WO2023242676A1

WO2023242676A1 - Lithium ion secondary battery

Info

Publication number: WO2023242676A1
Application number: PCT/IB2023/055788
Authority: WO
Inventors: 山崎舜平; 吉富修平; 秋元健吾; 田島亮太
Original assignee: 株式会社半導体エネルギー研究所
Priority date: 2022-06-17
Filing date: 2023-06-06
Publication date: 2023-12-21

Abstract

The present invention provides: a positive electrode active material for which a decrease in discharge capacity during charge and discharge cycles is reduced; and a secondary battery which uses this positive electrode active material. Alternatively, provided is a secondary battery having high safety. In this lithium ion secondary battery with a positive electrode: the positive electrode has a collector and a positive electrode active material layer; one surface of the collector has a first recessed portion, a second recessed portion, and a first portion located between the first recessed portion and the second recessed portion; the positive electrode active material layer is provided in the first recessed portion and the second recessed portion; and in a cross sectional view of the positive electrode, when the one surface of the collector faces upward, then with respect to the lowermost portion of the first recessed portion, the height of the uppermost portion of the positive electrode active material layer is substantially level with the height of the uppermost portion of the first portion.

Description

Lithium ion secondary battery

One embodiment of the present invention relates to a power storage device (also referred to as a 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. .

Among these, there is a high demand for lithium ion secondary batteries for mobile electronic devices, which have a large discharge capacity per weight and excellent cycle characteristics. In order to meet these demands, improvements in positive electrode active materials included in positive electrodes of lithium ion secondary batteries are actively being carried out (see, for example, Patent Documents 1 to 4).

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 5. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 6 can be referred to from ICSD. Furthermore, for example, the analysis program RIETAN-FP (Non-Patent Document 7) can be used for the Rietveld method analysis.

Further, as image processing software, for example, ImageJ (Non-Patent Documents 8 to 10) is known. By using this software, for example, the shape of the positive electrode active material can be analyzed.

Ultrafine electron beam 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 11) can be used to analyze the electron beam diffraction pattern.

Furthermore, fluorides such as fluorite (calcium fluoride) have been used as fluxing agents in iron manufacturing and the like for a long time, and their physical properties have been studied (Non-Patent Document 12).

It is known that when the temperature of a lithium ion secondary battery increases during charging, thermal runaway occurs through several states (Non-Patent Document 13).

Various research and developments are also being conducted on the reliability and safety of lithium ion secondary batteries. For example, Patent Document 14 describes the thermal stability of a positive electrode active material and an electrolyte.

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

Lithium ion secondary batteries and positive electrode active materials used therein still have room for improvement in various aspects such as cycle characteristics, capacity, charge/discharge characteristics, reliability, safety, and cost.

In order to identify the cause of deterioration of lithium-ion secondary batteries, we observed particles that are the positive electrode active material (lithium cobalt oxide (LiCoO ₂ )) and found multiple cracks (including cracks, cracks, etc.) in the particles. . Furthermore, it is thought that the particles contain many defects in addition to cracks that can be observed in SEM photographs.

The present inventors discovered that lithium ion It is speculated that this is one of the causes of deterioration of secondary batteries.

Furthermore, it has been observed that cobalt released or dissolved into the electrolyte is segregated on the negative electrode, and the segregated product grows. It has also been observed that cobalt segregates inside the separator.

Considering these circumstances, in order to prevent the segregation of cobalt in the negative electrode and separator, it is necessary to prevent the release of cobalt (Co) and oxygen (O) from the surface of the positive electrode active material particles, and to It is essential to reduce the occurrence of cracks and defects.

Furthermore, lithium cobalt oxide (LiCoO ₂ , sometimes referred to as LCO) shown in Patent Documents 1 to 3 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 lithium cobalt oxide is used, the oxygen released from the lithium cobalt oxide may react with the electrolyte, etc., leading to thermal runaway. In particular, when there are many particles of lithium cobalt oxide having cracks, it is dangerous because a large amount of oxygen (O) is released.

Patent Document 4 discloses a configuration in which a protective layer is provided between a positive electrode current collector and a positive electrode composite material layer in order to suppress an increase in battery temperature during nail penetration.

In light of these circumstances, it is important to prevent cobalt (Co) and oxygen (O) from being released from the surface of the positive electrode active material particles and to reduce the occurrence of cracks and defects in the positive electrode active material particles.

In view of the above, an object of one embodiment of the present invention is to provide a highly safe lithium ion secondary battery. Furthermore, an object of one embodiment of the present invention is to provide a lithium ion secondary 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 is a lithium ion secondary battery having a positive electrode, wherein the positive electrode includes a current collector and a positive electrode active material layer, and one surface of the current collector has a first recess. and a second recess, and a first portion located between the first recess and the second recess, and the positive electrode active material layer is provided in the first recess and the second recess. , in a cross-sectional view of the positive electrode, when one surface of the current collector faces upward, the height of the top of the positive electrode active material layer and the top of the first portion with respect to the bottom of the first recess. This is a lithium ion secondary battery whose heights are approximately the same.

Further, one embodiment of the present invention is a lithium ion secondary battery having a positive electrode, wherein the positive electrode includes a current collector, a first positive electrode active material layer, and a second positive electrode active material layer. , one surface of the current collector has a first recess, a second recess, and a first portion located between the first recess and the second recess; The other surface has a third recess, a fourth recess, and a second portion located between the third recess and the fourth recess, and the first positive electrode active material layer includes: The second positive electrode active material layer is provided in the first recess and the second recess, and the second positive electrode active material layer is provided in the third recess and the fourth recess. When, the height of the top of the first positive electrode active material layer and the height of the top of the first portion approximately match with respect to the bottom of the first recess, and in a cross-sectional view of the positive electrode. , when the other surface of the current collector faces upward, the height of the top of the second positive electrode active material layer and the height of the top of the second portion with respect to the bottom of the third recess. This is a lithium ion secondary battery, which roughly matches the above.

In any one of the above, the positive electrode active material layer includes a plurality of positive electrode active materials, a conductive material, and a binder, and the ratio of the positive electrode active material having cracks among the plurality of positive electrode active materials is 5%. It is preferable that it is below.

In any one of the above, the conductive material is preferably carbon black. Alternatively, the conductive material is preferably carbon nanotubes.

In 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 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, 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 and the distribution of magnesium overlap.

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.

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 the above, the positive electrode active material contains aluminum, and in the EDX-ray analysis intensity distribution of nickel, magnesium, and aluminum contained in the positive electrode active material, the position where the detected amount of aluminum is the maximum value is the position where the maximum value of the detected amount of nickel is found. When the peak width at a height of 1/5 of the maximum value of the detected amount of aluminum, which is located inside the position where the maximum value of the detected amount of magnesium is found, is divided into two by a perpendicular line drawn from the maximum value to the horizontal axis, It is preferable that the peak width Wc on the inside side is larger than the peak width Ws on the front side.

In any one of the above, a test battery using lithium as a positive electrode and a counter electrode was prepared, and when the test battery was charged to _4.6V , the positive electrode active material was When analyzed by diffraction, the diffraction pattern preferably has peaks at least at 2θ of 19.13 or more and less than 19.37 and at 45.37° or more and less than 45.57°.

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.

By reducing the size of the cracks in the active material particles, reducing the number of cracks and defects, and creating a structure in which cobalt and oxygen do not dissolve into the electrolyte, it is possible to further increase the stability of the crystal structure within the active material particles. charging/discharging cycle characteristics are further improved. Furthermore, it is possible to suppress the ignition of the lithium ion secondary battery in the event of an accident such as an internal short circuit.

According to one embodiment of the present invention, a highly safe lithium ion secondary battery can be provided. According to one embodiment of the present invention, a lithium ion secondary battery with little deterioration can be provided. Further, according to one embodiment of the present invention, a high capacity lithium ion secondary battery 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 positive electrode, and FIG. 1B is a cross-sectional view of the positive electrode.
2A to 2C are cross-sectional views of the positive electrode.
3A to 3D are cross-sectional views of the positive electrode active material layer.
FIG. 4A is a perspective view of the positive electrode, and FIG. 4B is a cross-sectional view of the positive electrode.
FIG. 5 is a cross-sectional view of the positive electrode.
FIG. 6 is a diagram illustrating a method for manufacturing a positive electrode current collector.
FIGS. 7A to 7E are diagrams illustrating a method for manufacturing a positive electrode current collector.
FIG. 8 is a diagram illustrating a method for manufacturing a positive electrode active material layer.
9A to 9E are diagrams illustrating a method for manufacturing a lithium ion secondary battery.
10A to 10C are diagrams illustrating a method for manufacturing a lithium ion secondary battery.
11A to 11C are cross-sectional views of the positive electrode active material.
12A to 12C are examples of the distribution of additive elements included in the positive electrode active material.
FIG. 13A is an example of the distribution of additive elements included in the positive electrode active material. FIG. 13B is a diagram illustrating the distribution of additive elements.
FIG. 14 is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and magnesium fluoride.
FIG. 15 is a diagram illustrating the results of DSC analysis.
FIG. 16 is an example of a TEM image in which the crystal orientations are approximately the same.
FIG. 17A is an example of a STEM image in which crystal orientations are approximately the same. FIG. 17B is an FFT pattern of a region of rock salt crystal RS, and FIG. 17C is an FFT pattern of a region of layered rock salt crystal LRS.
FIG. 18 is a diagram illustrating the crystal structure of the positive electrode active material.
FIG. 19 is a diagram illustrating the crystal structure of a conventional positive electrode active material.
FIG. 20 is a diagram illustrating the charging depth and lattice constant of the positive electrode active material.
FIG. 21 is a diagram showing an XRD pattern calculated from the crystal structure.
FIG. 22 is a diagram showing an XRD pattern calculated from the crystal structure.
FIGS. 23A and 23B are diagrams showing XRD patterns calculated from the crystal structure.
24A to 24C show lattice constants calculated from XRD.
25A to 25C show lattice constants calculated from XRD.
26A and 26B are cross-sectional views of the positive electrode active material.
FIGS. 27A to 27C are diagrams illustrating a method for manufacturing a positive electrode active material.
FIG. 28 is a diagram illustrating a method for producing a positive electrode active material.
29A to 29C are diagrams illustrating a method for manufacturing a positive electrode active material.
FIGS. 30A to 30H are diagrams illustrating an example of an electronic device.
31A to 31D are diagrams illustrating an example of an electronic device.
32A to 32C are diagrams illustrating an example of an electronic device.
33A to 33C are diagrams illustrating an example of a vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of embodiments for carrying out the present invention will be described below with reference to 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 and the like, space groups are expressed using short notation of international notation (or Hermann-Mauguin symbol). In addition, crystal planes and crystal directions are expressed using Miller indices. Individual planes indicating crystal planes are written using parentheses. 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.

Furthermore, the space group of a lithium ion secondary battery is identified by XRD, electron beam diffraction, neutron beam diffraction, or the like. 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.

Furthermore, if the anions have a structure in which three layers are shifted from each other and piled up like ABCABC, it will be referred to as 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 where the element is continuously detected within a non-noise range by a certain continuous analysis method.

Further, a positive electrode active material to which an additive element is added may be expressed as a composite oxide, a positive electrode material, a positive electrode material, a positive electrode material for a lithium ion secondary battery, or the like. 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.

Further, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments and the like, not all particles necessarily 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 the 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.

Note that unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) included in the lithium ion secondary battery 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 is sometimes referred to as a lithium ion secondary battery, and 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 lithium ion secondary battery that is one embodiment of the present invention will be described. A lithium ion secondary battery has a positive electrode, a negative electrode, a separator, and an electrolyte. In a lithium ion secondary battery, the positive electrode, negative electrode, and separator are preferably flat.

When the positive electrode, negative electrode, and separator are flat, the distance between the positive electrode and the negative electrode is constant, so charging and discharging can be performed uniformly anywhere inside the lithium ion secondary battery. Additionally, if the positive electrode, negative electrode, and separator are flat, it is possible to suppress stress concentration inside the lithium ion secondary battery when external force is applied to the lithium ion secondary battery. . Therefore, the positive electrode and the negative electrode are preferably flattened by roll press treatment or the like.

As described above, the positive electrode and the negative electrode are preferably flattened by roll press treatment, but roll press treatment may cause cracks in the active material included in the active material layer. In this case, the presence of cracks in the active material may increase deterioration associated with charging and discharging the lithium ion secondary battery. Moreover, there is a possibility that the safety of the lithium ion secondary battery may be reduced.

For example, in a positive electrode active material that has a surface layer that can suppress deterioration due to charging and discharging and increase safety, such as the positive electrode active material of one embodiment of the present invention, if a crack occurs, the surface layer This means that parts other than the original part will be exposed. This may cause deterioration and a decrease in safety due to charging and discharging of the lithium ion secondary battery. Specifically, in a fully charged cathode active material, oxygen is likely to be released from the cracked portions, making it easy for the lithium ion secondary battery to undergo thermal runaway and catch fire.

Therefore, it is desirable to suppress the occurrence of cracks in the positive electrode active material due to the roll press treatment of the positive electrode. Note that details of the positive electrode active material of one embodiment of the present invention will be described in Embodiment 2.

Thus, it is desirable that the positive electrode active material included in the positive electrode active material layer has few cracks. The positive electrode of one embodiment of the present invention has a structure in which a positive electrode current collector has irregularities and a positive electrode active material layer is embedded in the recesses, and cracks are suppressed from occurring in the positive electrode active material included in the positive electrode active material layer. can do. As a preferable state of the positive electrode active material layer in which the generation of cracks in the positive electrode active material is suppressed, for example, the ratio of the positive electrode active material having cracks among the plurality of positive electrode active materials (positive electrode active material particles) included in the positive electrode active material layer is preferably 10% or less, more preferably 5% or less, and even more preferably 1% or less based on the total of the plurality of positive electrode active materials. Although it is ideal to reduce the percentage of the positive electrode active material with cracks to 0%, in the actual process of manufacturing a positive electrode, it is difficult to completely eliminate the percentage of the positive electrode active material with cracks. The proportion of the positive electrode active material is greater than 0%.

[Positive electrode]
A positive electrode using a positive electrode current collector having irregularities that can suppress cracks in the positive electrode active material due to roll press treatment will be described with reference to FIGS. 1 to 8.

FIG. 1A is a perspective view showing a positive electrode 10 included in a lithium ion secondary battery that is one embodiment of the present invention. Note that the surface including the region B shown by the dashed-dotted line shows the cross section of the positive electrode 10. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12.

FIG. 1B is an enlarged cross-sectional view of a region B indicated by a dashed line in FIG. 1A.

In the cross-sectional view of region B shown in FIG. 1B, the positive electrode current collector 11 has a structure having two recesses 11A1 on one surface and a portion 11B1 located between the two recesses 11A1, and a structure on the other surface. shows a structure having two recesses 11A2 and a portion 11B2 located between the two recesses 11A2. The positive electrode active material layer 12 is provided in each of the recesses 11A1 and 11A2, and includes a positive electrode active material 100, a conductive material 15, and a binder (not shown). In the cross-sectional view of FIG. 1B, the positions of the recess 11A1 and the recess 11A2 existing in the depth direction are indicated by broken lines.

It is preferable to use a soft material as the material for the positive electrode current collector 11, and when using a metal material, for example, aluminum foil may be used. Moreover, when using the same structure as the positive electrode 10 in the negative electrode, it is good to use copper foil, preferably soft copper foil, as the negative electrode current collector.

Alternatively, as the material for the positive electrode current collector 11, a conductive film exhibiting rubber elasticity (conductive rubber-like film), a conductive resin film, or the like can be used.

The thickness T of the positive electrode current collector 11 having irregularities is preferably 10 μm or more and 100 μm or less, more preferably 15 μm or more and 50 μm or less, and more preferably 20 μm or more and 30 μm or less. Here, the thickness T of the positive electrode current collector 11 is, as shown in FIG. 1B, when one surface of the positive electrode current collector 11 is directed upward and the position of the lower surface of the portion 11B2 is taken as a reference. This is the height of the upper surface of the portion 11B1.

Note that a configuration in which the positive electrode current collector 11 has a recess 11A1 and a portion 11B1 on one surface and does not have a recess 11A2 and a portion 11B2 on the other surface, that is, the other surface is flat and the positive electrode active material layer 12 may be used. However, in order to increase the capacity of the lithium ion secondary battery, it is preferable that the positive electrode current collector 11 has concave portions and convex portions on both surfaces, and has the positive electrode active material layer 12 on both surfaces. In the following description, when explaining the content common to the recessed part 11A1 and the recessed part 11A2, these will be referred to as the recessed part 11A without distinguishing them. Similarly, when explaining the content common to the portion 11B1 and the portion 11B2, these will be referred to as the portion 11B without distinction. Note that the portion 11B can also be called a convex portion.

As shown in FIG. 1B, the positive electrode active material layer 12 is preferably embedded in the positive electrode current collector 11. When the positive electrode active material layer 12 is embedded in the positive electrode current collector 11, it is possible to suppress the occurrence of cracks in the positive electrode active material 100 included in the positive electrode active material layer 12 when the positive electrode 10 is roll-pressed. can.

The state in which the positive electrode active material layer 12 is embedded in the positive electrode current collector 11 means that when one surface of the positive electrode current collector 11 is directed upward in a cross-sectional view of the positive electrode as shown in FIG. This means that the height T1 of the top of the positive electrode active material layer 12 and the height T2 of the top of the portion 11B are approximately equal to each other with the bottom as a reference.

In this specification, "the heights are approximately the same" refers to a configuration in which the heights from a reference point (for example, the lowest part of the recess) are approximately equal in cross-sectional view. For example, when an electrode having an active material layer is subjected to roll press treatment, the heights of the surfaces to be treated are approximately the same. However, even if roll press processing is performed, the heights may not strictly match depending on the material of the active material layer, the film density of the active material layer, etc., but in this specification, etc., the "height" "approximately match". For example, when the height of the second portion is 80% or more and 120% or less of the height of the first portion on the treated surface of the electrode that has been subjected to roll press treatment, the height of the first portion It is said that the height and the height of the second part roughly match.

The recess 11A, the portion 11B, and the positive electrode active material layer 12 of the positive electrode 10 are not limited to the structure shown in FIG. 1B. For example, as shown in FIG. 2A, there may be a structure in which only one positive electrode active material 100 is present in the recess 11A, or as shown in FIG. 2B, a structure in which many positive electrode active materials 100 are present in the recess 11A. There may be.

Alternatively, as shown in FIG. 2C, a layer made of a conductive material 15 and a binder (not shown) is provided on the positive electrode active material layer 12 in the recess 11A so as to fill the gap between the positive electrode active materials 100. Good too. With such a structure, during the roll press treatment of the positive electrode 10, the force due to the roll press can be more uniformly applied to the positive electrode 10, and it is possible to suppress the occurrence of cracks in the positive electrode active material 100.

3A to 3D show other examples of the positive electrode active material layer 12. 3A to 3D are modified examples of the cross-sectional structure of the region C shown by the three-dot chain line in FIG. 2B.

FIG. 3A illustrates the carbon black 14, which is an example of the conductive material 15, and the electrolyte 20 contained in the voids located between the particles of the positive electrode active material 100. An example is shown in which the 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. 3A shows an example in which the positive electrode active material 100 is spherical, the shape 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. 3B shows an example in which the positive electrode active material 100 has a polygonal shape with rounded corners.

Further, in the positive electrode of FIG. 3B, graphene 16 is used as the carbon material used as the conductive material 15. In FIG. 3B, a positive electrode active material layer including a positive electrode active material 100, graphene 16, and carbon black 14 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 14.

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. 3C illustrates an example of a positive electrode using carbon fiber 17 instead of graphene. FIG. 3C shows an example different from FIG. 3B. When carbon fiber 17 is used, agglomeration of carbon black 14 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 14.

As the carbon fibers 17, carbon fibers such as mesophase pitch carbon fibers and isotropic pitch carbon fibers 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.

Moreover, FIG. 3D is illustrated as an example of another positive electrode. FIG. 3D 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 14 can be prevented and dispersibility can be further improved.

Note that 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. The content of the conductive material 15 with respect to the total amount of the positive electrode active material layer 12 is preferably 1 wt% or more and 10 wt% or less, and more preferably 1 wt% or more and 5 wt% or less.

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 two or more of the above binders.

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.

In addition, the solubility of cellulose derivatives such as carboxymethylcellulose is increased by converting them into salts such as sodium salts or ammonium salts of carboxymethylcellulose, making it easier to exhibit the effect as a viscosity modifier. 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.

The water-soluble polymer stabilizes the viscosity by dissolving in water, and other materials 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 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 also conduct lithium ions.

4A and 4B are other configuration examples of the positive electrode 10. FIG. 4B is an enlarged cross-sectional view of a region D indicated by a dashed line in FIG. 4A.

1A and 1B show an example in which the positive electrode current collector 11 has a portion 11B2 at a position on the other surface corresponding to the position of the recess 11A1 on one surface. The positive electrode of one embodiment of the present invention is not limited to this, and as shown in FIG. 4B, the positive electrode has a structure having a recess 11A2 on one surface of the positive electrode current collector at a position corresponding to the position of the recess 11A1 on the other surface. It's okay. At this time, as shown in FIG. 4B, a structure may be adopted in which a portion 11B2 is provided at a position on the other surface corresponding to the position of the portion 11B1 on one surface of the positive electrode current collector.

Alternatively, as shown in FIG. 5, a structure may be adopted in which the positive electrode current collector has a recess 11A2 and a portion 11B2 at a position on the other surface corresponding to the position of the recess 11A1 on one surface.

[Method for producing positive electrode]
6 to 8 are diagrams illustrating an example of a method for manufacturing the positive electrode 10 of one embodiment of the present invention.

[Method for producing current collector with unevenness]
Next, an example of a method for manufacturing a positive electrode current collector will be described. As the material for the current collector having irregularities (current collector material), it is preferable to use a soft material, and when using a metal material, for example, a metal foil such as aluminum foil may be used. In addition, when using the same structure as the positive electrode 10 in a negative electrode, it is good to use copper foil.

Alternatively, a conductive film exhibiting rubber elasticity (conductive rubber-like film), a conductive resin film, etc. can be used as the material of the current collector having irregularities.

<Conductive rubber film>
A conductive film exhibiting rubber elasticity can be used as the current collector material. A conductive film exhibiting rubber elasticity used as a current collector material is sometimes referred to as a conductive rubber film. Examples of conductive rubbery films include styrene-butadiene rubber, styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, butyl rubber, ethylene-propylene rubber, fluororubber, silicone rubber, It is possible to use a conductive rubber film having one or more rubber materials selected from the group consisting of urethane rubber and particulate or fibrous conductive material (also referred to as a conductive filler).

As for the physical properties of the conductive rubber film, for example, tensile strength, elongation at break, and volume resistivity are preferably within the following ranges. The tensile strength is preferably 0.1 MPa or more and 30 MPa or less, more preferably 1 MPa or more and 20 MPa or less, and more preferably 1 MPa or more and 10 MPa or less. Further, the elongation at break is preferably greater than 100% and less than 300%, more preferably greater than or equal to 130% and less than 200% (the length when no external force is applied is defined as 100%). Further, the volume resistivity is preferably 0.1 Ω·m or more and 30 Ω·m or less, more preferably 0.1 Ω·m or more and 20 Ω·m or less, and 0.1 Ω·m or more and 10 Ω·m or less. It is more preferable that it is, and it is more preferable that it is 0.1 Ω·m or more and 5 Ω·m or less.

As the conductive material of the conductive rubber film, one or more of conductive carbon materials and metal materials such as aluminum, titanium, stainless steel, gold, platinum, zinc, iron, and copper can be used. Examples of the conductive carbon 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, graphene, and graphene compounds. Two or more types 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.

Note that when the conductive rubber film is used as a positive electrode current collector, it may further contain an antioxidant such as a hindered phenol material.

In addition, when using a conductive rubber film as a negative electrode current collector, it is preferable that the metal material used as the conductive material is a material that does not alloy with carrier ions such as lithium.

<Conductive resin film>
Furthermore, a conductive resin film can be used as the current collector material. Examples of conductive resin films include resins such as polyolefin (polypropylene, polyethylene, etc.), nylon (polyamide), polyimide, vinylon, polyester, acrylic, polyurethane, and particulate or fibrous conductive materials (also called conductive fillers). A conductive resin film having the following can be used.

As the conductive material of the conductive resin film, one or more of a conductive carbon material and a metal material such as aluminum, titanium, stainless steel, gold, platinum, zinc, iron, copper, etc. can be used. Examples of the conductive carbon 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, graphene, and graphene compounds. Two or more types can be used. In addition, when using a conductive resin film as a positive electrode current collector, it is preferable to further include an antioxidant such as a hindered phenol-based material.

Note that the average particle size of the conductive material included in the conductive resin film can be 10 nm or more and 10 μm or less, and preferably 30 nm or more and 5 μm or less.

The above current collector material is prepared and embossing is performed on this current collector material. As a result, a current collector material having an uneven shape can be obtained. When aluminum is used as the current collector material, it can be used as the positive electrode current collector 11, and when annealed copper is used as the current collector material, it can be used as the negative electrode current collector.

<Emboss processing>
Embossing, which is a type of press working, will be explained below.

FIG. 6 is a cross-sectional view showing an example of embossing. Embossing is a type of press working, and is a process in which an embossing roll with an uneven surface is brought into pressure contact with a current collector material to form unevenness corresponding to the unevenness of the embossing roll on the current collector material. pointing. Note that the embossing roll is a roll with a pattern engraved on its surface.

Moreover, FIG. 6 is an example in which embossing is performed on both sides of the current collector material.

In FIG. 6, a current collector material 90 is sandwiched between an embossing roll 95 in contact with one surface of the current collector material and an embossing roll 96 in contact with the other surface, and the current collector material 90 is a current collector. It shows the material being fed out in the advancing direction 91. A pattern is formed on the surface of the current collector material by pressure or heat. Note that a pattern may be formed on the surface of the current collector material by both pressure and heat.

As the embossing roll, a metal roll, a ceramic roll, a plastic roll, a rubber roll, an organic resin roll, a wood roll, etc. can be used as appropriate.

In FIG. 6, embossing is performed using an embossing roll 96 that is a male embossing roll and an embossing roll 95 that is a female embossing roll. The male pattern embossing roll 96 has a plurality of convex portions 96a. The convex portion corresponds to a convex portion formed on the current collector material to be processed. The female pattern embossing roll 95 has a plurality of convex portions 95a. The adjacent convex portions 95a constitute a concave portion into which the convex portion 96a provided on the male embossing roll 96 fits into the convex portion formed on the current collector material.

By continuously performing embossing to raise a portion of the current collector material 90 and blank pressing to dent a portion of the current collector material 90, convex portions and flat portions can be continuously formed. . As a result, a pattern can be formed on the current collector material 90.

Next, a current collector having a plurality of convex portions having a shape different from that in FIG. 6 will be described with reference to FIGS. 7A to 7E. By changing the convex shapes of the embossing roll 95 and the embossing roll 96 in FIG. 6 to shapes different from those in FIG. 6, it is possible to embossing various cross-sectional shapes shown in FIGS. 7A to 7E.

FIG. 7A is a schematic cross-sectional view of an embossing having a wavy cross-sectional shape, and FIGS. 7B to 7E are modifications of FIG. 7A. 7B and 7C are diagrams showing an example of forming a wavy cross-sectional shape into a step-like shape, FIG. 7D is a diagram showing an example of forming a wavy cross-sectional shape into a rectangular shape, and FIG. 7E is a diagram showing an example of forming a wavy cross-sectional shape into a rectangular shape. FIG. 4 is a diagram illustrating an example in which a groove is formed with an acute valley shape and a trapezoidal peak shape.

Note that the production of the current collector having irregularities is not limited to the press working using the embossing roll described above, but may also be performed using a parallel flat plate having a pattern. Alternatively, it may be produced by plating using a mold with unevenness.

The positive electrode current collector 11 is obtained by any of the methods shown above.

[Preparation of active material layer]
The positive electrode 10 can be formed by applying a slurry onto the positive electrode current collector 11, drying it, and performing a press treatment after drying.

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 dispersion medium, and preferably further includes a conductive material mixed therein. 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. As a dispersion medium, for example, one or more of water, N-methylpyrrolidone (NMP), methanol, ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) can be used. .

FIG. 8 is a perspective view illustrating the process of applying slurry 12A to positive electrode current collector 11. By applying the slurry 12A on the positive electrode current collector 11 using the blade 80, a part of the slurry 12A can be embedded in the recess 11A of the positive electrode current collector 11. Thereafter, the slurry 12A embedded in the recess 11A becomes the positive electrode active material layer 12 by drying. The process from applying the slurry 12A to drying is called a coating process.

The slurry 12A includes a positive electrode active material 100, a conductive material 15, a binder, and a dispersion medium. As the binder and dispersion medium, those explained above can be used.

Through the coating process, the positive electrode active material layer 12 provided in the recess 11A of the positive electrode current collector 11 is not completely embedded in the positive electrode current collector 11 at this stage, and the positive electrode current collector 11 is not completely embedded in the positive electrode current collector 11. In some cases, the positive electrode active material layer 12 protrudes beyond the portion 11B. In such a structure, the flatness of the positive electrode 10 is low, and the capacity density per volume of the electrode is also reduced. Therefore, it is preferable to perform roll press treatment after the coating process. In the roll press treatment, the temperature of the roller may be 50°C or higher and 150°C or lower.

In the roll press treatment, the positive electrode 10 of one embodiment of the present invention has the positive electrode current collector 11 having irregularities, so that excessive pressing force is not applied to the positive electrode active material 100 included in the positive electrode active material layer 12. can be prevented from joining. Therefore, the positive electrode active material layer 12 can reduce the voids in the coating film and suppress the occurrence of cracks in the positive electrode active material 100 during the roll press treatment.

In the positive electrode 10 of one embodiment of the present invention manufactured in this manner, the ratio of the positive electrode active material 100 having cracks to the total of the plurality of positive electrode active materials can be 10% or less. Alternatively, in the positive electrode 10, the ratio of the positive electrode active material 100 having cracks to the total of the plurality of positive electrode active materials can be 5% or less. Alternatively, in the positive electrode 10, the ratio of the positive electrode active material 100 having cracks to the total of the plurality of positive electrode active materials can be set to 1% or less.

In this specification, a crack refers to a slit-like region and a region near the slit-like region when a cross section of a substance is observed. Also, when observing the surface of a substance, it refers to a deep valley-like region and the region near it.

A case will be described in which the number of substances having cracks, for example, the number of particles of a positive electrode active material having cracks, is measured. When the number of particles having cracks is measured by observing the cross section, when one or more cracks are confirmed in the cross section of the particle, it is counted as 1 as a particle having a crack. Further, when a particle is divided into two parts due to a crack, for example, when the end faces of adjacent particles have the same shape, the two parts are counted together as one particle having a crack. A SEM (Scanning Electron Microscope) or a TEM (Transmission Electron Microscope) may be used to observe the cross section. In addition, when measuring the abundance ratio of particles with cracks among a plurality of particles, it is necessary to observe the cross sections of at least 10 particles, preferably 20 or more particles, and more preferably 100 or more particles. good.

Similarly, in the case of measuring the number of particles having cracks by observing the surface, when one or more cracks are confirmed on the surface of a particle, it is counted as 1 as a particle having a crack. Further, when a particle is divided into two parts due to a crack, for example, when the end faces of adjacent particles have the same shape, the two parts are counted together as one particle having a crack. A SEM may be used to observe the surface. In addition, when measuring the abundance ratio of particles with cracks among a plurality of particles, it is necessary to observe the surfaces of at least 10 particles, preferably 20 or more particles, and more preferably 100 or more particles. good.

[Negative electrode]
The negative electrode has 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. Note that as the negative electrode current collector, conductive material, and binder, those explained in [Positive electrode] can be used. For example, as the negative electrode current collector, a current collector having the same structure as the positive electrode current collector 11 having unevenness as explained in FIG. 1 etc. may be used, or a general metal foil such as copper foil may be used. good.

As 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 includes an active material, a binder, and a solvent, and is 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 .

In addition, although one type of negative electrode active material can be used from among the negative electrode active materials shown above, it is also possible to use a combination of multiple types. For example, it can be a combination of a carbon material and silicon, or a combination of a carbon material and silicon monoxide.

Further, 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.

Moreover, when using a negative electrode that does not have a negative electrode active material, a negative electrode current collector having unevenness can be used. When using a negative electrode current collector with unevenness, the concave portions of the negative electrode current collector become cavities in which the lithium contained in the negative electrode current collector is likely to precipitate, so when lithium is precipitated, it is suppressed from forming a dendrite-like shape. can do.

[Method for manufacturing lithium ion secondary battery]
An example of a method for manufacturing a lithium ion secondary battery will be described below.

9A shows a top view of the positive electrode 10, FIG. 9B shows a separator 40, FIG. 9C shows a negative electrode 30, FIG. 9D shows a positive electrode lead 13 and a negative electrode lead 33, and FIG. 9E shows a top view of a film-like exterior body 50. The positive electrode lead 13 has a sealing part 75 and a lead metal 76a, and the negative electrode lead 33 has a sealing part 75 and a lead metal 76b.

The dimensions of each figure in FIG. 9 are approximately the same, and the region 41 surrounded by the dashed line in FIG. 9E is approximately the same as the dimension of the separator in FIG. 9B. Further, the area between the broken line 51 and the end portion in FIG. 9E becomes a sealing portion 52.

Further, the protruding portion of the positive electrode current collector 11 (dotted chain line in FIG. 9A) and the protruding portion of the negative electrode current collector 31 (dotted chain line in FIG. 9C) are referred to as tab portions. No active material layer is provided in the tab portion, and the current collector is exposed.

FIG. 10A is an example in which positive electrode active material layers 12 are provided on both sides of the positive electrode current collector 11. To explain in detail, the negative electrode current collector 31, the negative electrode active material layer 32, the separator 40, the positive electrode active material layer 12, the positive electrode current collector 11, the positive electrode active material layer 12, the separator 40, the negative electrode active material layer 32, the negative electrode current collector They are arranged in the order of body 31. A cross-sectional view of this laminated structure taken along a plane 70 is shown in FIG. 10B.

Note that although FIG. 10A 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 11 is stored between the two separators is shown. It is also possible to do this. A positive electrode active material layer 12 is formed on both sides of a positive electrode current collector 11 housed in a bag-shaped separator.

Further, it is also possible to provide the negative electrode active material layer 32 on both sides of the negative electrode current collector 31. In FIG. 10C, between two negative electrode current collectors 31 having negative electrode active material layers 32 on only one side, three negative electrode current collectors 31 having negative electrode active material layers 32 on both sides, and positive electrode active material layers on both sides. 12 shows an example in which a secondary battery is constructed in which four positive electrode current collectors 11 having a positive electrode current collector 12 and eight separators 40 are sandwiched between them. In this case as well, instead of using eight separators, four bag-shaped separators may be used.

Here, it is preferable that the positive electrode lead 13 and the plurality of positive electrode current collectors 11 are simultaneously fixed and electrically connected. Similarly, it is preferable that the negative electrode lead 33 and the plurality of negative electrode current collectors 31 are simultaneously fixed and electrically connected. By simultaneously connecting a plurality of current collectors and electrode leads in this manner, production can be performed efficiently.

Methods for fixing multiple current collectors and electrode leads include bonding and fixing using conductive resin (also called conductive adhesive), and fixing by sandwiching multiple current collectors and electrode leads with fixing members. A method of fixing the exposed portion of the metal foil by welding such as ultrasonic welding can be used.

Moreover, it is preferable that the separator 40 has a shape that makes it difficult for the positive electrode 10 and the negative electrode 30 to electrically short-circuit. For example, it is preferable to make the width of each separator 40 larger than the width of the positive electrode 10 and the negative electrode 30, since this makes it difficult for the positive electrode 10 and the negative electrode 30 to come into contact even when the relative positions of the positive electrode 10 and the negative electrode 30 are shifted.

In this way, a laminate of the positive electrode 10, the negative electrode 30, and the separator 40 is produced.

Thereafter, the laminate is housed inside the exterior body together with the electrolyte, and the exterior body is sealed. In this way, a lithium ion secondary battery is produced.

[Electrolytes]
Examples of electrolytes are explained below. As one form of the electrolyte, a liquid electrolyte (also referred to as an electrolytic solution) that includes a solvent and an electrolyte dissolved in the solvent can be used. The electrolyte is not limited to a liquid electrolyte (electrolyte solution) that is liquid at room temperature, and a solid electrolyte may also be used. Alternatively, it is also possible to use an electrolyte (semi-solid electrolyte) containing both a liquid electrolyte that is liquid at room temperature and a solid electrolyte that is solid at room temperature. Note that when a solid electrolyte or a semi-solid electrolyte is used in a bendable battery, the flexibility of the battery can be maintained by having a structure in which a part of the stack inside the battery includes the electrolyte.

When using a liquid electrolyte in a secondary battery, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, 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-dioxane, dimethoxy One or more of ethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or two or more of these may be used in any combination and ratio. can.

In addition, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the electrolyte solvent, the internal temperature of the secondary battery may increase due to short circuits or overcharging. This can prevent the secondary battery from exploding or catching fire. Ionic liquids are composed of cations and anions, and include organic cations and anions. Examples of organic cations 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, as the anion, monovalent amide anion, monovalent methide anion, fluorosulfonic acid anion, perfluoroalkylsulfonic acid anion, tetrafluoroborate anion, perfluoroalkylborate anion, hexafluorophosphate anion, or perfluorophosphate anion, Examples include alkyl phosphate anions.

For example, the secondary battery of one embodiment of the present invention uses alkali metal ions such as lithium ions, sodium ions, and potassium ions, and alkaline earth metal ions such as calcium ions, strontium ions, barium ions, beryllium ions, and magnesium ions as carrier ions. have as.

For example, when using lithium ions as carrier ions, the electrolyte contains a lithium salt. Examples of lithium salts 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 , _LiC4F9SO3 _, LiC( _CF3SO2 ₎ ₃ _, LiC( _C2F5SO2 ₎ ₃ _, LiN( _CF3SO2 ₎ ₂ _, LiN( _C4F9SO2 ₎ ( _CF3SO2 ₎ ), LiN(C ₂ F ₅ SO ₂ ) ₂ , etc. can be used.

As an example, the organic solvent described in this embodiment includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). When the total amount is 100 vol%, the volume ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is x:y:100-x-y (5≦x≦35, 0<y< 65) can be used. More specifically, an organic solvent containing EC, EMC, and DMC in a ratio of EC:EMC:DMC=30:35:35 (volume ratio) can be used.

Further, it is preferable that the electrolytic solution 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") and is highly purified. Specifically, it is preferable that the weight ratio of impurities to the electrolytic solution is 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

In addition, in order to form a film (Solid Electrolyte Interface) at the interface between the electrode (active material layer) and the electrolyte solution for the purpose of improving safety, vinylene carbonate (VC), propane sultone (PS) is added to the electrolyte solution. , tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or dinitrile compounds of succinonitrile or adiponitrile may be added. The concentration of the additive may be, for example, 0.1 wt% or more and 5 wt% or less based on the solvent.

Further, since the electrolyte includes a polymeric material that can be gelled, safety against leakage and the like is increased. Typical examples of polymeric materials to be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluoropolymer gel.

As the polymer material, 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. Moreover, the polymer formed may have a porous shape.

[Separator]
When the electrolyte contains an electrolytic solution, 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, which suppresses deterioration of the separator during high voltage charging and improves 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 polypropylene film may be coated on both sides with a mixed material of aluminum oxide and aramid. 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.

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

[Exterior body]
As the exterior body of the battery, for example, a metal material such as aluminum, stainless steel, titanium, etc. 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 in the laminate film can 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 nylon as an exterior body used in a highly flexible (bendable) battery. 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.

Furthermore, a graphene sheet may be used as the laminate film instead of the metal layer described above. 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.

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

(Embodiment 2)
In this embodiment, a positive electrode active material 100 of one embodiment of the present invention will be described using FIGS. 11 to 26.

11A to 11C are cross-sectional views of a positive electrode active material 100 that is one embodiment of the present invention. As shown in FIG. 11A, 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. 11B 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. 11C, a part of the grain boundary 101 is shown by a dashed line.

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 portion 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.

Further, it is assumed that the electrolyte, organic solvent, binder, conductive material, or compounds derived from these adhered 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 needs to contain a transition metal capable of redox in order to maintain charge neutrality even when lithium ions are intercalated and deintercalated. 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.

The additive element is preferably 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 above-mentioned advantages such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics are further enhanced. It is preferable that the weight of manganese contained in the positive electrode active material 100 is, for example, 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. 18 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 6).

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. 11 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 intensity distributions of characteristic X-rays corresponding to each additive element when EDX-ray analysis is performed along this arrow X1-X2 are shown in FIGS. 12A to 12C.

As shown in FIGS. 12A to 12C, it is preferable that the detected amount of at least magnesium and nickel among the additive elements is larger in the surface layer portion 100a than in the inner portion 100b. Furthermore, it is preferable that the detected amount has a narrow peak in a region closer to the surface within the surface layer portion 100a. For example, it is preferable that the detection amount peak is within 3 nm from the surface or the 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. 12B. 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 inner portion 100b may be very small compared to the surface layer portion 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 portion 100a is larger than the amount detected inside, similarly to magnesium or nickel. Moreover, it is preferable that the peak of the detected amount be in a region closer to the surface of the surface layer portion 100a. For example, it is preferable that the detection amount peak is 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. Moreover, it is preferable that the peak of the detected amount be in a region closer to the surface of the surface layer portion 100a. For example, it is preferable that the detection amount peak is within 3 nm from the surface or the reference point.

Moreover, it is preferable that at least aluminum among the additional elements has a detected amount peak inside the element compared to magnesium. The distributions of magnesium and aluminum may overlap as shown in FIG. 12A, or the distributions of magnesium and aluminum may not overlap as shown in FIG. 12C. 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. 13B, the peak width at 1/5 height (1/5 Max _Al ) of the maximum detected amount of aluminum (Max _Al ) was lowered from the maximum value to the horizontal axis. When divided into two by a perpendicular line, the peak width _Wc on the inside side may be larger than the peak width _Ws on the front side.

The reason why aluminum is distributed further into the interior than magnesium is considered 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 present as a solid solution 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 of 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 in the positive electrode active material 100, arrows Y1-Y2 are shown in FIG. FIG. 13A shows an example of the intensity distribution of characteristic X-rays corresponding to the additive elements 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 from the surface than in a surface with a non-(001) orientation. Specifically, the peaks of the detected amounts of magnesium and aluminum may be shallower than in other areas.

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 the surface layer portion 100a are important regions for maintaining the diffusion path of lithium ions, and at the same time are the regions from which lithium ions are first desorbed, so they tend to become unstable. Therefore, it is extremely important to reinforce the surface other than the (001) orientation and the 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 intensity distribution of characteristic X-rays corresponding to the additive elements on the surface other than the (001) orientation and the surface layer portion 100a is shown in any of FIGS. 12A to 12C. It is important that the distribution is as follows. 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 on 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. Furthermore, 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 insertion and desorption of lithium during charging and discharging, and the above advantages 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 present in a cobalt site, it has a lower redox potential than cobalt, so it can be said that it is easier to 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.

Also, the ionization tendency is smaller in the order of magnesium, aluminum, cobalt, and nickel. 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 in excess, there is a possibility 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. When the fluoride contains LiF and MgF ₂ , the eutectic point P of LiF and MgF ₂ is around 742°C (Ta), as shown in Figure 14 (quoted and added from Non-Patent Document 12, Figure 5), so the addition In the heating step after mixing the elements, the heating temperature is preferably 742° C. or higher. In addition, "Liquid" and "Liq." attached in the figure indicate "liquid", and "ss" indicates "solid".

Here, differential scanning calorimetry (DSC measurement) for fluorides and mixtures will be explained using FIG. 15. The fluoride in Figure 15 is a mixture of LiF and _MgF2 . They were mixed so that LiF:MgF ₂ =1:3 (molar ratio). The mixture in FIG. 15 is a mixture using lithium cobalt oxide as the lithium oxide and LiF and MgF ₂ as the fluorides. They were mixed so that LiCoO ₂ :LiF:MgF ₂ =100:0.33:1 (molar ratio).

As shown in FIG. 15, an endothermic peak is observed around 735° C. for 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. (Tb in FIG. 14) or higher, which is between these.

[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.

It is preferable that the positive electrode active material 100 contains phosphorus because the hydrogen fluoride generated by decomposition of the electrolyte or electrolyte may react with the phosphorus, thereby reducing the hydrogen fluoride concentration in the electrolyte.

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 additional 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.

Further, it is preferable to include additional elements having different distributions, as this can stabilize the crystal structure over a wider region. 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 are synergized and can 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 sufficiently secure a 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.

Further, some of the additive elements, particularly magnesium, nickel, and aluminum, preferably have a higher concentration in the surface layer 100a than in the interior 100b, but 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 interior 100b of a layered rock salt type toward the surface and surface layer portion 100a having characteristics of the rock salt type or both of 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.

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

Further, in a cross-sectional STEM image or the like, 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 approximately coincide. In addition, 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 , can be determined from electron diffraction patterns, 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. 16 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. 16) 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, contrast is obtained in proportion to the atomic number, 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. 17A 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. 17B shows the FFT pattern of the region of the rock salt crystal RS, and FIG. 17C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of FIGS. 17B and 17C shows the composition, the JCPDS card number, and the d value and angle calculated from the JCPDS card data. Actual values are shown on the right. Spots marked with O are 0th order diffraction.

The spots labeled A in FIG. 17B originate from the 11-1 reflection of the cubic crystal. The spots labeled A in FIG. 17C originate from layered rock salt type 0003 reflections. It can be seen from FIGS. 17B and 17C 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. 17B and the straight line passing through AO in FIG. 17C 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 spot labeled B in FIG. 17C is derived from the layered rock salt type 1014 reflection. 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 direction of the reciprocal lattice point (A in Figure 17C) 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 space different from the direction in which the 11-1 reflection of the cubic crystal is observed. For example, the spot labeled B in FIG. 17B is derived from 200 reflections of a cubic crystal. This is a diffraction spot at a location 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 direction of the reflection derived from cubic crystal 11-1 (A in Figure 17B). is sometimes 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, by carefully observing the shape of the positive electrode active material using a SEM, etc., you can make it easier to observe the (0003) plane. It is possible to process it into thin pieces. 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.

≪State 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 therefore 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, which means 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. 18 to 23 while comparing a conventional cathode active material and the cathode active material 100 of one embodiment of the present invention.

FIG. 19 shows changes in the crystal structure of a conventional positive electrode active material. The conventional positive electrode active material shown in FIG. 19 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 1 to 4.

FIG. 19 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 crystal structure of trigonal space group P-3m1, 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. 19, 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 3, 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 line and arrow in FIG. 19, in the H1-3 type crystal structure, the _CoO2 layer is largely shifted 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. 20 shows the change in the c-axis length of the conventional lithium cobalt oxide described in Non-Patent Document 4. 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 a change in the c-axis length of lithium cobalt oxide corresponds to a change in the angle at which, for example, a peak of the (003) plane of lithium cobalt oxide appears in the XRD pattern. In XRD using CuKα _1- ray, it is known that the peak of the (003) plane of lithium cobalt oxide occurs around 2θ of 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 so 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. 18, the change in crystal structure in the discharge state where x in Li _x CoO ₂ is 1 and in 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. 18 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. 18 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. 18 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. 18, 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 determined by ICSD coll. code. 172909 and 88721. Regarding H1-3, reference can be made to Non-Patent Document 3. O3' and monoclinic O1 (15) can be calculated from experimental values of XRD.

As described above, in the cathode active material 100 of one embodiment of the present invention, changes in the crystal structure when x in LixCoO ₂ is small, that is, when a large amount of lithium is released, are suppressed more than in conventional cathode active materials. There is. 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. Furthermore, all of the particles in the interior 100b of the positive electrode active material 100 do not have to have the O3' type and/or 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 having R-3mO3 symmetry even when charged at a high charging voltage, for example, 4.6 V or higher at 25° C., and therefore is preferable. It can be rephrased as In addition, it can be said that 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 case of 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.

In addition, 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 a similar crystal structure when the voltage obtained by subtracting the potential of graphite from the above voltage is applied.

Further, in O3' and monoclinic crystal O1 (15) in FIG. 18, lithium is shown to exist at all lithium sites with equal probability, but the present invention is not limited to this. It may be concentrated in some lithium sites, or it may have a symmetry such as monoclinic O1 (Li _0.5 CoO ₂ ) shown in FIG. 19, 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 means that the concentration of an element in a certain region is different from that in another region. 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 at the grain boundary 101 of the positive electrode active material 100 and its vicinity 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.

Grain boundaries 101 are one type of planar defect. 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.

Further, it is preferable to use a mixture of particles having different particle sizes in the positive electrode because 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. 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 contained in positive electrode active materials with high resolution, compare the height of crystallinity and crystal orientation, and analyze periodic lattice distortion 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 portion 100a, grain boundaries 101, etc. can be analyzed by electron beam diffraction or the like 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 in which a positive electrode active material, a conductive material, and a binder are mixed.

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 a charged state after charging and discharging a plurality of times, the conditions for charging and discharging the plurality of 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.

Furthermore, when analyzing the crystal structure in a discharged state after charging and discharging a plurality of times, constant current discharge can be performed at, for example, 2.5 V 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 21 and 22 show the ideal powder XRD patterns based on the CuKα ₁ line, which are 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. 23A and 23B. 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. 23A and 23B 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. 23A, the 2θ range is 18° or more. FIG. 23B 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 5). 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 3. 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. 21, 23A, and 23B, in the O3' type crystal structure, the diffraction pattern in powder X-ray diffraction is 2θ=19.25±0.12° (19.13° or more, 19.37° ), and a diffraction peak appears at 2θ = 45.47±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. 22, 23A, and 23B, 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, 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 less than 45.57° and/or Alternatively, it can be said that the appearance of a peak at 45.57° or more and 45.67° or less is a feature of the positive electrode active material 100 of one embodiment of the present invention.

This can also be said to be that the positive electrode active material 100 of one embodiment of the present invention has a crystal structure where x=1 and x≦0.24, and the positions where the XRD diffraction peaks appear are close to each other. 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.

Further, when similarly subjected to Rietveld analysis, it is preferable that the H1-3 type and O1 type crystal structures are 50% or less. Alternatively, it is 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.

Further, 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 of nickel and manganese and the range of lattice constants in which it is assumed that the influence of the Jahn-Teller effect is small.

FIG. 24 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. 24A shows the results for the a-axis, and FIG. 24B 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. 27 except that an aluminum source was not used.

FIG. 25 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. 25A shows the results for the a-axis, and FIG. 25B shows the results for the c-axis. Note that the lattice constant shown in FIG. 25 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. 27, except that a manganese source was used instead of a nickel source, and an aluminum source was not used.

FIG. 24C 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. 24A and 24B. FIG. 25C 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. 25A and 25B.

From FIG. 24C, there is a tendency for the a-axis/c-axis to change significantly when the nickel concentration is 5% and 7.5%, and the strain on 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. 25A, 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-ray source, 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, it is possible to quantitatively analyze the concentration of each element in a region approximately half of 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 according to one embodiment of the present invention, 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. 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 are chemically adsorbed after the positive electrode active material 100 is manufactured. 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.

Further, the concentration of the additive element may 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, it is preferable that the positive electrode active material 100 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, the surface or a region having a depth of 5 nm or more and 50 nm or less from a reference point. 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-ray source. 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 source: 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

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 fluorine and another element 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.

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 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 subjecting the cross section to 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 surface analysis (for example, elemental mapping), it is possible to quantitatively analyze 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. Further, the concentration distribution and maximum value of the added element can be analyzed by EDX-ray analysis. In addition, analysis in which the sample is sliced into thin sections, such as STEM-EDX, can analyze the concentration distribution in the depth direction from the surface of the positive electrode active material toward the center in a specific region without being affected by the distribution in the depth direction. 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 M _AVE of the detected amount of X-rays and the average value M _BG of the detected amount 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.

Moreover, the spatial resolution of STEM-EDX is about 1 nm. Therefore, the maximum value of the characteristic X-ray intensity distribution corresponding to the added element may deviate by about 1 nm. For example, even if the maximum value of the characteristic X-ray intensity distribution corresponding to an additive element such as magnesium exists outside the surface determined above, if the difference between the maximum value and the surface is less than 1 nm, it can be considered an error. .

In addition, the peak in STEM-EDX-ray analysis refers to the detected intensity in the intensity distribution of characteristic X-rays corresponding to each element, or the maximum value of characteristic X-rays for each element. 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 influence 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 intensity distribution of characteristic X-rays corresponding to 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 intensity distribution of characteristic X-rays corresponding to 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 the STEM-EDX-ray analysis, for example, a STEM device (HD-2700 manufactured by Hitachi High-Technology) may be used, and an EDAX Octane T Ultra W may be used as the EDX detector. The STEM device may be equipped with two EDX detectors. 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 each additive element, especially the additive element X, 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, it is preferable that the peak of the nickel concentration in the surface layer 100a exists within a depth of 3 nm from the surface of the positive electrode active material 100 or the reference point toward the center. 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. Further, 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.

Further, when the positive electrode active material 100 has aluminum as an additive element, 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 when subjected to EDX-ray analysis. 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 of 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 STEM image and the ultrafine electron beam diffraction pattern described below, the STEM image and the ultrafine electron beam diffraction pattern will be different 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. It cannot be detected as a bright spot in the 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 the ^case _of _cubic _CoO 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. When the surface of the cathode active material 100 is smooth and has few irregularities, when the cathode active material layer 12 is embedded in the recess 11A of the cathode current collector 11 shown in Embodiment 1, the surfaces of the cathode active material 100 are It is preferred because it is easy to slip and is difficult to crack even when subjected to press treatment.

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

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

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 8 to 10 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 by 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 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 (eg, "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 an embedded portion 102 as shown in FIG. 11B exists to embed these, elution of cobalt, etc. can be suppressed. Therefore, the positive electrode active material 100 can have excellent reliability and cycle characteristics.

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.

In addition, 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. 26A and 26B 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.

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 100, which is 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, and 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 and a lithium source simultaneously with an additive element source, 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 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.

Furthermore, 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. Note that 1×10 ⁻¹ nm=10 ⁻¹⁰ m.

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 (Shannon et al., Acta A 32 (1976) 751.), the ionic radius of six-coordinated aluminum is 0.535 x 10 ^-1 nm, and the ionic radius of six-coordinated oxygen is 0.535 x 10 -1 nm. is 1.4×10 ⁻¹ nm, and the sum of these is 1.935×10 ⁻¹ nm.

From the above, it is considered 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.

Furthermore, the initial heating can also be expected to have the effect of increasing the crystallinity of the layered rock salt type 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 that undergoes annealing and initial heating will be described with reference to FIGS. 27A to 27C.

<Step S11>
In step S11 shown in FIG. 27A, 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 having 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. 27A, 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, a bead mill, or the like can be used as a means for crushing 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. 27A, 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 time for cooling from the specified temperature to room temperature falls 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.

The 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 for 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.

Furthermore, 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 the 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. 27A 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. 27A, 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 described below, it may be called preheating or pretreatment.

Due to the initial heating, lithium is desorbed from a portion 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.

Furthermore, 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 lithium cobalt 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.

This initial heating does not require a lithium source. 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 in 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.

When the lithium cobalt oxide is heated in step S13, a temperature difference may occur between the surface and the inside of the lithium cobalt oxide. 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.

Further, the difference in shrinkage may cause micro-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.

When lithium cobalt oxide with a smooth surface is used as a positive electrode active material, there is 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. 27B and FIG. 27C.

<Step S21~Step S23>
The steps of preparing an additive element A source (A source) will be explained using FIGS. 27A and 27B. A lithium source may be prepared together with the additive element A source.

As the additive element A, the additive element 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. 27(B) 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. 27B, 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. 27B, 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. 27B will be explained using FIG. 27C. In step S21 shown in FIG. 27C, four types of additive element sources to be added to lithium cobalt oxide are prepared. That is, FIG. 27C differs from FIG. 27B 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. 27B. 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. 27C are similar to the steps described in FIG. 27B.

<Step S31>
Next, in step S31 shown in FIG. 27A, 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. 27A, the materials mixed above are collected to obtain a mixture 903. During recovery, sieving may be performed after crushing if necessary.

Note that although FIGS. 27A to 27C 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 a portion of 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. 27A, 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 (1130° C.) of lithium cobalt oxide. 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.

Furthermore, 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 will be reduced, and the diffusion path of additive elements (e.g. fluorine) will be inhibited. ) distribution may deteriorate.

Furthermore, it is believed that when an additive element (for example, fluorine) is uniformly distributed in the surface layer, a positive electrode active material that is smooth and has less 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.

Further, 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. 27A is about 12 μm, the heating temperature is preferably, for example, 650° C. or more and 950° C. or less. 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 more and 950° C. or less. 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. 27A, the heated material is collected and crushed as 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 one embodiment of the present invention and is different from method 1 for manufacturing a positive electrode active material, will be described with reference to FIGS. 28 to 29C. 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. 28, steps S11 to S15 are performed in the same manner as in FIG. 27A 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. 24(A).

<Step S21>
Step S21 shown in FIG. 29A 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. 27B. For example, as the additive element A1, one or more selected from magnesium, fluorine, and calcium can be suitably used. FIG. 29A shows an example in which a magnesium source (Mg source) and a fluorine source (F source) are prepared in step S21, assuming that magnesium and fluorine are selected as the additive element A1.

Steps S21 to S23 shown in FIG. 29A can be performed under the same conditions as steps S21 to S23 shown in FIG. 27B. As a result, an additive element A1 source (A1 source) can be obtained in step S23.

Further, steps S31 to S33 shown in FIG. 28 can be performed in the same steps as steps S31 to S33 shown in FIG. 27A.

<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. 28, 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. 29B and 29C.

<Step S41>
Step S41 shown in FIG. 29B 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. 27B. For example, as the additive element A2, one or more selected from nickel, titanium, boron, zirconium, and aluminum can be suitably used. FIG. 29B 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. 29B can be performed under the same conditions as steps S21 to S23 shown in FIG. 27B. As a result, an additive element A2 source (A2 source) can be obtained in step S43.

Further, FIG. 29C shows a modification of the steps described using FIG. 29B. In step S41 shown in FIG. 29C, 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 second additive element sources (A2 sources) are prepared. The step in FIG. 29C differs from that in FIG. 29B in that the added element is independently pulverized in step S42a.

<Step S51 to Step S53>
Next, steps S51 to S53 shown in FIG. 28 can be performed under the same conditions as steps S31 to S34 shown in FIG. 27A, and a mixture 904 is obtained in step S52. 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. 28 to 29C, 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 distribute the additive element A1 to have a higher concentration in the surface layer 100a than in the interior 100b, and to distribute the additive element A2 to have 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 described in this embodiment mode is performed on lithium cobalt oxide. Therefore, the conditions for the 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.

The positive electrode active material 100 with a smooth surface may be more resistant to physical destruction due to pressure, etc. than the positive electrode active material 100 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, an example in which a secondary battery, which is one embodiment of the present invention, is mounted in an electronic device will be described with reference to FIGS. 30A to 32C.

Examples of mounting a secondary battery having the positive electrode active material described in the previous embodiment in an electronic device are shown in FIGS. 30A to 30G. Electronic devices that use secondary batteries include, for example, television devices (also called televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (mobile phones, 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.

Furthermore, it is also possible to incorporate a secondary battery having a flexible shape along the curved surface of the inner or outer wall of a house, building, etc., or the interior or exterior of an automobile.

FIG. 30A shows an example of a mobile phone. The mobile phone 7400 includes a display section 7402 built into a housing 7401, as well as operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes a secondary battery 7407. By using the secondary battery of one embodiment of the present invention as the above-described secondary battery 7407, a lightweight and long-life mobile phone can be provided.

FIG. 30B shows the mobile phone 7400 in a curved state. When the mobile phone 7400 is deformed by an external force and curved as a whole, the secondary battery 7407 provided inside the mobile phone 7400 is also curved. Moreover, the state of the secondary battery 7407 bent at that time is shown in FIG. 30C. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. Note that the secondary battery 7407 has a lead electrode electrically connected to the current collector. The configuration is highly reliable even when the secondary battery 7407 is bent.

FIG. 30D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display portion 7102, operation buttons 7103, and a secondary battery 7104. Further, FIG. 30E shows a bent state of the secondary battery 7104. When the secondary battery 7104 is worn on the user's arm in a bent state, the housing deforms and the curvature of part or all of the secondary battery 7104 changes. Note that the degree of curvature at any point of a curve expressed by the value of the radius of the corresponding circle is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or all of the main surface of the casing or secondary battery 7104 changes within a radius of curvature of 40 mm or more and 150 mm or less. High reliability can be maintained as long as the radius of curvature on the main surface of the secondary battery 7104 is in the range of 40 mm or more and 150 mm or less. By using the secondary battery of one embodiment of the present invention as the above-described secondary battery 7104, a lightweight and long-life portable display device can be provided.

FIG. 30F shows an example of a wristwatch-type portable information terminal. The mobile information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, an operation button 7205, an input/output terminal 7206, and the like.

The portable information terminal 7200 can run various applications such as mobile telephony, e-mail, text viewing and creation, music playback, Internet communication, computer games, and so on.

The display portion 7202 is provided with a curved display surface, and can perform display along the curved display surface. Further, the display portion 7202 includes a touch sensor, and can be operated by touching the screen with a finger, a stylus, or the like. For example, by touching an icon 7207 displayed on the display portion 7202, an application can be started.

In addition to setting the time, the operation button 7205 can have various functions such as turning the power on and off, 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 7205 can be freely set using the operating system built into the mobile information terminal 7200.

Furthermore, the mobile information terminal 7200 can perform 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 portable information terminal 7200 is equipped with an input/output terminal 7206, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input/output terminal 7206. Note that the charging operation may be performed by wireless power supply without using the input/output terminal 7206.

The display portion 7202 of the mobile information terminal 7200 includes a secondary battery according to one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a portable information terminal that is lightweight and has a long life can be provided. For example, the secondary battery 7104 shown in FIG. 30E can be incorporated inside the housing 7201 in a curved state or inside the band 7203 in a bendable state.

Preferably, the mobile information terminal 7200 has a sensor. Preferably, the sensor includes, for example, a human body sensor such as a fingerprint sensor, a pulse sensor, and a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, and the like.

FIG. 30G shows an example of an armband-shaped display device. The display device 7300 includes a display portion 7304, and includes a secondary battery of one embodiment of the present invention. Further, the display device 7300 can include a touch sensor in the display portion 7304, and can also function as a mobile information terminal.

The display portion 7304 has a curved display surface, and can perform display along the curved display surface. Further, the display device 7300 can change the display status using short-range wireless communication based on communication standards.

Furthermore, the display device 7300 is equipped with an input/output terminal and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input/output terminals. Note that the charging operation may be performed by wireless power supply without using the input/output terminal.

By using the secondary battery of one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight and long-life display device can be provided.

Furthermore, an example in which the secondary battery with good cycle characteristics shown in the previous embodiment is mounted in an electronic device will be described using FIG. 30H, FIG. 31, and FIG. 32.

By using the secondary battery of one embodiment of the present invention as a secondary battery in everyday electronic devices, a product that is lightweight and has a long life can be provided. For example, everyday electronic devices include electric toothbrushes, electric shavers, electric beauty devices, etc., and the secondary batteries for these products are small, lightweight, and stick-shaped to make them easier for users to hold. , and a secondary battery with a large discharge capacity is desired.

FIG. 30H is a perspective view of a device also referred to as a cigarette containing smoking device (electronic cigarette). In FIG. 30H, an electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 including a liquid supply bottle, a sensor, and the like. To increase safety, a protection circuit that prevents overcharging and/or overdischarging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 30H has an external terminal so that it can be connected to a charging device. Since the secondary battery 7504 becomes a tip when held, it is desirable that the total length is short and the weight is light. Since the secondary battery of one embodiment of the present invention has a high discharge capacity and good cycle characteristics, it is possible to provide a small and lightweight electronic cigarette 7500 that can be used for a long time.

FIG. 31A shows an example of a wearable device. Wearable devices use secondary batteries as a power source. In addition, wearable devices that can be charged wirelessly in addition to wired charging with exposed connectors are being developed to improve splash-proof, water-resistant, and dust-proof performance when used in daily life or outdoors. desired.

For example, a secondary battery, which is one embodiment of the present invention, can be mounted in a glasses-type device 4000 as shown in FIG. 31A. Glasses-type device 4000 includes a frame 4000a and a display portion 4000b. By mounting a secondary battery on the temple portion of the curved frame 4000a, the eyeglass-type device 4000 can be lightweight, have good weight balance, and can be used for a long time. By including a secondary battery, which is one embodiment of the present invention, a configuration that can save space due to downsizing of the housing can be realized.

Further, a secondary battery, which is one embodiment of the present invention, can be mounted in the headset type device 4001. The headset type device 4001 includes at least a microphone section 4001a, a flexible pipe 4001b, and an earphone section 4001c. A secondary battery can be provided within the flexible pipe 4001b and/or within the earphone portion 4001c. By including a secondary battery, which is one embodiment of the present invention, a configuration that can save space due to downsizing of the housing can be realized.

Further, a secondary battery, which is one embodiment of the present invention, can be mounted in the device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. By including a secondary battery, which is one embodiment of the present invention, a configuration that can save space due to downsizing of the housing can be realized.

Further, a secondary battery, which is one embodiment of the present invention, can be mounted on the device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. By including a secondary battery, which is one embodiment of the present invention, a configuration that can save space due to downsizing of the housing can be realized.

Further, a secondary battery, which is one embodiment of the present invention, can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and a secondary battery can be mounted inside the belt portion 4006a. By including a secondary battery, which is one embodiment of the present invention, a configuration that can save space due to downsizing of the housing can be realized.

Further, the wristwatch-type device 4005 can be equipped with a secondary battery, which is one embodiment of the present invention. The wristwatch type device 4005 has a display portion 4005a and a belt portion 4005b, and a secondary battery can be provided in the display portion 4005a or the belt portion 4005b. By including a secondary battery, which is one embodiment of the present invention, a configuration that can save space due to downsizing of the housing can be realized.

The display section 4005a can display not only the time but also various information such as incoming mail and telephone calls.

Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly around the arm, it may be equipped with a sensor that measures the user's pulse, blood pressure, and the like. Data on the amount of exercise and health of the user can be accumulated to manage the user's health.

FIG. 31B shows a perspective view of the wristwatch type device 4005 removed from the wrist.

Further, a side view is shown in FIG. 31C. FIG. 31C shows a state in which a secondary battery 913 is built inside. Secondary battery 913 is the secondary battery shown in Embodiment 4. The secondary battery 913 is provided at a position overlapping the display portion 4005a, and is small and lightweight.

FIG. 31D shows an example of a wireless earphone. Although a wireless earphone having a pair of

main bodies

4100a and 4100b is illustrated here, the pair does not necessarily have to be a pair.

Main bodies

4100a and 4100b include a driver unit 4101, an antenna 4102, and a secondary battery 4103. It may also include a display section 4104. Further, it is preferable to have a board on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. It may also have a microphone.

Case 4110 includes a secondary battery 4111. Further, it is preferable to have a board on which circuits such as a wireless IC and a charging control IC are mounted, and a charging terminal. It may also have a display section, buttons, etc.

Main bodies

4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows the

main bodies

4100a and 4100b to reproduce sound data and the like sent from other electronic devices. Furthermore, if the

main bodies

4100a and 4100b have microphones, the sound acquired by the microphones can be sent to another electronic device, and the sound data processed by the electronic device can be sent again to the

main bodies

4100a and 4100b for playback. . This allows it to be used, for example, as a translator.

Further, the secondary battery 4111 included in the case 4110 can charge the secondary battery 4103 included in the main body 4100a. As the secondary battery 4111 and the secondary battery 4103, the coin type secondary battery, the cylindrical secondary battery, etc. of the previous embodiment can be used. A secondary battery using the positive electrode active material 100 obtained in Embodiment 1 as a positive electrode has a high energy density, and by using it for the secondary battery 4103 and the secondary battery 4111, it can save space as the wireless earphone becomes smaller. It is possible to realize a configuration that can accommodate.

FIG. 32A 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.

For example, 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 therein a secondary battery 6306 according to one embodiment of the present invention, and a semiconductor device or an electronic component. By using the secondary battery 6306 according to one embodiment of the present invention in the cleaning robot 6300, the cleaning robot 6300 can be an electronic device with a long operating time and high reliability.

FIG. 32B shows an example of a robot. The robot 6400 shown in FIG. 32B 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. Robot 6400 can communicate with a user using microphone 6402 and 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 therein a secondary battery 6409 according to one embodiment of the present invention, and a semiconductor device or an electronic component. By using the secondary battery according to one embodiment of the present invention in the robot 6400, the robot 6400 can be an electronic device with a long operating time and high reliability.

FIG. 32C shows an example of a flying object. The flying object 6500 shown in FIG. 32C includes a propeller 6501, a camera 6502, a secondary battery 6503, and the like, and has the ability to fly autonomously.

For example, image data captured by the camera 6502 is stored in the electronic component 6504. The electronic component 6504 can analyze image data and detect the presence or absence of obstacles during movement. Further, the remaining battery power can be estimated from the change in the storage capacity of the secondary battery 6503 by the electronic component 6504. The aircraft 6500 includes therein a secondary battery 6503 according to one embodiment of the present invention. By using the secondary battery according to one embodiment of the present invention in the flying object 6500, the flying object 6500 can be made into an electronic device with a long operating time and high reliability.

(Embodiment 5)
In this embodiment, an example is shown in which a secondary battery including a positive electrode active material of one embodiment of the present invention is mounted in a vehicle.

When a secondary battery is installed in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HV), electric vehicles (EV), or plug-in hybrid vehicles (PHV) can be realized.

FIG. 33 illustrates a vehicle using a secondary battery, which is one embodiment of the present invention. A car 8400 shown in FIG. 33A 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. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Further, the automobile 8400 has a secondary battery. For example, secondary battery modules can be lined up on the floor of a car. The secondary battery not only drives the electric motor 8406, but can also supply power to light emitting devices such as the headlight 8401 and a room light (not shown).

Further, the secondary battery can supply power to display devices such as a speedometer and a tachometer that the automobile 8400 has. Further, the secondary battery can supply power to a semiconductor device such as a navigation system included in the automobile 8400.

The automobile 8500 shown in FIG. 33B 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 8500. FIG. 33B shows a state where a ground-mounted charging device 8021 is charging a secondary battery 8024 mounted on a car 8500 via a cable 8022. When charging, a predetermined charging method and connector standard such as CHAdeMO (registered trademark) or combo may be used as appropriate. The charging device 8021 may be a charging station provided at a commercial facility, or may be a home power source. For example, using plug-in technology, the secondary battery 8024 mounted on the automobile 8500 can be charged by external power supply. 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 electrical power can be supplied from a ground power transmitting device in a non-contact manner for charging. In the case of this contactless power supply method, by incorporating a power transmission device into the road and/or the outer 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 vehicles using this non-contact 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 and/or when the vehicle is running. For such non-contact power supply, an electromagnetic induction method and/or a magnetic resonance method can be used.

Further, FIG. 33C is an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 33C includes a secondary battery 8602, a side mirror 8601, and a direction indicator light 8603. The secondary battery 8602 can supply electricity to the direction indicator light 8603.

Further, the scooter 8600 shown in FIG. 33C can store a secondary battery 8602 in an under-seat storage 8604. The secondary battery 8602 can be stored in the under-seat storage 8604 even if the under-seat storage 8604 is small. The secondary battery 8602 is removable, and when charging, the secondary battery 8602 can be carried indoors, charged, and stored before driving.

According to one aspect of the present invention, the cycle characteristics of the secondary battery can be improved, and the discharge capacity of the secondary battery can be increased. Therefore, the secondary battery itself can be made smaller and lighter. If the secondary battery itself can be made smaller and lighter, it will contribute to reducing the weight of the vehicle and improve its cruising distance. Further, a secondary battery mounted on a vehicle can also be used as a power supply source other than the vehicle. In this case, it is possible to avoid using a commercial power source, for example, at times of peak power demand. If it is possible to avoid using a commercial power source during times of peak power demand, it can contribute to energy conservation and reduction of carbon dioxide emissions. Furthermore, if the cycle characteristics are good, the secondary battery can be used for a long period of time, so the amount of rare metals such as cobalt used can be reduced.

10: positive electrode, 11A: recess, 11B: portion, 11: positive electrode current collector, 12A: slurry, 12: positive electrode active material layer, 13: positive electrode lead, 14: carbon black, 15: conductive material, 16: graphene, 17 : carbon fiber, 20: electrolyte, 30: negative electrode, 31: negative electrode current collector, 32: negative electrode active material layer, 33: negative electrode lead, 40: separator, 41: region, 52: sealing part, 70: plane, 75 : sealing part, 76a: lead metal, 76b: lead metal, 80: blade, 90: current collector material, 91: traveling direction, 95a: convex part, 95: embossing roll, 96a: convex part, 96: embossing roll , 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, 903: Mixture, 913: Secondary battery , 4000a: frame, 4000b: display section, 4000: glasses-type device, 4001a: microphone section, 4001b: flexible pipe, 4001c: earphone section, 4001: headset-type device, 4002a: housing, 4002b: secondary battery, 4002 : device, 4003a: housing, 4003b: secondary battery, 4003: device, 4005a: display section, 4005b: belt section, 4005: wristwatch type device, 4006a: belt section, 4006b: wireless power supply receiving section, 4006: belt type Device, 4100a: Main body, 4100b: Main body, 4101: Driver unit, 4102: Antenna, 4103: Secondary battery, 4104: Display section, 4110: Case, 4111: Secondary battery, 6300: Cleaning robot, 6301: Housing, 6302: Display section, 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 unit, 6406: Lower camera, 6407: Obstacle sensor, 6408: Movement mechanism, 6409: Secondary battery, 6500: Flying object, 6501: Propeller, 6502: Camera, 6503: Secondary battery, 6504: Electronic components , 7100: Mobile display device, 7101: Housing, 7102: Display unit, 7103: Operation button, 7104: Secondary battery, 7200: Mobile information terminal, 7201: Housing, 7202: Display unit, 7203: Band, 7204: Buckle, 7205: Operation button, 7206: Input/output terminal, 7207: Icon, 7300: Display device, 7304: Display section, 7400: Mobile phone, 7401: Housing, 7402: Display section, 7403: Operation button, 7404: External Connection port, 7405: Speaker, 7406: Microphone, 7407: Secondary battery, 7500: Electronic cigarette, 7501: Atomizer, 7502: Cartridge, 7504: Secondary battery, 8021: Charging device, 8022: Cable, 8024: Secondary battery , 8400: Car, 8401: Headlight, 8406: Electric motor, 8500: Car, 8600: Scooter, 8601: Side mirror, 8602: Secondary battery, 8603: Turn signal light, 8604: Under seat storage

Claims

A lithium ion secondary battery having a positive electrode,
The positive electrode includes a current collector and a positive electrode active material layer,
One surface of the current collector has a first recess, a second recess, and a first portion located between the first recess and the second recess,
The positive electrode active material layer is provided in the first recess and the second recess,
In a cross-sectional view of the positive electrode, when one surface of the current collector faces upward, the height of the top of the positive electrode active material layer and the height of the first A lithium-ion secondary battery whose heights at the top of the parts roughly match.
A lithium ion secondary battery having a positive electrode,
The positive electrode includes a current collector, a first positive electrode active material layer, and a second positive electrode active material layer,
One surface of the current collector has a first recess, a second recess, and a first portion located between the first recess and the second recess,
The other surface of the current collector has a third recess, a fourth recess, and a second portion located between the third recess and the fourth recess,
The first positive electrode active material layer is provided in the first recess and the second recess,
The second positive electrode active material layer is provided in the third recess and the fourth recess,
In a cross-sectional view of the positive electrode, when one surface of the current collector faces upward, the height of the top of the first positive electrode active material layer and the The heights of the tops of the first portions approximately match;
In a cross-sectional view of the positive electrode, when the other surface of the current collector faces upward, the height of the top of the second positive electrode active material layer, with respect to the bottom of the third recess, and the A lithium ion secondary battery in which the height of the top of the second portion approximately matches.
In claim 1,
The positive electrode active material layer includes a plurality of positive electrode active materials, a conductive material, and a binder,
A lithium ion secondary battery, wherein the proportion of the positive electrode active material having cracks among the plurality of positive electrode active materials is 5% or less.
In claim 3,
A lithium ion secondary battery, wherein the conductive material is carbon black.
In claim 3,
A lithium ion secondary battery in which the conductive material is a carbon nanotube.
In claim 1,
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 lithium ion secondary battery, in which a nickel distribution and a magnesium distribution overlap in a surface layer portion of the positive electrode active material.
In claim 6,
In the lithium ion secondary 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 7,
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 lithium ion secondary battery in which the difference in peak depth of the detected amount of magnesium is within 3 nm.
In claim 8,
The positive electrode active material contains aluminum,
In the intensity distribution in EDX-ray analysis of nickel, magnesium, and aluminum possessed by the positive electrode active material,
The position of the maximum detected amount of aluminum is located inside the position of the maximum detected amount of nickel and the maximum 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 lithium ion secondary battery with a large internal peak width Wc.
In any one of claims 6 to 9,
In the battery in which the positive electrode and the counter electrode are lithium, when the battery is charged to 4.6 V and the positive electrode is analyzed by powder X-ray diffraction using CuKα 1 ray, the diffraction pattern is at least 2θ. is 19.13 or more and less than 19.37,
Having a peak at 45.37° or more and less than 45.57°,
Lithium ion secondary battery.
In claim 10,
The positive electrode active material contains fluorine,
A lithium ion secondary 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.