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

Abstract

To provide a lithium ion secondary battery having excellent charge / discharge cycle characteristics.SOLUTION: A lithium ion secondary battery including a positive electrode and a negative electrode, in which the positive electrode includes a positive electrode active material particle containing magnesium, fluorine, and lithium cobalt oxide, and the positive electrode active material particle includes a region where magnesium is substituted for part of cobalt sites of a second layer to a sixth layer when a surface observed in a cross-sectional STEM image of a plane where lithium is inserted and extracted is a first layer. Magnesium has a function of relieving distortion between the layered rock-salt crystal structure and the rock-salt crystal structure, and fluorine has a function of promoting movement of magnesium into the positive electrode active material particle.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] One aspect 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. Another aspect 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. [Background technology]

[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. To meet this demand, there has been active research into improving the positive electrode active material of the positive electrode of secondary batteries (e.g., Patent Documents 1 and 2). Research is also being conducted on the crystalline structure of positive electrode active materials (Non-Patent Documents 1 to 3).

[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 4. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 5 can be referenced from ICSD. For Rietveld analysis, the analysis program RIETAN-FP (Non-Patent Document 6) can be used. VESTA (Non-Patent Document 7) can be used as software for drawing crystal structures.

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

[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 11) can be used to analyze the electron diffraction pattern. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2018-206747 [Patent Document 2] Japanese Patent Publication No. 2022-070247 [Non-patent literature]

[0009] [Non-Patent Document 1] 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.17340-17348 [Non-patent document 2] T. Motohashi, et al., “Electronic phase diagram of the layered cobalt oxide system LixCoO2(0.0≦x≦1.0)”, Physical Review B, 80(16);165114 [Non-patent document 3] Zhaohui Chen et al., “Staging Phase Transitions in LixCoO2”, Journal of The Electrochemical Society, 2002, 149(12)A1604-A1609 [Non-patent document 4] A. Belsky, et al., “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Cryst., (2002) B58 364-369. [Non-patent document 5] J. Akimoto, Y. Gotoh, Y. Oosawa, “Synthesis and structural refinement of LiCoO2 single crystals” Journal of Solid State Chemistry (1998) 141, p. 298-302. [Non-patent document 6] F.Izumi and K.Momma,“Three-Dimensional Visualization in Powder Diffraction” Solid State Phenom.130,15-20(2007);

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

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

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

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

[0014] One embodiment of the present invention is a lithium-ion secondary battery that includes a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide. When a first layer is defined as a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted, the positive electrode active material particles include a region in which magnesium is substituted at part of cobalt sites of second to sixth layers of the positive electrode active material particle.

[0015] In the above, the positive electrode active material particles preferably have a region where magnesium substitutes for a part of the cobalt sites of the fourth layer observed in a cross-sectional STEM image of the surface where lithium is inserted and desorbed.

[0016] In the above, it is preferable that the positive electrode active material particles have a layered rock salt type crystal structure inside and a rock salt type crystal structure on the surface layer, and that magnesium has the function of alleviating distortion between the layered rock salt type crystal structure and the rock salt type crystal structure.

[0017] Another embodiment of the present invention is a lithium-ion secondary battery that includes a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide. When a first layer is defined as a surface of the positive electrode active material particles observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted, the positive electrode active material particles include regions where magnesium is substituted at part of cobalt sites in second to sixth layers of the positive electrode active material particles, and the fluorine is present on the surface side of the regions.

[0018] In the above, fluorine preferably has the function of promoting the migration of magnesium into the inside of the positive electrode active material particles.

[0019] Another embodiment of the present invention is a lithium-ion secondary battery that includes a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide. When a first layer is defined as a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted, the positive electrode active material particles include regions in which magnesium is substituted at some of the cobalt sites of second to sixth layers of the positive electrode active material particle. Rock salt structures are scattered in the first to third layers observed in the cross-sectional STEM image of the surface where lithium is inserted and extracted.

[0020] Another embodiment of the present invention is a lithium-ion secondary battery including a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles containing magnesium, fluorine, aluminum, and lithium cobalt oxide. When a first layer is defined as a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted, the positive electrode active material particles include regions in which magnesium substitutes for some of the cobalt sites of second to sixth layers of the positive electrode active material particle. The positive electrode active material particles have a layered rock-salt crystal structure inside, and the aluminum is present inside the positive electrode active material particle.

[0021] In the above, aluminum preferably has a function of mitigating volume changes of the layered rock salt type crystal structure caused by charging and discharging.

[0022] Another embodiment of the present invention is a lithium-ion secondary battery including a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles containing magnesium, fluorine, nickel, and lithium cobalt oxide. When a first layer is defined as a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted, the positive electrode active material particles have regions in which magnesium substitutes for some of the cobalt sites of second to sixth layers of the positive electrode active material particle. The positive electrode active material particles have a layered rock-salt crystal structure inside and a rock-salt crystal structure in a surface layer portion, and the nickel is present in the surface layer portion of the positive electrode active material particle.

[0023] In the above, nickel preferably has a function of suppressing oxygen desorption and thereby suppressing the phase change from the layered rock salt type to the spinel type.

[0024] Another embodiment of the present invention is a lithium-ion secondary battery including a positive electrode and a negative electrode. The positive electrode includes positive electrode active material particles containing magnesium, fluorine, aluminum, nickel, and lithium cobalt oxide. The positive electrode active material particles have a layered rock-salt crystal structure inside and a rock-salt crystal structure on a surface layer portion. When a first layer is defined as a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted, the positive electrode active material particles have a region where magnesium is substituted at part of cobalt sites in second to sixth layers of the positive electrode active material particle. The fluorine is present on the surface side of the region. The aluminum is present inside the positive electrode active material particle. The nickel is present on the surface layer portion of the positive electrode active material particle.

[0025] In the above, when STEM-EDX ray analysis is performed on the positive electrode active material particles, the position at which the concentration of each element in the surface layer is maximum is preferably located closer to the center than the position at which the concentration of fluorine is maximum (atomic %).

[0026] Furthermore, in the above, the positive electrode active material particles have a layered rock salt type crystal structure belonging to the space group R-3m in a discharged state, and when the positive electrode active material particles are used for the positive electrode, lithium metal is used for the negative electrode, and a mixture of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and 2 wt % vinylene carbonate is used as an electrolyte, charging is performed under predetermined conditions in a 25°C environment, and then the charged positive electrode is analyzed by powder X-ray diffraction using CuKα1 radiation, the diffraction pattern preferably has a peak in the 2θ range of 19.13° or more and 19.37° or less, and a peak in the 2θ range of 45.37° or more and 45.57° or less. The charging under the specified conditions is performed by first charging at a constant current of 0.5 C (where 1 C = 137 mA / g) up to a voltage of 4.60 V, then charging at a constant voltage until the current value reaches 0.01 C, then resting for 30 minutes, then discharging at a constant current of 0.5 C up to a voltage of 2.5 V, then resting for 30 minutes, then charging at a constant current of 0.5 C up to a voltage of 4.6 V, and then charging at a constant voltage until the current value reaches 0.01 C.

[0027] In the above, it is preferable that, in EPMA of the positive electrode active material particles, the atomic ratio of magnesium to cobalt in the interior, Mg / Co, is 0.01 or more.

[0028] Another aspect of the present invention is a method for producing positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide, wherein the heating time at 650°C or higher exceeds 100 hours in total.

[0029] Another embodiment of the present invention is a method for producing positive electrode active material particles, the method including: mixing lithium cobalt oxide, a magnesium source, a fluorine source, and a lithium source to prepare a mixture; and heating the mixture at a temperature of 650°C to 950°C for a heating time exceeding 100 hours.

[0030] In the above, the heating temperature is preferably 826° C. or higher and 920° C. or lower, and the heating time is preferably more than 100 hours and 150 hours or shorter. [Effects of the Invention]

[0031] According to one embodiment of the present invention, it is possible to provide a positive electrode active material particle or a composite oxide that can be used in a lithium ion secondary battery and that exhibits a suppressed decrease in discharge capacity during charge-discharge cycles. Alternatively, it is possible to provide a positive electrode active material particle or a composite oxide that is less likely to lose its crystal structure even after repeated charge-discharge cycles. Alternatively, it is possible to provide a positive electrode active material particle or a composite oxide that exhibits a large discharge capacity. Alternatively, it is possible to provide a secondary battery or a vehicle that is highly safe or highly reliable.

[0032] According to one embodiment of the present invention, positive electrode active material particles, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0033] 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. [Brief explanation of the drawings]

[0034] [Figure 1] FIG. 1(A) is a cross-sectional view illustrating the internal structure of a secondary battery, and FIG. 1(B) is a cross-sectional view illustrating the positive electrode and electrolyte of the secondary battery. [Figure 2] 2A and 2B are cross-sectional views illustrating a positive electrode active material of one embodiment of the present invention. [Figure 3] 3A to 3F are cross-sectional views illustrating positive electrode active material particles of one embodiment of the present invention. [Figure 4] FIG. 4 is a diagram illustrating the crystal structure of a positive electrode active material particle according to one embodiment of the present invention. [Figure 5] FIG. 5 is a diagram illustrating the crystal structure of a conventional positive electrode active material particle. [Figure 6] FIG. 6 shows the XRD pattern calculated from the crystal structure. [Figure 7] FIG. 7 shows the XRD pattern calculated from the crystal structure. [Figure 8] 8(A) to 8(C) are diagrams illustrating a method for producing positive electrode active material particles. [Figure 9] FIG. 9 is a diagram illustrating a method for producing positive electrode active material particles. [Figure 10] 10(A) and 10(B) are diagrams illustrating a method for producing positive electrode active material particles. [Figure 11] 11A to 11C illustrate a lithium-ion secondary battery of one embodiment of the present invention. [Figure 12] 12A to 12C illustrate an electric vehicle according to one embodiment of the present invention. [Figure 13] 13A to 13E illustrate a vehicle and the like according to one embodiment of the present invention. [Figure 14] 14A to 14D illustrate electronic devices and the like according to one embodiment of the present invention. [Figure 15] 15(A) to 15(C) are diagrams illustrating the results of the STEM-EDX analysis. [Figure 16] 16(A) to 16(C) are diagrams illustrating the results of the STEM-EDX analysis. [Figure 17] 17(A) to 17(C) are diagrams illustrating the results of the STEM-EDX analysis. [Figure 18] 18(A) to 18(C) are diagrams illustrating the results of the STEM-EDX analysis. [Figure 19] 19(A) to 19(C) are diagrams illustrating the results of the STEM-EDX analysis. [Figure 20] 20(A) to 20(C) are diagrams illustrating the results of the STEM-EDX analysis. [Figure 21] 21(A) and 21(B) are diagrams illustrating the results of HAADF-STEM analysis. [Figure 22]22(A) and 22(B) are diagrams illustrating the results of HAADF-STEM analysis. [Figure 23] 23(A) and 23(B) are diagrams illustrating the results of HAADF-STEM analysis. [Figure 24] 24(A) and 24(B) are diagrams illustrating the results of HAADF-STEM analysis. [Figure 25] 25(A) and 25(B) are graphs illustrating the results of the charge-discharge cycle test. [Figure 26] FIG. 26 is a graph illustrating the results of the XRD analysis. [Figure 27] FIG. 27 is a graph illustrating the results of the XRD analysis. [Figure 28] FIG. 28 is a graph illustrating the results of the XRD analysis. DETAILED DESCRIPTION OF THE INVENTION

[0035] 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.

[0036] In addition, in the drawings, the size, layer thickness, or area may be exaggerated for clarity, and therefore are not necessarily limited to the scale.

[0037] Furthermore, ordinal numbers such as "first" and "second" used in this specification are used to avoid confusion between components, and do not indicate any order or ranking, such as the order of processes or stacking. Even if a term does not have an ordinal number in this specification, an ordinal number may be used in the claims to avoid confusion between components. Even if a term has an ordinal number in this specification, a different ordinal number may be used in the claims. Even if a term has an ordinal number in this specification, the ordinal number may be omitted in the claims.

[0038] In this specification, space groups are represented using short notation in the international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are represented using Miller indices. While space groups, crystal planes, and crystal directions are represented by superscript bars in crystallography, in this specification, due to formatting constraints, they may be represented by a minus sign (-) before the number instead of a bar above it. Individual orientations indicating directions within a crystal are represented by [ ], collective orientations indicating all equivalent directions are represented by < >, individual planes indicating crystal planes are represented by ( ), and collective planes with equivalent symmetry are represented by {}. For ease of understanding the structure, trigonal crystals represented by the space group R-3m are generally represented by a hexagonal composite hexagonal lattice. Unless otherwise specified, the space group R-3m will be represented by a composite hexagonal lattice. Miller indices may also be (hkil) rather than (hkl). Here, i is -(h+k).

[0039] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), but the cross-sectional shape of each particle may be an ellipse, a rectangle, a trapezoid, a triangle, a square with rounded corners, an asymmetric shape, or the like, and further, each particle may have an irregular shape.

[0040] The theoretical capacity of a positive electrode active material particle refers to the amount of electricity when all of the intercalable lithium contained in the positive electrode active material particle is deintercalated. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, that of LiNiO2 is 275 mAh / g, and that of LiMn2O4 is 148 mAh / g.

[0041] The amount of lithium remaining in the positive electrode active material particles that can be inserted and removed can be determined by the x in the composition formula, for example, Li xIt is indicated by x in M02. Note that M represents a transition metal, and in this specification and the like, unless otherwise specified, M is cobalt and / or nickel. In the case of positive electrode active material particles in a lithium ion secondary battery, x can be expressed as (theoretical capacity - charging capacity) / theoretical capacity. For example, when a lithium ion secondary battery using LiMO2 as the positive electrode active material is charged at 219.2 mAh / g, Li 0.2 It can be said that MO2 or x=0.2. Li x For example, x in MO2 is small, for example, 0.1 <x≦0.24をいう。

[0042] When properly synthesized lithium cobalt oxide before use in a positive electrode approximately satisfies the stoichiometric ratio, it is LiCoO2, where x = 1. It can also be said that lithium cobalt oxide contained in a lithium-ion secondary battery after discharge is complete is also LiCoO2, where x = 1. The state after discharge (discharged state) 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, for example.

[0043] Li x It is preferable that the charge capacity and / or discharge capacity used to calculate x in MO2 be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data from a lithium-ion secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0044] The space group of the positive electrode active material particles and the like is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0045] Furthermore, if the anions have a structure in which three layers are stacked with a mutually offset, such as ABCABC, it is called a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. At the same time, because real crystals always have defects, analytical results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM (transmission electron microscope) image, spots may appear at positions slightly different from the theoretical positions. For example, if the deviation from the theoretical positions and orientation is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0046] The distribution of a certain element refers to a region in which the element is continuously detected within a noise-free range using a certain continuous analytical method. A region in which the element is continuously detected within a noise-free range can also be defined as a region in which the element is always detected when the analysis is performed multiple times.

[0047] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode substance, a positive electrode material for secondary batteries, a positive electrode material for lithium ion secondary batteries, or the like.

[0048] In this specification, when simply referring to a positive electrode active material or positive electrode active material particles, the term may refer to multiple positive electrode active material particles or a single positive electrode active material particle, depending on the analytical method used. For example, in descriptions of scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDX) line analysis, STEM-electron energy loss spectroscopy (STEM-EELS), and electron diffraction, the term refers to a single positive electrode active material particle unless otherwise specified. On the other hand, in descriptions of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), various mass analyses, and the like, the term refers to multiple positive electrode active material particles unless otherwise specified.

[0049] 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 positive electrode active material particles have that characteristic, it can be said that there is a sufficient effect of improving the characteristics of the positive electrode active material particles and secondary batteries containing them.

[0050] Unless otherwise specified, the materials contained in secondary batteries (positive electrode active material particles, negative electrode active material, electrolyte, separator, etc.) are described as being in a state before degradation. Note that a decrease in discharge capacity due to aging and burn-in treatments during secondary battery manufacturing is not considered degradation. For example, a secondary battery consisting of a single cell or battery pack can be said to be in a state before degradation if it has a discharge capacity of 97% or more of its rated capacity. For secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other secondary batteries, the rated capacity conforms to not only the above JIS standard but also to various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0051] In this specification, the state of the materials in 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 an in-use product or in-use state, or a used product or used state.

[0052] In this specification, the (001) plane, the (003) plane, and the like may be collectively referred to as the (00l) plane. In this specification, the (00l) plane may also be referred to as the C plane, the basal plane, and the like. In addition, in lithium cobalt oxide, lithium has a two-dimensional diffusion path. In other words, it can be said that the lithium diffusion path exists along the plane. In this specification, the plane where the lithium diffusion path is exposed, that is, the plane where lithium is inserted and extracted (specifically, a plane other than the (00l) plane), may be referred to as the edge plane.

[0053] In this specification etc., the term "A and / or B" may be used, but this is an example of a description that includes only A, only B, or both A and B.

[0054] (Embodiment 1) In this embodiment, a battery and positive electrode active material particles of one embodiment of the present invention will be described with reference to FIGS.

[0055] [battery] A lithium-ion secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. When the electrolyte contains an electrolytic solution, the battery also includes a separator between the positive electrode and the negative electrode. The battery may further include an exterior covering that covers at least a portion of the periphery of the positive electrode, the negative electrode, and the electrolyte.

[0056] In this embodiment, the positive electrode and positive electrode active material particles of the battery according to one embodiment of the present invention will be mainly described. A method for manufacturing the positive electrode active material particles according to one embodiment of the present invention will be described in Embodiment 2, and details of the remaining components of the lithium-ion secondary battery according to one embodiment of the present invention will be described in Embodiment 3 and subsequent embodiments.

[0057] Fig. 1(A) is a cross-sectional schematic diagram illustrating the internal structure of a lithium-ion secondary battery 10. The lithium-ion secondary battery 10 has a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11 has a positive electrode current collector 21 and a positive electrode active material layer 22 on the positive electrode current collector 21, and the negative electrode 12 has a negative electrode current collector 31 and a negative electrode active material layer 32. As shown in the figure, the positive electrode active material layer 22 and the negative electrode active material layer 32 face each other with the separator 13 interposed therebetween. Although not shown in Fig. 1(A), electrolytes are present in the voids of the positive electrode active material layer 22, the voids of the separator 13, and the voids of the negative electrode active material layer 32.

[0058] 1A illustrates one positive electrode 11, one negative electrode 12, and one separator 13; however, the lithium-ion secondary battery of one embodiment of the present invention is not limited to this structure. The battery may have a structure including two positive electrodes 11, two negative electrodes 12, and two separators 13, or more than these may be stacked. Furthermore, the battery may have a wound structure instead of the stacked structure illustrated in FIG.

[0059] FIG. 1B is an enlarged view of a portion A enclosed by a dashed line in FIG.

[0060] The positive electrode active material layer 22 has positive electrode active material particles 100 (also referred to as a first positive electrode active material), a second positive electrode active material 200, and a conductive material 41. Although not shown, in addition to the positive electrode active material particles 100, the second positive electrode active material 200, and the conductive material 41, the layer 22 may also have a binder.

[0061] Furthermore, the voids in the positive electrode active material layer 22 are preferably filled with an electrolyte 51 as shown in the figure. For example, it is preferable that 60% or more of the voids in the positive electrode active material layer 22 be filled with the electrolyte 51, more preferably 70% or more of the voids, more preferably 80% or more of the voids, more preferably 90% or more of the voids, more preferably 95% or more of the voids, and most preferably 99% or more of the voids. The voids in the positive electrode active material layer 22 refer to regions in the positive electrode active material layer 22 other than those containing solid components (such as the positive electrode active material and conductive material).

[0062] [Positive electrode] The positive electrode 11 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 has positive electrode active material particles 100, and the positive electrode active material particles 100 are a particle group made up of a plurality of particles.

[0063] <Cathode active material particles 100> The positive electrode active material particles 100 have the function of absorbing and releasing lithium ions during charge and discharge. The positive electrode active material used in one embodiment of the present invention can be a material that exhibits minimal deterioration during charge and discharge (hereinafter also referred to as "charge and discharge"), even at high charge voltages (hereinafter also referred to as "high charge voltages"). Specifically, positive electrode active material particles (composite oxides) having the particle diameter (median diameter (D50)) described in this embodiment and having the characteristics described herein, and having a particle diameter of 10 μm to 50 μm, preferably 9 μm to 25 μm, can be used. The positive electrode active material particles contain one or more of additive element X, additive element Y, and additive element Z. The additive element X, additive element Y, and additive element Z will be described in detail in the section "Containing Elements." The additive element X, additive element Y, and additive element Z may be collectively referred to as additive element A.

[0064] The positive electrode active material particles 100 are the main constituent material of the positive electrode active material layer 22, and the weight of the positive electrode active material particles 100 is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more of the weight of the solid components of the positive electrode active material layer 22. If the particle diameter of the positive electrode active material particles 100 is too small, the surface area becomes too large, which may result in excessive reaction between the positive electrode active material surface and the electrolyte. For this reason, the particle diameter (median diameter (D50)) of the positive electrode active material is preferably 10 μm or more. Furthermore, if the particle diameter of the positive electrode active material is larger than the thickness of the active material layer (described later), the particle density of the active material layer cannot be increased, so the particle diameter of the largest particle is preferably 50 μm or less.

[0065] The second positive electrode active material 200 is a positive electrode active material having a particle diameter smaller than that of the positive electrode active material particles 100. For details other than the particle diameter, the description of the positive electrode active material particles 100 can be referred to. By mixing positive electrode active materials with different particle diameters, the particle density of the active material layer can be increased.

[0066] Particle size can be measured using a particle size distribution analyzer (laser diffraction particle size distribution analyzer) that uses a laser diffraction / scattering method. D50 is the particle size at which the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. Measurement of particle size is not limited to laser diffraction particle size distribution measurement, and the major axis of the particle cross section can also be measured by analysis such as SEM (Scanning Electron Microscope) or TEM. Note that, as a method for measuring D50 using analysis such as SEM or TEM, for example, 20 or more particles can be measured, a cumulative curve can be created, and the particle size at which the cumulative amount accounts for 50% can be taken as D50.

[0067] Unless otherwise specified in this specification, the "charge voltage" is expressed based on the potential of lithium metal. Furthermore, in this specification, the "high charge voltage" refers to a charge voltage of, for example, 4.5 V or higher, preferably 4.55 V or higher, more preferably 4.6 V or higher, 4.65 V or higher, or 4.7 V or higher.

[0068] As mentioned above, in this specification, a "high charge voltage" is defined as 4.6 V or higher based on the potential when the negative electrode is made of lithium metal, but when the potential when the negative electrode is made of a carbon material (e.g., graphite) is used as the reference, a "high charge voltage" is defined as 4.5 V or higher. In short, in the case of a half cell using lithium metal as the negative electrode, a charge voltage of 4.6 V or higher is defined as a high charge voltage, and in the case of a full cell using a carbon material (e.g., graphite) as the negative electrode, a charge voltage of 4.5 V or higher is defined as a high charge voltage.

[0069] The positive electrode active material particles 100 that are less susceptible to deterioration due to repeated charging and discharging at a high charging voltage will be described with reference to FIGS. 2(A) to 3(F).

[0070] 2(A) and 2(B) are cross-sectional views of a positive electrode active material particle 100 according to one embodiment of the present invention. Enlarged views of the vicinity of AB in FIG. 2(B) are shown in FIGS. 3(A) to 3(C). Enlarged views of the vicinity of CD in FIG. 2(B) are shown in FIGS. 3(D) to 3(F).

[0071] 2(A), the positive electrode active material particle 100 has a surface layer portion 100a and an interior portion 100b. In these drawings, the boundary between the surface layer portion 100a and the interior portion 100b is indicated by a dashed line.

[0072] The surface layer 100a of the positive electrode active material particle 100 refers to, for example, a region extending from the surface toward the interior, within 10 nm, perpendicular or approximately perpendicular from the surface. Note that "approximately perpendicular" refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be considered the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0073] The region deeper than the surface layer 100a of the positive electrode active material particle 100 is called the interior 100b, which is synonymous with the inner region or core.

[0074] Furthermore, when the positive electrode active material particle 100 has a layered rock-salt crystal structure of space group R-3m, the surface layer portion 100a has a lithium intercalation / deintercalation region 100a1 and a basal region 100a2, as shown in FIG. 2(B). In FIGS. 2(A) and 2(B), the line labeled (00l) represents the (00l) plane. The lithium intercalation / deintercalation region 100a1 has a surface exposed in a direction intersecting the (00l) plane, and the region extending from the surface to within 10 nm perpendicular or substantially perpendicular thereto is referred to as the lithium intercalation / deintercalation region 100a1. Here, "intersecting" refers to an angle between a perpendicular to the first plane (the (00l) plane) and a normal to the second plane (the surface of the positive electrode active material particle 100) of 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less.

[0075] The basal region 100a2 has a surface parallel to the (00l) plane, and the region extending from the surface to within 10 nm perpendicular or approximately perpendicular from the surface is referred to as the basal region 100a2. Note that "parallel" here means that the angle between the perpendicular to the first surface (the (00l) plane) and the normal to the second surface (the surface of the positive electrode active material particle 100) is 0 to 5 degrees, more preferably 0 to 2.5 degrees.

[0076] The surface of the positive electrode active material particle 100 refers to the surface of the composite oxide including the surface layer portion 100a and the interior portion 100b. Furthermore, the surface of the positive electrode active material particle 100 in a cross-sectional image, such as a scanning transmission electron microscope (STEM), refers to the surface closest to the exterior where a metal element with an atomic number greater than that of lithium is first observed. More specifically, the surface refers to the point where the atomic nucleus of the metal element with an atomic number greater than that of lithium first appears, i.e., where the brightness peak is present in a cross-sectional STEM image.

[0077] Therefore, the positive electrode active material particles 100 do not include metal oxides attached to the surface of aluminum oxide (Al2O3) or other materials that do not have lithium sites that can contribute to charge and discharge, nor do they include carbonates or hydroxyl groups that are chemically adsorbed after the production of the positive electrode active material particles 100. The attached metal oxides refer to, for example, metal oxides whose crystal orientation does not match that of the interior 100b.

[0078] It also does not include the electrolyte, decomposition products of the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material particles 100.

[0079] Since the positive electrode active material particle 100 is a compound containing oxygen and a transition metal capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material particle 100. Surfaces formed by slips, cracks, and / or fissures may also be considered to be the surface of the positive electrode active material particle 100. When the positive electrode active material particle 100 is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material particle 100. The protective film may be a single-layer or multi-layer film of carbon, metal, oxide, resin, etc.

[0080] <Contained elements> The positive electrode active material particles 100 contain lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material particles 100 may contain lithium cobalt oxide (LiCoO) to which the additive element has been added. Note that the composition of the lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0081] The positive electrode active material particles 100 of a lithium ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material particles 100 of one embodiment of the present invention preferably use cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, at least one or more selected from nickel and manganese may also be used. It is preferable for the positive electrode active material particles 100 to contain 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals contained therein, because of many advantages such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0082] The positive electrode active material particles 100 contain magnesium and fluorine as additive elements. In addition to these, it is preferable to use one or more selected from nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, calcium, barium, bromine, and beryllium. 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 %.

[0083] That is, the positive electrode active material particles 100 can be one or more of lithium cobalt oxide having magnesium and fluorine, lithium cobalt oxide having magnesium, fluorine and aluminum, lithium cobalt oxide having magnesium, fluorine and nickel, lithium cobalt oxide having magnesium, fluorine, nickel and aluminum, etc.

[0084] It can also be said that the positive electrode active material particles 100 can be any one or more of a positive electrode active material having cobalt, oxygen, magnesium, and fluorine, a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and aluminum, a positive electrode active material having cobalt, oxygen, magnesium, fluorine, and nickel, and a positive electrode active material having cobalt, oxygen, magnesium, fluorine, nickel, and aluminum, and the like, and can be used in a lithium ion secondary battery.

[0085] The additive element is preferably dissolved in the positive electrode active material particle 100. For example, when STEM-EDX line analysis is performed from the outside to the inside of the positive electrode active material particle 100, the position of the rising edge at which the additive element is detected in the depth direction is preferably located deeper than the position of the rising edge at which the transition metal M is detected, i.e., closer to the inside of the positive electrode active material particle 100.

[0086] These added elements further stabilize the crystal structure of the positive electrode active material particles 100 .

[0087] The additive element does not necessarily have to include nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, calcium, barium, bromine, or beryllium.

[0088] For example, if the positive electrode active material particles 100 are substantially free of titanium, the above-mentioned advantage of having excellent cycle characteristics will be further enhanced. The weight of titanium contained in the positive electrode active material particles 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less. Furthermore, when the positive electrode active material particles 100 are subjected to STEM-EDX analysis, it is preferable that characteristic X-rays attributable to titanium are not detected, i.e., the content is below the lower detection limit (e.g., less than 0.3 atomic %).

[0089] For example, if the positive electrode active material particles 100 are substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, as described above, will be enhanced. The weight of manganese contained in the positive electrode active material particles 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0090] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, it can be said that the atoms on the surface of the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 100a can be made sufficiently stable, Li x Even when x in CoO2 is small, for example, 0.24 or less, the layered structure consisting of cobalt and oxygen octahedra in the inner 100b can be made less likely to break.Furthermore, the displacement of the layers consisting of cobalt and oxygen octahedra in the inner 100b can be suppressed.

[0091] To give the surface layer portion 100a a stable composition and crystal structure, the surface layer portion 100a preferably contains an additive element, and more preferably contains multiple additive elements. Furthermore, the surface layer portion 100a preferably has a higher concentration of one or more selected from the additive elements than the interior portion 100b. Furthermore, the one or more selected from the additive elements contained in the positive electrode active material particles 100 preferably have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive elements in the positive electrode active material particles 100 differs depending on the additive element. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The concentration peak here refers to the maximum concentration value in the surface layer portion 100a or within 50 nm from the surface.

[0092] [distribution] The distribution of the added element will be described. Figures 3(A) to 3(C) are enlarged views of the vicinity of AB in Figure 2(B) and are views illustrating the region 100a1 where lithium is inserted and extracted in the positive electrode active material particle 100. Figures 3(D) to 3(F) are enlarged views of the vicinity of CD in Figure 2(B) and are views illustrating the basal region 100a2 of the positive electrode active material particle 100.

[0093] For example, some of the additive elements, such as magnesium, fluorine, silicon, phosphorus, boron, and calcium, preferably have a concentration gradient that increases from the interior 100b toward the surface, as shown by the gradation in Figures 3(A) and 3(D). An additive element having such a concentration gradient will be referred to as additive element X.

[0094] Other additive elements, such as aluminum and manganese, preferably have a concentration gradient as shown by the density of the hatching in Figures 3(B) and 3(E), and have a concentration peak in a region deeper than that of additive element X shown in Figures 3(A) and 3(D). The concentration peak is preferably present in the interior 100b. For example, it is preferable that the peak be in a region greater than 10 nm and not greater than 30 nm from the surface to the interior. An additive element having such a concentration gradient will be referred to as additive element Y.

[0095] As shown by the presence or absence of hatching and the density of the hatching in Figures 3(C) and 3(F), other additive elements, such as nickel and barium, may be clearly present in the lithium intercalation / deintercalation region 100a1 but substantially absent from the basal region 100a2. Here, "clearly present" refers to a case in which a characteristic X-ray energy spectrum of the element is detected in a cross-sectional STEM-EDX analysis of the positive electrode active material particle 100. "Substantially absent" refers to a case in which a characteristic X-ray energy spectrum of the element is not detected in a cross-sectional STEM-EDX analysis of the positive electrode active material particle 100. This also refers to the element being below the lower limit of detection in STEM-EDX analysis. An additive element having such a distribution is referred to as additive element Z.

[0096] The effects of each added element and the preferred concentrations thereof will be described in detail below.

[0097] 〔magnesium〕 A positive electrode active material particle 100 according to one embodiment of the present invention has a layered rock salt crystal structure inside and a rock salt crystal structure on the surface layer 100a. As will be described later, the crystal orientations of the two roughly match, but there is strain between the two due to lattice mismatch (which can also be considered a difference in inter-ionic distance). Magnesium, which is one of the additive elements X, has the function of alleviating this strain.

[0098] For example, if the rock-salt crystal structure of the surface layer portion 100a is formed only of cobalt oxide CoO, the lattice mismatch (difference in inter-ionic distance) with the lithium cobalt oxide of the interior portion 100b will be 5.9% or more. However, by dissolving magnesium in the surface layer portion 100a including the first layer, the lattice mismatch between the surface layer portion 100a and the lithium cobalt oxide of the interior portion 100b can be made smaller than the above. This is because the metal-oxygen distance in magnesium oxide MgO is closer to the average metal-oxygen distance in lithium cobalt oxide than the metal-oxygen distance in cobalt oxide CoO.

[0099] In order to efficiently exert the strain relaxation function, magnesium is preferably substituted at some of the cobalt sites or lithium sites of the second to sixth layers of the surface layer portion 100a, and particularly preferably at some of the cobalt sites of the fourth layer.

[0100] When magnesium substitutes for a portion of the cobalt site, it may induce lithium vacancies in the surrounding area to maintain cation balance. The presence of lithium vacancies accelerates lithium diffusion, resulting in a positive electrode active material particle 100 with reduced lithium diffusion resistance. Furthermore, when magnesium substitutes for a portion of the cobalt site, it is easy to maintain a layered rock salt or rock salt crystal structure even after charge and discharge. Therefore, it is possible to maintain the lithium diffusion path even after charge and discharge, resulting in a positive electrode active material particle 100 with little increase in lithium diffusion resistance even after charge and discharge.

[0101] However, if magnesium is present inside the seventh layer or higher, the disadvantage of blocking the lithium diffusion path by magnesium may outweigh the above-mentioned advantages. For this reason, it is preferable that the region substituted with magnesium is part of the cobalt site in the sixth layer or lower of the positive electrode active material particle.

[0102] In this specification, the surface, i.e., the row of atoms of a metal element with an atomic number higher than that of lithium, observed in a cross-sectional STEM image of the positive electrode active material particle 100, closest to the outside, is referred to as the first layer. The row of atoms of a metal element with an atomic number higher than that of lithium, observed next closest to the outside after the first layer, is referred to as the second layer. The same applies to the third layer and beyond. In a HAADF-STEM image, contrast proportional to the atomic number is obtained, and elements with higher atomic numbers are observed brighter. Furthermore, elemental analysis can be performed using STEM-energy dispersive X-ray spectroscopy (EDX). Therefore, by combining these analyses, if cobalt and magnesium are detected in the surface layer portion 100a by STEM-EDX analysis but other metal elements with atomic numbers close to that of magnesium are not detected, and an element with a lower brightness than cobalt is observed at the cobalt site in the HAADF-STEM image, it can be determined that magnesium has substituted for the cobalt site.

[0103] It is also possible that magnesium is in the position of a dangling bond.

[0104] In addition, 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 in the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is because the magnesium present at the lithium site functions as a pillar supporting the CoO2 layers. In addition, the presence of magnesium makes it easier to maintain the Li xWhen x in CoO2 is, for example, 0.24 or less, oxygen desorption from around magnesium can be suppressed. When magnesium is dispersed in a mesh-like pattern in the surface layer 100a, oxygen desorption can be suppressed more effectively without adversely affecting lithium intercalation and deintercalation. Since deterioration of the positive electrode active material particles 100 is thought to proceed first through oxygen desorption from the surface layer 100a, followed by cobalt elution, suppressing oxygen desorption is important for suppressing deterioration. Note that dispersed magnesium refers to distribution at an appropriate concentration.

[0105] To fully obtain the above-mentioned effects, it is preferable that a sufficient concentration of magnesium is present in the surface layer portion 100 a. For example, the maximum magnesium concentration in the surface layer portion 100 a as determined by STEM-EDX ray analysis is preferably 7 atomic % or more, and more preferably 10 atomic % or more.

[0106] However, excessive magnesium may adversely affect lithium insertion and extraction. Therefore, it is preferable that the positive electrode active material particle 100 contains an appropriate amount of magnesium. For example, the maximum magnesium concentration in the surface layer 100a as determined by STEM-EDX ray analysis is preferably 25 atomic% or less. That is, the maximum magnesium concentration in the surface layer 100a as determined by STEM-EDX ray analysis is preferably 7 atomic% or more and 25 atomic% or less, and more preferably 10 atomic% or more and 25 atomic% or less. Furthermore, when measured by EPMA, the atomic ratio Mg / Co of magnesium to cobalt in the interior is preferably greater than 0 and less than 0.02, and more preferably 0.005 or more and 0.015 or less.

[0107] [Fluorine] Fluorine has the function of promoting the migration of the additive element into the interior 100 b of the positive electrode active material particle 100 .

[0108] More specifically, when the melting point of a fluoride, such as lithium fluoride, is lower than that of the other additive element sources, it functions as a flux (also called a fluxing agent) that lowers the melting point of the other additive element sources. As a result, during the heating process, a liquid containing the fluoride and other additive elements spreads over the surface of the lithium cobalt oxide. This makes it easier to distribute the additive elements uniformly over the surface layer 100a. The uniform distribution of the additive elements over the surface layer 100a also facilitates their subsequent migration from the surface layer 100a to the interior. Furthermore, the presence of fluorine also functions as a diffusion aid for the diffusion of magnesium from the surface layer 100a to the interior.

[0109] Furthermore, fluorine is preferably present on the surface side (the region closest to the outside of the particle, which may be referred to as the outermost surface). For example, with regard to the position of maximum concentration of each element in the surface layer portion 100a when STEM-EDX line analysis is performed, the position of maximum fluorine concentration (atomic %) is preferably located closer to the surface than the positions of maximum concentrations (atomic %) of other added elements. In other words, the positions of maximum concentrations (atomic %) of other added elements such as magnesium are preferably located closer to the interior than the positions of maximum fluorine concentration (atomic %).

[0110] The presence of fluorine in the surface layer portion 100a, which has a surface that is in contact with the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material particles 100 and the electrolyte, and can also effectively improve corrosion resistance to hydrofluoric acid.

[0111] 〔nickel〕 Nickel has a stronger bond with oxygen than cobalt, so the presence of nickel suppresses oxygen desorption from the positive electrode active material particles 100 and suppresses the phase change from a layered rock salt crystal structure to a spinel crystal structure. Therefore, nickel is preferably present at a high concentration in the surface layer 100a, which is the region where the phase change to a spinel crystal structure is likely to occur. In particular, nickel is preferably present in the lithium sites up to the fourth layer in the region 100a1 where lithium is inserted and extracted.

[0112] Furthermore, nickel is Ni 2+ , Ni 3+ , Ni 4+ Of which Ni 2+ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. In this respect, nickel is also thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0113] Furthermore, when nickel is present at the lithium site, it can suppress the shift of the layered structure consisting of octahedra of cobalt and oxygen. It also suppresses volume changes during charging and discharging. This is presumably because the nickel present at the lithium site also functions as a pillar supporting the MO2 layers. Therefore, it is expected that the crystal structure will be more stable, especially in a charged state at high temperatures, for example, above 45°C, which is preferable. To achieve the above effects, it is preferable that nickel be present not only in the surface layer portion 100a but also in the interior portion 100b.

[0114] 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 have a negative effect on lithium insertion and extraction.

[0115] Therefore, it is preferable that the positive electrode active material particles 100 contain an appropriate amount of nickel. For example, in an EPMA, the atomic ratio Ni / Co of nickel to cobalt inside the particles is preferably greater than 0 and not greater than 0.05, more preferably 0.001 or more and 0.01 or less, and even more preferably 0.004 or more and 0.008 or less.

[0116] 〔aluminum〕 Aluminum has the function of mitigating volume changes in the layered rock-salt crystal structure due to charging and discharging. Aluminum can exist in the cobalt site of the 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 and volume change due to charging and discharging. Therefore, as described below, even if the positive electrode active material particle 100 is subjected to a force that causes it to expand and contract in the c-axis direction due to the insertion and desorption of lithium ions, deterioration of the positive electrode active material particle 100 can be suppressed.

[0117] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, because the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen from the aluminum's surroundings. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material particles 100 are used in secondary batteries. Furthermore, the positive electrode active material particles 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0118] On the other hand, excess aluminum can have adverse effects such as reduced capacity and / or increased lithium diffusion resistance.

[0119] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material particle 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material particle 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2% and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material particle 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material particle 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material particle 100.

[0120] [Roughly matching crystal orientation] 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.

[0121] For example, it is preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type.Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are roughly the same.

[0122] 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 distorted rock-salt crystal lattice.

[0123] 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 deficiencies are also acceptable.

[0124] Furthermore, the presence of both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc. The presence of both the characteristics of the layered rock salt type and the rock salt type crystal structure can also be said to mean that the rock salt structures are scattered within the layered rock salt structure. The region where the rock salt structures are scattered within the layered rock salt structure is preferably on the surface side of the surface layer portion 100a, and is preferably present, for example, in the first to third layers of the region 100a1 where lithium is inserted and extracted.

[0125] While the rock-salt type has no distinction between cation sites, the layered rock-salt type has two types of cation sites in its crystal structure: one is dominated by lithium and the other by transition metals. Both the rock-salt type and the layered rock-salt type share a layered structure, with alternating two-dimensional planes of cations and two-dimensional planes of anions. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, if the central spot (transmitted spot) is taken as the origin 000, the bright spot closest to the central spot would be, for example, the (111) plane in an ideal rock-salt type, and, for example, the (003) plane in a layered rock-salt type. For example, when comparing the electron diffraction patterns of rock-salt MgO and layered rock-salt LiCoO2, the distance between bright spots on the (003) plane of LiCoO2 is observed to be approximately half the distance between bright spots on the (111) plane of MgO. Therefore, if the analyzed region contains two phases, for example, rock-salt MgO and layered rock-salt LiCoO2, the electron diffraction pattern will show plane orientations in which bright spots of high brightness and bright spots of low brightness are arranged alternately. Bright spots common to both the rock-salt and layered rock-salt types will have high brightness, while bright spots occurring only in the layered rock-salt type will have low brightness.

[0126] Furthermore, when a layered rock-salt crystal structure is observed perpendicular to the c-axis in cross-sectional HAADF-STEM images, layers observed with high brightness and layers observed with low brightness are observed alternating. This characteristic is not seen 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 alternating in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in part of the low-brightness layers, i.e., the lithium layers.

[0127] Layered rock salt crystals and the anions in rock salt crystals form a cubic close-packed structure (face-centered cubic lattice structure). When layered rock salt crystals and rock salt crystals come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0128] However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group Fm-3m (the space group of general rock salt crystals) of rock salt crystals. Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and O3'-type crystals and rock salt crystals. In this specification, when the orientations of the cubic close-packed structures formed by anions in 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, the three-dimensional structural similarity in which the crystal orientations are approximately the same, or the same crystallographic orientation, is called topotaxis.

[0129] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and STEM images, etc. XRD, neutron diffraction, etc. can also be used as materials for determination.

[0130] <Li x Crystal structure of CoO2 with small x> The positive electrode active material particle 100 of one embodiment of the present invention has the above-described distribution of the additional element A and / or the crystal structure in a discharged state, and therefore, x It is preferable that the crystal structure when x in CoO2 is small is different from that of conventional positive electrode active materials. <x≦0.24をいうこととする。

[0131] Using Figs. 4 to 7, Li x The change in crystal structure due to the change in x in CoO2 will be described by comparing a conventional positive electrode active material with the positive electrode active material particle 100 according to one embodiment of the present invention.

[0132] The change in the crystal structure of a conventional positive electrode active material is shown in Figure 5. The conventional positive electrode active material shown in Figure 5 is lithium cobalt oxide (LiCoO2) that does not contain any additive element A. In Figure 5, R-3m O3 is added to indicate Li x The crystal structure of lithium cobalt oxide with x=1 in CoO2 is shown.

[0133] Furthermore, conventional lithium cobalt oxides with x = 0.12 or so have a crystal structure of the space group R-3m. This structure can be thought of as a structure in which a trigonal O1-type CoO2 structure and an R-3m O3-type LiCoO2 structure are alternately stacked. For this reason, this crystal structure is sometimes referred to as the H1-3-type crystal structure. In reality, the H1-3-type crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in Figure 5 and other parts of this specification, the c-axis of the H1-3-type crystal structure is shown as half the unit cell to facilitate comparison with other crystal structures.

[0134] As an example of an H1-3 type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co(0, 0, 0.42150±0.00016), O1(0, 0, 0.27671±0.00045), and O2(0, 0, 0.11535±0.00045). O1 and O2 are oxygen atoms. The unit cell that 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, the unit cell that results in the smallest GOF (goodness of fit) value should be used.

[0135] Li x When conventional lithium cobalt oxide is repeatedly charged and discharged so that x in CoO2 becomes 0.24 or less, the crystal structure changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0136] However, these two crystal structures have a large misalignment of the CoO2 layers. As shown by the dotted lines and arrows in Figure 5, in the H1-3 crystal structure, the CoO2 layers are significantly misaligned from those in the R-3m O3 discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0137] Furthermore, the difference in volume between these two crystal structures is large: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3m O3 crystal structure exceeds 3.5%, typically 3.9% or more.

[0138] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers like the trigonal O1 type, is likely to be unstable.

[0139] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide breaks down. This break in the crystal structure causes a deterioration in cycle characteristics. This is because the number of sites where lithium can exist stably decreases and it becomes difficult for lithium to be inserted and extracted.

[0140] On the other hand, in the positive electrode active material particle 100 according to one embodiment of the present invention shown in FIG. x The change in the crystal structure between the discharge state where x in CoO2 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 deviation of the CoO2 layer between the state where x is 1 and the state where x is 0.24 or less can be reduced. Furthermore, the change in volume compared per cobalt atom can be reduced. Therefore, the positive electrode active material particle 100 of one embodiment of the present invention is resistant to collapse of the crystal structure even when repeated charge and discharge in which x is 0.24 or less is performed, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material particle 100 of one embodiment of the present invention has a low crystalline structure even when Li x When x in CoO2 is 0.24 or less, the positive electrode active material particles 100 according to one embodiment of the present invention can have a more stable crystal structure than conventional positive electrode active materials. xWhen x in CoO2 is kept at 0.24 or less, short circuits are less likely to occur, which is preferable as it further improves the safety of the secondary battery.

[0141] Li x The crystal structure of the interior 100b of the positive electrode active material particle 100 when x in CoO2 is approximately 1 and 0.2 is shown in Figure 4. The interior 100b occupies the majority of the volume of the positive electrode active material particle 100 and is the part that contributes greatly to charge and discharge, so it can be said that the displacement of the CoO2 layer and changes in volume are the most problematic part.

[0142] When x=1, the positive electrode active material particles 100 have the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0143] However, the positive electrode active material particles 100 have a different crystal structure from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.12, which results in an H1-3 type crystal structure.

[0144] When x=0.2 or so, the positive electrode active material particle 100 of one embodiment of the present invention has a crystal structure belonging to the trigonal space group R-3m. This has the same symmetry as the CoO2 layer of O3. Therefore, this crystal structure is referred to as an O3'-type crystal structure. This crystal structure is shown in Figure 4, labeled R-3m O3'.

[0145] The O3' type crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), with the range of 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837(×10 -1 nm), and 2.807≦a≦2.827(×10 -1 nm) is more preferable, and typically a=2.817(×10 -1 nm). The c-axis is 13.681≦c≦13.881(×10 -1 nm), 13.751≦c≦13.811 is more preferable, and typically c=13.781(×10 -1 nm).

[0146] In the O3' crystal structure, ions of cobalt, nickel, magnesium, etc. occupy six oxygen coordination positions, while light elements such as lithium may occupy four oxygen coordination positions.

[0147] As shown by the dotted line in Figure 4, there is almost no deviation in the CoO2 layer between the R-3m(O3) in the discharged state and the O3'-type crystal structure.

[0148] 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, and typically 1.8%.

[0149] In this way, in the positive electrode active material particle 100 of one embodiment of the present invention, Li x When x in CoO2 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 per the same number of cobalt atoms is also suppressed. Therefore, the positive electrode active material particles 100 are resistant to collapse of their crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the positive electrode active material particles 100 suppress the decrease in charge / discharge capacity during charge / discharge cycles. Furthermore, because more lithium can be stably utilized than conventional positive electrode active materials, the positive electrode active material particles 100 have a large discharge capacity per weight and per volume. Therefore, the use of the positive electrode active material particles 100 enables the fabrication of secondary batteries with high discharge capacity per weight and per volume.

[0150] The positive electrode active material particles 100 are Li x It has been confirmed that when x in CoO2 is between 0.15 and 0.24, it may have an O3' type crystal structure, and it is also presumed that when x is between 0.24 and 0.27, it also has an O3' type crystal structure. However, the crystal structure is Li x Since it is affected not only by x in CoO2 but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it is not necessarily limited to the above range of x.

[0151] Therefore, the positive electrode active material particles 100 are Li x When x in CoO2 is more than 0.1 and not more than 0.24, the entire interior 100b of the positive electrode active material particle 100 does not have to have an O3'-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0152] Furthermore, the additive element A does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 100a of the positive electrode active material particle 100. For example, the distribution of the additive element A in the (001)-oriented surface may be different from that in the other surfaces. For example, the distribution of concentration peaks of one or more elements selected from additive element X and additive element Y in the (001)-oriented surface and its surface layer portion 100a may be limited to a shallower portion from the surface compared to surfaces other than the (001)-oriented surface. Alternatively, the (001)-oriented surface and its surface layer portion 100a may have a lower concentration of one or more elements selected from additive element X and additive element Y compared to other orientations. Alternatively, the (001)-oriented surface and its surface layer portion 100a may have one or more elements selected from additive element X and additive element Y below the lower detection limit.

[0153] <Analysis method> A certain positive electrode active material is Li x When x in CoO2 is small, whether it has an O3' type crystal structure or not depends on Li x This can be determined by analyzing a positive electrode having positive electrode active material particles with a small x in CoO2 using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0154] <Charging method> The composite oxide is Li x To determine whether a CoO2 alloy has an O3'-type crystal structure when x is small, a coin cell (CR2032 type, 20 mm diameter, 3.2 mm height) can be fabricated using the composite oxide as the positive electrode and lithium metal as the counter electrode. The coin cell contains an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0155] More specifically, the positive electrode may be prepared by coating a slurry of positive electrode active material particles, a conductive material, and a binder on a positive electrode current collector made of aluminum foil.

[0156] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the voltage value of the secondary battery and the potential value of the positive electrode will be different. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0157] The electrolyte used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution can be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC).

[0158] The separator can be a 25 μm thick porous polypropylene film.

[0159] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0160] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.60 V). The charging method is not particularly limited as long as the charging is performed at the desired voltage for a sufficient period of time. For example, when charging using CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be completed at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material particles, charging at such a low current value is desirable. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell is disassembled in an argon-atmosphere glove box and the positive electrode is removed to obtain positive electrode active material particles with the desired charge capacity. When various analyses are performed after this, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container in an argon atmosphere. Furthermore, it is preferable to remove the positive electrode promptly after charging is completed and perform the analysis. Specifically, within 1 hour after charging is completed, and more preferably within 30 minutes.

[0161] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, for example, charging can be performed by constant current charging at a current value of 20 mA / g to 100 mA / g up to an arbitrary voltage (e.g., 4.60 V), followed by constant voltage charging until the current value reaches 2 mA / g to 10 mA / g, and then discharging by constant current discharging at 2.5 V and 20 mA / g to 100 mA / g. Alternatively, discharging can be performed by constant current discharging at 3.0 V and 20 mA / g to 200 mA / g.

[0162] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed, for example, at 2.5 V and a current value of 20 mA / g or more and 200 mA / g or less, or at 3.0 V and a current value of 20 mA / g or more and 200 mA / g or less.

[0163] <xrd> The XRD measurement apparatus and conditions are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD equipment: Bruker AXS, D8 ADVANCE X-ray: CuKα1 Output: 40KV, 40mA Slit width: 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

[0164] 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.

[0165] Figures 6 and 7 show the ideal powder XRD patterns calculated from the O3'-type crystal structure and the H1-3-type crystal structure model using CuKα1 radiation. x The ideal XRD patterns calculated from the crystal structure of LiCoO2O3 with x = 1 in CoO2 and trigonal O1 with x = 0 are also shown. The patterns of LiCoO2(O3) and CoO2(O1) were created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), based on the crystal structure information obtained from ICSD. The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 = 1.540562 × 10 -10 m and λ2 were not set, and the monochromator was set to single. The XRD pattern of the H1-3 type crystal structure was created in the same manner as above, based on the information on the H1-3 type crystal structure shown in Figure 5. The XRD pattern of the O3' type crystal structure was estimated from the XRD pattern of the positive electrode active material particles of one embodiment of the present invention, and fitting was performed using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker), and the XRD pattern was created in the same manner as the others.

[0166] As shown in Figure 6, the O3' type crystal structure exhibits diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° to 19.37°) and 2θ = 45.47 ± 0.10° (45.37° to 45.57°).

[0167] However, as shown in Figure 7, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. x The appearance of diffraction peaks at 2θ=19.25±0.12° (19.13° or more and 19.37° or less) and 2θ=45.47±0.10° (45.37° or more and 45.57° or less) when x in CoO2 is small can be said to be a characteristic of the positive electrode active material particle 100 of one embodiment of the present invention.

[0168] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures with x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures with x = 1 and x ≦ 0.24 that appear at 2θ angles of 42° to 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0169] The positive electrode active material particles 100 according to one embodiment of the present invention are Li x When x in CoO2 is small, it is preferable that it has an O3'-type crystal structure, but it does not have to be entirely O3'-type. It may contain other crystal structures, or it may be partially amorphous. For example, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3'-type crystal structure is 5% or more, and more preferably 10% or more.

[0170] <xps> In XPS (X-ray Photoelectron Spectroscopy), in the case of inorganic oxides, using monochromatic aluminum Kα rays as X-rays allows analysis of a region from the surface to a depth of approximately 2 to 8 nm (usually 5 nm or less), allowing quantitative analysis of the concentration of each element in a region approximately half the depth of the surface layer 100a. Furthermore, narrow scan analysis allows analysis of the bonding state of elements. The quantitative accuracy of XPS is often approximately ±1 atomic %, and the lower detection limit is approximately 1 atomic %, depending on the element.

[0171] It is preferable that the concentration of one or more added elements selected from the surface layer portion 100a measured by XPS or the like is higher than the average concentration of the added elements in the entire positive electrode active material particle 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry) or the like.

[0172] The surface and surface layer 100a of the positive electrode active material particle 100 according to one embodiment of the present invention are assumed to be free of carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material particle 100. The surface of the positive electrode active material particle 100 is also assumed to be free of the electrolyte, binder, conductive material, and compounds derived therefrom that are attached to the surface of the positive electrode active material particle 100. Therefore, when quantifying the elements contained in the positive electrode active material particle 100, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0173] Furthermore, before being subjected to various analyses, the sample of the positive electrode active material particles 100 and the positive electrode active material layer may be washed or the like to remove the electrolyte, binder, conductive material, or compounds derived therefrom adhering to the surface of the positive electrode active material particles 100. At this time, lithium may dissolve in the solvent or the like used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0174] 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 comparison while reducing the influence of carbonates and the like that are chemically adsorbed after the production of the positive electrode active material particles 100. For example, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.400 to 1.20, more preferably 0.400 to 1.00, even more preferably 0.400 to 1.00, even more preferably 0.400 to 0.900, and even more preferably 0.400 to 0.700.

[0175] Furthermore, the ratio of the number of atoms of nickel to cobalt, Ni / Co, as determined by, for example, XPS analysis, is preferably 0.050 or more and 0.200 or less, more preferably 0.050 or more and 0.150 or less, even more preferably 0.050 or more and 0.100 or less, and even more preferably 0.050 or more and 0.070 or less.

[0176] Furthermore, the ratio of the number of fluorine atoms to the number of magnesium atoms, F / Mg, as determined by, for example, XPS analysis, is preferably 0.100 or more and 1.00 or less, more preferably 0.100 or more and 0.800 or less, even more preferably 0.100 or more and 0.500 or less, even more preferably 0.100 or more and 0.300 or less, and even more preferably 0.100 or more and 0.200 or less.

[0177] The above-described range indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material particle 100, but are widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material particle 100. In other words, as a result of XPS analysis of the positive electrode active material particle 100, the above-described range indicates that the crystal structure is not easily broken even when charging and discharging are repeated so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, good lithium insertion and extraction can be achieved in the positive electrode active material particle 100, and excellent rate characteristics can be achieved.

[0178] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. Furthermore, it is recommended to use an XPS instrument with an energy resolution such that the half-width of the Ag3d5 / 2 peak (112 eV) in the XPS spectrum of an Ag sample is 1.0 eV±0.1 eV. The take-off angle can be set to, for example, 45°. For example, the following XPS instrument and measurement conditions can be used for the measurement. Measurement equipment: PHI Quantera II X-ray: Monochromated Al Kα (1486.6 eV) Energy resolution: Half width of Ag3d5 / 2 peak is 1.0 eV ± 0.1 eV Detection area: 100 μmφ Detection depth: Approx. 4 to 5 nm (take-off angle 45°), Approx. 2 nm (take-off angle 15°) Measurement spectrum: Wide scan, narrow scan for each detected element

[0179] When the positive electrode active material particle 100 according to one embodiment of the present invention is analyzed by XPS, the peak (Mg1s peak) showing the bond energy between magnesium and other elements is preferably 1303.0 eV or more and less than 1305.0 eV, and more preferably about 1304.0 eV, which is different from the bond energy of magnesium fluoride, 1306.0 eV, and is closer to the bond energy of magnesium oxide.

[0180] In an XPS analysis of the positive electrode active material particles 100 according to one embodiment of the present invention, the measured XPS spectrum may be corrected so that the C1s peak is aligned with the reference value (284.8 eV), i.e., the entire spectrum may be shifted. This reduces the influence of differences in the XPS apparatus, differences in measurement conditions, etc., on the XPS measurement.

[0181] Furthermore, in an XPS analysis of the positive electrode active material particle 100 of one embodiment of the present invention, when the Mg1s peak is analyzed to analyze the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond, the peak component derived from the "O-Mg-F" bond is preferably below the lower limit of detection. Note that a peak component derived from the O-Mg-O bond may be included. Furthermore, a peak component derived from the "F-Mg-F" bond may also be included, but it is preferably 10% or less of the total, more preferably below the lower limit of detection.

[0182] That is, when analyzing the proportions of the peak component derived from the "O-Mg-O" bond, the peak component derived from the "O-Mg-F" bond, and the peak component derived from the "F-Mg-F" bond in an XPS analysis of the positive electrode active material particle 100 of one embodiment of the present invention, the peak component derived from the "O-Mg-O" bond is preferably 70% or more, more preferably 80% or more, even more preferably 90% or more, and particularly preferably 100%.

[0183] This section describes a method for analyzing the Mg1s peak of an XPS spectrum in XPS analysis. In analyzing the Mg1s peak, the peak component derived from the O-Mg-O bond is designated as fit peak 1, the peak component derived from the O-Mg-F bond as fit peak 2, and the peak component derived from the F-Mg-F bond as fit peak 3. These three fit peaks are then combined to calculate the peak combination ratio that minimizes the difference from the Mg1s peak of the XPS spectrum obtained by XPS analysis. The area ratio of fit peak 1, fit peak 2, and fit peak 3 can be assumed to represent the abundance ratio of O-Mg-O bonds, O-Mg-F bonds, and F-Mg-F bonds, and the analysis results can be output.

[0184] In the above-described XPS spectrum analysis method, the energy value (Ep1) at the maximum value (also referred to as the peak top) of fit peak 1 can refer to the energy value at the maximum value of the Mg1s peak when measured separately using MgO-coated LiCoO2 as a standard sample. Furthermore, the energy value (Ep3) at the maximum value of fit peak 3 can refer to the energy value at the maximum value of the Mg1s peak when measured separately using magnesium fluoride (MgF2, e.g., Kojundo Chemical Research Institute MGH18XB, purity 99.9% (3N up)) as a standard sample. Furthermore, the energy value (Ep2) at the maximum value of fit peak 2 can be set to a value intermediate between Ep1 and Ep3. Furthermore, Ep1 is located on the lower energy side compared to Ep3. The energy value at the maximum value of a peak is also referred to as the peak position.

[0185] In the XPS analysis of the positive electrode active material particle 100 according to one embodiment of the present invention, the Mg1s peak analysis result falling within the above-described preferred range can also be determined from the peak position and half-width of the peak. For example, the half-width of the Mg1s peak is preferably 1.0 eV to 3.0 eV, more preferably 1.0 eV to 2.8 eV, and particularly preferably 1.0 eV to 2.6 eV. Note that, in the above, the peak position of the Mg1s peak is on the lower energy side than the energy value of the maximum value of the Mg1s peak measured separately using magnesium fluoride as a standard sample.

[0186] <edx> It is preferable that one or more selected from the additive elements contained in the positive electrode active material particles 100 have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak differs depending on the additive element in the positive electrode active material particles 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material particle 100 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, EPMA (electron probe microanalysis), or the like.

[0187] 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 concentration within the positive electrode active material particle 100. 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.

[0188] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the additive element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material particle 100. Furthermore, EDX line analysis can analyze the concentration distribution and maximum value of the additive element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material particle 100 in a specific region without being affected by the distribution in the depth direction.

[0189] Since the positive electrode active material particle 100 is a compound containing a transition metal capable of inserting and desorbing lithium and oxygen, the surface of the positive electrode active material particle 100 is the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon insertion and desorption of lithium and oxygen are present and a region where they are not present.

[0190] In STEM-EDX ray analysis, the graph of the detected amount of characteristic X-rays of the element 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, the detected amount of characteristic X-rays of the transition metal M is the average value M of the detected amount of characteristic X-rays of the internal transition metal M. AVE and the average amount of the characteristic X-rays of the transition metal M in the background, M BG The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the two, or the detected amount of oxygen characteristic X-rays is the average value O AVE and the average amount of background oxygen characteristic X-rays detected, O BG The reference point is the point where the detected amount of the characteristic X-rays of the transition metal M is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the transition metal M inside and the average value of the detected amount of the characteristic X-rays of the transition metal M in the background. If the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of oxygen inside and the average value of the detected amount of the characteristic X-rays of oxygen in the background is different from the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of oxygen inside and the average value of the detected amount of the characteristic X-rays of oxygen in the background, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen adhering to the surface, and therefore, the detected amount of the characteristic X-rays of the transition metal M is considered to be 50% of the sum of the average value of the detected amount of the characteristic X-rays of the transition metal M inside. AVE and the average amount of the characteristic X-rays of the transition metal M in the background, M BG In the case of a positive electrode active material particle 100 having a plurality of transition metals M, the M of the transition metal element with the largest amount of characteristic X-rays detected inside can be used as the reference point. AVE and M BG The reference point can be determined using the following formula:

[0191] The average amount of characteristic X-rays of the above transition metals M detected in the background is M BG can be obtained by averaging the amount of characteristic X-rays of the transition metal M detected within the positive electrode active material particle 100 in a range of 2 nm or more, preferably 3 nm or more, avoiding the area where the amount of characteristic X-rays of the transition metal M begins 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 50 nm or more, from the region where the detected amount of characteristic X-rays of the transition metal M and oxygen saturates and stabilizes, for example, the region where the detected amount of characteristic X-rays of the transition metal M starts to increase. BG and the average value of the detected amount of characteristic X-rays of oxygen inside AVE can also be found in the same way.

[0192] Furthermore, the surface of the positive electrode active material particle 100 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystal structure of the positive electrode active material particle 100 is observed and an area where it is not observed, and is the outermost area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material particle 100 are confirmed.

[0193] In STEM-EDX analysis, a peak refers to a convex maximum value that appears in a graph of the characteristic X-ray intensity for each element, or the maximum value of the characteristic X-ray for each element. Note that noise in STEM-EDX analysis can be measured values ​​with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0194] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured after two scans can be used as the detection value for each element. The number of scans is not limited to two, and more scans can be performed and the integrated value can be used as the detection value for each element.

[0195] STEM-EDX line analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material particles 100. For example, carbon can be vapor-deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0196] Next, the positive electrode active material particles 100 are thinned to prepare a STEM cross-section sample. For example, the thinning process can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). In this case, pickup is performed using an MPS (micro-probing system), and the finishing process can be performed under conditions such as an acceleration voltage of 10 kV.

[0197] STEM-EDX analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) with an EDAX Octane T Ultra W EDX detector. An example of the conditions for EDX analysis using the Hitachi High-Tech HD-2700 is to set the emission current of the STEM device to between 6 μA and 10 μA, and measure a portion of the thinned sample with minimal depth and unevenness. The magnification is, for example, approximately 150,000 times. The conditions for EDX analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0198] In order to increase the spatial resolution in STEM-EDX line analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX line analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, in order to increase the analytical sensitivity in STEM-EDX line analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used for STEM-EDX line analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0199] In addition, in the positive electrode active material particles 100 containing magnesium and fluorine as additive elements, the fluorine distribution preferably has an overlapping region with the magnesium distribution. For example, the difference in depth between the peak of the fluorine concentration or detection amount and the peak of the magnesium concentration or detection amount is preferably within 10 nm, more preferably within 3 nm, even more preferably within 1 nm, and even more preferably within 0.5 nm.

[0200] In addition, in positive electrode active material particles 100 containing nickel as an additive element, the peak of the nickel concentration or detectable amount in the surface layer portion 100a is preferably present on the surface of the positive electrode active material particle 100 or at a depth of up to 3 nm from the reference point toward the center, and more preferably at a depth of up to 1 nm. In addition, in positive electrode active material particles 100 containing magnesium and nickel, the nickel distribution preferably has a region overlapping with the magnesium distribution. For example, the difference in depth between the peak of the nickel concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, and more preferably within 1 nm.

[0201] Furthermore, when the positive electrode active material particle 100 contains aluminum as an additive element, it is preferable that, upon EDX analysis, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 100a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 100a is located more inward than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material particle 100 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0202] Unless otherwise specified in this specification, the concentration of an element in EDX analysis is calculated by using the denominator as the sum of carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, and gallium.

[0203] <epma> The concentration of the additive element contained in the positive electrode active material particle 100 can be analyzed using EDX, but it can also be analyzed using EPMA. EPMA has higher detection capability (also referred to as a lower detection limit) than EDX when analyzing elements present in trace amounts in a sample. Therefore, it is preferable to use EPMA when analyzing a region where a trace amount of the additive element is present.

[0204] In the EPMA analysis, a cross section of the positive electrode active material particle 100 is exposed by mechanical polishing, ion polishing, FIB, or the like, and the cross section is analyzed. An EPMA device such as a JEOL JXA-iHP200F electron probe microanalyzer can be used. Measurement conditions can be, for example, an acceleration voltage of 10 kV, a beam diameter of approximately 3 μmφ, a probe current of 50 nA, and analytical crystal species of C(LDE2H), O(LDE1L), Mg(TAPL), Co(LIFH), Ni(TAPL), F(TAPL), Al(TAPL), or Ti(LIFH).

[0205] EPMA uses a wavelength-dispersive detector, and therefore has a higher ability to detect trace elements than EDX, which uses an energy-dispersive detector. On the other hand, the spatial resolution of EPMA analysis is inferior to that of EDX (especially STEM-EDX). Therefore, STEM-EDX is suitable for analysis focusing on the detailed distribution of additive elements in the surface layer 100a of the positive electrode active material particle 100, while EPMA analysis is suitable for analysis of trace amounts of additive elements in the interior 100b. Because EPMA and EDX use different analytical methods, the concentration values ​​obtained when analyzing the same region using each analytical method may not match.

[0206] <la-icp-ms> The concentration of the additive element contained in the positive electrode active material particle 100 can also be analyzed using LA-ICP-MS (laser ablation ICP-MS). LA-ICP-MS is capable of local analysis, and is therefore suitable for analyzing the concentration of the additive element only in the interior, excluding the surface layer portion.

[0207] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0208] (Embodiment 2) In this embodiment, an example of a method for manufacturing the positive electrode active material particles 100 of one embodiment of the present invention will be described with reference to FIGS.

[0209] Heating conditions are important for producing the positive electrode active material particles 100. The lower limit of the heating temperature is the temperature at which the reaction proceeds from the starting materials. The temperature at which the reaction proceeds may be any temperature at which mutual diffusion of the elements contained in the starting materials occurs, and may be lower than the melting temperature of the starting materials. An oxide will be used as an example for explanation. m 0.757 times (Tanman temperature T d ) solid-state diffusion occurs. Therefore, the heating temperature is preferably 650°C or higher.

[0210] Furthermore, in order to achieve the distribution of the additive elements as described above, it is preferable to provide a sufficient heating time. For example, the heating time is preferably more than 100 hours, and more preferably more than 100 hours but not more than 150 hours. Heating may be performed in multiple steps, in which case the total heating time to reach 650°C is preferably within the above range. For example, it is preferable that the total time during which the heating temperature of the furnace is set to 650°C or higher is within the above range. Note that the temperature drop time after heating is preferably, for example, 10 hours or more but not more than 50 hours.

[0211] <<Method 1 for Producing Positive Electrode Active Material Particles 100>> 8(A), a method 1 for producing the positive electrode active material particles 100 will be described. Here, magnesium and fluorine are used as the additive elements.

[0212] <Step S11> In step S11 shown in FIG. 8(A), a lithium source (Li source), a cobalt source (Co source), a magnesium source (Mg source), and a fluorine source (F source) are prepared as starting materials.

[0213] 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 for example, it is recommended to use a material with a purity of 99.99% or higher.

[0214] As the cobalt source, it is preferable to use a compound containing cobalt, such as tricobalt tetroxide or cobalt hydroxide.

[0215] The magnesium source may be magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. A plurality of the above-mentioned magnesium sources may also be used.

[0216] Examples of fluorine sources that can be used include lithium fluoride (LiF), magnesium fluoride (MgF), aluminum fluoride (AlF), cobalt fluoride (CoF, CoF), nickel fluoride (NiF), zirconium fluoride (ZrF), vanadium fluoride (VF), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF), calcium fluoride (CaF), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF), cerium fluoride (CeF, CeF), lanthanum fluoride (LaF), and sodium aluminum hexafluoride (NaAlF). 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.

[0217] <Steps S12 and S13> Next, the lithium source, cobalt source, magnesium source, and fluorine source are pulverized and mixed (step S12) to prepare a mixture 903 (step S13). The pulverization and mixing can be performed by either a dry or wet method. The wet method allows for finer particle pulverization and mixing. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is preferable to use an aprotic solvent that does not easily react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and cobalt source with dehydrated acetone with a purity of 99.5% or higher, with a water content of 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0218] 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. 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 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).

[0219] <Step S14> Next, in step S14, the mixture 903 is heated. As for the heating conditions, it is preferable that the total heating time at 650° C. or higher exceeds 100 hours, as described above.

[0220] <Step S15> In step S15, the heated material is recovered to obtain positive electrode active material particles 100. At this time, the recovered particles can be crushed by sieving, if necessary. Through the above steps, the positive electrode active material particles 100 of one embodiment of the present invention can be produced.

[0221] <<Method 2 for Producing Positive Electrode Active Material Particles 100>> 8(B), a description will be given of a second method for producing the positive electrode active material particles 100. Unlike the first method, this method uses lithium cobalt oxide as a starting material, which serves as both a lithium source and a cobalt source.

[0222] <Step S21> 8(B), lithium cobalt oxide, a magnesium source, and a fluorine source are prepared. The magnesium source and the fluorine source are collectively referred to as source A. For the magnesium source and the fluorine source, the description of step S11 can be referred to.

[0223] The magnesium source and the fluorine source may be added to the lithium cobalt oxide separately, or as shown in steps S21a to S21c of FIG. 8C, the magnesium source and the fluorine source may be mixed together and then added to the lithium cobalt oxide.

[0224] <Steps S22 and S23> Next, lithium cobalt oxide, a magnesium source, and a fluorine source are pulverized and mixed (step S22) to prepare a mixture 903 (step S23). For details of pulverization and mixing, the description of step S12 can be referred to.

[0225] <Step S24> Next, in step S24, the mixture 903 is heated. As for the heating conditions, it is preferable that the total heating time at 650° C. or higher exceeds 100 hours, as described above.

[0226] In addition, 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 mixture 903. For example, when LiF and MgF2 are used as the additive element source, the eutectic point of LiF and MgF2 is around 742°C, so the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0227] In addition, the mixture obtained by mixing LiCoO2:LiF:MgF2 = 100:0.33:1 (molar ratio) showed an initial melting temperature T im is 779℃, the melting peak temperature T pm is 815℃, and the melting end temperature T em Therefore, the lower limit of the heating temperature is more preferably 826°C or higher.

[0228] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0229] The upper limit of the heating temperature is below the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source, etc., within an appropriate range. If the temperature is too high, the fluoride will evaporate and decrease. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, a temperature of 1000°C or lower is more preferable, 950°C or lower is even more preferable, and 900°C or lower is even more preferable.

[0230] In consideration of these, the heating temperature in step S24 is preferably 650°C to 1130°C, more preferably 650°C to 1000°C, even more preferably 650°C to 950°C, and even more preferably 650°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 826°C to 1100°C, or 826°C to 1130°C, more preferably 826°C to 1000°C, even more preferably 826°C to 950°C, and even more preferably 826°C to 920°C.

[0231] <Step S25> In step S25, the heated material is recovered to obtain positive electrode active material particles 100. At this time, the recovered particles can be crushed by sieving, if necessary. Through the above steps, the positive electrode active material particles 100 of one embodiment of the present invention can be produced.

[0232] <<Method 3 for Producing Positive Electrode Active Material Particles 100>> 9 to 10(B) will be used to explain the manufacturing method 3 of the positive electrode active material particles 100. Unlike the manufacturing method 2, nickel and aluminum are used as additive elements in addition to magnesium and fluorine, and the additive elements are added in multiple batches and heated multiple times.

[0233] <Step S31> In step S31 shown in FIG. 9, lithium cobalt oxide is prepared as both a lithium source and a cobalt source, and an Al source is prepared as an additive element source.

[0234] The additive element A1 can be one or more selected from the additive elements described in the previous embodiments, such as magnesium, fluorine, nickel, aluminum, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, and calcium. Also, one or two selected from bromine and beryllium can be used.

[0235] A method for producing an Al source when magnesium and fluorine are used as the additional element Al will be described with reference to FIG.

[0236] <Step S31a> 10(A), a magnesium source and a fluorine source are first prepared. The magnesium source and the fluorine source can be determined by referring to the description of step S11.

[0237] <Step S31b> Next, in step S31b, the magnesium source and the fluorine source are pulverized and mixed. This step can be carried out under pulverization and mixing conditions selected from those described in step S12.

[0238] <Step S31b> Next, in step S31b, the material that has been crushed and mixed as described above is recovered to obtain a source of the additional element A1 (A1 source).

[0239] <Steps S32 to S35> Next, the mixture is mixed and heated in the same manner as in steps S22 to S25 to obtain a composite oxide.

[0240] <Step S41> Next, in step S41, an A2 source is prepared as an additive element source. The additive element A2 can be one or more of the additive elements described in the previous embodiment, such as magnesium, fluorine, nickel, aluminum, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, and calcium. Alternatively, one or more of bromine and beryllium can be used. However, it is preferable that the additive element A2 be an element not used as the additive element A1.

[0241] A method for producing an A2 source when nickel and aluminum are used as the additional element A2 will be described with reference to FIG. 10(B).

[0242] As the nickel source, for example, nickel hydroxide, nickel fluoride, etc. can be used, and as the aluminum source, for example, aluminum hydroxide, aluminum fluoride, etc. can be used.

[0243] The number of nickel atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0% of the number of cobalt atoms in the lithium cobalt oxide, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%. For example, when nickel hydroxide is used as the nickel source, when the number of moles of lithium cobalt oxide in step S10 is taken as 100, the number of moles of nickel hydroxide contained in the second additive element source is preferably 0.05 to 4.0 (0.05 mol% to 4.0 mol%), more preferably 0.20 to 2.0 (0.20 mol% to 2.0 mol%), and even more preferably 0.20 to 1.0 (0.20 mol% to 1.0 mol%).

[0244] The number of aluminum atoms contained in the second additive element source (A2 source) is preferably 0.05% to 4.0% of the number of cobalt atoms in the lithium cobalt oxide, more preferably 0.20% to 2.0%, and even more preferably 0.20% to 1.0%. For example, when aluminum hydroxide is used as the aluminum source, when the number of moles of lithium cobalt oxide in step S10 is taken as 100, the number of moles of aluminum hydroxide contained in the second additive element source is preferably 0.05 to 4.0 (0.05 mol% to 4.0 mol%), more preferably 0.20 to 2.0 (0.20 mol% to 2.0 mol%), and even more preferably 0.20 to 1.0 (0.20 mol% to 1.0 mol%).

[0245] 10(B), it is preferable to prepare a nickel source (Ni source) and an aluminum source (Al source) in step S41b and pulverize them in step S42a. As a result, an additional element source (A2 source) can be obtained in step S43. For pulverization conditions, the description of step S31b can be referred to.

[0246] <Steps S41 to S54> Next, the mixture is mixed and heated in the same manner as in steps S22 to S25, and the positive electrode active material particles 100 are obtained.

[0247] In the method 3 for producing the positive electrode active material particles 100, the total heating time in step S34 and step S44 preferably exceeds 100 hours, and more preferably exceeds 100 hours and is not more than 150 hours. Regarding the heating temperature, the description of the method 2 for producing the positive electrode active material particles 100 can be referred to.

[0248] The content of this embodiment can be freely combined with the content of other embodiment modes.

[0249] (Embodiment 3) In this embodiment, the configuration of a lithium ion secondary battery will be described.

[0250] [Positive electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes positive electrode active material particles and may further include at least one of a conductive additive and a binder. The positive electrode active material particles may be those described in the above embodiment.

[0251] <Cathode active material> As the positive electrode active material, the positive electrode active material particles 100 described in the previous embodiment may be mixed with other positive electrode active materials.

[0252] Other positive electrode active materials include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure, such as LiFePO4, LiFeO2, LiCoO2, LiNiO2, LiMnO2, and LiNi a Mn b Co c Examples include compounds such as O2 (a+b+c=1), LiMn2O4, V2O5, Cr2O5, and MnO2.

[0253] <Conductive additive> The conductive additive is also called a conductivity-imparting agent or a conductive material, and a carbon material can be used. By attaching the conductive additive between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, the term "attachment" does not only refer to physical adhesion between the active material and the conductive additive, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, the conductive additive covers part of the surface of the active material, the conductive additive is embedded in the surface irregularities of the active material, and electrical connection even when not in contact with each other.

[0254] Specific examples of carbon materials that can be used as the conductive additive include carbon black (furnace black, acetylene black, graphite, etc.), graphene, multi-graphene, graphene oxide, and / or reduced graphene oxide.

[0255] Furthermore, the use of a mixture of graphene and acetylene black is preferable because it allows for rapid charging, which is particularly effective when used in automotive lithium-ion secondary batteries.

[0256] <Binder> Examples of the binder include rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluorine rubber can also be used.

[0257] 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.

[0258] Alternatively, it is preferable to use 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 as the binder.

[0259] Additionally, graphene, multigraphene, graphene oxide, and / or reduced graphene oxide can function as a binder as well as a conductive additive.

[0260] The binder may be used in combination with two or more of the above.

[0261] <Positive electrode current collector> The current collector can be made of a highly conductive material, such as iron, gold, platinum, aluminum, titanium, or an alloy containing these metals, including stainless steel. The material used for the positive electrode current collector preferably 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 can 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. The current collector preferably has a thickness of 5 μm to 30 μm.

[0262] [Negative electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, and may further include a conductive additive and a binder.

[0263] <Negative electrode active material> As the negative electrode active material, for example, an alloy material and / or a carbon material can be used.

[0264] The carbon material used for the negative electrode active material may be one or more selected from graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, graphene compounds, carbon black, and the like.

[0265] 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.

[0266] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a low potential similar to that of metallic lithium (0.05 V to 0.3 V vs. Li / Li + ) This allows lithium-ion secondary batteries using graphite to exhibit high operating voltages. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0267] The negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, one or more materials selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium can be used. These elements have a higher capacity than carbon, and silicon, in particular, has a high theoretical capacity of 4200 mAh / g. Compounds containing these elements can also be used. Examples include titanium silicide, titanium silicon oxide, SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Here, elements capable of undergoing charge-discharge reactions through alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials. Compared to graphite, alloy-based materials, including silicon, may be preferable as negative electrode active materials for low-temperature secondary batteries because they may be able to suppress the decrease in charge-discharge capacity at low temperatures.

[0268] In this specification, "SiO" refers to, for example, silicon monoxide. Alternatively, SiO may refer to SiO x Here, x preferably has a value of 1 or close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0269] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO2, SiO x (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a D50 of 1 μm or less, more preferably 100 nm or less.

[0270] Alternatively, silicon particles covered with a graphene compound may be used as the negative electrode active material. In this case, it is more preferable that there is a space between the graphene compound and the silicon particles that can buffer structural changes.

[0271] In addition, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten dioxide (WO2), molybdenum dioxide (MoO2), and the like can be used.

[0272] In addition, the negative electrode active material is a nitride of lithium and a transition metal, Li3N-type Li 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N has a large discharge capacity (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0273] When a nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, which is preferable because it can be combined with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0274] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 It also occurs with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorides such as FeF3 and BiF3.

[0275] Furthermore, a combination of the above-mentioned negative electrode active materials may be used; for example, a negative electrode active material containing a mixture of graphite and silicon particles may be used. Silicon particles are silicon powders used as negative electrode active materials for lithium-ion secondary batteries. These particles have an average particle size distribution, i.e., an average particle size of approximately 100 nm, and are sometimes called nanosilicon particles. The silicon particles used are preferably prepared by pulverizing silicon raw materials to a uniform particle size. The silicon particles may contain at least one of silicon, silicon oxide, and silicon alloy. While laser diffraction particle size distribution measurement is typically used to measure particle size, the method is not limited to laser diffraction particle size distribution measurement. The major axis of the particle cross section may also be measured by analysis using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0276] Furthermore, as the conductive additive and binder that can be contained in the negative electrode active material layer, the same materials as the conductive additive and binder that can be contained in the positive electrode active material layer can be used.

[0277] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0278] [Electrolyte] The electrolytic solution contains an organic solvent, but the organic solvent of the electrolyte according to one embodiment of the present invention is not limited to being liquid at 25°C, and may be solid at 25°C or semi-solid at room temperature. Note that the organic solvent of the electrolyte according to one embodiment of the present invention is preferably liquid over a wide temperature range, including temperatures from below freezing to high temperatures, but is not limited thereto. The organic solvent may be liquid, solid, or semi-solid over a wide temperature range, including temperatures from below freezing to high temperatures.

[0279] The organic solvent is preferably an aprotic organic solvent, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,3-propane sultone (PS), fluoroethylene carbonate (FEC), methyl 3,3,3-trimethylpropane carbonate (MPC ... Any one of fluoropropionate (MTFP), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or any combination and ratio of two or more of these can be used.

[0280] Because PS has similar HOMO and LUMO levels to EC and DEC, it is resistant to oxidation and reduction even at high cutoff voltages, and when decomposed on the surface of the positive electrode active material particles, it tends to form polymers. This has the advantage of reducing the likelihood of gasification by forming low-molecular-weight decomposition products. Therefore, the electrolyte preferably contains 0.1 wt% to 10 wt%, and more preferably 0.25 wt% to 7.5 wt% of PS.

[0281] FEC is a cyclic carbonate with a high dielectric constant. When used in an organic solvent, it promotes the dissociation of lithium salts. However, because FEC contains electron-withdrawing substituents, it desolvates with lithium ions more easily than EC. Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. This facilitates the release of lithium ions from the surfaces of cathode active material particles and anode active material, thereby reducing the internal resistance of secondary batteries. Furthermore, FEC has a deep highest occupied molecular orbital (HOMO) level, making it less susceptible to oxidation and improving oxidation resistance. However, FEC's high viscosity is a concern. Therefore, it is recommended to use an organic solvent mixture containing MTFP in addition to FEC in the electrolyte. MTFP is a chain carbonate that can reduce the viscosity of the electrolyte or maintain the same viscosity at room temperature (typically 25°C) even at low temperatures (typically 0°C). Furthermore, although MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have electron-withdrawing substituents, it may still solvate with lithium ions when used in an electrolyte. When using a mixed organic solvent containing both FEC and MTFP, the volume ratio is FEC:MTFP=1:y, where y is preferably 2 or more and 20 or less, and more preferably 4 or more and 9 or less.

[0282] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the electricity storage device from exploding or catching fire even if the internal temperature rises due to an internal short circuit or overcharging of the electricity storage device. Ionic liquids are composed of cations and anions, including organic cations and anions. 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.

[0283] Examples of the electrolyte to be dissolved in the solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B 12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, lithium bis(oxalato)borate (Li(C2O4)2, LiBOB), etc. can be used alone or in any combination and ratio of two or more of these.

[0284] The electrolyte solution may also contain additives. The additives can suppress reactive decomposition of the electrolyte that may occur on the positive electrode surface or the negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives that can be used include propane sultone (PS), vinylene carbonate (VC), tert-butylbenzene (TBB), lithium bis(oxalato)borate (LiBOB), 1,3,6-hexanetricarbonitrile, ethyl 2-methylbutyrate, ethyl 2-methylvalerate, and propyl 2-methylbutyrate. PS is particularly preferred as an additive because it improves cycle characteristics.

[0285] The additive may be one or more dinitrile compounds, such as succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

[0286] Fluorobenzene may also be added to the organic solvent. The concentration of the additive is, for example, 0.1 wt% to 5 wt% of the total electrolyte solution. PS or EGBE are preferred because they form a good coating on the positive electrode during charge and discharge, improving cycle characteristics. Fluorobenzene (FB) is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes. Dinitrile compounds are preferred because their nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving voltage resistance. Furthermore, dinitrile compounds are preferred because they can prevent copper dissolution during overdischarge when a copper-containing current collector is used on the negative electrode. Considering the use of secondary batteries at high voltages, adding a nitrile compound is preferred.

[0287] The electrolyte used in the electricity storage device is preferably a highly purified electrolyte with a low content of granular waste or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0288] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0289] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0290] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel. For example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0291] In addition, the electrolyte can be a solid electrolyte containing inorganic materials such as sulfides or oxides, or a solid electrolyte containing polymeric materials such as polyethylene oxide (PEO). When a solid electrolyte is used, the installation of a separator or spacer is unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, dramatically improving safety.

[0292] [Separator] When the electrolyte contains a liquid electrolyte (also called an electrolyte solution), a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polypropylene (referred to as PP), polyimide (referred to as PI), polyester, acrylic, polyolefin, and polyurethane. The porosity of the separator can be 35% to 90%, preferably 60% to 85%. Separators using polypropylene can have a porosity of 35% to 45%. Separators using polyimide can have a porosity of 75% to 85%. The separator thickness is preferably 10 μm to 80 μm, more preferably 20 μm to 60 μm. A separator using polyimide is preferable because it can have a high porosity and can be made thick (typically, the thickness is 50 μm or more and 60 μm or less).

[0293] The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0294] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene may be coated with a ceramic material, a 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).

[0295] By using a multilayer separator, the safety of the lithium-ion battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the lithium-ion battery can be increased.

[0296] [Exterior body] The exterior of a lithium-ion battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior can be used. Examples of the film include a three-layer structure in which a thin, flexible metal film made of aluminum, stainless steel, copper, nickel, or the like is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film made of polyamide resin, polyester resin, or the like is further provided on the thin metal film as the outer surface of the exterior.

[0297] This embodiment can be used in combination with other embodiments.

[0298] (Fourth embodiment) In this embodiment, an example of a lithium-ion secondary battery will be described with reference to FIG.

[0299] Fig. 11(A) is a diagram illustrating a wound body 950a included in a lithium ion secondary battery 913, Fig. 11(B) is an exploded perspective view of the lithium ion secondary battery 913, and Fig. 11(C) is an external view of the lithium ion secondary battery 913. The lithium ion secondary battery 913 includes a positive electrode 932 having the positive electrode active material described in the previous embodiment, a negative electrode 931, an electrolyte layer, and a separator 933. The negative electrode 931 includes a negative electrode active material layer 931a. The positive electrode 932 includes a positive electrode active material layer 932a. These are wound as shown in Fig. 11(A).

[0300] The separator 933 has a width wider than the negative electrode active material layer 931a and the positive electrode active material layer 932a, and is wound so as to overlap the negative electrode active material layer 931a and the positive electrode active material layer 932a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931a be wider than the positive electrode active material layer 932a. A wound body 950a having such a shape is preferable because of its high safety and productivity.

[0301] 11B, the negative electrode 931 is electrically connected to a terminal 951. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952. The terminal 952 is electrically connected to a terminal 911b.

[0302] 11(C), the wound body 950a is covered with the housing 930 to form the lithium ion secondary battery 913. It is preferable that a safety valve, an overcurrent protection element, and the like are provided in the housing 930. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0303] 11B, the lithium ion secondary battery 913 may have a plurality of wound bodies 950a. By using a plurality of wound bodies 950a, the lithium ion secondary battery 913 can have a larger charge / discharge capacity.

[0304] By using the positive electrode active material particles of the present invention in the lithium ion secondary battery 913 having a wound body, it is possible to obtain a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics.

[0305] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0306] (Embodiment 5) In this embodiment, an example of application to an electric vehicle (EV) will be shown with reference to FIG.

[0307] 12(A), an electric vehicle is equipped with first batteries 1301a and 1301b as main driving lithium ion secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. By using the positive electrode active material particles of the present invention for the first batteries 1301a and 1301b, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0308] The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0309] The internal structure of the first battery 1301a may be a wound type or a stacked type. Alternatively, a lithium-ion secondary battery including the positive electrode active material particles of one embodiment of the present invention may be used for the first battery 1301a. By using a lithium-ion secondary battery including the positive electrode active material particles of one embodiment of the present invention for the first battery 1301a, an electric vehicle with a long driving range and usable in a wide range of ambient temperatures can be obtained.

[0310] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more may be connected in parallel. Also, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple lithium ion secondary batteries, it is possible to extract large amounts of power. The multiple lithium ion secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of lithium ion secondary batteries is also called a battery pack.

[0311] In addition, in an in-vehicle lithium ion secondary battery, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a in order to cut off power from multiple lithium ion secondary batteries.

[0312] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DC-DC circuit 1306. When a rear motor 1317 is provided on the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0313] Furthermore, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0314] The first battery 1301a will be described with reference to FIG. 12(B).

[0315] FIG. 12B shows an example in which nine prismatic lithium-ion secondary batteries 1300 are used as one battery pack 1415. Nine prismatic lithium-ion secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also called a casing). Because it is assumed that a vehicle is subjected to vibration or shaking from the outside (such as a road surface), it is preferable to fix multiple lithium-ion secondary batteries using the fixing portions 1413 and 1414 and the battery housing box. One electrode is electrically connected to a control circuit unit 1320 by a wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by a wiring 1422.

[0316] FIG. 12C shows an example of a block diagram of the battery pack 1415 shown in FIG. 12B.

[0317] The control circuit unit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit unit 1320 sets upper and lower voltage limits for the lithium-ion secondary battery used and limits the upper limit of the external current and the upper limit of the output current. The range between the lower limit and the upper limit of the lithium-ion secondary battery's voltage is within the recommended voltage range. If the voltage falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit unit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. A PTC element may also be provided in the charge / discharge path to provide a function for cutting off the current in response to a rise in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0318] The first batteries 1301a and 1301b mainly supply power to 42V (high-voltage) in-vehicle devices, while the second battery 1311 supplies power to 14V (low-voltage) in-vehicle devices. Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Using a lithium-ion secondary battery as the second battery 1311 offers the advantage of being maintenance-free, but after extended use, such as three years or more, there is a risk of abnormalities occurring that are not detectable at the time of manufacture. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b still have remaining capacity, the second battery 1311 is charged to maintain a full charge state by supplying power from the first battery to the second battery.

[0319] In this embodiment, an example in which lithium ion secondary batteries are used for both the first battery 1301a and the second battery 1311 is shown, but a lead acid battery, an all-solid-state battery, or an electric double layer capacitor may be used for the second battery 1311. By using the positive electrode active material particles of the present invention in the above-mentioned lithium ion secondary battery, it is possible to obtain a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics.

[0320] Furthermore, regenerated energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 from the motor controller 1303 and the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerated energy, it is desirable that the first batteries 1301a and 1301b be capable of being rapidly charged.

[0321] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions according to the charging characteristics of the lithium ion secondary battery used, and can perform rapid charging.

[0322] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a, 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the function of the battery controller 1302 is not used, it is preferable to charge the first batteries 1301a, 1301b via a control circuit unit 1320 to prevent overcharging. In some cases, the connection cable or the charger's connection cable is provided with a control circuit. The control circuit unit 1320 is also called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU includes a microcomputer. The ECU uses a CPU or a GPU.

[0323] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging is also possible by receiving power from external charging equipment using methods such as contactless power supply.

[0324] Next, an example in which the lithium-ion secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0325] Furthermore, installing lithium-ion secondary batteries in vehicles will enable next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), plug-in hybrid vehicles (PHVs), etc. Lithium-ion secondary batteries can also be installed in transportation vehicles such as agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

[0326] 13A to 13E show examples of vehicles and the like using the lithium-ion secondary battery of one embodiment of the present invention.

[0327] 13A shows an example of an electric bicycle using the lithium-ion secondary battery of one embodiment of the present invention. The lithium-ion secondary battery of one embodiment of the present invention can be applied to the electric bicycle 8700 shown in FIG. 13A. The lithium-ion secondary battery of one embodiment of the present invention may include a protection circuit.

[0328] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 can be detached from the electric bicycle 8700 and can be carried around. The power storage device 8702 includes a plurality of lithium-ion secondary batteries of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit. By using the positive electrode active material particles of the present invention for the lithium-ion secondary battery, the lithium-ion secondary battery can have large charge / discharge capacity and favorable cycle characteristics.

[0329] FIG. 13B illustrates an example of a motorcycle using a lithium-ion secondary battery of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 13B includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The scooter 8600 can store the power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can supply electricity to the turn signal light 8603. When the scooter has a motor, the power storage device 8602 can also supply electricity to the motor. By using the positive electrode active material particles of the present invention for the lithium-ion secondary battery included in the power storage device 8602, the lithium-ion secondary battery can have large charge / discharge capacity and favorable cycle characteristics.

[0330] The automobile 2001 shown in FIG. 13(C) is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a lithium ion secondary battery is mounted on a vehicle, an example of the lithium ion secondary battery shown in the above embodiment is installed in one or more locations. By using the positive electrode active material particles of the present invention in the lithium ion secondary battery mounted on a vehicle, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0331] 13C includes a battery pack 2200, which includes a battery module to which a plurality of lithium-ion secondary batteries are connected. The battery pack 2200 preferably further includes a charge control device electrically connected to the battery module.

[0332] Furthermore, automobile 2001 can charge its lithium-ion secondary battery by receiving power supply from an external charging facility using a plug-in method, a contactless power supply method, or the like. Charging can be performed using a predetermined charging method and connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. A charging station installed in a commercial facility, a household power source, or the like can be used as the external charging facility. For example, plug-in technology can be used to charge the power storage device installed in automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device such as an AC-DC converter.

[0333] Furthermore, although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the lithium-ion secondary battery while the vehicle is stopped and while moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0334] Figure 13(D) shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, in which more than 100 lithium ion secondary batteries with a nominal voltage of 3.0 V to 5.0 V are connected in series. Furthermore, except for the number of lithium ion secondary batteries constituting the battery module of the battery pack 2202, the same functions as those of Figure 13(C), and therefore a description thereof will be omitted. By using the positive electrode active material particles of the present invention for the lithium ion secondary batteries of the module, a lithium ion secondary battery with large charge / discharge capacity and good cycle characteristics can be obtained.

[0335] 13(E) shows, as an example, an aircraft 2004 having an engine that burns fuel. The aircraft 2004 has wheels for takeoff and landing and can therefore be considered a part of a transportation vehicle, and has a battery pack 2203 which includes a battery module formed by connecting multiple lithium ion secondary batteries and which includes the battery module and a charge control device.

[0336] The battery module of the aircraft 2004 is, for example, eight 4V lithium ion secondary batteries connected in series, with a maximum voltage of 32V. Other than the number of lithium ion secondary batteries constituting the battery module of the battery pack 2203, it has the same functions as those in Fig. 13(C), and therefore a description thereof will be omitted.

[0337] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0338] (Sixth embodiment) In this embodiment, an example of mounting a lithium-ion secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a lithium-ion secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0339] 14(A) shows an example of a mobile phone. The mobile phone 2100 includes a display unit 2102 built into a housing 2101, as well as operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. The mobile phone 2100 also includes a lithium ion secondary battery 2107. By using the positive electrode active material particles of the present invention in the lithium ion secondary battery, the lithium ion secondary battery can have a large charge / discharge capacity and good cycle characteristics.

[0340] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0341] The operation button 2103 can be provided with various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system built into the mobile phone 2100.

[0342] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0343] The mobile phone 2100 also has an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Charging may also be performed by wireless power supply without using the external connection port 2104.

[0344] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0345] FIG. 14B shows an unmanned aerial vehicle 2300 having multiple rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a lithium-ion secondary battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. By using the positive electrode active material particles of the present invention in the lithium-ion secondary battery, the lithium-ion secondary battery can have large charge / discharge capacity and favorable cycle characteristics.

[0346] Fig. 14(C) shows an example of a robot. The robot 6400 shown in Fig. 14(C) includes a lithium ion 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 moving mechanism 6408, a computing device, etc.

[0347] 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.

[0348] 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.

[0349] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0350] The robot 6400 includes a lithium-ion secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. By using the positive electrode active material particles of the present invention in the lithium-ion secondary battery, the lithium-ion secondary battery can have large charge / discharge capacity and favorable cycle characteristics.

[0351] 14(D) shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a lithium-ion 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.

[0352] 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, if an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a lithium-ion secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal space. By using the positive electrode active material particles of the present invention in the lithium-ion secondary battery, the lithium-ion secondary battery can have large charge / discharge capacity and favorable cycle characteristics.

[0353] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Example]

[0354] In this example, a positive electrode active material according to one embodiment of the present invention was prepared, and powder analysis and charge / discharge characteristics were evaluated.

[0355] <Preparation of positive electrode active material> The positive electrode active material samples prepared in this example will be described with reference to the preparation methods shown in Figures 8(B) and 8(C). As the positive electrode active material samples, Sample 1 and Sample 2 were prepared by changing the preparation conditions.

[0356] [Sample 1] As LiCoO2 in step S21 of FIG. 8(B), commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) containing no particular additive elements was prepared.

[0357] In this embodiment, an A source containing Mg and F as additive elements was prepared according to steps S21a to S21c shown in Fig. 8(C). First, according to step S21a shown in Fig. 8(C), lithium fluoride (LiF) was prepared as the F source, and magnesium fluoride (MgF2) was prepared as the Mg source. LiF and MgF2 were weighed so that the molar ratio was 1:3. Next, lithium fluoride and magnesium fluoride were mixed in dehydrated acetone and stirred at a rotation speed of 500 rpm for 20 hours. A ball mill was used for the mixing, and zirconium oxide balls were used as the media. After mixing, sieving was performed with a sieve having a mesh size of 300 μm to obtain the A source.

[0358] Next, as step S22, when the number of moles of lithium cobaltate was 100, lithium cobaltate and the A source were weighed so that the number of moles of magnesium fluoride contained in the A source was 1 (1 mol%), and lithium cobaltate and the A source were mixed with a ball mill. At this time, stirring was performed at a rotation speed of 150 rpm for 1 hour. This is a gentler condition than the stirring when obtaining the A source. Finally, sieving was performed with a sieve having a mesh size of 300 μm to obtain a mixture 903 with uniform particle size (step S23).

[0359] Next, as step S24, the mixture 903 was heated. The mixture 903 was evenly spread on the boat to a thickness of about 5 mm, covered, and heated. A muffler furnace was used for heating, and the inside of the muffler furnace and the boat were purged to create an oxygen atmosphere, and the gas inlet and outlet of the muffler furnace were blocked. The heating conditions were a heating temperature of 900 °C and a heating time of 10 hours. By heating, a positive electrode active material, which is a composite oxide having Mg and F, was obtained (step S25). The positive electrode active material thus obtained was designated as Sample 1.

[0360] [Sample 2] In step S24, a sample prepared in the same manner as Sample 1 except that the heating conditions were a heating temperature of 900 °C and a heating time of 120 hours was designated as Sample 2.

[0361] <XPS analysis results> XPS analysis was carried out to analyze the particle surfaces of the above-prepared Sample 1 and Sample 2. The XPS measurement device and conditions were as follows: Measurement equipment: ULVAC-PHI Quantera II X-ray source: Monochromatic Al Kα (1486.6eV) Detection area: 100 μmφ Detection depth (Condition 1): Range from the surface to approximately 5 nm (detector angle 45°) Detection depth (condition 2): Range from the surface to approximately 2 nm (detector angle 15°) Measurement spectrum: Wide scan, narrow scan for each detected element

[0362] The results of the XPS analysis are shown in Tables 1 and 2. In this specification and the like, atomic % and atomic % may be abbreviated to at%.

[0363] [Table 1]

[0364] [Table 2]

[0365] As shown in Tables 1 and 2, for both Sample 1 and Sample 2, there was no significant difference in the concentrations of each element analyzed under Condition 1 with a detector angle of 45° and Condition 2 with a detector angle of 15°.

[0366] Table 1 also shows the concentration of each element (at%: atomic concentration, sometimes referred to as atomic %) when the total concentration of lithium (Li), cobalt (Co), oxygen (O), carbon (C), fluorine (F), sulfur (S), calcium (Ca), magnesium (Mg), sodium (Na), and zirconium (Zr) is taken as 100 at%. Table 1 also shows the ratio of magnesium concentration to cobalt concentration (Mg / Co) and the ratio of fluorine concentration to magnesium concentration (F / Mg). In the table, "-" indicates that the value was below the detection limit.

[0367] In addition, the lithium concentration in the positive electrode active material of a lithium-ion secondary battery changes with charging and discharging. Therefore, Table 2 shows the concentration of each element when the total concentration of cobalt (Co), oxygen (O), carbon (C), fluorine (F), sulfur (S), calcium (Ca), magnesium (Mg), sodium (Na), and zirconium (Zr) is taken as 100 at% after excluding the lithium values ​​from the results in Table 1.

[0368] As shown in Tables 1 and 2, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) was 0.73 (45°) and 0.82 (15°) in Sample 1. The ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in Sample 2 was 0.48 (45°) and 0.64 (15°), both of which were in the range of 0.400 to 0.700.

[0369] Furthermore, as shown in Tables 1 and 2, the ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg) was 0.73 (45°) and 0.71 (15°) in Sample 1. The ratio of the number of fluorine atoms to the number of magnesium atoms (F / Mg) in Sample 2 was 0.17 (45°) and 0.12 (15°), both of which were within the range of 0.100 to 0.200.

[0370] Next, we focused on the Mg1s peak in the XPS spectrum during XPS analysis. In analyzing the Mg1s peak, we designated the peak component derived from the O-Mg-O bond as fit peak 1, the peak component derived from the O-Mg-F bond as fit peak 2, and the peak component derived from the F-Mg-F bond as fit peak 3. We then synthesized these three fit peaks and calculated the peak synthesis ratio that minimized the difference from the Mg1s peak in the XPS spectrum obtained by XPS analysis. The analysis results, obtained by assuming that the area ratios of fit peak 1, fit peak 2, and fit peak 3 were equal to the abundance ratios of O-Mg-O, O-Mg-F, and F-Mg-F bonds, are shown in Table 3.

[0371] In the XPS spectrum analysis method described above, the energy value (Ep1) at the maximum value (also referred to as the peak top) of fit peak 1 was determined by referring to the energy value at the maximum value of the Mg1s peak when a separate measurement was performed using MgO-coated LiCoO2 as a standard sample. The energy value (Ep3) at the maximum value of fit peak 3 was determined by referring to the energy value at the maximum value of the Mg1s peak when a separate measurement was performed using magnesium fluoride (MgF2, Kojundo Chemical Research Institute MGH18XB, purity 99.9% (3N) up) as a standard sample. The energy value (Ep2) at the maximum value of fit peak 2 was taken as the intermediate value between Ep1 and Ep3. The energy value at the maximum value of the peak is also referred to as the peak position.

[0372] In the XPS analysis, the energy axis of the XPS spectrum was corrected so that the maximum value of the C1s peak was 284.8 eV.

[0373] [Table 3]

[0374] As shown in Table 3, it was found that Sample 1 had peak components derived from O-Mg-O bonds and peak components derived from O-Mg-F bonds. Also, Sample 2 had peak components derived from O-Mg-O bonds, but it was found that the peak components derived from "O-Mg-F" bonds were below the detection limit. Note that even when it is set to 0.0% in the above analysis results, it does not mean that there is no corresponding bond at all. That is, the bond set to 0.0% may exist below the detection limit.

[0375] <EPMA Analysis Results> Next, for Sample 1 and Sample 2, the added elements were quantified by EPMA. The measuring apparatus and conditions of EPMA were as follows. Apparatus: JXA-iHP200F manufactured by JEOL Ltd. Acceleration voltage: 10 kV Beam diameter: approximately 3 μmφ

[0376] The samples were embedded in resin and processed by the ion polishing method to expose the particle cross-section. In EPMA, the measurement region was inside the positive electrode active material, that is, the center of the particle cross-section. Note that the cross-section is analyzed by EPMA using characteristic X-rays obtained from a region reaching a depth of about 1 μm from the surface of the analysis sample.

[0377] The EPMA results are shown in Table 4.

[0378]

Table 4

[0379] Table 4 shows the concentration of each element (at%: atomic concentration) when the total concentration of carbon (C), oxygen (O), magnesium (Mg), and cobalt (Co) is 100 at%, and the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) with respect to the number of cobalt atoms. For both Sample 1 and Sample 2, Mg / Co was 0.01, indicating that magnesium was also dissolved inside the positive electrode active material particles. In EPMA, no significant difference was observed between Sample 1 and Sample 2.

[0380] <STEM analysis> Cross-sectional STEM analysis of Sample 1 and Sample 2 was performed.

[0381] As a pretreatment before analysis, Sample 2 was thinned by the FIB method (μ-sampling method).

[0382] STEM and EDX were used with the following equipment and conditions. ≪STEM Observation≫ Scanning transmission electron microscope: JEM-ARM200F NEOARM manufactured by JEOL Observation conditions Acceleration voltage: 200 kV Magnification accuracy: ±10% ≪EDX≫ Analysis method: Energy dispersive X-ray spectroscopy (EDX) Scanning transmission electron microscope: JEM-ARM200F NEOARM manufactured by JEOL Acceleration voltage: 200 kV Measurement mode: STEM mode Elemental analyzer: JED-2300T X-ray detector: Si drift detector Energy resolution: approximately 140 eV X-ray extraction angle: approximately 30.5° Solid angle: 2.2 sr Number of pixels taken in: 256×256

[0383] [STEM-EDX] The results of cross-sectional HAADF-STEM analysis of Sample 1 and Sample 2 are shown in FIGS. 15 to 20.

[0384] Fig. 15(A) is a cross-sectional STEM image of Sample 1, Fig. 15(B) is a graph showing the results of STEM-EDX ray analysis at A-B in Fig. 15(A) with the vertical axis representing the count value of characteristic X-rays, and Fig. 15(C) is a graph showing the vertical axis of the graph in Fig. 15(B) as the quantitative value in atomic %. The quantitative value in atomic % in the STEM-EDX analysis was calculated by setting the sum of the detected amounts of carbon, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, and gallium to 100%.

[0385] Moreover, Fig. 16(A) is a graph in which the vertical axis of Fig. 15(B) is enlarged. Fig. 16(B) is a graph showing the graph of magnesium (Mg K) extracted from Fig. 16(A), and Fig. 16(C) is a graph showing the graph of fluorine (FK) extracted from Fig. 16(A).

[0386] Moreover, Fig. 17(A) is a graph in which the vertical axis of Fig. 15(C) is enlarged. Fig. 17(B) is a graph showing the graph of magnesium (Mg K) extracted from Fig. 17(A), and Fig. 17(C) is a graph showing the graph of fluorine (FK) extracted from Fig. 17(A).

[0387] FIG. 18(A) is a cross-sectional STEM image of Sample 2, FIG. 18(B) is a graph showing the results of STEM-EDX line analysis in CD of FIG. 18(A) as count values ​​of characteristic X-rays on the vertical axis, and FIG. 18(C) is a graph showing the quantitative value of atomic % on the vertical axis of the graph of FIG. 18(B).

[0388] Moreover, Fig. 19(A) is a graph in which the vertical axis of Fig. 18(B) is enlarged. Fig. 19(B) is a graph showing the graph of magnesium (Mg K) extracted from Fig. 19(A), and Fig. 19(C) is a graph showing the graph of fluorine (FK) extracted from Fig. 19(A).

[0389] Moreover, Fig. 20(A) is a graph in which the vertical axis of Fig. 18(C) is enlarged. Fig. 20(B) is a graph showing the graph of magnesium (Mg K) extracted from Fig. 20(A), and Fig. 20(C) is a graph showing the graph of fluorine (FK) extracted from Fig. 20(A).

[0390] As shown in Figures 19(A) to 20(C), the positions of the maximum fluorine concentration and the peak count values ​​were located closer to the surface than the positions of the maximum magnesium concentration and the peak count values. In other words, the positions of the maximum magnesium concentration and the peak count values ​​were located closer to the interior than the positions of the maximum fluorine concentration and the peak count values. Magnesium is a divalent cation and can substitute for cobalt sites. It can also be said that fluorine is located closer to the surface than the region where magnesium substitutes for some of the cobalt sites.

[0391] [HAADF-STEM] The results of HAADF-STEM analysis of Sample 1 and Sample 2 are shown in FIGS.

[0392] Fig. 21(A) is a cross-sectional STEM image of Sample 1, and Fig. 21(B) is a HAADF-STEM image of the area enclosed by a square in Fig. 21(A). Note that the area enclosed by the square includes the edge surface of Sample 1.

[0393] Fig. 22(A) is a cross-sectional STEM image of Sample 2, and Fig. 22(B) is a HAADF-STEM image of the area enclosed by a square in Fig. 22(A). Note that the area enclosed by the square includes the edge surface of Sample 2.

[0394] Figure 23(A) is an enlarged image of the area enclosed by a square in Figure 21(B), and Figure 23(B) is an enlarged image of the area enclosed by a square in Figure 22(B). A significant difference was confirmed between the HAADF-STEM image of Sample 1 shown in Figure 23(A) and the HAADF-STEM image of Sample 2 shown in Figure 23(B). This difference will be explained using Figures 24(A) and 24(B).

[0395] Figures 24(A) and 24(B) are HAADF-STEM images of Sample 2 in Figure 23(B) with brightness and contrast adjustments. The dashed lines in the figures are auxiliary lines drawn at locations where different crystal structures are presumed to exist on either side of the dashed lines. The left side of the dashed lines is presumed to be dominated by a rock salt structure (also referred to as scattered rock salt structures), while the right side of the dashed lines is presumed to be dominated by a layered rock salt structure. In other words, the first and second layers observed in the HAADF-STEM images, which are closest to the exterior, are presumed to be dominated by a rock salt structure, while the third and subsequent layers, which are closer to the interior, are presumed to be dominated by a layered rock salt structure. In Figures 24(A) and 24(B), the first through fifth layers are indicated by numbers 1 through 5. The black arrows on the right side of the figures indicate locations presumed to correspond to the Co site of the lithium cobalt oxide with a layered rock salt structure, and the open arrows on the right side of the figures indicate locations presumed to correspond to the Li site of the lithium cobalt oxide with a layered rock salt structure.

[0396] Focusing on the white circle in Figure 24(A), we can see that the brightness is low despite it being the location of the Co site. From the STEM-EDX results in Figures 18(A) to 20(C), cobalt and magnesium are detected in the surface layer of Sample 2, but no other metals with atomic numbers close to that of magnesium are detected. This suggests that Mg, an element with a smaller atomic weight than Co, is present at the Co site at the white circle. The location of the white circle, i.e., the location where Mg is thought to be present at the Co site, was the fourth layer in the HAADF-STEM image of Sample 2.

[0397] Next, focusing on the area indicated by the white triangle in Figure 24(B), we can see that the brightness is high at the position corresponding to the Li site. This is because an element with a larger atomic weight than Li is present at the Li site at the white triangle position, and based on the STEM-EDX results, it is thought that Mg or Co is present there. The position where Mg or Co is thought to be present at the Li site was the third layer in the HAADF-STEM image of Sample 2.

[0398] <Preparation of positive electrode> Samples 1 and 2 were prepared as the positive electrode active material, acetylene black (AB, Denka Li-400) as the conductive material, and polyvinylidene fluoride (PVDF, Solvay GE51305) as the binder. PVDF was dissolved in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 5%. Next, the positive electrode active material, AB, and PVDF were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum positive electrode current collector. NMP was used as the solvent for the slurry. After applying the slurry to a 20 μm aluminum current collector, the solvent was evaporated in a forced-air drying oven at 80°C for 1 hour.

[0399] Thereafter, in order to increase the density of the positive electrode active material layer on the positive electrode current collector, a pressing process was performed using a roll press. The pressing process was performed under a linear pressure of 210 kN / m. The upper and lower rolls of the roll press were both set at 120°C.

[0400] A positive electrode was obtained by the above steps. The amount of the positive electrode active material carried per area of ​​the positive electrode was about 7 mg / cm. 2 By this manufacturing method, a positive electrode having Sample 1 and a positive electrode having Sample 2 were manufactured.

[0401] <Half-cell fabrication> Coin-shaped half cells (also called coin cells) were fabricated using each of the above positive electrodes, lithium metal foil, a separator, an electrolyte, a coin cell positive electrode can, and a coin cell negative electrode can. The coin-shaped half cells were CR2032 type (diameter 20 mm, height 3.2 mm).

[0402] The electrolyte used was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, to which 1 mol / L of lithium hexafluorophosphate (LiPF6) was dissolved, to which 2 wt% of vinylene carbonate (VC) was added as an additive.

[0403] A porous polypropylene film was used as the separator.

[0404] In this manner, a coin cell having Sample 1 and a coin cell having Sample 2 were produced.

[0405] <Charge / discharge cycle test> A charge-discharge cycle test was carried out using the above half cell.

[0406] The conditions for the charge-discharge cycle test were as follows: charging at a constant current of 0.5 C up to 4.60 V, followed by constant voltage charging until the current value reached 0.05 C. Discharging was performed at a constant current of 0.5 C down to 2.50 V. Here, 1 C was set to 200 mA / g. The temperature of the measurement environment was 25°C. Charging and discharging were repeated 50 times in this manner. The results of the charge-discharge cycle test are shown in Figures 25(A) and 25(B).

[0407] Fig. 25(A) is a graph showing the discharge capacity and cycle number of a half cell using Sample 1 (dashed line) and a half cell using Sample 2 (solid line). Fig. 25(B) is a graph showing the discharge capacity of Fig. 25(A) as a discharge capacity retention rate. The discharge capacity retention rate was calculated by setting the discharge capacity value at the discharge where the maximum discharge capacity was obtained in the charge-discharge cycle test as 100%.

[0408] The results of these measurements are shown in Table 5. Table 5 shows the maximum discharge capacity values ​​for Sample 1 and Sample 2, as well as the discharge capacities and discharge capacity retention rates at the 1st, 2nd, 5th, 10th, 20th, 25th, 30th, 40th, and 50th cycles. The discharge capacity is a value per weight of the positive electrode active material in each half-cell.

[0409] [Table 5]

[0410] As shown in FIGS. 25(A) and 25(B) and Table 5, the cycling characteristics of the half-cell containing Sample 2 were superior to those of the half-cell containing Sample 1.

[0411] <XRD analysis of high voltage charging state> An experiment was conducted to investigate the crystal structures of Sample 1 and Sample 2 in a high-voltage charged state.

[0412] First, using a half-cell using Sample 1 and a half-cell using Sample 2, charging, discharging, disassembly of the half-cell, and XRD measurement were performed. The half-cells were fabricated as described above in <Fabrication of Half-Cells>, except that they were not pressed. Charging was performed at a constant current of 0.5 C up to 4.60 V, followed by constant voltage charging until the current value reached 0.01 C. Discharging was performed at a constant current of 0.5 C down to 2.5 V. For XRD analysis of the state of charge, 1 C was set to 137 mA / g. A 30-minute break was allowed between charging and discharging, and between charging and discharging.

[0413] Next, charging was performed before XRD analysis of the high-voltage charged state. Charging was performed by constant current charging at 0.5 C up to 4.60 V, and then constant voltage charging until the current value reached 0.01 C. After that, a rest period of 30 minutes was provided.

[0414] After the charging was completed, the half-cell was disassembled within one hour. To remove the positive electrode while it was still in a high-voltage charged state, the disassembly was carried out carefully using insulating tools to avoid short circuits. The disassembly was carried out in a glove box filled with argon, the dew point and oxygen concentration of which were controlled. The dew point of the glove box was preferably −70°C or lower, and the oxygen concentration was preferably 5 vol ppm or lower. Since the crystalline structure of the positive electrode active material may change if a long period of time has passed since the charging, it is preferable to disassemble and analyze the cell as soon as possible.

[0415] The positive electrode obtained by disassembling the half cell was attached to a 0.7 mm thick glass in the glove box and set in a hermetically sealed sample holder for XRD measurement (manufactured by Bruker, Part No. A100B33), thereby obtaining a positive electrode sealed on the stage for XRD measurement together with argon.

[0416] Within 15 minutes, the XRD measurement was started using the following XRD equipment and conditions. XRD equipment: Bruker AXS, D8 ADVANCE X-ray:CuKα1 ray Output: 40kV, 40mA Divergence slit: 0.6 mm Detector: LYNXEYE XE-T Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 75° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0417] The XRD patterns obtained by removing the CuKα2 peaks and the background due to the sample holder from the XRD measurement data of the positive electrodes (Sample 1 and Sample 2) in the high-voltage charged state measured above using the data analysis tool DIFFRAC.EVA under the following conditions are shown in Figures 26, 27, and 28. <Kα2 removal conditions> Max:1 Strength ratio: 0.5 Minimum: 0 <Background removal conditions> Curvature: 25 Threshold: 0.001

[0418] These figures also show the reference pattern (O3') for the O3' structure and the reference pattern (H1-3) for the H1-3 structure. Note that because the above measurements were performed using two half-cells, the measurement data for Sample 1 is listed as Sample 1-1 and Sample 1-2, and the measurement data for Sample 2 is listed as Sample 2-1 and Sample 2-2.

[0419] Fig. 27 shows the range of 2θ from 18° to 21° in XRD measurement, and Fig. 28 shows the range of 2θ from 42° to 47° in XRD measurement.

[0420] 26 to 28, only a few peaks matching the reference pattern (O3') of the O3' structure were observed in Sample 1 charged at a high voltage, whereas a clear peak matching the reference pattern (O3') of the O3' structure was observed in Sample 2 charged at a high voltage. [Explanation of symbols]

[0421] 10 Lithium-ion secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 21 Positive electrode current collector 22 Cathode active material layer 31 Negative electrode current collector 32 Negative electrode active material layer 41 Conductive materials 51 Electrolytes 100 Positive electrode active material particles 100a Surface layer 100b internal < / epma> < / edx> < / xps> < / xrd>

Claims

1. A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide; the positive electrode active material particle has a region in which magnesium is substituted for part of the cobalt site of a second layer to a sixth layer of the positive electrode active material particle, when the surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and extracted is defined as a first layer.

2. In claim 1, The positive electrode active material particles have a region where magnesium substitutes for a part of the cobalt site of the fourth layer observed in a cross-sectional STEM image of the surface where lithium is inserted and extracted.

3. In claim 1, the positive electrode active material particles have a layered rock salt type crystal structure inside and a rock salt type crystal structure on a surface layer, The magnesium has a function of alleviating distortion between the layered rock salt type crystal structure and the rock salt type crystal structure.

4. A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide; the positive electrode active material particle has a region in which magnesium is substituted for a part of cobalt sites in second to sixth layers of the positive electrode active material particle, when a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and desorbed is defined as a first layer; The lithium ion secondary battery, wherein the fluorine is present on the surface side of the region.

5. In claim 4, The lithium ion secondary battery, wherein the fluorine has a function of promoting the migration of the magnesium into the inside of the positive electrode active material particles.

6. A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide; the positive electrode active material particle has a region in which magnesium is substituted for a part of cobalt sites in second to sixth layers of the positive electrode active material particle, when a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and desorbed is defined as a first layer; The lithium ion secondary battery has rock salt structures scattered in first to third layers observed in a cross-sectional STEM image of the surface where lithium is inserted and extracted.

7. A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, aluminum, and lithium cobalt oxide; the positive electrode active material particle has a region in which magnesium is substituted for part of cobalt sites in second to sixth layers of the positive electrode active material particle, when a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and desorbed is defined as a first layer; the positive electrode active material particles have a layered rock salt type crystal structure therein, The lithium ion secondary battery, wherein the aluminum is present inside the positive electrode active material particles.

8. In claim 7, The aluminum has a function of mitigating volume changes of the layered rock salt crystal structure due to charging and discharging.

9. A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, nickel, and lithium cobalt oxide; the positive electrode active material particle has a region in which magnesium is substituted for a part of cobalt sites in second to sixth layers of the positive electrode active material particle, when a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and desorbed is defined as a first layer; the positive electrode active material particles have a layered rock salt type crystal structure inside and a rock salt type crystal structure on a surface layer, The lithium ion secondary battery, wherein the nickel is present in a surface layer portion of the positive electrode active material particles.

10. In claim 9, The nickel has a function of suppressing oxygen desorption, thereby suppressing the phase change from the layered rock salt type to the spinel type.

11. A positive electrode and a negative electrode are included. the positive electrode has positive electrode active material particles containing magnesium, fluorine, aluminum, nickel, and lithium cobalt oxide; the positive electrode active material particles have a layered rock salt type crystal structure inside and a rock salt type crystal structure on a surface layer, the positive electrode active material particle has a region in which magnesium is substituted for a part of cobalt sites in second to sixth layers of the positive electrode active material particle, when a surface of the positive electrode active material particle observed in a cross-sectional STEM image of a surface where lithium is inserted and desorbed is defined as a first layer; the fluorine is present on the surface side of the region, the aluminum is present inside the positive electrode active material particles, The lithium ion secondary battery, wherein the nickel is present in a surface layer portion of the positive electrode active material particles.

12. In any one of claims 1, 4, 6, 7, and 9, The maximum concentration of each element in the surface layer portion when the positive electrode active material particles were subjected to STEM-EDX ray analysis was as follows: A lithium-ion secondary battery in which the position of the maximum magnesium concentration (atomic %) is located more inward than the position of the maximum fluorine concentration (atomic %).

13. In any one of claims 1, 4, 6, 7, and 9, the positive electrode active material particles have a layered rock salt type crystal structure belonging to the space group R-3m in a discharged state, The positive electrode active material particles were used as the positive electrode, lithium metal was used as the negative electrode, and a mixture of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, and 2 wt % vinylene carbonate was used as the electrolyte. After charging under predetermined conditions in an environment of 25°C, The positive electrode in the charged state was １ When analyzed by powder X-ray diffraction using the It has a peak in the 2θ range of 19.13° or more and 19.37° or less, It has a peak in the 2θ range of 45.37° or more and 45.57° or less, The charging under the predetermined conditions is First, the battery was charged at a constant current of 0.5 C (where 1 C = 137 mA / g) up to a voltage of 4.60 V. Next, charge at a constant voltage until the current value reaches 0.01 C. Then take a 30-minute break, Next, the battery was discharged at a constant current of 0.5 C until the voltage reached 2.5 V. Then take a 30-minute break, Next, the battery was charged at a constant current of 0.5 C up to a voltage of 4.60 V. Next, the lithium ion secondary battery is charged at a constant voltage until the current value reaches 0.01C.

14. In any one of claims 1, 4, 6, 7, and 9, In the lithium ion secondary battery, in EPMA of the positive electrode active material particles, the atomic ratio Mg / Co of magnesium to cobalt inside is 0.01 or more.

15. To prepare positive electrode active material particles containing magnesium, fluorine, and lithium cobalt oxide, A method of A method for producing positive electrode active material particles, wherein the heating time at 650°C or higher exceeds 100 hours in total.

16. mixing lithium cobalt oxide, a magnesium source, a fluorine source, and a lithium source to form a mixture; and heating the mixture at a temperature of 650° C. to 950° C. for a heating time exceeding 100 hours.

17. In claim 16, The heating temperature is 826° C. or higher and 920° C. or lower, and the heating time is more than 100 hours and not more than 150 hours.

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