Lithium-ion secondary battery

Abstract

The purpose of the present invention is to provide a novel lithium-ion secondary battery. This lithium-ion secondary battery has a positive electrode and a negative electrode. The positive electrode has positive electrode active material particles. The positive electrode active material particles have lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum. The inside of the positive electrode active material particles has a layered rock-salt-type crystal structure of a space group R-3m. The positive electrode active material particles have an edge region in which a region of the layered rock-salt-type crystal structure, a region of a spinel-type crystal structure, and a region of the rock-salt-type crystal structure are arranged in this order from the inside to the outside of the positive electrode active material particles, and in XPS analysis of the positive electrode active material particles, a peak component derived from an O-Mg-O bond in an Mg1s peak is 90% or more.

Description

Lithium-ion rechargeable battery

[0001] One aspect of the present invention relates to a product, a method, or a method of making a product. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. Another aspect of the present invention relates to an energy storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a method of making the same.

[0002] In this specification, "electronic equipment" refers to all devices that have an energy storage device, and all electro-optical devices with an energy storage device, information terminal devices with an energy storage device, etc., are considered electronic equipment.

[0003] In recent years, there has been a great deal of development on various energy storage devices, including lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. In particular, the demand for lithium-ion secondary batteries, which offer high output and high capacity, has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable as a source of rechargeable energy in today's information society.

[0004] In particular, there is a high demand for secondary batteries for mobile electronic devices that have a large discharge capacity per unit weight and excellent cycle characteristics. To meet these demands, there is a great deal of research being done on improving the positive electrode active material of secondary batteries (for example, Patent Documents 1 and 2). Research is also being conducted on the crystal structure of positive electrode active material particles (Non-Patent Documents 1 to 3).

[0005] X-ray diffraction (XRD) is one of the methods used to analyze the crystal structure of positive electrode active material particles. By using ICSD (Inorganic Crystal Structure Database), which is introduced in Non-Patent Document 4, XRD data can be analyzed. For example, the lattice constant of lithium cobalt oxide, as described in Non-Patent Document 5, can be referenced from ICSD. For Rietveld method analysis, for example, the analysis program RIETAN-FP (Non-Patent Document 6) can be used. In addition, VESTA (Non-Patent Document 7) can be used as crystal structure plotting software.

[0006] Also, as image processing software, for example, ImageJ (Non-Patent Documents 8 to 10) is known. By using this software, for example, the shape of the positive electrode active material particles can be analyzed.

[0007] Electron microdiffraction is also effective for identifying the crystal structure of the positive electrode active material particles, particularly the crystal structure of the surface layer. For the analysis of electron diffraction patterns, for example, the analysis program Recipro (Non-Patent Document 11) can be used. In addition, STEM (Scanning Transmission Electron Microscope) - EDX (Energy Dispersive X-ray Spectroscopy) can be used for elemental analysis of the positive electrode active material. Non-Patent Document 12 is known as the detection limit (also referred to as the lower detection limit) when using STEM-EDX.

[0008] As lithium cobaltate, LiCoO₂ with a layered rock salt-type crystal structure of space group R-3m, which is often used as the positive electrode active material of secondary batteries

[0008] In addition, LiCoO₂ with a crystal structure of space group Fd-3m of LiTiO₂ ２ type is known (Non-Patent Documents 13, Non-Patent Document 14). LiCoO₂ with a crystal structure of space group Fd-3m of LiTiO₂ ２ is also known. ２ type of crystal structure of LiCoO₂ ２ is a structure that has lithium at the 16c position, cobalt at the 16d position, and oxygen at the 32e position in the Wyckoff positions (Non-Patent Document 15), and is also called the LiTiO₂ structure, LT-LiCoO₂ structure, etc.

[0009] Japanese Patent Application Laid-Open No. 2018-206747, Japanese Patent Application Laid-Open No. 2022-070247

[0010] ​​​​​​​​Toyoki Okumura et al.,“Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348T.Motohashi,et al.,“Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et al.,“Staging Phase Transitions in Li▲x▼CoO▲2▼”,Journal of The Electrochemical Society,2002,149(12)A1604−A1609A.Belsky,et al.,“New developments in the Inorganic Crystal Structure Database(ICSD):accessibility in support of materials research and design”,Acta Cryst.,(2002)B58 364−369.J.Akimoto,Y.Gotoh,Y.Oosawa,“Synthesis and structure refinement of LiCoO▲2▼ single crystals”Journal of Solid State Chemistry(1998)141,p.298−302.F.Izumi and K.Momma,“Three−Dimensional Visualization in Powder Diffraction”Solid State Phenom.130,15−20(2007)K.Momma and F.Izumi,”VESTA 3 for three−dimensional visualization of crystal,"Volumetric and morphology data" J. Appl. Cryst. (2011). 44, 1272−1276 Rasband, W. S., ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http: / / rsb.info.nih.gov / ij / , 1997−2012. Schneider, C. A., Rasband, W. S., Eliceiri, K. W. "NIH Image to ImageJ: 25 years of image analysis". Nature Methods 9, 671−675, 2012. Abramoff, M. D., Magelhães, P. J., Ram, S. J. "Image Processing with ImageJ". Biophotonics International, volume 11, issue 7, pp. 36−42, 2004. Seto, Y. & Ohtsuka, M. "Recipro: free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools" (2022). J. Appl. Cryst. 55. Keiichi Fukunaga, Yukito Kondo, "Detection Limit by TEM / STEM-EDS", Microscopy, 53.3, pp. 134−139. (2018) Antaya, M., Cearns, K., Preston, J. S., Reimers, J. N., & Dahn, J. R. In situ growth of layered, spinel, and rock-salt LiCoO▲2▼ by laser ablation deposition. Journal of applied physics, 76(5), 2799−2806, (1994). Maiyalagan, T., Jarvis, K. A., Therese, S., Ferreira, P. J., & Manthiram,A. Spinel-type lithium cobalt oxide as a bifunctional electrocatalyst for the oxygen evolution and oxygen reduction reactions. Nature communications, 5(1), 3949, (2014). W. Fischer, E. Koch, “14.2 Symbols and properties of lattice complexes”, International tables for crystallography Volume A, Fifth edition (ISBN: 0-7923-6590-9), Springer, (2005), pp. 848-872. ,

[0011] Lithium-ion secondary batteries still have room for improvement in various aspects, including output characteristics, discharge capacity, cycle characteristics, reliability, safety, and cost. For example, in order to suppress changes in the crystal structure of the surface of the positive electrode active material particles, the surface of the positive electrode active material particles is sometimes coated with an inert oxide, but this coating may inhibit the insertion and removal of lithium ions. If the insertion and removal of lithium ions is inhibited, there is a concern that the characteristics of the secondary battery will deteriorate, such as a decrease in discharge capacity during high-rate discharge (also called a decrease in output characteristics or a decrease in rate characteristics) and a decrease in charge and discharge capacity in low-temperature environments.

[0012] Therefore, one aspect of the present invention aims to provide positive electrode active material particles that can be used in lithium-ion secondary batteries and promote the insertion and removal of lithium ions. Alternatively, one aspect aims to provide positive electrode active material particles or composite oxides in which the decrease in discharge capacity during high-rate discharge is suppressed. Alternatively, one aspect aims to provide positive electrode active material particles or composite oxides in which the decrease in discharge capacity in low-temperature environments is suppressed. Alternatively, one aspect aims to provide positive electrode active material particles or composite oxides in which the decrease in discharge capacity during charge-discharge cycles is suppressed. Alternatively, one aspect aims to provide positive electrode active material particles or composite oxides in which the crystal structure is not easily disrupted even after repeated charge-discharge. Alternatively, one aspect aims to provide positive electrode active material particles or composite oxides with a large discharge capacity. Alternatively, one aspect aims to provide a secondary battery with high safety or reliability, an electronic device having said secondary battery, or a vehicle having said secondary battery.

[0013] Furthermore, one aspect of the present invention aims to provide positive electrode active material particles, composite oxides, energy storage devices, or methods for producing the same.

[0014] Furthermore, the description of these problems does not preclude the existence of other problems. Moreover, one aspect of the present invention does not need to solve all of these problems. It is possible to extract other problems from the description, drawings, and claims.

[0015] One aspect of the present invention is a lithium-ion secondary battery having a positive electrode and a negative electrode, wherein the positive electrode has positive electrode active material particles, the positive electrode active material particles contain lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum, the interior of the positive electrode active material particles has a layered rock salt type crystal structure of space group R-3m, the positive electrode active material particles have edge regions arranged in the order of layered rock salt type crystal structure region, spinel' type crystal structure region, and rock salt type crystal structure region from the interior to the exterior of the positive electrode active material particles, and in XPS analysis of the positive electrode active material particles, the peak component originating from the O-Mg-O bond in the Mg1s peak is 90% or more.

[0016] Alternatively, one aspect of the present invention is a lithium-ion secondary battery having a positive electrode and a negative electrode, wherein the positive electrode has positive electrode active material particles, the positive electrode active material particles contain lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum, the interior of the positive electrode active material particles has a layered rock salt type crystal structure of space group R-3m, the positive electrode active material particles have edge regions arranged in the order of layered rock salt type crystal structure region, spinel' type crystal structure region, and rock salt type crystal structure region from the interior to the exterior of the positive electrode active material particles, and in XPS analysis of the positive electrode active material particles, when the cobalt concentration is 1, the magnesium concentration (Mg / Co) is 0.50 or more and 0.90 or less, and in XPS analysis, the full width at half maximum of the Mg1s peak is 1.0 eV or more and 2.6 eV or less.

[0017] The spinel-type crystal structure described above is LiTiO with space group Fd-3m. ２ It is a type.

[0018] In the above, when the arrangement of bright spots on the surface of the positive electrode active material particles observed in the cross-sectional HAADF-STEM image of the edge region is defined as the first row, it is preferable that at least a portion of the fourth to ninth rows located in the interior direction of the positive electrode active material particles has a region of spinel'-type crystal structure, and that the region of spinel'-type crystal structure has lithium sites in which magnesium and nickel are detected in STEM-EELS analysis, cobalt sites in which magnesium and nickel are detected, and cobalt sites in which nickel is detected.

[0019] Alternatively, one aspect of the present invention comprises a positive electrode and a negative electrode, wherein the positive electrode has positive electrode active material particles comprising lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum, and the positive electrode active material particles have a surface portion with an edge region and an interior, the interior having a layered rock salt type crystalline structure of space group R-3m, and when the arrangement of bright spots on the surface of the positive electrode active material particles observed in a cross-sectional HAADF-STEM image of the edge region is defined as the first row, the fourth to ninth rows located in the interior direction of the positive electrode active material particles have STE This is a lithium-ion secondary battery in which, when M-EELS analysis is performed, magnesium and nickel are detected at a position where only the Wyckoff position 16c of space group Fd-3m overlaps in the <110> direction, nickel is detected at a position where only the Wyckoff position 16d overlaps in the <110> direction, and in XPS analysis of the positive electrode active material particles, the magnesium concentration (Mg / Co) is between 0.50 and 0.90 when the cobalt concentration is set to 1, and the full width at half maximum of the Mg1s peak in XPS analysis is between 1.0 eV and 2.6 eV.

[0020] In any one of the above, it is preferable that lithium metal is used as the negative electrode, and a mixture of lithium hexafluoride phosphate, ethylene carbonate, diethyl carbonate, and 2 wt% vinylene carbonate is used as the electrolyte, and that the first capacity value obtained when the battery is charged with a constant current of 0.5 C (provided that 1.0 C = 200 mA / g) up to a voltage of 4.60 V at 25°C, then charged with a constant voltage until the current value becomes 0.05 C, and then discharged with a constant current of 1.0 C to a voltage of 3.0 V is 96% or more of.

[0021] In any one of the above, it is preferable that lithium metal is used as the negative electrode, and a mixture of lithium hexafluoride phosphate, ethylene carbonate, diethyl carbonate, and 2 wt% vinylene carbonate is used as the electrolyte, and that the first capacity value obtained when the battery is charged with a constant current of 0.5 C (provided that 1.0 C = 200 mA / g) up to a voltage of 4.60 V at 25°C, then charged with a constant voltage until the current value becomes 0.05 C, and then discharged with a constant current of 0.1 C up to a voltage of 3.0 V is 75% or more of the second capacity value obtained when the battery is charged with a constant current of 0.5 C (provided that 1.0 C = 200 mA / g) up to a voltage of 4.60 V at 25°C, then charged with a constant voltage until the current value becomes 0.05 C, and then discharged with a constant current of 2.0 C up to a voltage of 3.0 V is 75% or more of the first capacity value obtained when the battery is charged with a constant current of 0.5 C (provided that 1.0 C = 200 mA / g) up to a voltage of 4.60 V at 25°C, then charged with a constant voltage until the current value becomes 0.05 C, and then discharged with a constant current of 2.0 C up to a voltage of 3.0 V.

[0022] In any one of the above, lithium metal is used as the negative electrode, and a mixture of lithium hexafluoride phosphate, ethylene carbonate, diethyl carbonate, and 2 wt% vinylene carbonate is used as the electrolyte. Constant current charging is performed at 25°C with a current value of 0.5C (provided that 1.0C = 200mA / g) until the voltage reaches 4.60V. After that, constant voltage charging is performed until the current value becomes 0.05C. When the positive electrode is analyzed by powder X-ray diffraction using CuKα1 rays in an argon atmosphere, it is preferable that the XRD pattern has diffraction peaks at least at 2θ = 19.25 ± 0.12° and 2θ = 45.47 ± 0.10°.

[0023] According to one aspect of the present invention, positive electrode active material particles can be used in lithium-ion secondary batteries and promote the insertion and removal of lithium ions. Alternatively, positive electrode active material particles or composite oxides can be provided in which the decrease in discharge capacity during high-rate discharge is suppressed. Alternatively, positive electrode active material particles or composite oxides can be provided in which the decrease in discharge capacity in low-temperature environments is suppressed. Alternatively, positive electrode active material particles or composite oxides can be provided in which the decrease in discharge capacity during charge-discharge cycles is suppressed. Alternatively, positive electrode active material particles or composite oxides can be provided in which the crystal structure is less likely to collapse even after repeated charge-discharge cycles. Alternatively, positive electrode active material particles or composite oxides can be provided in which the discharge capacity is large. Alternatively, a safe or highly reliable secondary battery, an electronic device having said secondary battery, or a vehicle having said secondary battery can be provided.

[0024] Furthermore, according to one aspect of the present invention, positive electrode active material particles, composite oxides, energy storage devices, or methods for producing the same can be provided.

[0025] Furthermore, the description of these effects does not preclude the existence of other effects. Moreover, one aspect of the present invention does not necessarily have to possess all of these effects. Other effects will naturally become apparent from the description in the specification, drawings, and claims, and it is possible to extract other effects from the description in the specification, drawings, and claims.

[0026] Figure 1A is a cross-sectional view illustrating the internal structure of a secondary battery, and Figure 1B is a cross-sectional view illustrating the positive electrode and electrolyte of the secondary battery. Figures 2A and 2B are cross-sectional views illustrating the positive electrode active material particles. Figures 3A, 3B, 3C, 3D, 3E, and 3F are cross-sectional views illustrating the positive electrode active material particles. Figure 4A is a HAADF-STEM image, and Figure 4B is an ABF-STEM image. Figures 5A and 5B are LiTiO ２ These are schematic diagrams of the crystal structure of LiTiO. Figures 6A, 6B, 6C, 6D, 6E, and 6F show LiTiO. ２This is a schematic diagram of the crystal structure of the type. Figure 7 is a diagram illustrating the crystal structure of the positive electrode active material particles. Figure 8A is a diagram illustrating the charging depth and lattice constant of the positive electrode active material, Figure 8B is a diagram illustrating the H1-3 type crystal structure, and Figure 8C is a diagram illustrating the O3' type crystal structure. Figure 9 is a diagram showing the XRD pattern calculated from the crystal structure. Figures 10A, 10B, 10C, 10D, 10E, 10F, and 10G are diagrams illustrating the positional relationship of the distribution in EDX radiation analysis. Figures 11A and 11B are diagrams illustrating the method for manufacturing positive electrode active material particles. Figures 12A, 12B, and 12C are diagrams illustrating the method for manufacturing positive electrode active material particles. Figure 13A is an exploded perspective view of a coin-type secondary battery, Figure 13B is a perspective view of a coin-type secondary battery, and Figure 13C is a cross-sectional perspective view thereof. Figure 14A shows an example of a cylindrical secondary battery. Figure 14B shows an example of a cylindrical secondary battery. Figure 14C shows an example of multiple cylindrical secondary batteries. Figure 14D shows an example of an energy storage system having multiple cylindrical secondary batteries. Figures 15A and 15B show the external appearance of a secondary battery. Figures 16A, 16B, and 16C illustrate a method for manufacturing a secondary battery. Figure 17A shows an electric bicycle, Figure 17B shows the secondary battery of an electric bicycle, and Figure 17C illustrates a scooter. Figures 18A, 18B, 18C, 18D, and 18E illustrate an example of an electronic device. Figure 19A shows an example of a wearable device, Figure 19B shows a perspective view of a wristwatch-type device, and Figure 19C illustrates a side view of a wristwatch-type device. Figure 20 is a cross-sectional HAADF-STEM image of positive electrode active material particles. Figures 21A and 21B illustrate the results of cross-sectional STEM-EDX analysis of positive electrode active material particles. Figures 22A and 22B illustrate the results of cross-sectional STEM-EDX analysis of positive electrode active material particles. Figures 23A and 23B are graphs illustrating the results of XPS analysis. Figures 24A and 24B are graphs illustrating the results of discharge rate tests. Figures 25A and 25B are graphs illustrating the XRD analysis results of the positive electrode in a high-voltage charged state. Figure 26A is a cross-sectional HAADF-STEM image of positive electrode active material particles, and Figure 26B is a black and white inverted version of the cross-sectional HAADF-STEM image.Figures 27A and 27B illustrate the results of cross-sectional STEM-EELS analysis of positive electrode active material particles. Figure 28 also illustrates the results of cross-sectional STEM-EELS analysis of positive electrode active material particles.

[0027] The following describes embodiments for carrying out the present invention with reference to drawings and other illustrations. However, the present invention is not limited to the following embodiments. It is possible to modify the embodiments for carrying out the invention without departing from the spirit of the present invention.

[0028] In this specification, space groups are expressed using international notation (or Hermann-Mauguin notation) in short notation. Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by superscripting numbers, but in this specification, due to formatting constraints, a minus sign (-) may be placed before the number instead of a superscript. Individual orientations indicating directions within a crystal are represented by [ ], collective orientations indicating all equivalent directions are represented by < >, individual crystal planes are represented by ( ), and collective planes with equivalent symmetry are represented by {}. Furthermore, for ease of understanding the structure, trigonal crystals represented by space group R-3m are generally represented as a composite hexagonal lattice, and in this specification, unless otherwise specified, space group R-3m will be represented as a composite hexagonal lattice. In addition, (hkl) as well as (hkil) may be used as Miller indices. Here, i is -(h+k).

[0029] In this specification, the term "particle" is not limited to spherical shapes (circular cross-sections), but may also refer to individual particles with elliptical, rectangular, trapezoidal, triangular, rounded quadrilateral, or asymmetrical cross-sections, and individual particles may also have irregular shapes.

[0030] Furthermore, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the insertable and detachable lithium contained in the positive electrode active material is detached. For example, LiCoO ２ Its theoretical capacity is 274 mAh / g, LiNiO ２ Its theoretical capacity is 275 mAh / g, LiMn ２ O ４ Its theoretical capacity is 148 mAh / g.

[0031] Furthermore, the amount of lithium remaining in the positive electrode active material can be determined by x in the composition formula, for example, Li. ｘ MO ２ The expression is denoted by x. Note that M represents a transition metal, and unless otherwise specified in this specification, M is cobalt and / or nickel. In the case of the positive electrode active material in a lithium-ion secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, Li ｘ CoO ２ When a lithium-ion secondary battery using as the positive electrode active material is charged to a charge capacity of 219.2 mAh / g, Li ０．２ CoO ２ Alternatively, we can say x = 0.2. Li ｘ MO ２ For x to be small, for example, 0.1 < x ≤ 0.24.

[0032] If properly synthesized lithium cobalt oxide, before being used as the positive electrode, approximately satisfies the stoichiometric ratio, then LiCoO ２ And x = 1. Also, lithium cobalt oxide contained in a lithium-ion secondary battery when discharge has finished is also LiCoO ２ Therefore, we can say that x = 1. The state in which discharge has ended (discharge state) referred to here is, for example, the state in which the voltage is 2.5V or less with a current of 100mA / g or less.

[0033] When determining the value of current per unit weight of positive electrode active material, if the weight of positive electrode active material contained in the positive electrode is unknown, the weight of positive electrode active material can be determined by the analysis described below. Generally, a positive electrode has a positive electrode current collector and a positive electrode active material layer, and the positive electrode active material layer has positive electrode active material, a binder, and a conductive material. If the positive electrode is a coated electrode in which the positive electrode active material layer is coated on the metal foil of the positive electrode current collector, the positive electrode current collector and the positive electrode active material layer can be easily separated. Therefore, the weight ratio of the positive electrode current collector and the positive electrode active material layer in the positive electrode can be calculated by separating the positive electrode current collector and the positive electrode active material layer and measuring their weights. In addition, the weight of positive electrode active material contained in the positive electrode active material layer can be calculated by determining the weight ratio of positive electrode active material, binder, and conductive material through analysis.

[0034] For analyzing the weight ratio of positive electrode active material in a positive electrode active material layer, thermogravimetric differential thermal analysis (TG-DTA) can be used, for example. Specifically, using the positive electrode active material layer as a sample, TG-DTA analysis is performed in an inert atmosphere, followed by analysis in an oxygen-containing atmosphere. By measuring the weight changes in each analysis, the weight ratio of positive electrode active material, binder, and conductive material in the positive electrode active material layer can be calculated. For example, in an inert atmosphere, the temperature is raised from 25°C to 600°C at a rate of 20°C / min in a 100% He atmosphere, and then the weight loss is measured when the temperature is lowered from 600°C to 300°C at a rate of 20°C / min. This weight loss in the inert atmosphere analysis can be considered to be due to the decomposition of organic compounds such as PVDF used as a binder. Furthermore, as an analysis in an oxygen-containing atmosphere, for example, after the completion of the analysis in the inert atmosphere described above, 80% He and 20% O ２ In this atmosphere, the weight loss is measured when the temperature is increased from 300°C to 800°C at a rate of 20°C / min. The weight loss in this oxygen-containing atmosphere can be considered to be mainly due to the combustion of the conductive material, which is composed mainly of carbon. In this way, the weight ratio of the positive electrode active material in the positive electrode active material layer can be determined.

[0035] Li ｘ MO ２ The charging and / or discharging capacities used to calculate x should preferably be measured under conditions that are free from or minimize the effects of short circuits and / or decomposition of the electrolyte. For example, data from lithium-ion secondary batteries that have experienced a sudden change in capacity, which may be attributed to a short circuit, should not be used to calculate x.

[0036] Furthermore, the space group of the positive electrode active material, etc., is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification, "belonging to a certain space group," "being part of a certain space group," or "being a certain space group" can be rephrased as "being identified to a certain space group."

[0037] Furthermore, if the anion has a structure in which three layers are stacked with a slight offset from each other, such as ABCABC, it will be called a cubic close-packed structure. Therefore, the anion does not have to be strictly a cubic lattice. At the same time, since real crystals always have defects, the analytical results do not necessarily conform to theory. For example, in FFT (Fast Fourier Transform) patterns such as electron diffraction patterns or TEM (Transmission Electron Microscope) images, spots may appear at positions slightly different from the theoretical positions.

[0038] In this specification and other documents, positive electrode active material particles may be referred to as positive electrode active material, composite oxide, positive electrode material, positive electrode material, positive electrode material for secondary batteries, positive electrode material for lithium-ion secondary batteries, etc.

[0039] Furthermore, when describing the characteristics of individual particles in the following embodiments, it is not necessary for all particles to possess those characteristics. For example, if 50% or more, preferably 66% or more, and more preferably 90% or more of three or more randomly selected positive electrode active material particles possess those characteristics, it can be said that this is sufficient to improve the properties of the positive electrode active material and the secondary battery having it.

[0040] Unless otherwise specified, the materials of a secondary battery (positive electrode active material particles, negative electrode active material, electrolyte, separator, etc.) will be described in their state before degradation. Note that a decrease in discharge capacity due to aging and burn-in treatments during the secondary battery manufacturing process is not considered degradation. For example, a secondary battery consisting of a single cell or a battery pack can be considered to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. The rated capacity for secondary batteries for portable devices conforms to JIS C 8711:2019. For secondary batteries other than those specified above, the rated capacity conforms to various JIS and IEC standards, including those for electric vehicle propulsion and industrial use, not limited to the above JIS standard.

[0041] In this specification, the state of a secondary battery before material degradation may be referred to as the initial product or initial state, and the state after degradation (the state in which the discharge capacity is less than 97% of the rated capacity of the secondary battery) may be referred to as a used product or in use, or a used product or a used state.

[0042] In this specification, the (001) plane and the (003) plane, etc., may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C plane, basal plane, etc. Furthermore, lithium in lithium cobalt oxide has a two-dimensional diffusion pathway. That is, the diffusion pathway of lithium can be said to exist along the plane. In this specification, the plane where the diffusion pathway of lithium is exposed, that is, the plane where lithium is inserted and removed (specifically, the plane other than the (00l) plane), may be referred to as the edge plane.

[0043] In this specification, secondary particles refer to particles formed by the aggregation of primary particles. Primary particles refer to particles that do not have visible grain boundaries. Single particles refer to particles that do not have visible grain boundaries. Single crystals refer to crystals in which grain boundaries do not exist within the particles, while polycrystalline crystals refer to crystals in which grain boundaries exist within the particles. Polycrystalline crystals can also be described as aggregates of multiple crystallites, and a grain boundary can be described as an interface between two or more crystallites. In polycrystalline crystals, it is preferable that the orientation of the crystallites is aligned.

[0044] In this specification, the phrase "A and / or B" may be used, but this is just one example of how A alone, B alone, or A and B may be included.

[0045] A short circuit in a secondary battery can not only cause malfunctions in the charging and / or discharging operations of the secondary battery, but can also lead to overheating and ignition. To realize a safe secondary battery, it is desirable that short circuits be suppressed even at high charging voltages. The positive electrode of a battery according to one aspect of the present invention suppresses short circuits even at high charging voltages. Therefore, it is possible to create a battery that achieves both high discharge capacity and safety.

[0046] In this specification, ordinal numbers such as "first," "second," etc., are used to avoid confusion of constituent elements and do not indicate any order or rank, such as sequence or layering order. Even if a term in this specification does not have an ordinal number, an ordinal number may be added in the claims to avoid confusion of constituent elements. Even if a term in this specification has an ordinal number, a different ordinal number may be added in the claims. Even if a term in this specification has an ordinal number, an ordinal number may be omitted in the claims.

[0047] (Embodiment 1) This embodiment describes a battery and positive electrode active material particles according to one aspect of the present invention.

[0048] [Battery] A lithium-ion battery according to one aspect of the present invention comprises a positive electrode, a negative electrode, and an electrolyte. If the electrolyte includes an electrolyte solution, a separator is provided between the positive electrode and the negative electrode. Furthermore, a lithium-ion battery according to one aspect of the present invention may have an outer casing that covers at least a portion of the positive electrode, negative electrode, and electrolyte.

[0049] This embodiment will primarily describe the positive electrode and positive electrode active material particles of a battery according to one aspect of the present invention. The method for producing the positive electrode active material particles will be described in Embodiment 2. The details of the remaining components of the lithium-ion battery according to one aspect of the present invention will be described in Embodiment 3.

[0050] Figure 1A is a schematic cross-sectional view illustrating the internal structure of a lithium-ion battery 10. The lithium-ion battery 10 has a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11 has a positive electrode current collector 21 and a positive electrode active material layer 22 on the positive electrode current collector 21, and the negative electrode 12 has a negative electrode current collector 31 and a negative electrode active material layer 32. As shown in the figure, the positive electrode active material layer 22 and the negative electrode active material layer 32 face each other with the separator 13 in between. Although not shown in Figure 1A, the electrolyte is contained in the voids of the positive electrode active material layer 22, the voids of the separator 13, and the voids of the negative electrode active material layer 32.

[0051] Although Figure 1A shows one positive electrode 11, one negative electrode 12, and one separator 13, the lithium-ion battery according to one embodiment of the present invention is not limited to this structure. The lithium-ion battery according to one embodiment of the present invention may have a structure having two positive electrodes 11, two negative electrodes 12, and two separators 13, and may have even more electrodes stacked. Furthermore, the lithium-ion battery according to one embodiment of the present invention may have a wound structure instead of the stacked structure shown in Figure 1A.

[0052] Figure 1B is an enlarged view of area A, which is enclosed by a dashed line in Figure 1A.

[0053] The positive electrode active material layer 22 comprises positive electrode active material particles 100 (also called first positive electrode active material particles), second positive electrode active material particles 200, and a conductive material 41. Although not shown in the figures, the positive electrode active material layer 22 may also have a binder in addition to the positive electrode active material particles 100, second positive electrode active material particles 200, and conductive material 41.

[0054] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with electrolyte 51, as shown in the figure. For example, it is preferable that 60% or more of the voids in the positive electrode active material layer 22 are filled with electrolyte 51, more preferably 70% or more, more preferably 80% or more, more preferably 90% or more, more preferably 95% or more, and most preferably 99% or more. Note that the voids in the positive electrode active material layer 22 refer to the areas in the positive electrode active material layer 22 other than the solid components (positive electrode active material particles, conductive material, etc.).

[0055] [Positive electrode] The positive electrode 11 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 has positive electrode active material particles 100.

[0056] <Positive Electrode Active Material Particles 100> The positive electrode active material particles 100 have the function of taking in lithium ions and releasing them during charging and discharging. The positive electrode active material particles used in one embodiment of the present invention can be made of a material that does not degrade much during charging and discharging (hereinafter also referred to as "charging and discharging"), even at high charging voltages (hereinafter also referred to as "high charging voltage"). Specifically, positive electrode active material particles (composite oxides) with a particle size (median diameter (D50)) of 10 μm or more and 50 μm or less, preferably 10 μm or more and 25 μm or less, obtained by the method for producing positive electrode active material particles described in Embodiment 2 can be used. The positive electrode active material particles 100 contain one or more of the additive elements X, Y, and Z. The additive elements X, Y, and Z will be described in detail in <Contained Elements>. Additive elements X, Y, and Z may be collectively referred to as additive element A.

[0057] The positive electrode active material particles 100 are the main constituent material of the positive electrode active material layer 22. The weight of the positive electrode active material particles 100 is preferably 50% or more, more preferably 60% or more, and more preferably 70% or more of the weight of the solid components of the positive electrode active material layer 22. If the particle size of the positive electrode active material particles 100 is too small, the surface area will become too large, which may lead to excessive reaction between the surface of the positive electrode active material particles and the electrolyte. For this reason, the particle size (median diameter (D50)) of the positive electrode active material particles is preferably 10 μm or more. Furthermore, if the particle size of the positive electrode active material particles is larger than the thickness of the positive electrode active material layer 22, which will be described later, the particle density of the positive electrode active material layer 22 cannot be increased. Therefore, it is preferable that the largest particle size be 50 μm or less.

[0058] Particle size can be measured using a particle size analyzer (laser diffraction particle size distribution analyzer) that employs laser diffraction and scattering methods. D50 is the particle size at which the cumulative amount of the cumulative curve of the particle size distribution measurement results reaches 50%. The measurement of particle size is not limited to laser diffraction particle size distribution measurement; the major axis of the particle cross-section may also be measured by analysis using SEM (Scanning Electron Microscope) or TEM. As a method of measuring D50 from analysis using SEM or TEM, for example, the particle sizes of 20 or more particles can be measured, a cumulative curve can be created, and the particle size at which the cumulative amount reaches 50% can be defined as D50.

[0059] Furthermore, it is preferable that all active material particles in the positive electrode active material layer 22 (i.e., positive electrode active material particles 100 and second positive electrode active material particles 200) have a small difference in charging depth during charging and discharging. This is because if the difference in charging depth is large, particles with a higher charging depth are more likely to deteriorate. To mitigate the difference in charging depth, the conductive material 41 and / or binder in the positive electrode active material layer 22 do not necessarily have to be uniformly distributed. For example, particles near the current collector may have a higher charging depth, while particles further away from the current collector may have a lower charging depth. Therefore, it is preferable that the conductive material 41 in the region far from the current collector is greater than in the region near the current collector.

[0060] Unless otherwise specified in this specification, "charging voltage" shall be expressed with reference to the potential of lithium metal. Furthermore, in this specification, "high charging voltage" refers to a charging voltage of 4.5V or higher, preferably 4.55V or higher, more preferably 4.6V or higher, 4.65V or higher, or 4.7V or higher.

[0061] Furthermore, as stated above, in this specification, "high charging voltage" is defined as 4.6V or higher based on the potential when the negative electrode is lithium metal. However, when the negative electrode is a carbon material (e.g., graphite), 4.5V or higher shall be referred to as "high charging voltage." In short, in the case of a half-cell using lithium metal as the negative electrode, a charging voltage of 4.6V or higher shall be called a high charging voltage, and in the case of a full-cell using a carbon material (e.g., graphite) as the negative electrode, a charging voltage of 4.5V or higher shall be called a high charging voltage.

[0062] The positive electrode active material particles 100, which degrade less with repeated charging and discharging at high charging voltages, will be explained using Figures 2A to 3F.

[0063] Figures 2A and 2B are cross-sectional views of positive electrode active material particles 100 according to one embodiment of the present invention.

[0064] The positive electrode active material particles 100 have a layered rock salt type crystal structure belonging to space group R-3m, and a rock salt type crystal structure located on the surface side of the layered rock salt type crystal structure. Furthermore, if a carbon-containing coating or the like exists outside the positive electrode active material particles 100, the positive electrode active material particles 100 can be said to have a layered rock salt type crystal structure and a rock salt type crystal structure between the layered rock salt type crystal structure and the carbon-containing coating. Note that lithium cobalt oxide (LiCoO) has a layered rock salt type crystal structure belonging to space group R-3m. ２ ) consists of a lithium layer and a layer made up of octahedrons of cobalt and oxygen (CoO ２ It has a layered structure in which layers (also called layers) are stacked alternately.

[0065] As shown in Figure 2A, the positive electrode active material particle 100 has a surface layer 100a and an interior 100b. In Figure 2A, the boundary between the surface layer 100a and the interior 100b is indicated by a dashed line.

[0066] The surface layer 100a of the positive electrode active material particles 100 refers to a region within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm, perpendicular or approximately perpendicular from the surface inward. Perpendicular or approximately perpendicular means an angle of 80° to 100°. Surfaces formed by cracks and / or fissures may also be considered the surface. The surface layer 100a is synonymous with the vicinity of the surface, the vicinity of the surface region, or the shell.

[0067] Furthermore, the region deeper than the surface layer 100a of the positive electrode active material particles is called the interior 100b. The interior 100b is synonymous with the interior region or core.

[0068] Furthermore, when the positive electrode active material particles 100 have a layered rock salt type crystalline structure of space group R-3m, the surface layer 100a has an edge region 100a1 and a basal region 100a2, as shown in Figure 2B. In Figures 2A and 2B, the line labeled (00l) represents the (00l) plane. Here, the edge region 100a1 has a surface exposed in a direction intersecting the (00l) plane, and the region extending perpendicularly or substantially perpendicularly inward from this surface within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm is called the edge region 100a1. Here, "intersecting" means that the angle formed by the perpendicular of the first plane ((00l) plane) and the normal of the second plane (the surface of the positive electrode active material particles 100) is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less.

[0069] Furthermore, the basal region 100a2 has a surface parallel to the (00l) plane, and the region extending perpendicularly or substantially perpendicularly inward from this surface within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm is called the basal region 100a2. Here, "parallel" means that the angle formed by the perpendicular of the first plane ((00l) plane) and the normal of the second plane (the surface of the positive electrode active material particles 100) is 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0070] The surface of the positive electrode active material particles 100 refers to the surface of the composite oxide, including the surface layer 100a and the interior 100b. The positive electrode active material particles 100 need to have a redox-capable transition metal M in order to maintain charge neutrality even when lithium ions are inserted and removed. In the positive electrode active material particles 100, the transition metal M is responsible for the redox reaction, and is redox-transformed as lithium ions are inserted and removed. When the positive electrode active material particles are subjected to analysis, a protective film may be applied to the surface, but this protective film is not included in the positive electrode active material particles. As a protective film, a single-layer or multi-layer film of carbon, metal, oxide, resin, etc., may be used.

[0071] The position of the surface of the positive electrode active material particles 100 can be determined, for example, using a High-Angle Annular Dark Field (HAADF) STEM image. In a HAADF STEM image, a contrast proportional to the square of the atomic number is obtained. Specifically, elements with larger atomic numbers have higher brightness (brighter), and elements with smaller atomic numbers have lower brightness (darker). Since lithium has a small atomic number of 3, lithium is observed with very low brightness in a HAADF STEM image, or it becomes difficult to observe lithium at all. The same is true for oxygen, which also has a small atomic number. On the other hand, transition metals M (e.g., cobalt) have large atomic numbers and are therefore observed with high brightness. Thus, in a HAADF STEM image of the cross-section of the positive electrode active material particles 100, high-brightness spots are thought to correspond to transition metals M, and the position of the high-brightness spot closest to the outside can be considered the surface of the positive electrode active material particles 100. Furthermore, the added element A is observed with brightness corresponding to its concentration and atomic number. For example, if an added element A with a low atomic number is substituted for a cobalt site, the brightness at the cobalt site decreases as the concentration of added element A increases. Similarly, if an added element A with a high atomic number is substituted for a lithium site, the brightness at the cobalt site increases as the concentration of added element A increases.

[0072] Furthermore, metal oxides (e.g., aluminum oxide) that adhere to the surface of the positive electrode active material particles 100 and do not have lithium sites that can contribute to charging and discharging are not included in the positive electrode active material particles 100. Examples of such metal oxides include metal oxides whose crystal orientation does not match that of the interior 100b.

[0073] The matching or approximate matching of crystal orientations in two regions can be determined from TEM images, STEM images, HAADF-STEM images, ABF (Annular Bright Field)-STEM images, electron diffraction patterns, etc. It can also be determined from the FFT patterns of TEM images and STEM images. Furthermore, XRD and neutron diffraction can also be used as indicators.

[0074] In this specification, electrolytes, electrolyte decomposition products, organic solvents, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material particles 100 are not treated as part of the positive electrode active material particles 100, but are considered separate from the positive electrode active material particles 100.

[0075] <Elements contained> The positive electrode active material particles 100 contain lithium, cobalt, oxygen, and additive element A. Alternatively, the positive electrode active material particles 100 contain lithium cobalt oxide (LiCoO ２ It is possible to have a material in which additive element A is added to ). Therefore, the composition of lithium cobaltate is not strictly limited to Li:Co:O = 1:1:2. Furthermore, it is preferable that the positive electrode active material particles 100 in one embodiment of the present invention have the crystalline structure described later.

[0076] The positive electrode active material particles of a lithium-ion secondary battery need to contain a redox-capable transition metal in order to maintain charge neutrality even when lithium ions are inserted and removed. In one embodiment of the present invention, it is preferable that the positive electrode active material particles 100 mainly use cobalt as the transition metal responsible for the redox reaction. In addition to cobalt, at least one or more selected from nickel and manganese may be used. It is preferable that the cobalt content of the transition metals in the positive electrode active material particles 100 is 75 atomic percent or more, preferably 90 atomic percent or more, and more preferably 95 atomic percent or more, as this offers many advantages, such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics.

[0077] Furthermore, if the cobalt content among the transition metals of the positive electrode active material particles 100 is 75 atomic percent or more, preferably 90 atomic percent or more, and more preferably 95 atomic percent or more, then lithium nickelate (LiNiO) ２ Compared to composite oxides in which nickel, such as ), makes up the majority of the transition metals, Li ｘ CoO ２ Stability is better when x is small. This is thought to be because cobalt is less affected by strain due to the Jahn-Teller effect than nickel. In transition metal compounds, the strength of the Jahn-Teller effect varies depending on the number of electrons in the d orbital of the transition metal. In layered rock salt type composite oxides in which octahedral low-spin nickel(III) ions, such as lithium nickelate, make up the majority of the transition metal, the Jahn-Teller effect is significant, and strain is likely to occur in the layers consisting of octahedra of nickel and oxygen. Therefore, there is a growing concern that the crystal structure may collapse during charge-discharge cycles. Also, nickel ions are larger than cobalt ions and are close in size to lithium ions. Therefore, in layered rock salt type composite oxides in which nickel makes up the majority of the transition metal, such as lithium nickelate, there is a problem that cation mixing of nickel and lithium is likely to occur.

[0078] It is preferable to use one or more elements selected from magnesium, fluorine, nickel, aluminum, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, and beryllium as the additive element A in the positive electrode active material particles 100. Furthermore, when the total amount of additive element A in the positive electrode active material particles 100 is set to 100 atomic%, if the sum of transition metals among the additive element A is too high, the chargeable and dischargeable capacity of the positive electrode active material particles 100 may decrease. Therefore, it is preferable that the sum be less than 25 atomic%, more preferably less than 10 atomic%, and even more preferably less than 5 atomic%.

[0079] In other words, the positive electrode active material particles 100 can be one or more of the following: lithium cobalt oxide having magnesium, lithium cobalt oxide having magnesium and aluminum, lithium cobalt oxide having magnesium and nickel, lithium cobalt oxide having magnesium, aluminum and nickel, lithium cobalt oxide having magnesium and fluorine, lithium cobalt oxide having magnesium, fluorine and nickel, lithium cobalt oxide having magnesium, fluorine, nickel and aluminum, etc.

[0080] Furthermore, it can be said that one or more of the following can be used as positive electrode active material particles 100: positive electrode active material particles having cobalt, oxygen, and magnesium; positive electrode active material particles having cobalt, oxygen, magnesium, and aluminum; positive electrode active material particles having cobalt, oxygen, magnesium, aluminum, and nickel; positive electrode active material particles having cobalt, oxygen, magnesium, and fluorine; positive electrode active material particles having cobalt, oxygen, magnesium, fluorine, and aluminum; positive electrode active material particles having cobalt, oxygen, magnesium, fluorine, and nickel; and positive electrode active material particles having cobalt, oxygen, magnesium, fluorine, nickel, and aluminum.

[0081] It is preferable that the additive element A is solid-dissolved in the positive electrode active material particles 100. For example, when performing STEM-EDX line analysis, it is preferable that the position where additive element A begins to be detected in the depth direction is deeper than the position where transition metal M begins to be detected, i.e., located inside the positive electrode active material particles 100. In STEM-EDX line analysis in the depth direction, the position where an element begins to be detected refers to the position where the amount of characteristic X-rays detected (count) due to that element begins to increase continuously.

[0082] These additive elements A make the crystal structure of the positive electrode active material particles 100 more stable, as will be described later.

[0083] Additive element A does not necessarily have to include magnesium, fluorine, nickel, aluminum, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, barium, bromine, or beryllium.

[0084] Furthermore, if the positive electrode active material particles 100 are substantially free of titanium, it becomes possible to enhance the effect of suppressing capacity degradation when repeatedly charging and discharging at high voltage. For example, the weight of titanium contained in the positive electrode active material particles 100 is preferably 600 ppm or less, and more preferably 100 ppm or less. Also, when the positive electrode active material particles 100 are subjected to STEM-EDX analysis, it is preferable that no characteristic X-rays attributable to titanium are detected, that is, they are below the detection limit (for example, less than 0.3 atomic percent).

[0085] The surface layer 100a is the region where lithium ions first desorb during charging, and the lithium concentration tends to be lower in this region than in the interior 100b. Furthermore, the atoms on the surface of the positive electrode active material particles 100 in the surface layer 100a can be described as having some of their bonds broken. Therefore, the surface layer 100a is prone to instability, and is a region where degradation of the crystal structure is likely to begin. On the other hand, if the surface layer 100a can be made sufficiently stable, Li ｘ CoO ２Even when the internal x is small, for example, when x is 0.24 or less, the layered structure consisting of octahedrons of cobalt and oxygen in the internal 100b can be made less prone to breakage. Furthermore, the displacement of the layers consisting of octahedrons of cobalt and oxygen in the internal 100b can be suppressed.

[0086] In order to ensure a stable composition and crystal structure for the surface layer 100a, it is preferable that the surface layer 100a contains additive element A, and more preferably that it contains multiple additive element A. It is also preferable that the surface layer 100a has a higher concentration of one or more selected additive element A than the interior layer 100b. Furthermore, it is preferable that the one or more selected additive element A present in the positive electrode active material particles 100 have a concentration gradient. It is also more preferable that the distribution of the positive electrode active material particles 100 differs depending on the additive element A. For example, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element A. Here, the concentration peak refers to the maximum value of concentration in the surface layer 100a or below 50 nm from the surface.

[0087] [Distribution] The distribution of additive element A will be explained. Figures 3A to 3C are enlarged views of the area around A-B in Figure 2B, illustrating the edge region 100a1 of the positive electrode active material particles 100. Figures 3D to 3F are enlarged views of the area around C-D in Figure 2B, illustrating the basal region 100a2 of the positive electrode active material particles 100.

[0088] For example, some of the additive elements A, such as magnesium, fluorine, silicon, phosphorus, and boron, preferably have a concentration gradient that increases from the interior 100b towards the surface, as shown by the density of the hatches in Figures 3A and 3D. Additive elements A having such a concentration gradient will be called additive elements X.

[0089] Another additive element A, such as aluminum or manganese, preferably has a concentration gradient as shown by the density of the hatches in Figures 3B and 3E, and has a concentration peak in a region deeper than that of additive element X shown in Figures 3A and 3D. The concentration peak may be located in the surface layer 100a or deeper than the surface layer 100a. For example, it is preferable that the peak is in a region of 5 nm to 30 nm from the surface inward. Additive element A having such a concentration gradient will be called additive element Y.

[0090] Other additive elements A, such as nickel and barium, may be clearly present in the edge region 100a1, but substantially absent in the basal region 100a2, as shown by the presence or absence and density of hatches in Figures 3C and 3F. Here, "clearly present" means that the characteristic X-ray energy spectrum of additive element A is detected in the cross-sectional STEM-EDX analysis of the positive electrode active material particles 100. "Substantially absent" means that the characteristic X-ray energy spectrum of additive element A is not detected in the cross-sectional STEM-EDX analysis of the positive electrode active material particles 100. This can also be said to mean that additive element A is below the detection limit in the STEM-EDX analysis. Additive element A having such a distribution will be called additive element Z.

[0091] For example, magnesium ions, one of the additive elements X, are divalent, and since magnesium ions are more stable in lithium sites than in cobalt sites in layered rock salt crystal structures, they readily occupy lithium sites. Furthermore, magnesium present in cobalt or lithium sites in layered rock salt crystal structures is oxygen-6 coordinate, with six oxygen atoms surrounding each magnesium atom. The same is true for rock salt-type magnesium oxide (MgO). Therefore, magnesium oxide is, for example, Mg·n(O) １／ｎ It can also be expressed as (n=6). The presence of magnesium at an appropriate concentration in the lithium site of the surface layer 100a makes it easier to maintain a layered rock salt type crystal structure. This is because the magnesium present in the lithium site is CoO ２It is presumed that this is because it functions as a pillar to support the layers. Also, the presence of magnesium means that Li ｘ CoO ２ When the internal x is, for example, 0.24 or less, the desorption of oxygen around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material particles 100. In addition, if the magnesium concentration in the surface layer 100a is high, it is expected that the corrosion resistance to hydrofluoric acid produced when the electrolyte reacts with water will improve. On the other hand, if the magnesium concentration is insufficient, the surface layer 100a may be eroded by hydrofluoric acid and dissolved into the electrolyte, which may prevent sufficient suppression of the phase change in the interior 100b.

[0092] At appropriate concentrations, magnesium does not adversely affect lithium insertion and removal during charging and discharging, and the above benefits can be enjoyed. However, excessive magnesium may inhibit lithium insertion and removal. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters not only lithium sites but also cobalt sites. In addition, excess magnesium compounds (oxides or fluorides, etc.) that do not substitute for lithium or cobalt sites may segregate on the surface of the positive electrode active material particles, potentially becoming a resistive component of the secondary battery. Moreover, as the magnesium concentration of the positive electrode active material particles increases, the discharge capacity of the positive electrode active material particles may decrease. This is thought to be because too much magnesium enters the lithium sites, reducing the amount of lithium that contributes to charging and discharging.

[0093] Therefore, it is preferable that the total amount of magnesium contained in the positive electrode active material particles 100 is appropriate. For example, the number of magnesium atoms is preferably 0.001 times or more and 0.1 times or less the number of cobalt atoms, more preferably greater than 0.01 times and less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium contained in the total positive electrode active material particles 100 referred to here may be the value obtained by performing an elemental analysis of the entire positive electrode active material particles 100 using, for example, GD-MS (Glow Discharge Mass Spectrometry) or ICP-MS (Inductively Coupled Plasma-Mass Spectrometry), or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material particles 100.

[0094] Furthermore, the thermal stability of the positive electrode active material particles 100 is improved due to the effect of aluminum, one of the additive elements Y. Therefore, the presence of aluminum as additive element Y improves the safety when the positive electrode active material particles 100 are used in secondary batteries. In addition, the positive electrode active material particles 100 can be made less susceptible to structural collapse even after repeated charging and discharging.

[0095] On the other hand, an excess of aluminum may negatively affect the insertion and removal of lithium.

[0096] Therefore, it is preferable that the total amount of aluminum in the positive electrode active material particles 100 is appropriate. For example, the total number of aluminum atoms in the positive electrode active material particles 100 is preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.3% to 1.5% of the total number of cobalt atoms. Alternatively, 0.05% to 2% is preferred. Alternatively, 0.1% to 4% is preferred. The total amount of aluminum in the positive electrode active material particles 100 referred to here may be, for example, the value obtained by elemental analysis of the entire positive electrode active material particles 100 using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material blend during the manufacturing process of the positive electrode active material particles 100.

[0097] Furthermore, nickel can be present in both the cobalt site and the lithium site of lithium cobalt oxide. When nickel is present in the cobalt site of lithium cobalt oxide, the reaction potential for lithium ion insertion and deinsertion is lower compared to when cobalt is present in the cobalt site, which leads to an increase in discharge capacity and is therefore preferable.

[0098] Furthermore, when nickel is present at the lithium site, the displacement of the layered structure consisting of octahedrons of cobalt and oxygen can be suppressed. Also, volume changes associated with charging and discharging are suppressed. Additionally, the elastic modulus increases, meaning the positive electrode active material particles 100 become harder. This is because nickel present at the lithium site also contributes to the CoO ２ This is presumed to be because it functions as a pillar supporting the layers. Therefore, it is desirable that the crystal structure becomes more stable, especially in the charged state at high temperatures, such as 45°C or higher.

[0099] On the other hand, an excess of nickel is undesirable because it intensifies the distortion caused by the Jahn-Teller effect. Furthermore, an excess of nickel may adversely affect lithium insertion and removal.

[0100] Therefore, it is preferable that the total amount of nickel in the positive electrode active material particles 100 is appropriate. For example, the number of nickel atoms in the positive electrode active material particles 100 is preferably more than 0% and 7.5% or less of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferable that it is more than 0% and 4% or less. Alternatively, it is preferable that it is more than 0% and 2% or less. Alternatively, it is preferable that it is more than 0% and 7.5% or less. Alternatively, it is preferable that it is more than 0% and 2% or less. Alternatively, it is preferable that it is more than 0% and 7.5% or less. Alternatively, it is preferable that it is more than 0% and 4% or less. The amount of nickel shown here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material particles using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material blend during the process of manufacturing the positive electrode active material particles.

[0101] Furthermore, as shown in Figures 3A and 3C, when the surface layer 100a contains both magnesium and nickel, divalent nickel may be able to exist more stably near divalent magnesium. Therefore, Li ｘ CoO ２ Even when the internal x is small, the elution of magnesium can be suppressed. Therefore, this can contribute to the stabilization of the surface layer 100a.

[0102] Furthermore, having multiple types of additive elements A with different distributions, such as additive elements X, Y, and Z, is preferable because it can stabilize the crystal structure over a wider area. For example, if the positive electrode active material particles 100 contain magnesium, which is one of the additive elements X, aluminum, which is one of the additive elements Y, and nickel, which is one of the additive elements Z, it can stabilize the crystal structure over a wider area than when it contains only one or two of the additive elements X, Y, and Z. Thus, when the positive electrode active material particles 100 contain three types of additive elements X, Y, and Z, surface stabilization can be sufficiently achieved by additive elements X such as magnesium and additive elements Z such as nickel, so additive element Y such as aluminum is not essential for the surface. Rather, it is preferable that aluminum is widely distributed slightly inward from the surface. For example, it is preferable that aluminum is continuously detected in the region from 1 nm to 25 nm in depth from the surface. It is preferable that aluminum is widely distributed in this way because it can stabilize the crystal structure over a wider area.

[0103] Furthermore, when the additive element Z is abundantly present in the edge region 100a1 (also referred to as preferentially present or selectively present) as shown in Figures 3C and 3F, it is preferable because it improves the stability of the crystal structure of the edge region 100a1 where lithium ions enter and exit the positive electrode active material particles 100 during charging and discharging of the lithium-ion battery. In addition, when the additive element Z has the distribution described above, it is preferable because it can minimize the effects of adding the additive element Z, such as a decrease in discharge voltage or a decrease in discharge capacity.

[0104] When the positive electrode active material particles 100 have multiple additive elements A as described above, the effects of each additive element A synergistically contribute to further stabilization of the surface layer 100a. In particular, the presence of magnesium, nickel, and aluminum is highly preferable as it is more effective in achieving a stable composition and crystal structure. Among these, it is preferable that the surface layer 100a of the positive electrode active material particles 100 has a region where magnesium is distributed closer to the surface than aluminum. Furthermore, in addition to the regions where magnesium and aluminum are distributed as described above, it is most preferable that the edge region 100a1 of the surface layer 100a of the positive electrode active material particles 100 has a region where the distribution of nickel and magnesium overlap.

[0105] <Crystal structure> <Li ｘ CoO ２ When x inside is 1 > The positive electrode active material particle 100 in one aspect of the present invention is in a discharge state, that is, Li ｘ CoO ２ When x = 1, it has a layered rock salt type crystal structure belonging to space group R-3m. Layered rock salt type composite oxides have high discharge capacity, possess a two-dimensional lithium ion diffusion pathway, and are suitable for lithium ion insertion / desorption reactions, making them excellent as positive electrode active material particles for secondary batteries. For this reason, it is particularly preferable that the interior 100b, which accounts for most of the volume of the positive electrode active material particle 100, has a layered rock salt type crystal structure.

[0106] On the other hand, in one embodiment of the present invention, it is preferable that the surface layer 100a of the positive electrode active material particles 100 has a function to reinforce the layered structure of the interior 100b, which consists of an octahedron of transition metal M and oxygen, so that it does not break down even if lithium is removed from the positive electrode active material particles 100 due to charging. Alternatively, it is preferable that the surface layer 100a functions as a barrier film for the positive electrode active material particles 100. Alternatively, it is preferable that the surface layer 100a, which is the outer periphery of the positive electrode active material particles 100, reinforces the positive electrode active material particles 100. Reinforcement as used here means suppressing structural changes in the surface layer 100a and interior 100b of the positive electrode active material particles 100, including the desorption of oxygen.

[0107] Therefore, it is preferable that the surface layer 100a has a different crystal structure from the interior 100b. Furthermore, it is preferable that the surface layer 100a has a more stable composition and crystal structure than the interior 100b. Specifically, in one embodiment of the present invention, at least a portion of the surface layer 100a of the positive electrode active material particle 100 has a rock salt type crystal structure. Furthermore, it is preferable that at least a portion of the surface layer 100a has the characteristics of a spinel type crystal structure. Specifically, in the positive electrode active material particle 100, it is preferable that the region with the rock salt type crystal structure has a crystal structure with a spinel type skeleton on the interior 100b side. More specifically, LiTiO with space group Fd-3m, which has a crystal structure similar to the spinel type crystal structure. ２ It is preferable that the crystal structure has the characteristics of the type (ICSD Collection code 48128). Furthermore, the surface layer 100a is of the layered rock salt type, LiTiO with space group Fd-3m. ２ It is more preferable to have the characteristics of three crystal structures: spinel type and rock salt type. Although crystal structures with a spinel-type skeleton are sometimes collectively referred to as spinel-type crystal structures, strictly speaking, it is preferable to have Co with metal atoms at the 8a and 16d sites at the Wyckoff positions. ３ O ４ Crystal structures such as these are called spinel-type crystal structures. Therefore, even if a material is described in this specification as having the characteristics of a spinel-type crystal structure, it does not necessarily mean that it is Co ３ O ４ This does not mean that the structure matches a spinel-type crystal structure like the one shown. Furthermore, in this specification, LiTiO with space group Fd-3m ２ The type of crystal structure is sometimes referred to as the spinel' type crystal structure. Note that in this specification, the space group Fd-3m LiTiO ２ Regarding the description of a type crystal structure, the symmetry of the atomic positions in the crystal structure is LiTiO ２ This description is used to indicate that it has a specific crystal structure, and is not intended to specify the constituent elements, such as "composed of lithium, titanium, and oxygen."

[0108] Furthermore, while it is preferable that some of the additive elements A, particularly magnesium, nickel, and aluminum, are present at higher concentrations in the surface layer 100a than in the interior 100b, it is also preferable that they be present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites of the interior 100b, it has the effect of making it easier to maintain a layered rock salt-type crystal structure. Also, when nickel is present at an appropriate concentration in the interior 100b, displacement of the layered structure consisting of octahedra of transition metal M and oxygen can be suppressed. Furthermore, even when both magnesium and nickel are present, divalent magnesium may be able to exist more stably near divalent nickel, so a synergistic effect of suppressing magnesium leaching can be expected. Also, aluminum can be present in the cobalt sites in the layered rock salt-type crystal structure. Since aluminum is a trivalent typical element and its valency does not change, lithium around aluminum does not easily move during charging and discharging. Therefore, aluminum and the surrounding lithium are CoO ２ It functions as a pillar supporting the layers and can suppress changes in the crystal structure. Furthermore, aluminum suppresses the leaching of surrounding cobalt, improving continuous charging resistance and / or storage resistance in a fully charged state. Also, because the Al-O bond is stronger than the Co-O bond, it can suppress the detachment of oxygen from around the aluminum.

[0109] Furthermore, it is preferable that the crystal structure changes continuously from the interior 100b toward the surface, as shown in the concentration gradient of additive element A in Figures 3A to 3E. Alternatively, it is preferable that the crystal orientations of the surface layer 100a and the interior 100b are roughly the same.

[0110] For example, it is preferable that the crystal structure changes sequentially from the interior 100b of the layered rock salt type toward the surface, from the characteristics of the layered rock salt type crystal structure to the characteristics of the spinel type crystal structure, and from the characteristics of the spinel type crystal structure to the characteristics of the rock salt type crystal structure. More specifically, in the edge region 100a1, the crystal structure changes sequentially from the interior 100b toward the outside of the positive electrode active material particle 100, to the region of the layered rock salt type crystal structure, LiTiO ２It is preferable to have a structure in which a region of the type crystal structure and a region having a rock salt type crystal structure are arranged in that order. In other words, it is preferable that in the edge region 100a1, from the interior 100b toward the outside of the positive electrode active material particles 100, a region of the layered rock salt type crystal structure, a region of the spinel' type crystal structure, and a region having a rock salt type crystal structure are arranged in that order. It is also preferable that this change in crystal structure is continuous. Furthermore, it is preferable that the crystal orientations of the three crystal structures, the layered rock salt type, the spinel' type, and the rock salt type, are roughly the same.

[0111] In this specification, the layered rock salt crystal structure belonging to space group R-3m of a composite oxide containing lithium and a transition metal M refers to a crystal structure having a rock salt-type ionic arrangement in which cations and anions are arranged alternately, and in which the transition metal M and lithium are regularly arranged to form a two-dimensional plane. Therefore, two-dimensional diffusion of lithium is possible in this crystal structure. Defects such as vacancies in cations or anions may be present. Furthermore, strictly speaking, the layered rock salt crystal structure may have a distorted lattice structure of the rock salt crystal.

[0112] Furthermore, a rock salt-type crystal structure refers to a structure that has a cubic crystal system or a crystal structure that can be considered cubic, in which cations and anions are arranged alternately. It is acceptable for there to be vacancies in either the cations or anions.

[0113] Furthermore, the characteristics of layered rock salt crystal structures, spinel crystal structures, and rock salt crystal structures can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc.

[0114] Figure 4A is a cross-sectional HAADF-STEM image of the vicinity of the surface of the positive electrode active material particle 100, and Figure 4B is a cross-sectional ABF-STEM image of the vicinity of the surface of the positive electrode active material particle 100.

[0115] In cross-sectional HAADF-STEM images, a layered rock salt type crystal structure (e.g., LiCoO) is observed. ２When the region is observed from a direction perpendicular to the c-axis, layers of high-luminosity bright spots and layers of low-luminosity bright spots are observed alternately in the direction of the c-axis (for example, <010> incidence), as shown in the region labeled as layered rock salt type in Figure 4A. In Figures 4A and 4B, bright spots aligned in the direction indicated by the block arrows are called layers, and bright spots aligned in the direction indicated by the dashed arrows are called rows. Similarly, in this specification, for the bright spots in the HAADF-STEM image of the edge region 100a1 of the positive electrode active material particles 100 observed from a direction perpendicular to the c-axis, bright spots aligned in the direction perpendicular to the c-axis of the region having a layered rock salt structure are called layers. Bright spots aligned in the direction intersecting the layers are called rows, and the rows are selected as the arrangement of bright spots that is closest to parallel to the edge surface of the edge region 100a1.

[0116] Rock salt crystal structures (e.g., CoO) do not exhibit these characteristics because there is no distinction between cation sites. Instead, only layers with rows of highly luminous spots are observed, as shown in the region labeled "rock salt type" in Figure 4A.

[0117] LiTiO in Figure 4A ２ In the region labeled "type," layers consisting only of high-luminosity bright spots and layers in which high-luminosity bright spots 110 and regions 111 where no distinct bright spots are observed alternately (block arrows in the figure; also called lithium layers) appear alternately in the column direction. When such characteristics are present, the region is said to have the characteristics of a spinel-type crystal structure. Furthermore, the region having the characteristics of a spinel-type crystal structure is LiTiO with space group Fd-3m. ２ In the case of a type crystal structure, in the region 111 in the HAADF-STEM image of Figure 4A where no clear bright spots are observed, atoms are present at the Wyckoff position 16c, not the Wyckoff position 8a.

[0118] When the region 111 in the HAADF-STEM image of Figure 4A, where no clear bright spots are observed, is examined in the ABF-STEM image shown in Figure 4B, no atomic-derived contrast was observed at the Wyckoff position 8a (solid circle in Figure 4B). In other words, in the positive electrode active material particles 100 of one embodiment of the present invention shown in Figures 4A and 4B, LiTiO ２ The region labeled "type" is LiTiO of space group Fd-3m. ２ It can be determined that it is a type crystal structure.

[0119] As shown in Figure 4B, the ABF-STEM image exhibits similar characteristics to the HAADF-STEM image, although the brightness and contrast are reversed. The reason for this reversal is that in the HAADF-STEM image, strong bright spots appear where elements with high atomic numbers (e.g., cobalt) are present, while in the ABF-STEM image, strong bright spots appear where elements with low atomic numbers (e.g., lithium) are present.

[0120] The characteristics of a spinel-type crystal structure can also be determined by electron diffraction or TEM-FFT patterns.

[0121] In this specification, the row of metallic elements with atomic numbers greater than lithium (also called a row of bright spots or arrangement of bright spots) observed in the cross-sectional HAADF-STEM image of the positive electrode active material particle 100, that is closest to the surface, i.e., the outside, is referred to as the first row. The row of metallic elements with atomic numbers greater than lithium observed in the next position closest to the outside after the first row is referred to as the second row. The same applies to the third row and subsequent rows. In other words, the rows proceed inward from the first row, second row, third row, fourth row, and so on.

[0122] For example, the first to third rows in the edge region 100a1 preferably have a rock salt type crystal structure. The fourth to ninth rows have LiTiO with space group Fd-3m. ２ It is preferable that the crystal structure is of the type. From the 10th row onward, it is preferable that the crystal structure is of the layered rock salt type. In the cross-sectional HAADF-STEM image, when the first row on the surface side is not clearly observed, it is possible that the region having the rock salt type crystal structure may be observed in the first and second rows, or only in the first row. Therefore, when the region having the rock salt type crystal structure is observed in the first and second rows, it is preferable that the third to eighth rows have the LiTiO2 type crystal structure with space group Fd-3m, and so on, the ordinal number of the rows may be increased. Alternatively, the width of the row in which the region having the LiTiO2 type crystal structure with space group Fd-3m is observed may be small (e.g., the fourth to eighth rows), or it may be large (e.g., the fourth to tenth rows).

[0123] An example of observing a region exhibiting the characteristics of a spinel-type crystal structure from the <110> direction in a cross-sectional HAADF-STEM image of the edge region 100a1 will be described. In this observation, in the region, the characteristics of the crystal structure change sequentially from those of a layered rock salt type to those of a spinel-type crystal structure, and then from those of a spinel-type crystal structure to those of a rock salt type crystal structure, in the direction from the inside to the outside of the particle (depth direction). At this time, in the region exhibiting the characteristics of a layered rock salt type crystal structure, a first layer (lithium layer) without clear bright spots and a second layer (cobalt layer) adjacent to the first layer with a row of clear bright spots can be observed. If the viewpoint is shifted along the extension of the first layer in the direction from the inside to the outside of the particle (direction of the block arrow in Figure 4A), a characteristic of alternating arrangement of high-luminosity bright spots 110 and regions 111 without clear bright spots can be observed in the region exhibiting the characteristics of a spinel-type crystal structure.

[0124] Furthermore, in the region exhibiting spinel-type crystal structure characteristics in the cross-sectional HAADF-STEM image, LiTiO with space group Fd-3m is observed. ２ It may have a crystal structure of the type Fd-3m. LiTiO ２ The existence of the type of crystal structure can be confirmed by ABF-STEM imaging, as mentioned above. LiTiO with space group Fd-3m ２ LiCoO having a crystal structure of type ２ In this case, having lithium (Li) at position 16c, cobalt (Co) at position 16d, and oxygen (O) at position 32e is preferable because the lithium at position 16c is linearly aligned in the <110> direction, which allows for smooth movement of lithium during charging and discharging. Note that since positions 16c and 16d are equivalent positions in terms of site symmetry, there are also cases where cobalt is at position 16c, lithium is at position 16d, and oxygen is at position 32e.

[0125] Figure 5A shows LiTiO ２A schematic diagram of the crystal structure of type 5A is shown. In Figure 5A, for example, region A is a position where only the Wyckoff position 16c of space group Fd-3m overlaps in the <110> direction, and in region A, lithium sites at position 16c are lined up. Also, for example, regions B and C are positions where the Wyckoff position 16c and the Wyckoff position 16d of space group Fd-3m overlap in the <110> direction, and in regions B and C, lithium sites at position 16c and cobalt sites at position 16d are lined up. Also, for example, region D is a position where only the Wyckoff position 16d of space group Fd-3m overlaps in the <110> direction, and in region D, cobalt sites at position 16d are lined up.

[0126] Region A shown in Figure 5A is a region where lithium sites are aligned in the <110> direction, and lithium diffusion is possible in this direction. LiTiO ２ In the crystal structure of this type, the lithium diffusion pathway exemplified in region A above exists in three dimensions. LiTiO ２ The three-dimensional lithium diffusion pathways of the crystal structure of this type will be explained using Figure 5B.

[0127] The <110> direction, which is the diffusion pathway for lithium, specifically exists in six directions:

[110] , [0-11],

[101] ,

[011] , [10-1], and [1-10]. In Figure 5B, LiTiO ２ Of the six directions of lithium diffusion pathways in the three-dimensional direction of the crystal structure of this type, three directions (direction 1, direction 2, and direction 3) are shown as representative. Direction 1 is shown as

[110] , direction 2 as

[011] , and direction 3 as [0-11]. In all directions, lithium sites are lined up in the thick lines labeled Li in the figure, indicating that there are lithium diffusion pathways in the three-dimensional direction.

[0128] Figures 6A to 6F show the six diffusion paths of lithium in more detail. In Figures 6A to 6F, LiTiO in space group Fd-3m ２ LiCoO with a crystal structure of the type ２The diagrams show schematic representations of different lithium diffusion pathways, indicated by dashed arrows. The arrow in Figure 6A indicates the

[110] direction, the arrow in Figure 6B indicates the [0-11] direction, the arrow in Figure 6C indicates the

[101] direction, the arrow in Figure 6D indicates the

[011] direction, the arrow in Figure 6E indicates the [10-1] direction, and the arrow in Figure 6F indicates the [1-10] direction.

[0129] The region exhibiting the characteristics of a spinel-type crystal structure is LiTiO with space group Fd-3m. ２ LiCoO having a crystal structure of type ２ To determine whether or not this is the case, for example, it can be determined by checking whether an atom is present at position 8a in an ABF-STEM image or a HAADF-STEM image. If an atom is not present at position 8a and an atom is present at position 16c, then it is LiTiO of space group Fd-3m. ２ It can be said that it is a type of crystal structure. Alternatively, in a region where the characteristics of a spinel-type crystal structure are confirmed by HAADF-STEM imaging, if STEM-EELS (Electron Energy-Loss Spectroscopy) analysis confirms an atomic column of lithium aligned in the <110> direction, an atomic column of lithium and cobalt aligned, and an atomic column of cobalt aligned, then it is a LiTiO with space group Fd-3m. ２ LiCoO having a crystal structure of type ２ It can be said that, in the region where the characteristics of a spinel-type crystal structure are confirmed by the HAADF-STEM image, LiTiO of space group Fd-3m ２ There are two methods for determining whether a crystal has a specific crystal structure: the ABF-STEM method and the STEM-EELS method. Note that LiTiO has a space group Fd-3m. ２ Either of the above methods may be used to obtain the type of crystal structure.

[0130] The above LiTio ２In a region having a ２ -type crystal structure, it is preferable to have magnesium and nickel of additive element A at the lithium site (Wyckoff position 16c). Further, it is preferable to have nickel of additive element A at the transition metal M site (cobalt site, Wyckoff position 16d). By having magnesium and nickel of additive element A at such positions,

[0131] the ２ -type crystal structure can be stabilized and the desorption of oxygen can be suppressed.

[0132] Here, the case of an R-3m layered rock salt type structure and the case of a ２ -type crystal structure of space group Fd-3m will be compared, and the results of simulations focusing on cobalt, nickel, and titanium to explain which has higher structural stability will be described. ２ In ２ LiCoO ２ [[ID= | comparing the case of an R-3m layered rock salt type structure and the case of a ２ -type crystal structure of space group Fd-3m, the case of the R-3m layered rock salt type structure has higher structural stability. Further, in ２ LiNiO ２ comparing the case of an R-3m layered rock salt type structure and the case of a

[0133] -type crystal structure of space group Fd-3m, the case of the [[ |-type crystal structure has higher structural stability. Further, in ２

[0134] comparing the case of an R-3m layered rock salt type structure and the case of a -type crystal structure of space group Fd-3m, the case of the R-3m layered rock salt type structure has higher structural stability. Note that all of the above calculations are based on a calculation model assuming a fully charged state where all Li has been desorbed.

[0134] As described above, in the case of a ２ -type crystal structure of space group Fd-3m, it can be said that having nickel at the transition metal M site (Wyckoff position 16d) can stabilize the structure more than when cobalt is at the same position and when titanium is at the same position.

[0134] Layered rock salt-type crystals, crystals having the characteristics of a spinel-type crystal structure (for example, LiTiO ２ type), and the anions of rock salt-type crystals take a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions of the O3'-type crystals described later also take a cubic close-packed structure. Therefore, when the layered rock salt-type crystal structure, the crystal structure of the LiTiO ２ type of space group Fd-3m, and the rock salt-type crystal structure are in contact with each other, there are crystal planes in which the orientations of the cubic close-packed structures composed of anions are aligned. For example, as shown in the schematic diagram of the crystal structure in FIG. 7, it is possible to take a structure in which the rock salt-type crystal structure, the crystal structure of the LiTiO ２ type of space group Fd-3m, and the cubic close-packed structure of the layered rock salt-type crystal structure are in contact with each other so that their orientations are aligned. Note that FIG. 7 shows the crystal structure of the edge region 100a1 of the positive electrode active material particles 100 of one aspect of the present invention considered from the examples described later. In the crystal structure of the LiTiO ２ type of space group Fd-3m (Non-Patent Documents 13 and 14), Co is trivalent, but in the positive electrode active material particles 100 of one aspect of the present invention, as shown in FIG. 7, in the cobalt layer, Co ２＋ and Co ３＋ are confirmed to be alternately arranged. This is considered to suggest that the added element A is contained in the region of the crystal structure of the LiTiO ２ type of space group Fd-3m in the positive electrode active material particles 100 of one aspect of the present invention.

[0135] The fact that the rock salt-type crystal structure, the crystal structure of the LiTiO ２ type of space group Fd-3m, and the layered rock salt-type crystal structure are in contact with each other so that they are aligned can also be explained as follows. The rock salt-type crystal structure and the LiTiO ２The cubic crystal structure is cubic, and the anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt crystal structure has a space group R-3m and is a rhombohedral structure, but to facilitate understanding of the structure, it can be represented by a composite hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice of the {111} plane of the cubic crystal has a similar atomic arrangement to the hexagonal lattice of the (0001) plane of the layered rock salt. The consistency between the two lattices can be described as the orientation of the cubic close-packed structure being aligned.

[0136] Thus, LiTiO of the space group Fd-3m ２ In this type of crystal structure, lithium can diffuse in three dimensions. Therefore, LiTiO with space group Fd-3m mainly exists between the rock salt crystal structure region and the layered rock salt crystal structure region. ２ Positive electrode active material particles 100 having a surface structure as shown in Figure 7, which includes a region of a specific type of crystal structure, are preferable because they have low lithium ion diffusion resistance and promote lithium ion insertion and deinsertion. Furthermore, using these positive electrode active material particles in a secondary battery is preferable because it suppresses the decrease in discharge capacity even during high-rate discharge and allows for the acquisition of a large discharge capacity.

[0137] However, the layered salt-type space group is R-3m, and the salt-type and LiTiO ２ Because it differs from the space group of the cubic crystal system, the Miller indices of crystal planes that satisfy the condition for the orientation of the cubic close-packed structure to be aligned are for layered rock salt type crystals and for rock salt type and LiTiO with space group Fd-3m. ２ The type differs. In this specification, etc., a layered rock salt type crystal structure, space group Fd-3m LiTiO ２ In the crystal structure of the type and the rock salt type, when the orientation of the cubic close-packed structure composed of anions is aligned within 5 degrees, preferably within 2.5 degrees, the crystal orientation is said to be approximately coincidental. In other words, in the layered rock salt type crystal structure, LiTiO with space group Fd-3m ２ In the crystal structure of the type and the rock salt type, when the orientation of the cubic close-packed structure composed of anions is aligned within 5 degrees, preferably within 2.5 degrees, it is said that the crystal orientation is approximately coincidental. Alternatively, in the layered rock salt type, LiTiO with space group Fd-3m２ In the rock salt and rock salt types, when the orientation of the cubic close-packed structure composed of anions is aligned within 5 degrees, preferably within 2.5 degrees, these crystal structures are said to be aligned. The orientation of the cubic close-packed structure composed of anions is, for example, the direction indicated by the block arrows in Figures 4A and 4B, and can also be called the layer direction.

[0138] <Li ｘ CoO ２ State where x is small > The positive electrode active material particles 100 of one aspect of the present invention have the above-described distribution and / or crystal structure of additive element A in the discharge state, Li ｘ CoO ２ The crystal structure when x is small differs from that of conventional positive electrode active material particles. Here, x is small when 0.1 < x ≤ 0.24.

[0139] Lithium cobalt oxide (LiCoO) without additive element A ２ Using the example of ), we will explain the change in the crystal structure of conventional positive electrode active material particles.

[0140] Figure 8A shows Li ｘ CoO ２ This shows the relationship between the x-value and the c-axis length. In conventional lithium cobalt oxide, the crystal structure at small x-values ​​is the H1-3 type crystal structure shown by the rhombic shape in the figure. The c-axis length decreases significantly during the phase transition to the H1-3 structure.

[0141] On the other hand, the positive electrode active material particles 100 in one aspect of the present invention are Li ｘ CoO ２ When x is small, the crystal structure is an O3' type crystal structure with little change in c-axis length, as shown by the gray circle in Figure 8A, labeled as the O3' structure. When x = 1, the positive electrode active material particle 100 has the same R-3m O3 crystal structure as conventional lithium cobalt oxide.

[0142] The O3' type crystal structure and the H1-3 type crystal structure are related to CoO ２ The interlayer distance changes significantly. Figure 8B shows the H1-3 type crystal structure, and Figure 8C shows the O3' type crystal structure, and the CoO in each of the structures shown in each figure. ２ The surface formed by the oxygen in the layer, and the next CoO２ This indicates the distance between the planes formed by oxygen in the layer. In the H1-3 type crystal structure, this interplanar spacing is 2.77 Å and 3.06 Å, while in the O3' type crystal structure, which will be described later, this interplanar spacing is 3.13 Å. When this interplanar spacing changes significantly, as in the H1-3 type crystal structure, degradation such as cracking may occur, especially in the surface layer where lithium diffusion pathways exist.

[0143] Therefore, repeated charging and discharging cycles that result in x being 0.24 or less cause the conventional lithium cobalt oxide crystal structure to break down. This breakdown of the crystal structure leads to a deterioration of the cycle characteristics. This is because the breakdown of the crystal structure reduces the number of sites where lithium can exist stably, and also makes it more difficult for lithium to be inserted and removed.

[0144] Conventional lithium cobalt oxide with a high x value of 0.24 or less, where many Li atoms have been removed, also readily removes oxygen. Along with the removal of oxygen, cobalt is also removed. ３＋ From Co ２＋ It changes from a layered rock salt type crystal structure to a degraded spinel type crystal structure (for example, LiCo ２ O ４ , or Co ３ O ４ ) may undergo a phase change. Furthermore, in the positive electrode active material particles 100 of one aspect of the present invention, there exists a space group Fd-3m LiTiO with the characteristics of a spinel-type crystal structure between the region of layered rock salt-type crystal structure and the region having a rock salt-type crystal structure. ２ The crystal structure of this type is different from the crystal structure of the degraded spinel type described above.

[0145] Furthermore, when oxygen is removed, cations such as cobalt that were bonded to the oxygen may dissolve into the electrolyte. If this progresses, it can lead to degradation in which holes are formed in the positive electrode active material particles, also known as pitting corrosion or pitting. At this time, strain and / or stress may occur in the positive electrode active material particles 100.

[0146] On the other hand, in the positive electrode active material particles 100, Li ｘ CoO ２The change in crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is less than that of conventional positive electrode active material particles. More specifically, the change in CoO between the state where x is 1 and the state where x is 0.24 or less ２ The displacement of the layers can be reduced. In one embodiment of the present invention, the positive electrode active material particle 100 has magnesium, one of the additive elements A, at a sufficient concentration in the surface layer 100a, even if lithium is desorbed from the positive electrode active material particle 100 and x becomes 0.24 or less, the phase change to an H1-3 type crystal structure can be suppressed due to the phase change suppression effect of magnesium. For example, in a rock salt type crystal structure having MgO, the internal 100b is CoO ２ It can also be described as functioning as a barrier that prevents layers from shifting.

[0147] The designation for the O3' type crystal structure will be explained. In one embodiment of the present invention, the positive electrode active material particle 100 at x = 0.24 has a crystal structure belonging to the trigonal space group R-3m. This is CoO ２ The layer symmetry is the same as that of O3. Therefore, this crystal structure is called the O3' type crystal structure.

[0148] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co(0,0,0.5), O(0,0,x), within the range of 0.20≦x≦0.25. Furthermore, the lattice constant of the unit cell is such that the a-axis is 2.797≦a≦2.837(×10⁻¹⁰). −１ (nm) is preferred, and 2.807 ≤ a ≤ 2.827 (×10 −１ nm is more preferable. The c-axis is 13.681 ≤ c ≤ 13.881 (×10 −１ A value of n(nm) is preferred, and 13.751 ≤ c ≤ 13.811 is more preferred.

[0149] The difference between the discharged state R-3m(O3) and the O3' type crystal structure is CoO ２ There is almost no layer displacement. On the other hand, the R-3m(O3) in the discharge state and the H1-3 type crystal structure are different in CoO ２ The layer displacement is large, and as a result, the c-axis length is significantly reduced, as shown in Figures 8A and 8B.

[0150] Furthermore, the difference in volume per unit number of cobalt atoms between the discharged state R-3m(O3) and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, and typically 1.8%. On the other hand, when comparing per unit number of cobalt atoms, the difference in volume between the H1-3 type crystal structure and the O3 type crystal structure of R-3m in the discharged state exceeds 3.5%, and typically 3.9% or more.

[0151] The positive electrode active material particles 100 are Li ｘ CoO ２ It has been confirmed that when x is between 0.15 and 0.24, it may have an O3' type crystal structure, and it is presumed that it also has an O3' type crystal structure when x is greater than 0.24 and less than or equal to 0.27. However, the crystal structure is Li ｘ CoO ２ Because x is affected not only by the internal value but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., x in the case of an O3' type crystal structure is not necessarily limited to the above range of x.

[0152] Therefore, the positive electrode active material particle 100 is Li ｘ CoO ２ When x is greater than 0.1 and less than or equal to 0.24, the entire interior 100b of the positive electrode active material particle 100 does not have to have an O3' type crystal structure. It may contain other crystal structures, or a part of it may be amorphous.

[0153] Also Li ｘ CoO ２ To make the internal x small, it is generally necessary to charge with a high charging voltage. Therefore Li ｘ CoO ２ A state where x is small can be rephrased as a state where the device is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6V or higher relative to the potential of lithium metal in an environment of 25°C, the H1-3 type crystal structure appears in conventional positive electrode active material particles. Therefore, a charging voltage of 4.6V or higher relative to the potential of lithium metal can be said to be a high charging voltage.

[0154] Therefore, the positive electrode active material particles 100 of one aspect of the present invention are preferable because they can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, a voltage of 4.6V or higher at 25°C. Furthermore, they are preferable because they can adopt an O3' type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65V or higher and 4.7V or lower at 25°C.

[0155] Even with the positive electrode active material particles 100, an H1-3 type crystal structure may only be observed when the charging voltage is further increased. Furthermore, as mentioned above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., so even at lower charging voltages, for example, when the charging voltage is 4.5V or more and less than 4.6V at 25°C, the positive electrode active material particles 100 of one embodiment of the present invention may take on an O3' type crystal structure.

[0156] Furthermore, when graphite is used as the negative electrode active material in a secondary battery, for example, the voltage of the secondary battery will decrease by the amount of the graphite's potential compared to the above. The potential of graphite is approximately 0.05V to 0.2V relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the potential of the positive electrode will be the secondary battery voltage minus the potential of graphite.

[0157] Note that the additive element A does not necessarily have to have a similar concentration gradient across the entire surface layer 100a of the positive electrode active material particles 100. An example of the distribution of additive element X near C-D in Figure 2B is shown in Figure 3D, and an example of the distribution of additive element Y near C-D is shown in Figure 3E.

[0158] Here, the region near C-D has a layered rock salt type crystal structure of R-3m, and the surface is (001) oriented. The distribution of additive element A in the surface layer 100a of a (001) oriented surface may differ from that of the surface layer 100a of other surfaces. For example, in the surface layer 100a of a (001) oriented surface, the distribution of one or more concentration peaks selected from additive element X and additive element Y may be limited to a shallower portion from the surface compared to the surface layer 100a of a surface other than a (001) oriented surface. Alternatively, the concentration of one or more elements selected from additive element X and additive element Y may be lower in the surface layer 100a of a (001) oriented surface compared to the surface layer 100a of other oriented surfaces. Alternatively, in the surface layer 100a of a (001) oriented surface, one or more elements selected from additive element X and additive element Y may be below the detection limit.

[0159] In the layered rock salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This is CoO ２ It can be said that the structure consists of layers and lithium layers stacked alternately parallel to the (001) plane. Therefore, the diffusion paths of lithium ions also exist parallel to the (001) plane.

[0160] CoO ２ Since the layer is relatively stable, the surface of the positive electrode active material particles 100 is more stable when it is oriented in the (001) direction. The main diffusion pathway of lithium ions during charging and discharging is not exposed on the (001) plane.

[0161] On the other hand, the lithium ion diffusion pathways are exposed on surfaces other than those oriented in the (001) direction. Therefore, the surfaces and surface layer 100a other than those oriented in the (001) direction are important regions for maintaining the lithium ion diffusion pathways, but at the same time, they are prone to instability because they are the regions where lithium ions first desorb. For this reason, reinforcing the surfaces and surface layer 100a other than those oriented in the (001) direction is extremely important for maintaining the overall crystal structure of the positive electrode active material particles 100.

[0162] Therefore, in another embodiment of the positive electrode active material particles 100 of the present invention, it is important that the distribution of the added element A in the surface layer 100a (edge ​​region 100a1) on surfaces other than (001) is as shown in Figures 3A to 3C. On the other hand, in the (001) surface and its surface layer 100a (basal region 100a2), the concentration of the added element A may be low or absent, as described above.

[0163] As will be explained in Embodiment 2 below, high-purity LiCoO ２ In a manufacturing method in which the additive element A is mixed in and heated after the initial manufacturing, the additive element A spreads mainly through the diffusion pathway of lithium ions. Therefore, it is easier to set the distribution of the additive element A in the surface layer 100a (edge ​​region 100a1) on surfaces other than (001) to a desirable range.

[0164] <Analysis Method> A certain positive electrode active material particle is Li ｘ CoO ２ Whether or not the positive electrode active material particle 100 of the present invention has an O3' type crystal structure when x is small depends on Li ｘ CoO ２ The positive electrode containing positive electrode active material particles with a small x value can be identified by analyzing it using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0165] XRD is particularly favored because it can analyze the symmetry of transition metals such as cobalt in the positive electrode active material particles with high resolution, compare the crystallinity and crystal orientation, analyze the periodic distortion of the lattice and crystallite size, and obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling a secondary battery. The XRD measurement method is not particularly limited, and for example, a method can be used in which the incident angle of the X-rays is changed and the angle of the X-ray detector is set to be the same as the incident angle to measure the X-ray diffraction intensity (hereinafter also referred to as 2θ / θ continuous scan or θ-2θ method). In XRD by 2θ / θ continuous scan, diffraction peaks are obtained that reflect the crystal structure of the interior 100b of the positive electrode active material particles 100, which occupies most of the volume of the positive electrode active material particles 100.

[0166] When analyzing crystallite size using XRD, it is preferable to measure while excluding the influence of the orientation of positive electrode active material particles due to pressure, etc. For example, positive electrode active material particles can be extracted from the positive electrode obtained by disassembling a secondary battery, and measured after being prepared as a powder sample. Alternatively, as part of the analysis of the XRD diffraction pattern, waveform separation may be performed using a reference diffraction pattern of the O3' structure, a reference diffraction pattern of the H1-3 structure, etc., and the proportion of the O3' structure may be calculated.

[0167] Furthermore, while it is preferable to obtain the XRD diffraction pattern for calculating the crystallite size using only the positive electrode active material particles, it may also be obtained in the positive electrode state, which includes a current collector, binder, and conductive material in addition to the positive electrode active material particles. However, in the positive electrode state, the positive electrode active material particles may be oriented due to the effects of pressurization during the manufacturing process. If the orientation is strong, it may not be possible to accurately calculate the crystallite size of the positive electrode active material particles. Therefore, the XRD diffraction pattern can be obtained by methods such as removing the positive electrode active material layer from the positive electrode, removing some of the binder and other components from the positive electrode active material layer using a solvent, and then filling it into a sample holder.

[0168] For calculating crystallite size, for example, a Bruker D8 ADVANCE can be used, with CuKα1 as the X-ray source, 2θ between 15° and 90°, increase of 0.005°, and a LYNXEYE XE-T detector. The diffraction pattern obtained with these settings and the literature value for lithium cobaltate (ICSD col. code. 172909) can be used. Crystal structure analysis can be performed using the Rietveld method with DIFFRAC. TOPAS ver. 6 as the crystal structure analysis software.

[0169] It is preferable to use the value of LVol-IB, which is the crystallite size corrected by the integral width criterion calculated using the above method, as the crystallite size.

[0170] Since lithium cobalt oxide is prone to (001) orientation, setting the preferred orientation to (001) may yield a better GOF (goodness of fit). The preferred orientation can be the March-Dollase function or Spherical Harmonics. Furthermore, in XRD analysis for determining crystallite size, the Rietveld method may not be suitable for determining crystallite size if the preferred orientation is less than 0.8 or greater than 1.2, as the sample orientation is too strong. In such cases where the Rietveld method is unsuitable, it is preferable to use other refinement techniques that yield a better GOF to determine the crystallite size. For example, the Whole-Powder Pattern Decomposition (WPPD) method can be employed.

[0171] Furthermore, with the full pattern resolution method, not only the crystallite size but also the lattice constant can be calculated without being affected by the orientation of the sample.

[0172] It is also important to note that simply adding element A may not result in the formation of an O3' type crystal structure. For example, even if lithium cobalt oxide has magnesium and fluorine, or lithium cobalt oxide has magnesium and aluminum, the structure may differ depending on the concentration and distribution of element A. ｘ CoO ２ There are two cases: one where x is 0.24 or less and the O3' type crystal structure accounts for 60% or more, and another where the H1-3 type crystal structure accounts for 50% or more.

[0173] Furthermore, even with the positive electrode active material particles 100 according to one aspect of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9V, an H1-3 type or trigonal O1 type crystal structure may occur. Therefore, in order to determine whether or not the positive electrode active material particles 100 are according to one aspect of the present invention, analysis of the crystal structure, including XRD, and information such as charging capacity or charging voltage are necessary.

[0174] However, positive electrode active material particles with a small x value may undergo a change in crystal structure when exposed to air. For example, they may change from an O3' type crystal structure to an H1-3 type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0175] On the other hand, even when handled in an argon atmosphere, the crystal structure of the positive electrode after disassembling a secondary battery may change over time. Therefore, it is preferable to employ a method that allows for rapid measurement as needed. It is also preferable to pre-dry a sealable sample holder for measurement in an argon atmosphere and to thoroughly remove any water adsorbed inside the sealed container. As a drying method, for example, it is preferable to use a vacuum drying oven and perform the process at a reduced pressure environment and 35°C for 12 hours.

[0176] In the case of XRD, the detector and / or measurement start angle can be appropriately changed to quickly observe the main peaks. If the crystal structure changes significantly over time, it is preferable to measure the main peak within 13 minutes of starting the measurement, for example.

[0177] Furthermore, whether or not the distribution of additive element A in the positive electrode active material particles is as described above can be determined by analyzing it using methods such as XPS, EDX, or EPMA (Electron Probe Micro Analyzer).

[0178] <Charging Method> To determine whether the composite oxide is a positive electrode active material particle 100 according to one aspect of the present invention, charging can be performed, for example, by manufacturing a coin cell (CR2032 type, 20 mm in diameter and 3.2 mm in height) using the composite oxide as the positive electrode and lithium metal as the counter electrode. The coin cell comprises an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0179] More specifically, the positive electrode can be made by coating an aluminum foil positive electrode current collector with a slurry containing positive electrode active material particles, a conductive material, and a binder.

[0180] When a material other than lithium metal is used for the counter electrode, the voltage value of the secondary battery and the potential value of the positive electrode will differ. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode relative to the lithium metal counter electrode.

[0181] As the electrolyte, a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7 was used, to which 1 mol / L of lithium hexafluoride phosphate (LiPF) was added. ６ A product having the above characteristics, to which 2 wt% vinylene carbonate (VC) is added as an additive, can be used.

[0182] A 25 μm thick porous polypropylene film can be used as the separator.

[0183] The positive electrode and negative electrode cans can be made of stainless steel (SUS).

[0184] The coin cell manufactured under the above conditions is charged with an arbitrary voltage (e.g., 4.50V, 4.55V, 4.58V, 4.60V, 4.62V, 4.65V, 4.70V, 4.75V, or 4.80V). The charging method is not particularly limited as long as it can be charged at any voltage for a sufficient amount of time. In this specification, when approximately 4.6V is mentioned, it refers to a voltage between 4.58V and 4.62V. For example, when charging with CCCV, the current in CC charging can be between 20mA / g and 100mA / g. CV charging can be terminated at 2mA / g and 10mA / g. Because the diffusion of lithium ions is slow inside the positive electrode active material particles, it is desirable to charge with such small current values ​​in order to observe the phase change of the positive electrode active material particles. The temperature should be 25°C or 45°C. After charging in this manner, the coin cell can be disassembled in a glove box under an argon atmosphere to remove the positive electrode, thereby obtaining positive electrode active material particles of any desired charge capacity. When performing various analyses thereafter, it is preferable to seal the removed positive electrode under an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the removed positive electrode in a sealable sample holder under an argon atmosphere. It is also preferable to remove the positive electrode as soon as possible after charging is complete and use it for analysis. To avoid the effects of self-discharge, it is preferable that the disassembly of the coin cell be completed within one hour after charging is complete, and more preferably within 30 minutes.

[0185] <XRD> The equipment and conditions for XRD measurement are not particularly limited. For example, XRD patterns can be measured with the following equipment and conditions: XRD equipment: Bruker D8 ADVANCE, X-ray: CuKα1, output: 40kV, 40mA, divergence slit: 0.6mm, detector: LYNXEYE XE-T, scanning method: 2θ / θ continuous scan, measurement range (2θ): 15° to 90°, step width (2θ): 0.01° setting, counting time: 1 second / step, sample stage rotation: 15 rpm.

[0186] If the sample to be measured is a powder, it can be set up by placing it in a glass sample holder or by sprinkling the sample onto a grease-coated silicon anti-reflective plate. If the sample to be measured is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set up to match the measurement surface required by the instrument. If the positive electrode active material layer is higher than the measurement surface required by the instrument, the diffraction pattern will shift overall to the higher angle side, and if it is lower, it will shift overall to the lower angle side. In this case, the shift in the diffraction pattern can be corrected using crystal structure analysis software or the like.

[0187] As shown in Figure 9, the O3' type crystal structure has diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° to 19.37°) and 2θ = 45.47 ± 0.10° (45.37° to 45.57°). However, these peaks do not appear in the H1-3 type crystal structure. Therefore, Li ｘ CoO ２ Having diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° to 19.37°) and 2θ = 45.47 ± 0.10° (45.37° to 45.57°) when x is small is a characteristic feature of the positive electrode active material particle 100 in one embodiment of the present invention.

[0188] Furthermore, when charging is performed with a voltage slightly lower than 4.60V (4.56V, 4.57V, 4.58V, or 4.59V) as the upper limit of the charging voltage, the above diffraction peaks appear shifted to the lower angle side. For example, when charging is performed with a charging voltage of 4.58V as the upper limit, the positive electrode active material particles 100 have diffraction peaks at 2θ = 18.85 ± 0.20° and 2θ = 45.15 ± 0.10°.

[0189] In one aspect of the present invention, the positive electrode active material particles 100 are Li ｘ CoO ２When x is small, it has an O3' type crystal structure, but not all of it has to be an O3' type crystal structure. It may contain other crystal structures, or part of it may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3' type crystal structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more. If the O3' type crystal structure is 50% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, it is possible to obtain cathode active material particles with sufficiently excellent cycle characteristics.

[0190] <XPS> In XPS (X-ray Photoelectron Spectroscopy), when using monochromatic aluminum Kα X-rays for inorganic oxides, it is possible to analyze regions from the surface to a depth of approximately 2 nm to 8 nm (usually less than 5 nm). Furthermore, narrow-scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is usually around ±1 atomic percent, and the detection limit is approximately 1 atomic percent, although this varies depending on the element.

[0191] In one embodiment of the present invention, it is preferable that the concentration of one or more selected additive elements A in the positive electrode active material particles 100 is higher in the surface layer 100a than in the interior 100b. This is equivalent to saying that it is preferable that the concentration of one or more selected additive elements A in the surface layer 100a is higher than the average concentration of the positive electrode active material particles 100 as a whole. For example, it can be said that it is preferable that the concentration of one or more selected additive elements A in the surface layer 100a, as measured by XPS, is higher than the average concentration of additive elements A in the positive electrode active material particles 100 as a whole, as measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, it is preferable that the concentration of magnesium in at least a part of the surface layer 100a, as measured by XPS, is higher than the average magnesium concentration of the positive electrode active material particles 100 as a whole. It is also preferable that the concentration of nickel in at least a part of the surface layer 100a is higher than the average nickel concentration of the positive electrode active material particles 100 as a whole. Furthermore, it is preferable that the concentration of aluminum in at least a portion of the surface layer 100a is higher than the average concentration of aluminum in the entire positive electrode active material particles 100. Also, it is preferable that the concentration of fluorine in at least a portion of the surface layer 100a is higher than the average concentration of fluorine in the entire positive electrode active material particles 100.

[0192] In one embodiment of the present invention, the surface and surface layer 100a of the positive electrode active material particles 100 do not contain carbonates, hydroxyl groups, etc., that have been chemically adsorbed after the production of the positive electrode active material particles 100. Furthermore, the surface of the positive electrode active material particles 100 does not contain electrolyte, binder, conductive material, or compounds derived therefrom. Therefore, when quantifying the elements contained in the positive electrode active material particles, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc., which can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds through analysis, and corrections may be made to exclude C-F bonds derived from the binder.

[0193] Before subjecting the samples to various analyses, the positive electrode active material particles 100 and the positive electrode active material layer 22 may be washed to remove electrolyte, binder, conductive material, and compounds derived therefrom that have adhered to the surface of the positive electrode active material particles 100. In this case, lithium may leach into the cleaning agent (e.g., solvent) used for washing. On the other hand, since additive element A does not easily leach, the effect of washing on the concentration and atomic ratio of additive element A is very small. For washing, it is preferable to use a method that causes little damage to the samples such as the positive electrode active material particles 100 and the positive electrode active material layer 22. As a cleaning agent, for example, a solvent that can be used in an electrolyte can be used. For solvents that can be used in an electrolyte, refer to the description of organic solvents listed in <Electrolyte> below. Dimethyl carbonate (DMC) or acetonitrile can be suitably used as a cleaning agent. Alternatively, a solvent that can be used during wet mixing or grinding (e.g., N-methyl-2-pyrrolidone (NMP)) can also be used as a cleaning agent. Additionally, cleaning agents can be combined with ultrasonic cleaning or filtration.

[0194] Furthermore, the concentration of additive element A may be compared in ratio to cobalt. Using the ratio to cobalt is preferable because it reduces the influence of carbonates and the like that chemically adsorbed after the production of the positive electrode active material particles. For example, the ratio of the number of magnesium atoms to the number of cobalt atoms, Mg / Co, as determined by XPS analysis, is preferably 0.400 or more and 1.20 or less, more preferably 0.500 or more and 1.00 or less, more preferably 0.500 or more and 0.900 or less, and more preferably 0.500 or more and 0.700 or less.

[0195] Furthermore, for example, the ratio of nickel atoms to cobalt atoms (Ni / Co) determined by XPS analysis is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, more preferably 0.050 or more and 0.100 or less, and more preferably 0.050 or more and 0.070 or less.

[0196] Furthermore, for example, the ratio of aluminum atoms to cobalt atoms (Al / Co) determined by XPS analysis is preferably 0.010 or more and 0.100 or less, more preferably 0.010 or more and 0.050 or less, and even more preferably 0.010 or more and 0.040 or less.

[0197] Furthermore, for example, the ratio of fluorine atoms to magnesium atoms, F / Mg, determined by XPS analysis, is preferably 0.100 or more and 1.00 or less, more preferably 0.100 or more and 0.800 or less, more preferably 0.100 or more and 0.500 or less, more preferably 0.100 or more and 0.300 or less, and more preferably 0.100 or more and 0.200 or less.

[0198] The fact that the concentrations are within the range described above indicates that these additive elements A are not adhering to a narrow area on the surface of the positive electrode active material particles 100, but are widely distributed at a favorable concentration in the surface layer 100a of the positive electrode active material particles 100. In other words, the fact that the XPS analysis results of the positive electrode active material particles 100 are within the range described above means that the crystal structure is less likely to collapse even when charging and discharging are repeated such that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, good lithium insertion and deinsertion is possible in the positive electrode active material particles 100, and excellent rate characteristics can be achieved.

[0199] For XPS analysis, monochromatic aluminum Kα rays can be used as the X-ray source. Furthermore, for energy resolution, it is preferable to use an XPS instrument with an energy resolution such that the full width at half maximum of the Ag3d5 / 2 peak (112 eV) in the XPS spectrum of an Ag sample is 1.0 eV ± 0.1 eV. The extraction angle can be, for example, 45°. For example, the following XPS instrument and measurement conditions can be used: Measurement instrument: ULVAC-PHIE Quantera II; X-ray: monochromatic Al Kα (1486.6 eV); Energy resolution: Full width at half maximum of the Ag3d5 / 2 peak is 1.0 eV ± 0.1 eV; Detection area: 100 μmφ; Detection depth: approximately 4 nm to 5 nm from the surface (extraction angle 45°); Measurement spectrum: wide scan, narrow scan for each detected element.

[0200] When XPS analysis is performed on the positive electrode active material particles 100 according to one embodiment of the present invention, the peak indicating the bond energy between magnesium and other elements (Mg1s peak) is preferably 1303.0 eV or more and less than 1305.0 eV, and more preferably around 1304.0 eV. This value is different from the bond energy of magnesium fluoride, which is 1306.0 eV, and is close to the bond energy of magnesium oxide.

[0201] In XPS analysis of positive electrode active material particles 100 according to one aspect of the present invention, the measured XPS spectrum is corrected so that the C1s peak matches a reference value (284.8 eV), that is, the entire spectrum is shifted. This reduces the influence of differences in XPS equipment, differences in measurement conditions, etc., on the XPS measurement.

[0202] Furthermore, in XPS analysis of positive electrode active material particles 100 according to one embodiment of the present invention, when analyzing the Mg1s peak and determining the ratio of peak components derived from O-Mg-O bonds, peak components derived from O-Mg-F bonds, and peak components derived from F-Mg-F bonds, it is preferable to have a peak component derived from O-Mg-O bonds. While peak components derived from O-Mg-F bonds may be present, it is preferable that they constitute 30% or less of the total three peak components, more preferably 20% or less, more preferably 10% or less, and even more preferably below the detection limit. Similarly, while peak components derived from F-Mg-F bonds may be present, it is preferable that they constitute 10% or less of the total, and even more preferably below the detection limit.

[0203] In other words, in XPS analysis of positive electrode active material particles 100 according to one aspect of the present invention, when the ratio of peak components derived from O-Mg-O bonds, peak components derived from O-Mg-F bonds, and peak components derived from F-Mg-F bonds is analyzed, it is preferable that the peak component derived from O-Mg-O bonds is 70% or more, more preferably 80% or more, even more preferably 90% or more, and particularly preferably 100%. However, when performing a similar analysis on the positive electrode after incorporating the positive electrode active material particles 100 according to one aspect of the present invention into a secondary battery, the peak component derived from O-Mg-O bonds may be detected due to the influence of fluorine contained in the binder or electrolyte.

[0204] This section describes a method for analyzing the Mg1s peak in an XPS spectrum during XPS analysis. In analyzing the Mg1s peak, the peak component derived from the O-Mg-O bond is designated as Fit Peak 1, the peak component derived from the O-Mg-F bond as Fit Peak 2, and the peak component derived from the F-Mg-F bond as Fit Peak 3. By synthesizing these three fit peaks, the ratio of peak synthesis that minimizes the difference between the resulting peak and the Mg1s peak in the XPS spectrum obtained from the XPS analysis can be calculated. The area ratio of Fit Peak 1, Fit Peak 2, and Fit Peak 3 at this time can be assumed to represent the relative abundance of the O-Mg-O bond, O-Mg-F bond, and F-Mg-F bond, and the analysis results can be output accordingly.

[0205] In the above XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also called the peak top) of fitted peak 1 is MgO-coated LiCoO ２ The energy value at the maximum value of the Mg1s peak when measured separately using magnesium fluoride (MgF) can be referenced. Furthermore, the energy value at the maximum value of the fitted peak 3 (Ep3) is the same as that of magnesium fluoride (MgF). ２For example, the energy value at the maximum value of the Mg1s peak when MGH18XB (purity 99.9% (3N) up) from the High Purity Chemical Laboratory is measured separately as a standard sample can be referenced. Also, the energy value at the maximum value of Fit Peak 2 (Ep2) can be an intermediate value between Ep1 and Ep3. Furthermore, Ep1 is located on the lower energy side compared to Ep3. The energy value at the maximum value of the peak is also called the peak position.

[0206] In XPS analysis of positive electrode active material particles 100 according to one embodiment of the present invention, the analysis result of the Mg1s peak being within the above preferred range can be determined from the peak position and the full width at half maximum of the peak. For example, the full width at half maximum of the Mg1s peak is preferably 1.0 eV or more and 3.0 eV or less, more preferably 1.0 eV or more and 2.8 eV or less, and particularly preferably 1.0 eV or more and 2.6 eV or less. In the above, the peak position of the Mg1s peak is on the lower energy side than the energy value at the maximum value of the Mg1s peak when magnesium fluoride is measured separately as a standard sample.

[0207] <EDX> It is preferable that one or more of the additive elements A present in the positive electrode active material particles 100 have a concentration gradient. It is even more preferable that the depth of the concentration peaks from the surface differs depending on the additive elements A of the positive electrode active material particles 100. The concentration gradient of the additive elements A can be evaluated, for example, by exposing the cross-section of the positive electrode active material particles 100 using FIB (Focused Ion Beam) and analyzing the cross-section using EDX, EPMA (Electron Probe Microanalysis), etc.

[0208] Within EDX analysis, measuring while scanning within a region and evaluating that region in two dimensions is called EDX surface analysis. Measuring while scanning linearly and evaluating the distribution of atomic concentration within positive electrode active material particles is called line analysis. Furthermore, sometimes line analysis is also used to refer to the extraction of linear region data from surface analysis. Measuring a region without scanning is called point analysis.

[0209] EDX surface analysis (e.g., elemental mapping) allows for quantitative analysis of the concentration of additive element A in the surface layer 100a, interior 100b, and near grain boundaries of the positive electrode active material particles 100. The vicinity of grain boundaries refers to the region within 10 nm of the grain boundary. Furthermore, EDX radiation analysis allows for the analysis of the concentration distribution and maximum value of additive element A. Analysis using thinned samples, such as STEM-EDX, is preferable because it allows for the analysis of the concentration distribution in the depth direction from the surface to the center of the positive electrode active material particles in a specific region, without being affected by the distribution in the depth direction. Furthermore, depth profiling techniques such as depth profiling SIMS (Secondary Ion Mass Spectrometry) and depth profiling XPS, which involve removing the sample surface by sputtering during measurement, do not yield the same results as STEM-EDX because the sputtering process can have an impact on elemental intrusion.

[0210] In STEM-EDX ray analysis, due to the principle or measurement errors, the graph of the detected characteristic X-rays of an element does not change sharply, making it difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX ray analysis, the detected amount of characteristic X-rays of the transition metal M is considered to be the average value M of the detected characteristic X-rays of the transition metal M inside the surface. ＡＶＥ And the average value M of the detected characteristic X-rays of the above transition metal M in the background. ＢＧ The point where the sum of the two is 50%, or where the amount of oxygen characteristic X-rays detected is equal to the average value of the amount of oxygen characteristic X-rays detected inside. ＡＶＥ And the average value of the detected characteristic X-rays of background oxygen O ＢＧThe reference point is the point at which the sum of the two values ​​becomes 50%. Note that if the point at which the detected amount of characteristic X-rays of the transition metal M becomes 50% of the sum of the average detected amount of characteristic X-rays of the internal transition metal M and the average detected amount of characteristic X-rays of the background transition metal M is different from the point at which the detected amount of characteristic X-rays of oxygen becomes 50% of the sum of the average detected amount of characteristic X-rays of internal oxygen and the average detected amount of characteristic X-rays of background oxygen, it is thought to be due to the influence of oxygen-containing metal oxides, carbonates, etc. adhering to the surface, and therefore the point at which the detected amount of characteristic X-rays of the transition metal M becomes 50% of the average detected amount of characteristic X-rays of internal transition metal M ＡＶＥ And the average value M of the detected characteristic X-rays of the above transition metal M in the background. ＢＧ The point at 50% of the sum of these can be adopted as the reference point. Also, in the case of positive electrode active material particles having multiple transition metals M, the transition metal element M that has the highest amount of characteristic X-rays detected inside is selected. ＡＶＥ and M ＢＧ The above reference points can be determined using this method.

[0211] The average value M of the detected characteristic X-rays of the above transition metal M in the background. ＢＧ For example, the average value M of the detected characteristic X-rays of the transition metal M can be determined by averaging the range of 2 nm or more, preferably 3 nm or more, outside the positive electrode active material particles, while avoiding the vicinity where the detection amount of characteristic X-rays of the transition metal M begins to increase. ＡＶＥ The average value of the background characteristic X-ray O is obtained by averaging a range of 2 nm or more, preferably 3 nm or more, from a depth of 30 nm or more, preferably more than 50 nm, in the region where the detection amount of characteristic X-rays of transition metal M and oxygen saturates and stabilizes, for example, from the region where the detection amount of characteristic X-rays of transition metal M begins to increase. ＢＧ and the average value of the detected amount of characteristic X-rays of the internal oxygen O ＡＶＥ This can be calculated in a similar manner.

[0212] Furthermore, the surface of the positive electrode active material particles 100 in cross-sectional STEM images, etc., is defined as the boundary between the region where an image originating from the crystal structure of the positive electrode active material particles is observed and the region where it is not observed, and is the outermost region where atomic columns originating from the nuclei of metal elements with atomic numbers greater than lithium among the metal elements constituting the positive electrode active material particles are confirmed.

[0213] In STEM-EDX analysis, a peak refers to a convex shape that appears on the graph of characteristic X-ray intensity or concentration for each element. Peak intensity refers to the maximum value of characteristic X-ray intensity or concentration for each element. Peak position refers to the position of the maximum value of characteristic X-ray intensity or concentration for each element. Noise in STEM-EDX analysis can include measurements with a full width at half maximum (FWHM) below the spatial resolution (R), for example, R / 2 or less.

[0214] Scanning the same location multiple times under the same conditions can reduce the impact of noise. For example, the cumulative value obtained from two scans can be used as the detected value for each element. The number of scans is not limited to two; more scans can be performed, and the cumulative value of those scans can be used as the detected value for each element.

[0215] For elemental quantification in EDX analysis, a standardless quantification method can be employed, using the k-factor built into the analytical instrument and / or analytical software. When determining elemental concentrations from the quantification results, it is preferable that the elements included include those used in the target material, raw materials, and pre-analysis treatment, such as carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, and gallium—a total of 14 elements. Since the quantification calculation is performed so that the sum of the quantification concentrations of these elements equals 100%, these elements are also called the denominator elements.

[0216] STEM-EDX radiation analysis can be performed, for example, as follows: First, a protective film is deposited on the surface of the positive electrode active material particles. For example, carbon can be deposited using an ion sputtering apparatus (Hitachi High-Tech MC1000).

[0217] Next, the positive electrode active material is thinned to prepare a STEM cross-sectional sample. For example, thinning can be performed using a FIB-SEM device (Hitachi High-Tech XVision 200TBS). In this case, pickup is performed using an MPS (microprobing system), and it is preferable to finish the processing by gradually decreasing the acceleration voltage from high to low in the order of 30kV, 15kV, 5kV, and 2kV.

[0218] STEM-EDX radiation analysis can be performed using, for example, a STEM instrument (Hitachi High-Tech HF5000) and an Oxford Ultim MAX TLE EDX detector (equipped with two detectors). During EDX radiation analysis, the emission current of the STEM instrument is set to 7 μA to 10 μA, and the area of ​​the thinned sample with little depth and unevenness is measured. STEM-EDX radiation analysis involves performing EDX surface analysis to evaluate the measurement target area in two dimensions, and then using analysis software to extract any area as EDX radiation analysis. The measurement magnification for EDX surface analysis is set to a magnification where the measurement pitch of the line analysis is approximately 0.3 nm, for example, around 160,000x (320,000x on the screen display due to drift correction). The conditions for EDX surface analysis can be set to 256 x 256 pixels, a Dwell Time of 50 μsec, and approximately 400 frames (with a target of approximately 150,000 Co counts in the qualitative spectrum). Furthermore, the extraction conditions for EDX line analysis can be set to a line width of 256 pixels to detect more characteristic X-rays. In addition, for EDX analysis, it is possible to arbitrarily select between Netcount (Counts), which indicates the number of detected element counts, and atomic%, which is obtained by multiplying Netcount by the element-specific coefficient, the k factor.

[0219] STEM-EDX radiation analysis is not limited to the above-mentioned STEM apparatus, but can also use other STEM apparatuses (such as JEOL's JEM-ARM200F NEOARM) and EDX detectors (such as JEOL's Dual SDD detector JED-2300T).

[0220] To achieve high spatial resolution in STEM-EDX ray analysis, a small beam diameter of the electron beam is preferable. The beam diameter in STEM-EDX ray analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the beam current of the electron beam. Therefore, the apparatus used for STEM-EDX ray analysis is preferably equipped with a spherical aberration correction device (Cs collector) that can reduce the beam diameter and increase the beam current.

[0221] Furthermore, in positive electrode active material particles 100 having magnesium and fluorine as additive element A, it is preferable that the distribution of fluorine has a region that overlaps with the distribution of magnesium. For example, it is preferable that the difference in depth direction between the peak of fluorine concentration or detected amount and the peak of magnesium concentration or detected amount is within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and still more preferably within 0.5 nm.

[0222] Furthermore, in positive electrode active material particles 100 having nickel as additive element A, the peak of nickel concentration or detected amount in the surface layer 100a is preferably located on the surface of the positive electrode active material particle 100, or within a depth of 3 nm from the reference point toward the center, and more preferably within a depth of 1 nm. Furthermore, in positive electrode active material particles 100 having magnesium and nickel, the distribution of nickel is preferably in a region that overlaps with the distribution of magnesium. For example, the difference in depth between the peak of nickel concentration or detected amount and the peak of magnesium concentration or detected amount is preferably within 3 nm, and more preferably within 1 nm.

[0223] Furthermore, when the positive electrode active material particles 100 contain aluminum as additive element A, when EDX radiation analysis is performed, it is preferable that the peaks of concentration or detection amount of magnesium, nickel, or fluorine are closer to the surface than the peaks of concentration or detection amount of aluminum in the surface layer 100a. In other words, it is preferable that the peak of concentration or detection amount of aluminum in the surface layer 100a is located further inward than the peaks of concentration or detection amount of magnesium, nickel, or fluorine. For example, it is preferable that the peak of concentration or detection amount of aluminum is located on the surface of the positive electrode active material particles 100, or at a depth of 0.5 nm to 50 nm from a reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0224] Here, we will explain how to represent the positional relationships of elemental distributions when performing EDX radiation analysis, using Figures 10A to 10G. Figures 10A to 10F are schematic diagrams showing the concentration distribution or detection amount distribution of the first element e1 and the second element e2. Figure 10G is a schematic diagram showing the concentration distribution or detection amount distribution of the first element e1, the second element e2, and the third element e3.

[0225] For example, if the concentration distribution or detection amount distribution of the first element e1 and the second element e2 is as shown in Figure 10A, the position where the concentration or detection amount of the second element e2 is maximum is located further inside than the position where the concentration or detection amount of the first element e1 is maximum. Also, for example, if the concentration distribution or detection amount distribution of the first element e1 and the second element e2 is as shown in Figure 10B, the position where the concentration or detection amount of the second element e2 is maximum is located further inside than the position where the concentration or detection amount of the first element e1 is maximum. Also, for example, if the concentration distribution or detection amount distribution of the first element e1 and the second element e2 is as shown in Figure 10C, the position where the concentration or detection amount of the first element e1 is maximum is located further inside than the position where the concentration or detection amount of the second element e2 is maximum. Furthermore, for example, if the concentration distribution or detection amount distribution of the first element e1 and the second element e2 is as shown in Figure 10D, the position where the concentration or detection amount of the second element e2 is maximum is located further inside than the position where the concentration or detection amount of the first element e1 is maximum. Furthermore, for example, if the concentration distribution or detection amount distribution of the first element e1 and the second element e2 is as shown in Figure 10E, the position where the concentration or detection amount of the first element e1 is maximum is located further inside than the position where the concentration or detection amount of the second element e2 is maximum. Furthermore, for example, if the concentration distribution or detection amount distribution of the first element e1 and the second element e2 is as shown in Figure 10F, the position where the concentration or detection amount of the second element e2 is maximum is located further inside than the position where the concentration or detection amount of the first element e1 is maximum.

[0226] The expression "having a region where the distributions overlap" will be explained using the example of a case where the concentration distributions or detection amount distributions of the first element e1, the second element e2, and the third element e3 are in the positional relationship shown in Figure 10G. In this specification, "having a region where the distributions of two elements overlap" means, for example, that the position where the maximum value occurs in the concentration distribution or detection amount distribution of at least one element is located within the range where the concentration or detection amount in the concentration distribution or detection amount distribution of the other element is 1 / 5 or more of the maximum value. Note that if the detection intensity of the background in EDX analysis is 1 / 5 or more of the above "maximum value", then "1 / 5 of the maximum value" in the above text shall be referred to as "background detection intensity (also called the detection limit)".

[0227] For example, in the positional relationship shown in Figure 10G, the position (p2) where the concentration distribution or detection amount distribution of the second element e2 is at its maximum value is located within the range (hatched area in the figure) where the concentration or detection amount of the first element e1 is at or above 1 / 5 of the maximum value (or the lower detection limit). Therefore, the first element e1 and the second element e2 have a region where their distributions overlap. On the other hand, the position (p3) where the concentration distribution or detection amount distribution of the third element e3 is at its maximum value is not located within the range (hatched area in the figure) where the concentration or detection amount of the first element e1 is at or above 1 / 5 of the maximum value (or the lower detection limit). Therefore, the first element e1 and the third element e3 cannot be said to have a region where their distributions overlap.

[0228] Furthermore, in the case of the positional relationship shown in Figure 10G, it can be said that the distribution of the second element e2 and the distribution of the third element e3 are located more inward than the distribution of the first element e1. Alternatively, it can be said that the distribution of the second element e2 and the distribution of the third element e3 are biased more towards the inward side than the distribution of the first element e1.

[0229] <Second Positive Electrode Active Material Particles> In one embodiment of the present invention, the positive electrode may have, in addition to the positive electrode active material particles 100, second positive electrode active material particles different from the positive electrode active material particles 100. The second positive electrode active material particles 200, like the positive electrode active material particles 100, have the function of taking in and releasing lithium ions during charging and discharging. The second positive electrode active material particles 200 used in one embodiment of the present invention can be made of a material that is similar to the positive electrode active material particles 100 except for the particle size, and which does not degrade as much during charging and discharging, even at high charging voltages. Specifically, in the method for producing positive electrode active material particles described in Embodiment 2, positive electrode active material particles (composite oxide) with a particle size (median diameter (D50)) of 0.1 μm or more and less than 9 μm, preferably 1 μm or more and 5 μm or less, obtained by reducing the particle size of the starting material and reducing the temperature or duration of the heat treatment, can be used. The second positive electrode active material particles 200 preferably contain one or more of the additive elements X, Y, and Z, similar to the positive electrode active material particles 100 described above.

[0230] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0231] (Embodiment 2) In this embodiment, an example of a method for producing positive electrode active material particles 100, which are positive electrode active material of a battery according to one aspect of the present invention, will be described. Figures 11A to 12C are diagrams illustrating the method for producing positive electrode active material particles 100.

[0232] The method of adding element A is crucial for producing the positive electrode active material particles 100. Simultaneously, good crystallinity within the positive electrode active material particles is also important.

[0233] Therefore, in the process of producing the positive electrode active material particles 100, it is preferable to first synthesize lithium cobalt oxide by mixing a first lithium source and a cobalt source and performing a heat treatment, then to mix the lithium cobalt oxide with a second lithium source and perform a heat treatment, and then to mix in an additive element source and perform a heat treatment.

[0234] In the method of synthesizing lithium cobalt oxide containing additive element A by mixing the first lithium source and cobalt source with additive element A source, it is difficult to increase the concentration of additive element A on the surface layer of the positive electrode active material particles. Furthermore, if the additive element A source is only mixed after the synthesis of lithium cobalt oxide without heating, the additive element A will only adhere to the lithium cobalt oxide without solid dissolving in it. Without sufficient heating, it is difficult to distribute the additive element A well. Therefore, it is preferable to synthesize lithium cobalt oxide first, then mix in the additive element A source, and then perform heat treatment.

[0235] However, if the heat treatment temperature is too high, cation mixing will occur, increasing the likelihood that the added element A, for example, magnesium, will enter the cobalt site. The magnesium present in the cobalt site is Li ｘ CoO ２ When x is small, the effect of maintaining the R-3m layered rock salt crystal structure is lost. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent cobalt and the evaporation of lithium.

[0236] [Initial Heating] Therefore, it is preferable to mix the lithium cobaltate with a second lithium source and heat it (also called initial heating) before mixing it with the additive element source and heating it. Here, it is preferable to use a material that functions as a flux as the second lithium source, for example lithium fluoride is suitable.

[0237] Furthermore, it is preferable to thoroughly mix the lithium cobalt oxide with the second lithium source. For example, mixing can be done using a ball mill.

[0238] In the process of thoroughly mixing lithium cobalt oxide with a second lithium source, the adhesion between lithium cobalt oxide particles is eliminated.

[0239] Furthermore, by adding a material that functions as a flux as a second lithium source, the re-adhesion of lithium cobalt oxide particles is suppressed in subsequent processes when the lithium cobalt oxide after initial heating is mixed with the added element source and heated.

[0240] If lithium cobalt oxide particles are stuck together and then additive element A is added and heated, there is a risk that additive element A will not be sufficiently distributed to the stuck areas. Therefore, when the sticking is resolved in a later process, such as a pressurizing process after coating the positive electrode current collector, surfaces where additive element A is not sufficiently added will be exposed, and degradation may progress from those surfaces when used in a secondary battery. For this reason, it is preferable to suppress the sticking of lithium cobalt oxide particles using the method described above before mixing in the additive element A source and heating.

[0241] Furthermore, by adding a flux during the initial heating process, when the lithium cobalt oxide after initial heating is mixed with the source of additive element A and heated in subsequent processes, a melting point depression occurs near the surface of the lithium cobalt oxide. This melting point depression makes it easier to distribute additive element A well at a temperature where cation mixing is less likely to occur.

[0242] Therefore, the heating temperature in the initial heating is preferably above the melting point of the second lithium source. For example, when lithium fluoride is used as the second lithium source, the temperature is preferably higher than the melting point of lithium fluoride, which is 848°C (for example, 850°C or higher), and more preferably 900°C or higher. Furthermore, the heating temperature in the initial heating is preferably below the temperature used when synthesizing lithium cobalt oxide (for example, 950°C or lower). In other words, it is preferable to mix lithium cobalt oxide and lithium fluoride and heat them at a temperature of 900°C to 950°C as the initial heating.

[0243] Furthermore, by using lithium cobalt oxide that has undergone the initial heating described above, it is possible to suppress lithium deficiency that occurs due to the evaporation of lithium during the subsequent process of mixing and heating the additive element A.

[0244] Lithium cobalt oxide that has undergone the initial heating described above preferably has a higher proportion of layered rock salt-type crystalline structure in the surface layer compared to lithium cobalt oxide before the initial heating. For example, in lithium cobalt oxide that has undergone the initial heating described above, it is preferable that the layered rock salt-type crystalline structure accounts for 35% or more, and more preferably 45% or more, in the portion within 2 nm from the surface. Furthermore, when EELS analysis is performed, it is preferable that the valence of cobalt is 2.35 or higher, and more preferably 2.45 or higher. A high proportion of layered rock salt-type crystalline structure is one factor indicating that lithium deficiency is suppressed.

[0245] On the other hand, it is preferable that not the entire surface layer of lithium cobalt oxide has a layered rock salt crystal structure. For example, the solid solubility limit of magnesium is extremely low in pure lithium cobalt oxide. In order to solid dissolve magnesium and other additive elements A at a sufficient concentration, it is preferable that the surface layer of lithium cobalt oxide has a rock salt crystal structure. Therefore, it is preferable that the lithium cobalt oxide that has undergone the above initial heating has a layered rock salt crystal structure of less than 100% within 2 nm from the surface, and more preferably 90% or less. Furthermore, when EELS analysis is performed, it is preferable that the valence of cobalt is less than 3.00, and more preferably 2.90 or less.

[0246] In other words, the lithium cobalt oxide that has undergone the initial heating described above preferably has a layered rock salt-type crystalline structure of 35% or more and less than 100% in the portion within 2 nm from the surface, more preferably 35% or more and 90%, and even more preferably 45% or more and 90%. Furthermore, when EELS analysis is performed, the valence of cobalt is preferably 2.35 or more and less than 3.00, more preferably 2.35 or more and 2.90, and even more preferably 2.45 or more and 2.90.

[0247] Method for producing positive electrode active material particles 100: Method 1 for producing positive electrode active material particles 100, which involves heat treatment and initial heating, will be explained using Figures 11A to 12C.

[0248] <Step S11> In step S11 shown in Figure 11B, a first lithium source (Li source 1) and a first cobalt source (Co source) are prepared as the starting materials, which are the first lithium and transition metal materials, respectively.

[0249] As the first lithium source, it is preferable to use a lithium-containing compound, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. The lithium source should preferably have high purity; for example, a material with a purity of 99.99% or higher is preferable.

[0250] As a cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide.

[0251] The cobalt source should preferably have high purity; for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N5 (99.995%) or higher, and even more preferably 5N (99.999%) or higher should be used. By using a high-purity material, impurities in the positive electrode active material particles can be controlled. As a result, the capacity of the secondary battery is increased and / or the reliability of the secondary battery is improved.

[0252] In addition, it is preferable that the cobalt source has high crystallinity, for example, that it has single crystal grains. The crystallinity of the cobalt source can be evaluated by TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, or by X-ray diffraction, electron diffraction, neutron diffraction, etc. The above methods for evaluating crystallinity can be applied not only to cobalt sources but also to the evaluation of other materials.

[0253] <Step S12> Next, as shown in Step S12 in Figure 11B, the first lithium source and cobalt source are crushed and mixed to produce a mixed material. Crushing and mixing can be done dry or wet. Wet crushing and mixing can produce finer particles. If wet crushing is used, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. It is more preferable to use an aprotic solvent that does not react easily with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the first lithium source and cobalt source with dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then crush and mix. By using dehydrated acetone of the above purity, the amount of impurities that may be mixed in can be reduced.

[0254] For grinding and mixing, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use aluminum oxide balls or zirconium oxide balls as the grinding media. Zirconium oxide balls are preferable because they produce less impurity. Also, when using a ball mill or a bead mill, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0255] <Step S13> Next, as step S13 shown in Figure 11B, the mixed material is heated. The heating is preferably carried out at 800°C to 1100°C, more preferably at 900°C to 1050°C, and even more preferably at 950°C to 1000°C. If the temperature is too low, the decomposition and melting of the first lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to the evaporation of lithium from the first lithium source and / or the excessive reduction of cobalt. For example, cobalt may change from trivalent to divalent, inducing oxygen defects, etc.

[0256] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably between 1 hour and 100 hours, and more preferably between 2 hours and 20 hours.

[0257] The heating rate depends on the target temperature, but a rate between 80°C / h and 250°C / h is preferable. For example, when heating to 1000°C for 10 hours, the heating rate can be 200°C / h.

[0258] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or lower, more preferably an atmosphere with a dew point of -80°C or lower. In this embodiment, heating will be carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, CH in the heating atmosphere is preferable. ４ CO, CO ２ , and H ２ It is preferable that the concentrations of these impurities be 5 ppb (parts per billion) or less.

[0259] An oxygen-containing atmosphere (oxygen atmosphere) is preferred as the heating atmosphere. For example, one method is to continuously introduce dry air into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flowing through the reaction chamber is called flow.

[0260] When the heating atmosphere is to be an oxygen-containing atmosphere, a method that does not involve flowing oxygen may be used. For example, the reaction chamber may be depressurized and then filled with oxygen (which can also be called purging), and the oxygen may be prevented from entering or leaving the reaction chamber to create an oxygen-containing atmosphere. For example, the reaction chamber may be depressurized to -970 hPa (differential pressure gauge) and then filled with oxygen to 50 hPa (differential pressure gauge).

[0261] After heating, natural cooling is acceptable, but it is preferable that the cooling time from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary; it is sufficient if it cools to a temperature acceptable for the next step.

[0262] Heating in this process may be carried out using a rotary kiln or a roller hearth kiln. When using a rotary kiln, heating can be performed while stirring, whether in a continuous or batch system.

[0263] When heating, the container holding the material to be heated is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that hardly allows impurities to mix with the material to be heated. In this embodiment, an aluminum oxide setter with a purity of 99.9% is used. When heating a mixture containing lithium fluoride (LiF), the LiF may volatilize due to heating, reducing the amount of LiF in the mixture. Therefore, when heating a mixture containing lithium fluoride, it is preferable to control the partial pressure of fluorine or fluoride in the atmosphere to an appropriate range. For example, it is preferable to cover the crucible or setter before heating because this can prevent the volatilization of the material to be heated. Mullite-cordierite may also be used as the material for the crucible and setter.

[0264] Furthermore, it is preferable to use a used crucible rather than a new one. In this specification, a new crucible is defined as one that has undergone the heating process of a material containing lithium, transition metal M, and / or additive element A two times or less. A used crucible is defined as one that has undergone the heating process of a material containing lithium, transition metal M, and / or additive element A three or more times. This is because, when a new crucible is used, there is a risk that some of the heated material, including lithium fluoride, may be absorbed, diffused, migrated, and / or adhered to the casing during heating. If some of the material is lost as a result, there is a growing concern that the distribution of elements on the surface of the positive electrode active material particles will not be within a desirable range. On the other hand, this risk is less with a used crucible.

[0265] After heating is complete, the material may be crushed and sieved as needed. When collecting the heated material, it may be transferred from the crucible to a mortar before collection. A zirconium oxide mortar is preferable for this purpose. Zirconium oxide mortars are made of a material that does not easily release impurities. Specifically, a zirconium oxide mortar with a purity of 90% or higher, preferably 99% or higher, should be used. The conditions described in step S13 can also be referred to in the heating steps described later, other than step S13.

[0266] <Step S14> Through the above steps, lithium cobalt oxide (LiCoO) shown in step S14 in Figure 11B is obtained. ２ ) can be synthesized. The lithium cobalt oxide (LiCoO) produced in this way can be synthesized. ２ ) is the starting material LiCoO in step S10 of Figure 11A. ２ It can be used as such.

[0267] <Step S10> The lithium cobalt oxide prepared in this way is used as the starting material LiCoO in step S10 of Figure 11A. ２ It can be used as such.

[0268] Although examples of producing lithium cobalt oxide by a solid-phase method have been shown as in steps S11 to S14, lithium cobalt oxide may also be produced by a coprecipitation method. Alternatively, lithium cobalt oxide may be produced by a hydrothermal method.

[0269] <Step S15> In step S15 shown in Figure 11A, a second lithium source (Li source 2) is prepared. It is preferable to use lithium fluoride as the second lithium source.

[0270] <Step S16> Next, in step S16 shown in Figure 11A, lithium cobalt oxide and the second lithium source are mixed. The mixing in step S16 is preferably carried out under milder conditions than the mixing in step S12 in order to avoid destroying the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter time than the mixing in step S12. Also, dry mixing is considered milder than wet mixing. For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, for example, it is preferable to use zirconium oxide balls as the media.

[0271] <Step S17> Next, as shown in Step S17 in Figure 11A, the lithium cobalt oxide and the second lithium source are heated. The heating is preferably carried out at 800°C to 1000°C, more preferably at 850°C to 950°C, and even more preferably at 900°C to 950°C. The heating time is preferably 1 hour to 60 hours, more preferably 2 hours to 20 hours, more preferably 2 hours to 10 hours, and even more preferably 2 hours to 6 hours. Because this is the first heating of the lithium cobalt oxide, the heating in Step S17 is sometimes called initial heating. Alternatively, because it is heated before Step S33 shown below, it may be called preheating or pretreatment. By carrying out Steps S16 and S17, lithium cobalt oxide with a smooth surface can be obtained.

[0272] In step S10, lithium cobalt oxide synthesized in step S14 may be used. In this case, steps S11 to S13 can be omitted. Even when using lithium cobalt oxide synthesized in advance, steps S16 and S17 can be performed to obtain lithium cobalt oxide with a smooth surface.

[0273] <Step S20a> Next, as shown in steps S20a to S33, it is preferable to add element A1 to the lithium cobalt oxide that has undergone initial heating. Adding element A1 to lithium cobalt oxide that has undergone initial heating allows for even addition of element A1. Therefore, it is preferable to add element A1 after initial heating. In step S20a, a first source of additive element (source A1) is prepared.

[0274] <Step S21> Details of step S20a are shown in Figure 12A. Steps S21 to S23 shown in Figure 12A are used to prepare a first additive element source (A1 source). As the first additive element A1, one or more selected from magnesium and fluorine can be suitably used. Figure 12A illustrates the case where a magnesium source (Mg source) and a fluorine source (F source) are used as the first additive element source.

[0275] Step S21 shown in Figure 12A will now be explained. When magnesium is selected as the additive element A, the additive element source can be called a magnesium source (Mg source). Magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate can be used as the magnesium source. Multiple magnesium sources may also be used.

[0276] When fluorine is selected as additive element A, the additive element source can be called a fluorine source (F source). Examples of such fluorine sources include lithium fluoride (LiF) and magnesium fluoride (MgF). ２ ), aluminum fluoride (AlF ３ ), cobalt fluoride (CoF ２ CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF) ２ ), cerium fluoride (CeF３ CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ ) and the like can be used. Among them, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating process described later. When lithium fluoride is used as the fluorine source, it can also be called the third lithium source.

[0277] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Another lithium source used in step S21 is lithium carbonate.

[0278] Furthermore, the fluorine source is fluorine (F ２ ), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ A gas such as F) may be used and mixed into the atmosphere during the heating process described later. Alternatively, multiple fluorine sources may be used.

[0279] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as both the fluorine source and the magnesium source. ２ Prepare the following: Lithium fluoride and magnesium fluoride are LiF:MgF ２ Mixing lithium fluoride and magnesium fluoride in a molar ratio of approximately 65:35 yields the greatest effect in lowering the melting point. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be in excess and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride should be LiF:MgF ２ Preferably, the ratio is x:1 (0 ≤ x ≤ 1.9), and LiF:MgF ２= x: 1 (0.1 ≤ x ≤ 0.5) is more preferable, and LiF: MgF ２ A more preferable value is x = 1 (x = 0.33 or its vicinity). In this specification, "nearby" means a value greater than 0.9 times the value and less than 1.1 times the value.

[0280] <Step S22> Next, in step S22 shown in Figure 12A, the magnesium source and the fluorine source are crushed and mixed. This step can be performed by selecting from the crushing and mixing conditions described in step S12.

[0281] <Step S23> Next, in step S23 shown in Figure 12A, the material that has been crushed and mixed above is recovered to obtain the additive element A1 source (A1 source). The additive element A1 source shown in step S23 has multiple starting materials and can be called a mixture.

[0282] The particle size of the above mixture is preferably such that the D50 (median diameter) is 600 nm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less. Even when one material is used as the additive element source, the D50 (median diameter) is preferably such that it is 600 nm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less.

[0283] When the mixture is finely powdered in this way (including cases where only one type of additive element A is present), it is easier to uniformly adhere the mixture to the surface of the lithium cobalt oxide particles when it is mixed with lithium cobalt oxide in a later process. When the mixture is uniformly adhered to the surface of the lithium cobalt oxide particles, it is preferable because it is easier to evenly distribute or diffuse the additive element A onto the surface layer of the positive electrode active material particles 100 after heating.

[0284] <Step S31> Next, in step S31 shown in Figure 11A, the lithium cobalt oxide that has undergone initial heating is mixed with the additive element A1 source (A1 source).

[0285] In this embodiment, the number of magnesium atoms contained in the additive element A1 source is preferably 0.50% to 3.0%, more preferably 0.75% to 2.0%, and even more preferably 0.75% to 1.0%, relative to the number of cobalt atoms in the lithium cobalt oxide after initial heating.

[0286] The mixing in step S31 is preferably carried out under milder conditions than the mixing in step S12 in order to avoid destroying the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter time than the mixing in step S12. Also, dry mixing is generally milder than wet mixing. For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0287] In this embodiment, the mixing is performed dry at 150 rpm for 1 hour using a ball mill with zirconium oxide balls with a diameter of 1 mm. The mixing is carried out in a dry room with a dew point of -100°C or higher and -10°C or lower.

[0288] <Step S32> Next, in step S32 of Figure 11A, the materials mixed above are collected to obtain mixture 903. Sieving may be performed during collection as necessary.

[0289] Although Figures 11A to 12A illustrate a manufacturing method in which additive element A is added only after initial heating, the present invention is not limited to the above method. Additive element A may be added at other times, or added multiple times. The timing may also be varied depending on the element.

[0290] <Step S33> Next, in step S33 shown in Figure 11A, the mixture 903 is heated.

[0291] For example, as additive element source A, MgF ２ If LiF and MgF are present, ２ Since the eutectic point is around 742°C, it is preferable that the heating temperature in step S33 be above the eutectic point (for example, 742°C or higher).

[0292] Also, LiCoO ２ : LiF : MgF ２ = 100:0.33:1 (molar ratio), the mixture obtained by mixing them shows an endothermic peak around 830°C in differential scanning calorimetry (DSC test). Since this endothermic peak is considered to be due to the melting of the mixture, a heating temperature of 830°C or higher is more preferable. Therefore, the heating in step S33 is preferably carried out at 800°C or higher and 1000°C or lower, more preferably at 830°C or higher and 950°C or lower, and still more preferably at 850°C or higher and 950°C or lower. Also, the heating time is preferably 1 hour or more and 60 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0293] In the manufacturing method described in this embodiment, LiF, which is the second lithium source added in steps S16 and S17 of FIG. 11A, may function as a flux.

[0294] Supplementary explanation about the heating time. The heating time varies depending on conditions such as the heating temperature, the size of lithium cobalt oxide in step S14, and the composition. When the lithium cobalt oxide is small, a lower heating temperature or a shorter heating time may be more preferable than when it is large.

[0295] <Step S34a> Next, the material heated in step S33 is recovered, and lithium cobalt oxide having the additive element A1 is produced. To distinguish it from the lithium cobalt oxide in step S14, it is called a composite oxide.

[0296] <Step S40> In step S40 shown in FIG. 11A, a second additive element source (A2 source) is prepared.

[0297] <Step S41> In Steps S41 to S43 shown in FIG. 12B, a second additive element source (A2 source) is prepared. As the additive element A2 included in the second additive element source, any one or more selected from nickel, boron, zirconium, and aluminum can be preferably used. In FIG. 12B, the case of using a nickel source (Ni source) and an aluminum source (Al source) as the second additive element source is exemplified. As the nickel source, for example, nickel hydroxide, nickel fluoride, etc. can be used. Also, as the aluminum source, for example, aluminum hydroxide, aluminum fluoride, etc. can be used.

[0298] The number of nickel atoms contained in the second additive element source (A2 source) is preferably 0.05% or more and 4.0% or less, more preferably 0.20% or more and 2.0% or less, and even more preferably 0.20% or more and 1.0% or less with respect to the number of cobalt atoms of the lithium cobalt oxide (composite oxide) having the additive element A1. For example, when nickel hydroxide is used as the nickel source, when the number of moles of lithium cobalt oxide in Step S10 is 100, the number of moles of nickel hydroxide contained in the second additive element source is preferably 0.05 or more and 4.0 or less (0.05 mol% or more and 4.0 mol% or less), more preferably 0.20 or more and 2.0 or less (0.20 mol% or more and 2.0 mol% or less), and even more preferably 0.20 or more and 1.0 or less (0.20 mol% or more and 1.0 mol% or less).

[0299] The number of aluminum atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0%, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%, relative to the number of cobalt atoms contained in the lithium cobalt oxide (composite oxide) having additive element A1. For example, when aluminum hydroxide is used as the aluminum source, when the number of moles of lithium cobalt oxide in step S10 is set to 100, the number of moles of aluminum hydroxide contained in the second additive element source is preferably 0.05 to 4.0 (0.05 mol% to 4.0 mol%), more preferably 0.20 to 2.0 (0.20 mol% to 2.0 mol%), and even more preferably 0.20 to 1.0 (0.20 mol% to 1.0 mol%).

[0300] Steps S41 to S43 shown in Figure 12B can be carried out under the same conditions as steps S21 to S23 shown in Figure 12A. As a result, an additive element source (A2 source) can be obtained in step S43.

[0301] Furthermore, Figure 12C shows a modified version of the steps described using Figure 12B. In step S41 shown in Figure 12C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, each is crushed. In other words, in step S43 shown in Figure 12C, multiple second additive element sources (A2 sources) are prepared.

[0302] <Steps S51 to S53> Next, steps S51 to S53 shown in Figure 11A can be carried out under the same conditions as steps S31 to S34. The heating in step S53 is preferably carried out at 800°C to 1000°C, more preferably at 800°C to 950°C, and even more preferably at 800°C to 900°C. The heating time is preferably 1 hour to 60 hours, more preferably 2 hours to 20 hours, and even more preferably 2 hours to 10 hours. It is preferable that the heating in step S53 is carried out at a lower heating temperature and for a shorter heating time than in step S33. Through the above steps, in step S54, positive electrode active material particles 100 according to one embodiment of the present invention can be produced. The positive electrode active material particles according to one embodiment of the present invention have a smooth surface.

[0303] As shown in Figures 11A to 12C, in the method for producing the positive electrode active material particles 100 described in this embodiment, the addition of additive element A to lithium cobalt oxide is carried out separately as additive element A1 and additive element A2. By adding additive element A1 and additive element A2 separately, the depth distribution of each additive element A can be changed. For example, it is possible to distribute additive element A1 at a higher concentration in the surface layer of the positive electrode active material particles compared to the interior, and to distribute additive element A2 at a higher concentration in the interior compared to the surface layer.

[0304] Furthermore, by performing steps S10 to S17 of the initial heating process shown in the method for producing positive electrode active material particles 100, the positive electrode active material particles 100 obtained in the final step S54 are LiTiO2 as explained in Figures 5A to 6F. ２ It becomes possible to have a crystal structure of a certain type.

[0305] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0306] (Embodiment 3) In this embodiment, each element constituting the battery will be described.

[0307] [Positive Electrode] The positive electrode comprises a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has positive electrode active material particles and may further have at least one of a conductive material and a binder. The positive electrode active material particles can be the positive electrode active material particles 100 described in Embodiment 1 and Embodiment 2.

[0308] The positive electrode current collector can be made of, for example, metal foil. The positive electrode can be formed by applying a slurry to the metal foil and drying it. Pressing may also be applied after drying. The positive electrode is formed by creating an active material layer on the positive electrode current collector.

[0309] A slurry is a liquid material used to form an active material layer on a positive electrode current collector. It contains an active material, a binder, and a solvent, and preferably also contains a conductive material. The slurry is sometimes called an electrode slurry or an active material slurry. When forming a positive electrode active material layer, a positive electrode slurry is used, and when forming a negative electrode active material layer, a negative electrode slurry is used.

[0310] The positive electrode active material particles have the function of taking in and releasing lithium ions during charging and discharging. The positive electrode active material particles 100 used in one embodiment of the present invention described above can be made of a material that does not degrade much during charging and discharging, even at high charging voltages.

[0311] <Binder> As a binder, it is preferable to use rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer. Fluororubber can also be used as a binder.

[0312] Furthermore, a water-soluble polymer may be used as the binder. Examples of water-soluble polymers include polysaccharides. Examples of polysaccharides include cellulose derivatives such as carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, diacetylcellulose, and regenerated cellulose, or starch. It is even more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0313] Alternatively, it is preferable to use materials such as polystyrene, methyl polyacrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, or nitrocellulose as the binder.

[0314] You may use a combination of several of the binders mentioned above.

[0315] <Conductive Materials> Conductive materials, also called conductivity imparters or conductivity enhancers, are typically made of carbon. By attaching conductive materials between multiple active materials, the multiple active materials are electrically connected to each other, increasing conductivity. Note that "attachment" does not only refer to physical contact between the active materials and the conductive material, but also includes cases where covalent bonds are formed, bonds are formed by van der Waals forces, the conductive material covers a portion of the surface of the active material, the conductive material fits into surface irregularities of the active material, or where they are electrically connected even without physical contact.

[0316] The active material layers, such as the positive electrode active material layer and the negative electrode active material layer, preferably contain a conductive material.

[0317] As conductive materials, one or more of the following can be used: carbon black such as acetylene black and furnace black; graphite such as artificial graphite and natural graphite; carbon fibers such as carbon nanofibers and carbon nanotubes; and graphene compounds.

[0318] As carbon fibers, for example, mesophase pitch carbon fibers and isotropic pitch carbon fibers can be used. Alternatively, carbon nanofibers or carbon nanotubes can be used. Carbon nanotubes can be fabricated, for example, by vapor deposition.

[0319] In this specification, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multigraphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multigraphene oxide, graphene quantum dots, etc. A graphene compound is defined as a material having carbon, having a plate-like or sheet-like shape, and possessing a two-dimensional structure formed by a six-membered carbon ring. This two-dimensional structure formed by a six-membered carbon ring may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a bent shape. Graphene compounds may also be rolled up to resemble carbon nanofibers.

[0320] The content of conductive material relative to the total amount of active material layer is preferably 1 wt% to 10 wt%, and more preferably 1 wt% to 5 wt%.

[0321] <Positive Electrode Current Collector> As the current collector, materials with high conductivity such as stainless steel, gold, platinum, aluminum, titanium, iron, and alloys thereof can be used. Furthermore, it is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. In addition, aluminum alloys to which elements that improve physical properties such as iron, silicon, titanium, neodymium, scandium, and molybdenum have been added can be used. Alternatively, it may be formed from a metallic element that reacts with silicon to form a silicide. Metallic elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in various shapes such as foil, plate, sheet, mesh, perforated metal, or expanded metal as appropriate. The current collector should preferably have a thickness of 5 μm or more and 30 μm or less.

[0322] [Negative electrode] The negative electrode comprises a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also contain a negative electrode active material, a conductive material, and a binder.

[0323] <Negative electrode active material> As the negative electrode active material, for example, alloy materials or carbon materials can be used.

[0324] Furthermore, the negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc., can be used. Such elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Compounds containing these elements may also be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ Mg ２ Sn, SnS ２ , V ２ Sn ３ FeSn ２ CoSn ２ Ni３ Sn ２ ,Cd ６ Sn ５ Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ LaSn ３ La ３ Co ２ Sn ７ CoSb ３ Examples include InSb and SbSn. For example, compounds of Si, SiO, or SiC with Ti may also be used. Here, elements capable of undergoing charge-discharge reactions through alloying and de-alloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloying materials.

[0325] In this specification, "SiO" refers to silicon monoxide, for example. Alternatively, SiO refers to SiO ｘ It can also be expressed as follows. Here, x preferably has a value of 1 or a value in the vicinity of 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0326] Carbon materials such as graphite, easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), carbon fibers (carbon nanotubes), graphene, and carbon black can be used.

[0327] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spheroidal graphite having a spherical shape can be used as artificial graphite. For example, MCMB may have a spherical shape and is therefore preferable. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, which may also be preferable. Examples of natural graphite include flake graphite and spheroidized natural graphite.

[0328] Graphite exhibits a potential as low as lithium metal (0.05V to 0.3V vs. Li / Li) when lithium ions are inserted into it (during the formation of lithium-graphite intercalation compounds). ＋). As a result, a lithium-ion battery using graphite can exhibit a high operating voltage. Furthermore, graphite is preferable because it has advantages such as a relatively high capacity per unit volume, a relatively small volume expansion, low cost, and high safety compared to lithium metal.

[0329] Also, as the negative electrode active material, titanium dioxide (TiO ２ ), lithium titanate (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and other oxides can be used.

[0330] As the conductive material and binder that the negative electrode active material layer can have, the same materials as those that the positive electrode active material layer can have can be used.

[0331] <Negative electrode current collector> In addition to the same materials as those of the positive electrode current collector, copper and the like can also be used for the negative electrode current collector. It is preferable to use a material that does not alloy with carrier ions such as lithium for the negative electrode current collector.

[0332] [Electrolyte] A secondary battery has an electrolyte containing carrier ions. In this specification and the like, the electrolyte is not limited to those containing an organic solvent that is liquid at room temperature, and includes a solid electrolyte, and also includes an electrolyte (semi-solid electrolyte) containing both an organic solvent that is liquid at room temperature and a solid electrolyte that is solid at room temperature. Note that a solution in which a lithium salt is dissolved in an organic solvent that is liquid at room temperature is sometimes referred to as an electrolyte solution.

[0333] <Organic solvent that is liquid at room temperature> An example of an organic solvent that is liquid at room temperature will be described below.

[0334] The organic solvent, which is liquid at room temperature, is preferably an aprotic organic solvent. For example, one or more of the following can be used: ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate (PP), methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc.

[0335] By using one or more flame-retardant and low-volatility ionic liquids (room-temperature molten salts) as organic solvents that are liquid at room temperature, it is possible to prevent the battery cell from rupturing and igniting even if the internal temperature rises due to an internal short circuit or overcharging. Ionic liquids consist of cations and anions, and include organic cations and anions. Examples of organic cations used in organic solvents include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, or aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in organic solvents include monovalent amide anions, monovalent methide anions, fluorosulfonic acid anions, perfluoroalkyl sulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, or perfluoroalkyl phosphate anions.

[0336] Furthermore, the lithium salt to be dissolved in the above organic solvent is, for example, LiPF ６ LiClO ４ LiAsF ６ LiBF ４ LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ Li２ B １０ Cl １０ Li ２ B １２ Cl １２ LiCF ３ SO ３ LiC ４ F ９ SO ３ LiC (CF ３ SO ２ ) ３ LiC(C ２ F ５ SO ２ ) ３ ,LiN(CF ３ SO ２ ) ２ ,LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), and LiN(C ２ F ５ SO ２ ) ２ You may use one or more selected from the above.

[0337] <Additives> The above organic solvent may contain additives. Additives can suppress the reaction decomposition of the electrolyte that may occur on the positive or negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives include vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate) borate (LiBOB). LiBOB is particularly preferred because it easily forms a good film. VC or FEC are preferred because they can form a good film on the negative electrode during aging or initial charging of the secondary battery, thereby improving cycle characteristics.

[0338] Dinitrile compounds can be used as additives. Specific examples of dinitrile compounds include succinonitrile, glutalonitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE). One or more of these dinitrile compounds can be used as additives.

[0339] Furthermore, fluorobenzene may be added to the above organic solvent. The concentration of the additive can be, for example, 0.1 wt% to 5 wt% of the total electrolyte. PS or EGBE is preferred because it can form a good film on the positive electrode during charging and discharging, improving cycle characteristics. FB is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes.

[0340] The electrolyte does not need to be liquid at room temperature; a semi-solid material called a polymer gel electrolyte may be used as the organic solvent. Using a polymer gel electrolyte enhances safety against leakage and other issues. It also allows for thinner and lighter battery cells.

[0341] As the polymer to be gelled, silicone gels, acrylic gels, acrylonitrile gels, polyethylene oxide-based gels, polypropylene oxide-based gels, fluorine-based polymer gels, and the like can be used.

[0342] Examples of polymers that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The resulting polymer may also have a porous structure.

[0343] [Separator] When the electrolyte contains an electrolyte solution, a separator is placed between the positive electrode and the negative electrode. As a separator, for example, materials such as paper and other cellulose fibers, nonwoven fabrics, glass fibers, ceramics, or porous films made of nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, polyimide, or polyurethane can be used. It is preferable that the separator be processed into a bag shape and placed so as to enclose either the positive electrode or the negative electrode.

[0344] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, hydroxide material, fluorine material, polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles, silicon oxide particles, and magnesium oxide. Examples of hydroxide materials include magnesium hydroxide and aluminum hydroxide. Examples of fluorine materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid and para-aramid).

[0345] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and improving the reliability of secondary batteries. Coating with fluorine-based materials facilitates better adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, particularly aramid, improves heat resistance, thereby enhancing the safety of secondary batteries.

[0346] For example, a polypropylene film may be coated on both sides with a mixture of aluminum oxide and aramid. Alternatively, the side of the polypropylene film in contact with the positive electrode may be coated with a mixture of aluminum oxide and aramid, and the side in contact with the negative electrode may be coated with a fluorine-based material.

[0347] By using a multi-layered separator, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, thus increasing the capacity per unit volume of the secondary battery.

[0348] [Outer Covering] The outer covering of a battery can be made of metal materials such as aluminum, stainless steel, or titanium, or resin materials. A film-like outer covering can also be used. As a film, for example, a three-layer film can be used, in which a highly flexible metal thin film or metal foil made of aluminum, stainless steel, titanium, copper, nickel, etc. is placed on a film made of polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., and an insulating synthetic resin film such as a polyamide resin or polyester resin is placed on the metal thin film as the outer surface of the outer covering. Such a multilayer film can be called a laminate film. In this case, the name of the material of the metal layer in the laminate film may be used to refer to it, such as aluminum laminate film, stainless steel laminate film, titanium laminate film, copper laminate film, nickel laminate film, etc.

[0349] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0350] (Embodiment 4) In this embodiment, an example of the shape of a secondary battery having a positive electrode manufactured by the manufacturing method described in the previous embodiment will be explained.

[0351] [Coin-type rechargeable battery] An example of a coin-type rechargeable battery is described below. Figure 13A is an exploded perspective view of a coin-type (single-layer flat type) rechargeable battery, Figure 13B is an external view, and Figure 13C is a cross-sectional view thereof. Coin-type rechargeable batteries are mainly used in small electronic devices.

[0352] Note that Figure 13A is a schematic diagram to show the overlapping of components (up / down relationship and positional relationship) for clarity. Therefore, Figures 13A and 13B are not perfectly identical corresponding diagrams.

[0353] In Figure 13A, the positive electrode 304, separator 310, negative electrode 307, spacer 322, and washer 312 are stacked. These are sealed with a gasket between the negative electrode can 302 and the positive electrode can 301. Note that the gasket for sealing is not shown in Figure 13A. The spacer 322 and washer 312 are used to protect the inside or fix the position within the can when the positive electrode can 301 and the negative electrode can 302 are pressed together. The spacer 322 and washer 312 are made of stainless steel or an insulating material.

[0354] The positive electrode 304 is a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305.

[0355] Figure 13B is a perspective view of the completed coin-type rechargeable battery.

[0356] The coin-type secondary battery 300 has a positive electrode casing 301, which also serves as the positive electrode terminal, and a negative electrode casing 302, which also serves as the negative electrode terminal, which are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with it. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with it. Furthermore, the negative electrode 307 is not limited to a laminated structure, and lithium metal foil or a lithium-aluminum alloy foil may be used.

[0357] Furthermore, the positive electrode 304 and negative electrode 307 used in the coin-type secondary battery 300 may each have the active material layer formed on only one side.

[0358] The positive electrode can 301 and the negative electrode can 302 can be made of metals such as nickel, aluminum, or titanium, or alloys thereof, or alloys of these with other metals (e.g., stainless steel), which are corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat them with nickel or aluminum to prevent corrosion caused by the electrolyte. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0359] The negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte solution, and as shown in Figure 13C, the positive electrode 304, separator 310, negative electrode 307, and negative electrode 302 are stacked in this order with the positive electrode 301 at the bottom, and the positive electrode 301 and negative electrode 302 are pressed together via a gasket 303 to manufacture a coin-type secondary battery 300.

[0360] By having the above configuration, a coin-type secondary battery 300 with high discharge capacity and excellent cycle characteristics can be obtained.

[0361] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Figure 14A. As shown in Figure 14A, the cylindrical secondary battery 616 has a positive electrode cap (battery cover) 601 on the top surface and a battery casing (outer casing) 602 on the sides and bottom. The positive electrode cap 601 and the battery casing (outer casing) 602 are insulated by a gasket (insulating packing) 610.

[0362] Figure 14B is a schematic diagram showing a cross-section of a cylindrical secondary battery. The cylindrical secondary battery shown in Figure 14B has a positive electrode cap (battery cover) 601 on the top surface and a battery casing (outer casing) 602 on the sides and bottom. The positive electrode cap and the battery casing (outer casing) 602 are insulated from each other by a gasket (insulating packing) 610.

[0363] Inside the hollow cylindrical battery can 602, a battery element is provided, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 in between. Although not shown, the battery element is wound around a central axis. The battery can 602 is closed at one end and open at the other end. The battery can 602 can be made of metals such as nickel, aluminum, or titanium, or alloys thereof, or alloys of these with other metals (for example, stainless steel), which are corrosion resistant to the electrolyte. Furthermore, it is preferable to coat the battery can 602 with nickel and aluminum, etc., to prevent corrosion by the electrolyte. Inside the battery can 602, the battery element in which the positive electrode, negative electrode, and separator are wound is sandwiched between a pair of opposing insulating plates 608 and insulating plate 609. In addition, a non-aqueous electrolyte (not shown) is injected into the inside of the battery can 602 in which the battery element is provided. The non-aqueous electrolyte can be the same as that used in coin-type secondary batteries.

[0364] Since the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form the active material on both sides of the current collector.

[0365] By using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode 604, a cylindrical secondary battery 616 with high capacity, high discharge capacity, and excellent cycle characteristics can be obtained.

[0366] A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Metal materials such as aluminum can be used for the positive electrode terminal 603, and copper can be used for the negative electrode terminal 607. The positive electrode terminal 603 is resistance-welded to the safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 disconnects the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the rise in the internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistance element whose resistance increases when the temperature rises, and it prevents abnormal heat generation by limiting the amount of current through the increase in resistance. PTC elements include barium titanate (BaTiO ３ ) semiconductor ceramics and the like can be used.

[0367] Figure 14C shows an example of an energy storage system 615. The energy storage system 615 has multiple secondary batteries 616. The positive electrode of each secondary battery is in contact with a conductor 624 separated by an insulator 625 and is electrically connected. The conductor 624 is electrically connected to a control circuit 620 via wiring 623. The negative electrode of each secondary battery is also electrically connected to the control circuit 620 via wiring 626. The control circuit 620 can be a charge / discharge control circuit that performs charging and / or discharging, or a protection circuit that prevents overcharging and / or over-discharging.

[0368] Figure 14D shows an example of an energy storage system 615. The energy storage system 615 has multiple secondary batteries 616, which are sandwiched between conductive plates 628 and 614. The multiple secondary batteries 616 are electrically connected to the conductive plates 628 and 614 by wiring 627. The multiple secondary batteries 616 may be connected in parallel, in series, or connected in parallel and then in series. By configuring an energy storage system 615 with multiple secondary batteries 616, a large amount of power can be extracted.

[0369] Multiple secondary batteries 616 may be connected in parallel and then further connected in series.

[0370] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When a secondary battery 616 overheats, it can be cooled by the temperature control device, and when a secondary battery 616 becomes too cold, it can be heated by the temperature control device. This makes the performance of the energy storage system 615 less susceptible to the influence of ambient temperature.

[0371] Furthermore, in Figure 14D, the energy storage system 615 is electrically connected to the control circuit 620 via wiring 621 and wiring 622. Wiring 621 is electrically connected to the positive terminals of the multiple secondary batteries 616 via conductive plate 628, and wiring 622 is electrically connected to the negative terminals of the multiple secondary batteries 616 via conductive plate 614.

[0372] <Laminated Secondary Battery> Next, an example of an external view of a laminated secondary battery is shown in Figures 15A and 15B. Figures 15A and 15B show a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0373] Figure 16A shows the external view of the positive electrode 503 and the negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and the positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as the tab region). The negative electrode 506 has a negative electrode current collector 504, and the negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., the tab region. Note that the area or shape of the tab regions of the positive and negative electrodes are not limited to the example shown in Figure 16A.

[0374] <Method for Manufacturing Laminated Secondary Batteries> An example of a method for manufacturing laminated secondary batteries, whose external appearance is shown in Figure 15A, will be explained using Figures 16B and 16C.

[0375] First, the negative electrode 506, separator 507, and positive electrode 503 are stacked. Figure 16B shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example using five sets of negative electrodes and four sets of positive electrodes is shown. This can also be called a laminate consisting of negative electrodes, separators, and positive electrodes. Next, the tab regions of the positive electrode 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For joining, for example, ultrasonic welding may be used. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0376] Next, the negative electrode 506, separator 507, and positive electrode 503 are placed on the outer casing 509.

[0377] Next, as shown in Figure 16C, the outer casing 509 is bent at the portion indicated by the dashed line. Then, the outer periphery of the outer casing 509 is joined. For joining, for example, heat compression bonding may be used. At this time, a region that is not joined (hereinafter referred to as an inlet) is provided on a part (or one side) of the outer casing 509 so that the electrolyte can be added later.

[0378] Next, the electrolyte is introduced into the inside of the outer casing 509 through an inlet provided in the outer casing 509. It is preferable to introduce the electrolyte under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is sealed. In this way, a laminate-type secondary battery 500 can be manufactured.

[0379] By using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode 503, a secondary battery 500 with high capacity, high discharge capacity, and excellent cycle characteristics can be obtained.

[0380] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0381] (Embodiment 5) In this embodiment, as an example of mounting a secondary battery in a vehicle, an example is shown in which a lithium-ion battery according to one aspect of the present invention is mounted in a motorcycle and a bicycle.

[0382] Figure 17A shows an example of an electric bicycle using a power storage device according to one aspect of the present invention. The power storage device according to one aspect of the present invention can be applied to the electric bicycle 8700 shown in Figure 17A. The power storage device according to one aspect of the present invention includes, for example, a plurality of batteries and a protection circuit.

[0383] The electric bicycle 8700 is equipped with a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists the rider. The power storage device 8702 is also portable, and Figure 17B shows it detached from the bicycle. The power storage device 8702 also has multiple storage batteries 8701, which are part of a power storage device according to one embodiment of the present invention, and the remaining battery level can be displayed on a display unit 8703. The power storage device 8702 also has a control circuit 8704 that can control the charging of the secondary battery or detect abnormalities. The control circuit 8704 is electrically connected to the positive and negative electrodes of the storage battery 8701. Furthermore, by combining it with a secondary battery that uses the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode, a synergistic effect on safety can be obtained. The secondary battery and control circuit 8704 that use the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0384] Figure 17C shows an example of a two-wheeled vehicle using a power storage device according to one embodiment of the present invention. The scooter 8600 shown in Figure 17C is equipped with a power storage device 8602, side mirrors 8601, and turn signals 8603. The power storage device 8602 can supply electricity to the turn signals 8603. Furthermore, the power storage device 8602, which houses multiple secondary batteries using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode, can have a high capacity and contribute to miniaturization.

[0385] Furthermore, the scooter 8600 shown in Figure 17C can accommodate the power storage device 8602 in the under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

[0386] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0387] (Embodiment 6) This embodiment describes an example in which a secondary battery, which is one aspect of the present invention, is mounted on an electronic device. Examples of electronic devices on which a secondary battery is mounted include television equipment (also called televisions or television receivers), monitors for computers, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game consoles, personal information terminals, sound playback devices, and large game machines such as pachinko machines. Personal information terminals include notebook personal computers, tablet terminals, e-book readers, and mobile phones.

[0388] Figure 18A shows an example of a mobile phone. The mobile phone 2100 includes a display unit 2102 built into the housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. The mobile phone 2100 also has a secondary battery 2107. By providing a secondary battery 2107 that uses the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode, a high capacity can be achieved, and a configuration that can accommodate space saving due to the miniaturization of the housing can be realized.

[0389] The mobile phone 2100 can run various applications such as making mobile phone calls, sending emails, reading and creating documents, playing music, communicating on the internet, and playing computer games.

[0390] The operation button 2103 can be assigned various functions, including time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, and power saving mode activation / deactivation. For example, the function of the operation button 2103 can be freely configured by the operating system built into the mobile phone 2100.

[0391] Furthermore, the mobile phone 2100 is capable of performing standardized short-range wireless communication. For example, it can communicate with a wireless communication-enabled headset to enable hands-free calling.

[0392] Furthermore, the mobile phone 2100 is equipped with an external connection port 2104, which allows it to directly exchange data with other information terminals via a connector. It can also be charged via the external connection port 2104. However, charging may be performed wirelessly without using the external connection port 2104.

[0393] Furthermore, it is preferable that the mobile phone 2100 has sensors. Preferably, the sensors include, for example, human body sensors such as fingerprint sensors, pulse sensors, and body temperature sensors, touch sensors, pressure sensors, or acceleration sensors.

[0394] Figure 18B shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 has a secondary battery 2301, a camera 2303, and an antenna (not shown), which are embodiments of the present invention. The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and high safety, so it can be used safely for a long period of time over a long period of time and is suitable as a secondary battery to be mounted on an unmanned aerial vehicle 2300.

[0395] Figure 18C shows an example of a robot. The robot 6400 shown in Figure 18C is equipped with a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, and the like.

[0396] Microphone 6402 has the function of detecting the user's voice and ambient sounds. Speaker 6404 has the function of emitting sound. Robot 6400 can communicate with the user using microphone 6402 and speaker 6404.

[0397] The display unit 6405 has the function of displaying various types of information. The robot 6400 can display the information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, and by installing it in a fixed position on the robot 6400, charging and data transfer can be made possible.

[0398] The upper camera 6403 and the lower camera 6406 have the function of imaging the area around the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize its surrounding environment and move safely using the upper camera 6403, the lower camera 6406 and the obstacle sensor 6407.

[0399] The robot 6400 is equipped with a secondary battery 6409 according to one aspect of the present invention and a semiconductor device or electronic components in its internal region. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and high safety, so it can be used safely for a long period of time over a long period of time and is suitable as a secondary battery 6409 to be mounted on the robot 6400.

[0400] The robot 6500 shown in Figure 18D includes a lithium-ion battery 6505, a computing unit 6510, an illuminance sensor 6501, a microphone 6502, an upper camera 6503, a speaker 6504, a lower camera 6506, and an obstacle sensor 6507. Note that a humanoid robot is shown here as an example.

[0401] In the robot 6500, the above-mentioned semiconductor devices and / or electronic components can be used in the computing device 6510, the illuminance sensor 6501, the upper camera 6503, the lower camera 6506, and the obstacle sensor 6507, etc.

[0402] The microphone 6502 has the function of detecting the user's voice and ambient sounds. The speaker 6504 has the function of emitting sound. The robot 6500 can communicate with the user using the microphone 6502 and speaker 6504.

[0403] The upper camera 6503 and the lower camera 6506 have the function of imaging the area around the robot 6500. In addition, the obstacle sensor 6507 can detect the presence or absence of obstacles in the direction of travel when the robot 6500 moves forward on two legs. The robot 6500 can recognize its surrounding environment and move safely using the upper camera 6503, the lower camera 6506 and the obstacle sensor 6507.

[0404] Figure 18E shows an example of a cleaning robot. The cleaning robot 6300 has a display unit 6302 located on the top surface of the housing 6301, multiple cameras 6303 located on the sides, a brush 6304, operation buttons 6305, a secondary battery 6306, and various sensors. Although not shown, the cleaning robot 6300 is equipped with wheels, a suction port, etc. The cleaning robot 6300 is self-propelled, can detect dirt 6310, and can suck up the dirt from a suction port located on the bottom surface.

[0405] The cleaning robot 6300 can analyze images captured by the camera 6303 to determine the presence or absence of obstacles such as walls, furniture, or steps. Furthermore, if the image analysis detects an object that may become entangled in the brush 6304, such as wiring, the rotation of the brush 6304 can be stopped. The cleaning robot 6300 is equipped with a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and high safety, so it can be used safely for a long period of time over a long period of time and is suitable as a secondary battery 6306 to be mounted on the cleaning robot 6300.

[0406] Figure 19A shows an example of a wearable device. Wearable devices use rechargeable batteries as a power source. Furthermore, in order to enhance splash resistance, water resistance, or dust resistance when used by users in daily life or outdoors, there is a demand for wearable devices that can be charged wirelessly in addition to wired charging with exposed connectors.

[0407] For example, a secondary battery according to one embodiment of the present invention can be mounted in a spectacle-type device 4000 as shown in Figure 19A. The spectacle-type device 4000 has a frame 4000a and a display unit 4000b. By mounting the secondary battery in the temple portion of the curved frame 4000a, a lightweight spectacle-type device 4000 with good weight balance and a long continuous usage time can be made. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and can realize a configuration that can accommodate space saving due to the miniaturization of the housing.

[0408] Furthermore, a secondary battery according to one aspect of the present invention can be mounted in the headset-type device 4001. The headset-type device 4001 has at least a microphone section 4001a, a flexible pipe 4001b, and an earphone section 4001c. The secondary battery can be provided in the flexible pipe 4001b or in the earphone section 4001c. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and can realize a configuration that can accommodate space saving due to the miniaturization of the housing.

[0409] Furthermore, a secondary battery according to one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. The secondary battery 4002b can be provided inside the thin housing 4002a of the device 4002. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and can realize a configuration that can accommodate space saving due to the miniaturization of the housing.

[0410] Furthermore, a secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. The secondary battery 4003b can be provided inside the thin housing 4003a of the device 4003. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and can realize a configuration that can accommodate space saving due to the miniaturization of the housing.

[0411] Furthermore, a secondary battery according to one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 has a belt portion 4006a and a wireless power supply and receiving portion 4006b, and a secondary battery can be mounted in the internal region of the belt portion 4006a. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and can realize a configuration that can accommodate space saving due to the miniaturization of the housing.

[0412] Furthermore, a secondary battery according to one embodiment of the present invention can be mounted in the wristwatch-type device 4005. The wristwatch-type device 4005 has a display unit 4005a and a belt unit 4005b, and the secondary battery can be provided in either the display unit 4005a or the belt unit 4005b. The secondary battery using the positive electrode active material particles 100 described in Embodiments 1 and 2 as the positive electrode has a high energy density and can realize a configuration that can accommodate space saving due to the miniaturization of the housing.

[0413] The display unit 4005a can display not only the time, but also various other information such as incoming emails or phone calls.

[0414] Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors to measure the user's pulse, blood pressure, etc. It can accumulate data on the user's exercise level and health, and manage their health.

[0415] Figure 19B shows a perspective view of the wristwatch-type device 4005 after it has been removed from the arm.

[0416] A side view is also shown in Figure 19C. Figure 19C shows how the secondary battery 913 is built into the internal region. The secondary battery 913 is the secondary battery shown in Embodiment 4. The secondary battery 913 is located in a position that overlaps with the display unit 4005a, and can be made high-density and high-capacity, as well as small and lightweight.

[0417] Since the wristwatch-type device 4005 is required to be small and lightweight, by using the positive electrode active material particles 100 described in Embodiments 1 and 2, etc., as the positive electrode of the secondary battery 913, a secondary battery 913 with high energy density and small size can be made.

[0418] The contents of this embodiment can be freely combined with the contents of other embodiments.

[0419] In this embodiment, lithium cobalt oxide that has undergone initial heating was mixed with magnesium, nickel, and aluminum to produce positive electrode active material particles, which were prepared in accordance with Embodiment 2, and their properties were evaluated.

[0420] <Preparation of positive electrode active material particles> The preparation of positive electrode active material particles in this embodiment will be described with reference to the preparation method shown in Figures 11A to 12B.

[0421] <Sample 1> LiCoO in step S10 of Figure 11A ２ As the starting material, commercially available lithium cobalt oxide (manufactured by Nippon Chemical Industrial Co., Ltd., Cellseed C-10N) containing cobalt as the transition metal M and not containing any additive element A was prepared. As the second lithium source (Li source 2) in step S15, lithium fluoride was prepared. In step S16, the lithium cobalt oxide and lithium fluoride were mixed, and then, as the initial heating in step S17, the mixture was placed in a crucible, covered, and heated in a muffle furnace at 850°C for 2 hours. After creating an oxygen atmosphere inside the muffle furnace, no flow occurred (O ２(Purge). In the mixing of step S16, lithium fluoride was weighed and mixed so that the number of moles of lithium cobaltate in step S10 was 0.33 (0.33 mol%), with the number of moles of lithium cobaltate in step S10 being 100. A ball mill was used for mixing, and zirconium oxide balls were used as the media. After mixing, the mixture was sieved through a sieve with a mesh size of 300 μm.

[0422] According to steps S21 to S23 shown in Figure 12A and steps S41 to S43 shown in Figure 12B, the additive element A was added in two parts: Mg and F, and Ni and Al.

[0423] In step S21 shown in Figure 12A, lithium fluoride (LiF) is prepared as the F source, and magnesium fluoride (MgF) is prepared as the Mg source. ２ I prepared the following: LiF:MgF ２ The elements were weighed in a 1:3 (molar ratio). Next, lithium fluoride and magnesium fluoride were mixed in dehydrated acetone (step S22), and the mixture was stirred at a rotational speed of 400 rpm for 12 hours to prepare the additive element source (A1 source). A ball mill was used for mixing, and zirconium oxide balls were used as the grinding medium. After mixing, the mixture was sieved through a sieve with a mesh size of 300 μm to obtain the A1 source (step S23).

[0424] Next, in step S31, when the number of moles of lithium cobalt oxide is set to 100, the amount of magnesium fluoride contained in source A1 is weighed so that the number of moles of magnesium fluoride is 1 (1 mol%), and it is mixed dry with the lithium cobalt oxide after initial heating. At this time, it is stirred at a rotational speed of 150 rpm for 1 hour. This is a condition in which the force applied to the powder is less than that applied when stirring to obtain source A1. Finally, it is sieved with a sieve having a mesh size of 300 μm to obtain a mixture 903 with uniform particle size (step S32).

[0425] Next, in step S33, the mixture 903 was heated. The heating conditions were 900°C for 20 hours. During heating, the crucible containing the mixture 903 was covered. The inside of the crucible was kept in an oxygen atmosphere, and the entry and exit of oxygen was blocked (purged). By heating, a composite oxide containing Mg and F was obtained (step S34a).

[0426] Next, in step S51, the composite oxide and the additive element source (A2 source) were mixed. Following step S41 shown in Figure 12B, nickel hydroxide was prepared as the Ni source and aluminum hydroxide was prepared as the Al source. When the number of moles of lithium cobaltate was set to 100, the number of moles of nickel hydroxide contained in the A2 source was 0.5 (0.5 mol%), and the number of moles of aluminum hydroxide was 0.5 (0.5 mol%). These were weighed and mixed dry with the composite oxide. At this time, the mixture was stirred at a rotational speed of 150 rpm for 1 hour. A ball mill was used for mixing, and zirconium oxide balls were used as the media. After mixing, the mixture was sieved through a sieve with a mesh size of 300 μm to obtain a mixture 904 with uniform particle size (step S52).

[0427] Next, in step S53, the mixture 904 was heated. The heating conditions were 850°C for 10 hours. During heating, the crucible containing the mixture 904 was covered. The crucible was kept in an oxygen-containing atmosphere, and the entry and exit of the oxygen was blocked (purged). By heating, lithium cobaltate containing Mg, F, Ni, and Al was obtained (step S54). The positive electrode active material particles (composite oxide) obtained in this way were designated as Sample 1.

[0428] <Sample 2> Sample 2 was prepared in the same manner as Sample 1, except that the heating temperature was changed to 875°C in step S17.

[0429] <Sample 3> Sample 3 was prepared in the same manner as Sample 1, except that the heating temperature was changed to 900°C in step S17.

[0430] <Sample 4> Sample 4 was prepared in the same manner as Sample 1, except that the heating temperature was changed to 925°C in step S17.

[0431] <Sample 5> Sample 5 was prepared in the same manner as Sample 1, except that the heating temperature was changed to 950°C in step S17.

[0432] <STEM-EDX Analysis> Cross-sectional STEM-EDX analysis was performed on the edge region of the surface layer of sample 3. Note that the above cross-sectional STEM-EDX analysis is a line analysis in the depth direction from the outside to the inside of the sample.

[0433] As a pretreatment before analysis, sample 3 was sectioned using the FIB method (μ-sampling method).

[0434] The following equipment and conditions were used for STEM and EDX. <<STEM Observation>> Scanning transmission electron microscope: JEOL JEM-ARM200F NEOARM Observation conditions: Acceleration voltage: 200kV Magnification accuracy: ±3% <<EDX>> Analysis method: Energy-dispersive X-ray spectroscopy (EDX) Scanning transmission electron microscope: JEOL JEM-ARM200F NEOARM Acceleration voltage: 200kV Measurement mode: STEM mode Elemental analyzer: JEOL Dual SDD detector JED-2300T X-ray detection area: 158mm ２ ×2 Measurement conditions Dell time: 50 μsec Number of frames: 400 Number of captured pixels: 256 × 256

[0435] Figure 20 shows the cross-sectional STEM-EDX analysis area of ​​sample 3. Figure 20 is a magnified HAADF-STEM image of the edge region of sample 3, and the surface of the edge region is presumed to be a (10-2) plane.

[0436] The results of the cross-sectional STEM-EDX analysis of sample 3 are shown in Figures 21A to 22B.

[0437] <Edge Region> Figure 21A is a graph showing the results of cross-sectional STEM-EDX radiation analysis in the edge region of sample 3, with the vertical axis representing the amount of characteristic X-rays detected (counts). Figure 21B is a graph showing the same data as in Figure 21A, but with the vertical axis representing the quantitative value of concentration (atomic%). In the horizontal axis of Figures 21A and 21B, the reference point on the particle surface is estimated to be at a position of 10 nm. The position of the reference point was determined by the method described in the previous embodiment (using 50% of the count of characteristic X-rays for oxygen).

[0438] Furthermore, Figure 22A is a graph with the vertical axis of Figure 21A magnified, and Figure 22B is a graph with the vertical axis of Figure 21B magnified.

[0439] Table 1 shows the peak value (also called the maximum value) and its position of the concentration of additive element A, based on the STEM-EDX analysis results shown in Figure 22B.

[0440] Note that the quantitative values ​​of concentrations (atomic %) in the above figure and Table 1 were calculated by setting the sum of the concentrations of carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, and gallium to 100%. Since fluorine was below the detection limit of 1.5 atomic percent, the maximum concentration and its position at that point are not indicated.

[0441]

[0442] As shown in Figures 21A to 22B and Table 1, the edge region of sample 3 was confirmed to contain magnesium, nickel, and aluminum. Furthermore, the concentration and detection peaks of magnesium had a steep peak shape, with most of its convex shape confined within a 10 nm range, indicating that a large amount of magnesium is present on the surface side of the surface layer of the particles in sample 3. Similarly, the concentration and detection peaks of nickel also had a steep peak shape, with most of its convex shape confined within a 10 nm range, indicating that a large amount of nickel is present on the surface side of the surface layer of the particles in sample 3. On the other hand, the concentration and detection peaks of aluminum were asymmetrical and had a gentler shape towards the interior of the particles, compared to those of magnesium and nickel. Specifically, the slope from the peak top towards the interior was gentler compared to the slope from the peak top towards the surface. In other words, it was confirmed that the detection amount and concentration of aluminum decreased gently from the surface layer towards the interior of the particles in sample 3.

[0443] In other words, magnesium and nickel were found to have steep peaks within 10 nm from the reference point of the Sample 3 particles toward the interior of the particles. Furthermore, aluminum was found to have asymmetrical, gently sloping peaks toward the interior of the particles beyond 10 nm from the reference point of the Sample 3 particles toward the interior of the particles.

[0444] Furthermore, the nickel distribution in the edge region of sample 3 largely overlapped with the magnesium distribution, and the difference in depth between the peak of nickel concentration or detected amount and the peak of magnesium concentration or detected amount was within 1 nm, specifically within 0.5 nm.

[0445] Furthermore, the magnesium concentration peaks in the edge region of sample 3, and the magnesium concentration peaks in the edge region, were located within a depth of 3 nm from the reference point toward the center. More specifically, the magnesium concentration peaks in the edge region of sample 3 were located within a depth of 1 nm from the reference point toward the center. Also, the nickel concentration peaks in the edge region of sample 3 were located within a depth of 1 nm from the reference point toward the center.

[0446] Furthermore, the peak in aluminum concentration in the edge region of sample 3 was located further inside the particles of sample 3 compared to those of magnesium and nickel.

[0447] <XPS Analysis> XPS analysis was performed on the particle surfaces of samples 1 to 5 prepared above. The XPS measurement equipment and conditions were as follows: Measurement equipment: ULVAC-PHIE Quantera II X-ray: Monochromatic Al Kα (1486.6 eV) Energy resolution: Full width at half maximum of Ag3d5 / 2 peak is 1.0 eV ± 0.1 eV or less Detection area: 100 μmφ Detector angle: 45° Measurement spectrum: Wide scan, narrow scan of each detected element

[0448] The results of the XPS analysis are shown in Table 2.

[0449]

[0450] Table 2 shows the concentrations of each element (at%: atomic concentration) when the total concentration of lithium (Li), cobalt (Co), titanium (Ti), oxygen (O), carbon (C), fluorine (F), sulfur (S), calcium (Ca), magnesium (Mg), sodium (Na), zirconium (Zr), nickel (Ni), silicon (Si), and aluminum (Al) is set to 100 at%. Note that in this example, the analysis results are rounded, so the total may not always equal 100%.

[0451] Table 2 also shows the concentrations of magnesium (Mg / Co), nickel (Ni / Co), and aluminum (Al / Co) when the cobalt concentration is set to 1. It also shows the concentration of fluorine (F / Mg) when the magnesium concentration is set to 1.

[0452] Figure 23A is a graph showing the above-mentioned Mg / Co and F / Mg values ​​for Samples 1 to 5. Figure 23B is a graph showing the above-mentioned Ni / Co and Al / Co values ​​for Samples 1 to 5. The horizontal axis of the graphs in Figures 23A and 23B represents the heating temperature conditions in step S17 for each sample, and the horizontal axis of the graphs represents the respective concentration ratios.

[0453] As shown in Figure 23A, compared to the samples where the heating temperature in step S17 was less than 900°C (Sample 1 and Sample 2), the samples where the heating temperature was 900°C or higher (Sample 3, Sample 4, and Sample 5) tended to have lower Mg / Co and F / Mg values. Furthermore, in the samples where the heating temperature in step S17 was 900°C or higher, the Mg / Co value was between 0.50 and 0.70, and the F / Mg value was between 0.10 and 0.20.

[0454] As shown in Figure 23B, the Ni / Co and Al / Co values ​​tended to decrease as the heating temperature in step S17 increased. Furthermore, the significant changes observed in the Ni / Co and Al / Co values ​​between the 875°C and 900°C conditions, as seen in the Mg / Co and F / Mg values, were not observed in the Ni / Co and Al / Co values.

[0455] Next, we focused on the Mg1s peak in the XPS spectrum obtained from XPS analysis and performed an analysis. In the analysis of the Mg1s peak, the peak component derived from the O-Mg-O bond was designated as Fit Peak 1, the peak component derived from the O-Mg-F bond as Fit Peak 2, and the peak component derived from the F-Mg-F bond as Fit Peak 3. These three fit peaks were synthesized, and the ratio of the synthesized peak that minimized the difference with the Mg1s peak in the XPS spectrum obtained from the XPS analysis was calculated. The analysis results obtained by assuming that the area ratios of Fit Peak 1, Fit Peak 2, and Fit Peak 3 represent the relative abundance of the O-Mg-O bond, O-Mg-F bond, and F-Mg-F bond are shown in Table 3.

[0456] In the above XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also called the peak top) of fitted peak 1 is MgO-coated LiCoO ２ The energy value at the maximum value of the Mg1s peak when measured separately using magnesium fluoride (MgF) was referenced. Furthermore, the energy value at the maximum value of fitted peak 3 (Ep3) was determined using magnesium fluoride (MgF). ２ The energy value at the maximum value of the Mg1s peak was referenced when MGH18XB, purity 99.9% (3N) up), from the High Purity Chemical Laboratory, was measured separately as a standard sample. Furthermore, the energy value at the maximum value of Fit Peak 2 (Ep2) was set to an intermediate value between Ep1 and Ep3. The energy value at the maximum value of the peak is also called the peak position.

[0457] Furthermore, the XPS spectrum used in the XPS analysis was corrected for the energy axis so that the maximum value of the C1s peak was 284.8 eV.

[0458]

[0459] Table 3 shows the heating temperature conditions in step S17 for samples 1 to 5, the Mg1s analysis results in XPS analysis, the peak position of the Mg1s peak for each sample, and the full width at half maximum of the peak.

[0460] As shown in Table 3, analysis of the XPS spectra of Samples 1 to 5 revealed that samples heated at temperatures below 900°C in step S17 (Sample 1 and Sample 2) had peak components derived from O-Mg-O bonds and O-Mg-F bonds, while samples heated at 900°C or higher (Sample 3, Sample 4, and Sample 5) had peak components derived from O-Mg-O bonds. It should be noted that even when the analysis results are shown as 0.0%, this does not mean that the corresponding bond is completely absent. In other words, the bond shown as 0.0% may be present below the detection limit.

[0461] Furthermore, as shown in Table 3, in the XPS spectra of Samples 1 to 5, the full width at half maximum of the Mg1s peak was 2.9 eV to 3.0 eV for the samples heated at less than 900°C in step S17 (Sample 1 and Sample 2), while it was 2.5 eV to 2.6 eV for the samples heated at 900°C or higher (Sample 3, Sample 4, and Sample 5).

[0462] In other words, the XPS analysis results showed that, compared to the samples heated at a temperature of less than 900°C in step S17 (Sample 1 and Sample 2), the samples heated at 900°C or higher (Sample 3, Sample 4, and Sample 5) had 100% of the peak components derived from O-Mg-O bonds, O-Mg-F bonds, and F-Mg-F bonds, when the sum of these components is set to 100. Furthermore, compared to the samples heated at a temperature of less than 900°C in step S17 (Sample 1 and Sample 2), the samples heated at 900°C or higher (Sample 3, Sample 4, and Sample 5) had a narrower full width at half maximum (FMAX) of the Mg1s peak, ranging from 2.5 eV to 2.6 eV.

[0463] From the above, it was revealed that the samples (Sample 3, Sample 4, and Sample 5) in which the heating temperature in step S17 was 900°C or higher fell within the preferred range described in Embodiment 1 (100% of the peak components originating from the O-Mg-O bond, and the full width at half maximum of the Mg1s peak was within the range of 1.0 eV to 2.6 eV).

[0464] In Example 2, discharge rate tests and charge-discharge cycle tests were performed using samples 1 to 5 prepared in Example 1.

[0465] <Preparation of the positive electrode> Samples 1 to 5 above were prepared as positive electrode active material particles, acetylene black (AB) was prepared as the conductive material, and polyvinylidene fluoride (PVDF) was prepared as the binder. The PVDF was prepared in advance by dissolving it in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, a slurry was prepared by mixing each of the positive electrode active material particles in a ratio of positive electrode active material particles:AB:PVDF = 95:3:2 (weight ratio), and this slurry was coated onto an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After coating the positive electrode current collector with the slurry, the solvent was evaporated.

[0466] Subsequently, to increase the density of the positive electrode active material layer on the positive electrode current collector, a press treatment was performed using a roll press machine. The press treatment conditions were as follows: a first press (linear pressure 210 kN / m) followed by a second press (linear pressure 1467 kN / m). The upper and lower rolls of the roll press machine were both set to 120°C.

[0467] The positive electrode was obtained through the above process. The amount of positive electrode active material particles supported per unit area of ​​the positive electrode was 14 mg / cm². ２ 15mg / cm or more ２ The following was observed. Using this manufacturing method, positive electrodes having sample 1 through sample 5 were produced.

[0468] <Half-cell fabrication> A coin-shaped half-cell was fabricated using the above-mentioned positive electrode, lithium metal foil, separator, electrolyte, coin cell positive electrode container, and coin cell negative electrode container. The shape of the coin-shaped half-cell was that of a CR2032 type (20 mm in diameter, 3.2 mm in height).

[0469] As the electrolyte, a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7 was used, to which 1 mol / L of lithium hexafluoride phosphate (LiPF) was added. ６A mixture containing 2 wt% vinylene carbonate (VC) as an additive was used.

[0470] A porous polypropylene film was used as the separator.

[0471] In this way, half-cells using the positive electrode containing Sample 1 to half-cells using the positive electrode containing Sample 5 were fabricated.

[0472] <Discharge Rate Test> Using the half-cell prepared above, a discharge rate test was conducted in a 25°C environment.

[0473] The conditions for the discharge rate test are as follows: In the first charge-discharge cycle, charging was performed at a constant current of 0.2C up to 4.60V, then at a constant voltage until the current value reached 0.05C, and then at a constant current of 0.2C up to 3.0V. In the second charge-discharge cycle, charging was performed at a constant current of 0.5C up to 4.60V, then at a constant voltage until the current value reached 0.05C, and then at a constant current of 0.1C up to 3.0V. In the third charge-discharge cycle, charging was performed at a constant current of 0.5C up to 4.60V, then at a constant voltage until the current value reached 0.05C, and then at a constant current of 1.0C up to 3.0V. In the fourth charge-discharge cycle, charging was performed at a constant current of 0.5C up to 4.60V, then at a constant voltage until the current value reached 0.05C, and then at a constant current of 2.0C up to 3.0V. In this study, 1.0C was defined as 200 mA / g per unit weight of positive electrode active material. The ambient temperature for the measurements was 25°C. The post-charge pause time (from completion of charging to the start of discharging) and the post-discharge pause time (from completion of discharging to the start of charging) were both set to 10 minutes. In addition, the charging and discharging cycles were each terminated for 20 hours, in conjunction with the termination conditions described above.

[0474] The results of the discharge rate test are shown in Figures 24A and 24B. In the graphs, the horizontal axis shows the "charge rate / discharge rate" of the measurement conditions, and the vertical axis shows the discharge capacity under each measurement condition. The discharge capacity values ​​shown above are obtained by dividing the discharge capacity value of each half-cell by the weight of the positive electrode active material particles contained in each half-cell. The results of the discharge rate test shown in Figures 24A and 24B are shown in Table 4.

[0475]

[0476] The discharge rate test results showed that at a discharge rate of 1.0C (charging 0.5C / discharging 1.0C), samples 3, 4, and 5 exhibited superior characteristics compared to samples 1 and 2. Specifically, samples 3, 4, and 5 showed discharge capacities of 200mAh / g or more even at a discharge rate of 1.0C, indicating that they have good discharge rate characteristics. Among them, sample 3 had particularly excellent discharge rate characteristics, showing a discharge capacity of over 160mAh / g at a discharge rate of 2.0C. This is more than 75% of the discharge capacity at a discharge rate of 0.1C, and approximately 80% of the discharge capacity, indicating particularly excellent discharge rate characteristics.

[0477] <XRD Analysis under High Voltage Charging State> An experiment was conducted to investigate the crystal structure of sample 3 under high voltage charging state.

[0478] <Preparation of the positive electrode> Sample 3 was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. The PVDF was prepared by dissolving it in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, a slurry was prepared by mixing each of the positive electrode active materials in the ratio of positive electrode active material:AB:PVDF = 95:3:2 (weight ratio), and this slurry was coated onto an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After coating the positive electrode current collector with the slurry, the solvent was evaporated. Note that in the preparation of the positive electrode, no pressing treatment was performed after the slurry dried.

[0479] <Half-cell preparation> Next, half-cells were prepared using the cathode containing Sample 3. The half-cells were prepared using the same method as in Example 2. The amount of cathode active material loaded in the cathode containing Sample 3 was 7 mg / cm³. ２ That was the case.

[0480] Next, the fabricated half-cells were used for charging, discharging, disassembly, and XRD measurement.

[0481] <Charge and Discharge Before Measurement> Charging was performed at a constant current of 0.2C up to 4.50V, and then at a constant voltage until the current value reached 0.05C. Discharging was performed at a constant current of 0.2C up to 3.0V. As with other tests, 1.0C was defined as 200mA / g per weight of positive electrode active material.

[0482] Next, charging was performed in a high-voltage charged state prior to XRD analysis. Charging was carried out at a constant current of 0.2C up to 4.60V, and then at a constant voltage until the current value reached 0.02C. The charging capacity at this stage was 212.9mAh / g.

[0483] <Disassembly and XRD Measurement of Half Cells> After charging to 4.60V was completed, the half cells were disassembled within one hour. During disassembly, in order to remove the positive electrode while it was still in a high-voltage charged state, insulating tools were used and the disassembly was carried out carefully to avoid short circuits. Disassembly was performed using a glove box filled with argon gas with controlled dew point and oxygen concentration. Preferably, the dew point inside the glove box is -70°C or lower, and the oxygen concentration is preferably 5 ppm or lower. Furthermore, since the crystal structure of the positive electrode active material may change due to self-discharge if a long time has passed since the above charging, it is preferable to disassemble and analyze the cells as soon as possible.

[0484] The positive electrode obtained by disassembling the half-cell was placed in a sealed sample holder inside the glove box.

[0485] XRD measurement was started within 15 minutes thereafter. The XRD equipment and conditions are as follows: XRD equipment: Bruker D8 ADVANCE X-ray: CuKα 1-ray Output: 40kV, 40mA Divergence angle: 0.6mm Detector: LYNXEYE XE-T Scanning method: 2θ / θ continuous scan Measurement range (2θ): 15° to 75° Step width (2θ): 0.01° Setting counting time: 1 second / step Sample stage rotation: 15 rpm

[0486] The XRD measurement data of the positive electrode (Sample 3) in the high-voltage charged state measured above is shown in Figures 25A and 25B. In Figures 25A and 25B, the reference patterns of the O3' structure (O3') and the H1-3 structure (H1-3) are shown together.

[0487] Figure 25A shows the range where 2θ is between 18° and 21° in the XRD measurement. Figure 25B shows the range where 2θ is between 42° and 48°. In addition, representative peak positions were extracted from the XRD measurement data of sample 3 and the reference pattern of the O3' structure, and are shown in Table 5. Furthermore, when the XRD measurement data showing a portion of the range in Figures 25A and 25B was analyzed using the crystal structure analysis software DIFFRAC.TOPAS, the lattice constant of the unit cell was found to be 2.81540 (10) for the a-axis. −１ (nm), and the c-axis is 13.7677 (10 −１ It was nm.

[0488]

[0489] As shown in Figures 25A, 25B and Table 5, the peak position of sample 3 in the high-voltage charged state in the XRD measurement roughly coincides with the peak position of the reference pattern (Ref) of the O3' structure. In other words, it can be said that sample 3 in the high-voltage charged state has an O3' structure. Furthermore, no peaks matching the reference pattern (Ref) of the H1-3 structure were observed in sample 3 in the high-voltage charged state. In other words, in sample 3 in the high-voltage charged state, it can be said that the region of the O3' structure is larger than the region of the H1-3 structure, the region of the O3' structure is much larger than the region of the H1-3 structure, there is almost no region of the H1-3 structure, more than 50% of the region is the O3' structure, more than 60 wt% of the region is the O3' structure, more than 70 wt% of the region is the O3' structure, more than 80 wt% of the region is the O3' structure, or more than 90 wt% of the region is the O3' structure.

[0490] In Example 4, cross-sectional HAADF-STEM analysis and cross-sectional STEM-EELS analysis were performed on sample 3 prepared in Example 1 to investigate the location of magnesium and nickel in the surface layer.

[0491] <STEM-EELS Analysis> Cross-sectional HAADF-STEM analysis was performed on the edge region of the surface layer of sample 3.

[0492] As a pretreatment before analysis, each of the three samples was sectioned using the FIB method (μ-sampling method).

[0493] HAADF-STEM analysis and STEM-EELS analysis were performed using the following equipment and conditions. <<STEM Observation>> Scanning transmission electron microscope: JEOL JEM-ARM200F NEOARM. Acceleration voltage: 200kV. <<EELS Analysis>> Analysis method: Electron energy loss spectroscopy (EELS). Scanning transmission electron microscope: JEOL JEM-ARM200F NEOARM. Acceleration voltage: 200kV. Measurement mode: Cs-STEM mode. Measurement pitch: 0.04nm. Thickness of measurement sample: approximately 25nm. EELS detector: Gatan ContinuumK3 Dwell. Dwell time: approximately 0.003sec. Number of frames: 1.

[0494] Figure 26A shows the HAADF-STEM image of the edge region of sample 3. In Figure 26A, the numbers 1 to 10 indicate the first to tenth columns. Figure 26B is a black-and-white inverted version of Figure 26A, created through image processing for improved visibility. In other words, in Figure 26A, areas where atoms are present are shown with high brightness (white), while in Figure 26B, areas where atoms are present are shown with low brightness (black). Furthermore, in Figure 26A, areas with a higher abundance of atoms with larger atomic numbers are shown with higher brightness, while the opposite trend is observed in Figure 26B.

[0495] As shown in Figures 26A and 26B, in the first to third columns, there was no difference in brightness between the locations of atoms in each column (vertical arrangement in the figure), exhibiting the characteristics of a rock salt crystal structure. Furthermore, from the tenth column onward, layers with high brightness (horizontal arrangement in the figure) and layers with low brightness were observed alternately in the vertical direction of the figure, exhibiting the characteristics of a layered rock salt crystal structure.

[0496] In Figure 26A, the high-luminosity layers from the 10th row onward can be considered layers of layered rock salt-type cobaltsite, while the low-luminosity layers can be considered layers of layered rock salt-type lithiumsite. Similarly, in Figure 26B, the low-luminosity layers from the 10th row onward can be considered layers of layered rock salt-type cobaltsite, while the high-luminosity layers can be considered layers of layered rock salt-type lithiumsite.

[0497] Furthermore, as shown in Figure 26A, in the range from the 4th to the 9th column, in the layered rock salt type lithium site layer, bright spots with high luminosity (5th, 7th, and 9th columns) and bright spots with low luminosity (4th, 6th, and 8th columns) were alternately arranged in the depth direction, exhibiting the characteristics of a spinel-type crystal structure.

[0498] Figures 27A to 28 show mapping images of STEM-EELS analysis in the same region as Figure 26A. Figure 27A is a mapping image in which the brightness of cobalt locations is shown as low (black), Figure 27B is a mapping image in which the brightness of magnesium locations is shown as low (black), and Figure 28 is a mapping image in which the brightness of nickel locations is shown as low (black).

[0499] Furthermore, in Figures 26B to 28, the rectangles enclosed by solid lines represent regions a, b, c, and d, respectively, and the dashed lines indicate the positions of the first to tenth columns.

[0500] Table 6 shows the results of the STEM-EELS analysis for regions a, b, c, and d. In Table 6, "Detected (Low)," "Detected (Medium)," and "Detected (High)" indicate detection, while "-" indicates that the detection limit was not reached. The notation in parentheses when an element is detected indicates the height of the EELS detection peak in three stages for each element; for example, "Detected (Medium)" for magnesium and "Detected (Medium)" for nickel do not necessarily indicate the same concentration.

[0501]

[0502] As shown in Table 6, the presence of magnesium was confirmed in regions a, b, and c. Nickel was also confirmed in regions a, b, c, and d. Furthermore, cobalt was confirmed in regions a, b, c, and d; however, in region a, considering the spatial resolution of the EELS analysis, it is possible that the detection was misinterpreted due to the influence of surrounding cobalt sites.

[0503] As described above, STEM-EELS analysis of the edge region of the positive electrode active material particles revealed a region between the rock salt structure region and the layered rock salt structure region, containing lithium sites (regions a and c) where magnesium and nickel are detected, cobalt sites (region b) where magnesium and nickel are detected, and cobalt sites (region d) where nickel is detected.

[0504] From the results in Figures 26B to 28 and Table 6, it can be inferred that a crystal structure of space group Fd-3m, as shown in Figures 5A to 6F, exists between the rock salt structure region and the layered rock salt structure region. In the schematic diagram of the crystal structure in Figure 5A, LiCoO of space group Fd-3m has lithium at position 16c, cobalt at position 16d, and oxygen at position 32e in Wyckoff notation. ２ This shows that the crystal structure is considered to be the same as that described in Non-Patent Documents 13 and 14. Therefore, the region shown in Figure 26A, etc., as having the characteristics of a spinel-type crystal structure is LiTiO with space group Fd-3m. ２ It was confirmed that it possesses a structure (spinel structure).

[0505] In the structure of Figure 5A, if magnesium and nickel are substituted in part of the lithium position at 16c, and nickel is substituted in part of the cobalt position at 16d, the results in Figures 26B to 28 and Table 6 can be explained without contradiction. Specifically, regions A, B, C, and D enclosed by dashed lines in Figure 5A correspond to regions a, b, c, and d in Figures 26B to 28, respectively, and the presence of lithium, magnesium, and nickel at the 16c position of space group Fd-3m, and cobalt and nickel at the 16d position can explain the results in Figures 26B to 28 and Table 6.

[0506] For example, in region A, only the Wyckoff position 16c of space group Fd-3m overlaps in the <110> direction, and in region a of the STEM-EELS analysis in the 4th to 9th columns of positive electrode active material particles corresponding to region A, magnesium and nickel are detected. Although lithium was not included in the detection target setting for this analysis, it is speculated that lithium is present in region a. Also, for example, in regions B and C, the Wyckoff position 16c and Wyckoff position 16d of space group Fd-3m overlap in the <110> direction, and in regions b and c of the STEM-EELS analysis in the 4th to 9th columns of positive electrode active material particles corresponding to regions C and D, magnesium and nickel are detected. Furthermore, for example, region D is a position where only the Wyckoff position 16d of space group Fd-3m overlaps in the <110> direction. In region d of the STEM-EELS analysis in the 4th to 9th columns of positive electrode active material particles corresponding to region D, nickel is detected, while magnesium is below the detection limit.

[0507] Furthermore, it can be considered that the cobalt present in region b of the STEM-EELS analysis includes not only trivalent cobalt but also divalent cobalt. Conversely, it can be considered that the cobalt present in region d of the STEM-EELS analysis is predominantly trivalent cobalt.

[0508] From the above, it has become clear that a positive electrode active material according to one aspect of the present invention has a region of space group Fd-3m between a region of rock salt structure and a region of layered rock salt structure in the edge region, and in this region, lithium, magnesium, and nickel are present at position 16c, and cobalt and nickel are present at position 16d.

[0509] In the case of the structure described above, compared to the case where the rock salt structure region and the layered rock salt structure region are in direct contact at the edge region, the internal stress is smaller, the surrounding crystal structure can be stabilized, and oxygen desorption can be suppressed. As a result, it can be used as a positive electrode active material with little capacity degradation even when repeatedly charged and discharged at high voltage. Furthermore, in the crystal structure shown in Figure 5A, lithium is arranged in region A, allowing for smooth movement of lithium during charging and discharging, and similar lithium pathways exist in three dimensions, so it can be used as a positive electrode active material that enables rapid charging and rapid discharging.

[0510] 10: Lithium-ion battery, 11: Positive electrode, 12: Negative electrode, 13: Separator, 21: Positive electrode current collector, 22: Positive electrode active material layer, 31: Negative electrode current collector, 32: Negative electrode active material layer, 41: Conductive material, 51: Electrolyte, 100: Positive electrode active material particles, 100a: Surface layer, 100b: Interior, 200: Second positive electrode active material particles

Claims

It has a positive electrode and a negative electrode, The positive electrode has positive electrode active material particles, The positive electrode active material particles contain lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum. The interior of the positive electrode active material particles has a layered rock salt type crystal structure with space group R-3m. The positive electrode active material particles have edge regions arranged in the following order from the inside outwards: a region of layered rock salt type crystal structure, a region of spinel' type crystal structure, and a region of rock salt type crystal structure. In the XPS analysis of the aforementioned positive electrode active material particles, the peak component originating from the O-Mg-O bond in the Mg1s peak is 90% or more. Lithium-ion rechargeable battery. 　 It has a positive electrode and a negative electrode, The positive electrode has positive electrode active material particles, The positive electrode active material particles contain lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum. The interior of the positive electrode active material particles has a layered rock salt type crystal structure with space group R-3m. The positive electrode active material particles have edge regions arranged in the following order from the inside outwards: a region of layered rock salt type crystal structure, a region of spinel' type crystal structure, and a region of rock salt type crystal structure. In the XPS analysis of the positive electrode active material particles, when the cobalt concentration is set to 1, the magnesium concentration ratio (Mg / Co) is 0.50 or more and 0.90 or less. In the aforementioned XPS analysis, the full width at half maximum of the Mg1s peak is between 1.0 eV and 2.6 eV. Lithium-ion rechargeable battery. 　 In claim 2, The aforementioned spinel'-type crystal structure is LiTiO with space group Fd-3m. ２ A type of battery called a lithium-ion rechargeable battery. 　 In claim 3, When the arrangement of bright spots on the surface of the positive electrode active material particles observed in the cross-sectional HAADF-STEM image of the edge region is defined as the first row, at least a portion of the fourth to ninth rows located in the interior direction of the positive electrode active material particles contains the LiTiO ２ It has a region of the type of crystal structure, The LiTo ２ The region of the crystal structure of this type has lithium sites in which magnesium and nickel are detected in STEM-EELS analysis, cobalt sites in which magnesium and nickel are detected, and cobalt sites in which nickel is detected. Lithium-ion rechargeable battery. 　 It has a positive electrode and a negative electrode, The positive electrode has positive electrode active material particles, The positive electrode active material particles contain lithium, cobalt, oxygen, magnesium, fluorine, nickel, and aluminum. The positive electrode active material particles have a surface portion having an edge region and an interior, the interior having a layered rock salt type crystalline structure of space group R-3m. When the arrangement of bright spots on the surface of the positive electrode active material particles observed in the cross-sectional HAADF-STEM image of the edge region is defined as the first row, STEM-EELS analysis is performed on the fourth to ninth rows located in the interior direction of the positive electrode active material particles, magnesium and nickel are detected at positions where only the Wyckoff position 16c of space group Fd-3m overlaps in the <110> direction, and nickel is detected at positions where only the Wyckoff position 16d overlaps in the <110> direction. In the XPS analysis of the positive electrode active material particles, when the cobalt concentration is set to 1, the magnesium concentration ratio (Mg / Co) is 0.50 or more and 0.90 or less. In the aforementioned XPS analysis, the full width at half maximum of the Mg1s peak is between 1.0 eV and 2.6 eV. Lithium-ion rechargeable battery. 　 In any one of claims 1 to 5 Lithium metal is used as the negative electrode, and the electrolyte is a mixture of lithium hexafluoride phosphate, ethylene carbonate, diethyl carbonate, and 2 wt% vinylene carbonate. When the first capacitance value is obtained after constant current charging at 25°C with a current value of 0.5C (satisfying 1.0C = 200mA / g) up to a voltage of 4.60V, then constant voltage charging until the current value becomes 0.05C, and then constant current discharge at 0.1C up to a voltage of 3.0V, The second capacity value obtained when the battery is charged with a constant current of 0.5C (while satisfying 1.0C = 200mA / g) to a voltage of 4.60V at 25°C, then charged with a constant voltage until the current reaches 0.05C, and finally discharged with a constant current of 1.0C to a voltage of 3.0V is 96% or more. Lithium-ion rechargeable battery. 　 In any one of claims 1 to 5 Lithium metal is used as the negative electrode, and the electrolyte is a mixture of lithium hexafluoride phosphate, ethylene carbonate, diethyl carbonate, and 2 wt% vinylene carbonate. When the first capacitance value is obtained after constant current charging at 25°C with a current value of 0.5C (satisfying 1.0C = 200mA / g) up to a voltage of 4.60V, then constant voltage charging until the current value becomes 0.05C, and then constant current discharge at 0.1C up to a voltage of 3.0V, The second capacity value obtained when the battery is charged with a constant current of 0.5C (satisfying 1.0C = 200mA / g) to a voltage of 4.60V at 25°C, then charged with a constant voltage until the current reaches 0.05C, and finally discharged with a constant current of 2.0C to a voltage of 3.0V is 75% or more. Lithium-ion rechargeable battery. 　 In any one of claims 1 to 5, Lithium metal is used as the negative electrode, and the electrolyte is a mixture of lithium hexafluoride phosphate, ethylene carbonate, diethyl carbonate, and 2 wt% vinylene carbonate. At 25°C, the battery is charged with a constant current of 0.5C (while satisfying 1.0C = 200mA / g) until the voltage reaches 4.60V, and then charged with a constant voltage until the current reaches 0.05C. When the positive electrode was analyzed by XRD using CuKα1 in an argon atmosphere, the XRD pattern had diffraction peaks at least at 2θ = 19.25 ± 0.12° and 2θ = 45.47 ± 0.10°. Lithium-ion rechargeable battery.

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