Battery

Abstract

To provide a battery with high safety.SOLUTION: A battery has: a positive electrode having a positive electrode active material; a negative electrode having a negative electrode active material; and an electrolyte. The positive electrode active material has a first region and a second region. The first region contains cobalt, magnesium, fluorine, and oxygen. The second region contains cobalt and oxygen. The first region is located further on a surface side of the positive electrode active material than the second region. The negative electrode active material contains graphite. The electrolyte contains a mixed organic solvent. When the battery in a fully charged state is subjected to a nailing test with a nail having a diameter of 3 mm at nailing speed of 5 mm / sec, a voltage of the battery drops from a first voltage Vb to a second voltage Vc, and then rises higher than the second voltage Vc.SELECTED DRAWING: Figure 5

Description

Technical field

[0001] One embodiment of the present invention relates to a battery, and specifically relates to a secondary battery. Further, the present invention is not limited to the above fields, but relates to semiconductor devices, display devices, light emitting devices, power storage devices, lighting devices, electronic devices, vehicles, and manufacturing methods thereof. The secondary battery of the present invention can be applied to the above-described semiconductor device, display device, light emitting device, power storage device, lighting device, electronic device, and vehicle as a necessary power source. For example, the above-mentioned electronic devices include information terminal devices equipped with secondary batteries. Furthermore, the above-mentioned power storage device includes a stationary power storage device and the like. [Background technology]

[0002] In recent years, demand for high-output, high-capacity lithium-ion secondary batteries (also referred to as lithium-ion batteries) has rapidly expanded, and they have become indispensable in modern society as a reusable energy source.

[0003] It is said that it is difficult to achieve both high capacity and safety in lithium ion secondary batteries. For example, a positive electrode active material having a layered rock salt crystal structure is expected to have a high capacity because a lithium ion diffusion path exists two-dimensionally within the crystal structure. However, positive electrode active materials with a layered rock-salt crystal structure are said to be susceptible to thermal runaway if too many lithium ions are desorbed during charging, causing the crystal structure to break, leading to safety issues. Safety tests include nail penetration tests, and in order to suppress the rise in battery temperature in abnormal situations such as nail penetration, for example, in Patent Document 1, a protective layer is installed between the positive electrode composite layer and the positive electrode current collector. A configuration has been proposed.

[0004] Lithium cobalt oxide (LiCoO 2 ) etc. are known. Lithium cobalt oxide has a layered rock salt crystal structure, and CoO 6 Since lithium ions can move two-dimensionally between the octahedral layers, the cycle characteristics are also good. However, lithium cobalt oxide has had the problem of phase changes during charging and discharging. For example, when lithium ions are released to some extent during charging, lithium cobalt oxide undergoes a phase change from hexagonal to monoclinic. Therefore, in order to use lithium cobalt oxide with good cycle characteristics, the amount of lithium ions released has been limited. In order to solve these problems, for example, Patent Documents 2 to 4 propose a structure in which additive elements are added to lithium cobalt oxide. Research on the crystal structure of positive electrode active materials has also been conducted (Non-Patent Documents 1 to 4).

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

[0006] Further, as image processing software, for example, ImageJ (Non-Patent Documents 8 to 10) is known. By using the software, it is possible to analyze, for example, the shape of a positive electrode active material.

[0007] Microelectron beam diffraction is also effective in identifying the crystal structure of the positive electrode active material, especially the crystal 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] Furthermore, fluorides such as fluorite (calcium fluoride) have been used as fluxing agents in iron manufacturing and the like for a long time, and their physical properties have been studied (Non-Patent Document 12).

[0009] It is known that when the temperature of a lithium ion secondary battery increases during charging, it goes through several states and reaches thermal runaway (Non-Patent Document 13).

[0010] Various research and developments are also being conducted regarding the reliability and safety of lithium ion secondary batteries. For example, Non-Patent Document 14 describes the thermal stability of a positive electrode active material and an electrolyte.

[0011] For example, Shannon's ionic radius is known as in Non-Patent Document 15. [Prior art documents] [Patent document]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 2019-129009 [Patent Document 2] Japanese Patent Application Publication No. 2019-179758 [Patent Document 3] WO2020 / 026078 issue [Patent Document 4] Japanese Patent Application Publication No. 2020-140954 [Non-patent literature]

[0013] [Non-patent document 1] Toyoki Okumura et al., “Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3-and O2-lithium cobalt oxides from first-principle calculation”, Journal of Materials Chemistry, 2012, 22, p.17340- 17348 [Non-patent document 2] Motohashi, T. et al., “Electronic phase diagram of the layered cobalt oxide system LixCoO2 (0.0≦x≦1.0)”, Physical Review B, 80(16);165114 [Non-patent document 3] Zhaohui Chen et al., “Staging Phase Transitions in LixCoO2”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609 [Non-patent document 4] G. G. Amatucci et al., “CoO2 , The End Member of the LixCoO2 Solid Solution” J. Electrochem. Soc. 143 (3) 1114 (1996). [Non-patent document 5] 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. [Non-patent document 6] Akimoto, J.; Gotoh, Y.; Oosawa, Y. “Synthesis and structural refinement of LiCoO2 single crystals” Journal of Solid State Chemistry (1998) 141, p. 298-302. [Non-patent document 7] F.Izumi and K.Momma,Solid State Phenom.,130,15-20(2007) [Non-patent document 8] Rasband, W.S., ImageJ, U.S. National Institutes of Health, Bethesda, Maryland, USA, http: / / rsb.info.nih.gov / ij / , 1997-2012. [Non-patent document 9] Schneider, C.A., Rasband, W.S., Eliceiri, K.W. “NIH Image to ImageJ: 25 years of image analysis”, Nature Methods 9, 671-675, 2012. [Non-patent document 10] Abramoff, M.D., Magelhaes, P.J., Ram, S.J., “Image Processing with ImageJ”, Biophotonics International, volume 11, issue 7, pp. 36-42, 2004. [Non-patent document 11] Seto, Y. & Ohtsuka, M., “ReciPro: free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools” (2022) J. Appl. Cryst., 55. [Non-patent document 12] W. E. Counts, R. Roy, and E. F. Osborn, “Fluoride Model Systems: II, The Binary Systems CaF2-BeF2, MgF2-BeF2, and LiF-MgF2”, Journal of the American Ceramic Society, 36 [1] 12-17 ( 1953). [Non-patent document 13] Nobuo Eda “2-4 Mechanism of heat generation” “His Li-ion battery charging and discharging technology learned from data” CQ Publishing, April 4, 2020, P.68-72 [Non-patent document 14] Shinya Kitano et al. GSYuasa Technical Report Vol. 2 No. 2 December 2015 P18-24 [Non-patent document 15] Shannon et al., Acta A, 32 (1976) 751. [Summary of the invention] [Problem to be solved by the invention]

[0014] Lithium cobalt oxide (LiCoO) shown in Patent Documents 2 to 4 2 , sometimes written as LCO) is said to have low thermal stability. In addition, when an internal short circuit occurs in a lithium-ion secondary battery due to a nail penetration test, Joule heat is generated, which causes the lithium cobalt oxide to reach a high temperature and release oxygen. Oxygen released from lithium cobalt oxide reacts with electrolyte, etc., which may lead to thermal runaway. Patent Document 1 discloses a configuration in which a protective layer is provided between a positive electrode current collector and a positive electrode composite material layer in order to suppress an increase in battery temperature during nail penetration.

[0015] In view of the above description, it is an object of one embodiment of the present invention to provide a highly safe battery. Furthermore, an object of one embodiment of the present invention is to provide a battery with high capacity and high safety.

[0016] Note that the description of these issues does not preclude the existence of other issues. Note that one embodiment of the present invention does not need to solve all of these problems. Note that problems other than these can be extracted from the description, drawings, and claims. [Means to solve the problem]

[0017] One embodiment of the present invention includes a positive electrode having a positive electrode active material, wherein the positive electrode active material has a first region and a second region, and the first region includes lithium, cobalt, magnesium, and oxygen. , the second region contains lithium, cobalt and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the thickness of the first region is 1 nm or more and 20 nm or less, The battery has a magnesium concentration of more than 0 and less than 10 atomic%.

[0018] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium, nickel, and The second region contains lithium, cobalt and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the first region has a thickness of 1 nm or more and 20 nm. or less, and the concentration of magnesium is greater than 0 and less than 10 atomic%.

[0019] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium, nickel, The second region contains fluorine and oxygen, the second region contains lithium, cobalt and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the thickness of the first region is 2 nm. The battery has a particle size of 20 nm or less, and a magnesium concentration of more than 0 and 10 atomic% or less.

[0020] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium, and oxygen. The second region contains lithium, cobalt, aluminum, and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the first region has a thickness of 2 nm or more and 20 nm. or less, and the concentration of magnesium is greater than 0 and less than 10 atomic%.

[0021] In another embodiment of the invention, the first region is preferably a region up to 5 nm from the surface.

[0022] In another embodiment of the present invention, the positive electrode active material has a volume resistivity of 1.0×10 at a pressure of 64 MPa in powder form. 5 It is preferable that it is Ω·cm or more.

[0023] In another aspect of the present invention, when a battery is subjected to a nail penetration test under the conditions of a battery voltage of 4.5V, a nail diameter of 3mm, and a nail penetration speed of 5mm / sec, the temperature rise ΔT of the battery is The temperature is preferably 50°C or lower.

[0024] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material having a first region and a second region, the first region containing a first lithium, cobalt, magnesium and oxygen, the second region has a second lithium, cobalt and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the surface of the positive electrode active material is , fluorine is adsorbed, the fluorine is combined with the first lithium, the thickness of the first region is 2 nm or more and 20 nm or less, and the concentration of magnesium is more than 0 and less than 10 atomic %.

[0025] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material having a first region and a second region, the first region containing a first lithium, cobalt, magnesium , nickel, and oxygen; the second region has a second lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region; is a battery in which fluorine is adsorbed, the fluorine is combined with the first lithium, the thickness of the first region is 2 nm or more and 20 nm or less, and the concentration of magnesium is greater than 0 and less than 10 atomic%. .

[0026] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material having a first region and a second region, the first region containing a first lithium, cobalt, magnesium , nickel, a first fluorine and oxygen, the second region has a second lithium, cobalt and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, The second fluorine is adsorbed on the surface of the positive electrode active material, the second fluorine is combined with the first lithium, the thickness of the first region is 2 nm or more and 20 nm or less, and the concentration of magnesium is 0. It is a battery that exceeds 10 atomic%.

[0027] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material having a first region and a second region, the first region containing a first lithium, cobalt, magnesium and oxygen, the second region has a second lithium, cobalt, aluminum, and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the first region is located on the surface of the positive electrode active material. is a battery in which fluorine is adsorbed, the fluorine is combined with the first lithium, the thickness of the first region is 2 nm or more and 20 nm or less, and the concentration of magnesium is greater than 0 and less than 10 atomic%. .

[0028] In another embodiment of the invention, the first region is preferably a region up to 5 nm from the surface.

[0029] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material having a first region and a second region, the first region containing a first lithium, cobalt, magnesium and oxygen, the second region has a second lithium, cobalt and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the surface of the positive electrode active material is , fluorine is adsorbed, the fluorine combines with the first lithium, and the volume resistivity of the positive electrode active material powder becomes 1.0 × 10 at a pressure of 64 MPa. 5 A battery with a resistance of Ω·cm or more.

[0030] Another embodiment of the present invention is a battery including a positive electrode having a positive electrode active material, wherein the positive electrode active material includes cobalt, nickel, and lithium. It has a first region including at least a part of the surface, and a second region which is an inner region than the first region, and the first region has the same number of nickel atoms as the first region. The ratio of the number of nickel atoms in the second region to the number of cobalt atoms in the second region is less than 1, and the ratio of the number of nickel atoms in the second region to the number of cobalt atoms in the first region is less than 1. is smaller than the ratio of cobalt atoms in the first region to the number of atoms of cobalt in the first region. is a battery that is carried out in a charged state in an environment of 25°C.

[0031] In another aspect of the present invention, in a nail penetration test under the conditions of a battery voltage of 4.5V, a nail diameter of 3mm, and a nail penetration speed of 5mm / sec, the temperature rise ΔT of the battery is 50°C or less. preferable.

[0032] Another embodiment of the present invention is a battery including a positive electrode having a positive electrode active material, wherein the positive electrode active material includes cobalt, nickel, and lithium. It has a first region including at least a part of the surface, and a second region which is an inner region than the first region, and the first region has the same number of nickel atoms as the first region. The ratio of the number of nickel atoms in the second region to the number of cobalt atoms in the second region is less than 1, and the ratio of the number of nickel atoms in the second region to the number of cobalt atoms in the first region is less than 1. is smaller than the ratio of the number of cobalt atoms in the first region to the number of atoms of cobalt, and after the battery is subjected to a charge / discharge cycle test where the number of cycles is 1 to 5 times, a nail penetration test is performed to short the battery. The battery did not ignite when tested, and the nail penetration test was conducted in a charged state in an environment of 23°C.

[0033] In another aspect of the present invention, in a nail penetration test under the conditions of a battery voltage of 4.6V, a nail diameter of 3mm, and a nail penetration speed of 5mm / sec, the temperature rise ΔT of the battery is 70°C or less. preferable.

[0034] In another aspect of the present invention, the battery preferably includes an electrolyte.

[0035] In another aspect of the invention, the resistance of the first region is preferably higher than the resistance of the second region.

[0036] In another embodiment of the present invention, the charge / discharge cycle test is preferably performed in an environment of 45° C., when charging is constant current-constant voltage charging, and when discharging is preferably constant current discharging.

[0037] In another embodiment of the present invention, it is preferable that the first region has lithium, fluorine is adsorbed on the surface, and the fluorine can bond with the lithium that the first region has.

[0038] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium , and oxygen; the second region contains lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region; After the test, the positive electrode active material has a ratio of oxygen atomic concentration to cobalt atomic concentration of less than 1.3 at locations less than 2 cm from the nail hole, and cobalt The battery includes a positive electrode active material in which the ratio of the atomic concentration of oxygen to the atomic concentration of oxygen is 1.3 or more.

[0039] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium , nickel, and oxygen; the second region contains lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region; After the nail penetration test, at a location where the distance from the nail hole is less than 2 cm, the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is less than 1.3, and at a location where the distance from the nail hole is 2 cm or more, The battery includes a positive electrode active material in which the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1.3 or more.

[0040] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium , nickel, fluorine, and oxygen; the second region contains lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region; The positive electrode active material has a ratio of oxygen atomic concentration to cobalt atomic concentration of less than 1.3 at a location where the distance from the nail hole is less than 2 cm after the nail penetration test, and a location where the distance from the nail hole is 2 cm or more. The battery includes a positive electrode active material in which the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1.3 or more.

[0041] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material has a first region and a second region, and the first region contains lithium, cobalt, magnesium , and oxygen, the second region contains lithium, cobalt, aluminum, and oxygen, the first region is located closer to the surface of the positive electrode active material than the second region, and the positive electrode is After the nail penetration test, at a location where the distance from the nail hole is less than 2 cm, the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is less than 1.3, and at a location where the distance from the nail hole is 2 cm or more, The battery includes a positive electrode active material in which the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1.3 or more.

[0042] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, and the positive electrode active material has a first region, a second region, , the first region has lithium, cobalt, magnesium, and oxygen, the second region has lithium, cobalt, and oxygen, and the first region has more Located on the surface side of the positive electrode active material, the negative electrode active material contains graphite, the electrolyte contains ethylene carbonate and diethyl carbonate, and the distance of the positive electrode from the nail hole after the nail penetration test is less than 2 cm. At the location where the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is less than 1.3, and at the location where the distance from the nail hole is 2 cm or more, the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1.3. This is a battery having the above positive electrode active material.

[0043] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, and the positive electrode active material has a first region, a second region, and the first region has lithium, cobalt, magnesium, nickel, and oxygen, the second region has lithium, cobalt, and oxygen, and the first region has lithium, cobalt, magnesium, nickel, and oxygen. The negative electrode active material contains graphite, the electrolyte contains ethylene carbonate and diethyl carbonate, and the positive electrode is located on the surface side of the positive electrode active material after the nail penetration test. A positive electrode active material in which the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is less than 1.3 at a distance of less than 2 cm, and the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt at a distance of 2 cm or more from the nail hole. is 1.3 or more.

[0044] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, and the positive electrode active material has a first region, a second region, the first region has lithium, cobalt, magnesium, nickel, fluorine, and oxygen; the second region has lithium, cobalt, and oxygen; the first region has lithium, cobalt, magnesium, nickel, fluorine, and oxygen; The negative electrode active material contains graphite, the electrolyte contains ethylene carbonate and diethyl carbonate, and the positive electrode is located on the surface side of the positive electrode active material than the area of The positive electrode active material has a ratio of the atomic concentration of oxygen to the atomic concentration of cobalt of less than 1.3 at a distance of less than 2 cm, and the atomic concentration of oxygen to the atomic concentration of cobalt at a location of 2 cm or more from the nail hole. and a positive electrode active material having a ratio of 1.3 or more.

[0045] Another embodiment of the present invention includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, and the positive electrode active material has a first region, a second region, the first region has lithium, cobalt, magnesium, and oxygen; the second region has lithium, cobalt, aluminum, and oxygen; the first region has lithium, cobalt, aluminum, and oxygen; The negative electrode active material contains graphite, the electrolyte contains ethylene carbonate and diethyl carbonate, and the positive electrode is located on the surface side of the positive electrode active material after the nail penetration test. A positive electrode active material in which the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is less than 1.3 at a distance of less than 2 cm, and the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt at a distance of 2 cm or more from the nail hole. is 1.3 or more.

[0046] In another embodiment of the present invention, it is preferable to perform the nail penetration test under the conditions of a battery voltage of 4.5 V, a nail diameter of 3 mm, and a nail penetration speed of 5 mm / sec.

[0047] Another embodiment of the present invention is a battery including a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, wherein the positive electrode active material has a first region and a second region. a region, the first region has cobalt, magnesium, fluorine and oxygen, the second region has cobalt and oxygen, and the first region has more than the second region. Located on the surface side of the positive electrode active material, the negative electrode active material has graphite, the electrolyte has a mixed organic solvent, the nail diameter is 3 mm, and the nail penetration speed is 5 mm / sec when the battery is fully charged. When a nail penetration test is performed under these conditions, the battery voltage drops from the first voltage Vb to the second voltage Vc, and then becomes higher than the second voltage Vc.

[0048] In another embodiment of the present invention, when the nail penetration test is carried out under conditions where the voltage of the battery is 4.5 V, it is preferable that the temperature rise ΔT of the battery is 50° C. or less.

[0049] Another embodiment of the present invention is a battery including a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte, wherein the positive electrode active material has a first region and a second region. a region, the first region has cobalt, magnesium, fluorine and oxygen, the second region has cobalt and oxygen, and the first region has more than the second region. Located on the surface side of the positive electrode active material, the negative electrode active material contains graphite, and the electrolyte contains a mixed organic solvent. After conducting a charge / discharge cycle test on the battery in a 45°C environment, When the battery is fully charged and a nail penetration test is performed under the conditions of a nail diameter of 3 mm and a nail penetration speed of 5 mm / sec, the battery voltage drops to Vc and maintains the value of Vc.

[0050] In another embodiment of the present invention, when the nail penetration test is conducted under the condition that the voltage of the battery is 4.6 V, it is preferable that the temperature rise ΔT of the battery is 70° C. or less.

[0051] In another embodiment of the present invention, the volume resistivity of the positive electrode active material powder is 1.0×10 at a pressure of 64 MPa. 5 It is preferable that it is Ω·cm or more.

[0052] In another aspect of the invention, the battery preferably does not ignite when subjected to a nail penetration test.

Effect of the invention

[0053] According to one embodiment of the present invention, a highly safe battery can be provided. Further, according to one embodiment of the present invention, a battery with high capacity and high safety can be provided.

[0054] 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 need to have all of these effects. Note that effects other than these will become obvious from the description, drawings, claims, etc., and effects other than these can be extracted from the description, drawings, claims, etc. It is. [Brief explanation of the drawing]

[0055]

Figure 1

Figure 2

[0056] DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of embodiments for carrying out the present invention will be described below with reference to drawings and the like. However, the present invention is not interpreted as being limited to the following embodiments. It is possible to change the mode of carrying out the invention without departing from the spirit of the invention.

[0057] In this specification, space groups are expressed using short notation of the international notation (or Hermann-Mauguin symbol). In addition, crystal planes and crystal directions are expressed using Miller indices. Space groups, crystal planes, and crystal directions are expressed in terms of crystallography by adding a superscript bar to the number, but in this specification etc., due to formatting constraints, instead of adding a bar above the number, they are written in front of the number. It is sometimes expressed by adding a - (minus sign) to it. Also, the individual orientation indicating the direction within the crystal is [ ], the collective orientation indicating all equivalent directions is < >, the individual plane indicating the crystal plane is ( ), and the collective plane with equivalent symmetry is {} Express each. In addition, the trigonal crystal represented by the space group R-3m is generally represented by a complex hexagonal lattice of hexagonal crystals for ease of understanding the structure, and unless otherwise mentioned in this specification, the space group R-3m is It is expressed as a complex hexagonal lattice. Also, not only (hkl) but also (hkil) may be used as the Miller index. Here i is -(h+k).

[0058] In this specification, etc., the term "particles" is not limited to only spherical shapes (circular cross-sectional shapes), but also includes particles whose cross-sectional shapes are elliptical, rectangular, trapezoidal, triangular, square with rounded corners, and asymmetrical. Examples include shape, and further, individual particles may be amorphous.

[0059] Further, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the lithium that can be intercalated and desorbed from the positive electrode active material is desorbed. For example, LiCoO 2 Theoretical capacity is 274mAh / g, LiNiO 2 Theoretical capacity is 274mAh / g, LiMn 2 O 4 The theoretical capacity of is 148mAh / g.

[0060] In addition, the amount of lithium that can be intercalated and deintercalated in the positive electrode active material is determined by x in the composition formula, for example, Li x M.O. 2 Indicated by the x in the middle. Note that M represents a transition metal, and unless otherwise specified in this specification, M is cobalt and / or nickel. In the case of a positive electrode active material in a lithium ion secondary battery, x=(theoretical capacity−charge capacity) / theoretical capacity. For example, Li x M.O. 2 When charging a lithium ion secondary battery using Li as the positive electrode active material at 219.2mAh / g, Li 0.2 M.O. 2 Or it can be said that x=0.2. Li x M.O. 2 When x is small, for example, 0.1<x≦0.24.

[0061] If properly synthesized lithium cobalt oxide satisfies approximately the stoichiometric ratio before being used in the positive electrode, LiCoO 2 and x=1. In addition, the lithium cobalt oxide contained in the lithium ion secondary battery that has finished discharging is also removed by LiCoO 2 Therefore, we can say that x=1. The term "discharge completed" as used herein refers to a state in which the voltage becomes 3.0 V or 2.5 V or less when discharging with a current of 100 mA / g or less, for example.

[0062] Li x M.O. 2 The charging capacity and / or discharging capacity used to calculate x in the equation is preferably measured under conditions where there is no or little influence of short circuits and / or decomposition of the electrolytic solution, etc. For example, data from a lithium-ion secondary battery that has undergone a sudden change in capacity that appears to be a short circuit must not be used to calculate x.

[0063] Further, the space group of the positive electrode active material and the like is identified by XRD, electron beam diffraction, neutron beam 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 identifying with a certain space group.

[0064] Furthermore, if the anions have a structure like ABCABC in which three layers are shifted from each other and stacked on top of each other, it is called a cubic close-packed structure. Therefore, the anion does not have to be strictly in a cubic lattice. At the same time, since real crystals always have defects, the analysis results do not necessarily have to match the theory. For example, in an FFT (fast Fourier transform) pattern such as an electron beam diffraction pattern or a TEM (Transmission Electron Microscope) image, a spot may appear at a position slightly different from the theoretical position. For example, if the deviation in orientation from the theoretical position is 5 degrees or less, or 2.5 degrees or less, it can be said that the structure has a cubic close-packed structure.

[0065] Furthermore, the distribution of an element refers to a region where the element is continuously detected within a non-noise range using a certain continuous analysis method.

[0066] Note that in this specification and the like, the surface layer portion of a positive electrode active material refers to a region within 20 nm or a region within 50 nm from the surface toward the inside in a direction perpendicular or substantially perpendicular to the surface. The surface layer portion has the same meaning as near-surface and near-surface region. Note that vertical or substantially vertical specifically refers to an angle between 80° and 100°. Further, a region deeper than the surface layer of the positive electrode active material is called the inside. Internal is synonymous with bulk or core.

[0067] In this specification and the like, the positive electrode active material may be expressed as a composite oxide, a positive electrode material, a positive electrode material, a positive electrode material for secondary batteries, a positive electrode material for lithium ion secondary batteries, etc. Further, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably contains a compound. Further, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably has a composition. Further, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably has a composite.

[0068] Further, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments and the like, not all particles necessarily have the characteristics. For example, if 50% or more, preferably 70% or more, more preferably 90% or more of three or more randomly selected positive electrode active material particles have the characteristic, it is sufficient to have the positive electrode active material and the same. It can be said that this has the effect of improving the characteristics of the secondary battery.

[0069] As the charging voltage of a secondary battery increases, the voltage applied to the positive electrode generally increases. Since the positive electrode active material of one embodiment of the present invention is stable in a charged state, it can be used as a secondary battery in which a decrease in discharge capacity due to repeated charging and discharging is suppressed.

[0070] Moreover, an internal short circuit or an external short circuit of the secondary battery not only causes problems in the charging operation and / or discharging operation of the secondary battery, but also may cause heat generation and ignition. In order to realize a safe secondary battery, it is preferable that internal short circuits or external short circuits be suppressed even at high charging voltages. In the positive electrode active material of one embodiment of the present invention, internal short circuits or external short circuits are suppressed even at high charging voltages. Therefore, it is possible to obtain a secondary battery that has both high discharge capacity and safety. Note that an internal short circuit in a secondary battery refers to contact between a positive electrode and a negative electrode inside the battery. Furthermore, an external short circuit of a secondary battery is assumed to occur due to misuse, and refers to contact between a positive electrode and a negative electrode outside the battery.

[0071] In this specification, etc., ignition in a nail penetration test means that flame is observed outside the exterior body within 1 minute after nail penetration, or that thermal runaway of the secondary battery has occurred. . For example, if thermal decomposition products of the positive and / or negative electrodes are observed at a distance of 2 cm or more from the nail penetration test after the nail penetration test, thermal runaway is said to have occurred. The thermal decomposition products of the positive electrode and / or negative electrode include, for example, aluminum oxide, which is obtained by oxidizing the aluminum of the positive electrode current collector, and copper oxide, which is obtained by oxidizing the copper of the negative electrode current collector.

[0072] For example, layered rock salt type LiMO is used as the positive electrode active material. 2 When using (M is Co and / or Ni), theoretically, the atomic ratio of O / the atomic ratio of M (hereinafter referred to as O / M ratio) is 2. On the other hand, due to thermal runaway, LiMO 2 When oxygen is released from the O / M ratio decreases. Therefore, for example, if the O / M ratio in Energy Dispersive X-ray Spectroscopy (EDX) analysis is less than 1.3 at a location 2 cm or more away from the nail penetration test after the nail penetration test, It is said that a thermal runaway occurred. Conversely, if the O / M ratio in EDX analysis is 1.3 or more at a location 2 cm or more from the puncture site, it can be said that thermal runaway has not occurred. Even if the battery voltage drops once and then rises after the nail penetration test, it can be said that thermal runaway has not occurred.

[0073] On the other hand, even if flames, sparks, and / or smoke are observed in a nail penetration test, if they remain at the puncture point, that is, the fire does not spread, and the secondary battery does not run away with heat, then it is not considered to have ignited. For example, even if a secondary battery is subjected to a nail penetration test, if the above-mentioned ignition does not occur, it can be said that the secondary battery does not ignite.

[0074] Note that unless otherwise specified, materials included in the secondary battery (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) will be described in terms of their state before deterioration. Note that a decrease in discharge capacity due to aging treatment and burn-in treatment in the secondary battery manufacturing stage is not called deterioration. For example, if a secondary battery consisting of a single cell or an assembled battery has a discharge capacity of 97% or more of the rated capacity, it can be said to be in a state before deterioration. The rated capacity complies with JIS C 8711:2019 for secondary batteries for portable devices. In the case of other secondary batteries, they comply not only with the JIS standards mentioned above, but also with JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0075] In this specification, etc., the state of the materials of a secondary battery before deterioration is referred to as the initial product or initial state, and the state after deterioration (when the secondary battery has a discharge capacity of less than 97% of its rated capacity) (state) is sometimes referred to as a used product or in-use state, or a used product or used state.

[0076] In this specification and the like, a lithium ion secondary battery refers to a battery using lithium ions as carrier ions, but the carrier ions of the present invention are not limited to lithium ions. For example, an alkali metal ion or an alkaline earth metal ion can be used as a carrier ion in the present invention, and specifically, a sodium ion or the like can be used. In this case, the present invention can be understood by reading lithium ions as sodium ions, etc. Furthermore, if there are no limitations on carrier ions, the battery may be referred to as a secondary battery.

[0077] In this specification, the (001) plane, the (003) plane, etc. are sometimes collectively referred to as the (00l) plane. Note that in this specification and the like, the (00l) plane is sometimes referred to as the C-plane, the basal plane, etc. Furthermore, in lithium cobalt oxide, lithium has a two-dimensional diffusion path. In other words, it can be said that the diffusion path of lithium exists along the surface. In this specification and the like, a surface other than the surface where the lithium diffusion path is exposed, that is, the surface where lithium is intercalated and deintercalated (specifically, the (00l) surface) may be referred to as an edge surface.

[0078] In this specification and the like, the supported amount is the weight of the active material per unit surface area of ​​the current collector. The amount of negative electrode active material supported can be adjusted according to the capacity of the positive electrode. In the case of double-sided coating in which a slurry containing an active material is coated on both sides of the current collector, the above-mentioned supported amount is preferably per one side. If the supported amount is small, the discharge capacity will be reduced. Therefore, the amount of positive electrode active material supported is 8.0mg / cm 2 The above is preferable.

[0079] In this specification and the like, secondary particles refer to particles formed by agglomeration of primary particles. Furthermore, in this specification and the like, primary particles refer to particles that do not have grain boundaries in their appearance. Furthermore, in this specification and the like, a single particle refers to a particle that does not have grain boundaries in its appearance. Furthermore, in this specification and the like, a single crystal particle refers to a crystal grain in which no grain boundary exists inside the particle, and a polycrystalline particle refers to a crystal grain in which a grain boundary exists inside the particle. A polycrystalline particle can be said to be an aggregate of multiple crystallites, and a grain boundary can be said to be an interface existing between two or more crystallites. Note that in the polycrystalline particles, it is preferable that the crystallites are oriented in the same direction.

[0080] In this specification and the like, "A and / or B" may be described, but this is an example of a description that includes only A, only B, and A and B.

[0081] (Embodiment 1) The nail penetration test is a test in which a rechargeable battery is fully charged and a nail having a predetermined diameter selected from 2 mm to 10 mm is inserted into the battery at a predetermined speed. In this embodiment, first, a nail penetration test device will be described. FIG. 1(A) shows a side view of the nail penetration testing device 1000, and FIG. 1(B) shows a perspective view of the nail penetration testing device 1000.

[0082] <Nail penetration test device> The nail penetration test device 1000 shown in FIG. 1(A) includes a stage 1001, a drive section 1002, a nail 1003, a voltage measurement device 1015, a temperature measurement device 1016, and a control section 1018. The drive unit 1002 has a drive mechanism 1012 that moves the nail 1003 in the direction of the arrow in the figure, and the drive mechanism 1012 operates so that the nail 1003 penetrates the secondary battery 1004 placed on the stage 1001. At this time, the secondary battery 1004 is kept in a fully charged state (State of Charge: a state equal to SOC 100%), and this operation is called a nail-piercing operation. Note that the broken line shown in FIG. 1(A) indicates a recessed portion of the stage 1001 provided for accommodating the nail 1003 after penetration during the nail piercing operation.

[0083] Information regarding the voltage of the secondary battery during the nail piercing operation is transmitted from the voltage measuring device 1015 to the control unit 1018. Specifically, the amount of voltage change, etc. is transmitted to the control unit 1018. Additionally, information regarding the temperature during the nail-penetrating operation is transmitted from the temperature measuring device 1016 to the control unit 1018. When controlling the operating conditions of the nail 1003, the control unit 1018 can transmit a control signal to the drive unit 1002.

[0084] FIG. 1(B) is a perspective view illustrating the vicinity of the upper part of the stage 1001 of the nail penetration testing apparatus 1000. A secondary battery 1004 installed on the stage 1001 is electrically connected to wiring 1005a and wiring 1005b. Note that the wiring 1005a and the wiring 1005b are included in the voltage measuring device 1015, and the wiring 1005a and the wiring 1005b are electrically connected to the positive electrode side tab and negative electrode side tab of the secondary battery 1004, respectively, and are connected to the secondary battery 1004. 1004 voltages can be measured. The voltage of the secondary battery 1004 is simply referred to as voltage, voltage value between positive and negative electrodes, battery voltage, cell voltage, or open circuit voltage. Further, when a temperature sensor is used as the temperature measuring device 1016, the temperature sensor is provided so as to be in contact with the surface of the exterior body of the secondary battery 1004.

[0085] Although FIG. 1(B) shows an example in which a first temperature sensor 1006a and a second temperature sensor 1006b are arranged, one or three or more temperature sensors may be arranged. In FIG. 1(B), the first temperature sensor 1006a is provided on the side where the wiring 1005a and the wiring 1005b are not arranged, and the second temperature sensor 1006b is provided on the side where the wiring 1005a and the wiring 1005b are arranged. It is preferable to arrange two or more temperature sensors because even if one temperature sensor becomes unusable due to expansion of the exterior body, another temperature sensor can be used.

[0086] Further, there is a welded area on the side where the wiring 1005a and the wiring 1005b are arranged, but there is no welded area on the side where the wiring 1005a and the wiring 1005b are not arranged because the exterior body is folded back. Therefore, even if the exterior body expands, the expansion on the side where wiring 1005a and wiring 1005b are not arranged is suppressed, and the first temperature sensor 1006a is preferably less likely to peel off than the second temperature sensor 1006b.

[0087] The broken-line ellipse shown in FIG. 1(B) is a region where the nail 1003 penetrates the secondary battery 1004 during the nail-piercing operation. The first temperature sensor 1006a and the second temperature sensor 1006b are preferably provided in a region equidistant from the region penetrated by the nail 1003. Typically, the first temperature sensor 1006a and the second temperature sensor 1006b may be provided within 5 cm, preferably within 2 cm, from the area through which the nail 1003 penetrates. It is preferable that the temperature change in the vicinity of the area penetrated by the nail 1003 can be ascertained. Note that when two or more temperature sensors are arranged, it is preferable to start the nail-penetration operation after confirming that the difference in temperature indicated by each temperature sensor is within ±5°C, preferably within ±2°C.

[0088] <Secondary battery in nail penetration test> Next, the state of the secondary battery in the nail penetration test will be explained again using FIG. 2(A), FIG. 2(B), etc. The nail penetration test is a test in which the secondary battery 1004 is fully charged and a nail 1003 having a predetermined diameter selected from 2 mm or more and 10 mm or less is inserted into the secondary battery 1004 at a predetermined speed. FIG. 2(A) shows a cross-sectional view of a secondary battery 1004 with a nail 1003 inserted therein. The secondary battery 1004 has a structure in which a positive electrode 503, a separator 508, a negative electrode 506, and an electrolyte 530 are housed in an exterior body 531. The positive electrode 503 has a positive electrode current collector 501 and a positive electrode active material layer 502 formed on both surfaces thereof, and the negative electrode 506 has a negative electrode current collector 511 and a negative electrode active material layer 512 formed on both surfaces thereof. Further, FIG. 2(B) shows an enlarged view of the vicinity of the nail 1003 and the positive electrode current collector 501, and also clearly shows the positive electrode active material 100 and the conductive material 553 included in the positive electrode active material layer 502.

[0089] As shown in FIGS. 2(A) and 2(B), when the nail 1003 is inserted into the secondary battery 1004, specifically, the nail 1003 penetrates the positive electrode 503 and the negative electrode 506, causing the internal A short circuit occurs. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode 506, and electrons (e - ) flows to the positive electrode 503, and Joule heat is generated at and near the internal short circuit. In addition, carrier ions, typically lithium ions (Li + ) is released into the electrolyte as indicated by the white arrow. However, before all lithium ions are released from the negative electrode, the battery temperature rapidly rises due to Joule heat generated by an internal short circuit, and the electrolyte begins to undergo reductive decomposition on the negative electrode surface. This is one of the electrochemical reactions and is called a reduction reaction of the electrolyte by the negative electrode.

[0090] In addition, when the temperature of the secondary battery 1004 increases due to Joule heat, when lithium cobalt oxide is used as the positive electrode active material, the lithium cobalt oxide undergoes a phase change from H1-3 type crystal structure to O1 type crystal structure ( In other words, structural changes) may occur, which may further generate heat. The crystal structure of the H1-3 type and the O1 type will be described later.

[0091] Then, as shown in FIGS. 2(A) and 2(B), the electrons (e - ), the tetravalent Co in the charged lithium cobalt oxide is reduced to trivalent or divalent Co, and this reduction reaction releases oxygen from the lithium cobalt oxide, and the electrolyte 530 undergoes an oxidation reaction due to the oxygen. decomposed by This is one of the electrochemical reactions and is called the oxidation reaction of the electrolyte by the positive electrode. The speed at which current flows into the positive electrode active material 100 etc. varies somewhat depending on the insulation properties of the positive electrode active material, and it is also believed that the speed at which the current flows affects the electrochemical reaction.

[0092] When an internal short circuit occurs in the secondary battery as described above, the temperature is thought to change as shown in the graph shown in FIG. 3. Figure 3 is a partially revised diagram quoting the graph shown on page 70 [Figure 2-12] of Non-Patent Document 13, and shows the temperature of the secondary battery (specifically, the internal temperature) versus time. It is a graph. If an internal short circuit occurs at (P0), the temperature of the secondary battery will rise over time. As shown in (P1), when the temperature of the secondary battery rises to around 100℃ due to Joule heat generated by an internal short circuit, the reference temperature (Ts), which is the limit temperature that does not cause thermal runaway of the secondary battery, increases. may exceed. Then, in (P2), the negative electrode (if graphite is used, the negative electrode is C 6 In (P3), the electrolyte is oxidized and heat is generated by the positive electrode, and in (P4), heat is generated due to thermal decomposition of the electrolyte. The secondary battery then goes into thermal runaway, resulting in fire or smoke.

[0093] At this time, in the positive electrode active material, cobalt becomes Co due to the electrons rapidly flowing into the positive electrode active material. 4+ FromCo 2+ A reaction occurs in which oxygen is released from the positive electrode active material. Since this reaction is exothermic, it accelerates thermal runaway. In other words, if this reaction can be suppressed, a safe secondary battery that is less prone to thermal runaway can be obtained.

[0094] In order to suppress the above reaction, it is preferable that, for example, the surface layer of the positive electrode active material contains an additive element that makes it difficult to release oxygen, and that the concentration of the additive element is higher than that inside. If oxygen is not released from the positive electrode active material, the above reduction reaction (e.g. Co 4+ FromCo 2+ reaction) is also suppressed. Examples of additive elements that make it difficult to release oxygen include magnesium, aluminum, and the like. Magnesium is suitable as an additive element that is less likely to release oxygen because the closer oxygen is to magnesium, the greater the energy for desorption becomes. Nickel is also considered to have the effect of suppressing oxygen release when present at the lithium site.

[0095] Furthermore, even if cobalt or the like is reduced, if lithium ions can be inserted into the positive electrode active material before oxygen is released, electrical neutrality will be maintained, and no oxygen will be released. Therefore, even if electrons suddenly flow into the positive electrode active material, the crystal structure of the positive electrode active material only needs to be kept stable until the lithium ions are inserted from the negative electrode through the electrolyte and into the positive electrode active material. I can say that.

[0096] In addition, in order to prevent smoke, heat generation, etc. from occurring during the nail penetration test, it is considered best to suppress the rise in temperature of the secondary battery, and to ensure that the negative electrode, positive electrode, and / or electrolyte have stable characteristics at high temperatures. . Specifically, it is preferable that the positive electrode active material 100 has a stable structure that does not release oxygen, especially when exposed to high temperatures. Alternatively, it is preferable that the positive electrode active material 100 has a structure in which the speed of current flowing to the positive electrode active material is slow. It is expected that this will have the remarkable effect of making it difficult for thermal runaway to occur and for fires to occur. As will be described later, the positive electrode active material 100, which is one embodiment of the present invention, can have both the stable structure described above and a structure that slows down the current speed.

[0097] <Thermal runaway of secondary batteries> Regarding the principle of thermal runaway in secondary batteries, the graph shown on page 69 [Fig. 2-11] of Non-Patent Document 13 is cited and a partially revised diagram is shown in Fig. 4. For example, when a secondary battery as described above rises in temperature (specifically, internal temperature) during charging, it goes through several states and reaches thermal runaway. FIG. 4 is a graph of the temperature of the secondary battery versus time. For example, when the temperature of the secondary battery reaches or near 100° C., (1) SEI (Solid Electrolyte Interphase) of the negative electrode collapses and heat is generated. In addition, if the temperature of the secondary battery exceeds 100℃, (2) the negative electrode (if graphite is used, the negative electrode 6 (3) At and around 150°C, the positive electrode oxidizes the electrolyte and generates heat. When the temperature of the secondary battery reaches 180°C or around 180°C, (4) thermal decomposition of the electrolyte occurs, and (5) oxygen is released from the positive electrode and thermal decomposition of the positive electrode occurs (the thermal decomposition includes the loss of positive electrode active material). (including structural changes) occur. After that, when the temperature of the secondary battery exceeds 200°C, (6) the negative electrode decomposes, and finally (7) the positive electrode and negative electrode come into direct contact. After passing through such a state, particularly state (5), state (6), or state (7), the secondary battery reaches thermal runaway.

[0098] In order to prevent thermal runaway, it is thought that it is better to suppress the rise in temperature of the secondary battery, and for the negative electrode, positive electrode, and / or electrolyte to have stable characteristics at high temperatures.

[0099] <Characteristics of secondary batteries in nail penetration test 1> After nail penetration, the voltage of the secondary battery may be 0 V, but the secondary battery that is one embodiment of the present invention exhibits a change in which the voltage drops and then rises, that is, the voltage drops and then returns. Further, the secondary battery that is one embodiment of the present invention shows a change in which when the voltage decreases, it does not reach 0V but maintains a low voltage value. The voltage change of the secondary battery will be explained using FIGS. 5(A) to 5(C).

[0100] FIG. 5(A) is an example graph showing the relationship between the position of the nail 1003 and the time of the nail piercing operation. The position of the nail 1003 refers to the tip of the nail 1003, and can also be said to be the depth from the surface of the secondary battery 1004. In FIG. 5(A), it can be determined that the nail 1003 is stuck in the secondary battery 1004 at time T0, and the value of the position of the nail 1003 increases toward La. Note that in FIG. 5(A), the position of the nail 1003 is constant at La, and at this time it can be considered that the nail 1003 has penetrated the secondary battery 1004.

[0101] FIG. 5(B) is an example of a graph showing the voltage of the secondary battery 1004, with the x-axis representing the time of the same nail-piercing operation as in FIG. 5(A). In FIG. 5(B), the voltage of the fully charged secondary battery 1004 is Vb, the nail 1003 is stuck into the secondary battery 1004 at time T0, and the voltage of the secondary battery 1004 drops at the subsequent time T1. . That is, at time T1, the nail 1003 is considered to be in contact with the positive and negative electrodes, and the voltage often drops rapidly. In the secondary battery that is one embodiment of the present invention, the voltage suddenly drops and then starts to rise. Such a change in the voltage of the secondary battery is a characteristic of the secondary battery that is one embodiment of the present invention, and is specifically thought to be caused by the positive electrode active material that is one embodiment of the present invention, which will be described later.

[0102] FIG. 5(C) is an example of a graph showing the voltage of the secondary battery 1004, with the x-axis representing the time of the same nail-penetration operation as in FIG. 5(A). In FIG. 5(C), the secondary battery 1004 is put into a deteriorated state by performing a charge / discharge cycle test or the like. The voltage of the secondary battery 1004 in a charged state is Vd, the nail 1003 is stuck into the secondary battery 1004 at time T0, and the voltage of the secondary battery 1004 drops at time T1. At time T1, the nail 1003 is considered to have come into contact with the positive and negative electrodes, and the voltage often drops rapidly. In the secondary battery that is one embodiment of the present invention, after the voltage suddenly drops, it does not reach 0V and maintains a low voltage of Vc. Note that Vc is preferably greater than 0V and less than 1V. Such a change in the voltage of the secondary battery is a characteristic of the secondary battery that is one embodiment of the present invention, and is specifically thought to be caused by the positive electrode active material that is one embodiment of the present invention, which will be described later.

[0103] <Characteristics of secondary batteries in nail penetration test 2> The temperature rise of the secondary battery when performing the nail penetration test, that is, the difference between the temperature before the nail penetration test and the maximum temperature reached after nail penetration (also called temperature rise ΔT), is preferably 130℃ or less, and 100℃ or less. is more preferable, 70°C or lower is more preferable, and even more preferably 50°C or lower. The temperature shall be the temperature within 5 cm, preferably within 2 cm from the nail hole, and specifically the value output using a temperature sensor placed within 5 cm, preferably within 2 cm from the nail hole. The temperature sensor is preferably provided in contact with the outer casing of the secondary battery.

[0104] Further, the maximum temperature during the nail penetration test is preferably 250°C or lower, more preferably 200°C or lower, and even more preferably 180°C or lower. More preferably, the temperature is lower than the temperature at which oxygen release from the positive electrode and thermal decomposition of the positive electrode occur.

[0105] Further, the maximum temperature during the nail penetration test is preferably 150°C or lower, more preferably 100°C or lower, and even more preferably 80°C or lower. More preferably, the temperature is lower than the temperature at which the electrolyte is oxidized by the positive electrode. More preferably, the maximum temperature is lower than the flash point of the mixed solvent used in the electrolytic solution. If the flash point of a mixed solvent is unknown, the flash point of each solvent can be used as a reference.

[0106] <Characteristics of secondary batteries in nail penetration test 3> The amount of positive electrode active material supported on the positive electrode of the secondary battery is 8 mg / cm 2 More than 25mg / cm 2 Less than or equal to 8mg / cm, preferably 8mg / cm 2 More than 23mg / cm 2 Less than or equal to 7mg / cm, preferably 7mg / cm 2 More than 21mg / cm 2 The following should be used. With such a supported amount, a highly safe secondary battery can be provided.

[0107] In the secondary battery, the positive and negative electrode capacity ratio is preferably 75% or more and 110% or less, preferably 75% or more and less than 100%. With such a positive and negative electrode capacity ratio, a highly safe secondary battery can be provided. The positive and negative electrode capacity ratio will be described in detail in Examples.

[0108] <Characteristics of secondary batteries in nail penetration test 4> It is preferable that the particles of the positive electrode active material included in the secondary battery have extremely few cracks. A crack that occurs in a particle may also be referred to as a region where the crystal plane of the particle is shifted or a region where the particle is cracked along the crystal plane, and often occurs along the (00l) plane. For example, when the positive electrode active material is observed by surface SEM or cross-sectional SEM, the number of cracks that can be observed per particle of positive electrode active material is preferably 0 or more and 5 or less.

[0109] Cracks may occur due to pressure applied after applying the positive electrode slurry to the positive electrode current collector. Therefore, in the manufacturing process of the positive electrode of the present invention, the pressure of the press should be set to, for example, a linear pressure of 500 kN / m or less, preferably a linear pressure of 300 kN / m or less, and more preferably a linear pressure of 250 kN / m or less.

[0110] <Electrode density> The electrode density of the positive electrode of the secondary battery is 3.0g / cm 3 More than 4.0g / cm 3 Below, preferably 3.0g / cm 3 More than 3.5g / cm 3 It is good if it is below. When the above-mentioned linear pressure is satisfied, the electrode density can be within this range. It is thought that a positive electrode having such an electrode density and a secondary battery having a positive electrode are unlikely to cause thermal runaway.

[0111] <The surface of the positive electrode active material must be smooth> The surface of the positive electrode active material included in the secondary battery is preferably smooth as a whole. In other words, it is preferable that the entire surface of the positive electrode active material be glossy. It can be said that such a positive electrode active material has no corners or is rounded.

[0112] Further, it is preferable that the positive electrode active material has no or very few microscopic particles attached to its surface. In this specification and the like, ultrafine particles refer to metal oxide particles having a particle size of 0.001 μm or more and 0.1 μm or less. The ultrafine particles may be fragments of the positive electrode active material and / or sources of additive elements that have not reacted.

[0113] The particle size of the ultrafine particles is the Feret diameter or the projection circle equivalent diameter measured from a surface SEM (Scanning Electron Microscope) image. For example, in the surface SEM image of the positive electrode, there are 10 ultrafine particles / cm 2 Below, preferably 5 pieces / cm 2 If it is below, it can be said that there are no or very few ultrafine particles.

[0114] <Heating using flux> In the manufacturing process of a positive electrode active material included in a secondary battery, it is preferable to add and heat a material that functions as a flux together with an additive element source. After the surface of the composite oxide and the additive element source are sufficiently melted by the flux, solidification begins. Therefore, even if ultrafine particles are attached to the surface of the composite oxide, they will be melted in these steps, so they will not remain on the surface or will be extremely small. In other words, the fact that there are no or very few ultrafine particles on the surface of the positive electrode active material can also be said to indicate that a material that functions as a flux was added and heated in the manufacturing process of the positive electrode active material.

[0115] <Initial heating> After initial heating, the positive electrode active material is smooth and glossy overall. Initial heating refers to heating of the composite oxide in the manufacturing process of the positive electrode active material. Initial heating also has the effect of alleviating distortions, crystal defects, etc. that the positive electrode active material has.

[0116] <Crystalline> The positive electrode active material included in the secondary battery preferably has high crystallinity, and is more preferably single crystal or polycrystalline. It is preferable to undergo the above initial heating because the crystallinity of the positive electrode active material increases. In particular, it is preferable that the positive electrode active material is a single crystal because cracks are less likely to occur even if a volume change occurs in the positive electrode active material due to charging and discharging. Furthermore, if the positive electrode active material is a single crystal, a secondary battery using the positive electrode active material is considered to be less likely to catch fire, and safety can be improved.

[0117] <Median diameter of positive electrode active material (D50)> The median diameter (D50) of the positive electrode active material of a highly safe secondary battery will be explained. If the positive electrode active material is too small, it may be difficult to apply the material during the preparation of the positive electrode. Furthermore, if the positive electrode active material is too small, the surface area becomes too large, and there is a risk that the reaction between the surface of the positive electrode active material and the electrolyte will be excessive. Furthermore, if the positive electrode active material is too small, it may be necessary to mix a large amount of conductive material, which may lead to a decrease in capacity. In these respects, the median diameter (D50) of the positive electrode active material is preferably 1 μm or more, preferably 5 μm or more, and more preferably 9 μm or more. A positive electrode active material with a small median diameter (D50) is preferable because it is less likely to cause a dislocation region. In addition, a positive electrode active material with a small median diameter (D50) is preferable because it is less likely to cause cracks even after the pressing process.

[0118] On the other hand, if the active materials are too small, there are concerns that the density of the positive electrode active material layer will decrease and side reactions with the electrolyte will increase. In this respect, the median diameter (D50) of the positive electrode active material is preferably 20 μm or less, preferably 18 μm or less, and more preferably 15 μm or less.

[0119] That is, the median diameter (D50) of the positive electrode active material can be determined by any combination of the above-mentioned upper and lower limits. For example, the median diameter is 1 μm or more and 20 μm or less, preferably 1 μm or more and 18 μm or less, and more preferably 1 μm or more and 15 μm or less.

[0120] Note that the above-mentioned median diameter (D50) can be measured, for example, by observation using SEM or TEM, or by a particle size distribution meter using a laser diffraction / scattering method. When measured using a particle size distribution analyzer using a laser diffraction / scattering method, the median diameter (D50) is the particle diameter when the cumulative amount accounts for 50% in the cumulative curve of particle size distribution measurement results. In addition, as a method to measure the median diameter (D50) from analysis such as SEM or TEM, for example, measure 20 or more particles, create a cumulative curve, and determine the particle diameter when the cumulative amount accounts for 50%. Bye.

[0121] <Laminated secondary battery> A secondary battery that is one embodiment of the present invention will be described. First, a typical laminate type secondary battery will be explained using FIG. 6(A), FIG. 6(B), etc.

[0122] As shown in FIG. 6(A), the secondary battery 1004 includes a plurality of positive electrodes 503, a plurality of negative electrodes 506, and a plurality of separators 508. Separator 508 is provided between positive electrode 503 and negative electrode 506, and in FIG. 6(A), separator 508 is shown with a dotted line for easier viewing. Separator 508 may contain an electrolyte, specifically a liquid electrolyte (also referred to as electrolyte). Note that when a solid electrolyte or a semi-solid electrolyte is used as the electrolyte, the secondary battery 1004 does not need to have the separator 508.

[0123] The positive electrode 503 and the negative electrode 506 each have a protruding tab portion and a portion other than the tab portion. The tab portion can be electrically connected to wiring 1005a, wiring 1005b, etc. in the nail penetration test device. The positive electrode 503 has a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer is preferably formed on both sides of the positive electrode current collector. The negative electrode 506 includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, and the negative electrode active material layer is preferably formed on both sides of the negative electrode current collector.

[0124] As shown in FIG. 6(B), a plurality of positive electrodes 503, a plurality of negative electrodes 506, and a plurality of separators 508 are stacked, and this is sometimes referred to as a laminate in this specification and the like. The tab portions of the plurality of negative electrodes 506 are joined together with the leads 512b at the joint portion 515b, and are electrically connected to each other. Further, the tab portions of the plurality of positive electrodes 503 are joined together with the leads 512a at a joint portion 515a, and are electrically connected to each other. Compared to the positive electrode current collector and the negative electrode current collector (these are simply called current collectors), the positive electrode active material layer and the negative electrode active material layer (these are simply called active material layers) have higher insulating properties, so the tab It is better not to form an active material layer on the portion. A material selected from aluminum, nickel, copper, titanium, or an alloy thereof can be used for the leads 512a and 512b. Ultrasonic bonding can be used for bonding at the bonding portion. Note that in the nail penetration test, it is not necessary to provide the leads 512a and 512b, but if they are provided, the wiring 1005a and the wiring 1005b are electrically connected to the leads 512a and 512b, respectively.

[0125] Furthermore, the secondary battery 1004 has an exterior body (not shown), and the stacked body shown in FIG. 6(A) is housed in the exterior body. After that, an electrolytic solution containing a lithium salt dissolved in the exterior body is injected. That is, the electrolyte has carrier ions, typically lithium ions. A secondary battery containing lithium ions is called a lithium ion secondary battery.

[0126] The exterior body is preferably film-shaped from the viewpoint of weight reduction, and a secondary battery having a film-like exterior body is called a laminate-type secondary battery. Further, from the viewpoint of excellent cooling performance, a laminated structure of a polymer and metal having excellent thermal conductivity may be used for the exterior body. Specifically, polypropylene may be used as the polymer and aluminum may be used as the metal, and nylon or the like may be further placed on the outside of the exterior body. Note that a metal can case may be applied to the exterior body, and when a circular can case is used, it is called a coin-shaped secondary battery.

[0127] [Cathode active material] Next, the positive electrode active material 100, which is one embodiment of the present invention, will be explained using FIG. The positive electrode active material 100 is a carrier ion, typically a lithium ion (Li + It is sufficient to use a compound containing a transition metal and oxygen that is capable of intercalating and deintercalating ). As the transition metal, one or more selected from cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), etc. can be used.

[0128] <Main component> In the positive electrode active material 100 of one embodiment of the present invention, cobalt is preferably used as the main component of the transition metal M responsible for redox reactions. Note that the main component of the transition metal M in this specification and the like refers to the transition metal M that has the highest atomic ratio among the transition metals M. For example, lithium cobalt oxide can be applied to the positive electrode active material 100 as a compound using Co as a transition metal. Alternatively, the positive electrode active material 100 is lithium cobalt oxide (LiCoO 2 ) is preferably added with an additive element. However, the positive electrode active material 100 according to one embodiment of the present invention may have a crystal structure described below. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0129] Further, it is preferable to use nickel in addition to cobalt for the positive electrode active material 100, and lithium cobalt nickelate can be used for the positive electrode active material 100. Alternatively, the positive electrode active material 100 is lithium cobalt nickelate (LiCo 1-y Ni y O 2 ) is preferably added with an additive element. However, the positive electrode active material 100 according to one embodiment of the present invention may have a crystal structure described below. Therefore, the composition of lithium cobalt nickelate is not strictly limited to Li:(Co+Ni):O=1:1:2.

[0130] In addition, in lithium cobalt nickelate, the ratio of nickel to the sum of cobalt and nickel is Ni / (Co+Ni), that is, LiCo 1-y Ni y O 2 y is preferably greater than 0 and less than 0.5. Further, it is more preferably 0.1 or more and 0.3 or less. Further, it is more preferable that it is more than 0.025 and less than 0.215.

[0131] Also LiCo 1-y Ni y O 2 When y is 0.1 or more and 0.3 or less, for example, Co:Ni=90:10 (atomic ratio), Co:Ni=80:20 (atomic ratio), or Co:Ni=70:30 (atomic ratio) Including cases where

[0132] It is preferable that the positive electrode active material 100 has high crystallinity. Further, the positive electrode active material 100 is preferably a single particle (also referred to as a primary particle) rather than a secondary particle. Further, it is more preferable that the particles of the positive electrode active material 100 are single crystal.

[0133] The positive electrode active material 100 according to one embodiment of the present invention preferably has an insulating region or a high resistance region. Note that this area may be referred to as a first area in order to distinguish it from other areas. The above region preferably exists in a narrow width of 1 nm or more and 20 nm or less, preferably 2 nm or more and 10 nm or less, and more preferably 2 nm or more and 5 nm or less when viewed in cross section of the positive electrode active material 100. It can be said to be thickness or width. The narrow region is sometimes referred to as a "shell" in this specification and the like. As the cross-sectional view, for example, a cross-sectional STEM (Scanning Transmission Electron Microscope) image can be used. FIG. 7(A) shows a positive electrode active material 100 having a shell 100s.

[0134] The shell 100s is preferably included in a surface layer portion 100a (see FIG. 7(G)) of the positive electrode active material 100, which will be described later. The positive electrode active material 100 having such a shell 100s is preferable because even when a nail penetration test is performed, the speed of current flowing into the positive electrode active material 100 can be slowed down, and ignition or smoke generation can be suppressed. . In order to slow down the speed of current flowing into the positive electrode active material 100, the shell is more preferably located outside or on the surface side of the surface layer of the positive electrode active material 100.

[0135] <Additional elements> The positive electrode active material 100 preferably includes an additive element. Additive elements include magnesium (Mg), fluorine (F), nickel (Ni), and aluminum (Al), and in addition to these, titanium (Ti), zirconium (Zr), vanadium (V), and iron (Fe). , manganese (Mn), chromium (Cr), niobium (Nb), arsenic (As), zinc (Zn), silicon (Si), sulfur (S), phosphorus (P), boron (B), bromine (Br) , and beryllium (Be).

[0136] Note that the additive elements do not necessarily include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium. .

[0137] For example, a positive electrode active material 100 that does not substantially contain manganese has greater advantages such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics. The weight of manganese contained in the positive electrode active material 100 is, for example, preferably 600 ppm or less, more preferably 100 ppm or less.

[0138] Magnesium is one of the preferred elements for Shell 100s. Therefore, in a cross-sectional view of the positive electrode active material 100, it is preferable that magnesium exists in a narrow width of 1 nm or more and 20 nm or less, preferably 2 nm or more and 10 nm or less, and more preferably 2 nm or more and 5 nm or less from the surface.

[0139] Note that when adding magnesium to the positive electrode active material 100, it is preferable to use magnesium fluoride as a magnesium source, and when magnesium fluoride is used, fluorine can also be added to the positive electrode active material 100. Furthermore, in a secondary battery having the positive electrode active material 100, lithium and fluorine may react during a nail penetration test or the like, but the amount of heat generated is suppressed to be smaller than when lithium reacts with oxygen. Therefore, it is preferable that the positive electrode active material 100 contains fluorine as an additive element. Furthermore, since the reaction between fluorine and lithium is thought to occur when the voltage of the secondary battery starts to rise during the nail penetration test, fluorine is useful in terms of the safety of secondary batteries.

[0140] Further, when LCO is applied to the positive electrode active material 100, which is one embodiment of the present invention, it is preferable to include nickel in addition to magnesium as an additive element. For example, it is preferable that a region containing magnesium and a region containing nickel overlap and connect on a surface where lithium can be inserted and extracted, that is, on an edge surface. In other words, it is preferable that nickel is also present in the shell. With this configuration, desorption of oxygen from the positive electrode active material or structural change of the positive electrode active material can be suppressed.

[0141] Note that the above-mentioned additional elements may be present in the shell 100s, or may be present in the surface layer portion 100a (see FIG. 7(G)), which will be described later. The additive element that contributes to the stability of the crystal structure of the positive electrode active material 100 is preferably present in the surface layer portion 100a where deterioration is likely to start.

[0142] Here, in order to confirm whether or not the shell 100s is formed in the positive electrode active material 100, it is preferable to measure the resistance of the powder that becomes the positive electrode active material (referred to as powder resistance). Specifically, if the powder resistance of the positive electrode active material containing additive elements is higher than that of the positive electrode active material without additive elements, a shell 100s is formed in the positive electrode active material 100. It is thought that there is a possibility that

[0143] It is preferable that the shell 100s also contains cobalt in addition to the additive elements. By having cobalt at least in the shell 100s, lithium ion (Li + ) can be inserted and removed, while also making it possible to slow down the speed of current flow due to internal short circuits. The surface layer portion 100a (see FIG. 7(G)), which will be described later, may also contain cobalt in addition to the additive element.

[0144] The shell 100s described above may be provided to sufficiently cover the entire positive electrode active material 100, or may be provided to cover a specific region of the positive electrode active material 100, for example, a surface other than the (00l) surface, as shown in FIG. 7(A). The shell 100s may be provided to be thick. Depending on the position of the shell, for example, if the shell is located on a plane other than the (00l) plane, oxygen release from there can be suppressed and thermal stability can be improved, resulting in a structure that is less prone to thermal runaway. . Also in the surface layer portion 100a (see FIG. 7(G)), which will be described later, the surface layer portion 100a where the additive element is present may be made thicker in a specific region. For example, it is preferable that the surface layer portion 100a exists thickly on a surface other than the (00l) surface where deterioration tends to begin.

[0145] However, as long as the shell 100s does not ignite in the nail penetration test, the shell 100s may be positioned in any position relative to the positive electrode active material 100, and the lithium ion (Li + Magnesium may be present in areas other than the shell, for example, in the entire surface layer, as long as it is possible to slow the flow of current due to internal short circuits while allowing the insertion and removal of magnesium.

[0146] Consider the concentration of added elements. For example, the concentration of magnesium, which is an additive element, is preferably greater than 0 and less than 10 atomic%, preferably greater than 0 and less than 5 atomic%, and more preferably greater than 0 and less than 2 atomic% in the shell 100s of lithium cobalt oxide. The magnesium concentration can be determined by energy dispersive X-ray spectroscopy (EDX) line analysis. If magnesium is present in the entire surface layer at a high concentration, the insulation properties will be high, making it difficult to obtain favorable battery characteristics in charge / discharge cycle tests and the like. On the other hand, the presence of magnesium in the surface layer, especially in an appropriate region such as the shell, and at an appropriate concentration can stabilize lithium cobalt oxide, preventing heat generation and smoke generation in the above-mentioned nail penetration test, etc. This is preferable because it can suppress. Furthermore, the presence of magnesium at an appropriate concentration in the shell 100s is also expected to increase the hardness of lithium cobalt oxide.

[0147] FIGS. 7(B) to 7(F) are conceptual diagrams in which the area B shown with squares in FIG. 7(A) is enlarged. Here, LCO containing Mg is exemplified as the positive electrode active material 100. As shown in FIG. 7(B), Mg, which is one of the additive elements, is preferably bonded to oxygen in the shell. Furthermore, the shell may contain Co, and Co may be bonded to oxygen. According to the shell shown in Figure 7(B), lithium ion (Li + It is thought that it is possible to slow down the speed of current flow due to internal short circuits while making insertion and removal possible.

[0148] Next, LCO containing Mg and F will be exemplified as the positive electrode active material 100. As shown in FIG. 7(C), F, which is one of the additive elements, does not need to be present in the shell and may be adsorbed on the surface of the positive electrode active material 100. Fluorine is highly electronegative and is known to easily form stable compounds with many elements. Inside the battery, the positive electrode active material 100 is impregnated with an electrolyte, and since fluorine is adsorbed on the surface of the positive electrode active material 100, it can react with the electrolyte near fluorine. Even if an internal short circuit occurs, thermal decomposition of the electrolytic solution, etc. can be suppressed.

[0149] Further, as the positive electrode active material 100, LCO containing Mg and F may have a fluorine compound 100f adsorbed on the surface of the positive electrode active material 100, as shown in FIG. 7(D). Fluorine is highly electronegative and is known to easily form stable compounds with many elements. The positive electrode active material 100 is impregnated with an electrolytic solution, and since the fluorine compound 100f is adsorbed on the surface of the positive electrode active material 100, it can react with the electrolytic solution near the fluorine compound 100f, and the internal Even if a short circuit occurs, thermal decomposition of the electrolytic solution, etc. can be suppressed.

[0150] The adsorption mentioned above includes chemisorption or physical adsorption. Chemical adsorption is the formation of a chemical bond by a chemical reaction between at least one of the additive elements and the surface of the positive electrode active material 100, and physical adsorption is the formation of a chemical bond between at least one of the additive elements and the surface of the positive electrode active material 100. This means that they are adsorbed due to intermolecular forces (van der Waals forces) that act between them.

[0151] Although not shown, the positive electrode active material 100 may contain fluorine in solid solution, for example, fluorine may substitute for a portion of the oxygen in lithium cobalt oxide. The solid-dissolved fluorine only needs to be present in the surface layer of the lithium cobalt oxide, and may be present in the shell. If there is sufficient fluorine in the positive electrode active material 100, both fluorine adsorbed on the surface and fluorine partially substituted for oxygen are present.

[0152] 7(E) and 7(F) are modified examples of the conceptual diagrams shown in FIG. 7(C) and FIG. 7(D), respectively, in which at least part of the F adsorbed on the surface of the positive electrode active material 100 is The figure shows an example in which the moiety is bonded to lithium (Li) present in the shell. Because fluorine is more electronegative than oxygen, lithium and fluorine bond more easily than lithium and oxygen. By combining fluorine with lithium, it is possible to suppress the lithium from combining with oxygen. In other words, combustion of lithium can be suppressed. Therefore, by adsorbing fluorine on the surface of the positive electrode active material 100, ignition, smoke generation, etc. of the secondary battery including the positive electrode active material 100 can be suppressed. For example, even if an internal short circuit occurs in a secondary battery having the positive electrode active material 100, ignition, smoke, etc. of the secondary battery can be suppressed. For example, even when a nail penetration test is performed on a secondary battery having the positive electrode active material 100, ignition, smoke generation, etc. of the secondary battery can be suppressed.

[0153] Further, by combining fluorine with lithium, the movement of the lithium can be suppressed. As a result, even if, for example, an internal short circuit occurs in a secondary battery having the positive electrode active material 100, the speed of the current flowing into the positive electrode active material 100 can be slowed down, and ignition, smoke generation, etc. can be suppressed. Furthermore, even when a nail penetration test is performed on a secondary battery having the positive electrode active material 100, for example, the speed of the current flowing into the positive electrode active material 100 can be slowed down, suppressing ignition, smoke generation, etc. can.

[0154] Examples of fluorides used in lithium ion secondary batteries include LiPF as a lithium salt, which will be described later. 6 , LiBF 4 There are binders such as polyvinylidene fluoride (PVDF). Fluorine from such fluorides may be adsorbed onto the surface of the positive electrode active material 100.

[0155] FIGS. 7(G) and 7(H) are examples of positive electrode active materials in which the boundary between the surface layer 100a and the interior 100b is indicated by a broken line. In this way, the surface layer portion 100a is distinguished from the shell, and the surface layer portion 100a includes the surface. As shown again, the surface layer portion 100a includes a shell.

[0156] FIG. 7(H) is an example of a positive electrode active material in which a crystal grain boundary 101 is added to FIG. 7(G) by a dashed line. Further, FIG. 7(H) also shows a crack formed on a part of the surface of the positive electrode active material, as well as a buried part 102 in contact with the part of the surface. The buried portion 102 preferably contains an additive element such as magnesium.

[0157] <Surface of positive electrode active material> 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 interior portion 100b. Such a surface can be confirmed in a cross-sectional view. Therefore, the surface of the positive electrode active material 100 is aluminum oxide (Al 2 O 3 ) and other metal oxides that do not have lithium sites that can contribute to charging and discharging, carbonates chemically adsorbed after the production of the positive electrode active material, and hydroxyl groups are not included. Note that the deposited metal oxide refers to a metal oxide whose crystal structure does not match that of the interior 100b, for example.

[0158] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen that are capable of intercalating and deintercalating lithium, it contains transition metals M (e.g. Co, Ni, Mn, Fe, etc.) and The interface between the region where oxygen exists and the region where oxygen does not exist is defined as the surface of the positive electrode active material. When a positive electrode active material is subjected to analysis, a protective film is sometimes attached to the surface, but the protective film is not included in the positive electrode active material. As the protective film, a single layer film or a multilayer film of carbon, metal, oxide, resin, etc. may be used.

[0159] Therefore, at the surface position of the positive electrode active material in STEM-EDX-ray analysis, etc., the detected amount of characteristic X-rays of the transition metal M is the average value M of the detected amount of characteristic X-rays of the transition metal M inside. AVE and the average value M of the detected amount of characteristic X-rays of the above transition metal M in the background. BG , or the detected amount of characteristic X-rays of oxygen is the average value of the detected amount of characteristic X-rays of internal oxygen. AVE and the average detected amount of background oxygen characteristic X-rays O BG The point that is 50% of the sum of Note that the detected amount of characteristic X-rays of the transition metal M is the sum of the average detected amount of characteristic X-rays of the internal transition metal M and the average detected amount of characteristic X-rays of the background transition metal M. The point where the detected amount of characteristic X-rays of oxygen is 50% of the sum of the average detected amount of characteristic X-rays of internal oxygen and the average detected amount of characteristic X-rays of background oxygen. If the point is different from the point where Average value M of detected amount of characteristic X-rays AVE and the average value M of the detected amount of characteristic X-rays of the above transition metal M in the background. BG The point that is 50% of the sum of can be adopted as the position of the surface of the positive electrode active material. In addition, in the case of a positive electrode active material containing multiple transition metals M, the M of the element with the largest amount of characteristic X-rays detected inside AVE and M BG can be used to find the surface.

[0160] Average value M of the detected amount of characteristic X-rays of the above transition metal M in the background BG can be determined by averaging the outer range of 2 nm or more, preferably 3 nm or more, avoiding the vicinity where the detected amount of characteristic X-rays of the transition metal M starts to increase, for example. Also, the average value M of the detected amount of characteristic X-rays of the internal transition metal M AVE is at a depth of 30 nm or more, preferably more than 50 nm, from a region where the detected amount of characteristic X-rays of transition metal M and oxygen is saturated and stable, for example, a region where the detected amount of characteristic X-rays of transition metal M starts to increase. , 2 nm or more, preferably 3 nm or more. Average value of detected amount of background oxygen characteristic X-rays O BG and the average value of the detected amount of characteristic X-rays of internal oxygen O AVE can be found in the same way.

[0161] In addition, the surface of the positive electrode active material 100 in a cross-sectional STEM image, etc. is the boundary between the area where an image derived from the crystal structure of the positive electrode active material is observed and the area where it is not observed, and the surface of the positive electrode active material 100 is the boundary between the area where an image derived from the crystal structure of the positive electrode active material is observed and the area where the image derived from the crystal structure of the positive electrode active material is not observed. The outermost region is defined as an atomic column originating from the nucleus of a metal element with a larger atomic number than lithium.

[0162] Furthermore, the spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the peak position (also referred to as maximum value) of the characteristic X-ray corresponding to the added element may deviate by about 1 nm. For example, even if the characteristic X-ray peak position corresponding to an additive element such as magnesium is located outside the surface determined above, if the difference between the peak and the surface is less than 1 nm, it can be considered as an error.

[0163] A peak in STEM-EDX-ray analysis refers to the local maximum value or maximum value of characteristic X-rays corresponding to each element. Note that noise in STEM-EDX ray analysis can be considered to be a measured value with a half-width below the spatial resolution (R), for example, below R / 2.

[0164] The influence of noise can be reduced by scanning the same location multiple times under the same conditions. For example, the integrated value obtained by measuring six scans can be used as a graph of the characteristic X-rays of each element. The number of scans is not limited to 6, and it is also possible to perform more than 6 scans and use the average as a graph of the characteristic X-rays of each element.

[0165] STEM-EDX ray analysis can be performed, for example, as follows. First, a protective film is deposited on the surface of the positive electrode active material. For example, carbon can be deposited using a carbon coating unit of an ion sputtering device (MC1000 manufactured by Hitachi High-Tech).

[0166] Next, the positive electrode active material is sliced ​​into thin sections to prepare a STEM cross-sectional sample. For example, thinning processing can be performed using a FIB-SEM device (XVision200TBS manufactured by Hitachi High-Tech). At that time, the pickup is performed using an MPS (micro probing system), and the finishing conditions can be, for example, an accelerating voltage of 10 kV.

[0167] For STEM-EDX ray analysis, for example, a STEM device (HD-2700 manufactured by Hitachi High-Tech) can be used, and an EDAX Octane T Ultra W (Dual EDS) can be used as an EDX detector. During EDX ray analysis, the accelerating voltage of the STEM device is set to 200 kV, the emission current is set to 6 μA or more and 10 μA or less, and the depth and unevenness of the part of the thin sectioned sample are measured. The magnification is, for example, about 150,000 times. The conditions for EDX ray analysis are a beam diameter of 0.2 nmφ, drift correction, a line width of 42 nm, a pitch of 0.2 nm, and a frame count of 6 or more.

[0168] In addition, the crystal grain boundaries 101 are, for example, areas where particles of the positive electrode active material 100 are stuck together, areas where the crystal orientation changes inside the positive electrode active material 100, in other words, the repetition of bright lines and dark lines in a STEM image etc. is discontinuous. This refers to areas with a large number of crystal defects, areas with a disordered crystal structure, etc. Furthermore, crystal defects refer to defects that can be observed in cross-sectional TEM, cross-sectional STEM images, etc., that is, structures where other atoms enter between lattices, cavities, etc. The grain boundary 101 can be said to be one of the planar defects. Further, the vicinity of the grain boundary 101 refers to a region within 10 nm from the grain boundary 101.

[0169] <Continuous change in crystal structure> Furthermore, it is preferable that the crystal structure changes continuously from the interior 100b toward the surface due to the concentration gradient of the additive element as described above. Alternatively, it is preferable that the crystal orientations of the surface layer portion 100a and the interior portion 100b are approximately the same.

[0170] For example, it is preferable that the crystal structure changes continuously from the interior 100b, which is a layered rock salt type, toward the surface and surface layer portion 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type. Alternatively, it is preferable that the crystal orientations of the surface layer portion 100a, which has the characteristics of a rock salt type, or both of a rock salt type and a layered rock salt type, and the crystal orientation of the layered rock salt type interior 100b are approximately the same.

[0171] In this specification, etc., the layered rock-salt crystal structure belonging to space group R-3m, which is possessed by composite oxides containing transition metals such as lithium and cobalt, refers to a structure in which cations and anions are arranged alternately. It has a rock salt-type ion arrangement, and the transition metal and lithium are regularly arranged to form a two-dimensional plane, so it is a crystal structure that allows two-dimensional diffusion of lithium. Note that there may be defects such as cation or anion deficiency. Strictly speaking, the layered rock salt crystal structure may have a structure in which the lattice of the rock salt crystal is distorted.

[0172] Further, the rock salt type crystal structure refers to a structure having a cubic system crystal structure, including a crystal structure belonging to the space group Fm-3m, in which cations and anions are arranged alternately. Note that there may be a deficiency of cations or anions.

[0173] Furthermore, the presence of both layered rock salt type and rock salt type crystal structure characteristics can be determined by electron beam diffraction, TEM images, cross-sectional STEM images, etc.

[0174] The rock salt type has no distinction in cation sites, but the layered rock salt type has two types of cation sites in its crystal structure, one mostly occupied by lithium and the other occupied by transition metals. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately is the same for both the rock salt type and the layered rock salt type. Among the bright spots of the electron beam diffraction pattern corresponding to the crystal planes forming this two-dimensional plane, when the center spot (transparent spot) is set as the origin 000, the bright point closest to the center spot is the ideal one. For example, a state rock salt type has a (111) plane, and a layered rock salt type has a (003) plane, for example. For example, rock salt type MgO and layered rock salt type LiCoO 2 When comparing the electron diffraction patterns of LiCoO 2 The distance between the bright spots on the (003) plane of MgO is observed to be approximately half the distance between the bright spots on the (111) plane of MgO. Therefore, in the analysis area, for example, rock salt type MgO and layered rock salt type LiCoO 2 When the electron beam diffraction pattern has two phases, there is a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to the halite type and layered halite type have strong brightness, and bright spots that occur only in the layered halite type have weak brightness.

[0175] Furthermore, in cross-sectional STEM images, etc., when a layered rock salt crystal structure is observed from a direction perpendicular to the c-axis, layers observed with strong brightness and layers observed with weak brightness are observed alternately. The rock salt type does not have these characteristics because there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both a rock salt type and a layered rock salt type, when observed from a specific crystal orientation, layers that are observed with strong brightness and layers that are observed with weak brightness are observed alternately in cross-sectional STEM images, etc. In addition, a metal with a higher atomic number than lithium exists in a part of the lithium layer, which has an even weaker brightness.

[0176] Layered rock salt crystals and anions of rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). In the O3' type and monoclinic O1(15) crystals described below, the anions are also presumed to have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there is a crystal plane in which the cubic close-packed structure composed of anions is oriented in the same direction.

[0177] Alternatively, it can also be explained as follows. Anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt type is in the space group R-3m and has a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a complex hexagonal lattice, and the (0001) plane of the layered rock salt type has a hexagonal lattice. The triangular lattice of the cubic {111} plane has an atomic arrangement similar to the hexagonal lattice of the (0001) plane of the layered rock salt type. When both lattices are consistent, it can be said that the orientations of the cubic close-packed structures are aligned.

[0178] However, the space group of layered rock salt crystals and O3' type crystals is R-3m, which is different from the space group Fm-3m of rock salt crystals (the space group of general rock salt crystals), so the above conditions are The Miller index of the crystal planes to satisfy is different between layered rock salt type crystals and O3' type crystals and rock salt type crystals. In this specification, in a layered rock salt type crystal, an O3' type crystal, and a rock salt type crystal, when the directions of the cubic close-packed structures composed of anions are aligned, it may be said that the orientations of the crystals approximately match. . In addition, having three-dimensional structural similarity such that the crystal orientations roughly match, or having the same crystallographic orientation, is called topotaxy.

[0179] The fact that the orientations of the crystals in the two regions roughly match means that TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field STEM, high-angle scattering annular dark-field scanning transmission electron microscopy) images, and ABF-STEM (Annular Bright -Field STEM, annular bright field scanning transmission electron microscope) images, electron diffraction patterns, TEM images, and FFT patterns such as STEM images. XRD, electron beam diffraction, neutron beam diffraction, etc. can also be used as material for judgment.

[0180] FIG. 8 shows an example of a TEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS roughly match. Images that reflect the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0181] For example, in a high-resolution TEM image, contrast derived from crystal planes can be obtained. Due to diffraction and interference of the electron beam, for example, when an electron beam is incident perpendicular to the c-axis of a layered rock-salt complex hexagonal lattice, the contrast originating from the (0003) plane is divided into bright bands (bright strips) and dark bands (dark strips). ) can be obtained by repeating. Therefore, repeating bright lines and dark lines are observed in TEM images, and bright lines (for example, L shown in Figure 8) RS and L LRS ) is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes roughly match, that is, the crystal orientations roughly match. 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 orientations of the crystals roughly match.

[0182] In HAADF-STEM images, contrast is proportional to the atomic number, and elements with higher atomic numbers are observed brighter. For example, in the case of layered rock salt type lithium cobalt oxide belonging to space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the position of the cobalt atoms, making the arrangement of the cobalt atoms clear. It is observed as a line or an array of bright points. Therefore, when lithium cobalt oxide, which has a layered rock salt crystal structure, is observed perpendicularly to the c-axis, the arrangement of cobalt atoms perpendicular to the c-axis is observed as a bright line or an array of strong bright points; The arrangement of lithium atoms and oxygen atoms is observed as dark lines or regions of low brightness. The same applies when lithium cobalt oxide contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0183] Therefore, in a HAADF-STEM image, repeating bright lines and dark lines are observed in two regions with different crystal structures, and if the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, the atomic arrangement roughly matches. In other words, it can be determined that the crystal orientations are approximately the same. 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 approximately match.

[0184] Note that in ABF-STEM, elements with smaller atomic numbers are observed brighter, but since it is similar to HAADF-STEM in that contrast is obtained according to the atomic number, crystal orientation can be determined in the same way as in HAADF-STEM images. be able to.

[0185] FIG. 9(A) shows an example of a STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS roughly match. The FFT pattern of the region of rock salt crystal RS is shown in FIG. 9(B), and the FFT pattern of the region of layered rock salt crystal LRS is shown in FIG. 9(C). The left side of FIGS. 9(B) and 9(C) shows the composition, the JCPDS card number, and the d value and angle calculated from the JCPDS card data. Actual measurements are shown on the right. Spots marked with O are 0th order diffraction.

[0186] The spots labeled A in FIG. 9(B) originate from the 11-1 reflection of the cubic crystal. The spots labeled A in FIG. 9(C) originate from layered rock salt type 0003 reflections. From FIG. 9(B) and FIG. 9(C), 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 match. That is, it can be seen that the straight line passing through AO in FIG. 9(B) and the straight line passing through AO in FIG. 9(C) are approximately parallel. As used herein, "approximately matching" and "approximately parallel" mean that the angle formed by the straight lines is 5 degrees or less, or 2.5 degrees or less.

[0187] In this way, in the FFT pattern and electron diffraction pattern, if the orientations of the layered rock salt type crystal and the rock salt type crystal roughly match, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type. , may roughly match. At this time, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. The fact that the reciprocal lattice points are spot-like and not continuous with other reciprocal lattice points means that the crystallinity is high.

[0188] In addition, if the direction of the 11-1 reflection of the cubic crystal and the direction of the 0003 reflection of the layered rock salt type are approximately the same as described above, depending on the incident direction of the electron beam, the direction of the 0003 reflection of the layered rock salt type may change. Spots that are not derived from layered rock salt 0003 reflections may be observed on the reciprocal lattice space that differs from the orientation. For example, the spot labeled B in FIG. 9(C) is derived from the 1014 reflection of the layered rock salt type. This is an angle of 52° or more and 56° or less from the direction of the reciprocal lattice point (A in Figure 9 (C)) derived from the 0003 reflection of the layered rock salt type (that is, ∠AOB is 52° or more and 56° or less). ), may be observed where d is 0.19 nm or more and 0.21 nm or less. Note that this index is just an example, and does not necessarily have to match this index. For example, reciprocal lattice points equivalent to 0003 and 1014 may be used.

[0189] Similarly, spots that are not derived from the cubic 11-1 reflection may be observed on a reciprocal space different from the direction in which the cubic 11-1 reflection is observed. For example, the spot labeled B in FIG. 9(B) is derived from 200 reflections of a cubic crystal. This is an angle of 54° or more and 56° or less (that is, ∠AOB is 54° or more and 56° or less) from the orientation of the reciprocal lattice point (A in Figure 9 (B)) derived from the 11-1 reflection of the cubic crystal. , and a diffraction spot may be observed at that location. Note that this index is just an example, and does not necessarily have to match this index. For example, reciprocal lattice points equivalent to 11-1 and 200 may be used.

[0190] In layered rock salt type positive electrode active materials such as lithium cobalt oxide, (0003) planes and planes equivalent to this, and (10-14) planes and planes equivalent to this tend to appear as crystal planes. Are known. Therefore, for example, when observing the (0003) plane with a TEM, first select a positive electrode active material particle in which a crystal plane expected to be the (0003) plane is observed with the SEM, etc. 10] It is preferable to process the positive electrode active material particles into thin sections using FIB (Focused Ion Beam) or the like so that the (0003) plane can be observed as the incident surface. When it is desired to judge whether the crystal orientation matches, it is preferable to thin the layered rock salt so that the (0003) plane can be easily observed.

[0191] <Crystal structure> The crystal structure of the positive electrode active material 100 according to one embodiment of the present invention will be explained while comparing it with a conventional positive electrode active material.

[0192] ≪Li x M.O. 2 When x inside is 1≫ FIG. 12 shows the crystal structure of the positive electrode active material 100 according to one embodiment of the present invention. The positive electrode active material 100 of one embodiment of the present invention is in a discharge state, that is, Li x M.O. 2 In the case where x=1 (M is a transition metal, specifically cobalt and / or nickel), it is preferable to have a layered rock salt type crystal structure belonging to space group R-3m. A layered rock salt type composite oxide has a high discharge capacity, has a two-dimensional lithium ion diffusion path, is suitable for inserting and extracting lithium ions, and is excellent as a positive electrode active material for secondary batteries. Therefore, it is particularly preferable that the interior 100b, which occupies most of the volume of the positive electrode active material 100, has a layered rock salt type crystal structure. Figure 13 shows the crystal structure of the layered rock salt type with R-3m O3 attached. R-3m O3 has lattice constants a=2.81610, b=2.81610, c=14.05360, α=90.0000, β=90.0000, 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 10). In Figure 13, O3 is added below the space group, but in this crystal structure, lithium occupies an octahedral site, and MO in the unit cell. 2 Because there are three layers, this crystal structure is sometimes called the O3 type crystal structure. In addition, M.O. 2 A layer refers to a structure in which an octahedral structure in which six oxygen atoms are coordinated with the transition metal M is continuous in a plane in an edge-sharing state. This is sometimes called a layer consisting of an octahedron of transition metal M and oxygen. Furthermore, although FIG. 13 shows that lithium ions are present in all lithium sites, as described above, ions of additional elements, such as magnesium ions, may be located in lithium sites.

[0193] On the other hand, the surface layer 100a of the positive electrode active material 100 according to one embodiment of the present invention is designed so that even if lithium is removed from the positive electrode active material 100 due to charging, the layered structure consisting of an octahedron of transition metal M and oxygen in the interior 100b will not be broken. It is preferable to have a reinforcing function. Alternatively, it is preferable that the surface layer portion 100a functions as a barrier film for the positive electrode active material 100. Alternatively, it is preferable that the surface layer portion 100a, which is the outer peripheral portion of the positive electrode active material 100, reinforces the positive electrode active material 100. Reinforcement here refers to suppressing structural changes in the surface layer 100a and interior 100b of the positive electrode active material 100, such as desorption of oxygen and / or displacement of the layered structure consisting of an octahedron of transition metal M and oxygen; and / or suppressing the decomposition of an organic electrolyte or the like on the surface of the positive electrode active material 100.

[0194] Therefore, it is preferable that the surface layer portion 100a has a crystal structure different from that of the interior portion 100b. Further, it is preferable that the surface layer portion 100a has a composition and crystal structure that are more stable at room temperature (25° C.) than the interior portion 100b. For example, it is preferable that at least a portion of the surface layer portion 100a of the positive electrode active material 100 according to one embodiment of the present invention has a rock salt crystal structure. Alternatively, the surface layer portion 100a preferably has both a layered rock salt type crystal structure and a rock salt type crystal structure. Alternatively, the surface layer portion 100a preferably has characteristics of both a layered rock salt type and a rock salt type crystal structure.

[0195] The surface layer portion 100a is a region where lithium ions are first desorbed during charging, and is a region where the lithium concentration tends to be lower than that in the interior portion 100b. Further, it can be said that some of the bonds of the atoms on the surface of the particles of the positive electrode active material 100 included in the surface layer portion 100a are in a state of being broken. Therefore, the surface layer portion 100a tends to become unstable, and can be said to be a region where the crystal structure tends to deteriorate. For example, if the crystal structure of the layered structure consisting of octahedrons of transition metal M and oxygen shifts in the surface layer 100a, the influence will chain to the interior 100b, causing the crystal structure of the layered structure to shift in the interior 100b as well, causing the positive electrode active material to shift. 100 is thought to lead to deterioration of the entire crystal structure. On the other hand, if the surface layer 100a can be made sufficiently stable, Li x CoO 2 Even when x inside is small, for example, even when x is 0.24 or less, the layered structure consisting of the octahedron of transition metal M and oxygen in the interior 100b can be made difficult to break. Furthermore, it is possible to suppress misalignment of the octahedral layer of transition metal M and oxygen in the interior 100b.

[0196] Furthermore, the interior 100b of the positive electrode active material 100 preferably has a low density of defects such as dislocations. Further, it is preferable that the positive electrode active material 100 has a large crystallite size as measured by XRD. In other words, it is preferable that the interior 100b has high crystallinity. Further, the surface of the positive electrode active material 100 is preferably smooth. These characteristics are important elements that support the reliability of the positive electrode active material 100 when used in a secondary battery. If the reliability of the positive electrode active material is high, the upper limit of the charging voltage of the secondary battery can be increased, and the secondary battery can have a high charge / discharge capacity.

[0197] Dislocations in the interior 100b can be observed by TEM, for example. If the density of defects such as dislocations is sufficiently low, no defects such as dislocations may be observed in a specific 1 μm square of the observation sample. Note that a dislocation is a type of crystal defect and is different from a point defect.

[0198] The crystallite size measured by XRD is preferably 300 nm or more, for example. The larger the crystallite size, the more Li x CoO 2 In a state where x is small, the O3' type crystal structure is likely to be maintained, and shortening of the c-axis length is likely to be suppressed.

[0199] It is thought that the fewer defects such as dislocations observed by TEM, the larger the crystallite size measured by XRD.

[0200] It is preferable to obtain the XRD diffraction pattern when calculating the crystallite size with only the positive electrode active material, but it is also possible to obtain it with the positive electrode containing a current collector, binder, conductive material, etc. in addition to the positive electrode active material. You may. However, in the state of the positive electrode, the particles of the positive electrode active material may be oriented such that the crystal planes of the particles of the positive electrode active material are aligned in one direction due to the influence of pressure etc. during the manufacturing process. If the orientation is strong, the crystallite size may not be calculated accurately, so take out the positive electrode active material layer from the positive electrode, remove some of the binder, etc. in the positive electrode active material layer using a solvent, etc., and then fill it into a sample holder. It is more preferable to obtain an XRD diffraction pattern by the method described below. There is also a method in which grease is applied onto a silicon non-reflective plate and a powder sample of a positive electrode active material or the like is adhered to the silicon non-reflective plate.

[0201] To calculate the crystallite size, for example, use Bruker D8 ADVANCE, use CuKα as X-ray, 2θ is 15° or more and 90° or less, increment 0.005, and the diffraction pattern obtained with LYNXEYE XE-T detector and lithium cobalt oxide. ICSD coll.code.172909 can be used as the document value. Analysis can be performed using DIFFRAC.TOPAS ver.6 as crystal structure analysis software, and the settings can be made as follows, for example. Emission Profile:CuKa5.lam Background:Chebychev polynomial, 5th order Instrument Primary radius:280mm Secondary radius:280mm Linear PSD 2Th angular range:2.9 FDS angle:0.3 Full Axial Convolution Filament length:12mm Sample length:15mm Receiving Slit length:12mm Primary Sollers:2.5 Secondary Sollers:2.5 Corrections Specimen displacement:Refine LP Factor:0

[0202] It is preferable to use the value of LVol-IB, which is the crystallite size calculated by the above method, as the crystallite size. Note that if the calculated Preferred Orientation is less than 0.8, the orientation of the sample is too strong and the sample may not be suitable for determining the crystallite size.

[0203] 〔distribution〕 The distribution of additive elements in the positive electrode active material 100 will be explained using a discharge state as an example. In order to make the surface layer portion 100a have a stable composition and crystal structure, the surface layer portion 100a preferably contains an additive element, and more preferably contains a plurality of additive elements. Further, it is preferable that the surface layer portion 100a has a higher concentration of one or more selected additive elements than the inside portion 100b. Further, it is preferable that one or more selected from the additive elements included in the positive electrode active material 100 have a concentration gradient. Further, it is more preferable that the positive electrode active material 100 has a different distribution depending on the added element. For example, it is more preferable that the depth of the detected amount peak from the surface differs depending on the added element. The peak of the detected amount here refers to the maximum value of the detected amount, and the peak of the detected amount in the surface layer portion 100a refers to the maximum value of the detected amount in the surface layer portion 100a or 50 nm or less from the surface. The detected amount refers to, for example, the count in EDX-ray analysis.

[0204] Among the additive elements, it is preferable that the detected amount of magnesium in the surface layer portion 100a is larger than the detected amount in the inner portion 100b. Furthermore, it is preferable that the detected amount of magnesium has a peak in a region closer to the surface in the surface layer portion 100a.

[0205] Among the additive elements, the amount of fluorine detected in the surface layer portion 100a is preferably larger than the amount detected in the interior portion 100b, similarly to magnesium. Furthermore, it is preferable that the detected amount of fluorine has a peak in a region closer to the surface in the surface layer portion 100a.

[0206] Among the additive elements, it is preferable that the amount of nickel detected in the surface layer portion 100a is larger than the amount detected in the interior portion 100b. Furthermore, it is preferable that the detected amount of nickel has a peak in a region closer to the surface within the surface layer portion 100a. For example, in the surface layer portion 100a, it is preferable that the detected amount of nickel in the shell is larger than the detected amount of nickel in the region inside the shell, and that the shell has a peak of the detected amount of nickel. Here, the ratio (Ni / Co) of the number of nickel Ni atoms to the number of cobalt Co atoms in the shell of the surface layer portion 100a is less than 1. In other words, the number of nickel Ni atoms included in the shell of the surface layer portion 100a is smaller than the number of cobalt Co atoms. Furthermore, the ratio (Ni / Co) of the number of nickel Ni atoms to the number of cobalt Co atoms at the peak of the detected amount of nickel is less than 1. Furthermore, the ratio of the number of nickel Ni atoms to the number of cobalt Co atoms in the region inside the shell (Ni / Co) is the ratio of the number of nickel Ni atoms in the shell to the number of cobalt Co atoms (Ni / Co). ) is smaller than Note that the amount of nickel detected in the interior 100b may be very small compared to that in the surface layer 100a. From the above, the number of nickel Ni atoms is smaller than the number of cobalt Co atoms in both the surface layer portion 100a and the interior 100b. Further, the number of nickel Ni atoms included in the positive electrode active material 100 is smaller than the number of cobalt Co atoms.

[0207] Further, when both magnesium and nickel are contained, it is preferable that the distributions of magnesium and nickel overlap. Note that in this specification and the like, the expression that the distributions of element A and element B overlap includes that the peaks of the detected amounts of element A and element B are at the same depth, and the peaks do not need to overlap entirely. For example, the peak of the detected amount of element A may be closer to the surface, and the peak of the detected amount of element B may be closer to the surface. However, the difference in depth between the peak of the detected amount of element B and the peak of the detected amount of element A is preferably within 3 nm. As a specific example, the expression that the distributions of magnesium and nickel overlap includes that the peaks of the detected amounts of magnesium and nickel are at the same depth, and the peaks do not need to overlap entirely. For example, the peak of the detected amount of magnesium may be closer to the surface, and the peak of the detected amount of nickel may be closer to the surface. However, the difference in depth between the peak of the detected amount of nickel and the peak of the detected amount of magnesium is preferably within 3 nm.

[0208] Among the additive elements, it is also preferable that the detected amount of titanium in the surface layer portion 100a is larger than the detected amount in the inner portion 100b. Furthermore, it is preferable that the detected amount of titanium has a peak in a region closer to the surface in the surface layer portion 100a.

[0209] Among the additional elements, silicon, phosphorus, boron, and / or calcium are also preferably detected in a larger amount in the surface layer 100a than in the interior 100b. Moreover, it is preferable that the detected amount of silicon, phosphorus, boron, and / or calcium has a peak in a region closer to the surface in the surface layer portion 100a.

[0210] Among the additive elements, it is preferable that aluminum has a detected amount peak inside the element compared to magnesium. The distributions of magnesium and aluminum may overlap, or the distributions of magnesium and aluminum may hardly overlap. The peak of the detected amount of aluminum may be present in the surface layer portion 100a, or may be deeper than the surface layer portion 100a. For example, it is preferable to have a peak in a region of 5 nm or more and 30 nm or less from the surface toward the inside.

[0211] The distribution of aluminum may not be a normal distribution. For example, set the aluminum distribution curve to the maximum value Max Al When separated, the length of the hem may differ between the front side and the inside side. Maximum amount of aluminum detected (Max Al ) height (1 / 5 Max Al ) is divided into two by a perpendicular line drawn from the maximum value to the horizontal axis, and the peak width on the surface side (W s ) than the inner peak width (W c ) may be large.

[0212] The reason why aluminum is distributed further into the interior than magnesium is considered to be because aluminum is easier to diffuse than magnesium. On the other hand, the reason why the amount of aluminum detected in the region closest to the surface is small is presumed to be because aluminum can exist more stably in regions where magnesium and the like are not present as a solid solution than in regions where magnesium and the like are dissolved in solid solution at a high concentration.

[0213] More specifically, in the layered rock salt type or cubic rock salt type region of space group R-3m, in the region where magnesium is dissolved in solid solution at a high concentration, the layered rock salt type LiAlO 2 Compared to , the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. In addition, around cobalt, Li + is Mg 2+ The valence change replaced by Co 3+ FromCo 2+ This can be compensated by oxidation and balance the cations. However, since Al can only be trivalent, it is considered difficult to coexist with magnesium in a rock salt type or layered rock salt type structure.

[0214] Among the additive elements, it is preferable that manganese, like aluminum, has a detection peak within the range compared to magnesium.

[0215] However, the additive elements do not necessarily have to have the same concentration gradient or distribution in all the surface layer portions 100a of the positive electrode active material 100.

[0216] Furthermore, the (001) oriented surface of the positive electrode active material 100 may have a different distribution of additive elements from other surfaces. For example, the (001) oriented surface and its surface layer portion 100a may have a lower detection amount of one or more selected additive elements compared to the non-(001) oriented surface. Specifically, the detected amount of nickel may be low. Particularly in the case of analysis methods that detect characteristic X-rays such as EDX, the energies of cobalt's Kβ and nickel's Kα are close to each other, making it difficult to detect trace amounts of nickel in materials where cobalt is the main element. Alternatively, in the (001) oriented surface and its surface layer portion 100a, the peak position of the detected amount of one or more selected from the additive elements may be shallower than in the surface other than the (001) oriented surface. Specifically, in the (001) oriented surface and its surface layer portion 100a, the peak positions of the detected amounts of magnesium and aluminum may be shallower than in the (001) oriented surface.

[0217] In the layered rock salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This is CoO 2 It can be said that it has a structure in which the lithium layer and the lithium layer are alternately stacked parallel to the (001) plane. Therefore, the lithium ion diffusion path also exists parallel to the (001) plane.

[0218] CoO 2 Since the layer is relatively stable, the surface of the positive electrode active material 100 is more stable if it has a (001) orientation. The main diffusion path of lithium ions during charging and discharging is not exposed on the (001) plane.

[0219] On the other hand, on surfaces other than the (001) orientation, lithium ion diffusion paths are exposed. Therefore, the surface other than the (001) orientation and its surface layer 100a are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are easily unstable because they are the regions where lithium ions are first desorbed. . Therefore, in order to maintain the crystal structure of the entire positive electrode active material 100, it is preferable to reinforce the surface other than the (001) orientation and its surface layer portion 100a.

[0220] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, attention may be paid to the concentration distribution of the additive element on the surface other than the (001) orientation and the surface layer portion 100a thereof. Among the additive elements, it is particularly preferable that nickel be detected on the surface other than the (001) orientation and on the surface layer portion 100a thereof. On the other hand, in the (001) oriented surface and its surface layer portion 100a, the concentration of the additive element may be low or absent as described above.

[0221] For example, the distribution of magnesium on the (001) oriented surface and its surface layer 100a preferably has a half width of 10 nm or more and 200 nm or less, more preferably 50 nm or more and 150 nm or less, and 80 nm or more and 120 nm or less. and even more preferable. Further, the distribution of magnesium on the non-(001) oriented surface and its surface layer 100a preferably has a half width of more than 200 nm and less than 500 nm, more preferably more than 200 nm and less than 300 nm, and more preferably more than 230 nm. More preferably, the wavelength is 270 nm or less.

[0222] Further, the half width of the distribution of nickel on the non-(001) oriented surface and its surface layer 100a is preferably 30 nm or more and 150 nm or less, more preferably 50 nm or more and 130 nm or less, and 70 nm or more and 110 nm or less. is even more preferable.

[0223] In a manufacturing method in which additive elements are mixed and then heated, which will be described in a later embodiment, the additive elements may spread mainly through the diffusion path of lithium ions. Therefore, in order to make the distribution of the additive elements on the surface other than the (001) orientation and its surface layer portion 100a within a preferable range, it is better to mix the additive elements after producing highly pure lithium cobalt oxide.

[0224] 〔magnesium〕 Magnesium is divalent, and in its layered rock salt crystal structure, aluminum or nickel, which is an additive element, stably exists in the cobalt site, so magnesium ions tend to exist in the lithium site rather than the cobalt site, that is, they enter the lithium site. Cheap. The presence of magnesium at an appropriate concentration in the lithium sites in the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This means that the magnesium present at the lithium site is CoO 2 It is assumed that this is because they function as pillars that support the layers. In addition, the presence of magnesium allows Li x CoO 2 When x is, for example, 0.24 or less, desorption of oxygen around magnesium can be suppressed, and thermal decomposition reactions can be suppressed. Furthermore, the presence of magnesium can be expected to increase the density of the positive electrode active material 100. Furthermore, when the magnesium concentration in the surface layer portion 100a is high, it can be expected that the corrosion resistance against hydrofluoric acid produced by decomposition of an organic electrolyte and the like will be improved.

[0225] If magnesium is at an appropriate concentration, it will not have an adverse effect on the insertion and desorption of lithium during charging and discharging, and the above benefits can be enjoyed. However, if magnesium is in excess, intercalation and deintercalation of lithium may be adversely affected. Furthermore, the effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters the cobalt site in addition to the lithium site. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) that do not substitute for either lithium sites or cobalt sites may segregate on the surface of the positive electrode active material and become a resistance component of the secondary battery. Furthermore, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the lithium site, reducing the amount of lithium that contributes to charging and discharging.

[0226] Therefore, it is preferable that the entire positive electrode active material 100 has an appropriate amount of magnesium. For example, the number of magnesium atoms is preferably 0.002 times or more and 0.06 times or less, more preferably 0.005 times or more and 0.03 times or less, and even more preferably about 0.01 times the number of cobalt atoms. Here, the amount of magnesium contained in the entire positive electrode active material 100 refers to the amount of magnesium contained in the entire positive electrode active material 100, using, for example, GD-MS (glow discharge mass spectrometry), ICP-MS (inductively coupled plasma mass spectrometry), etc. It may be a value obtained by analysis, or it may be a value based on the blending of raw materials in the process of producing the positive electrode active material 100.

[0227] [Fluorine] Fluorine is a monovalent anion, and when fluorine is adsorbed on the surface of the positive electrode active material 100, the energy for desorbing lithium from the positive electrode active material 100 becomes smaller. Fluorine may be substituted for part of the oxygen in the surface layer portion 100a as long as the energy is reduced. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. In other words, when fluorine is not present, cobalt ions change from trivalent to tetravalent as lithium is eliminated. On the other hand, when fluorine is present, the cobalt ion changes from divalent to trivalent as lithium is eliminated. The redox potential of cobalt ions is different between the two. Therefore, the desorption and insertion of lithium ions near fluorine tends to occur smoothly, and it is preferable that the positive electrode active material 100 has fluorine on the surface or surface layer portion. When the positive electrode active material 100 containing fluorine is used in a secondary battery, charging / discharging characteristics, large current characteristics, etc. can be improved. In addition, the presence of fluorine on the surface or surface layer that is in contact with the electrolyte, or the adsorption or attachment of fluoride to the surface, suppresses excessive reaction between the positive electrode active material 100 and the electrolyte. Can be done. Furthermore, corrosion resistance against hydrofluoric acid can be effectively improved.

[0228] In addition, when the melting point of a fluorine compound (sometimes called a fluoride) such as lithium fluoride is lower than the melting point of other additive element sources, the fluorine compound is a fluxing agent (sometimes called a fluoride) that lowers the melting point of the other additive element source. (also called a fluxing agent). Fluorine compounds are LiF and MgF 2 In the case where LiF and MgF are 2 Since the eutectic point P of is around 742°C (T1), it is preferable to set the heating temperature to 742°C or higher in the heating step after mixing the additive elements.

[0229] Here, differential scanning calorimetry (DSC) tests on fluorine compounds and mixtures will be explained using FIG. 11. The mixture in Figure 11 includes lithium cobalt oxide as the lithium oxide and LiF and MgF as the fluorine compounds. 2 It was mixed using More specifically, the mixture is LiCoO 2 :LiF:MgF 2 =100:0.33:1 (mol ratio). The fluorine compounds in Figure 11 are LiF and MgF. 2 It is a mixture of Specifically, the mixture is LiF:MgF 2 They were mixed at a ratio of =1:3 (mol ratio).

[0230] As shown in Figure 11, an endothermic peak is observed around 735°C for fluorine compounds. Also lithium cobalt oxide, LiF, and MgF 2 In the mixture, an endothermic peak is observed around 830℃. Therefore, the heating temperature after mixing the additive elements is preferably 742°C or higher, more preferably 830°C or higher. Further, the temperature may be 800° C. (T2 in FIG. 10) or higher, which is between these.

[0231] 〔nickel〕 Nickel can exist at both cobalt and lithium sites. When present in cobalt sites, the redox potential is lower than that of cobalt, so it can be said that it is easier to release lithium during charging, for example. Therefore, it can be expected that the charging and discharging speed will be faster.

[0232] Furthermore, when nickel is present at the lithium site, displacement of the layered structure consisting of octahedrons of cobalt and oxygen can be suppressed. Further, changes in volume due to charging and discharging are suppressed. Also, the elastic modulus becomes larger, that is, it becomes harder. This means that the nickel present at the lithium site is also CoO 2 It is assumed that this is because they function as pillars that support the layers. Therefore, it is expected that the crystal structure will become more stable especially in a charged state at a high temperature, for example, 45° C. or higher, which is preferable.

[0233] In addition, the distance between the cation and anion of nickel oxide (NiO) is smaller than that of MgO and CoO. 2 Close to the average distance between cations and anions of LiCoO 2 The orientation is easy to match.

[0234] In addition, the ionization tendency is smaller in the order of magnesium, aluminum, cobalt, and nickel. Therefore, it is considered that nickel is less eluted into the electrolyte than the other elements mentioned above during charging. Therefore, in the charged state, it is considered to be highly effective in stabilizing the crystal structure of the surface layer.

[0235] Furthermore, nickel is Ni 2+ , Ni3+ , Ni 4+ Of which Ni 2+ is the most stable, and nickel has a higher trivalent ionization energy than cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Therefore, nickel is considered to have the effect of suppressing the phase change from the layered rock salt type to the spinel type crystal structure.

[0236] On the other hand, if nickel is in excess, the influence of distortion due to the Jahn-Teller effect will be increased, which is undesirable. Moreover, if nickel is in excess, there is a possibility that intercalation and deintercalation of lithium will be adversely affected.

[0237] Therefore, it is preferable that the entire positive electrode active material 100 has an appropriate amount of nickel. For example, the number of nickel atoms in the positive electrode active material 100 is less than the number of cobalt atoms, preferably more than 0% and 7.5% or less of the number of cobalt atoms, preferably 0.05% or more and 4% or less, and 0.1% or more and 2%. The content is preferably 0.2% or more and 1% or less. Or more than 0% and 4% or less is preferable. Or preferably more than 0% and 2% or less. Or preferably 0.05% or more and 7.5% or less. Or preferably 0.05% or more and 2% or less. Or preferably 0.1% or more and 7.5% or less. Or preferably 0.1% or more and 4% or less. The amount of nickel shown here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, etc., or a value obtained by mixing raw materials in the process of producing the positive electrode active material. may be based on the value of

[0238] 〔aluminum〕 Aluminum can also be present in cobalt sites in a layered rock salt type crystal structure. Aluminum is a typical trivalent element and its valence does not change, so lithium around aluminum is difficult to move during charging and discharging. Therefore, aluminum and the lithium surrounding it function as pillars and can suppress changes in the crystal structure. Therefore, as will be described later, even if the positive electrode active material 100 is subjected to a force that expands and contracts in the c-axis direction due to insertion and desorption of lithium ions, that is, by changing the depth of charge or charging rate, the force that expands and contracts in the c-axis direction acts. However, deterioration of the positive electrode active material 100 can be suppressed.

[0239] Additionally, aluminum has the effect of suppressing the elution of surrounding cobalt and improving continuous charging resistance. Furthermore, since Al-O bonds are stronger than Co-O bonds, desorption of oxygen around aluminum can be suppressed. These effects improve thermal stability. Therefore, when aluminum is included as an additive element, safety can be improved when the positive electrode active material 100 is used in a secondary battery. Further, the positive electrode active material 100 can be made such that the crystal structure does not easily collapse even after repeated charging and discharging.

[0240] On the other hand, if aluminum is in excess, there is a possibility that insertion and deintercalation of lithium will be adversely affected.

[0241] Therefore, it is preferable that the entire positive electrode active material 100 has an appropriate amount of aluminum. For example, the number of aluminum atoms contained in the entire positive electrode active material 100 is preferably 0.05% or more and 4% or less of the number of cobalt atoms, preferably 0.1% or more and 2% or less, and more preferably 0.3% or more and 1.5% or less. Or preferably 0.05% or more and 2% or less. Or preferably 0.1% or more and 4% or less. The amount that the entire positive electrode active material 100 has here may be, for example, the value obtained by elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, etc., or the amount that the entire positive electrode active material 100 has. It may also be based on the value of the composition of raw materials during the production process.

[0242] Further, titanium oxides are known to have superhydrophilic properties. Therefore, by using the positive electrode active material 100 having titanium oxide in the surface layer portion 100a, the wettability with respect to a highly polar solvent may be improved. When used as a secondary battery, the interface between the positive electrode active material 100 and the highly polar electrolytic solution becomes good, and there is a possibility that an increase in internal resistance can be suppressed.

[0243] Furthermore, when phosphorus is present in the surface layer 100a, Li x CoO 2 It is preferable to maintain a small state in which x is small because short circuits can be suppressed in some cases. For example, it is preferable to exist in the surface layer portion 100a as a compound containing phosphorus and oxygen.

[0244] It is preferable that the positive electrode active material 100 contains phosphorus because the phosphorus reacts with hydrogen fluoride generated by decomposition of the electrolytic solution or lithium salt, and the hydrogen fluoride concentration in the electrolytic solution may be reduced.

[0245] Lithium salt is LiPF 6 If it has, hydrogen fluoride may be generated due to hydrolysis. Furthermore, there is a risk that hydrogen fluoride may be generated due to the 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 organic electrolyte, corrosion of the current collector and / or peeling of the coating portion 104 (see FIG. 19) may be suppressed. Further, it may be possible to suppress a decrease in adhesiveness due to gelation and / or insolubilization of PVDF.

[0246] When the positive electrode active material 100 has phosphorus together with magnesium, Li x CoO 2 The stability of the crystal structure in a state where x is small is extremely high, which is preferable. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% or more and 20% or less of the number of cobalt atoms, more preferably 2% or more and 10% or less, and even more preferably 3% or more and 8% or less. Or preferably 1% or more and 10% or less. Or preferably 1% or more and 8% or less. Or preferably 2% or more and 20% or less. Or preferably 2% or more and 8% or less. Or preferably 3% or more and 20% or less. Or preferably 3% or more and 10% or less. In addition, the number of magnesium atoms is preferably 0.1% or more and 10% or less of the number of cobalt atoms, more preferably 0.5% or more and 5% or less, and even more preferably 0.7% or more and 4% or less. Or preferably 0.1% or more and 5% or less. Or preferably 0.1% or more and 4% or less. Or preferably 0.5% or more and 10% or less. Or preferably 0.5% or more and 4% or less. Or preferably 0.7% or more and 10% or less. Or preferably 0.7% or more and 5% or less. The concentrations of phosphorus and magnesium shown here may be, for example, values ​​obtained by elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, etc., or values ​​obtained during the production process of the positive electrode active material 100. It may be based on the value of the raw material composition in .

[0247] In addition, when a crack is formed in a part of the surface of the positive electrode active material 100, phosphorus, more specifically, for example, phosphorus and oxygen is added to the embedded part 102 (see FIG. 7(H)) that is in contact with the part of the surface. The presence of the compound can suppress the progress of cracks.

[0248] [Synergistic effect of multiple 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. Therefore, Li x CoO 2 Even when x is small, the elution of magnesium can be suppressed. Therefore, it can contribute to stabilization of the surface layer portion 100a.

[0249] For the same reason, when adding additional elements to lithium cobalt oxide in the manufacturing process, it is preferable that magnesium be added in a step before nickel. Alternatively, it is preferable that magnesium and nickel are added in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide no matter what process it is added to, whereas nickel can diffuse widely into the interior of lithium cobalt oxide if magnesium is not present. Therefore, if nickel is added before magnesium, there is a concern that nickel will diffuse into the interior of lithium cobalt oxide and will not remain in the desired amount on the surface layer.

[0250] Further, it is preferable to include additional elements having different distributions, as this can stabilize the crystal structure over a wider region. For example, when the positive electrode active material 100 contains both magnesium, nickel, and aluminum, the crystal structure can be stabilized over a wider range than when it contains only one of them. In this way, when the positive electrode active material 100 also has additive elements with different distributions, aluminum is not essential for the surface because magnesium, nickel, etc. can sufficiently stabilize the surface. Rather, it is preferable that aluminum is widely distributed in a deeper region. For example, it is preferable that aluminum is continuously detected in a region from the surface in a depth direction of 1 nm or more and 25 nm or less. It is preferable that the crystal structure be widely distributed in a region of 0 nm or more and 100 nm or less from the surface, preferably 0.5 nm or more and 50 nm or less from the surface, since the crystal structure can be stabilized over a wider region.

[0251] When a plurality of additive elements are included as described above, the effects of each additive element are synergistic and can contribute to further stabilization of the surface layer portion 100a. In particular, it is preferable to contain magnesium, nickel, and aluminum because they are highly effective in providing a stable composition and crystal structure.

[0252] However, it is not preferable if the surface layer portion 100a is occupied only by the compound of the additive element and oxygen because it becomes difficult to insert and extract lithium. For example, it is not preferable that the surface layer portion 100a is occupied only by MgO, a structure in which MgO and NiO(II) are dissolved in solid solution, and / or a structure in which MgO and CoO(II) are dissolved in solid solution. Therefore, the surface layer portion 100a must contain at least cobalt, also contain lithium in the discharge state, and have a path for inserting and extracting lithium.

[0253] In order to ensure a sufficient path for insertion and desorption of lithium, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than magnesium. For example, when measured from the surface of the positive electrode active material 100 by XPS (X-ray photoelectron spectroscopy), the ratio Mg / Co of the number of atoms of magnesium to the number of atoms of cobalt, Co, is preferably 0.62 or less. Further, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than nickel. Further, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than aluminum. Further, it is preferable that the surface layer portion 100a has a higher concentration of cobalt than fluorine.

[0254] Furthermore, since too much nickel may inhibit the diffusion of lithium, it is preferable that the surface layer portion 100a has a higher concentration of magnesium than nickel. 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.

[0255] Further, some of the additive elements, particularly magnesium, nickel, and aluminum, preferably have a higher concentration in the surface layer 100a than in the interior 100b, but preferably also exist randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites in the interior 100b, there is an effect that the layered rock salt type crystal structure can be easily maintained as described above. Further, when nickel is present in the interior 100b at an appropriate concentration, the shift of the layered structure consisting of the octahedron of the transition metal M and oxygen can be suppressed in the same way as described above. Further, when magnesium and nickel are contained together, a synergistic effect of suppressing the elution of magnesium can be expected as described above.

[0256] ≪Li x M.O. 2 State where x inside is small≫ The positive electrode active material 100 of one embodiment of the present invention has the above-mentioned distribution of additive elements and / or crystal structure, and therefore Li x M.O. 2 The crystal structure in the state where x is small, that is, when charged at a high voltage, is different from that of conventional positive electrode active materials. Note that x is small here, meaning 0.1<x≦0.24. Furthermore, the high voltage in the charged state refers to 4.5V or higher, preferably 4.6V or higher, and more preferably 4.8V or higher.

[0257] Using Figures 12 and 13, Li x M.O. 2 Changes in the crystal structure due to changes in x will be explained while comparing a conventional positive electrode active material and the positive electrode active material 100 of one embodiment of the present invention.

[0258] Figure 13 shows changes in the crystal structure of conventional positive electrode active materials. The conventional cathode active material shown in Figure 13 is lithium cobalt oxide (LiCoO), which does not contain any additional elements. 2 ). In particular, changes in the crystal structure of lithium cobalt oxide without additive elements are described in Non-Patent Documents 1 to 3 and the like.

[0259] Figure 13 shows R-3m O3 in the discharge state, that is, Li x CoO 2 This shows the crystal structure of lithium cobalt oxide with x=1 in the figure. In the discharge state, conventional lithium cobalt oxide has the same crystal structure as the positive electrode active material 100 of one embodiment of the present invention.

[0260] Furthermore, it is known that conventional lithium cobalt oxide has a crystal structure in which the symmetry of lithium increases when x=0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO in the unit cell. 2 There is one layer. Therefore, it is sometimes called O1 type or monoclinic (indicated as monoclinic in the figure) O1 type.

[0261] Furthermore, when x=0, the positive electrode active material has a crystal structure of trigonal space group P-3m1, and CoOO is present in the unit cell. 2 There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal (denoted as trigonal in the figure) O1 type. In some cases, the trigonal crystal is converted to a complex hexagonal lattice and is called the hexagonal O1 type.

[0262] Further, conventional lithium cobalt oxide when x=0.12 has a crystal structure of space group R-3m. This structure is a trigonal O1-like CoO 2 structure and LiCoO like R-3m O3 2 It can also be said that this structure is made up of alternating layers of . Therefore, this crystal structure is sometimes called an H1-3 type crystal structure (denoted as H1-3 in the figure). Note that the actual intercalation and deintercalation of lithium does not necessarily occur uniformly within the positive electrode active material, and the lithium concentration may become mottled. Therefore, experimentally, an H1-3 type crystal structure is observed from approximately x = 0.25. be done. In fact, the H1-3 type crystal structure has twice as many cobalt atoms per unit cell as other structures. However, in this specification including FIG. 13, in order to facilitate comparison with other crystal structures, the c-axis of the H1-3 type crystal structure is shown in a diagram with half of the unit cell.

[0263] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.42150±0.00016), O1(0,0,0.27671± 0.00045), 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. For example, a unit cell with a GOF (goodness of fit) value close to 1 may be used.

[0264] Li, whose H1-3 type crystal structure can be confirmed experimentally x CoO 2 When charging and discharging are repeated such that x becomes 0.24 or less, the crystal structure of conventional lithium cobalt oxide changes between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state. (i.e. non-equilibrium phase change) is repeated.

[0265] These two crystal structures are CoO 2 There is a large discrepancy between the layers. As shown by the dotted line in Figure 13, in the H1-3 type crystal structure, CoO 2 The layer deviates greatly from the discharge state R-3m O3. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0266] Furthermore, there is a large difference in volume between these two crystal structures. When compared for the same number of cobalt atoms, the difference in volume between the H1-3 type crystal structure and the R-3m O3 type crystal structure in the discharge state exceeds 3.5%, typically 3.9% or more.

[0267] In addition, like the trigonal O1 type that the H1-3 type crystal structure has, CoO 2 Structures with successive layers are likely to be unstable.

[0268] Therefore, if charging and discharging are repeated so that x becomes 0.24 or less, the crystal structure of conventional lithium cobalt oxide collapses. The collapse of the crystal structure causes deterioration of cycle characteristics. This is because as the crystal structure collapses, the number of sites where lithium can exist stably decreases, and insertion and extraction of lithium becomes difficult.

[0269] On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in FIG. x M.O. 2 The crystal structure changes less in the discharge state where x is 1 and in the state where x is 0.24 or less than in conventional positive electrode active materials. More specifically, the MO in the state where x is 1 and the state where x is less than or equal to 0.24 2 Layer misalignment can be reduced. When comparing the positive electrode active material 100 having the same number of transition metal M atoms and the conventional positive electrode active material, the volume change of the positive electrode active material 100 is smaller than the volume change of the conventional positive electrode active material. Therefore, the cathode active material 100 according to one embodiment of the present invention has excellent properties because its crystal structure does not easily collapse even after repeated charging and discharging such that x becomes 0.24 or less, and sites where lithium can stably exist are maintained. It is possible to achieve excellent cycle characteristics.

[0270] In addition, the positive electrode active material 100 of one embodiment of the present invention includes Li x M.O. 2 When x is 0.24 or less, it can have a more stable crystal structure than conventional positive electrode active materials. Therefore, the positive electrode active material 100 according to one embodiment of the present invention contains Li x M.O. 2 Even when x is maintained at 0.24 or less, oxygen is difficult to desorb and thermal decomposition reactions can be suppressed. That is, a secondary battery using the positive electrode active material 100 of one embodiment of the present invention is preferable because safety is further improved.

[0271] Li x M.O. 2 FIG. 12 shows the crystal structure that the interior 100b of the positive electrode active material 100 has when x is approximately 1, 0.2, and 0.15. The interior 100b occupies most of the volume of the positive electrode active material 100 and is a part that greatly contributes to charging and discharging, so MO 2 It can be said that layer shift and volume change are the most problematic areas.

[0272] As described above, when x=1, the positive electrode active material 100 has the same R-3m O3 crystal structure as conventional lithium cobalt oxide. However, the positive electrode active material 100 has a crystal structure different from that of conventional lithium cobalt oxide when x is 0.24 or less, for example, about 0.2 and 0.15, which is the H1-3 type crystal structure.

[0273] The positive electrode active material 100 of one embodiment of the present invention when x=0.2 has a crystal structure belonging to the trigonal space group R-3m. This is M.O. 2 The layer symmetry is the same as O3. Therefore, this crystal structure will be referred to as an O3' type crystal structure. Alternatively, although it is not a spinel structure, a pattern resembling a spinel structure may appear in the XRD pattern, and this crystal structure is sometimes called a pseudo-spinel structure. This crystal structure is shown in Figure 12 with R-3m O3'. As will be described later, the positive electrode active material 100 may change to the H1-3 crystal structure after passing through the O3' type crystal structure, but even if it becomes the H1-3 type crystal structure, for example Since it can have the effect of suppressing oxygen release, it is estimated that even if a nail penetration test is performed on a lithium ion secondary battery using the positive electrode active material 100, ignition will be suppressed.

[0274] In the O3' type crystal structure, when M is cobalt, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,x), within the range of 0.20≦x≦0.25. It can be shown as Also, the lattice constant of the unit cell is 2.797×10 on the a-axis -10 ≦a≦2.837×10 -10 (m) is preferable, 2.807×10 -10 ≦a≦2.827×10 -10 (m) is more preferable, typically a=2.817×10 -10 (m). c-axis is 13.681×10 -10 ≦c≦13.881×10 -10 (m) is preferable, 13.751×10 -10 ≦c≦13.811×10 -10 (m) is more preferable, typically c=13.781×10 -10 (m).

[0275] Further, when x=0.15, the positive electrode active material 100 of one embodiment of the present invention has a crystal structure belonging to the monoclinic space group P2 / m. This is CoO in the unit cell. 2 There is one layer. Further, when x=about 0.15, it can be considered that lithium present in the positive electrode active material 100 is about 15 atomic % in the discharged state. Therefore, this crystal structure will be referred to as a monoclinic O1(15) type crystal structure. This crystal structure is shown in FIG. 12 with P2 / m monoclinic (indicated as monoclinic in the figure) O1 (15).

[0276] In the monoclinic O1(15) type crystal structure, when M is cobalt, the coordinates of cobalt and oxygen in the unit cell are 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 It can be shown within the range of ≦0.71. Also, the lattice constant of the unit cell is a=0.4880±0.005nm, b=0.2817±0.005nm, c=0.4839±0.005nm, α=90°, β=109.6±0.1°, γ=90°.

[0277] Note that this crystal structure can exhibit a lattice constant even in the space group R-3m if a certain degree of error is allowed in the Rietveld analysis. The coordinates of cobalt and oxygen in the unit cell in this case are Co(0,0,0.5), O(0,0,Z O ), 0.21≦Z O It can be shown within the range of ≦0.23. Also, the lattice constant of the unit cell is a=0.2817±0.002nm, c=1.368±0.01nm.

[0278] In both O3' type and monoclinic O1(15) type crystal structures, ions such as cobalt, nickel, and magnesium occupy six oxygen coordination positions. Note that light elements such as lithium and magnesium may occupy the 4-coordination position of oxygen.

[0279] As shown by the dotted line in Figure 12, the R-3m O3 in the discharge state and the O3' and monoclinic O1 (15) type crystal structures have a MO 2 There is almost no deviation between layers.

[0280] Further, the difference in volume per same number of cobalt atoms between R-3m O3 in the discharge state and the O3' type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0281] Furthermore, the difference in volume per same number of cobalt atoms between R-3m O3 in the discharge state and the monoclinic O1(15) type crystal structure is less than 3.3%, more specifically less than 3.0%, typically 2.5%. be.

[0282] Table 1 shows the difference in volume per cobalt atom between R-3m O3 in the discharge state and O3', monoclinic O1(15), H1-3 type, and trigonal O1. For the lattice constants of the crystal structures of R-3m O3 and trigonal O1 in the discharge state used in the calculations in Table 1, reference can be made to ICSD coll.code.172909 and 88721. Regarding H1-3, reference can be made to Non-Patent Document 3. O3' and monoclinic O1(15) can be calculated from experimental XRD values.

[0283]

table 1

[0284] In this way, in the positive electrode active material 100 of one embodiment of the present invention, Li x CoO 2 Changes in the crystal structure when x is small, that is, when a large amount of lithium is released, are suppressed more than in conventional cathode active materials. When comparing the positive electrode active material 100 having the same number of cobalt atoms and the conventional positive electrode active material, the volume change of the positive electrode active material 100 is smaller than that of the conventional positive electrode active material. Therefore, the crystal structure of the positive electrode active material 100 does not easily collapse even after repeated charging and discharging such that x becomes 0.24 or less. Therefore, in the positive electrode active material 100, a decrease in charge / discharge capacity during charge / discharge cycles is suppressed. In addition, since more lithium can be stably utilized than 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 high discharge capacity per weight and per volume can be manufactured.

[0285] Note that the positive electrode active material 100 is Li x M.O. 2 It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is estimated that even if x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. Also Li x M.O. 2 It has been confirmed that when x is greater than 0.1 and less than 0.2, typically x is greater than or equal to 0.15 and less than 0.17, it may have a monoclinic O1(15) type crystal structure. However, the crystal structure of Li x M.O. 2 The range of x is not necessarily limited to the above range because it is affected not only by x but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0286] Therefore, the positive electrode active material 100 is Li x M.O. 2 When x is more than 0.1 and less than 0.24, it may have only the O3' type, only the monoclinic O1(15) type, or have both crystal structures. Good too. Furthermore, all of the particles in the interior 100b of the positive electrode active material 100 do not have to have the O3' type and / or monoclinic O1(15) type crystal structure. It may contain other crystal structures, or may be partially amorphous.

[0287] Also Li x M.O. 2 In order to make x inside a small state, it is generally necessary to charge at a high charging voltage. Therefore, Li x M.O. 2 The state in which x is small can be rephrased as the state in which 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 more based on the potential of lithium metal in an environment of 25°C, an H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, based on the potential of lithium metal, a charging voltage of 4.6V or higher can be said to be a high charging voltage. Further, in this specification and the like, unless otherwise specified, the charging voltage is expressed based on the potential of lithium metal.

[0288] Therefore, the positive electrode active material 100 of one embodiment of the present invention is preferable because it can maintain a crystal structure with R-3m O3 symmetry even when charged at a high charging voltage, for example, 4.6 V or higher at 25°C. It can be rephrased. In addition, this can be said to be preferable because an O3' type crystal structure can be obtained when charged at a higher charging voltage, for example, a voltage of 4.65 V or more and 4.7 V or less at 25°C. In other words, it is preferable because a monoclinic O1(15) type crystal structure can be obtained when the battery is charged at a higher charging voltage, for example, at a voltage exceeding 4.7 V and not more than 4.8 V at 25°C.

[0289] Even in the positive electrode active material 100 of one embodiment of the present invention, when the charging voltage is further increased, the H1-3 type crystal structure may finally be observed. Furthermore, as mentioned above, the crystal structure is affected by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., so if the charging voltage is lower, for example, if the charging voltage is 4.5V or more and less than 4.6V at 25°C, In some cases, the positive electrode active material 100 according to one embodiment of the present invention can have an O3' type crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.7 V or less at 25° C., a monoclinic O1(15) type crystal structure may be obtained.

[0290] In addition, in a secondary battery, when graphite is used as a negative electrode active material, for example, the voltage of the secondary battery is lowered by the potential of graphite than the above. The potential of graphite is about 0.05V to 0.2V based on the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as a negative electrode active material, it has the same crystal structure as above when the voltage is obtained by subtracting the potential of graphite from the above voltage.

[0291] Furthermore, in O3' and monoclinic O1(15) in FIG. 12, lithium is shown to exist at all lithium sites with equal probability, but this is not restrictive. It may be present unevenly in some lithium sites, for example monoclinic O1 (Li 0.5 CoO 2 ) may have the symmetry of lithium. The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0292] In addition, the O3' and monoclinic O1(15) type crystal structures have lithium randomly between the layers, but CdCl 2 It can also be said that the crystal structure is similar to that of the type. This CdCl 2 The type-like crystal structure is similar to that of lithium nickelate. 0.06 NiO 2 Although the crystal structure is similar to that when charged to CdCl 2 It is known that it does not have a type crystal structure.

[0293] As described above, the conventional lithium cobalt oxide and the positive electrode active material 100 of one embodiment of the present invention are different from each other due to changes in the depth of charge, that is, Li x CoO 2 The crystal structure changes depending on the change in x inside. FIG. 14 shows the change in the c-axis length of the conventional lithium cobalt oxide described in Non-Patent Document 12. Round markers indicate hexagonal phase, and diamond-shaped markers indicate monoclinic phase.

[0294] Note that the change in the c-axis length of lithium cobalt oxide corresponds to the change in the angle at which the peak of, for example, the (003) plane of lithium cobalt oxide appears in the XRD pattern. CuKα 1 In line XRD, it is known that the peak of the (003) plane of lithium cobalt oxide occurs around 2θ of 19° to 20°.

[0295] ≪Grain boundaries≫ In addition to the above-mentioned distribution, it is more preferable that at least a portion of the additive elements included in the positive electrode active material 100 of one embodiment of the present invention be unevenly distributed in and near the grain boundaries 101.

[0296] Note that in this specification and the like, maldistribution refers to a concentration of an element in a certain region being different from that in another region. It has the same meaning as segregation, precipitation, non-uniformity, deviation, or a mixture of areas with high concentration and areas with low concentration.

[0297] For example, it is preferable that the magnesium concentration at the grain boundary 101 of the positive electrode active material 100 and its vicinity is higher than 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 in other regions of the interior 100b. It is also preferable that the nickel concentration at the grain boundary 101 and its vicinity is higher than in other regions of the interior 100b. Further, it is preferable that the aluminum concentration at the grain boundary 101 and its vicinity is also higher than in other regions of the interior 100b.

[0298] Grain boundary 101 is one of the planar defects. Therefore, like the particle surface, it tends to become unstable and the crystal structure tends to change. Therefore, if the concentration of the added element at the grain boundary 101 and its vicinity is high, changes in the crystal structure can be suppressed more effectively.

[0299] Further, when the magnesium concentration and fluorine concentration at and near the grain boundaries 101 are high, even if cracks occur along the grain boundaries 101 of the positive electrode active material 100 of one embodiment of the present invention, the surface caused by the cracks Magnesium and fluorine concentrations increase in the vicinity. Therefore, the corrosion resistance against hydrofluoric acid can be improved even in the positive electrode active material after cracks have occurred.

[0300] <Particle size> If the particle size of the positive electrode active material 100, which enables high voltage charging, is too large, there are problems such as difficulty in lithium diffusion and the surface of the active material layer becoming too rough when applied to a current collector. . On the other hand, if it is too small, problems such as excessive reaction with the electrolyte will 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. Or preferably 1 μm or more and 40 μm or less. Or preferably 1 μm or more and 30 μm or less. Or preferably 2 μm or more and 100 μm or less. Or preferably 2 μm or more and 30 μm or less. Or preferably 5 μm or more and 100 μm or less. Or preferably 5 μm or more and 40 μm or less.

[0301] Further, it is preferable to use a mixture of particles having different particle sizes in the positive electrode because the electrode density can be increased and a secondary battery with high energy density can be obtained. The positive electrode active material 100 having a relatively small particle size is expected to have high charge / discharge rate characteristics. A secondary battery using the positive electrode active material 100 having a relatively large particle size is expected to have high charge / discharge cycle characteristics and maintain a high discharge capacity.

[0302] Furthermore, when particles with different median diameters (D50) are mixed and used in a positive electrode, lithium is desorbed sequentially from the surface of the positive electrode active material. x CoO 2 The rate at which x in the positive electrode active material 100 with a relatively small particle size decreases is faster than that of the positive electrode active material 100 with a relatively large particle size. Therefore, when powder XRD measurement is performed on a positive electrode active material containing a mixture of particles with different particle sizes, both an O3' type crystal structure and a monoclinic O1(15) type crystal structure may be detected.

[0303] <Analysis method> A certain cathode active material is Li x CoO 2 When x is small, Li x CoO 2 This can be determined by analyzing a positive electrode containing a positive active material with a small x value using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0304] In particular, XRD can analyze the symmetry of transition metals such as cobalt in positive electrode active materials with high resolution, compare the height of crystallinity and crystal orientation, and analyze periodic lattice distortion and crystallite size. This is preferable because sufficient accuracy can be obtained even if the positive electrode obtained by disassembling the secondary battery is directly measured. Among XRD, powder XRD provides a diffraction peak that reflects the crystal structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0305] Note that when analyzing the crystallite size by powder XRD, it is preferable to perform the measurement without the influence of the orientation of the positive electrode active material particles due to pressurization or the like. For example, it is preferable to take out the positive electrode active material from a positive electrode obtained by disassembling a secondary battery and prepare a powder sample for measurement.

[0306] As described above, the positive electrode active material 100 of one embodiment of the present invention is Li x CoO 2 It is characterized by the fact that there is little change in the crystal structure between when x is 1 and when it is less than 0.24. A material in which 50% or more of the crystal structure changes significantly when charged at a high voltage is not preferable because it cannot withstand repeated high voltage charging and discharging.

[0307] Furthermore, it should be noted that simply adding additional elements may not result in an O3' type or monoclinic O1(15) type crystal structure. For example, even if lithium cobalt oxide contains magnesium and fluorine, or lithium cobalt oxide contains magnesium and aluminum, depending on the concentration and distribution of the added elements, Li x CoO 2When x is 0.24 or less, there are cases where the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and cases where the H1-3 type crystal structure accounts for 50% or more. .

[0308] In addition, even with the positive electrode active material 100 of one embodiment of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9V, an H1-3 type or trigonal O1 type crystal structure may occur. There is also. Therefore, in order to determine whether the cathode active material 100 is one embodiment of the present invention, analysis of the crystal structure such as XRD, and information such as charging capacity or charging voltage are required.

[0309] However, the positive electrode active material in a state where x is small may undergo a change in crystal structure when exposed to the atmosphere. For example, the O3' type and monoclinic O1(15) type crystal structures may change to the H1-3 type crystal structure. Therefore, it is preferable that all samples subjected to crystal structure analysis be handled in an inert atmosphere such as an argon atmosphere.

[0310] Further, whether or not the distribution of additive elements included in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, EDX, EPMA (electron probe microanalysis), or the like.

[0311] Further, the crystal structure of the surface layer 100a, grain boundaries 101, etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100.

[0312] ≪Powder resistance measurement≫ The volume resistivity of the powder of the positive electrode active material 100 according to one embodiment of the present invention will be explained.

[0313] In one embodiment of the present invention, the volume resistivity of the powder of the positive electrode active material 100 is 1.0×10 at a pressure of 64 MPa. 4 Preferably Ω cm or more, 1.0×10 5 More preferably Ω cm or more, 1.0×10 6 More preferably, it is Ω·cm or more. Also, at a pressure of 64MPa, 1.0×10 9 Preferably less than Ω cm, 1.0×10 8 More preferably Ω cm or less, 1.0×10 7 More preferably, it is Ω·cm or less. Fluorine is adsorbed on the surface of the positive electrode active material 100, and at least a portion of the fluorine combines with lithium contained in the positive electrode active material 100, thereby suppressing the movement of the lithium. Thereby, the volume resistivity of the powder of the positive electrode active material 100 can be increased, and specifically, the volume resistivity can be set to the above value. For example, the volume resistivity of a powder of positive electrode active material 100 is 1.0×10 at a pressure of 64 MPa. 5 It can be Ω·cm or more.

[0314] The positive electrode active material 100 having the above volume resistivity has a stable crystal structure even at high voltage. Therefore, the fact that the volume resistivity of the powder of the positive electrode active material 100 is within the above range allows the surface layer 100a to be formed well, which is important for the crystal structure of the positive electrode active material to be stable in the charged state. It can be used as an indicator to show what has been achieved. That is, in the positive electrode active material 100, it is preferable that at least the shell 100s has high resistance.

[0315] However, if a high resistance region exists thickly from the surface to the inside of the positive electrode active material 100, the battery reaction may be inhibited. Therefore, it is more preferable that only the thin region near the surface of the surface layer portion 100a has high resistance. That is, in the surface layer portion 100a, it is preferable that a high resistance region exist thinly from the surface toward the inside. For example, a region where Mg is present in a high concentration in the surface layer portion 100a can be a region with high resistance. Therefore, it is preferable that Mg be located in the surface layer portion 100a.

[0316] A method for measuring the volume resistivity of the powder of the positive electrode active material 100 according to one embodiment of the present invention will be described.

[0317] As shown in FIG. 20(A), the powder volume resistivity measuring device includes a first mechanism 10 having a resistance measurement terminal and applying pressure to a powder sample S (sample) to be measured. It is preferable to have a second mechanism 11. The second mechanism 11 preferably has a cylinder into which the powder sample S is introduced, and a piston that can move up and down inside the cylinder. A spring or the like is connected to the piston to apply pressure to the sample within the cylinder. The first mechanism 10 preferably has a measuring electrode in contact with the bottom surface of the cylinder. For example, MCP-PD51 manufactured by Mitsubishi Chemical Analytech Co., Ltd. can be used as a measuring device having such a measuring electrode and a mechanism for applying pressure to the powder to be measured. As the resistance meter, Loresta-GP or Hiresta-UP can be used. Loresta-GP can be used to measure low-resistance samples using the four-probe method as shown in Figure 20(B), while Hiresta-UP can be used to measure high-resistance samples using the two-probe method as shown in Figure 20(C). Can be used for measurement. The measurement environment is preferably a stable environment such as a dry room, but may be a general laboratory environment. The environment of the dry room is preferably, for example, a temperature environment of 20°C or more and 25°C or less, and a dew point environment of -40°C or less. A general laboratory environment may be a temperature environment of 15°C or more and 30°C or less, and a humidity environment of 30% or more and 70% or less.

[0318] Measurement of the volume resistivity of powder using the measuring device shown above will be explained. First, a powder sample is set in the second mechanism 11. The second mechanism 11 has a measurement section, in which the sample is put into a cylinder, and the bottom surface of the cylinder and the measurement electrode are in contact with each other, and it is possible to apply pressure to the sample. It has a structure with a piston etc. The measuring section also has a structure for measuring the thickness of the sample.

[0319] In measuring the volume resistivity of powder, the electrical resistance of the powder and the thickness of the powder are measured while pressure is applied to the powder. The pressure applied to the powder can be applied under multiple conditions. For example, the electrical resistance of the powder and the thickness of the powder can be measured under pressure conditions of 13 MPa, 25 MPa, 38 MPa, 51 MPa, and 64 MPa. The volume resistivity of the powder can be calculated from the measured electrical resistance of the powder and the thickness of the powder.

[0320] The method for calculating volume resistivity will be explained. When measuring with the two-terminal method using Hiresta-UP, the volume resistivity can be found by multiplying the electrical resistance of the powder by the area of ​​the measurement electrode in contact with the powder and dividing by the thickness of the powder. When measuring with the four-probe method using Loresta-GP, volume resistivity can be found by multiplying the electric resistance of the powder by a correction coefficient and by the thickness of the powder. The correction coefficient is a value that changes depending on the sample shape, dimensions, and measurement position, and can be determined using the calculation software built into Loresta-GP.

[0321] When performing the measurement as shown above, the volume resistivity of the powder of the positive electrode active material 100 of one embodiment of the present invention is 1.0 × 10 at a pressure of 64 MPa. 4 Preferably Ω cm or more, 1.0×10 5 More preferably Ω cm or more, 1.0×10 6 More preferably, it is Ω·cm or more. Also, at a pressure of 64MPa, 1.0×10 9 Preferably less than Ω cm, 1.0×10 8 More preferably Ω cm or less, 1.0×10 7 More preferably, it is Ω·cm or less. A battery having the positive electrode active material 100 exhibiting such a volume resistivity exhibits favorable cycle characteristics in a charge / discharge cycle test under high voltage conditions. Furthermore, the battery can be made less likely to catch fire in an internal short circuit test such as a nail penetration test.

[0322] ≪Charging method≫ Whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be determined by, for example, a coin cell (CR2032 type, diameter 20 mm, height This can be determined by making a battery (3.2 mm) and charging it. A coin cell has an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0323] More specifically, for the positive electrode, a slurry obtained by mixing the composite oxide as a positive electrode active material, a conductive material, and a binder can be applied to a positive electrode current collector made of aluminum foil.

[0324] As mentioned above, lithium metal can be used for the counter electrode, but materials other than lithium metal may also be used. When a material other than lithium metal is used, the potential of the secondary battery and the potential of the positive electrode are different. Voltages and potentials in this specification and the like are the potentials of the positive electrode unless otherwise mentioned.

[0325] The lithium salt contained in the electrolyte contains 1 mol / L lithium hexafluorophosphate (LiPF). 6 ), and the electrolyte is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC) at 2wt%. can.

[0326] A polypropylene porous film with a thickness of 25 μm can be used as the separator.

[0327] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0328] The coin cell produced under the above conditions is charged to an arbitrary voltage (for example, 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V or 4.8V). The charging method is not particularly limited as long as it can be charged to a desired voltage over a sufficient amount of time. For example, when charging with CCCV, the current in CC charging can be 20 mA / g or more and 100 mA / g or less. CV charging can be terminated at 2mA / g or more and 10mA / g or less. In order to observe the phase change of the positive electrode active material, it is desirable to perform charging at such a small current value. Since it is difficult to perform XRD measurements at temperatures below 0°C, the temperature is set at 25°C. Note that the temperature of 25°C is an example. After charging in this manner, the coin cell is disassembled in a glove box with an argon atmosphere and the positive electrode is taken out, thereby obtaining a positive electrode active material with an arbitrary charging capacity. When performing various analyzes after this, it is preferable to seal the chamber with an argon atmosphere in order to suppress reactions with external components. For example, XRD measurement can be performed with the positive electrode active material sealed in an airtight container with an argon atmosphere. Further, it is preferable to take out the positive electrode immediately after charging is completed and use it for analysis. Specifically, it is preferably within 1 hour, more preferably within 30 minutes after charging is completed.

[0329] Furthermore, when analyzing the crystal structure of a charged state after charging and discharging a plurality of times, the conditions for charging and discharging the plurality of times may be different from the above-mentioned charging conditions. For example, charging is performed by constant current charging to any voltage (e.g. 4.6V, 4.65V, 4.7V, 4.75V or 4.8V) at a current value of 20mA / g or more and 100mA / g or less, and then the current value is 2mA / g or more and 10mA or less. The battery can be charged at a constant voltage until it reaches / g or less, and discharged at a constant current of 2.5V and 20mA / g or more and 100mA / g or less.

[0330] Furthermore, when analyzing the crystal structure of the discharged state after charging and discharging multiple times, the discharge conditions for the multiple charging and discharging are, for example, constant current discharge at 2.5V and a current value of 20mA / g or more and 100mA / g or less. be able to.

[0331] ≪XRD≫ With appropriate adjustment and calibration, the equipment and conditions for XRD measurements are not particularly limited. For example, it can be measured using the following equipment and conditions. XRD device: Bruker AXS, D8 ADVANCE X-ray: Cu Kα 1 Output: 40kV, 40mA Divergence angle: Div.Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° or more and 90° or less Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15rpm As a standard sample used for adjustment and calibration, for example, a standard aluminum oxide sintered plate SRM 1976 of NIST (National Institute of Standards and Technology) can be used.

[0332] If the sample to be measured is a powder, it can be set by placing it on a glass sample holder or by sprinkling the sample on a greased silicone non-reflective plate. When 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 according to the measurement surface required by the apparatus.

[0333] A filter or the like may be used to make the characteristic X-ray monochromatic, or it may be performed using XRD data analysis software after obtaining the XRD pattern. For example, using DIFFRAC.EVA (Bruker XRD data analysis software), CuKα 2 Except for the peak due to the line, CuKα 1 Only line peaks can be extracted. The software can also be used to remove backgrounds.

[0334] In this specification and the like, data processing when referring to the 2θ value of a certain diffraction peak will be explained. First, a calculation model is fitted to an XRD pattern using crystal structure analysis software to obtain a calculated pattern. In the calculated pattern, the 2θ value at which the peak top of the diffraction peak appears is referred to as the 2θ value of the diffraction peak. Although the crystal structure analysis software used for fitting is not particularly limited, for example, TOPASver.3 (crystal structure analysis software manufactured by Bruker) can be used.

[0335] Figure 15 shows CuKα in X-rays. 1 The XRD patterns corresponding to the O3 type crystal structure, O3' type crystal structure, and monoclinic O1(15) type crystal structure are shown when using . Figure 16 also shows the CuKα calculated from the H1-3 type crystal structure model. 1 CuKα calculated from the ideal powder XRD pattern by line and the crystal structure of trigonal O1 with x=0 1 An ideal XRD pattern with lines is shown. FIGS. 17(A) and 17(B) show all of the above-mentioned XRD patterns. However, the 2θ range is 18° or more and 21° or less, and the 2θ range is 42° or more and 46° or less. In addition, LiCoO 2 (O3) and CoO 2 The pattern (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). At this time, the range of 2θ is 15° to 75°, step size=0.01, wavelength λ=1.54×10 -10 m, 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 crystal structure patterns of the O3' type and the monoclinic O1(15) type were determined by estimating the crystal structure from the XRD pattern of the positive electrode active material 100 of one embodiment of the present invention, and using TOPAS ver.3 (Bruker Crystal Structure Analysis). Fitting was performed using software (software).

[0336] As shown in Fig. 15, Fig. 17(A), and Fig. 17(B), in the O3' type crystal structure, 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°), and 2θ = 45.47 ± 0.10° ( A diffraction peak appears at 45.37° or more and less than 45.57°).

[0337] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear at 2θ = 19.47 ± 0.10° (19.37° to 19.57°) and 2θ = 45.62 ± 0.05° (45.57° to 45.67°). .

[0338] However, as shown in FIGS. 16, 17(A), and 17(B), no peaks appear at these positions in the H1-3 crystal structure and trigonal O1. Therefore, Li x CoO 2 When x is small, peaks should appear at positions where 2θ is 19.13° or more and less than 19.37° and / or 19.37° or more and 19.57° or less, and 45.37° or more and less than 45.57° and / or 45.57° or more and 45.67° or less. This can be said to be a feature of the positive electrode active material 100 of one embodiment of the present invention.

[0339] This can be said to be a crystal structure where x=1 and x≦0.24, and the positions where the XRD diffraction peaks appear are close to each other. More specifically, among the main diffraction peaks of crystal structures with x=1 and x≦0.24, the difference in 2θ is more preferably 0.7° or less for peaks that appear at 2θ of 42° or more and 46° or less. can be said to be less than 0.5°.

[0340] Note that the positive electrode active material 100 of one embodiment of the present invention is made of Li x CoO 2 When x in the particle is small, it has an O3' type and / or monoclinic O1(15) type crystal structure; You don't have to. It may contain other crystal structures, or may be partially amorphous. However, when performing Rietveld analysis 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, More preferably, it is 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 still more preferably 66% or more, the positive electrode active material has sufficiently excellent cycle characteristics. be able to.

[0341] In addition, even after 100 cycles or more of charging and discharging from the start of measurement, it is preferable that the O3' type and / or monoclinic O1(15) type crystal structure remains 35% or more when Rietveld analysis is performed. % or more, more preferably 43% or more.

[0342] Further, when Rietveld analysis is similarly performed, it is preferable that the H1-3 type and O1 type crystal structures are 50% or less. or more preferably 34% or less. Or, more preferably, it is not substantially observed.

[0343] Furthermore, the sharpness of the diffraction peak in the XRD pattern indicates the high degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging be sharp, that is, have a narrow half-width, for example, a full width at half-maximum. The half width varies depending on the XRD measurement conditions and the 2θ value even for peaks generated from the same crystal phase. In the case of the above measurement conditions, the full width at half maximum of a peak observed at 2θ=43° or more and 46° or less 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 satisfy this requirement. If some peaks satisfy this requirement, it can be said that the crystallinity of the crystal phase is high. Such high crystallinity contributes to sufficient stabilization of the crystal structure after charging.

[0344] In addition, the crystallite size of the O3' type and monoclinic O1 (15) crystal structure possessed by the positive electrode active material 100 is 2 It decreases only to about 1 / 20 of (O3). Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, Li x CoO 2 When x is small, a clear O3' type and / or monoclinic O1 (15) crystal structure peak can be confirmed. On the other hand, conventional LiCoO 2 Then, even if a part of the crystal structure can be similar to the O3' type and / or monoclinic O1(15), the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half width of the XRD peak.

[0345] In the positive electrode active material 100 of one embodiment of the present invention, as described above, it is preferable that the influence of the Jahn-Teller effect is small. In addition to cobalt, transition metals such as nickel and manganese may be included as additive elements, as long as the influence of the Jahn-Teller effect is small.

[0346] In the positive electrode active material, we will use XRD analysis to discuss the proportions of nickel and manganese and the range of lattice constants that are assumed to have a small influence from the Jahn-Teller effect.

[0347] FIG. 18 shows the results of calculating the a-axis and c-axis lattice constants using XRD when the positive electrode active material 100 of one embodiment of the present invention has a layered rock salt crystal structure and contains cobalt and nickel. show. Figure 18(A) shows the results for the a-axis, and Figure 18(B) shows the results for the c-axis. Note that the XRD pattern used for these calculations is the powder after the synthesis of the positive electrode active material, but before it is incorporated into the positive electrode. The nickel concentration on the horizontal axis indicates the nickel concentration when the sum of the numbers of cobalt and nickel atoms is taken as 100%.

[0348] Figure 18(C) shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c axis).

[0349] As shown in FIG. 18(C), when the nickel concentration is 5% and 7.5%, there is a tendency for the a-axis / c-axis to change significantly, and at the nickel concentration of 7.5%, the distortion of the a-axis becomes large. This distortion can be attributed to the Jahn-Teller distortion of trivalent nickel. It is suggested that when the nickel concentration is less than 7.5%, an excellent positive electrode active material with small Jahn-Teller distortion can be obtained.

[0350] Note that the above range of nickel concentration does not necessarily apply to the surface layer portion 100a. That is, in the surface layer portion 100a, the concentration may be higher than the above concentration.

[0351] Based on the above, we considered the preferable range of the lattice constant, and found that in the positive electrode active material of one embodiment of the present invention, the layered structure of the positive electrode active material 100 in the non-charging and discharging state or in the discharge state, which can be estimated from the XRD pattern. In the rock salt crystal structure, the a-axis lattice constant is 2.814×10 -10 greater than m 2.817×10 -10 smaller than m, and the c-axis lattice constant is 14.05×10 -10 greater than m 14.07×10 -10 It has been found that it is preferable to be smaller than m. The state where charging and discharging are not performed may be, for example, the state of the powder before producing the positive electrode of the secondary battery.

[0352] Alternatively, in the layered rock-salt crystal structure of the positive electrode active material 100 in a non-charged and discharged state or in a discharged state, the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) is It is preferably larger than 0.20000 and smaller than 0.20049.

[0353] Alternatively, when XRD analysis is performed on the layered rock salt crystal structure of the cathode active material 100 in a state where no charging and discharging is performed or in a discharged state, a first peak is observed at a 2θ of 18.50° or more and 19.30° or less, And a second peak may be observed at 2θ of 38.00° or more and 38.80° or less.

[0354] ≪XPS≫ With XPS, in the case of inorganic oxides, if monochromatic aluminum Kα rays are used as the X-rays, it is possible to analyze a region from the surface to a depth of approximately 2 to 8 nm (usually 5 nm or less). The concentration of each element can be quantitatively analyzed in a region approximately half the depth. Furthermore, by performing narrow scan analysis, the bonding state of elements can be analyzed.

[0355] In the positive electrode active material 100 according to one embodiment of the present invention, it is preferable that the concentration of one or more selected additive elements is higher in the surface layer 100a than in the interior 100b. This is synonymous with the fact that the concentration of one or more selected additive elements in the surface layer portion 100a is preferably higher than the average of the entire positive electrode active material 100. Therefore, for example, the concentration of one or more additive elements selected from the surface layer 100a measured by It can be said that it is preferable that the concentration is higher than that of . For example, it is preferable that the magnesium concentration in at least a portion of the surface layer portion 100a measured by XPS or the like is higher than the magnesium concentration in the entire positive electrode active material 100. Further, it is preferable that the nickel concentration in at least a portion of the surface layer portion 100a is higher than the nickel concentration in the entire positive electrode active material 100. Further, it is preferable that the aluminum concentration in at least a portion of the surface layer portion 100a is higher than the aluminum concentration in the entire positive electrode active material 100. Further, it is preferable that the fluorine concentration in at least a portion of the surface layer portion 100a is higher than the fluorine concentration in the entire positive electrode active material 100.

[0356] Note that the surface and surface layer portion 100a of the positive electrode active material 100 according to one embodiment of the present invention do not contain carbonate, hydroxyl groups, etc. that are chemically adsorbed after the positive electrode active material 100 is produced. It is also assumed that the electrolytic solution, binder, conductive material, or compounds derived from these adhered to the surface of the positive electrode active material 100 are not included. Therefore, when quantifying the elements contained in the positive electrode active material, correction may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc. that can be detected by surface analysis such as XPS. For example, in XPS, it is possible to separate the types of bonds by analysis, and correction may be performed to exclude binder-derived C-F bonds.

[0357] Furthermore, in order to remove the electrolyte, binder, conductive material, or compounds derived from these adhering to the surface of the cathode active material, the sample of the cathode active material and the cathode active material layer must be cleaned before being subjected to various analyses. You may do so. At this time, lithium may dissolve into the solvent used for cleaning, but even in that case, the additive elements are difficult to dissolve, so the atomic ratio of the additive elements is not affected.

[0358] Further, the concentration of the additive element may be compared in terms of its ratio to cobalt. By using the ratio to cobalt, it is possible to reduce the influence of carbonate, etc. chemically adsorbed after the positive electrode active material is produced, and to make a comparison, which is preferable. For example, the ratio Mg / Co of magnesium to the number of cobalt atoms as determined by XPS analysis is preferably 0.4 or more and 1.5 or less. On the other hand, Mg / Co as determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0359] Similarly, it is preferable that the positive electrode active material 100 has a higher concentration of lithium and cobalt than each additive element in the surface layer portion 100a in order to sufficiently secure a path for insertion and desorption of lithium. This means that it is preferable that the concentration of lithium and cobalt in the surface layer 100a is higher than the concentration of one or more of the additive elements selected from the additive elements contained in the surface layer 100a measured by XPS etc. can. For example, it is preferable that the concentration of cobalt in at least a portion of the surface layer portion 100a measured by XPS or the like is higher than the concentration of magnesium in at least a portion of the surface layer portion 100a measured by XPS or the like. Similarly, it is preferable that the concentration of lithium is higher than the concentration of magnesium. Further, it is preferable that the concentration of cobalt is higher than the concentration of nickel. Similarly, it is preferable that the concentration of lithium is higher than the concentration of nickel. Further, it is preferable that the concentration of cobalt is higher than the concentration of aluminum. Similarly, it is preferable that the concentration of lithium is higher than the concentration of aluminum. Further, it is preferable that the concentration of cobalt is higher than the concentration of fluorine. Similarly, it is preferable that the concentration of lithium is higher than the concentration of fluorine.

[0360] Further, it is more preferable that the additive elements such as aluminum are widely distributed in a deep region, for example, in a region with a depth of 5 nm or more and 50 nm or less from the surface. Therefore, in the analysis of the entire cathode active material 100 using ICP-MS, GD-MS, etc., additive elements such as aluminum are detected, but the analysis result by XPS etc. targeting about 5 nm from the surface and the concentration may be different.

[0361] Further, when XPS analysis is performed on the positive electrode active material 100 of one embodiment of the present invention, the number of magnesium atoms is preferably 0.4 times or more and 1.2 times or less, and more preferably 0.65 times or more and 1.0 times or less, relative to the number of cobalt atoms. Further, the number of nickel atoms is preferably 0.15 times or less, more preferably 0.03 times or more and 0.13 times or less relative to the number of cobalt atoms. Furthermore, the number of aluminum atoms is preferably 0.12 times or less, more preferably 0.09 times or less, relative to the number of cobalt atoms. The number of fluorine atoms is preferably 0.1 times or more and 1.1 times or less, more preferably 0.3 times or more and 0.9 times or less, relative to the number of cobalt atoms. The above range indicates 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 in the surface layer 100a of the positive electrode active material 100 at a preferable concentration. It can be said that it shows.

[0362] When performing XPS analysis, for example, monochromatic aluminum Kα rays can be used as the X-rays. Further, the take-out angle may be, for example, 45°. For example, it can be measured using the following equipment and conditions. Measuring device: PHI, Quantera II X-ray: Monochromatic Al Kα (1486.6eV) Detection area: 100μmφ Detection depth: approx. 4~5nm (extraction angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0363] Furthermore, when the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably about 684.3 eV. This value is different from both the binding energy of lithium fluoride, 685 eV, and the binding energy of magnesium fluoride, 686 eV.

[0364] Further, when the positive electrode active material 100 of one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably about 1303 eV. This value is different from the binding energy of magnesium fluoride, 1305 eV, and is close to the binding energy of magnesium oxide.

[0365] ≪EDX≫ It is preferable that one or more selected additive elements included in the positive electrode active material 100 have a concentration gradient. Further, it is more preferable that the depth of the concentration peak from the surface of the positive electrode active material 100 differs depending on the added element. The concentration gradient of the additive element can be evaluated by, for example, exposing a cross section of the positive electrode active material 100 using FIB or the like, and analyzing the cross section using EDX, EPMA, or the like.

[0366] Among EDX measurements, measuring while scanning the area and evaluating the area two-dimensionally is called EDX surface analysis. Also, measuring while scanning linearly and evaluating the distribution of atomic concentration within the positive electrode active material is called line analysis. Furthermore, data extracted from linear areas from EDX surface analysis is sometimes called line analysis. Furthermore, measuring a certain area without scanning it is called point analysis.

[0367] By EDX surface analysis (for example, elemental mapping), it is possible to quantitatively analyze the concentration of added elements in the surface layer 100a, interior 100b, vicinity of grain boundaries 101, etc. of the positive electrode active material 100. Furthermore, the concentration distribution and maximum value of the added element can be analyzed by EDX ray analysis. In addition, analysis using a thin sample like STEM-EDX can analyze the concentration distribution in the depth direction from the surface of the positive electrode active material toward the center in a specific region without being affected by the distribution in the depth direction. , is more suitable.

[0368] Therefore, when the positive electrode active material 100 of one embodiment of the present invention is subjected to EDX surface analysis or EDX point analysis, it is preferable that the concentration of the additive element such as magnesium in the surface layer portion 100a is higher than that in the interior portion 100b.

[0369] For example, when the positive electrode active material 100 having magnesium as an additive element is subjected to EDX surface analysis or EDX point analysis, it is preferable that the magnesium concentration in the surface layer portion 100a is higher than the magnesium concentration in the interior portion 100b. Furthermore, when performing EDX ray analysis, the peak of magnesium concentration in the surface layer 100a preferably exists within a depth of 3 nm from the surface of the positive electrode active material 100 toward the center, and more preferably exists within a depth of 1 nm. Preferably, it is more preferably present at a depth of 0.5 nm. Further, it is preferable that the magnesium concentration attenuates to 60% or less of the peak at a depth of 1 nm from the peak top. Further, it is preferable that the attenuation decreases to 30% or less of the peak at a depth of 2 nm from the peak top. Note that the peak of concentration herein refers to the maximum value of concentration.

[0370] Furthermore, 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 fluorine concentration and the peak of magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0371] Furthermore, when performing EDX ray analysis, the peak of fluorine concentration in the surface layer 100a preferably exists within a depth of 3 nm from the surface of the positive electrode active material 100 toward the center, and more preferably exists within a depth of 1 nm. Preferably, it is more preferably present at a depth of 0.5 nm. Further, it is more preferable that the peak of the fluorine concentration be present slightly closer to the surface than the peak of the magnesium concentration, since this increases resistance to hydrofluoric acid. For example, the peak of fluorine concentration is more preferably 0.5 nm or more closer to the surface than the peak of magnesium concentration, and even more preferably 1.5 nm or more closer to the surface.

[0372] In addition, in the positive electrode active material 100 having nickel as an additive element, it is preferable that the peak of nickel concentration in the surface layer 100a exists within a depth of 3 nm from the surface of the positive electrode active material 100 toward the center, and up to a depth of 1 nm. It is more preferable that it exists within a depth of 0.5 nm, and even more preferably that it exists within a depth of 0.5 nm. Furthermore, in the positive electrode active material 100 containing magnesium and nickel, the distribution of nickel preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of nickel concentration and the peak of magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0373] In addition, when the positive electrode active material 100 has aluminum as an additive element, it is preferable that the peak of the concentration of magnesium, nickel, or fluorine is closer to the surface than the peak of the aluminum concentration in the surface layer 100a when EDX ray analysis is performed. . For example, the aluminum concentration peak preferably exists at a depth of 0.5 nm or more and 50 nm or less from the surface of the positive electrode active material 100 toward the center, and more preferably at a depth of 5 nm or more and 50 nm or less.

[0374] Further, when EDX ray analysis, area analysis, or point analysis was performed on the positive electrode active material 100, the ratio of magnesium Mg to the number of cobalt Co atoms (Mg / Co) at the peak of magnesium concentration was less than 1, and 0.05 or more and 0.6 The following are preferable, and 0.1 or more and 0.4 or less are more preferable. The ratio of the number of atoms of aluminum Al to cobalt Co (Al / Co) at the peak of the aluminum concentration is less than 1, preferably 0.05 or more and 0.6 or less, and 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 less than 1, preferably 0 or more and 0.2 or less, and more preferably 0.01 or more and 0.1 or less. Alternatively, Ni / Co is preferably 0.1 or more and 0.5 or less. Further, the ratio of the number of atoms of cobalt Co and nickel Ni is preferably Co:Ni=90:10, Co:Ni=80:20, Co:Ni=70:30, or a ratio between these. The ratio of the number of atoms of fluorine F to cobalt Co (F / Co) at the peak of the fluorine concentration is less than 1, preferably 0 or more and 1.6 or less, and more preferably 0.1 or more and 1.4 or less.

[0375] Note that the surface of the positive electrode active material 100 in the EDX ray analysis results can be estimated as follows, for example. Regarding an element uniformly present in the interior 100b of the positive electrode active material 100, such as oxygen or cobalt, the point at which the amount detected in the interior 100b is 1 / 2 is defined as the surface.

[0376] Since the positive electrode active material 100 is a composite oxide, the surface can be estimated using the detected amount of oxygen. Specifically, first, calculate the average oxygen concentration O from the area where the detected amount of oxygen inside 100b is stable. ave seek. At this time, oxygen O is thought to be due to chemical adsorption or background in an area that can be clearly judged to be outside the surface. bg If detected, O bg The average value of oxygen concentration O ave It can be done. This average value O ave The value of 1 / 2 of O ave The measurement point that showed the measurement value closest to / 2 can be estimated to be the surface of the positive electrode active material.

[0377] Furthermore, the surface can be estimated in the same manner as above using the detected amount of cobalt. Alternatively, similar estimation can be made using the sum of detected amounts of a plurality of transition metals. Detected amounts of transition metals such as cobalt are suitable for surface estimation because they are not easily affected by chemisorption.

[0378] Further, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of the additive element A to cobalt Co (A / Co) near the grain boundary 101 is preferably 0.020 or more and 0.50 or less. More preferably, it is 0.025 or more and 0.30 or less. More preferably, it is 0.030 or more and 0.20 or less. Or preferably 0.020 or more and 0.30 or less. Or preferably 0.020 or more and 0.20 or less. Or preferably 0.025 or more and 0.50 or less. Or preferably 0.025 or more and 0.20 or less. Or preferably 0.030 or more and 0.50 or less. Or preferably 0.030 or more and 0.30 or less.

[0379] For example, when the additive element is magnesium, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium to cobalt (Mg / Co) near the grain boundary 101 is preferably 0.020 or more and 0.50 or less. . More preferably, it is 0.025 or more and 0.30 or less. More preferably, it is 0.030 or more and 0.20 or less. Or preferably 0.020 or more and 0.30 or less. Or preferably 0.020 or more and 0.20 or less. Or preferably 0.025 or more and 0.50 or less. Or preferably 0.025 or more and 0.20 or less. Or preferably 0.030 or more and 0.50 or less. Or preferably 0.030 or more and 0.30 or less. In addition, when the above range is present at multiple locations of the positive electrode active material 100, for example, three or more locations, the additive element is not attached to a narrow range on the surface of the positive electrode active material 100, but is preferable to the surface layer 100a of the positive electrode active material 100. This can be said to indicate that the concentration is widely distributed.

[0380] ≪EPMA≫ EPMA is also capable of quantifying elements. Area analysis allows analysis of the distribution of each element.

[0381] When EPMA surface analysis is performed on the cross section of the positive electrode active material 100 of one embodiment of the present invention, it is preferable that one or more selected additive elements have a concentration gradient, similar to the EDX analysis results. . Further, it is more preferable that the depth of the concentration peak from the surface differs depending on the added element. The preferred range of the concentration peak of each additive element is also the same as in the case of EDX.

[0382] However, EPMA analyzes the area from the surface to a depth of about 1 μm. Therefore, the quantitative value of each element may differ from the measurement results using other analysis methods. For example, when the surface of the positive electrode active material 100 is analyzed by EPMA, the concentration of each additive element present in the surface layer portion 100a may be lower than the result of XPS.

[0383] ≪Raman spectroscopy≫ As described above, in the positive electrode active material 100 of one embodiment of the present invention, at least a portion of the surface layer portion 100a preferably has a rock salt crystal structure. Therefore, when the positive electrode active material 100 and the positive electrode containing the same are analyzed by Raman spectroscopy, it is preferable that not only the layered rock salt crystal structure but also the cubic crystal structure including the rock salt type is observed. In the HAADF-STEM images and ultrafine electron diffraction patterns described below, the HAADF-STEM images cannot be obtained without cobalt, which is substituted at the lithium position at a certain frequency in the depth direction at the time of observation, and cobalt, which is present at the 4-coordination position of oxygen. and cannot be detected as a bright spot in the ultrafine electron diffraction pattern. On the other hand, since Raman spectroscopy is an analysis that captures the vibrational modes of bonds such as Co-O, it may be possible to observe the wavenumber peak of the corresponding vibrational mode even if the amount of the relevant Co-O bond is small. be. Furthermore, Raman spectroscopy can be used to measure the surface area of ​​several μm. 2 Since it is possible to measure a range of about 1 μm in depth, it is possible to detect states that exist only on the particle surface with high sensitivity.

[0384] For example, when the laser wavelength is 532 nm, layered rock salt type LiCoO 2 So, 470cm -1 ~490cm -1 , 580cm -1 ~600cm -1 peak (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 ), 665cm -1 ~685cm -1 peak (vibration mode: A 1g ) is observed.

[0385] Therefore, of the integrated intensity of each peak, 470cm -1 ~490cm -1 I1, 580cm -1 ~600cm -1 I2, 665cm -1 ~685cm -1 When is I3, the value of I3 / I2 is preferably 1% or more and 10% or less, more preferably 3% or more and 9% or less.

[0386] If a cubic crystal structure including a rock salt type is observed in the above range, it can be said that the surface layer portion 100a of the positive electrode active material 100 has a rock salt type crystal structure within a preferable range.

[0387] ≪Ultrafine electron diffraction pattern≫ It is preferable that the characteristics of the rock salt type crystal structure as well as the layered rock salt crystal structure be observed in the ultrafine electron diffraction pattern as well as in Raman spectroscopy. However, in STEM images and microelectron diffraction patterns, taking into account the difference in sensitivity mentioned above, it is important to note that the characteristics of the rock salt crystal structure do not become too strong at the surface layer 100a, especially at the outermost surface (for example, at a depth of 1 nm from the surface). preferable. Rather than having the outermost surface covered with a rock-salt-type crystal structure, it is better to have an additive element such as magnesium in the lithium layer while maintaining the layered rock-salt-type crystal structure. This is because the stabilizing function becomes stronger.

[0388] Therefore, for example, when obtaining an ultrafine electron diffraction pattern in a region with a depth of 1 nm or less from the surface and an ultrafine electron diffraction pattern in a region with a depth of 3 nm or more and 10 nm or less, the difference in the lattice constants calculated from them is Smaller is preferable.

[0389] 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 or more and 10 nm or less is preferably 0.01 nm or less for the a-axis, and 0.1 nm or less for the c-axis. It is preferable to have one. Further, the a-axis is more preferably 0.005 nm or less, and the c-axis is more preferably 0.06 nm or less. Further, it is more preferable that the a-axis is 0.004 nm or less, and even more preferable that the c-axis is 0.03 nm or less.

[0390] <Additional features> The positive electrode active material 100 may have a recess, a crack, a depression, a V-shaped cross section, or the like. These are one type of defects, and when charging and discharging are repeated, there is a risk that cobalt will be eluted, the crystal structure will collapse, the positive electrode active material 100 will crack, and oxygen will be eliminated. However, if a buried portion 102 as shown in FIG. 7(H) exists to bury these, elution of cobalt, etc. can be suppressed. Therefore, the reliability and cycle characteristics of a secondary battery using the positive electrode active material 100 can be improved.

[0391] As described above, if the additive element contained in the positive electrode active material 100 is in excess, there is a possibility that insertion and desorption of lithium will be adversely affected. Furthermore, when the positive electrode active material 100 is used in a secondary battery, there is a possibility that an increase in internal resistance, a decrease in charge / discharge capacity, etc. may occur. On the other hand, if it is insufficient, it may not be distributed throughout the surface layer 100a, and the effect of suppressing the deterioration of the crystal structure may become insufficient. As described above, it is necessary that the additive element has an appropriate concentration in the positive electrode active material 100, but it is not easy to adjust the concentration.

[0392] Therefore, if the positive electrode active material 100 has a region where the additive element is unevenly distributed, some of the atoms of the excessive additive element are removed from the interior 100b of the positive electrode active material 100, and the concentration of the additive element is maintained at an appropriate concentration in the interior 100b. It can be done. This can suppress an increase in internal resistance, a decrease in charge / discharge capacity, etc. when used as a secondary battery. Being able to suppress the increase in internal resistance of a secondary battery is an extremely desirable characteristic, particularly when charging and discharging at a large current, for example, at 400 mA / g or more.

[0393] Further, in the positive electrode active material 100 having a region where the additive element is unevenly distributed, it is permissible to mix the additive element in excess to some extent during the manufacturing process. Therefore, the margin in production becomes wider, which is preferable.

[0394] Further, a coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. 19(A) and 19(B) respectively show structures in which the coating portion 104 is attached to the positive electrode active material 100 shown in FIG. 7(G) and FIG. 7(H).

[0395] The covering portion 104 is preferably formed by depositing decomposition products such as lithium salt and organic electrolyte during charging and discharging, for example. Especially Li x CoO 2When charging is repeated such that x becomes 0.24 or less, it is expected that the charge / discharge cycle characteristics will be improved by having a coating portion derived from the organic electrolyte on the surface of the positive electrode active material 100. This is for reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing elution of cobalt. Preferably, the covering portion 104 contains carbon, oxygen, and fluorine, for example. Furthermore, when LiBOB and / or SUN (suberonitrile) is used as the electrolyte, it is easy to obtain a high-quality coating. Therefore, the coating portion 104 containing one or more selected from boron, nitrogen, sulfur, and fluorine is preferable because it may be a high-quality coating portion. Further, the covering portion 104 does not need to cover all of the positive electrode active material 100. For example, it is sufficient if it covers 50% or more of the surface of the positive electrode active material 100, more preferably 70% or more, and even more preferably 90% or more. Fluorine may be adsorbed on the surface of the positive electrode active material 100 in areas where the coating portion 104 is not present.

[0396] This embodiment can be used in combination with other embodiments.

[0397] (Embodiment 2) In this embodiment, an example of a method for manufacturing a positive electrode active material 100, which is one embodiment of the present invention, will be described.

[0398] In order to produce the positive electrode active material 100 having the distribution, composition, and / or crystal structure of the additive elements as described in the previous embodiment, how to add the additive elements is important. At the same time, it is also important that the interior 100b has good crystallinity.

[0399] In the manufacturing process of the positive electrode active material 100, there is a method in which lithium cobalt oxide is synthesized, and then an additive element source is mixed and a heat treatment is performed. Alternatively, a method may be used in which lithium cobalt oxide having an additive element is synthesized by mixing a cobalt source, a lithium source, and an additive element source at the same time. Further, it is preferable not only to mix lithium cobalt oxide and the additive element source but also to perform heating so that the additive element can be solid-dissolved in the lithium cobalt oxide. Furthermore, in order to distribute the additive elements well, it is preferable to perform sufficient heating. Therefore, heat treatment after mixing the additive element source is important. The heat treatment after mixing the additive element source is sometimes called firing or annealing.

[0400] However, if the heating temperature is too high, cation mixing will occur, increasing the possibility that additional elements, such as magnesium, will enter the cobalt site. Magnesium present in cobalt sites is Li x CoO 2 When x is small, there is no effect in maintaining the layered rock salt crystal structure of R-3m. Furthermore, if the temperature of the heat treatment is too high, there are concerns that there will be negative effects such as cobalt being reduced to become divalent and lithium evaporating.

[0401] Therefore, it is preferable to mix a material that functions as a flux together with the additive element source or as an additive element source. As a material that functions as a fluxing agent, a substance having a melting point lower than that of lithium cobalt oxide can be used. For example, fluorine compounds such as lithium fluoride are suitable as the flux. Addition of the flux lowers the melting point of the additive element source and the lithium cobalt oxide. By lowering the melting point, it becomes easier to distribute the additive element well at a temperature at which cation mixing is less likely to occur.

[0402] [Initial heating] Furthermore, it is more preferable to perform heating after synthesizing lithium cobalt oxide and before mixing additional elements. This heating may be called initial heating.

[0403] Due to the initial heating, lithium is desorbed from a part of the surface layer 100a of lithium cobalt oxide, so that the distribution of the added elements becomes even better.

[0404] More specifically, it is thought that the initial heating makes it easier to vary the distribution depending on the added element through the following mechanism. First, due to the initial heating, lithium is desorbed from a part of the surface layer portion 100a. Next, this lithium cobalt oxide having the surface layer portion 100a deficient in lithium and additional element sources including a nickel source, an aluminum source, and a magnesium source are mixed and heated. Among the additive elements, magnesium is a typical divalent element, and nickel is a transition metal but tends to become divalent ions. Therefore, in a part of the surface layer 100a, Mg 2+ and Ni 2+ and Co reduced due to lithium deficiency. 2+ A rock-salt-type phase is formed. However, since this phase is formed in a part of the surface layer portion 100a, it may not be clearly visible in an electron microscope image such as STEM or in an electron beam diffraction pattern.

[0405] Among the additive elements, nickel is likely to form a solid solution when the surface layer 100a is layered rock salt type lithium cobalt oxide and diffuses to the interior 100b, but when part of the surface layer 100a is rock salt type, it tends to remain in the surface layer 100a. . Therefore, by performing initial heating, divalent additive elements such as nickel can be easily retained in the surface layer portion 100a. The effect of this initial heating is particularly large on the surface of the positive electrode active material 100 other than the (001) orientation and its surface layer portion 100a.

[0406] In addition, 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.

[0407] For example, rock salt type Ni 0.5 Mg 0.5 Me-O distance at O ​​is 2.09×10 -10 m, the Me-O distance in rock salt MgO is 2.11×10 -10 m. Furthermore, even if a spinel-type phase is formed in a part of the surface layer 100a, spinel-type NiAl 2 O 4 The Me-O distance of is 2.0125×10 -10 m, spinel type MgAl 2 O 4 The Me-O distance of is 2.02×10 -10 m. In both cases, the Me-O distance is 2×10 -10 Exceeds m.

[0408] 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 is 1.905×10 -10 m(Li-O distance is 2.11×10 -10 m). Also, layered rock salt type LiCoO 2 The Co-O distance in is 1.9224×10 -10 m(Li-O distance is 2.0916×10 -10 m).

[0409] According to Shannon's ionic radius described in Non-Patent Document 15, the ionic radius of six-coordinated aluminum is 0.535×10 -10 m, the ionic radius of six-coordinated oxygen is 1.4×10 -10 m, and their sum is 1.935×10 -10 m.

[0410] From the above, it is considered that aluminum exists more stably at sites other than lithium in the layered rock salt type than in the rock salt type. Therefore, aluminum is more likely to be distributed in a deeper region having a layered rock salt phase and / or inside 100b than in a region near the surface having a rock salt phase in the surface layer 100a.

[0411] In addition, the initial heating can be expected to have the effect of increasing the crystallinity of the layered rock salt crystal structure of the interior 100b.

[0412] Therefore, especially Li x CoO 2 In order to produce the positive electrode active material 100 having a monoclinic O1(15) type crystal structure when x is, for example, 0.15 or more and 0.17 or less, it is preferable to perform this initial heating.

[0413] However, initial heating does not necessarily have to be performed. In other heating processes, by controlling the atmosphere, temperature, time, etc., Li x CoO 2 When x is small, the positive electrode active material 100 having O3' type and / or monoclinic O1(15) type can be produced in some cases.

[0414] 《Cathode active material production method 1》 Method 1 for producing the positive electrode active material 100 that undergoes initial heating will be explained using FIGS. 21(A) to 22(C).

[0415] <Step S11> In step S11 shown in FIG. 21(A), a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal materials, respectively.

[0416] 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; for example, a material with a purity of 99.99% or more may be used.

[0417] As the cobalt source, it is preferable to use a compound containing cobalt, and for example, cobalt oxide such as tricobalt tetroxide, cobalt hydroxide, etc. can be used.

[0418] 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 4N5 (99.995%) or higher, still more preferably 5N (99.999%) or higher. Good to use. By using high-purity materials, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery increases and / or the reliability of the secondary battery improves.

[0419] In addition, it is preferable that the cobalt source has high crystallinity, for example having single crystal grains. The crystallinity of the cobalt source can be evaluated using TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc., or evaluations using XRD, electron beam diffraction, neutron beam diffraction, etc. Note that the above method for evaluating crystallinity can be applied not only to cobalt sources but also to evaluating other crystallinities.

[0420] <Step S12> Next, in step S12 shown in FIG. 21(A), a lithium source and a cobalt source are ground and mixed to produce a mixed material. Grinding and mixing can be done dry or wet. The wet method is preferable because it can crush the particles into smaller pieces. If using a wet method, prepare a solvent. As the solvent, ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), and the like can be used. It is more preferable to use an aprotic solvent that hardly reacts with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or more is used. It is preferable to mix the lithium source and the cobalt source with dehydrated acetone having a purity of 99.5% or more and suppressing the water content to 10 ppm or less, and perform the pulverization and mixing. By using dehydrated acetone of the purity described above, possible impurities can be reduced.

[0421] A ball mill, a bead mill, or the like can be used as a means for crushing and mixing. When using a ball mill, aluminum oxide balls or zirconium oxide balls may be used as the grinding media. Zirconium oxide balls are preferable because they emit fewer impurities. Further, when using a ball mill, bead mill, etc., the peripheral speed is preferably 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 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0422] <Step S13> Next, in step S13 shown in FIG. 21(A), the mixed material is heated. The heating is preferably performed at a temperature of 800°C or more and 1100°C or less, more preferably 900°C or more and 1000°C or less, and even more preferably about 950°C. If the temperature is too low, the lithium source and cobalt source may be insufficiently decomposed and melted. On the other hand, if the temperature is too high, defects may occur due to evaporation of lithium from the lithium source and / or excessive reduction of cobalt. For example, cobalt changes from trivalent to divalent, which can induce oxygen defects.

[0423] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if the heating time is too long, productivity will decrease. For example, the heating time is preferably 1 hour or more and 100 hours or less, more preferably 2 hours or more and 20 hours or less.

[0424] The temperature increase rate depends on the temperature reached by the heating temperature, but is preferably 80°C / h or more and 250°C / h or less. For example, when heating at 1000°C for 10 hours, the temperature increase rate is preferably 200°C / h.

[0425] 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 -80°C or less. In this embodiment, heating is performed in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, CH 4 , CO, CO 2 , and H 2 It is preferable that the concentration of impurities such as the following is 5 ppb (parts per billion) or less.

[0426] As the heating atmosphere, an atmosphere containing oxygen is preferable. For example, there is a method of continuously introducing dry air into the reaction chamber. In this case, the flow rate of drying air is preferably 10 L / min. The method in which oxygen is continuously introduced into the reaction chamber and the oxygen flows within the reaction chamber is called flow.

[0427] When the heating atmosphere is an atmosphere containing oxygen, a method without flow may be used. For example, a method may be used in which the reaction chamber is depressurized and then filled with oxygen (also referred to as purging) to prevent the oxygen from leaving the reaction chamber. For example, the reaction chamber may be depressurized to -970 hPa and then filled with oxygen to 50 hPa.

[0428] Cooling after heating may be allowed to cool naturally, but it is preferable that the temperature drop time from the specified temperature to room temperature be within 10 hours or more and 50 hours or less; for example, the cooling rate (hereinafter also referred to as cooling rate) should be 80°C / h or more and 250°C / h or more. C / h or less is preferable, and 180 C / h or more and 210 C / h or less is more preferable. However, cooling to room temperature is not necessarily required, and cooling to a temperature permitted by the next step is sufficient.

[0429] The heating in this step may be performed using a rotary kiln or a roller hearth kiln. Heating with a rotary kiln can be carried out while stirring in either a continuous type or a batch type.

[0430] The crucible used for heating is preferably an aluminum oxide crucible. An aluminum oxide crucible is 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 it. Volatilization or sublimation of the material can be prevented. Providing a lid is sufficient as long as it can prevent volatilization or sublimation of the material from the time of temperature rise to the time of temperature fall in this step, and the crucible does not necessarily need to be hermetically sealed with a lid. For example, as described above, by filling the reaction chamber with oxygen, it is also possible to carry out this step without sealing the crucible.

[0431] Further, it is preferable to use a used crucible rather than a new one. In this specification, etc., a new crucible refers to a crucible that has undergone the process of charging and heating materials containing lithium, transition metal M, and / or additive elements two or fewer times. In addition, a used crucible is defined as one that has undergone the process of charging and heating materials containing lithium, transition metal M, and / or additive elements three or more times. This is because if a new crucible is used, there is a risk that some of the material, including lithium fluoride, may be absorbed, diffused, moved and / or attached to the sheath during heating. If a part of the material is lost due to these factors, there is a growing concern that the distribution of elements, particularly in the surface layer of the positive electrode active material, will not be within the preferred range. On the other hand, this fear is less with second-hand crucibles.

[0432] After the heating is completed, it may be crushed and further sieved if necessary. When recovering the heated material, it may be transferred from the crucible to the mortar and then recovered. Further, it is preferable to use an aluminum oxide mortar or a zirconium oxide mortar as the mortar. Aluminum oxide mortar is 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. Note that the same heating conditions as in step S13 can be applied to heating steps other than step S13, which will be described later.

[0433] <Step S14> Through the above steps, lithium cobalt oxide (LiCoO) shown in step S14 of FIG. 21(A) is 2 ) can be synthesized. When using the median diameter (D50) as the particle size of lithium cobalt oxide, it is preferable to crush the lithium cobalt oxide in order to obtain the positive electrode active material 100 with a relatively small median diameter (D50).

[0434] Although an example has been shown in which the composite oxide is produced by a solid phase method as in steps S11 to S14, the composite oxide may also be produced by a coprecipitation method. Alternatively, the composite oxide may be produced by a hydrothermal method.

[0435] <Step S15> Next, in step S15 shown in FIG. 21(A), lithium cobalt oxide is heated. Since the lithium cobalt oxide is first heated, the heating in step S15 may be referred to as initial heating. Alternatively, since it is heated before step S20 shown below, it may be called preheating or pretreatment. The crucible and / or lid used in this step are the same as those used in step S13. Although the following effects are expected from the initial heating, the initial heating is not essential to obtain the positive electrode active material that is one embodiment of the present invention.

[0436] Due to the initial heating, lithium is desorbed from a portion of the surface layer portion 100a of lithium cobalt oxide as described above. Further, it can be expected to have the effect of increasing the crystallinity of the interior 100b. Further, impurities may be mixed in the lithium source and / or cobalt source prepared in step S11 and the like. It is possible to reduce impurities from the lithium cobalt oxide completed in step S14 by initial heating.

[0437] Furthermore, initial heating has the effect of smoothing the surface of lithium cobalt oxide. When the surface of the composite oxide is smooth, it means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a state in which there are few foreign substances attached to the surface is called smooth. Foreign matter is considered to be a cause of unevenness, and it is preferable that it does not adhere to the surface.

[0438] This initial heating does not require the provision of a lithium compound source. Alternatively, it is not necessary to prepare an additional element source. Alternatively, there is no need to prepare a material that functions as a flux.

[0439] If the heating time in this step is too short, a sufficient effect will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from the heating conditions explained in step S13. Adding to the heating conditions, the heating temperature in this step is preferably lower than the temperature in step S13 in order to maintain the crystal structure of the composite oxide. Further, the heating time in this step is preferably shorter than the time in step S13 in order to maintain the crystal structure of the composite oxide. For example, heating is preferably performed at a temperature of 700° C. or higher and 1000° C. or lower for 2 hours or more and 20 hours or less.

[0440] The effect of increasing the crystallinity of the interior 100b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the shrinkage difference of the lithium cobalt oxide produced in step S13.

[0441] When the lithium cobalt oxide is heated in step S13, a temperature difference may occur between the surface and the inside of the lithium cobalt oxide. Temperature differences can induce differential shrinkage. It is also thought that a difference in shrinkage occurs because the fluidity between the surface and the inside differs due to the temperature difference. The energy associated with differential shrinkage imparts differential internal stress to lithium cobalt oxide. The difference in internal stress is also called strain, and the energy is sometimes called strain energy. It is considered that the internal stress is removed by the initial heating in step S15, and in other words, the strain energy is homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in the lithium cobalt oxide is relaxed. As a result, the surface of lithium cobalt oxide may become smooth. It is also said that the surface has been improved. In other words, it is considered that after step S15, the difference in shrinkage that occurs in the lithium cobalt oxide is alleviated, and the surface of the composite oxide becomes smooth.

[0442] Further, the difference in shrinkage may cause micro-shifts in the lithium cobalt oxide, such as crystal shifts. This step may also be carried out in order to reduce the deviation. Through this step, it is possible to equalize the deviation of the composite oxide. If the misalignment is made uniform, the surface of the composite oxide may become smooth. It is also said that crystal grains have been aligned. In other words, it is considered that after step S15, the displacement of crystals, etc. that occurs in the composite oxide is alleviated, and the surface of the composite oxide becomes smooth.

[0443] When lithium cobalt oxide with a smooth surface is used as a positive electrode active material, there is less deterioration during charging and discharging as a secondary battery, and cracking of the positive electrode active material can be prevented.

[0444] Note that lithium cobalt oxide synthesized in advance may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on lithium cobalt oxide synthesized in advance, lithium cobalt oxide with a smooth surface can be obtained.

[0445] <Step S20> Next, as shown in step S20, it is preferable to add additive element A to the lithium cobalt oxide that has undergone initial heating. When additive element A is added to lithium cobalt oxide that has undergone initial heating, additive element A can be added evenly. Therefore, it is preferable to add the additive element A after the initial heating. The step of adding additive element A will be explained using FIG. 21(B) and FIG. 21(C).

[0446] <Step S21~Step S23> The steps of preparing the additive element A source (A source) will be explained using FIG. 21(B) and FIG. 21(C). A lithium source may be prepared together with the additive element A source.

[0447] As the additive element A, the additive elements explained in the previous embodiment, such as magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and One or more selected from boron can be used. Moreover, one or two selected from bromine and beryllium can also be used.

[0448] <Step S21> Step S21 shown in FIG. 21(B) will be explained. When magnesium is selected as the additive element, the additive element source can be called a magnesium source (Mg source). As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Further, a plurality of the above-mentioned magnesium sources may be used.

[0449] When fluorine is selected as the additive element, the additive element source can be called a fluorine source (F source). Examples of the fluorine source include lithium fluoride (LiF) and magnesium fluoride (MgF). 2 ), aluminum fluoride (AlF 3 ), titanium fluoride (TiF 4 ), cobalt fluoride (CoF) 2 , CoF 3 ), nickel fluoride (NiF 2 ), zirconium fluoride (ZrF 4 ), vanadium fluoride (VF 5 ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF 2 ), calcium fluoride (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 ) etc. can be used. Among these, 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.

[0450] Magnesium fluoride can be used both as a fluorine source and as a magnesium source. Furthermore, lithium fluoride can be used both as a fluorine source and as a lithium source. Other lithium sources used in step S21 include lithium carbonate.

[0451] In addition, the fluorine source may be a gas, and fluorine (F 2 ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF 2 , O 2 F 2 , O 3 F 2 , O 4 F 2 , O 5 F 2 , O 6 F 2 , O 2 F) etc. may be used and mixed in the atmosphere in the heating step described later. Further, a plurality of the above-mentioned fluorine sources may be used.

[0452] In this embodiment, lithium fluoride (LiF) is prepared as a fluorine source, and magnesium fluoride (MgF) is prepared as a fluorine source and a magnesium source. 2 ) to prepare. Lithium fluoride and magnesium fluoride are LiF:MgF 2 Mixing at a ratio of about 65:35 (mol ratio) will have the highest effect of lowering the melting point. On the other hand, if the amount of lithium fluoride increases, there is a concern that the amount of lithium will be too much and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride and magnesium fluoride is LiF:MgF 2 =x:1(0≦x≦1.9), LiF:MgF 2 =x:1(0.1≦x≦0.5) is more preferable, LiF:MgF 2 =x:1 (x=0.33 or its vicinity) is more preferable. Note that in this specification and the like, the term "nearby" means a value greater than 0.9 times and smaller than 1.1 times the value.

[0453] <Step S22> Next, in step S22 shown in FIG. 21(B), the magnesium source and the fluorine source are ground and mixed. This step can be carried out by selecting from the pulverization and mixing conditions explained in step S12.

[0454] <Step S23> Next, in step S23 shown in FIG. 21(B), the materials crushed and mixed above can be recovered to obtain an additive element A source (A source). Note that the additive element A source shown in step S23 has a plurality of starting materials and can be called a mixture.

[0455] The particle size of the above mixture preferably has a median diameter (D50) of 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less. Even when one type of material is used as the additive element source, the median diameter (D50) is preferably 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less.

[0456] When such a finely powdered mixture (including cases where only one type of additive element is added) is mixed with lithium cobalt oxide in a later process, it is difficult to uniformly adhere the mixture to the surface of the lithium cobalt oxide particles. Cheap. It is preferable that the mixture is uniformly adhered to the surface of the lithium cobalt oxide particles because it is easy to uniformly distribute or diffuse the additive element in the surface layer portion 100a of the composite oxide after heating.

[0457] <Step S21> A process different from that in FIG. 21(B) will be explained using FIG. 21(C). In step S21 shown in FIG. 21(C), four types of additive element sources to be added to lithium cobalt oxide are prepared. In other words, FIG. 21(C) differs from FIG. 21(B) in the type of additive element source. A lithium source may be prepared together with the additive element source.

[0458] A magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared as four types of additional element sources. Note that the magnesium source and the fluorine source can be selected from the compounds described in FIG. 21(B). As the nickel source, nickel oxide, nickel hydroxide, etc. can be used. As the aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0459] <Step S22 and Step S23> Step S22 and step S23 shown in FIG. 21(C) are the same as the steps described in FIG. 21(B).

[0460] <Step S31> Next, in step S31 shown in FIG. 21(A), lithium cobalt oxide and an additive element A source (A source) are mixed. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg included in the additive element A source is preferably Co:Mg=100:y (0.1≦y≦6), and Co: More preferably, Mg=100:y (0.3≦y≦3).

[0461] The mixing in step S31 is preferably performed under milder conditions than the mixing in step S12 so as not to destroy the shape of the lithium cobalt oxide particles. For example, it is preferable that the rotation speed is lower or the time is shorter than the mixing in step S12. It can also be said that the dry method has milder conditions than the wet method. For example, a ball mill, a bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use, for example, zirconium oxide balls as the media.

[0462] In this embodiment, dry mixing is performed at 150 rpm for 1 hour using a ball mill using zirconium oxide balls with a diameter of 1 mm. The mixing shall be performed in a dry room with a dew point of -100°C or higher and -10°C or lower.

[0463] <Step S32> Next, in step S32 of FIG. 21(A), the materials mixed above are collected to obtain a mixture 903. During recovery, sieving may be performed after crushing if necessary.

[0464] Note that although FIGS. 21(A) to 21(C) describe a manufacturing method in which additive elements are added after initial heating, the present invention is not limited to the above method. The additive element may be added at other timings or may be added multiple times. The timing may be changed depending on the added element.

[0465] For example, as shown in FIGS. 22(A) to 22(C), the additive element may be added to the lithium source and the cobalt source at the stage of step S11, that is, at the stage of the starting material of the composite oxide. FIG. 22(A) shows a flow of adding a magnesium source to a lithium source and a cobalt source. FIG. 22(B) shows a flow of adding a magnesium source and an aluminum source to a lithium source and a cobalt source. FIG. 22(C) shows a flow of adding a magnesium source and a nickel source to a lithium source and a cobalt source. The additive element sources shown in FIGS. 22(A) to 22(C) are examples.

[0466] Thereafter, the process continues to step S12, passes through step S13, and in step S14, lithium cobalt oxide having an additive element can be obtained. It is also possible to control the distribution of additive elements according to the timing of adding the additive elements. The additive elements added as shown in FIGS. 22(A) to 22(C) are expected to be located inside the positive electrode active material 100. In addition, in the case of the flow shown in FIGS. 22(A) to 22(C), there is no need to separate the steps S11 to S14 and the steps S21 to S23, which is a simple and highly productive method. You can say that. Of course, even in the flow shown in FIGS. 22(A) to 22(C), a new additive element may be added in step S20.

[0467] Alternatively, lithium cobalt oxide having a portion of additive elements in advance may be used. For example, if lithium cobalt oxide to which magnesium and fluorine are added is used, some of the steps S11 to S14 and S20 can be omitted. It can be said that this is a simple and highly productive method.

[0468] Further, after heating the lithium cobalt oxide to which magnesium and fluorine have been added in advance in step S15, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source are added as in step S20. may be added.

[0469] <Step S33> Next, in step S33 shown in FIG. 21(A), the mixture 903 is heated. The heating can be performed by selecting from the heating conditions explained 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 are reduced, and lithium cobalt oxide and the like may not be able to maintain a layered rock salt crystal structure.

[0470] Here, we will add some additional information about heating temperature. The lower limit of the heating temperature in step S33 needs to be equal to or higher than the temperature at which the reaction between lithium cobalt oxide and the additive element source progresses. The temperature at which the reaction proceeds may be any temperature at which interdiffusion occurs between lithium cobalt oxide and the element contained in the additive element source, and may be lower than the melting temperature of these materials. To explain using an oxide as an example, the melting temperature T m 0.757 times (Tamman temperature T d ), it is known that solid phase diffusion occurs. Therefore, the heating temperature in step S33 may be 650° C. or higher.

[0471] Of course, if the temperature is higher than the temperature at which one or more of the materials selected from the mixture 903 melts, the reaction will proceed more easily. For example, LiF and MgF as additive element sources 2 , LiF and MgF 2 Since the eutectic point of is around 742°C (see eutectic point P in FIG. 10), the lower limit of the heating temperature in step S33 is preferably 742°C or higher.

[0472] Also, LiCoO 2 :LiF:MgF 2 In the mixture 903 obtained by mixing at a molar ratio of =100:0.33:1, an endothermic peak is observed at around 830°C in a DSC test. Therefore, the lower limit of the heating temperature is more preferably 830°C or higher.

[0473] A higher heating temperature is preferable because the reaction progresses more easily, heating time is shorter, and productivity is higher.

[0474] The upper limit of the heating temperature is lower than the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is concern that lithium cobalt oxide will decompose, albeit in a small amount. Therefore, the upper limit of the heating temperature is more preferably 1000°C or less, even more preferably 950°C or less, and even more preferably 900°C or less.

[0475] Considering these, the heating temperature in step S33 is preferably 650°C or more and 1130°C or less, more preferably 650°C or more and 1000°C or less, even more preferably 650°C or more and 950°C or less, and even more preferably 650°C or more and 900°C or less. preferable. Further, the temperature is preferably 742°C or more and 1130°C or less, more preferably 742°C or more and 1000°C or less, even more preferably 742°C or more and 950°C or less, and even more preferably 742°C or more and 900°C or less. Furthermore, the temperature is preferably 830°C or more and 1130°C or less, more preferably 830°C or more and 1000°C or less, even more preferably 830°C or more and 950°C or less, and even more preferably 830°C or more and 900°C or less. Note that the heating temperature in step S33 is preferably lower than the heating temperature in step S13.

[0476] Here, an example of the heating furnace used in this step S33 will be explained using FIG. 25.

[0477] A heating furnace 220 shown in FIG. 25 includes a heating furnace interior space 202, a hot plate 204, a pressure gauge 221, a heater section 206, and a heat insulating material 208. Preferably, a container 216 corresponding to a crucible or pod is heated with a lid 218 placed thereon. With this configuration, it is possible to create an atmosphere containing fluoride in the space 219 formed by the container 216 and the lid 218. During heating, by keeping the concentration of gasified fluoride in space 219 constant or not decreasing by keeping it covered, fluorine and magnesium can be included near the particle surfaces of mixture 903. Since the space 219 has a smaller volume than the heating furnace interior space 202, a small amount of fluoride evaporates, thereby creating an atmosphere containing fluoride. That is, the atmosphere of the reaction system can be made into an atmosphere containing fluoride without significantly reducing the amount of fluoride contained in the mixture 903. Further, by using the lid 218, the mixture 903 can be easily and inexpensively heated in an atmosphere containing fluoride.

[0478] Furthermore, before heating in the heating furnace interior space 202, a step of making the heating furnace interior space 202 an atmosphere containing oxygen, and a step of installing a container 216 containing the mixture 903 in the heating furnace interior space 202 are performed. conduct. With this order of steps, the mixture 903 can be heated in an atmosphere containing oxygen and fluoride. For example, heating is performed while gas is flowing (flow). Gas can be introduced from the lower surface of the heating furnace interior space 202 and exhausted to the upper surface. Further, during heating, the heating furnace interior space 202 can be sealed to be a closed space so that gas is not carried outside (purge).

[0479] There are no particular restrictions on the method of creating an oxygen-containing atmosphere in the heating furnace interior space 202, but one example is a method in which a gas containing oxygen such as oxygen gas or dry air is introduced after exhausting the heating furnace interior space 202; Examples include a method in which a gas containing oxygen, such as gas or dry air, is introduced for a certain period of time. Among these, it is preferable to introduce oxygen gas (oxygen replacement) after evacuating the heating furnace interior space 202. Note that the atmosphere in the heating furnace interior space 202 may be regarded as an atmosphere containing oxygen.

[0480] Furthermore, fluoride and the like that have adhered to the inner walls of the container 216 and the lid 218 can be re-floated by heating and adhered to the mixture 903.

[0481] There is no particular restriction on the process of heating the heating furnace 220. Heating may be performed using a heating mechanism provided in the heating furnace 220.

[0482] Furthermore, there is no particular restriction on how the mixture 903 is arranged when placed in the container 216, but as shown in FIG. It is preferable to arrange the mixture 903 so that the height of the upper surface of the mixture 903 is uniform.

[0483] The heating in step S33 is preferably performed while controlling the pressure inside the furnace using the pressure gauge 221. The inside of the furnace is preferably at atmospheric pressure or pressurized. For example, it is believed that the surface of lithium cobalt oxide melts when exposed to pressurized conditions. Therefore, LiF and MgF 2 The surface of the lithium cobalt oxide that is heated at the same time can be melted by applying pressure.

[0484] Cooling after heating in step S33 above may be allowed to cool naturally, but it is preferable that the temperature drop time from the specified temperature to room temperature falls within 10 hours or more and 50 hours or less, for example, the temperature drop rate is 80°C / h or more and 250°C / h or less. The temperature is preferably 180°C / h or more and 210°C / h or less. The cooling rate in this step S33 is preferably faster than that in step S13. A fast cooling rate is called rapid cooling. By performing rapid cooling after the above-mentioned melting, a shell can be appropriately produced. Specifically, it becomes possible to produce a narrow shell. Note that the temperature at the time of completion of cooling does not necessarily need to be room temperature, as long as the temperature is cooled to a temperature that is allowed by the next step.

[0485] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluorine compounds caused by a fluorine source or the like within an appropriate range. It is also possible to control the partial pressure by placing a lid on the crucible used in this step and heating it. As mentioned above, the lid can prevent the material from volatilizing or sublimating. That is, it is only necessary to prevent volatilization or sublimation of the material from the time of temperature rise to the time of temperature fall in this step, and the crucible does not necessarily need to be sealed with a lid. For example, by filling the reaction chamber in which the crucible is placed with oxygen, it is also possible to carry out this step without sealing the crucible. A positive electrode active material containing an appropriate amount of fluorine or a fluorine compound is preferable because it can suppress heat generation and smoke generation even when an internal short circuit occurs.

[0486] In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, may function as a flux. With this function, the heating temperature can be lowered to below the decomposition temperature of lithium cobalt oxide, for example from 742°C to 950°C, and additive elements such as magnesium are distributed in the surface layer, creating a positive electrode active material with good characteristics. can.

[0487] However, since LiF has a lower specific gravity in a gaseous state than oxygen, LiF may volatilize or sublimate due to heating, and if volatilized, LiF in the mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat LiF while suppressing its volatilization. Note that even if LiF is not used as a fluorine source, LiCoO 2 There is also a possibility that LiF on the surface reacts with F, the fluorine source, to generate LiF and volatilize it. Therefore, even if a fluorine compound with a higher melting point than LiF is used, it is necessary to suppress volatilization in the same way.

[0488] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF in the heating furnace is high. Such heating can suppress volatilization of LiF in the mixture 903. It is a good idea to place a lid on the crucible to suppress the volatilization of LiF.

[0489] The heating in this step is preferably performed so that the particles of the mixture 903 do not stick to each other. If the particles of the mixture 903 stick to each other during heating, the contact area with oxygen in the atmosphere decreases and the diffusion path of the added elements (e.g. fluorine) is inhibited, thereby preventing the addition of the added elements (e.g. magnesium and fluorine) to the surface layer. Fluorine) distribution may deteriorate. In order to promote the reaction with oxygen in the atmosphere, it is not necessary to seal the crucible with a lid.

[0490] It is also believed that when the additive element (eg, fluorine) is uniformly distributed in the surface layer, a positive electrode active material that is smooth and has less irregularities can be obtained. Therefore, in order to maintain the smooth state of the surface after the heating in step S15 or to make it even smoother in this step, it is preferable that the particles of the mixture 903 do not stick to each other.

[0491] Further, when heating is performed using a rotary kiln, it is preferable to control the flow rate of the atmosphere containing oxygen in the kiln. For example, it is preferable to reduce the flow rate of the atmosphere containing oxygen, to purge the atmosphere first, and to not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is not preferable for maintaining surface smoothness.

[0492] When heating with a roller hearth kiln, for example, by placing a lid on the container containing the mixture 903, the mixture 903 can be heated in an atmosphere containing LiF. It is similar to the lid placed on a crucible.

[0493] A note about heating time. The heating time varies depending on conditions such as the heating temperature, the size of the lithium cobalt oxide obtained in step S14, and the composition. When the lithium cobalt oxide is small, lower temperatures or shorter times may be more preferred than when the lithium cobalt oxide is large.

[0494] When the median diameter (D50) of the lithium cobalt oxide obtained in step S14 of FIG. 21(A) 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. Note that the time for cooling down after heating is preferably, for example, 10 hours or more and 50 hours or less.

[0495] On the other hand, when the median diameter (D50) of the lithium cobalt oxide obtained in step S14 is about 5 μm, the heating temperature is preferably, for example, 650° C. or more and 950° C. or less. The heating time is preferably, for example, 1 hour or more and 10 hours or less, and more preferably about 5 hours. Note that the time for cooling down after heating is preferably, for example, 10 hours or more and 50 hours or less.

[0496] <Step S34> Next, in step S34 shown in FIG. 21(A), the heated material is collected and crushed if necessary to obtain the positive electrode active material 100. At this time, the collected particles may be further sieved. Through the above steps, the positive electrode active material 100 of one embodiment of the present invention can be manufactured. The positive electrode active material of one embodiment of the present invention has a smooth surface.

[0497] 《Cathode active material production method 2》 Next, a method 2 for producing a positive electrode active material, which is an embodiment of the present invention and is different from the method 1 for producing a positive electrode active material, will be described with reference to FIGS. 23 to 24(C). Manufacturing method 2 of the positive electrode active material differs from manufacturing method 1 mainly in the number of times the additive element is added and the mixing method. For other descriptions, the description of production method 1 can be referred to.

[0498] In FIG. 23, steps S11 to S15 are performed in the same manner as in FIG. 21(A) to prepare lithium cobalt oxide that has undergone initial heating.

[0499] <Step S20a> Next, in step S20a, a source of additive element A1 is prepared to be used for adding additive element A1 to lithium cobalt oxide that has undergone initial heating. The process of preparing the additive element A1 source will be explained using FIG. 24(A).

[0500] <Step S21> Step S21 shown in FIG. 24(A) will be explained. The additive element A1 can be selected from the elements exemplified as the additive element A explained in step S21 shown in FIG. 21(B). For example, as the additive element A1, one or more selected from magnesium, fluorine, and calcium can be suitably used. FIG. 24(A) shows an example in which a magnesium source (Mg source) and a fluorine source (F source) are prepared in step S21, assuming that magnesium and fluorine are sel...

Claims

1. A battery comprising a positive electrode having a positive electrode active material and a negative electrode containing graphite, and further comprising a film-like outer casing housing the positive electrode and the negative electrode, The positive electrode active material comprises nickel, magnesium, fluorine, aluminum, and lithium cobalt oxide. The positive electrode active material has a shell that includes at least a portion of the surface of the positive electrode active material. In the aforementioned shell, the distribution of detected nickel is superimposed on the distribution of detected magnesium. A battery that does not ignite when subjected to a nail-piercing test to short-circuit it under the following conditions. Nail-piercing test conditions: The battery is set to a voltage of 4.5V in an environment of 25°C, and a nail-piercing test is performed using a 3mm diameter nail at a speed of 5mm / sec.

2. A battery comprising a positive electrode having a positive electrode active material and a negative electrode containing graphite, and further comprising a film-like outer casing that houses the positive electrode and the negative electrode, The positive electrode active material comprises nickel, magnesium, fluorine, aluminum, and lithium cobalt oxide. The positive electrode active material has a first region which includes at least a portion of the surface other than the (001) surface of the positive electrode active material. In the first region, the distribution of detected nickel is superimposed on the distribution of detected magnesium. A battery that does not ignite when subjected to a nail-piercing test to short-circuit it under the following conditions. Nail-piercing test conditions: The battery is set to a voltage of 4.5V in an environment of 25°C, and a nail-piercing test is performed using a 3mm diameter nail at a speed of 5mm / sec.

3. The battery according to claim 1 or claim 2, wherein in the nail-piercing test, the temperature rise ΔT of the battery is 130°C or less.

4. The battery according to claim 1 or claim 2, wherein the distribution of the detected amount of magnesium and the distribution of the detected amount of nickel are distributions shown by count values ​​in EDX radiation analysis.

5. The battery according to claim 1 or claim 2, further comprising a polymer gel electrolyte.

6. A battery comprising a positive electrode having a positive electrode active material and a negative electrode containing graphite, and further comprising a film-like outer casing that houses the positive electrode and the negative electrode, The positive electrode active material comprises nickel, magnesium, fluorine, aluminum, and lithium cobalt oxide. The positive electrode active material has a first region including at least a part of the surface of the positive electrode active material, and a second region which is an interior region of the first region. In the first region having at least a surface other than the (001) surface, the ratio of the number of nickel atoms to the number of cobalt atoms (Ni1 / Co1) is less than 1. The ratio of nickel atoms to cobalt atoms in the second region (Ni2 / Co2) is smaller than the ratio of nickel atoms to cobalt atoms in the region having a surface other than the (001) surface (Ni1 / Co1). A battery that does not ignite when subjected to a nail-piercing test to short-circuit it under the following conditions. Nail-piercing test conditions: The battery is set to a voltage of 4.5V in an environment of 25°C, and a nail-piercing test is performed using a 3mm diameter nail at a speed of 5mm / sec.

7. The battery according to claim 6, wherein in the nail-piercing test, the temperature rise ΔT of the battery is 50°C or less.

8. The battery according to claim 6, wherein, after performing a charge-discharge cycle test with a number of cycles of 1 to 5 cycles, the battery does not ignite when the nail-piercing test is performed.

9. The battery according to claim 6, wherein, in an EDX analysis of the positive electrode at a distance of less than 2 cm from the nail hole after the nail-piercing test, the ratio of the number of oxygen atoms to the number of cobalt atoms (O1 / Co1) is less than 1.

3.

10. The battery according to claim 6, wherein, in EDX analysis of a location on the positive electrode at a distance of 2 cm or more from the nail hole after the nail-piercing test, the ratio of the number of oxygen atoms to the number of cobalt atoms (O2 / Co2) is 1.3 or more.

11. A battery comprising a positive electrode having a positive electrode active material and a negative electrode containing graphite, and further comprising a film-like outer casing that houses the positive electrode and the negative electrode, The positive electrode active material comprises nickel, magnesium, fluorine, aluminum, and lithium cobalt oxide. The positive electrode active material has a first region including at least a part of the surface of the positive electrode active material, and a second region which is an interior region of the first region. In the first region having at least a surface other than the (001) surface, the ratio of the number of nickel atoms to the number of cobalt atoms (Ni1 / Co1) is less than 1. The ratio of nickel atoms to cobalt atoms in the second region (Ni2 / Co2) is smaller than the ratio of nickel atoms to cobalt atoms in the region having a surface other than the (001) surface (Ni1 / Co1). A battery that does not ignite when subjected to a nail-piercing test to short-circuit it under the following conditions. Nail-piercing test conditions: The battery is set to a voltage of 4.6V in an environment of 25°C, and a nail-piercing test is performed using a 3mm diameter nail at a speed of 5mm / sec.

12. The battery according to claim 11, wherein in the nail-piercing test, the temperature rise ΔT of the battery is 70°C or less.

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