Secondary battery

Abstract

Provided are a positive electrode active material and a secondary battery using same, wherein a decrease in discharge capacity during charge / discharge cycles is suppressed. Alternatively, provided is a highly safe secondary battery. The secondary battery has a positive electrode active material which has a lithium cobalt oxide containing nickel and magnesium, wherein the amount of nickel and magnesium detected in the surface layer of the positive electrode active material is greater than the amount of nickel and magnesium detected inside the positive electrode active material, and the distribution of nickel and the distribution of magnesium overlap in the surface layer of the positive electrode active material. It is preferable that the positive electrode active material further contains fluorine, and the amount of fluorine detected in the surface layer of the positive electrode active material is greater than the amount of fluorine detected inside the positive electrode active material.

Description

secondary battery

[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 to 3). 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 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] Furthermore, fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and other processes, and their physical properties have been studied (Non-Patent Document 13).

[0009] Various research and development efforts are also being made on the reliability and safety of lithium ion secondary batteries. For example, Patent Document 14 describes the thermal stability of positive electrode active materials and electrolyte solutions.

[0010] Japanese Patent Application Laid-Open No. 2019-179758 WO2020 / 026078 Pamphlet Japanese Patent Application Laid-Open No. 2020-140954

[0011] 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.17340−17348Motohashi,T.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−A1609G.G.Amatucci et.al.,“CoO▲2▼,The End Member of the Lix CoO▲2▼ Solid Solution”J.Electrochem.Soc.143(3)1114(1996).Belsky,A.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.Akimoto,J.;Gotoh,Y.;Oosawa,Y.“Synthesis and structure refinement of Li CoO2 single crystals”Journal of Solid State Chemistry(1998)141,p.298−302.F.Izumi and K.Momma,Solid State Phenom.,130,15−20(2007)K.Momma and F.Izumi,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. W. E. Counts, R. Roy, and E. F. Osborn, "Fluoride Model Systems: II, The Binary Systems CaF2-BeF2, MgF2-BeF2, and LiF-MgF2", Journal of the American Ceramic Society, 36[1] 12-17 (1953). Shinya Kitano et al., GSYuasa Technical Report, Vol. 2, No. 2, December 2015, pp. 18-24.

[0012] Lithium ion secondary batteries still have room for improvement in various aspects, such as discharge capacity, cycle characteristics, reliability, safety, and cost.

[0013] Therefore, there is a demand for positive electrode active materials that can improve issues such as discharge capacity, cycle characteristics, reliability, safety, and cost when used in secondary batteries.

[0014] An object of one embodiment of the present invention is to provide a positive electrode active material or 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 or 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 or composite oxide that exhibits a large discharge capacity.Another object is to provide a secondary battery that is safe or highly reliable.

[0015] Another object of one embodiment of the present invention is to provide a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof.

[0016] 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.

[0017] To solve the above problems, one embodiment of the present invention provides a lithium cobalt oxide having magnesium, nickel, and aluminum in a surface layer. In particular, nickel is preferably present on a surface (edge ​​surface, also referred to as a surface other than the (001) surface of the lithium cobalt oxide) where a lithium diffusion path is exposed. Furthermore, a structure in which a magnesium-containing region and a nickel-containing region are overlapped, linked, or connected on a surface where lithium can be inserted and removed, i.e., a surface other than the (001) surface, is preferred. This structure can suppress oxygen desorption from the positive electrode active material or suppress structural changes in the positive electrode active material. In other words, providing a shell on a surface other than the (001) surface can sometimes suppress oxygen desorption from surfaces other than the (001) surface. The (001) surface and the (003) surface may also be collectively referred to as the (001) surface. The (001) surface may also be referred to as the C-plane, basal plane, or the like. In addition, lithium has a two-dimensional diffusion path in lithium cobalt oxide. In other words, the lithium diffusion path can be said to exist along the surface. In this specification, a surface where the lithium diffusion path is exposed, that is, a surface where lithium is inserted and extracted, that is, a surface other than the (001) surface, may be referred to as an edge surface.

[0018] The surface layer portion refers to a region extending from the surface to a certain depth inside. In the lithium cobalt oxide according to one embodiment of the present invention, nickel is preferably present particularly in the surface portion, where the surface is an edge surface.

[0019] In lithium cobalt oxide, lithium has a two-dimensional diffusion path, which can also be expressed as the lithium diffusion path along the surface.

[0020] The lithium diffusion path is exposed at the edge surface. In other words, the edge surface is a surface that is not parallel to the surface along which the lithium diffusion path runs, and that intersects with the surface along which the lithium diffusion path runs.

[0021] The edge plane can be, for example, a plane other than the (001) plane of lithium cobalt oxide. In the lithium cobalt oxide according to one embodiment of the present invention, nickel is preferably present in the surface layer, particularly in a portion where the surface is a plane other than the (001) plane.

[0022] In addition to the above, in one embodiment of the present invention, it is preferable that the surface layer portion has fluorine.

[0023] One aspect of the present invention is a lithium ion secondary battery having a positive electrode, the positive electrode having a positive electrode active material, the positive electrode active material having lithium cobalt oxide containing nickel and magnesium, the amount of nickel detected in a surface layer portion of the positive electrode active material is greater than the amount of nickel detected inside the positive electrode active material, the amount of magnesium detected in the surface layer portion of the positive electrode active material is greater than the amount of magnesium detected inside the positive electrode active material, and the nickel distribution and the magnesium distribution overlap in the surface layer portion of the positive electrode active material.

[0024] In the above, nickel is preferably detected on a surface other than the (001) plane of the lithium cobalt oxide in the surface layer portion of the positive electrode active material.

[0025] In the above, in EDX analysis, the difference between the peak depth of the detected amount of nickel and the peak depth of the detected amount of magnesium in the surface layer portion of the positive electrode active material is preferably within 3 nm.

[0026] In the above, the positive electrode active material contains aluminum, and in an EDX-ray analysis profile of nickel, magnesium, and aluminum contained in the positive electrode active material, the maximum detected amount of aluminum is located inside the maximum detected amount of nickel and the maximum detected amount of magnesium, and when the peak width at a height ⅕ of the maximum detected amount of aluminum is divided in half by a perpendicular line drawn from the maximum to the horizontal axis, the peak width W ｓ The inner peak width W ｃ is preferably large.

[0027] In the above-described battery having lithium in the positive electrode and counter electrode, when the battery is charged to 4.6 V and the positive electrode active material is analyzed by powder X-ray diffraction using CuKα1 radiation, the diffraction pattern preferably has peaks at 2θ of at least 19.13 and less than 19.37° and at 2θ of at least 45.37° and less than 45.57°.

[0028] In the above, the positive electrode active material preferably contains titanium, and the amount of titanium detected in the surface layer of the positive electrode active material is greater than the amount of titanium detected inside the positive electrode active material.

[0029] In the above, the positive electrode active material preferably contains fluorine, and the amount of fluorine detected in the surface layer of the positive electrode active material is greater than the amount of fluorine detected inside the positive electrode active material.

[0030] According to one embodiment of the present invention, a positive electrode active material or composite oxide that can be used in a lithium ion secondary battery and exhibits a suppressed decrease in discharge capacity during charge-discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide that does not easily lose its crystal structure even after repeated charge-discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide that exhibits a large discharge capacity can be provided. Alternatively, a secondary battery with high safety or reliability can be provided.

[0031] According to one embodiment of the present invention, a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0032] 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.

[0033] FIGS. 1A and 1B are cross-sectional views of a positive electrode active material. FIGS. 2A to 2C are examples of the distribution of additive elements contained in a positive electrode active material. FIG. 3A is an example of the distribution of additive elements contained in a positive electrode active material. FIG. 3B is a diagram illustrating the distribution of additive elements. FIG. 4 is a phase diagram showing the relationship between the composition of lithium fluoride and magnesium fluoride and temperature. FIG. 5 is a diagram illustrating the results of DSC analysis. FIG. 6 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 7A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 7B is an FFT pattern of the region of the rock salt crystal RS, and FIG. 7C is an FFT pattern of the region of the layered rock salt crystal LRS. FIG. 8 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 9 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIG. 10 is a diagram illustrating the depth of charge and lattice constant of a positive electrode active material. FIG. 11 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 12 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIGS. 13A and 13B are diagrams showing XRD patterns calculated from the crystal structure. FIGS. 14A to 14C are lattice constants calculated from XRD. FIGS. 15A to 15C are lattice constants calculated from XRD. FIGS. 16A and 16B are cross-sectional views of a positive electrode active material. FIGS. 17A to 17C are diagrams explaining a method for producing a positive electrode active material. FIG. 18 is a diagram explaining a method for producing a positive electrode active material. FIGS. 19A to 19C are diagrams explaining a method for producing a positive electrode active material. FIG. 20 is a diagram showing the appearance of a secondary battery. FIGS. 21A to 21C are diagrams explaining a method for producing a secondary battery. FIGS. 22A to 22H are diagrams explaining an example of an electronic device. FIGS. 23A to 23D are diagrams explaining an example of an electronic device. FIGS. 24A to 24C are diagrams explaining an example of an electronic device. FIGS. 25A to 25C are diagrams explaining an example of a vehicle. FIG. 26 is a graph showing the temperature rise of a secondary battery. Figures 27A and 27B are diagrams illustrating a nail penetration test. Figure 28 is a graph showing the temperature rise of a secondary battery when an internal short circuit occurs. Figures 29A and 29B are HAADF-STEM images of the positive electrode active material. Figures 30A and 30B are micro-electron diffraction patterns. Figures 31A and 31B are micro-electron diffraction patterns.FIGS. 32A and 32B are electron microbeam diffraction patterns. FIG. 33A is an HAADF-STEM image of the positive electrode active material, FIG. 33B is a cobalt mapping image, FIG. 33C is an oxygen mapping image, FIG. 33D is a magnesium mapping image, FIG. 33E is an aluminum mapping image, and FIG. 33F is a silicon mapping image. FIG. 34A is a diagram showing the scanning method for STEM-EDX ray analysis, and FIG. 34B is a STEM-EDX ray analysis profile. FIG. 35 is an enlarged view of a portion of FIG. 34B. FIG. 36 is an excerpt from FIG. 35. FIG. 37 is an excerpt from FIG. 35. FIGS. 38A and 38B are HAADF-STEM images of the positive electrode active material. FIGS. 39A and 39B are electron microbeam diffraction patterns. FIGS. 40A and 40B are electron microbeam diffraction patterns. FIGS. 41A and 41B are electron microbeam diffraction patterns. FIG. 42A is a HAADF-STEM image of the positive electrode active material, FIG. 42B is a silicon mapping image, FIG. 42C is a cobalt mapping image, FIG. 42D is a magnesium mapping image, FIG. 42E is an aluminum mapping image, and FIG. 42F is a nickel mapping image. FIG. 43A is a diagram showing the scanning method for STEM-EDX-ray analysis, and FIG. 43B is a STEM-EDX-ray analysis profile. FIG. 44 is an enlarged view of a portion of FIG. 43B. FIG. 45 is an excerpt from FIG. 44. FIG. 46 is an excerpt from FIG. 44. FIG. 47 is an excerpt from FIG. 44. FIGS. 48A and 48B are HAADF-STEM images. FIG. 49 is an XRD pattern of the positive electrode active material after charging. FIGS. 50A and 50B are enlarged XRD patterns of a portion of FIG. 49. FIG. 51 is an XRD pattern of the positive electrode active material after charging. Figures 52A and 52B are enlarged XRD patterns of a portion of Figure 51. Figures 53A and 53B are diagrams illustrating a nail penetration test device. Figures 54A to 54C are diagrams showing the results of a nail penetration test. Figures 55A to 55C are diagrams showing the results of a nail penetration test. Figure 56 is a diagram showing the results of a DSC test.

[0034] 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.

[0035] 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 {}. Trigonal crystals expressed in the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice to facilitate understanding of the structure. Miller indices may also be expressed using (hkil) instead of (hkl). Here, i is -(h+k). In this specification and the like, for the space group R-3m, unless otherwise specified, crystal planes and the like are expressed as a composite hexagonal lattice.

[0036] 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.

[0037] The theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. For example, LiCoO ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 274mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0038] The amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ CoO ２In the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ In the above formula, "x" is small, for example, when 0.1<x≦0.24. The extent to which lithium has been released from the positive electrode active material relative to the theoretical capacity is sometimes referred to as the depth of charge. In this specification and the like, the depth of charge = 1 - x.

[0039] 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 secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The completion of discharge here refers to a state where the voltage is 3.0 V or 2.5 V or less at a current of 100 mA / g or less.

[0040] Li ｘ CoO ２ It 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 on a 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.

[0041] The space group of a crystal structure 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."

[0042] Furthermore, if the anion arrangement is roughly close to cubic close packing, it can be considered cubic close packing. Cubic close packing anion arrangement refers to a state in which the second layer of anions is arranged above the voids of the first layer of anions, and the third layer of anions is arranged directly above the voids of the second layer of anions, but not directly above the first layer of anions. Therefore, the anions do not necessarily have to be arranged in a cubic lattice. Furthermore, since real crystals always have defects, analytical results do not necessarily have to be theoretical. For example, in FFT (fast Fourier transform) patterns such as electron diffraction patterns or TEM images, spots may appear at positions slightly different from the theoretical positions. For example, a cubic close packing structure can be said to be present if the orientation from the theoretical positions is 5 degrees or less, or 2.5 degrees or less.

[0043] The distribution of a certain element refers to a region in which the element is continuously detected within a range that is not a noise by a certain continuous analytical method. A region in which the element is continuously detected within a range that is not a noise can also be referred to as a region in which the element is always detected when the analysis is performed multiple times.

[0044] A cathode active material to which an additive element is added may be referred to as a composite oxide, a cathode material, a cathode ingredient, a cathode material for a secondary battery, or the like. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a compound. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composition. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composite.

[0045] 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 70% or more, and more preferably 90% or more of three or more randomly selected particles of the positive electrode active material have that characteristic, it can be said that the effect of sufficiently improving the characteristics of the positive electrode active material and a secondary battery containing it is achieved.

[0046] As the charging voltage of a secondary battery increases, the voltage of the positive electrode generally increases. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge.

[0047] Furthermore, a short circuit in a secondary battery not only causes problems in the charging and / or discharging operations of the secondary battery, but may also lead to heat generation and fire. To achieve a safe secondary battery, it is preferable that the short circuit current be suppressed even at a high charging voltage. The positive electrode active material of one embodiment of the present invention suppresses the short circuit current even at a high charging voltage. Therefore, a secondary battery that achieves both high discharge capacity and safety can be obtained.

[0048] In this specification, ignition in a nail penetration test refers to the observation of a flame outside the exterior body within one minute of the nail being inserted. Alternatively, it refers to the occurrence of thermal runaway in a secondary battery. For example, if the temperature of a secondary battery rises above 130°C, thermal runaway can be said to have occurred. The temperature at this time can be measured using a temperature sensor attached to the exterior body of the secondary battery. Furthermore, if solid thermal decomposition products derived from the positive electrode and / or negative electrode are observed at a distance of 2 cm or more from the insertion point after the nail penetration test, it can also be said that ignition has occurred.

[0049] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in secondary batteries are described in their pre-degradation state. Note that 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 lithium-ion secondary cell or lithium-ion secondary battery pack (hereinafter referred to as a lithium-ion secondary battery) 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 lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0050] 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 in use state, or a used product or used state.

[0051] Embodiment 1 In this embodiment, a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS.

[0052] 1A and 1B are cross-sectional views of a cathode active material 100 according to one embodiment of the present invention. As shown in FIG. 1A, the cathode active material 100 has a surface layer 100a and an interior 100b. In these figures, the boundary between the surface layer 100a and the interior 100b is indicated by a dashed line. In addition, in FIG. 1B, a portion of a grain boundary 105 is indicated by a dashed-dotted line. Also, FIG. 1B shows the cathode active material 100 having an embedded portion 102. In the figure, (001) indicates the (001) plane of lithium cobalt oxide. LiCoO ２ belongs to the space group R-3m.

[0053] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region extending from the surface toward the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface toward the interior. Note that "approximately perpendicular" refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0054] The region of the positive electrode active material deeper than the surface layer 100a is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0055] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the inner portion 100b. ２ O ３This does not include metal oxides having no lithium sites that can contribute to charging and discharging, such as those having metal oxides attached thereto, carbonates chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. Note that the attached metal oxides refer to metal oxides whose crystal structure does not match that of the interior 100b, for example.

[0056] It also does not include the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 100.

[0057] The crystal grain boundary 105 refers to, for example, a portion where particles of the positive electrode active material 100 are adhered to each other, a portion where the crystal orientation changes inside the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. The crystal defect refers to a defect that can be observed in a cross-sectional TEM (Transmission Electron Microscope), a cross-sectional STEM (Scanning Transmission Electron Microscope) image, etc., that is, a deviation in the crystal structure, a structure in which other atoms have entered between the lattices, a cavity, etc. The crystal grain boundary 105 can be said to be one of the planar defects. The vicinity of the crystal grain boundary 105 refers to a region within 10 nm from the crystal grain boundary 105.

[0058] <Containing Elements> The positive electrode active material 100 contains lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 contains lithium cobalt oxide (LiCoO ２ However, the positive electrode active material 100 of one embodiment of the present invention may have any of the crystal structures described below. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0059] The positive electrode active material of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction. This is to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 100 of one embodiment of the present invention preferably uses cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or two selected from nickel and manganese may also be used. When the transition metals contained in the positive electrode active material 100 are 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, cobalt is relatively easy to synthesize and handle, making it a preferred secondary battery. Furthermore, secondary batteries using this positive electrode active material have many advantages, such as excellent cycle characteristics.

[0060] Furthermore, when the cobalt content of the transition metals in the positive electrode active material 100 is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal, ｘ CoO ２ The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) accounts for the majority of the transition metal, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel accounts for the majority of the transition metal, are prone to cation mixing between nickel and lithium.

[0061] The additive element contained in the positive electrode active material 100 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. 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 %.

[0062] That is, the positive electrode active material 100 may include lithium cobalt oxide to which magnesium and fluorine are added, lithium cobalt oxide to which magnesium, fluorine and titanium are added, lithium cobalt oxide to which magnesium, fluorine and aluminum are added, lithium cobalt oxide to which magnesium, fluorine and nickel are added, lithium cobalt oxide to which magnesium, fluorine, nickel and aluminum are added, and the like.

[0063] The additive element is preferably dissolved in the positive electrode active material 100. Therefore, for example, when a line analysis is performed using STEM-energy dispersive X-ray spectroscopy (EDX), the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the transition metal M detected increases, i.e., closer to the interior of the positive electrode active material 100.

[0064] In this specification and the like, the depth at which the amount of a certain element detected increases in STEM-EDX line analysis refers to the depth at which measurement values ​​that can be determined not to be noise in terms of intensity, spatial resolution, etc. are continuously obtained.

[0065] As will be described later, these additive elements further stabilize the crystal structure of the positive electrode active material 100. In this specification and the like, the additive element has the same meaning as a mixture or a part of a raw material.

[0066] The additive element does not necessarily have to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0067] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, are further enhanced. The weight of manganese contained in the cathode active material 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0068] <Crystal structure> <Li ｘ CoO ２ When x is 1 in the positive electrode active material 100 of one embodiment of the present invention is in a discharged state, that is, Li ｘ CoO ２ When x = 1 in the formula, it is preferable that 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 most of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure. Figure 8 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).

[0069] On the other hand, the surface layer portion 100a of the cathode active material 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 cobalt and oxygen octahedra is not destroyed even when lithium is removed from the cathode active material 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and inner portion 100b of the cathode active material 100, such as oxygen desorption and / or shifting of the layered structure of cobalt and oxygen octahedra. And / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0070] 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 (e.g., 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 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 characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0071] 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, the atoms on the surface of the particles of the positive electrode active material 100 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. For example, if the crystal structure of the layered structure consisting of octahedra of cobalt and oxygen in the surface layer 100a is displaced, this influence is 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 100. On the other hand, if the surface layer 100a can be sufficiently stabilized, Li ｘ CoO ２Even when x is small, for example, 0.24 or less, the layered structure of cobalt and oxygen octahedra in the interior 100b can be made less likely to break.Furthermore, displacement of the layers of cobalt and oxygen octahedra in the interior 100b can be suppressed.

[0072] [Distribution] To provide the surface layer 100a with a stable composition and crystalline 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 100 preferably have a concentration gradient. Furthermore, the distribution of the positive electrode active material 100 preferably differs depending on the additive element. For example, it is more preferable that the depth of the peak of the detected amount in the surface layer from the surface or the reference point in EDX-ray analysis described below differs depending on the additive element. Here, the detected amount peak refers to the maximum value of the detected amount in the surface layer 100a or within 50 nm from the surface. The detected amount refers to, for example, the count in EDX-ray analysis.

[0073] 1A shows an example of the depth direction of a crystal plane other than the (001) plane of lithium cobalt oxide in positive electrode active material 100 of one embodiment of the present invention, indicated by arrows X1-X2. Examples of profiles of each added element obtained by EDX analysis along arrows X1-X2 are shown in FIGS. 2A to 2C .

[0074] As shown in Figures 2A to 2C, it is preferable that the detected amount of at least magnesium and nickel among the added elements is greater in the surface layer portion 100a than in the interior portion 100b. Furthermore, it is preferable that the detected amount peak is narrower in the surface layer portion 100a in a region closer to the surface. For example, it is preferable that the detected amount peak is on the surface or within 3 nm from the reference point. It is also preferable that the distributions of magnesium and nickel overlap. The detected amount peaks of magnesium and nickel may be at the same depth, or the magnesium peak may be closer to the surface, or the nickel peak may be closer to the surface as shown in Figure 2B. The difference in depth between the detected amount peak of nickel and the detected amount peak of magnesium is preferably within 3 nm, and more preferably within 1 nm.

[0075] Furthermore, the amount of nickel detected in the inner portion 100b is very small compared to the surface portion 100a, or may not be detected, or may be 1 atomic % or less.

[0076] Although not shown, it is preferable that the detectable amount of fluorine in the surface layer portion 100a is greater than the detectable amount inside, similar to magnesium or nickel. It is also preferable that the detectable amount peak in the surface layer portion 100a is closer to the surface. For example, it is preferable that the detectable amount peak is on the surface or within 3 nm from the reference point. Similarly, it is preferable that the detectable amount of titanium, silicon, phosphorus, boron, and / or calcium is greater than the detectable amount inside the surface layer portion 100a. It is also preferable that the detectable amount peak is on the surface or within 3 nm from the reference point.

[0077] Furthermore, it is preferable that at least aluminum, among the added elements, has a peak of detected amount further inward than magnesium. The distributions of magnesium and aluminum may overlap as shown in Figure 2A, or there may be little overlap between the distributions of magnesium and aluminum as shown in Figure 2C. The peak of detected amount of aluminum may be present in the surface layer 100a, or may be deeper than the surface layer 100a. For example, it is preferable that the peak be present on the surface, or in a region of 5 nm to 30 nm from the reference point toward the interior.

[0078] The distribution of aluminum may not be a normal distribution. For example, if the distribution of aluminum is expressed as a maximum value Max Ａｌ When the sample is divided by the maximum value of the detected amount of aluminum (Max Ａｌ ) 1 / 5 height (1 / 5 Max Ａｌ ) is divided in half by a perpendicular line drawn from the maximum value to the horizontal axis, the peak width W ｓ The inner peak width W ｃ may be large.

[0079] The reason why aluminum is more distributed to the interior than magnesium is thought to be because aluminum diffuses faster than magnesium. On the other hand, the amount of aluminum detected in the region closest to the surface is low, presumably because aluminum exists more stably in regions where magnesium and other elements are not present in solid solution at high concentrations than in regions where they are not.

[0080] More specifically, in the region of the layered rock salt type of space group R-3m or the cubic rock salt type where magnesium is dissolved at a high concentration, layered rock salt type LiAlO ２ Compared to the case of cobalt, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋ From Co ２＋However, since Al can only be trivalent, it is thought that it is difficult for it to coexist with magnesium in a rock salt or layered rock salt structure.

[0081] Although not shown, it is preferable that manganese has a peak of detectable amount inside magnesium, similar to aluminum.

[0082] However, the additive element does not necessarily have to have the same concentration gradient or distribution throughout the entire surface layer 100a of the positive electrode active material 100. Arrows Y1-Y2 are shown in Figure 1 as an example of the depth direction of the (001) plane of lithium cobalt oxide in the positive electrode active material 100. An example of the profile of the additive element along arrows Y1-Y2 is shown in Figure 3A.

[0083] The (001)-oriented surface may have a different distribution of additive elements than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a lower detectable amount of one or more additive elements compared to surfaces other than the (001)-oriented surface. Specifically, the detectable amount of nickel may be low. Alternatively, the (001)-oriented surface and its surface layer 100a may have no detectable amount of one or more additive elements selected from the additive elements, or the detectable amount may be 1 atomic % or less. Specifically, nickel may be no detectable amount of nickel, or 1 atomic % or less. In particular, with analytical methods that detect characteristic X-rays, such as EDX, the Kβ of cobalt and the Kα of nickel are close in energy, making it difficult to detect trace amounts of nickel in materials where cobalt is the primary element. Alternatively, the (001)-oriented surface and its surface layer 100a may have a peak of detectable amount of one or more additive elements shallower from the surface than surfaces other than the (001)-oriented surface. Specifically, the peaks of detectable amount of magnesium and aluminum may be shallower than those of other surfaces.

[0084] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and the diffusion path of lithium ions is also parallel to the (001) plane.

[0085] CoO ２ Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0086] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface and the surface layer 100a other than the (001) orientation are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and the surface layer 100a other than the (001) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0087] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, it is important that the profile of the additive element in the surface other than the (001) orientation and in the surface layer 100a thereof has a distribution such as that shown in any one of Figures 2A to 2C. Among the additive elements, nickel is particularly preferably detected in the surface other than the (001) orientation and in the surface layer 100a thereof. On the other hand, the concentration of the additive element in the (001)-oriented surface and in the surface layer 100a thereof may be low or absent, as described above.

[0088] For example, the magnesium distribution in the (001)-oriented surface and its surface layer 100a preferably has a half-width of 10 nm to 200 nm, more preferably 50 nm to 150 nm, and even more preferably 80 nm to 120 nm. Also, the magnesium distribution in the non-(001)-oriented surface and its surface layer 100a preferably has a half-width of more than 200 nm to 500 nm, more preferably more than 200 nm to 300 nm, and even more preferably 230 nm to 270 nm.

[0089] Furthermore, the nickel distribution in the surface that is not (001) oriented and in the surface layer 100a thereof preferably has a half-width of 30 nm or more and 150 nm or less, more preferably 50 nm or more and 130 nm or less, and even more preferably 70 nm or more and 110 nm or less.

[0090] High purity LiCoO, which will be described in a later embodiment ２ In the manufacturing method of mixing the additive element after manufacturing the silicon nitride film and then heating the mixture, the additive element spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additive element in the surface other than the (001) orientation and in the surface layer 100a thereof can be easily controlled to a preferred range.

[0091] [Magnesium] Magnesium is 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 of the surface layer 100a has the effect of suppressing contraction of the c-axis length even when a force that expands and contracts in the c-axis direction due to the insertion and desorption of lithium ions acts. It also 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 It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２ When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. Furthermore, a high magnesium concentration in the surface layer portion 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0092] At an appropriate concentration, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying both the lithium and cobalt sites. Furthermore, unnecessary magnesium compounds (e.g., oxides and fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become resistance components in secondary batteries. Furthermore, as the magnesium concentration in the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0093] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.002 to 0.06 times the number of cobalt atoms, more preferably 0.005 to 0.03 times, and even more preferably about 0.01 times. The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, 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 100.

[0094] [Nickel] Nickel is LiMeO ２ In the layered rock salt crystal structure of the present invention, nickel can exist on either the cobalt site or the lithium site. When present on the cobalt site, its redox potential is lower than that of cobalt, making it easier to release lithium and electrons during charging. This can be expected to result in faster charge and discharge speeds. Therefore, even at the same charge voltage, a greater charge and discharge capacity can be obtained when the transition metal M is nickel than when it is cobalt.

[0095] 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 caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site can also be prevented from shifting to CoO ２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0096] In addition, the distance between the cations and anions of nickel oxide (NiO) is closer to that of LiCoO than that of rock salt MgO and rock salt CoO. ２ The average distance between the cations and anions is close to that of LiCoO ２ The orientation is likely to match.

[0097] In addition, the order of ionization tendency is greatest for magnesium, aluminum, cobalt, and nickel. Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements mentioned above during charging. Therefore, nickel is thought to be highly effective in stabilizing the crystalline structure of the surface layer when in a charged state.

[0098] 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. Therefore, nickel is thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0099] 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.

[0100] Therefore, it is preferable that the entire positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 100 is preferably more than 0% but not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, more than 0% but not more than 4% is preferable. Alternatively, more than 0% but not more than 2% is preferable. Alternatively, 0.05% to 7.5% is preferable. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 7.5% is preferable. Alternatively, 0.1% to 4% is preferable. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 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.

[0101] [Aluminum] Aluminum can also be present at the cobalt site in a layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and the lithium around it function as pillars, which can suppress changes in the crystal structure. As a result, as will be described later, the positive electrode active material 100 has the effect of maintaining the c-axis length even when a force that expands and contracts in the c-axis direction due to the insertion and desorption of lithium ions is applied. This can suppress deterioration of the positive electrode active material 100.

[0102] 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 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0103] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0104] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 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 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 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 100.

[0105] [Fluorine] Fluorine is a monovalent anion, and when a portion of the oxygen in the surface layer portion 100a is replaced by fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. That is, in the absence of fluorine, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two. Therefore, when a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 100 is replaced by fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material 100 and the electrolyte. Furthermore, corrosion resistance to hydrofluoric acid can be effectively improved.

[0106] Furthermore, when the melting point of a fluoride such as lithium fluoride is lower than the melting point of the other additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source. ２ When the crystalline structure has a liquid phase, as shown in FIG. 4 (cited and revised from FIG. 5 of Non-Patent Document 13, "Liquid" in the figure indicates the liquid phase), LiF and MgF ２Since the eutectic point P of the alloy is around 742°C (T1), it is preferable to set the heating temperature to 742°C or higher in the heating step after mixing the additive elements.

[0107] Here, differential scanning calorimetry (DSC) measurements of fluorides and mixtures will be explained using Fig. 5. The vertical axis of Fig. 5 represents heat flow, and the horizontal axis represents temperature. The fluorides in Fig. 5 are LiF and MgF ２ It is a mixture of LiF:MgF ２ The mixture in FIG. 5 contains lithium cobalt oxide as the lithium oxide, and LiF and MgF as the fluorides. ２ It is a mixture of LiCoO ２ :LiF:MgF ２ The components were mixed so that the molar ratio was 100:0.33:1.

[0108] As shown in Fig. 5, an endothermic peak is observed in the vicinity of 735°C for the fluoride. Also, an endothermic peak is observed in the vicinity of 830°C for the mixture. Therefore, the heating temperature after mixing the additive element is preferably 742°C or higher, more preferably 830°C or higher. Alternatively, it may be 800°C (T2 in Fig. 4) or higher, which is between these temperatures.

[0109] Although the flux effect of fluoride is extremely effective in the heating process, the effect of fluoride in the secondary battery is limited. Therefore, it is not a problem if at least a portion of the fluoride has evaporated at the end of the heating process. In other words, the amount of fluorine remaining in the completed positive electrode active material 100 may be very small, or even undetectable.

[0110] [Other Additive Elements] Titanium oxide is known to have superhydrophilic properties. Therefore, by providing the cathode active material 100 with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 has good wettability with highly polar solvents. When used in a secondary battery, this improves the contact at the interface between the cathode active material 100 and a highly polar electrolyte, which may suppress an increase in internal resistance.

[0111] Furthermore, when phosphorus is present in the surface layer 100a, Li ｘCoO ２ When the value of x in the graph is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that the graphite oxide is present in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0112] When the positive electrode active material 100 contains phosphorus, the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0113] The electrolyte is LiPF ６ In the case where the electrolyte contains PVDF, there is a risk of hydrogen fluoride being generated by hydrolysis. Furthermore, hydrogen fluoride may also be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 may be suppressed. Furthermore, there is a risk of suppressing a decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0114] When the positive electrode active material 100 contains phosphorus together with magnesium, Li ｘ CoO ２This is preferable because stability is extremely high when x is small in the positive electrode active material 100. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, 1% to 10% is preferable. Alternatively, 1% to 8% is preferable. Alternatively, 2% to 20% is preferable. Alternatively, 2% to 8% is preferable. Alternatively, 3% to 20% is preferable. Alternatively, 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Alternatively, 0.1% to 5% is preferable. Alternatively, 0.1% to 4% is preferable. Alternatively, 0.5% to 10% is preferable. Alternatively, 0.5% to 4% is preferable. Alternatively, 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 100.

[0115] Furthermore, when the positive electrode active material 100 has a crack, the progression of the crack can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the crack on the surface, for example, in the embedded portion 102.

[0116] [Synergistic Effect of Multiple Added Elements] Furthermore, when the surface layer portion 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0117] For the same reason, when adding an additive element to lithium cobalt oxide in the manufacturing process, it is preferable to add magnesium in a step before adding nickel. Alternatively, it is preferable to add magnesium and nickel in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of the step in which it is added, whereas nickel can diffuse widely into the interior of lithium cobalt oxide in the absence of magnesium. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of the lithium cobalt oxide and not remain in the surface layer in the desired amount.

[0118] Furthermore, having additive elements with different distributions in combination is preferable because it can stabilize the crystal structure in a wider region. For example, if the positive electrode active material 100 has both magnesium and nickel distributed in a region closer to the surface of the surface layer 100a and aluminum distributed in a deeper region, it can stabilize the crystal structure in a wider region than if it had only one of these elements. In this way, when the positive electrode active material 100 has additive elements with different distributions in combination, aluminum is not essential to the surface because surface stabilization can be sufficiently achieved by magnesium, nickel, etc. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable for aluminum to be continuously detected in a region from 1 nm to 25 nm in the depth direction from the surface. A wide distribution in a region from 0 nm to 100 nm from the surface, preferably a region from 0.5 nm to 50 nm from the surface, is preferable because it can stabilize the crystal structure in a wider region.

[0119] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100 a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0120] However, if the surface layer 100a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO, a structure in which MgO and NiO(II) are solid-solved, and / or a structure in which MgO and CoO(II) are solid-solved. Therefore, the surface layer 100a must contain at least cobalt, and in the discharged state, it must also contain lithium, and it must have a path for the insertion and extraction of lithium.

[0121] In order to ensure sufficient paths for lithium insertion and desorption, the surface layer portion 100a preferably has a higher cobalt concentration than magnesium. For example, when measured from the surface of the positive electrode active material 100 by XPS, the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is preferably 0.62 or less. The surface layer portion 100a preferably has a higher cobalt concentration than nickel. The surface layer portion 100a preferably has a higher cobalt concentration than aluminum. The surface layer portion 100a preferably has a higher cobalt concentration than fluorine.

[0122] Furthermore, since an excessive amount of nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 100 a. For example, when measured from the surface of the positive electrode active material 100 by XPS, the number of nickel atoms is preferably 1 / 6 or less of the number of magnesium atoms.

[0123] Furthermore, while it is preferable that some of the added elements, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it can suppress the shift in the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0124] It is preferable that the crystal structure continuously changes from the interior 100b toward the surface due to the concentration gradient of the added element as described above, or that the crystal orientation of the surface layer 100a and the interior 100b roughly coincide.

[0125] 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 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 approximately the same.

[0126] 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.

[0127] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0128] The fact that it has 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, and the like.

[0129] 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. ２ In 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.

[0130] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the 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 observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0131] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3'-type and monoclinic O1(15) crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is aligned.

[0132] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0133] 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.

[0134] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, FFT patterns of TEM images, STEM images, etc. XRD (X-ray diffraction), electron beam diffraction, neutron beam diffraction, etc. can also be used as materials for the judgment.

[0135] 6 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and the rock salt crystals RS are roughly the same. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0136] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in Figure 6) ＲＳ and L ＬＲＳ When the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0137] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the cobalt atoms, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate with a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0138] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0139] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0140] Figure 7A shows an example of an STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS are roughly the same. Figure 7B shows the FFT pattern of the region of the rock salt crystal RS, and Figure 7C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of Figures 7B and 7C shows the composition, JCPDS card number, and the d value, angle, and incidence calculated from these. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0141] The spot marked A in Figure 7B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 7C is derived from the 0003 reflection of the layered rock salt type. From Figures 7B and 7C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 7B is roughly parallel to the line passing through AO in Figure 7C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0142] In this way, in the FFT pattern and the electron beam diffraction pattern, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0143] Furthermore, as described above, when the orientation of the cubic 11-1 reflection and the orientation of the layered rock salt 0003 reflection are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the layered rock salt 0003 reflection may be observed in a reciprocal lattice space different from the orientation of the layered rock salt 0003 reflection. For example, the spot marked B in FIG. 7C is originating from the layered rock salt 1014 reflection. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point originating from the layered rock salt 0003 reflection (A in FIG. 7C ) and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be identical. For example, reciprocal lattice points equivalent to 0003 and 1014 may also be used.

[0144] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 7B originates from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 7B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to match this. For example, a reciprocal lattice point equivalent to 11-1 and 200 may also be used.

[0145] It is known that layered rock-salt type positive electrode active materials, including lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material using an SEM or the like, it is possible to thin-section the observation sample using an FIB or the like so that the electron beam is [12-10] incident in a TEM or the like, making the (0003) plane easier to observe. When determining whether the crystal orientation is consistent, it is preferable to thin-section the layered rock-salt type so that the (0003) plane can be easily observed.

[0146] <Li ｘ CoO ２ The positive electrode active material 100 of one embodiment of the present invention has the above-described distribution of the additive element and / or the crystal structure in a discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0147] 8 to 13, Li ｘ CoO ２ The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0148] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 9. The conventional positive electrode active material shown in FIG. 9 is a lithium cobalt oxide (LiCoO ２ In particular, changes in the crystal structure of lithium cobalt oxide that does not contain any added elements are described in Non-Patent Documents 1 to 4, etc.

[0149] In Figure 9, R-3m O3 is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0150] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0151] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a complex hexagonal lattice, it is sometimes called hexagonal O1 type.

[0152] 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 the structure of and the structure of are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 crystal structure. Note that actual lithium insertion / extraction does not necessarily occur uniformly within the positive electrode active material, and the lithium concentration may be uneven, so experimentally, an H1-3 crystal structure is observed from about x = 0.25. In fact, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 9 and other parts of this specification, to facilitate comparison with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0153] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in a 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 represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0154] 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.

[0155] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 9, in the H1-3 type crystal structure, CoO ２ The 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.

[0156] Furthermore, the difference in volume between these two crystal structures is large. Lithium cobalt oxide exhibits a significant change in the depth of charge, i.e., Li ｘ CoO ２ The crystal structure and unit cell volume change depending on the change in x in the matrix. Figure 10 shows the change in the c-axis length of conventional lithium cobalt oxide described in Non-Patent Document 4. The round markers represent hexagonal phases, and the diamond markers represent monoclinic phases. In the H1-3 phase, the c-axis length shrinks, as shown by the diamond markers in Figure 10. The phase transition from the O3 to the H1-3 phase is accompanied by the desorption of lithium ions, so it is thought that the phase transition occurs from the surface of the positive electrode active material, which is the region where lithium ions are first desorbed, but it can eventually spread to the entire positive electrode active material.

[0157] The change in the c-axis length of lithium cobalt oxide corresponds to the change in the angle at which a peak of, for example, the (003) plane of lithium cobalt oxide appears in the XRD pattern. It is known that in XRD using CuKα1 radiation, the peak of the (003) plane of lithium cobalt oxide appears at 2θ of around 19° to 20°.

[0158] Therefore, when compared per the same number of cobalt atoms, the difference in volume between the H1-3 type crystal structure and the discharged R-3m O3 type crystal structure exceeds 3.5%, typically 3.9% or more.

[0159] 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.

[0160] 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 for lithium to be inserted and extracted.

[0161] On the other hand, in the positive electrode active material 100 of 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. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 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 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. ｘ CoO ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２When 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.

[0162] Li ｘ CoO ２ The crystal structure of the inner portion 100b of the positive electrode active material 100 when x is 1, approximately 0.2, and approximately 0.15 is shown in FIG. 8. The inner portion 100b occupies the majority of the volume of the positive electrode active material 100 and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0163] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0164] However, the positive electrode active material 100 has a different crystal structure from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.15, which results in an H1-3 type crystal structure.

[0165] The positive electrode active material 100 according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The symmetry of the layers 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 8 with the notation R-3m O3'.

[0166] 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. Furthermore, the lattice constant of the unit cell is preferably 2.797≦a≦2.837 (Å), more preferably 2.807≦a≦2.827 (Å), typically a=2.817 (Å). The c-axis is preferably 13.681≦c≦13.881 (Å), more preferably 13.751≦c≦13.811 (Å), typically c=13.781 (Å).

[0167] Furthermore, the positive electrode active material 100 of one embodiment of the present invention when x is about 0.15 has a crystal structure belonging to the monoclinic space group P2 / m.２ There is one layer. In addition, the lithium present in the positive electrode active material 100 at this time is about 15 atomic % in the discharged state. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. This crystal structure is shown in Figure 8 with the P2 / m monoclinic O1(15) attached.

[0168] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ The lattice constants of the unit cell are a = 4.880 ± 0.05 Å, b = 2.817 ± 0.05 Å, c = 4.839 ± 0.05 Å, α = 90°, β = 109.6 ± 0.1°, and γ = 90°.

[0169] This crystal structure can also show the lattice constant in the space group R-3m if some error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constants of the unit cell are a = 2.817 ± 0.02 Å and c = 13.68 ± 0.1 Å.

[0170] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy hexacoordinated oxygen sites, although lighter elements such as lithium and magnesium may occupy tetracoordinated oxygen sites.

[0171] As shown by the dotted line in FIG. 8, the CoO ２ There is almost no layer misalignment.

[0172] 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%.

[0173] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the monoclinic O1(15) type crystal structure is 3.3% or less, more specifically 3.0% or less, typically 2.5%.

[0174] Table 1 shows the difference in volume per cobalt atom between discharged R-3m O3, O3', monoclinic O1(15), H1-3, and trigonal O1. Regarding the lattice constants of each crystal structure used in the calculations in Table 1, literature values ​​can be referenced for discharged R-3m O3 and trigonal O1 (ICSDcoll.code.172909 and 88721). For H1-3, Non-Patent Document 3 can be referenced. For O3' and monoclinic O1(15), the lattice constants can be calculated from experimental XRD values.

[0175]

[0176] As described above, in the positive electrode active material 100 according to 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 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0177] The positive electrode active material 100 is Li ｘ CoO ２It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be present, and it is presumed that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ CoO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically when x is 0.15 or more and less than 0.17, the crystal structure may be monoclinic O1(15) type. However, the crystal structure is Li 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.

[0178] Therefore, the positive electrode active material 100 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material 100 may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or some may be amorphous.

[0179] Also Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO ２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when a conventional positive electrode active material is charged at a voltage of 4.6 V or more relative to the potential of lithium metal in a 25°C environment using CCCV (constant current constant voltage), an H1-3 crystal structure appears. Therefore, a charging voltage of 4.6 V or more relative to the potential of lithium metal can be said to be a high charging voltage. Unless otherwise specified, in this specification, charging voltages are expressed relative to the potential of lithium metal.

[0180] Therefore, the positive electrode active material 100 of one embodiment of the present invention can be said to be preferable because it can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it can be said to be preferable because it can adopt an O3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C. In other words, it can be said to be preferable because it can adopt an even higher monoclinic O1(15)-type crystal structure when charged at a higher charging voltage, for example, a voltage of more than 4.7 V and 4.8 V or lower at 25° C.

[0181] Even with the positive electrode active material 100, an H1-3 crystal structure may be finally observed when the charge voltage is further increased. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or more and less than 4.6 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt an O3′ crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.7 V or less at 25° C., the positive electrode active material 100 may be able to adopt a monoclinic O1(15) crystal structure.

[0182] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0183] In addition, in O3' and monoclinic O1(15) in FIG. 8, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, for example, in monoclinic O1(Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0184] The crystal structure of O3' and monoclinic O1(15) type has random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0185] <Grain Boundary> In addition to the above distribution, at least a part of the additional element contained in the positive electrode active material 100 of one embodiment of the present invention is preferably unevenly distributed in and around the grain boundary 105 .

[0186] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of areas with high concentration and areas with low concentration.

[0187] For example, it is preferable that the magnesium concentration at and near the grain boundaries 105 of the positive electrode active material 100 is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries 105 is higher than that in other regions of the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries 105 is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries 105 is higher than that in other regions of the interior 100b.

[0188] The grain boundaries 105 are one type of planar defect. Therefore, like the grain surfaces, they tend to become unstable and are prone to change in the crystal structure. Therefore, if the concentration of the added element at and near the grain boundaries 105 is high, the change in the crystal structure can be more effectively suppressed.

[0189] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries 105, even if cracks occur along the grain boundaries 105 in the positive electrode active material 100 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the cracks have occurred. Furthermore, a side reaction between the electrolyte and the positive electrode active material can be suppressed even in the positive electrode active material after the cracks have occurred.

[0190] <Single Particle> The positive electrode active material 100 preferably has high crystallinity, and more preferably is a single crystal. That is, the positive electrode active material 100 preferably has a single particle. When the positive electrode active material 100 according to one embodiment of the present invention is a single particle, cracks are less likely to occur even when a volume change occurs in the positive electrode active material 100 due to charge and discharge, which is preferable. Furthermore, when the positive electrode active material 100 is a single particle, a secondary battery using the positive electrode active material 100 is thought to be less likely to ignite, thereby improving safety.

[0191] <Crystallite Size> For example, the lower limit of the crystallite size of the positive electrode active material 100 calculated from the half-width of the XRD diffraction pattern is preferably 250 nm or more, and more preferably 420 nm or more. The crystallite size can be calculated, for example, by the following Scherrer formula.

[0192]

[0193] However, to enlarge the crystallites, excess lithium can be added and heated. However, excess lithium may cause gelation of the binder during electrode fabrication, such as the positive electrode. To avoid this disadvantage, it is advisable to set an upper limit on the crystallite size. For example, by setting the crystallite size calculated from the XRD diffraction pattern to 600 nm or less, preferably 500 nm or less, it is possible to avoid the above disadvantages. This upper limit value can be arbitrarily combined with the above-mentioned lower limit on the crystallite size.

[0194] The XRD diffraction pattern for calculating the half-width is preferably obtained from the positive electrode active material alone, but may also be obtained from the positive electrode including the positive electrode active material, a current collector, a binder, a conductive material, etc. However, in the positive electrode state, the positive electrode active material may be oriented due to the influence of pressure, etc., during the manufacturing process. If the orientation is strong, accurate calculation of the crystallites may not be possible. 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., in the positive electrode active material layer using a solvent, etc., and then filling the sample into a sample holder.

[0195] <Particle size> If the particle size of the positive electrode active material 100 of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector may occur. On the other hand, if the particle size is too small, problems such as excessive reaction with the electrolyte may occur. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0196] Furthermore, using a mixture of particles with different particle sizes in the positive electrode can increase the electrode density, which is preferable because it allows for a secondary battery with high energy density. Positive electrode active material 100 with a relatively small particle size is expected to have high charge / discharge rate characteristics. Positive electrode active material 100 with a relatively large particle size is expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0197] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100 of one embodiment of the present invention has an O3′-type and / or monoclinic O1(15)-type crystal structure by Li ｘ CoO ２This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0198] 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 the positive electrode obtained by disassembling the 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 100, which occupies the majority of the volume of the positive electrode active material 100.

[0199] When analyzing the crystallite size by powder XRD, it is preferable to measure the crystallite size while excluding the influence of orientation 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 sample.

[0200] As described above, the positive electrode active material 100 according to one embodiment of the present invention is ｘ CoO 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 voltages accounts for 50% or more of the crystal structure are not preferable because they cannot withstand high-voltage charging and discharging.

[0201] 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 CoO 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.

[0202] Furthermore, even in the positive electrode active material 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the positive electrode active material 100 of one embodiment of the present invention is the positive electrode active material 100, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0203] However, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, the crystal structure may change from O3'-type or monoclinic O1(15)-type to H1-3-type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0204] 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, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0205] The crystal structure of the surface layer 100 a, the grain boundaries 105 , etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0206] <Charging Method> Charging for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed by preparing a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and charging the coin cell.

[0207] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0208] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0209] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0210] The separator may be a 25 μm thick porous polypropylene film.

[0211] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0212] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). The charging method is not particularly limited as long as charging can be performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the current during CC (constant current) charging can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material, it is desirable to charge at such a small current value. On the other hand, if the current does not reach 2 mA / g or more and 10 mA / g or less even after long-term CV charging, it is considered that the current is being consumed not for charging the positive electrode active material but for decomposing the electrolyte. Therefore, CV charging may be terminated after a sufficient time has elapsed since the start of charging. In this case, a sufficient time can be, for example, 1.5 hours or more and 3 hours or less. 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 a positive electrode active material with a desired charge capacity. When various analyses are performed thereafter, it is preferable to seal the cell in an argon atmosphere to suppress 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 quickly remove the positive electrode and subject it to analysis after charging is complete. Specifically, it is preferable to do so within one hour after charging is complete, and more preferably within 30 minutes.

[0213] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge / discharge. For example, charging may be performed by constant current charging at a current value of 20 mA / g or more and 100 mA / g or less up to a given voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 2 mA / g or more and 10 mA / g or less, and then constant current discharging at 2.5 V and 20 mA / g or more and 100 mA / g or less.

[0214] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at 2.5 V with a current value of 20 mA / g or more and 100 mA / g or less.

[0215] Unless otherwise specified in this specification, charge / discharge capacity and charge / discharge current are expressed per weight of the positive electrode active material.

[0216] <<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 AXS X-ray source: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0217] If the measurement sample is a powder, it can be set by placing it in a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0218] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ The ideal powder XRD patterns of the lines are shown in Figures 11, 12, 13A and 13B.ｘ CoO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 13A and 13B show the XRD patterns of the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, and the H1-3-type crystal structure, with Figure 13A showing an enlarged view of the region in which 2θ is in the range of 18° (degrees) or more and 21° or less, and Figure 13B showing an enlarged view of the region in which 2θ is in the range of 42° or more and 46° or less. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from the Inorganic Crystal Structure Database (ICSD) (see Non-Patent Document 5). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０ m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same manner as the others.

[0219] As shown in Figures 11, 13A and 13B, in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0220] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear at 2θ=19.47±0.10° (19.37° or more and 19.57° or less) and 2θ=45.62±0.05° (45.57° or more and 45.67° or less).

[0221] However, as shown in Figures 12, 13A and 13B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of peaks at 19.13° or more and less than 19.37° and / or 19.37° or more and less than 19.57°, and at 45.37° or more and less than 45.57° and / or 45.57° or more and less than 45.67° when x is small can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0222] This can also be said to be because, in the positive electrode active material 100 of one embodiment of the present invention, the positions at which XRD diffraction peaks appear are close between the crystal structures where x = 1 and where x ≦ 0.24. More specifically, among the main diffraction peaks of the crystal structures where x = 1 and where x ≦ 0.24, the difference in 2θ between peaks that appear at 2θ of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0223] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２ When x in the formula is small, the particles have an O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the particles have an O3'-type and / or monoclinic O1(15)-type crystal structure. Other crystal structures may be included, or some may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0224] Similarly, when Rietveld analysis is performed, the H1-3 type and O1 type crystal structures are preferably 50% or less, more preferably 34% or less, and even more preferably substantially not observed.

[0225] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0226] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. For example, a narrow full width at half maximum is preferable. Even for peaks arising from the same crystalline phase, the half-width varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for peaks observed between 2θ = 43° and 46°, the full width at half maximum is preferably, for example, 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of that crystalline phase is high. Such high crystallinity contributes to sufficient stabilization of the crystal structure after charging.

[0227] The crystallite size of the O3'-type and monoclinic O1(15) crystal structures of the positive electrode active material 100 is approximately equal to that of LiCoO in a discharged state. ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the formula is small, the peaks of the O3' type and / or monoclinic O1(15) crystal structure can be clearly observed. ２ In this case, even if some of the crystal structure resembles the O3' type and / or monoclinic O1(15) crystal structure, the crystallite size will be small and the peaks will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0228] As described above, the positive electrode active material 100 of one embodiment of the present invention preferably has a small influence of the Jahn-Teller effect. As long as the influence of the Jahn-Teller effect is small, the positive electrode active material 100 may contain a transition metal such as nickel or manganese as an additive element in addition to cobalt.

[0229] In the positive electrode active material, the range of the nickel and manganese ratios and lattice constants in which the influence of the Jahn-Teller effect is presumed to be small is considered using XRD analysis.

[0230] FIG. 14 shows the results of calculating the a-axis and c-axis lattice constants using XRD for a positive electrode active material 100 according to one embodiment of the present invention, which has a layered rock-salt crystal structure and contains cobalt and nickel. FIG. 14A shows the a-axis result, and FIG. 14B shows the c-axis result. Note that the XRD patterns used in these calculations are for powder after synthesis of the positive electrode active material, but before incorporation into the positive electrode. The nickel concentration on the horizontal axis represents the nickel concentration when the sum of the number of cobalt and nickel atoms is taken as 100%. The positive electrode active material was prepared according to the preparation method shown in FIG. 17, except that an aluminum source was not used.

[0231] FIG. 15 shows the results of estimating the a-axis and c-axis lattice constants using XRD for a positive electrode active material 100 according to one embodiment of the present invention, which has a layered rock-salt crystal structure and contains cobalt and manganese. FIG. 15A shows the a-axis result, and FIG. 15B shows the c-axis result. Note that the lattice constants shown in FIG. 15 were obtained by XRD measurement of a powder obtained after synthesis of the positive electrode active material and before incorporation into a positive electrode. The manganese concentration on the horizontal axis indicates the manganese concentration when the sum of the number of cobalt and manganese atoms is taken as 100%. The positive electrode active material was prepared according to the preparation method shown in FIG. 17 , except that a manganese source was used instead of a nickel source and no aluminum source was used.

[0232] Fig. 14C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in Fig. 14A and Fig. 14B. Fig. 15C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in Fig. 15A and Fig. 15B.

[0233] 14C shows that the a-axis / c-axis tends to change significantly at nickel concentrations of 5% and 7.5%, with the a-axis distortion increasing at a nickel concentration of 7.5%. This distortion may be due to the Jahn-Teller distortion of trivalent nickel. This suggests that an excellent positive electrode active material with small Jahn-Teller distortion can be obtained at nickel concentrations of less than 7.5%.

[0234] Next, Figure 15A suggests that when the manganese concentration is 5% or more, the behavior of the change in lattice constant is different and does not follow Vegard's law. Therefore, it is suggested that when the manganese concentration is 5% or more, the crystal structure is different. Therefore, the manganese concentration is preferably 4% or less, for example.

[0235] The above ranges of nickel concentration and manganese concentration do not necessarily apply to the surface layer 100a, that is, the concentrations in the surface layer 100a may be higher than the above ranges.

[0236] As a result of considering a preferable range of the lattice constant from the above, it was found that in the positive electrode active material of one embodiment of the present invention, the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge and discharge are performed or in a discharged state, which can be estimated from the XRD pattern, has an a-axis lattice constant of 2.814 × 10 −１０ m is greater than 2.817 x 10 −１０ m and the lattice constant of the c-axis is 14.05 × 10 −１０ m or larger, 14.07 x 10 −１０ It has been found that the value is preferably smaller than m. The state in which no charge and discharge are performed may be, for example, the state of powder before the positive electrode of the secondary battery is produced.

[0237] Alternatively, in the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, it is preferable that the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is greater than 0.20000 and smaller than 0.20049.

[0238] Alternatively, when XRD analysis is performed on the layered rock salt type crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, a first peak may be observed at 2θ of not less than 18.50° and not more than 19.30°, and a second peak may be observed at 2θ of not less than 38.00° and not more than 38.80°.

[0239] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements. The quantitative accuracy of XPS is often about ±1 atomic %, with a lower limit of about 1 atomic %, depending on the element.

[0240] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion 100a than in the interior portion 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the average magnesium concentration throughout the cathode active material 100. Furthermore, the nickel concentration in at least a portion of the surface layer portion 100a is preferably higher than the average nickel concentration throughout the cathode active material 100. It is also preferable that the aluminum concentration in at least a part of the surface layer portion 100a is higher than the average aluminum concentration in the entire positive electrode active material 100. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 100a is higher than the average fluorine concentration in the entire positive electrode active material 100.

[0241] Note that the surface and surface layer 100a of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 100. Furthermore, the surface of the positive electrode active material 100 also does not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, 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.

[0242] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent 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.

[0243] The concentration of the added element may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances chemisorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 to 1.5. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 to 0.06.

[0244] Similarly, in order to ensure sufficient lithium insertion / extraction paths, the positive electrode active material 100 preferably has a higher concentration of lithium and cobalt in the surface layer portion 100a than the concentrations of the respective additive elements. This means that the concentrations of lithium and cobalt in the surface layer portion 100a are preferably higher than the concentrations of one or more additive elements selected from the additive elements contained in the surface layer portion 100a as measured by XPS or the like. For example, the concentration of cobalt in at least a portion of the surface layer portion 100a as measured by XPS or the like is preferably higher than the concentration of magnesium in at least a portion of the surface layer portion 100a as measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. Furthermore, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. Furthermore, the concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. Furthermore, the concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

[0245] Furthermore, it is more preferable that aluminum be widely distributed in a deep region, for example, the surface or a region at a depth of 5 nm to 50 nm from the reference point. Therefore, although aluminum is detected in an analysis of the entire cathode active material 100 using ICP-MS, GD-MS, or the like, it is more preferable that the concentration of aluminum is not detected by XPS or the like, or is 1 atomic % or less.

[0246] Furthermore, when XPS analysis was performed on the positive electrode active material 100 of one embodiment of the present invention, the number of magnesium atoms was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times, relative to the number of cobalt atoms. The number of nickel atoms was preferably 0.15 to 0.15 times, more preferably 0.03 to 0.13 times, relative to the number of cobalt atoms. The number of aluminum atoms was preferably 0.12 to 0.09 times, more preferably 0.09 times, relative to the number of cobalt atoms. The number of fluorine atoms was preferably 0.3 to 0.9 times, more preferably 0.1 to 1.1 times, relative to the number of cobalt atoms. The above ranges indicate that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100 but are widely distributed at preferred concentrations in the surface layer 100a of the positive electrode active material 100.

[0247] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions: Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0248] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0249] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0250] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. More preferably, the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0251] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0252] 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 boundary 105 of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis that thins the 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 in a specific region without being affected by the distribution in the depth direction.

[0253] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen 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. When the positive electrode active material 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. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, etc.

[0254] In STEM-EDX ray analysis or the like, the element profile does not change sharply in principle or due to measurement errors, 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, it is important to consider whether the transition metal M is greater than the average value M of the amount detected inside. ＡＶＥ and the average background value M ＢＧ The point where the oxygen concentration is 50% of the sum of the average value of the detected amount of oxygen inside the ＡＶＥ and the average background value O ＢＧ The reference point is the point where the sum of the internal and background is 50%. If the transition metal M and oxygen are different from each other in terms of the 50% point of the sum of the internal and background, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen adhering to the surface. Therefore, the average value M of the internal detected amount of the transition metal M is used. ＡＶＥ and the average background value M ＢＧ In the case of a positive electrode active material having a plurality of transition metals M, the M of the element with the largest count in the inner portion 100b can be used. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0255] The average value M of the background of the transition metal M ＢＧ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of 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 counts of the transition metal M and oxygen are saturated and stable, for example, the region where the detected amount of the transition metal M starts to increase. ＢＧ and the average value of the amount of oxygen detected inside O ＡＶＥ can also be found in the same way.

[0256] The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between the region where an image derived from the crystalline structure of the positive electrode active material is observed and the region where it is not observed, and is the outermost region 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 are observed. Alternatively, it is the intersection of the tangent line drawn to the brightness profile from the surface toward the bulk of the STEM image and the axis in the depth direction. The surface in a STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0257] Furthermore, the spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the maximum value of the additive element profile may deviate by approximately 1 nm. For example, even if the maximum value of the additive element profile of magnesium or the like is outside the surface obtained above, if the difference between the maximum value and the surface is less than 1 nm, this can be considered an error.

[0258] In addition, a peak in STEM-EDX-ray analysis refers to the detected intensity in each element profile or the maximum value of the characteristic X-rays for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0259] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated values ​​measured over six scans can be used as the profile of each element. The number of scans is not limited to six; more scans can be performed and the average can be used as the profile of each element.

[0260] The 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. For example, carbon can be vapor-deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0261] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0262] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W (two-pronged) EDX detector. During EDX ray analysis, the emission current of the STEM device is set to 6 μA or more and 10 μA or less, and a portion of the thinned sample with minimal depth and unevenness is measured. The magnification is, for example, approximately 150,000 times. The conditions for EDX ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0263] When EDX area analysis or EDX point analysis is performed on positive electrode active material 100 of one embodiment of the present invention, the concentration of each additional element, particularly additional element X, in surface layer portion 100a is preferably higher than that in inner portion 100b.

[0264] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, it is preferable that the magnesium concentration in the surface layer 100a is higher than that in the interior 100b. Furthermore, when EDX analysis is performed, the magnesium concentration peak in the surface layer 100a is preferably present on the surface of the cathode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, it is preferable that the magnesium concentration decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it is preferable that the magnesium concentration decays to 30% or less of the peak at a depth of 2 nm from the peak top. Note that the "peak concentration" referred to here refers to the maximum concentration value.

[0265] Furthermore, when EDX-ray analysis is performed, the magnesium concentration in the surface layer portion 100a (detected amount of magnesium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, and silicon) is preferably 0.5 atomic % or more and 10 atomic % or less, and more preferably 1 atomic % or more and 5 atomic % or less.

[0266] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0267] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer portion 100a preferably exists on the surface of the positive electrode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, if the fluorine concentration peak exists slightly closer to the surface than the magnesium concentration peak, resistance to hydrofluoric acid is increased, which is more preferable. For example, the fluorine concentration peak is more preferably 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferably 1.5 nm or more closer to the surface.

[0268] In the cathode active material 100 containing nickel as an additive element, the nickel concentration peak in the surface layer 100a preferably exists at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. In the cathode active material 100 containing magnesium and nickel, the nickel distribution preferably overlaps with the magnesium distribution. For example, the difference in depth between the nickel concentration peak and the magnesium concentration peak is preferably within 3 nm, more preferably within 1 nm.

[0269] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100 a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present on the surface of the positive electrode active material 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.

[0270] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0271] Furthermore, when the positive electrode active material 100 is subjected to linear or area analysis, the ratio of the number of atoms of the additional element A to the cobalt Co in the vicinity of the crystal grain boundary 105 (A / Co) is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0272] For example, when the added element is magnesium, when line analysis or area analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in the vicinity of the grain boundary 105 is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less. Furthermore, when the above-mentioned range is achieved at multiple locations, for example, three or more locations, of the positive electrode active material 100, it can be said that this indicates that the added element is not attached to a narrow area on the surface of the positive electrode active material 100, but is widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material 100.

[0273] <EPMA> EPMA (Electron Probe Microanalysis) can also quantify elements. Area analysis can analyze the distribution of each element.

[0274] When EPMA surface analysis is performed on a cross section of the positive electrode active material 100 according to one embodiment of the present invention, it is preferable that one or more selected from the additive elements have a concentration gradient, similar to the EDX analysis results. It is also more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The preferred range of the concentration peak for each additive element is also the same as in the case of EDX.

[0275] However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from the measurement results obtained using other analytical methods. For example, when the surface of the positive electrode active material 100 is analyzed using EPMA, the concentration of each added element present in the surface layer 100 a may be lower than the result obtained using XPS.

[0276] <Raman Spectroscopy> As described above, in the positive electrode active material 100 according to one embodiment of the present invention, at least a portion of the surface layer 100a preferably has a rock salt crystal structure. Therefore, when the positive electrode active material 100 and a positive electrode including the positive electrode active material 100 are analyzed by Raman spectroscopy, it is preferable to observe not only the layered rock salt crystal structure but also cubic crystal structures such as rock salt crystal structure. In the STEM image and the electron microbeam diffraction pattern described below, unless cobalt is substituted at lithium positions with a certain frequency in the depth direction during observation, or cobalt is present at the oxygen tetracoordination positions, they cannot be detected as bright spots in the STEM image and the electron microbeam diffraction pattern. On the other hand, because Raman spectroscopy is an analysis that captures the vibrational modes of bonds such as Co—O, peaks of the wavenumbers of the corresponding vibrational modes may be observed even when the amount of the corresponding Co—O bond is small. Furthermore, Raman spectroscopy can be performed on a surface layer having an area of ​​several μm ２ Since it is possible to measure a range of about 1 μm in depth, it is possible to capture with high sensitivity the state that exists only on the particle surface.

[0277] For example, when the laser wavelength is 532 nm, layered rock salt LiCoO ２ So, 470 cm −１ ~490cm −１ , 580 cm −１ ~600cm −１ Peak at (vibration mode: E ｇ , A １ｇ ) is observed. On the other hand, cubic CoO ｘ (0<x<1) (rock salt type Co １−ｙ O (0<y<1) or spinel type Co ３ O ４ ) is 665 cm −１ ~685cm −１ Peak at (Vibration mode: A １ｇ ) is observed.

[0278] Therefore, the integrated intensity of each peak was calculated at 470 cm −１ ~490cm −１ I1,580cm −１ ~600cm −１ I2, 665 cm −１ ~685cm −１When I3 is taken as I3, the value of I3 / I2 is preferably 1% or more and 10% or less, and more preferably 3% or more and 9% or less.

[0279] If a cubic crystal structure such as a rock salt type is observed within the above range, it can be said that the surface layer 100a of the positive electrode active material 100 has a rock salt type crystal structure within a preferred range.

[0280] <<Electron Diffraction Pattern>> As with Raman spectroscopy, it is preferable that the characteristics of the rock salt-type crystal structure are observed in the electron diffraction pattern as well as the layered rock salt crystal structure. However, in the STEM image and the electron diffraction pattern, taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock salt-type crystal structure are not too strong in the surface layer portion 100a, particularly in the outermost surface (for example, 1 nm deep from the surface). This is because, rather than the outermost surface being covered with a rock salt-type crystal structure, it is preferable that the lithium layer has a layered rock salt-type crystal structure and an additive element such as magnesium is present in the lithium layer, which can ensure a lithium diffusion path and have a stronger function of stabilizing the crystal structure.

[0281] Therefore, for example, when a micro-electron beam diffraction pattern is obtained from a region having a depth of 1 nm or less from the surface and a micro-electron beam diffraction pattern from a region having a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constant calculated from these patterns is small.

[0282] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is preferably 0.1 Å or less for the a-axis and 1.0 Å or less for the c-axis. It is more preferably 0.05 Å or less for the a-axis and 0.6 Å or less for the c-axis. It is even more preferably 0.04 Å or less for the a-axis and 0.3 Å or less for the c-axis.

[0283] <Surface Roughness and Specific Surface Area> The positive electrode active material 100 of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness indicates that the effect of the flux described below is sufficiently exerted, and the surface of the additive element source and the lithium cobalt oxide are melted. Therefore, this is one factor indicating that the distribution of the additive element in the surface layer portion 100 a is good.

[0284] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of the positive electrode active material 100, the specific surface area of ​​the positive electrode active material 100, or the like.

[0285] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0286] First, the cathode active material 100 is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 100 with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material 100 is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 100 is selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness at least within 400 nm of the outer periphery of the particle of the cathode active material.

[0287] On the particle surfaces of the positive electrode active material 100 of this embodiment, the root mean square (RMS) surface roughness, which is an index of roughness, is preferably less than 3 nm, more preferably less than 1 nm, and even more preferably less than 0.5 nm.

[0288] The image processing software used for noise processing, interface extraction, etc. is not particularly limited, but for example, "ImageJ" described in Non-Patent Documents 9 to 11 can be used. Spreadsheet software, etc. is also not particularly limited, but for example, Microsoft Office Excel can be used.

[0289] For example, the actual specific surface area S measured by the gas adsorption method using the constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 100 can also be quantified from the ratio of

[0290] Ideal specific surface area S ｉ is calculated assuming that all particles have the same diameter as D50, the same weight, and an ideal spherical shape.

[0291] The median diameter D50 can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. The specific surface area can be measured by a specific surface area measuring device using a gas adsorption method based on a constant volume method, for example.

[0292] The positive electrode active material 100 according to one embodiment of the present invention has an ideal specific surface area S calculated from the median diameter D50. ｉ and the actual specific surface area S Ｒ The ratio S Ｒ / S ｉ is preferably 2.1 or less.

[0293] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0294] First, a surface SEM image of the positive electrode active material 100 is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area are the same.

[0295] Next, image processing software (for example, "ImageJ") is used to convert the SEM image into, for example, an 8-bit image (called a grayscale image). The grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 gradations. Dark areas have lower gradations, and bright areas have higher gradations. The luminance change can be quantified in relation to the number of gradations. This numerical value is called the grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material as a numerical value.

[0296] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram is a three-dimensional representation of the gradation distribution in the target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0297] In the positive electrode active material 100 of one embodiment of the present invention, the difference between the maximum and minimum values ​​of the grayscale value is preferably 120 or less, more preferably 115 or less, and even more preferably 70 to 115. The standard deviation of the grayscale value is preferably 11 or less, more preferably 8 or less, and even more preferably 4 to 8.

[0298] <Additional Features> The positive electrode active material 100 may have recesses, cracks, dents, V-shaped cross sections, and the like. These are defects, and repeated charge and discharge may cause cobalt elution, collapse of the crystalline structure, cracking of the positive electrode active material 100, oxygen desorption, and the like. However, if the embedded portion 102 shown in FIG. 1B is present to bury these defects, cobalt elution and the like can be suppressed. This allows the positive electrode active material 100 to have excellent reliability and cycle characteristics.

[0299] As described above, an excess of the additive element contained in the positive electrode active material 100 may adversely affect the insertion and desorption of lithium. Furthermore, when the positive electrode active material 100 is used in a secondary battery, it may result in an increase in internal resistance and a decrease in charge / discharge capacity. On the other hand, if the additive element is insufficient, it may not be distributed throughout the entire surface layer portion 100a, and the effect of suppressing deterioration of the crystalline structure may be insufficient. Thus, the additive element needs to be present at an appropriate concentration in the positive electrode active material 100, but adjusting this concentration is not easy.

[0300] Therefore, when the positive electrode active material 100 has a region where the additive element is unevenly distributed, some of the excess atoms of the additive element are removed from the interior 100b of the positive electrode active material 100, and an appropriate additive element concentration can be achieved in the interior 100b. This makes it possible to suppress an increase in internal resistance and a decrease in charge / discharge capacity when the positive electrode active material 100 is used as a secondary battery. The ability to suppress an increase in internal resistance of a secondary battery is an extremely desirable characteristic, particularly in charge / discharge at large currents, for example, at 400 mA / g or more.

[0301] Furthermore, in the positive electrode active material 100 having a region where the additive element is unevenly distributed, it is permissible to mix an excess amount of the additive element to some extent in the manufacturing process, which is preferable because it widens the margin in production.

[0302] A coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. Figures 16A and 16B show examples of positive electrode active material 100 to which a coating portion 104 is attached.

[0303] The coating portion 104 is preferably formed by the accumulation of decomposition products of the electrolyte and the organic electrolytic solution during charging and discharging. ｘ CoO ２When repeated charging is performed such that x in the formula is 0.24 or less, the presence of a coating portion derived from the electrolyte on the surface of the positive electrode active material 100 is expected to improve charge-discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing cobalt elution. The coating portion 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as the electrolyte, a high-quality coating portion is easily obtained. Therefore, a coating portion 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating portion and is therefore preferred. Furthermore, the coating portion 104 does not have to cover the entire positive electrode active material 100. For example, it is sufficient for the coating portion 104 to cover 50% or more of the surface of the positive electrode active material 100, with 70% or more being more preferable and 90% or more being even more preferable.

[0304] This embodiment can be used in combination with other embodiments.

[0305] Embodiment 2 In this embodiment, an example of a method for manufacturing a positive electrode active material 100, which is one embodiment of the present invention, will be described.

[0306] In order to prepare the positive electrode active material 100 having the distribution, composition, and / or crystal structure of the additive elements as described in the previous embodiment, the method of adding the additive elements is important. At the same time, it is also important that the crystallinity of the inner portion 100b is good.

[0307] Therefore, in the process of producing the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, and then mix in the additive element source and perform a heat treatment.

[0308] In the method of synthesizing lithium cobalt oxide containing an additive element by mixing an additive element source simultaneously with a cobalt source and a lithium source, it is difficult to increase the concentration of the additive element in the surface layer portion 100a. Furthermore, if the additive element source is simply mixed without heating after synthesizing lithium cobalt oxide, the additive element will simply adhere to the lithium cobalt oxide without dissolving in the lithium cobalt oxide. Without sufficient heating, it is also difficult to achieve a good distribution of the additive element. Therefore, it is preferable to mix the additive element source after synthesizing lithium cobalt oxide and then perform a heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

[0309] However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that an added element, such as magnesium, enters the cobalt site. ｘ CoO ２ When the value of x in the matrix is ​​small, the layered rock salt crystal structure of R-3m cannot be maintained. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent and the evaporation of lithium.

[0310] Therefore, it is preferable to mix a material that functions as a flux with the additive element source. Any material with a lower melting point than lithium cobalt oxide can function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting points of the additive element source and lithium cobalt oxide. Lowering the melting point makes it easier to distribute the additive elements well at a temperature where cation mixing is unlikely to occur.

[0311] [Initial Heating] It is more preferable to heat the lithium cobalt oxide after synthesis and before mixing with the additive element. This heating is sometimes called initial heating.

[0312] The initial heating causes lithium to be released from a part of the surface layer 100a of the lithium cobalt oxide, resulting in a more favorable distribution of the additive elements.

[0313] More specifically, it is believed that initial heating facilitates different distributions depending on the additive element through the following mechanism. First, lithium is released from a portion of the surface layer 100a by initial heating. Next, this lithium-deficient surface layer 100a is mixed with an additive element source, such as a nickel source, an aluminum source, or a magnesium source, and the mixture is heated. Of the additive elements, magnesium is a divalent typical element, and nickel is a transition metal, but it easily becomes a divalent ion. Therefore, Mg is released from a portion of the surface layer 100a. ２＋ and Ni ２＋ and Co reduced by lithium deficiency. ２＋ However, since this phase is formed only in a part of the surface layer portion 100a, it may not be clearly observed in an electron microscope image such as an STEM or an electron beam diffraction pattern.

[0314] Among the additive elements, nickel is likely to dissolve and diffuse into the interior 100b when the surface layer 100a is a layered rock-salt lithium cobaltate, but when part of the surface layer 100a is a rock-salt lithium cobaltate, nickel tends to remain in the surface layer 100a. Therefore, initial heating can make it easier for divalent additive elements such as nickel to remain in the surface layer 100a. The effect of this initial heating is particularly significant on the surface other than the (001)-oriented surface of the positive electrode active material 100 and on the surface layer 100a.

[0315] Furthermore, in these rock salt types, the bond distance between the metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt type.

[0316] For example, rock salt type Ni ０．５ Mg ０．５ The Me-O distance in rock salt MgO is 2.09 Å, and the Me-O distance in rock salt MgO is 2.11 Å. Even if a spinel phase is formed in a part of the surface layer 100a, the spinel phase is not formed in the spinel NiAl ２ O ４ The Me-O distance is 2.0125 Å, and the spinel type MgAl ２ O ４ The Me-O distance is 2.02 Å. In both cases, the Me-O distance exceeds 2 Å. Note that 1 Å = 10 −１０ m.

[0317] On the other hand, in the layered rock salt type, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, layered rock salt type LiAlO ２ The Al-O distance in the layered rock salt LiCoO is 1.905 Å (the Li-O distance is 2.11 Å). ２ The Co—O distance in this case is 1.9224 Å (the Li—O distance is 2.0916 Å).

[0318] According to Shannon's ionic radii (Shannon et al., Acta A 32 (1976) 751), the ionic radius of hexacoordinated aluminum is 0.535 Å, the ionic radius of hexacoordinated oxygen is 1.4 Å, and the sum of these is 1.935 Å.

[0319] From the above, it is believed that aluminum exists more stably in non-lithium sites in the layered rock-salt structure than in the rock-salt structure, and therefore aluminum is more likely to be distributed in the deeper region having the layered rock-salt structure and / or the interior 100b than in the region close to the surface having the rock-salt structure in the surface layer portion 100a.

[0320] Furthermore, the initial heating is expected to reduce defects including dislocations in the inner portion 100b and improve the crystallinity of the layered rock-salt crystal structure. The low level of defects in the inner portion 100b is also thought to be related to the ease of forming the O3' type and / or monoclinic O1(15) type.

[0321] Therefore, especially Li ｘ CoO ２ When x is, for example, 0.15 or more and 0.17 or less, it is preferable to perform this initial heating in order to produce a positive electrode active material 100 having a monoclinic O1(15) type crystal structure.

[0322] However, initial heating is not necessarily required. In other heating steps, such as annealing, the atmosphere, temperature, time, etc. can be controlled to prevent Li ｘ CoO ２ When x in the formula (I) is small, it may be possible to prepare a positive electrode active material 100 having an O3′ type and / or a monoclinic O1(15) type.

[0323] <<Method 1 for Producing Positive Electrode Active Material>> Method 1 for producing positive electrode active material 100, which undergoes annealing and initial heating, will be described with reference to FIGS. 17A to 17C.

[0324] <Step S11> In step S11 shown in FIG. 17A, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0325] 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.

[0326] As the cobalt source, it is preferable to use a compound containing cobalt, such as cobalt oxide or cobalt hydroxide.

[0327] The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity and / or reliability of the secondary battery can be increased.

[0328] In addition, the cobalt source preferably has high crystallinity, for example, single crystal grains. The crystallinity of the cobalt source can be evaluated using a TEM (transmission electron microscope) image, a STEM (scanning transmission electron microscope) image, a HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) image, an ABF-STEM (annular bright-field scanning transmission electron microscope) image, or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, or the like. Note that the above-mentioned methods for evaluating crystallinity can be applied not only to the evaluation of cobalt sources but also to the evaluation of the crystallinity of other sources.

[0329] <Step S12> Next, in step S12 shown in FIG. 17A, the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller fragments. 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 more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the 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 them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0330] 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).

[0331] <Step S13> Next, in step S13 shown in FIG. 17A, the mixed material is heated. Heating is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably at approximately 950°C. If the temperature is too low, the decomposition and melting of the lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, resulting in defects. For example, cobalt may change from trivalent to divalent, inducing oxygen defects.

[0332] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably from 1 hour to 100 hours, and more preferably from 2 hours to 20 hours.

[0333] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0334] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0335] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and allowing oxygen to flow through the reaction chamber is called flow.

[0336] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen (which may also be called purging) may be used to prevent the oxygen from entering or leaving the reaction chamber. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0337] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0338] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0339] The crucible used for heating is preferably an aluminum oxide crucible. Aluminum oxide crucibles are made of a material that does not easily release impurities. In this embodiment, an aluminum oxide crucible with a purity of 99.9% is used. It is preferable to heat the crucible with a lid on, as this can prevent the material from volatilizing.

[0340] Furthermore, it is preferable to use a used crucible rather than a new one. In this specification, a new crucible refers to one that has undergone two or fewer heating processes with materials containing lithium, transition metal M, and / or additive elements. A used crucible refers to one that has undergone three or more heating processes with materials containing lithium, transition metal M, and / or additive elements. This is because using a new crucible may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the sheath during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, may not fall within the desired range. On the other hand, this risk is less likely with a used crucible.

[0341] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use an agate or partially stabilized zirconium oxide mortar for the mortar. Heating conditions equivalent to those of step S13 can also be applied to heating steps other than step S13, which will be described later.

[0342] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized.

[0343] Although the example of producing the composite oxide by the solid phase method in steps S11 to S14 has been shown, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0344] <Step S15> Next, in step S15 shown in Fig. 17A, the lithium cobalt oxide is heated. Because this is the first heating of the lithium cobalt oxide, the heating in step S15 is sometimes called initial heating. Alternatively, because this heating is performed before step S20 described below, it is sometimes called preheating or pretreatment.

[0345] As described above, the initial heating causes lithium to be desorbed from a portion of the surface layer 100a of the lithium cobalt oxide. It is also expected to have the effect of improving the crystallinity of the interior 100b. Furthermore, impurities may be mixed into the lithium source and / or cobalt source prepared in step S11, etc. The initial heating can reduce the amount of impurities in the lithium cobalt oxide completed in step S14.

[0346] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. A smooth lithium cobalt oxide surface means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a smooth surface means that there is little foreign matter adhering to the surface. Foreign matter is thought to be a cause of irregularities, so it is preferable that it does not adhere to the surface.

[0347] For this initial heating, it is not necessary to prepare a lithium compound source, an additive element source, or a material that functions as a flux.

[0348] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. In addition to the heating conditions, the heating temperature in this step should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in this step should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0349] The effect of increasing the crystallinity of the inner portion 100b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the difference in shrinkage of the lithium cobalt oxide produced in step S13.

[0350] The heating in step S13 may cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference may induce a shrinkage difference. It is also believed that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is believed that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. Homogenizing the strain energy relieves the strain in the lithium cobalt oxide. This may result in a smoother surface for the lithium cobalt oxide. This is also referred to as an improved surface. In other words, it is believed that the shrinkage difference in the lithium cobalt oxide is relieved after step S15, resulting in a smoother surface for the composite oxide.

[0351] Furthermore, the difference in shrinkage may cause microscopic misalignment, such as crystal misalignment, in the lithium cobalt oxide. This step is preferably carried out to reduce this misalignment. This step makes it possible to equalize the misalignment in the composite oxide. When the misalignment is equalized, the surface of the composite oxide may become smooth. This is also referred to as the alignment of crystal grains. In other words, it is believed that step S15 reduces the misalignment of crystals and other particles in the composite oxide, resulting in a smoother surface.

[0352] When lithium cobalt oxide with a smooth surface is used as the positive electrode active material, deterioration during charge and discharge in a secondary battery is reduced and cracking of the positive electrode active material can be prevented.

[0353] Note that pre-synthesized lithium cobalt oxide may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on pre-synthesized lithium cobalt oxide, lithium cobalt oxide with a smooth surface can be obtained.

[0354] <Step S20> Next, as shown in step S20, it is preferable to add the additional element A to the lithium cobalt oxide that has undergone the initial heating. When the additional element A is added to the lithium cobalt oxide that has undergone the initial heating, the additional element A can be added evenly. Therefore, it is preferable to add the additional element A after the initial heating. The step of adding the additional element A will be described with reference to FIGS. 17B and 17C.

[0355] 17B, ​​a source of an additional element A (A source) to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the additional element A source.

[0356] The additive element A can be any of the additive elements described in the previous embodiment. Specifically, one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. Also, one or two elements selected from bromine and beryllium can be used.

[0357] When magnesium is selected as the additive element, the source of the additive element can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0358] When fluorine is selected as the additive element, the source of the additive element can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (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.

[0359] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0360] The fluorine source may also be gaseous, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F) or the like may be used and mixed into the atmosphere in the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0361] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and magnesium source. ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF２ The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = near 0.33) is more preferable. In this specification, "near" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0362] 17B, ​​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.

[0363] 17B, ​​the pulverized and mixed materials are collected to obtain a source of the additional element A (A source). The source of the additional element A shown in step S23 includes a plurality of starting materials and can be called a mixture.

[0364] The particle size of the mixture is preferably D50 (median diameter) of 600 nm to 10 μm, more preferably 1 μm to 5 μm. Even when a single material is used as the additive element source, the D50 (median diameter) is preferably 600 nm to 10 μm, more preferably 1 μm to 5 μm.

[0365] Such a finely powdered mixture (including the case where only one type of additive element is included) can be easily adhered to the surfaces of the lithium cobalt oxide particles when mixed with the lithium cobalt oxide in a later step. If the mixture is evenly adhered to the surfaces of the lithium cobalt oxide particles, it is preferable because the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating.

[0366] <Step S21> A step different from that shown in FIG. 17B will be described with reference to FIG. 17C. In step S21 shown in FIG. 17C, four types of additive element sources to be added to lithium cobalt oxide are prepared. That is, the types of additive element sources shown in FIG. 17C are different from those shown in FIG. 17B. A lithium source may be prepared together with the additive element sources.

[0367] As sources of four additive elements, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described with reference to FIG. 17B. Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0368] <Steps S22 and S23> Steps S22 and S23 shown in FIG. 17C are similar to the steps described with reference to FIG. 17B.

[0369] 17A , lithium cobalt oxide and a source of additional element A (A source) are mixed together. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the source of additional element A is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0370] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter mixing time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium.

[0371] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0372] <Step S32> Next, in step S32 of Fig. 17A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0373] 17A to 17C illustrate a fabrication method in which the additive element is added only after initial heating, but the present invention is not limited to this method. The additive element may be added at other timings or multiple times. The timing may vary depending on the element.

[0374] For example, an additive element may be added to the lithium source and the cobalt source in step S11, i.e., in the stage of the starting material for the composite oxide. Then, in step S13, lithium cobalt oxide containing the additive element can be obtained. In this case, it is not necessary to separate steps S11 to S14 from steps S21 to S23. This method can be said to be simple and highly productive.

[0375] Alternatively, lithium cobalt oxide containing some of the additive elements may be used. For example, if lithium cobalt oxide containing magnesium and fluorine is used, steps S11 to S14 and some of step S20 can be omitted. This method is simple and has high productivity.

[0376] Alternatively, lithium cobalt oxide to which magnesium and fluorine have been added in advance may be heated in step S15, and then a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added as in step S20.

[0377] <Step S33> Next, in step S33 shown in FIG. 17A, the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may exceed atmospheric pressure in order to increase the oxygen partial pressure in the heating atmosphere. This is because if the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and the like may be reduced, and the layered rock salt type crystal structure of lithium cobalt oxide and the like may not be maintained.

[0378] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 650° C. or higher.

[0379] Of course, the reaction is more likely to proceed if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. 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.

[0380] Also, LiCoO ２ :LiF:MgF ２ A mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830° C. in differential scanning calorimetry (DSC). Therefore, the lower limit of the heating temperature is more preferably 830° C. or higher.

[0381] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0382] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 900°C or lower.

[0383] In consideration of these, the heating temperature in step S33 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 800°C or higher and 1100°C or lower, or 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, and even more preferably 830°C or higher and 900°C or lower. The heating temperature in step S33 is preferably higher than that in step S13.

[0384] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0385] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered below the decomposition temperature of lithium cobalt oxide, for example, to 742° C. or higher and 950° C. or lower, and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0386] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF will volatilize when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, etc., LiCoO ２There is a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0387] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 903.

[0388] The heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additive elements (e.g., fluorine) diffuse is blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0389] Furthermore, it is believed that uniform distribution of an additive element (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the particles of mixture 903 do not adhere to each other.

[0390] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0391] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 903 .

[0392] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of the lithium cobalt oxide in step S14, etc. When the lithium cobalt oxide is small, a lower temperature or a shorter heating time may be preferable than when the lithium cobalt oxide is large.

[0393] 17A, when the median diameter (D50) of the lithium cobalt oxide is about 12 μm, the heating temperature is preferably, for example, 650° C. or more and 950° C. or less. The heating time is preferably, for example, 3 hours or more and 60 hours or less, more preferably 10 hours or more and 30 hours or less, and even more preferably about 20 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or more and 50 hours or less.

[0394] On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 5 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0395] <Step S34> Next, in step S34 shown in Fig. 17A, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered particles. Through the above steps, the positive electrode active material 100 according to one embodiment of the present invention can be produced. The positive electrode active material according to one embodiment of the present invention has a smooth surface.

[0396] 18 to 19C , a positive electrode active material preparation method 2, which is one embodiment of the present invention and differs from positive electrode active material preparation method 1, will be described. The positive electrode active material preparation method 2 differs from preparation method 1 mainly in the number of times the additive elements are added and the mixing method. The rest of the description can be taken into consideration as in preparation method 1.

[0397] In FIG. 18, steps S11 to S15 are performed in the same manner as in FIG. 17A to prepare lithium cobalt oxide that has undergone initial heating.

[0398] <Step S20a> Next, as shown in step S20a, it is preferable to add an additional element A1 to the lithium cobalt oxide that has undergone the initial heating.

[0399] <Step S21> In step S21 shown in Fig. 19A, a first additive element source is prepared. The first additive element source can be selected from the additive elements A described in step S21 shown in Fig. 17B and used. For example, the additive element A1 can be one or more selected from magnesium, fluorine, and calcium. Fig. 19A illustrates an example in which a magnesium source (Mg source) and a fluorine source (F source) are used as the first additive element source.

[0400] Steps S21 to S23 shown in Fig. 19A can be performed under the same conditions as steps S21 to S23 shown in Fig. 17B. As a result, an additional element source (A1 source) can be obtained in step S23.

[0401] Furthermore, steps S31 to S33 shown in FIG. 18 can be performed in the same manner as steps S31 to S33 shown in FIG. 17A.

[0402] <Step S34a> Next, the material heated in step S33 is recovered to produce lithium cobalt oxide containing the additional element A1. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0403] <Step S40> In step S40 shown in Fig. 18, the additional element A2 is added. This will be described with reference to Figs. 19B and 19C.

[0404] <Step S41> In step S41 shown in Fig. 19B, a second additive element source is prepared. The second additive element source can be selected from the additive elements A described in step S21 shown in Fig. 17B and used. For example, the additive element A2 can be one or more selected from nickel, titanium, boron, zirconium, and aluminum. Fig. 19B illustrates an example in which a nickel source (Ni source) and an aluminum source (Al source) are used as the second additive element source.

[0405] Steps S41 to S43 shown in Fig. 19B can be performed under the same conditions as steps S21 to S23 shown in Fig. 17B. As a result, an additional element source (A2 source) can be obtained in step S43.

[0406] 19C shows a modified example of the steps described with reference to FIG. 19B. In step S41 shown in FIG. 19C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are independently pulverized. As a result, in step S43, a plurality of second additive element sources (A2 sources) are prepared. The steps in FIG. 19C differ from those in FIG. 19B in that the additive elements are independently pulverized in step S42a.

[0407] <Steps S51 to S53> Next, steps S51 to S53 shown in Fig. 18 can be performed under the same conditions as steps S31 to S34 shown in Fig. 17A. The conditions for step S53, which is a heating step, may be a lower temperature and a shorter time than those for step S33. Through the above steps, in step S54, the positive electrode active material 100 according to one embodiment of the present invention can be produced. The positive electrode active material according to one embodiment of the present invention has a smooth surface.

[0408] 18 to 19C , in fabrication method 2, an additional element A1 and an additional element A2 are introduced into lithium cobalt oxide separately. By introducing the additional elements separately, the concentration distribution of each additional element in the depth direction can be changed. For example, it is possible to distribute the additional element A1 so that its concentration is higher in the surface layer portion 100a than in the interior portion 100b, and to distribute the additional element A2 so that its concentration is higher in the interior portion 100b than in the surface layer portion 100a.

[0409] After the initial heating described in this embodiment, a positive electrode active material with a smooth surface can be obtained.

[0410] The initial heating shown in this embodiment is performed on lithium cobalt oxide. Therefore, the initial heating is preferably performed at a temperature lower than that required for obtaining lithium cobalt oxide and for a shorter heating time than that required for obtaining lithium cobalt oxide. The step of adding an additive element to lithium cobalt oxide is preferably performed after the initial heating. This addition step can be performed in two or more steps. Following this order of steps is preferable because it maintains the surface smoothness obtained by the initial heating.

[0411] Positive electrode active material 100 with a smooth surface may be more resistant to physical destruction due to pressure, etc. than positive electrode active material that does not have a smooth surface. For example, positive electrode active material 100 is less likely to be destroyed in a test involving pressure, such as a nail penetration test, and as a result, safety may be improved.

[0412] This embodiment can be used in combination with other embodiments.

[0413] Embodiment 3 In this embodiment, an example of a secondary battery of one embodiment of the present invention will be described with reference to FIGS. 20 and 21 .

[0414] <Configuration Example of Secondary Battery> Hereinafter, a secondary battery shown in FIG. 20 will be described as an example, in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body.

[0415] [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 a positive electrode active material, and may include a conductive material (synonymous with a conductive additive) and a binder. The positive electrode active material is the positive electrode active material prepared by the preparation method described in the previous embodiment.

[0416] The positive electrode active material described in the above embodiment may be mixed with another positive electrode active material.

[0417] 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 , LiNiO ２ , LiMn ２ O ４ , V ２ O ５, Cr ２ O ５ , MnO ２ The following compounds are exemplified:

[0418] Other positive electrode active materials include LiMn ２ O ４ Lithium-containing materials having a spinel-type crystal structure containing manganese, such as lithium nickel oxide (LiNiO ２ or LiNi １−ｘ M ｘ O ２ It is preferable to mix (0<x<1) (M=Co, Al, etc.) This configuration can improve the characteristics of the secondary battery.

[0419] The conductive material may be a carbon-based material such as acetylene black, or may be a carbon nanotube, graphene, or a graphene compound.

[0420] In this specification and the like, graphene compounds include multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings is sometimes called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0421] In this specification and the like, graphene oxide refers to a material that contains carbon and oxygen, has a sheet shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0422] In this specification, reduced graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. A single reduced graphene oxide can function, but multiple sheets may be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % or more and 15 atomic % or less. By achieving these carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0423] Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds may also have a sheet-like shape. Graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. Even thin graphene compounds may have very high conductivity, allowing a small amount of graphene to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene compound covers 80% or more of the active material. It is preferable that the graphene compound clings to at least a portion of the active material particles. It is also preferable that the graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the unevenness of a single active material particle or the unevenness formed by multiple active material particles. It is also preferable that the graphene compound surrounds at least a portion of the active material particles. The graphene compound may also have holes.

[0424] When active material particles having a small particle size, for example, 1 μm or less, are used, the specific surface area of ​​the active material particles is large, and more conductive paths connecting the active material particles are required. In such cases, it is preferable to use a graphene compound that can efficiently form conductive paths even in a small amount.

[0425] Because of the properties described above, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and rapid discharging. For example, rapid charging and rapid discharging characteristics may be required for secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. Rapid charging characteristics may also be required for mobile electronic devices, etc. Rapid charging and discharging refers to charging and discharging at, for example, 200 mA / g, 400 mA / g, or 1000 mA / g or more.

[0426] The plurality of graphenes or graphene compounds are formed so as to partially cover the plurality of granular positive electrode active material particles or to be attached to the surfaces of the plurality of granular positive electrode active material particles, and are therefore preferably in surface contact with each other.

[0427] Here, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphenes or graphene compounds together. When an active material is covered with a graphene net, the graphene net can also function as a binder that bonds the active materials together. Therefore, the amount of binder can be reduced or no binder can be used, thereby improving the ratio of active material to the electrode volume and electrode weight. In other words, the discharge capacity of a secondary battery can be increased.

[0428] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer 200. 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 median diameter (D50) of 1 μm or less, more preferably 100 nm or less.

[0429] [Binder] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0430] 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.

[0431] 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.

[0432] The binder may be used in combination with two or more of the above.

[0433] [Current Collector] The current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, 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 elements that improve heat resistance, such as 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.

[0434] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also have a conductive material and a binder.

[0435] [Negative Electrode Active Material] As the negative electrode active material, for example, an alloy-based material and / or a carbon-based material can be used.

[0436] As the negative electrode active material, an element capable of undergoing a charge-discharge reaction by alloying / de-alloying reaction with lithium can be used. For example, a material containing one or more elements selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger charge-discharge capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, a compound containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni３ Sn ２ , 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. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0437] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO ｘ Here, x preferably has a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, x is preferably 0.2 or more and 1.2 or less. Alternatively, x is preferably 0.3 or more and 1.5 or less.

[0438] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0439] 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 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.

[0440] 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 secondary batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high charge / discharge capacity per unit volume, relatively little volume expansion, low cost, and greater safety than lithium metal.

[0441] 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 oxide (WO ２ ), molybdenum oxide (MoO ２ ) and other oxides can be used.

[0442] In addition, as the negative electrode active material, a composite 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 ３ has a large charge / discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0443] When a composite 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 composite nitride of lithium and a transition metal can be used as the negative electrode active material, even when a material containing lithium ions is used as the positive electrode active material, by first removing the lithium ions contained in the positive electrode active material.

[0444] 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 ３ This also occurs with fluorides such as

[0445] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0446] [Negative electrode current collector] The negative electrode current collector can be made of the same material as the positive electrode current collector. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0447] [Electrolyte] The electrolyte contains a solvent and an electrolyte. The solvent for the electrolyte 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), 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 sultone, or any combination and ratio of two or more of these.

[0448] Fluorine-containing solvents such as FEC have a low HOMO, which is preferable since they can withstand high voltages.

[0449] 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 secondary battery from exploding and / or catching fire even if the internal temperature of the secondary battery rises due to an internal short circuit, overcharging, or the like. 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.

[0450] The electrolyte to be dissolved in the solvent is, for example, LiPF ６ , LiClO４ , 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 ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ These lithium salts may be used alone or in any combination and ratio of two or more thereof.

[0451] The electrolyte used in the secondary battery is preferably a highly purified electrolyte with a low content of granular dust 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.

[0452] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), dinitrile compounds such as succinonitrile and adiponitrile, fluorobenzene, and ethyleneglycolbis(propionitrile)ether may be added to the electrolyte solution. The concentration of the added material may be, for example, 0.1 wt % to 5 wt % of the total solvent. VC and LiBOB are particularly preferred because they are likely to form a good coating portion.

[0453] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0454] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0455] 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.

[0456] Examples of polymers that can be used include polymers having a polyalkylene oxide structure, such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0457] In addition, instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a solid electrolyte containing a polymer material such as a PEO (polyethylene oxide) can be used. When a solid electrolyte is used, the installation of a separator and / or spacer is unnecessary. Furthermore, since the entire battery can be solidified, the risk of leakage is eliminated, dramatically improving safety.

[0458] It is also preferable that the material used for the electrolyte contains few impurities.

[0459] [Separator] The secondary battery preferably has a separator. Examples of separators that can be used include paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into an envelope shape and disposed so as to encase either the positive electrode or the negative electrode.

[0460] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0461] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0462] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0463] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the discharge capacity per volume of the secondary battery can be increased.

[0464] [Exterior Body] The exterior body of the secondary battery can be made of, for example, a metal material such as aluminum and / 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.

[0465] <Laminated Secondary Battery and Method for Producing the Same> An example of an external view of a laminated secondary battery 500 is shown in Figures 20 and 21. Figures 20 and 21 include a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511. If the laminated secondary battery has a flexible configuration, when it is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent to match the deformation of the electronic device. An example of a method for producing the laminated secondary battery will be described with reference to Figures 21A to 21C.

[0466] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 21B shows the stacked negative electrode 506, the separator 507, and the positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0467] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0468] Next, as shown in Fig. 21C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0469] Next, an electrolyte solution (not shown) is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0470] By using the positive electrode active material described in the above embodiment for the positive electrode 503, the secondary battery 500 can have a high discharge capacity and excellent cycle characteristics.

[0471] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0472] Embodiment 4 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on an electronic device will be described with reference to FIGS. 22A to 24C. FIG.

[0473] 22A to 22G show examples in which the secondary battery having the positive electrode active material described in the above embodiment is mounted in an electronic device. Examples of electronic devices to which the secondary battery is applied include television devices (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 machines, personal digital assistants, sound reproduction devices, and large game machines such as pachinko machines.

[0474] Furthermore, a secondary battery having a flexible shape can be incorporated along the curved surfaces of the inner or outer walls of houses, buildings, etc., and the interior or exterior of automobiles.

[0475] 22A illustrates an example of a mobile phone. The mobile phone 7400 includes a display portion 7402 built into a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes a secondary battery 7407. By using the secondary battery of one embodiment of the present invention as the secondary battery 7407, a lightweight mobile phone with a long lifetime can be provided.

[0476] FIG. 22B shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 installed therein is also bent. FIG. 22C shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. The secondary battery 7407 has a lead electrode electrically connected to the current collector. For example, the current collector is copper foil, and a portion of the current collector is alloyed with gallium to improve adhesion with the active material layer in contact with the current collector, resulting in a configuration with high reliability when the secondary battery 7407 is bent.

[0477] FIG. 22D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display portion 7102, operation buttons 7103, and a secondary battery 7104. FIG. 22E shows a bent secondary battery 7104. When the secondary battery 7104 is worn on a user's arm in a bent state, the housing deforms, causing a change in the curvature of part or the entire secondary battery 7104. Note that the degree of curvature at any point on the curve, expressed as the radius of the corresponding circle, is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or the entire main surface of the housing or the secondary battery 7104 changes when the radius of curvature is in the range of 40 mm to 150 mm. High reliability can be maintained when the radius of curvature of the main surface of the secondary battery 7104 is in the range of 40 mm to 150 mm. By using the secondary battery of one embodiment of the present invention as the secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0478] 22F shows an example of a wristwatch-type portable information terminal 7200. The portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input / output terminal 7206, and the like.

[0479] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0480] The display surface of the display portion 7202 is curved, and a display can be performed along the curved display surface. The display portion 7202 also includes a touch sensor, and can be operated by touching the screen with a finger or a stylus. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0481] The operation button 7205 can have 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 7205 can be freely set by an operating system incorporated in the portable information terminal 7200.

[0482] The portable information terminal 7200 is also capable of performing standardized short-range wireless communication. For example, hands-free conversation is possible by communicating with a wirelessly enabled headset.

[0483] The portable information terminal 7200 also includes an input / output terminal 7206, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal 7206. Note that charging may be performed by wireless power supply without using the input / output terminal 7206.

[0484] The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a lightweight portable information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 22E can be incorporated into the housing 7201 in a curved state or into the band 7203 in a bendable state.

[0485] The portable information terminal 7200 preferably has a sensor. For example, a fingerprint sensor, a pulse sensor, a human body sensor such as a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted as the sensor.

[0486] 22G illustrates an example of an armband-type display device. The display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can also be provided with a touch sensor in the display portion 7304 and can also function as a portable information terminal.

[0487] The display surface of the display portion 7304 is curved, and display can be performed along the curved display surface. The display state of the display device 7300 can be changed by short-range wireless communication according to a communication standard.

[0488] The display device 7300 also includes an input / output terminal, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal. Note that charging may be performed by wireless power supply without using the input / output terminal.

[0489] By using the secondary battery of one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight display device with a long lifetime can be provided.

[0490] An example in which the secondary battery having good cycle characteristics shown in the above embodiment is mounted on an electronic device will be described with reference to FIGS. 22H, 23, and 24. FIG.

[0491] By using a secondary battery of one embodiment of the present invention as a secondary battery in daily electronic devices, products that are lightweight and have a long life can be provided. For example, daily electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. For secondary batteries in these products, a stick-shaped secondary battery that is easy for users to hold, small, lightweight, and has a large discharge capacity is desired.

[0492] FIG. 22H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 22H, the electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 that includes a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and / or over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 22H has external terminals so that it can be connected to a charging device. Because the secondary battery 7504 is the tip portion when held, it is desirable that the total length be short and the weight be light. The secondary battery of one embodiment of the present invention has a high discharge capacity and good cycle characteristics, making it possible to provide a compact and lightweight electronic cigarette 7500 that can be used for a long period of time.

[0493] 23A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0494] For example, the secondary battery of one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 23A . The eyeglasses-type device 4000 includes a frame 4000a and a display unit 4000b. Mounting the secondary battery on the temples of the curved frame 4000a makes it possible to provide the eyeglasses-type device 4000 with a lightweight design, a good weight balance, and a long continuous use time. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to a smaller housing can be realized.

[0495] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b and / or the earphone unit 4001c. By including the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to a miniaturized housing can be realized.

[0496] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0497] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. By providing the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0498] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted inside the belt portion 4006a. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0499] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided on the display portion 4005a or the belt portion 4005b. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0500] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0501] Furthermore, since the wristwatch device 4005 is a wearable device that is worn directly on the arm, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0502] FIG. 23B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0503] 23C shows a side view of the display portion 4005a. FIG. 23C shows a state in which a secondary battery 913 is built inside the display portion 4005a. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and is small and lightweight.

[0504] 23D shows an example of a wireless earphone. Here, the wireless earphone is shown having a pair of main bodies 4100a and 4100b, but this does not necessarily have to be a pair.

[0505] The main bodies 4100a and 4100b each have a driver unit 4101, an antenna 4102, and a secondary battery 4103. They may also have a display unit 4104. They also preferably have a substrate on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. They may also have a microphone.

[0506] The case 4110 has a secondary battery 4111. It is preferable that the case 4110 also has a board on which circuits such as a wireless IC and a charge control IC are mounted, and a charging terminal. It may also have a display unit, buttons, and the like.

[0507] The main units 4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows sound data and the like sent from other electronic devices to be played back on the main units 4100a and 4100b. If the main units 4100a and 4100b have microphones, the sound picked up by the microphones can be sent to the other electronic device, and the sound data after processing by the electronic device can be sent back to the main units 4100a and 4100b for playback. This allows the devices to be used as, for example, translation devices.

[0508] The secondary battery 4103 included in the main body 4100a can be charged from the secondary battery 4111 included in the case 4110. The coin-type secondary battery, the cylindrical secondary battery, or the like described in the above embodiments can be used as the secondary battery 4111 and the secondary battery 4103. A secondary battery using the positive electrode active material 100 obtained in Embodiment 1 as a positive electrode has high energy density. By using the secondary battery 4103 and the secondary battery 4111 as the secondary battery, a configuration that can accommodate space saving associated with miniaturization of wireless earphones can be realized.

[0509] 24A 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 secondary 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.

[0510] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to 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 rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component therein. By using the secondary battery 6306 according to one embodiment of the present invention in the cleaning robot 6300, the cleaning robot 6300 can be a highly reliable electronic device with a long operating time.

[0511] Fig. 24B shows an example of a robot. A robot 6400 shown in Fig. 24B includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0512] 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.

[0513] 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.

[0514] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. Furthermore, 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.

[0515] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component inside the robot 6400. By using the secondary battery according to one embodiment of the present invention in the robot 6400, the robot 6400 can be a highly reliable electronic device with a long operating time.

[0516] Fig. 24C shows an example of an air vehicle. An air vehicle 6500 shown in Fig. 24C has a propeller 6501, a camera 6502, a secondary battery 6503, and the like, and has the function of flying autonomously.

[0517] For example, image data captured by the camera 6502 is stored in the electronic component 6504. The electronic component 6504 can analyze the image data and detect the presence or absence of an obstacle when moving. Furthermore, the electronic component 6504 can estimate the remaining battery charge from a change in the storage capacity of the secondary battery 6503. The flying object 6500 includes the secondary battery 6503 according to one embodiment of the present invention therein. By using the secondary battery according to one embodiment of the present invention in the flying object 6500, the flying object 6500 can be an electronic device with a long operating time and high reliability.

[0518] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0519] Embodiment 5 In this embodiment, an example in which a secondary battery including the positive electrode active material of one embodiment of the present invention is mounted on a vehicle will be described.

[0520] When a secondary battery is installed in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), or plug-in hybrid vehicles (PHVs) can be realized.

[0521] FIG. 25 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 shown in FIG. 25A is an electric automobile using 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. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Furthermore, the automobile 8400 includes a secondary battery. For example, secondary battery modules can be arranged on the floor of the interior of the vehicle. The secondary battery not only drives the electric motor 8406 but also supplies power to light-emitting devices such as a headlight 8401 and a room light (not shown).

[0522] The secondary battery can also supply power to display devices such as a speedometer and a tachometer included in the automobile 8400. The secondary battery can also supply power to semiconductor devices such as a navigation system included in the automobile 8400.

[0523] The automobile 8500 shown in FIG. 25B can charge its secondary battery by receiving power from an external charging facility using a plug-in system and / or a wireless power supply system. FIG. 25B shows a state in which a ground-mounted charging device 8021 charges a secondary battery 8024 mounted on the automobile 8500 via a cable 8022. The charging method and connector specifications may be determined as appropriate using a predetermined system, such as CHAdeMO (registered trademark) or Combo. The charging device 8021 may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge the secondary battery 8024 mounted on the automobile 8500 using external power supply. Charging can be performed by converting AC power to DC power using a conversion device, such as an AC-DC converter.

[0524] 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 the road and / or 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 vehicles. Furthermore, a solar cell can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped and / or moving. For such contactless power supply, an electromagnetic induction method and / or a magnetic field resonance method can be used.

[0525] 25C shows an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 25C includes a secondary battery 8602, a side mirror 8601, and a turn signal light 8603. The secondary battery 8602 can supply electricity to the turn signal light 8603.

[0526] 25C is capable of storing a secondary battery 8602 in under-seat storage 8604. Even if under-seat storage 8604 is small, secondary battery 8602 can be stored in under-seat storage 8604. Secondary battery 8602 is removable, and when charging, secondary battery 8602 can be carried indoors, charged, and then stored before riding.

[0527] According to one aspect of the present invention, the cycle characteristics of the secondary battery are improved, and the discharge capacity of the secondary battery can be increased. Therefore, the secondary battery itself can be made smaller and lighter. If the secondary battery itself can be made smaller and lighter, it will contribute to reducing the weight of the vehicle, thereby improving the cruising range. Furthermore, the secondary battery installed in the vehicle can also be used as a power supply source for purposes other than the vehicle. In this case, for example, it is possible to avoid using a commercial power source during peak power demand. Avoiding the use of a commercial power source during peak power demand can contribute to energy conservation and reduction of carbon dioxide emissions. Furthermore, if the cycle characteristics are good, the secondary battery can be used for a long period of time, and the amount of rare metals used, such as cobalt, can be reduced.

[0528] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0529] (Embodiment 6) In this embodiment, thermal runaway of a secondary battery and a nail penetration test will be described, and the principle that a secondary battery using the positive electrode active material 100 of one embodiment of the present invention is unlikely to catch fire when subjected to a nail penetration test will be described.

[0530] <Thermal runaway of secondary batteries> The graph shown on page 69 [Figure 2-11] of Non-Patent Document 4 is quoted and partially modified to be shown in Figure 26. Figure 26 is a graph of the internal temperature (hereinafter simply referred to as temperature) of a secondary battery versus time, and shows that as the temperature rises, it passes through several states before reaching thermal runaway.

[0531] When the temperature of the secondary battery reaches or approaches 100°C, (1) the SEI (Solid Electrolyte Interface) of the negative electrode collapses and heat is generated. Also, when the temperature of the secondary battery exceeds 100°C, (2) the negative electrode (when graphite is used, the negative electrode becomes C ６Li) and heat generation occurs, and (3) oxidation of the electrolyte by the positive electrode occurs and heat generation occurs. Then, when the temperature of the secondary battery reaches or near 180°C, (4) thermal decomposition of the electrolyte occurs, and (5) oxygen release and thermal decomposition from the positive electrode (this thermal decomposition includes a structural change of the positive electrode active material) occurs. Thereafter, when the temperature of the secondary battery exceeds 200°C, (6) decomposition of the negative electrode occurs, and finally (7) direct contact between the positive electrode and the negative electrode occurs. The secondary battery reaches thermal runaway through the above-mentioned states (5), (6), and (7). That is, to prevent thermal runaway, it is preferable to suppress the temperature rise of the secondary battery and maintain a stable state of the negative electrode, positive electrode, and / or electrolyte at high temperatures exceeding 100°C.

[0532] The cathode active material 100 according to one embodiment of the present invention has a stable crystal structure and exhibits the effect of suppressing oxygen desorption. Therefore, a secondary battery using the cathode active material 100 is considered to suppress the temperature rise of the secondary battery without reaching at least the state (5) and subsequent states, thereby exhibiting the remarkable effect of being less likely to experience thermal runaway. This can be evaluated, for example, by differential scanning calorimetry (DSC) measurements of a sample in which an electrolyte solution is added to a cathode having the cathode active material 100, or by a nail penetration test of a secondary battery having the cathode active material 100.

[0533] The peak appearing between 250°C and 300°C in DSC measurement is presumed to be due to oxygen release from the positive electrode active material and subsequent thermal decomposition. The higher the temperature at which this peak appears and the higher the temperature at which the maximum value is shown, the higher the thermal stability. For example, in DSC measurement of a sample in which a positive electrode having the positive electrode active material 100 is placed in an electrolyte, the peak appearing between 250°C and 300°C shows a maximum temperature of preferably 260°C or higher, more preferably 270°C or higher. Furthermore, it is preferable that the heat flow per weight of the positive electrode active material at the time of the maximum value is small.

[0534] Furthermore, the temperature rise of the secondary battery when the nail penetration test is conducted, i.e., the difference between the temperature before the nail penetration test and the maximum temperature reached after the nail penetration test, is preferably 130°C or less, more preferably 100°C or less, even more preferably 80°C or less, and even more preferably 60°C or less.

[0535] <Nail Penetration Test> Next, the nail penetration test will be described with reference to FIGS. 27A to 27C . The nail penetration test is a test in which a secondary battery 500 is fully charged (States of Charge: a state equivalent to 100% SOC) and a nail 1003 having a predetermined diameter selected from 2 mm to 10 mm is inserted into the secondary battery 500 at a predetermined speed selected from 1 mm / s to 20 mm / s. FIG. 27A shows a cross-sectional view of the secondary battery 500 with the nail 1003 inserted. The secondary battery 500 has a structure in which a positive electrode 503, a separator 507, a negative electrode 506, and an electrolyte 530 are housed in an exterior case 531. The positive electrode 503 has a positive electrode current collector 501 and positive electrode active material layers 502 formed on both sides thereof. The negative electrode 506 has a negative electrode current collector 504 and negative electrode active material layers 505 formed on both sides thereof. FIG. 27B shows an enlarged view of nail 1003 and positive electrode current collector 501, and clearly shows positive electrode active material 100 which is one embodiment of the present invention and conductive material 553 contained in positive electrode active material layer 502.

[0536] As shown in Figures 27A and 27B, when the nail 1003 penetrates the positive electrode 503 and the negative electrode 506, an internal short circuit occurs. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode, and electrons (e − ) flows to the positive electrode 503, and Joule heat is generated at the internal short circuit location and its vicinity. In addition, due to the internal short circuit, carrier ions, typically lithium ions (Li ＋ ) are released into the electrolyte as shown by the white arrow. However, before all the lithium ions are released from the negative electrode, the battery temperature rises rapidly due to Joule heat generated by the internal short circuit, and the electrolyte begins to reduce and decompose on the surface of the negative electrode. This is called the reduction reaction of the electrolyte by the negative electrode. Then, the electrons (e −), the transition metal M, which was tetravalent in the NCM in the charged state, is reduced to trivalent or divalent, and oxygen is released from the NCM by this reduction reaction, and the electrolyte 530 is further decomposed by the released oxygen, etc. This is called an oxidation reaction of the electrolyte by the positive electrode.

[0537] Furthermore, when an internal short circuit occurs in a secondary battery, the temperature changes as shown in the graph in Figure 28. Figure 28 is a partially modified version of the graph shown on page 70 [Figure 2-12] of Non-Patent Document 4, and is a graph of the temperature of the secondary battery against time, showing that when an internal short circuit occurs at (P0), the temperature of the secondary battery rises over time. Specifically, as shown in (P1), heat generation due to Joule heat continues, and when the temperature of the secondary battery reaches or near 100°C, it exceeds the reference temperature (Ts) of the secondary battery. Then, at (P2), the negative electrode (when graphite is used, the negative electrode is C ６ In (P3), the electrolyte is oxidized by the positive electrode and heat is generated, and in (P4), heat is generated due to thermal decomposition of the electrolyte. This causes thermal runaway in the secondary battery, leading to fire, etc.

[0538] At this time, in the positive electrode active material, the transition metal M is reduced (for example, cobalt is reduced to Co ４＋ From Co ２＋ This reaction causes oxygen to be released from the positive electrode active material. Because this reaction is exothermic, it acts as a positive feedback loop for thermal runaway. In other words, if this reaction can be suppressed, it will be possible to create a positive electrode active material that is less susceptible to thermal runaway.

[0539] Therefore, it is preferable that the surface layer of the positive electrode active material, which is likely to be the site of the above reaction, has a high concentration of metal that does not easily release oxygen. ４＋ From Co ２＋ The reaction that leads to oxygen release is also suppressed. Metals that do not readily release oxygen are those that form stable metal oxides, such as magnesium and aluminum. Nickel is also thought to have the effect of suppressing oxygen release when present at the lithium site.

[0540] When a nail penetration test was performed on a secondary battery using the positive electrode active material 100 according to one embodiment of the present invention, the positive electrode active material 100 exhibited the unique effect of suppressing oxygen release due to the presence of the barrier film described above, which is believed to suppress the oxidation reaction of the electrolyte and also suppress heat generation. Furthermore, with the positive electrode active material 100, the barrier film on the surface layer has properties similar to an insulator, which is believed to slow the rate at which current flows into the positive electrode in the event of an internal short circuit. This is expected to result in significant effects such as reduced thermal runaway and reduced risk of fire.

[0541] Furthermore, even if a transition metal M such as cobalt is reduced, if lithium ions can be inserted into the positive electrode active material before oxygen is released, electrical neutrality is maintained, and an exothermic reaction accompanied by oxygen release does not occur. Therefore, even if electrons suddenly flow into the positive electrode active material, it is sufficient if the crystal structure of the positive electrode active material is kept stable until the lithium ions are inserted into the positive electrode active material from the negative electrode via the electrolyte.

[0542] In this example, a positive electrode active material 100 according to one embodiment of the present invention was fabricated and its characteristics were analyzed.

[0543] <Preparation of Positive Electrode Active Material> Samples prepared in this example will be described with reference to the preparation method shown in FIGS. 18 and 19.

[0544] LiCoO in step S14 of FIG. ２ As the initial heating step in step S15, a commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) containing cobalt as the transition metal M and no additional elements was prepared. The lithium cobalt oxide was placed in a crucible, which was then covered with a lid and heated in a muffle furnace at 850°C for 2 hours. After the muffle furnace was placed in an oxygen atmosphere, no flow (O ２ (Purge). Checking the amount recovered after the initial heating revealed that the weight had decreased slightly. This decrease in weight may have been due to the removal of impurities such as lithium carbonate from the lithium cobalt oxide.

[0545] 19A and 19C, Mg, F, Ni, and Al were added separately as additive elements. According to step S21 shown in FIG. 19A, LiF was prepared as the F source, and MgF was prepared as the Mg source. ２ LiF:MgF was prepared. ２ were weighed out so that the molar ratio was 1:3. Next, LiF and MgF were dissolved in dehydrated acetone. ２ The above ingredients were mixed and stirred at a rotation speed of 400 rpm for 12 hours to prepare an additive element source (Al source). A ball mill was used for mixing, and zirconium oxide balls were used as the grinding medium. 20 mL of dehydrated acetone, 22 g of zirconium oxide balls (1 mm diameter), and a total of approximately 9 g of F source and Mg source were added to a 45 mL capacity ball mill and mixed. The mixture was then sieved through a 300 μm mesh sieve to obtain the Al source.

[0546] Next, in step S31, the Al source was weighed out so that the cobalt content was 1 mol % and dry-mixed with the initially heated lithium cobalt oxide. The mixture was stirred at a rotation speed of 150 rpm for 1 hour, which was a gentler stirring condition than when obtaining the Al source. Finally, the mixture was sieved with a 300 μm mesh to obtain a mixture 903 with a uniform particle size (step S32).

[0547] Next, in step S33, the mixture 903 was heated. The heating conditions were 900°C and 20 hours. During heating, a lid was placed on the crucible containing the mixture 903. The crucible was filled with an oxygen-containing atmosphere, and the inflow and outflow of oxygen was blocked (purging). A composite oxide containing Mg and F was obtained by heating (step S34a).

[0548] Next, in step S51, the composite oxide and the additive element source (A2 source) were mixed. Following step S41 shown in FIG. 19B, nickel hydroxide was prepared as the nickel source by pulverization, and aluminum hydroxide was prepared as the aluminum source by pulverization. Nickel hydroxide and aluminum hydroxide were weighed out so that they constituted 0.5 mol% of the lithium cobalt oxide and 0.5 mol% of the lithium cobalt oxide, respectively, and then dry-mixed with the composite oxide. The mixture was stirred for one hour at a rotation speed of 150 rpm. A ball mill was used for mixing, and zirconium oxide balls were used as the milling media. Approximately 7.5 g of the composite oxide, nickel source, and aluminum source were mixed with 22 g of zirconium oxide balls (1 mm diameter) in a 45 mL capacity ball mill. These stirring conditions were gentler than those used to obtain the A1 source. Finally, the mixture was sieved through a 300 μm mesh sieve to obtain a mixture 904 with uniform particle size (step S52).

[0549] Next, in step S53, the mixture 904 was heated. The heating conditions were 850°C and 10 hours. During heating, a lid was placed on the crucible containing the mixture 904. The crucible was filled with an oxygen-containing atmosphere, and the inflow and outflow of the oxygen was blocked (purging). By heating, lithium cobalt oxide containing Mg, F, Ni, and Al was obtained (step S54). The positive electrode active material (composite oxide) obtained in this manner was designated as Sample 1-1.

[0550] The sample that was not subjected to the heating in step S15 was designated as Sample 2. In Sample 2, the flow rate of oxygen was set to 10 L / min during the heating in step S53.

[0551] As a comparative example, sample 10 was prepared using lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) that had not undergone any particular treatment.

[0552] Further, sample 11 was prepared by simply subjecting lithium cobalt oxide to the heating in step S15.

[0553] The preparation conditions for Samples 1-1, 10 and 11 are shown in Table 2.

[0554]

[0555] <STEM and EDX (Energy Dispersive X-ray Analysis)> Next, area analysis (e.g., element mapping) and electron beam diffraction were performed by STEM-EDX for Sample 10, Sample 11, and Sample 1-1. Electron beam diffraction was also performed for Sample 2.

[0556] As a pretreatment before analysis, each sample was thinned by the FIB method (μ-sampling method).

[0557] STEM and EDX were performed using the following equipment and conditions.

[0558] <STEM observation> Scanning transmission electron microscope: JEOL JEM-ARM200F Observation conditions Acceleration voltage: 200 kV Magnification accuracy: ±10%

[0559] EDX Analysis method: Energy dispersive X-ray spectroscopy (EDX) Scanning transmission electron microscope: JEOL JEM-ARM200F Acceleration voltage: 200 kV Beam diameter: Approximately 0.1 nmφ Elemental analyzer: JED-2300T X-ray detector: Si drift detector Energy resolution: Approximately 140 eV X-ray take-off angle: 21.9° Solid angle: 0.98 sr Number of captured pixels: 128 x 128

[0560] HAADF-STEM images of sample 10 are shown in Figures 29A and 29B. Figure 29A shows the surface and surface layer with a (001) orientation, while Figure 29B shows the surface and surface layer with a non-(001) orientation. Both were observed to have a layered rock salt crystal structure. Microelectron beam diffraction patterns were obtained at points 1-1 to 1-3 and points 2-1 to 2-3 in the figures. The d values, plane angles, and lattice constants calculated for the space group R-3m are shown in Table 4.

[0561] Similarly, HAADF-STEM images of sample 11 are shown in Figures 30A and 30B. Figure 30A shows the surface and surface layer with a (001) orientation, while Figure 30B shows the surface and surface layer with a non-(001) orientation. Both were observed to have a layered rock salt crystal structure. Microelectron beam diffraction patterns were obtained at points 3-1 to 3-3 and points 4-1 to 4-3 in the figures. The d values, plane angles, and lattice constants calculated for the space group R-3m are shown in Table 4.

[0562] Fig. 31A shows an HAADF-STEM image of the (001) oriented surface and surface layer of Sample 1-1. The points in Fig. 31A where the electron microbeam diffraction patterns were obtained are indicated by points 3-1 to 3-3 in Fig. 31B.

[0563] FIG. 30A shows the electron microbeam diffraction pattern of point 3-1 in FIG. 31B, and the diffraction spots used to determine the d value and plane angle are circled in FIG. 30B. The literature values ​​for lithium cobalt oxide (rhombohedral) are also shown. FIG. 31A shows the electron microbeam diffraction pattern of point 3-2 in FIG. 31B, and the diffraction spots used to determine the d value and plane angle are circled in FIG. 31B. FIG. 32A shows the electron microbeam diffraction pattern of point 3-3 in FIG. 31B, and the diffraction spots used to determine the d value and plane angle are circled in FIG. 32B. The lattice constants calculated for these d values, plane angles, and space group R-3m are shown in Table 4.

[0564] 33A shows a HAADF-STEM image of the (001) oriented surface and surface layer of Sample 1-1. EDX area analysis of this region detected C, O, F, Mg, Al, Si, Ca, Co, and Ga. Ga was thought to be derived from FIB processing. Si and Ca were found to be the LiCoO ２ It is thought that trace amounts of elements contained in the SiO2 layer have become unevenly distributed on the surface. Mapping images of the major elements cobalt and oxygen, as well as magnesium, aluminum, and silicon, which were confirmed to be unevenly distributed, are shown in Figures 33B to 33F.

[0565] FIG. 34A shows a HAADF-STEM image of the (001)-oriented surface and surface layer of sample 1-1, with the arrow indicating the scanning direction of the STEM-EDX ray analysis. FIG. 34B shows a profile of the STEM-EDX ray analysis of the region. The vertical axis represents counts, and the horizontal axis represents distance. FIG. 35 shows a diagram enlarged in the vertical direction from FIG. 34B. Furthermore, excerpts of the cobalt and magnesium profiles from FIG. 35 are shown in FIG. 36, and excerpts of the cobalt, aluminum, and fluorine profiles are shown in FIG. 37.

[0566] From the profiles in Figures 34B to 37, the reference point was estimated to be a point at a distance of 7.95 nm. Specifically, the region avoiding the vicinity where the detected amount of cobalt begins to increase was set at a distance of 0.25 to 3.49 nm in Figures 34B and 35. The region where the counts of cobalt and oxygen become saturated and stable was set at a distance of 56.1 to 59.3 nm. Co, which is a transition metal M, was adopted, and M ＡＶＥ and M ＢＧ Calculating the 50% point of the sum of these values ​​gives 1408.1 Counts, and estimating the reference point by finding a regression line gives 7.95 nm. An error of 1 nm is considered.

[0567] Next, a HAADF-STEM image of the surface and surface layer portion of sample 1-1 that is not (001) oriented is shown in Fig. 38A. The points in Fig. 38A where the electron microbeam diffraction patterns were obtained are shown as points 4-1 to 4-3 in Fig. 38B.

[0568] Figure 39A shows the electron microbeam diffraction pattern of point 4-1 in Figure 38B, and the diffraction spots used to determine the d value and plane angle are circled in Figure 39B. Literature values ​​for lithium cobalt oxide are also shown. Figure 40A shows the electron microbeam diffraction pattern of point 4-2 in Figure 38B, and the diffraction spots used to determine the d value and plane angle are circled in Figure 40B. Figure 41A shows the electron microbeam diffraction pattern of point 4-3 in Figure 38B, and the diffraction spots used to determine the d value and plane angle are circled in Figure 41B. Table 4 shows the lattice constants calculated for these d values, plane angles, and space group R-3m.

[0569] 42A shows a HAADF-STEM image of the non-(001) oriented surface and surface layer of Sample 1-1. EDX area analysis of this region detected C, O, F, Mg, Al, Si, Co, Ni, and Ga. Mapping images of the major element cobalt, as well as silicon, magnesium, aluminum, and nickel, which were confirmed to be significantly distributed unevenly, are shown in FIGS. 42B to 42F.

[0570] Furthermore, Figure 43A shows a HAADF-STEM image of the non-(001) oriented surface and surface layer of Sample 1-1, with the arrow indicating the scanning direction of the STEM-EDX ray analysis. Figure 43B shows a STEM-EDX ray analysis profile of the region. Figure 44 shows an enlarged view of Figure 43B in the vertical direction. Furthermore, excerpts of the cobalt and magnesium profiles from Figure 44 are shown in Figure 45, excerpts of the cobalt and nickel profiles are shown in Figure 46, and excerpts of the cobalt, aluminum, and fluorine profiles are shown in Figure 47.

[0571] From the profiles in Figures 43B to 47, the reference point was estimated to be a distance of 7.45 nm. Specifically, the region avoiding the vicinity where the detected amount of cobalt begins to increase was set to 0.25 to 3.49 nm in Figures 43B and 44. The region where the counts of cobalt and oxygen became saturated and stable was set to a distance of 56.1 to 59.3 nm. Co, a transition metal M, was adopted, and M ＡＶＥ and M ＢＧ The 50% point of the sum of these values ​​is calculated to be 1749.0 Counts, and the regression line is calculated to estimate the reference point to be 7.45 nm. An error of 1 nm is considered.

[0572] Comparing the above-mentioned (001) oriented surface and the surface not oriented in (001) revealed the following.

[0573] Nickel was not detected on the (001)-oriented surface, but was detected on the non-(001)-oriented surface. The ratio of manganese and aluminum to cobalt differed between the (001)-oriented surface and the non-(001)-oriented surface.

[0574] More specifically, the intensity ratio of the additive element to cobalt was Mg / Co=0.07 and Al / Co=0.06 on the (001) oriented surface, and the half-width of the magnesium distribution was 1.38 nm.

[0575] On the other hand, in the planes not oriented in the (001) direction, the intensity ratios were Mg / Co = 0.14, Al / Co = 0.04, and Ni / Co = 0.05. The half-width of the magnesium distribution was 1.90 nm, and the half-width of the nickel distribution was 1.67 nm.

[0576] Furthermore, on the surface that was not (001) oriented, nickel was more abundant than aluminum, and magnesium was more abundant than nickel, distributed closer to the surface.

[0577] Furthermore, since the Al / Co intensity ratio is smaller on the non-(001) oriented surface than on the (001) oriented surface, it was presumed that aluminum diffused into the positive electrode active material on the non-(001) oriented surface.

[0578] In addition, magnesium was more distributed toward the surface than aluminum on the surface with either orientation. As is clear from the half-width above, the magnesium distribution had a sharper shape than the aluminum distribution. Fluorine was also detected on the surface with either orientation.

[0579] Next, HAADF-STEM images of the (001)-oriented surface and surface layer portion of Sample 2 are shown in Figures 48A and 48B. In these figures, points 1 and 2 indicate the locations where the electron microbeam diffraction patterns were obtained. Although not shown, an electron microbeam diffraction pattern was also obtained from the internal region of Sample 2. The d values, plane angles, and lattice constants calculated as the space group R-3m obtained from these are shown in Table 3.

[0580]

[0581] <<Electron microbeam diffraction pattern>> The lattice constants shown in Table 3 were calculated from the electron microbeam diffraction patterns and cannot be directly compared with the lattice constants calculated from the XRD patterns. However, it is possible to compare the lattice constants calculated from the electron microbeam diffraction patterns, and it can be said that they show the characteristics of each sample.

[0582] As shown in Table 3, the lattice constant was large at point 1, which is closest to the surface of sample 2. Therefore, there was a large difference in the lattice constant between the measurement point closest to the surface and the measurement point further inside. This is thought to be because the characteristics of rock-salt crystal structures, including magnesium oxide, are strongly observed in the surface layer.

[0583] On the other hand, for sample 1-1, no significant difference in lattice constant was observed at any measurement point, and the ultrafine electron beam diffraction pattern showed strong characteristics of a layered rock salt type even at the measurement point closest to the surface. This is presumed to be because the rock salt type cobalt oxide (CoO) and other elements were restored to a layered rock salt type crystal structure by the initial heating.

[0584] More specifically, in sample 2, the lattice constant at point 1 (measurement point at a depth of 1 nm or less from the surface) was larger by 0.13 Å for the a-axis and 1.14 Å for the c-axis than that at point 2 (measurement point at a depth of 3 nm to 10 nm).

[0585] On the other hand, for sample 1-1, the difference in lattice constant between the measurement point at a depth of 1 nm or less from the surface and the measurement point at a depth of 3 nm to 10 nm was 0.04 Å or less for the a-axis and 0.3 Å or less for the c-axis.

[0586] Even in the ultrafine electron diffraction pattern of a region 1 nm or less deep from the surface, such as sample 1-1, it was shown that by maintaining the same lattice constant and characteristics of the layered rock-salt crystal structure as the interior, the function of stabilizing the crystal structure of the surface layer became stronger. This was presumed to be due to the effective insertion of added elements such as magnesium into the lithium sites of the surface layer.

[0587] In this example, a positive electrode active material was prepared, and its crystal structure after charging was analyzed by XRD.

[0588] <Preparation of Positive Electrode Active Material> Samples prepared in this example will be described with reference to the preparation method shown in FIGS.

[0589] First, LiCoO in step S14 of FIG. ２As the starting material, commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) containing cobalt as the transition metal M and not particularly containing magnesium, fluorine, aluminum, etc. was prepared. No initial heating was performed.

[0590] As in step S20 shown in FIG. 17B, LiCoO ２ Nickel and aluminum were added as additive elements to the lithium cobalt oxide (step S31). Nickel hydroxide was prepared as the nickel source, and aluminum hydroxide was prepared as the aluminum source. The nickel hydroxide and aluminum hydroxide were weighed out so that they accounted for 0.5 mol % of the lithium cobalt oxide, and the aluminum hydroxide was weighed out so that they accounted for 0.5 mol % of the lithium cobalt oxide, and the nickel hydroxide and aluminum hydroxide were dry-mixed with the composite oxide (step S31), to obtain a mixture 903 (step S32).

[0591] Next, in step S33, mixture 903 was heated. The heating conditions were 850°C and 10 hours. During heating, a lid was placed on the crucible containing mixture 904. Lithium cobalt oxide containing nickel and aluminum was obtained by heating (step S34). The other production conditions were the same as in Example 1. The positive electrode active material (composite oxide) obtained in this manner was designated Sample 21.

[0592] Sample 22 was prepared in the same manner as Sample 21 except that only aluminum was added as an additive element.

[0593] Sample 23 was prepared in the same manner as Sample 21 except that only nickel was added as an additive element.

[0594] In addition, fluorine and magnesium are used as additive elements, LiF is used as the fluorine source, and MgF is used as the magnesium source. ２ The above was prepared to prepare an additive element source (A source), and the A source was mixed to be 0.5 mol % of lithium cobalt oxide. Sample 24 was prepared in the same manner as Sample 21, except that the heating conditions were 850° C. and 60 hours.

[0595] Sample 25 was prepared in the same manner as Sample 21, except that only magnesium was used as the additive element, magnesium hydroxide was prepared as the magnesium source, and the magnesium hydroxide was mixed so as to account for 0.5 mol % of the lithium cobalt oxide.

[0596] Sample 26 was prepared in the same manner as Sample 21, except that only fluorine was used as the additive element, lithium fluoride was prepared as the fluorine source, and the lithium fluoride was mixed so that the lithium fluoride accounted for 1.17 mol % of the lithium cobalt oxide.

[0597] Next, in step S20a shown in Fig. 18, a magnesium source and a fluorine source were added, and in step S40, a nickel source and an aluminum source were added to obtain sample 27. Specifically, LiF was used as the fluorine source and MgF was used as the magnesium source. ２ was prepared to prepare an additive element source (A source), and LiCoO was added to the A source so that the A source was 2 mol % of lithium cobalt oxide. ２ After mixing the resulting composite oxide with nickel hydroxide, the mixture was heated at 850°C for 60 hours. ９ H ２１ AlO ３ The mixture was mixed with an isopropanol solution containing 1,000,000 sachets of sachets, and the sol-gel reaction was carried out in an air atmosphere for 17 hours. The mixture was then dried in a ventilated oven at 80°C for 3 hours. The heating conditions were 850°C and 2 hours. The other conditions were the same as those for Sample 21.

[0598] The preparation conditions for Samples 21 to 27 are shown in Table 4.

[0599]

[0600] A half cell was assembled using the positive electrode active material prepared above and a positive electrode active material prepared in the same manner as in Sample 1-1 of Example 1. The conditions for the half cell are described below.

[0601] The positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum current collector using NMP as a solvent.

[0602] The slurry was applied to a current collector and dried to obtain a positive electrode. No pressure was applied. The amount of active material carried on the positive electrode was approximately 7 mg / cm. ２ It was decided.

[0603] The electrolyte solution used was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, to which 2 wt% vinylene carbonate (VC) was added as an additive. The electrolyte contained 1 mol / L of lithium hexafluorophosphate (LiPF ６ ) was used. Polypropylene was used as the separator.

[0604] A lithium metal counter electrode was prepared, and a coin-shaped half cell was formed with the above-mentioned positive electrode, etc. XRD measurement was carried out using this after charging.

[0605] The coin cell was charged and subjected to XRD measurement. The charging conditions were CC charging (0.05 C or 0.2 C, voltage 4.6 V, 0.02 C cut), and the charging temperature was 25°C. 1 C = 200 mA / g. The positive electrode of the coin cell charged under the above conditions was sealed in an airtight sample holder (manufactured by Bruker) under an argon atmosphere, and XRD measurement was performed.

[0606] The XRD measurement conditions were as follows: XRD device: D8 ADVANCE manufactured by Bruker AXS X-ray source: Cu Output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15-75° Step width (2θ): set to 0.01° Counting time: 1 second / step Sample stage rotation: 15 rpm

[0607] The obtained XRD patterns were analyzed using the analysis software DIFFRAC.EVA to separate the background and CuKα２ The peaks of the lines were removed under the following conditions: Curvature: 25, Threshold: 1E-5, and Intensity Ratio: 0.5.

[0608] The XRD pattern of Sample 1-1 when charged to 4.7 V is shown in FIG. ２ The patterns of (O3), O3', H1-3, and O1-type crystal structure are also shown. An enlarged view of the range of 18° (deg) or more and 21.5° or less in FIG. 49 is shown in FIG. 50A. An enlarged view of the range of 36° or more and 46° or less in FIG. 49 is shown in FIG. 50B. Charging was performed using CCCV (upper limit voltage 4.7 V, constant current 0.2 C, cut-off current 0.02 C (1 C = 200 mA / g)) at an ambient temperature of 25°C. The charge capacity was 215.3 mAh / g.

[0609] FIG. 51 shows the XRD patterns of Samples 21 to 27 when charged to 4.6 V. The patterns of the H1-3 crystal structure and the O3' crystal structure are also shown. FIG. 52A shows an enlarged view of the range from 18° to 21° in FIG. 51 . FIG. 52B shows an enlarged view of the range from 43° to 47° in FIG. 51 . Samples 22, 23, 25, and 26 were charged using CC charging (constant current of 0.05 C, cut-off voltage of 4.6 V, 1 C = 200 mA / g). Sample 24 was charged using CCCV (constant current of 0.34 C, upper limit voltage of 4.6 V, cut-off current of 0.0068 C, 1 C = 200 mA / g), and...

Claims

1. A lithium-ion secondary battery having a positive electrode, The positive electrode has a positive electrode active material, The positive electrode active material has lithium cobalt oxide containing nickel and magnesium. The amount of nickel detected in the surface layer of the positive electrode active material is greater than the amount of nickel detected inside the positive electrode active material. The amount of magnesium detected in the surface layer of the positive electrode active material is greater than the amount of magnesium detected inside the positive electrode active material. The (001) surface of the positive electrode active material has a lower nickel detection amount compared to other surfaces of the positive electrode active material's surface. A lithium-ion secondary battery in which the distribution of nickel and magnesium superimposes on the surface layer of the positive electrode active material.

2. A lithium-ion secondary battery having a positive electrode, The positive electrode has a positive electrode active material, The positive electrode active material has lithium cobalt oxide containing nickel, magnesium, and fluorine. The amount of nickel detected in the surface layer of the positive electrode active material is greater than the amount of nickel detected inside the positive electrode active material. The amount of magnesium detected in the surface layer of the positive electrode active material is greater than the amount of magnesium detected inside the positive electrode active material. The amount of fluorine detected in the surface layer of the positive electrode active material is greater than the amount of fluorine detected inside the positive electrode active material. The (001) surface of the positive electrode active material has a lower nickel detection amount compared to other surfaces of the positive electrode active material's surface. In the surface layer of the positive electrode active material, the distribution of nickel and the distribution of magnesium are superimposed. A lithium-ion secondary battery in which the distribution of fluorine and magnesium superimposes on the surface layer of the positive electrode active material.

3. In claim 1 or claim 2, In EDX radiation analysis, the surface layer of the positive electrode active material, The depth of the peak in the amount of nickel detected, The difference in peak depth of detected magnesium is within 3 nm in a lithium-ion secondary battery.

4. In claim 3, The positive electrode active material contains aluminum, In the EDX radiation analysis profiles of nickel, magnesium, and aluminum present in the positive electrode active material, The maximum detected amount of aluminum is within the range of the maximum detected amounts of nickel and magnesium. When the peak width at 1 / 5 of the maximum detected aluminum level is divided in half by a perpendicular line drawn from the maximum value to the horizontal axis, Peak width W on the surface side ｓ Rather, Internal peak width W ｃ Large lithium-ion rechargeable batteries.

5. In any one of claims 1 to 4, In a battery in which lithium is used for both the positive and counter electrodes, when the battery is charged to 4.6V, the positive electrode active material is such that when the positive electrode is analyzed by powder X-ray diffraction using CuKα1 rays, the diffraction pattern is at least 2θ 19.13 or higher and less than 19.37, The peak is between 45.37° and less than 45.57°. Lithium-ion rechargeable battery.