JP6090247B2 - Positive electrode active material and lithium ion secondary battery - Google Patents

Description

The present invention relates to a positive electrode active material and a lithium ion secondary battery. Specifically, the present invention relates to a positive electrode active material containing particles of a lithium transition metal composite oxide.

  Due to the remarkable development of portable electronic technology in recent years, electronic devices such as mobile phones and notebook computers are recognized as fundamental technologies that support an advanced information society. In addition, research and development related to the enhancement of the functions of these electronic devices have been energetically advanced, and the power consumption of these electronic devices has been increasing proportionally. On the other hand, these electronic devices are required to be driven for a long time, and a high energy density of a secondary battery as a driving power source is inevitably desired. Moreover, the higher the energy density of the battery, the more desirable from the viewpoint of the occupied volume and mass of the battery built in the electronic device. For this reason, at present, lithium ion secondary batteries having an excellent energy density have been built into most devices.

Lithium transition metal composite oxide particles such as LiCoO ₂ and LiNiO ₂ are widely used as positive electrode active materials for lithium ion secondary batteries. In recent years, in order to improve the characteristics of lithium transition metal composite oxide particles, various technologies have been proposed to modify the particle surface by forming a coating layer on the particle surface or diffusing materials from the particle surface. Has been. The conventionally proposed technologies are shown below.

  Patent Documents 1 and 2 disclose a method of coating a metal oxide on the surface of a positive electrode active material or a positive electrode. Patent Documents 3 to 9 disclose a method of uniformly coating a lithium transition metal composite oxide on the particle surface and a method of diffusing from the surface.

  Patent Document 10 discloses a positive electrode active material in which a lump of metal oxide is attached on a metal oxide layer. Patent Document 11 discloses a positive electrode active material in which one or more surface treatment layers containing two or more coating elements are formed on the surface of a core containing a lithium compound. Patent Document 12 discloses a material having a high affinity for lithium and a reaction between a compound that supplies a cation and a lithium transition metal composite oxide.

Japanese Patent No. 3172388 Japanese Patent No. 3691279 JP 7-235292 A JP 2000-149950 A JP 2000-156227 A JP 2000-164214 A JP 2000-195517 A JP 2001-196063 A JP 2002-231227 A Japanese Patent Laid-Open No. 2001-256969 JP 2002-164053 A U.S. Pat. No. 7,364,793

  However, the high-temperature storage characteristics are insufficient only with the coating elements, coating methods, and coating forms disclosed in Patent Documents 1 and 2. For example, in order to improve the characteristics of lithium transition metal composite oxide particles, increasing the amount of metal oxide coating inhibits the diffusion of lithium ions. Cannot be obtained.

  When the methods disclosed in Patent Documents 3 to 9 are used, a high capacity can be maintained, but depending on the diffusion state of the transition metal from the surface, the cycle characteristics are improved and the high-temperature storage characteristics are insufficient.

  When the present inventors produced a positive electrode active material using the production method disclosed in Patent Document 10, sufficient cycle characteristics were not obtained, resulting in a significant decrease in discharge capacity.

  When the present inventors produced a positive electrode active material using the technique disclosed in Patent Document 11, a uniform multilayer was formed, and good high-temperature storage characteristics were not recognized.

  When the present inventors produced a positive electrode active material using the manufacturing method disclosed in Patent Document 12, unevenness and dropping of the material added as a coating material occurred, and inactive materials such as oxide and lithium fluoride were produced. Since the compound is formed, the function of improving cycle characteristics and high-temperature storage characteristics due to coating could not be sufficiently exhibited. Moreover, since the movement of lithium ions during charging and discharging is inhibited at the solid-liquid interface, the discharge capacity and the cycle characteristics cannot be improved at a practical level. Moreover, since lithium was deprived from the lithium transition metal composite oxide, the discharge capacity tended to decrease.

In view of the foregoing, while suppressing the decrease in the discharge capacity, it is to provide a positive active material, and a lithium ion secondary battery can be obtained that excellent cycle characteristics, and high-temperature storage characteristics.

In order to solve the above-mentioned problem, the first invention
Particles of lithium transition metal composite oxide containing lithium, main transition metal element M1, and metal element M2 different from main transition metal element M1,
The metal element M2 is Mg,
The surface concentration of the metal element M2 is higher than that inside the particle, and the concentration of the metal element M2 continuously changes from the surface,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%, the molar fraction r is in the range of 0.55 ≦ r <1.00,
In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r decreases from the particle surface toward the depth direction,
Non-aqueous electrolysis in which the molar fraction r decreases in the depth direction from the particle surface within a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%. It is a positive electrode active material for lithium ion secondary batteries containing a liquid.
(However, the ratio d (%) = {included in the whole from the particle surface to a predetermined depth [(mass of main transition metal element M1) + (mass of metal element M2)]} / (total mass of particles), mole fraction Rate r = (substance amount of metal element M2) / [(substance amount of main transition metal element M1) + (substance amount of metal element M2)])

The second invention is
An electrolyte including a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode includes particles of lithium transition metal composite oxide containing lithium, a main transition metal element M1, and a metal element M2 different from the main transition metal element M1,
The metal element M2 is Mg,
The surface concentration of the metal element M2 is higher than that inside the particle, and the concentration of the metal element M2 continuously changes from the surface,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%, the molar fraction r is in the range of 0.55 ≦ r <1.00,
In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r decreases from the particle surface toward the depth direction,
In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%, the molar fraction r decreases in the depth direction from the particle surface. Next battery.
(However, ratio d (%) = {included in the whole from the particle surface to a predetermined depth [(mass of main transition metal element M1) + (mass of metal element M2)] / (total mass of particle), molar fraction] r = (amount of metal element M2)} / [(amount of main transition metal element M1) + (amount of metal element M2)])

  In the present invention, the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80 within the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%. Therefore, the reaction at the interface between the positive electrode active material particles and the electrolytic solution can be suppressed.

  As described above, according to the present invention, excellent cycle characteristics and high-temperature storage characteristics can be obtained while suppressing a decrease in discharge capacity.

FIG. 1 is a cross-sectional view showing a configuration example of a nonaqueous electrolyte secondary battery according to the first embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view illustrating a part of the spirally wound electrode body illustrated in FIG. FIG. 3 is an exploded perspective view showing a configuration example of the nonaqueous electrolyte secondary battery according to the second embodiment of the present invention. 4 is a cross-sectional view showing a cross-sectional structure taken along line IV-IV of the spirally wound electrode body shown in FIG.

  The present invention has been devised as a result of extensive studies to solve the above-described problems of the prior art. The outline will be described below.

Lithium-containing transition metal oxides such as LiCoO ₂ and LiNiO ₂ are widely used as positive electrode active materials for non-aqueous electrolyte secondary batteries. It will cause deterioration of battery characteristics. That is, by increasing the reactivity at the interface between the positive electrode active material and the electrolyte, the transition metal component is eluted from the positive electrode, the active material is deteriorated, and Li occlusion and release due to precipitation of the eluted metal on the negative electrode side occurs. Causes inhibition. In addition, the decomposition reaction of the electrolyte solution at the interface is accelerated, a film is formed on the surface, and gas is generated.

  As a result of further detailed investigation of the deterioration mechanism of the battery characteristics by the present inventors, it was confirmed that the deterioration progressed also outside the positive electrode active material particles, that is, in the electrolyte solution, the separator film, the binder, and the like. It was done. Moreover, characteristic deterioration was recognized also in the positive electrode active material which performed the conventional surface treatment out of particle | grains.

  In addition, in a state in which the positive / negative electrode ratio is appropriately designed, charging is performed so that the maximum charging voltage is 4.20 V or higher, preferably 4.35 V or higher, more preferably 4.40 V or higher. It is possible to improve the energy density of the water electrolyte secondary battery. However, as a result of repeated studies on the non-aqueous electrolyte secondary battery in which the energy density is improved in this way, the present inventors have found that this battery has the following problems. That is, when charging / discharging is repeated at a high charge voltage state of 4.25 V or more, deterioration of the above-described active material and electrolyte solution is accelerated, leading to a decrease in charge / discharge cycle life and performance deterioration after high-temperature storage. I came to find.

Therefore, as a result of intensive studies to solve the above-mentioned problems, the present inventors contain lithium, a main transition metal element M1, and a metal element M2 different from the main transition metal element M1. In the lithium transition metal composite oxide particles, the inventors have found that the concentration distribution in the depth direction of M2 near the surface is controlled within a specific range.
The present invention has been devised based on the above studies.

Embodiments of the present invention will be described in the following order with reference to the drawings.
(1) First embodiment (example of cylindrical battery)
(2) Second embodiment (example of flat battery)

<1. First Embodiment>
[Battery configuration]
FIG. 1 is a cross-sectional view showing a configuration example of a nonaqueous electrolyte secondary battery according to the first embodiment of the present invention. This non-aqueous electrolyte secondary battery is a so-called lithium ion secondary battery in which the capacity of the negative electrode is represented by a capacity component due to insertion and extraction of lithium (Li) as an electrode reactant. This non-aqueous electrolyte secondary battery is a so-called cylindrical type, and a pair of strip-like positive electrode 21 and strip-like negative electrode 22 are stacked and wound inside a substantially hollow cylindrical battery can 11 via a separator 23. The wound electrode body 20 is rotated. The battery can 11 is made of, for example, iron (Fe) plated with nickel (Ni), and has one end closed and the other end open. Inside the battery can 11, for example, an electrolytic solution is injected as an electrolyte, and the separator 23 is impregnated. In addition, a pair of insulating plates 12 and 13 are respectively disposed perpendicular to the winding peripheral surface so as to sandwich the winding electrode body 20.

  At the open end of the battery can 11, a battery lid 14, a safety valve mechanism 15 and a thermal resistance element (PTC element) 16 provided inside the battery lid 14 are provided via a sealing gasket 17. It is attached by caulking. Thereby, the inside of the battery can 11 is sealed. The battery lid 14 is made of, for example, the same material as the battery can 11. The safety valve mechanism 15 is electrically connected to the battery lid 14, and the disk plate 15 </ b> A is reversed and wound around the battery lid 14 when the internal pressure of the battery exceeds a certain level due to an internal short circuit or external heating. The electrical connection with the electrode body 20 is cut off. The sealing gasket 17 is made of, for example, an insulating material, and the surface is coated with asphalt.

  For example, a center pin 24 is inserted in the center of the wound electrode body 20. A positive electrode lead 25 made of aluminum (Al) or the like is connected to the positive electrode 21 of the spirally wound electrode body 20, and a negative electrode lead 26 made of nickel or the like is connected to the negative electrode 22. The positive electrode lead 25 is electrically connected to the battery lid 14 by being welded to the safety valve mechanism 15, and the negative electrode lead 26 is welded to and electrically connected to the battery can 11.

  FIG. 2 is an enlarged cross-sectional view showing a part of the spirally wound electrode body 20 shown in FIG. Hereinafter, the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte constituting the secondary battery will be sequentially described with reference to FIG.

(Positive electrode)
The positive electrode 21 has, for example, a structure in which a positive electrode active material layer 21B is provided on both surfaces of a positive electrode current collector 21A. Although not shown, the positive electrode active material layer 21B may be provided only on one surface of the positive electrode current collector 21A. The positive electrode current collector 21A is made of, for example, a metal foil such as an aluminum foil. The positive electrode active material layer 21B includes, for example, one or more positive electrode materials capable of occluding and releasing an electrode reactant as a positive electrode active material, and a conductive agent such as graphite and the like as necessary. A binder such as polyvinylidene fluoride is included.

As the positive electrode material capable of occluding and releasing the electrode reactant, a compound capable of occluding and releasing lithium (Li) is used. This compound includes particles of lithium transition metal composite oxide containing lithium, a main transition metal element M1, and a metal element M2 different from the main transition metal element M1. Here, the main transition metal constituting the particles of the lithium transition metal composite oxide means a transition metal having the largest ratio among the transition metals constituting the particles. For example, when lithium cobaltate having an average composition of LiCo _0.98 Al _0.01 Mg _0.01 O ₂ is used as the lithium transition metal composite oxide, the main transition metal element M1 is cobalt (Co). The metal element M2 has a concentration gradient from the particle center toward the particle surface at the particle surface. By having such a concentration gradient, a lithium ion diffusion path can be secured. The molar fraction r is in the range of 0.20 ≦ r ≦ 0.80 in the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%. However, the ratio d and the molar fraction r are defined by the following formulas (I) and (II).
Ratio d (%) = [(mass of main transition metal element M1) + (mass of metal element M2)] / (total particle mass) (I)
Molar fraction r = (amount of metal element M2) / [(amount of main transition metal element M1) + (amount of metal element M2)] (II)

  The mass of the main transition metal element M1 and the mass of the metal element M2 are obtained by dissolving the particle surface of the lithium transition metal composite oxide with a buffer solution and containing the main transition metal element M1 and the metal element M2 dissolved in the buffer solution. The quantity is obtained by mass spectrometry.

  Specifically, the ratio d (%) and the mole fraction r are obtained as follows. First, a buffer solution is added to the lithium transition metal composite oxide particles and stirred. Next, a buffer solution in which the particle surface of the lithium transition metal composite oxide is dissolved is collected every predetermined time and filtered through a filter. The mass of the main transition metal element M1 and the metal element M2 in the buffer solution collected every time is measured by inductively coupled plasma (ICP) analysis. Next, the substance amount [mol] of each genus element M1, M2 is calculated using the measured mass, and the ratio d and the mole fraction r are obtained from the above formulas (I) and (II). Here, it is assumed that the particle is spherical, and the calculation is performed by assuming that the particle changes in a state of decreasing the radius while maintaining a similar spherical shape by dissolution.

  When the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r is within the range of 0.20 ≦ r ≦ 0.80. , Cycle characteristics, and high-temperature storage characteristics can be improved.

  On the other hand, in the range where the ratio d (%) from the particle surface to the predetermined depth does not satisfy 0.020% ≦ d ≦ 0.050%, the molar fraction r is set to 0.20 ≦ r ≦ 0.80. However, the effect of improving the high-temperature storage characteristics and the cycle characteristics does not necessarily appear.

  In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020 ≦ d ≦ 0.050, it is preferable that the molar fraction r decreases from the particle surface toward the depth direction. Thereby, the fall of a capacity | capacitance maintenance factor and a high temperature storage characteristic can be suppressed, and especially the fall of a capacity | capacitance maintenance factor can be suppressed.

  Further, within the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r is within the range of 0.20 ≦ r ≦ 0.80. In addition, within the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.02%, the molar fraction r is within the range of 0.55 ≦ r <1.0. It is more preferable that Thereby, while being able to suppress the fall of discharge capacity, cycling characteristics and high temperature storage characteristics can be improved more. In addition, the molar fraction r decreases from the particle surface toward the depth direction even when the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%. Is preferred. Thereby, the capacity | capacitance maintenance factor and the fall of a high temperature storage characteristic can be suppressed more.

  It is preferable that at least one element X selected from sulfur (S), phosphorus (P), and fluorine (F) is mainly contained on the particle surface in an aggregated form. This is because the active molecule can be stabilized.

  The surface concentration of the metal element M2 is preferably increased. Specifically, the concentration of the metal element M2 is preferably increased from the particle center toward the particle surface in the particle surface portion of the lithium transition metal composite oxide. The lithium transition metal composite oxide particles whose surface concentration of the metal element M2 is thus increased are preferably lithium transition metal composite oxide particles containing lithium, the main transition metal element M1, and the metal element M2. , Sulfur (S), phosphorus (P), and fluorine (F) are obtained by reacting with a compound containing at least one element X. In this reaction, it is preferable to react in the presence of a compound containing lithium (Li). This is because it can be expected that Li in the core material is prevented from seeping out to the surface during the reaction by coexisting such a compound.

  Metal element M2 is manganese (Mn), magnesium (Mg), nickel (Ni), aluminum (Al), boron (B), zirconium (Zr), barium (Ba), molybdenum (Mo), titanium (Ti), It is preferably at least one selected from niobium (Nb), tungsten (W), and iron (Fe), and among these elements, magnesium (Mg) is particularly preferable. This is because the use of these elements as the metal element M2 can be expected to improve the structural stability of the active material when the cycle is repeated.

  The main transition metal element M1 is preferably one selected from nickel (Ni), cobalt (Co), manganese (Mn), and iron (Fe). Specifically, as the positive electrode material, for example, a lithium-containing compound such as lithium oxide, lithium phosphorus oxide, lithium sulfide, or an intercalation compound containing lithium is suitable, and a mixture of two or more of these is used. May be. In order to increase the energy density, a lithium transition metal composite oxide containing at least lithium (Li) and one or more transition metal elements is preferable. Among them, lithium cobaltate, lithium nickelate, nickel cobalt manganese composite lithium oxide A lithium-containing compound having a layered structure such as a product is more preferable from the viewpoint of increasing the capacity. In particular, a lithium cobaltate-containing transition metal oxide mainly composed of lithium cobaltate is preferable because of its high filling property and high discharge voltage. The lithium cobaltate-containing transition metal oxide may be substituted with at least one element selected from Groups 2 to 15 or subjected to fluorination treatment.

  Examples of such lithium-containing compounds include lithium composite oxides having a layered rock salt structure, lithium composite oxides having a spinel structure, and lithium composite phosphates having an olivine structure.

(Negative electrode)
The negative electrode 22 has, for example, a structure in which a negative electrode active material layer 22B is provided on both surfaces of a negative electrode current collector 22A. Although not shown, the negative electrode active material layer 22B may be provided only on one surface of the negative electrode current collector 22A. The anode current collector 22A is made of, for example, a metal foil such as a copper foil.

  The negative electrode active material layer 22B includes one or more negative electrode materials capable of inserting and extracting lithium as the negative electrode active material, and the positive electrode active material layer 21B as necessary. It is comprised including the binder similar to. In this secondary battery, the electrochemical equivalent of the negative electrode material capable of inserting and extracting lithium is larger than the electrochemical equivalent of the positive electrode 21, and lithium metal is deposited on the negative electrode 22 during charging. It is supposed not to.

  Examples of the negative electrode material capable of inserting and extracting lithium include non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, and fired organic polymer compounds And carbon materials such as carbon fiber and activated carbon. Among these, examples of coke include pitch coke, needle coke, and petroleum coke. An organic polymer compound fired body refers to a carbonized material obtained by firing a polymer material such as phenol resin or furan resin at an appropriate temperature, and part of it is non-graphitizable carbon or graphitizable carbon. Some are classified as: Examples of the polymer material include polyacetylene and polypyrrole. These carbon materials are preferable because the change in crystal structure that occurs during charge and discharge is very small, a high charge and discharge capacity can be obtained, and good cycle characteristics can be obtained. In particular, graphite is preferable because it has a high electrochemical equivalent and can provide a high energy density. Further, non-graphitizable carbon is preferable because excellent characteristics can be obtained. Furthermore, those having a low charge / discharge potential, specifically, those having a charge / discharge potential close to that of lithium metal are preferable because a high energy density of the battery can be easily realized.

  Examples of the negative electrode material capable of inserting and extracting lithium include materials capable of inserting and extracting lithium and containing at least one of a metal element and a metalloid element as a constituent element. This is because a high energy density can be obtained by using such a material. In particular, the use with a carbon material is more preferable because a high energy density can be obtained and excellent cycle characteristics can be obtained. The negative electrode material may be a single element, alloy or compound of a metal element or metalloid element, or may have at least a part of one or more of these phases. In the present invention, the alloy includes an alloy containing one or more metal elements and one or more metalloid elements in addition to an alloy composed of two or more metal elements. Moreover, the nonmetallic element may be included. Some of the structures include a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or two or more of them.

  Examples of metal elements or metalloid elements constituting the negative electrode material include magnesium (Mg), boron (B), aluminum (Al), gallium (Ga), indium (In), silicon (Si), and germanium (Ge). ), Tin (Sn), lead (Pb), bismuth (Bi), cadmium (Cd), silver (Ag), zinc (Zn), hafnium (Hf), zirconium (Zr), yttrium (Y), palladium (Pd) ) Or platinum (Pt). These may be crystalline or amorphous.

  Among these, as the negative electrode material, a material containing a 4B group metal element or a semimetal element in the short-period type periodic table as a constituent element is preferable, and at least one of silicon (Si) and tin (Sn) is particularly preferable. It is included as an element. This is because silicon (Si) and tin (Sn) have a large ability to occlude and release lithium (Li), and a high energy density can be obtained.

  As an alloy of tin (Sn), for example, as a second constituent element other than tin (Sn), silicon (Si), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr) The thing containing at least 1 sort is mentioned. As an alloy of silicon (Si), for example, as a second constituent element other than silicon (Si), tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr). The thing containing 1 type is mentioned.

  Examples of the tin (Sn) compound or silicon (Si) compound include those containing oxygen (O) or carbon (C). In addition to tin (Sn) or silicon (Si), the above-described compounds are used. Two constituent elements may be included.

Examples of the negative electrode material capable of inserting and extracting lithium further include other metal compounds or polymer materials. Examples of other metal compounds include oxides such as MnO ₂ , V ₂ O ₅ , and V ₆ O ₁₃ , sulfides such as NiS and MoS, and lithium nitrides such as LiN ₃ , and polymer materials include polyacetylene. , Polyaniline or polypyrrole.

(Separator)
The separator 23 separates the positive electrode 21 and the negative electrode 22 and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. As the separator 23, for example, a porous film made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramic may be used as a single layer, or a laminate of a plurality of them. In particular, the separator 23 is preferably a polyolefin porous film. It is because it is excellent in the short-circuit prevention effect and can improve the safety of the battery by the shutdown effect. Further, as the separator 23, a separator in which a porous resin layer such as polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE) is formed on a microporous film such as polyolefin may be used.

(Electrolytes)
As the electrolyte, any of a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent, a solid electrolyte in which an electrolyte salt is contained, and a gel electrolyte in which an organic polymer is impregnated with a nonaqueous solvent and an electrolyte salt are used. Can do.

  The non-aqueous electrolyte is prepared by appropriately combining an organic solvent and an electrolyte, and any organic solvent can be used as long as it is used for this type of battery. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4 Examples thereof include methyl 1,3 dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetic acid ester, butyric acid ester, and propionic acid ester.

  As the solid electrolyte, for example, any inorganic solid electrolyte or polymer solid electrolyte can be used as long as the material has lithium ion conductivity. Examples of the inorganic solid electrolyte include lithium nitride and lithium iodide. The polymer solid electrolyte includes, for example, an electrolyte salt and a polymer compound that dissolves the electrolyte salt. As the polymer compound, for example, an ether polymer such as poly (ethylene oxide) or a crosslinked product thereof, a poly (methacrylate) ester polymer, an acrylate polymer or the like may be used alone or copolymerized or mixed in the molecule. Can do.

  As the matrix of the gel electrolyte, various polymers can be used as long as they can be gelated by absorbing the non-aqueous electrolyte. For example, fluorine-based polymers such as poly (vinylidene fluoride) and poly (vinylidene fluoride-co-hexafluoropropylene), ether-based polymers such as poly (ethylene oxide) and cross-linked products, poly (acrylonitrile), etc. Can be used. In particular, it is preferable to use a fluorine-based polymer from the viewpoint of redox stability. By containing an electrolyte salt, ionic conductivity is imparted.

Any electrolyte salt can be used as long as it is used for this type of battery. Illustrative examples include lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiCl, and LiBr. These lithium salts may be used individually by 1 type, and 2 or more types may be mixed and used for them.

[Battery manufacturing method]
Next, an example of a method for manufacturing the nonaqueous electrolyte secondary battery according to the first embodiment of the present invention will be described.

  First, lithium transition metal composite oxide particles containing lithium, a main transition metal element M1, and a metal element M2, and at least one element X selected from sulfur (S), phosphorus (P), and fluorine (F) And a compound containing At this time, it is preferable to further mix a compound containing lithium (Li). Next, the mixed material is subjected to mechanochemical treatment, and at least one element selected from sulfur (S), phosphorus (P), and fluorine (F) is formed on the surface of the lithium transition metal composite oxide particles. A compound containing X and preferably a compound containing lithium (Li) are deposited. The mechanochemical treatment time is preferably 5 minutes or more and 2 hours or less. This is because if it is less than 5 minutes, the coating treatment tends to be insufficient, and if it exceeds 2 hours, the particles tend to physically break and become a small particle size. Next, the lithium transition metal composite oxide particles, which are firing precursors, are fired. The firing temperature (heat treatment temperature) is preferably 500 ° C. or higher and 1500 ° C. or lower. When the temperature is lower than 500 ° C., the coating treatment tends to be insufficient. When the temperature exceeds 1500 ° C., secondary particles are formed, and the coating property tends to deteriorate. As described above, lithium transition metal composite oxide particles in which the surface concentration of the metal element M2 is increased can be obtained. Further, the particles mainly contain at least one element X selected from sulfur (S), phosphorus (P), and fluorine (F) in an aggregated form on the particle surface.

  In general, when the molar fraction r is changed by the above-described coating technique, addition of a compound containing at least one element X selected from sulfur (S), phosphorus (P), and fluorine (F) as a coating material It is possible to change the molar fraction r by adjusting the amount. When the addition amount is too small, the reaction amount on the surface of the positive electrode active material is too small, so that the coating is not formed well, and the molar fraction r becomes low. Increasing the amount added increases the molar fraction r, but in principle does not exceed 1.00. Since the reaction proceeds from the surface of the positive electrode active material, when the amount of addition increases, the ratio d (%) from the particle surface to the predetermined depth is large, that is, from the surface of the positive electrode active material to the inside, a high molar content. You will get a rate.

  When the mixed state of the covering material and the base material is not good, the molar fraction r may be lowered even when the same amount of the same covering material is added. For example, when the particle diameter after pulverization of the coating material is 100 μm or more, the average particle diameter of a general positive electrode active material is larger than 5 to 30 μm, the dispersion state is sparse, and as a result a good coating state cannot be obtained, The mole fraction r may be low. As for the mixing method, it is sufficient that the positive electrode active material base material and the covering material are sufficiently mixed. In addition to the mechanochemical method described above, a planetary mixer or a disper is used, or a plastic bag is shaken. Even the rudimentary technique can be effective.

  Next, for example, a positive electrode active material containing particles of a lithium transition metal composite oxide produced as described above, a conductive agent, and a binder are mixed to prepare a positive electrode mixture, and this positive electrode mixture Is dispersed in a solvent such as N-methyl-2-pyrrolidone to prepare a paste-like positive electrode mixture slurry. Next, this positive electrode mixture slurry is applied to the positive electrode current collector 21 </ b> A, the solvent is dried, and the positive electrode active material layer 21 </ b> B is formed by compression molding with a roll press or the like, thereby forming the positive electrode 21.

  Further, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and this negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to obtain a paste-like negative electrode mixture slurry Is made. Next, the negative electrode mixture slurry is applied to the negative electrode current collector 22A, the solvent is dried, and the negative electrode active material layer 22B is formed by compression molding using a roll press or the like, and the negative electrode 22 is manufactured.

  Next, the positive electrode lead 25 is attached to the positive electrode current collector 21A by welding or the like, and the negative electrode lead 26 is attached to the negative electrode current collector 22A by welding or the like. Next, the positive electrode 21 and the negative electrode 22 are wound through the separator 23. Next, the front end of the positive electrode lead 25 is welded to the safety valve mechanism 15, and the front end of the negative electrode lead 26 is welded to the battery can 11, and the wound positive electrode 21 and negative electrode 22 are joined with the pair of insulating plates 12 and 13. It is housed inside the sandwiched battery can 11. Next, after the positive electrode 21 and the negative electrode 22 are accommodated in the battery can 11, for example, an electrolytic solution is injected into the battery can 11 and impregnated in the separator 23. Next, the battery lid 14, the safety valve mechanism 15, and the heat sensitive resistance element 16 are fixed to the opening end of the battery can 11 by caulking through a sealing gasket 17. Thereby, the secondary battery shown in FIG. 1 is obtained.

  The upper limit charging voltage of the secondary battery may be, for example, 4.20V, but it is preferable that the battery is designed to be higher than 4.20V. Specifically, it is preferable that the battery is designed to be in the range of 4.25V to 4.80V, and is 4.35V or more from the viewpoint of discharge capacity and 4.65V or less from the viewpoint of safety. It is more preferable that the design is made within the range. Moreover, it is preferable that a minimum discharge voltage shall be 2.00V or more and 3.30V or less. Thus, the energy density can be increased by increasing the battery voltage.

  In the secondary battery according to the first embodiment, when charged, lithium ions are released from the positive electrode active material layer 21B, and occlude and store lithium contained in the negative electrode active material layer 22B via an electrolyte such as an electrolytic solution. Occluded by the negative electrode material that can be released. Next, when discharge is performed, lithium ions occluded in the negative electrode material that can occlude and release lithium in the negative electrode active material layer 22B are released, and the positive electrode active material layer 21B passes through an electrolyte such as an electrolytic solution. Occluded.

  In the first embodiment, at least a part of the particle surface of the lithium transition metal composite oxide as the positive electrode active material contains at least one metal element M2 different from the main transition metal element M1 inside the particle. The metal element M2 has a concentration gradient from the particle center to the particle surface at the particle surface. The particles of the lithium-containing transition metal oxide mainly contain at least one element X selected from sulfur (S), phosphorus (P), and fluorine (F) in an aggregated form on the particle surface. As a result, cycle deterioration and internal resistance increase due to charging / discharging in a high temperature environment are less than conventional, and both high capacity and battery characteristics can be achieved.

  Although details of improvement in cycle characteristics and the like by using such a metal element M2 as a coating material are not clear, it is presumed to be due to the following mechanism. Inside the non-aqueous electrolyte secondary battery in a charged state, the positive electrode is in a state having a strong oxidizing property, and the electrolytic solution in contact with the positive electrode is in an environment in which oxidative decomposition easily proceeds particularly in a high temperature environment. As decomposition of the electrolytic solution proceeds, an inactive film is formed on the positive electrode active material particles, thereby inhibiting electron transfer and lithium ion transfer. In addition, the decomposed components generate very active molecules in the electrolyte in the electrode pores, accelerating the deterioration of the electrolyte or attacking the positive electrode active material to dissolve the constituent elements and reduce the capacity I will let you. That is, in order to suppress such a phenomenon, it is not sufficient to stabilize the interface between the positive electrode active material particles and the electrolyte, and the active molecules are stabilized in the vicinity of the particles outside the positive electrode active material particles. It is preferable to act synergistically.

  In the lithium-containing transition metal oxide particles according to the first embodiment, the metal element M2 different from the main transition metal inside the particles is arranged on the particle surface, thereby stabilizing the interface between the positive electrode active material particles and the electrolyte solution. Plan. Further, the active molecule is stabilized by arranging a lithium transition metal composite oxide containing at least one element X selected from S, P and F in an aggregated form in the vicinity of the particles. And, it is considered that a very high level of battery performance improvement is realized by inducing a synergistic effect of both. Furthermore, when the concentration of the metal element M2 on the particle surface is low, there is a tendency that sufficient cycle characteristics and high temperature storage characteristics cannot be improved. On the other hand, when the concentration is too high, the diffusion of Li ions is hindered. It seems that there is a tendency that a large capacity cannot be obtained.

  In the method for analyzing the surface state of the positive electrode active material particles according to the first embodiment, the covering state can be guaranteed three-dimensionally (three-dimensionally), and quantitative concentration gradient analysis can be performed. On the other hand, it is difficult to perform the above-described analysis by the conventionally proposed method for analyzing the surface state of the positive electrode active material particles.

  For example, Japanese Patent Application Laid-Open No. 2001-196063 describes that the concentration gradient of Al from the particle surface to the inside is calculated from the components by sequentially dissolving the particle surface using sulfuric acid. . However, the Al concentration gradient in the particle depth direction disclosed in JP-A-2001-196063 does not affect the cycle characteristics or battery safety. Further, in the method disclosed in Japanese Patent Application Laid-Open No. 2001-196063, the pH of sulfuric acid changes during the measurement, and this pH change causes a difference in the dissolution rate of each metal species, so that quantitative concentration gradient analysis can be performed. Not.

  In JP 2007-335331 A, JP 2008-135279 A, and WO 06/123572, a transition metal other than the main metal constituting the active material is coated on the particle surface, and XPS or TOF -It is described that the covering state is analyzed using SIMS. However, these analytical methods can measure only one point (one-dimensional) on the surface layer, and the covering state cannot be guaranteed three-dimensionally (three-dimensional). Further, the depth of measurement is about several nanometers, and the coating depth effective for high temperature storage characteristics and cycle characteristics is not quantitatively observed.

<2. Second Embodiment>
[Battery configuration]
FIG. 3 is an exploded perspective view showing a configuration example of the nonaqueous electrolyte secondary battery according to the second embodiment of the present invention. In this secondary battery, a wound electrode body 30 to which a positive electrode lead 31 and a negative electrode lead 32 are attached is accommodated in a film-like exterior member 40, and can be reduced in size, weight, and thickness. ing.

  Each of the positive electrode lead 31 and the negative electrode lead 32 is led out from the inside of the exterior member 40 to the outside, for example, in the same direction. The positive electrode lead 31 and the negative electrode lead 32 are each made of, for example, a metal material such as aluminum, copper, nickel, or stainless steel, and each have a thin plate shape or a mesh shape.

  The exterior member 40 is made of, for example, a rectangular aluminum laminated film in which a nylon film, an aluminum foil, and a polyethylene film are bonded together in this order. For example, the exterior member 40 is disposed so that the polyethylene film side and the wound electrode body 30 face each other, and the outer edge portions are in close contact with each other by fusion bonding or an adhesive. An adhesive film 41 is inserted between the exterior member 40 and the positive electrode lead 31 and the negative electrode lead 32 to prevent intrusion of outside air. The adhesion film 41 is made of a material having adhesion to the positive electrode lead 31 and the negative electrode lead 32, for example, a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene.

  The exterior member 40 may be made of a laminated film having another structure, a polymer film such as polypropylene, or a metal film instead of the above-described aluminum laminated film.

  4 is a cross-sectional view taken along line IV-IV of the spirally wound electrode body 30 shown in FIG. The wound electrode body 30 is obtained by stacking and winding a positive electrode 33 and a negative electrode 34 with a separator 35 and an electrolyte layer 36 interposed therebetween, and the outermost peripheral portion is protected by a protective tape 37.

  The positive electrode 33 has a structure in which a positive electrode active material layer 33B is provided on one or both surfaces of a positive electrode current collector 33A. The negative electrode 34 has a structure in which a negative electrode active material layer 34B is provided on one surface or both surfaces of a negative electrode current collector 34A, and the negative electrode active material layer 34B and the positive electrode active material layer 33B are arranged to face each other. Yes. The configurations of the positive electrode current collector 33A, the positive electrode active material layer 33B, the negative electrode current collector 34A, the negative electrode active material layer 34B, and the separator 35 are respectively the positive electrode current collector 21A, the positive electrode active material layer 21B, and the negative electrode in the first embodiment. This is the same as the current collector 22A, the negative electrode active material layer 22B, and the separator 23.

  The electrolyte layer 36 includes an electrolytic solution and a polymer compound serving as a holding body that holds the electrolytic solution, and has a so-called gel shape. The gel electrolyte layer 36 is preferable because high ion conductivity can be obtained and battery leakage can be prevented. The configuration of the electrolytic solution (that is, the solvent, the electrolyte salt, and the like) is the same as that of the secondary battery according to the first embodiment. Examples of the polymer compound include polyacrylonitrile, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide, polypropylene oxide, polyphosphazene, and polysiloxane. , Polyvinyl acetate, polyvinyl alcohol, polymethyl methacrylate, polyacrylic acid, polymethacrylic acid, styrene-butadiene rubber, nitrile-butadiene rubber, polystyrene or polycarbonate. In particular, polyacrylonitrile, polyvinylidene fluoride, polyhexafluoropropylene, or polyethylene oxide is preferable from the viewpoint of electrochemical stability.

[Battery manufacturing method]
Next, an example of a method for manufacturing the nonaqueous electrolyte secondary battery according to the second embodiment of the present invention will be described.

  First, a precursor solution containing a solvent, an electrolyte salt, a polymer compound, and a mixed solvent is applied to each of the positive electrode 33 and the negative electrode 34, and the mixed solvent is volatilized to form the electrolyte layer 36. After that, the positive electrode lead 31 is attached to the end of the positive electrode current collector 33A by welding, and the negative electrode lead 32 is attached to the end of the negative electrode current collector 34A by welding. Next, the positive electrode 33 and the negative electrode 34 on which the electrolyte layer 36 is formed are laminated via a separator 35 to form a laminated body, and then the laminated body is wound in the longitudinal direction, and a protective tape 37 is attached to the outermost peripheral portion. The wound electrode body 30 is formed by bonding. Finally, for example, the wound electrode body 30 is sandwiched between the exterior members 40, and the outer edges of the exterior members 40 are sealed and sealed by heat fusion or the like. At that time, the adhesion film 41 is inserted between the positive electrode lead 31 and the negative electrode lead 32 and the exterior member 40. Thereby, the secondary battery shown in FIG. 3 and FIG. 4 is obtained.

  Further, this secondary battery may be manufactured as follows. First, the positive electrode 33 and the negative electrode 34 are produced as described above, and the positive electrode lead 31 and the negative electrode lead 32 are attached to the positive electrode 33 and the negative electrode 34. Next, the positive electrode 33 and the negative electrode 34 are laminated and wound via the separator 35, and a protective tape 37 is adhered to the outermost peripheral portion to form a wound body that is a precursor of the wound electrode body 30. Next, the wound body is sandwiched between the exterior members 40, and the outer peripheral edge except for one side is heat-sealed to form a bag shape, which is then stored inside the exterior member 40. Next, an electrolyte composition including a solvent, an electrolyte salt, a monomer that is a raw material of the polymer compound, a polymerization initiator, and other materials such as a polymerization inhibitor as necessary is prepared, and the exterior member Inject into 40.

  After injecting the electrolyte composition, the opening of the exterior member 40 is heat-sealed and sealed in a vacuum atmosphere. Next, the gelled electrolyte layer 36 is formed by applying heat to polymerize the monomer to obtain a polymer compound. Thus, the secondary battery shown in FIG. 3 is obtained.

  The operation and effect of the nonaqueous electrolyte secondary battery according to the second embodiment are the same as those of the nonaqueous electrolyte secondary battery according to the first embodiment.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these examples.

  In this example, the ratio d and the molar fraction r are determined as follows.

(Ratio d, molar fraction r)
10 ml of a pH 5.0 buffer solution prepared by mixing citric acid and sodium citrate with 0.2 g of lithium transition metal composite oxide was added, and the mixture was stirred every 1/20 minutes. The product was filtered through a 0.2 μm filter. The mass / volume concentration of the main transition metal element M1 (= Co) and the metal element M2 (= Mg, Mn, Ni) in each solution obtained was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Emission Spectrometry) [HORIBA JY238 ULTRACE], and the masses of M1 and M2 dissolved in 10 ml of buffer solution were calculated. Furthermore, the mass [mol] of each genus element M1 and M2 was computed using the mass, and the ratio d and the mole fraction r were calculated | required from the following formula (I) and formula (II). Here, it was assumed that the particles were spherical, and the calculation was performed assuming that the particles changed in a state of decreasing the radius while maintaining a similar spherical shape by dissolution.
Ratio d (%) = [(mass of main transition metal element M1) + (mass of metal element M2)] / (total mass of particles) × 100 (I)
Molar fraction r = (amount of metal element M2) / [(amount of main transition metal element M1) + (amount of metal element M2)] (II)

Covering state ratio d is 0.02 <d <0.08 M2 of the range of is considered to be effective in the cycle characteristics, and high-temperature storage characteristics. In the following examples, samples in which the value of the molar fraction r is changed in a range where the ratio d considered to be particularly effective is in the range of 0.02% and 0.05%, and their battery characteristics are compared. did.

  In this example, the distribution state of the metal element M2 and the element X was determined as follows.

(Distribution state of metal element M2 and element X)
By SEM / EDX, it was confirmed whether Mg was uniformly distributed on the particle surface, and further, it was observed whether P was scattered on the particle surface. Further, the cross section of the particle was cut and the elemental distribution in the radial direction was measured by Auger electron spectroscopy, and the state in which the Mg concentration continuously changed from the surface was observed.

< Reference Example 1>
A positive electrode active material was prepared as follows.
Lithium carbonate (Li ₂ CO ₃ ), cobalt carbonate (CoCO ₃ ), aluminum hydroxide (Al (OH) ₃ ), and magnesium carbonate (MgCO ₃ ) were added at 0.5: 0.98: 0.01: 0.01. After being mixed at a molar ratio of 5, the mixture was fired in air at 900 ° C. for 5 hours to obtain a lithium-cobalt composite oxide (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) as a base material. Next, lithium carbonate (Li ₂ CO ₃ ) and ammonium dihydrogen phosphate (NH ₄ H ₂ ) with respect to lithium cobalt oxide (LiCo0.98Al _0.01 Mg _0.01 O ₂ ) having an average particle diameter of 13 μm (measured by a laser scattering method) PO ₄ ) was weighed and mixed so that the atomic ratio was Co: Li: P = 98: 1: 1.

Next, the mixed material was treated with a mechanochemical apparatus for 1 hour, and lithium carbonate and ammonium dihydrogen phosphate were deposited on the surface using lithium cobaltate particles as a central material. The calcined precursor was heated at a rate of 3 ° C. per minute, held at 900 ° C. for 3 hours, and then gradually cooled to obtain a lithium transition metal composite oxide. Lithium in which magnesium (Mg) is uniformly distributed on the particle surface, the surface concentration of magnesium (Mg) is higher than that inside the particle, and lithium phosphate (Li ₃ PO ₄ ) is scattered on the particle surface. A transition metal composite oxide was obtained. As a result of confirming the details of the surface concentration gradient of magnesium (Mg), the molar fractions r at the ratio d = 0.02% and 0.05% were 0.32 and 0.30, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.46 and 0.25, respectively. Further, when the obtained powder was observed with SEM / EDX, it was confirmed that Mg was uniformly distributed on the particle surface and P was scattered on the particle surface. Further, this was measured powder X-ray diffraction pattern using CuKα the powder diffraction peaks of Li ₃ PO ₄ was confirmed in addition to a diffraction peak corresponding to LiCoO ₂ having a layered rock salt structure. Further, when the particle cross section was cut and the elemental distribution in the radial direction was measured by Auger electron spectroscopy, it was observed from this measurement that the Mg concentration continuously changed from the surface.

  Using the lithium transition metal composite oxide particles obtained as described above as the positive electrode active material, a nonaqueous electrolyte secondary battery was fabricated as described below.

  A positive electrode was produced as follows. First, a positive electrode mixture was prepared by mixing 98% by weight of the positive electrode active material, 0.8% by weight of amorphous carbon powder (Ketjen Black) and 1.2% by weight of polyvinylidene fluoride (PVdF). Next, this positive electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry, and then this positive electrode mixture slurry was coated on both surfaces of a belt-like positive electrode current collector made of a belt-like aluminum foil. Was applied uniformly. The obtained coated product was dried with hot air, and then compression molded with a roll press to form a positive electrode mixture layer. Thereby, a strip-like positive electrode was obtained.

  A negative electrode was produced as follows. First, 95% by weight of graphite powder and 5% by weight of PVdF were mixed to prepare a negative electrode mixture. Next, this negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode mixture slurry, and then the negative electrode mixture slurry was uniformly applied to both surfaces of a strip-shaped negative electrode current collector made of a strip-shaped copper foil. Furthermore, a negative electrode mixture layer was formed by hot press molding this. Thereby, a strip-shaped negative electrode was obtained.

  The strip-shaped positive electrode and the strip-shaped negative electrode fabricated as described above were wound many times through a porous polyolefin film, to fabricate a spiral electrode body. This electrode body was housed in a nickel-plated iron battery can, and insulating plates were arranged on both upper and lower surfaces of the electrode body. Next, the aluminum positive electrode lead is led out from the positive electrode current collector and welded to the protrusion of the safety valve that is electrically connected to the battery lid, and the nickel negative electrode lead is led out from the negative electrode current collector. Welded to the bottom of the can.

Ethylene carbonate and methyl ethyl carbonate were mixed so that the volume mixing ratio was 1: 1, thereby preparing a mixed solution. Next, LiPF ₆ was dissolved in this mixed solution to a concentration of 1 mol / dm ³ to prepare a nonaqueous electrolytic solution.

  Finally, after injecting the electrolyte into the battery can incorporating the above electrode body, the safety can, the PTC element and the battery lid are fixed by caulking the battery can through the insulating sealing gasket, and the outer diameter is A cylindrical nonaqueous electrolyte secondary battery having a height of 18 mm and a height of 65 mm was produced.

(Evaluation of initial discharge capacity and capacity maintenance rate)
The initial discharge capacity and capacity retention rate of the non-aqueous electrolyte secondary battery produced as described above were determined as follows.
First, after charging under conditions of an environmental temperature of 45 ° C., a charging voltage of 4.35 V, a charging current of 1.5 A, and a charging time of 2.5 hours, discharging is performed under the conditions of a discharging current of 2.0 A and a final voltage of 3.0 V. The initial discharge capacity (discharge capacity at the first cycle) was measured. Next, after repeating charge / discharge under the above-described charge / discharge conditions, the discharge capacity at the 300th cycle was measured. Next, using the discharge capacity at the first cycle and the discharge capacity at the 300th cycle, the capacity retention rate after 300 cycles was obtained from the following equation.
Capacity maintenance ratio [%] after 300 cycles = (discharge capacity at 300th cycle / discharge capacity at the first cycle) × 100

(High temperature storage characteristics)
The high-temperature storage characteristics of the non-aqueous electrolyte secondary battery produced as described above were determined as follows.
First, after charging under conditions of an environmental temperature of 45 ° C., a charging voltage of 4.35 V, a charging current of 1.5 A, and a charging time of 2.5 hours, discharging is performed under the conditions of a discharging current of 2.0 A and a final voltage of 3.0 V The initial capacity was measured. Next, the battery was charged under conditions of an environmental temperature of 45 ° C., a charging voltage of 4.35 V, a charging current of 1.5 A, and a charging time of 2.5 hours, and then stored in an environment of high temperature of 60 ° C. for 300 hours. Next, discharge was performed under the condition of 0.2 C, and the discharge capacity after high temperature storage was measured. Next, using the initial capacity and the discharge capacity after high temperature storage, the capacity retention rate (high temperature storage characteristics) after high temperature storage was determined from the following equation.
Capacity retention rate after storage at high temperature [%] = (discharge capacity after storage at high temperature / initial capacity) × 100

< Reference Example 2>
A nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1. Next, the initial discharge capacity, the capacity retention rate, and the high-temperature storage characteristics were evaluated in the same manner as in Reference Example 1 except that the charging voltage was 4.20V.

< Reference Example 3>
A nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1. Next, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated in the same manner as in Reference Example 1 except that the charging voltage was set to 4.50V.

< Reference Example 4>
A positive electrode active material was produced in the same manner as in Reference Example 1 except that the second baking temperature was 950 ° C. and the holding time was 30 minutes. The molar fractions r of this positive electrode active material ratio d = 0.02% and 0.05% were 0.22 and 0.21, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.38 and 0.16, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Example 5>
A positive electrode active material was produced in the same manner as in Reference Example 1 except that LiCo _0.95 Al _0.01 Mg _0.04 O ₂ was used as the base material lithium-cobalt composite oxide. The molar fractions r of this positive electrode active material ratio d = 0.02% and 0.05% were 0.73 and 0.52, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.86 and 0.44, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

< Reference Example 6>
A positive electrode active material was prepared in the same manner as in Reference Example 1 except that LiCo _0.97 Al _0.01 Mg _0.02 O ₂ was used as the base material lithium-cobalt composite oxide. The molar fractions r of this positive electrode active material ratio d = 0.02% and 0.05% were 0.31 and 0.30, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.56 and 0.25, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

< Reference Example 7>
The base metal lithium-cobalt composite oxide is LiCoO _2, and lithium-cobalt composite oxide (LiCoO ₂ ), lithium carbonate (Li ₂ CO ₃ ), magnesium carbonate (MgCO ₃ ), and ammonium dihydrogen phosphate (NH _A positive electrode active material was produced in the same manner as in Reference Example 1 except that ₄ H ₂ PO ₄ ) was weighed and mixed so that Co: Li: Mg: P = 100: 1: 2: 1 by atomic ratio. The molar fractions r at the positive electrode active material ratios d = 0.02% and 0.05% were 0.46 and 0.40, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.55 and 0.44, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

< Reference Example 8>
Lithium / cobalt composite oxide as a base material was LiCoO _2, and lithium / cobalt composite oxide (LiCoO ₂ ) was coated with nickel hydroxide and manganese phosphate as coating materials. At this time, the total composition ratio of Ni: Co: Mn is 5: 2: 3, the ratio d = 0.02%, and the molar fraction r (Ni + Mn / Co + Ni + Mn) at 0.05% is 0.35 and 0.34, respectively. The molar fraction r at the ratio d = 0.01% and 0.10% was adjusted to 0.56 and 0.25, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 1>
Lithium cobaltate (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) that was not coated was used as the positive electrode active material. The molar fractions at the positive electrode active material ratios d = 0.02% and 0.05% were 0.01 and 0.01, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.01 and 0.01, respectively. A nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative example 2>
The base metal lithium-cobalt composite oxide is LiCoO _2, and lithium cobaltate (LiCoO ₂ ), lithium carbonate (Li ₂ CO ₃ ), magnesium carbonate (MgCO ₃ ), and ammonium dihydrogen phosphate (NH ₄ H _2). The same procedure as in Reference Example 1 was carried out to prepare a positive electrode active material, except that PO ₄ ) was weighed and mixed so that the atomic ratio was Co: Li: Mg: P = 100: 1: 0.5: 1. did. Next, the molar fractions r at the positive electrode active material ratios d = 0.02% and 0.05% were 0.18 and 0.10, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.25 and 0.08, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 3>
Lithium cobaltate (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ), lithium carbonate (Li ₂ CO ₃ ), magnesium carbonate (MgCO ₃ ), and ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) in atomic ratio Co: A positive electrode active material was produced in the same manner as in Reference Example 1 except that weighing and mixing were performed so that Li: Mg: P = 100: 1: 1: 4. The molar fractions r at the positive electrode active material ratios d = 0.02% and 0.05% were 0.82 and 0.83, respectively. Further, the molar fractions r at the ratios d = 0.01% and 0.10% were 0.80 and 0.85, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative example 4>
A positive electrode active material was produced in the same manner as in Reference Example 1 except that the second firing temperature was 750 ° C. The molar fraction r at 0.02% and 0.05% in the ratio d of the positive electrode active material was 0.81 and 0.75, respectively. The molar fractions r at the ratios d = 0.01% and 0.10% were 0.81 and 0.70, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 5>
A positive electrode active material was produced in the same manner as in Reference Example 1 except that the second firing temperature was 850 ° C. The molar fractions r of this positive electrode active material ratio d = 0.02% and 0.05% were 0.62 and 0.71, respectively. Further, the molar fractions r at the ratios d = 0.01% and 0.10% were 0.55 and 0.75, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 6>
A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 5. Next, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated in the same manner as in Reference Example 1 except that the charging voltage was 4.20V.

<Comparative Example 7>
A nonaqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 5. Next, the initial discharge capacity, the capacity retention rate, and the high-temperature storage characteristics were evaluated in the same manner as in Reference Example 1 except that the charging voltage was 4.50V.

<Comparative Example 8>
A positive electrode active material was produced in the same manner as in Reference Example 1 except that the second baking treatment was not performed. The molar fractions r at the positive electrode active material ratios d = 0.02% and 0.05% were 0.80 and 0.81, respectively. In addition, the molar fractions r at the ratios d = 0.01% and 0.10% were 0.82 and 0.79, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 9>
A positive electrode active material was produced in the same manner as in Reference Example 1 except that the mechanochemical treatment was performed for 15 minutes. The molar fractions r at the positive electrode active material ratios d = 0.02% and 0.05% were 0.21 and 0.16, respectively. In addition, the molar fractions r at the ratios d = 0.01% and 0.10% were 0.31 and 0.14, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 10>
As the lithium transition metal composite oxide, a positive electrode active material was prepared by performing a coating treatment using lithium hydroxide and manganese phosphate as a coating material on lithium cobalt oxide (LiCoO ₂ ). At this time, the overall composition ratio of Ni: Co: Mn is 1: 1: 1, the ratio d = 0.02, and the molar fraction r (Ni + Mn / Co + Ni + Mn) at 0.05% is 0.25, 0.17, respectively. The molar fraction r at the ratio d = 0.01% and 0.10% was adjusted to be 0.30 and 0.15, respectively. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 except that this positive electrode active material was used. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

Tables 1 and 2 show the configurations of the positive electrode active materials of the nonaqueous electrolyte secondary batteries of Example 5, Reference Examples 1 to 4 , 6 to 8, and Comparative Examples 1 to 10, and the evaluation results thereof.

Table 1 shows the following.
In Example 5, Reference Examples 1 to 4 , and 6 to 8, excellent cycle characteristics and high-temperature storage characteristics can be obtained while suppressing a decrease in discharge capacity, while in Comparative Examples 1 to 10, Such characteristics cannot be obtained.

In Example 5 and Reference Examples 1 to 4 and 6 to 8, the molar fraction r in the ratio d of 0.02% ≦ d ≦ 0.05% is in the range of 0.20 ≦ r ≦ 0.80. Further, when the ratio d (%) from the particle surface to the predetermined depth is in the range of 0.020 ≦ d ≦ 0.050, the molar fraction r tends to decrease from the particle surface toward the depth direction.

In Comparative Examples 1-2, Comparative Example 4, and Comparative Examples 9-10, the ratio d (%) from the particle surface to the predetermined depth is in the depth direction from the particle surface in the range of 0.020 ≦ d ≦ 0.050. The mole fraction r tends to be constant or decrease toward. However, when the ratio d is 0.02% ≦ d ≦ 0.05%, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80.

In Comparative Example 1, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 because no coating material is used. In Comparative Example 2, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 because the mixing amount of the base material and the coating material when producing the positive electrode active material is not appropriate. . In Comparative Example 4, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 because the second firing is 750 ° C. In Comparative Example 9, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 because the mechanochemical treatment time is 15 minutes, which is very short compared to Reference Example 1. is there. In Comparative Example 10, the molar fraction is not in the range of 0.20 ≦ r ≦ 0.80 because the mixing amount of the base material and the coating material when producing the positive electrode active material is not appropriate.

In Comparative Example 3 and Comparative Example 8, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 when the ratio d is 0.02% ≦ d ≦ 0.05%. Further, when the ratio d (%) from the particle surface to the predetermined depth is in the range of 0.020 ≦ d ≦ 0.050, r tends to increase from the particle surface toward the depth direction.

  In Comparative Example 3, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 because the mixing amount of the base material and the coating material when producing the positive electrode active material is not appropriate. . In Comparative Example 8, the molar fraction r is not in the range of 0.20 ≦ r ≦ 0.80 because the second baking treatment is not performed.

In Comparative Examples 5 to 7, the molar fraction r in the ratio d of 0.02% ≦ d ≦ 0.05% is in the range of 0.20 ≦ r ≦ 0.80. However, when the ratio d (%) from the particle surface to the predetermined depth is in the range of 0.020% ≦ d ≦ 0.050%, the molar fraction r tends to increase from the particle surface toward the depth direction. .

  In Comparative Examples 5 to 7, the molar fraction r tends to increase because the second firing is 850 ° C.

  As described above, the molar fraction r is within the range of 0.20 ≦ r ≦ 0.80 within the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%. As a result, excellent cycle characteristics and high-temperature storage characteristics can be obtained while suppressing a decrease in discharge capacity.

Moreover, as a manufacturing method of a positive electrode active material, the manufacturing method which pulls out the metal element M2 from the base material to the surface like Example 5, Reference Examples 1-4, and 6 is preferable. This is because such a manufacturing method is simple in process, has a more uniform distribution on the surface, and easily maintains the structure, and thus has excellent cycle characteristics and high-temperature storage characteristics.

Table 2 shows the following.
When the ratio d is out of the range of 0.02% ≦ d ≦ 0.05%, the high temperature storage characteristics and the cycle are not limited even if the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80. The characteristics are not improved.

  Conventionally, X-ray photoelectron spectroscopy (XPS) or time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used as an analytical method for confirming the surface state of the positive electrode active material. Secondary Ion Mass Spectroscopy) is used. Table 3 shows the ratio d = 0.010% corresponding to the region in the depth direction measured by these analysis methods (corresponding to a region several nm from the particle surface), and the ratio d = 0 corresponding to a deeper region. The measurement result of the molar fraction r at 100% (corresponding to a region of 100 nm or more from the particle surface) is shown.

< Reference Example 9>
The nonaqueous electrolyte secondary battery produced in Reference Example 4 was disassembled, the positive electrode current collector was peeled off from the positive electrode, immersed in NMP, the binder was removed, the conductive agent was burned off, and the positive electrode active material was taken out. The molar fraction r in this positive electrode active material ratio d = 0.02% and 0.05% was 0.290.22.

< Reference Example 10>
In Reference Example 5, the produced nonaqueous electrolyte secondary battery was disassembled, the positive electrode current collector was peeled off from the positive electrode, immersed in NMP, the binder was removed, the conductive material was burned out, and the positive electrode active material was taken out. The molar fractions r of this positive electrode active material ratio d = 0.02% and 0.05% were 0.79 and 0.53, respectively.

Table 3 shows the configurations of the positive electrode active materials of the nonaqueous electrolyte secondary batteries of Reference Example 9 and Comparative Example 10, and the evaluation results thereof.

Table 3 shows the following.
Even when the produced non-aqueous electrolyte secondary battery is disassembled to obtain a positive electrode active material, the ratio d (%) from the particle surface to the predetermined depth is within a range satisfying 0.020% ≦ d ≦ 0.050%. It can be seen that the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80.

<Example 11>
A positive electrode active material was prepared as follows.
Lithium carbonate (Li ₂ CO ₃ ), cobalt carbonate (CoCO ₃ ), aluminum hydroxide (Al (OH) ₃ ), and magnesium carbonate (MgCO ₃ ) were added at 0.5: 0.98: 0.01: 0.01. After being mixed at a molar ratio of 5, the mixture was fired in air at 900 ° C. for 5 hours to obtain a lithium-cobalt composite oxide (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) as a base material. Next, with respect to lithium cobaltate (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) having an average particle diameter of 13 μm (measured by a laser scattering method), an average particle diameter of 6 μm (measured by a laser scattering method) pulverized by a jet mill. Ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) was weighed and mixed so that the atomic ratio was Co: P = 99: 1.

Next, the mixed material was treated with a mechanochemical apparatus for 1 hour to deposit lithium dihydrogen phosphate on the surface with lithium cobaltate particles as a central material. The calcined precursor was heated at a rate of 3 ° C. per minute, held at 900 ° C. for 3 hours, and then gradually cooled to obtain a lithium transition metal composite oxide. That is, magnesium (Mg) is uniformly distributed on the particle surface, the surface concentration of magnesium (Mg) is higher than that inside the particle, and lithium phosphate (Li ₃ PO ₄ ) is scattered on the particle surface. Lithium transition metal composite oxide was obtained. As a result of confirming the details of the surface concentration gradient of magnesium (Mg), the molar fractions r at the ratio d = 0.01%, 0.15%, 0.02%, and 0.05% were 0.82 and 0.00, respectively. 73, 0.62, and 0.40. Further, when the obtained powder was observed with SEM / EDX, it was confirmed that Mg was uniformly distributed on the particle surface and P was scattered on the particle surface. Further, this was measured powder X-ray diffraction pattern using CuKα the powder diffraction peaks of Li ₃ PO ₄ was confirmed in addition to a diffraction peak corresponding to LiCoO ₂ having a layered rock salt structure. Further, when the particle cross section was cut and the elemental distribution in the radial direction was measured by Auger electron spectroscopy, it was observed from this measurement that the Mg concentration continuously changed from the surface.

A nonaqueous electrolyte secondary battery was produced in the same manner as in Reference Example 1 using the lithium transition metal composite oxide particles obtained as described above as the positive electrode active material. Next, in the same manner as in Reference Example 1, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Example 12>
Ammonium dihydrogen phosphate (NH ₄ H ₂ ) having an average particle size of 6 μm (measured by a laser scattering method) pulverized by a jet mill against lithium cobalt oxide (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) having an average particle size of 6 μm. A positive electrode active material was prepared in the same manner as in Example 11 except that PO ₄ ) was weighed and mixed so that the atomic ratio was Co: P = 98.8: 1.2. The molar fractions r at the positive electrode active material ratios d = 0.01%, 0.015%, 0.02%, and 0.05% are 0.92, 0.85, 0.80, and 0.65, respectively. there were. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 11 except that this positive electrode active material was used. Next, in the same manner as in Example 11, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Example 13>
Lithium cobaltate (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) with an average particle size of 6 μm is an ammonium sulfate (NH ₄ ) ₂ SO ₄ ) with an average particle size of 3 μm (measured by a laser scattering method) pulverized by a jet mill. They were weighed and mixed so that Co: S = 99: 1 by atomic ratio. The mixed material was processed for 30 minutes by a planetary mixer, and ammonium sulfate was deposited on the surface with lithium cobaltate particles as a central material. A positive electrode active material was produced in the same manner as in Example 11 except for the above. The molar fractions r at the positive electrode active material ratios d = 0.01%, 0.015%, 0.02%, and 0.05% are 0.80, 0.71, 0.58, and 0.38, respectively. there were. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 11 except that this positive electrode active material was used. Next, in the same manner as in Example 11, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

< Reference Example 14>
Ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) having an average particle diameter of 100 μm (measured by a laser scattering method) pulverized by a jet mill was weighed and mixed so that the atomic ratio was Co: P = 99: 1. A positive electrode active material was produced in the same manner as Example 11 except for the above. The molar fractions r at the positive electrode active material ratios d = 0.01%, 0.015%, 0.02%, and 0.05% are 0.62, 0.53, 0.44, and 0.25, respectively. there were. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 11 except that this positive electrode active material was used. Next, in the same manner as in Example 11, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 11>
Ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) having an average particle diameter of 6 μm (measured by a laser scattering method) pulverized by a jet mill was weighed and mixed so that the atomic ratio was Co: P = 95: 5. A positive electrode active material was produced in the same manner as Example 11 except for the above. The molar fractions r at the positive electrode active material ratios d = 0.01%, 0.015%, 0.02%, and 0.05% are 0.98, 0.95, 0.92, and 0.85, respectively. there were. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 11 except that this positive electrode active material was used. Next, in the same manner as in Example 11, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

<Comparative Example 12>
Lithium cobaltate (LiCo _0.98 Al _0.01 Mg _0.01 O ₂ ) with an average particle size of 6 μm is an ammonium sulfate (NH ₄ ) ₂ SO ₄ ) with an average particle size of 3 μm (measured by a laser scattering method) pulverized by a jet mill. Weighing was performed so that Co: S = 99: 1 by atomic ratio. A positive electrode active material was produced in the same manner as in Example 13 except that this material was fired without mixing. The molar fractions r at the positive electrode active material ratios d = 0.01%, 0.015%, 0.02%, and 0.05% are 0.33, 0.25, 0.20, and 0.15, respectively. there were. Next, a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 13 except that this positive electrode active material was used. Next, in the same manner as in Example 13, the initial discharge capacity, capacity retention rate, and high-temperature storage characteristics were evaluated.

In Table 4, the structure of the positive electrode active material of the nonaqueous electrolyte secondary battery of Example 11-Example 1 3 , Reference example 14, and Comparative example 11-Comparative example 12 and the evaluation result are shown.

  Table 4 shows the following.

  In Example 11, Example 12, and Example 13, the molar fraction r is 0.20 ≦ r ≦ 0.80 within a range where the ratio d (%) satisfies 0.020% ≦ d ≦ 0.050%. In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.01% ≦ d <0.02%, the molar fraction r is 0.55 ≦ r <1. It was within the range of 0. As a result, it was possible to obtain extremely excellent cycle characteristics and high-temperature storage characteristics while suppressing a decrease in discharge capacity.

In Reference Example 14, the molar fraction r in the ratio d of 0.02% ≦ d ≦ 0.05% is in the range of 0.20 ≦ r ≦ 0.80, but the ratio d is 0.01% ≦ d. The molar fraction r at <0.02% was outside the range of 0.55 ≦ r <1.00, and very excellent cycle characteristics and high temperature storage characteristics could not be obtained. This is because the average particle size of ammonium dihydrogen phosphate, which is a coating material, is 100 μm, so that a good mixed state with the base material cannot be obtained, and as a result, a good covering state on the surface of the base material can be achieved. It is thought that it is impossible.

  In Comparative Example 11, the molar fraction r in the ratio d of 0.01% ≦ d <0.02% is in the range of 0.55 ≦ r <1.00, but the ratio d is 0.02% ≦ d. The molar fraction r at ≦ 0.05% was out of the range of 0.20 ≦ r ≦ 0.80. This is because the coating thickness is too thick because the amount of the coating material added is too large, and the positive electrode active material contributing to the charge / discharge capacity is reduced. Therefore, in Comparative Example 11, the initial discharge capacity was small.

  In Comparative Example 12, the molar fraction r when the ratio d is 0.01% ≦ d <0.02% is outside the range of 0.55 ≦ r <1.00, and the ratio d is 0.02% ≦ d ≦ The molar fraction r at 0.05% was out of the range of 0.20 ≦ r ≦ 0.80. This is because the mixed state was insufficient and good coating was not achieved. Therefore, in Comparative Example 12, good cycle characteristics and high temperature storage characteristics could not be obtained.

  As mentioned above, although embodiment of this invention was described concretely, this invention is not limited to the above-mentioned embodiment, The various deformation | transformation based on the technical idea of this invention is possible.

  For example, the configurations, methods, shapes, materials, numerical values, and the like given in the above-described embodiments are merely examples, and different configurations, methods, shapes, materials, numerical values, and the like may be used as necessary.

  The configurations of the above-described embodiments can be combined with each other without departing from the gist of the present invention.

  Further, in the above-described embodiments and examples, the case where the present invention is applied to the flat type and the cylindrical type secondary battery has been described. The same applies to a secondary battery.

  In the above-described embodiment, an example in which the present invention is applied to a secondary battery having a winding structure has been described, but the structure of the secondary battery is not limited to this. For example, the present invention can be applied to a secondary battery having a structure in which a positive electrode and a negative electrode are folded or a secondary battery having a stacked structure.

DESCRIPTION OF SYMBOLS 11 Battery can 12, 13 Insulation board 14 Battery cover 15 Safety valve mechanism 15A Disk board 16 Heat sensitive resistance element 17 Gasket 20, 30 Winding electrode body 21, 33 Positive electrode 21A, 33A Positive electrode collector 21B, 33B Positive electrode active material layer 22 , 34 Negative electrode 22A, 34A Negative electrode current collector 22B, 34B Negative electrode active material layer 23, 35 Separator 24 Center pin 25, 31 Positive electrode lead 26, 32 Negative electrode lead 36 Electrolyte layer 37 Protective tape 40 Exterior member 41 Adhesive film

Claims (4)

Particles of lithium transition metal composite oxide containing lithium, main transition metal element M1, and metal element M2 different from main transition metal element M1,
The metal element M2 is Mg,
The surface concentration of the metal element M2 is higher than that inside the particles, and the concentration of the metal element M2 continuously changes from the surface,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%, the molar fraction r is in the range of 0.55 ≦ r <1.00,
In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r decreases from the particle surface toward the depth direction,
Non-aqueous electrolysis in which the molar fraction r decreases in the depth direction from the particle surface within a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%. The positive electrode active material for lithium ion secondary batteries containing a liquid.
(However, the ratio d (%) = {included in the whole from the particle surface to a predetermined depth [(mass of main transition metal element M1) + (mass of metal element M2)]} / (total mass of particles), mole fraction Rate r = (substance amount of metal element M2) / [(substance amount of main transition metal element M1) + (substance amount of metal element M2)])   The positive electrode active material according to claim 1, comprising at least one element X selected from S, P and F in an aggregated form mainly on the particle surface. An electrolyte including a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode includes particles of lithium transition metal composite oxide containing lithium, a main transition metal element M1, and a metal element M2 different from the main transition metal element M1,
The metal element M2 is Mg,
The surface concentration of the metal element M2 is higher than that inside the particles, and the concentration of the metal element M2 continuously changes from the surface,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r is in the range of 0.20 ≦ r ≦ 0.80,
In the range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%, the molar fraction r is in the range of 0.55 ≦ r <1.00,
In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.020% ≦ d ≦ 0.050%, the molar fraction r decreases from the particle surface toward the depth direction,
In a range where the ratio d (%) from the particle surface to the predetermined depth satisfies 0.010% ≦ d <0.020%, the molar fraction r decreases in the depth direction from the particle surface. Next battery.
(However, ratio d (%) = {included in the whole from the particle surface to a predetermined depth [(mass of main transition metal element M1) + (mass of metal element M2)] / (total mass of particle), molar fraction] r = (amount of metal element M2)} / [(amount of main transition metal element M1) + (amount of metal element M2)])   The lithium ion secondary battery according to claim 3, wherein the upper limit charging voltage is in the range of 4.25V to 4.80V and the lower limit discharging voltage is in the range of 2.00V to 3.30V.

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