battery

Abstract

Provided is a novel positive electrode active material. Also provided is a battery with excellent charge and discharge characteristics. This battery has a positive electrode. The positive electrode has a positive electrode active material layer. The positive electrode active material layer has positive electrode active material particles containing magnesium, titanium, nickel, aluminum, and lithium cobalt oxide. XPS analysis of the plurality of positive electrode active material particles in the positive electrode active material layer shows a peak in the range from 850 eV to 860 eV inclusive and at a position at least 0.5 eV higher than the peak position in XPS analysis of nickel(II) oxide.

Description

battery

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

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

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

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

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

[0009] Toyoki Okumura et.al.,“Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348T.Motohashi,et.al.,“Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et.al.,“Staging Phase Transitions in Li▲x▼CoO▲2▼”,Journal of The Electrochemical Society,2002,149(12)A1604−A1609Belsky,A.et.al.,“New developments in the Inorganic Crystal Structure Database(ICSD):accessibility in support of materials research and design”,Acta Cryst.,(2002)B58 364−369.Akimoto,J.;Gotoh,Y.;Oosawa,Y.“Synthesis and structure refinement of LiCoO▲2▼ single crystals”Journal of Solid State Chemistry(1998)141,p.298−302.F.Izumi and K.Momma,,“Three−Dimensional Visualization in Powder Diffraction”Solid State Phenom.130,15−20(2007)K.Momma and F.Izumi,”VESTA 3 for three−dimensional visualization of crystal,volumetric and morphology data”J.Appl.Cryst.(2011).44,1272−1276Rasband,W.S.,ImageJ,U.S.National Institutes of Health,Bethesda,Maryland,USA,http: / / rsb.info.nih.gov / ij / ,1997−2012.Schneider,C.A.,Rasband,W.S.,Eliceiri,K.W.”NIH Image to ImageJ:25 years of image analysis”.Nature Methods 9,671−675,2012.Abramoff,M.D.,Magelhaes,P.J.,Ram,S.J.”Image Processing with ImageJ”.Biophotonics International,volume 11,issue 7,pp.36−42,2004.Seto,Y.&Ohtsuka,M.”ReciPro:free and open−source multipurpose crystallographic software integrating a crystal model database and viewer,diffraction and microscopy simulators,and diffraction data analysis tools“(2022).J.Appl.Cryst.55.;

[0010] Lithium-ion secondary batteries have room for improvement in various aspects, such as discharge capacity, cycle characteristics, reliability, safety, and cost. For example, to suppress changes in the crystalline structure of the surface of the positive electrode active material, the surface of the positive electrode active material may be coated with an inert oxide, but this coating may inhibit lithium intercalation and deintercalation. If the intercalation and deintercalation of lithium ions is inhibited, there is concern that the secondary battery's characteristics may deteriorate, such as reduced rate characteristics and reduced charge / discharge capacity in low-temperature environments.

[0011] In view of the above, an object of one embodiment of the present invention is to provide a positive electrode active material that can be used in a lithium-ion secondary battery and that promotes lithium ion insertion and desorption. Another object is to provide a positive electrode active material or a composite oxide in which a decrease in discharge capacity in a low-temperature environment is suppressed. Another object is to provide a positive electrode active material or a composite oxide in which a decrease in discharge capacity during high-rate discharge is suppressed. Another object is to provide a positive electrode active material or a composite oxide in which a decrease in discharge capacity during charge-discharge cycles is suppressed. Another object is to provide a positive electrode active material or a composite oxide whose crystal structure is not easily destroyed even after repeated charge-discharge. Another object is to provide a positive electrode active material or a composite oxide that has a large discharge capacity. Another object is to provide a secondary battery or a vehicle that is safe or highly reliable.

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

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

[0014] One embodiment of the present invention is a battery having a positive electrode, wherein the positive electrode includes a positive electrode active material layer. The positive electrode active material layer includes positive electrode active material particles containing magnesium, titanium, nickel, aluminum, and lithium cobalt oxide. When a plurality of positive electrode active material particles included in the positive electrode active material layer are subjected to XPS analysis, the battery has a first peak in a range of 850 eV or more and 860 eV or less.

[0015] In the battery, the first peak preferably includes a peak derived from an O-Ni-O-Ti-O bond, or the peak position of the first peak is preferably 0.5 eV or more higher than the peak position of nickel(II) oxide in XPS analysis.

[0016] Alternatively, in the battery, when XPS analysis is performed, it is preferable that a second peak be present in the range of 1300 eV to 1310 eV. In this case, it is preferable that the second peak include a peak derived from an O-Mg-F bond. Alternatively, it is preferable that the peak position of the second peak be higher than the peak position in XPS analysis of magnesium oxide and lower than the peak position in XPS analysis of magnesium fluoride.

[0017] In addition, in any one of the batteries described above, when cracks in the positive electrode active material particles are analyzed using a cross-sectional SEM image of the positive electrode active material layer, the number of cracks in the positive electrode active material particles is 0.06 / μm ２ Below, the total length of the cracks is 0.06 μm / μm ２ It is preferable that:

[0018] Alternatively, in any one of the batteries described above, it is preferable that, in XPS analysis, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) is 0.40 or more and 1.50 or less, the ratio of the number of titanium atoms to the number of cobalt atoms (Ti / Co) is 0.05 or more and 0.20 or less, and the ratio of the number of nickel atoms to the number of cobalt atoms (Ni / Co) is 0.05 or more and 0.20 or less.

[0019] Alternatively, in any one of the batteries described above, when the powder resistance of the positive electrode active material particles contained in the positive electrode active material layer is measured, the volume resistivity is 1.0 × 10 at a pressure of 64 MPa.８ Ω・cm or more 1.0×10 １０ It is preferably Ω·cm or less.

[0020] Alternatively, in any one of the batteries described above, when the particle size distribution of the positive electrode active material particles is measured, it is preferable that the particle size distribution of the positive electrode active material particles has one peak.

[0021] In addition, a battery using any one of the above positive electrodes and lithium metal as the negative electrode and 1 mol / L of lithium hexafluorophosphate (LiPF ６ When a test battery is produced using a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 (EC:DEC) and 2 wt % vinylene carbonate (VC) containing 100% ethylene carbonate (EC) as a lithium salt, and the test battery is charged at a constant current of 100 mA / g up to a voltage of 4.60 V in a 45°C environment, and then charged at a constant voltage until the current reaches 10 mA / g, and then discharged at a current of 6 mA / g for one second, it is preferable that the resistance DCIR, calculated by dividing the difference between the voltage before discharge and the voltage during discharge by the current, be 35 Ω or less.

[0022] In addition, when a test battery is used to carry out 50 charge-discharge cycle tests, it is preferable that the DCIR is 35Ω or less after each charge in the charge-discharge cycle test.

[0023] Alternatively, any one of the above positive electrodes and lithium metal are used as the negative electrode, and 1 mol / L of lithium hexafluorophosphate (LiPF ６ When a test battery was produced using a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 (EC:DEC) and 2 wt % vinylene carbonate (VC) with 100% ethylene carbonate (EC) as the lithium salt, the test battery was subjected to 50 charge-discharge cycle tests at 45°C and a charge voltage of 4.55 V or more and 4.65 V or less, and then the positive electrode was removed and a cross-sectional SEM image of the positive electrode was taken. The number of pits in the positive electrode active material particles was 0.085 / μm ２ It is preferable that:

[0024] According to one embodiment of the present invention, a positive electrode active material or composite oxide that can be used in a lithium ion secondary battery and that exhibits a suppressed decrease in discharge capacity in a low-temperature environment can be provided. Alternatively, a positive electrode active material or composite oxide that exhibits a suppressed decrease in discharge capacity during high-rate discharge can be provided. Alternatively, a positive electrode active material or composite oxide that exhibits a suppressed decrease in discharge capacity during charge-discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide that does not easily lose its crystal structure even after repeated charge-discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide that exhibits a large discharge capacity can be provided. Alternatively, a secondary battery or vehicle that is highly safe or highly reliable can be provided.

[0025] According to one embodiment of the present invention, a positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0026] 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.

[0027] FIG. 1A is a cross-sectional view of a positive electrode active material. FIGS. 1B and 1C are diagrams illustrating the positions of nickel and titanium in the crystal structure. FIGS. 2A and 2B are diagrams illustrating the results of DSC analysis. FIG. 3 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 4A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 4B is an FFT pattern of the rock salt crystal RS region, and FIG. 4C is an FFT pattern of the layered rock salt crystal LRS region. FIG. 5 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 6 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIG. 7 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 8 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIGS. 9A and 9B are diagrams illustrating XRD patterns calculated from the crystal structure. FIG. 10 is a cross-sectional view of a positive electrode active material. FIG. 11 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 12A to 12C are diagrams illustrating a method for preparing a positive electrode active material. FIGS. 13A and 13B are diagrams illustrating a method for fabricating a positive electrode active material. FIG. 14 is a diagram illustrating the appearance of a secondary battery. FIGS. 15A to 15C are diagrams illustrating a method for fabricating a secondary battery. FIGS. 16A to 16H are diagrams illustrating an example of an electronic device. FIGS. 17A to 17D are diagrams illustrating an example of an electronic device. FIGS. 18A to 18C are diagrams illustrating an example of an electronic device. FIGS. 19A to 19C are diagrams illustrating an example of a vehicle. FIGS. 20A to 20G are narrow scan graphs of XPS analysis of the positive electrode active material. FIGS. 21A to 21G are narrow scan graphs of XPS analysis of the positive electrode active material. FIGS. 22A and 22B are graphs illustrating the analysis results of the XPS analysis of the positive electrode active material. FIG. 23 is a graph showing the results of a discharge rate test of Example 2. FIGS. 24A and 24B are graphs showing the results of a discharge rate test of Example 2. 25A and 25B are graphs showing the results of the charge-discharge cycle test of Example 2. FIGS. 26A and 26B are graphs showing the results of the charge-discharge cycle test of Example 2. FIG. 27 is a graph showing the results of the charge-discharge cycle test of Example 2. FIG. 28 is a diagram showing the results of cross-sectional SEM and image analysis of the positive electrode. FIG. 29 is a diagram showing the results of cross-sectional SEM and image analysis of the positive electrode.FIG. 30 shows the results of STEM-EDX ray analysis of the positive electrode active material. FIG. 31 shows the results of STEM-EDX ray analysis of the positive electrode active material. FIGS. 32A to 32C are enlarged cross-sectional SEM images of the positive electrode. FIGS. 33A to 33C are enlarged cross-sectional SEM images of the positive electrode. FIG. 34A is an HAADF-STEM image of the positive electrode active material. FIG. 34B is an enlarged view of a portion of FIG. 34A. FIGS. 34C and 34D are enlarged views of a portion of FIG. 34B. FIG. 35A is an HAADF-STEM image of the positive electrode active material. FIG. 35B is an enlarged view of a portion of FIG. 35A. FIGS. 35C and 35D are enlarged views of a portion of FIG. 35B. FIG. 36A is an HAADF-STEM image of the positive electrode active material. FIG. 36B is an enlarged view of a portion of FIG. 36A. FIG. 36C is an enlarged view of a portion of FIG. 36B.

[0028] 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.

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

[0030] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, the terms "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."

[0031] Furthermore, if the arrangement of anions is roughly close to cubic close packing, it can be considered cubic close packing. Cubic close packing of anions refers to a state in which the second layer of anions is arranged above the voids of the anions packed in the first layer, and the third layer of anions is arranged directly above the voids of the second layer of anions, but not directly above the anions in the first layer. Therefore, the anions do not necessarily have to be in a cubic lattice. Furthermore, since real crystals always have defects, the analysis results do not necessarily conform to theory. 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 orientation from the theoretical positions is 5 degrees or less, or 2.5 degrees or less, it can be said that a cubic close packing structure is formed.

[0032] In this specification etc., the term "particle" may be used to refer to the shape of an active material or conductive material, and includes a spherical shape (a circular cross-sectional shape). In this specification etc., the shape of an active material or conductive material is not limited to a spherical shape, and may include cross-sectional shapes such as an ellipse, rectangle, trapezoid, triangle, square with rounded corners, and asymmetric shapes, and individual particles may also have an irregular shape.

[0033] In this specification and the like, when simply referring to a positive electrode active material, there are cases where a description is given of a plurality of positive electrode active material particles or a single positive electrode active material particle, depending on the analytical method, etc. For example, in the case of descriptions relating to various observations using a TEM, various observations using a STEM (Scanning Transmission Electron Microscope), electron beam diffraction, STEM-EDX (Energy Dispersive X-ray Spectroscopy), and STEM-EELS (Electron Energy Loss Spectroscopy), the description refers to a single positive electrode active material particle unless otherwise specified. On the other hand, in the case of analyses such as XPS, XRD, ICP-MS (Inductively Coupled Plasma Mass Spectrometry), GD-MS (Glow Discharge Mass Spectrometry), and the like, unless a particular object of analysis is mentioned, the description refers to a plurality of positive electrode active material particles. Note that the above descriptions of the positive electrode active material and positive electrode active material particles are the same as when mentioning a substance name such as lithium cobalt oxide, and depending on the analytical method, etc., there are cases where a description is given of a plurality of lithium cobalt oxide particles and cases where a description is given of a single lithium cobalt oxide particle.

[0034] The theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. For example, LiCoO ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 275mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0035] In addition, the amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ MO ２In the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiMO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ MO ２ Or we can say x = 0.2. ｘ MO ２ In the above, "x" is small means, for example, 0.1<x≦0.24. In addition, the extent to which lithium is released from the positive electrode active material relative to the theoretical capacity may be referred to as the depth of charge. ｘ MO ２ The charge depth of the battery is "1-x".

[0036] For example, if properly synthesized lithium cobalt oxide before use in the positive electrode approximately satisfies the stoichiometric ratio, LiCoO ２ and x = 1. The lithium cobalt oxide contained in the secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The completion of discharge here refers to a state where the voltage is 3.00 V or 2.50 V or less at a current of 100 mA / g or less.

[0037] In addition, Li ｘ MO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data on a secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0038] The distribution of a certain element refers to a region in which the element is continuously detected within a range that is not a noise by a certain continuous analytical method. The region in which the element is continuously detected within a range that is not a noise can also be referred to as a region in which the element is always detected when the analysis is performed multiple times.

[0039] Furthermore, the positive electrode active material to which the additive element is added may be referred to as a composite oxide, a positive electrode material, a positive electrode material, a positive electrode material for secondary batteries, or the like.

[0040] Furthermore, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, etc., it is not necessary for all particles to have that characteristic. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of the positive electrode active material have that characteristic, it can be said that the effect of sufficiently improving the characteristics of the positive electrode active material and a secondary battery containing it is achieved.

[0041] As the charging voltage of a secondary battery increases, the voltage (potential) of the positive electrode generally increases. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge.

[0042] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in the secondary battery are described in their pre-degradation state. Furthermore, a decrease in discharge capacity due to aging and burn-in treatments during secondary battery manufacturing is not considered to be degradation. For example, a lithium-ion secondary cell or lithium-ion secondary battery pack (hereinafter referred to as a lithium-ion secondary battery) can be said to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0043] In this specification and the like, the state of the materials of a secondary battery before deterioration is sometimes referred to as an initial product or initial state, and the state after deterioration (the state when the secondary battery has a discharge capacity of less than 97% of the rated capacity) is sometimes referred to as a product in use or in use state, or a used product or used state.

[0044] Embodiment 1 In this embodiment, a positive electrode active material of one embodiment of the present invention will be described with reference to FIGS. 1A to 10. FIG.

[0045] A positive electrode active material of one embodiment of the present invention includes any one or more of “lithium cobalt oxide to which magnesium and titanium are added,” “lithium cobalt oxide to which magnesium, titanium, and fluorine are added,” “lithium cobalt oxide to which magnesium, nickel, and titanium are added,” “lithium cobalt oxide to which magnesium, nickel, titanium, and fluorine are added,” “lithium cobalt oxide to which magnesium, nickel, aluminum, and titanium are added,” and “lithium cobalt oxide to which magnesium, nickel, aluminum, titanium, and fluorine are added.” In other words, a positive electrode active material of one embodiment of the present invention includes any one or more of “lithium cobalt oxide having magnesium and titanium,” “lithium cobalt oxide having magnesium, titanium, and fluorine,” “lithium cobalt oxide having magnesium, nickel, titanium, and fluorine,” “lithium cobalt oxide having magnesium, nickel, aluminum, and titanium,” and “lithium cobalt oxide having magnesium, nickel, aluminum, titanium, and fluorine.” In particular, the positive electrode active material of one embodiment of the present invention more preferably contains lithium cobalt oxide to which magnesium, nickel, aluminum, titanium, and fluorine are added (lithium cobalt oxide having magnesium, nickel, aluminum, titanium, and fluorine). Note that in this specification, "lithium cobalt oxide to which magnesium and titanium are added" may also be referred to as "lithium cobalt oxide having magnesium and titanium" as described above, and the same may also be referred to as "positive electrode active material having magnesium, titanium, and lithium cobalt oxide," "positive electrode active material particles having magnesium, titanium, and lithium cobalt oxide," etc. The positive electrode active material LiMO of one embodiment of the present invention ２ The region in which the additional element is present will be described with reference to FIGS. 1A and 1B. FIG.

[0046] Fig. 1A is a cross-section of a positive electrode active material 100 according to one embodiment of the present invention. Fig. 1B is a schematic diagram of element distribution when measurement is performed from the surface toward the interior in a cross-sectional analysis including the surface and a surface layer portion of the positive electrode active material 100. Measurement from the surface toward the interior is also referred to as measurement in the depth direction, and the X1-X2 and Y1-Y2 arrows in Fig. 1A are examples of the depth direction.

[0047] As shown in FIG. 1A, the positive electrode active material 100 has a surface layer 100a and an inner portion 100b. In FIG. 1A, the boundary between the surface layer 100a and the inner portion 100b is indicated by a dashed line. (001) in the figure indicates LiMO. ２ This shows the (001) plane of LiMO. ２ belongs to the space group R-3m.

[0048] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region extending from the surface toward the interior within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm perpendicular or approximately perpendicular from the surface toward the interior. Note that approximately perpendicular refers to an angle of 80° to 100°. Surfaces resulting from cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0049] The region deeper than the surface layer 100a of the positive electrode active material is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0050] Furthermore, when the positive electrode active material 100 has a layered rock salt crystal structure of the space group R-3m, the surface layer portion 100a has a basal region and an edge region.

[0051] The basal region has a surface parallel to the (001) plane (also referred to as a (001)-oriented surface). The basal region is a region extending from the surface to the interior, perpendicular or substantially perpendicular to the surface, within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm. Note that "parallel" here means that the angle between the perpendicular to the first surface (the (001) plane) and the normal to the second surface (the surface of the positive electrode active material 100) is 0 degrees or more and less than 10 degrees, preferably 0 degrees or more and 5 degrees or less, more preferably 0 degrees or more and 2.5 degrees or less.

[0052] The edge region is a region of the surface layer portion 100a other than the basal region. In other words, the edge region has a surface exposed in a direction intersecting with the (001) plane (also referred to as a surface other than the (001) orientation), and the region extending from the surface toward the interior, perpendicular or substantially perpendicular to the surface, is referred to as the edge region. Note that, intersecting here means that the angle formed between the perpendicular to the first surface (the (001) plane) and the normal to the second surface (the surface of the positive electrode active material 100) is 10 degrees or more and 90 degrees or less, more preferably 30 degrees or more and 90 degrees or less, and even more preferably 50 degrees or more and 90 degrees or less.

[0053] The surface of the positive electrode active material 100 refers to the surface of the positive electrode active material particles, including the surface layer 100a and the interior 100b. Therefore, the positive electrode active material 100 does not include metal oxides, such as aluminum oxide, that do not have lithium sites that can contribute to charging and discharging and that are attached to the particle surface, carbonates that are chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. The attached metal oxide refers to, for example, a metal oxide whose crystal structure orientation does not match that of the interior 100b, i.e., a metal oxide that does not have continuity in terms of the ordered atomic arrangement. In other words, the oxide region (e.g., a region having a rock salt-type crystal structure) that is continuous with the interior 100b of the positive electrode active material 100 in terms of the ordered atomic arrangement is included in the positive electrode active material 100.

[0054] Furthermore, the positive electrode active material 100 does not include electrolytes, organic solvents, binders, conductive materials, or compounds derived from these materials that are attached to the positive electrode active material 100 .

[0055] Although not shown, the positive electrode active material 100 may have a crystal grain boundary. The crystal grain boundary refers to, for example, a portion where particles of the positive electrode active material 100 adhere to each other, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. Furthermore, the crystal defect refers to a defect that can be observed in a cross-sectional TEM image, a cross-sectional STEM image, etc., i.e., a structure in which other atoms have entered between the lattices, a cavity, etc. The crystal grain boundary can be considered to be one type of planar defect. Furthermore, the vicinity of the crystal grain boundary refers to a region within 10 nm from the crystal grain boundary.

[0056] <Containing Elements> The positive electrode active material 100 contains lithium, a transition metal M, oxygen, and an additive element. The transition metal M is at least one selected from cobalt, nickel, and manganese. Alternatively, the positive electrode active material 100 may be a composite oxide of lithium and the transition metal M (LiMO). ２ ) to which an additive element is added. However, the positive electrode active material 100 of one embodiment of the present invention can have a distribution or a crystal structure of the additive element described later. Therefore, the composition is not strictly limited to Li:M:O=1:1:2 (atomic ratio).

[0057] The positive electrode active material 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 100 of one embodiment of the present invention preferably uses cobalt as the transition metal M responsible for the oxidation and reduction reaction. It is preferable that the positive electrode active material 100 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals M contained in the positive electrode active material 100, because of many advantages such as relatively easy synthesis and handling, and excellent cycle characteristics.

[0058] Furthermore, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metal M in the positive electrode active material 100, lithium nickel oxide (LiNiO２ Compared to composite oxides in which nickel accounts for the majority of the transition metal, such as lithium nickel oxide, the stability is superior when a large amount of lithium is released upon charging. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides in which octahedral-coordinated low-spin nickel(III) accounts for the majority of the transition metal M, such as lithium nickel oxide, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen more susceptible to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are similar in size to lithium ions. Therefore, layered rock-salt composite oxides in which nickel accounts for the majority of the transition metal M, such as lithium nickel oxide, have the problem of prone to cation mixing between nickel and lithium.

[0059] The additive elements contained in the positive electrode active material 100 are preferably one or more selected from magnesium, titanium, nickel, aluminum, fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0060] That is, the positive electrode active material 100 can have "lithium cobalt oxide to which magnesium and titanium have been added," "lithium cobalt oxide to which magnesium, titanium, and fluorine have been added," "lithium cobalt oxide to which magnesium, nickel, and titanium have been added," "lithium cobalt oxide to which magnesium, nickel, titanium, and fluorine have been added," "lithium cobalt oxide to which magnesium, nickel, aluminum, and titanium have been added," "lithium cobalt oxide to which magnesium, nickel, aluminum, titanium, and fluorine have been added," and the like.

[0061] The additive element is preferably dissolved in the positive electrode active material 100. Therefore, for example, when a line analysis is performed using STEM-EDX (Energy Dispersive X-ray Spectroscopy), the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the transition metal M detected increases, i.e., closer to the interior of the positive electrode active material 100.

[0062] In this specification and the like, the depth at which the amount of a certain element detected increases in STEM-EDX line analysis refers to the depth at which measurement values ​​that can be determined not to be noise in terms of intensity, spatial resolution, etc. are continuously obtained.

[0063] These added elements further stabilize the crystal structure of the positive electrode active material 100, as will be described later.

[0064] The additive elements do not necessarily have to include all of fluorine, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0065] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, are further enhanced. The weight of manganese contained in the cathode active material 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0066] The positive electrode active material 100 has the above-described additive element in the surface layer portion 100a. It is more preferable that the positive electrode active material 100 has a plurality of additive elements. It is also preferable that the surface layer portion 100a has a higher concentration of one or more selected from the additive elements than the inner portion 100b. Alternatively, it is preferable that the surface layer portion 100a has a higher detected amount of one or more selected from the additive elements than the inner portion 100b. It is also preferable that the one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient.

[0067] In addition, it may be more preferable that the distribution of the positive electrode active material 100 differs depending on the added element. For example, it may be more preferable that the depth of the peak of the detected amount in the surface layer 100a from the surface or from a reference point in EDX-ray analysis described below differs depending on the added element. The peak of the detected amount here refers to the maximum value (maximum value) of the detected amount at a depth of 50 nm or less from the surface of the surface layer 100a. The detected amount refers to, for example, the number of characteristic X-rays counted in EDX-ray analysis.

[0068] [Titanium] The presence of titanium as an additive element in the surface layer portion 100 a can promote the insertion and desorption of lithium ions into and from the positive electrode active material 100 .

[0069] Titanium exists stably in the octahedral sites of hexavalent oxygen atoms in the oxide, but titanium oxide and lithium titanate cannot form a stable layered rock salt type crystal structure. For example, TiO ２ The rutile crystal structure is the most stable, and Li ４ Ti ５ O １２ has a spinel-type crystal structure. Therefore, when a small amount of titanium is dissolved in the surface layer 100a having a layered rock salt type or rock salt type crystal structure, defects occur in part of the crystal structure of the surface layer 100a.

[0070] Lithium cobalt oxide is in a discharged state (i.e., Li ｘ CoO ２ When x is 1 in the formula (1), the band gap is large and the electrical resistance is high. Therefore, by introducing titanium defects into a part of the surface layer 100a, the band gap can be narrowed and the resistance can be reduced to a desirable degree.

[0071] Cobalt is also present in the oxide. ３＋ is the most stable, but titanium is Ti ４＋ Therefore, the charge on the lithium ion around titanium is relatively lower than that around cobalt. ４＋Defects may be induced in the cation sites near titanium, and such defects reduce the diffusion resistance of cations, especially lithium ions. Therefore, the diffusion resistance of lithium ions is reduced around titanium. In addition, some of the cobalt around titanium is converted into Co due to the presence of titanium. ２＋ It may be the case.

[0072] Therefore, the presence of titanium in the surface layer portion 100a can reduce the diffusion resistance of lithium ions at the interface between the electrolyte and the positive electrode active material 100. Furthermore, it is believed that the titanium present in the surface layer portion 100a does not significantly reduce the electrical resistance (e.g., powder resistance) of the positive electrode active material 100. A relatively high electrical resistance of the positive electrode active material 100 is preferable because it contributes to safety in the event of a short circuit in the secondary battery.

[0073] If the amount of titanium is too small, the effect of promoting the insertion and desorption of lithium ions described above is not fully exhibited. On the other hand, if the amount of titanium is too large, it may form a different phase from other added elements such as magnesium (for example, MgTiO with an ilmenite-type crystal structure). ３ Furthermore, magnesium is taken away during the formation of the heterophase, which may reduce the magnesium concentration in the surface layer 100a, whose crystal structure is roughly the same as that of the interior 100b. ＋ For the positive electrode active material 100 of one embodiment of the present invention, which is intended for high-voltage charging exceeding 100 V, magnesium present at a high concentration in the surface layer portion 100a is essential for suppressing phase change. Therefore, the loss of magnesium due to heterophase formation is a significant disadvantage. Furthermore, if there are too many titanium defects, there is a concern that oxygen may be more likely to be desorbed from the surface. Therefore, titanium, together with magnesium, fluorine, and the like, is preferably present at a lower concentration than magnesium in the surface and near-surface regions of the surface layer portion 100a.

[0074] Specifically, the Ti / Co ratio of the entire positive electrode active material is preferably 0.0001 (atomic ratio) or more, more preferably 0.001 (atomic ratio) or more, and is preferably less than 0.0075 (atomic ratio), more preferably 0.004 (atomic ratio) or less, and even more preferably 0.003 (atomic ratio) or less.

[0075] Furthermore, in EDX ray analysis of the surface layer portion 100a, when the denominator is the sum of carbon, oxygen, fluorine, magnesium, sodium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, gallium, zirconium, and molybdenum, the maximum titanium concentration is preferably 0.3 at% or more and 3 at% or less, and more preferably 0.5 at% or more and 2 at% or less.

[0076] Furthermore, in an XPS analysis for analyzing the surfaces of a plurality of particles of the positive electrode active material 100, when the denominator is the sum of carbon, oxygen, fluorine, magnesium, sodium, aluminum, silicon, phosphorus, sulfur, calcium, titanium, iron, cobalt, nickel, gallium, zirconium, and molybdenum, the titanium concentration is preferably 0.5 at% or more and 5.0 at% or less, and more preferably 1.0 at% or more and 2.0 at% or less.

[0077] In addition, in an EPMA analysis of the particle cross section of the positive electrode active material 100, it is preferable that titanium is not detected (below the lower detection limit) or is present at a level similar to the lower detection limit in the inner portion 100b.

[0078] The titanium source is not particularly limited, but may be, for example, TiO ２ Titanium oxide such as Li ２ TiO ３ , LiTiO ２ , Li ４ Ti ５ O １２ In particular, when a heating step is carried out after mixing the titanium source, it is possible that part of the lithium evaporates during heating, and therefore it is preferable to use the lithium titanate described above that contains lithium.

[0079] Furthermore, the element that can be added to promote the insertion and desorption of lithium ions is not limited to titanium. Other elements may also be added. When using elements other than titanium, it is preferable that the element stably exists at the hexacoordinated octahedral oxygen site in the oxide, but that the oxide of lithium and the element does not form a stable layered rock-salt type crystal structure. Furthermore, elements that are stable at a higher oxidation number than cobalt are preferable. For example, niobium, manganese, tungsten, chromium, molybdenum, rhenium, tantalum, etc. can be used.

[0080] [Distribution] To achieve the above-described effects, it is preferable that the concentration of at least magnesium (Mg), nickel (Ni), and titanium (Ti) among the additive elements is higher in the surface layer 100a than in the interior 100b, as shown in FIG. 1B . Alternatively, it is preferable that the detected amount in the surface layer 100a is greater than that in the interior 100b. Furthermore, it is preferable that the detected amount peak in the surface layer 100a be closer to the surface. For example, it is preferable that the detected amount peak be on the surface or within 3 nm from the reference point. It is also preferable that the distributions of magnesium and nickel have an overlapping region. Note that in FIG. 1B , the distributions of magnesium, nickel, and titanium are shown as a single curve to avoid cluttering the drawing, but this is not intended to limit the configuration in which the distribution positions and concentrations of the above elements coincide. For example, the detected amount peaks of magnesium and nickel may be at the same depth, or the magnesium peak may be closer to the interior, or the nickel peak may be closer to the interior.

[0081] The difference in depth between the peak of detected nickel and the peak of detected magnesium is preferably within 3 nm, more preferably within 1 nm. The half-width of the detected amounts is preferably narrow. In this specification, the half-width refers to the full width at half maximum.

[0082] Similarly, it is preferable that the distributions of magnesium and titanium overlap. The peaks of the detected amounts of magnesium and titanium may be at the same depth, or the magnesium peak may be closer to the center, or the titanium peak may be closer to the center. The difference in depth between the peak of detected amount of titanium and the peak of detected amount of magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount is narrow.

[0083] Similarly, it is preferable that the distributions of titanium and nickel overlap. The peaks of the detected amounts of titanium and nickel may be at the same depth, or the titanium peak may be closer to the center, or the nickel peak may be closer to the center. The difference in depth between the peak of the detected amount of titanium and the peak of the detected amount of nickel is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount is narrow.

[0084] In other words, it is preferable that the distributions of magnesium, nickel, and titanium overlap. The difference in depth between the peaks of the detected amount of titanium, nickel, and magnesium is preferably within 3 nm, and more preferably within 1 nm. It is also preferable that the half-width of the detected amount is narrow.

[0085] The above-mentioned regions where the distributions of nickel and titanium overlap, the region where the distributions of magnesium and nickel overlap, the region where the distributions of magnesium and titanium overlap, or the region where the distributions of magnesium, nickel, and titanium overlap are preferably located in the edge region of the surface layer portion 100a where lithium ions are inserted and desorbed. In other words, the distribution of the additive element shown in the schematic diagram of FIG. 1B is preferably observed in the direction of the X1-X2 arrow in FIG. 1A. On the other hand, the above-mentioned overlapping regions are not necessarily required in the basal region of the surface layer portion 100a. The horizontal axis of FIG. 1B indicates the distance from X1 (direction of the arrow).

[0086] 1C , when nickel (Ni) and titanium (Ti) are present in the same cobalt (Co) layer and in adjacent positions in lithium cobalt oxide having a layered rock-salt crystal structure of space group R-3m, it has been confirmed by first-principles calculations that the structure is stabilized. Therefore, having a region where the distributions of nickel and titanium overlap is preferable because it can stabilize the structure of the positive electrode active material 100.

[0087] Furthermore, the amount of nickel detected in the inner portion 100b is very small compared to the surface portion 100a, or may not be detected, or may be 1 atomic % or less.

[0088] Similarly to magnesium, nickel, and titanium, it is preferable that the amount of fluorine detected in the surface layer 100a is greater than the amount detected inside. It is also preferable that the peak of the detection amount be in a region closer to the surface of the surface layer 100a. For example, it is preferable that the peak of the detection amount be on the surface or within 3 nm from the reference point.

[0089] Furthermore, as shown in FIG. 1B , it is preferable that at least aluminum (Al) among the additive elements has a maximum detectable amount closer to the interior of the positive electrode active material particle than magnesium and titanium. As shown in FIG. 1B , the distributions of magnesium and titanium and aluminum may have overlapping regions, but the overlapping region may be minimal. The maximum detectable amount of aluminum may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. For example, it is preferable that the peak be present on the surface or in a region of 5 nm to 30 nm from the reference point toward the interior.

[0090] The distribution of aluminum as described above can further stabilize the layered rock-salt crystal structure of the positive electrode active material 100. For example, it is expected that the change from the layered rock-salt crystal structure to the spinel crystal structure in the surface layer 100a of the positive electrode active material 100 can be suppressed. The spinel crystal structure generated in the layered rock-salt crystal structure may move or expand due to the transfer of charge of the transition metal M. Furthermore, defects such as grain boundaries in the positive electrode active material 100 can serve as diffusion paths for lithium ions in the c-axis direction. Additional elements such as aluminum may be present near the defects.

[0091] The reason why aluminum is distributed deeper than magnesium and titanium is thought to be because aluminum has a faster diffusion rate than magnesium. On the other hand, the amount of aluminum detected in the region closest to the surface is low, presumably because aluminum exists more stably in regions where magnesium and other elements are not present in solid solution at high concentrations than in regions where they are not.

[0092] More specifically, in the region of the layered rock salt type of space group R-3m or the cubic rock salt type where magnesium is dissolved at a high concentration, layered rock salt type LiAlO ２ Compared to the case of cobalt, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋ From Co ２＋ This compensates for the cation balance and allows for a balance. However, since aluminum can only be trivalent, it is thought that it is difficult for it to exist stably near magnesium in a layered rock salt structure. For the same reason that aluminum can only be trivalent, it is thought that it is difficult for aluminum to exist stably in a rock salt structure.

[0093] However, the added element does not necessarily have to have the same concentration gradient or distribution throughout the entire surface layer portion 100 a of the positive electrode active material 100 .

[0094] The surface of the basal region of the positive electrode active material 100 (also referred to as the (001)-oriented surface) may have a different distribution of additive elements than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a lower detectable amount of one or more elements selected from the additive elements compared to surfaces other than the (001)-oriented surface. Specifically, the detectable amount of one or more elements selected from magnesium, nickel, and titanium may be low. Alternatively, the (001)-oriented surface and its surface layer 100a may have no detectable amount of one or more elements selected from the additive elements, or the detectable amount of such elements may be 1 atomic % or less. Specifically, the detectable amount of nickel may be no detectable amount or 1 atomic % or less. In particular, in the case of an analytical method that detects characteristic X-rays such as EDX, the energy of Kβ of cobalt and Kα of nickel are close, making it difficult to detect trace amounts of nickel in a material in which cobalt is the primary element. Alternatively, the (001) oriented surface and its surface layer 100a may have peaks of one or more detected elements selected from the additive elements that are shallower from the surface than those of surfaces other than the (001) oriented surface. Specifically, the peaks of the detected amounts of magnesium and aluminum may be shallower than those of other surfaces.

[0095] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and the diffusion path of lithium ions is also parallel to the (001) plane.

[0096] MO ２ Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0097] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface and the surface layer 100a other than the (001) orientation are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and the surface layer 100a other than the (001) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0098] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, it is important that the distribution of the additive elements in the surface and the surface layer 100a other than the (001) orientation is, for example, as shown in FIG. 1B . Of the additive elements, nickel is particularly preferably detected in the surface and the surface layer 100a other than the (001) orientation. On the other hand, the concentration of the additive element in the (001)-oriented surface and the surface layer 100a may be low or absent, as described above.

[0099] High-purity LiMO, which will be described in a later embodiment ２ In the manufacturing method in which the additive element is mixed and heated after the (001) orientation, the additive element spreads mainly through the diffusion path of lithium ions, and therefore the distribution of the additive element in the surface other than the (001) orientation and in the surface layer portion 100a thereof can be easily controlled to a preferred range.

[0100] [Magnesium] Magnesium is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by the MO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen from around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. Furthermore, a high magnesium concentration in the surface layer portion 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0101] At appropriate concentrations, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying cobalt sites in addition to lithium sites. Furthermore, excess magnesium compounds (e.g., oxides and fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become resistance components in secondary batteries. Furthermore, increasing the magnesium concentration of the positive electrode active material can decrease the discharge capacity of the positive electrode active material. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium available for charging and discharging.

[0102] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.002 to 0.06 times (0.2% to 6.0%) the number of cobalt atoms, more preferably 0.005 to 0.03 times (0.5% to 3.0%), and even more preferably about 0.01 times (0.1%). The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0103] [Nickel] Nickel is LiMO ２In the layered rock salt crystal structure of the present invention, nickel can exist on either the cobalt site or the lithium site. When present on the cobalt site, its redox potential is lower than that of cobalt, making it easier to release lithium and electrons during charging. This can be expected to result in faster charge and discharge speeds. Therefore, even at the same charge voltage, a greater charge and discharge capacity can be obtained when the transition metal M is nickel than when it is cobalt.

[0104] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the change in volume caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site also acts as a catalyst for MO ２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0105] In addition, the distance between the cations and anions of nickel oxide (NiO) is closer to that of LiCoO than that of rock salt MgO and rock salt CoO. ２ The average distance between the cations and anions is close to that of LiCoO ２ The orientation is likely to match.

[0106] In addition, the order of ionization tendency is lowest for magnesium, aluminum, cobalt, and nickel (Mg>Al>Co>Ni). Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements listed above during charging. Therefore, it is thought to be highly effective in stabilizing the crystalline structure of the surface layer in the charged state.

[0107] Furthermore, nickel is Ni ２＋ , Ni ３＋ , Ni ４＋ Of which Ni ２＋ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Therefore, nickel is thought to have the effect of suppressing the phase change from the layered rock salt type to the spinel-type crystal structure.

[0108] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also adversely affect the insertion and extraction of lithium.

[0109] Therefore, it is preferable that the entire positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 100 is preferably more than 0% but not more than 7.5% of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, more than 0% but not more than 4% is preferable. Alternatively, more than 0% but not more than 2% is preferable. Alternatively, 0.05% to 7.5% is preferable. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 7.5% is preferable. Alternatively, 0.1% to 4% is preferable. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0110] [Aluminum] Aluminum can also be present at the cobalt site in a layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Therefore, as described below, even if the positive electrode active material 100 experiences a force that causes it to expand and contract in the c-axis direction due to the insertion and desorption of lithium ions, i.e., even if a force that causes it to expand and contract in the c-axis direction due to changes in the charge depth or charge rate occurs, deterioration of the positive electrode active material 100 can be suppressed.

[0111] Aluminum also has the effect of suppressing the elution of surrounding cobalt and improving continuous charge durability. Furthermore, since the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen from the aluminum's surroundings. These effects improve thermal stability. Therefore, the presence of aluminum as an additive element can improve safety when the positive electrode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is resistant to collapse even after repeated charge and discharge.

[0112] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0113] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2%, and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire positive electrode active material 100 referred to here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0114] [Fluorine] Fluorine is a monovalent anion, and when a portion of the oxygen in the surface layer portion 100a is replaced by fluorine, the lithium desorption energy decreases. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. That is, in the absence of fluorine, cobalt ions change from trivalent to tetravalent upon lithium desorption. On the other hand, when fluorine is present, cobalt ions change from divalent to trivalent upon lithium desorption. The redox potential of cobalt ions differs between the two. Therefore, when a portion of the oxygen in the surface layer portion 100a of the positive electrode active material 100 is replaced by fluorine, it can be said that desorption and insertion of lithium ions near the fluorine easily occurs. Therefore, when the positive electrode active material 100 is used in a secondary battery, the charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer portion 100a, which has a surface that contacts the electrolyte, or the adhesion of fluoride to the surface, can suppress excessive reaction between the positive electrode active material 100 and the electrolyte. Furthermore, corrosion resistance to hydrofluoric acid can be effectively improved.

[0115] Here, differential scanning calorimetry (DSC measurement) for fluorides and mixtures will be described with reference to Figures 2A and 2B. In both figures, the horizontal axis represents the temperature of the sample, and the vertical axis represents the difference in heat flow from the reference. The DSC measurement device and conditions are not particularly limited, but in this embodiment, measurements were performed using the following device and conditions. DSC device: EVO2 DSC8271 manufactured by Rigaku Corporation Heating rate: 20°C / min The obtained measurement results were subjected to background correction using the analysis software Thermo Plus EVO, and the extrapolated initial melting temperature T ｉｍ , melting peak temperature T ｐｍ and the melting end temperature T ｅｍ The heat flow was calculated as the heat flow per sample weight.

[0116] FIG. 2A shows the fluoride mixture of lithium fluoride and magnesium fluoride, LiF:MgF ２ 2B shows the DSC measurement results of the mixture obtained by adding lithium cobalt oxide (C-10N manufactured by Nippon Chemical Industry Co., Ltd.) to the above mixture. ２ :LiF:MgF ２ The components were mixed so that the molar ratio was 100:0.33:1.

[0117] As shown in FIG. 2A, the initial melting temperature T ｉｍ is 731 ° C, the melting peak temperature T ｐｍ is 739°C, and the melting end temperature T ｅｍ The T of the mixture of lithium cobalt oxide, lithium fluoride, and magnesium fluoride was 746°C. ｉｍ is 779°C, T ｐｍ is 815℃, T ｅｍ The temperature was 826°C.

[0118] Therefore, the heating temperature after mixing the additive element is preferably 731°C or higher, and more preferably 826°C or higher. It can also be set to 800°C or higher, which is between these temperatures. A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is high. However, if the temperature is too high, there is a risk of decomposition of lithium cobalt oxide. Therefore, the upper limit of the heating temperature is set to be lower than 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 in a small amount. Therefore, a temperature of 1000°C or lower is more preferable.

[0119] [Other additive elements] When phosphorus is contained in the surface layer portion 100a, Li ｘ CoO ２ When the value of x in the graph is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that the graphite oxide is present in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0120] When the positive electrode active material 100 contains phosphorus, the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0121] The electrolyte is LiPF ６ In the case where the electrolyte contains the above-mentioned compound, hydrogen fluoride may be generated by hydrolysis. Furthermore, hydrogen fluoride may be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 may be suppressed. Furthermore, a decrease in adhesion due to gelation and / or insolubilization of PVDF may be suppressed.

[0122] When the positive electrode active material 100 contains phosphorus together with magnesium, Li ｘ CoO ２This is preferable because stability is extremely high when x is small in the positive electrode active material 100. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, 1% to 10% is preferable. Alternatively, 1% to 8% is preferable. Alternatively, 2% to 20% is preferable. Alternatively, 2% to 8% is preferable. Alternatively, 3% to 20% is preferable. Alternatively, 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Alternatively, 0.1% to 5% is preferable. Alternatively, 0.1% to 4% is preferable. Alternatively, 0.5% to 10% is preferable. Alternatively, 0.5% to 4% is preferable. Alternatively, 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 100.

[0123] Furthermore, when the positive electrode active material 100 has cracks, the progression of the cracks can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on the surface, for example, in the embedded portion.

[0124] [Synergistic Effect of Multiple Added Elements] When the surface layer portion 100a contains both titanium and nickel, that is, when the surface layer portion 100a has an O—Ti—O—Ni—O bond, LiCoO ２ Compared to when the compound is used alone, the bond with oxygen is stronger, and oxygen desorption is suppressed. The suppression of oxygen desorption suppresses the transformation of the layered rock salt type crystal structure into a spinel type crystal structure or a rock salt type crystal structure.

[0125] The titanium and nickel are preferably present in an intermediate region (also referred to as a boundary region or interface) between the rock salt type crystal structure and the layered rock salt type crystal structure in surface layer portion 100a. That is, positive electrode active material 100 has a layered rock salt structure inside, a rock salt type crystal structure containing magnesium in a portion of the surface layer, and an intermediate region between the rock salt type crystal structure containing magnesium and the layered rock salt type crystal structure, and this intermediate region preferably contains nickel and titanium.

[0126] Furthermore, when the surface layer 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ MO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0127] For the same reason, when adding an additive element to lithium cobalt oxide in the manufacturing process, it is preferable to add magnesium in a step before adding nickel. Alternatively, it is preferable to add magnesium and nickel in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of the step in which it is added, whereas nickel can diffuse widely into the interior of lithium cobalt oxide in the absence of magnesium. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of the lithium cobalt oxide and not remain in the surface layer in a desired amount.

[0128] Furthermore, when the surface layer 100a contains both nickel and titanium, the presence of tetravalent titanium near divalent nickel may further stabilize the surrounding crystal structure. ｘ MO ２ Even when x in the positive electrode active material 100 is small, the elution of oxygen (O) can be suppressed. This can contribute to the stabilization of the surface layer portion 100a. When the positive electrode active material 100 is analyzed by XPS, it is preferable that there is a bond between nickel and titanium in close proximity, and it is more preferable that there is, for example, a bond between nickel and titanium via oxygen (Ni—O—Ti bond).

[0129] The excitation X-rays used in XPS measurements can be monochromated Al, and the detection area can be 100 μmφ. The take-off angle (the angle between the tilt of the sample stage and the detection direction of the detector) can be 45° or 15°. The detection depth is approximately 4 to 5 nm when the take-off angle is 45°, and approximately 2 nm when the take-off angle is 15°. The correction standard can be C1s.

[0130] The obtained Ni2p ３／２ In the XPS spectrum of the present invention, when an O-Ni-O-Me(metal)-O bond is present, the XPS peak resulting from the bond energy is observed shifted to a higher energy side than the peak position of NiO (the peak position of the O-Ni-O-Ni-O bond) when a reference sample of NiO (nickel (II) oxide) is measured. In this specification, the XPS peak position refers to the energy value at the maximum value of the convex shape that appears in the XPS spectrum.

[0131] For example, when XPS analysis of the positive electrode active material 100 shows a peak in the range of 850 eV to 860 eV and shifted to a higher energy side than the peak position of NiO, the positive electrode active material 100 can be considered to have an O-Ni-O-Me (metal)-O bond. In this case, Me can be any one or more of titanium, cobalt, magnesium, and lithium. More specifically, it is considered to have any one or more of an O-Ni-O-Ti-O bond, an O-Ni-O-Co-O bond, an O-Ni-O-Mg-O bond, and an O-Ni-O-Li-O bond. These bonds are expected to be effective in suppressing pits, as described below. They are also expected to have the effect of facilitating the formation of an O3'-type crystal structure in a charged state. The shift amount is preferably 0.2 eV to 2.0 eV, and more preferably 0.5 eV to 1.5 eV.

[0132] Furthermore, when the surface layer 100a contains magnesium, nickel, and titanium, the presence of nickel results in the formation of MgTiO with an ilmenite-type crystal structure. ３The formation of heterophases such as MgTiO is suppressed. The ilmenite crystal structure has hexagonal close-packed anions, which differs from the rock salt and layered rock salt crystal structures, which have cubic close-packed anions, and therefore requires a certain amount of activation energy for the phase transition. NiO(II) is a compound with low chemical activity, so MgTiO ３ It has the effect of suppressing the production of

[0133] For the above reasons, when adding an additional element to lithium cobalt oxide in the manufacturing process, if titanium is added in a process later than magnesium and nickel, the effect of nickel in suppressing the formation of heterophases is particularly strong.

[0134] Furthermore, having additive elements with different distributions in combination is preferable because it can stabilize the crystal structure in a wider region. For example, if the positive electrode active material 100 has magnesium, nickel, and titanium distributed in a region closer to the surface of the surface layer 100a, and aluminum distributed in a deeper region, it can stabilize the crystal structure in a wider region than if it had only one of these elements. In this way, when the positive electrode active material 100 has additive elements with different distributions in combination, aluminum is not essential to the surface because surface stabilization can be sufficiently achieved by magnesium, nickel, etc. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable that aluminum be continuously detected in a region from 0 nm to 100 nm in the depth direction from the surface, preferably a region from 0.5 nm to 50 nm from the surface, and more preferably a region from 1 nm to 25 nm from the surface. In this way, a wide distribution of aluminum is preferable because it can stabilize the crystal structure in a wider region.

[0135] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100 a. In particular, when magnesium, nickel, aluminum, and titanium are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0136] However, if the surface layer 100a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO or a structure in which MgO and MO(II) are solid-solved. Therefore, the surface layer 100a must contain at least a transition metal M such as cobalt, and also contain lithium in the discharged state, so that it has a path for the insertion and extraction of lithium.

[0137] In order to ensure sufficient lithium insertion / extraction paths, the surface layer portion 100a preferably has a higher cobalt concentration than magnesium. For example, when measured from the surface of the positive electrode active material 100 using XPS, the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is preferably 0.4 or more and 1.5 or less. The surface layer portion 100a preferably has a higher cobalt concentration than titanium, and the ratio Ti / Co of the number of titanium atoms Ti to the number of cobalt atoms Co is preferably 0.05 or more and 0.2 or less. The surface layer portion 100a preferably has a higher cobalt concentration than nickel, and the ratio Ni / Co of the number of nickel atoms Ni to the number of cobalt atoms Co is preferably 0.05 or more and 0.2 or less. The surface layer portion 100a preferably has a higher cobalt concentration than aluminum. The surface layer portion 100a preferably has a higher cobalt concentration than fluorine.

[0138] Furthermore, since an excessive amount of nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 100 a. For example, when measured from the surface of the positive electrode active material 100 by XPS, the number of nickel atoms is preferably 1 / 6 or less of the number of magnesium atoms.

[0139] Furthermore, it is preferable that some of the added elements, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, but they are also preferably present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it can suppress the shift in the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0140] Furthermore, in the surface layer portion 100a, it is preferable that magnesium is bonded to oxygen and fluorine. In particular, it is preferable that Mg—F bonds derived from magnesium fluoride remain. As a result, when the positive electrode active material 100 is analyzed by XPS, it is preferable that bonds between magnesium and fluorine exist, and it is even more preferable that bonds between magnesium and fluorine and oxygen (O—Mg—F bonds) exist.

[0141] The excitation X-rays used in XPS measurements can be monochromated Al, and the detection area can be 100 μmφ. The take-off angle (the angle between the tilt of the sample stage and the detection direction of the detector) can be 45° or 15°. The detection depth is approximately 4 to 5 nm when the take-off angle is 45°, and approximately 2 nm when the take-off angle is 15°. The correction criteria can be Mg1s and C1s.

[0142] In the obtained XPS spectrum of Mg1s, if an O-Mg-F bond is present, the bond energy of the Mg is ２ The lower limit of detection for O-Mg-F bonds is about 10%, and the error in the waveform analysis results is about ±2%.

[0143] 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.

[0144] 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 a rock salt type or both a rock salt type and a layered rock salt type crystal structure. Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has the 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.

[0145] 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, the layered rock-salt crystal structure may be considered to be a structure in which the lattice of a rock-salt crystal is distorted.

[0146] The rock salt crystal structure refers to a cubic crystal structure, such as a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately, and may contain cation or anion defects.

[0147] The fact that it has both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0148] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is taken as the origin (000), the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２The distance between the bright spots on the (003) plane of LiCoO is observed to be about half the distance between the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0149] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, metals with atomic numbers higher than that of lithium are present in some of the low-brightness layers, i.e., the lithium layers.

[0150] Layered rock salt crystals and the anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3'-type and monoclinic O1(15) crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure formed by the anions is aligned.

[0151] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0152] However, the space group of the layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group Fm-3m (space group of general rock salt crystals) of the rock salt crystals, and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt crystals and O3'-type crystals and the rock salt crystals. In this specification, when the orientations of the cubic close-packed structures formed by anions in the layered rock salt crystals, O3'-type and rock salt crystals are aligned, it may be said that the crystal orientations are approximately the same. In addition, having a three-dimensional structural similarity such that the crystal orientations are approximately the same, or having the same crystallographic orientation, is called topotaxis.

[0153] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns, etc. It can also be determined from FFT patterns of TEM images and FFT patterns of STEM images, etc. Furthermore, XRD, neutron diffraction, etc. can also be used as materials for determination.

[0154] 3 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and the rock salt crystals RS are roughly the same. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0155] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in Figure 3) ＲＳ and L ＬＲＳWhen the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0156] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers appearing brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the cobalt atoms, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate with a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0157] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0158] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to the atomic number is obtained, so the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0159] Figure 4A shows an example of an STEM image in which the orientations of the layered rock-salt crystal LRS and the rock-salt crystal RS are roughly the same. Figure 4B shows the FFT pattern of the region of the rock-salt crystal RS, and Figure 4C shows the FFT pattern of the region of the layered rock-salt crystal LRS. The left side of Figures 4B and 4C shows the composition, JCPDS card number, and d-values ​​and angles calculated from the JCPDS card data. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0160] The spot marked A in Figure 4B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 4C is derived from the 0003 reflection of the layered rock salt type. From Figures 4B and 4C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 4B is roughly parallel to the line passing through AO in Figure 4C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0161] In this way, in the FFT pattern and the electron beam diffraction pattern, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0162] Furthermore, as described above, when the orientation of the cubic 11-1 reflection and the orientation of the layered rock salt 0003 reflection are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the layered rock salt 0003 reflection may be observed in a reciprocal lattice space different from the orientation of the layered rock salt 0003 reflection. For example, the spot marked B in Figure 4C is originating from the layered rock salt 10-14 reflection. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point originating from the layered rock salt 0003 reflection (A in Figure 4C), and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be identical. For example, a reciprocal lattice point equivalent to 0003 and 10-14 may also be used.

[0163] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 4B originates from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 4B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to match this. For example, reciprocal lattice points equivalent to the 11-1 and 200 reflections of the cubic crystal may also be used.

[0164] It is known that layered rock salt type positive electrode active materials, such as lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, for example, when observing the (0003) plane using a TEM or the like, first select positive electrode active material particles in which a crystal plane expected to be the (0003) plane is observed using a SEM or the like, and then, for example, the positive electrode active material particles can be thinned using a FIB (Focused Ion Beam) or the like so that the (0003) plane can be observed in a TEM or the like with an electron beam [12-10] incident. When determining whether the crystal orientation is consistent, it is preferable to thin the layered rock salt type (0003) plane so that it is easy to observe.

[0165] <Crystal structure> <Liｘ MO ２ When x is 1 in the positive electrode active material 100 of one embodiment of the present invention is in a discharged state, that is, Li ｘ MO ２ When x = 1 in the formula, the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies the majority of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure. Figure 5 shows the layered rock-salt type crystal structure, labeled R-3m O3. R-3m O3 has lattice constants a = 2.81610, b = 2.81610, c = 14.05360, α = 90.0000, β = 90.0000, and γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), and O(0,0,0.23951) (Non-Patent Document 5).

[0166] On the other hand, the surface layer portion 100a of the cathode active material 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b so that the layered structure of the transition metal M and oxygen octahedra is not destroyed even when lithium is removed from the cathode active material 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and inner portion 100b of the cathode active material 100, such as oxygen desorption and / or shifting of the layered structure of the transition metal M and oxygen octahedra. And / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0167] Therefore, the surface layer portion 100a preferably has a different crystal structure from the interior portion 100b. Furthermore, the surface layer portion 100a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0168] 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 particles of the positive electrode active material 100 in the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. For example, if the crystal structure of the layered structure consisting of transition metal M and oxygen octahedra in the surface layer 100a is displaced, this effect will be transmitted to the interior 100b, causing the layered crystal structure in the interior 100b to also be displaced, which is thought to lead to deterioration of the crystal structure of the entire positive electrode active material 100. On the other hand, if the surface layer 100a can be sufficiently stabilized, Li ｘ MO ２ Even when x is small, for example, 0.24 or less, the layered structure of the inner portion 100b made of the transition metal M and oxygen octahedra can be made less likely to break. Furthermore, displacement of the layer made of the transition metal M and oxygen octahedra in the inner portion 100b can be suppressed.

[0169] Furthermore, the interior 100b of the positive electrode active material 100 preferably has a low density of defects, including dislocations. Furthermore, the positive electrode active material 100 preferably has a large crystallite size as measured by XRD. In other words, the interior 100b preferably has high crystallinity. Furthermore, the surface of the positive electrode active material 100 preferably has a smooth surface. These characteristics are important factors supporting the reliability of the positive electrode active material 100 when used in a secondary battery. If the reliability of the positive electrode active material is high, the upper limit of the charging voltage of the secondary battery can be increased, resulting in a secondary battery with a high charge / discharge capacity.

[0170] Dislocations in the interior 100b can be observed, for example, by a TEM. If the density of defects, including dislocations, is sufficiently low, they may not be observed within a specific 1 μm square of the observation sample. Note that dislocations are a type of crystal defect and are different from vacancy defects.

[0171] The larger the crystallite size, the greater the amount of Li ｘ CoO ２ When x is small, the O3' type crystal structure is easily maintained, and contraction of the c-axis length is easily suppressed.

[0172] It is believed that the fewer defects, including dislocations, observed by TEM, the larger the crystallite size measured by XRD.

[0173] When calculating the crystallite size, the XRD diffraction pattern is preferably obtained from the positive electrode active material alone. However, it may also be obtained from the positive electrode, which includes the positive electrode active material, a current collector, a binder, a conductive material, and the like. However, in the positive electrode state, the positive electrode active material may be oriented due to the influence of pressure, etc., during the manufacturing process. Strong orientation may prevent accurate calculation of the crystallite size. Therefore, it is more preferable to obtain the pattern by removing the positive electrode active material layer from the positive electrode, removing some of the binder, etc., in the positive electrode active material layer using a solvent, etc., and then loading the sample into a sample holder. Another method is to apply grease to a silicon anti-reflective plate and then attach the powder sample.

[0174] The crystallite size is calculated using, for example, a Bruker D8 ADVANCE and CuKα X-rays. １The diffraction pattern obtained using a LYNXEYEXE-T detector with a 2θ angle of 15° to 90° increments of 0.005 and ICSD Coll. Code 172909 as the literature value for lithium cobalt oxide can be used. Analysis can be performed using DIFFRAC. TOPAS ver. 6 crystal structure analysis software, with the following settings, for example: Emission Profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9 FDS angle: 0.3 Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5 Secondary Sollers: 2.5 Corrections Specimen displacement: Refine LP Factor: 0

[0175] It is preferable to use the value of LVol-IB, which is the crystallite size corrected by the integral width standard calculated by the above method, as the crystallite size. Note that if the calculated preferred orientation is less than 0.8, the orientation of the sample may be too strong and it may not be suitable for determining the crystallite size.

[0176] <Li ｘ MO ２ The positive electrode active material 100 of one embodiment of the present invention has the above-described distribution of the additive element and / or the crystal structure in a discharged state, and therefore, ｘ MO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0177] 5 to 9B, Li ｘ MO ２The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0178] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 6. The conventional positive electrode active material shown in FIG. 6 is lithium cobalt oxide (LiCoO ２ In particular, changes in the crystal structure of lithium cobalt oxide that does not contain any added elements are described in Non-Patent Documents 1 to 3, etc.

[0179] In Figure 6, R-3m O3 is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a plane with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0180] It is also known that conventional lithium cobalt oxide has a crystal structure that belongs to the monoclinic space group P2 / m when x is about 0.5, and the lithium ions are aligned to increase the symmetry. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0181] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0182] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２It can also be said that the structure of and the structure of are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 crystal structure. Note that, since the actual insertion and desorption of lithium does not necessarily occur uniformly within the positive electrode active material and the lithium concentration may become uneven, the H1-3 crystal structure is experimentally observed from about x = 0.25. In fact, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 6 and other parts of this specification, to facilitate comparison with other crystal structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0183] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co (0,0,0.42150±0.00016), O1 (0,0,0.27671±0.00045), and O2 (0,0,0.11535±0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to 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, a unit cell with a small GOF (goodness of fit) value can be adopted.

[0184] Li ｘ CoO ２ When charging and discharging are repeated so that x in the formula is 0.24 or less, conventional lithium cobalt oxide undergoes repeated changes in crystal structure (i.e., non-equilibrium phase changes) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0185] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 6, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m O3 in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0186] Furthermore, the difference in volume between these two crystal structures is large, so that, when compared per equal number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3mO3 crystal structure exceeds 3.5%, typically 3.9% or more.

[0187] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0188] Therefore, when charging and discharging are repeated so that x is 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0189] On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. ｘ MO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ MO ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ MO ２ When the value of x in the formula (1) is kept at 0.24 or less, short circuits are unlikely to occur. In such a case, the safety of the secondary battery is further improved, which is preferable.

[0190] Li ｘ MO ２The crystal structure of the inner portion 100b of the positive electrode active material 100 when x is 1, approximately 0.2, and approximately 0.15 is shown in FIG. 5. The inner portion 100b occupies the majority of the volume of the positive electrode active material 100 and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0191] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0192] However, the positive electrode active material 100 has a different crystal structure from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 or 0.15, which results in an H1-3 type crystal structure.

[0193] The positive electrode active material 100 according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The layer symmetry is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 5 with the notation R-3m O3'.

[0194] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１０ m), and 2.807≦a≦2.827(×10 −１０ m) is more preferable, and typically a=2.817(×10 −１０ The c-axis is 13.681 ≤ c ≤ 13.881 (× 10 −１０ m), and 13.751≦c≦13.811 (×10 −１０ m) is more preferable, and typically c=13.781 (×10 −１０ m).

[0195] The positive electrode active material 100 of one embodiment of the present invention when x is about 0.15 may have a crystal structure belonging to the monoclinic space group P2 / m. ２There is one layer. In this case, the amount of lithium present in the positive electrode active material 100 is about 15 atomic % in the discharged state. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. This crystal structure is shown in Figure 5 with the P2 / m monoclinic O1(15) symbol.

[0196] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ The lattice constant of the unit cell is a = 4.880 ± 0.05 (× 10 −１０ m), b=2.817±0.05(×10 −１０ m), c=4.839±0.05(×10 −１０ m), α = 90°, β = 109.6 ± 0.1°, γ = 90°.

[0197] This crystal structure can also show the lattice constant in the space group R-3m if some error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constant of the unit cell is a = 2.817 ± 0.02 (× 10 −１０ m), c=13.68±0.1(×10 −１０ m).

[0198] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy hexacoordinated oxygen sites, although lighter elements such as lithium and magnesium may occupy tetracoordinated oxygen sites.

[0199] As shown by the dotted line in FIG. 5, the CoO ２ There is almost no layer misalignment.

[0200] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0201] The difference in volume per the same number of cobalt atoms between R-3m O3 in a discharged state and the monoclinic O1(15) type crystal structure is 3.3% or less, more specifically 3.0% or less, typically 2.5%.

[0202] Table 1 shows the difference in volume per cobalt atom between discharged R-3m O3, O3', monoclinic O1(15), H1-3, and trigonal O1. For the lattice constants of the crystal structures of discharged R-3m O3 and trigonal O1 used in the calculations of Table 1, ICSD Coll. Code. 172909 and 88721 can be referenced. For H1-3, Non-Patent Document 3 can be referenced. For O3' and monoclinic O1(15), calculations can be made from experimental XRD values.

[0203]

[0204] As described above, in the positive electrode active material 100 according to one embodiment of the present invention, Li ｘ MO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0205] The positive electrode active material 100 is Liｘ MO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be present, and it is presumed that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ MO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically when x is 0.15 or more and less than 0.17, the crystal structure may be monoclinic O1(15) type. However, the crystal structure is Li ｘ MO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0206] Therefore, the positive electrode active material 100 is Li ｘ MO ２ When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material 100 may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or some may be amorphous.

[0207] Also Li ｘ MO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ MO ２ A state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.60 V or higher relative to the potential of lithium metal in an environment of 25°C, a H1-3 crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.60 V or higher relative to the potential of lithium metal can be said to be a high charging voltage. Unless otherwise specified, in this specification and the like, charging voltages are expressed relative to the potential of lithium metal.

[0208] Therefore, in other words, the positive electrode active material 100 of one embodiment of the present invention is preferable because it can maintain a crystal structure having the symmetry of R-3m O3 even when charged at a high charging voltage, for example, a voltage of 4.60 V or higher at 25°C.

[0209] Even with the positive electrode active material 100, an H1-3 crystal structure may be finally observed when the charge voltage is further increased. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, when the charge voltage is lower, for example, even when the charge voltage is 4.50 V or more and less than 4.60 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt an O3′ crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.70 V or less at 25° C., the positive electrode active material 100 may be able to adopt a monoclinic O1(15) crystal structure.

[0210] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.20 V relative to the potential of lithium metal. Therefore, a secondary battery using graphite as the negative electrode active material has a similar crystal structure at a voltage obtained by subtracting the potential of graphite from the voltage when lithium metal is used as the negative electrode active material.

[0211] In addition, in O3' and monoclinic O1(15) in FIG. 5, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, for example, in monoclinic O1(Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0212] The O3' and monoclinic O1(15) crystal structures have random lithium between the layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0213] <Grain Boundaries> In addition to the above distribution, at least a portion of the additive element contained in the positive electrode active material 100 of one embodiment of the present invention is preferably unevenly distributed in and near the grain boundaries.

[0214] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of areas with high concentration and areas with low concentration.

[0215] For example, it is preferable that the magnesium concentration at and near the grain boundaries of the positive electrode active material 100 is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the nickel concentration at and near the grain boundaries is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundaries is higher than that in other regions of the interior 100b.

[0216] Grain boundaries are a type of planar defect. Therefore, like particle surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the concentration of added elements at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0217] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the positive electrode active material 100 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the surface where the cracks occur. Therefore, even after the cracks occur, the corrosion resistance to hydrofluoric acid of the positive electrode active material can be improved. Furthermore, even after the cracks occur, the side reaction between the electrolyte and the positive electrode active material can be suppressed.

[0218] <Particle diameter> If the particle diameter of the positive electrode active material 100 of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium, excessive roughness of the surface of the active material layer when applied to a current collector, etc. On the other hand, if the particle diameter is too small, problems such as excessive reaction with the electrolyte solution may occur.

[0219] The particle diameter of the positive electrode active material 100 can be measured, for example, by a laser diffraction particle size distribution analyzer. The particle diameter (D50) of the positive electrode active material 100 measured by the laser diffraction particle size distribution analyzer is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0220] D50 is the particle size at which the integrated amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. Similarly, D10 is the particle size at which the integrated amount in the cumulative curve of the particle size distribution measurement results accounts for 10%. D90 is the particle size at which the integrated amount in the cumulative curve of the particle size distribution measurement results accounts for 90%.

[0221] Furthermore, when particle size distribution measurement is performed, it is preferable that the particle size distribution of the positive electrode active material particles has one peak. Furthermore, it is preferable that the values ​​of D10, D50, and D90 in the particle size distribution measurement are close, that is, the particles are single particles with a uniform particle size.

[0222] <Analysis method> A certain positive electrode active material is ｘ MO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100 of one embodiment of the present invention has an O3′-type and / or monoclinic O1(15)-type crystal structure by Li ｘ MO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0223] In particular, XRD is preferable in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies the majority of the volume of the positive electrode active material 100.

[0224] When analyzing the crystallite size by powder XRD, it is preferable to measure the size while excluding the influence of the orientation of the positive electrode active material particles due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the size.

[0225] As described above, the positive electrode active material 100 according to one embodiment of the present invention is ｘ MO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltage accounts for 50% or more of the crystal structure are not preferable because they cannot withstand repeated high-voltage charging and discharging.

[0226] It should also be noted that simply adding an additive element may not result in an O3' or monoclinic O1(15) crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li ｘ MO ２ In some cases, x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases, the H1-3 type crystal structure accounts for 50% or more.

[0227] Furthermore, even in the positive electrode active material 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be formed when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.90 V. Therefore, to determine whether or not the positive electrode active material 100 of one embodiment of the present invention is a material, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0228] However, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, the crystal structure may change from O3'-type or monoclinic O1(15)-type to H1-3-type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0229] Furthermore, whether or not the distribution of the added elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, electron probe microanalyzer (EPMA), or the like.

[0230] The crystal structure of the surface layer 100 a and the grain boundaries can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0231] <Charging Method> To determine whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention, for example, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) using the composite oxide as a positive electrode and lithium metal as a counter electrode can be fabricated as a test battery and charged. The coin cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0232] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0233] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0234] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0235] The separator may be a 25 μm thick porous polypropylene film.

[0236] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0237] Note that the above coin cell (test battery) may be fabricated using the positive electrode removed by disassembling a battery (for example, a battery in an initial state mounted on a commercially available product), and charging may be performed to determine whether or not the coin cell is the positive electrode active material 100 of one embodiment of the present invention described below.

[0238] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.60 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging is performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the CC charging current can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at 2 mA / g or more and 10 mA / g or less. To observe the phase change of the positive electrode active material, it is desirable to charge at such a small current value. On the other hand, if the current does not reach 2 mA / g or more and 10 mA / g or less even after long-term CV charging, it is considered that the current is being consumed not for charging the positive electrode active material but for decomposing the electrolyte. Therefore, CV charging may be terminated after a sufficient time has elapsed since the start of the charging. In this case, a sufficient time can be, for example, 1.5 hours or more and 3 hours or less. The temperature is set to 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with a desired charge capacity. When various analyses are performed thereafter, it is preferable to seal the cell in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to quickly remove the positive electrode and subject it to analysis after charging is complete. Specifically, it is preferable to do so within one hour after charging is complete, and more preferably within 30 minutes.

[0239] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge / discharge. For example, charging may be performed by constant current charging at a current value of 20 mA / g or more and 100 mA / g or less up to a given voltage (e.g., 4.60 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V), followed by constant voltage charging until the current value reaches 2 mA / g or more and 10 mA / g or less, and then discharging by constant current discharging at 2.50 V and 20 mA / g or more and 100 mA / g or less.

[0240] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at 2.50 V with a current value of 20 mA / g or more and 100 mA / g or less.

[0241] <XRD> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0242] 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.

[0243] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ The ideal powder XRD patterns of the lines are shown in Figures 7, 8, 9A and 9B. ｘ MO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 9A and 9B show the XRD patterns of the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, and the H1-3-type crystal structure, with Figure 9A showing an enlarged view of the region where 2θ is in the range of 18° to 21°, and Figure 9B showing an enlarged view of the region where 2θ is in the range of 42° to 46°. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from the crystal structure information obtained from ICSD (see Non-Patent Document 4). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of the positive electrode active material, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same way as the others.

[0244] As shown in Figures 7, 9A and 9B, in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0245] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear at 2θ=19.47±0.10° (19.37° or more and 19.57° or less) and 2θ=45.62±0.05° (45.57° or more and 45.67° or less).

[0246] However, as shown in Figures 8, 9A and 9B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of peaks at 19.13° or more and less than 19.37° and / or 19.37° or more and less than 19.57°, and at 45.37° or more and less than 45.57° and / or 45.57° or more and less than 45.67° when x is small can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0247] This can also be said to be because, in the positive electrode active material 100 of one embodiment of the present invention, the positions at which XRD diffraction peaks appear are close between the crystal structures where x = 1 and where x ≦ 0.24. More specifically, among the main diffraction peaks of the crystal structures where x = 1 and where x ≦ 0.24, the difference in 2θ between peaks that appear at 2θ of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0248] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２When x in the formula is small, the particles may have an O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the particles may have an O3'-type and / or monoclinic O1(15)-type crystal structure. Other crystal structures may be included, or some may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0249] Similarly, when Rietveld analysis is performed, the H1-3 type and O1 type crystal structures are preferably less than 50%, more preferably 34% or less, and even more preferably substantially not observed.

[0250] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0251] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. For example, a narrow half-width is preferable. Even for peaks arising from the same crystalline phase, the half-width varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for peaks observed between 2θ = 43° and 46°, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0252] The crystallite size of the O3'-type and monoclinic O1(15) crystal structures of the positive electrode active material 100 is approximately equal to that of LiCoO in a discharged state. ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the formula is small, the peaks of the O3' type and / or monoclinic O1(15) crystal structure can be clearly observed. ２ In this case, even if some of the crystal structure resembles the O3' type and / or monoclinic O1(15) crystal structure, the crystallite size will be small and the peaks will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0253] As described above, the positive electrode active material 100 of one embodiment of the present invention preferably has a small influence of the Jahn-Teller effect. As long as the influence of the Jahn-Teller effect is small, the positive electrode active material 100 may contain a transition metal such as nickel or manganese as an additive element in addition to cobalt.

[0254] For example, when the nickel concentration is 5% or 7.5%, the a-axis / c-axis tends to change significantly, and when the nickel concentration is 7.5%, the distortion of the a-axis increases. This distortion may be due to Jahn-Teller distortion of trivalent nickel. Therefore, it is preferable that the nickel content of the transition metal M contained in the positive electrode active material 100 is less than 7.5 atomic %.

[0255] Furthermore, when the manganese concentration is 5% or more, the behavior of the change in lattice constant is different and it is suggested that it does not follow Vegard's law. Therefore, of the transition metals M contained in the positive electrode active material 100, manganese is preferably 4 atomic % or less.

[0256] The above ranges of nickel concentration and manganese concentration do not necessarily apply to the surface layer 100a, that is, the concentrations in the surface layer 100a may be higher than the above ranges.

[0257] As a result of considering a preferable range of the lattice constant from the above, it was found that in the positive electrode active material of one embodiment of the present invention, the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge and discharge are performed or in a discharged state, which can be estimated from the XRD pattern, has an a-axis lattice constant of 2.814 × 10 −１０ m is greater than 2.817 x 10 −１０ m and the lattice constant of the c-axis is 14.05 × 10 −１０ m or larger, 14.07 x 10 −１０ It has been found that the value is preferably smaller than m. The state in which no charge and discharge are performed may be, for example, the state of powder before the positive electrode of the secondary battery is produced.

[0258] Alternatively, in the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, it is preferable that the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is greater than 0.20000 and smaller than 0.20049.

[0259] Alternatively, when XRD analysis is performed on the layered rock salt type crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, a first peak may be observed at 2θ of not less than 18.50° and not more than 19.30°, and a second peak may be observed at 2θ of not less than 38.00° and not more than 38.80°.

[0260] <<Resistance DCIR>> In the above <<Charging Method>>, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) is prepared with a lithium counter electrode and charged as a charging method for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention. Using a coin cell prepared in a similar manner, it is possible to measure the low resistance DCIR, which is one of the characteristics of the positive electrode active material 100 of one embodiment of the present invention.

[0261] A coin cell prepared under the same conditions as in the above charging method is charged at a desired voltage (e.g., 4.50 V, 4.55 V, 4.60 V, 4.65 V, 4.70 V, 4.75 V, or 4.80 V). The charging method is not particularly limited as long as charging can be performed at the desired voltage for a sufficient period of time. For example, when charging by CCCV, the 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.

[0262] The charged coin cell is left in a resting state (a state in which neither charging nor discharging is performed) for 10 minutes, and then the resistance DCIR (Direct Current Internal Resistance, also referred to as DC resistance) is measured. The resistance DCIR is measured by discharging for 1 second at a current of 2 mA / g to 10 mA / g (for example, 6 mA / g) per weight of the positive electrode active material (lithium cobalt oxide). The DCIR can be calculated by dividing the difference between the open circuit voltage before discharge and the voltage during discharge by the current (for example, 6 mA / g).

[0263] For example, when the open circuit voltage before discharge (after charge) is 4.50 V or more and 4.60 V or less, the resistance DCIR of a coin cell including the positive electrode active material 100 of one embodiment of the present invention is preferably 50 Ω or less, more preferably 40 Ω or less, and even more preferably 35 Ω or less. Note that a series of measurements of the resistance DCIR are preferably performed in an environment of 45°C.

[0264] XPS: In the case of inorganic oxides, XPS can analyze a region from the surface to a depth of approximately 1 to 8 nm (usually 5 nm or less) using monochromatic aluminum Kα rays as the X-ray source. This allows quantitative analysis of the concentration of each element in a region approximately half the depth of the surface layer 100a. Furthermore, the depth of the analysis region can be changed by changing the photoelectron take-off angle (also called the take-off angle or detector angle) relative to the surface to be measured. For example, a take-off angle of 45° can analyze a region from the surface to a depth of approximately 5 nm, and a take-off angle of 15° can analyze a region from the surface to a depth of approximately 2 nm. By performing measurements under multiple conditions with different detection depths in XPS analysis, it is possible to estimate the distribution of the additive elements in the depth direction of the surface layer 100a. Furthermore, by combining this with digging into the surface to be measured using Ar ion sputtering, it is also possible to measure the distribution of the additive elements in the depth direction up to several μm. As the XPS apparatus, for example, an apparatus such as Quantera II or Quantera SXM manufactured by ULVAC-PHI, Inc. can be used.

[0265] Furthermore, narrow scan analysis can be used in the above measurements to analyze the bonding state of elements.The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 0.1 to 1 atomic %, depending on the element.

[0266] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion 100a than in the interior portion 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS, GD-MS, or the like. For example, the magnesium concentration of at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the average magnesium concentration throughout the cathode active material 100. Furthermore, the titanium concentration of at least a portion of the surface layer portion 100a is preferably higher than the average titanium concentration throughout the cathode active material 100. Furthermore, the nickel concentration of at least a portion of the surface layer portion 100a is preferably higher than the average nickel concentration throughout the cathode active material 100. It is also preferable that the aluminum concentration in at least a part of the surface layer portion 100a is higher than the average aluminum concentration in the entire positive electrode active material 100. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 100a is higher than the average fluorine concentration in the entire positive electrode active material 100.

[0267] Note that the surface and surface layer 100a of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 100. Furthermore, the surface of the positive electrode active material 100 also does not contain the electrolyte, binder, conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0268] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the ratio of the number of atoms of the added element is not affected.

[0269] The concentration of the added element may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances chemisorbed after the preparation of the positive electrode active material. For example, the atomic ratio Mg / Co of magnesium (Mg) to cobalt (Co) determined by XPS analysis is preferably 0.4 to 1.5. The atomic ratio Ti / Co of titanium (Ti) to cobalt (Co) determined by XPS analysis is preferably 0.05 to 0.20. The atomic ratio Ni / Co of nickel (Ni) to cobalt (Co) determined by XPS analysis is preferably 0.05 to 0.20. Meanwhile, the atomic ratio Mg / Co determined by ICP-MS analysis is preferably 0.002 to 0.06.

[0270] Furthermore, it is more preferable that aluminum is widely distributed in a deep region, for example, the surface or a region at a depth of 5 nm to 50 nm from the reference point. Therefore, it is more preferable that aluminum is detected in an analysis of the entire cathode active material 100 using ICP-MS, GD-MS, etc., but the aluminum concentration is not detected by XPS, etc., or the aluminum concentration is 1 atomic % or less.

[0271] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. The take-off angle can be, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: Quantera II manufactured by ULVAC-PHI Inc. X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: range of approximately 4 to 5 nm from the surface (detector angle 45°) Measurement spectrum: wide scan, narrow scan of each detected element

[0272] When the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 688 eV.

[0273] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1306 eV, and more preferably about 1304 eV.

[0274] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak showing the bond energy between titanium and other elements is preferably greater than or equal to 455 eV and less than 460 eV, and the valence of Ti is preferably tetravalent.

[0275] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak showing the bond energy between nickel and another element is preferably greater than or equal to 855 eV and less than 859 eV. ２＋ It is preferable that the energy is shifted to a higher energy side than 854 eV, which is the bond energy of —O.

[0276] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. It is more preferable that the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using an FIB or the like and analyzing the cross section using EDX, EPMA, or the like.

[0277] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Linear analysis is also used to refer to data extracted from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0278] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundaries of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis using a thinned sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0279] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen capable of lithium insertion / extraction, the interface between a region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) that is oxidized and reduced upon lithium insertion / extraction and oxygen is present and a region where it is not present is defined as the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be attached to the surface, but the protective film is not included in the positive electrode active material. The protective film may be a single-layer or multilayer film of carbon, metal, oxide, resin, etc.

[0280] In STEM-EDX-ray analysis or the like, in principle or due to measurement errors, the graph of the detected amount of characteristic X-rays of the element does not change sharply, and it may be difficult to precisely determine the surface. Therefore, when referring to the depth direction in STEM-EDX-ray analysis or the like, the detected amount of characteristic X-rays of the transition metal M is the average value M of the detected amount of characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the detected amount of oxygen characteristic X-rays and the average value O ＡＶＥ and the average value O of the detected amount of characteristic X-rays of oxygen in the background ＢＧThe reference point is the point where the detected amount of the characteristic X-rays of the transition metal M is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M and the average value of the detected amount of the characteristic X-rays of the background transition metal M. If the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen is different from the point where the detected amount of the characteristic X-rays of oxygen is 50% of the sum of the average value of the detected amount of the characteristic X-rays of the internal oxygen and the average value of the detected amount of the characteristic X-rays of the background oxygen, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen attached to the surface, and therefore, the detected amount of the characteristic X-rays of the transition metal M is different from the point where ... oxygen is different from the sum of the average value of the detected amount of the characteristic X-rays of the internal transition metal M. ＡＶＥ and the average M of the detected amount of characteristic X-rays of the transition metal M in the background. ＢＧ In the case of a positive electrode active material containing a plurality of transition metals M, the M of the 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:

[0281] The average value M of the detected amount of characteristic X-rays of the transition metal M in the background ＢＧ can be obtained by averaging, for example, a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＡＶＥ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the detected amount of characteristic X-rays of the transition metal M and oxygen becomes saturated and stable, for example, the region where the detected amount of characteristic X-rays of the transition metal M starts to increase. ＢＧ and the average value O of the detected amount of characteristic X-rays of oxygen inside ＡＶＥ can also be found in the same way.

[0282] Furthermore, the surface of the positive electrode active material 100 in a cross-sectional STEM image or the like is the boundary between an area where an image derived from the crystalline structure of the positive electrode active material is observed and an area where an image is not observed, and is the outermost area of ​​an area where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are confirmed.

[0283] In addition, a peak in STEM-EDX-ray analysis 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-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0284] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value measured over six scans can be used as the detection value for each element. The number of scans is not limited to six, and more scans can be performed, and the average can be used as the detection value for each element.

[0285] The STEM-EDX analysis can be performed, for example, as follows: First, a protective film is vapor-deposited on the surface of the positive electrode active material. For example, carbon can be vapor-deposited using an ion sputtering device (MC1000 manufactured by Hitachi High-Technologies).

[0286] Next, the positive electrode active material is sliced ​​to prepare a STEM cross-section sample. For example, the slice processing can be performed using an FIB-SEM device (Hitachi High-Tech XVision 200TBS). Pickup is performed using an MPS (microprobing system), and the finishing conditions can be, for example, an acceleration voltage of 10 kV.

[0287] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W EDX detector. An example of conditions for EDX ray analysis using the Hitachi High-Tech HD-2700 is to set the emission current of the STEM device to 6 μA or more and 10 μA or less, 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 ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0288] In order to increase the spatial resolution in STEM-EDX ray analysis, it is preferable that the beam diameter of the electron beam (also referred to as beam diameter, probe diameter, or probe diameter) is small. The beam diameter in STEM-EDX ray analysis is preferably 0.3 nm or less, more preferably 0.2 nm or less, and even more preferably 0.1 nm or less. Furthermore, in order to increase the analytical sensitivity in STEM-EDX ray analysis, it is preferable to increase the beam current of the electron beam (also referred to as probe current). Therefore, it is preferable that the device used in STEM-EDX ray analysis is equipped with a spherical aberration corrector (Cs corrector) that can reduce the beam diameter and increase the beam current.

[0289] When EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, the concentration of each additive element, particularly any one or more of magnesium, nickel, and titanium, in the surface layer portion 100a is preferably higher than the concentration of the element in the interior portion 100b. Alternatively, the detected amount of each additive element, particularly any one or more of magnesium, nickel, and titanium, in the surface layer portion 100a is preferably higher than the detected amount of the element in the interior portion 100b.

[0290] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, the magnesium concentration in the surface layer 100a is preferably higher than the magnesium concentration in the interior 100b. Furthermore, when EDX ray analysis is performed, the peak of the magnesium concentration in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, the magnesium concentration preferably decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it is preferable that the magnesium concentration decays to 30% or less of the peak at a depth of 2 nm from the peak top. Alternatively, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, the detected amount of characteristic X-rays of magnesium in the surface layer 100a is preferably higher than the detected amount of characteristic X-rays of magnesium in the interior 100b. Furthermore, when EDX-ray analysis is performed, the peak of the detected amount of characteristic X-rays of magnesium in the surface layer portion 100a preferably exists on the surface of the positive electrode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, the detected amount of characteristic X-rays of magnesium preferably decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it preferably decays to 30% or less of the peak at a depth of 2 nm from the peak top. The concentration peak top here refers to the maximum value or the position where the maximum value appears in a convex shape that appears in the concentration distribution graph for each element. Furthermore, the detected amount peak top refers to the maximum value or the position where the maximum value appears in a convex shape that appears in the detection amount distribution graph for each element.

[0291] Furthermore, when EDX-ray analysis is performed, the maximum value of the magnesium concentration (detected amount of magnesium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.5 Atomic% or more and 10 Atomic% or less, and more preferably 1 Atomic% or more and 7 Atomic% or less, or 1 Atomic% or more and 5 Atomic% or less.

[0292] Furthermore, when EDX-ray analysis is performed, the maximum value of the titanium concentration (detected amount of titanium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.2 Atomic% or more and 10 Atomic% or less, and more preferably 0.5 Atomic% or more and 6 Atomic% or less, or 0.5 Atomic% or more and 5 Atomic% or less.

[0293] Furthermore, when EDX-ray analysis is performed, the maximum value of the nickel concentration (detected amount of nickel / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.2 Atomic% or more and 5 Atomic% or less, and more preferably 0.5 Atomic% or more and 4 Atomic% or less, or 0.5 Atomic% or more and 3 Atomic% or less.

[0294] Furthermore, when EDX-ray analysis is performed, the maximum value of the fluorine concentration (detected amount of fluorine / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, titanium, and nickel) in the surface layer portion 100a is preferably 0.5 Atomic% or more and 10 Atomic% or less, and more preferably 1 Atomic% or more and 6 Atomic% or less.

[0295] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably has a region overlapping with the distribution of magnesium. 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, even more preferably within 0.5 nm, and even more preferably within 0.3 nm.

[0296] In the positive electrode active material 100 containing nickel as an additive element, the peak of the nickel concentration or detectable amount in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the positive electrode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. In the positive electrode active material 100 containing magnesium and nickel, the nickel distribution preferably has an overlapping region 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, more preferably within 1 nm.

[0297] Furthermore, in the cathode active material 100 containing titanium as an additive element, the peak of the titanium concentration or detectable amount in the surface layer 100a is preferably present at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, in the cathode active material 100 containing magnesium and titanium, the distribution of titanium preferably has a region overlapping with the distribution of magnesium. For example, the difference in depth between the peak of the titanium concentration or detectable amount and the peak of the magnesium concentration or detectable amount is preferably within 3 nm, more preferably within 1 nm.

[0298] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that, when EDX-ray analysis is performed, the peak of the magnesium, nickel, or fluorine concentration or detected amount is closer to the surface than the peak of the aluminum concentration or detected amount in the surface layer portion 100a. In other words, it is preferable that the peak of the aluminum concentration or detected amount in the surface layer portion 100a is located more inward than the peak of the magnesium, nickel, or fluorine concentration or detected amount. For example, it is preferable that the peak of the aluminum concentration or detected amount is present on the surface of the positive electrode active material 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.

[0299] When the positive electrode active material 100 contains magnesium, titanium, nickel, and aluminum as described above, a battery using the positive electrode active material 100 can achieve both "high cycle characteristics" that can suppress deterioration in discharge capacity due to repeated high-voltage charging (for example, charging with an upper limit of 4.60 V) and discharging, and "high low-temperature characteristics" that can obtain a large discharge capacity at low temperatures (for example, 0°C, -20°C, -40°C).

[0300] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0301] Furthermore, when the positive electrode active material 100 is subjected to linear or area analysis, the ratio of the number of atoms of the additional element A to the cobalt Co in the vicinity of the grain boundaries (A / Co) is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0302] For example, when the additive element is magnesium, when linear or area analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) near the grain boundaries is preferably 0.020 or more and 0.50 or less. It is even more preferably 0.025 or more and 0.30 or less. It is even more preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less. Furthermore, when the ratio is within the above range at multiple locations, for example, three or more locations, on the positive electrode active material 100, it can be said that this indicates that the additive element is not attached to a narrow area on the surface of the positive electrode active material 100, but is widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material 100.

[0303] <<EPMA>> The concentration of the additive element in the positive electrode active material 100 can be analyzed using EDX, but it can also be analyzed using EPMA. EPMA has a higher detection capability (also referred to as a lower detection limit) than EDX when analyzing elements present in trace amounts in a sample. Therefore, it is preferable to use EPMA when analyzing a region where a trace amount of the additive element is present.

[0304] In the EPMA analysis, a cross section of the positive electrode active material 100 is exposed by mechanical polishing, ion polishing, FIB, etc., and the cross section is analyzed. As an EPMA device, for example, an electron probe microanalyzer JXA-iHP200F manufactured by JEOL Ltd. can be used.

[0305] EPMA uses a wavelength-dispersive detector, so it has a higher ability to detect trace elements than EDX, which uses an energy-dispersive detector. On the other hand, the spatial resolution of EPMA analysis is inferior to that of EDX (especially STEM-EDX). Therefore, STEM-EDX is suitable for analysis focusing on the detailed distribution of added elements in the surface layer 100a of the positive electrode active material 100, while EPMA analysis is suitable for analysis of trace amounts of added elements in the interior 100b. Note that EPMA and EDX use different analytical methods, so the concentration values ​​obtained when analyzing the same region using each analytical method may not match.

[0306] The additive elements in the interior 100b of the positive electrode active material 100 may preferably contain trace amounts of magnesium and nickel. For example, in EPMA analysis, the ratio Mg / Co of the number of magnesium (Mg) and cobalt (Co) atoms in the interior 100b is preferably 0.003 or more and 0.01 or less. Furthermore, in EPMA analysis, the ratio Ni / Co of the number of nickel (Ni) and cobalt (Co) atoms in the interior 100b is preferably 0.003 or more and 0.01 or less. Meanwhile, in EPMA analysis, the number of titanium atoms in the interior 100b is preferably less than the lower limit of detection. The lower limit of detection for the number of titanium atoms is 0.2 at% when analyzing the interior of lithium cobalt oxide using, for example, a JXA-iHP200F.

[0307] <Raman Spectroscopy> As described above, in the positive electrode active material 100 according to one embodiment of the present invention, at least a portion of the surface layer 100a preferably has a rock salt crystal structure. Therefore, when the positive electrode active material 100 and a positive electrode including the positive electrode active material 100 are analyzed by Raman spectroscopy, it is preferable to observe not only the layered rock salt crystal structure but also cubic crystal structures such as rock salt crystal structure. In the STEM image and the electron microbeam diffraction pattern described below, unless cobalt is substituted at lithium positions with a certain frequency in the depth direction during observation, or cobalt is present at the oxygen tetracoordination positions, they cannot be detected as bright spots in the STEM image and the electron microbeam diffraction pattern. On the other hand, because Raman spectroscopy is an analysis that captures the vibrational modes of bonds such as Co—O, peaks of the wavenumbers of the corresponding vibrational modes may be observed even when the amount of the corresponding Co—O bond is small. Furthermore, Raman spectroscopy can be performed on a surface layer having an area of ​​several μm ２ Since it is possible to measure a range of about 1 μm in depth, it is possible to capture with high sensitivity the state that exists only on the particle surface.

[0308] For example, when the laser wavelength is 532 nm, layered rock salt LiCoO ２ So, 470 cm −１ ~490cm −１ , 580 cm −１ ~600cm −１ Peak at (vibration mode: E ｇ , A １ｇ ) is observed. On the other hand, cubic CoO ｘ (0<x<1) (rock salt type Co １−ｙ O (0<y<1) or spinel type Co ３ O ４ ) is 665 cm −１ ~685cm −１ Peak at (Vibration mode: A １ｇ ) is observed.

[0309] Therefore, 470 cm −１ ~490cm −１ The integrated intensity of I1, 580 cm −１ ~600cm −１ The integrated intensity of I2, 665 cm −１ ~685cm −１When the integrated intensity of the above is I3, the value of I3 / I2 is preferably 1% or more and 10% or less, and more preferably 3% or more and 9% or less.

[0310] If a cubic crystal structure such as a rock salt type is observed within the above range, it can be said that the surface layer 100a of the positive electrode active material 100 has a rock salt type crystal structure within a preferred range.

[0311] <<Electron Diffraction Pattern>> As with Raman spectroscopy, it is preferable that the characteristics of the rock salt-type crystal structure are observed in the electron diffraction pattern as well as the layered rock salt crystal structure. However, in the STEM image and the electron diffraction pattern, taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock salt-type crystal structure are not too strong in the surface layer portion 100a, particularly in the outermost surface (for example, 1 nm deep from the surface). This is because, rather than the outermost surface being covered with a rock salt-type crystal structure, it is preferable that the lithium layer has a layered rock salt-type crystal structure and an additive element such as magnesium is present in the lithium layer, which can ensure a lithium diffusion path and have a stronger function of stabilizing the crystal structure.

[0312] Therefore, for example, when a micro-electron beam diffraction pattern is obtained from a region having a depth of 1 nm or less from the surface and a micro-electron beam diffraction pattern from a region having a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constant calculated from these patterns is small.

[0313] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is 0.1 × 10 for the a-axis. −１０ m or less, and the c-axis is preferably 1.0 × 10 −１０ It is preferable that the a-axis is 0.05×10 −１０ m or less, and the c-axis is preferably 0.6×10 −１０ It is more preferable that the a-axis is 0.04×10 −１０ m or less, and the c-axis is more preferably 0.3 × 10 −１０ It is more preferable that the length is m or less.

[0314] <Surface Roughness and Specific Surface Area> The positive electrode active material 100 of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness indicates that the effect of the flux described below is sufficiently exerted, and the surface of the additive element source and the lithium cobalt oxide are melted. Therefore, this is one factor indicating that the distribution of the additive element in the surface layer portion 100 a is good.

[0315] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of the positive electrode active material 100, the specific surface area of ​​the positive electrode active material 100, or the like.

[0316] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0317] First, the cathode active material 100 is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 100 with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material 100 is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 100 is selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness at least within 400 nm of the outer periphery of the particle of the cathode active material.

[0318] On the particle surfaces of the positive electrode active material 100 of this embodiment, the root mean square (RMS) surface roughness, which is an index of roughness, is preferably less than 3 nm, more preferably less than 1 nm, and even more preferably less than 0.5 nm.

[0319] The image processing software used for noise processing, interface extraction, etc. is not particularly limited, but for example, "ImageJ" described in Non-Patent Documents 8 to 10 can be used. Spreadsheet software, etc. is also not particularly limited, but for example, Microsoft Office Excel can be used.

[0320] For example, the actual specific surface area S measured by the gas adsorption method using the constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 100 can also be quantified from the ratio of

[0321] Ideal specific surface area S ｉ is calculated assuming that all particles have the same diameter as D50, the same weight, and an ideal spherical shape.

[0322] The median diameter D50 can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. The specific surface area can be measured by a specific surface area measuring device using a gas adsorption method based on a constant volume method, for example.

[0323] The positive electrode active material 100 according to one embodiment of the present invention has an ideal specific surface area S calculated from the median diameter D50. ｉ and the actual specific surface area S Ｒ The ratio S Ｒ / S ｉ is preferably 2.1 or less.

[0324] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0325] First, a surface SEM image of the positive electrode active material 100 is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area are the same.

[0326] Next, image processing software (for example, "ImageJ") is used to convert the SEM image into, for example, an 8-bit image (called a grayscale image). The grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 gradations. Dark areas have lower gradations, and bright areas have higher gradations. The luminance change can be quantified in relation to the number of gradations. This numerical value is called the grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material as a numerical value.

[0327] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram shows the gradation distribution in the target area and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0328] When evaluating the positive electrode active material 100 of one embodiment of the present invention, the difference between the maximum and minimum grayscale values ​​is preferably 120 or less, more preferably 115 or less, and even more preferably 70 to 115. The standard deviation of the grayscale values ​​is preferably 11 or less, more preferably 8 or less, and even more preferably 4 to 8.

[0329] <Particle Size Distribution Analysis Using Cross-Sectional SEM Image of Positive Electrode> The particle size distribution of the positive electrode active material 100 can also be calculated from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0330] First, an analysis region is cut out from the acquired cross-sectional SEM image. A range having a sufficient area for image analysis can be cut out, for example, a range of 50 μm or more × 100 μm or more, but this is not limited thereto. Depending on factors such as the size of the positive electrode active material, a smaller or larger area may be cut out.

[0331] The cross-sectional SEM image may be cut out using a function of image processing software, for example, ImageJ, which may be used as the image processing software, and cut out using its crop function.

[0332] Next, the cut-out first image is binarized using image processing software, and particle analysis is performed.

[0333] ImageJ, for example, can be used as the image processing software. The binarization process will be described below. A first image displayed on a 256-value grayscale is used as a frequency graph excluding black (value 0) and white (value 255), and the low-value side (HWHM_L) and high-value side (HWHM_H) are determined as the half-width at half maximum (HWHM) of the maximum peak in the frequency graph. Next, a minimum value a in a range twice the width of HWHM_L on the low-value side from the value corresponding to the peak top (maximum frequency) of the maximum peak, and a maximum value b in a range twice the width of HWHM_H on the high-value side are determined.

[0334] Next, binarization is performed so that values ​​less than a are white, values ​​greater than a and less than b are black, and values ​​greater than b are white. Specifically, the threshold function of ImageJ is used to perform binarization as Threshold (a, b). After that, random bright spots thought to be caused by the conductive material are removed using the Gray Morphology (radius = 3, operator = open, type = circle) and Gray Morphology (radius = 1, operator = close, type = circle) conditions, and a second image can be obtained.

[0335] Next, using the second image, the particle size (projected area) was determined to be 0.5 μm using the Analyze Particles function of ImageJ. ２ 700 μm or more ２ The following particles are detected, and the area S of each particle is obtained. Next, the diameter r of each particle is calculated based on the area S of each particle (Equation 1).

[0336]

[0337] In this way, the particle size distribution of each particle can be calculated from the cross-sectional SEM image. Performing the above analysis is called performing particle size distribution analysis using a cross-sectional SEM image of the positive electrode.

[0338] A coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. Fig. 10 shows an example of a positive electrode active material 100 to which a coating portion 104 is attached.

[0339] The coating portion 104 is preferably formed by the accumulation of decomposition products of the electrolyte and the organic electrolytic solution during charging and discharging. ｘ CoO ２ When repeated charging is performed such that x in the formula is 0.24 or less, the presence of a coating portion derived from the electrolyte on the surface of the positive electrode active material 100 is expected to improve charge-discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing cobalt elution. The coating portion 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as the electrolyte, a high-quality coating portion is easily obtained. Therefore, a coating portion 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating portion and is therefore preferred. Furthermore, the coating portion 104 does not have to cover the entire positive electrode active material 100. For example, it is sufficient for the coating portion 104 to cover 50% or more of the surface of the positive electrode active material 100, with 70% or more being more preferable and 90% or more being even more preferable.

[0340] <<Powder Resistivity Measurement>> The positive electrode active material 100 according to one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge. The positive electrode active material 100 having the above-described excellent properties is characterized in that, in the above-described <<XRD>>, Li ｘ CoO ２ It has been explained that when x in the formula is small, the cathode active material 100 has an O3'-type and / or monoclinic O1(15)-type crystal structure. Furthermore, in the above section <<EDX>>, a preferred distribution of the added elements when STEM-EDX analysis is performed on the cathode active material 100 has been explained. Furthermore, the cathode active material 100 according to one embodiment of the present invention is also characterized by the volume resistivity of the powder.

[0341] A feature of the positive electrode active material 100 according to one embodiment of the present invention is that the volume resistivity of the powder of the positive electrode active material 100 is 1.0×10 ８ Ω・cm or more 1.0×10 １０ It is preferable that the resistance is Ω cm or less, and 5.0 × 10 ８ Ω・cm or more 1.5×10 ９ It is more preferable that the resistivity is Ω·cm or less.

[0342] The positive electrode active material 100 having the above volume resistivity has a stable crystal structure even at high voltages. Therefore, the volume resistivity of the powder of the positive electrode active material 100 being within the above range can be used as an indicator that the surface layer portion 100 a, which is important for the stability of the crystal structure of the positive electrode active material in a charged state, has been successfully formed.

[0343] A method for measuring the volume resistivity of the powder of the positive electrode active material 100 according to one embodiment of the present invention will be described.

[0344] The volume resistivity of a powder is preferably measured using an instrument with a resistance measurement terminal and a mechanism for applying pressure to the powder to be measured. The resistance measurement terminal preferably has four terminals (also referred to as four-point probes). For example, the MCP-PD51 manufactured by Mitsubishi Chemical Analytech Co., Ltd. can be used as a measurement device having a resistance measurement terminal and a mechanism for applying pressure to the powder (sample) to be measured. The resistance measurement device can be the Loresta-GP low resistance meter or the Hiresta-GP high resistance meter. The Loresta-GP can be used to measure low resistance samples, and the Hiresta-GP can be used to measure high resistance samples. The measurement environment is preferably a stable environment such as a dry room. A dry room environment, for example, with a temperature of 25°C and a dew point of -40°C or less, is preferable. When performing measurements in a humid environment, the electrical resistance may decrease due to the influence of water in the atmosphere, potentially preventing the actual physical property values ​​from being obtained.

[0345] Measurement of the volume resistivity of powder using the measuring device shown above will be described. First, a powder sample is set in the measuring unit. The measuring unit is structured so that the powder sample and a terminal for resistance measurement are in contact with each other and can apply pressure to the powder sample. The measuring unit also has a structure for measuring the volume of the powder sample. Specifically, the measuring unit has a cylindrical space in which the powder sample is set. The structure for measuring the volume of the powder sample described above can measure the volume occupied by the powder at that time by measuring the height of the powder set in the space.

[0346] In measuring the volume resistivity of a powder, the electrical resistance of the powder and the volume of the powder are measured while pressure is applied to the powder. The pressure applied to the powder can be measured under a variety of conditions. For example, the electrical resistance and volume of the powder can be measured under pressure conditions of 16 MPa, 25 MPa, 38 MPa, 51 MPa, and 64 MPa. The volume resistivity of the powder can be calculated from the measured electrical resistance and volume of the powder.

[0347] When the above-described measurement is performed, the volume resistivity of the powder of the positive electrode active material 100 according to one embodiment of the present invention is 1.0×10 when measured under a pressure of 64 MPa. ８ Ω・cm or more 1.0×10 １０ When the capacitance is 5.0×10 Ω cm or less, favorable cycle characteristics are exhibited in a charge-discharge cycle test under high charge voltage conditions. ８ Ω・cm or more 1.5×10 ９ When the electrical resistance is Ω·cm or less, more preferable cycle characteristics are exhibited in a charge / discharge cycle test under high voltage conditions.

[0348] Unless otherwise specified in the present specification, the volume resistivity measured as above is the volume resistivity of the powder.

[0349] <Ion Chromatography> A method for measuring the powder of the positive electrode active material 100 according to one embodiment of the present invention by ion chromatography will be described. The ion chromatography measurement involves a pretreatment step of dissolving the powder of the positive electrode active material 100 in acid to obtain a solution for measurement, and a measurement step of measuring the solution.

[0350] The ion chromatography apparatus and conditions are not particularly limited. For example, the measurement can be performed using the apparatus and conditions described below. As the ion chromatography apparatus, for example, a Dionex ICS-2100 ion chromatography system manufactured by Thermo Fisher Scientific can be used.

[0351] An example of pretreatment for ion chromatography will be described. 250 mg of powder of the positive electrode active material 100 and 0.05 M H ２ SO ４ 2 ml of the aqueous solution is prepared, placed in a glass container with a lid, and mixed to obtain a first mixed solution. Ultrasonic waves can be applied for approximately one hour during the mixing process. The container is then left to stand at room temperature for at least 12 hours. The first mixed solution is then filtered, and 1 ml of the filtrate is mixed with 9 ml of pure water to obtain a second mixed solution. In this manner, the powder of the positive electrode active material 100 can be pretreated.

[0352] Next, the second mixed solution obtained by the pretreatment is subjected to ion chromatography, which may be used to perform anion analysis and cation analysis.

[0353] An example of anion analysis conditions is shown below. Anion analysis can be performed at 35°C using a Dionex IonPac AG20 (2 x 50 mm) or Dionex IonPac AS20 (2 x 250 mm) column. A KOH aqueous solution is used as the eluent, and the flow rate can be set to 0.44 ml / min. It is preferable to perform gradient measurement so that the concentration of the KOH aqueous solution gradually increases. An electrical conductivity detector is used as the detector, and a mixed anion standard solution manufactured by Kanto Chemical Co., Ltd. can be used to create a calibration curve.

[0354] An example of the conditions for cation analysis is shown below. Cation analysis can be performed at 30°C using a Dionex IonPac CG16 (3 x 50 mm) or Dionex IonPac CS16 (3 x 250 mm) column. The eluent is an aqueous methanesulfonic acid (MSA) solution, and the flow rate can be set to 0.36 ml / min. It is recommended to perform isocratic measurements with the concentration of the MSA aqueous solution kept constant. An electrical conductivity detector is used as the detector, and a cation mixed standard solution manufactured by Kanto Chemical Co., Ltd. can be used to create a calibration curve.

[0355] The ion chromatography measurement described above allows quantitative measurement of anions such as fluorine (F) and chlorine (Cl), and cations such as lithium (Li), magnesium (Mg), cobalt (Co), and nickel (Ni).

[0356] In ion chromatography measurement of the powder of the positive electrode active material 100 of one embodiment of the present invention, the weight of fluorine relative to the weight of the powder is preferably 100 ppm to 1000 ppm, more preferably 100 ppm to 200 ppm.

[0357] In addition, when the positive electrode active material 100 is charged at a high depth of charge (such as at 4.5 V or higher) or charged and discharged at high temperatures (45°C or higher), progressive defects (also called pits or pitting corrosion) may occur in the positive electrode active material. Furthermore, defects such as fissures (also called cracks) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. The opening shape of the pits is not circular but has a deep, groove-like shape. The source of the pits may be point defects. Furthermore, it is believed that the crystal structure of the LCO collapses near the pits, resulting in a crystal structure different from that of the layered rock salt type. The collapse of the crystal structure may inhibit the diffusion and release of lithium ions, which are carrier ions, and pits are thought to be a factor in deteriorating cycle characteristics.

[0358] In this specification, cracks and pits are different. Immediately after the preparation of positive electrode active material particles, cracks may be present, but pits do not. Pits can be described as holes formed by the loss of several layers of cobalt and oxygen due to charging and discharging under conditions of high depth of charge, such as high voltage conditions of 4.5 V or higher, or high temperature (45°C or higher), or as locations where cobalt has dissolved. Cracks refer to new surfaces formed by the application of physical pressure, or fissures caused by grain boundaries. Cracks may also occur due to the expansion and contraction of the positive electrode active material caused by charging and discharging. Pits may also occur from cracks and / or cavities within the positive electrode active material.

[0359] This embodiment can be used in combination with other embodiments.

[0360] Embodiment 2 In this embodiment, an example of a method for manufacturing a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS. 11A to 13B.

[0361] In order to prepare the positive electrode active material 100 having the distribution, composition, and / or crystal structure of the additive elements as described in the previous embodiment, the method of adding the additive elements is important. At the same time, it is also important that the crystallinity of the inner portion 100b is good.

[0362] Therefore, in the process of producing the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, and then mix in the additive element source and perform a heat treatment.

[0363] In the method of synthesizing lithium cobalt oxide containing an additive element by mixing an additive element source simultaneously with a cobalt source and a lithium source, it is difficult to increase the concentration of the additive element in the surface layer portion 100a. Furthermore, if the additive element source is simply mixed without heating after synthesizing lithium cobalt oxide, the additive element will simply adhere to the lithium cobalt oxide without dissolving in the lithium cobalt oxide. Without sufficient heating, it is difficult to achieve a good distribution of the additive element. Therefore, it is preferable to mix the additive element source after synthesizing lithium cobalt oxide and then perform a heat treatment. This heat treatment after mixing the additive element source is sometimes called annealing.

[0364] However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that an added element, such as magnesium, enters the cobalt site. ｘ CoO ２ When the value of x in the matrix is ​​small, the layered rock salt crystal structure of R-3m cannot be maintained. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent and the evaporation of lithium.

[0365] Therefore, it is preferable to mix a material that functions as a flux with the additive element source. Any material with a lower melting point than lithium cobalt oxide can function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting points of the additive element source and lithium cobalt oxide. Lowering the melting point makes it easier to distribute the additive elements well at a temperature where cation mixing is unlikely to occur.

[0366] <<Method 1 for Producing Positive Electrode Active Material>> Method 1 for producing the positive electrode active material 100 will be described with reference to FIGS. 11 to 12C.

[0367] <Step S11> In step S11 shown in FIG. 11, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0368] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0369] As the cobalt source, it is preferable to use a compound containing cobalt, for example, cobalt oxide such as tricobalt tetroxide, cobalt hydroxide, or the like.

[0370] The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity and / or reliability of the secondary battery can be increased.

[0371] In addition, the cobalt source preferably has high crystallinity, for example, preferably has single crystal grains. The crystallinity of the cobalt source can be evaluated using TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc., or by X-ray diffraction, electron diffraction, neutron diffraction, etc. The above-mentioned methods for evaluating crystallinity can be applied not only to the evaluation of cobalt sources but also to the evaluation of other crystallinity.

[0372] <Step S12> Next, in step S12 shown in FIG. 11 , the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller fragments. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and the cobalt source with dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0373] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as pulverizing media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, it is preferable to set the peripheral speed to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0374] <Step S13> Next, in step S13 shown in FIG. 11 , the mixed material is heated. Heating is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably at approximately 950°C. If the temperature is too low, the decomposition and melting of the lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, resulting in defects. For example, cobalt may change from trivalent to divalent, inducing oxygen defects, etc.

[0375] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0376] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate is preferably 200° C. / h.

[0377] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２It is preferable that the impurity concentrations of the above are each 5 ppb (parts per billion) or less.

[0378] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and allowing oxygen to flow through the reaction chamber is called flow.

[0379] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen (which may also be called purging) may be used to prevent the oxygen from entering or leaving the reaction chamber. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0380] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0381] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0382] The container for accommodating the object to be heated during heating is preferably an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). An aluminum oxide crucible is a material that is almost free of impurities. In this embodiment, a setter made of aluminum oxide with a purity of 99.9% is used. It is preferable to heat the crucible or setter with a lid, as this prevents the material from volatilizing. Mullite-cordierite may also be used as the material for the crucible and setter.

[0383] Furthermore, it is preferable to use a crucible that has been used multiple times rather than a new one. In this specification, a new crucible refers to one that has undergone two or fewer heating processes with materials containing lithium, transition metal M, and / or additive elements. A multiple-use crucible refers to one that has undergone three or more heating processes with materials containing lithium, transition metal M, and / or additive elements. This is because using a new crucible may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the sheath during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, may not fall within the desired range. On the other hand, this risk is less likely with a multiple-use crucible.

[0384] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use an aluminum oxide mortar. Aluminum oxide mortars are made of a material that does not easily release impurities. Specifically, an aluminum oxide mortar with a purity of 90% or more, preferably 99% or more, is used. Heating conditions equivalent to those of step S13 can also be applied to heating steps other than step S13, which will be described later.

[0385] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized.

[0386] Although the example of producing the composite oxide by the solid phase method in steps S11 to S14 has been shown, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0387] Note that pre-synthesized lithium cobalt oxide may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on pre-synthesized lithium cobalt oxide, lithium cobalt oxide with a smooth surface can be obtained.

[0388] <Step S20> Next, as shown in step S20, it is preferable to add an additive element to the lithium cobalt oxide. In the method for producing a positive electrode active material described in this embodiment, the additive element is added in multiple steps, so in the flow shown in Figure 11, the additive element added first will be referred to as A1, the additive element added second time will be referred to as A2, and the additive element added third time will be referred to as A3. The step of adding additive element A1 will be described with reference to Figure 12A.

[0389] 12A, an additive element source (Al source) to be added to lithium cobalt oxide is prepared. A lithium source may be prepared together with the Al source.

[0390] The additive element A1 can be any of the additive elements described in the previous embodiment, specifically, one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron.

[0391] When magnesium is selected as the additive element, the source of the additive element can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0392] When fluorine is selected as the additive element, the source of the additive element can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF ２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0393] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S21 is lithium carbonate.

[0394] The fluorine source can be a gas, and fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F), nitrogen trifluoride (NF ３ ) or the like may be used and mixed in the atmosphere during the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0395] 11 and 12C, magnesium and fluorine are used as the additive element A1. Lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = 0.33 or its vicinity) is more preferable. In this specification, "or its vicinity" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0396] 12A, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0397] 12A, the pulverized and mixed materials are collected to obtain the Al source. The Al source shown in step S23 contains multiple starting materials and can be called a mixture.

[0398] The particle size of the mixture is preferably D50 (median diameter) of 600 nm to 10 μm, more preferably 1 μm to 5 μm. Even when a single material is used as the additive element source, the D50 (median diameter) is preferably 600 nm to 10 μm, more preferably 1 μm to 5 μm.

[0399] Such a finely powdered mixture (including the case where only one type of additive element is included) can be easily adhered to the surfaces of the lithium cobalt oxide particles when mixed with the lithium cobalt oxide in a later step. If the mixture is evenly adhered to the surfaces of the lithium cobalt oxide particles, it is preferable because the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating.

[0400] 11, lithium cobalt oxide and an Al source are mixed together. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the Al source is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0401] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or a shorter mixing time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium.

[0402] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0403] <Step S32> Next, in step S32 of Fig. 11, the mixed materials are collected to obtain a mixture 901. When collecting the materials, they may be crushed and then sieved, if necessary.

[0404] <Step S33> Next, in step S33 shown in Fig. 11, the mixture 901 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may exceed atmospheric pressure in order to increase the oxygen partial pressure in the heating atmosphere. This is because if the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and the like may be reduced, and lithium cobalt oxide and the like may not be able to maintain a layered rock salt type crystal structure.

[0405] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds refers to the temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 can be set to 650° C. or higher.

[0406] Of course, the reaction is more likely to proceed if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 901. For example, LiF and MgF are used as the additive element source. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0407] Also, LiCoO ２ :LiF:MgF ２ The mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 showed an initial melting temperature T ｉｍ is 779 ° C, the melting peak temperature T ｐｍ is 815°C, and the melting end temperature T ｅｍ Therefore, the lower limit of the heating temperature is more preferably 826°C or higher.

[0408] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0409] The upper limit of the heating temperature is set to be below the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source, etc., within an appropriate range. If the temperature is too high, fluoride will decrease due to evaporation. For example, the vapor pressure of lithium fluoride rises sharply from 900°C. Therefore, a temperature of 1000°C or less is more preferable, 950°C or less is even more preferable, 900°C or less is even more preferable, and 850°C or less is even more preferable. If the evaporation of lithium fluoride is suppressed, the surface layer 100a can be made to have high concentrations of fluorine and lithium. If the surface layer 100a contains a sufficient amount of lithium, a heterophase (MgTiO ３ Another advantage is that it makes it harder for problems like these to occur.

[0410] In consideration of these, the heating temperature in step S33 is preferably 650°C or higher and 1130°C or lower, more preferably 650°C or higher and 1000°C or lower, even more preferably 650°C or higher and 950°C or lower, and even more preferably 650°C or higher and 900°C or lower. Also, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Also, it is preferably 826°C or higher and 1100°C or lower, or 826°C or higher and 1130°C or lower, more preferably 826°C or higher and 1000°C or lower, even more preferably 826°C or higher and 950°C or lower, and even more preferably 826°C or higher and 900°C or lower.

[0411] Furthermore, when the mixture 901 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0412] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered below the decomposition temperature of lithium cobalt oxide, for example, to 742° C. or higher and 950° C. or lower, and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0413] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF may volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, etc., LiCoO ２ There is a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0414] Therefore, it is preferable to heat the mixture 901 in an atmosphere containing LiF, that is, to heat the mixture 901 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 901.

[0415] The heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 901. If the particles of the mixture 901 adhere to each other during heating, the contact area with oxygen in the atmosphere decreases and the route along which the additive elements (e.g., fluorine) diffuse is blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0416] Furthermore, it is believed that uniform distribution of the additive element (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the particles of mixture 901 do not adhere to each other.

[0417] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0418] When heating is performed using a roller hearth kiln, the mixture 901 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 901 .

[0419] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of the lithium cobalt oxide in step S14, etc. When the lithium cobalt oxide is small, a lower temperature or a shorter heating time may be preferable than when the lithium cobalt oxide is large.

[0420] 11, when the median diameter (D50) of the lithium cobalt oxide is about 12 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably, for example, 3 hours or higher and 60 hours or lower, more preferably 10 hours or higher and 30 hours or lower, and even more preferably about 20 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0421] On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 7 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 5 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0422] <Step S34> Next, in step S34 shown in FIG. 11, the heated material is recovered and crushed as necessary to obtain a composite oxide 902.

[0423] <Step S40> Next, in step S40 shown in FIG. 11 , an additive element source (A2 source) is prepared. The additive element A2 can be the additive element described in step S21. In method 1 for producing a positive electrode active material described in FIGS. 11 to 12C , nickel and aluminum are used as the additive element A2. Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source. As shown in steps S41 to S43 of FIG. 12B , the nickel source and the aluminum source can be pulverized to obtain the A2 source. The pulverization conditions can refer to the conditions in step S22.

[0424] 11, the composite oxide 902 and the A2 source are mixed. The mixing conditions can be found in the description of step S31.

[0425] 11, the mixed materials are recovered to obtain a mixture 903. When recovering the materials, they may be crushed and then sieved, if necessary.

[0426] 11, the mixture 903 is heated. The heating conditions can be found in the description of step S33.

[0427] <Step S54> Next, in step S54 shown in FIG. 11, the heated material is recovered and crushed as necessary to obtain a composite oxide 904.

[0428] <Step S60> Next, in step S60 shown in FIG. 11 , an additive element source (A3 source) is prepared. The additive element described in step S21 can be used as the additive element A3. In method 1 for producing a positive electrode active material described in FIGS. 11 to 12C , titanium is used as the additive element A3. Lithium titanate, titanium oxide, titanium hydroxide, etc. can be used as the titanium source. As shown in steps S61 to S63 of FIG. 12C , the titanium source can be pulverized to obtain the A3 source. The pulverization conditions can refer to the conditions in step S22.

[0429] <Step S71> In step S71 shown in Fig. 11, the composite oxide 904 and the A3 source are mixed. The mixing conditions can be found in the description of step S31.

[0430] 11, the mixed materials are recovered to obtain a mixture 905. When recovering the materials, they may be crushed and then sieved, if necessary.

[0431] 11, the mixture 905 is heated. The heating conditions can be found in the description of step S33.

[0432] <Step S74> Next, in step S74 shown in Fig. 11, the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered particles. Through the above steps, the positive electrode active material 100 of one embodiment of the present invention can be produced. The positive electrode active material of one embodiment of the present invention has a smooth surface.

[0433] Positive electrode active material 100 with a smooth surface may be more resistant to physical destruction due to pressure, etc. than positive electrode active material that does not have a smooth surface. For example, positive electrode active material 100 is less likely to be destroyed in a test involving pressure, such as a nail penetration test, and as a result, safety may be improved.

[0434] [Initial Heating] In the above-described production method, it may be more preferable to perform heating after synthesizing lithium cobalt oxide and before mixing in the additive element. This heating is called initial heating.

[0435] The initial heating causes lithium to be released from a part of the surface layer 100a of the lithium cobalt oxide, resulting in a more favorable distribution of the additive elements.

[0436] More specifically, it is believed that initial heating facilitates different distributions depending on the additive element through the following mechanism. First, lithium is released from a portion of the surface layer 100a by initial heating. Next, this lithium-deficient surface layer 100a is mixed with an additive element source, such as a nickel source, an aluminum source, or a magnesium source, and the mixture is heated. Of the additive elements, magnesium is a divalent typical element, and nickel is a transition metal, but it easily becomes a divalent ion. Therefore, Mg is released from a portion of the surface layer 100a. ２＋ and Ni ２＋ and Co reduced by lithium deficiency. ２＋ However, since this phase is formed only in a part of the surface layer portion 100a, it may not be clearly observed in an electron microscope image such as an STEM or an electron beam diffraction pattern.

[0437] Among the additive elements, nickel is likely to dissolve and diffuse into the interior 100b when the surface layer 100a is a layered rock-salt lithium cobaltate, but when part of the surface layer 100a is a rock-salt lithium cobaltate, nickel tends to remain in the surface layer 100a. Therefore, initial heating can make it easier for divalent additive elements such as nickel to remain in the surface layer 100a. The effect of this initial heating is particularly significant on the surface other than the (001)-oriented surface of the positive electrode active material 100 and on the surface layer 100a.

[0438] Furthermore, in these rock salt types, the bond distance between the metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt type.

[0439] For example, rock salt type Ni ０．５ Mg ０．５ The Me-O distance in O is 2.09 × 10 −１０ m, and the Me-O distance in rocksalt MgO is 2.11 × 10 −１０ Even if a spinel-type phase is formed in a part of the surface layer 100a, the spinel-type NiAl ２ O ４ The Me-O distance is 2.0125 × 10 −１０ m, spinel-type MgAl ２ O ４ The Me-O distance is 2.02 × 10 −１０In both cases, the Me-O distance is 2 × 10 −１０ Exceeds m.

[0440] On the other hand, in the layered rock salt type, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, layered rock salt type LiAlO ２ The Al-O distance in −１０ m (Li-O distance is 2.11 × 10 −１０ m). Also, layered rock salt LiCoO ２ The Co-O distance in −１０ m (Li-O distance is 2.0916 × 10 −１０ m).

[0441] According to Shannon's ionic radius, the ionic radius of hexacoordinated aluminum is 0.535 × 10 −１０ The ionic radius of hexacoordinated oxygen is 1.4 × 10 −１０ m, and the sum of these is 1.935 × 10 −１０ m.

[0442] From the above, it is believed that aluminum exists more stably in non-lithium sites in the layered rock-salt structure than in the rock-salt structure, and therefore aluminum is more likely to be distributed in the deeper region having the layered rock-salt structure and / or the interior 100b than in the region close to the surface having the rock-salt structure in the surface layer portion 100a.

[0443] Furthermore, the initial heating is expected to have the effect of increasing the crystallinity of the layered rock salt type crystal structure in the inner portion 100b.

[0444] However, initial heating is not necessarily required. In other heating steps, such as annealing, the atmosphere, temperature, time, etc. can be controlled to prevent Li ｘ CoO ２ When x in the formula is small, a positive electrode active material 100 having an O3' type can sometimes be produced.

[0445] <<Preparation Method 2 of Positive Electrode Active Material>> Next, a preparation method 2 of a positive electrode active material, which is one embodiment of the present invention and differs from Preparation Method 1 of a positive electrode active material, will be described with reference to Figures 13A and 13B. Preparation Method 2 of a positive electrode active material differs from Preparation Method 1 mainly in the number of times an additive element is added. For other descriptions, the description of Preparation Method 1 can be referred to.

[0446] In the positive electrode active material preparation method 1, the additive element is added only after LiMO2 is prepared. However, the present invention is not limited to the above method. The additive element may be added at another timing or multiple times. The timing may be changed depending on the element.

[0447] For example, the additive element may be added when mixing the lithium source (Li source) and the cobalt source (Co source) in step S11, i.e., in the stage of the starting materials for the composite oxide. The additive element source added at this time is shown as the A0 source in FIG. 13A. Then, in step S13, lithium cobalt oxide containing the additive element can be obtained. In this case, it is not necessary to separate steps S11 to S14 from steps S21 to S23. This method can be said to be simple and highly productive.

[0448] Alternatively, lithium cobalt oxide containing some of the additive elements may be used. For example, if lithium cobalt oxide containing magnesium and fluorine is used, steps S11 to S14 and some of step S20 can be omitted. This method is simple and has high productivity.

[0449] Alternatively, an additional element may be added to lithium cobalt oxide to which magnesium and fluorine have been added in advance.

[0450] In addition, in the positive electrode active material preparation method 1, magnesium and fluorine are used as A1, and nickel and aluminum are used as A2, and a preparation method in which these are added at different times has been described. However, magnesium, fluorine, nickel, and aluminum may also be added simultaneously. Figures 13A and 13B show a method in which these are added simultaneously as step S20a. This preparation method can also be said to be a simple and highly productive method because it can reduce the mixing and heating steps.

[0451] This embodiment can be used in combination with other embodiments.

[0452] Embodiment 3 In this embodiment, an example of a secondary battery of one embodiment of the present invention will be described with reference to FIGS. 14 to 15C.

[0453] <Configuration Example of Secondary Battery> Hereinafter, a secondary battery shown in FIG. 14 will be described as an example, in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body.

[0454] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and may include a conductive material (synonymous with a conductive additive) and a binder. The positive electrode active material is the positive electrode active material prepared by the preparation method described in the previous embodiment.

[0455] The positive electrode active material described in the above embodiment may be mixed with another positive electrode active material.

[0456] Other examples of the positive electrode active material include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, LiFePO ４ , LiFeO ２ , LiNiO ２ , LiMn ２ O ４ , V ２ O ５ , Cr ２ O ５ , MnO ２ The following compounds are exemplified:

[0457] Other positive electrode active materials include LiMn ２ O ４Lithium-containing materials having a spinel-type crystal structure containing manganese, such as lithium nickel oxide (LiNiO ２ or LiNi １−ｘ M ｘ O ２ It is preferable to mix (0<x<1) (M=Co, Al, etc.) This configuration can improve the characteristics of the secondary battery.

[0458] <Conductive Material> The conductive material is also called a conductivity imparting agent or a conductivity aid, and is made of a carbon material. By attaching the conductive material between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. Note that the term "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive material covers part of the surface of the active material, where the conductive material is embedded in the surface irregularities of the active material, and where the materials are electrically connected even when not in contact with each other.

[0459] The active material layers such as the positive electrode active material layer and the negative electrode active material layer preferably contain a conductive material.

[0460] As the conductive material, for example, one or more of carbon black such as acetylene black and furnace black, graphite such as artificial graphite and natural graphite, carbon fibers such as carbon nanofibers and carbon nanotubes, and graphene compounds can be used.

[0461] Examples of carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Carbon nanofibers and carbon nanotubes can also be used as carbon fibers. Carbon nanotubes can be produced by vapor phase growth, for example.

[0462] In this specification and the like, graphene compounds include graphene, multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0463] In this specification and the like, graphene oxide refers to a material that contains carbon and oxygen, has a sheet shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0464] In this specification and the like, reduced graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. A single sheet of reduced graphene oxide can function, but multiple sheets may also be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 Atomic% and the oxygen concentration is 2 Atomic% or more and 15 Atomic% or less. By achieving such carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0465] Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds may also have a sheet-like shape. Graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. Even with a thin film, they may have very high conductivity, allowing a small amount of material to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene compound covers 80% or more of the active material. It is preferable that the graphene compound clings to at least a portion of the active material particles. It is also preferable that the graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the unevenness of a single active material particle or the unevenness formed by multiple active material particles. It is also preferable that the graphene compound surrounds at least a portion of the active material particles. The graphene compound may also have holes.

[0466] When active material particles having a small particle size, for example, 1 μm or less, are used, the specific surface area of ​​the active material particles is large, and more conductive paths connecting the active material particles are required. In such cases, it is preferable to use a graphene compound that can efficiently form conductive paths even in a small amount.

[0467] Because of the properties described above, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and rapid discharging. For example, rapid charging and rapid discharging characteristics may be required for secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. Rapid charging characteristics may also be required for mobile electronic devices, etc. Rapid charging and discharging refers to, for example, charging and discharging at 200 mA / g, 400 mA / g, or 1000 mA / g or more per weight of the positive electrode active material.

[0468] The plurality of graphenes or graphene compounds are formed so as to partially cover the plurality of granular positive electrode active material particles or to be attached to the surfaces of the plurality of granular positive electrode active material particles, and are therefore preferably in surface contact with each other.

[0469] Here, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphenes or graphene compounds together. When an active material is covered with a graphene net, the graphene net can also function as a binder that bonds the active materials together. Therefore, the amount of binder can be reduced or no binder can be used, thereby improving the ratio of active material to the electrode volume and electrode weight. In other words, the discharge capacity of a secondary battery can be increased.

[0470] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO ２ , SiO ｘ (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a median diameter (D50) of 1 μm or less, more preferably 100 nm or less.

[0471] The content of the conductive material relative to the total amount of the active material layer is preferably 1 wt % or more and 10 wt % or less, and more preferably 1 wt % or more and 5 wt % or less.

[0472] Unlike granular conductive materials such as carbon black, which make point contact with the active material, graphene compounds enable surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular active material and the graphene compound with a smaller amount than that of ordinary conductive materials. This allows the ratio of the active material in the active material layer to be increased, thereby increasing the discharge capacity of the battery.

[0473] Particulate carbon-containing compounds such as carbon black and graphite, or fibrous carbon-containing compounds such as carbon nanotubes, tend to fill microscopic spaces. Microscopic spaces refer to, for example, the spaces between multiple active materials. By combining a carbon-containing compound that easily fills microscopic spaces with a sheet-like carbon-containing compound such as graphene, which can impart conductivity across multiple particles, the density of the electrode can be increased, resulting in the formation of an excellent conductive path. The battery obtained by the manufacturing method of one embodiment of the present invention has high capacity density and is stable, making it effective as an in-vehicle battery.

[0474] [Binder] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0475] 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.

[0476] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0477] The binder may be used in combination with two or more of the above.

[0478] [Current Collector] The current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable to use a current collector with a thickness of 5 μm to 30 μm.

[0479] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also have a conductive material and a binder.

[0480] [Negative Electrode Active Material] As the negative electrode active material, for example, an alloy-based material and / or a carbon-based material can be used.

[0481] As the negative electrode active material, an element capable of undergoing a charge-discharge reaction by alloying / de-alloying reaction with lithium can be used. For example, a material containing one or more elements selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger charge-discharge capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, a compound containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0482] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO ｘ Here, x preferably has a value of 1 or a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, x is preferably 0.2 or more and 1.2 or less. Alternatively, x is preferably 0.3 or more and 1.5 or less.

[0483] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0484] 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.

[0485] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of lithium metal (0.05 V or more and 0.30 V or less vs. Li / Li ＋ This allows lithium-ion secondary batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high charge / discharge capacity per unit volume, relatively little volume expansion, low cost, and greater safety than lithium metal.

[0486] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten oxide (WO ２ ), molybdenum oxide (MoO ２ ) and other oxides can be used.

[0487] In addition, as the negative electrode active material, a composite nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N ３ has a large charge / discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0488] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the composite nitride of lithium and a transition metal can be used as the negative electrode active material, even when a material containing lithium ions is used as the positive electrode active material, by first removing the lithium ions contained in the positive electrode active material.

[0489] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ Fluorides such as:

[0490] The conductive material and binder that can be contained in the negative electrode active material layer can be the same as the conductive material and binder that can be contained in the positive electrode active material layer.

[0491] [Negative electrode current collector] The negative electrode current collector can be made of the same material as the positive electrode current collector. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0492] [Electrolyte] One form of electrolyte can be an electrolytic solution having a solvent and an electrolyte dissolved in the solvent. The electrolytic solution contains a solvent and a lithium salt. The solvent for the electrolytic solution is preferably an aprotic organic solvent, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone, or any combination and ratio of two or more of these.

[0493] When the electrolytic solution contains ethylene carbonate (EC) and diethyl carbonate (DEC), when the total content of ethylene carbonate and diethyl carbonate is 100 vol %, the volume ratio of ethylene carbonate to diethyl carbonate can be x:100-x (where 20≦x≦40). More specifically, a mixed organic solvent containing EC and DEC in a volume ratio of EC:DEC=30:70 can be used.

[0494] Furthermore, when the electrolyte solution contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), when the total content of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol%, the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate can be x:y:100-x-y (where 5≦x≦35 and 0<y<65). More specifically, a mixed organic solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 can be used.

[0495] Furthermore, a mixed organic solvent containing a fluorinated cyclic carbonate (sometimes referred to as a fluorinated cyclic carbonate) or a fluorinated chain carbonate (sometimes referred to as a fluorinated chain carbonate) can be used as the electrolyte. Furthermore, it is preferable that the mixed organic solvent contains both a fluorinated cyclic carbonate and a fluorinated chain carbonate. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have electron-withdrawing substituents, which are preferable because they lower the solvation energy of lithium ions. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable for the electrolyte, and a mixed organic solvent containing these is also suitable.

[0496] Examples of fluorinated cyclic carbonates that can be used include fluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC has isomers such as cis-4,5 and trans-4,5. Since all fluorinated cyclic carbonates have electron-withdrawing substituents, they are considered to have low solvation energy for lithium ions. The electron-withdrawing substituent in FEC is an F group.

[0497] Methyl 3,3,3-trifluoropropionate is an example of a fluorinated chain carbonate. The abbreviation for methyl 3,3,3-trifluoropropionate is "MTFP." In MTFP, the electron-withdrawing substituent is CF ３ It is the base.

[0498] FEC is a cyclic carbonate with a high dielectric constant, and when used in an organic solvent, it promotes the dissociation of lithium salts. On the other hand, because FEC has electron-withdrawing substituents, it is more likely to desolvate with lithium ions than ethylene carbonate (EC). Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. Therefore, it is easier to separate lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Furthermore, because FEC has a deep highest occupied molecular orbital (HOMO), it is less susceptible to oxidation and has improved oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC alone in the electrolyte. MTFP is a type of chain carbonate, and can have the effect of reducing the viscosity of the electrolyte solution or maintaining the viscosity at room temperature (typically 25° C.) even at low temperatures (typically 0° C.). Furthermore, MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, but may form a solvate with lithium ions when used in the electrolyte solution.

[0499] The organic solvent described above is free from granular dust or molecules other than the constituent molecules of the organic solvent (hereinafter simply referred to as "impurities"), and oxygen (O ２ ) or water (H ２ O). It is preferable that the content of ) is low and highly purified. It is also preferable that reaction by-products during synthesis are suppressed through appropriate purification. Specifically, the impurities in the electrolyte are 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The concentration of water among the impurities can be detected by Karl Fischer titration.

[0500] Furthermore, it is preferable that the above-mentioned organic solvent has almost no peaks due to impurities that can be confirmed by NMR measurement or the like. "Almost no peaks can be confirmed" means that the ratio of the integrated area of ​​the peak due to the impurity to the integrated area of ​​the peak due to the main component (simply referred to as "integral ratio") is 0.005 or less, preferably 0.002 or less. The device used for NMR measurement is not particularly limited, but for example, Bruker's "AVANCE III 400" can be used. Furthermore, among the five peaks of acetonitrile derived from acetonitrile-d3 used as a solvent in 1H-NMR measurement, the central peak can be located at 1.94 ppm.

[0501] For example, in the case of MTFP, it is known that when 1H-NMR is measured using acetonitrile-d3 solvent, four peaks appear at δ between 3.29 ppm and 3.43 ppm. However, if other peaks appear in this vicinity, for example, if a peak appears at δ between 3.24 ppm and 3.29 ppm, the peak is considered to be derived from impurities. Therefore, if the ratio (integral ratio) of the peak area between 3.24 ppm and 3.29 ppm to the peak area between 3.29 ppm and 3.43 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks due to impurities are almost impossible to confirm.

[0502] It is preferable to mix and use FEC having such physical properties and MTFP in a volume ratio of x:100-x (where 5≦x≦30, preferably 10≦x≦20), where the total content of the mixed organic solvent containing FEC and MTFP is 100 vol %. In other words, it is preferable to mix and use MTFP in a larger amount than FEC in the mixed organic solvent.

[0503] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding and / or catching fire even if the internal temperature of the secondary battery rises due to an internal short circuit, overcharging, or the like. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0504] [Lithium Salt] The lithium salt (also called an electrolyte) to be dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in any combination and ratio of two or more of these. The lithium salt is preferably used in an amount of 0.5 mol / L or more and 3.0 mol / L or less relative to the solvent. ６ , LiBF ４ The use of such improves the safety of lithium ion secondary batteries.

[0505] The above-mentioned electrolyte is preferably a highly purified electrolyte with a low content of granular dust or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is 1 wt % or less, preferably 0.1 wt % or less, and more preferably 0.01 wt % or less.

[0506] [Additives] The electrolyte may 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 vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), and lithium bis(oxalate)borate (LiBOB). LiBOB is particularly preferred because it easily forms a good coating. VC or FEC is preferred because it can form a good coating on the negative electrode during battery aging or charging in the early stages of use, thereby improving cycle characteristics.

[0507] The additive may be one or more dinitrile compounds, such as succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

[0508] Fluorobenzene may also be added to the organic solvent. The concentration of the additive can be, for example, 0.1 wt% or more and 5 wt% or less relative to 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. FB is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes. Dinitrile compounds are preferred because their nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving high-voltage resistance. Furthermore, dinitrile compounds are preferred because they prevent copper dissolution during overdischarge when a copper-containing current collector is used on the negative electrode. Considering the use of secondary batteries at high voltages, adding a nitrile compound is preferred.

[0509] [Gel Electrolyte] A polymer gel in which a polymer is swollen with an electrolytic solution may be used as the gel electrolyte. By using a polymer gel electrolyte, a semi-solid electrolyte layer can be provided, and safety against leakage and the like can be improved. In addition, it is possible to reduce the thickness and weight of the secondary battery.

[0510] 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.

[0511] Examples of polymers that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0512] [Solid Electrolyte] Instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a solid electrolyte containing a polymer material such as a PEO (polyethylene oxide) can be used. When a solid electrolyte is used, the installation of a separator and / or spacer is unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, and safety is dramatically improved.

[0513] [Separator] The secondary battery preferably has a separator. Examples of separators that can be used include paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into an envelope shape and disposed so as to encase either the positive electrode or the negative electrode.

[0514] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0515] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0516] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0517] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the discharge capacity per volume of the secondary battery can be increased.

[0518] [Exterior Body] The exterior body of the secondary battery can be made of, for example, a metal material such as aluminum and / or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0519] <Laminated Secondary Battery and Fabrication Method> An example of an external view of a laminated secondary battery 500 is shown in FIGS. 14 to 15C. FIGS. 14 to 15C show a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511. The positive electrode 503 has a positive electrode current collector 501 and a positive electrode active material layer 502, and the negative electrode 506 has a negative electrode current collector 504 and a negative electrode active material layer 505. If the laminated secondary battery has a flexible configuration, when it is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent to accommodate deformation of the electronic device. An example of a method for fabricating the laminated secondary battery will be described with reference to FIGS. 15A to 15C.

[0520] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 15B shows the stacked negative electrode 506, the separator 507, and the positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0521] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0522] Next, as shown in Fig. 15C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0523] Next, an electrolyte solution (not shown) is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0524] By using the positive electrode active material described in the above embodiment for the positive electrode 503, the secondary battery 500 can have a high discharge capacity and excellent cycle characteristics.

[0525] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0526] Embodiment 4 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted on an electronic device will be described with reference to FIGS. 16A to 18C.

[0527] 16A to 16G show examples in which the secondary battery having the positive electrode active material described in the above embodiment is mounted in an electronic device. Examples of electronic devices to which the secondary battery is applied include television devices (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0528] Furthermore, a secondary battery having a flexible shape can be incorporated along the curved surfaces of the inner or outer walls of houses, buildings, etc., and the interior or exterior of automobiles.

[0529] 16A illustrates an example of a mobile phone. The mobile phone 7400 includes a display portion 7402 built into a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 includes a secondary battery 7407. By using the secondary battery of one embodiment of the present invention as the secondary battery 7407, a lightweight mobile phone with a long lifetime can be provided.

[0530] Figure 16B shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 provided inside is also bent. Figure 16C shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. The secondary battery 7407 has lead electrodes electrically connected to the current collectors.

[0531] FIG. 16D shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display portion 7102, operation buttons 7103, and a secondary battery 7104. FIG. 16E shows a bent secondary battery 7104. When the secondary battery 7104 is worn on a user's arm in a bent state, the housing deforms, causing a change in the curvature of part or the entire secondary battery 7104. Note that the degree of curvature at any point on the curve, expressed as the radius of the corresponding circle, is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or the entire main surface of the housing or the secondary battery 7104 changes when the radius of curvature is in the range of 40 mm to 150 mm. High reliability can be maintained when the radius of curvature of the main surface of the secondary battery 7104 is in the range of 40 mm to 150 mm. By using the secondary battery of one embodiment of the present invention as the secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0532] 16F shows an example of a wristwatch-type portable information terminal 7200. The portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input / output terminal 7206, and the like.

[0533] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0534] The display surface of the display portion 7202 is curved, and a display can be performed along the curved display surface. The display portion 7202 also includes a touch sensor, and can be operated by touching the screen with a finger or a stylus. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0535] The operation button 7205 can have various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 7205 can be freely set by an operating system incorporated in the portable information terminal 7200.

[0536] The portable information terminal 7200 is also capable of performing standardized short-range wireless communication. For example, hands-free conversation is possible by communicating with a wirelessly enabled headset.

[0537] The portable information terminal 7200 also includes an input / output terminal 7206, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal 7206. Note that charging may be performed by wireless power supply without using the input / output terminal 7206.

[0538] The display portion 7202 of the portable information terminal 7200 includes the secondary battery of one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a lightweight portable information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 16E can be incorporated into the housing 7201 in a curved state or into the band 7203 in a bendable state.

[0539] The portable information terminal 7200 preferably has a sensor. For example, a fingerprint sensor, a pulse sensor, a human body sensor such as a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted as the sensor.

[0540] 16G illustrates an example of an armband-type display device. The display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can also be provided with a touch sensor in the display portion 7304 and can also function as a portable information terminal.

[0541] The display surface of the display portion 7304 is curved, and display can be performed along the curved display surface. The display state of the display device 7300 can be changed by short-range wireless communication according to a communication standard.

[0542] The display device 7300 also includes an input / output terminal, and can directly exchange data with another information terminal via a connector. Charging can also be performed via the input / output terminal. Note that charging may be performed by wireless power supply without using the input / output terminal.

[0543] By using the secondary battery of one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight display device with a long lifetime can be provided.

[0544] An example in which the secondary battery having good cycle characteristics described in the above embodiment is mounted on an electronic device will be described with reference to FIGS. 16H to 18C. FIG.

[0545] By using a secondary battery of one embodiment of the present invention as a secondary battery in daily electronic devices, products that are lightweight and have a long life can be provided. For example, daily electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. For secondary batteries in these products, a stick-shaped secondary battery that is easy for users to hold, small, lightweight, and has a large discharge capacity is desired.

[0546] FIG. 16H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 16H, the electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 that includes a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and / or over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 16H has external terminals so that it can be connected to a charging device. Because the secondary battery 7504 is the tip portion when held, it is desirable that the total length be short and the weight be light. The secondary battery of one embodiment of the present invention has a high discharge capacity and good cycle characteristics, making it possible to provide a compact and lightweight electronic cigarette 7500 that can be used for a long period of time.

[0547] 17A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0548] For example, the secondary battery of one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 17A . The eyeglasses-type device 4000 includes a frame 4000 a and a display unit 4000 b. Mounting the secondary battery on the temples of the curved frame 4000 a makes it possible to provide the eyeglasses-type device 4000 with a lightweight design, a good weight balance, and a long continuous use time. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to a smaller housing can be realized.

[0549] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b and / or the earphone unit 4001c. By including the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to a miniaturized housing can be realized.

[0550] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0551] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. By providing the secondary battery according to one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0552] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted inside the belt portion 4006a. By including the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0553] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided on the display portion 4005a or the belt portion 4005b. By providing the secondary battery of one embodiment of the present invention, a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0554] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0555] Furthermore, since the wristwatch device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0556] FIG. 17B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0557] 17C shows a side view of the display portion 4005a. FIG. 17C shows a state in which a secondary battery 913 is built inside the display portion 4005a. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and is small and lightweight.

[0558] 17D shows an example of a wireless earphone. Here, the wireless earphone is shown as having a pair of main bodies 4100a and 4100b, but this does not necessarily have to be a pair.

[0559] The main bodies 4100a and 4100b each have a driver unit 4101, an antenna 4102, and a secondary battery 4103. They may also have a display unit 4104. They also preferably have a substrate on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. They may also have a microphone.

[0560] The case 4110 has a secondary battery 4111. It is preferable that the case 4110 also has a board on which circuits such as a wireless IC and a charge control IC are mounted, and a charging terminal. It may also have a display unit, buttons, and the like.

[0561] The main units 4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows sound data and the like sent from other electronic devices to be played back on the main units 4100a and 4100b. If the main units 4100a and 4100b have microphones, the sound picked up by the microphones can be sent to the other electronic device, and the sound data after processing by the electronic device can be sent back to the main units 4100a and 4100b for playback. This allows the devices to be used as, for example, translation devices.

[0562] The secondary battery 4103 included in the main body 4100a can be charged from the secondary battery 4111 included in the case 4110. The coin-type secondary battery, the cylindrical secondary battery, or the like described in the above embodiments can be used as the secondary battery 4111 and the secondary battery 4103. A secondary battery using the positive electrode active material 100 obtained in Embodiment 1 as a positive electrode has high energy density. By using the secondary battery 4103 and the secondary battery 4111 as the secondary battery, a configuration that can accommodate space saving associated with miniaturization of wireless earphones can be realized.

[0563] 18A shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0564] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component therein. By using the secondary battery 6306 according to one embodiment of the present invention in the cleaning robot 6300, the cleaning robot 6300 can be a highly reliable electronic device with a long operating time.

[0565] Fig. 18B shows an example of a robot. A robot 6400 shown in Fig. 18B includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0566] 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.

[0567] 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.

[0568] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. Furthermore, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0569] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component inside the robot 6400. By using the secondary battery according to one embodiment of the present invention in the robot 6400, the robot 6400 can be a highly reliable electronic device with a long operating time.

[0570] Fig. 18C shows an example of an aircraft 6500. The aircraft 6500 shown in Fig. 18C has a propeller 6501, a camera 6502, a secondary battery 6503, and the like, and has the function of flying autonomously.

[0571] For example, image data captured by the camera 6502 is stored in the electronic component 6504. The electronic component 6504 can analyze the image data and detect the presence or absence of an obstacle when moving. Furthermore, the electronic component 6504 can estimate the remaining battery charge from a change in the storage capacity of the secondary battery 6503. The flying object 6500 includes the secondary battery 6503 according to one embodiment of the present invention therein. By using the secondary battery according to one embodiment of the present invention in the flying object 6500, the flying object 6500 can be an electronic device with a long operating time and high reliability.

[0572] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0573] Embodiment 5 In this embodiment, an example in which a secondary battery including the positive electrode active material of one embodiment of the present invention is mounted on a vehicle will be described.

[0574] When a secondary battery is installed in a vehicle, next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), or plug-in hybrid vehicles (PHVs) can be realized.

[0575] FIG. 19 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 illustrated in FIG. 19A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Furthermore, the automobile 8400 includes a secondary battery. For example, secondary battery modules can be arranged on the floor of the interior of the vehicle. The secondary battery not only drives the electric motor 8406 but also supplies power to light-emitting devices such as a headlight 8401 and a room light (not shown).

[0576] The secondary battery can also supply power to display devices such as a speedometer and a tachometer included in the automobile 8400. The secondary battery can also supply power to semiconductor devices such as a navigation system included in the automobile 8400.

[0577] The automobile 8500 shown in FIG. 19B can charge ...

Claims

A battery having a positive electrode, The positive electrode has a positive electrode active material layer, the positive electrode active material layer has positive electrode active material particles having magnesium, titanium, nickel, aluminum, and lithium cobalt oxide; the positive electrode active material layer has a first peak in a range of 850 eV or more and 860 eV or less when the positive electrode active material particles are subjected to XPS analysis. 　 In claim 1, The first peak includes a peak derived from an O—Ni—O—Ti—O bond. 　 In claim 1, The peak position of the first peak is 0.5 eV or more higher than the peak position of nickel(II) oxide in an XPS analysis. 　 In claim 1, The battery has a second peak in the range of 1300 eV or more and 1310 eV or less when analyzed by XPS. 　 In claim 4, The second peak includes a peak derived from an O-Mg-F bond. 　 In claim 4, a peak position of the second peak being higher than a peak position in an XPS analysis of magnesium oxide and lower than a peak position in an XPS analysis of magnesium fluoride. 　 In any one of claims 1 to 6, When cracks in the positive electrode active material particles are analyzed using a cross-sectional SEM image of the positive electrode active material layer, the number of cracks in the positive electrode active material particles is 0.06 / μm ２ Hereinafter, the total length of the cracks is 0.06 μm / μm. ２ Below is the battery. 　 In any one of claims 1 to 6, a ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) is 0.40 or more and 1.50 or less, a ratio of the number of titanium atoms to the number of cobalt atoms (Ti / Co) is 0.05 or more and 0.20 or less, and a ratio of the number of nickel atoms to the number of cobalt atoms (Ni / Co) is 0.05 or more and 0.20 or less, in the XPS analysis. 　 In any one of claims 1 to 6, When the powder resistance of the positive electrode active material particles in the positive electrode active material layer is measured, the volume resistivity is 1.0×10 ８ Ω・cm or more 1.0×10 １０ A battery having a resistance of less than Ω cm. 　 In any one of claims 1 to 6, A battery, wherein when a particle size distribution of the positive electrode active material particles is measured, the particle size distribution of the positive electrode active material particles has one peak. 　 The positive electrode according to any one of claims 1 to 6 and lithium metal are used as a negative electrode, The electrolyte was 1 mol / L lithium hexafluorophosphate (LiPF ６ When a test battery was prepared using a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of EC:DEC = 3:7 and vinylene carbonate (VC) at 2 wt%, A battery in which, when the test battery is charged at a constant current of 100 mA / g up to a voltage of 4.60 V in an environment of 45° C., and then charged at a constant voltage until the current value reaches 10 mA / g, and then discharged at a current of 6 mA / g for 1 second, the resistance DCIR calculated by dividing the difference between the voltage before the discharge and the voltage during the discharge by the current is 35 Ω or less. 　 In claim 11, A battery, wherein when the test battery is used to perform 50 charge / discharge cycle tests, the DCIR is 35Ω or less after any charging in the charge / discharge cycle tests. 　 The positive electrode according to any one of claims 1 to 6 and lithium metal are used as a negative electrode, The electrolyte was 1 mol / L lithium hexafluorophosphate (LiPF ６ When a test battery was prepared using a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of EC:DEC = 3:7 and vinylene carbonate (VC) at 2 wt%, The test battery was subjected to a charge-discharge cycle test 50 times at 45° C. and a charge voltage of 4.55 V or more and 4.65 V or less, and then the positive electrode was taken out. When a cross-sectional SEM image of the positive electrode was obtained, The number of pits in the positive electrode active material particles is 0.085 / μm ２ That is, the battery.

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