Method for producing secondary battery

Abstract

Provided are: a positive electrode active material by which a decrease in discharge capacity during charge and discharge cycles is suppressed; and a secondary battery using the same. Also provided is a secondary battery having high safety. The secondary battery includes: a positive electrode having a positive electrode active material; a negative electrode; and an electrolyte. The positive electrode active material is formed through a first step for mixing a composite oxide containing lithium, cobalt, and oxygen, a magnesium source and a nickel source to form a mixture, and a second step for heating the mixture, wherein the nickel source is a nickel fluoride, and in the second step, magnesium, nickel, and fluorine are segregated on a surface of the composite oxide.

Description

Method for manufacturing secondary batteries

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

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

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

[0004] In particular, there is a high demand for secondary batteries with a large discharge capacity per weight and excellent cycle characteristics for mobile electronic devices, etc. To meet this demand, improvements to the positive electrode active materials of the positive electrodes of secondary batteries have been actively pursued (e.g., Patent Documents 1 to 3). Research on the crystalline structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 4).

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

[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 beam 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 beam diffraction pattern.

[0008] Fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing and other processes, and their physical properties have been studied (Non-Patent Document 12). Non-Patent Document 13 also describes the physical properties of fluorides such as nickel fluoride.

[0009] It is known that when the temperature of a lithium ion secondary battery rises during charging, it goes through several states and eventually reaches thermal runaway (Non-Patent Document 15).

[0010] Various research and development efforts are also being conducted on the reliability and safety of lithium-ion secondary batteries. For example, Non-Patent Document 14 describes the thermal stability of positive electrode active materials and electrolyte solutions.

[0011] Japanese Patent Application Laid-Open No. 2019-179758 WO2020 / 026078 Pamphlet Japanese Patent Application Laid-Open No. 2020-140954

[0012] Toyoki Okumura et al,”Correlation of lithium ion distribution and X−ray absorption near−edge structure in O3−and O2−lithium cobalt oxides from first−principle calculation”,Journal of Materials Chemistry,2012,22,p.17340−17348Motohashi,T.et al,”Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”,Physical Review B,80(16);165114Zhaohui Chen et al,“Staging Phase Transitions in Li▲x▼CoO▲2▼”,Journal of The Electrochemical Society,2002,149(12)A1604−A1609G.G.Amatucci et.al.,“CoO2,The End Member of the Lix CoO2 Solid Solution”J.Electrochem.Soc.143(3)1114(1996).Belsky,A.et al.,“New developments in the Inorganic Crystal Structure Database(ICSD):accessibility in support of materials research and design”,Acta Cryst.,(2002)B58 364−369.Akimoto,J.;Gotoh,Y.;Oosawa,Y.“Synthesis and structure refinement of Li CoO2 single crystals”,Journal of Solid State Chemistry(1998)141,p.298−302.F.Izumi and K.Momma,Solid State Phenom.,130,15−20(2007)Rasband,W.S.,ImageJ,U.S.National Institutes of Health,Bethesda,Maryland,USA,http: / / rsb.info.nih.gov / ij / ,1997−2012.Schneider,C.A.,Rasband,W.S.,Eliceiri,K.W.“NIH Image to ImageJ:25 years of image analysis”.Nature Methods 9,671−675,2012.Abramoff,M.D.,Magelhaes,P.J.,Ram,S.J.“Image Processing with ImageJ”.Biophotonics International,volume 11,issue 7,pp.36−42,2004.Seto,Y.& Ohtsuka,M.“ReciPro:free and open−source multipurpose crystallographic software integrating a crystal model database and viewer,diffraction and microscopy simulators,and diffraction data analysis tools”,J.Appl.Cryst.55,2022.W.E.Counts,R.Roy,and E.F.Osborn,“Fluoride Model Systems:II,The Binary Systems CaF▲2▼−BeF▲2▼,MgF▲2▼−BeF▲2▼,and LiF−MgF▲2▼”,Journal of the American Ceramic Society,36[1]12−17,1953.J.A.Ocadiz−Flores et al.,“Thermodynamic assessment of the LiF−NiF▲2▼,NaF−NiF▲2▼ and KF−NiF▲2▼ systems”,The Journal of Chemical Thermodynamics,121,17−26,2018. Shinya Kitano et al. GSYuasa Technical Report Vol. 2 No. 2 December 2015 P. 18-24. Nobuo Eda 2-4 Heat Generation Mechanism Learning from Data Li-ion Battery Charging and Discharging Technology CQ Publishing April 4, 2020 pp. 68-72.

[0013] There is still room for improvement in various aspects of secondary batteries, including lithium ion secondary batteries, such as discharge capacity, cycle characteristics, reliability, safety, and cost.

[0014] Therefore, there is a demand for positive electrode active materials that can improve issues such as discharge capacity, cycle characteristics, reliability, safety, and cost when used in secondary batteries.

[0015] An object of one embodiment of the present invention is to provide a positive electrode active material or composite oxide that can be used in a secondary battery and that exhibits a suppressed decrease in discharge capacity during charge-discharge cycles.Another object is to provide a positive electrode active material or composite oxide that is less likely to lose its crystal structure even after repeated charge-discharge cycles.Another object is to provide a positive electrode active material or composite oxide that exhibits a large discharge capacity.Another object is to provide a safe or highly reliable secondary battery.

[0016] 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.

[0017] 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.

[0018] One aspect of the present invention is a method for manufacturing a secondary battery including a positive electrode having a positive electrode active material, a negative electrode, and an electrolyte. The positive electrode active material is formed by a first step of mixing a composite oxide having lithium, cobalt, and oxygen with a magnesium source and a nickel source to form a mixture, and a second step of heating the mixture. The nickel source is nickel fluoride, and magnesium, nickel, and fluorine are segregated on the surface of the composite oxide by the second step.

[0019] In the above embodiment, the magnesium source is preferably magnesium fluoride.

[0020] Another embodiment of the present invention is a method for manufacturing a secondary battery including a positive electrode having a positive electrode active material, a negative electrode, and an electrolyte. The positive electrode active material is formed through a first step of mixing a composite oxide having lithium, cobalt, and oxygen with magnesium fluoride, nickel fluoride, and lithium fluoride to form a mixture, and a second step of heating the mixture. In the second step, magnesium, nickel, and fluorine are segregated on a surface of the composite oxide.

[0021] In the above embodiment, the positive electrode active material has a layered rock salt type crystal structure of R-3m, and the positive electrode active material is CoO ２ The magnesium segregated on the surface of the composite oxide is ２ It is preferable to prevent the layers from shifting.

[0022] In the above embodiment, the heating in the second step is preferably carried out at a temperature of 650°C or higher and 1000°C or lower.

[0023] In the above embodiment, the heating in the second step is preferably carried out at a temperature at which cation mixing is unlikely to occur.

[0024] Alternatively, one embodiment of the present invention is a method for manufacturing a secondary battery including a positive electrode having a positive electrode active material, a negative electrode, and an electrolyte. The positive electrode active material is formed by: a first step of mixing a first composite oxide, a magnesium source, and a nickel source to form a first mixture; a second step of performing a first heating on the first mixture to form a second composite oxide; a third step of mixing the second composite oxide and an aluminum source to form a second mixture; and a fourth step of performing a second heating on the second mixture. The first composite oxide includes lithium, cobalt, and oxygen, and the nickel source is nickel fluoride. Magnesium, nickel, and fluorine are segregated on a surface of the second composite oxide by the second step.

[0025] In the above embodiment, the magnesium source is preferably magnesium fluoride.

[0026] In the above embodiment, the positive electrode active material has a layered rock salt type crystal structure of R-3m, and the positive electrode active material is CoO ２ The magnesium segregated on the surface of the second composite oxide is a CoO ２ It is preferable to prevent the layers from shifting.

[0027] In the above aspect, it is preferable that the first heating in the second step is performed at a temperature of 650°C or higher and 1000°C or lower, and the second heating in the fourth step is performed at a temperature lower than the temperature in the first heating.

[0028] One aspect of the present invention is a method for manufacturing a secondary battery including a positive electrode having a positive electrode active material, a negative electrode, and an electrolyte, in which the positive electrode active material is formed by a first step of mixing a first composite oxide, a first nickel source, and lithium fluoride to form a first mixture, a second step of performing a first heating on the first mixture to form a second composite oxide, a third step of mixing the second composite oxide and a second nickel source to form a second mixture, and a fourth step of performing a second heating on the second mixture, in which the first composite oxide includes lithium and cobalt.

[0029] In the above embodiment, the first nickel source is preferably nickel fluoride.

[0030] In the above aspect, it is preferable that in the first step, a magnesium source is mixed in addition to the first composite oxide, the first nickel source, and lithium fluoride to form the first mixture.

[0031] In the above embodiment, it is preferable that the first nickel source is nickel fluoride and the magnesium source is magnesium fluoride.

[0032] Another embodiment of the present invention is a method for manufacturing a secondary battery including a positive electrode having a positive electrode active material, a negative electrode, and an electrolyte. The positive electrode active material is formed through a first step of mixing a first composite oxide and a first nickel source to form a first mixture, a second step of performing a first heating on the first mixture to form a second composite oxide, a third step of mixing the second composite oxide, a second nickel source, and lithium fluoride to form a second mixture, and a fourth step of performing a second heating on the second mixture. The first composite oxide includes lithium, cobalt, and oxygen.

[0033] In the above embodiment, the second nickel source is preferably nickel fluoride.

[0034] In the above aspect, it is preferable that in the third step, a magnesium source is mixed in addition to the second composite oxide, the second nickel source, and lithium fluoride to form a second mixture.

[0035] In the above embodiment, it is preferable that the first nickel source is nickel fluoride and the magnesium source is magnesium fluoride.

[0036] In the above aspect, it is preferable that the first heating in the second step is performed at a temperature of 650°C or higher and 1000°C or lower, and the second heating in the fourth step is performed at a temperature lower than the temperature in the first heating.

[0037] According to one embodiment of the present invention, a positive electrode active material or composite oxide that can be used in a secondary battery and exhibits a suppressed decrease in discharge capacity during charge-discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide that is less likely to lose its crystal structure even after repeated charge-discharge cycles can be provided. Alternatively, a positive electrode active material or composite oxide that exhibits a large discharge capacity can be provided. Alternatively, a secondary battery with high safety or reliability can be provided.

[0038] 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.

[0039] 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.

[0040] FIGS. 1A and 1B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 2 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 3A and 3B are diagrams illustrating a method for preparing a positive electrode active material. FIGS. 4A and 4B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 5 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 6A and 6B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 7 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 8A and 8B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 9 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 10A and 10B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 11 is a diagram illustrating a method for preparing a positive electrode active material. FIGS. 12A and 12B are diagrams illustrating a method for preparing a positive electrode active material. FIG. 13 is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and magnesium fluoride. FIG. 14 is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and nickel fluoride. FIG. 15 is an example of a cross-sectional process diagram illustrating one embodiment of the present invention. FIGS. 16A and 16B are cross-sectional views of a positive electrode active material. FIG. 17 is a diagram illustrating the crystal planes of a positive electrode active material. FIGS. 18A to 18C are examples of the distribution of additive elements contained in a positive electrode active material. FIGS. 19A to 19C are examples of the distribution of additive elements contained in a positive electrode active material. FIG. 20 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 21A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 21B is an FFT pattern of the rock salt crystal RS region, and FIG. 21C is an FFT pattern of the layered rock salt crystal LRS region. FIG. 22 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 23 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIG. 24 is a diagram illustrating the depth of charge and lattice constant of a positive electrode active material. FIG. 25 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 26 is a diagram illustrating an XRD pattern calculated from the crystal structure. Figures 27A and 27B are diagrams showing XRD patterns calculated from the crystal structure. Figures 28A to 28C are lattice constants calculated from XRD. Figures 29A to 29C are lattice constants calculated from XRD. Figures 30A and 30B are cross-sectional views of positive electrode active material. Figure 31 is a diagram showing the appearance of a secondary battery.Figures 32A to 32C are diagrams illustrating a method for manufacturing a secondary battery. Figure 33 is a graph showing the temperature rise of a secondary battery. Figures 34A and 34B are diagrams illustrating a nail penetration test. Figure 35 is a graph showing the temperature rise of a secondary battery when an internal short circuit occurs. Figures 36A to 36H are diagrams illustrating an example of an electronic device. Figures 37A to 37D are diagrams illustrating an example of an electronic device. Figures 38A to 38C are diagrams illustrating an example of an electronic device. Figures 39A to 39C are diagrams illustrating an example of a vehicle.

[0041] 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.

[0042] In this specification, space groups are expressed using shortnotation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. Individual planes indicating crystal planes are expressed using ( ). In crystallography, space groups, crystal planes, and crystal directions are expressed with a superscript bar. However, due to formatting constraints, in this specification, numbers may be expressed with a minus sign (-) before them instead of a bar above them. 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 represented by the space group R-3m are often expressed as a hexagonal composite hexagonal lattice to facilitate understanding of the structure. Miller indices may also use (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.

[0043] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), and examples of the cross-sectional shape of individual particles include ellipsoids, rectangles, trapezoids, triangles, squares with rounded corners, and asymmetric shapes, and further, individual particles may have an irregular shape.

[0044] The theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. For example, LiCoO ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 274mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0045] The amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ CoO ２ In the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, LiCoO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ In this specification, "x" is small means, for example, that 0.1<x≦0.24. The extent to which lithium has been released from the positive electrode active material relative to the theoretical capacity is sometimes referred to as the depth of charge. In this specification and elsewhere, the depth of charge is expressed as 1−x.

[0046] When properly synthesized lithium cobalt oxide is used in the positive electrode, the stoichiometric ratio is approximately satisfied. ２ and x = 1. The lithium cobalt oxide contained in the secondary battery after discharge is also LiCoO ２ It can be said that x = 1. The completion of discharge here refers to a state where the voltage is 3.0 V or 2.5 V or less at a current of 100 mA / g or less.

[0047] Li ｘ CoO ２ It is preferable that the charge capacity and / or discharge capacity used to calculate x in the above should be measured under conditions that are free of or minimally affected by short-circuiting and / or decomposition of the electrolyte, etc. For example, data on a secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit should not be used to calculate x.

[0048] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0049] Furthermore, if the anions have a structure in which three layers are stacked with a skew, such as ABCABC, it is called a cubic close-packed structure. Therefore, the anions do not need to be strictly cubic lattices. At the same time, since real crystals always have defects, the analysis results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM image, spots may appear at positions slightly different from the theoretical positions. For example, if the orientation from the theoretical positions is less than 5 degrees, or less than 2.5 degrees, it can be said to have a cubic close-packed structure.

[0050] The distribution of a certain element refers to a region in which the element is continuously detected within a range that is not a noise by a certain continuous analytical method. A region in which the element is continuously detected within a range that is not a noise can also be referred to as a region in which the element is always detected when the analysis is performed multiple times.

[0051] A cathode active material to which an additive element is added may be referred to as a composite oxide, a cathode material, a cathode ingredient, a cathode material for a secondary battery, or the like. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a compound. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composition. In this specification and the like, the cathode active material of one embodiment of the present invention preferably includes a composite.

[0052] 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.

[0053] As the charging voltage of a secondary battery increases, the voltage of the positive electrode generally increases. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in charge / discharge capacity due to repeated charge / discharge.

[0054] Furthermore, a short circuit in a secondary battery not only causes problems in the charging and / or discharging operation of the secondary battery, but may also lead to heat generation and fire. To achieve a safe secondary battery, it is preferable to suppress short-circuit current even at high charging voltages. The positive electrode active material of one embodiment of the present invention suppresses short-circuit current even at high charging voltages. Therefore, a secondary battery can be obtained that achieves both high discharge capacity and safety. Furthermore, the positive electrode active material of one embodiment of the present invention exhibits the unique effect of suppressing oxygen desorption, which is thought to suppress the oxidation reaction of the electrolyte and reduce heat generation. Furthermore, the positive electrode active material of one embodiment of the present invention is expected to have the remarkable effect of slowing the rate of current flowing into the positive electrode during an internal short circuit, making it less likely to cause thermal runaway and fire.

[0055] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in secondary batteries are described in their pre-degradation state. Note that a decrease in discharge capacity due to aging and burn-in treatments during secondary battery manufacturing is not considered to be degradation. For example, a lithium-ion secondary cell or lithium-ion secondary battery pack (hereinafter referred to as a lithium-ion secondary battery) can be said to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0056] 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.

[0057] Embodiment 1 In this embodiment, a method for manufacturing a positive electrode active material of one embodiment of the present invention will be described.

[0058] A positive electrode active material according to one embodiment of the present invention contains lithium, cobalt, oxygen, and an additive element.

[0059] In the positive electrode active material of one embodiment of the present invention, the additive element is preferably present in a solid solution in the positive electrode active material. The positive electrode active material of one embodiment of the present invention preferably has the additive element in a surface layer portion. The surface layer portion is synonymous with the surface vicinity, the surface vicinity region, or the shell.

[0060] In the positive electrode active material of one embodiment of the present invention, the concentration of the additive element in the surface layer is preferably higher than the concentration in the interior. The interior is synonymous with the interior region or the core. In the positive electrode active material of one embodiment of the present invention, the additive element may not be detected in the interior.

[0061] Note that in the positive electrode active material of one embodiment of the present invention, the depth of the peak of the detected amount in the surface layer portion may differ depending on the added element.

[0062] The positive electrode active material of one embodiment of the present invention can be prepared, for example, by preparing a composite oxide containing lithium, cobalt, and oxygen, and then adding an additive element to the composite oxide. The additive element can be added, for example, by preparing a mixture of the prepared composite oxide and a source of the additive element and heating the mixture. In this case, the additive element that diffuses rapidly in the composite oxide may be added deep from the surface of the positive electrode active material.

[0063] When preparing a composite oxide containing lithium, cobalt, and oxygen, an additive element can also be added at the same time to prepare a positive electrode active material. Specifically, for example, a lithium source, a cobalt source, and an additive element source can be mixed to prepare a composite oxide containing the additive element as a positive electrode active material. However, in such a method, it is difficult to increase the concentration of the additive element in the surface layer compared to the concentration in the interior.

[0064] On the other hand, a method of preparing a composite oxide and then adding an additive element to the composite oxide is more preferable as a method of preparing a positive electrode active material according to one embodiment of the present invention, since it is easy to increase the concentration of the additive element in the surface layer portion compared to the concentration in the interior portion.

[0065] When adding multiple additive elements, the addition of different additive elements can be performed in different steps. For example, after preparing a composite oxide containing lithium, cobalt, and oxygen, the composite oxide is mixed with a first additive element source and heated. Then, a second additive element source is added and heated. Note that the number of additive element sources is not limited to two.

[0066] Alternatively, different additive elements may be added in the same process. For example, after preparing a composite oxide containing lithium, cobalt, and oxygen, the composite oxide, a first additive element source, and a second additive element source are mixed and heated. The number of additive element sources is not limited to two, and three or more additive element sources may be mixed.

[0067] Here, it is preferable to use a composite oxide having a layered rock salt type crystal structure of R-3m as the composite oxide containing lithium, cobalt, and oxygen.

[0068] Furthermore, it is preferable to use one or more additive elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium.

[0069] [Magnesium and Nickel] The positive electrode active material of one embodiment of the present invention preferably contains magnesium and nickel.

[0070] In a method for producing a positive electrode active material according to one embodiment of the present invention, for example, a composite oxide containing lithium, cobalt, and oxygen is produced, and then the composite oxide, a first additive element source, and a second additive element source are mixed and heated. In this case, when the composite oxide containing lithium, cobalt, and oxygen has an R-3m layered rock salt crystal structure and magnesium is used as the first additive element and nickel is used as the second additive element, for example, the magnesium source and the nickel source are preferably added in the same step.

[0071] In this case, it is more preferable to mix a fluoride that functions as a flux together with the additive element source. That is, the process for producing a positive electrode active material of one embodiment of the present invention preferably includes the steps of: preparing a composite oxide having lithium, cobalt, and oxygen; mixing the composite oxide, a magnesium source, a nickel source, and a fluoride to prepare a mixture; and heating the mixture, and the composite oxide having lithium, cobalt, and oxygen preferably has an R-3m layered rock salt type crystal structure.

[0072] Here, the composite oxide containing lithium, cobalt, and oxygen is, for example, LiMeO ２ (Me is a metal). Here, the metal Me is, for example, cobalt. That is, a composite oxide having lithium, cobalt, and oxygen is, for example, LiCoO ２ The lithium cobalt oxide is represented by the formula: Alternatively, a transition metal may be used in addition to cobalt as the metal Me. For example, one or two metals selected from nickel and manganese may be used in addition to cobalt as the metal Me. When the metal Me contains 75 atomic % or more, preferably 90 atomic % or more of cobalt, this is preferable as it has many advantages, such as being relatively easy to synthesize, being easy to handle in the synthesis process, and having excellent cycle characteristics.

[0073] Furthermore, the positive electrode active material of one embodiment of the present invention, which is produced by adding an additive element to a composite oxide, preferably has a layered rock-salt crystal structure belonging to the space group R-3m in a discharged state.２ The composition is not strictly limited to Li:Me:O=1:1:2.

[0074] The distribution and composition of the additive elements in the positive electrode active material change depending on how the additive elements are added in the manufacturing process. In addition, by devising a manufacturing method for the positive electrode active material, a positive electrode active material with good internal crystallinity can be obtained.

[0075] In the process for producing a positive electrode active material according to one embodiment of the present invention, first, LiMeO ２ It is preferable to synthesize a composite oxide represented by the following formula: and then mix the additive element source and perform a heat treatment. As a specific example, the case where the metal Me is cobalt will be described below.

[0076] In the method of synthesizing lithium cobalt oxide containing an additive element by simultaneously mixing a cobalt source and a lithium source with an additive element source, it is difficult to increase the concentration of the additive element in the surface layer. 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, mixing the additive element source after synthesizing lithium cobalt oxide and then performing a heat treatment is preferable, as it allows the additive element concentration to be higher in the surface layer than in the interior. This heat treatment after mixing the additive element source is sometimes called annealing.

[0077] 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 tend to occupy the lithium site. However, if the amount of magnesium added is large, magnesium can occupy the cobalt site in addition to the lithium site.

[0078] Magnesium present at the lithium site acts as a charge source for the secondary battery. ｘ CoO ２ When x in the R-3m is small, it has the effect of maintaining the layered rock salt type crystal structure of R-3m. On the other hand, magnesium present in the cobalt site has the effect of maintaining the layered rock salt type crystal structure of R-3m. ｘ CoO ２When x in the matrix becomes small, the effect of maintaining the layered rock salt type crystal structure of R-3m becomes small.

[0079] Nickel is LiMeO ２ In the layered rock-salt crystal structure of the cathode, magnesium can exist on either the cobalt site or the lithium site. By adding nickel to the region where magnesium is added, it is expected that magnesium will be prevented from entering the cobalt site, thereby maintaining the stability of the crystal structure of the cathode active material.

[0080] In this way, it is preferable that nickel is also added to the region where magnesium is added. Therefore, it is preferable that the distributions of magnesium and nickel overlap. In the method for producing a positive electrode active material, for example, by adding magnesium and nickel in the same process, the concentration distributions of magnesium and nickel in the depth direction can be overlapped. Here, the concentration distribution of the additive in the depth direction can be evaluated, for example, by performing EDX-ray analysis on a cross section of the positive electrode active material from the surface toward the inside. The overlap of the concentration distributions of magnesium and nickel in the depth direction refers, for example, to the overlap of the region where magnesium is detected and the region where nickel is detected in the above-mentioned EDX-ray analysis.

[0081] As will be described in detail later, when nickel is present at the cobalt site, a larger charge / discharge capacity can be obtained. Furthermore, when nickel is present at the lithium site, the layered structure consisting of octahedra of cobalt and oxygen is suppressed from shifting, contributing to the stability of the crystal structure. Thus, whether nickel is added to the cobalt site or the lithium site, it can be said that it contributes to improving the performance of the positive electrode active material.

[0082] Here, nickel is preferred because its high concentration in the surface layer, together with magnesium, is expected to stabilize the crystal structure of the positive electrode active material. Meanwhile, nickel may be present at a high concentration in the surface layer of the positive electrode active material, as well as being widely distributed in deeper regions, for example, throughout the interior of the positive electrode active material. For example, as shown in FIG. 18A (described later), nickel may have a distribution represented by the sum of a distribution D1 having a narrow peak in the detection amount in the surface layer and a distribution D2 that is widely distributed in a region deeper than distribution D1, for example, extending to the interior of the positive electrode active material. By distributing nickel widely throughout the interior, for example, it is expected that the charge / discharge capacity of the secondary battery will be increased.

[0083] In addition, simultaneous addition of two or more additive elements may be preferable from the viewpoint of lowering the heating temperature during the preparation of the positive electrode active material. After mixing the additive element source with lithium cobalt oxide, sufficient heating is required to achieve a good distribution of the additive element. However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that the additive element, such as magnesium, will enter the cobalt site. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as cobalt being reduced to divalent and lithium evaporating.

[0084] In the process of producing a positive electrode active material, when a mixture of lithium cobalt oxide, a magnesium source, and a nickel source is prepared and the mixture is heated, the mixture of the magnesium source and the nickel source may cause a drop in the melting point of the lithium cobalt oxide, etc. The drop in the melting point makes it easier to favorably distribute magnesium and nickel in the lithium cobalt oxide at a temperature where cation mixing is unlikely to occur.

[0085] Furthermore, divalent nickel may be able to exist more stably in the vicinity of divalent magnesium. By adding the magnesium source and the nickel source in the same process, for example, magnesium and nickel may be disposed close to each other, allowing divalent magnesium to exist stably, and thus elution of magnesium may be suppressed.

[0086] It is preferable to use a fluoride as the additive element source, which, when mixed with other additive element sources, can function as a flux (also called a fluxing agent) that lowers the melting points of the other additive element sources and the lithium cobalt oxide.

[0087] Furthermore, a material that functions as a flux may be further mixed with the additive element source. If the melting point is lower than that of lithium cobalt oxide, the material can be said to function as a flux. For example, it is preferable to mix a fluoride such as lithium fluoride with the magnesium source and nickel source. Adding a fluoride that functions as a flux lowers the melting points of the additive element source and lithium cobalt oxide. Lowering the melting point makes it easier to achieve good distribution of the additive element at a temperature where cation mixing is unlikely to occur.

[0088] In the method for producing a positive electrode active material according to one embodiment of the present invention, magnesium fluoride can be used as the magnesium source, nickel fluoride can be used as the nickel source, and lithium fluoride can be used as the flux.

[0089] The eutectic point between magnesium fluoride, which is a magnesium source, and lithium fluoride, which is a flux, will be explained. As shown in Figure 13 (quoted and added from Figure 5 of Non-Patent Document 12), LiF and MgF ２ The eutectic point P of is around 742°C (T1).

[0090] The eutectic point of nickel fluoride, which is a nickel source, and lithium fluoride, which is a flux, will be explained. As shown in Figure 14 (quoted from Figure 2 of Non-Patent Document 13), LiF and NiF ２ The eutectic point of, for example, NiF ２ When the content (x on the horizontal axis in FIG. 14) is 0.2, the temperature is in the range of 1060 [K] to 1065 [K] (in the range of 787° C. to 792° C.).

[0091] Although FIGS. 13 and 14 show the eutectic points of the respective additive element sources and the fluxes, when the three materials, magnesium fluoride, nickel fluoride, and lithium fluoride, are mixed together in a method for producing a positive electrode active material according to one embodiment of the present invention, a further decrease in melting point may occur.

[0092] The depth to which the additive element is added also depends on the orientation of the surface exposed on the surface of the lithium cobalt oxide.

[0093] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The diffusion path of lithium ions is parallel to the (001) plane.

[0094] In the layered rock salt type crystal structure of R-3m, magnesium and nickel tend to diffuse along paths parallel to the (001) plane.

[0095] When the (001) plane is exposed on the surface of lithium cobalt oxide, the paths parallel to the (001) plane are not exposed on the surface, and therefore it can be said that magnesium and nickel have difficulty entering the lithium cobalt oxide.

[0096] On the other hand, when a plane other than the (001) orientation is exposed on the surface of lithium cobalt oxide, paths parallel to the (001) plane are exposed on the surface. Therefore, it can be said that magnesium and nickel can more easily enter lithium cobalt oxide and diffuse deeper when a plane other than the (001) orientation is exposed on the surface than when the (001) plane is exposed on the surface.

[0097] Here, the (001) plane and the (003) plane may be collectively referred to as the (00l) plane. The (00l) plane may also be referred to as the basal plane or C-plane. Furthermore, planes other than the (00l) plane may also be referred to as edge planes.

[0098] CoO ２ Since the layer is relatively stable, when the surface of the positive electrode active material is the (001) plane, the surface is highly stable. On the other hand, the diffusion path of lithium ions is exposed on surfaces other than the (001) orientation. Surfaces other than the (001) orientation are prone to instability because they are the regions from which lithium ions first desorb.

[0099] Therefore, it is extremely important to increase the concentrations of magnesium and nickel on the surface and surface layer portions other than the (001) orientation to prevent the crystal structure from collapsing due to lithium elimination during charging.

[0100] By using the method for producing a positive electrode active material according to one embodiment of the present invention, the concentrations of magnesium and nickel can be significantly increased in the surface layer portion when the surface has a plane other than the (001) orientation. Furthermore, in the method for producing a positive electrode active material according to one embodiment of the present invention, by adding magnesium and nickel in the same process, the concentration of nickel can also be increased in the region where the magnesium concentration is high. This is expected to prevent magnesium from entering the cobalt site and maintain the stability of the crystalline structure of the positive electrode active material, thereby significantly reducing the disruption of the crystalline structure of the positive electrode active material due to repeated charge and discharge of the secondary battery.

[0101] [Aluminum] The positive electrode active material of one embodiment of the present invention may contain aluminum. Aluminum may be present at the cobalt site in the layered rock salt crystal structure. As will be described in detail later, aluminum can contribute to suppressing changes in the crystal structure, preventing cobalt elution, and the like.

[0102] In the method for producing a positive electrode active material according to one embodiment of the present invention, the mixing of the aluminum source and the subsequent heating are performed in a step separate from the mixing of the magnesium source and the nickel source and the subsequent heating. Alternatively, the aluminum source may be mixed together with the magnesium source and the nickel source.

[0103] [Fluorine] In the method for producing a cathode active material according to one embodiment of the present invention, it is preferable to use a fluoride as an additive element source. Furthermore, in addition to the additive element source, a fluoride capable of functioning as a flux can be used. By producing a cathode active material using a fluoride, a fluorine-containing cathode active material can be obtained. The inclusion of fluorine in the cathode active material can facilitate the desorption and insertion of lithium ions, suppress excessive reaction with the electrolyte, and the like, as will be described in detail later. Furthermore, in the method for producing a cathode active material according to one embodiment of the present invention, fluorine may be used instead of or in addition to a fluoride. Furthermore, by incorporating fluorine into the surface layer of the cathode active material according to one embodiment of the present invention, ignition or overheating of the lithium ion secondary battery can be suppressed, further improving thermal safety. Furthermore, by incorporating fluorine into the surface layer of the cathode active material according to one embodiment of the present invention, oxygen desorption from the cathode active material is less likely, and thermal decomposition reactions may be suppressed.

[0104] [Other Additive Elements] The positive electrode active material of one embodiment of the present invention may contain titanium. By using a positive electrode active material having titanium oxide in the surface layer portion, wettability with a highly polar solvent may be improved.

[0105] Furthermore, the positive electrode active material of one embodiment of the present invention can contain phosphorus. When phosphorus is present in the surface layer portion, short circuits can be suppressed when the positive electrode active material is maintained in a charged state. Hydrogen fluoride generated by decomposition of the electrolytic solution or electrolyte may react with phosphorus contained in the positive electrode active material, thereby reducing the hydrogen fluoride concentration in the electrolyte. Furthermore, when the positive electrode active material has cracks, the presence of a phosphorus-containing compound inside the positive electrode active material with the cracks on the surface can suppress the progression of the cracks.

[0106] [Initial Heating] After the lithium cobalt oxide is synthesized, it is more preferable to heat it before mixing the additive element. This heating is sometimes called initial heating.

[0107] The initial heating causes lithium to be released from a portion of the surface layer of the lithium cobalt oxide, resulting in a more favorable distribution of the added elements.

[0108] More specifically, it is believed that initial heating facilitates different distributions depending on the added element through the following mechanism. First, lithium is removed from part of the surface layer by initial heating. Next, this lithium-deficient surface layer is mixed with an added element source, such as a nickel source, an aluminum source, or a magnesium source, and heated. Of the added 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 part of the surface layer. 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, it may not be clearly observed in electron microscope images such as STEM (scanning transmission electron microscope) and electron beam diffraction patterns.

[0109] Among the additive elements, nickel is easily dissolved and diffuses into the interior when the surface layer is a layered rock salt type lithium cobalt oxide, but tends to remain in the surface layer when part of the surface layer is a rock salt type. Therefore, initial heating can make it easier for divalent additive elements such as nickel to remain in the surface layer. The effect of this initial heating is particularly significant on the surface other than the (001)-oriented surface of the positive electrode active material and its surface layer.

[0110] 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.

[0111] For example, rock salt type Ni ０．５ Mg ０．５ The Me-O distance in MgO is 2.09 Å, and the Me-O distance in rock salt MgO is 2.11 Å. Even if a spinel phase is formed in a part of the surface layer, the spinel phase ２ O ４ The Me-O distance is 2.0125 Å, and the spinel type MgAl ２ O ４ The Me-O distance is 2.02 Å. In both cases, the Me-O distance exceeds 2 Å. Note that 1 Å = 10 −１０ m.

[0112] On the other hand, in the layered rock salt type, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, layered rock salt type LiAlO ２ The Al-O distance in the layered rock salt LiCoO is 1.905 Å (the Li-O distance is 2.11 Å). ２ The Co—O distance in this case is 1.9224 Å (the Li—O distance is 2.0916 Å).

[0113] According to Shannon's ionic radii (Shannon et al., Acta A 32 (1976) 751), the ionic radius of hexacoordinated aluminum is 0.535 Å, the ionic radius of hexacoordinated oxygen is 1.4 Å, and the sum of these is 1.935 Å.

[0114] From the above, it is thought that aluminum exists more stably in non-lithium sites in the layered rocksalt phase than in the rocksalt phase, and therefore aluminum is more likely to be distributed in the deeper and / or more internal regions of the layered rocksalt phase than in the regions closer to the surface of the rocksalt phase.

[0115] In addition, the initial heating is expected to have the effect of increasing the crystallinity of the internal layered rock salt type crystal structure.

[0116] Therefore, especially Li ｘ CoO ２ When x is, for example, 0.15 or more and 0.17 or less, in order to prepare a positive electrode active material having a monoclinic O1(15) type crystal structure, it is preferable to carry out this initial heating.

[0117] However, initial heating is not necessarily required. In other heating steps, such as annealing, the atmosphere, temperature, time, etc. can be controlled to prevent Li ｘ CoO ２ When x in the formula (I) is small, it may be possible to prepare a positive electrode active material 100 having an O3′ type and / or a monoclinic O1(15) type.

[0118] <<Method 1 for Producing Positive Electrode Active Material>> Method 1 for producing a positive electrode active material 100 that undergoes annealing and initial heating will be described with reference to FIGS. 1A and 1B. FIG.

[0119] <Step S11> In step S11 shown in FIG. 1A, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0120] 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.

[0121] As the cobalt source, it is preferable to use a compound containing cobalt, such as cobalt oxide or cobalt hydroxide.

[0122] 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.

[0123] In addition, the cobalt source preferably has high crystallinity, for example, single crystal grains. The crystallinity of the cobalt source can be evaluated using a TEM (transmission electron microscope) image, a STEM image, a HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) image, an ABF-STEM (annular bright-field scanning transmission electron microscope) image, or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, or the like. Note that the above-mentioned methods for evaluating crystallinity can be applied not only to the evaluation of cobalt sources but also to the evaluation of the crystallinity of other sources.

[0124] <Step S12> Next, in step S12 shown in FIG. 1A, 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 pulverization. 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.

[0125] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as pulverizing media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed: 400 rpm, ball mill diameter: 40 mm).

[0126] <Step S13> Next, in step S13 shown in FIG. 1A, 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, defects may occur due to lithium evaporation from the lithium source and / or excessive reduction of cobalt. For example, cobalt may change from trivalent to divalent, which may induce oxygen defects.

[0127] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time is preferably from 1 hour to 100 hours, and more preferably from 2 hours to 20 hours.

[0128] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0129] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] The crucible used for heating is preferably an aluminum oxide crucible. Aluminum oxide crucibles are made of a material that does not easily release impurities. In this embodiment, an aluminum oxide crucible with a purity of 99.9% is used. It is preferable to heat the crucible with a lid on, as this can prevent the material from volatilizing.

[0135] 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 used crucible refers to one that has undergone three or more heating processes with materials containing lithium, transition metal M, and / or additive elements. This is because using a new crucible may result in some of the materials, including lithium fluoride, being absorbed, diffused, migrated, and / or adhered to the crucible during heating. If some of the materials are lost as a result of this, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, may not fall within the desired range. On the other hand, this risk is less likely with a used crucible.

[0136] 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 a mortar made of zirconium oxide or agate. It is also possible 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.

[0137] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized.

[0138] 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.

[0139] 1A, the lithium cobalt oxide is heated. Because this is the first heating of the lithium cobalt oxide, the heating in step S15 may be referred to as initial heating. Alternatively, because this heating is performed before step S20 described below, it may be referred to as preheating or pretreatment.

[0140] As described above, the initial heating causes lithium to be desorbed from a portion of the surface layer of the lithium cobalt oxide. This is also expected to have the effect of improving the internal crystallinity. Furthermore, impurities may be present in the lithium source and / or cobalt source prepared in step S11, etc. The initial heating can reduce the amount of impurities in the lithium cobalt oxide completed in step S14.

[0141] Furthermore, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. A smooth lithium cobalt oxide surface means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a smooth surface means that there is little foreign matter adhering to the surface. Foreign matter is thought to be a cause of irregularities, so it is preferable that it does not adhere to the surface.

[0142] For this initial heating, it is not necessary to prepare a lithium compound source, an additive element source, or a material that functions as a flux.

[0143] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. In addition to the heating conditions, the heating temperature in this step should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in this step should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0144] The effect of increasing the internal crystallinity is, for example, the effect of alleviating distortion, displacement, etc. resulting from differential shrinkage, etc., of the lithium cobalt oxide produced in step S13.

[0145] The heating in step S13 may cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference may induce a shrinkage difference. It is also believed that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is believed that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. Homogenizing the strain energy relieves the strain in the lithium cobalt oxide. This may result in a smoother surface for the lithium cobalt oxide. This is also referred to as an improved surface. In other words, it is believed that the shrinkage difference in the lithium cobalt oxide is relieved after step S15, resulting in a smoother surface for the composite oxide.

[0146] Furthermore, the difference in shrinkage may cause microscopic misalignment, such as crystal misalignment, in the lithium cobalt oxide. This step is preferably carried out to reduce this misalignment. This step makes it possible to equalize the misalignment in the composite oxide. When the misalignment is equalized, the surface of the composite oxide may become smooth. This is also referred to as the alignment of crystal grains. In other words, it is believed that step S15 reduces the misalignment of crystals and other particles in the composite oxide, resulting in a smoother surface.

[0147] When lithium cobalt oxide with a smooth surface is used as the positive electrode active material, deterioration during charge and discharge in a secondary battery is reduced and cracking of the positive electrode active material can be prevented.

[0148] 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.

[0149] <Step S20> Next, as shown in step S20, an additive element A source (A source) is added to the lithium cobalt oxide that has undergone the initial heating. Here, it is preferable to add magnesium and nickel as the additive element A. When the additive element is added to the lithium cobalt oxide that has undergone the initial heating, the additive element can be added evenly. Therefore, it is preferable to add the additive element after the initial heating. The step of adding the additive element A will be described with reference to FIG. 1B.

[0150] 1B, a magnesium source (Mg source) and a nickel source (Ni source) to be added to lithium cobalt oxide are prepared. It is preferable to prepare a fluoride together with the magnesium source and the nickel source.

[0151] The magnesium source may be magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. A plurality of the above-mentioned magnesium sources may also be used.

[0152] The nickel source may be nickel fluoride, nickel oxide, nickel hydroxide, etc. A plurality of the above nickel sources may also be used.

[0153] Examples of fluorides include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), nickel fluoride (NiF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among them, lithium fluoride is preferable because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below. Fluorides can function as a fluorine source.

[0154] Magnesium fluoride can be used as both a magnesium source and a fluorine source. Nickel fluoride can be used as both a nickel source and a fluorine source. Lithium fluoride can be used as both a lithium source and a fluorine source. Another lithium source that can be used in step S21 is lithium carbonate.

[0155] The fluorine source may also be gaseous, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F) or the like may be used and mixed into the atmosphere in the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0156] In this embodiment, lithium fluoride (LiF) is prepared as the fluoride, and magnesium fluoride (MgF ２ ) was prepared as a nickel source. ２ ) to prepare.

[0157] Lithium fluoride and magnesium fluoride are LiF:MgF ２When the molar ratio is about 65:35, the effect of lowering the melting point is maximized.

[0158] Lithium fluoride and nickel fluoride are also known as LiF:NiF ２ At a molar ratio of 0.8:0.2 or a ratio in the vicinity thereof, the melting point is suitably low and NiF ２ can exist stably as a liquid without becoming excessive.

[0159] On the other hand, if the amount of lithium fluoride is too large, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate.

[0160] The molar ratio of lithium fluoride, magnesium fluoride, and nickel fluoride was LiF:MgF ２ : NiF ２ = x:1:y, where x is, for example, 0 to 10, or 0 to 5, or 0 to 3, or 0 to 2, or 0 to 1, or 0 to 0.5. Also, y is, for example, greater than 0 and less than 3, or 0.2 to 2.

[0161] 1B, the magnesium source, the nickel source, and the fluoride are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0162] 1B, the pulverized and mixed materials are collected to obtain a source of the additional element A (A source). Note that the source of the additional element A shown in step S23 contains a plurality of starting materials and can be called a mixture.

[0163] 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.

[0164] Such a finely powdered mixture (including the case where only one kind of additive element is contained) can be easily adhered to the surface of lithium cobalt oxide particles when mixed with lithium cobalt oxide in a later step. If the mixture is evenly adhered to the surface of lithium cobalt oxide particles, it is easy to distribute or diffuse the additive element evenly in the surface layer portion of the composite oxide after heating, which is preferable.

[0165] 1A , lithium cobalt oxide is mixed with a source of additional element A. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the source of additional element A is preferably Co:Mg=100:z (0.1≦z≦6), and more preferably M:Mg=100:z (0.3≦z≦3).

[0166] 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.

[0167] 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.

[0168] 1A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0169] 1A and 1B illustrate a fabrication method in which an additive element is added only after initial heating, but the present invention is not limited to the above method. The additive element may be added at a different timing or multiple times. The timing may vary depending on the element.

[0170] For example, an additive element may be added to the lithium source and the cobalt source in step S11, i.e., in the stage of the starting material for the composite oxide. Then, in step S13, lithium cobalt oxide containing the additive element can be obtained. In this case, it is not necessary to separate steps S11 to S14 from steps S21 to S23. This method can be said to be simple and highly productive.

[0171] 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.

[0172] Alternatively, lithium cobalt oxide to which magnesium and fluorine have been added in advance may be heated in step S15, and then a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added as in step S20.

[0173] <Step S33> Next, in step S33 shown in FIG. 1A, the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may be higher than 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.

[0174] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 650° C. or higher.

[0175] Of course, the reaction is more likely to proceed if the temperature is higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF, MgF ２ and NiF ２ When LiF and MgF ２ The eutectic point of LiF and NiF is around 742°C. ２ For example, LiF and MgF have an extreme eutectic point near 790°C. ２ and NiF ２ When these three compounds are mixed, a further decrease in the eutectic point is expected, and therefore the lower limit of the heating temperature in step S33 is preferably set to, for example, 700°C or higher, and may be 740°C or higher, or 790°C or higher.

[0176] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0177] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is more preferably 1000°C or lower, and even more preferably 950°C or lower.

[0178] The heating temperature in step S33 is preferably higher than that in step S13.

[0179] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0180] In the manufacturing method described in this embodiment, some materials, such as lithium fluoride, which is a fluorine source, may function as a flux, which allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, and allows magnesium and other additive elements to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0181] However, since lithium fluoride has a lower specific gravity in a gaseous state than oxygen, there is a possibility that lithium fluoride will volatilize when heated, and if it volatilizes, the amount of lithium fluoride 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 lithium fluoride. Even if lithium fluoride is not used as a fluorine source, etc., LiCoO ２ There is also a possibility that the Li on the surface reacts with the F in the fluorine source to produce lithium fluoride, which then volatilizes. Therefore, even if a fluoride with a higher melting point than lithium fluoride is used, it is still necessary to suppress volatilization.

[0182] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing lithium fluoride, that is, to heat the mixture 903 in a state where the partial pressure of lithium fluoride is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of lithium fluoride in the mixture 903.

[0183] FIG. 15 shows an example of a heating furnace that can be used for heating. The heating furnace 220 shown in FIG. 15 includes a furnace space 202, a hot plate 204, a pressure gauge 221, a heater unit 206, and a heat insulator 208. It is more preferable to perform annealing with a lid 218 attached to the container 216. This configuration allows the space 219 defined by the container 216 and the lid 218 to be an atmosphere containing fluoride. By maintaining the lid during annealing so that the concentration of gasified fluoride in the space 219 remains constant or does not decrease, fluorine and magnesium can be contained near the particle surface. Since the space 219 has a smaller volume than the furnace space 202, a small amount of fluoride volatilizes, creating an atmosphere containing fluoride. In other words, the reaction system can be made into an atmosphere containing fluoride without significantly reducing the amount of fluoride contained in the mixture 903. Therefore, LiMeO ２ Furthermore, by using the lid 218, the mixture 903 can be annealed in an atmosphere containing fluoride easily and inexpensively.

[0184] Furthermore, before heating in the heating furnace space 202, a step of creating an oxygen-containing atmosphere in the heating furnace space 202 and a step of placing the container 216 containing the mixture 903 in the heating furnace space 202 are performed. By performing these steps in this order, the mixture 903 can be annealed in an atmosphere containing oxygen and fluoride. For example, the annealing is performed while a gas is flowing. Furthermore, the heating furnace space 202 can be sealed during the annealing to form a closed space so that the gas is not transported to the outside.

[0185] There are no particular limitations on the method for creating an oxygen-containing atmosphere in the heating furnace space 202, but examples include a method of evacuating the heating furnace space 202 and then introducing an oxygen-containing gas such as oxygen gas or dry air, or a method of infusing an oxygen-containing gas such as oxygen gas or dry air for a certain period of time. Among these, it is preferable to evacuate the heating furnace space 202 and then introduce oxygen gas (oxygen substitution). Note that the air in the heating furnace space 202 may be considered to be an oxygen-containing atmosphere.

[0186] Furthermore, fluoride and the like adhering to the inner walls of the container 216 and the lid 218 can be re-emitted by heating and attached to the mixture 903 .

[0187] There is no particular limitation on the process for heating the heating furnace 220. Heating may be performed using a heating mechanism provided in the heating furnace 220.

[0188] Furthermore, there are no particular restrictions on how the mixture 903 is arranged when placed in the container 216, but it is preferable to arrange the mixture 903 so that the upper surface of the mixture 903 is flat with respect to the bottom surface of the container 216, in other words, so that the height of the upper surface of the mixture 903 is uniform, as shown in Figure 15.

[0189] The heating in step S33 is preferably performed while controlling the pressure inside the furnace with a pressure gauge 221. The inside of the furnace is preferably kept at atmospheric pressure or under pressure. If the furnace is kept under pressure during heating, the surface of the positive electrode active material 100 becomes smooth, and the positive electrode active material 100 may be more resistant to physical destruction due to pressure or the like than a positive electrode active material that is not smooth. For example, the positive electrode active material 100 is less likely to be destroyed in a test that involves pressure, such as a nail penetration test, and as a result, safety may be improved.

[0190] The heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additive elements (e.g., fluorine) diffuse is blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0191] Furthermore, it is believed that uniform distribution of an additive element (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the particles of mixture 903 do not adhere to each other.

[0192] 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.

[0193] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing lithium fluoride by placing a lid on a container containing the mixture 903, for example.

[0194] 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.

[0195] 1A, when the median diameter (D50) of the lithium cobalt oxide is about 12 μm, the heating temperature is preferably, for example, 650° C. or more and 950° C. or less. The heating time is, for example, preferably, 3 hours or more and 60 hours or less, more preferably, 10 hours or more and 30 hours or less, and even more preferably, about 20 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or more and 50 hours or less.

[0196] On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 5 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0197] <Step S34> Next, in step S34 shown in FIG. 1A, 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.

[0198] 2 to 3B , a positive electrode active material preparation method 2, which is one embodiment of the present invention and differs from positive electrode active material preparation method 1, will be described. The positive electrode active material preparation method 2 differs from preparation method 1 mainly in the number of times the additive elements are added and the mixing method. The rest of the description can be taken into consideration as in preparation method 1.

[0199] In FIG. 2, steps S11 to S15 are carried out in the same manner as in FIG. 1A to prepare lithium cobalt oxide that has undergone initial heating.

[0200] <Step S20a> Next, as shown in step S20a, it is preferable to add an additional element A1 to the lithium cobalt oxide that has undergone the initial heating.

[0201] <Step S21> In step S21 shown in Fig. 3A, a magnesium source, a nickel source, and a fluoride are prepared. Steps S21 to S23 shown in Fig. 3A can be performed under the same conditions as steps S21 to S23 shown in Fig. 1B. As a result, an additional element source (A1 source) can be obtained in step S23.

[0202] Furthermore, steps S31 to S33 shown in FIG. 2 can be performed in the same manner as steps S31 to S33 shown in FIG. 1A.

[0203] <Step S34a> Next, the material heated in step S33 is recovered, and in step S34a, lithium cobalt oxide containing the additional element A1 is produced. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0204] <Step S40> In step S40 shown in Fig. 2, the additional element A2 is added. This will be described with reference to Fig. 3B as well.

[0205] <Step S41> In step S41 shown in FIG. 3B, a second additive element source is prepared. As the second additive element source, one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, and beryllium can be used. Alternatively, one or more elements selected from bromine and beryllium can be used. For example, as the additive element A2, one or more elements selected from aluminum, titanium, boron, and zirconium can be preferably used.

[0206] 3B illustrates an example in which an aluminum source (Al source) is used as the second additive element source. Examples of the aluminum source that can be used include aluminum fluoride, aluminum hydroxide, and aluminum oxide.

[0207] Here, it is preferable that Ni:Al=w:1 (2≦w≦5).

[0208] Steps S41 to S43 shown in Fig. 3B can be performed under the same conditions as steps S21 to S23 shown in Fig. 1B. As a result, an additional element source (A2 source) can be obtained in step S43.

[0209] The A2 source may contain the same additive element as the Al source.

[0210] 2 can be performed under the same conditions as steps S31 to S34 shown in FIG. 1A. The heating conditions of step S53 may be a lower temperature and a shorter time than those of step S33. Through the above steps, in step S54, the positive electrode active material 100 of one embodiment of the present invention can be produced.

[0211] The concentration distribution of the additive element in the depth direction varies depending on, for example, the heating temperature, the heating time, the rate of temperature increase, the rate of temperature decrease, and the like.

[0212] 2 to 3B, in the manufacturing method 2, the additive element is introduced into the lithium cobalt oxide separately as additive element A1 and additive element A2. By introducing the additive elements separately, the concentration distribution of each additive element in the depth direction can be changed. For example, the additive element A1 can be distributed so that the concentration is higher in the surface layer portion than in the interior, and the additive element A2 can be distributed widely to a deeper region.

[0213] Specifically, for example, magnesium and nickel can be distributed at high concentrations in the surface layer, and aluminum can be distributed widely to a deeper region.

[0214] Alternatively, for example, the additional element A2 can be distributed so that the concentration is higher in the surface layer portion than in the interior, and the additional element A1 can be distributed widely to a deeper region.

[0215] Furthermore, when the additive element A1 and the additive element A2 contain the same element, the element can be distributed at a high concentration in the surface layer portion and at a wider depth, for example, from the surface to the interior of the positive electrode active material. For example, when the additive element A1 and the additive element A2 both contain nickel, the nickel can be distributed at a high concentration in the surface layer portion and at a wider depth, for example, from the surface to the interior of the positive electrode active material.

[0216] 4A and 4B , a cathode active material preparation method 3, which is an embodiment of the present invention and differs from cathode active material preparation method 1, will be described below. The cathode active material preparation method 3 differs from preparation method 1 mainly in that step S20 in step S20 is the step shown in FIG. 4B instead of the step shown in FIG. 1B .

[0217] 4A, steps S11 to S15 are performed to prepare lithium cobalt oxide that has undergone initial heating. For steps S11 to S15, the description of FIG. 1A can be referred to.

[0218] <Step S20b> Next, as shown in step S20b, it is preferable to add an additional element A3 to the lithium cobalt oxide that has undergone the initial heating.

[0219] 4B , the elements added to the lithium cobalt oxide are magnesium, nickel, and one or more selected from aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. Also, one or more selected from bromine and beryllium may be used.

[0220] Here, a magnesium source (Mg source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. It is also preferable to prepare a fluoride together with the magnesium source, nickel source, and aluminum source.

[0221] Lithium fluoride is used as the fluoride, magnesium fluoride is used as the magnesium source, and nickel fluoride is used as the nickel source. For the mixing ratio of lithium fluoride, magnesium fluoride, and nickel fluoride, see, for example, the description of FIG. 1B.

[0222] Steps S21 to S23 shown in Fig. 4B can be performed under the same conditions as steps S21 to S23 shown in Fig. 1B. As a result, an additional element source (A3 source) can be obtained in step S23.

[0223] 4A, steps S31 to S34 are performed. For steps S31 to S34, the description of FIG. 1A can be referred to. As a result, in step S34, the positive electrode active material 100 can be obtained.

[0224] By using the flow shown in Fig. 4A, the process can be simplified compared to when using the process shown in Fig. 2. On the other hand, when using the process shown in Fig. 2, the process of adding the Al source and the A2 source as the additive element can be separated, and the concentration distribution of each additive element can be made different.

[0225] <<Preparation Method 4 of Positive Electrode Active Material>> Next, a description will be given of a preparation method 4 of a positive electrode active material, which is one embodiment of the present invention and which differs from Preparation Method 3 of a positive electrode active material, with reference to FIGS. 5 to 6B . Preparation Method 4 of a positive electrode active material differs mainly in the number of times that additive elements are added and the mixing method. For the other descriptions, the descriptions of Preparation Method 1 can be referred to.

[0226] In FIG. 5, steps S11 to S15 are carried out in the same manner as in FIG. 1A to prepare lithium cobalt oxide that has undergone initial heating.

[0227] <Step S20c> Next, as shown in step S20c, it is preferable to add an additional element A4 to the lithium cobalt oxide that has undergone the initial heating.

[0228] 6A, the elements added to the lithium cobalt oxide are magnesium, nickel, and one or more selected from aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. One or more selected from bromine and beryllium may also be used.

[0229] Here, a magnesium source (Mg source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. It is also preferable to prepare a fluoride together with the magnesium source, nickel source, and aluminum source.

[0230] Steps S21 to S23 shown in Fig. 6A can be performed under the same conditions as steps S21 to S23 shown in Fig. 1B. As a result, an additional element source (A4 source) can be obtained in step S23.

[0231] Furthermore, steps S31 to S33 shown in FIG. 5 can be performed in the same manner as steps S31 to S33 shown in FIG. 1A.

[0232] <Step S34b> Next, the material heated in step S33 is recovered, and in step S34b, lithium cobalt oxide containing the additional element A4 is produced. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0233] <Step S40b> In step S40b shown in Fig. 5, an additional element A5 is added. This will be described with reference to Fig. 6B.

[0234] <Step S41> In step S41 shown in Figure 6B, a second additive element source is prepared. As the second additive element source, one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. Also, one or more elements selected from bromine and beryllium can be selected and used. For example, as the additive element A5, one or more elements selected from titanium, boron, and zirconium can be preferably used.

[0235] Steps S41 to S43 shown in Fig. 6B can be performed under the same conditions as steps S21 to S23 shown in Fig. 1B. As a result, an additional element source (A5 source) can be obtained in step S43.

[0236] The A5 source may contain the same additive elements as the A4 source.

[0237] 5 can be performed under the same conditions as steps S31 to S34 shown in FIG. 1A. The heating conditions of step S53 may be a lower temperature and a shorter time than those of step S33. Through the above steps, in step S54, the positive electrode active material 100 of one embodiment of the present invention can be produced.

[0238] 5 to 6B , in fabrication method 2, the additive element A4 and the additive element A5 are introduced separately into lithium cobalt oxide. By introducing the additive elements separately, the concentration distribution of each additive element in the depth direction can be changed. For example, the additive element A4 can be distributed so that its concentration is higher in the surface layer portion than in the interior, and the additive element A5 can be distributed widely to a deeper region. Alternatively, for example, the additive element A5 can be distributed so that its concentration is higher in the surface layer portion than in the interior, and the additive element A4 can be distributed widely to a deeper region.

[0239] Furthermore, when the additional element A4 and the additional element A5 contain the same element, the element can be distributed at a high concentration in the surface layer portion and also widely distributed in a deeper region.

[0240] <<Method A for Producing Positive Electrode Active Material>> One embodiment of the present invention will be described with reference to FIGS. 7 to 11B. FIG.

[0241] 7, steps S11 to S15 are performed to prepare lithium cobalt oxide that has undergone initial heating. For steps S11 to S15, the description of FIG. 1A can be referred to.

[0242] <Step S20d> Next, in step S20d, it is preferable to add an additional element A6 to the lithium cobalt oxide that has undergone the initial heating. Details of step S20d are shown in FIG. 8A.

[0243] 8A, the elements added to the lithium cobalt oxide are magnesium, nickel, and one or more selected from aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. One or more selected from bromine and beryllium may also be used.

[0244] Here, a nickel source (Ni source) is prepared. It is preferable to prepare a fluoride together with the nickel source.

[0245] The nickel source may be nickel fluoride, nickel oxide, nickel hydroxide, etc. A plurality of the above nickel sources may also be used.

[0246] Examples of fluorides include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), nickel fluoride (NiF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among them, lithium fluoride is preferable because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below. Fluorides can function as a fluorine source.

[0247] Nickel fluoride can be used as both a nickel source and a fluorine source, and lithium fluoride can be used as both a lithium source and a fluorine source. Another lithium source that can be used in step S21 is lithium carbonate.

[0248] The fluorine source may also be gaseous, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F) or the like may be used and mixed into the atmosphere in the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0249] In this embodiment, lithium fluoride (LiF) is prepared as the fluoride, and nickel fluoride (NiF ２ ) to prepare.

[0250] Lithium fluoride and nickel fluoride are LiF:NiF ２ At a molar ratio of 0.8:0.2 or a ratio in the vicinity thereof, the melting point is suitably low and NiF ２ can exist stably as a liquid without becoming excessive.

[0251] On the other hand, if the amount of lithium fluoride is too large, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate.

[0252] The molar ratio of lithium fluoride and nickel fluoride is LiF:NiF ２ = k: 1. For example, k is 0 to 10, or 0 to 5, or 0 to 3, or 0 to 2, or 0 to 1, or 0 to 0.5.

[0253] 7, the magnesium source, the nickel source, and the fluoride are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0254] 7, the pulverized and mixed materials are collected to obtain a source of the additional element A (A source). The source of the additional element A shown in step S23 includes a plurality of starting materials and can be called a mixture.

[0255] 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.

[0256] Such a finely powdered mixture (including the case where only one kind of additive element is contained) can be easily adhered to the surface of lithium cobalt oxide particles when mixed with lithium cobalt oxide in a later step. If the mixture is evenly adhered to the surface of lithium cobalt oxide particles, it is easy to distribute or diffuse the additive element evenly in the surface layer portion of the composite oxide after heating, which is preferable.

[0257] Through the above steps, the additive element source (A6 source) can be obtained in step S23.

[0258] 7, lithium cobalt oxide and an additional element A source (A source) are mixed together. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of nickel atoms Ni in the additional element A source is, for example, Co:Ni=100:m (0.1≦m≦20).

[0259] 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.

[0260] 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.

[0261] <Step S32> Next, in step S32 of Fig. 7, the mixed materials are recovered to obtain a mixture 903. When recovering the materials, they may be crushed and then sieved, if necessary.

[0262] 7 illustrates a fabrication method in which the additive element is added only after the initial heating, but the present invention is not limited to this method. The additive element may be added at a different timing or multiple times. The timing may vary depending on the element.

[0263] For example, an additive element may be added to the lithium source and the cobalt source in step S11, i.e., in the stage of the starting material for the composite oxide. Then, in step S13, lithium cobalt oxide containing the additive element can be obtained. In this case, it is not necessary to separate steps S11 to S14 from steps S21 to S23. This method can be said to be simple and highly productive.

[0264] <Step S33> Next, in step S33 shown in FIG. 7, the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, the pressure inside the furnace may exceed atmospheric pressure in order to increase the oxygen partial pressure in the heating atmosphere. This is because if the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and the like may be reduced, and lithium cobalt oxide and the like may no longer be able to maintain a layered rock salt type crystal structure.

[0265] Here, a supplementary note about the heating temperature will be provided. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 650° C. or higher.

[0266] Of course, the reaction is more likely to proceed if the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF and NiF are used as the additive element source. ２ When LiF and NiF ２ has an extreme value of the eutectic point, for example, near 790° C. The lower limit of the heating temperature in step S33 is preferably set to, for example, 700° C. or higher, and may be 790° C. or higher.

[0267] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0268] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is more preferably 1000°C or lower, and even more preferably 950°C or lower.

[0269] The heating temperature in step S33 is preferably higher than that in step S13.

[0270] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0271] In the manufacturing method described in this embodiment, some materials, such as lithium fluoride, which is a fluorine source, may function as a flux, which allows the heating temperature to be lowered to a temperature lower than the decomposition temperature of lithium cobalt oxide, and allows magnesium and other additive elements to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0272] However, since lithium fluoride has a lower specific gravity in a gaseous state than oxygen, there is a possibility that lithium fluoride will volatilize when heated, and if it volatilizes, the amount of lithium fluoride 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 lithium fluoride. Even if lithium fluoride is not used as a fluorine source, etc., LiCoO ２ There is also a possibility that the Li on the surface reacts with the F in the fluorine source to produce lithium fluoride, which then volatilizes. Therefore, even if a fluoride with a higher melting point than lithium fluoride is used, it is still necessary to suppress volatilization.

[0273] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing lithium fluoride, that is, to heat the mixture 903 in a state where the partial pressure of lithium fluoride is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of lithium fluoride in the mixture 903.

[0274] FIG. 15 shows an example of a heating furnace that can be used for heating. The heating in step S33 is preferably performed while controlling the pressure inside the furnace with a pressure gauge 221. The inside of the furnace is preferably atmospheric pressure or pressurized. If the pressure is applied during heating, the surface of the positive electrode active material 100 becomes smooth, and the positive electrode active material 100 may be more resistant to physical damage caused by pressure than a positive electrode active material that is not smooth. For example, the positive electrode active material 100 may be less likely to be damaged in a test that involves pressure, such as a nail penetration test, which may result in increased safety.

[0275] The heating in this step is preferably performed so as not to cause adhesion between particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additive elements (e.g., fluorine) diffuse is blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0276] Furthermore, it is believed that uniform distribution of an additive element (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the particles of mixture 903 do not adhere to each other.

[0277] 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.

[0278] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing lithium fluoride by placing a lid on a container containing the mixture 903, for example.

[0279] 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.

[0280] 7, 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.

[0281] On the other hand, when the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 650° C. or higher and 950° C. or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 5 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0282] <Step S34c> Next, the material heated in step S33 is recovered, and in step S34c, lithium cobalt oxide containing the additional element A6 is produced. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0283] <Step S40c> In step S40c shown in Fig. 7, an additional element A7 is added. This will be described with reference to Fig. 8B as well.

[0284] <Step S41> In step S41 shown in Figure 8B, a second additive element source is prepared. As the second additive element source, one or more elements selected from magnesium, nickel, aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. Also, one or more elements selected from bromine and beryllium can be selected and used. For example, as the additive element A7, one or more elements selected from titanium, boron, and zirconium can be preferably used.

[0285] 8B illustrates an example in which a magnesium source (Mg source) is prepared as the second additive element source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. A plurality of the above-mentioned magnesium sources may also be used. Here, magnesium fluoride is used as an example. Note that magnesium fluoride can be used both as a magnesium source and a fluorine source.

[0286] In addition to the magnesium source, a fluoride may be prepared. For example, magnesium fluoride may be used as the magnesium source, and lithium fluoride may be used as the fluoride. Lithium fluoride and magnesium fluoride may be used in the form of LiF:MgF２ When the molar ratio is about 65:35, the effect of lowering the melting point is maximized.

[0287] In addition to magnesium, an aluminum source (Al source) may be prepared. As the aluminum source, aluminum fluoride, aluminum hydroxide, aluminum oxide, etc. may be used.

[0288] Steps S41 to S43 can be performed under the same conditions as steps S21 to S23, as a result of which an additional element source (A7 source) can be obtained in step S43.

[0289] The A7 source may contain the same additive elements as the A6 source.

[0290] <Steps S51 to S53> Next, steps S51 to S53 can be performed under the same conditions as steps S31 to S34. The conditions for the heating step in step S53 may be a lower temperature and a shorter time than those in step S33. Through the above steps, in step S54, the positive electrode active material 100 according to one embodiment of the present invention can be produced. For example, LiF, MgF, etc. can be used as the additive element source. ２ and NiF ２ When LiF and MgF ２ The eutectic point of the alloy is around 742° C. Therefore, the lower limit of the heating temperature in step S53 is preferably set to, for example, 700° C. or higher, and may be 740° C. or higher.

[0291] In fabrication method A, the additive element into lithium cobalt oxide is introduced separately as additive element A6 and additive element A7. By introducing them separately, the concentration distribution of each additive element in the depth direction can be changed. For example, additive element A6 can be distributed so that its concentration is higher in the surface layer portion than in the interior, and additive element A7 can be distributed widely to a deeper region. Alternatively, for example, additive element A7 can be distributed so that its concentration is higher in the surface layer portion than in the interior, and additive element A6 can be distributed widely to a deeper region.

[0292] Specifically, for example, nickel can be distributed widely to a deeper region, and magnesium can be concentrated at a higher concentration in the surface layer.

[0293] Furthermore, when the additional element A6 and the additional element A7 contain the same element, the element can be distributed at a high concentration in the surface layer portion and can be widely distributed in a deeper region, for example, from the surface to the interior of the positive electrode active material. For example, when the additional element A6 and the additional element A7 both contain nickel, the nickel can be distributed at a high concentration in the surface layer portion and can be widely distributed in a deeper region, for example, from the surface to the interior of the positive electrode active material.

[0294] <<Method B for Producing Positive Electrode Active Material>> Next, a method B for producing a positive electrode active material, which is one embodiment of the present invention and is different from method A for producing a positive electrode active material, will be described with reference to FIGS. 9 , 10A , 10B , 11A , and 11B .

[0295] In FIG. 9, steps S11 to S15 are carried out in the same manner as in FIG. 7 to prepare lithium cobalt oxide that has undergone initial heating.

[0296] <Step S20e> Next, as shown in step S20e, it is preferable to add an additional element A8 to the lithium cobalt oxide that has undergone the initial heating.

[0297] 10A, the elements added to the lithium cobalt oxide are magnesium, nickel, and one or more selected from aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. One or more selected from bromine and beryllium may also be used.

[0298] Here, a magnesium source (Mg source) and a nickel source (Ni source) are prepared. It is also preferable to prepare a fluoride together with the magnesium source and the nickel source.

[0299] Steps S21 to S23 shown in Fig. 9 can be performed under the same conditions as steps S21 to S23 shown in Fig. 7. As a result, an additional element source (A8 source) can be obtained in step S23.

[0300] 7. Steps S31 to S33 shown in FIG. 9 can be performed in the same manner as steps S31 to S33 shown in FIG.

[0301] <Step S34e> Next, the material heated in step S33 is recovered, and in step S34e, lithium cobalt oxide containing the additional element A8 is produced. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0302] <Step S40d> In step S40d shown in Fig. 9, an additional element A9 is added. This will be described with reference to Fig. 10B.

[0303] <Step S41> In step S41 shown in Figure 10B, a second additive element source is prepared. As the second additive element source, one or more elements selected from magnesium, nickel, aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. Also, one or more elements selected from bromine and beryllium can be selected and used. For example, as the additive element A5, one or more elements selected from titanium, boron, and zirconium can be preferably used.

[0304] Steps S41 to S43 shown in Fig. 10B can be performed under the same conditions as steps S21 to S23 shown in Fig. 7. As a result, an additional element source (A9 source) can be obtained in step S43.

[0305] Here, it is preferable that at least one of the same elements as the additional element A8 is selected as the additional element A9. Here, nickel is selected as the additional element A9. Furthermore, in addition to nickel, aluminum or the like may also be selected as the additional element A9.

[0306] In step S41, a nickel source is prepared. Here, nickel fluoride is used as the nickel source. In step S41, a fluoride such as lithium fluoride may be prepared in addition to the nickel source.

[0307] 9 can be performed under the same conditions as steps S31 to S34 shown in FIG. 7. The heating conditions of step S53 may be a lower temperature and a shorter time than those of step S33. Through the above steps, in step S54, the positive electrode active material 100 of one embodiment of the present invention can be produced.

[0308] 9 , in fabrication method B, the additive element into lithium cobalt oxide is introduced separately as additive element A8 and additive element A9. By introducing the additive elements separately, the concentration distribution of each additive element in the depth direction can be changed. For example, additive element A8 can be distributed so that its concentration is higher in the surface layer portion than in the interior, and additive element A9 can be distributed widely to a deeper region. Alternatively, for example, additive element A9 can be distributed so that its concentration is higher in the surface layer portion than in the interior, and additive element A8 can be distributed widely to a deeper region.

[0309] Here, since both the additive element A8 and the additive element A9 contain nickel, in the positive electrode active material of one embodiment of the present invention, for example, nickel can be distributed so that the concentration is high in the surface layer portion and can also be widely distributed to a deeper region.

[0310] <<Method C of Producing Positive Electrode Active Material>> FIG. 11 shows an example in which step S20e_2 is used instead of step S20e and step S40d_2 is used instead of step S40d in FIG.

[0311] By applying step S20e_2 shown in FIG. 12A as step S20e_2 and step S40d_2 shown in FIG. 12B as step S40d_2, nickel can be selected as the additive element A8 and magnesium and nickel can be selected as the additive element A9.

[0312] 12A, the elements added to the lithium cobalt oxide are magnesium, nickel, and one or more selected from aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron. One or more selected from bromine and beryllium may also be used.

[0313] Here, a nickel source (Ni source) is prepared. It is also preferable to prepare a fluoride such as lithium fluoride together with the nickel source.

[0314] <Step S41> In step S41 shown in Figure 12B, a second additive element source is prepared. As the second additive element source, one or more elements selected from magnesium, nickel, aluminum, fluorine, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. Also, one or more elements selected from bromine and beryllium can be selected and used. For example, as the additive element A5, one or more elements selected from titanium, boron, and zirconium can be preferably used.

[0315] Here, it is preferable that at least one of the same elements as the additional element A8 is selected as the additional element A9. Here, nickel and magnesium are selected as the additional element A9. Furthermore, in addition to nickel and magnesium, aluminum or the like may also be selected as the additional element A9. Furthermore, in step S41, a fluoride such as lithium fluoride may be prepared.

[0316] The positive electrode active material of one embodiment of the present invention has a smooth surface.

[0317] Furthermore, after the initial heating described in this embodiment, a positive electrode active material with a smooth surface can be obtained.

[0318] The initial heating shown in this embodiment is performed on lithium cobalt oxide. Therefore, the initial heating is preferably performed at a temperature lower than that required for obtaining lithium cobalt oxide and for a shorter heating time than that required for obtaining lithium cobalt oxide. The step of adding an additive element to lithium cobalt oxide is preferably performed after the initial heating. This addition step can be performed in two or more steps. Following this order of steps is preferable because it maintains the surface smoothness obtained by the initial heating.

[0319] A positive electrode active material with a smooth surface may be more resistant to physical destruction due to pressure, etc. than a positive electrode active material that does not have a smooth surface. For example, the positive electrode active material 100 may be less likely to be destroyed in a test involving pressure, such as a nail penetration test, which may result in increased safety.

[0320] This embodiment can be used in combination with other embodiments.

[0321] Embodiment 2 In this embodiment, a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS.

[0322] FIGS. 16A and 16B are cross-sectional views of a cathode active material 100 according to one embodiment of the present invention. As shown in FIG. 16A , the cathode active material 100 has a surface layer 100a and an interior 100b. In these figures, a dashed line indicates the boundary between the surface layer 100a and the interior 100b. In FIG. 16A , arrows X1-X2 are shown as an example of the depth direction of a crystal plane other than the (001) plane of lithium cobalt oxide, and arrows Y1-Y2 are shown as an example of the depth direction of a crystal plane that is the (001) plane. FIG. 16B also shows a portion of a grain boundary 101 with a dashed-dotted line. FIG. 16B also shows a cathode active material 100 having an embedded portion 102. E-F in FIG. 16B is a line drawn so as to intersect with the newly generated surface layer 100a due to displacement along the arrow direction. (001) in the figure indicates the (001) plane of lithium cobalt oxide. LiCoO ２ belongs to the space group R-3m.

[0323] FIG. 17 shows LiMeO in the space group R-3m. ２The planes in the crystal structure of the present invention are the (003) plane, (104) plane, (012) plane, (1-12) plane, (101) plane, (110) plane, (2-10) plane, (01-1) plane, (10-2) plane, and (01-4) plane. Examples of surfaces other than the (001) orientation include the (104) plane, (012) plane, (1-12) plane, (101) plane, (110) plane, (2-10) plane, (01-1) plane, (10-2) plane, and (01-4) plane, as well as planes parallel to these planes and equivalent planes. The amount of additional elements detected may be lower in surfaces other than the (001) orientation and in the surface layer 100a thereof compared to the (001) orientation surface.

[0324] 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.

[0325] The region of the positive electrode active material deeper than the surface layer 100a is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0326] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer portion 100a and the inner portion 100b. ２ O ３ This does not include metal oxides having no lithium sites that can contribute to charging and discharging, such as those having metal oxides attached thereto, carbonates chemically adsorbed after the preparation of the positive electrode active material, hydroxyl groups, etc. Note that the attached metal oxides refer to metal oxides whose crystal structure does not match that of the interior 100b, for example.

[0327] It also does not include the electrolyte, organic solvent, binder, conductive material, or compounds derived from these that are attached to the positive electrode active material 100.

[0328] The crystal grain boundary 101 refers to, for example, a portion where particles of the positive electrode active material 100 are adhered to each other, a portion where the crystal orientation changes 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. The crystal defect refers to a defect that can be observed in a cross-sectional TEM (transmission electron microscope), a cross-sectional STEM image, etc., that is, a structure in which other atoms have entered between the lattices, a cavity, etc. The crystal grain boundary 101 can be said to be one type of planar defect. The vicinity of the crystal grain boundary 101 refers to a region within 10 nm of the crystal grain boundary 101.

[0329] <Contained Elements> The positive electrode active material 100 contains lithium, cobalt, oxygen, and an additive element.

[0330] The positive electrode active material 100 is LiMeO ２ (Me is a metal) to which an additive element is added. Alternatively, the positive electrode active material 100 may be LiMeO ２ (Me is a metal) to which an additive element is added. Here, the metal Me is, for example, cobalt. That is, a composite oxide having lithium, cobalt, and oxygen is, for example, LiCoO ２ Alternatively, the metal Me may contain one or two elements selected from nickel and manganese in addition to cobalt. In the metal Me, cobalt preferably accounts for 75 atomic % or more, and more preferably 90 atomic % or more. The positive electrode active material 100 may have a crystal structure as described below, and may be LiMeO ２ The composition is not strictly limited to Li:Me:O=1:1:2.

[0331] More specifically, for example, the positive electrode active material 100 is lithium cobalt oxide (LiCoO ２ ) to which an additive element has been added can be used. Alternatively, for example, the positive electrode active material 100 includes lithium cobalt oxide to which an additive element has been added. Here, the composition of the lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0332] 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 responsible for the oxidation and reduction reaction. In addition to cobalt, one or two metals selected from nickel and manganese may also be used. 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 contained therein, because of its many advantages, such as relatively easy synthesis, ease of handling in the synthesis process, and excellent cycle characteristics.

[0333] Furthermore, when the cobalt content of the transition metals in the positive electrode active material 100 is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal, ｘ CoO ２ The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) accounts for the majority of the transition metal, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel accounts for the majority of the transition metal, are prone to cation mixing between nickel and lithium.

[0334] The additive element contained in the positive electrode active material 100 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. The sum of the transition metals among the additive elements is preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0335] That is, the positive electrode active material 100 may include lithium cobalt oxide to which magnesium and fluorine are added, lithium cobalt oxide to which magnesium, fluorine and titanium are added, lithium cobalt oxide to which magnesium, fluorine and aluminum are added, lithium cobalt oxide to which magnesium, fluorine and nickel are added, lithium cobalt oxide to which magnesium, fluorine, nickel and aluminum are added, and the like.

[0336] The additive element is preferably in a solid solution in the positive electrode active material 100. When the additive element is in a solid solution in the positive electrode active material 100, for example, the additive element is also detected at a depth where the metal Me is detected as a main component. Therefore, for example, when performing a linear analysis using STEM-EDX, the depth at which the amount of the additive element detected increases is preferably located deeper than the depth at which the amount of the metal Me detected increases, i.e., closer to the interior of the positive electrode active material 100. Here, when analysis is performed from an arbitrary position outside the surface of the positive electrode active material 100 toward the inside of the positive electrode active material 100, when the surface is reached, the metal Me, which is the main component, begins to be detected, that is, the amount detected increases.

[0337] 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.

[0338] As will be described later, these additive elements further stabilize the crystal structure of the positive electrode active material 100. In this specification and the like, the additive element has the same meaning as a mixture or a part of a raw material.

[0339] Note that the cathode active material of one embodiment of the present invention may not contain any of the additive elements exemplified above, or the concentration of the additive element may be extremely low. 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 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.

[0340] <Crystal structure> <Li ｘ CoO ２ When x is 1 in the positive electrode active material 100 of one embodiment of the present invention is in a discharged state, that is, Li ｘ CoO ２ When x = 1 in the formula, it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have high discharge capacity, have two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies the majority of the volume of the positive electrode active material 100, has a layered rock-salt type crystal structure. Figure 22 shows the layered rock-salt type crystal structure, labeled R-3m O3. R-3m O3 has lattice constants a = 2.81610, b = 2.81610, c = 14.05360, α = 90.0000, β = 90.0000, and γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0,0,0), Co(0,0,0.5), and O(0,0,0.23951) (Non-Patent Document 6).

[0341] On the other hand, the surface layer portion 100a of the cathode active material 100 according to one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b so that the layered structure of cobalt and oxygen octahedra is not destroyed even when lithium is removed from the cathode active material 100 upon charging. Alternatively, the surface layer portion 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer portion 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer portion 100a and inner portion 100b of the cathode active material 100, such as oxygen desorption and / or shifting of the layered structure of cobalt and oxygen octahedra. And / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0342] 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.

[0343] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the interior 100b. In addition, the atoms on the surface of the particles of the positive electrode active material 100 in the surface layer 100a can be said to be in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and can be said to be a region where deterioration of the crystal structure is likely to begin. For example, if the crystal structure of the layered structure consisting of octahedra of cobalt and oxygen in the surface layer 100a is displaced, this influence is transmitted to the interior 100b, causing the layered crystal structure in the interior 100b to also be displaced, which is thought to lead to deterioration of the crystal structure of the entire positive electrode active material 100. On the other hand, if the surface layer 100a can be sufficiently stabilized, Li ｘ CoO ２Even when x is small, for example, 0.24 or less, the layered structure of cobalt and oxygen octahedra in the interior 100b can be made less likely to break.Furthermore, displacement of the layers of cobalt and oxygen octahedra in the interior 100b can be suppressed.

[0344] [Distribution] To provide the surface layer 100a with a stable composition and crystalline structure, the surface layer 100a preferably contains an additive element, and more preferably contains multiple additive elements. Furthermore, the surface layer 100a preferably has a higher concentration of one or more selected additive elements than the interior 100b. Furthermore, the one or more selected additive elements contained in the positive electrode active material 100 preferably have a concentration gradient. Furthermore, the distribution of the positive electrode active material 100 preferably differs depending on the additive element. For example, it is more preferable that the depth of the peak of the detected amount in the surface layer from the surface or the reference point in EDX-ray analysis described below differs depending on the additive element. Here, the detected amount peak refers to the maximum value of the detected amount in the surface layer 100a or within 50 nm from the surface. The detected amount refers to, for example, the count in EDX-ray analysis.

[0345] 16 shows an example of the depth direction of a crystal plane other than the (001) plane of lithium cobalt oxide in positive electrode active material 100 of one embodiment of the present invention, indicated by arrows X1-X2. Examples of profiles of each added element obtained by EDX analysis along arrows X1-X2 are shown in FIGS.

[0346] 18A shows distribution D1 (dashed line) and distribution D2 (dash-dotted line) as examples of additive element profiles. Distribution D1 has a narrow peak of detected amounts in a region closer to the surface of the surface layer portion 100a. Distribution D2 has a wider distribution of detected amounts than distribution D2. Distribution D2 has a peak of detected amounts, for example, in a region deeper than distribution D1. The distribution of detected amounts in distribution D2 may extend to the interior 100b. Distribution D2 may have a peak of detected amounts in the surface layer portion 100a or in the interior 100b.

[0347] 18B and 18C show examples of magnesium and nickel profiles. As shown in FIGS. 18B and 18C, the detected amount of magnesium and nickel is preferably greater in the surface layer portion 100a than in the interior portion 100b. Furthermore, it is preferable that the detected amount peak be narrower in the surface layer portion 100a in a region 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. Here, the "surface" corresponds to the position where the spectrum of metal Me, the main component of lithium cobalt oxide, rises in EDX line analysis from any position outside the surface of the positive electrode active material 100 toward the interior of the positive electrode active material 100.

[0348] Furthermore, it is preferable that the distributions of magnesium and nickel overlap. The peaks of the detected amounts of magnesium and nickel may be at the same depth, or the magnesium peak may be closer to the surface, or the nickel peak may be closer to the surface as shown in Figure 18C. The difference in depth between the peak of detected amount of nickel and the peak of detected amount of magnesium is preferably within 3 nm, and more preferably within 1 nm.

[0349] Furthermore, the amount of nickel detected in the inner portion 100b may be very small compared to the surface portion 100a, or may not be detected, i.e., may be below the lower detection limit.

[0350] Although not shown, it is preferable that the detectable amount of fluorine in the surface layer portion 100a is greater than the detectable amount inside, similar to magnesium or nickel. It is also preferable that the detectable amount peak in the surface layer portion 100a is closer to the surface. For example, it is preferable that the detectable amount peak is on the surface or within 3 nm from the reference point. Similarly, it is preferable that the detectable amount of titanium, silicon, phosphorus, boron, and / or calcium is greater than the detectable amount inside the surface layer portion 100a. It is also preferable that the detectable amount peak is on the surface or within 3 nm from the reference point.

[0351] Furthermore, it is preferable that at least aluminum, among the added elements, has a peak of detected amount further inside than magnesium. The distributions of magnesium and aluminum may overlap, or there may be little overlap between the distributions of magnesium and aluminum. The peak of detected amount of aluminum may be present in the surface layer 100a, or may be deeper than the surface layer 100a. For example, it is preferable that the peak is present in a region of 5 nm to 30 nm from the surface or the reference point toward the inside. For example, distribution D2 shown in FIG. 18A can be referred to as the distribution of aluminum.

[0352] The distribution of aluminum may not be a normal distribution. For example, if the distribution of aluminum is expressed as a maximum value Max Ａｌ When the sample is divided by the maximum amount of aluminum detected, the length of the skirt may differ between the surface side and the inner side. Ａｌ ) 1 / 5 height (1 / 5 Max Ａｌ ) is divided in half by a perpendicular line drawn from the maximum value to the horizontal axis, the peak width W ｓ The inner peak width W ｃ may be large.

[0353] It is speculated that aluminum has a faster diffusion rate than magnesium, and that aluminum is distributed deeper into the interior than magnesium. On the other hand, the amount of aluminum detected in the region closest to the surface is low because aluminum can exist 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.

[0354] 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 Li, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. ＋ is Mg ２＋ The valence change due to substitution to Co ３＋ From Co ２＋However, since Al can only be trivalent, it is thought that it is difficult for it to coexist with magnesium in a rock salt or layered rock salt structure.

[0355] Although not shown, it is preferable that manganese has a peak of detectable amount inside magnesium, similar to aluminum.

[0356] The additive element may have a peak of detectable amount in the surface layer and also be widely distributed in the interior. The additive element may have a first peak of detectable amount in the surface layer and also be widely distributed in the interior. Figure 19A shows an example in which nickel has a first peak of detectable amount in the surface layer and is widely distributed in the interior, with a second peak of detectable amount in the interior. The nickel profile shown in Figure 19A can be represented, for example, by the sum of distribution D1 and distribution D2 shown in Figure 18A.

[0357] In addition, the additive element may have a first peak of detectable amount in the surface layer portion and also be widely distributed in the interior, with the detectable amount gradually decreasing from the surface layer portion and no clear peak in the interior. Figure 19B shows an example in which nickel has a first peak of detectable amount in the surface layer portion and is widely distributed in the interior, with the detectable amount gradually decreasing from the surface layer portion and no clear peak in the interior.

[0358] In addition, the additive element may have a first peak of detectable amount in the surface layer portion, and may be widely distributed in the interior, with little change in concentration or a roughly uniform concentration. Figure 19C shows an example in which nickel has a first peak of detectable amount in the surface layer portion, and is widely distributed in the interior, with little change in concentration or a roughly uniform concentration.

[0359] The magnesium may have a first peak of detectable amount in the surface layer and a second peak of detectable amount in the interior. Alternatively, the magnesium may have a first peak of detectable amount in the surface layer and be widely distributed in the interior, with the detectable amount gradually decreasing from the surface layer and no clear peak in the interior. Alternatively, the magnesium may have a first peak of detectable amount in the surface layer and be widely distributed in the interior, with little change in concentration or a roughly uniform concentration in the interior.

[0360] Alternatively, magnesium may be widely distributed inside, with only a small amount of magnesium detected in the region closest to the surface.

[0361] Alternatively, nickel may be widely distributed inside, with only a small amount of nickel detected in the region closest to the surface.

[0362] Furthermore, aluminum may have a first peak of detectable amount in the surface layer and a second peak of detectable amount in the interior. Alternatively, aluminum may have a first peak of detectable amount in the surface layer and be widely distributed in the interior, with the detectable amount gradually decreasing from the surface layer and no clear peak in the interior. Alternatively, aluminum may have a first peak of detectable amount in the surface layer and be widely distributed in the interior, with little change in concentration or a roughly uniform concentration in the interior.

[0363] The (001)-oriented surface may have a different distribution of additive elements than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a lower detectable amount of one or more additive elements compared to surfaces other than the (001)-oriented surface. Specifically, the detectable amount of nickel may be low. Alternatively, the (001)-oriented surface and its surface layer 100a may have a detectable amount of one or more additive elements below the lower detection limit. Specifically, the detectable amount of nickel may be below the lower detection limit. In particular, with analytical methods that detect characteristic X-rays such as EDX, the Kβ of cobalt and the Kα of nickel are close in energy, making it difficult to detect trace amounts of nickel in materials where cobalt is the primary element. Alternatively, the (001)-oriented surface and its surface layer 100a may have a peak of the detectable amount of one or more additive elements shallower from the surface than surfaces other than the (001)-oriented surface. Specifically, the peaks of the detectable amounts of magnesium and aluminum may be shallower than those of other surfaces.

[0364] 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 therefore the diffusion path of lithium ions is also parallel to the (001) plane.

[0365] CoO ２ Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0366] 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.

[0367] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, it is preferable that the additional element is detected on the surface other than the (001) orientation and on the surface layer 100a thereof. Among the additional elements, it is particularly preferable that nickel be detected on the surface other than the (001) orientation and on the surface layer 100a thereof. On the other hand, the concentration of the additional element may be low or absent on the (001)-oriented surface and on the surface layer 100a thereof, as described above.

[0368] For example, the magnesium distribution in the (001)-oriented surface and its surface layer 100a preferably has a half-width of 10 nm to 200 nm, more preferably 50 nm to 150 nm, and even more preferably 80 nm to 120 nm. Also, the magnesium distribution in the non-(001)-oriented surface and its surface layer 100a preferably has a half-width of more than 200 nm to 500 nm, more preferably more than 200 nm to 300 nm, and even more preferably 230 nm to 270 nm.

[0369] Furthermore, the nickel distribution in the surface that is not (001) oriented and in the surface layer 100a thereof preferably has a half-width of 30 nm or more and 150 nm or less, more preferably 50 nm or more and 130 nm or less, and even more preferably 70 nm or more and 110 nm or less.

[0370] [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 CoO 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.

[0371] At an appropriate concentration, magnesium does not adversely affect the lithium intercalation and deintercalation processes during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect lithium intercalation and deintercalation. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying both the lithium and cobalt sites. Furthermore, unnecessary magnesium compounds (e.g., oxides and fluorides) that do not substitute for either the lithium or cobalt sites may segregate on the surface of the positive electrode active material and become resistance components in secondary batteries. Furthermore, as the magnesium concentration in the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0372] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.002 to 0.06 times the number of cobalt atoms, more preferably 0.005 to 0.03 times, and even more preferably about 0.01 times. The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0373] [Nickel] Nickel is LiMeO ２In the layered rock salt crystal structure of , nickel can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, its redox potential is lower than that of cobalt, so it is easier to release lithium and electrons during charging, for example. This can be expected to result in faster charge and discharge speeds. Therefore, even at the same charge voltage, a larger charge capacity can be obtained when the transition metal M is nickel than when it is cobalt.

[0374] Furthermore, when nickel exists at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the volume change caused by charging and discharging is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel existing at the lithium site can also be prevented from shifting to CoO ２ This is presumably because they function as pillars supporting the layers together, which is preferable because it is expected that the crystal structure will be more stable especially in a charged state at high temperatures, for example, 45° C. or higher.

[0375] 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.

[0376] In addition, the order of ionization tendency is greatest for magnesium, aluminum, cobalt, and nickel. Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements mentioned above during charging. Therefore, nickel is thought to be highly effective in stabilizing the crystalline structure of the surface layer when in a charged state.

[0377] Furthermore, nickel is Ni ２＋ , Ni ３＋ , Ni ４＋ Of which Ni ２＋ is the most stable, and nickel has a lower 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.

[0378] On the other hand, excessive nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and excessive nickel may also adversely affect lithium insertion and extraction.

[0379] 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.

[0380] [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.

[0381] 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.

[0382] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0383] 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.

[0384] [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.

[0385] [Other Additive Elements] Titanium oxide is known to have superhydrophilic properties. Therefore, by providing the cathode active material 100 with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 has good wettability with highly polar solvents. When used in a secondary battery, this improves the contact at the interface between the cathode active material 100 and a highly polar electrolyte, which may suppress an increase in internal resistance.

[0386] Furthermore, when phosphorus is present in the surface layer 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.

[0387] 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.

[0388] The electrolyte is LiPF ６ In the case where the electrolyte contains PVDF, there is a risk of hydrogen fluoride being generated by hydrolysis. Furthermore, hydrogen fluoride may also be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 may be suppressed. Furthermore, there is a risk of suppressing a decrease in adhesion due to gelation and / or insolubilization of PVDF.

[0389] 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.

[0390] Furthermore, when the positive electrode active material 100 has a crack, the progression of the crack can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the crack on the surface, for example, in the embedded portion 102.

[0391] [Synergistic Effect of Multiple Added Elements] Furthermore, when the surface layer portion 100a contains both magnesium and nickel, there is a possibility that divalent nickel can exist more stably near divalent magnesium. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0392] Furthermore, having additive elements with different distributions in combination is preferable because it can stabilize the crystal structure in a wider region. For example, if the positive electrode active material 100 has both magnesium and nickel distributed in a region closer to the surface of the surface layer 100a and aluminum distributed in a deeper region, it can stabilize the crystal structure in a wider region than if it had only one of these elements. In this way, when the positive electrode active material 100 has additive elements with different distributions in combination, aluminum is not essential to the surface because surface stabilization can be sufficiently achieved by magnesium, nickel, etc. Rather, it is preferable for aluminum to be widely distributed in a deeper region. For example, it is preferable for aluminum to be continuously detected in a region from 1 nm to 25 nm in the depth direction from the surface. A wide distribution in a region from 0 nm to 100 nm from the surface, preferably a region from 0.5 nm to 50 nm from the surface, is preferable because it can stabilize the crystal structure in a wider region.

[0393] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100 a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0394] However, if the surface layer 100a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO, a structure in which MgO and NiO(II) are solid-solved, and / or a structure in which MgO and CoO(II) are solid-solved. Therefore, the surface layer 100a must contain at least cobalt, and in the discharged state, it must also contain lithium, and it must have a path for the insertion and extraction of lithium.

[0395] In order to ensure sufficient paths for lithium insertion and desorption, the surface layer portion 100a preferably has a higher cobalt concentration than magnesium. For example, when measured from the surface of the positive electrode active material 100 by XPS, the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is preferably 0.62 or less. The surface layer portion 100a preferably has a higher cobalt concentration than nickel. The surface layer portion 100a preferably has a higher cobalt concentration than aluminum. The surface layer portion 100a preferably has a higher cobalt concentration than fluorine.

[0396] 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.

[0397] Furthermore, although it is preferable that some of the added elements, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they may also be present randomly and in small amounts 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.

[0398] 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.

[0399] For example, it is preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type.Alternatively, it is preferable that the orientation of the surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are approximately the same.

[0400] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as vacancies of cations or anions may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a structure in which the lattice of the rock-salt crystal is distorted.

[0401] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0402] 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.

[0403] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO 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.

[0404] 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.

[0405] 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.

[0406] 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.

[0407] 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.

[0408] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, FFT patterns of TEM images, STEM images, etc. XRD (X-ray diffraction), electron beam diffraction, neutron beam diffraction, etc. can also be used as materials for the judgment.

[0409] 20 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.

[0410] 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 FIG. 20) ＲＳ and L ＬＲＳ When the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0411] 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.

[0412] 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.

[0413] 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.

[0414] Figure 21A shows an example of an STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS roughly coincide. Figure 21B shows the FFT pattern of the region of the rock salt crystal RS, and Figure 21C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of Figures 21B and 21C shows the composition, JCPDS card number, and the d value and angle calculated from these. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0415] The spot marked A in Figure 21B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 21C is derived from the 0003 reflection of the layered rock salt type. From Figures 21B and 21C, 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 21B is roughly parallel to the line passing through AO in Figure 21C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5 degrees or less, or 2.5 degrees or less.

[0416] 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.

[0417] Furthermore, as described above, when the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt crystal may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock salt crystal. For example, the spot marked B in FIG. 21C originates from the 1014 reflection of the layered rock salt crystal. 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 (A in FIG. 21C) originating from the 0003 reflection of the layered rock salt crystal, and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be identical. For example, reciprocal lattice points equivalent to 0003 and 1014 may also be used.

[0418] 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. 21B is originating 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. 21B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to match this. For example, a reciprocal lattice point equivalent to 11-1 and 200 may also be used.

[0419] It is known that layered rock-salt type positive electrode active materials, including lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material using an SEM or the like, it is possible to thin-section the observation sample using an FIB or the like so that the electron beam is [12-10] incident in a TEM or the like, making the (0003) plane easier to observe. When determining whether the crystal orientation is consistent, it is preferable to thin-section the layered rock-salt type so that the (0003) plane can be easily observed.

[0420] <Li ｘ CoO ２The positive electrode active material 100 of one embodiment of the present invention has the above-described distribution of the additive element and / or the crystal structure in a discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0421] Using Figures 22 to 27, Li ｘ CoO ２ The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0422] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 23. The conventional positive electrode active material shown in FIG. 23 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 4, etc.

[0423] Figure 23 shows the R-3m O3 and 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.

[0424] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5, and belongs to the monoclinic (denoted as monoclinic in the figure) space group P2 / m. This structure has CoO ２ There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0425] When x = 0, the positive electrode active material has a crystal structure of the trigonal (denoted as trigonal in the figure) space group P-3m1, and also contains CoO in the unit cell. 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.

[0426] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m O ２ It can also be said that the structure of and the structure of are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 crystal structure. Note that, 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 23 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.

[0427] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in a unit cell can be expressed as Co (0,0,0.42150±0.00016), O1 (0,0,0.27671±0.00045), and O2 (0,0,0.11535±0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0428] Li ｘ CoO ２When 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.

[0429] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 23, 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.

[0430] Furthermore, the difference in volume between these two crystal structures is large. Lithium cobalt oxide exhibits a significant change in the depth of charge, i.e., Li ｘ CoO ２ The crystal structure and the volume of the unit cell change according to the change in x in the matrix. Figure 24 shows the change in the c-axis length of conventional lithium cobalt oxide described in Non-Patent Document 4. The round markers indicate the hexagonal phase, and the diamond markers indicate the monoclinic phase. In the H1-3 phase, the c-axis length contracts, as shown by the diamond marker in Figure 24. The phase transition from O3 to H1-3 phase is a phase transition accompanied by the release of lithium ions, so it is thought that the phase transition occurs from the surface of the positive electrode active material, which is the region where lithium ions are first released, but it can eventually spread to the entire positive electrode active material. Furthermore, even if an additive element is present, if its distribution is insufficient, Li ｘ CoO ２ When x in the formula is about 0.2, it becomes H1-3. For example, if the maximum magnesium concentration in the surface layer is less than 1 atomic %, it is thought that the c-axis length shrinks and the alloy does not become O3'.

[0431] The change in the c-axis length of lithium cobalt oxide corresponds to the change in the angle at which a peak of, for example, the (003) plane of lithium cobalt oxide appears in the XRD pattern. It is known that in XRD using CuKα1 radiation, the peak of the (003) plane of lithium cobalt oxide appears at 2θ of around 19° to 20°.

[0432] Therefore, when compared per the same number of cobalt atoms, the difference in volume between the H1-3 type crystal structure and the discharged R-3m O3 type crystal structure exceeds 3.5%, typically 3.9% or more.

[0433] 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.

[0434] 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.

[0435] On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２ The layer misalignment can be reduced. Furthermore, the change in volume per cobalt atom can be reduced. Therefore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and excellent cycle characteristics can be achieved. Furthermore, the positive electrode active material 100 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. ｘ CoO ２ When x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO 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.

[0436] Li ｘ CoO ２The crystal structures of the interior 100b of the positive electrode active material 100 when x is 1, approximately 0.2, and approximately 0.15 are shown in Fig. 22. The interior 100b occupies the majority of the volume of the positive electrode active material 100 and is the part that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0437] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m O3 as conventional lithium cobalt oxide.

[0438] 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.

[0439] The positive electrode active material 100 according to one embodiment of the present invention when x is about 0.2 has a crystal structure belonging to the trigonal space group R-3m. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 22 with the notation R-3m O3'.

[0440] In the O3'-type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. Furthermore, the lattice constant of the unit cell is preferably 2.797≦a≦2.837 (Å), more preferably 2.807≦a≦2.827 (Å), typically a=2.817 (Å). The c-axis is preferably 13.681≦c≦13.881 (Å), more preferably 13.751≦c≦13.811 (Å), typically c=13.781 (Å).

[0441] Furthermore, the positive electrode active material 100 of one embodiment of the present invention when x is about 0.15 has a crystal structure belonging to the monoclinic space group P2 / m. ２ There is one layer. In addition, the 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 22 with the P2 / m monoclinic O1(15) symbol.

[0442] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ The lattice constants of the unit cell are a = 4.880 ± 0.05 Å, b = 2.817 ± 0.05 Å, c = 4.839 ± 0.05 Å, α = 90°, β = 109.6 ± 0.1°, and γ = 90°.

[0443] This crystal structure can also show the lattice constant in the space group R-3m if some error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constants of the unit cell are a = 2.817 ± 0.02 Å and c = 13.68 ± 0.1 Å.

[0444] 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.

[0445] As shown by the dotted line in FIG. 22, the CoO ２ There is almost no layer misalignment.

[0446] 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%.

[0447] 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%.

[0448] Table 1 shows the difference in volume per cobalt atom between discharged R-3m O3, O3', monoclinic O1(15), H1-3, and trigonal O1. Regarding the lattice constants of each crystal structure used in the calculations in Table 1, literature values ​​can be referenced for discharged R-3m O3 and trigonal O1 (ICSDcoll.code.172909 and 88721). For H1-3, Non-Patent Document 3 can be referenced. For O3' and monoclinic O1(15), the lattice constants can be calculated from experimental XRD values.

[0449]

[0450] As described above, in the positive electrode active material 100 according to one embodiment of the present invention, Li ｘ CoO ２ When x is small, i.e., when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material 100 is resistant to collapse of its crystal structure even when repeatedly charged and discharged so that x is 0.24 or less. Therefore, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0451] The positive electrode active material 100 is Li ｘ CoO 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 CoO ２It has been confirmed that when x is greater than 0.1 and less than 0.2, typically when x is 0.15 or more and less than 0.17, the crystal structure may be monoclinic O1(15) type. However, the crystal structure is Li ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0452] Therefore, the positive electrode active material 100 is Li ｘ CoO ２ When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material 100 may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or some may be amorphous.

[0453] Also Li ｘ CoO ２ To make the x in the cell small, it is generally necessary to charge the cell at a high charging voltage. ｘ CoO ２ The state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6 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 4.6 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, charging voltages are expressed relative to the potential of lithium metal.

[0454] Therefore, the positive electrode active material 100 of one embodiment of the present invention can be said to be preferable because it can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher at 25° C. In other words, it can be said to be preferable because it can adopt an O3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower at 25° C. In other words, it can be said to be preferable because it can adopt an even higher monoclinic O1(15)-type crystal structure when charged at a higher charging voltage, for example, a voltage of more than 4.7 V and 4.8 V or lower at 25° C.

[0455] Even with the positive electrode active material 100, an H1-3 crystal structure may be finally observed when the charge voltage is further increased. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or more and less than 4.6 V at 25° C., the positive electrode active material 100 of one embodiment of the present invention may be able to adopt an O3′ crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.7 V or less at 25° C., the positive electrode active material 100 may be able to adopt a monoclinic O1(15) crystal structure.

[0456] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0457] In addition, in O3' and monoclinic O1(15) in FIG. 22, 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.

[0458] The crystal structure of O3' and monoclinic O1(15) type has random lithium between layers, but CdCl２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0459] <Grain Boundary> In addition to the above distribution, at least a part of the additive element contained in the positive electrode active material 100 of one embodiment of the present invention is preferably unevenly distributed in the grain boundary 101 and its vicinity.

[0460] 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.

[0461] For example, it is preferable that the magnesium concentration at and near the grain boundaries 101 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 101 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 101 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 101 is higher than that in other regions of the interior 100b.

[0462] The grain boundaries 101 are one type of planar defect. Therefore, like the grain surfaces, they tend to become unstable and are prone to change in the crystal structure. Therefore, if the concentration of the added element at and near the grain boundaries 101 is high, the change in the crystal structure can be more effectively suppressed.

[0463] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundary 101, even if cracks occur along the grain boundary 101 in the positive electrode active material 100 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the cracks occur.

[0464] <Particle size> If the particle size of the positive electrode active material 100 of one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector may occur. On the other hand, if the particle size is too small, problems such as excessive reaction with the electrolyte may occur. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0465] Furthermore, using a mixture of particles with different particle sizes in the positive electrode can increase the electrode density, which is preferable because it allows for a secondary battery with high energy density. Positive electrode active material 100 with a relatively small particle size is expected to have high charge / discharge rate characteristics. Positive electrode active material 100 with a relatively large particle size is expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0466] <Analysis method> A certain positive electrode active material is ｘ CoO ２ When x in the formula (I) is small, it can be determined whether the positive electrode active material 100 of one embodiment of the present invention has an O3′-type and / or monoclinic O1(15)-type crystal structure by Li ｘ CoO 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.

[0467] 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.

[0468] When analyzing the crystallite size by powder XRD, it is preferable to measure the crystallite size while excluding the influence of orientation due to pressure, etc. For example, it is preferable to take out the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, prepare a powder sample, and then measure the sample.

[0469] As described above, the positive electrode active material 100 according to one embodiment of the present invention is ｘ CoO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. Materials in which the crystal structure that undergoes large changes when charged at high voltages accounts for 50% or more of the crystal structure are not preferable because they cannot withstand high-voltage charging and discharging.

[0470] It should also be noted that simply adding an additive element may not result in an O3' or monoclinic O1(15) crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additive element, Li ｘ CoO ２ In some cases, x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases, the H1-3 type crystal structure accounts for 50% or more.

[0471] Furthermore, even in the positive electrode active material 100 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the positive electrode active material 100 of one embodiment of the present invention is the positive electrode active material 100, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0472] 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.

[0473] Furthermore, whether or not the distribution of the additive elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0474] The crystal structure of the surface layer 100 a, the grain boundaries 101 , etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0475] <Charging Method> Charging for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed by preparing a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and charging the coin cell.

[0476] 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.

[0477] 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.

[0478] 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).

[0479] The separator may be a 25 μm thick porous polypropylene film.

[0480] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0481] The coin cell prepared under the above conditions is charged at a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). The charging method is not particularly limited as long as charging 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.

[0482] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge / discharge. For example, charging may be performed by constant current charging at a current value of 20 mA / g or more and 100 mA / g or less up to a given voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 2 mA / g or more and 10 mA / g or less, and then constant current discharging at 2.5 V and 20 mA / g or more and 100 mA / g or less.

[0483] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at 2.5 V with a current value of 20 mA / g or more and 100 mA / g or less.

[0484] <<XRD>> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １ Line output: 40 kV, 40 mA Divergence angle: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0485] 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.

[0486] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ Ideal powder XRD patterns based on the line are shown in Figures 25, 26, 27A, and 27B. ｘ CoO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 27A and 27B 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 27A showing an enlarged view of the region where 2θ is in the range of 18° to 21°, and Figure 27B showing an enlarged view of the region where 2θ is in the range of 42° to 46°. ２ (O3) and CoO ２The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from the Inorganic Crystal Structure Database (ICSD) (see Non-Patent Document 5). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０ m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of the positive electrode active material of one embodiment of the present invention, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same manner as the others.

[0487] As shown in Figures 25, 27A and 27B, 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°).

[0488] 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).

[0489] However, as shown in Figures 26, 27A and 27B, 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.

[0490] 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.

[0491] The positive electrode active material 100 according to one embodiment of the present invention is Li ｘ CoO ２ When x in the formula is small, the particles have an O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the particles have an O3'-type and / or monoclinic O1(15)-type crystal structure. Other crystal structures may be included, or some may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0492] Similarly, when Rietveld analysis is performed, the H1-3 type and O1 type crystal structures are preferably 50% or less, more preferably 34% or less, and even more preferably substantially not observed.

[0493] 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.

[0494] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. For example, a narrow full width at half maximum is preferable. Even for peaks arising from the same crystalline phase, the half-width varies depending on the XRD measurement conditions and the value of 2θ. Under the above-mentioned measurement conditions, for peaks observed between 2θ = 43° and 46°, the full width at half maximum is preferably, for example, 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of that crystalline phase is high. Such high crystallinity contributes to sufficient stabilization of the crystal structure after charging.

[0495] 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.

[0496] 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.

[0497] In the positive electrode active material, the range of the nickel and manganese ratios and lattice constants in which the influence of the Jahn-Teller effect is presumed to be small is considered using XRD analysis.

[0498] FIG. 28 shows the results of calculating the a-axis and c-axis lattice constants using XRD for a positive electrode active material 100 of one embodiment of the present invention having a layered rock-salt crystal structure and containing cobalt and nickel. FIG. 28A shows the a-axis result, and FIG. 28B shows the c-axis result. Note that the XRD patterns used in these calculations are for powder after synthesis of the positive electrode active material, but before incorporation into the positive electrode. The nickel concentration on the horizontal axis represents the nickel concentration when the sum of the number of cobalt and nickel atoms is taken as 100%. The positive electrode active material was prepared according to the preparation method shown in FIG. 23, except that an aluminum source was not used.

[0499] FIG. 29 shows the results of estimating the a-axis and c-axis lattice constants using XRD for a positive electrode active material 100 according to one embodiment of the present invention, which has a layered rock-salt crystal structure and contains cobalt and manganese. FIG. 29A shows the a-axis result, and FIG. 29B shows the c-axis result. Note that the lattice constants shown in FIG. 29 were obtained by XRD measurement of a powder obtained after synthesis of the positive electrode active material and before incorporation into a positive electrode. The manganese concentration on the horizontal axis indicates the manganese concentration when the sum of the number of cobalt and manganese atoms is taken as 100%. The positive electrode active material was prepared according to the preparation method shown in FIG. 23, except that a manganese source was used instead of a nickel source and no aluminum source was used.

[0500] Figure 28C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in Figures 28A and 28B. Figure 29C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in Figures 29A and 29B.

[0501] 28C shows that the a-axis / c-axis tends to change significantly at nickel concentrations of 5% and 7.5%, with the a-axis distortion increasing at 7.5% nickel. This distortion may be due to the Jahn-Teller distortion of trivalent nickel. This suggests that an excellent positive electrode active material with small Jahn-Teller distortion can be obtained at nickel concentrations of less than 7.5%.

[0502] Next, Figure 29A suggests that when the manganese concentration is 5% or more, the behavior of the change in lattice constant is different and does not follow Vegard's law. Therefore, it is suggested that when the manganese concentration is 5% or more, the crystal structure is different. Therefore, the manganese concentration is preferably 4% or less, for example.

[0503] 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.

[0504] 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.

[0505] 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.

[0506] 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°.

[0507] <<XPS>> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), so the concentration of each element can be quantitatively analyzed in a region about half the depth of the surface layer 100a. Furthermore, narrow scan analysis can be used to analyze the bonding state of the elements. The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, depending on the element.

[0508] In the cathode active material 100 according to one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer portion 100a than in the interior portion 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average concentration throughout the cathode active material 100. Therefore, for example, it can be said that the concentration of one or more selected additive elements in the surface layer portion 100a measured by XPS or the like is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer portion 100a measured by XPS or the like is preferably higher than the magnesium concentration throughout the cathode active material 100. Furthermore, the nickel concentration in at least a portion of the surface layer portion 100a is preferably higher than the 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 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 fluorine concentration in the entire positive electrode active material 100.

[0509] 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.

[0510] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0511] The concentration of the added element may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances chemisorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 to 1.5. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 to 0.06.

[0512] Similarly, in order to ensure sufficient lithium insertion / extraction paths, the positive electrode active material 100 preferably has a higher concentration of lithium and cobalt in the surface layer portion 100a than the concentrations of the respective additive elements. This means that the concentrations of lithium and cobalt in the surface layer portion 100a are preferably higher than the concentrations of one or more additive elements selected from the additive elements contained in the surface layer portion 100a as measured by XPS or the like. For example, the concentration of cobalt in at least a portion of the surface layer portion 100a as measured by XPS or the like is preferably higher than the concentration of magnesium in at least a portion of the surface layer portion 100a as measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. Furthermore, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. Furthermore, the concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. Furthermore, the concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

[0513] Furthermore, it is more preferable that aluminum be widely distributed in a deep region, for example, the surface or a region at a depth of 5 nm to 50 nm from the reference point. Therefore, although aluminum is detected in an analysis of the entire cathode active material 100 using ICP-MS, GD-MS, or the like, it is more preferable that the concentration of aluminum is below the lower limit of detection using XPS, etc.

[0514] Furthermore, when XPS analysis was performed on the positive electrode active material 100 of one embodiment of the present invention, the number of magnesium atoms was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times, relative to the number of cobalt atoms. The number of nickel atoms was preferably 0.15 to 0.15 times, more preferably 0.03 to 0.13 times, relative to the number of cobalt atoms. The number of aluminum atoms was preferably 0.12 to 0.09 times, more preferably 0.09 times, relative to the number of cobalt atoms. The number of fluorine atoms was preferably 0.3 to 0.9 times, more preferably 0.1 to 1.1 times, relative to the number of cobalt atoms. The above ranges indicate that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100 but are widely distributed at preferred concentrations in the surface layer 100a of the positive electrode active material 100.

[0515] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions: Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0516] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between fluorine and another element is preferably greater than or equal to 682 eV and less than 685 eV, and more preferably about 684.3 eV, which is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV).

[0517] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is analyzed by XPS, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV, which is a value different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide.

[0518] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. More preferably, the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated, for example, by exposing a cross section of the positive electrode active material 100 using a focused ion beam (FIB) or the like and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0519] 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.

[0520] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundary 101 of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis that thins the sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0521] 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.

[0522] Therefore, when referring to the depth direction in STEM-EDX ray analysis, the transition metal M is determined based on the average value M of the amount detected inside. ＡＶＥ and the average background value M ＢＧ The point where the oxygen concentration is 50% of the sum of the average value of the detected amount of oxygen inside the ＡＶＥ and the average background value O ＢＧ The reference point is the point where the sum of the internal and background is 50%. If the transition metal M and oxygen differ in the 50% point, this is considered to be due to the influence of metal oxides, carbonates, etc. containing oxygen adhering to the surface. Therefore, the average value M of the detected amount inside the transition metal M is used. ＡＶＥ and the average background value M ＢＧ In the case of a positive electrode active material having a plurality of transition metals M, the M of the element with the largest count in the inner portion 100b can be used. ＡＶＥ and M ＢＧ The reference point can be determined using the following formula:

[0523] The average value M of the background of the transition metal M ＢＧ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, from the outside of the positive electrode active material, avoiding the vicinity where the detected amount of transition metal M starts to increase. ＡＶＥ can be obtained by averaging a range of 2 nm or more, preferably 3 nm or more, at a depth of 30 nm or more, preferably more than 50 nm, from the region where the counts of the transition metal M and oxygen are saturated and stable, for example, the region where the detected amount of the transition metal M starts to increase. ＢＧ and the average value of the amount of oxygen detected inside O ＡＶＥ can also be found in the same way.

[0524] The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between the region where an image derived from the crystalline structure of the positive electrode active material is observed and the region where it is not observed, and is the outermost region where atomic columns derived from the atomic nuclei of metal elements having atomic numbers larger than that of lithium among the metal elements constituting the positive electrode active material are observed. Alternatively, it is the intersection of the tangent line drawn to the brightness profile from the surface toward the bulk of the STEM image and the axis in the depth direction. The surface in a STEM image or the like may be determined in conjunction with an analysis with higher spatial resolution.

[0525] Furthermore, the spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the maximum value of the additive element profile may deviate by approximately 1 nm. For example, even if the maximum value of the additive element profile of magnesium or the like is outside the surface obtained above, the difference between the maximum value and the surface can be considered an error if it is less than 1 nm.

[0526] In addition, a peak in STEM-EDX-ray analysis refers to the detected intensity in each element profile or the maximum value of the characteristic X-rays for each element. Note that noise in STEM-EDX-ray analysis may be a measured value with a half-width less than the spatial resolution (R), for example, R / 2 or less.

[0527] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated values ​​measured over six scans can be used as the profile of each element. The number of scans is not limited to six; more scans can be performed and the average can be used as the profile of each element.

[0528] 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).

[0529] 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.

[0530] STEM-EDX ray analysis can be performed using, for example, a STEM device (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W (two-pronged) EDX detector. During EDX ray analysis, the emission current of the STEM device is set to 6 μA or more and 10 μA or less, and a portion of the thinned sample with minimal depth and unevenness is measured. The magnification is, for example, approximately 150,000 times. The conditions for EDX ray analysis can be drift correction, a line width of 42 nm, a pitch of 0.2 nm, and six or more frames.

[0531] When EDX area analysis or EDX point analysis is performed on positive electrode active material 100 of one embodiment of the present invention, the concentration of each additional element, particularly additional element X, in surface layer portion 100a is preferably higher than that in inner portion 100b.

[0532] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, it is preferable that the magnesium concentration in the surface layer 100a is higher than that in the interior 100b. Furthermore, when EDX analysis is performed, the magnesium concentration peak in the surface layer 100a is preferably present on the surface of the cathode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, it is preferable that the magnesium concentration decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it is preferable that the magnesium concentration decays to 30% or less of the peak at a depth of 2 nm from the peak top. Note that the "peak concentration" referred to here refers to the maximum concentration value.

[0533] Furthermore, when EDX-ray analysis is performed, the magnesium concentration in the surface layer portion 100a (detected amount of magnesium / (sum of detected amounts of magnesium, oxygen, cobalt, fluorine, aluminum, and silicon) is preferably 0.5 atomic % or more and 10 atomic % or less, and more preferably 1 atomic % or more and 5 atomic % or less.

[0534] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0535] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer portion 100a preferably exists on the surface of the positive electrode active material 100 or at a depth of 3 nm from the reference point toward the center, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, if the fluorine concentration peak exists slightly closer to the surface than the magnesium concentration peak, resistance to hydrofluoric acid is increased, which is more preferable. For example, the fluorine concentration peak is more preferably 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferably 1.5 nm or more closer to the surface.

[0536] In the cathode active material 100 containing nickel as an additive element, the nickel concentration peak in the surface layer 100a preferably exists at a depth of 3 nm from the surface or reference point toward the center of the cathode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. In the cathode active material 100 containing magnesium and nickel, the nickel distribution preferably overlaps with the magnesium distribution. For example, the difference in depth between the nickel concentration peak and the magnesium concentration peak is preferably within 3 nm, more preferably within 1 nm.

[0537] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100 a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present on the surface of the positive electrode active material 100 or at a depth of 0.5 nm to 50 nm from the reference point toward the center, and more preferably at a depth of 5 nm to 50 nm.

[0538] 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.

[0539] Furthermore, when the positive electrode active material 100 is subjected to linear or area analysis, the ratio of the number of atoms of the additional element A to the cobalt Co in the vicinity of the crystal grain boundary 101 (A / Co) is preferably 0.020 or more and 0.50 or less. It is further preferably 0.025 or more and 0.30 or less. It is further 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.

[0540] For example, when the added element is magnesium, when line analysis or area analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in the vicinity of the grain boundary 101 is preferably 0.020 or more and 0.50 or less. It is further preferably 0.025 or more and 0.30 or less. It is further preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less. Furthermore, when the above range is found in multiple locations, for example, three or more locations, of the positive electrode active material 100, it can be said that this indicates that the added element is not attached to a narrow area on the surface of the positive electrode active material 100, but is widely distributed at a preferred concentration in the surface layer portion 100a of the positive electrode active material 100.

[0541] <EPMA> EPMA (Electron Probe Microanalysis) can also quantify elements. Area analysis can analyze the distribution of each element.

[0542] When EPMA surface analysis is performed on a cross section of the positive electrode active material 100 according to one embodiment of the present invention, it is preferable that one or more selected from the additive elements have a concentration gradient, similar to the EDX analysis results. It is also more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The preferred range of the concentration peak for each additive element is also the same as in the case of EDX.

[0543] However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from the measurement results obtained using other analytical methods. For example, when the surface of the positive electrode active material 100 is analyzed using EPMA, the concentration of each added element present in the surface layer 100 a may be lower than the result obtained using XPS.

[0544] <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.

[0545] 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.

[0546] Therefore, the integrated intensity of each peak was calculated at 470 cm −１ ~490cm −１ I1,580cm −１ ~600cm −１ I2, 665 cm −１ ~685cm −１When I3 is taken as I3, the value of I3 / I2 is preferably 1% or more and 10% or less, and more preferably 3% or more and 9% or less.

[0547] 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.

[0548] <<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.

[0549] 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.

[0550] For example, the difference in lattice constant calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is preferably 0.1 Å or less for the a-axis and 1.0 Å or less for the c-axis. It is more preferably 0.05 Å or less for the a-axis and 0.6 Å or less for the c-axis. It is even more preferably 0.04 Å or less for the a-axis and 0.3 Å or less for the c-axis.

[0551] <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.

[0552] 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.

[0553] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0554] 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.

[0555] 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.

[0556] 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.

[0557] 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

[0558] 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.

[0559] 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.

[0560] 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.

[0561] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0562] 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.

[0563] 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.

[0564] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram is a three-dimensional representation of the gradation distribution in the target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0565] In the positive electrode active material 100 of one embodiment of the present invention, the difference between the maximum and minimum values ​​of the grayscale value is preferably 120 or less, more preferably 115 or less, and even more preferably 70 to 115. The standard deviation of the grayscale value is preferably 11 or less, more preferably 8 or less, and even more preferably 4 to 8.

[0566] <Additional Features> The positive electrode active material 100 may have recesses, cracks, dents, V-shaped cross sections, and the like. These are defects, and repeated charge and discharge may cause cobalt elution, collapse of the crystalline structure, cracking of the positive electrode active material 100, oxygen desorption, and the like. However, if the embedded portion 102 shown in FIG. 1B is present to bury these defects, cobalt elution and the like can be suppressed. This allows the positive electrode active material 100 to have excellent reliability and cycle characteristics.

[0567] As described above, an excess of the additive element contained in the positive electrode active material 100 may adversely affect the insertion and desorption of lithium. Furthermore, when the positive electrode active material 100 is used in a secondary battery, it may result in an increase in internal resistance and a decrease in charge / discharge capacity. On the other hand, if the additive element is insufficient, it may not be distributed throughout the entire surface layer portion 100a, and the effect of suppressing deterioration of the crystalline structure may be insufficient. Thus, the additive element needs to be present at an appropriate concentration in the positive electrode active material 100, but adjusting this concentration is not easy.

[0568] Therefore, when the positive electrode active material 100 has a region where the additive element is unevenly distributed, some of the excess atoms of the additive element are removed from the interior 100b of the positive electrode active material 100, and an appropriate additive element concentration can be achieved in the interior 100b. This makes it possible to suppress an increase in internal resistance and a decrease in charge / discharge capacity when the positive electrode active material 100 is used as a secondary battery. The ability to suppress an increase in internal resistance of a secondary battery is an extremely desirable characteristic, particularly in charge / discharge at large currents, for example, at 400 mA / g or more.

[0569] Furthermore, in the positive electrode active material 100 having a region where the additive element is unevenly distributed, it is permissible to mix an excess amount of the additive element to some extent in the manufacturing process, which is preferable because it widens the margin in production.

[0570] A coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. Figures 30A and 30B show examples of positive electrode active material 100 to which a coating portion 104 is attached.

[0571] The coating portion 104 is preferably formed by the accumulation of decomposition products of the electrolyte and the organic electrolytic solution during charging and discharging. ｘ CoO ２When repeated charging is performed such that x in the formula is 0.24 or less, the presence of a coating portion derived from the electrolyte on the surface of the positive electrode active material 100 is expected to improve charge-discharge cycle characteristics. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing cobalt elution. The coating portion 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as the electrolyte, a high-quality coating portion is easily obtained. Therefore, a coating portion 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating portion and is therefore preferred. Furthermore, the coating portion 104 does not have to cover the entire positive electrode active material 100. For example, it is sufficient for the coating portion 104 to cover 50% or more of the surface of the positive electrode active material 100, with 70% or more being more preferable and 90% or more being even more preferable.

[0572] This embodiment can be used in combination with other embodiments.

[0573] 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. 31 and 32 .

[0574] <Configuration Example of Secondary Battery> Hereinafter, a secondary battery shown in FIG. 31 will be described as an example, in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body.

[0575] [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.

[0576] The positive electrode active material described in the above embodiment may be mixed with another positive electrode active material.

[0577] 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 , V２ O ５ , Cr ２ O ５ , MnO ２ The following compounds are exemplified:

[0578] Other positive electrode active materials include LiMn ２ O ４ Lithium-containing materials having a spinel-type crystal structure containing manganese, such as lithium nickel oxide (LiNiO ２ or LiNi １−ｘ M ｘ O ２ It is preferable to mix (0<x<1) (M=Co, Al, etc.) This configuration can improve the characteristics of the secondary battery.

[0579] The conductive material may be a carbon-based material such as acetylene black, or may be a carbon nanotube, graphene, or a graphene compound.

[0580] In this specification and the like, graphene compounds include multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings is sometimes called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0581] 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.

[0582] In this specification, reduced graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. A single reduced graphene oxide can function, but multiple sheets may be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is 2 atomic % or more and 15 atomic % or less. By achieving these carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D of the G band to the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0583] Graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds may also have a sheet-like shape. Graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. Even thin graphene compounds may have very high conductivity, allowing a small amount of graphene to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene compound covers 80% or more of the active material. It is preferable that the graphene compound clings to at least a portion of the active material particles. It is also preferable that the graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the unevenness of a single active material particle or the unevenness formed by multiple active material particles. It is also preferable that the graphene compound surrounds at least a portion of the active material particles. The graphene compound may also have holes.

[0584] 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.

[0585] Because of the properties described above, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and rapid discharging. For example, rapid charging and rapid discharging characteristics may be required for secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. Rapid charging characteristics may also be required for mobile electronic devices, etc. Rapid charging and discharging refers to charging and discharging at, for example, 200 mA / g, 400 mA / g, or 1000 mA / g or more.

[0586] 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.

[0587] 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.

[0588] Furthermore, a material used in forming the graphene compound may be mixed with the graphene compound and used in the active material layer 200. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO ２ , SiO ｘ(x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a median diameter (D50) of 1 μm or less, more preferably 100 nm or less.

[0589] [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.

[0590] 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.

[0591] 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.

[0592] The binder may be used in combination with two or more of the above.

[0593] [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.

[0594] [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.

[0595] [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.

[0596] As the negative electrode active material, an element capable of undergoing a charge-discharge reaction by alloying / de-alloying reaction with lithium can be used. For example, a material containing one or more elements selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger charge-discharge capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, a compound containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３Sn 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.

[0597] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO ｘ Here, x preferably has a value close to 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, x is preferably 0.2 or more and 1.2 or less. Alternatively, x is preferably 0.3 or more and 1.5 or less.

[0598] 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.

[0599] 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.

[0600] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋This allows lithium-ion secondary batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high charge / discharge capacity per unit volume, relatively little volume expansion, low cost, and greater safety than lithium metal.

[0601] 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.

[0602] 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.

[0603] 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.

[0604] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ , CuO, Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ This also occurs with fluorides such as

[0605] 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.

[0606] [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.

[0607] [Electrolyte] The electrolyte contains a solvent and an electrolyte. The solvent for the electrolyte is preferably an aprotic organic solvent, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone, or any combination and ratio of two or more of these.

[0608] 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.

[0609] The electrolyte to be dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ These lithium salts may be used alone or in any combination and ratio of two or more thereof.

[0610] The electrolyte used in the secondary battery is preferably a highly purified electrolyte with a low content of granular dust or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0611] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte solution. The concentration of the added material may be, for example, 0.1 wt % to 5 wt % of the total solvent. VC and LiBOB are particularly preferred because they are likely to form a good coating portion.

[0612] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0613] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0614] 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.

[0615] Examples of polymers that can be used include polymers having a polyalkylene oxide structure, such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0616] In addition, instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a solid electrolyte containing a polymer material such as a PEO (polyethylene oxide) can be used. When a solid electrolyte is used, the installation of a separator and / or spacer is unnecessary. Furthermore, since the entire battery can be solidified, the risk of leakage is eliminated, dramatically improving safety.

[0617] [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.

[0618] 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).

[0619] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide-based materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0620] 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.

[0621] 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.

[0622] [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.

[0623] <Laminated Secondary Battery and Method for Fabricating the Same> An example of an external view of a laminated secondary battery 500 is shown in Figures 31 and 32. Figures 31 and 32 include a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511. If the laminated secondary battery has a flexible configuration, when it is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent to match the deformation of the electronic device. An example of a method for fabricating the laminated secondary battery will be described with reference to Figures 32A to 32C.

[0624] FIG. 32A shows the appearance of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501 and a positive electrode active material layer 502 provided on the positive electrode current collector 501. The negative electrode 506 has a negative electrode current collector 504 and a negative electrode active material layer 505 provided on the negative electrode current collector 504. First, the negative electrode 506, a separator 507, and a positive electrode 503 are stacked. FIG. 32B shows the stacked negative electrode 506, separator 507, and 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 a positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For the joining, ultrasonic welding or the like may be used, for example. Similarly, the tab regions of the negative electrodes 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the negative electrode on the outermost surface.

[0625] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are arranged on the outer casing 509 .

[0626] Next, as shown in Fig. 32C, 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 the electrolyte solution 508 can be introduced later.

[0627] Next, the electrolyte solution 508 (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 508 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.

[0628] 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.

[0629] <Thermal runaway of secondary batteries> The graph shown on page 69 [Figure 2-11] of Non-Patent Document 15 is quoted and partially modified to be shown in Figure 33. Figure 33 is a graph of the internal temperature (hereinafter simply referred to as temperature) of a secondary battery versus time, and shows that as the temperature rises, it passes through several states before reaching thermal runaway.

[0630] Generally, when the temperature of a secondary battery reaches or approaches 100°C, (1) the SEI (Solid Electrolyte Interface) of the negative electrode collapses and heat is generated. Also, when the temperature of the secondary battery exceeds 100°C, (2) the negative electrode (when graphite is used, the negative electrode becomes C ６ Li) and heat generation occurs, and (3) oxidation of the electrolyte by the positive electrode occurs and heat generation occurs. Then, when the temperature of the secondary battery reaches or near 180°C, (4) thermal decomposition of the electrolyte occurs, and (5) oxygen release and thermal decomposition from the positive electrode (this thermal decomposition includes a structural change of the positive electrode active material) occurs. Thereafter, when the temperature of the secondary battery exceeds 200°C, (6) decomposition of the negative electrode occurs, and finally (7) direct contact between the positive electrode and the negative electrode occurs. The secondary battery reaches thermal runaway through the above-mentioned states (5), (6), and (7). That is, to prevent thermal runaway, it is preferable to suppress the temperature rise of the secondary battery and maintain a stable state of the negative electrode, positive electrode, and / or electrolyte at high temperatures exceeding 100°C.

[0631] The cathode active material 100 according to the first embodiment of the present invention has a stable crystal structure and exhibits the effect of suppressing oxygen desorption. Therefore, a secondary battery using the cathode active material 100 is considered to be able to suppress the temperature rise of the secondary battery without reaching at least the state (5) and subsequent states, thereby exhibiting the remarkable effect of being less likely to experience thermal runaway.

[0632] <Nail Penetration Test> Next, the nail penetration test will be described using Figures 34A and 34B, etc. In the nail penetration test, a secondary battery 500 is fully charged (States of Charge: a state equivalent to 100% SOC) and a nail 1003 having a predetermined diameter selected from 2 mm to 10 mm is inserted into the secondary battery 500 at a predetermined speed selected from 1 mm / s to 20 mm / s, etc. Figure 34A shows a cross-sectional view of the secondary battery 500 with the nail 1003 inserted. The secondary battery 500 has a structure in which a positive electrode 503, a separator 507, a negative electrode 506, and an electrolyte 530 are housed in an exterior case 531. The positive electrode 503 has a positive electrode current collector 501 and positive electrode active material layers 502 formed on both sides thereof. The negative electrode 506 has a negative electrode current collector 504 and negative electrode active material layers 505 formed on both sides thereof. FIG. 34B shows an enlarged view of nail 1003 and positive electrode current collector 501, and clearly shows positive electrode active material 100 which is one embodiment of the present invention and conductive material 553 contained in positive electrode active material layer 502.

[0633] Generally, as shown in Figures 34A and 34B, when a nail 1003 penetrates the positive electrode 503 and the negative electrode 506, an internal short circuit occurs. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode, and electrons (e − ) flows to the positive electrode 503, and Joule heat is generated at the internal short circuit location and its vicinity. In addition, due to the internal short circuit, carrier ions, typically lithium ions (Li ＋ ) are released into the electrolyte as shown by the white arrow. If there are insufficient anions in the electrolyte 530 at this time, the electrolyte 530 cannot absorb all the lithium ions released from the negative electrode 506, and the electrolyte 530 begins to decompose. This is one of the electrochemical reactions, and is called a reduction reaction of the electrolyte by the negative electrode. Then, the electrons (e −), the tetravalent cobalt in the charged lithium cobalt oxide is reduced to trivalent or divalent, and this reduction reaction causes oxygen to be released from the lithium cobalt oxide, and the electrolyte 530 is further decomposed by the released oxygen, etc. This is one of the electrochemical reactions, and is called an oxidation reaction of the electrolyte by the positive electrode.

[0634] Generally, when an internal short circuit occurs in a secondary battery, the temperature changes as shown in the graph in Figure 35. Figure 35 is a partially modified version of the graph shown on page 70 [Figure 2-11] of Non-Patent Document 15, and shows the temperature of the secondary battery versus time. When an internal short circuit occurs at (P0), the temperature ...

Claims

1. A method for manufacturing a secondary battery having a positive electrode with a positive electrode active material, a negative electrode, and an electrolyte, The positive electrode active material is A first step of mixing a composite oxide having lithium, cobalt, and oxygen, a magnesium source, a nickel source, and lithium fluoride to form a mixture, The mixture is formed by a second step of heating the mixture, The nickel source is nickel fluoride. In the second step described above, magnesium, nickel, and fluorine are segregated on the surface of the composite oxide. Method for manufacturing a secondary battery.

2. A method for manufacturing a secondary battery having a positive electrode with a positive electrode active material, a negative electrode, and an electrolyte, The positive electrode active material is A first step is to mix a composite oxide having lithium, cobalt, and oxygen with magnesium fluoride, nickel fluoride, and lithium fluoride to form a mixture. The mixture is formed by a second step of heating the mixture, In the second step described above, magnesium, nickel, and fluorine are segregated on the surface of the composite oxide. Method for manufacturing a secondary battery.

3. In claim 1 or claim 2, The positive electrode active material has a layered rock salt type crystalline structure of R-3m, The positive electrode active material is CoO ２ It has a structure in which layers and lithium layers are stacked alternately. The magnesium segregated on the surface of the aforementioned composite oxide is CoO ２ Suppresses layer displacement, Method for manufacturing a secondary battery.

4. In claim 1 or claim 2, The heating in the second step is carried out at a temperature of 650°C or higher and 1000°C or lower. Method for manufacturing a secondary battery.

5. In claim 1 or claim 2, The heating in the second step is carried out at a temperature at which cation mixing is unlikely to occur. Method for manufacturing a secondary battery.

6. A method for manufacturing a secondary battery having a positive electrode with a positive electrode active material, a negative electrode, and an electrolyte, The positive electrode active material is A first step is to mix a first composite oxide, a magnesium source, a nickel source, and lithium fluoride to form a first mixture. A second step involves subjecting the first mixture to a first heating to form a second composite oxide, A third step of mixing the second composite oxide and an aluminum source to form a second mixture, The second mixture is subjected to a second heating in a fourth step, and the mixture is formed by this, The first composite oxide comprises lithium, cobalt, and oxygen. The nickel source is nickel fluoride. The second step involves segregating magnesium, nickel, and fluorine on the surface of the second composite oxide. Method for manufacturing a secondary battery.

7. In claim 6, The magnesium source is magnesium fluoride. Method for manufacturing a secondary battery.

8. In claim 6 or claim 7, The positive electrode active material has a layered rock salt type crystalline structure of R-3m, The positive electrode active material is CoO ２ It has a structure in which layers and lithium layers are stacked alternately. The magnesium segregated on the surface of the second composite oxide is CoO ２ Suppresses layer displacement, Method for manufacturing a secondary battery.

9. In claim 6 or claim 7, The first heating in the second step is carried out at a temperature of 650°C or higher and 1000°C or lower. The second heating in the fourth step is performed at a temperature lower than the temperature in the first heating. Method for manufacturing a secondary battery.

10. A method for manufacturing a secondary battery having a positive electrode with a positive electrode active material, a negative electrode, and an electrolyte, The positive electrode active material is A first step is to mix a first composite oxide, a first nickel source, and lithium fluoride to form a first mixture, A second step involves subjecting the first mixture to a first heating to form a second composite oxide, A third step of mixing the second composite oxide and a second nickel source to form a second mixture, The second mixture is subjected to a second heating in a fourth step, and the mixture is formed by this, The first composite oxide comprises lithium and cobalt. Method for manufacturing a secondary battery.

11. In claim 10, The first nickel source is nickel fluoride. Method for manufacturing a secondary battery.

12. In claim 10, In the first step, a magnesium source is mixed with the first composite oxide, the first nickel source, and the lithium fluoride to form the first mixture. Method for manufacturing a secondary battery.

13. In claim 10, In the first step, a magnesium source is mixed with the first composite oxide, the first nickel source, and the lithium fluoride to form the first mixture. The first nickel source is nickel fluoride. The magnesium source is magnesium fluoride. Method for manufacturing a secondary battery.

14. A method for manufacturing a secondary battery having a positive electrode with a positive electrode active material, a negative electrode, and an electrolyte, The positive electrode active material is A first step of mixing a first composite oxide and a first nickel source to form a first mixture, A second step involves subjecting the first mixture to a first heating to form a second composite oxide, A third step of mixing the second composite oxide, a second nickel source, and lithium fluoride to form a second mixture, The second mixture is subjected to a second heating in a fourth step, and the mixture is formed by this, The first composite oxide comprises lithium and cobalt. Method for manufacturing a secondary battery.

15. In claim 14, The first nickel source is nickel fluoride. Method for manufacturing a secondary battery.

16. In claim 14, In the third step, a magnesium source is mixed with the second composite oxide, the second nickel source, and the lithium fluoride to form the second mixture. Method for manufacturing a secondary battery.

17. In claim 14, In the third step, a magnesium source is mixed with the second composite oxide, the second nickel source, and the lithium fluoride to form the second mixture. The first nickel source is nickel fluoride. The magnesium source is magnesium fluoride. Method for manufacturing a secondary battery.

18. In any one of claims 10 to 17, The first heating in the second step is carried out at a temperature of 650°C or higher and 1000°C or lower. The second heating in the fourth step is performed at a temperature lower than the temperature in the first heating. Method for manufacturing a secondary battery.