds 100°C, (2) the electrolyte is reduced and heat generated by the negative electrode (if graphite is used, the negative electrode becomes C ₆ Li), and (3) the electrolyte is oxidized and heat generated by the positive electrode. occurs. When the temperature of the battery reaches 180°C or around 180°C, (4) thermal decomposition of the electrolyte occurs, and (5) oxygen is released from the positive electrode and thermal decomposition of the positive electrode occurs (the thermal decomposition involves structural changes in the positive electrode active material). ) occurs. Thereafter, when the temperature of the battery exceeds 200° C., (6) decomposition of the negative electrode occurs, and finally (7) direct contact between the positive and negative electrodes occurs. After passing through such a state, particularly state (5), state (6), or state (7), the battery goes into thermal runaway.

Furthermore, according to Figure 1 of Non-Patent Document 15, when the temperature of the battery exceeds 660°C, aluminum used as the positive electrode current collector melts, and a thermite reaction with the oxide used as the positive electrode active material occurs. It is stated that the temperature at some points is as high as 1000°C or more.

In order to prevent thermal runaway, it is considered best to suppress the temperature rise of the battery and to ensure that the components that make up the battery (negative electrode, positive electrode, electrolyte, etc.) are stable at high temperatures.

In a battery according to one embodiment of the present invention in which the laminated metal sheet described above is used for the positive electrode current collector 22, when the battery reaches a high temperature, the composite oxide that is the positive electrode active material and the positive electrode current collector 22 This is preferable because there is less possibility that the thermite reaction will occur. Alternatively, even if a thermite reaction occurs between the composite oxide that is the positive electrode active material and the positive electrode current collector 22, the amount of heat generated by the thermite reaction can be reduced, which is preferable.

[Nail insertion test]
Next, a nail penetration test for a general lithium ion battery will be explained using FIGS. 3A, 3B, and the like. The nail penetration test is a test in which the battery 500 is fully charged (States of Charge: equivalent to 100% SOC) and a nail 1003 having a predetermined diameter selected from 2 mm to 10 mm is inserted into the battery at a predetermined speed. . The speed at which the nail is inserted can be, for example, 1 mm/s or more and 20 mm/s or less. FIG. 3A shows a cross-sectional view of the battery 500 with a nail 1003 inserted therein. The battery 500 has a structure in which a positive electrode 503, a separator 508, a negative electrode 506, and an electrolyte 530 are housed in an exterior body 531. The positive electrode 503 has a positive electrode current collector 501 and positive electrode active material layers 502 formed on both surfaces thereof, and the negative electrode 506 has a negative electrode current collector 504 and negative electrode active material layers 505 formed on both surfaces thereof. Further, FIG. 3B shows an enlarged view of the nail 1003 and the positive electrode current collector 501, and also clearly shows the positive electrode active material 100 and the conductive material 553 included in the positive electrode active material layer 502.

As shown in FIGS. 3A and 3B, when nail 1003 penetrates positive electrode 503 and negative electrode 506, an internal short circuit occurs. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode, and electrons (e ⁻ ) flow through the nail 1003 and the like to the positive electrode 503 as shown by the arrow, and Joule heat is generated at the internal short circuit and its vicinity. . Furthermore, due to the internal short circuit, carrier ions, typically lithium ions (Li ⁺ ), released from the negative electrode 506 are released into the electrolytic solution as indicated by the white arrow. Here, if there is a shortage of anions in the electrolytic solution 530, if lithium ions are desorbed from the negative electrode 506 to the electrolytic solution 530, the electrical neutrality of the electrolytic solution 530 will not be maintained. Start disassembling it to keep it accurate. This is one of the electrochemical reactions and is called a reduction reaction of the electrolyte by the negative electrode.

Additionally, the temperature of the battery 500 may rise due to Joule heat. At this time, when lithium cobalt oxide is used as the positive electrode active material, a change in the crystal structure of the lithium cobalt oxide may occur, and further heat generation may occur.

Then, the electrons (e ^- ) flowing to the positive electrode 503 reduce the tetravalent Co in the charged lithium cobalt oxide to become trivalent or divalent, and this reduction reaction releases oxygen from the lithium cobalt oxide. be done. Further, the electrolytic solution 530 is decomposed by an oxidation reaction caused by the oxygen. This is one of the electrochemical reactions and is called the oxidation reaction of the electrolyte by the positive electrode. The speed at which current flows into the positive electrode active material 100 and the like varies depending on the insulation properties of the positive electrode active material, and it is also believed that the speed at which the current flows affects the electrochemical reaction.

When an internal short circuit occurs in the battery, the temperature is considered to change as shown in the graph shown in FIG. 2B. FIG. 2B is a partially revised diagram based on the graph shown on page 70 [FIG. 2-12] of Non-Patent Document 14, and is a graph of battery temperature (specifically, internal temperature) versus time. be. When an internal short circuit occurs at (P0), the temperature of the battery increases over time. As shown in (P1), when the temperature of the battery rises to around 100℃ due to Joule heat generated by an internal short circuit, it exceeds the reference temperature (Ts), which is the limit temperature that does not lead to thermal runaway of the battery. There are cases. Then, in (P2), the electrolyte is reduced and heat is generated by the negative electrode (when graphite is used, the negative electrode becomes C ₆ Li), in (P3), the electrolyte is oxidized and heat is generated by the positive electrode, and in (P4), the electrolyte is oxidized and heat is generated by the positive electrode. Heat generation occurs due to thermal decomposition of the electrolyte. The battery then goes into thermal runaway, resulting in fire or smoke.

In order to prevent smoke, heat generation, etc. from occurring during the nail penetration test, it is considered best to suppress the temperature rise of the battery and to ensure that the components that make up the battery (negative electrode, positive electrode, electrolyte, etc.) are stable at high temperatures. Specifically, it is preferable that the positive electrode active material has a stable structure that does not release oxygen even when exposed to high temperatures. Alternatively, the positive electrode active material preferably has a structure in which the speed of current flowing into the active material is slow. As will be described later in Embodiment 2, the positive electrode active material 100, which is one embodiment of the present invention, can have both the stable structure described above and a structure that slows down the current speed.

The positive electrode can be formed by applying a slurry onto the positive electrode current collector and drying it. Note that pressing may be applied after drying. The positive electrode has an active material layer formed on a current collector.

The slurry is a material liquid used to form an active material layer on a current collector, and contains an active material, a binder, and a solvent, preferably further mixed with a conductive material. Note that the slurry is sometimes called an electrode slurry or an active material slurry, and when forming a positive electrode active material layer, it is also called a positive electrode slurry.

Materials that can be used as the binder 13 and the conductive material 14 used in producing the positive electrode 20 will be described below.

[Binder]
As the binder, it is preferable to use rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Furthermore, fluororubber can be used as the binder.

Further, as the binder, it is preferable to use, for example, a water-soluble polymer. As the water-soluble polymer, for example, polysaccharides can be used. As the polysaccharide, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, or starch can be used. Further, it is more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

Or, as a binder, polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride It is preferable to use materials such as polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, nitrocellulose, etc. .

The binder may be used in combination of more than one of the above.

For example, a material with particularly excellent viscosity adjusting effect may be used in combination with other materials. For example, although rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, for example, it is preferable to mix with a material that is particularly effective in controlling viscosity. As a material having a particularly excellent viscosity adjusting effect, for example, a water-soluble polymer may be used. In addition, as water-soluble polymers having particularly excellent viscosity adjusting effects, the above-mentioned polysaccharides, such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch are used. be able to.

Note that by converting a cellulose derivative such as carboxymethyl cellulose into a salt such as a sodium salt or an ammonium salt of carboxymethyl cellulose, the solubility is increased and the effect as a viscosity modifier is more easily exerted. The increased solubility can also improve the dispersibility with the active material or other components when preparing an electrode slurry. In this specification and the like, cellulose and cellulose derivatives used as binders for electrodes include salts thereof.

By dissolving the water-soluble polymer in water, the viscosity is stabilized, and other materials to be combined as the active material and binder, such as styrene-butadiene rubber, can be stably dispersed in the aqueous solution. Furthermore, since it has a functional group, it is expected that it will be easily adsorbed stably on the surface of the active material. In addition, many cellulose derivatives such as carboxymethylcellulose have functional groups such as hydroxyl or carboxyl groups, and because of these functional groups, polymers interact with each other and may exist widely covering the surface of the active material. Be expected.

When the binder forms a film that covers or is in contact with the surface of the active material, it is expected to serve as a passive film and suppress the decomposition of the electrolyte. Here, the "passive film" is a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of an active material, the battery reaction potential In this case, decomposition of the electrolytic solution can be suppressed. Further, it is more desirable that the passive film suppresses electrical conductivity and can conduct lithium ions.

[Conductive material]
The conductive material is also called a conductivity imparting agent or a conductivity aid, and a carbon material is used. By attaching a conductive material between the plurality of active materials, the plurality of active materials are electrically connected to each other, thereby increasing conductivity. Note that "adhesion" does not only mean that the active material and the conductive material are in close physical contact with each other, but also when a covalent bond occurs or when they bond due to van der Waals forces, the surface of the active material The concept includes cases where a conductive material covers a part of the active material, cases where the conductive material fits into the unevenness of the surface of the active material, cases where the active material is electrically connected even if they are not in contact with each other.

It is preferable that the active material layers, such as the positive electrode active material layer and the negative electrode active material layer, include a conductive material.

Examples of the conductive material include carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fibers such as carbon nanofibers and carbon nanotubes, and graphene compounds. More than one species can be used.

As the carbon fiber, carbon fibers such as mesophase pitch carbon fiber and isotropic pitch carbon fiber can be used. Furthermore, carbon nanofibers, carbon nanotubes, or the like can be used as the carbon fibers. Carbon nanotubes can be produced, for example, by a vapor phase growth method.

The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt% or more and 10 wt% or less, more preferably 1 wt% or more and 5 wt% or less.

Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance. It is possible to improve electrical conductivity with Therefore, the ratio of active material in the active material layer can be increased. Thereby, the discharge capacity of the battery can be increased.

Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, easily enter minute spaces. The minute space refers to, for example, a region between a plurality of active materials. By using a combination of carbon-containing compounds that can easily enter tiny spaces and sheet-like carbon-containing compounds such as graphene that can impart conductivity across multiple particles, we can increase electrode density and create excellent conductive paths. can be formed. A battery obtained by the manufacturing method of one embodiment of the present invention has a high capacity density per volume, can be stable, and is effective as a vehicle-mounted battery.

4A to 4D show configuration examples of the positive electrode 20. 4A to 4D are modifications of the positive electrode 20 shown in FIG. 1C.

FIG. 4A uses carbon black 15 as the carbon material used as the conductive material 14 and illustrates the electrolyte 60 contained in the voids located between the particles of the positive electrode active material 100. An example is shown in which the second positive electrode active material 110 is further included.

A binder (resin) may be mixed in order to fix the positive electrode current collector 11 and the positive electrode active material 100 as a positive electrode of a secondary battery. A binder is also called a binding agent. The binder is a polymeric material, and when a large amount of the binder is included, the proportion of the active material in the positive electrode decreases, and the discharge capacity of the secondary battery decreases. Therefore, it is preferable to mix the amount of binder to a minimum.

Although FIG. 4A shows an example in which the positive electrode active material 100 is spherical, it is not particularly limited. For example, the cross-sectional shape of the positive electrode active material 100 may be an ellipse, a rectangle, a trapezoid, a triangle, a polygon with rounded corners, or an asymmetric shape. For example, FIG. 4B shows an example in which the positive electrode active material 100 has a polygonal shape with rounded corners.

Furthermore, in the positive electrode of FIG. 4B, graphene 16 is used as the carbon material used as the conductive material 14. In FIG. 4B, a positive electrode active material layer including a positive electrode active material 100, graphene 16, and carbon black 15 is formed on the positive electrode current collector 11. Note that graphene 16 alone may be used as the conductive material without using carbon black 15.

As the graphene 16, for example, graphene, multilayer graphene, multigraphene, reduced graphene oxide, reduced multilayer graphene oxide, reduced multilayer graphene oxide, or the like can be used.

FIG. 4C illustrates an example of a positive electrode using carbon fiber 17 instead of graphene. FIG. 4C shows an example different from FIG. 4B. When carbon fiber 17 is used, agglomeration of carbon black 15 can be prevented and dispersibility can be improved. Note that the carbon fiber 17 alone may be used as the conductive material without using the carbon black 15.

As the carbon fiber 17, carbon fibers such as mesophase pitch carbon fiber and isotropic pitch carbon fiber can be used. Furthermore, carbon nanofibers, carbon nanotubes, or the like can be used as the carbon fibers. Carbon nanotubes can be produced, for example, by a vapor phase growth method.

Further, FIG. 4D is illustrated as an example of another positive electrode. FIG. 4D shows an example in which carbon fiber 17 is used in addition to graphene 16. When both graphene 16 and carbon fiber 17 are used, agglomeration of carbon black such as carbon black 15 can be prevented and dispersibility can be further improved.

[Negative electrode]
The negative electrode included in the battery of one embodiment of the present invention includes a negative electrode active material layer and a negative electrode current collector. Further, the negative electrode active material layer includes a negative electrode active material, and may further include a conductive material and a binder.

For the current collector, for example, metal foil such as copper foil can be used. The negative electrode can be formed by applying a slurry onto a metal foil and drying it. Note that pressing may be applied after drying. The negative electrode has an active material layer formed on a current collector.

The slurry is a material liquid used to form an active material layer on a current collector, and contains an active material, a binder, and a solvent, preferably further mixed with a conductive material. Note that the slurry is sometimes called an electrode slurry or an active material slurry, and when forming a negative electrode active material layer, it is also called a negative electrode slurry.

<Negative electrode active material>
For example, a carbon material or an alloy-based material can be used as the negative electrode active material.

As the carbon material, for example, graphite (natural graphite, artificial graphite), graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, etc. can be used. can.

Examples of graphite include artificial graphite and natural graphite. Examples of the artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape, which is preferred. Furthermore, it is relatively easy to reduce the surface area of MCMB, which may be preferable. Examples of natural graphite include flaky graphite and spheroidized natural graphite.

Graphite exhibits a potential as low as that of lithium metal (0.05 V or more and 0.3 V or less vs. Li/Li ⁺ ) when lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is generated). This allows lithium ion batteries using graphite to exhibit high operating voltage. Furthermore, graphite is preferable because it has advantages such as a relatively high capacity per unit volume, a relatively small volumetric expansion, low cost, and higher safety than lithium metal.

Non-graphitizable carbon can be obtained, for example, by firing synthetic resins such as phenol resins or organic substances derived from plants. The non-graphitizable carbon included in the negative electrode active material of the lithium ion battery according to one embodiment of the present invention has a (002) plane spacing of 0.34 nm or more and 0.50 nm or less, as measured by X-ray diffraction (XRD). It is preferably 0.35 nm or more and 0.42 nm or less.

Further, as the negative electrode active material, an element that can perform a charge/discharge reaction by alloying/dealloying reaction with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh/g. For this reason, it is preferable to use silicon as the negative electrode active material. Further, compounds having these elements may also be used. For example, SiO, _Mg2Si _, _Mg2Ge , _SnO , _SnO2 , _Mg2Sn , _SnS2 , _V2Sn3 , _FeSn2 , _CoSn2 , _Ni3Sn2 , _Cu6Sn5 _, _Ag3Sn , Ag _3Sb , _Ni2MnSb _, _CeSb3 , _LaSn3 , _La3Co2Sn7 , _CoSb3 _, InSb, SbSn, and the like. Here, an element that can perform a charging/discharging reaction by alloying/dealloying reaction with lithium, a compound having the element, etc. may be referred to as an alloy-based material.

In this specification and the like, "SiO" refers to silicon monoxide, for example. Alternatively, SiO can also be expressed as SiO _x . Here, x preferably has a value of 1 or a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less.

In addition, as negative electrode active materials, titanium dioxide (TiO ₂ ), lithium titanium oxide (Li ₄ Ti ₅ O ₁₂ ), lithium-graphite intercalation compound (Li _x C ₆ ), niobium pentoxide (Nb ₂ O ₅ ), oxidized Oxides such as tungsten (WO ₂ ) and molybdenum oxide (MoO ₂ ) can be used.

Further, as the negative electrode active material, Li _3-x M _x N (M=Co, Ni, Cu) having a Li ₃ N type structure, which is a double nitride of lithium and a transition metal, can be used. For example, Li _2.6 Co _0.4 N ₃ is preferable because it exhibits a large discharge capacity (900 mAh/g, 1890 mAh/cm ³ ).

When a double nitride of lithium and a transition metal is used, since the negative electrode active material contains lithium ions, it can be combined with materials such as V ₂ O ₅ and Cr ₃ O ₈ that do not contain lithium ions as the positive electrode active material, which is preferable. . Note that even when a material containing lithium ions is used as the positive electrode active material, a double nitride of lithium and a transition metal can be used as the negative electrode active material by removing lithium ions contained in the positive electrode active material in advance.

Furthermore, a material that causes a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used as the negative electrode active material. Materials that cause conversion reactions include oxides such as Fe ₂ O ₃ , CuO, Cu ₂ O, RuO ₂ , and Cr ₂ O ₃ , sulfides such as CoS _0.89 , NiS, and CuS, and Zn ₃ N ₂ , nitrides such as Cu ₃ N and Ge ₃ N ₄ , phosphides such as NiP ₂ , FeP ₂ and CoP ₃ , and fluorides such as FeF ₃ and BiF ₃ .

Note that one type of negative electrode active material can be used from among the negative electrode active materials shown above, but a combination of multiple types can also be used. For example, it can be a combination of a carbon material and silicon, or a combination of a carbon material and silicon monoxide.

In addition, as another form of the negative electrode, it may be a negative electrode that does not have a negative electrode active material at the time of completion of battery production. An example of a negative electrode that does not have a negative electrode active material is a negative electrode that has only a negative electrode current collector at the end of battery production, and the lithium ions that are released from the positive electrode active material when the battery is charged are deposited on the negative electrode current collector. It can be a negative electrode that is precipitated as lithium metal to form a negative electrode active material layer. A battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

When using a negative electrode that does not have a negative electrode active material, a film may be provided on the negative electrode current collector to uniformly deposit lithium. For example, a solid electrolyte having lithium ion conductivity can be used as a membrane for uniformly depositing lithium. As the solid electrolyte, sulfide-based solid electrolytes, oxide-based solid electrolytes, polymer-based solid electrolytes, and the like can be used. Among these, a polymer solid electrolyte is suitable as a film for uniformly depositing lithium because it is relatively easy to form a uniform film on the negative electrode current collector. Further, as a film for uniformizing lithium precipitation, for example, a metal film that forms an alloy with lithium can be used. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Since lithium and magnesium form a solid solution over a wide composition range, it is suitable as a film for uniformizing the precipitation of lithium.

[Electrolytes]
As one form of electrolyte, an electrolytic solution including a solvent and an electrolyte dissolved in the solvent can be used. The electrolyte includes a solvent and a lithium salt. As the solvent for the electrolytic solution, aprotic organic solvents are preferred, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, and dimethyl carbonate. (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4 - One or more of dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or any combination and ratio of two or more of these. It can be used in

When the organic solvent contained in the electrolytic solution contains ethylene carbonate (EC) and diethyl carbonate (DEC), when ethylene carbonate and diethyl carbonate are 100 vol%, the volume ratio of ethylene carbonate and diethyl carbonate is A mixed organic solvent in which x: 100-x (20≦x≦40) can be used. More specifically, a mixed organic solvent containing EC and DEC in a ratio of EC:DEC=30:70 (volume ratio) can be used.

In addition, when the organic solvent contained in the electrolytic solution contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are combined at 100 vol%. When the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is x:y:100-x-y (5≦x≦35 and 0<y<65). Organic solvents can be used. More specifically, a mixed organic solvent containing EC, EMC, and DMC in a ratio of EC:EMC:DMC=30:35:35 (volume ratio) can be used.

Furthermore, as the organic solvent contained in the electrolytic solution, a fluorinated cyclic carbonate or a mixed organic solvent containing a fluorinated chain carbonate can be used. Furthermore, it is preferable that the mixed organic solvent contains both a fluorinated cyclic carbonate and a fluorinated chain carbonate. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have a substituent that exhibits electron-withdrawing properties, and are preferred because they lower the solvation energy of lithium ions. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable for the electrolytic solution, and a mixed organic solvent thereof is suitable.

As the fluorinated cyclic carbonate, for example, fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), or tetrafluoroethylene carbonate (F4EC) can be used. Can be done. Note that DFEC has isomers such as cis-4,5 and trans-4,5. Since any of the fluorinated cyclic carbonates has a substituent that exhibits electron-withdrawing properties, it is thought that the solvation energy of lithium ions is low. In FEC, the electron-withdrawing substituent is the F group.

An example of the fluorinated chain carbonate is

methyl

3,3,3-trifluoropropionate. The abbreviation for

methyl

3,3,3-trifluoropropionate is "MTFP". In MTFP, the electron-withdrawing substituent is the _CF3 group.

FEC is one of the cyclic carbonates and has a high dielectric constant, so when used in an organic solvent, it has the effect of promoting the dissociation of lithium salt. On the other hand, since FEC has a substituent that exhibits electron-withdrawing properties, desolvation with lithium ions progresses more easily than ethylene carbonate (EC). Specifically, the solvation energy of lithium ions in FEC is smaller than that in ethylene carbonate (EC), which does not have a substituent that exhibits electron-withdrawing properties. Therefore, lithium ions are easily released on the surface of the positive electrode active material and the surface of the negative electrode active material, and the internal resistance of the secondary battery can be lowered. Furthermore, since FEC has a deep highest occupied molecular orbital (HOMO) level, a deep HOMO level is less likely to be oxidized and improves oxidation resistance. On the other hand, there is concern that FEC has a high viscosity. Therefore, it is preferable to use a mixed organic solvent containing not only FEC but also MTFP in the electrolytic solution. MTFP is one of the chain carbonates and has the effect of lowering the viscosity of the electrolytic solution or maintaining the viscosity at room temperature (typically 25°C) even at low temperatures (typically 0°C). is also possible. Furthermore, although MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, it is difficult to solvate lithium ions when used in an electrolyte. may be generated.

Assuming that the mixed organic solvent containing FEC and MTFP having such physical properties is 100 vol%, the volume ratio is x:100-x (5≦x≦30, preferably 10≦x≦20). It is best to mix them and use them. In other words, in the mixed organic solvent, it is preferable to mix the organic solvents so that MTFP is larger than FEC. The negative electrode of one embodiment of the present invention has a coating layer on the surface of the negative electrode active material, and when metallic titanium is used for the coating layer, when fluorine ions are generated in the electrolytic solution containing the above FEC or MTFP, It is also possible to form a passive film on the surface of titanium metal. Therefore, the negative electrode of one embodiment of the present invention can be suitably combined with an electrolytic solution containing a mixed organic solvent containing FEC and MTFP.

In addition, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as a solvent for the electrolyte, internal temperature increases due to internal short circuits or overcharging of the secondary battery can be avoided. It is also possible to prevent secondary batteries from bursting and/or catching fire. Ionic liquids are composed of cations and anions, and include organic cations and anions. Examples of the organic cation used in the electrolytic solution include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. In addition, examples of anions used in the electrolytic solution include monovalent amide anions, monovalent methide anions, fluorosulfonic acid anions, perfluoroalkylsulfonic acid anions, tetrafluoroborate anions, perfluoroalkylborate anions, and hexafluorophosphate anions. , or perfluoroalkyl phosphate anion. For example, lithium bis(fluorosulfonyl)imide (also referred to as EMI-FSI) can be used.

[Lithium salt]
Examples of the lithium salt (also called electrolyte) to be dissolved in the above solvent include LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiAlCl ₄ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC(CF ₃ SO ₂ ) ₃ , LiC(C ₂ F ₅ SO ₂ ) ₃ , LiN(SO ₂ F) ₂ (also referred to as LiFSI) , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₄ F ₉ SO ₂ )(CF ₃ SO ₂ ), LiN(C ₂ F ₅ SO ₂ ) ₂ , etc., or two of these. The above can be used in any combination and ratio. The amount of the lithium salt relative to the solvent is preferably 0.5 mol/L or more and 3.0 mol/L or less. Use of fluorides such as LiPF ₆ and LiBF ₄ improves the safety of lithium ion secondary batteries.

As the electrolytic solution mentioned above, it is preferable to use a highly purified electrolytic solution that has a low content of particulate dust or elements other than the constituent elements of the electrolytic solution (hereinafter also simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolytic solution is 1 wt% or less, preferably 0.1 wt% or less, more preferably 0.01 wt% or less.

[Additive]
The electrolyte may contain additives. The additive can suppress reaction decomposition of the electrolyte that may occur on the surface of the positive electrode or the negative electrode when operating the secondary battery at high voltage and/or high temperature. As additives, for example, vinylene carbonate (VC), propane sultone (PS), TerT-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate)borate (LiBOB) may be used. LiBOB is particularly preferred because it easily forms a good film. VC or FEC is preferable because it forms a good film on the negative electrode during aging of the secondary battery or during charging at the initial stage of use and improves cycle characteristics.

As the additive, any one type or two or more types of dinitrile compounds can be used. Specific examples of dinitrile compounds include, for example, succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

Furthermore, fluorobenzene (FB) may be added to the above organic solvent. The concentration of the additive may be, for example, 0.1 wt% or more and 5 wt% or less based on the entire electrolytic solution. PS or EGBE is preferable because it forms a good film on the positive electrode during charging and discharging and improves cycle characteristics. FB is preferable because it improves the wettability of the organic solvent to the positive electrode and negative electrode. A dinitrile compound is preferable because the nitrile group is oriented toward the positive electrode and the negative electrode, inhibiting oxidative decomposition of the organic solvent, and thus improving high voltage resistance. Furthermore, dinitrile compounds are preferable because they can prevent dissolution of copper during overdischarge when a current collector containing copper is used in the negative electrode. Considering the use of secondary batteries at high voltages, it is preferable to add a nitrile compound.

[Gel electrolyte]
As the gel electrolyte, a polymer gel obtained by swelling a polymer with an electrolytic solution may be used. By using a polymer gel electrolyte, a semi-solid electrolyte layer can be provided, increasing safety against leakage and the like. Further, it is possible to make the secondary battery thinner and lighter.

As the polymer to be gelled, silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, fluoropolymer gel, etc. can be used.

As the polymer, for example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing them can be used. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[Solid electrolyte]
Instead of the electrolyte, a solid electrolyte containing an inorganic material such as a sulfide or oxide, a solid electrolyte containing a polymeric material such as PEO (polyethylene oxide), or the like can be used. When a solid electrolyte is used, it is not necessary to install a separator and/or spacer. Additionally, since the entire battery can be solidified, there is no risk of leakage, dramatically improving safety.

[Separator]
Next, the separator 40 shown in FIG. 1B etc. will be explained. In the case of a battery having an electrolyte, a separator is placed between the positive electrode and the negative electrode. Examples of separators include fibers containing cellulose such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic materials using nylon (polyamide), polyimide, vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. A material made of fiber or the like can be used. It is preferable that the separator is processed into a bag shape and arranged so as to surround either the positive electrode or the negative electrode.

The separator may have a multilayer structure. For example, a film of an organic material such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine material, a polyamide material, a polyimide material, or a mixture thereof. As the ceramic material, for example, aluminum oxide particles (alumina, boehmite, etc.), silicon oxide particles, etc. can be used. As the fluorine-based material, for example, PVDF, polytetrafluoroethylene, etc. can be used. As the polyamide material, for example, nylon, aramid (meta-aramid, para-aramid), etc. can be used.

Coating with a ceramic material improves oxidation resistance, so it is possible to suppress deterioration of the separator during high voltage charging and improve battery reliability. Furthermore, coating with a fluorine-based material makes it easier for the separator and electrode to come into close contact with each other, thereby improving output characteristics. Coating with a polyamide-based material, especially aramid, improves heat resistance, which can improve the safety of the battery.

For example, a mixed material of aluminum oxide and aramid may be coated on both sides of a polypropylene film. Alternatively, the surface of the polypropylene film in contact with the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface in contact with the negative electrode may be coated with a fluorine-based material.

By using a separator with a multilayer structure, the safety of the battery can be maintained even if the overall thickness of the separator is thin, so the capacity per volume of the battery can be increased.

[Exterior body]
Next, the exterior body 50 shown in FIG. 1A etc. will be explained. As the exterior body of the battery, for example, a metal material such as aluminum, stainless steel, or titanium, or a resin material can be used. Moreover, a film-like exterior body can also be used. As a film, for example, a highly flexible metal thin film or metal foil such as aluminum, stainless steel, titanium, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide. A three-layer film can be used in which an insulating synthetic resin film such as polyamide resin or polyester resin is provided on a thin metal film as the outer surface of the exterior body. A film with such a multilayer structure can be called a laminate film. At this time, the laminate film may be called an aluminum laminate film, a stainless steel laminate film, a titanium laminate film, a copper laminate film, a nickel laminate film, etc. using the name of the material of the metal layer that the laminate film has.

The material or thickness of the metal layer included in the laminate film may affect the flexibility of the battery. For example, it is preferable to use an aluminum laminate film having a polypropylene layer, an aluminum layer, and a nylon layer as an exterior body used in a battery where flexibility or weight reduction is important. Here, the thickness of the aluminum layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Note that if the aluminum layer is thinner than 10 μm, there is a concern that the gas barrier properties will deteriorate due to pinholes in the aluminum layer, so the thickness of the aluminum layer is preferably 10 μm or more.

It is preferable to use, for example, a stainless steel laminate film having a polypropylene layer, a stainless steel layer, and a nylon layer as an exterior body for a battery where physical strength or safety is important. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the stainless steel layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Note that if the stainless steel layer is thinner than 10 μm, there is a concern that the gas barrier properties may be deteriorated due to pinholes in the stainless steel layer, so the thickness of the stainless steel layer is preferably 10 μm or more. Note that stainless steel in this specification refers to steel (an alloy of iron and carbon) containing about 12% or more of chromium, and can be roughly classified into martensitic, ferritic, or austenitic based on composition. Note that stainless steel to which one or more selected from Ti, Nb, Mo, Cu, Ni, or Si is added is also included.

Alternatively, for example, it is preferable to use a titanium laminate film having a polypropylene layer, a titanium layer, and a nylon layer. Furthermore, a polyethylene terephthalate layer may be provided on the nylon layer. Here, the thickness of the titanium layer is preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. Note that if the titanium layer is thinner than 10 μm, there is a concern that the gas barrier properties will deteriorate due to pinholes in the titanium layer, so the thickness of the titanium layer is preferably 10 μm or more.

Furthermore, a graphene sheet may be used as the laminate film instead of the above metal layer. As the graphene sheet, a multilayer graphene sheet with a size of 100 nm or more and 30 μm or less, preferably 200 nm or more and 20 μm or less can be used. Since the graphene sheet is flexible, has an interlayer distance of 0.34 nm, and has gas barrier properties, it is suitable as a film for use in the exterior of a secondary battery.

[Example of electrode laminate]
Below, a configuration example of a laminate having a plurality of stacked electrodes will be described.

5A shows the positive electrode current collector 22, FIG. 5B shows the separator 40, FIG. 5C shows the negative electrode current collector 32, FIG. 5D shows the positive electrode lead 21 and negative electrode lead 31, and FIG. 5E shows the film-like exterior body 50. A top view (also called a plan view) is shown. The positive electrode lead 21 has a sealing part 24 and a lead metal 76a, and the negative electrode lead 31 has a sealing part 34 and a lead metal 76b.

The dimensions of each of the figures in FIGS. 5A to 5E are approximately the same, and the region 41 surrounded by the dashed line in FIG. 5E is approximately the same as the dimension of the separator in FIG. 5B. Further, the area between the broken line and the end in FIG. 5E becomes the sealing part 51.

Further, the protruding portion of the positive electrode current collector 22 (the broken line portion in FIG. 5A) and the protruding portion of the negative electrode current collector 32 (the broken line portion in FIG. 5C) are referred to as tab portions.

FIG. 6A is an example in which positive electrode active material layers 23 are provided on both sides of the positive electrode current collector 22. To explain in detail, the negative electrode current collector 32, the negative electrode active material layer 33, the separator 40, the positive electrode active material layer 23, the positive electrode current collector 22, the positive electrode active material layer 23, the separator 40, the negative electrode active material layer 33, the negative electrode current collector They are arranged in the order of body 32. A cross-sectional view of this laminated structure taken along a plane 70 is shown in FIG. 6B. Note that the positive electrode active material layer 23 can be provided on both sides of the positive electrode current collector 22, except for the tab portion described in FIG. 5A. Further, the negative electrode active material layer 33 can be provided on one side of the negative electrode current collector 32, excluding the tab portion described in FIG. 5C.

Note that although FIG. 6A shows an example in which two separators are used, a structure in which one separator is bent and sealed at both ends to form a bag shape, and the positive electrode current collector 22 is stored between the two separators is shown. It is also possible to do this. A positive electrode active material layer 23 is formed on both sides of a positive electrode current collector 22 housed in a bag-shaped separator.

It is also possible to provide the negative electrode active material layer 33 on both sides of the negative electrode current collector 32. In FIG. 6C, between two negative electrode current collectors 32 having negative electrode active material layers 33 on only one side, three negative electrode current collectors 32 having negative electrode active material layers 33 on both sides, and positive electrode active material layers on both sides. An example is shown in which a secondary battery is constructed by sandwiching four positive electrode current collectors 22 having a diameter of 23 and eight separators 40. In FIG. 6C, the negative electrode active material layer 33 can be provided on both sides of the negative electrode current collector 32, except for the tab portion explained in FIG. 5C. In this case as well, instead of using eight separators, four bag-shaped separators may be used.

The capacity of the secondary battery can be increased by increasing the number of layers. Furthermore, by providing the positive electrode active material layers 23 on both sides of the positive electrode current collector 22 and the negative electrode active material layers 33 on both sides of the negative electrode current collector 32, the thickness of the secondary battery can be reduced.

FIG. 7A shows a diagram of a secondary battery formed by providing the positive electrode active material layer 23 on only one side of the positive electrode current collector 22 and providing the negative electrode active material layer 33 on only one side of the negative electrode current collector 32. To explain in detail, a negative electrode active material layer 33 is provided on one side of the negative electrode current collector 32, and a separator 40 is laminated so as to be in contact with the negative electrode active material layer 33. The surface of the separator 40 on the side not in contact with the negative electrode active material layer 33 is in contact with the positive electrode active material layer 23 of the positive electrode current collector 22 with the positive electrode active material layer 23 formed on one side. The surface of the positive electrode current collector 22 is in contact with the positive electrode current collector 22 on which another positive electrode active material layer 23 is formed on one side. At this time, the positive electrode current collector 22 is arranged so that the surfaces on which the positive electrode active material layer 23 is not formed face each other. Then, a separator 40 is further formed, and the negative electrode active material layer 33 of the negative electrode current collector 32 having the negative electrode active material layer 33 formed on one side thereof is laminated so as to be in contact with the separator.

A cross-sectional view of the laminated structure of FIG. 7A taken along a plane 71 is shown in FIG. 7B.

In FIG. 7A, two separators are used, but one separator is bent and both ends are sealed to form a bag shape, and between them, two positive electrode current collectors 22 with a positive electrode active material layer 23 arranged on one side are inserted. You can also sandwich it.

FIG. 7C shows a diagram in which a plurality of the laminated structures of FIG. 7A are laminated. In FIG. 7C, the surfaces of the negative electrode current collector 32 on which the negative electrode active material layer 33 is not formed are arranged facing each other. FIG. 7C shows a state in which 12 positive electrode

current collectors

22, 12 negative electrode

current collectors

32, and 12 separators 40 are stacked.

In the case of the configuration shown in FIGS. 7A to 7C, the positive electrode active material layer 23 can be provided on one side of the positive electrode current collector 22, except for the tab portion described in FIG. 5A. Further, the negative electrode active material layer 33 can be provided on one side of the negative electrode current collector 32, excluding the tab portion described in FIG. 5C.

After stacking as shown in FIGS. 6 and 7, the plurality of positive electrode current collectors 22 are all fixed at the tab portions and electrically connected. Similarly, all of the plurality of negative electrode current collectors 32 are fixed and electrically connected at the tab portion.

Here, it is preferable that the positive electrode lead 21 and the plurality of positive electrode current collectors 22 are fixed at the same time using tab portions and electrically connected. Similarly, it is preferable that the negative electrode lead 31 and the plurality of negative electrode current collectors 32 are simultaneously fixed and electrically connected using tab portions. By simultaneously connecting a plurality of current collectors and electrode leads in this manner, production can be performed efficiently.

As a method of fixing the plurality of current collectors and electrode leads, a method of fixing them by welding such as ultrasonic welding, etc. can be used.

FIGS. 8A to 8C are cross-sectional schematic diagrams showing an example of a laminate in which a positive electrode 20, a negative electrode 30, and a separator 40 are laminated. The schematic cross-sectional views shown in FIGS. 8A to 8C are schematic cross-sectional views taken along Y1-Y2 of the battery 10 shown in FIG. 1A, and the exterior body 50 is omitted. Further, regarding the positive electrode 20 and the negative electrode 30, illustration of the current collector and the active material layer is omitted to avoid complication of the drawings.

As shown in FIGS. 8A to 8C, the separator 40 has a region located between the positive electrode 20 and the negative electrode 30. In other words, the positive electrode 20 and the negative electrode 30 have an overlapping region with the separator 40 in between. Note that the separator 40 may have a region located between the positive electrode 20 and the exterior body 50, or may have a region located between the negative electrode 30 and the exterior body 50.

Like the laminate shown in FIG. 8A, a laminate including a plurality of positive electrodes 20, a plurality of negative electrodes 30, and a plurality of separators 40 can be formed.

Also, like the laminate shown in FIGS. 8B and 8C, a laminate including a plurality of positive electrodes 20, a plurality of negative electrodes 30, and one separator 40 can be formed.

As in the laminate shown in FIG. 8B, the separator 40 has a meandering shape and can be positioned between the plurality of positive electrodes 20 and the plurality of negative electrodes 30.

Furthermore, as in the laminate shown in FIG. 8C, the separator 40 can have a wound shape and be positioned between the plurality of positive electrodes 20 and the plurality of negative electrodes 30.

This embodiment can be implemented in combination with other embodiments as appropriate.

(Embodiment 2)
In this embodiment, a positive electrode active material that can be used for a positive electrode of a battery according to one embodiment of the present invention will be described with reference to FIGS. 9 to 24.

9A to 9C are cross-sectional views of a positive electrode active material 100 that is one embodiment of the present invention. As shown in FIG. 9A, the positive electrode active material 100 has a surface layer portion 100a and an interior portion 100b. In these figures, the boundary between the surface layer portion 100a and the interior portion 100b is indicated by a broken line. Further, FIG. 9B shows a positive electrode active material 100 having a buried part 102. (001) in the figure indicates the (001) plane of lithium cobalt oxide. LiCoO ₂ belongs to space group R-3m. Further, in FIG. 9C, a part of the grain boundary 101 is shown by a dashed line. Note that the surface layer portion 100a can be referred to as a barrier film, and the lithium cobalt oxide having the surface layer portion 100a may be referred to as lithium cobalt oxide having a barrier film.

In this specification and the like, the surface layer 100a of the positive electrode active material 100 is, for example, within 50 nm from the surface toward the inside, more preferably within 35 nm from the surface toward the inside, and still more preferably 20 nm from the surface toward the inside. most preferably refers to a region within 10 nm perpendicularly or substantially perpendicularly from the surface toward the inside. Note that "substantially perpendicular" is defined as 80° or more and 100° or less. Cracks and/or surfaces caused by cracks may also be referred to as surfaces. The surface layer portion 100a has the same meaning as near-surface, near-surface region, or shell.

Further, a region deeper than the surface layer portion 100a of the positive electrode active material is referred to as an interior 100b. Interior 100b is synonymous with interior region or core.

The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the interior portion 100b. Therefore, the positive electrode active material 100 is made of materials to which metal oxides such as aluminum oxide (Al ₂ O ₃ ) that do not have lithium sites that can contribute to charging and discharging are attached, and carbonates chemically adsorbed after the production of the positive electrode active material. , hydroxyl group, etc. are not included. Note that the deposited metal oxide refers to a metal oxide whose crystal structure does not match that of the interior 100b, for example.

It is also assumed that the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that adhere to the positive electrode active material 100 are not included.

In addition, the crystal grain boundaries 101 are, for example, areas where particles of the positive electrode active material 100 are fixed to each other, areas where the crystal orientation changes inside the positive electrode active material 100, in other words, the repetition of bright lines and dark lines in a STEM image etc. is discontinuous. This refers to areas where the crystal structure is disordered, areas with many crystal defects, areas where the crystal structure is disordered, etc. Furthermore, crystal defects refer to defects that can be observed in cross-sectional TEM (transmission electron microscopy), cross-sectional STEM images, etc., that is, structures where other atoms enter between lattices, cavities, etc. The grain boundary 101 can be said to be one of the planar defects. Further, the vicinity of the grain boundary 101 refers to a region within 10 nm from the grain boundary 101.

<Contained elements>
The positive electrode active material 100 includes lithium, cobalt, oxygen, and additional elements. Alternatively, the positive electrode active material 100 includes lithium cobalt oxide (LiCoO ₂ ) to which an additive element is added. However, the positive electrode active material 100 according to one embodiment of the present invention may have a crystal structure described below. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

The positive electrode active material of a lithium ion secondary battery must contain a transition metal capable of redox in order to maintain charge neutrality even when lithium ions are intercalated and desorbed. It is preferable that the positive electrode active material 100 of one embodiment of the present invention mainly uses cobalt as the transition metal responsible for the redox reaction. In addition to cobalt, at least one or two selected from nickel and manganese may be used. Among the transition metals contained in the positive electrode active material 100, if cobalt accounts for 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, it is relatively easy to synthesize, easy to handle, and has excellent cycle characteristics. It is preferable as it has many advantages.

In addition, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metals in the positive electrode active material 100, nickel such as lithium nickelate (LiNiO ₂ ) accounts for the majority of the transition metals. The stability is better when x in Li x CoO 2 is small compared to complex oxides in which the amount of x in Li _x CoO ₂ is small. This is thought to be because cobalt is less affected by distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds differs depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt complex oxides, such as lithium nickelate, in which octahedral-coordinated low-spin nickel (III) accounts for the majority of the transition metal, are strongly influenced by the Jahn-Teller effect, and are separated from the octahedral structure of nickel and oxygen. Distortion is likely to occur in the layers. Therefore, there is a growing concern that the crystal structure will collapse during charge/discharge cycles. Also, nickel ions are larger than cobalt ions and are close to the size of lithium ions. Therefore, in layered rock salt type composite oxides in which nickel accounts for the majority of the transition metal, such as lithium nickelate, there is a problem that cation mixing of nickel and lithium tends to occur.

The additive elements included in the positive electrode active material 100 include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. It is preferable to use one or more selected ones. Moreover, the sum of transition metals among the additional elements is preferably less than 25 atom %, more preferably less than 10 atom %, and even more preferably less than 5 atom %.

In other words, the positive electrode active material 100 includes lithium cobalt oxide to which magnesium and fluorine are added, lithium cobalt oxide to which magnesium, fluorine and titanium are added, lithium cobalt oxide to which magnesium, fluorine and aluminum are added, magnesium, fluorine and nickel. It can have added lithium cobalt oxide, lithium cobalt oxide added with magnesium, fluorine, nickel and aluminum, and the like.

It is preferable that the additive element is dissolved in the positive electrode active material 100. Therefore, for example, when performing STEM-EDX line analysis, the depth at which the amount of added elements increases is deeper than the depth at which the amount of transition metal M is detected, that is, the positive electrode active area. Preferably, it is located inside the substance 100.

In this specification, etc., the depth at which the amount of a certain element detected in STEM-EDX line analysis increases is defined as the depth at which measurement values that can be determined not to be noise from the viewpoint of intensity, spatial resolution, etc. are continuously obtained. This refers to the depth at which it becomes like this.

These additional elements further stabilize the crystal structure of the positive electrode active material 100, as described below. Note that in this specification and the like, the additive element has the same meaning as a mixture or a part of raw materials.

Note that the additive elements do not necessarily include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium. .

For example, if the positive electrode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis, ease of handling, and excellent cycle characteristics will be greater. The weight of manganese contained in the positive electrode active material 100 is, for example, preferably 600 ppm or less, more preferably 100 ppm or less.

<Crystal structure>
≪When x in Li _x CoO ₂ is 1≫
The positive electrode active material 100 of one embodiment of the present invention preferably has a layered rock-salt crystal structure belonging to space group R-3m in a discharge state, that is, when x=1 in Li _x CoO ₂ . Layered rock salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, are suitable for lithium ion insertion/extraction reactions, and are excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the interior 100b, which occupies most of the volume of the positive electrode active material 100, has a layered rock salt crystal structure. FIG. 16 shows the layered rock salt type crystal structure with R-3m O3 attached. R-3m O3 has lattice constants a = 2.81610, b = 2.81610, c = 14.05360, α = 90.0000, β = 90.0000, γ = 120.0000, and in the unit cell. The coordinates of lithium, cobalt, and oxygen are Li (0, 0, 0), Co (0, 0, 0.5), and O (0, 0, 0.23951) (Non-Patent Document 7).

On the other hand, the surface layer 100a of the positive electrode active material 100 according to one embodiment of the present invention is reinforced so that the layered structure made of octahedrons of cobalt and oxygen in the interior 100b will not be broken even if lithium is removed from the positive electrode active material 100 due to charging. It is preferable to have a function. Alternatively, it is preferable that the surface layer portion 100a functions as a barrier film for the positive electrode active material 100. Alternatively, it is preferable that the surface layer portion 100a, which is the outer peripheral portion of the positive electrode active material 100, reinforces the positive electrode active material 100. Reinforcement here refers to suppressing structural changes in the surface layer portion 100a and interior portion 100b of the positive electrode active material 100, such as desorption of oxygen and/or displacement of the layered structure consisting of an octahedron of cobalt and oxygen. and/or suppressing oxidative decomposition of the electrolyte on the surface of the positive electrode active material 100.

Therefore, it is preferable that the surface layer portion 100a has a crystal structure different from that of the interior portion 100b. Further, it is preferable that the surface layer portion 100a has a composition and crystal structure that are more stable at room temperature (25° C.) than the interior portion 100b. For example, it is preferable that at least a portion of the surface layer portion 100a of the positive electrode active material 100 according to one embodiment of the present invention has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt type crystal structure and a rock salt type crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt type and a rock salt type crystal structure.

The surface layer portion 100a is a region where lithium ions are first desorbed during charging, and is a region where the lithium concentration tends to be lower than that in the interior portion 100b. Further, it can be said that some of the bonds of the atoms on the surface of the particles of the positive electrode active material 100 included in the surface layer portion 100a are in a state of being broken. Therefore, the surface layer portion 100a tends to become unstable, and can be said to be a region where the crystal structure tends to deteriorate. For example, if the crystal structure of the layered structure made of octahedrons of cobalt and oxygen shifts in the surface layer 100a, the influence will be chained to the interior 100b, causing the crystal structure of the layered structure to shift in the interior 100b as well. This is thought to lead to deterioration of the crystal structure. On the other hand, if the surface layer 100a can be made sufficiently stable, even when x in Li _x CoO ₂ is small, for example, even when x is 0.24 or less, the layered structure consisting of cobalt and oxygen octahedrons in the interior 100b will be difficult to break. Can be done. Furthermore, it is possible to suppress misalignment of the octahedral layer of cobalt and oxygen in the interior 100b.

〔distribution〕
In order to make the surface layer portion 100a have a stable composition and crystal structure, the surface layer portion 100a preferably contains an additive element, and more preferably contains a plurality of additive elements. Further, it is preferable that the concentration of one or more selected additive elements is higher in the surface layer portion 100a than in the interior portion 100b. Further, it is preferable that one or more selected from the additive elements included in the positive electrode active material 100 have a concentration gradient. Further, it is more preferable that the distribution of the positive electrode active material 100 differs depending on the added element. For example, it is more preferable that the depth of the detected amount peak in the surface layer from the surface or a reference point in EDX-ray analysis described below differs depending on the added element. The peak of the detected amount here refers to the maximum value of the detected amount in the

surface layer portion

100a or 50 nm or less from the surface. The detected amount refers to, for example, a count in EDX-ray analysis.

Arrows X1-X2 are shown in FIG. 9 as an example of the depth direction of a crystal plane other than the (001) plane of lithium cobalt oxide in the positive electrode active material 100 of one embodiment of the present invention. Examples of characteristic X-ray intensity distributions (also referred to as EDX-ray analysis profiles) of each additive element when EDX-ray analysis is performed along arrows X1-X2 are shown in FIGS. 10A to 10C.

As shown in FIGS. 10A to 10C, it is preferable that the amount of at least magnesium and nickel among the added elements detected in the surface layer portion 100a is larger than the amount detected in the interior portion 100b. Furthermore, it is preferable that the detected amounts of magnesium and nickel have a narrow peak in a region closer to the surface in the surface layer portion 100a. For example, it is preferable to have peaks of detected amounts of magnesium and nickel within 3 nm from the surface or a reference point. Moreover, it is preferable that the distributions of magnesium and nickel overlap. The peaks of the detected amounts of magnesium and nickel may be at the same depth, the peak of magnesium may be closer to the surface, and the peak of nickel may be closer to the surface as shown in FIG. 10B. The difference in depth between the peak of the detected amount of nickel and the peak of the detected amount of magnesium is preferably within 3 nm, and more preferably within 1 nm.

Further, the amount of nickel detected in the interior 100b may be very small compared to the surface layer 100a, or may not be detected, that is, it may be below the lower limit of detection.

Although not shown, it is preferable that the amount of fluorine detected in the surface layer 100a is larger than the amount detected inside, similar to magnesium or nickel. Moreover, it is preferable that the detected amount of fluorine has a peak in a region closer to the surface of the surface layer portion 100a. For example, it is preferable that the detected amount of fluorine has a peak within 3 nm from the surface or the reference point. Similarly, it is preferable that the amount of titanium, silicon, phosphorus, boron, and/or calcium detected in the surface layer portion 100a is larger than the amount detected inside. Furthermore, it is preferable that the detected amounts of titanium, silicon, phosphorus, boron, and/or calcium have a peak in a region closer to the surface of the surface layer portion 100a. For example, it is preferable to have a peak of the detected amount of titanium, silicon, phosphorus, boron, and/or calcium within 3 nm from the surface or a reference point.

Also, among the additive elements, it is preferable that at least aluminum has a detected amount peak inside the element compared to magnesium. The distributions of magnesium and aluminum may overlap as shown in FIG. 10A, or the distributions of magnesium and aluminum may not overlap as shown in FIG. 10C. The peak of the detected amount of aluminum may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. For example, it is preferable to have a peak in a region of 5 nm or more and 30 nm or less from the surface or the reference point toward the inside.

Further, the distribution of aluminum may not be a normal distribution. For example, when the distribution of aluminum is divided by the maximum value Max _Al , the length of the hem may differ between the front side and the inside side. More specifically, as shown in FIG. 11B, the peak width at a height (1/5 Max _Al ) of the maximum value (Max _Al ) of the detected amount of aluminum is plotted from the maximum value on the horizontal axis. When divided into two by a downward perpendicular line, the peak width _Wc on the inner side may be larger than the peak width _Ws on the surface side.

The reason why aluminum is distributed further into the interior than magnesium is thought to be because the diffusion rate of aluminum is higher than that of magnesium. On the other hand, the reason why the amount of aluminum detected in the region closest to the surface is small is presumed to be because aluminum can exist more stably in regions where magnesium and the like are not dissolved than in regions where magnesium and the like are dissolved in solid solution at a high concentration.

More specifically, in the layered rock salt type or cubic rock salt type region of space group R-3m, in the region where magnesium is dissolved in solid solution at a high concentration, compared to the layered rock salt type _LiAlO2 , Because the distance between the cation and oxygen is long, it is difficult for aluminum to exist stably. Further, in the vicinity of cobalt, the change in valence caused by the substitution of Li ⁺ with Mg ²⁺ can be compensated for by changing from Co ³⁺ to Co ²⁺ , thereby achieving cation balance. However, since Al can only be trivalent, it is considered difficult to coexist with magnesium in a rock salt type or layered rock salt type structure.

Although not shown, it is preferable that manganese, like aluminum, has a detection peak within the range compared to magnesium.

However, the additive elements do not necessarily have to have the same concentration gradient or distribution in all the surface layer portions 100a of the positive electrode active material 100. As an example of the depth direction of the (001) plane of lithium cobalt oxide of the positive electrode active material 100, arrows Y1-Y2 are shown in FIG. FIG. 11A shows an example of the intensity distribution of the characteristic X-ray of the added element along the arrow Y1-Y2.

The (001) oriented surface may have a different distribution of additive elements from other surfaces. For example, the (001) oriented surface and its surface layer portion 100a may have a lower detected amount of one or more selected additive elements than the surface other than the (001) oriented surface. Specifically, the detected amount of nickel may be low. Alternatively, in the (001) oriented surface and its surface layer portion 100a, the detected amount of one or more selected from the additive elements may be below the lower detection limit. Specifically, the detected amount of nickel may be below the lower limit of detection. Particularly in the case of an analysis method that detects characteristic X-rays such as EDX, it is difficult to detect trace amounts of nickel in materials whose main element is cobalt because the energies of Kβ of cobalt and Kα of nickel are close. Alternatively, in the (001) oriented surface and its surface layer portion 100a, the peak of the detected amount of one or more selected from the additive elements may be shallower than in the (001) oriented surface. Specifically, in the (001) oriented surface and its surface layer portion 100a, the peak positions of the detected amounts of magnesium and aluminum may be shallower than in the (001) oriented surface.

In the layered rock salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This can be said to be a structure in which _two CoO layers and a lithium layer are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane.

Since the CoO ₂ layer is relatively stable, the surface of the positive electrode active material 100 is more stable if it has a (001) orientation. The main diffusion path of lithium ions during charging and discharging is not exposed on the (001) plane.

On the other hand, lithium ion diffusion paths are exposed on surfaces other than the (001) orientation. Therefore, the surface other than the (001) orientation and its surface layer portion 100a are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are easily unstable because they are the regions from which lithium ions are first desorbed. Therefore, it is extremely important to reinforce the surface other than the (001) orientation and its surface layer portion 100a in order to maintain the crystal structure of the entire positive electrode active material 100.

Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, the characteristic X-ray intensity distribution of the additive element on the surface other than the (001) orientation and the surface layer portion 100a is as shown in any of FIGS. 10A to 10C. It is important that the distribution is consistent. Among the additive elements, it is particularly preferable that nickel is detected on the surface other than the (001) orientation and on the surface layer portion 100a thereof. On the other hand, in the (001) oriented surface and its surface layer portion 100a, the concentration of the additive element may be low as described above, or may be absent.

For example, the distribution of magnesium in the (001) oriented surface and its surface layer 100a preferably has a half width of 10 nm or more and 200 nm or less, more preferably 50 nm or more and 150 nm or less, and 80 nm or more and 120 nm or less. and even more preferable. Further, the distribution of magnesium on the non-(001) oriented surface and its surface layer 100a preferably has a half width of more than 200 nm and less than 500 nm, more preferably more than 200 nm and less than 300 nm, and more preferably more than 230 nm and 270 nm. It is more preferable that it is the following.

Further, the half width of the distribution of nickel on the non-(001) oriented surface and its surface layer 100a is preferably 30 nm or more and 150 nm or less, more preferably 50 nm or more and 130 nm or less, and 70 nm or more and 110 nm or less. is even more preferable.

In the manufacturing method described in the later embodiment, in which high-purity LiCoO ₂ is manufactured, an additive element is mixed and heated later, the additive element spreads mainly through the diffusion path of lithium ions. Therefore, it is easy to set the distribution of additive elements on the surface other than the (001) orientation and the surface layer portion 100a within a preferable range.

〔magnesium〕
Magnesium is divalent, and magnesium ions are more stable in lithium sites than in cobalt sites in a layered rock salt crystal structure, so they easily enter lithium sites. The presence of magnesium at an appropriate concentration in the lithium sites of the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is presumed to be because the magnesium present at the lithium site functions as a pillar that supports the _two CoO layers. Furthermore, the presence of magnesium can suppress desorption of oxygen around magnesium when x in Li _x CoO ₂ is, for example, 0.24 or less. Furthermore, the presence of magnesium can be expected to increase the density of the positive electrode active material 100. Furthermore, when the magnesium concentration in the surface layer portion 100a is high, it can be expected that the corrosion resistance against hydrofluoric acid produced by decomposition of the electrolytic solution will be improved.

If magnesium is at an appropriate concentration, it will not adversely affect the insertion and desorption of lithium during charging and discharging, and the above benefits can be enjoyed. However, an excess of magnesium may have an adverse effect on lithium intercalation and deintercalation. Furthermore, the effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters the cobalt site in addition to the lithium site. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) that do not substitute for either lithium sites or cobalt sites may segregate on the surface of the positive electrode active material and become a resistance component of the secondary battery. Furthermore, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the lithium site, reducing the amount of lithium that contributes to charging and discharging.

Therefore, it is preferable that the entire positive electrode active material 100 has an appropriate amount of magnesium. For example, the number of magnesium atoms is preferably 0.002 times or more and 0.06 times or less, more preferably 0.005 times or more and 0.03 times or less, and even more preferably about 0.01 times the number of cobalt atoms. The amount of magnesium contained in the entire positive electrode active material 100 herein may be a value obtained by elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, etc. It may be based on the value of the composition of raw materials in the process of producing the substance 100.

〔nickel〕
Nickel can exist at both cobalt sites and lithium sites in the layered rock salt crystal structure of LiMeO ₂ . When it exists in a cobalt site, it can be said to have a lower oxidation-reduction potential than cobalt, so it can easily give up lithium and electrons during charging, for example. Therefore, it can be expected that the charging and discharging speed will be faster. Therefore, even at the same charging voltage, a larger charge/discharge capacity can be obtained when the transition metal M is nickel than when the transition metal M is cobalt.

Further, when nickel exists at the lithium site, displacement of the layered structure consisting of octahedrons of cobalt and oxygen can be suppressed. Further, changes in volume due to charging and discharging are suppressed. Also, the elastic modulus becomes larger, that is, it becomes harder. This is presumably because nickel present at the lithium site also functions as a pillar supporting the _two CoO layers. Therefore, it is expected that the crystal structure will become more stable especially in a charged state at a high temperature, for example, 45° C. or higher, which is preferable.

In addition, the distance between the cation and anion of nickel oxide (NiO) is closer to the average distance between the cation and anion of LiCoO ₂ than that of rock salt-type MgO and rock salt-type CoO, and the orientation matches that of LiCoO ₂ . Cheap.

Additionally, magnesium, aluminum, cobalt, and nickel have a greater tendency to ionize in that order. Therefore, it is thought that nickel is less eluted into the electrolyte than the other elements mentioned above during charging. Therefore, it is considered to be highly effective in stabilizing the crystal structure of the surface layer in the charged state.

Further, among nickel, Ni ²⁺ , Ni ³⁺ , and Ni ⁴⁺ , Ni ²⁺ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Therefore, nickel is considered to have the effect of suppressing the phase change from a layered rock salt type crystal structure to a spinel type crystal structure.

On the other hand, if nickel is present in excess, the influence of distortion due to the Jahn-Teller effect will be increased, which is undesirable. Moreover, if nickel is in excess, there is a possibility that intercalation and deintercalation of lithium will be adversely affected.

Therefore, it is preferable that the entire positive electrode active material 100 has an appropriate amount of nickel. For example, the number of nickel atoms in the positive electrode active material 100 is preferably more than 0% and less than 7.5% of the number of cobalt atoms, preferably 0.05% or more and 4% or less, and preferably 0.1% or more and 2% or less. is preferable, and more preferably 0.2% or more and 1% or less. Alternatively, it is preferably more than 0% and 4% or less. Alternatively, it is preferably more than 0% and 2% or less. Or preferably 0.05% or more and 7.5% or less. Or preferably 0.05% or more and 2% or less. Or preferably 0.1% or more and 7.5% or less. Or preferably 0.1% or more and 4% or less. The amount of nickel shown here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, etc., or a value obtained by mixing raw materials in the process of producing the positive electrode active material. may be based on the value of

〔aluminum〕
Aluminum can also exist in cobalt sites in a layered rock salt type crystal structure. Aluminum is a typical trivalent element and its valence does not change, so lithium around aluminum is difficult to move during charging and discharging. Therefore, aluminum and the lithium surrounding it function as pillars and can suppress changes in the crystal structure. Therefore, as will be described later, even if the positive electrode active material 100 is subjected to a force that expands and contracts in the c-axis direction due to insertion and desorption of lithium ions, that is, even if a force that expands and contracts in the c-axis direction is applied by changing the charging depth or charging rate. , deterioration of the positive electrode active material 100 can be suppressed.

Additionally, aluminum has the effect of suppressing the elution of surrounding cobalt and improving continuous charging resistance. Furthermore, since the Al--O bond is stronger than the Co--O bond, desorption of oxygen around aluminum can be suppressed. These effects improve thermal stability. Therefore, when aluminum is included as an additive element, safety can be improved when the positive electrode active material 100 is used in a secondary battery. Moreover, the positive electrode active material 100 can be made such that the crystal structure does not easily collapse even after repeated charging and discharging.

On the other hand, if aluminum is present in excess, there is a risk that insertion and deintercalation of lithium will be adversely affected.

Therefore, it is preferable that the entire positive electrode active material 100 has an appropriate amount of aluminum. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% or more and 4% or less, preferably 0.1% or more and 2% or less, and 0.3% or more and 1.5% or less of the number of cobalt atoms. % or less is more preferable. Or preferably 0.05% or more and 2% or less. Or preferably 0.1% or more and 4% or less. The amount that the entire positive electrode active material 100 has here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, etc., or the amount that the entire positive electrode active material 100 has. It may also be based on the value of the composition of raw materials during the production process.

[Fluorine]
Fluorine is a monovalent anion, and when part of the oxygen in the surface layer portion 100a is replaced with fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. In other words, when fluorine is not present, cobalt ions change from trivalent to tetravalent as lithium is eliminated. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent as lithium is eliminated. The redox potential of cobalt ions is different between the two. Therefore, if part of the oxygen in the surface layer 100a of the positive electrode active material 100 is replaced with fluorine, it can be said that desorption and insertion of lithium ions near fluorine are likely to occur smoothly. Therefore, when the positive electrode active material 100 is used in a secondary battery, charging/discharging characteristics, large current characteristics, etc. can be improved. In addition, the presence of fluorine in the surface layer portion 100a, which has a surface that is in contact with the electrolytic solution, or the adhesion of fluoride to the surface suppresses excessive reaction between the positive electrode active material 100 and the electrolytic solution. Can be done. Furthermore, corrosion resistance against hydrofluoric acid can be effectively improved.

Further, when the melting point of fluoride such as lithium fluoride is lower than the melting point of other additive element sources, it can function as a fluxing agent (also referred to as a fluxing agent) that lowers the melting point of the other additive element sources. As shown in FIG. 12 (quoted and added from Non-Patent Document 13, FIG. 5), the eutectic point P of LiF and MgF ₂ is around 742° C. (T1). Therefore, when a mixed fluoride containing LiF and MgF ₂ as the fluoride is used as an additive element source, it is preferable to set the heating temperature to 742° C. or higher in the heating step after mixing the additive elements.

Here, differential scanning calorimetry (DSC measurement) for mixed fluorides and mixtures will be explained using FIG. 13. The curve labeled mixed fluoride in FIG. 13 is the result of a DSC measurement of a mixture of LiF and _MgF2 . The mixed fluoride was prepared by mixing LiF:MgF ₂ at a molar ratio of 1:3. The curve labeled “Mixture” in FIG. 13 is the result of DSC measurement of a mixture of lithium cobalt oxide, LiF and MgF ₂ . The mixture was prepared by mixing LiCoO ₂ :LiF:MgF ₂ =100:0.33:1 (molar ratio).

As shown in FIG. 13, an endothermic peak is observed around 735°C in the mixed fluoride. In addition, an endothermic peak is observed in the mixture at around 830°C. Therefore, the heating temperature after mixing the additive elements is preferably 742°C or higher, more preferably 830°C or higher. Further, the temperature may be 800° C. (T2 in FIG. 12) or higher, which is between these values.

[Other additive elements]
Titanium oxides are known to have superhydrophilic properties. Therefore, by using the positive electrode active material 100 having titanium oxide in the surface layer portion 100a, the wettability with respect to a highly polar solvent may be improved. When used as a secondary battery, the interface between the positive electrode active material 100 and the highly polar electrolytic solution becomes good, and there is a possibility that an increase in internal resistance can be suppressed.

Further, it is preferable to have phosphorus in the surface layer portion 100a because short circuits may be suppressed when x in Li _x CoO ₂ is maintained in a small state. For example, it is preferable to exist in the surface layer portion 100a as a compound containing phosphorus and oxygen.

When the positive electrode active material 100 contains phosphorus, it is preferable because the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte, and there is a possibility that the hydrogen fluoride concentration in the electrolyte can be reduced.

When the electrolyte contains LiPF ₆ , hydrogen fluoride may be generated due to hydrolysis. Furthermore, there is a possibility that hydrogen fluoride may be generated due to the reaction between polyvinylidene fluoride (PVDF) used as a component of the positive electrode and an alkali. By reducing the concentration of hydrogen fluoride in the electrolyte, corrosion of the current collector and/or peeling of the coating portion 104 may be suppressed. Further, it may be possible to suppress a decrease in adhesiveness due to gelation and/or insolubilization of PVDF.

It is preferable that the positive electrode active material 100 contains phosphorus together with magnesium because stability in a state where x in Li _x CoO ₂ is small is extremely high. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% or more and 20% or less of the number of cobalt atoms, more preferably 2% or more and 10% or less, and even more preferably 3% or more and 8% or less. Or preferably 1% or more and 10% or less. Or preferably 1% or more and 8% or less. Or preferably 2% or more and 20% or less. Or preferably 2% or more and 8% or less. Or preferably 3% or more and 20% or less. Or preferably 3% or more and 10% or less. In addition, the number of magnesium atoms is preferably 0.1% or more and 10% or less of the number of cobalt atoms, more preferably 0.5% or more and 5% or less, and more preferably 0.7% or more and 4% or less. Or preferably 0.1% or more and 5% or less. Or preferably 0.1% or more and 4% or less. Or preferably 0.5% or more and 10% or less. Or preferably 0.5% or more and 4% or less. Or preferably 0.7% or more and 10% or less. Or preferably 0.7% or more and 5% or less. The concentrations of phosphorus and magnesium shown here may be, for example, values obtained by elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, etc., or values obtained during the manufacturing process of the positive electrode active material 100. It may be based on the value of the raw material composition in .

Further, when the positive electrode active material 100 has a crack, the crack progresses due to the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the crack as the surface, for example, in the embedded part 102. can be suppressed.

[Synergistic effect of multiple additive elements]
Furthermore, when the surface layer portion 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. Therefore, elution of magnesium can be suppressed even when x in Li _x CoO ₂ is small. Therefore, it can contribute to stabilization of the surface layer portion 100a.

For the same reason, when adding additive elements to lithium cobalt oxide in the manufacturing process, it is preferable that magnesium be added in a step before nickel. Alternatively, it is preferable that magnesium and nickel are added in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide no matter what process it is added to, whereas nickel can diffuse widely into the interior of lithium cobalt oxide if magnesium is not present. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of lithium cobalt oxide and will not remain in the desired amount on the surface layer.

It is also preferable to have additional elements with different distributions, as this can stabilize the crystal structure over a wider area. For example, if the positive electrode active material 100 has both magnesium and nickel distributed in a region closer to the surface in the surface layer portion 100a, and aluminum distributed in a deeper region than these, the positive electrode active material 100 is more It is possible to stabilize the crystal structure in a wide range. In this way, when the positive electrode active material 100 has additional elements with different distributions, aluminum is not essential for the surface because the surface can be sufficiently stabilized by magnesium, nickel, etc. Rather, it is preferable that aluminum is widely distributed in a deeper region. For example, it is preferable that aluminum is continuously detected in a region from the surface in a depth direction of 1 nm or more and 25 nm or less. It is preferable that the crystal structure be widely distributed in a region of 0 nm or more and 100 nm or less from the surface, preferably 0.5 nm or more and 50 nm or less from the surface, since the crystal structure can be stabilized over a wider region.

When a plurality of additive elements are included as described above, the effects of each additive element can be synergized and contribute to further stabilization of the surface layer portion 100a. In particular, magnesium, nickel and aluminum are highly effective in providing a stable composition and crystal structure.

However, if the surface layer portion 100a is occupied only by the compound of the additive element and oxygen, it is not preferable because it becomes difficult to insert and extract lithium. For example, it is not preferable that the surface layer portion 100a is occupied only by MgO, a structure in which MgO and NiO(II) are dissolved in solid solution, and/or a structure in which MgO and CoO(II) are dissolved in solid solution. Therefore, the surface layer portion 100a must contain at least cobalt, also contain lithium in the discharge state, and have a path for inserting and extracting lithium.

In order to ensure a sufficient path for insertion and desorption of lithium, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than magnesium. For example, when measured from the surface of the positive electrode active material 100 by XPS, the ratio Mg/Co of the number of atoms of magnesium to the number of atoms of cobalt, Co, is preferably 0.62 or less. Further, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than nickel. Further, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than aluminum. Further, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than fluorine.

Furthermore, since too much nickel may inhibit the diffusion of lithium, it is preferable that the surface layer portion 100a has a higher concentration of magnesium than nickel. For example, when measured from the surface of the positive electrode active material 100 by XPS, the number of nickel atoms is preferably 1/6 or less of the number of magnesium atoms.

Although some of the additive elements, particularly magnesium, nickel, and aluminum, are preferably present in a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably randomly and dilutely present in the interior 100b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites in the interior 100b, there is an effect that the layered rock salt type crystal structure can be easily maintained, similar to the above. Furthermore, if nickel exists in the interior 100b at an appropriate concentration, the shift of the layered structure consisting of octahedrons of cobalt and oxygen can be suppressed in the same manner as described above. Further, when magnesium and nickel are contained together, a synergistic effect of suppressing the elution of magnesium can be expected as described above.

It is preferable that the crystal structure changes continuously from the interior 100b toward the surface due to the concentration gradient of the additive element as described above. Alternatively, it is preferable that the crystal orientations of the surface layer portion 100a and the interior portion 100b are approximately the same.

For example, it is preferable that the crystal structure changes continuously from the layered rock salt-type interior 100b toward the surface and surface layer portion 100a that has the characteristics of the rock salt type or both the rock salt type and the layered rock salt type. Alternatively, it is preferable that the crystal orientations of the surface layer portion 100a, which has the characteristics of a rock salt type or both of a rock salt type and a layered rock salt type, and the crystal orientation of the layered rock salt type interior 100b are generally the same.

In this specification, etc., the layered rock salt type crystal structure belonging to space group R-3m, which is possessed by a composite oxide containing transition metals such as lithium and cobalt, refers to a structure in which cations and anions are arranged alternately. It has a rock salt-type ion arrangement, and the transition metal and lithium are regularly arranged to form a two-dimensional plane, so it is a crystal structure that allows two-dimensional diffusion of lithium. Note that there may be defects such as cation or anion deficiency. Strictly speaking, the layered rock salt crystal structure may have a structure in which the lattice of the rock salt crystal is distorted.

Further, the term "rock salt type crystal structure" refers to a structure having a cubic system crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately. Note that there may be a deficiency of cations or anions.

Furthermore, the presence of both layered rock salt type and rock salt type crystal structure characteristics can be determined by electron beam diffraction, TEM images, cross-sectional STEM images, etc.

The rock salt type has no distinction in cation sites, but the layered rock salt type has two types of cation sites in its crystal structure, one mostly occupied by lithium and the other occupied by transition metals. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately is the same for both the rock salt type and the layered rock salt type. Among the bright spots of the electron beam diffraction pattern corresponding to the crystal planes forming this two-dimensional plane, when the central spot (transparent spot) is set as the origin 000, the bright spot closest to the central spot is the ideal one. For example, a state rock salt type has a (111) plane, and a layered rock salt type has a (003) plane, for example. For example, when comparing the electron diffraction patterns of rock salt type MgO and layered rock salt type LiCoO ₂ , the distance between the bright spots on the (003) plane of LiCoO ₂ is approximately half the distance between the bright spots on the (111) plane of MgO. observed at a distance of about Therefore, when the analysis region has two phases, for example, rock salt type MgO and layered rock salt type _LiCoO2 , the electron diffraction pattern has a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. do. Bright spots common to the halite type and layered halite type have strong brightness, and bright spots that occur only in the layered halite type have weak brightness.

In addition, in a cross-sectional STEM image, etc., when a layered rock salt crystal structure is observed from a direction perpendicular to the c-axis, layers observed with strong brightness and layers observed with weak brightness are observed alternately. The rock salt type does not have these characteristics because there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both a rock salt type and a layered rock salt type, when observed from a specific crystal orientation, layers that are observed with strong brightness and layers that are observed with weak brightness are observed alternately in cross-sectional STEM images, etc. In addition, a metal with a higher atomic number than lithium exists in a part of the lithium layer, which has an even weaker brightness.

Layered rock salt crystals and anions of rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). In the O3'-type and monoclinic O1(15) crystals described below, the anions are also presumed to have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there is a crystal plane in which the cubic close-packed structure composed of anions is oriented in the same direction.

Alternatively, it can also be explained as follows. Anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt type has a space group R-3m and has a rhombohedral structure, but to facilitate understanding of the structure, it is generally expressed as a complex hexagonal lattice, and the (0001) plane of the layered rock salt type has a hexagonal lattice. The triangular lattice of the cubic {111} plane has an atomic arrangement similar to the hexagonal lattice of the (0001) plane of the layered rock salt type. When both lattices are consistent, it can be said that the orientations of the cubic close-packed structures are aligned.

However, the space group of layered rock salt crystals and O3' type crystals is R-3m, which is different from the space group Fm-3m of rock salt crystals (the space group of general rock salt crystals), so the above conditions are The Miller index of the crystal planes to be satisfied is different between a layered rock salt type crystal and an O3' type crystal and a rock salt type crystal. In this specification, in a layered rock salt type crystal, an O3' type crystal, and a rock salt type crystal, when the directions of the cubic close-packed structures constituted by anions are aligned, it may be said that the orientations of the crystals are approximately the same. Furthermore, having three-dimensional structural similarity such that the crystal orientations roughly match, or having the same crystallographic orientation is called topotaxy.

The fact that the orientations of the crystals in the two regions roughly match can be seen in TEM (Transmission Electron Microscope) images and STEM (Scanning Transmission Electron Microscope) images. , HAADF-STEM (High-angle Annular Dark Field Scanning TEM (high-angle scattering annular dark-field scanning transmission electron microscope) image, ABF-STEM (annular bright-field scanning transmission electron microscope) image , an electron beam diffraction pattern, etc. It can also be determined based on FFT patterns of TEM images, STEM images, etc. Furthermore, XRD (X-ray diffraction), neutron beam diffraction, etc. can also be used as materials for judgment.

FIG. 14 shows an example of a TEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS are approximately the same. A TEM image, a STEM image, a HAADF-STEM image, an ABF-STEM image, etc., provide images that reflect the crystal structure.

For example, in a high-resolution TEM image, contrast derived from crystal planes can be obtained. Due to electron beam diffraction and interference, for example, when an electron beam is incident perpendicularly to the c-axis of a layered rock-salt complex hexagonal lattice, the contrast originating from the (0003) plane is divided into bright bands (bright strips) and dark bands (dark strips). ) is obtained by repeating. Therefore, repeating bright lines and dark lines are observed in the TEM image, and if the angle between the bright lines (for example, L _RS and L _LRS shown in FIG. 14) is 5 degrees or less or 2.5 degrees or less, the crystal plane is approximately It can be determined that they match, that is, the crystal orientations approximately match. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the orientations of the crystals approximately match.

Furthermore, in the HAADF-STEM image, a contrast proportional to the atomic number is obtained, and elements with larger atomic numbers are observed brighter. For example, in the case of layered rock salt type lithium cobalt oxide belonging to space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the position of the cobalt atom, making the arrangement of the cobalt atoms clear. It can be observed as a line or as an array of bright points. Therefore, when lithium cobalt oxide, which has a layered rock salt crystal structure, is observed perpendicular to the c-axis, the arrangement of cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of strong bright points, and lithium atoms and oxygen atoms are observed perpendicularly to the c-axis. The arrangement is observed as a dark line or region of low brightness. The same applies to the case where lithium cobalt oxide contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

Therefore, in a HAADF-STEM image, repeating bright lines and dark lines are observed in two regions with different crystal structures, and if the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, the atomic arrangement is approximately It can be determined that they match, that is, the crystal orientations approximately match. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the orientations of the crystals approximately match.

Note that in ABF-STEM, elements with smaller atomic numbers are observed brighter, but since it is similar to HAADF-STEM in that contrast depending on the atomic number can be obtained, the crystal orientation is determined in the same way as in HAADF-STEM images. be able to.

FIG. 15A shows an example of a STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS are approximately the same. FIG. 15B shows the FFT pattern of the region of the rock salt crystal RS, and FIG. 15C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of FIGS. 15B and 15C shows the composition, the JCPDS card number, and the d value and angle calculated from the JCPDS card data. Actual measurements are shown on the right. Spots marked with O are 0th order diffraction.

The spots labeled A in FIG. 15B originate from the 11-1 reflection of the cubic crystal. The spots labeled A in FIG. 15C are derived from layered rock salt type 0003 reflections. It can be seen from FIGS. 15B and 15C that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly match. That is, it can be seen that the straight line passing through AO in FIG. 15B and the straight line passing through AO in FIG. 15C are approximately parallel. As used herein, "approximately matching" and "approximately parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

In this way, in the FFT pattern and the electron diffraction pattern, if the orientations of the layered rock salt type crystal and the rock salt type crystal roughly match, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type. , may roughly match. At this time, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. The fact that the reciprocal lattice points are spot-like and not continuous with other reciprocal lattice points means that the crystallinity is high.

Furthermore, if the direction of the 11-1 reflection of the cubic crystal and the direction of the 0003 reflection of the layered rock salt type are approximately the same as described above, depending on the incident direction of the electron beam, the direction of the 0003 reflection of the layered rock salt type may vary. On a reciprocal lattice space different from the orientation, a spot that is not derived from layered rock salt type 0003 reflection may be observed. For example, the spots labeled B in FIG. 15C are derived from layered rock salt type 1014 reflections. This is an angle of 52° or more and 56° or less (that is, ∠AOB is 52° or more and 56° or less) from the orientation of the reciprocal lattice point (A in Figure 15C) derived from the 0003 reflection of the layered rock salt type, and d may be observed at a location of 0.19 nm or more and 0.21 nm or less. Note that this index is just an example, and does not necessarily have to match this index. For example, reciprocal lattice points equivalent to 0003 and 1014 may be used.

Similarly, a spot that is not derived from the 11-1 reflection of the cubic crystal may be observed on a reciprocal lattice space different from the direction in which the 11-1 reflection of the cubic crystal was observed. For example, the spot labeled B in FIG. 15B is derived from 200 reflections of a cubic crystal. This is a point that is at an angle of 54° or more and 56° or less (that is, ∠AOB is 54° or more and 56° or less) from the orientation of the reciprocal lattice point (A in Figure 15B) derived from the 11-1 reflection of the cubic crystal. Diffraction spots may be observed. Note that this index is just an example, and does not necessarily have to match this index. For example, reciprocal lattice points equivalent to 11-1 and 200 may be used.

In layered rock salt type positive electrode active materials such as lithium cobalt oxide, (0003) plane and planes equivalent to this, and (10-14) plane and planes equivalent to this tend to appear as crystal planes. Are known. Therefore, for example, when observing a (0003) plane using a TEM, etc., first select a positive electrode active material particle in which a crystal plane expected to be a (0003) plane is observed using a SEM, etc., and when an electron beam is observed using a TEM etc. ] It is preferable to process the positive electrode active material particles into thin sections using FIB or the like so that the (0003) plane can be observed as the incident light. When it is desired to judge whether the crystal orientation matches, it is preferable to thin the layered rock salt so that the (0003) plane can be easily observed.

≪Status where x in Li _x CoO ₂ is small≫
The positive electrode active material 100 of one embodiment of the present invention has the above-described distribution of additive elements and/or crystal structure in a discharge state, and thus has a crystal structure in a state where x in Li _x CoO ₂ is small. However, it is different from conventional positive electrode active materials. Note that x is small here, meaning 0.1<x≦0.24.

A change in the crystal structure due to a change in x in Li _x CoO ₂ will be explained using FIGS. 16 to 21 while comparing a conventional cathode active material and the cathode active material 100 of one embodiment of the present invention.

FIG. 17 shows changes in the crystal structure of a conventional positive electrode active material. The conventional positive electrode active material shown in FIG. 17 is lithium cobalt oxide (LiCoO ₂ ) without any particular additive element. In particular, changes in the crystal structure of lithium cobalt oxide without additive elements are described in Non-Patent Documents 2 to 5.

FIG. 17 shows the crystal structure of lithium cobalt oxide with x=1 in Li _x CoO ₂ with R-3m O3. In this crystal structure, lithium occupies octahedral sites, and three CoO ₂ layers exist in the unit cell. Therefore, this crystal structure is sometimes called an O3 type crystal structure. Note that the CoO ₂ layer refers to a structure in which an octahedral structure in which six oxygen atoms are coordinated with cobalt is continuous in a plane in a shared edge state. This is sometimes referred to as a layer consisting of an octahedron of cobalt and oxygen.

Furthermore, it is known that conventional lithium cobalt oxide has a crystal structure in which the symmetry of lithium increases when x=0.5 and belongs to the monoclinic space group P2/m. In this structure, one CoO ₂ layer exists in the unit cell. Therefore, it is sometimes called O1 type or monoclinic O1 type.

Further, when x=0, the positive electrode active material has a trigonal space group P-3m1 crystal structure, and one CoO ₂ layer is also present in the unit cell. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, the trigonal crystal is sometimes converted into a complex hexagonal lattice and is called the hexagonal O1 type.

Furthermore, conventional lithium cobalt oxide when x=about 0.12 has a crystal structure of space group R-3m. This structure can also be said to be a structure in which a CoO ₂ structure like trigonal O1 type and a LiCoO ₂ structure like R-3m O3 are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 type crystal structure. Note that the actual intercalation and desorption of lithium does not necessarily occur uniformly within the positive electrode active material, and the lithium concentration may become mottled. is observed. In fact, the H1-3 type crystal structure has twice the number of cobalt atoms per unit cell as other structures. However, in this specification including FIG. 17, in order to facilitate comparison with other crystal structures, the c-axis of the H1-3 type crystal structure is shown as 1/2 of the unit cell.

As an example of the H1-3 type crystal structure, as described in Non-Patent Document 4, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.42150±0.00016), O1(0, 0,0.27671±0.00045), O2 (0,0,0.11535±0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, a unit cell with a small GOF (goodness of fit) value may be used.

When charging and discharging are repeated such that x in Li _x CoO ₂ becomes 0.24 or less, conventional lithium cobalt oxide has an H1-3 type crystal structure, an R-3m O3 structure in a discharged state, The crystal structure changes (that is, non-equilibrium phase changes) repeatedly between the two.

However, these two crystal structures have a large misalignment of the CoO ₂ layers. As shown by the dotted lines and arrows in FIG. 17, in the H1-3 type crystal structure, the _CoO2 layer is largely deviated from the R-3mO3 in the discharge state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

Furthermore, there is a large difference in volume between these two crystal structures. The crystal structure and unit cell volume of lithium cobalt oxide change depending on the change in the depth of charge, that is, the change in x in Li _x CoO ₂ . FIG. 18 shows the change in the c-axis length of the conventional lithium cobalt oxide described in Non-Patent Document 5. Round markers indicate hexagonal phase, and diamond-shaped markers indicate monoclinic phase. In the H1-3 phase, the c-axis length contracts, as shown by the diamond-shaped marker in FIG. Since the phase transition from O3 to H1-3 phase is a phase transition accompanying the desorption of lithium ions, it is thought that the phase transition occurs from the surface of the positive electrode active material, which is the region from which lithium ions first escape, but eventually the positive electrode It can extend to the entire active material.

Note that the change in the c-axis length of lithium cobalt oxide corresponds to the change in the angle at which the peak of, for example, the (003) plane of lithium cobalt oxide appears in the XRD pattern. It is known that in XRD using CuKα1 rays, the peak of the (003) plane of lithium cobalt oxide occurs at a 2θ of around 19° to 20°.

Therefore, when comparing the same number of cobalt atoms, the difference in volume between the H1-3 type crystal structure and the R-3m O3 type crystal structure in the discharge state exceeds 3.5%, typically 3.9% or more. It is.

In addition, a structure in which _two CoO layers are continuous, such as the trigonal O1 type, which the H1-3 type crystal structure has, is likely to be unstable.

Therefore, if charging and discharging are repeated such that x becomes 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. The collapse of the crystal structure causes deterioration of cycle characteristics. This is because as the crystal structure collapses, the number of sites where lithium can exist stably decreases, and insertion and extraction of lithium becomes difficult.

On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. 16, the change in crystal structure between the discharge state where x in Li _x CoO ₂ is 1 and the state where x is 0.24 or less is different from that of the conventional positive electrode active material. less than. More specifically, the deviation between _{the two} CoO layers between the state where x is 1 and the state where x is 0.24 or less can be reduced. Further, the change in volume when compared per cobalt atom can be reduced. Therefore, in the cathode active material 100 of one embodiment of the present invention, even if charging and discharging are repeated such that x becomes 0.24 or less, the crystal structure does not easily collapse, and excellent cycle characteristics can be achieved. Further, the positive electrode active material 100 of one embodiment of the present invention can have a more stable crystal structure than conventional positive electrode active materials when x in Li _x CoO ₂ is 0.24 or less. Therefore, in the cathode active material 100 of one embodiment of the present invention, short circuits are unlikely to occur when x in Li _x CoO ₂ is maintained at 0.24 or less. In such a case, the safety of the secondary battery is further improved, which is preferable.

FIG. 16 shows the crystal structure that the interior 100b of the positive electrode active material 100 has when x in Li _x CoO ₂ is about 1, 0.2, and about 0.15. Since the interior 100b occupies most of the volume of the positive electrode active material 100 and is a part that greatly contributes to charging and discharging, it can be said that the displacement of the CoO ₂ layer and the change in volume are the most problematic part.

When x=1, the positive electrode active material 100 has the same R-3mO3 crystal structure as conventional lithium cobalt oxide.

However, when x is 0.24 or less, for example, about 0.2 and 0.15, where conventional lithium cobalt oxide has an H1-3 type crystal structure, the positive electrode active material 100 forms a crystal with a different structure. have

The positive electrode active material 100 of one embodiment of the present invention when x=0.2 has a crystal structure belonging to the trigonal space group R-3m. This is because the symmetry of the CoO ₂ layer is the same as that of O3. Therefore, this crystal structure will be referred to as an O3' type crystal structure. This crystal structure is shown in FIG. 16 with R-3m O3'.

The crystal structure of the O3' type has the coordinates of cobalt and oxygen in the unit cell within the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. It can be shown as Further, the lattice constant of the unit cell is preferably 2.797≦a≦2.837 (×10 ⁻¹ nm) on the a-axis, more preferably 2.807≦a≦2.827 (×10 ⁻¹ nm), Typically, a=2.817 (×10 ⁻¹ nm). The c-axis preferably has 13.681≦c≦13.881 (×10 ⁻¹ nm), more preferably 13.751≦c≦13.811 (×10 ⁻¹ nm), and typically c=13. 781 (×10 ⁻¹ nm).

Further, when x=about 0.15, the positive electrode active material 100 of one embodiment of the present invention has a crystal structure belonging to a monoclinic space group P2/m. In this case, one CoO ₂ layer exists in the unit cell. Further, at this time, the amount of lithium present in the positive electrode active material 100 is about 15 atomic % in the discharged state. Therefore, this crystal structure will be referred to as a monoclinic O1(15) type crystal structure. This crystal structure is shown in FIG. 16 with P2/m monoclinic O1 (15).

The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as
Co1(0.5,0,0.5),
Co2 (0, 0.5, 0.5),
O1 (X _O1 , 0, Z _O1 ),
0.23≦X _O1 ≦0.24, 0.61≦Z _O1 ≦0.65,
O2( _XO2,0.5 , _ZO2 ),
It can be shown within the range of 0.75≦X _O2 ≦0.78, 0.68≦Z _O2 ≦0.71. Also, the lattice constant of the unit cell is
a=4.880±0.05 (×10 ⁻¹ nm),
b=2.817±0.05 (×10 ⁻¹ nm),
c=4.839±0.05 (×10 ⁻¹ nm),
α=90°,
β=109.6±0.1°,
γ=90°.

Note that this crystal structure can exhibit a lattice constant even in the space group R-3m if a certain degree of error is allowed. The coordinates of cobalt and oxygen in the unit cell in this case are
Co(0,0,0.5),
O(0,0,Z _O ),
It can be shown within the range of 0.21≦Z _O ≦0.23.
Also, the lattice constant of the unit cell is
a=2.817±0.02 (×10 ⁻¹ nm),
c=13.68±0.1 (×10 ⁻¹ nm).

In both the O3' type and monoclinic O1 (15) type crystal structures, ions such as cobalt, nickel, and magnesium occupy six oxygen coordination positions. Note that light elements such as lithium and magnesium may occupy the 4-coordination position of oxygen.

As shown by the dotted line in FIG. 16, there is almost no displacement of the CoO ₂ layer between the R-3m O3 in the discharge state and the O3' and monoclinic O1(15) type crystal structures.

Also, the difference in volume per same number of cobalt atoms between R-3m O3 in the discharge state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%. It is.

In addition, the difference in volume per the same number of cobalt atoms between R-3m O3 in the discharge state and the monoclinic O1 (15) type crystal structure is less than 3.3%, more specifically less than 3.0%, typically is 2.5%.

Table 1 shows the difference in volume per cobalt atom between R-3m O3 in the discharge state and O3', monoclinic O1 (15), H1-3 type, and trigonal O1. The lattice constants of the crystal structures of R-3m O3 and trigonal O1 in the discharge state used in the calculations in Table 1 are given in ICSD coll. code. 172909 and 88721. Regarding H1-3, reference can be made to Non-Patent Document 4. O3' and monoclinic O1 (15) can be calculated from experimental values of XRD. Note that 1 Å=10 ⁻¹⁰ m.

As described above, in the cathode active material 100 of one embodiment of the present invention, changes in the crystal structure when x in Li _x CoO ₂ is small, that is, when a large amount of lithium is released, are suppressed more than in conventional cathode active materials. has been done. In addition, changes in volume are also suppressed when comparing the same number of cobalt atoms. Therefore, the crystal structure of the positive electrode active material 100 does not easily collapse even after repeated charging and discharging such that x becomes 0.24 or less. Therefore, in the positive electrode active material 100, a decrease in charge/discharge capacity during charge/discharge cycles is suppressed. Furthermore, since more lithium can be stably utilized than conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with high discharge capacity per weight and per volume can be manufactured.

It has been confirmed that the positive electrode active material 100 may have an O3' type crystal structure when x in Li _x CoO ₂ is 0.15 or more and 0.24 or less, and when x exceeds 0.24 and 0. It is estimated that even if it is less than .27, it has an O3' type crystal structure. In addition, when x in Li _x CoO ₂ exceeds 0.1 and is 0.2 or less, typically x is 0.15 or more and 0.17 or less, it has a monoclinic O1 (15) type crystal structure. It has been confirmed that there is. However, since the crystal structure is influenced not only by x in Li _x CoO ₂ but also by the number of charge/discharge cycles, charge/discharge current, temperature, electrolyte, etc., it is not necessarily limited to the above range of x.

Therefore, when x in Li _x CoO ₂ exceeds 0.1 and is 0.24 or less, the positive electrode active material 100 may have only the O3' type or only the monoclinic O1 (15) type. or may have both crystal structures. Further, all of the particles in the interior 100b of the positive electrode active material 100 do not have to have an O3' type and/or a monoclinic O1(15) type crystal structure. It may contain other crystal structures or may be partially amorphous.

Furthermore, in order to make x in Li _x CoO ₂ small, it is generally necessary to charge at a high charging voltage. Therefore, a state in which x in Li _x CoO ₂ is small can be rephrased as a state in which the battery is charged at a high charging voltage. For example, when CC/CV charging is performed in an environment of 25° C. at a voltage of 4.6 V or more based on the potential of lithium metal, an H1-3 type crystal structure appears in a conventional positive electrode active material. Therefore, a charging voltage of 4.6 V or more can be said to be a high charging voltage with reference to the potential of lithium metal. In this specification and the like, unless otherwise specified, charging voltage is expressed based on the potential of lithium metal.

Therefore, the positive electrode active material 100 of one embodiment of the present invention can maintain a crystal structure with R-3mO3 symmetry even when charged at a high charging voltage, for example, 4.6 V or higher at 25° C., and is therefore preferable. It can be rephrased as In other words, it is preferable because an O3' type crystal structure can be obtained when charged at a higher charging voltage, for example, a voltage of 4.65 V or more and 4.7 V or less at 25° C. In other words, it is preferable because a monoclinic O1 (15) type crystal structure can be obtained when the battery is charged at a higher charging voltage, for example, a voltage exceeding 4.7 V and not more than 4.8 V at 25°C.

Even in the positive electrode active material 100, the H1-3 type crystal structure may be finally observed when the charging voltage is further increased. Furthermore, as mentioned above, the crystal structure is affected by the number of charge/discharge cycles, charge/discharge current, temperature, electrolyte, etc., so if the charging voltage is lower, for example, if the charging voltage is 4.5 V or more and less than 4.6 V at 25°C, In some cases, the positive electrode active material 100 of one embodiment of the present invention can have an O3' type crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.7 V or less at 25° C., a monoclinic O1 (15) type crystal structure may be obtained.

Note that when graphite is used as a negative electrode active material in a secondary battery, for example, the voltage of the secondary battery is lowered by the potential of graphite than the above. The potential of graphite is about 0.05V to 0.2V based on the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as a negative electrode active material, it has the same crystal structure as above when the voltage is obtained by subtracting the potential of graphite from the above voltage.

Further, in O3' and monoclinic O1 (15) in FIG. 16, lithium is shown to exist at all lithium sites with equal probability, but the present invention is not limited to this. It may exist biasedly at some lithium sites, or it may have symmetry such as monoclinic O1 (Li _0.5 CoO ₂ ) shown in FIG. 17, for example. The distribution of lithium can be analyzed, for example, by neutron diffraction.

It can also be said that the O3' and monoclinic O1(15) type crystal structures are similar to the CdCl ₂ type crystal structure, although they have lithium randomly between the layers. This crystal structure similar to CdCl type ₂ is close to the crystal structure when lithium nickelate is charged to Li _0.06 NiO ₂ , but pure lithium cobalt oxide or a layered rock salt type positive electrode active material containing a large amount of cobalt is It is known that CdCl does not normally have a type ₂ crystal structure.

≪Grain boundaries≫
In addition to the above-mentioned distribution, it is more preferable that at least a portion of the additive elements included in the positive electrode active material 100 of one embodiment of the present invention be unevenly distributed in and near the grain boundaries 101.

Note that in this specification and the like, maldistribution refers to the concentration of an element in a certain region being different from that in other regions. It has the same meaning as segregation, precipitation, non-uniformity, deviation, or a mixture of areas with high concentration and areas with low concentration.

For example, it is preferable that the magnesium concentration in and around the grain boundaries 101 of the positive electrode active material 100 is higher than in other regions of the interior 100b. Further, it is preferable that the fluorine concentration in the grain boundaries 101 and the vicinity thereof is also higher than in other regions of the interior 100b. Further, it is preferable that the nickel concentration in the grain boundaries 101 and the vicinity thereof is also higher than in other regions of the interior 100b. Further, it is preferable that the aluminum concentration in the grain boundaries 101 and the vicinity thereof is also higher than in other regions of the interior 100b.

The grain boundary 101 is one of the planar defects. Therefore, like the particle surface, it tends to become unstable and the crystal structure tends to change. Therefore, if the concentration of the additive element at and near the grain boundaries 101 is high, changes in the crystal structure can be suppressed more effectively.

Further, when the magnesium concentration and fluorine concentration at the grain boundary 101 and the vicinity thereof are high, even if a crack occurs along the grain boundary 101 of the positive electrode active material 100 of one embodiment of the present invention, the surface Magnesium and fluorine concentrations increase in the vicinity. Therefore, the corrosion resistance against hydrofluoric acid can be improved even in the positive electrode active material after cracks have occurred.

<Particle size>
If the particle size of the positive electrode active material 100 of one embodiment of the present invention is too large, there are problems such as difficulty in lithium diffusion and the surface of the active material layer becoming too rough when applied to a current collector. On the other hand, if it is too small, problems such as excessive reaction with the electrolyte will occur. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, the thickness is preferably 1 μm or more and 40 μm or less. Alternatively, the thickness is preferably 1 μm or more and 30 μm or less. Alternatively, the thickness is preferably 2 μm or more and 100 μm or less. Or preferably 2 μm or more and 30 μm or less. Alternatively, the thickness is preferably 5 μm or more and 100 μm or less. Alternatively, the thickness is preferably 5 μm or more and 40 μm or less.

Furthermore, it is preferable to use a mixture of particles with different particle sizes in the positive electrode, since the electrode density can be increased and a secondary battery with high energy density can be obtained. The positive electrode active material 100 having a relatively small particle size is expected to have high charge/discharge rate characteristics. A secondary battery using the positive electrode active material 100 having a relatively large particle size is expected to have high charge/discharge cycle characteristics and maintain a high discharge capacity.

<Analysis method>
Whether the positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention having an O3' type and/or monoclinic O1(15) type crystal structure when x in Li _x CoO ₂ is small _, _Li can.

In particular, XRD can analyze the symmetry of transition metals such as cobalt in the positive electrode active material with high resolution, compare the height of crystallinity and crystal orientation, and analyze the periodic strain of the lattice and crystallite size. This is preferable because sufficient accuracy can be obtained even if the positive electrode obtained by disassembling the secondary battery is directly measured. Among XRD, powder XRD provides a diffraction peak that reflects the crystal structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

Note that when analyzing the crystallite size by powder XRD, it is preferable to perform the measurement without the influence of the orientation of the positive electrode active material particles due to pressurization or the like. For example, it is preferable to take out the positive electrode active material from a positive electrode obtained by disassembling a secondary battery and prepare a powder sample for measurement.

As described above, the positive electrode active material 100 of one embodiment of the present invention is characterized by a small change in crystal structure between when x in Li _x CoO ₂ is 1 and when x is 0.24 or less. A material in which 50% or more of the crystal structure changes significantly when charged at a high voltage is not preferable because it cannot withstand repeated high voltage charging and discharging.

Furthermore, it should be noted that there are cases where the O3' type or monoclinic O1 (15) type crystal structure is not achieved simply by adding additional elements. For example, even if lithium cobalt oxide has magnesium and fluorine, or lithium cobalt oxide has magnesium and aluminum, depending on the concentration and distribution of the added elements, x in Li _x CoO ₂ may be 0.24 or less. In some cases, the O3' type and/or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases, the H1-3 type crystal structure accounts for 50% or more.

In addition, even with the positive electrode active material 100 of one embodiment of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9 V, the crystal structure of the H1-3 type or trigonal O1 type will change. This may occur in some cases. Therefore, in order to determine whether the positive electrode active material 100 of one embodiment of the present invention is used, analysis of the crystal structure such as XRD, and information such as charging capacity or charging voltage are required.

However, the positive electrode active material in a state where x is small may undergo a change in crystal structure when exposed to the atmosphere. For example, the O3' type and monoclinic O1(15) type crystal structures may change to the H1-3 type crystal structure. Therefore, it is preferable that all samples subjected to crystal structure analysis be handled in an inert atmosphere such as an argon atmosphere.

In addition, whether the distribution of additive elements in the positive electrode active material is in the state described above can be determined by, for example, XPS, energy dispersive X-ray spectroscopy (EDX), EPMA ( This can be determined by analysis using methods such as electronic probe microanalysis.

Further, the crystal structure of the surface layer 100a, grain boundaries 101, etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100.

≪Charging method≫
Charging to determine whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention is performed using, for example, a coin cell (CR2032 type) using the composite oxide for the positive electrode and lithium metal for the counter electrode. , 20 mm in diameter and 3.2 mm in height) can be made and charged. A coin cell has an electrolyte, a separator, a positive electrode can, and a negative electrode can.

More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

Lithium metal can be used for the counter electrode. Note that when a material other than lithium metal is used for the counter electrode, the potential of the secondary battery and the potential of the positive electrode are different. Voltages and potentials in this specification and the like are the potentials of the positive electrode unless otherwise mentioned.

The electrolytic solution contains 1 mol/L lithium hexafluorophosphate (LiPF ₆ ), and the electrolytic solution contains ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 ( (volume ratio), vinylene carbonate (VC) mixed at 2 wt % can be used.

A polypropylene porous film with a thickness of 25 μm can be used as the separator.

The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

The coin cell produced under the above conditions is charged at an arbitrary voltage (for example, 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V or 4.8V). The charging method is not particularly limited as long as it can be charged at any voltage for a sufficient amount of time. For example, when charging with CCCV, the current in CC charging can be 20 mA/g or more and 100 mA/g or less. CV charging can be completed at 2 mA/g or more and 10 mA/g or less. In order to observe the phase change of the positive electrode active material, it is desirable to perform charging at such a small current value. On the other hand, if the current does not decrease to 2 mA/g or more and 10 mA/g or less even after long-term CV charging, it is thought that the current is being consumed to decompose the electrolyte rather than charging the positive electrode active material, so make sure that the current is sufficient from the start. CV charging may be terminated when a certain amount of time has elapsed. The sufficient time at this time can be, for example, 1.5 hours or more and 3 hours or less. The temperature is 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box with an argon atmosphere and the positive electrode is taken out, thereby obtaining a positive electrode active material with an arbitrary charging capacity. When performing various analyzes after this, it is preferable to seal the chamber with an argon atmosphere in order to suppress reactions with external components. For example, XRD can be performed in a sealed container with an argon atmosphere. Further, it is preferable to take out the positive electrode immediately after charging is completed and use it for analysis. Specifically, it is preferably within 1 hour, more preferably within 30 minutes after charging is completed.

Furthermore, when analyzing the crystal structure of the charged state after charging and discharging multiple times, the conditions for charging and discharging the multiple times may be different from the above-mentioned charging conditions. For example, charging is performed by constant current charging at a current value of 20 mA/g or more and 100 mA/g or less to an arbitrary voltage (for example, 4.6 V, 4.65 V, 4.7 V, 4.75 V or 4.8 V), and then the current value is Constant voltage charging can be performed until the voltage is 2 mA/g or more and 10 mA/g or less, and discharging can be performed at a constant current of 2.5 V and 20 mA/g or more and 100 mA/g or less.

Further, when analyzing the crystal structure in the discharged state after charging and discharging multiple times, constant current discharge can be performed at, for example, 2.5V and a current value of 20 mA/g or more and 100 mA/g or less.

≪XRD≫
The equipment and conditions for XRD measurement are not particularly limited. For example, it can be measured using the following equipment and conditions.
XRD device: Bruker AXS, D8 ADVANCE
X-ray: CuKα _1- ray output: 40kV, 40mA
Divergence angle: Div. Slit, 0.5°
Detector: LynxEye
Scan method: 2θ/θ continuous scan Measurement range (2θ): 15° or more and 90° or less Step width (2θ): 0.01° Setting Counting time: 1 second/step Sample table rotation: 15 rpm

If the sample to be measured is a powder, it can be set by placing it in a glass sample holder or by sprinkling the sample on a greased silicone non-reflective plate. When the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set according to the measurement surface required by the apparatus.

Figures 19 and 20 show the ideal powder XRD pattern using the CuKα ₁ line, which is calculated from the models of the O3' type crystal structure, the monoclinic O1 (15) type crystal structure, and the H1-3 type crystal structure. , shown in FIGS. 21A and 21B. For comparison, ideal XRD patterns calculated from the crystal structures of LiCoO ₂ O3 with x=1 in Li _x CoO ₂ and trigonal O1 with x=0 are also shown. 21A and 21B show the XRD patterns of the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure, and in FIG. 21A, the 2θ range is 18° or more. FIG. 21B is an enlarged view of the region where the 2θ range is 42° or more and 46° or less. Note that the patterns of LiCoO ₂ (O3) and CoO ₂ (O1) are one of the modules of Materials Studio (BIOVIA) from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 6). It was created using Reflex Powder Diffraction. The range of 2θ was 15° to 75°, Step size=0.01, wavelength λ1=1.540562×10 ⁻¹⁰ m, λ2 was not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 4. The crystal structure patterns of the O3' type and the monoclinic O1 (15) type were estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, and the crystal structures were estimated using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and an XRD pattern was created in the same manner as the others.

As shown in FIGS. 19, 21A, and 21B, in the O3' type crystal structure, 2θ=19.25±0.12° (19.13° or more and less than 19.37°), and 2θ=45.47 A diffraction peak appears at ±0.10° (45.37° or more and less than 45.57°).

In addition, in the monoclinic O1 (15) type crystal structure, 2θ = 19.47 ± 0.10° (19.37° or more and 19.57° or less), and 2θ = 45.62 ± 0.05° (45 A diffraction peak appears at .57° or more and 45.67° or less).

However, as shown in FIGS. 20, 21A, and 21B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. Therefore, when x in Li _x CoO ₂ is small, 2θ is 19.13° or more and less than 19.37° and/or 19.37° or more and 19.57° or less, and 45.37° or more and 45.57° It can be said that the appearance of a peak at a position of 45.57° or more and 45.67° or less is a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

This can also be said to be that in the positive electrode active material 100 of one embodiment of the present invention, the positions where the XRD diffraction peaks appear are close to each other in the crystal structure where x=1 and x≦0.24. More specifically, among the main diffraction peaks of the crystal structure where x=1 and x≦0.24, the difference in 2θ is 0.7° or less between the peaks that appear at 2θ of 42° or more and 46° or less. , more preferably 0.5° or less.

Note that the positive electrode active material 100 of one embodiment of the present invention has an O3' type and/or monoclinic O1(15) type crystal structure when x in Li _x CoO ₂ is small; however, all of the particles are O3' type and/or monoclinic O1 (15) type crystal structure. It may contain other crystal structures, or may be partially amorphous. However, when performing Rietveld analysis on the XRD pattern, the O3' type and/or monoclinic O1 (15) type crystal structure is preferably 50% or more, more preferably 60% or more, More preferably, it is 66% or more. If the O3' type and/or monoclinic O1(15) type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material has sufficiently excellent cycle characteristics. be able to.

Furthermore, when Rietveld analysis is similarly performed, it is preferable that the H1-3 type and O1 type crystal structures are 50% or less. Or, it is more preferably 34% or less. Or, it is more preferable that it is substantially not observed.

In addition, even after 100 cycles or more of charging and discharging from the start of measurement, it is preferable that the O3' type and/or monoclinic O1(15) type crystal structure remains 35% or more when Rietveld analysis is performed. % or more, more preferably 43% or more.

Furthermore, the sharpness of the diffraction peak in the XRD pattern indicates the high degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging be sharp, that is, have a narrow half-width. For example, it is preferable that the full width at half maximum is narrower. The half width varies depending on the XRD measurement conditions and the 2θ value even for peaks generated from the same crystal phase. In the case of the measurement conditions described above, in the peak observed at 2θ=43° or more and 46° or less, the full width at half maximum is preferably 0.2° or less, more preferably 0.15° or less, and 0.12° or less. More preferred. Note that not all peaks necessarily satisfy this requirement. If some peaks satisfy this requirement, it can be said that the crystallinity of the crystal phase is high. Such high crystallinity contributes to sufficient stabilization of the crystal structure after charging.

Further, the crystallite size of the O3' type and monoclinic O1 (15) crystal structure that the positive electrode active material 100 has decreases only to about 1/20 of LiCoO ₂ (O3) in the discharge state. Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, when x in Li _x CoO ₂ is small, a clear O3' type and/or monoclinic O1(15) crystal structure peak is confirmed. can. On the other hand, in conventional LiCoO ₂ , even if a part of the crystal structure can be similar to the O3' type and/or monoclinic O1 (15), the crystallite size is small and the peak is broad and small. The crystallite size can be determined from the half width of the XRD peak.

In the positive electrode active material 100 of one embodiment of the present invention, it is preferable that the influence of the Jahn-Teller effect is small as described above. In addition to cobalt, transition metals such as nickel and manganese may be included as additive elements, as long as the influence of the Jahn-Teller effect is small.

In the positive electrode active material, using XRD analysis, we will discuss the proportions and lattice constant ranges of nickel and manganese, which are assumed to have a small influence from the Jahn-Teller effect.

FIG. 22 shows the results of calculating the a-axis and c-axis lattice constants using XRD when the positive electrode active material 100 according to one embodiment of the present invention has a layered rock salt crystal structure and contains cobalt and nickel. show. FIG. 22A shows the results for the a-axis, and FIG. 22B shows the results for the c-axis. Note that the XRD pattern used for these calculations is the powder after the synthesis of the positive electrode active material, but before it is incorporated into the positive electrode. The nickel concentration on the horizontal axis indicates the nickel concentration when the sum of the numbers of cobalt and nickel atoms is taken as 100%. The positive electrode active material was manufactured according to the manufacturing method shown in FIG. 25, except that an aluminum source was not used.

FIG. 23 shows the results of estimating the a-axis and c-axis lattice constants using XRD when the positive electrode active material 100 according to one embodiment of the present invention has a layered rock salt crystal structure and contains cobalt and manganese. shows. FIG. 23A shows the results for the a-axis, and FIG. 23B shows the results for the c-axis. Note that the lattice constant shown in FIG. 23 is for the powder after the synthesis of the positive electrode active material, and is based on XRD measurement before incorporating it into the positive electrode. The manganese concentration on the horizontal axis indicates the manganese concentration when the sum of the numbers of cobalt and manganese atoms is taken as 100%. The positive electrode active material was manufactured according to the manufacturing method shown in FIG. 25, except that a manganese source was used instead of a nickel source and an aluminum source was not used.

FIG. 22C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis/c-axis) for the positive electrode active materials whose lattice constant results are shown in FIGS. 22A and 22B. FIG. 23C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis/c-axis) for the positive electrode active materials whose lattice constant results are shown in FIGS. 23A and 23B.

From FIG. 22C, there is a tendency for the a-axis/c-axis to change significantly when the nickel concentration is 5% and 7.5%, and the distortion of the a-axis becomes large when the nickel concentration is 7.5%. This distortion may be due to the Jahn-Teller distortion of trivalent nickel. It is suggested that when the nickel concentration is less than 7.5%, an excellent positive electrode active material with low Jahn-Teller distortion can be obtained.

Next, from FIG. 23A, it is suggested that when the manganese concentration is 5% or more, the behavior of the change in the lattice constant is different and does not follow Vegard's law. Therefore, it is suggested that the crystal structure is different when the manganese concentration is 5% or more. Therefore, the concentration of manganese is preferably 4% or less, for example.

Note that the above ranges of nickel concentration and manganese concentration do not necessarily apply to the surface layer portion 100a. That is, in the surface layer portion 100a, the concentration may be higher than the above concentration.

Based on the above, we considered the preferable range of the lattice constant, and found that in the positive electrode active material of one embodiment of the present invention, the layered structure of the positive electrode active material 100 in a state in which no charging and discharging is performed or in a discharged state, which can be estimated from the XRD pattern. In the rock salt type crystal structure, the a-axis lattice constant is greater than 2.814×10 ⁻¹⁰ m and smaller than 2.817×10 ⁻¹⁰ m, and the c-axis lattice constant is less than 14.05×10 ⁻¹⁰ m. It was found that it is preferable that the diameter be larger than 14.07×10 ⁻¹⁰ m. The state where charging and discharging are not performed may be, for example, the state of the powder before producing the positive electrode of the secondary battery.

Alternatively, in the layered rock salt crystal structure of the cathode active material 100 in a state where no charging/discharging is performed or in a discharged state, the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis/c-axis) is It is preferably greater than 0.20000 and smaller than 0.20049.

Alternatively, when XRD analysis is performed on the layered rock salt crystal structure of the cathode active material 100 in a state where no charging/discharging is performed or in a discharged state, a first peak is observed at 2θ of 18.50° or more and 19.30° or less. is observed, and a second peak may be observed at 2θ of 38.00° or more and 38.80° or less.

≪XPS≫
With X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, if monochromatic aluminum Kα rays are used as the X-rays, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less). Therefore, the concentration of each element can be quantitatively analyzed in a region that is approximately half the depth of the surface layer 100a. Additionally, narrow scan analysis allows the bonding state of elements to be analyzed. Note that the quantitative accuracy of XPS is about ±1 atomic % in most cases, and the lower limit of detection is about 1 atomic %, although it depends on the element.

In the positive electrode active material 100 of one embodiment of the present invention, it is preferable that the concentration of one or more selected from the additive elements is higher in the surface layer portion 100a than in the interior portion 100b. This is synonymous with the fact that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average of the entire positive electrode active material 100. Therefore, for example, the concentration of one or more additive elements selected from the surface layer 100a measured by It can be said that it is preferable that the concentration of the added element be higher than the average concentration of the added element of the entire positive electrode active material 100 measured by . For example, it is preferable that the magnesium concentration of at least a portion of the surface layer portion 100a measured by XPS or the like is higher than the average magnesium concentration of the entire positive electrode active material 100. Further, it is preferable that the nickel concentration in at least a portion of the surface layer portion 100a is higher than the nickel concentration in the entire positive electrode active material 100. Further, it is preferable that the aluminum concentration in at least a portion of the surface layer portion 100a is higher than the aluminum concentration in the entire positive electrode active material 100. Further, it is preferable that the fluorine concentration in at least a portion of the surface layer portion 100a is higher than the fluorine concentration in the entire positive electrode active material 100.

Note that the surface and surface layer portion 100a of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonate, hydroxyl groups, etc. that were chemically adsorbed after the positive electrode active material 100 was produced. It is also assumed that the electrolytic solution, binder, conductive material, or compounds derived from these adhered to the surface of the positive electrode active material 100 are not included. Therefore, when quantifying the elements contained in the positive electrode active material, correction may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc. that can be detected by surface analysis such as XPS. For example, in XPS, it is possible to separate the types of bonds by analysis, and correction may be performed to exclude binder-derived C-F bonds.

Furthermore, before subjecting to various analyses, samples such as the positive electrode active material and the positive electrode active material layer are washed to remove the electrolyte, binder, conductive material, or compounds derived from these that have adhered to the surface of the positive electrode active material. You may do so. At this time, lithium may dissolve into the solvent used for cleaning, but even in that case, the additive elements are difficult to dissolve, so the atomic ratio of the additive elements is not affected.

The concentration of the added element may also be compared in terms of its ratio to cobalt. By using the ratio to cobalt, it is possible to reduce the influence of carbonate, etc. chemically adsorbed after the positive electrode active material is produced, and to make a comparison, which is preferable. For example, the ratio Mg/Co of the number of atoms of magnesium and cobalt as determined by XPS analysis is preferably 0.4 or more and 1.5 or less. On the other hand, Mg/Co as determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

Similarly, the positive electrode active material 100 preferably has a higher concentration of lithium and cobalt than each additive element in the surface layer portion 100a in order to sufficiently secure a path for insertion and desorption of lithium. This means that the concentration of lithium and cobalt in the surface layer 100a is preferably higher than the concentration of one or more of the additive elements selected from the additive elements contained in the surface layer 100a, which is measured by XPS or the like. can. For example, it is preferable that the concentration of cobalt in at least a portion of the surface layer portion 100a measured by XPS or the like is higher than the concentration of magnesium in at least a portion of the surface layer portion 100a measured by XPS or the like. Similarly, it is preferable that the concentration of lithium is higher than the concentration of magnesium. Further, it is preferable that the concentration of cobalt is higher than the concentration of nickel. Similarly, it is preferable that the concentration of lithium is higher than the concentration of nickel. Further, it is preferable that the concentration of cobalt is higher than that of aluminum. Similarly, it is preferable that the concentration of lithium is higher than the concentration of aluminum. Further, it is preferable that the concentration of cobalt is higher than that of fluorine. Similarly, it is preferable that the concentration of lithium is higher than that of fluorine.

Furthermore, it is more preferable that aluminum is widely distributed in a deep region, for example, in a region where the depth from the surface or the reference point is 5 nm or more and 50 nm or less. Therefore, although aluminum is detected in the analysis of the entire positive electrode active material 100 using ICP-MS, GD-MS, etc., it is more preferable that the concentration of aluminum is below the detection limit in XPS etc.

Furthermore, when performing XPS analysis on the positive electrode active material 100 of one embodiment of the present invention, the number of magnesium atoms is preferably 0.4 times or more and 1.2 times or less, and 0.65 times or more and 1 times or less, relative to the number of cobalt atoms. More preferably, it is .0 times or less. Further, the number of nickel atoms is preferably 0.15 times or less, more preferably 0.03 times or more and 0.13 times or less relative to the number of cobalt atoms. Furthermore, the number of aluminum atoms is preferably 0.12 times or less, more preferably 0.09 times or less, relative to the number of cobalt atoms. The number of fluorine atoms is preferably 0.3 times or more and 0.9 times or less, more preferably 0.1 times or more and 1.1 times or less, relative to the number of cobalt atoms. The above range indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100, but are widely distributed in the surface layer 100a of the positive electrode active material 100 at a preferable concentration. It can be said that it shows.

When performing XPS analysis, for example, monochromatic aluminum Kα rays can be used as the X-rays. Further, the take-out angle may be, for example, 45°. For example, it can be measured using the following equipment and conditions.
Measuring device: Quantera II manufactured by PHI
X-ray: Monochromatic Al Kα (1486.6eV)
Detection area: 100μmφ
Detection depth: Approximately 4~5 nm (takeout angle 45°)
Measurement spectrum: wide scan, narrow scan for each detected element

Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably about 684.3 eV. This value is different from both the binding energy of lithium fluoride, 685 eV, and the binding energy of magnesium fluoride, 686 eV.

Further, when the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably about 1303 eV. This value is different from the binding energy of magnesium fluoride, 1305 eV, and is close to the binding energy of magnesium oxide.

≪EDX≫
It is preferable that one or more selected from the additive elements included in the positive electrode active material 100 have a concentration gradient. Further, it is more preferable that the depth of the concentration peak from the surface of the positive electrode active material 100 differs depending on the added element. The concentration gradient of the additive element can be determined by, for example, exposing a cross section of the positive electrode active material 100 using FIB (Focused Ion Beam) or the like, and then using the cross section by energy dispersive X-ray spectroscopy (EDX) or EPMA (electronic electron beam). It can be evaluated by analysis using probe microanalysis).

Among EDX measurements, measuring while scanning the area and evaluating the area two-dimensionally is called EDX surface analysis. Also, measuring while scanning linearly and evaluating the distribution of atomic concentration within the positive electrode active material is called line analysis. Furthermore, data on a linear region extracted from the EDX surface analysis is sometimes called line analysis. Also, measuring a certain area without scanning it is called point analysis.

By EDX plane analysis (for example, elemental mapping), the concentration of added elements in the surface layer 100a, interior 100b, vicinity of crystal grain boundaries 101, etc. of the positive electrode active material 100 can be quantitatively analyzed. Further, the concentration distribution and maximum value of the added element can be analyzed by EDX-ray analysis. In addition, analysis using a thin sample like STEM-EDX can analyze the concentration distribution in the depth direction from the surface of the cathode active material toward the center in a specific region without being affected by the distribution in the depth direction. , is more suitable.

Since the positive electrode active material 100 is a compound containing a transition metal and oxygen that are capable of intercalating and deintercalating lithium, the transition metal M (for example, Co, Ni, Mn, Fe, etc.) and oxygen that undergo oxidation and reduction as lithium intercalates and deintercalates. The interface between the region where is present and the region where is not is defined as the surface of the positive electrode active material. When a positive electrode active material is subjected to analysis, a protective film is sometimes attached to the surface, but the protective film is not included in the positive electrode active material. As the protective film, a single layer film or a multilayer film of carbon, metal, oxide, resin, etc. may be used.

Therefore, when referring to the depth direction in STEM-EDX-ray analysis, etc., the detected amount of characteristic X-rays of the transition metal M is the average value M _AVE of the detected amount of characteristic X-rays of the transition metal M inside, The point where the detected amount of characteristic X-rays of the above-mentioned transition metal M in the background becomes 50% of the sum with the average value M _BG , or the detected amount of characteristic X-rays of oxygen is the detected amount of characteristic X-rays of internal oxygen. The reference point is a point that is 50% of the sum of the average value _OAVE and the average value _OBG of the detected amount of characteristic X-rays of background oxygen. The detected amount of characteristic X-rays of the transition metal M is the sum of the average detected amount of characteristic X-rays of the internal transition metal M and the average detected amount of characteristic X-rays of the background transition metal M. and the detected amount of characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of characteristic X-rays of internal oxygen and the average value of the detected amount of characteristic X-rays of background oxygen. If the point differs from that of , it is considered to be due to the influence of oxygen-containing metal oxides, carbonates, etc. adhering to the surface, so the detected amount of characteristic X-rays of the transition metal M differs from the characteristic of the transition metal M inside. A point that is 50% of the sum of the average value _MAVE of detected amounts of X-rays and the average value _MBG of detected amounts of characteristic X-rays of the background transition metal M can be adopted as the reference point. Further, in the case of a positive electrode active material having a plurality of transition metals M, the reference point can be determined using M _AVE and M _BG of the elements whose internal characteristic X-rays are detected in the largest amount.

The average value _MBG of the detected amount of characteristic X-rays of the transition metal M in the background is preferably 2 nm or more outside the positive electrode active material, avoiding the vicinity where the detected amount of characteristic X-rays of the transition metal M starts to increase. can be determined by averaging over a range of 3 nm or more. In addition, the average value _MAVE of the detected amount of characteristic X-rays of the internal transition metal M is a region where the detected amounts of characteristic X-rays of transition metal M and oxygen are saturated and stable, for example, the detected amount of characteristic X-rays of transition metal M A range of 2 nm or more, preferably 3 nm or more can be determined on average at a depth of 30 nm or more, preferably more than 50 nm from the region where . The average value O _BG of the detected amount of characteristic X-rays of background oxygen and the average value O _AVE of the detected amount of characteristic X-rays of internal oxygen can be similarly determined.

In addition, the surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image, etc. is the boundary between a region where an image derived from the crystal structure of the positive electrode active material is observed and a region where it is not observed. This is the outermost region in which an atomic column originating from the nucleus of a metal element with a higher atomic number than lithium among the metal elements constituting the substance is confirmed.

Additionally, the spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the position where the detected amount of characteristic X-rays of the added element reaches its maximum value may be shifted by about 1 nm. For example, even if there is a position where the detected amount of characteristic X-rays of an additive element such as magnesium has a maximum value outside the surface determined above, if the difference between the maximum value and the surface is less than 1 nm, it can be considered as an error. can.

In addition, the peak in STEM-EDX-ray analysis refers to the detected intensity of characteristic X-rays of each element, or the position where the maximum value is reached. Note that noise in STEM-EDX-ray analysis may include a measured value of half-width that is less than the spatial resolution (R), for example, less than R/2.

The effects of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value obtained by measuring six scans can be used as the detected amount of characteristic X-rays of each element. The number of scans is not limited to six, and it is also possible to perform more scans and use the average as the detected amount of characteristic X-rays of each element.

STEM-EDX-ray analysis can be performed, for example, as follows. First, a protective film is deposited on the surface of the positive electrode active material. For example, carbon can be deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Tech).

Next, the positive electrode active material is cut into thin pieces to prepare a STEM cross-sectional sample. For example, thinning processing can be performed using a FIB-SEM device (XVision 200TBS manufactured by Hitachi High-Technology). At that time, the pickup is performed using an MPS (micro probing system), and the finishing conditions can be, for example, an accelerating voltage of 10 kV.

For STEM-EDX-ray analysis, for example, a STEM device (HD-2700 manufactured by Hitachi High-Tech) may be used, and an EDAX Octane T Ultra W may be used as the EDX detector. At the time of EDX-ray analysis, the emission current of the STEM device is set to be 6 μA or more and 10 μA or less, and the depth and portions of the thin sectioned sample with few irregularities are measured. The magnification is, for example, about 150,000 times. The conditions for the EDX-ray analysis may include drift correction, line width of 42 nm, pitch of 0.2 nm, and number of frames of 6 or more.

When the positive electrode active material 100 of one embodiment of the present invention is subjected to EDX plane analysis or EDX point analysis, it is preferable that the concentration of the additive element such as magnesium in the surface layer portion 100a is higher than that in the interior portion 100b.

For example, when EDX plane analysis or EDX point analysis is performed on the positive electrode active material 100 having magnesium as an additive element, it is preferable that the magnesium concentration in the surface layer portion 100a is higher than the magnesium concentration in the interior portion 100b. Furthermore, when EDX-ray analysis is performed, the peak of the magnesium concentration in the surface layer 100a preferably exists within a depth of 3 nm from the surface or reference point toward the center of the positive electrode active material 100, and preferably exists within a depth of 1 nm. It is more preferable to do so, and it is still more preferable to exist at a depth of 0.5 nm. Further, it is preferable that the magnesium concentration attenuates to 60% or less of the peak at a depth of 1 nm from the peak. Further, it is preferable that the attenuation decreases to 30% or less of the peak at a depth of 2 nm from the peak. Note that the concentration peak (also referred to as peak top) herein refers to the maximum value of concentration.

Further, when EDX-ray analysis was performed, the magnesium concentration (detected amount of magnesium/(sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, and silicon) in the surface layer 100a was 0.5 at.% or more and 10 at.% or less). It is preferably at least 1 atomic % and at most 5 atomic %.

In the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of fluorine concentration and the peak of magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

In addition, when performing EDX-ray analysis, the peak of fluorine concentration in the surface layer 100a preferably exists within a depth of 3 nm from the surface or reference point toward the center of the positive electrode active material 100, and preferably exists within a depth of 1 nm. It is more preferable to do so, and it is still more preferable to exist at a depth of 0.5 nm. Further, it is more preferable that the peak of the fluorine concentration be present slightly closer to the surface than the peak of the magnesium concentration, since this increases resistance to hydrofluoric acid. For example, the peak of fluorine concentration is more preferably 0.5 nm or more closer to the surface than the peak of magnesium concentration, and even more preferably 1.5 nm or more closer to the surface.

In the positive electrode active material 100 having nickel as an additive element, the peak of the nickel concentration in the surface layer 100a is preferably present within a depth of 3 nm from the surface or reference point toward the center of the positive electrode active material 100. It is more preferable to exist within a depth of 1 nm, and even more preferably to exist within a depth of 0.5 nm. In the positive electrode active material 100 containing magnesium and nickel, the distribution of nickel preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of nickel concentration and the peak of magnesium concentration is preferably within 3 nm, more preferably within 1 nm.

In addition, when the positive electrode active material 100 has aluminum as an additive element, when EDX-ray analysis is performed, it is preferable that the peak of the concentration of magnesium, nickel, or fluorine is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100a. For example, the peak of aluminum concentration preferably exists at a depth of 0.5 nm or more and 50 nm or less from the surface of the positive electrode active material 100 or the reference point toward the center, and more preferably at a depth of 5 nm or more and 50 nm or less.

Further, when the positive electrode active material 100 is subjected to EDX-ray analysis, area analysis, or point analysis, the ratio of the number of atoms of magnesium Mg and cobalt Co (Mg/Co) at the peak of magnesium concentration is preferably 0.05 or more and 0.6 or less. , more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum Al and cobalt Co (Al/Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel Ni and cobalt Co (Ni/Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine F and cobalt Co (F/Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

Further, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of the additive element A to cobalt Co (A/Co) in the vicinity of the grain boundary 101 is preferably 0.020 or more and 0.50 or less. More preferably, it is 0.025 or more and 0.30 or less. More preferably, it is 0.030 or more and 0.20 or less. Or preferably 0.020 or more and 0.30 or less. Or preferably 0.020 or more and 0.20 or less. Or preferably 0.025 or more and 0.50 or less. Or preferably 0.025 or more and 0.20 or less. Or preferably 0.030 or more and 0.50 or less. Or preferably 0.030 or more and 0.30 or less.

For example, when the additive element is magnesium, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium and cobalt atoms (Mg/Co) near the grain boundary 101 is 0.020 or more and 0.50. The following are preferred. More preferably, it is 0.025 or more and 0.30 or less. More preferably, it is 0.030 or more and 0.20 or less. Or preferably 0.020 or more and 0.30 or less. Or preferably 0.020 or more and 0.20 or less. Or preferably 0.025 or more and 0.50 or less. Or preferably 0.025 or more and 0.20 or less. Or preferably 0.030 or more and 0.50 or less. Or preferably 0.030 or more and 0.30 or less. Furthermore, if the above range is present at multiple locations of the positive electrode active material 100, for example at three or more locations, the additive element will not adhere to a narrow area on the surface of the positive electrode active material 100, but will preferably be applied to the surface layer 100a of the positive electrode active material 100. This can be said to indicate that the concentration is widely distributed.

≪EPMA≫
EPMA (Electron Probe Microanalysis) is also capable of quantifying elements. Area analysis allows analysis of the distribution of each element.

When EPMA surface analysis is performed on a cross section of the positive electrode active material 100 according to one embodiment of the present invention, it is preferable that one or more selected additive elements have a concentration gradient, similar to the EDX analysis results. Further, it is more preferable that the depth of the concentration peak from the surface differs depending on the added element. The preferred range of the concentration peak of each additive element is also the same as in the case of EDX.

However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative value of each element may differ from the measurement results using other analysis methods. For example, when the surface of the positive electrode active material 100 is analyzed by EPMA, the concentration of each additive element present in the surface layer portion 100a may be lower than the result of XPS.

≪Raman spectroscopy≫
As described above, in the positive electrode active material 100 of one embodiment of the present invention, at least a portion of the surface layer portion 100a preferably has a rock salt crystal structure. Therefore, when the positive electrode active material 100 and the positive electrode containing the same are analyzed by Raman spectroscopy, it is preferable that not only the layered rock salt crystal structure but also the cubic crystal structure including the rock salt type is observed. In the HAADF-STEM image and ultrafine electron diffraction pattern described below, it is found that if there is no cobalt substituted at the lithium position with a certain frequency in the depth direction at the time of observation, and cobalt present at the 4-coordination position of oxygen, the HAADF-STEM image will be different. and cannot be detected as a bright spot in the ultrafine electron diffraction pattern. On the other hand, since Raman spectroscopy is an analysis that captures the vibrational mode of bonds such as Co-O, even if the amount of the corresponding Co-O bond is small, it may be possible to observe the wavenumber peak of the corresponding vibrational mode. be. Furthermore, since Raman spectroscopy can measure a surface area of several μm ² and a depth of about 1 μm, it is possible to sensitively capture states that exist only on the particle surface.

For example, when the laser wavelength is 532 nm, peaks (vibration modes: E _g , A _1g ) are observed at 470 cm ⁻¹ to 490 cm ⁻¹ and 580 cm ⁻¹ to 600 cm ⁻¹ in layered rock salt type LiCoO ₂ . On the other hand, in cubic CoO _x (0<x<1) (rock salt Co _1-y O (0<y<1) or ^spinel Co ₃ O ₄ ), the ^peak (vibration mode: A _1g ) is observed.

Therefore, when the integrated intensity of each peak is 470 cm ^-1 to 490 cm ^-1 as I1, 580 ^{cm -1} to 600 cm ^-1 as I2, and 665 cm ^-1 to 685 cm ^-1 as I3, the value of I3/I2 is 1% or more. It is preferably 10% or less, and more preferably 3% or more and 9% or less.

If a cubic crystal structure including a rock salt type is observed in the above range, it can be said that the surface layer 100a of the positive electrode active material 100 has a rock salt type crystal structure in a preferable range.

≪Ultrafine electron diffraction pattern≫
It is preferable that the characteristics of the rock salt type crystal structure as well as the layered rock salt crystal structure be observed in the ultrafine electron diffraction pattern as well as in Raman spectroscopy. However, in the STEM image and ultrafine electron diffraction pattern, taking into account the above-mentioned difference in sensitivity, the characteristics of the rock salt crystal structure should not be too strong at the surface layer 100a, especially at the outermost surface (for example, at a depth of 1 nm from the surface). is preferred. Rather than having the outermost surface covered with a rock-salt-type crystal structure, it is better to have an additive element such as magnesium in the lithium layer while maintaining the layered rock-salt-type crystal structure. This is because the stabilizing function becomes stronger.

Therefore, for example, when obtaining a microelectron diffraction pattern in a region with a depth of 1 nm or less from the surface and a microelectron diffraction pattern in a region with a depth of 3 nm or more and 10 nm or less, the difference in the lattice constants calculated from them is Smaller is preferable.

For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm or more and 10 nm or less is preferably 0.1 × 10 ^-1 nm or less about the a-axis, and c It is preferable that the diameter is 1.0×10 ⁻¹ nm or less along the axis. Moreover, it is more preferable that the a-axis is 0.05×10 ⁻¹ nm or less, and the c-axis is more preferably 0.6×10 ⁻¹ nm or less. Further, it is more preferable that the a-axis is 0.04×10 ⁻¹ nm or less, and even more preferable that the c-axis is 0.3×10 ⁻¹ nm or less.

≪Surface roughness and specific surface area≫
The positive electrode active material 100 according to one embodiment of the present invention preferably has a smooth surface with few irregularities. The fact that the surface is smooth and has few irregularities indicates that the effect of the flux described below was sufficiently exerted and the surfaces of the additive element source and lithium cobalt oxide were melted. Therefore, this is one factor indicating that the distribution of the additive elements in the surface layer portion 100a is good.

The fact that the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or a cross-sectional TEM image of the positive electrode active material 100, the specific surface area of the positive electrode active material 100, etc.

For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as shown below.

First, the positive electrode active material 100 is processed by FIB or the like to expose a cross section. At this time, it is preferable to cover the positive electrode active material 100 with a protective film, a protective agent, or the like. Next, a SEM image of the interface between the protective film and the like and the positive electrode active material 100 is taken. Noise processing is performed on the SEM image using image processing software. For example, after Gaussian blurring (σ=2) is performed, binarization is performed. Furthermore, interface extraction is performed using image processing software. Furthermore, the interface line between the protective film or the like and the positive electrode active material 100 is selected using an automatic selection tool or the like, and the data is extracted into spreadsheet software or the like. Using functions such as spreadsheet software, perform correction from the regression curve (quadratic regression), obtain parameters for roughness calculation from the data after slope correction, and obtain root mean square surface roughness (RMS) with standard deviation calculated. . The surface roughness of the positive electrode active material is at least 400 nm around the outer periphery of the particles.

In the particle surface of the positive electrode active material 100 of this embodiment, the root mean square (RMS) surface roughness, which is an index of roughness, is less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. Roughness (RMS) is preferred.

Note that the image processing software that performs noise processing, interface extraction, etc. is not particularly limited, but for example, "ImageJ" described in Non-Patent Documents 9 to 11 can be used. Further, spreadsheet software and the like are not particularly limited, but Microsoft Office Excel can be used, for example.

For example, the surface smoothness of the positive electrode active material 100 can also be quantified from the ratio of the actual specific surface area S _R measured by a gas adsorption method using a constant volume method and the ideal specific surface area S _i . can.

The ideal specific surface area S _i is calculated by assuming that all particles have the same diameter as D50, the same weight, and an ideal spherical shape.

The median diameter D50 can be measured using a particle size distribution meter using a laser diffraction/scattering method. The specific surface area can be measured by, for example, a specific surface area measuring device using a gas adsorption method using a constant volume method.

In the positive electrode active material 100 of one embodiment of the present invention, the ratio S _R /S _i of the ideal specific surface area S _i determined from the median diameter D50 and the actual specific surface area S _R is preferably 2.1 or less. .

Alternatively, the surface smoothness can also be quantified from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

First, a surface SEM image of the positive electrode active material 100 is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. Preferably, the observation plane is perpendicular to the electron beam. When comparing multiple samples, use the same measurement conditions and observation area.

Next, an image obtained by converting the above SEM image into, for example, 8 bits (this is called a grayscale image) is obtained using image processing software (for example, "ImageJ"). A grayscale image includes luminance (brightness information). For example, in an 8-bit gray scale image, brightness can be represented by 2 to the 8th power = 256 gradations. The number of gradations is low in dark areas, and the number of gradations is high in bright areas. Luminance changes can be quantified in association with the number of gradations. This numerical value is called a grayscale value. By obtaining the gray scale value, it becomes possible to evaluate the unevenness of the positive electrode active material numerically.

Furthermore, it is also possible to represent the brightness changes in the target area using a histogram. A histogram is a three-dimensional representation of the gradation distribution in a target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually understand and evaluate the unevenness of the positive electrode active material.

In the positive electrode active material 100 of one embodiment of the present invention, the difference between the maximum value and the minimum value of the gray scale value is preferably 120 or less, more preferably 115 or less, and 70 or more and 115 or less. is even more preferable. Further, the standard deviation of the gray scale value is preferably 11 or less, more preferably 8 or less, and even more preferably 4 or more and 8 or less.

<Additional features>
The positive electrode active material 100 may have a recess, a crack, a depression, a V-shaped cross section, or the like. These are one type of defects, and when charging and discharging are repeated, cobalt may be eluted, the crystal structure may collapse, the positive electrode active material 100 may be cracked, and oxygen may be eliminated. However, if there is an embedded part 102 as shown in FIG. 9B to embed these elements, elution of cobalt, etc. can be suppressed. Therefore, the reliability and cycle characteristics of a secondary battery using the positive electrode active material 100 can be improved.

As described above, if the additive element contained in the positive electrode active material 100 is in excess, there is a risk that insertion and desorption of lithium will be adversely affected. Furthermore, when the positive electrode active material 100 is used in a secondary battery, there is a possibility that an increase in internal resistance, a decrease in charge/discharge capacity, etc. may occur. On the other hand, if it is insufficient, it may not be distributed throughout the surface layer portion 100a, and the effect of suppressing the deterioration of the crystal structure may become insufficient. As described above, it is necessary that the additive element has an appropriate concentration in the positive electrode active material 100, but it is not easy to adjust the concentration.

Therefore, if the positive electrode active material 100 has a region where the additive element is unevenly distributed, some of the atoms of the excessive additive element are removed from the interior 100b of the positive electrode active material 100, and the concentration of the additive element is adjusted to an appropriate concentration in the interior 100b. can do. This can suppress an increase in internal resistance, a decrease in charge/discharge capacity, etc. when used as a secondary battery. Being able to suppress an increase in the internal resistance of a secondary battery is an extremely desirable characteristic, particularly when charging and discharging at a large current, for example, at 400 mA/g or more.

Furthermore, in the positive electrode active material 100 having a region where the additive element is unevenly distributed, it is permissible to mix the additive element in a certain amount of excess during the manufacturing process. Therefore, the production margin is wide, which is preferable.

Further, a coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. FIGS. 24A and 24B show an example of the positive electrode active material 100 to which the coating portion 104 is attached.

The covering portion 104 is preferably formed by, for example, depositing decomposition products of an electrolyte and an organic electrolyte during charging and discharging. In particular, when charging is repeated such that x in Li _x CoO ₂ is 0.24 or less, it is expected that the charge-discharge cycle characteristics will be improved by having a coating derived from the electrolyte on the surface of the positive electrode active material 100. be done. This is for reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing elution of cobalt. Preferably, the covering portion 104 contains carbon, oxygen, and fluorine, for example. Furthermore, when LiBOB and/or SUN (suberonitrile) is used as the electrolyte, it is easy to obtain a high-quality coating. Therefore, the coating portion 104 containing one or more selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating portion and is therefore preferable. Further, the covering portion 104 does not need to cover all of the positive electrode active material 100. For example, it is sufficient to cover 50% or more of the surface of the positive electrode active material 100, more preferably 70% or more, and even more preferably 90% or more.

≪Powder resistance≫
The positive electrode active material 100 of one embodiment of the present invention has a stable crystal structure even at high voltage. Since the crystal structure of the positive electrode active material is stable in the charged state, it is possible to suppress a decrease in charge/discharge capacity due to repeated charging/discharging. As a feature of the positive electrode active material 100 having excellent properties as described above, in the above <<XRD>>, when x in Li _x CoO ₂ is small, O3' type and/or monoclinic O1 (15) type It was explained that it has a crystal structure of Moreover, in the above <<EDX>>, a preferable distribution of the additive elements in the case where the positive electrode active material 100 is subjected to STEM-EDX analysis was explained. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is also characterized by the volume resistivity of the powder.

As a feature of the positive electrode active material 100 of one embodiment of the present invention, the volume resistivity of the powder of the positive electrode active material 100 is preferably 1.0×10 ⁴ Ω·cm or more at a pressure of 64 MPa, and 1.0 It is more preferably ×10 ⁵ Ω·cm or more, and more preferably 1.0 × 10 ⁶ Ω·cm or more. Moreover, at a pressure of 64 MPa, it is preferably 1.0×10 ⁹ Ω・cm or less, more preferably 1.0×10 ⁸ Ω・cm or less, and 1.0×10 ⁷ Ω・cm or less It is more preferable that there be.

The positive electrode active material 100 having the above volume resistivity has a stable crystal structure even at high voltage. Therefore, the fact that the volume resistivity of the powder of the positive electrode active material 100 is within the above range allows the surface layer portion 100a to be formed well, which is important for the crystal structure of the positive electrode active material to be stable in the charged state. It can be used as an indicator to show what has been achieved. That is, it is preferable that the surface layer portion 100a has high resistance.

However, if a high-resistance region exists thickly from the surface of the positive electrode active material 100 toward the inside, the battery reaction may be inhibited. Therefore, it is more preferable that only a thin region near the surface of the surface layer portion 100a has high resistance. That is, in the surface layer portion 100a, it is preferable that a high resistance region exist thinly from the surface toward the inside.

This embodiment can be used in combination with other embodiments.

(Embodiment 3)
In this embodiment, an example of a method for manufacturing a positive electrode active material that can be used for a positive electrode of a battery according to one embodiment of the present invention will be described.

In order to produce the positive electrode active material 100 having the distribution, composition, and/or crystal structure of the additive elements as described in the previous embodiment, how to add the additive elements is important. At the same time, it is also important that the interior 100b has good crystallinity.

Therefore, in the manufacturing process of the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, then mix the additive element source and perform heat treatment.

In the method of synthesizing lithium cobalt oxide having an additive element by mixing a cobalt source, a lithium source, and an additive element source at the same time, it is difficult to increase the additive element concentration in the surface layer portion 100a. Further, after synthesizing lithium cobalt oxide, if the additive element source is only mixed and no heating is performed, the additive element will simply adhere to the lithium cobalt oxide without being dissolved in solid form. Without sufficient heating, it is difficult to distribute the additive elements well. Therefore, it is preferable to synthesize lithium cobalt oxide, mix the additive element source, and perform heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

However, if the annealing temperature is too high, cation mixing will occur, increasing the possibility that additional elements, such as magnesium, will enter the cobalt sites. Magnesium present in the cobalt site has no effect on maintaining the layered rock salt type crystal structure of R-3m when x in Li _x CoO ₂ is small. Furthermore, if the temperature of the heat treatment is too high, there are concerns that there will be adverse effects such as cobalt being reduced to become divalent and lithium evaporating.

Therefore, it is preferable to mix a material that functions as a flux together with the additive element source. If it has a lower melting point than lithium cobalt oxide, it can be said to be a material that functions as a fluxing agent. For example, fluorine compounds such as lithium fluoride are suitable. Addition of the flux lowers the melting point of the additive element source and the lithium cobalt oxide. By lowering the melting point, it becomes easier to distribute the additive element well at a temperature at which cation mixing is less likely to occur.

[Initial heating]
Furthermore, it is more preferable to perform heating after synthesizing lithium cobalt oxide and before mixing additional elements. This heating may be called initial heating.

Due to the initial heating, lithium is desorbed from a part of the surface layer 100a of lithium cobalt oxide, so that the distribution of the added elements becomes even better.

More specifically, it is thought that the initial heating makes it easier to vary the distribution depending on the added element through the following mechanism. First, lithium is desorbed from a portion of the surface layer portion 100a due to initial heating. Next, this lithium cobalt oxide having the surface layer portion 100a deficient in lithium and additional element sources including a nickel source, an aluminum source, and a magnesium source are mixed and heated. Among the additive elements, magnesium is a typical divalent element, and nickel is a transition metal but tends to become a divalent ion. Therefore, a rock salt-type phase containing Mg ²⁺ , Ni ²⁺ , and Co ²⁺ reduced due to lithium deficiency is formed in a part of the surface layer 100a. However, since this phase is formed in a part of the surface layer portion 100a, it may not be clearly visible in an electron microscope image such as STEM or in an electron beam diffraction pattern.

Among the additive elements, nickel tends to form a solid solution when the surface layer 100a is layered rock salt type lithium cobalt oxide and diffuses to the interior 100b, but when a part of the surface layer 100a is rock salt type, it tends to stay in the surface layer 100a. . Therefore, by performing initial heating, divalent additive elements such as nickel can be easily retained in the surface layer portion 100a. The effect of this initial heating is particularly large on the surface of the positive electrode active material 100 other than the (001) orientation and the surface layer portion 100a thereof.

In addition, in these rock salt types, the bond distance between metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt type.

For example, the Me-O distance in rock salt-type Ni _0.5 Mg _0.5 O is 2.09×10 ⁻¹ nm, and the Me-O distance in rock salt-type MgO is 2.11×10 ⁻¹ nm. Furthermore, even if a spinel-type phase is formed in a part of the surface layer 100a, the Me-O distance of spinel-type NiAl ₂ O ₄ is 2.0125×10 ⁻¹ nm, and the Me-O distance of spinel-type MgAl ₂ O ₄ is 2.0125×10 −1 nm. The O distance is 2.02×10 ⁻¹ nm. In both cases, the Me-O distance exceeds 2×10 ⁻¹ nm.

On the other hand, in the layered rock salt type, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, the Al-O distance in layered rock salt type LiAlO ₂ is 1.905×10 ⁻¹ nm (Li—O distance is 2.11×10 ⁻¹ nm). Moreover, the Co-O distance in layered rock salt type LiCoO ₂ is 1.9224×10 ⁻¹ nm (Li—O distance is 2.0916×10 ⁻¹ nm).

According to Shannon's ionic radius (Non-Patent Document 1), the ionic radius of six-coordinated aluminum is 0.535×10 ⁻¹ nm, and the ionic radius of six-coordinated oxygen is 1.4×10 ⁻¹ nm. and the sum of these is 1.935×10 ⁻¹ nm.

From the above, it is thought that aluminum exists more stably at sites other than lithium in the layered rock salt type than in the rock salt type. Therefore, aluminum is more likely to be distributed in a deeper region having a layered rock salt phase and/or inside 100b than in a region near the surface having a rock salt phase in the surface layer 100a.

Further, the initial heating can also be expected to have the effect of increasing the crystallinity of the layered rock salt crystal structure of the interior 100b.

Therefore, in order to produce the positive electrode active material 100 having a monoclinic O1 (15) type crystal structure especially when x in Li _x CoO ₂ is, for example, 0.15 or more and 0.17 or less, this initial heating is required. It is preferable.

However, initial heating does not necessarily have to be performed. In other heating steps, such as annealing, by controlling the atmosphere, temperature, time, etc., when x _in _Li In some cases, the substance 100 can be produced.

《Method for producing positive electrode active material 1》
A method 1 for manufacturing the positive electrode active material 100 through annealing and initial heating will be described with reference to FIGS. 25A to 25C.

<Step S11>
In step S11 shown in FIG. 25A, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal materials, respectively.

As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity; for example, a material with a purity of 99.99% or more may be used.

As the cobalt source, it is preferable to use a compound containing cobalt, and for example, cobalt oxide such as tricobalt tetroxide, cobalt hydroxide, etc. can be used.

The cobalt source preferably has a high purity, for example, the purity is 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N5 (99.995%) or higher, and even more preferably 5N (99%) or higher. .999%) or more is preferably used. By using high-purity materials, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery increases and/or the reliability of the secondary battery improves.

In addition, it is preferable that the cobalt source has high crystallinity, for example, it may have single crystal grains. For evaluation of the crystallinity of the cobalt source, TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high angle scattering annular dark field scanning transmission electron microscope) images, ABF-STEM (annular bright field scanning electron microscope) images, Evaluations include scanning transmission electron microscopy) images, X-ray diffraction (XRD), electron diffraction, neutron diffraction, and the like. Note that the above method for evaluating crystallinity can be applied not only to cobalt sources but also to evaluating other crystallinities.

<Step S12>
Next, in step S12 shown in FIG. 25A, a lithium source and a cobalt source are ground and mixed to produce a mixed material. Grinding and mixing can be done dry or wet. The wet method is preferable because it can be crushed into smaller pieces. If using a wet method, prepare a solvent. As the solvent, ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. can be used. It is more preferable to use an aprotic solvent that hardly reacts with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or more is used. It is preferable to mix the lithium source and the cobalt source with dehydrated acetone having a purity of 99.5% or more and suppressing the water content to 10 ppm or less, and perform the pulverization and mixing. By using dehydrated acetone of the purity described above, possible impurities can be reduced.

A ball mill, bead mill, or the like can be used as a means for grinding and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls may be used as the grinding media. Zirconium oxide balls are preferable because they emit fewer impurities. Furthermore, when using a ball mill, bead mill, or the like, the circumferential speed is preferably 100 mm/s or more and 2000 mm/s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is 838 mm/s (rotation speed 400 rpm, ball mill diameter 40 mm).

<Step S13>
Next, in step S13 shown in FIG. 25A, the mixed material is heated. The heating is preferably performed at a temperature of 800°C or more and 1100°C or less, more preferably 900°C or more and 1000°C or less, and even more preferably about 950°C. If the temperature is too low, the lithium source and cobalt source may be insufficiently decomposed and melted. On the other hand, if the temperature is too high, defects may occur due to evaporation of lithium from the lithium source and/or excessive reduction of cobalt. For example, cobalt changes from trivalent to divalent, which may induce oxygen defects.

If the heating time is too short, lithium cobalt oxide will not be synthesized, but if the heating time is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

The temperature increase rate depends on the temperature reached by the heating temperature, but is preferably 80°C/h or more and 250°C/h or less. For example, when heating at 1000°C for 10 hours, the temperature increase rate is preferably 200°C/h.

Heating is preferably carried out in an atmosphere with little water such as dry air, for example an atmosphere with a dew point of -50°C or less, more preferably -80°C or less. In this embodiment, heating is performed in an atmosphere with a dew point of -93°C. Further, in order to suppress impurities that may be mixed into the material, the concentration of impurities such as CH ₄ , CO, CO ₂ , H _{2 ,} etc. in the heating atmosphere is preferably set to 5 ppb (parts per billion) or less.

An atmosphere containing oxygen is preferable as the heating atmosphere. For example, there is a method of continuously introducing dry air into the reaction chamber. In this case, the flow rate of dry air is preferably 10 L/min. The method in which oxygen is continuously introduced into the reaction chamber and the oxygen flows within the reaction chamber is called flow.

When the heating atmosphere is an atmosphere containing oxygen, a method without flow may be used. For example, a method may be used in which the reaction chamber is depressurized and then filled with oxygen (also referred to as purging) to prevent the oxygen from entering or exiting the reaction chamber. For example, the reaction chamber may be depressurized to -970 hPa and then filled with oxygen to 50 hPa.

Cooling after heating may be allowed to cool naturally, but it is preferable that the temperature drop time from the specified temperature to room temperature is within 10 hours or more and 50 hours or less. However, cooling to room temperature is not necessarily required, and cooling to a temperature permitted by the next step is sufficient.

Heating in this step may be performed using a rotary kiln or a roller hearth kiln. Heating with a rotary kiln can be carried out while stirring in either a continuous type or a batch type.

The crucible used during heating is preferably an aluminum oxide crucible. An aluminum oxide crucible is a material that does not easily release impurities. In this embodiment, an aluminum oxide crucible with a purity of 99.9% is used. It is preferable to heat the crucible with a lid on it. It can prevent material volatilization.

Also, it is preferable to use a used crucible rather than a new one. In this specification, etc., a new crucible refers to one in which a material containing lithium, a transition metal M, and/or an additive element is charged and heated twice or less. Furthermore, a used crucible is one that has undergone the step of charging and heating materials containing lithium, transition metal M, and/or additive elements three or more times. This is because if a new crucible is used, there is a risk that some of the material, including lithium fluoride, will be absorbed, diffused, moved and/or attached to the sheath during heating. If a part of the material is lost due to these factors, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, will not be within the preferred range. On the other hand, this fear is less with second-hand crucibles.

After heating is completed, it may be crushed and further sieved if necessary. When recovering the heated material, it may be transferred from the crucible to the mortar and then recovered. Further, it is preferable to use an aluminum oxide mortar as the mortar. Aluminum oxide mortar is a material that does not easily release impurities. Specifically, an aluminum oxide mortar with a purity of 90% or more, preferably 99% or more is used. Note that the same heating conditions as in step S13 can be applied to heating steps other than step S13, which will be described later.

<Step S14>
Through the above steps, lithium cobalt oxide (LiCoO ₂ ) shown in step S14 shown in FIG. 25A can be synthesized.

Although an example has been shown in which the composite oxide is produced by a solid phase method as in steps S11 to S14, the composite oxide may also be produced by a coprecipitation method. Alternatively, the composite oxide may be produced by a hydrothermal method.

<Step S15>
Next, in step S15 shown in FIG. 25A, lithium cobalt oxide is heated. Since the lithium cobalt oxide is first heated, the heating in step S15 may be referred to as initial heating. Alternatively, since it is heated before step S20 shown below, it may be called preheating or pretreatment.

Due to the initial heating, lithium is desorbed from a part of the surface layer portion 100a of lithium cobalt oxide as described above. Moreover, the effect of increasing the crystallinity of the interior 100b can be expected. Further, impurities may be mixed in the lithium source and/or cobalt source prepared in step S11 and the like. It is possible to reduce impurities from the lithium cobalt oxide completed in step S14 by initial heating.

Additionally, initial heating has the effect of smoothing the surface of lithium cobalt oxide. When the surface of lithium cobalt oxide is smooth, it means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a state in which there are few foreign substances attached to the surface is called smooth. Foreign matter is considered to be a cause of unevenness, and it is preferable that it does not adhere to the surface.

There is no need to prepare a lithium source for this initial heating. Alternatively, it is not necessary to prepare an additive element source. Alternatively, there is no need to prepare a material that functions as a flux.

If the heating time of this step is too short, a sufficient effect will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. Adding to the heating conditions, the heating temperature in this step is preferably lower than the temperature in step S13 in order to maintain the crystal structure of the composite oxide. Further, the heating time in this step is preferably shorter than the time in step S13 in order to maintain the crystal structure of the composite oxide. For example, heating is preferably performed at a temperature of 700° C. or more and 1000° C. or less for 2 hours or more and 20 hours or less.

The effect of increasing the crystallinity of the interior 100b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the shrinkage difference of the lithium cobalt oxide produced in step S13.

In the lithium cobalt oxide, a temperature difference may occur between the surface and the inside of the lithium cobalt oxide due to the heating in step S13. Temperature differences can induce differential shrinkage. It is also thought that the temperature difference causes a difference in shrinkage due to the difference in fluidity between the surface and the inside. The energy associated with differential shrinkage imparts differential internal stress to lithium cobalt oxide. The difference in internal stress is also called strain, and the energy is sometimes called strain energy. It is considered that the internal stress is removed by the initial heating in step S15, and in other words, the strain energy is homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in lithium cobalt oxide is relaxed. As a result, the surface of lithium cobalt oxide may become smooth. It is also said that the surface has been improved. In other words, it is considered that after step S15, the shrinkage difference that occurs in the lithium cobalt oxide is alleviated, and the surface of the composite oxide becomes smooth.

Furthermore, the difference in shrinkage may cause microscopic shifts in the lithium cobalt oxide, such as crystal shifts. This step may also be carried out in order to reduce the deviation. Through this step, it is possible to equalize the deviation of the composite oxide. If the misalignment is made uniform, the surface of the composite oxide may become smooth. It is also said that crystal grains have been aligned. In other words, it is considered that after step S15, the displacement of crystals, etc. that occurs in the composite oxide is alleviated, and the surface of the composite oxide becomes smooth.

If lithium cobalt oxide with a smooth surface is used as a positive electrode active material, there will be less deterioration during charging and discharging as a secondary battery, and cracking of the positive electrode active material can be prevented.

Note that lithium cobalt oxide synthesized in advance may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on lithium cobalt oxide synthesized in advance, lithium cobalt oxide with a smooth surface can be obtained.

<Step S20>
Next, as shown in step S20, it is preferable to add additive element A to the lithium cobalt oxide that has undergone initial heating. When the additive element A is added to the lithium cobalt oxide that has undergone initial heating, the additive element A can be added evenly. Therefore, it is preferable to add the additive element A after the initial heating. The step of adding additive element A will be explained using FIG. 25B and FIG. 25C.

<Step S21~Step S23>
The steps of preparing the additive element A source (A source) will be explained using FIGS. 25B and 25C. A lithium source may be prepared together with the additive element A source.

As the additive element A, the additive elements described in the previous embodiment can be used. Specifically, one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. . Moreover, one or two selected from bromine and beryllium can also be used.

<Step S21>
Step S21 shown in FIG. 25B will be explained. When magnesium is selected as the additive element, the additive element source can be called a magnesium source (Mg source). As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Further, a plurality of the above-mentioned magnesium sources may be used.

When fluorine is selected as the additive element, the additive element source can be called a fluorine source (F source). Examples of the fluorine source include lithium fluoride (LiF), magnesium fluoride (MgF ₂ ), aluminum fluoride (AlF ₃ ), titanium fluoride (TiF ₄ ), cobalt fluoride (CoF ₂ , CoF ₃ ), and fluorine. Nickel fluoride (NiF ₂ ), zirconium fluoride (ZrF ₄ ), vanadium fluoride (VF ₅ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ₂ ), calcium fluoride (CaF ₂ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ₂ ), cerium fluoride (CeF ₃ , CeF ₄ ), lanthanum fluoride (LaF ₃ ), or hexafluoride Sodium aluminum (Na ₃ AlF ₆ ) or the like can be used. Among these, lithium fluoride is preferable because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

Magnesium fluoride can be used both as a fluorine source and as a magnesium source. Lithium fluoride can also be used as a lithium source. Another lithium source used in step S21 is lithium carbonate.

Further, the fluorine source may be a gas, such as fluorine (F ₂ ), fluorocarbon, sulfur fluoride, or fluorinated oxygen (OF ₂ , O ₂ F ₂ , O ₃ F ₂ , O ₄ F ₂ , O ₅ F ₂ , O ₆ F ₂ , O ₂ F) or the like may be used and mixed in the atmosphere in the heating step described later. Further, a plurality of the above-mentioned fluorine sources may be used.

In this embodiment, lithium fluoride (LiF) is prepared as a fluorine source, and magnesium fluoride (MgF ₂ ) is prepared as a fluorine source and a magnesium source. When lithium fluoride and magnesium fluoride are mixed at a molar ratio of about 65:35 (LiF:MgF ₂ ), the effect of lowering the melting point is maximized. On the other hand, if the amount of lithium fluoride increases, there is a concern that the amount of lithium will be too much and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride and magnesium fluoride is preferably LiF:MgF ₂ =x:1 (0≦x≦1.9), and LiF:MgF ₂ =x:1 (0.1≦ x≦0.5) is more preferable, and LiF:MgF ₂ =x:1 (x=0.33 or its vicinity) is even more preferable. Note that in this specification and the like, the term "near" means a value greater than 0.9 times and less than 1.1 times that value.

<Step S22>
Next, in step S22 shown in FIG. 25B, the magnesium source and the fluorine source are ground and mixed. This step can be carried out by selecting from the pulverization and mixing conditions described in step S12.

<Step S23>
Next, in step S23 shown in FIG. 25B, the materials crushed and mixed above can be recovered to obtain an additive element A source (A source). Note that the additive element A source shown in step S23 has a plurality of starting materials and can be called a mixture.

The particle size of the above mixture preferably has a D50 (median diameter) of 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less. Even when one type of material is used as the additive element source, the D50 (median diameter) is preferably 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less.

When such a finely powdered mixture (including cases where only one type of additive element is added) is mixed with lithium cobalt oxide in a later step, it is difficult to uniformly adhere the mixture to the surface of the lithium cobalt oxide particles. Cheap. It is preferable that the mixture is uniformly adhered to the surface of the lithium cobalt oxide particles because it is easy to uniformly distribute or diffuse the additive element in the surface layer portion 100a of the composite oxide after heating.

<Step S21>
A process different from that in FIG. 25B will be explained using FIG. 25C. In step S21 shown in FIG. 25C, four types of additive element sources to be added to lithium cobalt oxide are prepared. That is, FIG. 25C differs from FIG. 25B in the type of additive element source. A lithium source may be prepared together with the additive element source.

A magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared as four types of additional element sources. Note that the magnesium source and the fluorine source can be selected from the compounds described in FIG. 25B. As the nickel source, nickel oxide, nickel hydroxide, etc. can be used. As the aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

<Step S22 and Step S23>
Steps S22 and S23 shown in FIG. 25C are similar to the steps described in FIG. 25B.

<Step S31>
Next, in step S31 shown in FIG. 25A, lithium cobalt oxide and an additive element A source (A source) are mixed. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg included in the additive element A source is preferably Co:Mg=100:y (0.1≦y≦6), It is more preferable that M:Mg=100:y (0.3≦y≦3).

The mixing in step S31 is preferably performed under milder conditions than the mixing in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable that the rotational speed is lower or the time is shorter than the mixing in step S12. It can also be said that the dry method has milder conditions than the wet method. For example, a ball mill, a bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use, for example, zirconium oxide balls as the media.

In this embodiment, dry mixing is performed at 150 rpm for 1 hour using a ball mill using zirconium oxide balls with a diameter of 1 mm. Further, the mixing is performed in a dry room with a dew point of -100°C or more and -10°C or less.

<Step S32>
Next, in step S32 of FIG. 25A, the materials mixed above are collected to obtain a mixture 903. During recovery, sieving may be performed after crushing if necessary.

Although FIGS. 25A to 25C describe a manufacturing method in which additive elements are added only after initial heating, the present invention is not limited to the above method. The additive element may be added at other timings or may be added multiple times. The timing may be changed depending on the element.

For example, the additive element may be added to the lithium source and the cobalt source at the stage of step S11, that is, at the stage of the starting material of the composite oxide. Thereafter, in step S13, lithium cobalt oxide having additive elements can be obtained. In this case, there is no need to separate the steps S11 to S14 from the steps S21 to S23. It can be said that this is a simple and highly productive method.

Alternatively, lithium cobalt oxide having some of the additive elements in advance may be used. For example, if lithium cobalt oxide to which magnesium and fluorine are added is used, steps S11 to S14 and a part of step S20 can be omitted. It can be said that this is a simple and highly productive method.

Further, after heating the lithium cobalt oxide to which magnesium and fluorine have been added in advance in step S15, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source are added as in step S20. may be added.

<Step S33>
Next, in step S33 shown in FIG. 25A, the mixture 903 is heated. The heating conditions can be selected from the heating conditions explained in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may exceed atmospheric pressure in order to increase the oxygen partial pressure in the heating atmosphere. This is because if the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and the like are reduced, and lithium cobalt oxide and the like may not be able to maintain a layered rock salt crystal structure.

Here is some additional information about heating temperature. The lower limit of the heating temperature in step S33 needs to be equal to or higher than the temperature at which the reaction between lithium cobalt oxide and the additive element source progresses. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of the elements of the lithium cobalt oxide and the additional element source occurs, and may be lower than the melting temperature of these materials. This will be explained using an oxide as an example, and it is known that solid phase diffusion occurs from 0.757 times the melting temperature T _m (Tammann temperature T _d ). Therefore, the heating temperature in step S33 may be 650° C. or higher.

Of course, if the temperature is higher than the temperature at which one or more of the materials selected from the mixture 903 melts, the reaction will more easily proceed. For example, when LiF and MgF ₂ are used as the additive element source, the eutectic point of LiF and MgF ₂ is around 742°C, so the lower limit of the heating temperature in step S33 is preferably 742°C or higher.

In addition, the mixture 903 obtained by mixing LiCoO ₂ :LiF:MgF ₂ =100:0.33:1 (molar ratio) has an endothermic peak at around 830°C in differential scanning calorimetry (DSC measurement). is observed. Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

A higher heating temperature is preferable because the reaction progresses more easily, heating time is shorter, and productivity is higher.

The upper limit of the heating temperature is lower than the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is concern that lithium cobalt oxide will decompose, albeit in a small amount. Therefore, the temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 900°C or lower.

Based on these, the heating temperature in step S33 is preferably 650°C or more and 1130°C or less, more preferably 650°C or more and 1000°C or less, even more preferably 650°C or more and 950°C or less, and even more preferably 650°C or more and 900°C or less. preferable. Further, the temperature is preferably 742°C or more and 1130°C or less, more preferably 742°C or more and 1000°C or less, even more preferably 742°C or more and 950°C or less, and even more preferably 742°C or more and 900°C or less. Further, the temperature is preferably 800°C or more and 1100°C or less, 830°C or more and 1130°C or less, more preferably 830°C or more and 1000°C or less, even more preferably 830°C or more and 950°C or less, and even more preferably 830°C or more and 900°C or less. Note that the heating temperature in step S33 is preferably higher than that in step S13.

Further, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride caused by a fluorine source or the like within an appropriate range.

In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, may function as a flux. With this function, the heating temperature can be lowered to below the decomposition temperature of lithium cobalt oxide, for example, from 742°C to 950°C, and additive elements such as magnesium are distributed in the surface layer, creating a positive electrode active material with good characteristics. can.

However, since LiF has a lower specific gravity than oxygen in a gaseous state, there is a possibility that LiF will volatilize or sublimate due to heating, and if it volatilizes, LiF in the mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat LiF while suppressing its volatilization. Note that even if LiF is not used as a fluorine source, there is a possibility that Li on the surface of LiCoO ₂ and F of the fluorine source react to generate LiF and volatilize. Therefore, even if a fluoride having a higher melting point than LiF is used, it is necessary to suppress volatilization in the same way.

Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF in the heating furnace is high. Such heating can suppress volatilization of LiF in the mixture 903.

The heating in this step is preferably performed so that the particles of the mixture 903 do not stick to each other. If mixture 903 particles stick to each other during heating, the contact area with oxygen in the atmosphere decreases, and the diffusion path of added elements (e.g. fluorine) is inhibited, thereby preventing the addition of added elements (e.g. magnesium and fluorine) to the surface layer. ) distribution may deteriorate.

It is also believed that if the additive element (for example, fluorine) is uniformly distributed in the surface layer, a positive electrode active material that is smooth and has few irregularities can be obtained. Therefore, in order for the surface to remain smooth or to become even smoother after the heating in step S15 in this process, it is better that the particles of the mixture 903 do not stick to each other.

Furthermore, when heating is performed using a rotary kiln, it is preferable to control the flow rate of the atmosphere containing oxygen in the kiln. For example, it is preferable to reduce the flow rate of the atmosphere containing oxygen, to purge the atmosphere first, and to not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is not preferable for maintaining surface smoothness.

In the case of heating with a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by placing a lid on the container containing the mixture 903, for example.

A note about heating time. The heating time varies depending on conditions such as the heating temperature, the size of the lithium cobalt oxide in step S14, and the composition. If the lithium cobalt oxide is small, lower temperatures or shorter times may be more preferred than if it is larger.

When the median diameter (D50) of the lithium cobalt oxide in step S14 of FIG. 25A is about 12 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably 3 hours or more and 60 hours or less, more preferably 10 hours or more and 30 hours or less, and even more preferably about 20 hours. Note that the time for cooling down after heating is preferably 10 hours or more and 50 hours or less, for example.

On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably 1 hour or more and 10 hours or less, and more preferably about 5 hours. Note that the time for cooling down after heating is preferably 10 hours or more and 50 hours or less, for example.

<Step S34>
Next, in step S34 shown in FIG. 25A, the heated material is collected and crushed if necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the collected particles. Through the above steps, the positive electrode active material 100 of one embodiment of the present invention can be manufactured. The positive electrode active material of one embodiment of the present invention has a smooth surface.

《Method for producing positive electrode active material 2》
Next, a method 2 for manufacturing a positive electrode active material, which is an embodiment of the present invention and is different from the method 1 for manufacturing a positive electrode active material, will be described with reference to FIGS. 26 to 27C. Manufacturing method 2 of the positive electrode active material differs from manufacturing method 1 mainly in the number of times of addition of additive elements and the mixing method. For other descriptions, the description of Production Method 1 can be referred to.

In FIG. 26, steps S11 to S15 are performed in the same manner as in FIG. 25A to prepare lithium cobalt oxide that has undergone initial heating.

<Step S20a>
Next, in step S20a, a source of additive element A1 is prepared to be used for adding additive element A1 to lithium cobalt oxide that has undergone initial heating. The process of preparing the additive element A1 source will be described using FIG. 27A.

<Step S21>
Step S21 shown in FIG. 27A will be explained. The additive element A1 can be selected from the elements exemplified as the additive element A explained in step S21 shown in FIG. 25B. For example, as the additive element A1, one or more selected from magnesium, fluorine, and calcium can be suitably used. In FIG. 27A, a case where a magnesium source (Mg source) and a fluorine source (F source) are used is illustrated as a case where magnesium and fluorine are selected as the additive elements A1.

Steps S21 to S23 shown in FIG. 27A can be performed under the same conditions as steps S21 to S23 shown in FIG. 25B. As a result, an additive element A1 source (A1 source) can be obtained in step S23.

Furthermore, steps S31 to S33 shown in FIG. 26 can be performed in the same steps as steps S31 to S33 shown in FIG. 25A.

<Step S34a>
Next, the material heated in step S33 is recovered, and lithium cobalt oxide having the additive element A1 is produced. It is also referred to as a second composite oxide to distinguish it from the composite oxide in step S14.

<Step S40>
In step S40 shown in FIG. 26, a source of additive element A2 used for adding additive element A2 to the second composite oxide is prepared. The steps of preparing the additive element A2 source will be explained using FIGS. 27B and 27C.

<Step S41>
Step S41 shown in FIG. 27B will be explained. The additive element A2 can be selected from the elements exemplified as the additive element A explained in step S21 shown in FIG. 25B. For example, as the additive element A2, one or more selected from nickel, titanium, boron, zirconium, and aluminum can be suitably used. FIG. 27B shows an example in which a nickel source (Ni source) and an aluminum source (Al source) are prepared when nickel and aluminum are selected as the additive element A2.

Steps S41 to S43 shown in FIG. 27B can be performed under the same conditions as steps S21 to S23 shown in FIG. 25B. As a result, an additive element A2 source (A2 source) can be obtained in step S43.

Further, FIG. 27C shows a modification of the process of preparing the additive element A2 source described using FIG. 27B. In step S41 shown in FIG. 27C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are each pulverized independently. As a result, in step S43, a plurality of additive element A2 sources (A2 sources) are prepared. The step in FIG. 27C differs from that in FIG. 27B in that the added element is independently pulverized in step S42a.

<Step S51 to Step S53>
Next, steps S51 to S53 shown in FIG. 26 can be performed under the same conditions as steps S31 to S34 shown in FIG. 25A, and a mixture 904 is obtained in step S52. Further, the conditions for step S53 regarding the heating step may be lower temperature and shorter time than step S33. Through the above steps, in step S54, the positive electrode active material 100 of one embodiment of the present invention can be manufactured. The positive electrode active material of one embodiment of the present invention has a smooth surface.

As shown in FIGS. 26 to 27C, in manufacturing method 2, the additive elements to lithium cobalt oxide are introduced separately into additive element A1 and additive element A2. By introducing each element separately, the distribution of each additive element in the depth direction can be changed. For example, it is also possible to add the additive element A1 to a higher concentration in the surface layer 100a than in the interior 100b, and to add the additive element A2 to a higher concentration in the interior 100b than in the surface layer 100a. .

After the initial heating shown in this embodiment mode, a positive electrode active material with a smooth surface can be obtained.

The initial heating shown in this embodiment mode is performed on lithium cobalt oxide. Therefore, the conditions for initial heating are preferably lower than the heating temperature for obtaining lithium cobalt oxide and shorter than the heating time for obtaining lithium cobalt oxide. The step of adding additional elements to lithium cobalt oxide is preferably performed after initial heating. The addition step can be divided into two or more steps. It is preferable to follow this process order because the smoothness of the surface obtained by the initial heating is maintained.

A positive electrode active material 100 with a smooth surface may be more resistant to physical destruction due to pressure or the like than a positive electrode active material with a smooth surface. For example, the positive electrode active material 100 is less likely to be destroyed in a test involving pressurization such as a nail penetration test, which may result in increased safety.

This embodiment can be used in combination with other embodiments.

(Embodiment 4)
In this embodiment, a positive electrode active material that can be used for a positive electrode of a battery according to one embodiment of the present invention will be described.

In Embodiment 1, lithium cobalt oxide having a barrier film (surface layer portion 100a) was described as the positive electrode active material 100 that can be used for the positive electrode of a battery according to one embodiment of the present invention. The positive electrode active material used in the battery of one embodiment of the present invention is not limited to lithium cobalt oxide having a barrier film, and a positive electrode active material represented by Li _x MO ₂ having a barrier film can be used. Note that M is one or more selected from Co, Ni, Mn, and Al. Further, the positive electrode active material represented by Li _x MO ₂ has a layered rock salt type crystal structure belonging to space group R-3m.

Examples of the positive electrode active material represented by Li _x MO ₂ include lithium cobalt oxide, lithium cobalt-nickelate, lithium nickel-cobalt-manganate, lithium nickel-cobalt-aluminate, and lithium nickel-manganese-aluminate. Any one or more of these can be used.

As the lithium cobalt-nickelate, for example, lithium cobalt-nickelate to which magnesium and fluorine are added can be used. Moreover, it is preferable to use cobalt-lithium nickelate to which magnesium, fluorine, and aluminum are added. Note that in cobalt-lithium nickelate, the number of cobalt atoms is greater than the number of nickel atoms.

For example, as nickel-cobalt-lithium manganate, the ratio of the number of nickel atoms, the number of cobalt atoms, and the number of manganese atoms is nickel: cobalt: manganese = 1:1:1, nickel: cobalt: manganese = 5:2 :3, nickel: cobalt: manganese = 6:2:2, nickel: cobalt: manganese = 8:1:1, and nickel: cobalt: manganese = 9:0.5:0.5, and the ratios in their vicinity nickel-cobalt-lithium manganate can be used.

As a feature of Li _x MO ₂ having a barrier film, the volume resistivity of the powder is preferably 1.0×10 ⁴ Ω・cm or more, and 1.0×10 ⁵ Ω・cm or more at a pressure of 64 MPa. It is more preferable that it is, and it is more preferable that it is 1.0×10 ⁶ Ω·cm or more. Moreover, at a pressure of 64 MPa, it is preferably 1.0×10 ⁹ Ω・cm or less, more preferably 1.0×10 ⁸ Ω・cm or less, and 1.0×10 ⁷ Ω・cm or less It is more preferable that there be.

The positive electrode active material having the above volume resistivity has a stable crystal structure even at high voltages, and can form a good surface layer, which is important for the crystal structure of the positive electrode active material to be stable in the charged state. It can be used as an indicator to show that In other words, the surface layer portion preferably has high resistance.

However, if a high-resistance region exists thickly from the surface of the positive electrode active material toward the inside, the battery reaction may be inhibited. Therefore, it is more preferable that only a thin region near the surface of the surface layer portion has high resistance. That is, in the surface layer portion, it is preferable that a high resistance region exist thinly from the surface toward the inside.

This embodiment can be used in combination with other embodiments.

(Embodiment 5)
In this embodiment, an example of a vehicle including a secondary battery according to one embodiment of the present invention will be described.

As a vehicle, a secondary battery can typically be applied to an automobile. Examples of automobiles include next-generation clean energy vehicles such as hybrid vehicles (HV), electric vehicles (EV), and plug-in hybrid vehicles (PHEV or PHV). A secondary battery can be applied. Vehicles are not limited to automobiles. For example, vehicles include trains, monorails, ships, submersibles (deep sea exploration vehicles, unmanned submarines), flying vehicles (helicopters, unmanned aerial vehicles (drones), airplanes, rockets, artificial satellites), electric bicycles, electric motorcycles, etc. The secondary battery of one embodiment of the present invention can be applied to these vehicles.

As shown in FIG. 28C, the electric vehicle is equipped with

first batteries

1301a and 1301b as main drive secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. ing. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have a high output, and a large capacity is not required, and the capacity of the second battery 1311 is smaller than that of the

first batteries

1301a and 1301b.

The internal structure of the first battery 1301a may be a wound type shown in FIG. 6C or FIG. 7A, or a stacked type shown in FIG. 8A or FIG. 8B.

Although this embodiment shows an example in which two

first batteries

1301a and 1301b are connected in parallel, three or more may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may not be necessary. By configuring a battery pack that includes a plurality of secondary batteries, a large amount of electric power can be extracted. A plurality of secondary batteries may be connected in parallel, may be connected in series, or may be connected in parallel and then further connected in series. A plurality of secondary batteries is also called an assembled battery.

In addition, in order to cut off power from multiple secondary batteries in a vehicle-mounted secondary battery, the first battery 1301a has a service plug or circuit breaker that can cut off high voltage without using tools. provided.

The electric power of the

first batteries

1301a and 1301b is mainly used to rotate the motor 1304, but it is also used to power 42V-based in-vehicle components (electric power steering 1307, heater 1308, defogger 1309, etc.) via a DCDC circuit 1306. ). Even when the rear motor 1317 is provided on the rear wheel, the first battery 1301a is used to rotate the rear motor 1317.

Further, the second battery 1311 supplies power to 14V vehicle components (audio 1313, power window 1314, lamps 1315, etc.) via the DCDC circuit 1310.

Next, the first battery 1301a will be explained using FIG. 28A.

FIG. 28A shows an example in which nine square secondary batteries 1300 are used as one battery pack 1415. Further, nine prismatic secondary batteries 1300 are connected in series, one electrode is fixed by a fixing part 1413 made of an insulator, and the other electrode is fixed by a fixing part 1414 made of an insulator. Although this embodiment shows an example in which the battery is fixed using the fixing

parts

1413 and 1414, it may also be configured to be housed in a battery housing box (also referred to as a housing). Since it is assumed that vibrations or shaking are applied to the vehicle from the outside (road surface, etc.), it is preferable to fix the plurality of secondary batteries using fixing

parts

1413, 1414, a battery housing box, and the like. Further, one electrode is electrically connected to the control circuit section 1320 by a wiring 1421. The other electrode is electrically connected to the control circuit section 1320 by a wiring 1422.

Next, FIG. 28B shows an example of a block diagram of the battery pack 1415 shown in FIG. 28A.

The control circuit section 1320 includes a switch section 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarge, a control circuit 1322 for controlling the switch section 1324, and a voltage measuring section for the first battery 1301a. has. The control circuit section 1320 has an upper limit voltage and a lower limit voltage set for the secondary battery to be used, and limits the upper limit of the current from the outside or the upper limit of the output current to the outside. The range of the secondary battery's lower limit voltage to upper limit voltage is within the recommended voltage range, and when the voltage is outside of that range, the switch section 1324 is activated and functions as a protection circuit. Furthermore, the control circuit section 1320 can also be called a protection circuit because it controls the switch section 1324 to prevent over-discharging and/or over-charging. For example, when the control circuit 1322 detects a voltage that is likely to cause overcharging, the switch section 1324 is turned off to cut off the current. Furthermore, a PTC element may be provided in the charging/discharging path to provide a function of cutting off the current in response to a rise in temperature. Further, the control circuit section 1320 has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

The switch section 1324 can be configured by combining n-channel transistors or p-channel transistors. The switch section 1324 is not limited to a switch having an Si transistor using single crystal silicon, but includes, for example, Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (phosphide). The switch portion 1324 may be formed using a power transistor including indium (indium), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), GaOx (gallium oxide; x is a real number greater than 0), or the like.

The

first batteries

1301a and 1301b mainly supply power to 42V system (high voltage HV) in-vehicle equipment, and the second battery 1311 supplies power to 14V system (low voltage LV) in-vehicle equipment. . As the second battery 1311, a lead-acid battery is often used because it is advantageous in terms of cost.

In this embodiment, an example is shown in which lithium ion batteries are used as both the first battery 1301a and the second battery 1311. The second battery 1311 may be a lead-acid battery, an all-solid-state battery, or an electric double layer capacitor.

Furthermore, regenerated energy due to the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and charged to the second battery 1311 from the motor controller 1303 or the battery controller 1302 via the control circuit section 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerated energy, it is desirable that the

first batteries

1301a and 1301b can be rapidly charged.

The battery controller 1302 can set the charging voltage, charging current, etc. of the

first batteries

1301a and 1301b. The battery controller 1302 can set charging conditions according to the charging characteristics of the secondary battery to be used and perform rapid charging.

Although not shown, when connecting the electric vehicle to an external charger, the outlet of the charger or the connection cable of the charger is electrically connected to the battery controller 1302. Power supplied from an external charger charges the

first batteries

1301a and 1301b via the battery controller 1302. Also, depending on the charger, a control circuit is provided and the function of the battery controller 1302 is not used in some cases, but in order to prevent overcharging, the

first batteries

1301a and 1301b are charged via the control circuit section 1320. It is preferable. In some cases, the charger outlet or the charger connection cable is provided with a control circuit. The control circuit section 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. Further, the ECU includes a microcomputer. Further, the ECU uses a CPU or a GPU.

External chargers installed at charging stands etc. include 100V outlet-200V outlet, or 3-phase 200V and 50kW. It is also possible to charge the battery by receiving power from an external charging facility using a non-contact power supply method or the like.

When performing rapid charging, a secondary battery that can withstand high voltage charging is desired in order to perform charging in a short time.

In addition, by using graphene as a conductive material, the capacity decrease is suppressed even when the electrode layer is made thicker and the loading amount is increased, and the synergistic effect of maintaining high capacity has resulted in a secondary battery with significantly improved electrical characteristics. can. It is particularly effective for secondary batteries used in vehicles, and provides a vehicle with a long cruising range, specifically a cruising range of 500 km or more on one charge, without increasing the weight ratio of the secondary battery to the total vehicle weight. be able to.

In particular, the secondary battery of the present embodiment described above can provide a secondary battery for vehicles with excellent safety and reliability by using the battery 10 described in Embodiment 1.

Next, an example in which a secondary battery, which is one embodiment of the present invention, is mounted in a vehicle, typically a transportation vehicle, will be described.

When the secondary battery shown in any one of FIG. 5D, FIG. 7C, and FIG. 28A is installed in a vehicle, next-generation clean energy such as a hybrid vehicle (HV), electric vehicle (EV), or plug-in hybrid vehicle (PHV) can be used. A car can be realized. We also install secondary batteries in agricultural machinery, motorized bicycles including electric assist bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, or spacecraft. It can also be installed. The secondary battery of one embodiment of the present invention can be a high capacity secondary battery. Therefore, the secondary battery of one embodiment of the present invention is suitable for reduction in size and weight, and can be suitably used for transportation vehicles.

29A to 29D illustrate a transportation vehicle using one embodiment of the present invention. A car 2001 shown in FIG. 29A is an electric car that uses an electric motor as a power source for driving. Alternatively, it is a hybrid vehicle that can appropriately select and use an electric motor and an engine as a power source for driving. When a secondary battery is mounted on a vehicle, the example of the secondary battery shown in Embodiment 6 is installed at one or more locations. An automobile 2001 shown in FIG. 29A includes a battery pack 2200, and the battery pack includes a secondary battery module to which a plurality of secondary batteries are connected. Furthermore, it is preferable to include a charging control device electrically connected to the secondary battery module.

Further, the automobile 2001 can be charged by receiving power from an external charging facility using a plug-in method, a non-contact power supply method, or the like to a secondary battery of the automobile 2001. When charging, a predetermined charging method or connector standard such as CHAdeMO (registered trademark) or combo may be used as appropriate. The charging equipment may be a charging station provided at a commercial facility or may be a home power source. For example, using plug-in technology, it is possible to charge the power storage device mounted on the vehicle 2001 by supplying power from the outside. Charging can be performed by converting AC power into DC power via a conversion device such as an ACDC converter.

Although not shown, a power receiving device can be mounted on a vehicle, and power can be supplied from a ground power transmitting device in a non-contact manner for charging. In the case of this non-contact power supply method, by incorporating a power transmission device into the road or outside wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is running. Further, electric power may be transmitted and received between two vehicles using this contactless power supply method. Furthermore, a solar cell may be provided on the exterior of the vehicle, and the secondary battery may be charged when the vehicle is stopped or traveling. For such non-contact power supply, an electromagnetic induction method or a magnetic resonance method can be used.

FIG. 29B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 has a maximum voltage of 170V, for example, in which four secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less are connected in series, and 48 cells are connected in series. Except for the difference in the number of secondary batteries constituting the secondary battery module of the battery pack 2201, it has the same functions as those in FIG. 29A, so a description thereof will be omitted.

FIG. 29C shows, as an example, a large transport vehicle 2003 with an electrically controlled motor. The secondary battery module of the transportation vehicle 2003 has a maximum voltage of 600 V, for example, by connecting in series one hundred or more secondary batteries with a nominal voltage of 3.0 V or more and 5.0 V or less. Therefore, a secondary battery with small variations in characteristics is required. Further, except for the difference in the number of secondary batteries constituting the secondary battery module of the battery pack 2202, etc., it has the same functions as those in FIG. 31A, so a description thereof will be omitted.

FIG. 29D shows an example aircraft 2004 with an engine that burns fuel. Since the aircraft 2004 shown in FIG. 29D has wheels for takeoff and landing, it can be said to be a type of transportation vehicle, and a plurality of secondary batteries are connected to form a secondary battery module, and the secondary battery module and charging control are performed. It has a battery pack 2203 that includes a device.

The maximum voltage of the secondary battery module of the aircraft 2004 is 32V, which is obtained by connecting eight 4V secondary batteries in series, for example. Except for the difference in the number of secondary batteries constituting the secondary battery module of the battery pack 2203, etc., it has the same functions as those in FIG. 29A, so a description thereof will be omitted.

FIG. 29E shows an artificial satellite 2005 equipped with a secondary battery 2204 as an example. Since the artificial satellite 2005 is used in outer space, it is desired that there be no failure due to ignition, and it is preferable to include the secondary battery 2204, which is an aspect of the present invention and has excellent safety. Furthermore, it is more preferable that the secondary battery 2204 is mounted inside the artificial satellite 2005 while being covered with a heat insulating member.

This embodiment can be freely combined with other embodiments.

(Embodiment 6)
In this embodiment, as an example of mounting a secondary battery on a vehicle, an example will be shown in which a lithium ion battery, which is an embodiment of the present invention, is mounted on a two-wheeled vehicle or a bicycle.

FIG. 30A is an example of an electric bicycle using the power storage device of one embodiment of the present invention. The power storage device of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 30A. A power storage device according to one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists the driver. Further, the power storage device 8702 is portable, and is shown in a state removed from the bicycle in FIG. 30B. Further, the power storage device 8702 has a plurality of built-in storage batteries 8701 included in the power storage device of one embodiment of the present invention, and can display the remaining battery level and the like on a display portion 8703. Power storage device 8702 also includes a control circuit 8704 that can control charging or detect abnormality of a secondary battery, an example of which is shown in Embodiment 7. The control circuit 8704 is electrically connected to the positive and negative electrodes of the storage battery 8701. By combining it with the battery 10 described in Embodiment 1, a synergistic effect regarding safety can be obtained. The battery 10 and control circuit 8704 described in Embodiment 1 are highly safe and can greatly contribute to eradicating accidents such as fires caused by secondary batteries.

FIG. 30C is an example of a two-wheeled vehicle using the power storage device of one embodiment of the present invention. A scooter 8600 shown in FIG. 30C includes a power storage device 8602, a side mirror 8601, and a direction indicator light 8603. The power storage device 8602 can supply electricity to the direction indicator light 8603. Further, the power storage device 8602 that houses a plurality of batteries 10 obtained in Embodiment 1 can have a high capacity and can contribute to miniaturization.

Further, the scooter 8600 shown in FIG. 30C can store a power storage device 8602 in an under-seat storage 8604. The power storage device 8602 can be stored in the under-seat storage 8604 even if the under-seat storage 8604 is small.

This embodiment can be freely combined with other embodiments.

(Embodiment 7)
In this embodiment, an example in which a secondary battery, which is one embodiment of the present invention, is mounted in an electronic device will be described. Examples of electronic devices equipped with secondary batteries include television devices (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, and mobile phones (mobile phones, etc.). Examples include mobile phone devices (also referred to as mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines. Examples of portable information terminals include notebook personal computers, tablet terminals, electronic book terminals, and mobile phones.

FIG. 31A shows an example of a mobile phone. The mobile phone 2100 includes a display section 2102 built into a housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 includes a secondary battery 2107. By providing the battery 10 described in Embodiment 1 as the secondary battery 2107, the configuration can have a high capacity, can accommodate space savings due to the miniaturization of the housing, and has a configuration with excellent safety and reliability. can be realized.

The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, text viewing and creation, music playback, Internet communication, computer games, etc.

In addition to setting the time, the operation button 2103 can have various functions such as turning on and off the power, turning on and off wireless communication, executing and canceling silent mode, and executing and canceling power saving mode. . For example, the functions of the operation buttons 2103 can be freely set using the operating system built into the mobile phone 2100.

Furthermore, the mobile phone 2100 is capable of performing short-range wireless communication according to communication standards. For example, by communicating with a headset capable of wireless communication, it is also possible to make hands-free calls.

Furthermore, the mobile phone 2100 is equipped with an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that the charging operation may be performed by wireless power supply without using the external connection port 2104.

Furthermore, it is preferable that the mobile phone 2100 has a sensor. As the sensor, it is preferable to include, for example, a human body sensor such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

FIG. 31B is an unmanned aircraft 2300 with multiple rotors 2302. Unmanned aerial vehicle 2300 is sometimes called a drone. Unmanned aircraft 2300 includes a secondary battery 2301, which is one embodiment of the present invention, a camera 2303, and an antenna (not shown). Unmanned aerial vehicle 2300 can be remotely controlled via an antenna. The battery 10 obtained in Embodiment 1 has a high energy density and is highly safe, so it can be used safely for a long time and is suitable as a secondary battery to be mounted on the unmanned aircraft 2300.

FIG. 31C shows an example of a robot. The robot 6400 shown in FIG. 31C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display portion 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a calculation device, and the like.

The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, and the like. Furthermore, the speaker 6404 has a function of emitting sound. The robot 6400 can communicate with a user using a microphone 6402 and a speaker 6404.

The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display section 6405. The display unit 6405 may include a touch panel. Further, the display unit 6405 may be a removable information terminal, and by installing it at a fixed position on the robot 6400, charging and data exchange are possible.

The upper camera 6403 and the lower camera 6406 have a function of capturing images around the robot 6400. Further, the obstacle sensor 6407 can detect the presence or absence of an obstacle in the direction of movement of the robot 6400 when the robot 6400 moves forward using the moving mechanism 6408. The robot 6400 uses an upper camera 6403, a lower camera 6406, and an obstacle sensor 6407 to recognize the surrounding environment and can move safely.

The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal area. The battery 10 obtained in Embodiment 1 has a high energy density and is highly safe, so it can be used safely for a long time and is suitable as the secondary battery 6409 mounted on the robot 6400.

FIG. 31D shows an example of a portable electric fan. The portable electric fan 6200 includes a secondary battery 6209 according to one embodiment of the present invention, an operation button 6205, a fan 6202, an external connection port 6204, and the like in a housing 6201. The secondary battery 6209 is charged via the external connection port 6204. Note that the fan 6202 is rotated by operating a motor using electric power supplied from the secondary battery 6209. Although the secondary battery 6209 is an example of a cylindrical secondary battery, the shape is not particularly limited. The battery 10 obtained in Embodiment 1 has a high energy density and a stable crystal structure, is highly reliable, and is suitable as the secondary battery 6209 mounted in the portable electric fan 6200.

FIG. 31E shows an example of a cleaning robot. The cleaning robot 6300 includes a display portion 6302 placed on the top surface of a housing 6301, a plurality of cameras 6303 placed on the side, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is equipped with tires, a suction port, and the like. The cleaning robot 6300 is self-propelled, detects dirt 6310, and can suck the dirt from a suction port provided on the bottom surface.

The cleaning robot 6300 can analyze the image taken by the camera 6303 and determine the presence or absence of obstacles such as walls, furniture, or steps. Furthermore, if an object such as wiring that is likely to become entangled with the brush 6304 is detected through image analysis, the rotation of the brush 6304 can be stopped. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal area. The battery 10 obtained in Embodiment 1 has a high energy density and a stable crystal structure, is highly reliable, and is suitable as the secondary battery 6306 mounted on the cleaning robot 6300.

This embodiment can be freely combined with other embodiments.

10: Battery, 11: Positive electrode current collector, 13: Binder, 14: Conductive material, 15: Carbon black, 16: Graphene, 17: Carbon fiber, 20: Positive electrode, 21: Positive electrode lead, 22a: Metal foil, 22b: Covering layer, 22: positive electrode current collector, 23: positive electrode active material layer, 24: sealing part, 30: negative electrode, 31: negative electrode lead, 32: negative electrode current collector, 33: negative electrode active material layer, 34: sealing part, 40: separator, 41: region, 50: exterior body, 51: sealing part, 60: electrolyte, 70: plane, 71: plane, 76a: lead metal, 76b: lead metal, 100a: surface layer part, 100b: Inside, 100: positive electrode active material, 101: grain boundary, 102: embedded part, 104: covering part, 110: second positive electrode active material, 500: battery, 501: positive electrode current collector, 502: positive electrode active material layer , 503: positive electrode, 504: negative electrode current collector, 505: negative electrode active material layer, 506: negative electrode, 508: separator, 530: electrolyte, 531: exterior body, 553: conductive material, 903: mixture, 1003: nail, 1300: square secondary battery, 1301a: first battery, 1301b: first battery, 1302: battery controller, 1303: motor controller, 1304: motor, 1305: gear, 1306: DCDC circuit, 1307: electric power steering , 1308: Heater, 1309: Defogger, 1310: DCDC circuit, 1311: Second battery, 1312: Inverter, 1313: Audio, 1314: Power window, 1315: Lamps, 1316: Tire, 1317: Rear motor, 1320: Control Circuit section, 1321: Control circuit section, 1322: Control circuit, 1324: Switch section, 1413: Fixed section, 1414: Fixed section, 1415: Battery pack, 1421: Wiring, 1422: Wiring, 2001: Automobile, 2002: Transport vehicle , 2003: Transport vehicle, 2004: Aircraft, 2005: Satellite, 2100: Mobile phone, 2101: Housing, 2102: Display section, 2103: Operation button, 2104: External connection port, 2105: Speaker, 2106: Microphone, 2107 : Secondary battery, 2200: Battery pack, 2201: Battery pack, 2202: Battery pack, 2203: Battery pack, 2204: Secondary battery, 2300: Unmanned aircraft, 2301: Secondary battery, 2302: Rotor, 2303: Camera, 6200: Portable electric fan, 6201: Housing, 6202: Fan, 6204: External connection port, 6205: Operation button, 6209: Secondary battery, 6300: Cleaning robot, 6301: Housing, 6302: Display unit, 6303: Camera , 6304: Brush, 6305: Operation button, 6306: Secondary battery, 6310: Dust, 6400: Robot, 6401: Illuminance sensor, 6402: Microphone, 6403: Upper camera, 6404: Speaker, 6405: Display section, 6406: Lower part Camera, 6407: Obstacle sensor, 6408: Movement mechanism, 6409: Secondary battery, 8600: Scooter, 8601: Side mirror, 8602: Power storage device, 8603: Turn signal light, 8604: Under seat storage, 8700: Electric bicycle, 8701: Storage battery, 8702: Power storage device, 8703: Display unit, 8704: Control circuit

Claims

has a positive electrode,
The positive electrode includes a positive electrode current collector and a positive electrode active material layer,
The positive electrode active material layer is provided on the positive electrode current collector,
The battery, wherein the positive electrode current collector includes a metal foil and a coating layer that covers at least a part of the surface of the metal foil.
In claim 1,
The metal foil is stainless steel foil,
The battery, wherein the coating layer includes aluminum.
In claim 2,
The thickness of the metal foil is 1 μm or more and 30 μm or less,
A battery, wherein the thickness of the coating layer is 1 nm or more and 1 μm or less.
In claim 3,
The battery has an exterior body containing the positive electrode,
The battery, wherein the exterior body is a stainless steel laminate film.
In any one of claims 1 to 4,
The positive electrode active material layer includes a positive electrode active material,
The positive electrode active material has lithium cobalt oxide containing nickel and magnesium,
The detected amount of nickel in the surface layer of the positive electrode active material is larger than the detected amount of nickel inside the positive electrode active material,
The detected amount of magnesium in the surface layer of the positive electrode active material is larger than the detected amount of magnesium inside the positive electrode active material,
A battery, wherein a surface layer portion of the positive electrode active material has a region where a nickel distribution and a magnesium distribution overlap.
In claim 5,
In the battery, nickel is detected on a surface other than the (001) surface of lithium cobalt oxide in the surface layer portion of the positive electrode active material.
In claim 6,
In the EDX-ray analysis, in the surface layer of the positive electrode active material,
The depth of the peak of the detected amount of nickel,
A battery in which the difference in peak depth of the detected amount of magnesium is within 3 nm.
In claim 7,
The positive electrode active material contains aluminum,
In the EDX-ray analysis of nickel, magnesium and aluminum contained in the positive electrode active material,
The maximum value of the detected amount of aluminum is within the maximum value of the detected amount of nickel and the maximum value of the detected amount of magnesium.
When the peak width at a height of 1/5 of the maximum value of the detected amount of aluminum is divided into two by a perpendicular line drawn from the maximum value to the horizontal axis,
From the peak width Ws on the surface side,
A battery with a large internal peak width Wc.
In claim 5,
When the battery using lithium as the positive electrode and the counter electrode is charged to 4.6 V and the positive electrode is analyzed by powder X-ray diffraction using CuKα1 ray, the diffraction pattern of the positive electrode active material is at least 2θ of 19.13. a first peak that is greater than or equal to 19.37;
having a second peak of 45.37° or more and less than 45.57°;
battery.
In claim 9,
The positive electrode active material contains fluorine,
A battery, wherein the detected amount of fluorine in the surface layer of the positive electrode active material is larger than the detected amount of fluorine inside the positive electrode active material.
In claim 1,
The metal foil is stainless steel foil,
The battery, wherein the coating layer includes aluminum and titanium.