Lithium-ion secondary cell and method for producing positive electrode active material particles

Abstract

The present invention provides a lithium-ion secondary cell with good charge–discharge cycle characteristics. The lithium-ion secondary cell has a positive electrode and a negative electrode. The positive electrode has positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide. In XPS analysis of the positive electrode active material particles, the atomic ratio Mg / Co of magnesium and cobalt is 1.00 or more, and when the Mg1s peak is analyzed, the O-Mg-F bond–derived peak component is 10% or more. When the positive electrode active material particles are analyzed by STEM-EDX, there is a region of a surface layer portion where the magnesium concentration is 7 atomic percent or more.

Description

Lithium-ion secondary battery and method for producing positive electrode active material particles

[0001] One embodiment of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One embodiment of the present invention relates to a power storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a manufacturing method thereof.

[0002] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like.

[0003] In recent years, there has been active development of various types of energy storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. Demand for high-power, high-capacity lithium-ion secondary batteries has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0004] In particular, there is a high demand for secondary batteries with a large discharge capacity per weight and excellent cycle characteristics for mobile electronic devices, etc. To meet this demand, improvements to the positive electrode active materials of the positive electrodes of secondary batteries have been actively pursued (e.g., Patent Documents 1 and 2). Research on the crystalline structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 4).

[0005] X-ray diffraction (XRD) is one of the techniques used to analyze the crystal structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) introduced in Non-Patent Document 5. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 6 can be referenced from the ICSD. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 7) can be used. VESTA (Non-Patent Document 8) can be used as software for drawing crystal structures.

[0006] Also, image processing software such as ImageJ (Non-Patent Documents 9 to 11) is known. By using this software, for example, the shape of the positive electrode active material can be analyzed.

[0007] Microelectron diffraction is also effective for identifying the crystalline structure of the positive electrode active material, particularly the crystalline structure of the surface layer. For example, the analysis program ReciPro (Non-Patent Document 12) can be used to analyze the electron diffraction pattern.

[0008] JP 2018-206747 A JP 2022-070247 A

[0009] 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−A1609Mikami,M.,Saito,J.,Ochiai,T.et al.,“Controlling lithium cobalt oxide phase transition using molten fluoride salt for improved lithium−ion batteries”,Commun Mater 5,108(2024).A.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. , Magelhaes, 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. ,

[0010] There is still room for improvement in various aspects of lithium ion secondary batteries, such as output characteristics, discharge capacity, cycle characteristics, reliability, safety, and cost.

[0011] In view of the above, an object of one embodiment of the present invention is to provide a positive electrode active material particle or a composite oxide that can be used in a lithium ion secondary battery and that exhibits a suppressed decrease in discharge capacity during charge-discharge cycles.Another object is to provide a positive electrode active material particle or a composite oxide that is less likely to lose its crystal structure even after repeated charge-discharge cycles.Another object is to provide a positive electrode active material particle or a composite oxide that exhibits a large discharge capacity.Another object is to provide a secondary battery or a vehicle that is highly safe or highly reliable.

[0012] Another object of one embodiment of the present invention is to provide positive electrode active material particles, a composite oxide, a power storage device, or a manufacturing method thereof.

[0013] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims.

[0014] One aspect of the present invention is a lithium-ion secondary battery that includes a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles that include magnesium, fluorine, and lithium cobalt oxide. In XPS analysis of the positive electrode active material particles, an atomic ratio of magnesium to cobalt (Mg / Co) is 1.00 or greater, and analysis of the Mg1s peak shows that a peak component derived from an O-Mg-F bond accounts for 10% or greater. In STEM-EDX ray analysis of the positive electrode active material particles, a maximum magnesium concentration in a surface layer portion is 7 atomic% or greater.

[0015] In the above, the positive electrode active material particles have a layered rock salt type crystal structure belonging to the space group R-3m in a discharged state, the positive electrode active material particles are used in the positive electrode, lithium metal is used in the negative electrode, polypropylene is used for the separator, and an electrolyte solution containing 1 mol / L of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and 2 wt % vinylene carbonate is used. After charging to a voltage of 4.60 V in an environment of 25° C., the positive electrode in the charged state is subjected to CuKα １When analyzed by X-ray diffraction using X-rays, if the ranges in which peaks occur within 13 minutes from the start of measurement differ from those after 45 minutes or more have elapsed, it is preferable that, when measured within 13 minutes from the start of measurement, a peak occurs in the range of 2θ of 19.13° or more and 19.37° or less, and a peak occurs in the range of 2θ of 45.37° or more and 45.57° or less.

[0016] In the above, the X-ray diffraction analysis is carried out in an argon atmosphere, and the oxygen content of the argon atmosphere is preferably 5 ppm or less.

[0017] In the above, it is preferable that, in an EPMA analysis of the positive electrode active material particles, the atomic ratio of magnesium to cobalt in the interior, Mg / Co, is 0.005 or more and 0.015 or less.

[0018] In the above, the positive electrode active material particles have a layered rock salt type crystal structure belonging to the space group R-3m in a discharged state, the positive electrode active material particles are used for the positive electrode, lithium metal is used for the negative electrode, polypropylene is used for the separator, and a mixture of 1 mol / L of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and 2 wt % vinylene carbonate is used as an electrolyte. After charging under predetermined conditions in an environment of 25°C, the positive electrode in the charged state is subjected to CuKα １ When analyzed by X-ray diffraction using tandem scanning electron microscope, a peak appears in the 2θ range of 19.13° to 19.37°, and a peak appears in the 2θ range of 45.37° to 45.57°. The charging under the predetermined conditions is performed by first constant-current charging at a current value of 0.5 C (where 1 C = 137 mA / g) up to a voltage of 4.60 V, then constant-voltage charging until the current value reaches 0.01 C, then resting for 30 minutes, then constant-current discharging at a current value of 0.5 C down to a voltage of 2.5 V, then resting for 30 minutes, then constant-current charging at a current value of 0.5 C up to a voltage of 4.6 V, and then constant-voltage charging until the current value reaches 0.01 C.

[0019] Another aspect of the present invention is a method for producing positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide, in which the total heating time at 650°C or higher is 30 hours or more and 60 hours or less.

[0020] Another embodiment of the present invention is a method for producing positive electrode active material particles, the method including: mixing lithium cobalt oxide, a magnesium source, a fluorine source, and a lithium source to produce a mixture; and heating the mixture at a heating temperature of 650° C. or higher and 950° C. or lower for a total heating time of 30 hours or higher and 60 hours or lower to produce positive electrode active material particles.

[0021] In the above, the heating temperature is preferably 826° C. or higher and 920° C. or lower, and the total heating time is preferably 30 hours or higher and 60 hours or lower.

[0022] According to one embodiment of the present invention, it is possible to provide a positive electrode active material particle or a composite oxide that can be used in a lithium ion secondary battery and that exhibits a suppressed decrease in discharge capacity during charge-discharge cycles. Alternatively, it is possible to provide a positive electrode active material particle or a composite oxide that is less likely to lose its crystal structure even after repeated charge-discharge cycles. Alternatively, it is possible to provide a positive electrode active material particle or a composite oxide that exhibits a large discharge capacity. Alternatively, it is possible to provide a secondary battery or a vehicle that is highly safe or highly reliable.

[0023] According to one embodiment of the present invention, positive electrode active material particles, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0024] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0025] FIG. 1A is a cross-sectional view illustrating the internal structure of a secondary battery, and FIG. 1B is a cross-sectional view illustrating a positive electrode and an electrolyte of the secondary battery. FIGS. 2A and 2B are cross-sectional views illustrating a positive electrode active material according to one embodiment of the present invention. FIGS. 3A, 3B, 3C, 3D, 3E, and 3F are cross-sectional views illustrating positive electrode active material particles according to one embodiment of the present invention. FIG. 4 is a diagram illustrating the crystal structure of a positive electrode active material particle according to one embodiment of the present invention. FIG. 5 is a diagram illustrating the crystal structure of a conventional positive electrode active material particle. FIG. 6 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 7 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIGS. 8A, 8B, and 8C are diagrams illustrating a method for preparing positive electrode active material particles. FIG. 9 is a diagram illustrating a method for preparing positive electrode active material particles. FIGS. 10A and 10B are diagrams illustrating a method for preparing positive electrode active material particles. FIGS. 11A, 11B, and 11C are diagrams illustrating a lithium-ion battery according to one embodiment of the present invention. FIGS. 12A, 12B, and 12C are diagrams illustrating an electric vehicle according to one embodiment of the present invention. FIGS. 13A, 13B, 13C, 13D, and 13E are diagrams illustrating a vehicle according to one embodiment of the present invention. FIGS. 14A, 14B, 14C, and 14D are diagrams illustrating an electronic device according to one embodiment of the present invention. FIGS. 15A, 15B, and 15C are diagrams illustrating the results of STEM-EDX analysis. FIGS. 16A, 16B, and 16C are diagrams illustrating the results of STEM-EDX analysis. FIGS. 17A, 17B, and 17C are diagrams illustrating the results of STEM-EDX analysis. FIGS. 18A, 18B, and 18C are diagrams illustrating the results of STEM-EDX analysis. FIGS. 19A, 19B, and 19C are diagrams illustrating the results of STEM-EDX analysis. 20A, 20B, and 20C are diagrams illustrating the results of STEM-EDX analysis. FIGS. 21A and 21B are diagrams illustrating the results of HAADF-STEM analysis. FIGS. 22A and 22B are diagrams illustrating the results of HAADF-STEM analysis. FIGS. 23A and 23B are diagrams illustrating the results of HAADF-STEM analysis. FIGS. 24A and 24B are graphs illustrating the results of a charge-discharge cycle test. FIG. 25 is a graph illustrating the results of XRD analysis. FIG. 26 is a graph illustrating the results of XRD analysis.Figure 27 is a graph illustrating the results of the XRD analysis. Figure 28 is a graph illustrating the results of the XRD analysis.

[0026] Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings, etc. However, the present invention should not be construed as being limited to the following examples. The embodiments of the present invention can be modified within the scope of the spirit of the present invention.

[0027] In addition, in the drawings, the size, layer thickness, or area may be exaggerated for clarity, and therefore are not necessarily limited to the scale.

[0028] Furthermore, ordinal numbers such as "first" and "second" used in this specification are used to avoid confusion between components and do not indicate any order or ranking, such as order of placement or stacking. Even if a term is not used in this specification, an ordinal number may be used in the claims to avoid confusion between components. Even if a term is used in this specification, a different ordinal number may be used in the claims. Even if a term is used in this specification, the ordinal number may be omitted in the claims.

[0029] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. For ease of understanding the structure, trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice. Unless otherwise specified, the space group R-3m will also be expressed as a composite hexagonal lattice in this specification. Miller indices may also be expressed as (hkil) rather than (hkl). Here, i is −(h+k).

[0030] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), and examples of the cross-sectional shape of individual particles include ellipsoids, rectangles, trapezoids, triangles, squares with rounded corners, and asymmetric shapes, and further, individual particles may have an irregular shape.

[0031] The theoretical capacity of the positive electrode active material particles refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material particles is deintercalated. ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 275mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0032] The amount of lithium remaining in the positive electrode active material particles that can be inserted and removed can be determined by x in the composition formula, for example, Li ｘ MO ２ In the above formula, M represents a transition metal, and unless otherwise specified in this specification, M is cobalt and / or nickel. In the case of positive electrode active material particles in a lithium ion secondary battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, LiMO ２ When a lithium ion secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ MO ２ Or we can say x = 0.2. ｘ MO ２ The small value of x in the formula means, for example, 0.1<x≦0.24.

[0033] When properly synthesized lithium cobalt oxide is used in the positive electrode, the stoichiometric ratio is approximately satisfied. ２ and x = 1. The lithium cobalt oxide contained in the lithium ion secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The state where discharge is completed (discharged state) here refers to a state where the voltage is 2.5 V or less at a current of 100 mA / g or less, for example.

[0034] Li ｘ MO ２It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data from a lithium-ion secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0035] The space group of the positive electrode active material particles and the like is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0036] Furthermore, if the anions have a structure in which three layers are stacked with a shift between them, such as ABCABC, it is called a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. At the same time, since real crystals always have defects, the analysis results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, if the deviation from the theoretical position and orientation is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0037] The distribution of a certain element refers to a region in which the element is continuously detected within a noise-free range using a certain continuous analytical method. A region in which the element is continuously detected within a noise-free range can also be defined as a region in which the element is always detected when the analysis is performed multiple times.

[0038] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode substance, a positive electrode material for secondary batteries, a positive electrode material for lithium ion secondary batteries, or the like.

[0039] Furthermore, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, etc., it is not necessary for all particles to have that characteristic. 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 have that characteristic, it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material particles and secondary batteries containing them.

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

[0041] In this specification and the like, the state of the materials of a secondary battery before deterioration is sometimes referred to as an initial product or initial state, and the state after deterioration (the state when the secondary battery has a discharge capacity of less than 97% of the rated capacity) is sometimes referred to as a product in use or a state in use, or a used product or a used state.

[0042] In this specification, the (001) plane and the (003) plane may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C plane, the basal plane, or the like. In addition, in lithium cobalt oxide, lithium has a two-dimensional diffusion path. In other words, it can be said that the lithium diffusion path exists along the plane. In this specification, a plane where the lithium diffusion path is exposed, that is, a plane where lithium is inserted and extracted (specifically, a plane other than the (00l) plane), may be referred to as an edge plane.

[0043] In this specification etc., the term "A and / or B" may be used, but this is an example of a description that includes only A, only B, or both A and B.

[0044] Embodiment 1 In this embodiment, a battery and positive electrode active material particles of one embodiment of the present invention will be described with reference to FIGS.

[0045] [Battery] A lithium ion secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. When the electrolyte contains an electrolytic solution, the battery also includes a separator between the positive electrode and the negative electrode. The battery may further include an exterior body that covers at least a portion of the periphery of the positive electrode, the negative electrode, and the electrolyte.

[0046] In this embodiment, the positive electrode and positive electrode active material particles of the battery according to one embodiment of the present invention will be mainly described. A method for manufacturing the positive electrode active material particles according to one embodiment of the present invention will be described in Embodiment 2, and other details of the lithium-ion battery according to one embodiment of the present invention will be described in Embodiment 3 and subsequent embodiments.

[0047] FIG. 1A is a cross-sectional schematic diagram 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 interposed therebetween. Although not shown in FIG. 1A , electrolytes are present 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.

[0048] 1A illustrates one positive electrode 11, one negative electrode 12, and one separator 13, the lithium ion battery of one embodiment of the present invention is not limited to this structure. The lithium ion battery may have a structure including two positive electrodes 11, two negative electrodes 12, and two separators 13, or may have more than two electrodes stacked. Furthermore, the lithium ion battery may have a wound structure instead of the stacked structure illustrated in FIG. 1A.

[0049] FIG. 1B is an enlarged view of a portion A enclosed by a dashed line in FIG. 1A.

[0050] The positive electrode active material layer 22 includes positive electrode active material particles 100 (also referred to as a first positive electrode active material), a second positive electrode active material 200 having a smaller particle diameter than the positive electrode active material particles 100, and a conductive material 41. Although not shown, the positive electrode active material layer 22 may include a binder in addition to the positive electrode active material particles 100, the second positive electrode active material 200, and the conductive material 41.

[0051] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with an 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 be filled with the electrolyte 51, more preferably 70% or more of the voids, even more preferably 70% or more of the voids, even more preferably 80% or more of the voids, even more preferably 90% or more of the voids, even more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the positive electrode active material layer 22 refer to regions in the positive electrode active material layer 22 other than the solid components (positive electrode active material, conductive material, etc.).

[0052] [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, and the positive electrode active material particles 100 are a particle group made up of a plurality of particles.

[0053] <Positive Electrode Active Material Particles 100> The positive electrode active material particles 100 have the function of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material used in one embodiment of the present invention can be a material that exhibits minimal deterioration during charge and discharge (hereinafter also referred to as "charge and discharge"), even at high charge voltages (hereinafter also referred to as "high charge voltages"). Specifically, positive electrode active material particles (composite oxides) having the particle diameter (median diameter (D50)) described in this embodiment and having the characteristics described herein, and having a particle diameter of 10 μm to 50 μm, preferably 9 μm to 25 μm, can be used. The positive electrode active material particles contain one or more of an additive element X, an additive element Y, and an additive element Z. The additive element X, the additive element Y, and the additive element Z will be described in detail in the section <Containing Elements>. The additive element X, the additive element Y, and the additive element Z may be collectively referred to as additive element A.

[0054] The positive electrode active material particles 100 are the main constituent material of the positive electrode active material layer 22, and the weight of the positive electrode active material particles 100 is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more of the weight of the solid components of the positive electrode active material layer 22. If the particle diameter of the positive electrode active material particles 100 is too small, the surface area becomes too large, which may result in excessive reaction between the positive electrode active material surface and the electrolyte. For this reason, the particle diameter (median diameter (D50)) of the positive electrode active material is preferably 10 μm or more. Furthermore, if the particle diameter of the positive electrode active material is larger than the thickness of the active material layer described below, the particle density of the active material layer cannot be increased, so the particle diameter of the largest particle is preferably 50 μm or less.

[0055] The second positive electrode active material 200 is a positive electrode active material having a particle diameter smaller than that of the positive electrode active material particles 100. For details other than the particle diameter, the description of the positive electrode active material particles 100 can be referred to. By using a mixture of positive electrode active materials having different particle diameters, the particle density of the active material layer can be increased.

[0056] The particle size can be measured using a particle size distribution meter (laser diffraction particle size distribution measuring device) using a laser diffraction / scattering method. D50 is the particle size when the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. The measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section may be measured by analysis such as SEM (Scanning Electron Microscope) or TEM. Note that, as a method for measuring D50 by analysis such as SEM or TEM, for example, 20 or more particles are measured, a cumulative curve is created, and the particle size when the cumulative amount accounts for 50% can be taken as D50.

[0057] Unless otherwise specified in this specification, the "charging voltage" is expressed based on the potential of lithium metal. Furthermore, in this specification, the "high charging voltage" refers to a charging voltage of, for example, 4.5 V or higher, preferably 4.55 V or higher, more preferably 4.6 V or higher, 4.65 V or higher, or 4.7 V or higher.

[0058] As mentioned above, in this specification, a "high charging voltage" is defined as 4.6 V or higher based on the potential when the negative electrode is made of lithium metal, but when the potential when the negative electrode is made of a carbon material (e.g., graphite) is used as the reference, a "high charging voltage" is defined as 4.5 V or higher. In short, in the case of a half cell using lithium metal as the negative electrode, a charging voltage of 4.6 V or higher is defined as 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 4.5 V or higher is defined as a high charging voltage.

[0059] Positive electrode active material particles 100 that are less susceptible to deterioration due to repeated charging and discharging at a high charging voltage will be described with reference to FIGS. 2A to 3F. FIG.

[0060] 2A and 2B are cross-sectional views of a positive electrode active material particle 100 according to one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 2B are shown in FIGS. 3A to 3C. Enlarged views of the vicinity of C-D in FIG. 2B are shown in FIGS. 3D to 3F.

[0061] 2A, the positive electrode active material particle 100 has a surface layer portion 100a and an inner portion 100b. In these drawings, the boundary between the surface layer portion 100a and the inner portion 100b is indicated by a dashed line.

[0062] The surface layer 100a of the positive electrode active material particle 100 refers to, for example, a region extending from the surface toward the interior, within 10 nm, perpendicular or approximately perpendicular from the surface. Note that "approximately perpendicular" refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be considered the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0063] The region deeper than the surface layer 100a of the positive electrode active material particle 100 is called the interior 100b, which is synonymous with the inner region or core.

[0064] Furthermore, when the positive electrode active material particles 100 have a layered rock salt crystal structure of space group R-3m, as shown in FIG. 2B , the surface layer portion 100a has a region 100a1 where lithium is inserted and extracted, and a basal region 100a2.

[0065] The basal region 100a2 has a surface parallel to the (00l) plane, and the region extending from the surface to within 10 nm perpendicular or approximately perpendicular to the surface is referred to as the basal region 100a2. Note that "parallel" here means that the angle between the perpendicular to the first surface (the (00l) plane) and the normal to the second surface (the surface of the positive electrode active material particle 100) is 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0066] 2A and 2B, the straight line labeled (001) represents the (001) plane. Here, the lithium intercalation / deintercalation region 100a1 has a surface exposed in a direction intersecting with the (001) plane, and the region extending from the surface perpendicularly or substantially perpendicularly to the surface within 10 nm is referred to as the lithium intercalation / deintercalation region 100a1. Here, "intersecting" refers to an angle between the perpendicular to the first surface (the (001) plane) and the normal to the second surface (the surface of the positive electrode active material particle 100) that is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less.

[0067] The surface of the positive electrode active material particle 100 refers to the surface of the composite oxide including the surface layer portion 100a and the interior portion 100b. Furthermore, the surface of the positive electrode active material particle 100 in a cross-sectional image, such as a scanning transmission electron microscope (STEM), refers to the surface closest to the outside, where a metal element with an atomic number greater than that of lithium is first observed. More specifically, the surface refers to the point where the atomic nucleus of the metal element with an atomic number greater than that of lithium is first observed in the HAADF-STEM image, i.e., the point where the brightness peak is present in the cross-sectional STEM image.

[0068] Therefore, the positive electrode active material particles 100 are made of aluminum oxide (Al ) that does not have lithium sites that can contribute to charge and discharge. ２ O ３ ), carbonates chemically adsorbed after the production of the positive electrode active material particles 100, hydroxyl groups, etc. The attached metal oxide refers to, for example, a metal oxide whose crystal orientation does not match that of the interior 100b.

[0069] It also does not include the electrolyte, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material particles 100 .

[0070] Since the positive electrode active material particle 100 is a compound containing oxygen and a transition metal capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material particle 100. Surfaces formed by slips, cracks, and / or fissures may also be considered to be the surface of the positive electrode active material particle 100. When the positive electrode active material particle 100 is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material particle 100. As the protective film, a single-layer film or a multilayer film of carbon, metal, oxide, resin, etc. may be used.

[0071] <Additive Element> The positive electrode active material particles 100 contain lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material particles 100 contain lithium cobalt oxide (LiCoO ２ The composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0072] The positive electrode active material particles 100 contain magnesium and fluorine as additive elements, and in addition thereto, it is preferable to use one or more selected from nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, calcium, barium, bromine, and beryllium.

[0073] The positive electrode active material particles 100 of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material particles 100 of one embodiment of the present invention preferably use cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or more selected from nickel and manganese may also be used. It is preferable for the positive electrode active material particles 100 to contain 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt among the transition metals contained in the positive electrode active material particles 100, as this has many advantages, such as relatively easy synthesis, ease of handling, and excellent cycle characteristics. Therefore, the sum of the transition metals among the additive elements is preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %, of the total amount of transition metals contained in the positive electrode active material particles 100.

[0074] That is, the positive electrode active material particles 100 can be made of one or more of lithium cobalt oxide having magnesium and fluorine, lithium cobalt oxide having magnesium, fluorine and aluminum, lithium cobalt oxide having magnesium, fluorine and nickel, lithium cobalt oxide having magnesium, fluorine, nickel and aluminum, etc.

[0075] It can also be said that the positive electrode active material particles 100 can be any one or more of a positive electrode active material having cobalt, oxygen, magnesium, and fluorine, a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and aluminum, a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and nickel, and a positive electrode active material having cobalt, oxygen, magnesium, fluorine, nickel, and aluminum, and the like, and can be used in a lithium ion battery.

[0076] The additive element is preferably dissolved in the positive electrode active material particle 100. For example, when a line analysis using STEM-EDX (scanning transmission electron microscope-energy dispersive X-ray spectroscopy) is performed from the outside to the inside of the positive electrode active material particle 100, the position in the depth direction at which the additive element begins to be detected is preferably located deeper than the position at which the transition metal M begins to be detected, i.e., closer to the inside of the positive electrode active material particle 100. In the line analysis using STEM-EDX in the depth direction, the position at which a certain element begins to be detected refers to the position at which the detected amount of characteristic X-rays attributable to that element begins to increase continuously.

[0077] These added elements further stabilize the crystal structure of the positive electrode active material particles 100 .

[0078] The additive element does not necessarily have to include nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, calcium, barium, bromine, or beryllium.

[0079] For example, if the positive electrode active material particles 100 are substantially free of titanium, the above-mentioned advantage of having excellent cycle characteristics will be further enhanced. The weight of titanium contained in the positive electrode active material particles 100 is preferably, for example, 600 ppm or less, and more preferably 100 ppm or less. Furthermore, when the positive electrode active material particles 100 are analyzed by STEM-EDX, it is preferable that characteristic X-rays attributable to titanium are not detected, that is, the amount is below the lower detection limit (for example, less than 0.3 atomic %).

[0080] For example, if the cathode active material particles 100 are substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, as described above, will be enhanced. The weight of manganese contained in the cathode active material particles 100 is preferably, for example, 600 ppm or less, and more preferably 100 ppm or less.

[0081] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 100b. In addition, the atoms on the surface of the positive electrode active material particle 100 that are in the surface layer 100a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and can be said to be a region where deterioration 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 ２ Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of cobalt and oxygen octahedra can be made less likely to break.Furthermore, displacement of the layers made of cobalt and oxygen octahedra in the inner portion 100b can be suppressed.

[0082] To provide the surface layer 100a with a stable composition and crystal structure, the surface layer 100a preferably contains an additive element, and more preferably contains multiple additive elements. Furthermore, the surface layer 100a preferably has a higher concentration of one or more selected additive elements than the interior 100b. Furthermore, the one or more selected additive elements contained in the positive electrode active material particles 100 preferably have a concentration gradient. Furthermore, the positive electrode active material particles 100 preferably have different distributions depending on the additive elements. For example, the concentration peaks of each additive element are preferably detected at different depths from the surface. The position of the concentration peak refers to the position where the concentration reaches its maximum value in the surface layer 100a or within a range of 50 nm or less from the surface.

[0083] [Distribution] The distribution of the added element will be described. Figures 3A to 3C are enlarged views of the vicinity of A-B in Figure 2B, and are views illustrating the region 100a1 where lithium is inserted and extracted in the positive electrode active material particle 100. Figures 3D to 3F are enlarged views of the vicinity of C-D in Figure 2B, and are views illustrating the basal region 100a2 of the positive electrode active material particle 100.

[0084] For example, some of the additive elements, such as magnesium, fluorine, silicon, phosphorus, boron, and calcium, preferably have a concentration gradient that increases from the interior 100b toward the surface, as shown by the gradation in Figures 3A and 3D. An additive element having such a concentration gradient will be referred to as an additive element X.

[0085] As shown by the density of the hatching in Figures 3B and 3E, other additive elements, such as aluminum and manganese, preferably have a concentration gradient and a concentration peak in a region deeper than that of additive element X shown in Figures 3A and 3D. The concentration peak is preferably present in the interior 100b. For example, it is preferable that the peak be in a region greater than 10 nm and not greater than 30 nm from the surface to the interior. An additive element having such a concentration gradient will be referred to as additive element Y.

[0086] As shown by the presence or absence of hatching and the density of the hatching in Figures 3C and 3F, other additive elements, such as nickel and barium, may be clearly present in the lithium insertion / extraction region 100a1 but substantially absent from the basal region 100a2. Here, "clearly present" refers to a case in which a characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material particle 100. Furthermore, "substantially absent" refers to a case in which a characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material particle 100. This also refers to the element being below the lower limit of detection in the STEM-EDX analysis. In this case, this also refers to the element being below the lower limit of detection in the STEM-EDX analysis. An additive element having such a distribution is referred to as additive element Z.

[0087] The effects of each added element and the preferred concentrations thereof will be described in detail below.

[0088] [Magnesium] The positive electrode active material particle 100 according to one embodiment of the present invention has a layered rock salt crystal structure in the interior and a rock salt crystal structure in the surface layer portion 100a. As will be described later, the crystal orientations of the two roughly match, but there is strain between the two due to lattice mismatch (which can also be considered as a difference in inter-ionic distance). Magnesium, which is one of the additive elements X, has the function of alleviating this strain.

[0089] For example, if the rock-salt crystal structure of the surface layer 100a is formed only of cobalt oxide CoO, the lattice mismatch (difference in inter-ionic distance) with the lithium cobalt oxide in the interior 100b will be 5.9% or more. However, by dissolving magnesium in the surface layer 100a, the lattice mismatch between the surface layer 100a and the lithium cobalt oxide in the interior 100b can be made smaller than the above. This is because the metal-oxygen distance in magnesium oxide MgO is closer to the average metal-oxygen distance in lithium cobalt oxide than the metal-oxygen distance in cobalt oxide CoO.

[0090] In order to efficiently exert the function of relaxing strain, magnesium is preferably substituted for a part of the cobalt sites or lithium sites in the surface layer portion 100a.

[0091] In the HAADF-STEM image, contrast proportional to the atomic number is obtained, and elements with higher atomic numbers are observed brighter. Furthermore, elemental analysis is possible with STEM-EDX. Therefore, by combining these analyses, if cobalt and magnesium are detected in the surface layer 100a by STEM-EDX analysis but no other metal elements with atomic numbers close to that of magnesium are detected, and an element with a lower brightness than cobalt is observed at the cobalt site in the HAADF-STEM image, it can be determined that magnesium has substituted for the cobalt site.

[0092] It is also possible that magnesium is in the position of a dangling bond.

[0093] In addition, magnesium ions are divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ This is because it functions as a pillar that supports the layers. ｘ CoO ２ When x in the formula is, for example, 0.24 or less, oxygen desorption from the periphery of magnesium can be suppressed. When magnesium is dispersed in a mesh-like pattern in the surface layer portion 100a, oxygen desorption can be suppressed more effectively without adversely affecting the intercalation and deintercalation of lithium. Since deterioration of the positive electrode active material particles 100 is thought to proceed first through oxygen desorption from the surface layer portion 100a, followed by cobalt elution, suppressing oxygen desorption is important for suppressing deterioration. Dispersed magnesium refers to distribution at a moderate concentration.

[0094] In order to fully obtain the above-described effects, it is preferable that a sufficient concentration of magnesium is present in the surface layer portion 100 a. For example, the maximum concentration of magnesium in the surface layer portion 100 a as determined by STEM-EDX ray analysis is preferably 7 atomic % or more, and more preferably 10 atomic % or more.

[0095] However, excessive magnesium may adversely affect lithium insertion and desorption. Therefore, it is preferable that the positive electrode active material particle 100 contains an appropriate amount of magnesium. For example, the maximum magnesium concentration in the surface layer portion 100a when subjected to STEM-EDX ray analysis is preferably 25 atomic% or less. That is, the maximum magnesium concentration in the surface layer portion 100a when subjected to STEM-EDX ray analysis is preferably 7 atomic% or more and 25 atomic% or less, and more preferably 10 atomic% or more and 25 atomic% or less. Furthermore, in EPMA, the atomic ratio Mg / Co of magnesium to cobalt in the interior is preferably greater than 0 and 0.02 or less, and more preferably 0.005 or more and 0.015 or less.

[0096] [Fluorine] Fluorine has the function of promoting the migration of the additive element into the interior 100 b of the positive electrode active material particle 100 .

[0097] More specifically, when the melting point of a fluoride, such as lithium fluoride, is lower than that of the other additive element source, the fluoride functions as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source. This allows a liquid containing the fluoride and other additive elements to spread over the surface of the lithium cobalt oxide during the heating process. This facilitates uniform distribution of the additive elements in the surface layer 100a. The uniform distribution of the additive elements in the surface layer 100a also facilitates subsequent migration from the surface layer 100a to the interior. Furthermore, the presence of fluorine also functions as an aid for the diffusion of magnesium from the surface layer 100a to the interior.

[0098] Furthermore, fluorine is preferably present on the surface side (which may be referred to as the region closest to the outside of the particle, the outermost surface, etc.). For example, with regard to the position of maximum concentration of each element in the surface layer portion 100a when STEM-EDX-ray analysis is performed, the position of maximum fluorine concentration (atomic %) is preferably located closer to the surface than the positions of maximum concentrations (atomic %) of other added elements. In other words, the positions of maximum concentrations (atomic %) of other added elements including magnesium are preferably located closer to the interior than the positions of maximum fluorine concentration (atomic %).

[0099] The presence of fluorine in the surface layer portion 100a, which has a surface that is in contact with the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material particles 100 and the electrolyte, and can also effectively improve corrosion resistance to hydrofluoric acid.

[0100] [Nickel] Nickel has a stronger bond with oxygen than cobalt, and therefore the presence of nickel suppresses oxygen desorption from the positive electrode active material particles 100 and suppresses the phase change from a layered rock salt crystal structure to a spinel crystal structure. Therefore, nickel is preferably present at a high concentration in the surface layer portion 100a, which is a region where the phase change to spinel is likely to occur. In particular, nickel is preferably substituted for lithium sites up to the fourth layer in the region 100a1 where lithium is inserted and extracted.

[0101] Furthermore, nickel is Ni ２＋ , Ni ３＋ , Ni ４＋ Of which Ni ２＋ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. In this respect, nickel is also thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0102] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the volume change due to charging and discharging is prevented. This is because nickel existing at the lithium site can also be prevented from shifting. ２ This is presumably because nickel functions as a pillar supporting the layers. Therefore, it is expected that the crystal structure will be more stable, particularly in a charged state at high temperatures, for example, 45°C or higher, which is preferable. To achieve the above effect, nickel is preferably present not only in the surface layer portion 100a but also in the interior portion 100b.

[0103] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also adversely affect lithium insertion and extraction.

[0104] Therefore, it is preferable that the positive electrode active material particles 100 contain an appropriate amount of nickel. For example, in EPMA, the atomic ratio of nickel to cobalt inside the particles, Ni / Co, is preferably greater than 0 and not greater than 0.05, more preferably 0.001 or more and 0.01 or less, and even more preferably 0.004 or more and 0.008 or less.

[0105] [Aluminum] Aluminum has the function of mitigating volume changes due to charge and discharge of the layered rock-salt crystal structure. Aluminum can exist at the cobalt site in the layered rock-salt crystal structure. Since aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charge and discharge. Therefore, the aluminum and the lithium around it function as pillars, which can suppress changes in the crystal structure and volume change due to charge and discharge. Therefore, as described below, even if the positive electrode active material particle 100 is subjected to a force that causes it to expand and contract in the c-axis direction due to the insertion and desorption of lithium ions, deterioration of the positive electrode active material particle 100 can be suppressed.

[0106] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, since the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen from the aluminum's surroundings. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material particles 100 are used in secondary batteries. Furthermore, the positive electrode active material particles 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0107] On the other hand, excessive aluminum may have adverse effects on the intercalation and deintercalation of lithium, such as a decrease in charge / discharge capacity and a decrease in lithium ion mobility.

[0108] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material particle 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material particle 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material particle 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material particle 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material particle 100.

[0109] [Crystal orientation is roughly the same] It is preferable that the crystal structure changes continuously from the interior 100b toward the surface due to the concentration gradient of the added element as described above. Alternatively, it is preferable that the crystal orientation of the surface layer 100a and the interior 100b are roughly the same.

[0110] For example, it is preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type. Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are roughly the same.

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

[0112] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately, and may contain cation or anion defects.

[0113] Furthermore, the presence of both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc. The presence of both the characteristics of the layered rock salt type and the rock salt type crystal structure can also be said to mean that the rock salt structure is scattered within the layered rock salt structure. The region where the rock salt structure is scattered within the layered rock salt structure is preferably on the surface side of the surface layer portion 100a.

[0114] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２ The distance between the bright spots on the (003) plane of LiCoO is observed to be about half the distance between the bright spots on the (111) plane of MgO. ２In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0115] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in a cross-sectional HAADF-STEM image, layers observed with high brightness and layers observed with low brightness are alternately observed. Such characteristics are not observed in the rock-salt structure, since there is no distinction between cation sites. In the case of a crystal structure having characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are alternately observed in a cross-sectional STEM image, and furthermore, a metal with an atomic number greater than that of lithium is present in part of the low-brightness layer, i.e., the lithium layer.

[0116] Layered rock salt crystals and the anions in rock salt crystals form a cubic close-packed structure (face-centered cubic lattice structure). When layered rock salt crystals and rock salt crystals come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0117] However, the space group of the layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group Fm-3m (space group of general rock salt crystals) of the rock salt crystals, and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt crystals and O3'-type crystals and the rock salt crystals. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt crystals, O3'-type and rock salt crystals are aligned, it may be said that the crystal orientations are approximately the same. In addition, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0118] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and FFT patterns of STEM images, etc. Furthermore, XRD, neutron diffraction, etc. can also be used as materials for determination.

[0119] <Li ｘ MO ２ When x is 1 in the positive electrode active material particle 100 according to one embodiment of the present invention, the positive electrode active material particle 100 is in a discharged state, that is, Li ｘ MO ２ When x = 1 in the formula, the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies the majority of the volume of the positive electrode active material particle 100, has a layered rock-salt type crystal structure. Figure 5 shows the layered rock-salt type crystal structure, labeled R-3m O3. R-3m O3 has lattice constants a = 2.81610, b = 2.81610, c = 14.05360, α = 90.0000, β = 90.0000, and γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), and O(0,0,0.23951) (Non-Patent Document 6).

[0120] On the other hand, the surface layer portion 100a of the cathode active material particle 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b so that the layered structure of the transition metal M and oxygen octahedra is not destroyed even when lithium is removed from the cathode active material particle 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material particle 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material particle 100, preferably reinforces the cathode active material particle 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and inner portion 100b of the cathode active material particle 100, such as oxygen desorption and / or shifting of the layered structure of the transition metal M and oxygen octahedra. And / or to suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material particle 100.

[0121] Therefore, the surface layer portion 100a preferably has a different crystal structure from the interior portion 100b. Furthermore, the surface layer portion 100a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material particle 100 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has the characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0122] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, it can be said that the atoms on the surface of the particle of the positive electrode active material particle 100 that are in the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. For example, if the crystal structure of the layered structure consisting of the transition metal M and oxygen octahedrons in the surface layer 100a is displaced, the effect will be transmitted to the interior 100b, causing the layered crystal structure in the interior 100b to also be displaced, which is thought to lead to deterioration of the crystal structure of the entire positive electrode active material particle 100. On the other hand, if the surface layer 100a can be sufficiently stabilized, Li ｘ MO ２Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of the transition metal M and oxygen octahedra can be made less likely to break. Furthermore, displacement of the layer made of the transition metal M and oxygen octahedra in the inner portion 100b can be suppressed.

[0123] Furthermore, the interior 100b of the positive electrode active material particle 100 preferably has a low density of defects, including dislocations. Furthermore, the positive electrode active material particle 100 preferably has a large crystallite size as measured by XRD. In other words, the interior 100b preferably has high crystallinity. Furthermore, the surface of the positive electrode active material particle 100 preferably has a smooth surface. These characteristics are important factors that support the reliability of the positive electrode active material particle 100 when used in a secondary battery. If the reliability of the positive electrode active material is high, the upper limit of the charging voltage of the secondary battery can be increased, resulting in a secondary battery with a high charge / discharge capacity.

[0124] Dislocations in the interior 100b can be observed, for example, by a TEM. If the density of defects, including dislocations, is sufficiently low, they may not be observed within a specific 1 μm square of the observation sample. Note that dislocations are a type of crystal defect and are different from vacancy defects.

[0125] The larger the crystallite size, the greater the amount of Li ｘ CoO ２ When x is small, the O3' type crystal structure is easily maintained, and contraction of the c-axis length is easily suppressed.

[0126] It is believed that the fewer defects such as dislocations observed by TEM, the larger the crystallite size measured by XRD.

[0127] When calculating the crystallite size, the XRD diffraction pattern is preferably obtained from the positive electrode active material alone. However, it may also be obtained from the positive electrode, which includes the positive electrode active material, a current collector, a binder, a conductive material, and the like. However, in the positive electrode state, the positive electrode active material may be oriented due to the influence of pressure during the manufacturing process. Strong orientation may make it difficult to accurately calculate the crystallite size. Therefore, it is more preferable to obtain the pattern by removing the positive electrode active material layer from the positive electrode, removing some of the binder, etc., from the positive electrode active material layer using a solvent, etc., and then loading the sample into a sample holder. Another method is to apply grease to a silicon non-reflective plate and then attach the powder sample.

[0128] The crystallite size can be calculated using, for example, a Bruker D8 ADVANCE, a CuKα X-ray source, a 2θ angle of 15° to 90° increments of 0.005, and a LYNXEYE XE-T detector, and a diffraction pattern obtained using ICSD Coll. Code. 172909 as the literature value for lithium cobaltate. Analysis can be performed by the Rietveld method using DIFFRAC. TOPAS ver. 6 as crystal structure analysis software, and can be set, for example, as follows: Emission Profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9 FDS angle: 0.3 Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5 Secondary Sollers: 2.5 Corrections Specimen displacement: Refine LP Factor: 0

[0129] It is preferable to use the value of LVol-IB, which is the crystallite size corrected based on the integral width standard calculated by the above method, as the crystallite size.

[0130] Since lithium cobalt oxide tends to be (001) oriented, setting the preferred orientation to (001) may result in a better GOF (goodness of fit). The March-Dollase function or spherical harmonics can be used as the preferred orientation. Furthermore, when the Rietveld method is used, if the preferred orientation is less than 0.8 or more than 1.2, the sample may be too strongly oriented and may not be suitable for determining the crystallite size. In such cases where the Rietveld method is not suitable, it is preferable to determine the crystallite size using another refinement method that can obtain a better GOF. For example, the full pattern decomposition (WPPD) method can be used.

[0131] In addition, with the full pattern decomposition method, it is possible to calculate not only the crystallite size but also the lattice constant without being affected by the orientation of the sample.

[0132] <Li ｘ CoO ２ Crystal structure in which x is small in the positive electrode active material particle 100 according to one embodiment of the present invention has the above-described distribution of the additional element A and / or the crystal structure in the discharged state, and therefore, ｘ CoO ２ It is preferable that the crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means that 0.1<x≦0.24.

[0133] 4 to 7, Li ｘ CoO ２ The change in the crystal structure due to the change in x in the positive electrode active material particle 100 of one embodiment of the present invention will be described by comparing a conventional positive electrode active material with the positive electrode active material particle 100 of one embodiment of the present invention.

[0134] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 5. The conventional positive electrode active material shown in FIG. 5 is a lithium cobalt oxide (LiCoO２ ) In Figure 5, R-3m O3 is added to Li ｘ CoO ２ 1 shows the crystal structure of lithium cobalt oxide with x=1.

[0135] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２ It can also be said that this crystal structure is a structure in which the structure of and the structure of are stacked alternately. For this reason, this crystal structure is sometimes called an H1-3 crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 5 and other parts of this specification, to make it easier to compare with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the size of the unit cell.

[0136] As an example of the H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to express the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt the unit cell that results in the smallest GOF value.

[0137] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0138] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 5, in the H1-3 type crystal structure, CoO ２The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0139] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure is greater than 3.5%, typically 3.9% or more.

[0140] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0141] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult to insert and extract lithium.

[0142] On the other hand, in the positive electrode active material particle 100 according to one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Also, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material particle 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material particle 100 of one embodiment of the present invention has a Li ｘ CoO ２ When x is 0.24 or less, the positive electrode active material particles 100 according to one embodiment of the present invention can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0143] Li ｘ CoO ２ The crystal structure of the interior 100b of the positive electrode active material particle 100 when x is approximately 1 or 0.2 is shown in FIG. 4. The interior 100b occupies the majority of the volume of the positive electrode active material particle 100 and is the part that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0144] When x=1, the positive electrode active material particles 100 have the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0145] However, the positive electrode active material particles 100 have a different crystal structure from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.12, which results in an H1-3 type crystal structure.

[0146] When x=0.24, the positive electrode active material particle 100 according to one embodiment of the present invention has a crystal structure belonging to the trigonal space group R-3m. ２ The layer symmetry is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 4 with the notation R-3m O3'.

[0147] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ nm). The c-axis is 13.681≦c≦13.881 (×10 −１ nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 −１ nm).

[0148] In the O3' type crystal structure, ions of cobalt, nickel, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0149] As shown by the dotted line in FIG. 4, the difference between R-3m(O3) in the discharged state and the O3′-type crystal structure is ２ There is almost no layer misalignment.

[0150] The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0151] In this way, in the positive electrode active material particle 100 of one embodiment of the present invention, Li ｘ CoO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material particles 100 are resistant to collapse of their crystal structure even when repeatedly charged and discharged such that x is 0.24 or less. Therefore, the positive electrode active material particles 100 are suppressed from decreasing in charge / discharge capacity during charge / discharge cycles. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material particles 100 have a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material particles 100, secondary batteries with high discharge capacity per weight and per volume can be fabricated.

[0152] The positive electrode active material particles 100 are Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is presumed that even when x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. However, the crystal structure is ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0153] Therefore, the positive electrode active material particles 100 are Li ｘ CoO ２When x is greater than 0.1 and equal to or less than 0.24, the entire interior 100b of the positive electrode active material particle 100 does not have to have an O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0154] Furthermore, the additive element A does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 100a of the positive electrode active material particle 100. For example, the distribution of the additive element A in the (001)-oriented surface may be different from that in the other surfaces. For example, the distribution of one or more concentration peaks selected from the additive element X and the additive element Y in the (001)-oriented surface and its surface layer portion 100a may be limited to a shallower portion from the surface compared to surfaces other than the (001)-oriented surface. Alternatively, the (001)-oriented surface and its surface layer portion 100a may have a lower concentration of one or more elements selected from the additive element X and the additive element Y compared to the other orientations. Alternatively, the (001)-oriented surface and its surface layer portion 100a may have one or more elements selected from the additive element X and the additive element Y below the lower detection limit.

[0155] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x is small, whether or not the crystal structure is O3' type depends on Li ｘ CoO ２ This can be determined by analyzing a positive electrode having positive electrode active material particles with a small x using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0156] In particular, XRD is preferable in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material particle 100, which occupies the majority of the volume of the positive electrode active material particle 100.

[0157] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the size.

[0158] As described above, the positive electrode active material particles 100 according to one embodiment of the present invention are ｘ MO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0159] It should also be noted that simply adding an additive element may not result in an O3' or monoclinic O1(15) crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li ｘ MO ２ In some cases, x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases, the H1-3 type crystal structure accounts for 50% or more.

[0160] Furthermore, even in the case of the positive electrode active material particle 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated if x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.90 V. Therefore, to determine whether or not the positive electrode active material particle 100 is one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0161] However, positive electrode active materials with a small x value may undergo changes in crystal structure when exposed to air. For example, the crystal structure may change from an O3'-type or monoclinic O1(15)-type 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, and to maintain the inert atmosphere during XRD measurement. For example, when argon gas is used as the inert atmosphere, the oxygen content of the argon gas is preferably 5 ppm or less, more preferably 3 ppm or less, and even more preferably 2 ppm or less. Furthermore, by adjusting the dew point of the argon gas to -60°C or less, preferably -70°C or less, accurate XRD measurement can be performed. Furthermore, to avoid changes in the crystal structure due to unintended migration of lithium ions, the environmental temperature during handling and measurement is preferably 25°C or less, more preferably 20°C or less.

[0162] However, even if handled in an argon atmosphere, the crystal structure of the positive electrode may change over time after the secondary battery is disassembled. Therefore, it is preferable to adopt a method that allows for rapid measurement as needed.

[0163] In the case of XRD, the detector and / or measurement start angle can be appropriately changed in order to quickly observe the main peak. For example, it is preferable to measure the main peak within 13 minutes from the start of measurement.

[0164] Furthermore, whether or not the distribution of the additive elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX (energy dispersive X-ray spectroscopy), electron probe microanalysis (EPMA), or the like.

[0165] The crystal structure of the surface layer 100 a and the grain boundaries can be analyzed by electron diffraction or the like of a cross section of the positive electrode active material particle 100 .

[0166] <Charging method> The composite oxide is Li ｘ CoO ２When x is small, charging to determine whether the composite oxide has an O3'-type crystal structure can be performed by fabricating a coin cell (CR2032 type, 20 mm diameter, 3.2 mm height) using the composite oxide as the positive electrode and lithium metal as the counter electrode. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0167] More specifically, the positive electrode may be prepared by coating a slurry of positive electrode active material particles, a conductive material, and a binder on a positive electrode current collector made of aluminum foil.

[0168] Lithium metal can be used for the counter electrode. 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 are different. Unless otherwise specified, the voltage and potential in this specification and the like refer to the potential of the positive electrode (vs. Li / Li ＋ )

[0169] The electrolyte was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 1 mol / L of lithium hexafluorophosphate (LiPF ６ A solution in which 2 wt % of vinylene carbonate (VC) is added as an additive to the solution in which 2 wt % of vinylene carbonate (VC) is dissolved can be used.

[0170] The separator may be a 25 μm thick porous polypropylene film.

[0171] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0172] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.60 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be completed at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material particles, charging at such a low current value is desirable. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain positive electrode active material particles with the desired charge capacity. When various analyses are performed after this, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to remove the positive electrode promptly after charging is completed and perform the analysis. Specifically, within one hour after charging is completed, and more preferably within 30 minutes.

[0173] Furthermore, when analyzing the crystal structure in the charged state after multiple charge / discharge cycles, for example, charging can be performed by constant current charging at a current value of 20 mA / g to 100 mA / g up to an arbitrary voltage (e.g., 4.60 V), followed by constant voltage charging until the current value reaches 2 mA / g to 10 mA / g, and discharging can be performed by constant current discharging at 2.5 V and 20 mA / g to 100 mA / g, or by constant current discharging at 3.0 V and 20 mA / g to 200 mA / g.

[0174] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed, for example, at 2.5 V and a current value of 20 mA / g or more and 200 mA / g or less, or at 3.0 V and a current value of 20 mA / g or more and 200 mA / g or less.

[0175] <XRD> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker Corporation, X-ray: CuKα １Output: 40 kV, 40 mA, Divergence slit: 0.6 mm, Detector: LYNXEYE XE-T, Scan method: 2θ / θ continuous scan, Measurement range (2θ): 15° to 90°, Step width (2θ): Set to 0.01°, Counting time: 1 second / step, Sample stage rotation: 15 rpm.

[0176] If the measurement sample is a powder, it can be placed in a glass sample holder or sprinkled on a greased silicone anti-reflective plate. If the measurement sample is a positive electrode rather than a powder, the positive electrode can be attached to a substrate of appropriate thickness with double-sided tape, and the positive electrode active material layer can be set to fit the required measurement surface of the device. If the positive electrode active material layer is higher than the required measurement surface of the device, the diffraction pattern will be shifted overall to the higher angle side; if it is lower, the diffraction pattern will be shifted overall to the lower angle side. In this case, the diffraction pattern shift can be corrected using crystal structure analysis software, etc.

[0177] CuKα calculated from the O3' type crystal structure and the H1-3 type crystal structure model １ The ideal powder XRD patterns by the line are shown in Figures 6 and 7. For comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structure of LiCoO3 and the trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD. The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The XRD pattern of the H1-3 type crystal structure was created in the same manner as above, based on the information on the H1-3 type crystal structure shown in Fig. 5. The XRD pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material particles of one embodiment of the present invention, fitting using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating an XRD pattern in the same manner as the others.

[0178] As shown in FIG. 6, the O3′ type crystal structure has diffraction peaks at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less).

[0179] However, as shown in FIG. 7, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ It can be said that the presence of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and 19.37° or less) and 2θ = 45.47 ± 0.10° (45.37° or more and 45.57° or less) when x in the particle is small is a characteristic of the positive electrode active material particle 100 of one embodiment of the present invention.

[0180] This can also be said to mean that the positions of the XRD diffraction peaks are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0181] The positive electrode active material particles 100 according to one embodiment of the present invention are ｘ CoO ２When x in the formula is small, it is preferable to have an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. For example, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0182] <XPS> XPS (X-ray Photoelectron Spectroscopy) can analyze an area from the surface to a depth of about 2 to 8 nm (usually 5 nm or less) in the case of inorganic oxides by using monochromatic aluminum Kα rays as X-rays. Furthermore, narrow scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, although it depends on the element.

[0183] It is preferable that the concentration of one or more added elements selected from the surface layer portion 100a measured by XPS or the like is higher than the average concentration of the added elements in the entire positive electrode active material particle 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry) or the like.

[0184] Note that the surface and surface layer portion 100a of the positive electrode active material particle 100 according to one embodiment of the present invention are assumed to be free of carbonates, hydroxyl groups, and the like that are chemically adsorbed after the production of the positive electrode active material particle 100. Furthermore, they are assumed to be free of the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material particle 100. Therefore, when quantifying the elements contained in the positive electrode active material particle 100, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0185] Furthermore, before being subjected to various analyses, the sample, such as the positive electrode active material particles 100 and the positive electrode active material layer, may be washed or the like to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surfaces of the positive electrode active material particles 100. At this time, lithium may dissolve in the solvent or the like used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0186] The concentration of the added element may be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparison while reducing the influence of carbonates and the like that are chemically adsorbed after the production of the positive electrode active material particles 100. For example, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 or more and 1.40 or less, more preferably 0.600 or more and 1.40 or less, even more preferably 0.600 or more and 1.20 or less, even more preferably 0.800 or more and 1.200 or less, and even more preferably 1.000 or more and 1.200 or less.

[0187] Furthermore, the ratio of the number of atoms of nickel to cobalt, Ni / Co, as determined by, for example, XPS analysis, is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, even more preferably 0.050 or more and 0.100 or less, and even more preferably 0.050 or more and 0.070 or less.

[0188] Furthermore, the ratio of the number of fluorine atoms to the number of magnesium atoms, F / Mg, as determined by, for example, XPS analysis, is preferably 0.100 or more and 1.00 or less, more preferably 0.100 or more and 0.800 or less, even more preferably 0.300 or more and 0.800 or less, even more preferably 0.300 or more and 0.700 or less, and even more preferably 0.500 or more and 0.700 or less.

[0189] The above range indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material particle 100, but are widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material particle 100. In other words, as a result of XPS analysis of the positive electrode active material particle 100, the above range indicates that the crystal structure is less likely to collapse even when charging and discharging are repeated so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, good lithium insertion and desorption is possible in the positive electrode active material particle 100, and excellent rate characteristics can be achieved.

[0190] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. Furthermore, it is preferable to use an XPS apparatus having 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 take-off angle can be, for example, 45° or 15°. For example, the measurement can be performed using the following XPS apparatus and measurement conditions: Measurement apparatus: PHI Quantera II X-ray: Monochromated 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 5 nm (take-off angle 45°), approximately 2 nm (take-off angle 15°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0191] When the positive electrode active material particle 100 according to one embodiment of the present invention is subjected to XPS analysis, the peak (Mg1s peak) representing the bond energy between magnesium and other elements is preferably equal to or greater than 1303.0 eV and less than 1305.0 eV, and more preferably about 1304.0 eV, which is a value different from the bond energy of magnesium fluoride, 1306.0 eV, and is close to the bond energy of magnesium oxide.

[0192] In the XPS analysis of the positive electrode active material particle 100 according to one embodiment of the present invention, the measured XPS spectrum may be corrected so that the C1s peak is aligned with the reference value (284.8 eV), i.e., the entire spectrum may be shifted. This can reduce the influence of differences in the XPS apparatus, differences in measurement conditions, and the like on the XPS measurement.

[0193] Furthermore, by analyzing the Mg1s peak in XPS analysis, the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond can be calculated. When XPS analysis of the positive electrode active material particle 100 of one embodiment of the present invention is performed, it is preferable that the peak component derived from the "O-Mg-F" bond is less than the peak component derived from the "O-Mg-O" bond. It is also preferable that the particle has both the peak component derived from the "O-Mg-O" bond and the peak component derived from the "O-Mg-F" bond.

[0194] That is, in XPS analysis of the positive electrode active material particle 100 of one embodiment of the present invention, when the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond are analyzed so that the total is 100%, the peak component derived from the "O-Mg-F" bond is preferably 10% or more, more preferably 20% or more. Also, the peak component derived from the "O-Mg-O" bond is preferably 60% or more, more preferably 70% or more.

[0195] A method for analyzing the Mg1s peak of an XPS spectrum in XPS analysis will be described. 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 is designated as fit peak 2, and the peak component derived from the F-Mg-F bond is designated as fit peak 3. These three fit peaks are synthesized, and the peak synthesis ratio that minimizes the difference from the Mg1s peak of the XPS spectrum obtained by XPS analysis is calculated. The analysis results can be output by assuming that the area ratio of fit peak 1, fit peak 2, and fit peak 3 is equal to the abundance ratio of O-Mg-O bonds, O-Mg-F bonds, and F-Mg-F bonds.

[0196] In the above-mentioned XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also called peak top) of fit peak 1 is determined by the MgO-coated LiCoO ２ The energy value at the maximum value of the Mg1s peak when measured separately as a standard sample can be referenced. The energy value at the maximum value of fit peak 3 (Ep3) can be referenced to the energy value at the maximum value of magnesium fluoride (MgF ２ For example, the energy value at the maximum value of the Mg1s peak when a standard sample (for example, MGH18XB manufactured by Kojundo Chemical Research Institute, purity 99.9% (3N up)) is separately measured can be referenced. The energy value at the maximum value of fit peak 2 (Ep2) can be set to an intermediate value between Ep1 and Ep3. EP1 is located on the lower energy side compared to EP3. The energy value at the maximum value of a peak is also referred to as the peak position.

[0197] <EDX> It is preferable that one or more selected from the additive elements contained in the positive electrode active material particles 100 have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak differs depending on the additive element in the positive electrode active material particles 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material particle 100 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, electron probe microanalysis (EPMA), or the like.

[0198] Among EDX measurements, EDX area analysis refers to measuring while scanning an area and evaluating the area two-dimensionally. EDX area analysis refers to measuring while scanning linearly and evaluating the distribution of atomic concentration within the positive electrode active material particle 100. Furthermore, data extracted from a linear area of ​​EDX area analysis may also be called line analysis. Measurement of a certain area without scanning is called point analysis.

[0199] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material particle 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material particle 100 in a specific region without being affected by the distribution in the depth direction.

[0200] Since the positive electrode active material particle 100 is a compound containing a transition metal capable of inserting and detaching lithium and oxygen, the surface of the positive electrode active material particle 100 is the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon insertion and detachment of lithium and oxygen are present and a region where they are not present.

[0201] In STEM-EDX-ray analysis or the like, in principle or due to measurement errors, the graph of the detected amount of characteristic X-rays of the element does not change sharply, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX-ray analysis or the like, the detected amount of characteristic X-rays of the transition metal M is the average value M of the detected amount of characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the detected amount of oxygen characteristic X-rays and the average value O ＡＶＥ and the average value O of the detected amount of characteristic X-rays of oxygen in the background ＢＧThe reference point is the point where the detected amount of the characteristic X-rays of the transition metal M is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M and the average value of the detected amount of the characteristic X-rays of the background transition metal M. If the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen is different from the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen attached to the surface, and therefore, the detected amount of the characteristic X-rays of the transition metal M is different from the point where ... oxygen is different from the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ In the case of the positive electrode active material particle 100 having a plurality of transition metals M, the M of the transition metal element with the largest amount of characteristic X-rays detected inside can be used as the reference point. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0202] The average value M of the detected amount of characteristic X-rays of the transition metal M in the background ＢＧ can be obtained by averaging, for example, a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material particle 100, avoiding the vicinity where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＡＶＥ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the detected amount of characteristic X-rays of the transition metal M and oxygen becomes saturated and stable, for example, the region where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＢＧ and the average value O of the detected amount of characteristic X-rays of oxygen inside ＡＶＥ can also be found in the same way.

[0203] Furthermore, the surface of the positive electrode active material particle 100 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystal structure of the positive electrode active material particle 100 is observed and an area where an image is not observed, and is the outermost area of ​​an area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material particle 100 are confirmed.

[0204] In STEM-EDX-ray analysis, a peak refers to a convex shape that appears on a graph of the characteristic X-ray intensity or concentration of each element. The peak intensity refers to the maximum value of the convex shape or the maximum value of the characteristic X-ray for each element. The peak position refers to the maximum value of the convex shape or the position of the maximum value of the characteristic X-ray for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a full width at half maximum that is less than the spatial resolution (R), for example, less than R / 2.

[0205] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured after two scans can be used as the detection value for each element. The number of scans is not limited to two, and more scans can be performed, and the average can be used as the detection value for each element.

[0206] STEM-EDX analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material particles 100. For example, carbon can be vapor-deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0207] Next, the positive electrode active material is thinned to prepare a STEM cross-section sample. For example, thinning can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the processing conditions are preferably, for example, gradually decreasing the acceleration voltage from high to low, for example, 30 kV, 15 kV, 5 kV (up to 2 kV).

[0208] STEM-EDX analysis can be performed using, for example, a STEM device (Hitachi High-Tech HF5000) and an OXFORD Ultim MAX TLE (two devices installed) as the EDX detector. During EDX analysis, the emission current of the STEM device is set to 7 μA or more and 10 μA or less, and a portion of the thinned sample with minimal depth and unevenness is measured. STEM-EDX analysis involves performing EDX area analysis to evaluate the measurement target area in two dimensions, after which any desired region can be extracted for EDX analysis using analysis software. The measurement magnification for EDX area analysis is set to a magnification that results in a linear analysis measurement pitch of approximately 0.3 nm, for example, approximately 160,000 times (320,000 times the magnification on the screen due to drift correction, which is doubled). The conditions for EDX area analysis can be 256 x 256 captured pixels, 50 μsec dwell time, and approximately 400 frames (a total count of Co in a qualitative spectrum is approximately 150,000 counts). The extraction conditions for EDX ray analysis can be a line width of 256 pixels to detect as many characteristic X-rays as possible. For EDX analysis, the Netcount, which indicates the total count of the detected element, and the atomic % obtained by multiplying the Netcount by the k factor, a coefficient specific to the element, can be selected at will.

[0209] The STEM-EDX ray analysis is not limited to the above-mentioned STEM device, and other STEM devices (JEM-ARM200F NEOARM manufactured by JEOL Ltd.) and EDX detectors (Dual SDD detector JED-2300T manufactured by JEOL Ltd.) can also be used. The EDX ray analysis can be performed under the same conditions as those described above.

[0210] In order to increase the spatial resolution in STEM-EDX ray analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. 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, in order to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used in STEM-EDX ray analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0211] In addition, in the positive electrode active material particles 100 containing magnesium and fluorine as additive elements, the fluorine distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the fluorine concentration or detection amount and the peak of the magnesium concentration or detection amount is preferably within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and even more preferably within 0.5 nm.

[0212] Furthermore, in the positive electrode active material particle 100 having nickel as an additive element, the peak of the nickel concentration or detectable amount in the surface layer portion 100a is preferably present on the surface of the positive electrode active material particle 100 or at a depth of up to 3 nm from the reference point toward the center, and more preferably at a depth of up to 1 nm. Furthermore, in the positive electrode active material particle 100 having magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the nickel concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, and more preferably within 1 nm.

[0213] Furthermore, when the positive electrode active material particle 100 contains aluminum as an additive element, it is preferable that, when EDX-ray analysis is performed, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 100a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 100a is located more inward than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material particle 100 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0214] Unless otherwise specified in this specification, the concentration of an element in EDX analysis is calculated by using the denominator as the sum of carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, and gallium.

[0215] <EPMA> The concentration of the additive element contained in the positive electrode active material particle 100 can be analyzed using EDX, but it can also be analyzed using EPMA. EPMA has a higher detection capability (also referred to as a lower detection limit) than EDX when analyzing elements present in trace amounts in a sample. Therefore, it is preferable to use EPMA when analyzing a region where a trace amount of the additive element is present.

[0216] In the EPMA analysis, a cross section of the positive electrode active material particle 100 is exposed by mechanical polishing, ion polishing, FIB, or the like, and the cross section is analyzed. An EPMA device such as the JXA-iHP200F electron probe microanalyzer manufactured by JEOL Ltd. can be used. Measurement conditions can be, for example, an acceleration voltage of 10 kV, a beam diameter of approximately 3 μmφ, an irradiation current of 50 nA, and analytical crystal species of C(LDE2H), O(LDE1L), Mg(TAPL), Co(LIFH), Ni(TAPL), F(TAPL), Al(TAPL), or Ti(LIFH).

[0217] EPMA uses a wavelength-dispersive detector, so it has a higher ability to detect trace elements than EDX, which uses an energy-dispersive detector. On the other hand, the spatial resolution of EPMA analysis is inferior to that of EDX (especially STEM-EDX). Therefore, STEM-EDX is suitable for analysis focusing on the detailed distribution of added elements in the surface layer 100a of the positive electrode active material particle 100, while EPMA analysis is suitable for analysis of trace amounts of added elements in the interior 100b. Note that EPMA and EDX use different analytical methods, so the concentration values ​​obtained when analyzing the same region using each analytical method may not match.

[0218] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0219] Embodiment 2 In this embodiment, an example of a method for manufacturing the positive electrode active material particle 100 of one embodiment of the present invention will be described with reference to FIGS. 8A to 10B.

[0220] Heating conditions are important for producing the positive electrode active material particles 100. The lower limit of the heating temperature is the temperature at which a reaction proceeds from the starting materials. The temperature at which the reaction proceeds may be any temperature at which mutual diffusion of elements contained in the starting materials occurs, and may be lower than the melting temperature of the starting materials. An oxide will be used as an example for explanation. ｍ 0.757 times (Tammann temperature T ｄ It is known that solid-state diffusion occurs from 0°C. Therefore, the heating temperature is preferably 650°C or higher.

[0221] Furthermore, in order to achieve the distribution of the additive elements as described above, it is preferable to set an appropriate heating time. The heating time is, for example, preferably 30 hours or more and 100 hours or less, more preferably 30 hours or more and 80 hours or less, more preferably 30 hours or more and 70 hours or less, and even more preferably 30 hours or more and 60 hours or less. Heating may be performed in multiple stages, in which case the total heating time required to reach 650°C is preferably within the above range. For example, it is preferable that the total time during which the furnace heating temperature is set to 650°C or more is within the above range. The temperature-lowering time after heating is, for example, preferably 10 hours or more and 50 hours or less.

[0222] <<Method 1 for Producing Positive Electrode Active Material Particles 100>> Method 1 for producing positive electrode active material particles 100 will be described with reference to Fig. 8A. Here, magnesium and fluorine are used as additive elements.

[0223] <Step S11> In step S11 shown in FIG. 8A, a lithium source (Li source), a cobalt source (Co source), a magnesium source (Mg source), and a fluorine source (F source) are prepared as starting materials.

[0224] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0225] As the cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide.

[0226] The magnesium source may be magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. A plurality of the above-mentioned magnesium sources may also be used.

[0227] Examples of fluorine sources include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), cobalt fluoride (CoF ２ , CoF３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0228] <Steps S12 and S13> Next, the lithium source, cobalt source, magnesium source, and fluorine source are pulverized and mixed (step S12) to prepare a mixture 903 (step S13). The pulverization and mixing can be performed by either a dry or wet method. The wet method allows for finer pulverization and mixing of particles. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is preferable to use an aprotic solvent that does not easily react 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 pulverize and mix the mixture. Using dehydrated acetone with the above purity can reduce potential impurities.

[0229] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as pulverizing media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, the peripheral speed should be set 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).

[0230] <Step S14> Next, in step S14, the mixture 903 is heated. As for the heating conditions, as described above, the total heating time to reach 650°C or higher is preferably 30 hours or more and 100 hours or less, more preferably 30 hours or more and 80 hours or less, still more preferably 30 hours or more and 70 hours or less, and still more preferably 30 hours or more and 60 hours or less.

[0231] <Step S15> In step S15, the heated material is recovered to obtain the positive electrode active material particles 100. At this time, the recovered particles can be crushed by sieving as necessary. Through the above steps, the positive electrode active material particles 100 of one embodiment of the present invention can be produced.

[0232] <<Method 2 for Producing Positive Electrode Active Material Particles 100>> Method 2 for producing positive electrode active material particles 100 will be described with reference to Fig. 8B. Unlike Method 1, this method uses lithium cobalt oxide as a starting material, which serves as both a lithium source and a cobalt source.

[0233] 8B, lithium cobalt oxide, a magnesium source, and a fluorine source are prepared. The magnesium source and the fluorine source are collectively referred to as Source A. For the magnesium source and the fluorine source, the description of Step S11 can be referred to.

[0234] The magnesium source and the fluorine source may be added to the lithium cobalt oxide, or the magnesium source and the fluorine source may be mixed together and then added to the lithium cobalt oxide as shown in steps S21a to S21c of FIG. 8C.

[0235] <Steps S22 and S23> Next, lithium cobalt oxide, a magnesium source, and a fluorine source are pulverized and mixed (step S22) to prepare a mixture 903 (step S23). For details of pulverization and mixing, the description of step S12 can be referred to.

[0236] <Step S24> Next, in step S24, the mixture 903 is heated. As for the heating conditions, as described above, it is preferable that the total heating time at 650° C. or higher exceeds 100 hours.

[0237] In addition, if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903, the reaction is more likely to proceed. For example, LiF and MgF are used as the additive element source. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0238] Also, LiCoO ２ :LiF:MgF ２ The mixture obtained by mixing the components in a molar ratio of 100:0.33:1 had an initial melting temperature T ｉｍ is 779 ° C, the melting peak temperature T ｐｍ is 815°C, and the melting end temperature T ｅｍ Therefore, the lower limit of the heating temperature is more preferably 826°C or higher.

[0239] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0240] The upper limit of the heating temperature is set to be below the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range. If the temperature is too high, the fluoride will decrease due to evaporation. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, a temperature of 1000°C or lower is more preferable, a temperature of 950°C or lower is even more preferable, and a temperature of 920°C or lower is even more preferable.

[0241] In consideration of these, the heating temperature in step S24 is preferably 650°C or higher and 1130°C or lower, more preferably 650°C or higher and 1000°C or lower, even more preferably 650°C or higher and 950°C or lower, and even more preferably 650°C or higher and 900°C or lower. Also, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Also, it is preferably 826°C or higher and 1100°C or lower, or 826°C or higher and 1130°C or lower, more preferably 826°C or higher and 1000°C or lower, even more preferably 826°C or higher and 950°C or lower, and even more preferably 826°C or higher and 920°C or lower.

[0242] <Step S25> In step S25, the heated material is recovered to obtain the positive electrode active material particles 100. At this time, the recovered particles can be crushed by sieving as necessary. Through the above steps, the positive electrode active material particles 100 of one embodiment of the present invention can be produced.

[0243] 9 to 10B , a description will be given of a method 3 for producing the positive electrode active material particles 100. Unlike the method 2 for producing the positive electrode active material particles 100, nickel and aluminum are used as additive elements in addition to magnesium and fluorine, and the additive elements are added in multiple batches and heated multiple times.

[0244] <Step S31> In step S31 shown in FIG. 9, lithium cobalt oxide is prepared as both a lithium source and a cobalt source, and an Al source is prepared as an additive element source.

[0245] The additive element A1 can be one or more selected from the additive elements described in the previous embodiments, such as magnesium, fluorine, nickel, aluminum, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, and calcium. Also, one or two selected from bromine and beryllium can be used.

[0246] A method for producing an Al source when magnesium and fluorine are used as the additional element Al will be described with reference to FIG. 10A.

[0247] 10A, a magnesium source and a fluorine source are prepared. The magnesium source and the fluorine source may be prepared by referring to the description of step S11.

[0248] <Step S31b> Next, in step S31b, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0249] <Step S31b> Next, in step S31b, the material that has been pulverized and mixed as described above is recovered, and an additional element A1 source (A1 source) can be obtained.

[0250] <Steps S32 to S35> Next, similarly to steps S22 to S25, the mixture is mixed and heated to obtain a composite oxide.

[0251] <Step S41> Next, in step S41, an A2 source is prepared as an additive element source. The additive element A2 can be one or more of the additive elements described in the previous embodiment, such as magnesium, fluorine, nickel, aluminum, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, and calcium. Alternatively, one or two elements selected from bromine and beryllium can also be used. However, it is preferable that the additive element A2 be an element not used as the additive element A1.

[0252] A method for producing an A2 source when nickel and aluminum are used as the additional element A2 will be described with reference to FIG. 10B.

[0253] As the nickel source, for example, nickel hydroxide, nickel fluoride, etc. can be used, and as the aluminum source, for example, aluminum hydroxide, aluminum fluoride, etc. can be used.

[0254] 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, relative to the number of cobalt atoms in the lithium cobalt oxide. For example, when nickel hydroxide is used as the nickel source, when the number of moles of lithium cobalt oxide in step S10 is taken as 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).

[0255] The number of aluminum atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0% of the number of cobalt atoms in the lithium cobalt oxide, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%. For example, when aluminum hydroxide is used as the aluminum source, when the number of moles of lithium cobalt oxide in step S10 is taken as 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%).

[0256] 3B, it is preferable to prepare a nickel source (Ni source) and an aluminum source (Al source) in step S41b and then pulverize them in step S42a. As a result, an additional element source (A2 source) can be obtained in step S43. The pulverization conditions can be determined by referring to the description of step S31b.

[0257] <Steps S41 to S54> Next, similarly to steps S22 to S25, the mixture is mixed and heated to obtain positive electrode active material particles 100.

[0258] In the method 3 for producing the positive electrode active material particles 100, the total heating time in step S34 and step S44 is preferably 30 hours or more and 100 hours or less, more preferably 30 hours or more and 80 hours or less, more preferably 30 hours or more and 70 hours or less, and still more preferably 30 hours or more and 60 hours or less. With regard to the heating temperature, the description of the method 2 for producing the positive electrode active material particles 100 can be referred to.

[0259] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0260] Embodiment 3 In this embodiment, a structure of a lithium ion battery will be described.

[0261] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes positive electrode active material particles, and may further include at least one of a conductive additive and a binder. The positive electrode active material particles may be those described in the previous embodiment.

[0262] <Positive Electrode Active Material> As the positive electrode active material, a mixture of the positive electrode active material particles 100 described in the above embodiment and another positive electrode active material may be used.

[0263] Other examples of the positive electrode active material include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, LiFePO ４ , LiFeO ２ , LiCoO ２ , LiNiO ２ , LiMnO２ , LiNi ａ Mn ｂ Co ｃ O ２ (a+b+c=1), LiMn ２ O ４ , V ２ O ５ , Cr ２ O ５ , MnO ２ The following compounds are exemplified:

[0264] <Conductive additive> The conductive additive is also called a conductivity imparting agent or conductive material, and a carbon material can be used. By attaching the conductive additive between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, the term "attachment" does not only refer to physical adhesion between the active material and the conductive additive, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive additive covers part of the surface of the active material, where the conductive additive is embedded in the surface irregularities of the active material, and where the active material is electrically connected even when not in contact with each other.

[0265] Specific examples of carbon materials that can be used as the conductive additive include carbon black (furnace black, acetylene black, graphite, etc.), graphene, multi-graphene, graphene oxide, and / or reduced graphene oxide.

[0266] Furthermore, the use of a mixture of graphene and acetylene black is preferable because it allows for rapid charging, which is particularly effective when used in automotive lithium-ion batteries.

[0267] <Binder> Examples of the binder that can be used include rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluorine rubber can also be used.

[0268] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include one or more of cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, and starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0269] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, 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, and nitrocellulose.

[0270] Additionally, graphene, multi-graphene, graphene oxide, and / or reduced graphene oxide can function not only as a conductive additive but also as a binder.

[0271] The binder may be used in combination with two or more of the above.

[0272] <Positive Electrode Current Collector> The current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, titanium, or iron, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing an element that improves physical properties, such as iron, silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal 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 the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable to use a current collector with a thickness of 5 μm to 30 μm.

[0273] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may further have a conductive additive and a binder.

[0274] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material and / or a carbon material can be used.

[0275] The carbon material used for the negative electrode active material may be one or more selected from graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, graphene compounds, carbon black, and the like.

[0276] 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, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0277] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety compared to lithium metal.

[0278] The negative electrode active material may be an element capable of undergoing a charge-discharge reaction through alloying and dealloying reactions with lithium. For example, one or more materials selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. may be used. These elements have a larger capacity than carbon, and silicon, in particular, has a high theoretical capacity of 4200 mAh / g. 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 , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３, InSb, SbSn, etc. Also, for example, a compound of Ti with Si, SiO or SiC may be used. Here, elements capable of undergoing charge / discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, may be referred to as alloy-based materials. Compared to graphite, alloy-based materials such as silicon may be preferred as negative electrode active materials for low-temperature secondary batteries because they may be less susceptible to a decrease in charge / discharge capacity at low temperatures.

[0279] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or close to 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.

[0280] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO ２ , SiO ｘ (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a D50 of 1 μm or less, more preferably 100 nm or less.

[0281] Alternatively, silicon particles covered with a graphene compound may be used as the negative electrode active material. In this case, it is more preferable that there is a space between the graphene compound and the silicon particles that can buffer structural changes.

[0282] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２), molybdenum dioxide (MoO ２ ) and the like can be used.

[0283] In addition, as the negative electrode active material, a nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0284] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0285] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３Fluorides such as:

[0286] Furthermore, a combination of the above-mentioned negative electrode active materials may be used; for example, a negative electrode active material containing a mixture of graphite and silicon particles may be used. Silicon particles are silicon powders used as negative electrode active materials for lithium-ion secondary batteries. These particles have an average particle size distribution, i.e., an average particle size of approximately 100 nm, and are sometimes referred to as nanosilicon particles. The silicon particles used are preferably prepared by pulverizing silicon raw materials to a uniform particle size. The silicon particles may include at least one of silicon, silicon oxide, and silicon alloy. While laser diffraction particle size distribution measurement is typically used to measure particle size, the measurement 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 a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0287] Another example of a negative electrode is one that does not have a negative electrode active material at the end of the battery fabrication. A negative electrode that does not have a negative electrode active material can be, for example, a negative electrode that has only a negative electrode current collector at the end of the battery fabrication, in which lithium ions released from the positive electrode active material upon charging the battery are deposited as lithium metal on the negative electrode current collector to form a negative electrode active material layer. A battery using such a negative electrode is sometimes called a negative electrode-free (anode-free) battery, a negative electrode-less (anode-less) battery, or the like.

[0288] When a negative electrode without a negative electrode active material is used, a film for uniforming lithium deposition may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniforming lithium deposition. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as films for uniforming lithium deposition because they are relatively easy to form uniformly on the negative electrode current collector. Furthermore, for example, a metal film that forms an alloy with lithium can be used as the film for uniforming lithium deposition. For example, a magnesium metal film can be used as the metal film that forms an alloy with lithium. Lithium and magnesium form a solid solution over a wide composition range, making them suitable as films for uniforming lithium deposition.

[0289] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0290] Furthermore, as the conductive additive and binder that can be contained in the negative electrode active material layer, the same materials as the conductive additive and binder that can be contained in the positive electrode active material layer can be used.

[0291] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0292] [Electrolyte Solution] The electrolyte solution contains an organic solvent, but the organic solvent of the electrolyte according to one embodiment of the present invention is not limited to being liquid at 25°C, and may be solid at 25°C or semi-solid at room temperature. The organic solvent of the electrolyte according to one embodiment of the present invention is preferably liquid over a wide temperature range, including temperatures from below freezing to high temperatures, but is not limited thereto. The organic solvent may be liquid, solid, or semi-solid over a wide temperature range, including temperatures from below freezing to high temperatures.

[0293] The organic solvent is preferably an aprotic organic solvent, and examples thereof include 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), 1,3-propane sultone (PS), fluoroethylene carbonate (FEC), methyl 3,3,3-trifluoropropionate (MTFP), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultones, and any combination and ratio of two or more of these may be used.

[0294] Because PS has HOMO and LUMO levels equivalent to those of EC and DEC, it is resistant to oxidation and reduction even at high cutoff voltages, and when decomposed on the surface of the positive electrode active material particles, it tends to form polymers. Therefore, it has the advantage of being less likely to gasify into small molecular weight decomposition products. Therefore, the electrolyte preferably contains 0.1 wt% to 10 wt%, more preferably 0.25 wt% to 7.5 wt% of PS.

[0295] FEC is a cyclic carbonate with a high dielectric constant, and when used in an organic solvent, it promotes the dissociation of lithium salts. On the other hand, because FEC has electron-withdrawing substituents, it is more likely to desolvate with lithium ions than EC. Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. Therefore, FEC easily releases lithium ions from the surfaces of the positive electrode active material particles and the negative electrode active material, thereby reducing the internal resistance of the secondary battery. Furthermore, because FEC has a deep highest occupied molecular orbital (HOMO) level, it is less susceptible to oxidation and has improved oxidation resistance. However, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC alone in the electrolyte. MTFP is a type of chain carbonate, and can reduce the viscosity of the electrolyte solution or maintain the viscosity at room temperature (typically 25°C) even at low temperatures (typically 0°C). Furthermore, although MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, it may form a solvation with lithium ions when used in an electrolyte solution. When a mixed organic solvent containing both FEC and MTFP is used, when the volume ratio is FEC:MTFP=1:y, y is preferably 2 or more and 20 or less, more preferably 4 or more and 9 or less.

[0296] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the electricity storage device from exploding or catching fire even if the internal temperature rises due to an internal short circuit or overcharging of the electricity storage device. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0297] The electrolyte to be dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (Li(C) ２ O ４ ) ２ , LiBOB), or two or more of these can be used in any combination and ratio.

[0298] The electrolyte solution may also contain additives. The additives can suppress reactive decomposition of the electrolyte that may occur on the positive electrode surface or the negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives that can be used include propane sultone (PS), vinylene carbonate (VC), tert-butylbenzene (TBB), lithium bis(oxalato)borate (LiBOB), 1,3,6-hexanetricarbonitrile, ethyl 2-methylbutyrate, ethyl 2-methylvalerate, and propyl 2-methylbutyrate. PS is particularly preferred as an additive because it improves cycle characteristics.

[0299] The additive may be one or more dinitrile compounds, such as succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

[0300] Fluorobenzene may also be added to the organic solvent. The concentration of the additive is, for example, 0.1 wt% to 5 wt% of the total electrolyte solution. PS or EGBE are preferred because they form a good coating on the positive electrode during charge and discharge, improving cycle characteristics. Fluorobenzene (FB) is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes. Dinitrile compounds are preferred because their nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving high-voltage resistance. Furthermore, dinitrile compounds are preferred because they prevent copper dissolution during overdischarge when a copper-containing current collector is used on the negative electrode. Considering the use of secondary batteries at high voltages, adding a nitrile compound is preferred.

[0301] The electrolyte used in the electricity storage device is preferably a highly purified electrolyte with a low content of granular waste or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0302] If the impurities contained in the electrolyte solution are oxygen and / or water, the crystal structure in the charged state described in the previous embodiment may become unstable. Therefore, the water content in the electrolyte solution is preferably 30 ppm or less, and more preferably 20 ppm or less.

[0303] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0304] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0305] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel. For example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0306] In addition, the electrolyte can be a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a polymer material such as a polyethylene oxide (PEO)-based solid electrolyte. When a solid electrolyte is used, the installation of a separator or spacer is unnecessary. Furthermore, since the entire battery can be solidified, the risk of leakage is eliminated, dramatically improving safety.

[0307] [Separator] When the electrolyte contains a liquid electrolyte (also called an electrolyte solution), a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those formed from cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or porous films using nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polypropylene (referred to as PP), polyimide (referred to as PI), polyester, acrylic, polyolefin, and polyurethane. The porosity of the separator can be 35% to 90%, preferably 60% to 85%. Separators using polypropylene can have a porosity of 35% to 45%. Separators using polyimide can have a porosity of 75% to 85%. The separator thickness is preferably 5 μm to 50 μm, more preferably 5 μm to 30 μm.

[0308] The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0309] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene may be coated with a ceramic material, a hydroxide material, a fluorine material, a polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles, silicon oxide particles, and magnesium oxide particles. Examples of hydroxide materials include magnesium hydroxide particles and aluminum hydroxide particles. Examples of fluorine materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid and para-aramid).

[0310] By using a multilayer separator, the safety of the lithium-ion battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the lithium-ion battery can be increased.

[0311] [Exterior Body] The exterior body of a lithium-ion battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0312] This embodiment can be used in combination with other embodiments.

[0313] Embodiment Mode 4 In this embodiment mode, an example of a lithium ion battery will be described with reference to FIG.

[0314] Fig. 11A is a diagram illustrating a wound body 950a of a lithium ion battery 913, Fig. 11B is an exploded perspective view of the lithium ion battery 913, and Fig. 11C is an external view of the lithium ion battery 913. The lithium ion battery 913 has a positive electrode 932 having the positive electrode active material described in the previous embodiment, a negative electrode 931, an electrolyte layer, and a separator 933, and the negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a. These are wound as shown in Fig. 11A.

[0315] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0316] 11B, the negative electrode 931 is electrically connected to a terminal 951. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952. The terminal 952 is electrically connected to a terminal 911b.

[0317] 11C , wound body 950 a is covered with housing 930 to form lithium ion battery 913. Housing 930 is preferably provided with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0318] 11B, the lithium ion battery 913 may have a plurality of wound bodies 950a. By using a plurality of wound bodies 950a, the lithium ion battery 913 can have a larger charge / discharge capacity.

[0319] By using the positive electrode active material particles of the present invention in the lithium ion battery 913 having a wound body, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0320] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0321] Fifth Embodiment In this embodiment, an example of application to an electric vehicle (EV) will be described with reference to FIG.

[0322] 12A , the electric vehicle is equipped with first batteries 1301 a and 1301 b as main driving lithium ion batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. By using the positive electrode active material particles of the present invention in the first batteries 1301 a and 1301 b, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0323] The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have a high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0324] The internal structure of the first battery 1301a may be a wound type or a stacked type. Alternatively, a lithium-ion battery including the positive electrode active material particles of one embodiment of the present invention may be used for the first battery 1301a. By using a lithium-ion battery including the positive electrode active material particles of one embodiment of the present invention for the first battery 1301a, an electric vehicle with a long driving range and usable in a wide range of ambient temperatures can be obtained.

[0325] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple lithium ion batteries, it is possible to extract large amounts of power. The multiple lithium ion batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of lithium ion batteries is also called a battery pack.

[0326] In addition, in an in-vehicle lithium-ion battery, in order to cut off power from multiple lithium-ion batteries, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0327] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0328] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0329] The first battery 1301a will be described with reference to FIG. 12B.

[0330] FIG. 12B shows an example in which nine prismatic lithium-ion batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic lithium-ion batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also referred to as a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (e.g., from the road surface), it is preferable to fix multiple lithium-ion batteries using the fixing portions 1413 and 1414 and the battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0331] FIG. 12C shows an example of a block diagram of the battery pack 1415 shown in FIG. 12B.

[0332] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the lithium-ion battery used and limits the upper limit of the external current and the upper limit of the output current. The range between the lower and upper voltage limits for the lithium-ion battery is within the recommended voltage range. If the voltage falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0333] The first batteries 1301a and 1301b mainly supply power to on-board equipment in the 42V system (high-voltage HV system), while the second battery 1311 supplies power to on-board equipment in the 14V system (low-voltage LV system). Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Using a lithium-ion battery as the second battery 1311 has the advantage of being maintenance-free, but after prolonged use, e.g., three years or more, there is a risk of abnormalities occurring that cannot be detected at the time of manufacture. In particular, if the second battery 1311 that starts the inverter becomes inoperable, even if the first batteries 1301a and 1301b have remaining capacity, the second battery 1311 is charged to maintain a full charge state by supplying power from the first battery to the second battery.

[0334] In this embodiment, an example in which lithium ion batteries are used for both the first battery 1301 a and the second battery 1311 is shown, but a lead-acid battery, an all-solid-state battery, or an electric double layer capacitor may be used for the second battery 1311. By using the positive electrode active material particles of the present invention in the above-mentioned lithium ion battery, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0335] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 from the motor controller 1303 and the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0336] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the lithium ion batteries used, and can perform rapid charging.

[0337] Although not shown, when the electric vehicle is connected to an external charger, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a, 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a, 1301b via the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU also includes a microcomputer. The ECU uses a CPU or a GPU.

[0338] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0339] Next, an example in which a lithium-ion battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0340] Furthermore, installing lithium-ion batteries in vehicles will enable next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), plug-in hybrid vehicles (PHVs), etc. Lithium-ion batteries can also be installed in transportation vehicles such as agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

[0341] 13A to 13E show examples of vehicles and the like using the lithium-ion battery of one embodiment of the present invention.

[0342] 13A illustrates an example of an electric bicycle using the lithium-ion battery of one embodiment of the present invention. The lithium-ion battery of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 13A. The lithium-ion battery of one embodiment of the present invention may include a protection circuit.

[0343] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 can be detached from the main body of the electric bicycle 8700 and can be carried around. The power storage device 8702 includes a plurality of lithium-ion batteries of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit. By using the positive electrode active material particles of the present invention in the lithium-ion battery, a lithium-ion secondary battery with large charge / discharge capacity and favorable cycle characteristics can be obtained.

[0344] 13B illustrates an example of a motorcycle using a lithium-ion battery of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 13B includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The scooter 8600 can store the power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can supply electricity to the turn signal light 8603. When the scooter has a motor, the power storage device 8602 can also supply electricity to the motor. By using the positive electrode active material particles of the present invention for the lithium-ion battery included in the power storage device 8602, a lithium-ion secondary battery with large charge / discharge capacity and favorable cycle characteristics can be obtained.

[0345] The automobile 2001 shown in Figure 13C is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a lithium ion battery is mounted on a vehicle, an example of the lithium ion battery shown in the above embodiment is installed in one or more locations. By using the positive electrode active material particles of the present invention in the lithium ion battery mounted on the vehicle, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0346] 13C includes a battery pack 2200, which includes a battery module to which a plurality of lithium-ion batteries are connected. The battery pack 2200 further preferably includes a charge control device electrically connected to the battery module.

[0347] Furthermore, the automobile 2001 can charge its lithium-ion battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. The charging method and connector standards may be appropriately determined using a predetermined system such as CHAdeMO (registered trademark) or Combo. The external charging facility may be a charging station installed in a commercial facility, a household power source, or the like. For example, plug-in technology can be used to charge the power storage device installed in the automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device such as an AC-DC converter.

[0348] Furthermore, although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the lithium-ion battery while the vehicle is stopped and while moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0349] Figure 13D shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The battery module of transport vehicle 2003 has, for example, one hundred or more lithium ion batteries with a nominal voltage of 3.0 V to 5.0 V connected in series to produce a maximum voltage of 600 V. Furthermore, except for the number of lithium ion batteries constituting the battery module of battery pack 2202, the battery module has the same functions as that shown in Figure 13C, and therefore a description thereof will be omitted. By using the positive electrode active material particles of the present invention in the lithium ion batteries of the module, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0350] 13E shows, as an example, an aircraft 2004 having an engine that burns fuel. The aircraft 2004 can be considered a type of transport vehicle because it has wheels for takeoff and landing, and has a battery pack 2203 that includes a battery module formed by connecting multiple lithium-ion batteries and includes the battery module and a charge control device.

[0351] The battery module of the aircraft 2004 has, for example, eight 4 V lithium ion batteries connected in series, with a maximum voltage of 32 V. Other than the number of lithium ion batteries constituting the battery module of the battery pack 2203, it has the same functions as those in Fig. 13C, and therefore a description thereof will be omitted.

[0352] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0353] (Embodiment 6) In this embodiment, an example of mounting a lithium-ion battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a lithium-ion battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0354] 14A shows an example of a mobile phone. The mobile phone 2100 includes a display unit 2102 built into a housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. The mobile phone 2100 also includes a lithium ion battery 2107. By using the positive electrode active material particles of the present invention in the lithium ion battery, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0355] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0356] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0357] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0358] The mobile phone 2100 also includes an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0359] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0360] 14B shows an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a lithium-ion battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. By using the positive electrode active material particles of the present invention in the lithium-ion battery, a lithium-ion secondary battery with large charge / discharge capacity and favorable cycle characteristics can be obtained.

[0361] Fig. 14C shows an example of a robot. A robot 6400 shown in Fig. 14C includes a lithium ion 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, etc.

[0362] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0363] The display unit 6405 has a function of displaying various information. The robot 6400 can display 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, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0364] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of 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 the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0365] The robot 6400 includes a lithium-ion battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. By using the positive electrode active material particles of the present invention in the lithium-ion battery, the lithium-ion secondary battery can have large charge / discharge capacity and favorable cycle characteristics.

[0366] 14D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a lithium-ion battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0367] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 and determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a lithium-ion battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. By using the positive electrode active material particles of the present invention in the lithium-ion battery, a lithium-ion secondary battery with large charge / discharge capacity and favorable cycle characteristics can be obtained.

[0368] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0369] In this example, a positive electrode active material according to one embodiment of the present invention was prepared, and powder analysis and charge / discharge characteristics were evaluated.

[0370] <Preparation of Positive Electrode Active Material> The positive electrode active material samples prepared in this example will be described with reference to the preparation method shown in Figure 8B and Figure 8C. Samples 1 and 2 were prepared as positive electrode active material samples by changing the preparation conditions.

[0371] [Sample 1] LiCoO in step S21 of FIG. ２ As the lithium cobalt oxide, commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) containing no particular additive elements was prepared.

[0372] In this example, an A source containing Mg and F as additive elements was prepared according to steps S21a to S21c shown in FIG. 8C. First, according to step S21a shown in FIG. 8C, lithium fluoride (LiF) was prepared as the F source, and magnesium fluoride (MgF ２ ) was prepared. ２ were weighed out so that the molar ratio was 1:3. Next, lithium fluoride and magnesium fluoride were mixed in dehydrated acetone and stirred at a rotation speed of 500 rpm for 20 hours. A ball mill was used for mixing, and zirconium oxide balls were used as the medium. After mixing, the mixture was sieved through a sieve with 300 μm openings to obtain the A source.

[0373] Next, in step S22, magnesium fluoride was weighed out so that the number of moles of magnesium fluoride contained in the A source was 1 (1 mol%) when the number of moles of lithium cobalt oxide was 100, and mixed with lithium cobalt oxide in a ball mill. The mixture was stirred for 1 hour at a rotation speed of 150 rpm. This condition applies a smaller force to the powder than when stirring to obtain the A source. Finally, the mixture was sieved with a sieve having 300 μm openings, and a mixture 903 with a uniform particle size was obtained (step S23).

[0374] Next, in step S24, mixture 903 was heated. The heating conditions were a heating temperature of 900°C and a heating time of 10 hours. During heating, mixture 903 was uniformly placed in a sheath to a thickness of approximately 5 mm, and a lid was placed over the sheath. An oxygen-containing atmosphere was created inside the sheath, and the inflow and outflow of oxygen was blocked (purging). A cathode active material, which was a composite oxide containing Mg and F, was obtained by heating (step S25). The cathode active material obtained in this manner was designated Sample 1.

[0375] [Sample 2] Sample 2 was prepared in the same manner as Sample 1, except that in step S24, the heating conditions were a heating temperature of 900°C and a heating time of 40 hours.

[0376] <XPS Analysis Results> XPS analysis was performed on the particle surfaces of Sample 1 and Sample 2 prepared above. The XPS measurement device and conditions were as follows. Measurement device: Quantera II manufactured by ULVAC-PHI, Inc. X-ray source: Monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth (Condition 1): Range from the surface to approximately 5 nm (take-off angle 45°) Detection depth (Condition 2): Range from the surface to approximately 2 nm (take-off angle 15°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0377] The results of the XPS analysis are shown in Tables 1 and 2. In this specification and the like, atomic % may be abbreviated to at%.

[0378]

[0379]

[0380] Table 1 shows the concentration of each element (at%: atomic concentration, sometimes referred to as Atomic%) when the total concentration of lithium (Li), cobalt (Co), oxygen (O), carbon (C), fluorine (F), sulfur (S), calcium (Ca), magnesium (Mg), sodium (Na), and zirconium (Zr) is taken as 100 at%. Note that the concentration values ​​of each element in Table 1 are rounded off, so the total may not equal 100.0. Table 1 also shows the ratio of magnesium concentration to cobalt concentration (Mg / Co) and the ratio of fluorine concentration to magnesium concentration (F / Mg). Note that in the table, "-" indicates that the concentration was below the lower limit of detection.

[0381] Furthermore, the lithium concentration of the positive electrode active material of a lithium-ion secondary battery changes during charging and discharging. Therefore, Table 2 shows the concentrations of each element when the total concentration of cobalt (Co), oxygen (O), carbon (C), fluorine (F), sulfur (S), calcium (Ca), magnesium (Mg), sodium (Na), and zirconium (Zr) is taken as 100 at% after excluding the lithium values ​​from the results of Table 1. Note that the values ​​for the concentrations of each element in Table 2 are rounded off, so the total may not equal 100.0.

[0382] As shown in Tables 1 and 2, for both Sample 1 and Sample 2, there was no significant difference in the concentrations of each element analyzed under Condition 1 with a take-off angle of 45° and Condition 2 with a take-off angle of 15°, except for magnesium (Mg) in Sample 2. Meanwhile, the magnesium (Mg) concentration in Sample 2 was higher (15.8 at%) under Condition 2 (15°) than under Condition 1 (45°) (13.6 at%). This analysis result indicates that Mg is distributed in large amounts in the surface layer, extending from the surface to approximately 2 nm.

[0383] As shown in Tables 1 and 2, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in Sample 1 was 0.73 (45°) and 0.82 (15°). The ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in Sample 2 was 0.83 (45°) and 1.15 (15°), both in the range of 0.800 to 1.200. In particular, in Sample 2, under Condition 2 (15°), in which analysis was performed at a depth of approximately 2 nm from the surface, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) was 1.15, which was 1.00 or more, and was a high value.

[0384] Furthermore, as shown in Tables 1 and 2, the ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg) was 0.73 (45°) and 0.71 (15°) in Sample 1. The ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg) in Sample 2 was 0.60 (45°) and 0.54 (15°), both of which were within the range of 0.500 or more and 0.700 or less.

[0385] Next, analysis was performed focusing on the Mg1s peak of the XPS spectrum in the XPS analysis. In the analysis of the Mg1s peak, the peak component derived from the O-Mg-O bond was designated fit peak 1, the peak component derived from the O-Mg-F bond was designated fit peak 2, and the peak component derived from the F-Mg-F bond was designated fit peak 3. These three fit peaks were synthesized, and the peak synthesis ratio that minimized the difference from the Mg1s peak of the XPS spectrum obtained by 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 were equal to the abundance ratios of O-Mg-O bonds, O-Mg-F bonds, and F-Mg-F bonds are shown in Table 3.

[0386] In the above-mentioned XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also called peak top) of fit peak 1 is determined by the MgO-coated LiCoO ２ The energy value at the maximum value of the Mg1s peak when measured separately as a standard sample was referenced. The energy value at the maximum value of fit peak 3 (Ep3) was calculated based on the energy value at the maximum value of magnesium fluoride (MgF ２ The energy value at the maximum value of the Mg1s peak when a standard sample (Kojundo Chemical Laboratory MGH18XB, purity 99.9% (3N up)) was separately measured was referenced. The energy value at the maximum value of fit peak 2 (Ep2) was set to the intermediate value between Ep1 and Ep3. The energy value at the maximum value of the peak is also referred to as the peak position.

[0387] The XPS spectrum in the XPS analysis was corrected on the energy axis so that the maximum value of the C1s peak was 284.8 eV.

[0388]

[0389] As shown in Table 3, Sample 1 and Sample 2 were found to have peak components derived from O-Mg-O bonds and peak components derived from O-Mg-F bonds. Furthermore, both Sample 1 and Sample 2 had 10% or more peak components derived from O-Mg-F bonds. Note that even when the above analysis results indicate 0.0%, this does not mean that the corresponding bond is completely absent. In other words, a bond indicated as 0.0% may exist below the detection limit.

[0390] <EPMA Results> Next, the amounts of the added elements were quantified by EPMA for Sample 1 and Sample 2. The EPMA measurement device and conditions were as follows: Device: JXA-iHP200F manufactured by JEOL Ltd. Acceleration voltage: 10 kV Beam diameter: approximately 3 μmφ

[0391] The samples were embedded in resin and processed by ion polishing to expose the particle cross-section. The measurement area for EPMA was the interior of the positive electrode active material, i.e., the center of the particle cross-section. These cross-sections were analyzed using characteristic X-rays obtained from a region extending to a depth of approximately 1 μm from the surface of the analyzed sample.

[0392] The EPMA results are shown in Table 4.

[0393]

[0394] Table 4 shows the concentration (at%: atomic concentration) of each element when the total concentration of carbon (C), oxygen (O), magnesium (Mg), and cobalt (Co) is taken as 100 at%, and the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co). Note that the values ​​for the concentration of each element in Table 4 are rounded off, so the total may not equal 100.0. The Mg / Co ratio for both Sample 1 and Sample 2 was 0.01, falling within the range of 0.005 to 0.015. This indicates that magnesium is also dissolved in the positive electrode active material particles. No significant differences were observed between Sample 1 and Sample 2 in EPMA.

[0395] <STEM Analysis> Cross-sectional STEM analysis of Sample 1 and Sample 2 was carried out.

[0396] As a pretreatment before analysis, each of Sample 1 and Sample 2 was thin-sectioned by FIB (μ-sampling method).

[0397] The following equipment and conditions were used for STEM and EDX. <STEM observation> Scanning transmission electron microscope: JEOL Ltd. JEM-ARM200F NEOARM Observation conditions Acceleration voltage: 200 kV Magnification accuracy: ±10% <EDX> Analysis method: Energy dispersive X-ray spectroscopy (EDX) Scanning transmission electron microscope: JEOL Ltd. JEM-ARM200F NEOARM Acceleration voltage: 200 kV Measurement mode: STEM mode Elemental analyzer: JED-2300T X-ray detector: Si drift detector Energy resolution: Approximately 140 eV X-ray take-off angle: Approximately 30.5° Solid angle: 2.2 sr Number of captured pixels: 256 x 256

[0398] [STEM-EDX] The results of cross-sectional STEM-EDX analysis of Sample 1 and Sample 2 are shown in FIGS.

[0399] Fig. 15A is a cross-sectional STEM image of Sample 1, Fig. 15B is a graph showing the results of STEM-EDX analysis at A-B in Fig. 15A, with the vertical axis representing the number of counts of characteristic X-rays, and Fig. 15C is a graph showing the vertical axis of the graph in Fig. 15B as a quantitative value of Atomic %. Note that the quantitative value of Atomic % in the STEM-EDX analysis was calculated by setting the sum of the detected amounts of carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, and gallium to 100%.

[0400] Fig. 16A is a graph in which the vertical axis of Fig. 15B is enlarged. Fig. 16B is a graph showing the graph of magnesium (Mg K) extracted from Fig. 16A, and Fig. 16C is a graph showing the graph of fluorine (F K) extracted from Fig. 16A.

[0401] Fig. 17A is a graph in which the vertical axis of Fig. 15C is enlarged. Fig. 17B is a graph showing the graph of magnesium (Mg K) extracted from Fig. 17A, and Fig. 17C is a graph showing the graph of fluorine (F K) extracted from Fig. 17A.

[0402] 18A is a cross-sectional STEM image of Sample 2, FIG. 18B is a graph showing the results of STEM-EDX-ray analysis at C-D in FIG. 18A , with the vertical axis representing the count number of characteristic X-rays, and FIG. 18C is a graph showing the vertical axis of the graph in FIG. 18B as a quantitative value of Atomic %.

[0403] Fig. 19A is a graph in which the vertical axis of Fig. 18B is enlarged. Fig. 19B is a graph showing magnesium (Mg K) extracted from Fig. 19A, and Fig. 19C is a graph showing fluorine (F K) extracted from Fig. 19A.

[0404] Fig. 20A is a graph in which the vertical axis of Fig. 18C is enlarged. Fig. 20B is a graph showing the graph of magnesium (Mg K) extracted from Fig. 20A, and Fig. 20C is a graph showing the graph of fluorine (F K) extracted from Fig. 20A.

[0405] [HAADF-STEM] The results of HAADF-STEM analysis of Sample 1 and Sample 2 are shown in FIGS.

[0406] Fig. 21A is an HAADF-STEM image of Sample 1, and Fig. 21B is an HAADF-STEM image of the region enclosed by a square in Fig. 21A. The region enclosed by the square includes the edge surface of Sample 1.

[0407] Fig. 22A is an HAADF-STEM image of Sample 2, and Fig. 22B is an HAADF-STEM image of the region enclosed by a square in Fig. 22A. The region enclosed by the square includes the edge surface of Sample 2.

[0408] Fig. 23A is an enlarged image of the area surrounded by a square in Fig. 21B, and Fig. 23B is an enlarged image of the area surrounded by a square in Fig. 22B. No significant difference was observed between the HAADF-STEM image of Sample 1 shown in Fig. 23A and the HAADF-STEM image of Sample 2 shown in Fig. 23B.

[0409] <Preparation of Positive Electrode> Samples 1 and 2 were prepared as the positive electrode active material, acetylene black (AB) was prepared as the conductive material, and polyvinylidene fluoride (PVDF) was prepared as the binder. PVDF was prepared by dissolving it in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After applying the slurry to a 20 μm-thick positive electrode current collector, the solvent was evaporated in a ventilated oven at 80°C.

[0410] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process was performed using a roll press machine. The pressing process was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press machine were both set at 120°C.

[0411] A positive electrode was obtained by the above steps. The amount of the positive electrode active material supported per area of ​​the positive electrode was about 7 mg / cm. ２ By this manufacturing method, a positive electrode having Sample 1 and a positive electrode having Sample 2 were manufactured.

[0412] <Fabrication of Half Cells> Coin-shaped half cells (also referred to as coin cells) were fabricated using each of the above-described positive electrodes, lithium metal foil, a separator, an electrolyte, a coin cell positive electrode can, and a coin cell negative electrode can. The coin-shaped half cells were CR2032 type (diameter 20 mm, height 3.2 mm).

[0413] The electrolyte was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 1 mol / L of lithium hexafluorophosphate (LiPF ６) was dissolved in a solution to which 2 wt % of vinylene carbonate (VC) was added as an additive.

[0414] A porous polypropylene film was used as the separator.

[0415] In this manner, a coin cell having Sample 1 and a coin cell having Sample 2 were fabricated.

[0416] <Charge-Discharge Cycle Test> A charge-discharge cycle test was carried out using the above half cell.

[0417] The conditions for the charge-discharge cycle test were that the battery was charged at a constant current of 0.5 C up to 4.60 V, followed by constant voltage charging until the current value reached 0.05 C. The battery was discharged at a constant current of 0.5 C down to 2.50 V. The rest period between the end of charge and the start of discharge was 10 minutes. Here, 1 C was set to 200 mA / g. The temperature of the measurement environment was 25°C. Charge and discharge were repeated 50 times in this manner. The results of the charge-discharge cycle test are shown in Figures 24A and 24B.

[0418] Fig. 24A is a graph showing the discharge capacity versus cycle number for a half cell using Sample 1 (dashed line) and a half cell using Sample 2 (solid line). Fig. 24B is a graph showing the discharge capacity of Fig. 24A in terms of discharge capacity retention. The discharge capacity retention was calculated by setting the discharge capacity value at which the maximum discharge capacity was obtained in the charge-discharge cycle test as 100%.

[0419] The results of these measurements are shown in Table 5. Table 5 shows the discharge capacities and discharge capacity retention rates of Sample 1 and Sample 2. The third row of Table 5 shows the maximum values ​​for each, and the fourth row and subsequent rows show values ​​corresponding to the number of cycles listed in the first column. Note that the discharge capacity is a value per weight of the positive electrode active material in each half-cell, and the discharge capacity retention rate is set to 100% as the maximum discharge capacity in the charge-discharge cycle test.

[0420]

[0421] As shown in FIGS. 24A and 24B and Table 5, the cycling characteristics of the half-cell containing Sample 2 were superior to those of the half-cell containing Sample 1.

[0422] <XRD Analysis in High-Voltage Charging State> Experiments were carried out to investigate the crystal structures of Sample 1 and Sample 2 in the high-voltage charging state.

[0423] First, using a half cell using Sample 1 and a half cell using Sample 2, charging, discharging, disassembly of the half cell, and XRD measurement were performed. The half cells were fabricated as described above in <Fabrication of Half Cell>. No pressing was performed during fabrication of the positive electrode. Charging was performed at a constant current of 0.5 C up to 4.60 V, followed by constant voltage charging until the current value reached 0.01 C. Discharging was performed at a constant current of 0.5 C down to 2.5 V. In the XRD analysis of the state of charge, 1 C was set to 137 mA / g. A 30-minute pause was allowed between charging and discharging, and between discharging and charging.

[0424] Next, charging was performed before XRD analysis of the high-voltage charged state. Charging was performed by constant current charging at 0.5 C up to 4.60 V, and then constant voltage charging until the current value reached 0.01 C. After charging, the battery was rested for 30 minutes.

[0425] After that, the half-cell was disassembled within one hour after the end of the charging and post-charging rest periods. During disassembly, the positive electrode was removed while still in a high-voltage charged state. In order to avoid short-circuiting, insulating tools were used to carefully disassemble the half-cell. The positive electrode removed from the disassembled half-cell was washed with DMC. Disassembly was performed in a glove box filled with argon, the dew point and oxygen concentration of which were controlled. The glove box dew point was −60°C or less, and the oxygen concentration was 5 ppm or less. Since the crystalline structure of the positive electrode active material may change if a long period of time has passed since the charging, the cell was disassembled and analyzed as soon as possible.

[0426] The positive electrode obtained by disassembling the half cell was attached to a glass plate and set in a hermetically sealed sample holder for XRD measurement (manufactured by Bruker, Part No. A100B33) in the glove box to obtain a positive electrode sealed in the sample holder for XRD measurement together with argon. The temperature of the laboratory used for disassembling the half cell, removing the positive electrode, setting it in the sample holder, and performing XRD measurement was 25°C or lower.

[0427] Thereafter, XRD measurement was started within 15 minutes after disassembly. The XRD device and conditions were as follows: XRD device: D8 ADVANCE manufactured by Bruker Corporation X-ray: CuKα １ Line output: 40 kV, 40 mA Divergence slit: 0.6 mm Detector: LYNXEYE XE-T Scan 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

[0428] The XRD measurement data of the positive electrodes (Sample 1 and Sample 2) in the high-voltage charged state measured above are shown in Figures 25, 26, and 27. The XRD measurement data shown in the figures was obtained by removing CuKα2 and processing with a Curvature of 25 and a Threshold of 0.001. These figures also show a reference pattern (O3') for the O3'-type crystal structure and a reference pattern (H1-3) for the H1-3 structure. Because the above measurements were performed using two half cells, the measurement data for Sample 1 is listed as Sample 1-1 and Sample 1-2, and the measurement data for Sample 2 is listed as Sample 2-1 and Sample 2-2.

[0429] Fig. 26 shows the range of 2θ from 18° to 21° in XRD measurement, and Fig. 27 shows the range of 2θ from 42° to 47° in XRD measurement.

[0430] 25 to 27, only a few peaks matching the reference pattern (O3') of the O3' structure were confirmed in Sample 1 in the high-voltage charging state. On the other hand, in Sample 2 in the high-voltage charging state, peaks matching the reference pattern (O3') of the O3' structure, namely, a peak in the 2θ range of 19.13° to 19.37° and a peak in the 2θ range of 45.37° to 45.57°, were clearly observed, confirming the presence of the O3' structure.

[0431] According to the above examples, the differences between Sample 1 and Sample 2 include that in XPS analysis measured under conditions of a take-off angle of 15°, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) is 1.15, which is 1.00 or more; in STEM-EDX analysis, the Mg peak top concentration (maximum concentration) is 23 atomic%, which is 7 atomic% or more; and in a high-voltage charging state, Sample 2 has an O3' structure.

[0432] Further, as other analytical results, in the EPMA analysis, Sample 2 was found to have a ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) of 0.01, which is within a range of 0.005 or more and 0.015 or less, and in the XPS analysis of the Mg1s peak, it was found to have 10% or more of "O-Mg-F bonds."

[0433] From these results, it can be considered that Sample 2 has the characteristics of having 10% or more of "O-Mg-F bonds" in the analysis of the Mg1s peak in the XPS analysis, and that the maximum Mg concentration is 7 atomic% or more in the STEM-EDX analysis, and therefore has an O3' structure in the high-voltage charging state, and as a result, excellent charge-discharge cycle characteristics can be obtained.

[0434] Alternatively, Sample 2 has the following characteristics: in the analysis of the Mg1s peak in XPS analysis, it has 10% or more of "O-Mg-F bonds"; in the STEM-EDX analysis, it has a maximum Mg concentration of 7 atomic% or more; and in the XPS analysis measured under conditions of a take-off angle of 15°, it has a ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) of 1.00 or more; therefore, it can be considered that Sample 2 has an O3' structure in a high-voltage charged state, and as a result, it has been able to obtain excellent charge-discharge cycle characteristics.

[0435] Alternatively, sample 2 has the following characteristics: in an analysis of the Mg1s peak in an XPS analysis, it has 10% or more "O-Mg-F bonds"; in a STEM-EDX analysis, it has a maximum Mg concentration of 7 atomic% or more; in an XPS analysis measured under conditions of a take-off angle of 15°, it has a ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) of 1.00 or more; and in an EPMA analysis, it has a ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) of 0.005 or more and 0.015 or less; and therefore, it can be considered that sample 2 has an O3' structure in a high-voltage charged state, and as a result, it has been possible to obtain excellent charge-discharge cycle characteristics.

[0436] In this example, a positive electrode active material of one embodiment of the present invention was prepared, and the change in the crystal structure over time in a charged state was evaluated.

[0437] <Preparation of Positive Electrode Active Material> A positive electrode active material was prepared in the same manner as in Sample 1 of Example 1, except that the heating conditions were a heating temperature of 950° C. and a heating time of 40 hours. This was designated Sample 3.

[0438] <XRD Analysis in High-Voltage Charging State> An experiment was carried out to investigate the crystal structure of Sample 3 in a high-voltage charging state.

[0439] First, using a half-cell using Sample 3, charging, discharging, disassembly of the half-cell, and XRD measurement were performed. The half-cell was fabricated as described in Example 1, <Fabrication of Half-Cell>. Charging was performed by constant current charging at 0.5 C up to 4.60 V, followed by constant voltage charging until the current value reached 0.01 C. Discharging was performed by constant current discharging at 0.5 C down to 2.5 V. In the XRD analysis of the state of charge, 1 C was set to 137 mA / g. A 30-minute pause was allowed between charging and discharging, and between discharging and charging.

[0440] Next, charging was performed before XRD analysis of the high-voltage charged state. Charging was performed by constant current charging at 0.5 C up to 4.60 V, and then constant voltage charging until the current value reached 0.01 C. After charging, the battery was rested for 10 minutes. The charge capacity at this time was 215.7 mAh / g.

[0441] Thereafter, the half cell was disassembled in the same manner as in Example 1, and the removed positive electrode was subjected to XRD measurement. The measurement range (2θ) was set to 42° or more and 47° or less. The time required for each measurement within this range was 13 minutes. After the measurement, a rest period of 2 minutes was allowed. Measurement within this range was repeated 14 times. The measurement conditions other than the measurement range (2θ) and repetition were the same as in Example 1.

[0442] The XRD measurement data for the positive electrode (Sample 3) in the high-voltage charged state measured above is shown in Figure 28. The diffraction patterns obtained 14 times were normalized, and the number of times and the time from the start of the measurement were added. It was confirmed that the peak near 45°, which corresponds to the 104 diffraction of the O3'-type crystal structure, broadened over time and shifted to a lower angle. For example, in the fourth measurement taken 45 minutes after the start of the measurement, the maximum value in the range of 42° to 47° was 2θ = 45.3104°.

[0443] On the other hand, the maximum value in the range of 42° or more and 47° or less in the first measurement was observed at 2θ = 45.5250°. This is within the range of 2θ = 45.47 ± 0.10° (45.37° or more and 45.57° or less) where a diffraction peak appears in the O3' type crystal structure as stated in embodiment 1. Therefore, in the case of sample 3, it was shown that the 104 diffraction peak of the O3' type crystal structure can be accurately measured within 13 minutes from the start of measurement.

[0444] 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 portion, 100b: Interior

Claims

A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide; In an XPS analysis of the positive electrode active material particles, an atomic ratio of magnesium to cobalt (Mg / Co) is 1.00 or more, and when an Mg1s peak is analyzed, 10% or more of a peak component derived from an O-Mg-F bond is present; The XPS analysis was carried out under the condition of a take-off angle of 15°. a lithium ion secondary battery, wherein, when the positive electrode active material particles are subjected to STEM-EDX ray analysis, the maximum concentration of magnesium in a surface layer portion of the positive electrode active material particles is 7 atomic % or more; 　 A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide; In an XPS analysis of the positive electrode active material particles, an atomic ratio of magnesium to cobalt (Mg / Co) is 1.00 or more, and when an Mg1s peak is analyzed, 10% or more of a peak component derived from an O-Mg-F bond is present; The XPS analysis was carried out under the condition of a take-off angle of 15°. when the positive electrode active material particles are subjected to STEM-EDX ray analysis, the maximum concentration of magnesium in a surface layer portion of the positive electrode active material particles is 7 atomic % or more; the positive electrode active material particles have a layered rock salt crystal structure belonging to the space group R-3m in a discharged state, The positive electrode active material particles were used as a positive electrode, lithium metal was used as a negative electrode, polypropylene was used as a separator, and an electrolyte solution containing 1 mol / L of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and 2 wt % of vinylene carbonate was used. After charging to a voltage of 4.60 V in an environment of 25° C., The positive electrode in the charged state was １ When analyzing by X-ray diffraction using X-rays, if the ranges in which peaks occur within 13 minutes from the start of measurement are different from those after 45 minutes or more have elapsed, when measuring within 13 minutes from the start of measurement, A peak occurs in the 2θ range of 19.13° or more and 19.37° or less, A peak occurs in the 2θ range of 45.37° or more and 45.57° or less. Lithium-ion secondary battery. 　 In claim 2, The X-ray diffraction analysis is carried out in an argon atmosphere, The lithium ion secondary battery, wherein the argon atmosphere contains 5 ppm or less of oxygen. 　 In any one of claims 1 to 3, a ratio of the number of atoms of magnesium to cobalt, Mg / Co, within the positive electrode active material particles is 0.005 or more and 0.015 or less, as determined by EPMA analysis of the positive electrode active material particles; 　 A method for producing positive electrode active material particles having magnesium, fluorine, and lithium cobalt oxide, comprising the steps of: A method for producing positive electrode active material particles, wherein the total heating time at 650°C or higher is 30 hours or more and 60 hours or less. 　 mixing lithium cobalt oxide, a magnesium source, a fluorine source, and a lithium source to form a mixture; and heating the mixture at 650° C. or higher and 950° C. or lower for 30 hours or higher and 60 hours or lower to produce positive electrode active material particles. 　 In claim 6, The heating temperature is 826°C or higher and 920°C or lower.

Images