Secondary battery

Abstract

To provide a battery with high safety.SOLUTION: A battery includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolytic solution. The positive electrode active material includes 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 positioned closer to a surface side of the positive electrode active material than the second region. The negative electrode active material contains blacklead. The electrolytic solution has mixed organic solvents. When a nail penetration test is conducted under conditions that the battery is in a fully charged state; a diameter of a nail is 3 mm; and nailing speed is 5 mm / sec, a voltage of the battery decreases from a first voltage Vb to a second voltage Vc, and then becomes higher than the second voltage Vc.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] One aspect of the present invention relates to a battery, and more particularly to a secondary battery. The present invention is not limited to semiconductor devices, display devices, light-emitting devices, power storage devices, lighting devices, electronic devices, vehicles, and The present invention relates to the semiconductor device, display device, light emitting device, power storage device, lighting device, and a manufacturing method thereof. The secondary battery of the present invention can be applied as a necessary power source to devices, electronic devices, and vehicles. For example, the electronic device described above includes an information terminal device equipped with a secondary battery. Furthermore, the above-mentioned power storage device includes a stationary power storage device. [Background technology]

[0002] In recent years, high-power, high-capacity lithium-ion secondary batteries (also referred to as lithium-ion batteries) have Demand is rapidly increasing, and it has become an essential reusable energy source in modern society. It has become.

[0003] It is difficult to achieve both high capacity and safety in lithium-ion secondary batteries. For example, a positive electrode active material with a layered rock salt crystal structure has Since the diffusion paths of lithium ions exist two-dimensionally, high capacity is expected. Therefore, the positive electrode active material with a layered rock salt crystal structure loses lithium ions when it is charged. However, the crystal structure is destroyed, which is said to lead to thermal runaway, and this poses safety issues. Safety tests include nail penetration tests, and in abnormal situations such as when a nail is penetrated, the battery temperature rises. In order to suppress the increase, for example, Patent Document 1 discloses a method for providing a protective layer between the positive electrode mixture layer and the positive electrode current collector. A configuration has been proposed.

[0004] Lithium cobalt oxide (LiCoO2) is known as a positive electrode active material with a layered rock salt crystal structure. Lithium cobalt oxide has a layered rock-salt crystal structure, with layers of CoO6 octahedra. Since lithium ions can move two-dimensionally between the electrodes, the cycle characteristics are also good. However, lithium cobalt oxide has a problem of phase change during charging and discharging. When some lithium ions are released, the lithium cobalt oxide changes from a hexagonal crystal to a monoclinic crystal. Therefore, lithium cobalt oxide is used with good cycle characteristics. In order to use the battery, the amount of lithium ions released must be limited. For example, Patent Documents 2 to 4 propose a configuration in which an additive element is added to lithium cobalt oxide. Furthermore, research into the crystal structure of positive electrode active materials is also being conducted (Non-Patent Documents 1 to 5). 4).

[0005] In addition, XRD (X-ray Diffraction) was used to determine the crystalline structure of the positive electrode active material. This is one of the methods used for analyzing the structure of semiconductors. Organic Crystal Structure Database For example, in Non-Patent Document 6, The lattice constant of lithium cobalt oxide can be found from ICSD. For example, the analysis program RIETAN-FP (Non-Patent Document 7) is used for the LB method analysis. This can be done.

[0006] As image processing software, for example, ImageJ (Non-Patent Documents 8 to 10) is used. It is known that the shape of the positive electrode active material can be analyzed by using this software. This can be done.

[0007] Microelectron beam diffraction is also effective for identifying the crystal structure of the positive electrode active material, particularly the crystal structure of the surface layer. The electron beam diffraction pattern is analyzed using, for example, the analysis program ReciPro (Non-Patent Document 11 ) can be used.

[0008] Fluorides such as fluorite (calcium fluoride) have long been used as fluxes in iron manufacturing. Its physical properties have been studied (Non-Patent Document 12).

[0009] When the temperature of a lithium-ion secondary battery rises during charging, it goes through several states before it goes into thermal runaway. It is known that this leads to

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

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

[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 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] GG 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 Structure Refinement of LiCoO2 Single Crystals.” Journal of Solid State Chemistry (1998) 141, p. 298-3

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[0014] Lithium cobalt oxide (LiCoO2, abbreviated as LCO) disclosed in Patent Documents 2 to 4 Lithium-ion secondary batteries are known to have poor thermal stability. If an internal short circuit occurs during the test, Joule heat is generated, causing the lithium cobalt oxide to become hot. The oxygen released from the lithium cobalt oxide reacts with the electrolyte, etc. In Patent Document 1, in order to suppress the rise in battery temperature when a nail is pierced, To this end, a configuration is disclosed in which a protective layer is provided between the positive electrode current collector and the positive electrode mixture layer.

[0015] In view of the above, an object of one embodiment of the present invention is to provide a highly safe battery. Another object of one embodiment of the present invention is to provide a battery that has a high capacity and is highly safe. do.

[0016] The description of these problems does not preclude the existence of other problems. It is not necessary for the embodiments to solve all of these problems. It is possible to extract other problems from the description of the claim. [Means for solving the problem]

[0017] One aspect 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. a first region having lithium, cobalt, magnesium, and oxygen, and a second region having The first region has lithium, cobalt, and oxygen, and the second region has a higher positive electrode active material than the second region. The thickness of the first region is 1 nm or more and 20 nm or less, and the thickness of the first region is 1 nm or more and 20 nm or less. The concentration is greater than 0 and less than 10 atomic %.

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

[0019] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material comprising a first region and a second region. and a second region, the first region containing lithium, cobalt, magnesium, nickel, and fluorine. and oxygen, the second region having lithium, cobalt, and oxygen, and the first region having 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 or more and 20 nm or less. and the magnesium concentration is greater than 0 and less than or equal to 10 atomic %.

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

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

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

[0023] In another embodiment of the present invention, the battery voltage is 4.5 V, the nail diameter is 3 mm, and When a nail penetration test was conducted under the conditions of a nail penetration speed of 5 mm / sec, the battery temperature rise ΔT It is preferable that the temperature is 50°C or lower.

[0024] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material comprising a first region and a second region. and two regions, the first region containing lithium, cobalt, magnesium, and oxygen. the second region has a second lithium, cobalt, and oxygen; the first region has a second lithium, cobalt, and oxygen; The positive electrode active material is located on the surface side of the positive electrode active material. Fluorine is adsorbed on the surface of the positive electrode active material. The first region is bonded to lithium, and the thickness of the first region is 2 nm or more and 20 nm or less, and the first region is bonded to magnesium. The concentration of sodium is greater than 0 and less than 10 atomic %.

[0025] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material comprising a first region and a second region. and a second region, the first region containing a first lithium, cobalt, magnesium, nickel and and oxygen, the second region having a second lithium, cobalt, and oxygen, and the first region is located closer to the surface of the positive electrode active material than the second region, and fluorine is adsorbed on the surface of the positive electrode active material. The fluorine is bonded to the first lithium, and the thickness of the first region is 2 nm or more and 20 nm or less. The magnesium concentration is greater than 0 and less than 10 atomic %.

[0026] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material comprising a first region and a second region. and a second region, the first region being a first lithium, cobalt, magnesium, nickel, The first region has fluorine and oxygen, and the second region has 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 provided with The second fluorine is adsorbed, the second fluorine is bonded to the first lithium, and the thickness of the first region is The thickness is 2 nm or more and 20 nm or less, and the magnesium concentration is greater than 0 and less than 10 atomic %. Below is the battery.

[0027] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material comprising a first region and a second region. and two regions, the first region containing lithium, cobalt, magnesium, and oxygen. the second region has a second lithium, cobalt, aluminum, and oxygen; The region is located closer to the surface of the positive electrode active material than the second region, and fluorine is absorbed on the surface of the positive electrode active material. The fluorine is bonded to the first lithium, and the thickness of the first region is 2 nm or more and 20 nm or less. and the magnesium concentration is greater than 0 and less than or equal to 10 atomic %.

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

[0029] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material comprising a first region and a second region. and two regions, the first region containing lithium, cobalt, magnesium, and oxygen. the second region has a second lithium, cobalt, and oxygen; the first region has a second lithium, cobalt, and oxygen; The positive electrode active material is located on the surface side of the positive electrode active material. Fluorine is adsorbed on the surface of the positive electrode active material. The volume resistivity of the powder of the positive electrode active material is increased by bonding with the first lithium at a pressure of 64 MPa. at 1.0×10 5 A battery with a resistance of Ω·cm or more.

[0030] Another aspect of the present invention is a battery including a positive electrode having a positive electrode active material, the positive electrode active material comprising: The positive electrode active material contains cobalt, nickel, and lithium, and the positive electrode active material has a small surface area. and a second region that is an area inside the first region. , the ratio of the number of nickel atoms in the first region to the number of cobalt atoms in the first region The ratio of the number of nickel atoms in the first region to the number of nickel atoms in the second region is less than 1. The ratio of the number of nickel atoms in the first region to the number of nickel atoms in the first region is The ratio of the number of cobalt atoms to the number of cobalt atoms is smaller than the number of cobalt atoms in the battery. When a nail penetration test was conducted to short-circuit the battery, no fire occurred. The battery is tested in a charged state under an environment of 0.5 °C.

[0031] In another embodiment of the present invention, the battery voltage is 4.5 V, the nail diameter is 3 mm, and the nail penetration speed is In a nail penetration test at 5mm / sec, the battery temperature rise ΔT is 50°C or less. It is preferable to do so.

[0032] Another aspect of the present invention is a battery including a positive electrode having a positive electrode active material, the positive electrode active material comprising: The positive electrode active material contains cobalt, nickel, and lithium, and the positive electrode active material has a small surface area. and a second region that is an area inside the first region. , the ratio of the number of nickel atoms in the first region to the number of cobalt atoms in the first region The ratio of the number of nickel atoms in the first region to the number of nickel atoms in the second region is less than 1. The ratio of the number of nickel atoms in the first region to the number of nickel atoms in the first region is The ratio of the number of cobalt atoms to the number of cobalt atoms is smaller than the number of cobalt atoms in the battery. After conducting the following charge / discharge cycle test, a nail penetration test was conducted to short-circuit the battery. The nail penetration test is carried out in a charged state at 23°C. do.

[0033] In another embodiment of the present invention, the battery voltage is 4.6 V, the nail diameter is 3 mm, and the nail penetration speed is In a nail penetration test at 5mm / sec, the battery temperature rise ΔT is 70°C or less. It is preferable to do so.

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

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

[0036] In another embodiment of the present invention, the charge-discharge cycle test is performed in a 45° C. environment, and charging is performed at a constant current. It is preferable to charge at a constant voltage and discharge at a constant current.

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

[0038] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material including a first region and and a second region, the first region containing lithium, cobalt, magnesium, and oxygen. the second region has lithium, cobalt, and oxygen, and the first region has The positive electrode was located closer to the surface of the positive electrode active material than the nail hole after the nail penetration test. At the point where the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is less than 1.3 The atomic concentration of cobalt in the positive electrode active material and at a point 2 cm or more away from the nail hole was measured. and 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 aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material including a first region and and a second region, the first region being composed of lithium, cobalt, magnesium, nickel, and oxygen, the second region having lithium, cobalt, and oxygen, and the first region having The positive electrode is located closer to the surface of the positive electrode active material than the second region, and the positive electrode is At a distance of less than 2 cm, the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1.3 The positive electrode active material is less than 2 cm away from the nail hole. and a positive electrode active material having a ratio of oxygen atomic concentration to oxygen atomic concentration of 1.3 or more. do.

[0040] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material including a first region and and a second region, the first region being composed of lithium, cobalt, magnesium, nickel, the first region having fluorine and oxygen, the second region having lithium, cobalt, and oxygen, The region is located closer to the surface of the positive electrode active material than the second region, and the positive electrode is The ratio of the atomic concentration of oxygen to the atomic concentration of cobalt at a distance of less than 2 cm from the The positive electrode active material with a value of less than 1.3 and the cobalt at a point 2 cm or more away from the nail hole and a positive electrode active material having a ratio of the atomic concentration of oxygen to the atomic concentration of ethene of 1.3 or more. It's a battery.

[0041] Another aspect of the present invention includes a positive electrode having a positive electrode active material, the positive electrode active material including a first region and and a second region, the first region containing lithium, cobalt, magnesium, and oxygen. the second region comprises lithium, cobalt, aluminum, and oxygen; and the first region The positive electrode is located closer to the surface of the positive electrode active material than the second region, and the positive electrode is At a distance of less than 2 cm, the ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1 The positive electrode active material is less than 0.3 and the cobalt is present at a distance of 2 cm or more from the nail hole. a positive electrode active material having a ratio of oxygen atomic concentration to oxygen atomic concentration of 1.3 or more. is.

[0042] Another aspect of the present invention is a battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution and the positive electrode active material has a first region and a second region, and the first region is lithium. the second region has lithium, cobalt, magnesium, and oxygen; and oxygen, the first region being located closer to the surface of the positive electrode active material than the second region, The material includes graphite, and the electrolyte includes ethylene carbonate and diethyl carbonate. However, the positive electrode had no cobalt atoms at a point less than 2 cm away from the nail hole after the nail penetration test. The ratio of the oxygen atomic concentration to the oxygen molecular concentration is less than 1.3. The ratio of the atomic concentration of oxygen to the atomic concentration of cobalt is 1.3 or more at a point 2 cm or more away. and a positive electrode active material.

[0043] Another aspect of the present invention is a battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution and the positive electrode active material has a first region and a second region, and the first region is lithium. the second region having lithium, cobalt, magnesium, nickel, and oxygen; The first region contains cobalt and oxygen, and is located closer to the surface of the positive electrode active material than the second region. The negative electrode active material contains graphite, and the electrolyte contains ethylene carbonate and diethyl carbonate. The positive electrode has a nate, and after a nail penetration test, the positive electrode has a contact point that is less than 2 cm away from the nail hole. The ratio of the atomic concentration of oxygen to the atomic concentration of barium is less than 1.3. The ratio of the atomic concentration of oxygen to the atomic concentration of cobalt at a distance of 2 cm or more from the and a positive electrode active material in which the σ is 1.3 or more.

[0044] Another aspect of the present invention is a battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution and the positive electrode active material has a first region and a second region, and the first region is lithium. the first region having lithium, cobalt, magnesium, nickel, fluorine, and oxygen; The first region has a surface of the positive electrode active material that is closer to the surface of the positive electrode active material than the second region. The negative electrode active material is graphite, and the electrolyte is ethylene carbonate and diethylene glycol. The positive electrode has a polycarbonate, and after a nail penetration test, the positive electrode has a and 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. , the atomic concentration of oxygen relative to the atomic concentration of cobalt at a distance of 2 cm or more from the nail hole and a positive electrode active material having a concentration ratio of 1.3 or more.

[0045] Another aspect of the present invention is a battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution and the positive electrode active material has a first region and a second region, and the first region is lithium. the second region has lithium, cobalt, magnesium, and oxygen; The first region contains aluminum and oxygen, and is located closer to the surface of the positive electrode active material than the second region. The negative electrode active material is graphite, and the electrolyte is ethylene carbonate and diethyl carbonate. The positive electrode has a carbon dioxide gas at a point less than 2 cm away from the nail hole after a 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; The atomic concentration of oxygen relative to the atomic concentration of cobalt at a distance of 2 cm or more from the hole and a positive electrode active material in which the ratio of

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

[0047] Another aspect of the present invention is a battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution and a battery having the cathode active material having a first region and a second region, The first region has cobalt, magnesium, fluorine and oxygen, and the second region has cobalt, and oxygen, the first region being located closer to the surface of the positive electrode active material than the second region, The material contains graphite, the electrolyte contains a mixed organic solvent, and the battery is fully charged. When a nail penetration test was conducted with a diameter of 3 mm and a nail penetration speed of 5 mm / sec, the battery The voltage drops from the first voltage Vb to the second voltage Vc, and then becomes higher than the second voltage Vc. It's a battery.

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

[0049] Another aspect of the present invention is a battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution and a battery having the cathode active material having a first region and a second region, The first region has cobalt, magnesium, fluorine and oxygen, and the second region has cobalt, and oxygen, the first region being located closer to the surface of the positive electrode active material than the second region, The material contains graphite, the electrolyte contains a mixed organic solvent, and the battery is charged in a 45°C environment. After the discharge cycle test, the battery was fully charged and subjected to a nail test with a diameter of 3 mm and a nail penetration test. When a nail penetration test is performed at a speed of 5 mm / sec, the battery voltage drops to Vc. It is a battery that maintains the value of Vc.

[0050] In another embodiment of the present invention, the nail penetration test was carried out under the condition that the battery voltage was 4.6 V. In this case, 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 powder of the positive electrode active material is When force is 1.0 x 10 5 It is preferable that the resistivity is Ω·cm or more.

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

[0053] According to one embodiment of the present invention, a highly safe battery can be provided. By using this method, it is possible to provide a battery with high capacity and high safety.

[0054] The description of these effects does not preclude the existence of other effects. The embodiment does not necessarily have to have all of these effects. , the specification, drawings, claims, etc., and It is possible to extract other effects from the claims and other descriptions. [Brief explanation of the drawings]

[0055] [Figure 1] 1(A) and 1(B) are diagrams illustrating the nail penetration test. [Figure 2] 2(A) and 2(B) are diagrams illustrating the nail penetration operation. [Figure 3] FIG. 3 is a graph showing the change in the internal temperature when the internal temperature of a secondary battery in which an internal short circuit occurs increases. [Figure 4] FIG. 4 is a graph showing changes when the internal temperature of the secondary battery increases. [Figure 5] 5(A) to 5(C) are diagrams for explaining changes in the voltage of the secondary battery during a nail penetration test. [Figure 6] 6(A) and 6(B) are diagrams illustrating a laminated secondary battery. [Figure 7] 7(A) to 7(H) are cross-sectional views of the positive electrode active material. [Figure 8] Figure 8 is an example of a TEM image in which the crystal orientations are roughly consistent. [Figure 9] Figure 9(A) is an example of a STEM image in which the crystal orientations are roughly consistent, Figure 9(B) is an FFT pattern of the rock-salt-type crystal RS region, and Figure 9(C) is an FFT pattern of the layered rock-salt-type crystal LRS region. [Figure 10] FIG. 10 is a phase diagram showing the relationship between the composition of lithium fluoride and magnesium fluoride and the temperature. [Figure 11] FIG. 11 is a diagram illustrating the results of the DSC test. [Figure 12] FIG. 12 is a diagram illustrating the crystal structure of the positive electrode active material. [Figure 13]FIG. 13 is a diagram illustrating the crystal structure of a conventional positive electrode active material. [Figure 14] FIG. 14 is a diagram illustrating the state of charge and the lattice constant of the positive electrode active material. [Figure 15] FIG. 15 shows an XRD pattern calculated from the crystal structure. [Figure 16] FIG. 16 shows the XRD pattern calculated from the crystal structure. [Figure 17] 17(A) and 17(B) are diagrams showing XRD patterns calculated from the crystal structure. [Figure 18] 18(A) to 18(C) show the lattice constants calculated from XRD. [Figure 19] 19(A) and 19(B) are cross-sectional views of the positive electrode active material. [Figure 20] 20(A) to 20(C) are explanatory diagrams relating to powder resistance measurement. [Figure 21] 21A to 21C are diagrams illustrating a method for manufacturing a positive electrode active material. [Figure 22] 22A to 22C are diagrams illustrating a method for manufacturing a positive electrode active material. [Figure 23] FIG. 23 is a diagram illustrating a method for manufacturing a positive electrode active material. [Figure 24] 24A to 24C are diagrams illustrating a method for manufacturing a positive electrode active material. [Figure 25] FIG. 25 is a diagram illustrating the heating furnace and heating conditions. [Figure 26] 26(A) and 26(B) are diagrams illustrating the positive electrode active material layer. [Figure 27] 27(A) to 27(C) are diagrams illustrating a coin-type secondary battery. [Figure 28] 28(A) to 28(D) are diagrams illustrating a cylindrical secondary battery. [Figure 29] 29(A) and 29(B) are diagrams illustrating a wound type secondary battery. [Figure 30]FIG. 30 is a diagram illustrating a wound type secondary battery. [Figure 31] 31A to 31D are diagrams illustrating electronic devices. [Figure 32] 32A to 32C are diagrams illustrating electronic devices. [Figure 33] 33(A) to 33(C) are diagrams for explaining a vehicle. [Figure 34] Figures 34(A) to 34(F) are photographs illustrating the results of the nail penetration test. [Figure 35] Figures 35(A) to 35(F) are photographs illustrating the results of the nail penetration test. [Figure 36] 36(A) to 36(F) are graphs illustrating the results of the nail penetration test. [Figure 37] 37(A) to 37(F) are graphs illustrating the results of the nail penetration test. [Figure 38] Figures 38(A) to 38(D) are photographs illustrating the results of the nail penetration test. [Figure 39] Figures 39(A) to 39(E) are photographs illustrating the results of the nail penetration test. [Figure 40] Figures 40(A) to 40(E) are photographs illustrating the results of the nail penetration test. [Figure 41] Figures 41(A) and 41(B) are photographs illustrating the results of the nail penetration test. [Figure 42] Figures 42(A) to 42(D) are photographs illustrating the results of the nail penetration test. [Figure 43] Figures 43(A) to 43(C) are photographs illustrating the results of the nail penetration test. [Figure 44] 44(A) and 44(B) are graphs illustrating the results of the nail penetration test. [Figure 45] 45(A) and 45(B) are graphs illustrating the results of the nail penetration test. [Figure 46] Figures 46(A) and 46(B) are energy diagrams illustrating the results of the nail penetration test. [Figure 47] FIG. 47 is a photograph illustrating the results of the nail penetration test. [Figure 48] 48(A) and 48(B) are graphs illustrating the results of the nail penetration test. [Figure 49] FIG. 49 is a graph illustrating the results of the nail penetration test. [Figure 50] FIG. 50 is a graph showing the results of the DSC test. [Figure 51] Figure 51(A) is a cross-sectional STEM image, Figure 51(B) is an EDX mapping image, and Figure 51(C) is a graph showing the results of EDX line analysis. [Figure 52] Figure 52(A) is a cross-sectional STEM image, Figure 52(B) is an EDX mapping image, and Figure 52(C) is a graph showing the results of EDX line analysis. [Figure 53] Figure 53(A) is a cross-sectional STEM image, Figure 53(B) is an EDX mapping image, and Figure 53(C) is a graph showing the results of EDX line analysis. [Figure 54] Figure 54(A) is a cross-sectional STEM image, and Figure 54(B) is an EDX mapping image. [Figure 55] FIG. 55 is a graph showing the results of the cycle test. [Figure 56] FIG. 56 is a graph showing the results of powder resistivity measurements. [Figure 57] 57(A) and 57(B) are graphs showing the results of the cycle test. [Figure 58] FIG. 58 is a graph showing the results of powder resistance measurements. [Figure 59] Figure 59 is a model diagram of the calculation. [Figure 60] FIG. 60(A) is a model diagram of the calculation, and FIG. 60(B) is the calculation result. [Figure 61] Figure 61(A) and Figure 61(B) are model diagrams of the calculation. [Figure 62] Figures 62(A) and 62(B) are photographs illustrating the results of the nail penetration test. [Figure 63] Figures 63(A) and 63(B) are photographs illustrating the results of the nail penetration test. [Figure 64] Figure 64 is a photograph illustrating the results of the nail penetration test. [Figure 65] 65(A) and 65(B) are graphs showing the results of the impedance test, and FIG. 65(C) is a diagram showing the equivalent circuit used in the analysis. [Figure 66] FIG. 66 is a graph showing the results of the DSC test. DETAILED DESCRIPTION OF THE INVENTION

[0056] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The present invention should not be construed as being limited to the following embodiments. It is possible to change the mode of carrying out the invention.

[0057] In this specification, the space group is expressed in Sho notation (or Hermann-Mauguin notation). The crystal plane and crystal direction are expressed using rt notation. The notation of space groups, crystal planes, and crystal directions is expressed as follows: However, due to formatting restrictions in this specification, instead of placing a bar above the number, a -( In addition, individual orientations that indicate directions within a crystal are expressed as [ ], collective orientations indicating all equivalent directions are < >, individual faces indicating crystal faces are ( ), etc. The set planes with valence symmetry are expressed as {}. Also, they are expressed in the space group R-3m. For ease of understanding the structure, the trigonal crystal is generally represented by a complex hexagonal lattice of the hexagonal crystal. Unless otherwise specified, the space group R-3m is represented by a compound hexagonal lattice. The exponent may be (hkil) instead of (hkl). Here, i is -(h+ k).

[0058] In this specification, particles are not limited to those having a spherical shape (a circular cross section), The cross-sectional shape of each particle may be elliptical, rectangular, trapezoidal, triangular, square with rounded corners, or asymmetrical. Furthermore, each particle may have an irregular shape.

[0059] The theoretical capacity of the positive electrode active material is the capacity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. For example, the theoretical capacity of LiCoO2 is 274mAh / g, The theoretical capacity of iO2 is 274mAh / g, and that of LiMn2O4 is 148mAh / g. be.

[0060] The amount of lithium remaining in the positive electrode active material that can be inserted or removed is determined by the x in the composition formula, For example, Li x It is represented by x in MO2. M represents a transition metal, and in this specification etc. Unless otherwise specified, M is cobalt and / or nickel. In the case of the positive electrode active material, x = (theoretical capacity - charging capacity) / theoretical capacity. BaLi x A lithium-ion secondary battery using MO2 as the positive electrode active material was charged at 219.2mAh / g. If the battery is charged, Li 0.2 It can be said that MO2 or x=0.2. Li x x in MO2 is small, for example, 0.1 <x≦0.24をいう。

[0061] When properly synthesized lithium cobalt oxide is used in the positive electrode, the stoichiometric ratio is approximately , LiCoO2 and x = 1. Also, the lithium ion secondary battery contains The lithium cobalt oxide used in this study is LiCoO2, so x = 1. The end of the test is when the voltage drops to 3.0 V or less when the discharge current is 100 mA / g or less. This refers to the state where the voltage drops below 2.5V.

[0062] Li x The charge capacity and / or discharge capacity used to calculate x in MO2 is determined by the short circuit and / or electrolytic It is preferable to measure under conditions that are minimally affected by decomposition of the liquid, etc. Data on lithium-ion secondary batteries that have experienced a sudden change in capacity should not be used to calculate x. Not possible.

[0063] The space group of the positive electrode active material is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," Alternatively, being in a certain space group can be rephrased as being identified with a certain space group.

[0064] If the anions have a structure like ABCABC, where three layers are stacked with each other, then the cubic This is called a close-packed structure. Therefore, the anions do not need to be in a cubic lattice. Since real crystals always have defects, analytical results may not always be consistent with theory. For example, electron diffraction patterns or TEM (Transmission Electron FFT (Fast Fourier Transform) patterns of microscope and transmission electron microscope images In this case, the spot may appear at a position slightly different from the theoretical position. If the deviation of the orientation from the cubic structure is less than 5 degrees or less than 2.5 degrees, it is said to have a cubic close-packed structure. good.

[0065] The distribution of a certain element is the continuous distribution of the element in a range that is not noise in a certain continuous analysis method. This refers to the area that is actively detected.

[0066] In this specification, the surface layer of the positive electrode active material refers to the layer extending from the surface to the inside. The area is defined as a region within 20 nm or 50 nm in a direction perpendicular or nearly perpendicular to the surface. The layer portion is synonymous with the surface vicinity and the surface vicinity region. The angle formed with the surface is 80° or more and 100° or less. The inner region is referred to as the interior, which is synonymous with the bulk or core.

[0067] In this specification, the positive electrode active material is referred to as a composite oxide, a positive electrode material, a positive electrode material for a secondary battery, a lithium In this specification and the like, the present invention may be referred to as a cathode material for lithium ion secondary batteries. In one embodiment, the positive electrode active material preferably includes a compound. The positive electrode active material of one embodiment of the present invention preferably has a composition. Therefore, the positive electrode active material of one embodiment of the present invention preferably includes a composite.

[0068] In addition, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, etc., However, not all particles need to have this characteristic. For example, three or more randomly selected positive particles At least 50%, preferably at least 70%, more preferably at least 90% of the particles of the electrode active material If the positive electrode active material has this characteristic, the characteristics of the positive electrode active material and the secondary battery containing the positive electrode active material can be sufficiently improved. It can be said that this has the effect of

[0069] As the charging voltage of the secondary battery increases, the voltage applied to the positive electrode generally increases. In one embodiment, the positive electrode active material is stable in a charged state, and therefore, the positive electrode active material is stable during charging and discharging. A secondary battery can be obtained in which the decrease in discharge capacity due to repeated discharge is suppressed.

[0070] In addition, an internal or external short circuit of the secondary battery may occur during charging and / or discharging of the secondary battery. This not only causes malfunctions, but also may lead to heat generation and fire. In order to realize this, it is preferable that an internal short circuit or an external short circuit is suppressed even at a high charging voltage. The positive electrode active material of one embodiment of the present invention is characterized in that it is free from internal short circuits or external short circuits even at high charging voltages. Therefore, it is possible to provide a secondary battery that has both a high discharge capacity and safety. An internal short circuit in a secondary battery refers to contact between the positive and negative electrodes inside the battery. An external short circuit of a secondary battery is assumed to occur in the event of misuse, and occurs when the positive and negative electrodes come into contact with each other outside the battery. It refers to doing something.

[0071] In this specification, ignition in the nail penetration test means that a flame occurs within one minute after the nail is inserted. This refers to the occurrence of thermal runaway in a secondary battery, or the occurrence of a thermal runaway in a secondary battery. After the nail penetration test is completed, the positive and / or negative electrodes are Thermal decomposition of the positive and / or negative electrodes is observed. For example, aluminum oxide is formed when the aluminum in the positive electrode current collector is oxidized, and aluminum in the negative electrode current collector is This includes copper oxide, which is copper oxidized.

[0072] For example, when layered rock salt LiMO2 (M is Co and / or Ni) is used as the positive electrode active material, Theoretically, the ratio of the number of O atoms to the number of M atoms (hereinafter referred to as the O / M ratio) is 2. On the other hand, when oxygen is released from LiMO2 due to thermal runaway, the O / M ratio decreases. For example, after the nail penetration test, the energy Energy Dispersive X-ray Spectroscopy (EDX) If the O / M ratio in the fluoroscopy analysis is less than 1.3, thermal runaway occurs. Conversely, in EDX analysis, the If the O / M ratio is 1.3 or higher, it can be said that thermal runaway has not occurred. After the test is completed, even if the battery voltage drops once and then rises again, thermal runaway has not occurred. It can be said that there is no such person.

[0073] On the other hand, in the nail penetration test, even if flames, sparks and / or smoke are observed, they do not remain at the point of penetration. In other words, if the fire does not spread and the secondary battery does not go into thermal runaway, it is not considered a fire. If the above fire does not occur when a secondary battery is subjected to a nail penetration test, it is a non-igniting secondary battery. It can be said that.

[0074] Unless otherwise specified, the materials contained in the secondary battery (positive electrode active material, negative electrode active material, electrolyte, The description of the condition before deterioration is given for secondary battery manufacturing stage. The decrease in discharge capacity due to aging and burn-in treatment is not called deterioration. For example, a discharge capacity of 97% or more of the rated capacity of a secondary battery consisting of a single cell or a battery pack is not If the battery has a certain amount of capacity, it can be said to be in a state before deterioration. For secondary batteries, comply with JIS C 8711:2019. For other secondary batteries, In addition to the above JIS standards, we also comply with various JIS and IEC standards for electric vehicle propulsion, industrial use, etc. do.

[0075] In this specification, the state of the materials of a secondary battery before deterioration is referred to as an initial product or initial state. This is called the deteriorated state (when the discharge capacity of the secondary battery is less than 97% of the rated capacity). ) when referring to a product in use or in a used state, or a used product or in a used state There is.

[0076] In this specification, a lithium ion secondary battery is a battery that uses lithium ions as carrier ions. However, the carrier ions of the present invention are not limited to lithium ions. The use of alkali metal ions or alkaline earth metal ions as carrier ions is Specifically, sodium ions and the like can be applied. In this case, lithium The present invention can be understood by replacing the ions with sodium ions, etc. When there is no limitation on the ions, it may be referred to as a secondary battery.

[0077] In this specification, the (001) plane, the (003) plane, etc. are collectively referred to as the (001) plane. In this specification, the (001) plane is sometimes referred to as the C plane, the basal plane, etc. In addition, 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 case, the surface where the lithium diffusion path is exposed, that is, the surface where lithium is inserted and desorbed (specifically Faces other than the (00l) face are sometimes called edge faces.

[0078] In this specification and the like, the amount of supported active material is the weight of the active material per unit surface area of ​​the current collector. The amount of negative electrode active material carried can be adjusted according to the capacity of the positive electrode. In the case of double-sided coating in which a slurry having a desired quality is applied, the above-mentioned amount of support may be determined per one side. If the amount of the supported material is small, the discharge capacity will be small. Therefore, the amount of the supported positive electrode active material is set to 8.0 mg / cm 2 The above is preferable.

[0079] In this specification, etc., the term "secondary particles" refers to particles formed by aggregation of primary particles. In this specification, the term "primary particles" refers to particles that do not have grain boundaries visible on the surface. In this specification etc., a single particle refers to a particle that does not have a grain boundary on its appearance. In this case, a single crystal particle refers to a crystal particle in which there is no grain boundary inside the particle, and a polycrystalline particle refers to a crystal particle in which there is no grain boundary inside the particle. A particle refers to a crystalline particle with a grain boundary inside the particle. A polycrystalline particle is a particle made up of multiple crystallites. It may be called an aggregate, and a grain boundary may be called an interface that exists between two or more crystallites. In the polycrystalline particles, it is preferable that the crystallites are oriented in the same direction.

[0080] In this specification, etc., there may be a description of "A and / or B", but this does not mean only A and only B. This is an example of a description that includes A and B.

[0081] (Embodiment 1) The nail penetration test is a test in which a secondary battery is fully charged and the nail is penetrated to a specified depth selected from 2 mm to 10 mm. This is a test in which a nail having a diameter of 100 mm is driven into a secondary battery at a predetermined speed. First, the nail penetration test device will be described. Fig. 1(A) shows a side view of the nail penetration test device 1000. FIG. 1(B) shows a perspective view of the nail penetration test device 1000.

[0082] <Nail penetration test device> The nail penetration test device 1000 shown in FIG. 1(A) includes a stage 1001, a driving unit 1002, and A nail 1003, a voltage measuring device 1015, a temperature measuring device 1016, and a control unit 1018 are included. The driving unit 1002 has a driving mechanism 1012 that moves the nail 1003 in the direction of the arrow in the figure. Then, the drive mechanism 1012 drives the nail 1003 to the secondary battery 1 At this time, the secondary battery 1004 is in a fully charged state (State Of Charge: State equivalent to 100% SOC) and The broken line in FIG. 1(A) indicates the length of the nail 10 after penetration during the nail insertion operation. 10 shows a recess in stage 1001 provided to accommodate stage 1003.

[0083] The voltage measuring device 1015 sends information about the voltage of the secondary battery during the nail penetration operation to the control unit 1018. Specifically, the amount of voltage change and the like are transmitted to the control unit 1018. Information about the temperature during the nail penetration operation is sent from the control unit 1016 to the control unit 1018. 18 transmits a control signal to the driving unit 1002 when controlling the operating conditions of the nail 1003. This can be done.

[0084] FIG. 1B is a perspective view illustrating the vicinity of the upper part of the stage 1001 of the nail penetration test device 1000. The secondary battery 1004 placed on the stage 1001 is connected to a wiring 1005a and a wiring 1005b. The wiring 1005a and the wiring 1005b are electrically connected to each other. The measuring instrument 1015 has the wiring 1005a and the wiring 1005b. , and are electrically connected to the positive electrode tab and the negative electrode tab of the secondary battery 1004, The voltage of the secondary battery 1004 can be measured simply by measuring the voltage between the positive and negative electrodes. Also, the temperature sensor 1016 is called the temperature sensor. When a temperature sensor is used, the temperature sensor is provided so as to be in contact with the surface of the exterior body of the secondary battery 1004. .

[0085] In FIG. 1B, a first temperature sensor 1006a and a second temperature sensor 1006b are arranged. As an example, one temperature sensor may be disposed, or three or more temperature sensors may be disposed. A first temperature sensor 1006a is provided on the side where the wiring 1005a and the wiring 1005b are not arranged, A second temperature sensor 1006b is provided on the side where the wiring 1005a and the wiring 1005b are arranged. If two or more temperature sensors are installed, one of the temperature sensors may not be used due to the expansion of the exterior body, etc. This is preferable because even if one temperature sensor becomes unavailable, another temperature sensor can be used. .

[0086] Furthermore, there is a welding area on the side where the wiring 1005a and the wiring 1005b are arranged, but The exterior body is folded back on the side where the wiring 1005a and the wiring 1005b are not arranged. Therefore, even if the exterior body expands, the wiring 1005a and the wiring 1005b are The expansion of the side where the first temperature sensor 1006a is not arranged is suppressed, and the first temperature sensor 1006b detects the second temperature It is more difficult to peel off than sensor 1006b and is therefore preferable.

[0087] The dashed ellipse shown in FIG. 1B indicates the distance between the nail 1003 and the secondary battery 1004 during the nail penetration operation. The first temperature sensor 1006a and the second temperature sensor 1006b are , and may be provided in an area equidistant from the area through which the nail 1003 penetrates. The first temperature sensor 100 is located within 5 cm, preferably within 2 cm, of the area penetrated by the 03. It is preferable to provide a second temperature sensor 1006a and a second temperature sensor 1006b. It is preferable to be able to grasp the temperature change. When two or more temperature sensors are installed, Check that the temperature difference indicated by the temperature sensor is within ±5°C, preferably within ±2°C. It is advisable to start the nailing action from this point.

[0088] <Secondary battery in nail penetration test> Next, the state of the secondary battery in the nail penetration test was revised using Figures 2(A) and 2(B), etc. The nail penetration test was carried out with the secondary battery 1004 in a fully charged state, with the secondary battery 1004 being penetrated by a nail from 2 mm to 10 mm. A nail 1003 having a predetermined diameter selected from below is inserted into a secondary battery 1004 at a predetermined speed. FIG. 2(A) shows a cross-sectional view of a secondary battery 1004 with a nail 1003 stuck in it. The secondary battery 1004 includes a positive electrode 503, a separator 508, a negative electrode 506, and an electrolyte 530. The positive electrode 503 has a structure in which the positive electrode current collector 501 and the positive electrode current collector 502 are housed in an exterior body 531. The negative electrode 506 has a negative electrode current collector 511 and a positive electrode active material layer 502 formed on both sides thereof. 2B shows the structure of the negative electrode active material layer 512 formed on the nail 1003 and the positive electrode current collector 512. 5 shows an enlarged view of the vicinity of 01, and shows the positive electrode active material 100 and the conductive material contained in the positive electrode active material layer 502. Material 553 is also specified.

[0089] As shown in FIGS. 2A and 2B, a nail 1003 is driven into a secondary battery 1004. Specifically, the nail 1003 penetrates the positive electrode 503 and the negative electrode 506, causing an internal short circuit. Then, the potential of the nail 1003 becomes equal to the potential of the negative electrode 506, and As shown by the black arrow, electrons (e - ) flows to the positive electrode 503, and the internal short circuit point and its vicinity In addition, Joule heat is generated in the negative electrode 506 due to an internal short circuit. Lithium ion (Li + ) is released into the electrolyte as shown by the white arrow. However, before all the lithium ions are released from the negative electrode, the battery is destroyed by Joule heat generated by an internal short circuit. The battery temperature rises rapidly, causing the electrolyte to begin to decompose at the surface of the negative electrode. This is one of the reactions, and is called the reduction reaction of the electrolyte by the negative electrode.

[0090] In addition, when the temperature of the secondary battery 1004 rises due to Joule heat, the positive electrode active material contains cobalt oxide. When lithium is used, lithium cobalt oxide has a H1-3 type crystal structure and an O1 type crystal structure. This can cause a phase change (i.e., structural change) to the structure, which can also generate heat. The crystal structures of the -3 type and the O1 type will be described later.

[0091] As shown in FIGS. 2A and 2B, the electrons (e - ) As a result, the tetravalent Co in the charged lithium cobalt oxide is reduced to trivalent or divalent. This reduction reaction releases oxygen from the lithium cobalt oxide and 0 is decomposed by an oxidation reaction with the oxygen. This is one of the electrochemical reactions, and The rate at which current flows into the positive electrode active material 100 etc. is called the oxidation reaction of the electrolyte by the positive electrode. The rate at which the current flows affects the electrochemical reaction, depending on the insulating properties of the electrode active material. It could also be a boss.

[0092] As mentioned above, when an internal short circuit occurs in a secondary battery, the temperature changes as shown in the graph in Figure 3. Figure 3 is based on the graph shown on page 70 [Figures 2-12] of Non-Patent Document 13. This is a partially revised figure using the graph of the temperature (specifically, the internal temperature) of the secondary battery over time. When an internal short circuit occurs at (P0), the temperature of the secondary battery rises over time. As shown in (P1), the temperature of the secondary battery increases due to Joule heat generated by an internal short circuit. When the temperature rises to around 100°C, the reference temperature, which is the limit temperature at which the secondary battery does not go into thermal runaway, is reached. In this case, the negative electrode (graphite) may exceed the temperature (Ts). At (P3), the electrolyte is reduced and heat is generated by the negative electrode (C6Li). Oxidation of the electrolyte occurs and heat is generated, and in (P4), heat is generated due to thermal decomposition of the electrolyte. The battery may go into thermal runaway, catching fire or emitting smoke.

[0093] At this time, the electrons that suddenly flow into the positive electrode active material convert the cobalt to Co 4+ From Co 2+ This reaction causes oxygen to be released from the positive electrode active material. Since this is an exothermic reaction, it accelerates thermal runaway. This makes it possible to provide a safe secondary battery that is less prone to runaway.

[0094] In order to suppress the above reaction, for example, the surface layer of the positive electrode active material is made of an additive that does not easily release oxygen. It is preferable that the positive electrode active material has a higher concentration of the added element than the inside. If oxygen is not released from the 4+ From Co 2+ (Reaction resulting in As additive elements that do not easily release oxygen, for example, magnesium, aluminum, Magnesium is released more easily from oxygen atoms closer to the magnesium. Since the energy of Ni is large, it is suitable as an additive element that does not easily release oxygen. Nickel is also thought to have the effect of suppressing oxygen release when present at the lithium site. .

[0095] In addition, even if cobalt or other substances are reduced, lithium ions are inserted into the positive electrode active material before oxygen is released. If oxygen can be absorbed into the positive electrode active material, electrical neutrality is maintained and oxygen release does not occur. Even if electrons suddenly flow in, lithium ions are inserted into the positive electrode active material through the electrolyte from the negative electrode. It is sufficient if the crystal structure of the positive electrode active material is kept stable during this period.

[0096] In addition, in order to prevent smoking, heat generation, etc. from occurring during the nail penetration test, the temperature rise of the secondary battery must be suppressed. It is considered desirable that the negative electrode, positive electrode and / or electrolyte have stable properties at high temperatures. Specifically, the positive electrode active material 100 does not release oxygen, especially when exposed to high temperatures. It is preferable that the positive electrode active material has a stable structure in which the rate of current flow to the positive electrode active material is slow. It is preferable that the positive electrode active material 100 has such a structure. In this case, thermal runaway is unlikely to occur, and fire, etc. As will be described later, a significant effect is expected in that the positive electrode active material according to one embodiment of the present invention is unlikely to reach the above-mentioned state. The substance 100 can have both the above stable structure and a structure that slows down the speed of current. .

[0097] <Thermal runaway of secondary batteries> The principle of thermal runaway in secondary batteries is shown on page 69 [Figure 2-11] of Non-Patent Document 13. The graph is quoted and partially revised as shown in Figure 4. For example, when charging the secondary battery described above, As the temperature (specifically the internal temperature) rises, it goes through several stages and eventually reaches thermal runaway. is a graph of the temperature of the secondary battery against time, and for example, when the temperature of the secondary battery is 100°C or When the temperature is close to 1000°C, (1) the SEI (Solid Electrolyte Interlayer) of the negative electrode When the temperature of the secondary battery exceeds 100°C, the (2 ) The negative electrode (if graphite is used, the negative electrode will be C6Li) reduces the electrolyte and generates heat. (3) At or near 150°C, oxidation of the electrolyte occurs at the positive electrode and heat is generated. When the battery temperature reaches or approaches 180°C, (4) thermal decomposition of the electrolyte occurs, and (5) the positive electrode Oxygen is released from the cathode and the cathode is thermally decomposed (this thermal decomposition involves structural changes in the cathode active material). Then, when the temperature of the secondary battery exceeds 200°C, (6) decomposition of the negative electrode occurs, and finally , (7) The positive and negative electrodes come into direct contact. In this state, especially in the state of (5) and the state of (6), After passing through state (1) or state (7), the secondary battery reaches thermal runaway.

[0098] To prevent thermal runaway, it is necessary to suppress the temperature rise of the secondary battery and It is also considered desirable that the electrolyte has stable properties at high temperatures.

[0099] <Characteristics of secondary batteries in nail penetration test 1> After the nail is pierced, the voltage of the secondary battery may become 0V. This indicates a change in voltage, where the voltage drops and then rises, i.e., the voltage drops and then returns. In the secondary battery according to one embodiment of the present invention, when the voltage drops, it does not reach 0 V and maintains a low voltage value. The voltage change of the secondary battery is shown in Figs. 5(A) to 5(C). This will be explained using:

[0100] FIG. 5(A) is an example of a graph showing the relationship between the position of the nail 1003 and the time of the nail insertion operation. The position of the nail 1003 refers to the tip of the nail 1003, and is the depth from the surface of the secondary battery 1004. In FIG. 5A, at time T0, a nail 1003 pierces a secondary battery 1004. It can be determined that the nail 1003 has been struck, and the value of the position of the nail 1003 increases toward La. At this time, the position of the nail 1003 is fixed at La, and the nail 1003 penetrates the secondary battery 1004. It can be considered as having passed.

[0101] In FIG. 5B, the time of the nail penetration operation, which is the same as in FIG. 5A, is plotted on the x-axis, and the voltage of the secondary battery 1004 is plotted on the x-axis. 5B is a graph showing the voltage of the secondary battery 1004 in a fully charged state. Vb, and at time T0, the nail 1003 is stuck into the secondary battery 1004, and at time T1 At time T1, the voltage of the secondary battery 1004 drops. It is believed that the negative electrode is in contact with the electrode, and the voltage often drops suddenly. In a secondary battery, the voltage drops sharply and then starts to rise. Such a change in the voltage of a secondary battery is This is a feature of the secondary battery according to one embodiment of the present invention, and is specifically an embodiment of the present invention described later. It is believed to be caused by the positive electrode active material.

[0102] In FIG. 5C, the time of the nail penetration operation, which is the same as in FIG. 5A, is plotted on the x-axis, and the voltage of the secondary battery 1004 is plotted on the x-axis. 5C is a graph showing an example of the voltage applied to the secondary battery 1004 after a charge-discharge cycle test. The voltage of the secondary battery 1004 in the charged state is Vd, and At time T0, a nail 1003 is stuck in the secondary battery 1004, and at time T1, the secondary battery 1004 At time T1, the nail 1003 is considered to be in contact with the positive and negative poles, and the voltage drops. In the secondary battery according to one embodiment of the present invention, after the voltage drops sharply, The voltage Vc does not reach 0V and remains low at Vc. Such a change in the voltage of the secondary battery is a feature of the secondary battery according to one embodiment of the present invention. Specifically, this is believed to be caused by the positive electrode active material according to one embodiment of the present invention, which will be described later. do.

[0103] <Characteristics of secondary batteries in nail penetration test 2> Temperature rise of the secondary battery when a nail penetration test is performed, i.e., temperature before and after the nail penetration test The difference in the maximum temperature reached by the ° C. or less, more preferably 70° C. or less, and even more preferably 50° C. or less. The temperature should be within 5 cm, preferably within 2 cm, of the nail hole. The value output by a temperature sensor placed within the temperature range, preferably within 2 cm. The capacitor should be provided so as to be in contact with the exterior body of the secondary battery.

[0104] The maximum temperature during the nail penetration test is preferably 250°C or less, more preferably 200°C or less. More preferably, the temperature is 180° C. or lower. It is preferable that the temperature is lower than the temperature at which the reaction occurs.

[0105] Furthermore, the maximum temperature during the nail penetration test is preferably 150°C or less, and more preferably 100°C or less. More preferably, the temperature is a temperature at which oxidation of the electrolyte occurs by the positive electrode. It is more preferable that the maximum temperature is lower than the flammability of the mixed solvent used in the electrolyte. If the flash point of the mixed solvent is unknown, the flash point of each solvent can be used as a reference. can.

[0106] <Characteristics of secondary batteries in nail penetration test 3> The amount of the positive electrode active material carried in the positive electrode of the secondary battery is 8 mg / cm 2 More than 25mg / c m 2 Less than 8 mg / cm, preferably 2 More than 23mg / cm 2 Less than 7mg, preferably / cm 2 More than 21mg / cm 2 With such a loading amount, safety is ensured. High-performance secondary batteries can be provided.

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

[0108] <Characteristics of secondary batteries in nail penetration tests 4> It is preferable that the particles of the positive electrode active material in the secondary battery have very few cracks. A rack may be referred to as an area where the crystal planes of a grain are displaced, or an area where the grain is broken by a crystal plane. For example, when the positive electrode active material is observed by surface SEM or When observed by cross-section SEM, there were 0 to 5 observable cracks per particle of positive electrode active material. The following is good:

[0109] The cracks are caused by pressure applied after the positive electrode slurry is applied to the positive electrode current collector. Therefore, in the manufacturing process of the positive electrode of the present invention, the pressure of the press is set to, for example, a linear pressure of 500 kJ. N / m or less, preferably a linear pressure of 300 kN / m or less, more preferably a linear pressure of 250 kN / m or less It's better to put it below.

[0110] <Electrode density> The electrode density of the positive electrode of the secondary battery is 3.0 g / cm 3 More than 4.0g / cm 3 Below, I prefer Or 3.0g / cm 3 More than 3.5g / cm 3 It is preferable that the linear pressure is less than or equal to the above. The electrode density can be in the range. Secondary batteries with this structure are thought to be less likely to experience thermal runaway.

[0111] <The surface of the positive electrode active material must be smooth> It is preferable that the surface of the positive electrode active material of the secondary battery is smooth overall. It is preferable that the surface of the cathode active material is glossy overall. It can be said that the shape is rounded or not.

[0112] Furthermore, it is preferable that the positive electrode active material has no or very few microparticles attached to its surface. In this specification, the term "ultrafine particles" refers to metal oxide particles having a particle size of 0.001 μm or more and 0.1 μm or less. The term "microparticles" refers to fragments of the positive electrode active material and / or unreacted particles. It may be a source of added elements.

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

[0114] <Heating using a flux> In the manufacturing process of the positive electrode active material of a secondary battery, a material that functions as a flux together with an additive element source is used. The flux is preferably added to the surface of the composite oxide and the source of the additive element to sufficiently bond the surface of the composite oxide and the source of the additive element to each other. After melting, solidification begins. Therefore, if extremely small particles are attached to the surface of the composite oxide, Even if there is any residue, it will melt during these processes, so there will be very little or no residue remaining on the surface. The absence or very few microscopic particles on the surface of the positive electrode active material is due to the manufacturing process of the positive electrode active material. This can also be said to indicate that a material that functions as a flux was added and heated.

[0115] <Initial heating> The positive electrode active material that has undergone initial heating is smooth and shiny overall. This refers to the heating of the composite oxide during the manufacturing process of the active material. This also has the effect of alleviating distortion and crystal defects that the positive electrode active material has.

[0116] <Crystalline> The positive electrode active material of the secondary battery preferably has high crystallinity, and is preferably single crystal or polycrystalline. This is more preferable. The crystallinity of the positive electrode active material increases after the initial heating. If the active material is a single crystal, even if the volume of the positive electrode active material changes due to charging and discharging, cracks will not form. Furthermore, when the positive electrode active material is a single crystal, the secondary battery using the positive electrode active material Ponds are thought to be less likely to catch fire, which could improve safety.

[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 described. If the positive electrode active material is too small, it may be difficult to apply it when manufacturing the positive electrode. If the surface area is too small, the reaction between the positive electrode active material surface and the electrolyte will be too large. In addition, if the positive electrode active material is too small, it is necessary to mix a large amount of conductive material. In this case, there is a risk of a decrease in capacity. The median diameter (D50) of the positive electrode active material is 1 μm or more, preferably 5 μm or more, more preferably Preferably, the median diameter (D50) is 9 μm or more. In addition, a positive electrode active material with a small median diameter (D50) is preferable because it is less likely to produce a detached region. This is preferable because cracks are less likely to occur even after the pressing process.

[0118] On the other hand, if the active material is too small, the density of the positive electrode active material layer will decrease, and side reactions with the electrolyte will occur. In this regard, the median diameter (D50) of the positive electrode active material is The thickness is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less.

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

[0120] The above-mentioned median diameter (D50) can be determined by, for example, observation using an SEM or TEM, or Alternatively, it can be measured by a particle size distribution analyzer that uses the laser diffraction and scattering method. When measuring with a particle size distribution analyzer using the diffraction / scattering method, the median diameter (D50) is The particle size when the cumulative amount in the cumulative curve of particle size distribution measurement results accounts for 50% The median diameter (D50) can be measured by SEM or TEM analysis. For example, measure 20 or more particles, create a cumulative curve, and calculate the cumulative amount that accounts for 50%. The particle diameter at the time of

[0121] <Laminated secondary battery> A secondary battery according to one embodiment of the present invention will be described. First, a typical laminated secondary battery This will be explained with reference to FIGS. 6(A) and 6(B) and so on.

[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 and a plurality of separators 508. The separators 508 are The separator 508 is shown by a dotted line in FIG. 6A for ease of viewing. Separator 508 may contain an electrolyte, specifically a liquid electrolyte (also called an electrolyte solution). When a solid electrolyte or a semi-solid electrolyte is used as the electrolyte, the secondary battery 100 4 may not have 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 is electrically connected to the wiring 1005a and the wiring 1005b in the nail penetration test device. The positive electrode 503 is a positive electrode current collector and a positive electrode active material formed on the positive electrode current collector. The positive electrode active material layer is preferably formed on both sides of the positive electrode current collector. The negative electrode active material layer is formed on the negative electrode current collector. It is best to form it on both sides of the body.

[0124] As shown in FIG. 6(B), a plurality of positive electrodes 503, a plurality of negative electrodes 506, and a plurality of separators 5 The negative electrodes 506 are stacked, and in this specification, this may be referred to as a stack. The lead portion 512b is joined at a joint 515b, and is electrically connected to each other. The tab portions of the positive electrodes 503 are connected to the leads 512a at the joints 515. The positive electrode current collector and the negative electrode current collector (these are joined at a) and electrically connected to each other. Compared with the positive electrode active material layer and the negative electrode active material layer (which are simply called the active material layer), Since the lead 51 has high insulating properties, it is better not to form an active material layer on the tab portion. 2a, and lead 512b are made of aluminum, nickel, copper, titanium, or alloys thereof. Selected materials can be used. Ultrasonic welding can be used for joining at the joints. In the nail penetration test, it is not necessary to provide leads 512a and 512b. When provided, the lead 512a and the lead 512b are connected to the wiring 1005a and The wiring 1005b is electrically connected.

[0125] Furthermore, the secondary battery 1004 has an exterior body (not shown), and the laminated body shown in FIG. 6(A) is attached to the exterior. The battery is then housed in a housing. Then, an electrolyte solution containing dissolved lithium salts is poured into the housing. The electrolyte contains carrier ions, typically lithium ions. A secondary battery having this structure is called a lithium ion secondary battery.

[0126] The exterior body is preferably in a film form from the viewpoint of weight reduction, and a secondary battery having a film-shaped exterior body is The battery is called a laminated secondary battery. A laminated structure of a polymer with excellent conductivity and a metal may be used. It is preferable to use polyethylene terephthalate (PE) and aluminum as the metal, and further use nylon on the outside of the outer casing. The outer casing may be a metal case, and a circular case may be used. When a battery using this material is used, it is called a coin-type secondary battery.

[0127] [Cathode active material] Next, a positive electrode active material 100 according to one embodiment of the present invention will be described with reference to FIG. The mass of the carrier ion is typically a lithium ion (Li + ) can be inserted and removed, A compound containing a transition metal and oxygen may be used. The transition metal may be cobalt (Co), nickel (Ni), or the like. Nickel (Ni), manganese (Mn), iron (Fe), etc. are used. It is possible.

[0128] <Main ingredient> The positive electrode active material 100 of one embodiment of the present invention is a transition metal M that is responsible for the oxidation-reduction reaction. It is preferable to use a transition metal M as the main component in this specification. The transition metal is Co, and the atomic ratio is the highest in the group M. As a mixture, lithium cobalt oxide can be applied to the positive electrode active material 100. The electrode active material 100 is lithium cobalt oxide (LiCoO2) with added elements. However, the positive electrode active material 100 according to one embodiment of the present invention has a crystal structure as described below. Therefore, the composition of lithium cobalt oxide must be exactly Li:Co:O=1:1:2. It is not limited to:

[0129] In addition, it is preferable to use nickel in addition to cobalt as the positive electrode active material 100. Cobalt nickel lithium oxide can be applied to the positive electrode active material 100. 0 is lithium cobalt nickel oxide (LiCo 1-y Ni yO2) with added elements However, the positive electrode active material 100 of one embodiment of the present invention preferably has a crystal structure described later. Therefore, the composition of lithium cobalt nickel oxide is strictly Li:(Co +Ni):O=1:1:2.

[0130] In addition, in lithium cobalt nickel oxide, the proportion of nickel in the sum of cobalt and nickel is The ratio Ni / (Co+Ni), i.e. LiCo 1-y Ni y y in O2 exceeds 0 It is preferably less than 0.5, and more preferably 0.1 or more and 0.3 or less. It is more preferable that the ratio is more than 0.025 and not more than 0.215.

[0131] Also LiCo 1-y Ni y For example, y in O2 is 0.1 or more and 0.3 or less. Ni=90:10 (atomic ratio), Co:Ni=80:20 (atomic ratio), or Co:N This includes the case where i=70:30 (atomic ratio).

[0132] The positive electrode active material 100 preferably has high crystallinity. It is preferable that the positive electrode active material 100 is a single particle (also called a primary particle) rather than a single particle. More preferably, the particles are single crystals.

[0133] The positive electrode active material 100 according to one embodiment of the present invention has an insulating region or a region with high resistance. It is advisable to do so. In order to distinguish this area from other areas, it is sometimes called the first area. The region has a cross section of the positive electrode active material 100 of 1 nm or more and 20 nm or less, preferably 2 nm or more. It is preferable that the width is narrow, such as 10 nm or less, and more preferably 2 nm to 5 nm. The above numerical value can be said to be the thickness or width of the first region in cross section. In some cases, it is called a "shell." For cross-sectional views, for example, cross-sectional STEM (Scanning Electron Microscopy) ng Transmission Electron Microscope, Scanning Transmission Electron microscope images can be used. In Figure 7(A), a positive electrode active material having a shell 100s is shown. Shows quality 100.

[0134] The shell 100s is contained in a surface layer 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 Even if this is done, the speed of the current flowing into the positive electrode active material 100 can be slowed down. This is preferable because it can prevent the occurrence of fire, smoke, etc. In order to make the surface layer of the positive electrode active material 100 softer, the shell is located on the outer or surface side of the surface layer of the positive electrode active material 100. It is more preferable to do so.

[0135] <Additional elements> The positive electrode active material 100 may contain an additive element. The additive element may be magnesium (Mg), fluorine (F), or the like. Fluorine (F), nickel (Ni), and aluminum (Al) are examples. Ti, Zirconium (Zr), Vanadium (V), Iron (Fe), Manganese (Mn) , chromium (Cr), niobium (Nb), arsenic (As), zinc (Zn), silicon (Si), sulfur Examples include yellow (S), phosphorus (P), boron (B), bromine (Br), and beryllium (Be). can be done.

[0136] The added elements do not necessarily include magnesium, fluorine, nickel, aluminum, titanium, etc. , zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur It may be free of yellow, phosphorus, boron, bromine, or beryllium.

[0137] For example, if the positive electrode active material 100 does not substantially contain manganese, it is relatively easy to synthesize and obtain. The advantages of ease of handling and excellent cycle characteristics are even greater. The weight of manganese contained in 00 is preferably, for example, 600 ppm or less, It is more preferable that the concentration is 0 ppm or less.

[0138] Magnesium is one of the elements suitable for Shell 100s. In the cross-sectional view of 00, magnesium is 1 nm or more and 20 nm or less from the surface, preferably 2 It exists in a narrow width of 2 nm to 10 nm, and more preferably 2 nm to 5 nm. This is preferable.

[0139] When magnesium is added to the positive electrode active material 100, magnesium fluoride is used as the magnesium source. It is preferable to use magnesium, and when magnesium fluoride is used, fluorine is also used as the positive electrode active material. Furthermore, in a secondary battery having the positive electrode active material 100, During testing, lithium may react with fluorine, but this is because lithium reacts with oxygen. Therefore, the amount of heat generated in the positive electrode active material 100 is smaller than that generated when the positive electrode active material 100 is reacted with the additive element. Furthermore, the reaction between fluorine and lithium is such that, in a nail penetration test, It is thought that fluorine is generated when the voltage of a secondary battery starts to rise due to the It is useful in terms of sex.

[0140] In addition, when LCO is used in the positive electrode active material 100 according to one embodiment of the present invention, ma In addition to magnesium, nickel is preferably included. For example, a surface that can insert and remove lithium, That is, on the edge surface, the region containing magnesium and the region containing nickel overlap each other. In other words, it is preferable that nickel is also in the shell. By forming the cathode active material as a cathode active material, it is possible to suppress the desorption of oxygen from the cathode active material or to prevent the structural change of the cathode active material. It can be suppressed.

[0141] The additive elements may be present in the shell 100s or in the surface layer 100 described later. a (see FIG. 7(G)). The additive element that contributes to the deterioration should be present in the surface layer portion 100a where the deterioration is likely to start.

[0142] Here, in order to check whether or not the shell 100s is formed on the positive electrode active material 100, It is advisable to measure the resistance of the powder that will be the electrode active material (called powder resistance). The powder resistance of the positive electrode active material containing the additive element is higher than the powder resistance of the positive electrode active material containing no additive element. If the value is higher than 100%, it is considered that the shell 100s may be formed on the positive electrode active material 100. can be done.

[0143] Shell 100s should contain cobalt in addition to the additive elements. By containing cobalt, lithium ions (Li + ) while allowing for insertion and removal of the internal It is possible to slow down the speed at which current flows into the surface layer 10 due to a short circuit. In the case of 0a (see FIG. 7(G)), it is also preferable to have cobalt in addition to the additional element.

[0144] The above-mentioned shell 100s may be provided so as to sufficiently cover the entire positive electrode active material 100. 7(A), a specific region of the positive electrode active material 100, for example, a region other than the (001) plane, The shell 100s may be provided so as to be thicker at the surface. For example, if the shell is located on a plane other than the (00l) plane, oxygen release from the shell is suppressed. This improves thermal stability and makes the structure less susceptible to thermal runaway. 100a (see FIG. 7(G)), the surface layer 100a where the additive element is present is a specific For example, the surface layer 100a may be thicker in a region other than the (001) plane. It is best if it is thick on the surface where it is most likely to start.

[0145] However, as long as the shell 100s does not ignite in the nail penetration test, the positive electrode active material 100 The lithium ion (Li + ) while allowing insertion and removal of If the speed at which current flows due to an internal short circuit can be slowed down, magnesium will It may be present in areas other than the core, for example, it may be present in the entire surface layer.

[0146] The concentration of the added element is examined. For example, the concentration of magnesium, which is an added element, is In the lithium shell 100s, more than 0 and 10 atomic % or less, preferably 0 More preferably, it is more than 0 and less than 2 atomic %. The magnesium concentration can be measured by energy dispersive X-ray spectroscopy (EDX). Dispersive X-ray Spectroscopy (DXS) When magnesium is present throughout the entire surface layer and at a high concentration, the insulating properties Therefore, it is difficult to obtain desirable battery characteristics in charge / discharge cycle tests. On the other hand, magnesium is needed in the surface layer, especially in the appropriate areas like the shell, and in the appropriate places. The presence of lithium cobalt oxide at a suitable concentration can stabilize the lithium cobalt oxide, and the above-mentioned nail penetration This is preferable because it can suppress heat generation and smoke generation during tests etc. The presence of iron in the shell at the appropriate concentration increases the hardness of lithium cobalt oxide. It is also expected that

[0147] 7(B) to 7(F) are enlarged conceptual diagrams of the boxed area B in FIG. 7(A). Here, LCO containing Mg is used as the positive electrode active material 100. As shown in FIG. As shown above, Mg, one of the additive elements, is preferably bonded to oxygen in the shell. It is preferable that Co is bonded to oxygen. If so, lithium ion (Li + ) while allowing for insertion and removal of the current due to an internal short circuit. It is thought that the loading speed can be slowed down.

[0148] Next, LCO containing Mg and F will be exemplified as the positive electrode active material 100. As shown in FIG. In addition, F, which is one of the additive elements, does not need to be present in the shell, and is present on the surface of the positive electrode active material 100. Fluorine has a high electronegativity and forms stable compounds with many elements. It is known that the positive electrode active material 100 is easily dissolved in the electrolyte solution inside the battery. The fluorine is adsorbed on the surface of the positive electrode active material 100, and thus the fluorine is in the vicinity Even if an internal short circuit occurs, the heat of the electrolyte can react with the surrounding electrolyte. Solutions etc. can be suppressed.

[0149] Furthermore, as shown in FIG. 7(D), the positive electrode active material 100 is LCO containing Mg and F. The element compound 100f may be adsorbed on the surface of the positive electrode active material 100. It is known that it has a high affinity for many elements and easily forms stable compounds with them. 100 is impregnated in an electrolyte, and a fluorine compound 100f is formed on the surface of the positive electrode active material 100. By adsorbing to the surface, it can react with the electrolyte in the vicinity of the fluorine compound 100f. Even if an internal short circuit occurs, thermal decomposition of the electrolyte can be suppressed.

[0150] The adsorption mentioned above includes chemical adsorption and physical adsorption. Chemical adsorption is the adsorption of at least one of the added elements. A chemical bond is formed by a chemical reaction between the surface of the positive electrode active material 100 and the surface of the positive electrode active material 100. Physical adsorption is an intermolecular force acting between at least one of the additive elements and the surface of the positive electrode active material 100. The reason is that they are adsorbed by van der Waals forces.

[0151] Although not shown, the positive electrode active material 100 may contain fluorine in solid solution, for example, cobalt. The oxygen in the lithium cobalt oxide may be partially substituted with fluorine. It is sufficient that the lithium nitrate is present in the surface layer of the lithium nitrate, or it may be present in the shell. If there is enough fluorine for 0, some of the fluorine and oxygen adsorbed on the surface will be replaced. Fluorine is present in both.

[0152] 7(E) and 7(F) are modifications of the conceptual diagrams shown in FIGS. 7(C) and 7(D), respectively. At least a part of the F adsorbed on the surface of the positive electrode active material 100 is present in the shell. This shows an example of fluorine bonding with lithium (Li), which has a higher electronegativity than oxygen. Therefore, lithium and fluorine bond more easily than lithium and oxygen. By combining with oxygen, the lithium can be prevented from combining with oxygen. Therefore, the adsorption of fluorine onto the surface of the positive electrode active material 100 can suppress the combustion of the positive electrode active material 100. It is possible to suppress the occurrence of fire, smoke, etc. in a secondary battery having the positive electrode active material 100. For example, Even if an internal short circuit occurs in a secondary battery having 100, the secondary battery will not catch fire or explode. For example, a nail penetration test was performed on a secondary battery having the positive electrode active material 100. Even if the secondary battery is damaged, it is possible to prevent the secondary battery from catching fire or emitting smoke.

[0153] Furthermore, by bonding with lithium, fluorine can suppress the movement of the lithium. Therefore, even if 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 fire, smoke, etc. can be suppressed. Furthermore, for example, a nail penetration test was performed on a secondary battery having the positive electrode active material 100. Even in this case, the speed of the current flowing into the positive electrode active material 100 can be slowed down, and the generation Fire and smoke can be suppressed.

[0154] Examples of fluorides used in lithium ion secondary batteries include lithium salts, which will be described later. Examples of suitable binders include LiPF6 and LiBF4, and polyvinylidene fluoride (P Fluorine from such fluorides is absorbed onto the surface of the positive electrode active material 100. You may wear it.

[0155] 7(G) and 7(H) show the boundary between the surface layer 100a and the interior 100b of the positive electrode. In this way, the surface layer 100a is distinguished from the shell, and the surface layer 100a is Again, the surface layer 100a includes a shell.

[0156] FIG. 7(H) is an example of a positive electrode active material in which the crystal grain boundary 101 is added by a dashed line to FIG. 7(G). Furthermore, in FIG. 7(H), cracks formed on a part of the surface of the positive electrode active material and the Also shown is a buried portion 102 that contacts a portion of the surface. The buried portion 102 is made of magnesium or the like. It is preferable that the alloy contains the following additional elements.

[0157] <Surface of the positive electrode active material> The surface of the positive electrode active material 100 is a composite oxide including the surface layer 100a and the inner layer 100b. This surface can be confirmed by cross-sectional view. The surface of the active material 100 is made of aluminum oxide (Al2O3) and other materials that contribute to charging and discharging. The positive electrode active material is made by chemical absorption after the production of a metal oxide that does not have a lithium site. It does not include adsorbed carbonates, hydroxyl groups, etc. The adsorbed metal oxides are, for example, This refers to a metal oxide whose crystal structure does not match that of part 100b.

[0158] The positive electrode active material 100 is a compound containing a transition metal and oxygen capable of inserting and extracting lithium. Therefore, transition metals M (e.g., Co, Ni, Mn, F) that are oxidized and reduced with the insertion and desorption of lithium are The interface between the region where oxygen is present and the region where oxygen is not present is defined as the surface of the positive electrode active material. When a positive electrode active material is subjected to analysis, a protective film may be attached to the surface. The protective film is a single layer or multilayer film of carbon, metal, oxide, resin, etc. It may be used.

[0159] Therefore, the surface position of the positive electrode active material in STEM-EDX ray analysis etc. is determined by the above transition metal The detected amount of characteristic X-rays of M is the average value M of the detected amount of characteristic X-rays of the above transition metal M inside AVE and , the average amount of detected characteristic X-rays of the transition metal M above in the background, M BG 50% of the sum of or the point where the detected amount of oxygen characteristic X-rays is equal to the average value O A VE and the average amount of background oxygen characteristic X-rays detected, O BG It is 50% of the sum of The detected amount of characteristic X-rays of the transition metal M is the characteristic X-rays of the internal transition metal M. The average amount of detected X-rays and the average amount of detected X-rays characteristic of the transition metal M in the background The point where the detected amount of oxygen characteristic X-rays is 50% of the sum of the two points, and the detected amount of oxygen characteristic X-rays is the average of the detected amount of oxygen characteristic X-rays inside the sample. The difference is that the value is 50% of the sum of the average detected amount of characteristic X-rays of oxygen in the background. This is thought to be due to the influence of metal oxides, carbonates, etc. containing oxygen that adhere to the surface. The detected amount of characteristic X-rays of the transition metal M is the average value of the detected amount of characteristic X-rays of the transition metal M inside M AVE and the average amount of the characteristic X-rays of the transition metal M in the background, M BG with The point where the sum of the two is 50% can be used as the surface position of the positive electrode active material. In the case of a positive electrode active material having multiple elements of the element M, the element M with the highest amount of characteristic X-rays detected inside is AVE and M BG The surface can be found using

[0160] The average amount of characteristic X-rays of the above transition metals M detected in the background is M BG is, for example, a transition metal Avoid the area where the amount of detected characteristic X-rays of group M begins to increase, and focus on the area 2 nm or more outside, preferably 3 nm. The amount of detected characteristic X-rays of the internal transition metal M can be calculated by averaging the range above m. The average value of M AVE is the region where the detected amount of characteristic X-rays of transition metals M and oxygen is saturated and stable, e.g. For example, the depth is preferably 30 nm or more from the region where the amount of detected characteristic X-rays of the transition metal M begins to increase. is calculated by averaging the area over 50 nm, preferably over 2 nm, and more preferably over 3 nm. The average amount of background oxygen characteristic X-rays detected is O BG and internal acid Average detected amount of characteristic X-rays of the element O AVE can also be found in the same way.

[0161] The surface of the positive electrode active material 100 in a cross-sectional STEM image or the like is a surface that is formed due to the crystalline structure of the positive electrode active material. The boundary between the area where the image is observed and the area where it is not observed is the area that constitutes the positive electrode active material. Among metallic elements, the atomic columns derived from the nuclei of metallic elements with atomic numbers larger than that of lithium are It is the outermost area to be checked.

[0162] In addition, the spatial resolution of STEM-EDX is about 1 nm. The peak position (also called the maximum value) of the characteristic X-rays may be shifted by about 1 nm. The peak position of the characteristic X-rays corresponding to the added elements such as magnesium outside the surface obtained above Even if there is a difference between the peak and the surface, it can be considered an error if the difference is less than 1 nm.

[0163] A peak in STEM-EDX ray analysis is the maximum or lowest value of the characteristic X-rays corresponding to each element. The noise in STEM-EDX line analysis is referred to as the spatial resolution. The measured value of the half-width is less than the maximum (R), for example, less than R / 2.

[0164] The effect of noise can be reduced by scanning the same area multiple times under the same conditions. The integrated value of the scan measurement can be used to create a graph of the characteristic X-rays of each element. It is not limited to 6, and it is also possible to go further and average the results to create a graph of the characteristic X-rays of each element. do.

[0165] STEM-EDX analysis can be carried out, for example, as follows. First, the surface of the positive electrode active material is For example, a protective film is deposited on the surface of the ion sputtering device (Hitachi High-Tech MC1000). Carbon can be vapor deposited in a carbon coating unit.

[0166] Next, the positive electrode active material is sliced ​​and a STEM cross-section sample is prepared. Thinning can be done using the Hitachi High-Tech XVision 200TBS. In this case, pickup is performed using an MPS (micro-probing system), and the conditions for finishing are can be set to, for example, an acceleration voltage of 10 kV.

[0167] STEM-EDX analysis is performed using, for example, a STEM device (Hitachi High-Tech HD-2700). The EDX detector is the Octane T Ultra W (Dual ED) from EDAX. During EDX analysis, the accelerating voltage of the STEM device is set to 200 kV. The emission current was set to be between 6 μA and 10 μA. The EDX measures areas with minimal depth and unevenness. The magnification is, for example, about 150,000 times. The conditions for line analysis were a beam diameter of 0.2 nm, drift correction, line width of 42 nm, and pitch of 0. .2nm, and the number of frames can be 6 or more.

[0168] The grain boundary 101 is, for example, a portion where particles of the positive electrode active material 100 adhere to each other. The part where the crystal orientation changes inside the active material 100, that is, the repetition of bright and dark lines in the STEM image, etc. The areas where the repeats are discontinuous, areas containing many crystal defects, areas where the crystal structure is disordered, etc. Crystal defects are defects that can be observed in cross-sectional TEM and cross-sectional STEM images, i.e., defects in the lattice The grain boundary 101 is a plane defect. The vicinity of the grain boundary 101 is the region within 10 nm from the grain boundary 101. This shall mean:

[0169] <Continuous change in crystal structure> Furthermore, due to the concentration gradient of the added element as described above, the crystals are formed from the inside 100b toward the surface. It is preferable that the crystal structure of the surface layer 100a and the inner layer 100b change continuously. It is preferable that the orientations are roughly the same.

[0170] For example, from the inside 100b of the layered rock salt type, characteristics of the rock salt type or both the rock salt type and the layered rock salt type can be obtained. It is preferable that the crystal structure changes continuously toward the surface and the surface layer portion 100a. or a surface layer 100a having characteristics of both rock salt type and layered rock salt type, and layered rock salt It is preferred that the crystal orientations in the interior 100b of the mold are roughly consistent.

[0171] In this specification and the like, a composite oxide containing lithium and a transition metal such as cobalt The layered rock salt type crystal structure of the cations and anions belongs to the space group R-3m. It has a rock-salt type ion arrangement in which transition metals and lithium are regularly arranged in a two-dimensional plane. This refers to a crystal structure that allows two-dimensional diffusion of lithium to form a plane. Strictly speaking, the layered rock salt crystal structure may have defects such as a lack of anions. For example, the lattice of the rock salt crystal may have a distorted structure.

[0172] The rock salt crystal structure is a cubic crystal structure, including the crystal structure belonging to the space group Fm-3m. It has a crystal structure in which cations and anions are arranged alternately. Alternatively, there may be a deficiency of an anion.

[0173] In addition, the presence of both layered and rock salt crystal structure characteristics was confirmed by electron diffraction and TEM images. This can be determined by cross-sectional STEM images, etc.

[0174] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, the cation sites in the crystal structure are There are two types, one dominated by lithium and the other by transition metals. The layered structure in which two-dimensional planes and two-dimensional planes of anions are alternately arranged is called the rock salt type and the layered rock salt type. The number of bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane is also the same. When the central spot (transparent spot) is set as the origin 000, the bright spot closest to the central spot is The points are, for example, the (111) plane in the ideal rock salt type, and the (003) plane in the layered rock salt type. For example, comparing the electron diffraction patterns of rock-salt MgO and layered rock-salt LiCoO2, In this case, the distance between the bright spots on the (003) surface of LiCoO2 is approximately equal to the distance between the bright spots on the (111) surface of MgO. Therefore, if the analysis area is a region of interest, for example, rock salt MgO In the case of the layered rock salt LiCoO2 phase, the electron diffraction pattern shows a strong bright spot. There is a plane orientation where bright spots and weakly bright spots are arranged alternately. This is common to both rock salt and layered rock salt types. The bright spots that occur only in the layered rock salt type have a strong brightness, while the bright spots that occur only in the layered rock salt type have a weak brightness.

[0175] In cross-sectional STEM images, when the 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. Since there is no distinction in the ion sites, such characteristics are not observed. In the case of a crystal structure with specific crystal orientations, when observed from a specific crystal orientation, cross-sectional STEM images etc. In the case of the luminance layer, layers observed with high brightness and layers observed with low brightness are observed alternately, and then layers observed with even lower brightness are observed. In the layer of the lithium, i.e., in part of the lithium layer, there is a metal with an atomic number higher than that of lithium.

[0176] Layered rock salt crystals and the anions of rock salt crystals form a cubic close-packed structure (face-centered cubic lattice structure). The O3' type and monoclinic O1(15) crystals described later also have a cubic close-packed structure. Therefore, when the layered rock salt crystals come into contact with each other, the anions There are crystal planes along which the cubic close-packed structure is oriented.

[0177] Alternatively, it can be explained as follows: The negative charge on the {111} plane of the cubic crystal structure The ions have a triangular lattice. The layered rock salt type has a space group of R-3m and a rhombohedral structure. To make the structure easier to understand, it is generally represented as a complex hexagonal lattice, and the layered rock salt type (0001 ) planes have a hexagonal lattice. The triangular lattice of the cubic {111} planes is similar to that of the layered rock salt type (0001) The atomic arrangement is similar to that of a hexagonal lattice on the surface. The compatibility of the two lattices is called cubic close-packed. This means that the orientation of the filling structure is aligned.

[0178] However, the space group of the layered rock salt crystal and the O3' type crystal is R-3m, and the space group of the rock salt type crystal is Since it is different from the group Fm-3m (the space group of general rock salt crystals), crystals that satisfy the above conditions The Miller indices of the planes are different between the layered rock salt crystal and the O3' type crystal and the rock salt type crystal. In the layered rock salt crystal, O3' type crystal and rock salt type crystal, When the orientations of the cubic closest-packed structures are aligned, it is sometimes said that the crystal orientations are roughly aligned. In addition, they have a three-dimensional structural similarity such that the crystal orientations are roughly the same, or The same crystallographic orientation is called topotaxy.

[0179] The crystalline orientation of the two regions roughly coincides with each other, as can be seen from TEM images, STEM images, and HAADF-S TEM (High-angle Annular Dark Field STEM) Angular scattering annular dark-field scanning transmission electron microscope (ABF-STEM) image, ABF-STEM (Annular Scattering Bright-Field STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns This can be determined from the FFT patterns of TEM and STEM images, etc. Electron diffraction, neutron diffraction, etc. can also be used as materials for judgment.

[0180] Figure 8 shows an example of a TEM image in which the orientation of layered rock salt crystals LRS and RS roughly coincides. In TEM, STEM, HAADF-STEM, and ABF-STEM images, An image that reflects the crystal structure is obtained.

[0181] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. Due to diffraction and interference, for example, when an electron beam is incident perpendicular to the c-axis of a layered rock salt type composite hexagonal lattice, In this case, the contrast originating from the (0003) plane appears as a bright strip and a dark strip. This is observed as a repetition of bands (dark strips). Therefore, in the TEM image, there are bright and dark lines. The repetition of this phenomenon is observed, and the bright lines (e.g., the L RS and L LRS ) is an angle of 5 If the difference is less than 2.5 degrees, the crystal planes are roughly aligned, i.e., the crystal orientation is Similarly, if the angle between the dark lines is less than 5 degrees, If the difference is 2.5 degrees or less, it can be determined that the crystal orientations are roughly the same. .

[0182] In addition, the contrast in the HAADF-STEM image is proportional to the atomic number. The larger the element, the brighter it is observed. For example, layered rocksalt cobalt, which belongs to the space group R-3m In the case of lithium nitrate, cobalt (atomic number 27) has the highest atomic number, so cobalt The electron beam is strongly scattered at the atomic positions, and the arrangement of cobalt atoms appears as bright lines or points of high brightness. Therefore, lithium cobalt oxide with a layered rock salt crystal structure is When observed perpendicular to the axis, the arrangement of cobalt atoms perpendicular to the c axis appears as bright lines or points of strong brightness. The arrangement of lithium and oxygen atoms perpendicular to the c-axis is observed as dark or bright lines. The fluorine (atomic number 9) added to lithium cobalt oxide is observed as a low-energy region. ) and magnesium (atomic number 12).

[0183] Therefore, in the HAADF-STEM image, bright and dark lines appear in two regions with different crystal structures. If repetition is observed and the angle between the bright lines is less than 5 degrees or less than 2.5 degrees, the atom It can be determined that the arrangements are roughly the same, that is, the crystal orientations are roughly the same. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, the crystal orientation is It can be determined that they roughly match.

[0184] In ABF-STEM, elements with smaller atomic numbers are observed brighter. Similar to HAADF-STEM, the contrast obtained is similar to that of HAADF-STEM. -The orientation of the crystals can be determined in the same way as with STEM images.

[0185] Figure 9(A) shows a STEM image of the layered rock salt crystal LRS and the rock salt crystal RS, showing that their orientations are roughly the same. The FFT pattern of the rock salt type crystal RS region is shown in Figure 9(B), and the FFT pattern of the layered rock salt type crystal RS region is shown in Figure 9(C). The FFT pattern of the LRS region is shown in Figure 9(C). Composition, JCPDS card number, and d value calculated from JCPDS card data The angle and the measured values ​​are shown on the right. The spot marked with O is the zeroth diffraction order.

[0186] The spot marked A in Figure 9(B) is due to the 11-1 reflection of the cubic crystal. The spot marked A in Figure 9(B) is derived from the 0003 reflection of the layered rock salt type. ) and Fig. 9(C), the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type are In other words, the line passing through AO in FIG. 9(B) and the line passing through AO in FIG. It can be seen that the line passing through AO in (C) is roughly parallel to the line passing through AO in (C). "Approximately parallel" means that the angle between the straight lines is 5 degrees or less, or 2.5 degrees or less.

[0187] Thus, the FFT pattern and electron diffraction pattern show that the layered rock salt crystal and the rock salt crystal are different. If the orientations are roughly the same, the layered rock salt type has a <0003> orientation and the rock salt type has a <11-1> orientation. In this case, these reciprocal lattice points are spot-shaped. It is preferable that the reciprocal lattice points are not continuous with other reciprocal lattice points. The fact that the reciprocal lattice points are not continuous means that the crystallinity is high.

[0188] As mentioned above, the 11-1 reflection of the cubic crystal and the 0003 reflection of the layered rock salt crystal are , roughly coincide, depending on the incident direction of the electron beam, the layered rock salt type 0003 reflection A spot not originating from the layered rock salt type 0003 reflection is observed in the reciprocal lattice space different from the orientation. For example, the spot marked B in Figure 9(C) is a layered rock salt type 1014 reaction. This is due to the reciprocal lattice point (Fig. 9(C)) originating from the 0003 reflection of the layered rock salt type. ) from the direction of A) of the 56° or less), and d is observed in the area between 0.19 nm and 0.21 nm. Note that this index is just an example and does not necessarily have to match. For example, Reciprocal lattice points equivalent to 0003 and 1014 may also be used.

[0189] Similarly, the 11-1 reflection of the cubic crystal is observed in a different reciprocal lattice space from the 11-1 reflection of the cubic crystal. Spots that are not due to single reflections may be observed. For example, the spot marked with B in Figure 9(B) The spot is due to the 200 reflection of the cubic crystal. This is due to the 11-1 reflection of the cubic crystal. The angle between 54° and 56° (i.e., The angle of the AOB is between 54° and 56°, and no diffraction spots are observed at these locations. This index is an example and does not necessarily have to match the actual index. For example, the reciprocal lattice points equivalent to 11-1 and 200 may be used.

[0190] In addition, layered rock salt type positive electrode active materials such as lithium cobalt oxide have (0003) and The (10-14) plane and its equivalents are likely to appear as crystal faces. Therefore, when observing the (0003) plane with a TEM, First, select particles of the positive electrode active material in which a crystal plane expected to be the (0003) plane is observed using an SEM or the like. For example, in a TEM, the electron beam is incident on [12-10] so that the (0003) plane can be observed. The positive electrode active material particles are thinned using a FIB (Focused Ion Beam) or similar. If you want to judge the alignment of the crystal orientation, use the (0003) plane of the layered rock salt type. It is preferable to slice it into thin sections so that it 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 was compared with that of a conventional positive electrode active material. explain.

[0192] <Li x When x in MO2 is 1≫ 12 shows the crystal structure of a positive electrode active material 100 according to one embodiment of the present invention. The active material 100 is in a discharged state, i.e., Li x When x=1 in MO2 (M is a transition metal, Specifically, cobalt and / or nickel) and layered rock salt type crystals belonging to the space group R-3m. Layered rock salt type composite oxides have a high discharge capacity and a two-dimensional structure. It has a diffusion path for lithium ions, making it suitable for the insertion and desorption of lithium ions, and is suitable for use in secondary batteries. Therefore, the positive electrode active material 100 is particularly excellent as a positive electrode active material. It is preferable that 100b has a layered rock salt type crystal structure. The crystal structure is indicated by R-3m O3. R-3m O3 has a lattice constant of a = 2.8161. 0, b=2.81610, c=14.05360, α=90.0000, β=90.00 00, γ=120.0000, and the lithium, cobalt, and oxygen in the unit cell The coordinates of are Li(0,0,0), Co(0,0,0.5), O(0,0,0.23951 ) (Non-Patent Document 10). In Figure 13, O3 is added under the space group. The structure is such that lithium occupies octahedral sites and MO Because there are three double layers, this crystal structure is sometimes called an O3 type crystal structure. The two-layer structure is an octahedral structure in which oxygen atoms are six-coordinated to the transition metal M, and the two layers are connected to each other in a plane with edge-sharing. This is sometimes called a layer consisting of transition metal M and oxygen octahedra. In addition, in Figure 13, all lithium sites are shown as having lithium ions, but As mentioned above, ions of the added element, such as magnesium ions, are located at the lithium site. This sometimes happens.

[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 Even if lithium is removed from 00, the layer structure consisting of the transition metal M and oxygen octahedrons in the inner 100b remains. It is preferable that the surface layer 100a has a function of reinforcing the structure so that it does not break. It is preferable that the positive electrode active material 100 functions as a barrier film for the positive electrode active material 100. It is preferable that the surface layer 100a reinforces the positive electrode active material 100. Positive electrode active material such as oxygen elimination and / or deviation of the layer structure consisting of the transition metal M and oxygen octahedron Suppressing structural changes in the surface layer 100a and the inside 100b of the substrate 100, and / or This means that the decomposition of the electrolyte solution or the like on the surface of the positive electrode active material 100 is suppressed.

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

[0195] The surface layer 100a is the region from which lithium ions are first desorbed during charging, and the inner layer 100b The surface layer 100a is a region where the lithium concentration is likely to be lower than that of the positive electrode active material. The atoms on the surface of a particle with a quality of 100 can be said to be in a state where some of the bonds are broken. The portion 100a is likely to become unstable, and is a region where the crystal structure is likely to start to deteriorate. In the surface layer portion 100a, the crystal structure of the layered structure consisting of the transition metal M and oxygen octahedra is displaced. When this happens, the influence spreads to the inner part 100b, and the layered crystal structure also appears in the inner part 100b. This is thought to lead to a deterioration of the crystal structure of the entire positive electrode active material 100. If 100a can be made stable enough, Li x Even when x in CoO2 is small, e.g., Even if the ratio is 0.24 or less, the layer structure consisting of the inner 100b transition metal M and oxygen octahedron is hard to break. Furthermore, the layer consisting of the transition metal M and oxygen octahedron in the inner 100b can be The deviation can be suppressed.

[0196] In addition, it is preferable that the interior 100b of the positive electrode active material 100 has a low density of defects such as dislocations. In addition, the positive electrode active material 100 preferably has a large crystallite size as measured by XRD. In other words, it is preferable that the inner portion 100b has high crystallinity. These characteristics are important for the positive electrode active material when used in a secondary battery. High reliability of the positive electrode active material is an important factor supporting the reliability of the secondary battery. The upper limit of the charge voltage can be increased, and a secondary battery with a high charge / discharge capacity can be obtained.

[0197] The dislocations in the interior 100b can be observed, for example, by TEM. If the number is sufficiently small, defects such as dislocations may not be observed within a specific 1 μm square of the observation sample. Dislocations are a type of crystal defect and are different from point defects.

[0198] The crystallite size measured by XRD is preferably 300 nm or more. The larger the noise, the more Li x O3' type in CoO2 when x is small The crystal structure is easily maintained, and shortening of the c-axis length is easily suppressed.

[0199] The fewer defects such as dislocations observed by TEM, the smaller the crystallite size measured by XRD. The noise is expected to increase.

[0200] The XRD diffraction pattern used to calculate the crystallite size is obtained from the positive electrode active material alone. It is preferable that the positive electrode contains a current collector, a binder, a conductive material, etc. in addition to the positive electrode active material. However, in the positive electrode state, the positive electrode activity may be affected by pressure applied during the manufacturing process. The particles of the positive electrode active material may be oriented so that the crystal planes of the particles are aligned in one direction. If the orientation is strong, the crystallite size may not be calculated accurately. The active material layer is taken out, and the binder and the like in the positive electrode active material layer are removed to some extent using a solvent or the like. It is more preferable to obtain an XRD diffraction pattern by filling the sample holder with the sample. In addition, grease was applied to a silicon non-reflective plate, and a powder sample of the positive electrode active material was placed on the silicon. Another method is to attach it to a non-reflective conductive plate.

[0201] The crystallite size is calculated using, for example, a Bruker D8 ADVANCE, and X-ray CuKα, 2θ is 15° to 90°, increment 0.005, detection The diffraction pattern obtained using the LYNXEYE XE-T detector and the structure of lithium cobalt oxide were ICSD coll.code.172909 can be used as the reference value. The structural analysis software used was DIFFRAC.TOPAS ver.6. For example, it can be set as follows: Emission Profile:CuKa5.lam Background:Chebychev polynomial, 5th order Instrument Primary radius: 280mm Secondary radius: 280 mm Linear PSD 2Th angular range:2.9 FDS angle: 0.3 Full Axial Convolution Filament length: 12mm Sample length: 15 mm Receiving Slit Length: 12mm Primary Sollers: 2.5 Secondary Sollers: 2.5 Corrections Specimen displacement:Refine LP Factor: 0

[0202] The value of LVol-IB, which is the crystallite size calculated by the above method, was used as the crystallite size. It is preferable to adopt the calculated Preferred Orientation. If the value is less than 0.8, the sample is too strongly oriented and the crystallite size of the sample cannot be determined. It may not be suitable for

[0203] 〔distribution〕 The distribution of the additive elements in the positive electrode active material 100 will be described using the discharged state as an example. In order to make the surface layer 100a have a stable composition and crystal structure, the surface layer 100a may contain an additive element. It is preferable that the surface layer portion 100a has a plurality of additive elements. It is preferable that the concentration of one or more selected from the additive elements is higher than that of 0b. One or more selected from the additive elements contained in the substance 100 may have a concentration gradient. It is more preferable that the distribution of the positive electrode active material 100 varies depending on the added element. For example, depending on the added element, the depth from the surface of the peak of the detected amount may differ. The peak of the detected amount here is the maximum value of the detected amount, and the surface layer 100a The peak of the detected amount at the surface layer 100a or at 50 nm or less from the surface The detected amount refers to, for example, the count in EDX analysis.

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

[0205] Of the additive elements, fluorine, like magnesium, has a detected amount in the surface layer 100a that is greater than that in the interior 100. In addition, it is preferable that the detected amount of b is larger than that of b. It is preferable that the detected amount of fluorine has a peak in this region.

[0206] Of the additive elements, nickel was detected in a greater amount in the surface layer 100a than in the interior 100b. Furthermore, it is preferable that nickel is detected in the region closer to the surface of the surface layer portion 100a. For example, it is preferable that the surface layer 100a has a peak of the amount of the elution of the shell. The amount of nickel detected in the shell was greater than that in the area inside the shell. It is preferable that the detected amount of nickel is peaked in the shell. The ratio of the number of nickel atoms (Ni) to the number of cobalt atoms (Co) in the well (Ni / Co) is less than 1. In other words, the number of nickel Ni atoms in the shell of the surface layer portion 100a is The number of atoms is less than that of cobalt Co. Also, the number of atoms of nickel Ni at the peak of nickel detection amount is The ratio of the number of Ni atoms to the number of cobalt atoms (Ni / Co) is less than 1. The ratio of the number of nickel atoms (Ni) to the number of cobalt atoms (Co) in the region inside the shell The ratio (Ni / Co) is the ratio of the number of nickel atoms (Ni) to the number of cobalt atoms (Co) in the shell. The ratio of nickel to cobalt (Ni / Co) is smaller than that of the surface layer. In this case, the surface layer 100a and the inner layer 100b are very small compared to each other. In both of the above, the number of nickel atoms is less than the number of cobalt atoms. In addition, the number of nickel atoms (Ni) contained in the positive electrode active material 100 is smaller than the number of cobalt atoms (Co). do not have.

[0207] Furthermore, when both magnesium and nickel are present, the distributions of magnesium and nickel overlap. It is preferable that the distributions of element A and element B overlap. The peaks of the detected amounts of elements A and B are at the same depth, and the entire peaks do not overlap. 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 peak of the detected amount of element B and the peak of the detected amount of element A may be closer to the surface. The difference in the depth of the peak of the amount of magnesium is preferably within 3 nm. The overlap of the magnesium and nickel distributions means that the peaks of the detected amounts of magnesium and nickel are at the same depth. The peaks may not overlap entirely, including the amount of magnesium detected. The peak 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 detected nickel and the peak of detected magnesium is It is preferable that the thickness is within 3 nm.

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

[0209] Among the added elements, silicon, phosphorus, boron and / or calcium are also detected in the surface layer portion 100a. It is preferable that the amount of the surface layer 100a is larger than the amount of the inner layer 100b. The peaks of detected amounts of silicon, phosphorus, boron and / or calcium are in the region closer to the surface. It is preferable that

[0210] Of the added elements, aluminum has a peak of detected amount closer to the center than magnesium. The distribution of magnesium and aluminum may overlap, or The peak of the detected amount of aluminum is It may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. It is preferable that the peak is in a region of 5 nm or more and 30 nm or less from the center toward the inside.

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

[0212] The reason why aluminum is distributed deeper inside than magnesium is because This is thought to be because aluminum diffuses more easily than the surface. The reason why the amount of aluminum detected is less than in the area where magnesium and other elements are dissolved in solid solution at high concentrations is that It is presumed that this is because aluminum is more stable in regions where the temperature is not high.

[0213] More specifically, in the region of the layered rock salt type or cubic rock salt type of space group R-3m In the region where magnesium is dissolved at a high concentration, the layered rock salt type LiAlO2 In comparison, the distance between the cation and oxygen is long, making it difficult for aluminum to exist stably. , and Li around Cobalt + Mg 2+ The valence change due to substitution with Co 3+ From Co 2+ to However, Al can only take on a trivalent state. Therefore, it is thought that it is difficult for magnesium to coexist with it in rock salt or layered rock salt structures. .

[0214] Of the added elements, manganese, like aluminum, has a detectable amount of peaks inside magnesium. It is preferable to have a

[0215] However, it is not always necessary that the additive element is the same in the entire surface layer portion 100a of the positive electrode active material 100. The concentration gradient or distribution may not be the same.

[0216] Furthermore, the (001) oriented surface of the positive electrode active material 100 has a distribution of the additive elements different from the other surfaces. For example, the (001) oriented surface and its surface layer 100a may be (00 1) It is acceptable if the amount of one or more added elements detected is lower than that of the surface other than the orientation. Specifically, the amount of nickel detected may be low. In the analytical method using cobalt, the Kβ of cobalt and the Kα of nickel are close in energy, so cobalt It is difficult to detect trace amounts of nickel in materials where nickel is the main element. The surface and its surface layer 100a have one or more peaks of detected amounts selected from the added elements. The position of may be shallower than the surface other than the (001) orientation. The oriented surface and its surface layer 100a have peaks of detected amounts of magnesium and aluminum. The position of the surface may be shallow compared to the surface other than the (001) orientation.

[0217] In the layered rock-salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. This is a structure in which CoO2 layers and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion path of lithium ions also exists parallel to the (001) plane. do.

[0218] Since the CoO2 layer is relatively stable, the surface of the positive electrode active material 100 tends to have a (001) orientation. The (001) plane is the most stable surface. The main diffusion path of lithium ions during charging and discharging is the exposed Not yet.

[0219] On the other hand, on surfaces other than the (001) orientation, the diffusion paths of lithium ions are exposed. Therefore, the surface other than the (001) orientation and the surface layer 100a thereof are diffusion paths for lithium ions. This is an important region for maintaining the temperature, and at the same time, it is the region 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) oriented surface and the surface layer 100a thereof.

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

[0221] For example, the distribution of magnesium in the (001) oriented surface and its surface layer 100a is as follows: The half width is preferably 10 nm or more and 200 nm or less, and more preferably 50 nm or more and 150 nm or less. It is more preferable that the thickness is 80 nm or less and 120 nm or less. In addition, the distribution of magnesium in the surface that is not (001) oriented and in the surface layer 100a thereof is The half width is preferably more than 200 nm and not more than 500 nm, and more preferably less than 200 nm. More preferably, it is between 230 nm and 270 nm. It is even more preferable that:

[0222] The distribution of nickel in the surface and the surface layer 100a that is not (001) oriented is The value width is preferably 30 nm or more and 150 nm or less, and more preferably 50 nm or more and 130 nm or less. It is more preferable that the thickness is 70 nm or more and 110 nm or less, and further more preferable that the thickness is 70 nm or more and 110 nm or less.

[0223] In the manufacturing method in which the additive elements are mixed and then heated, which will be described in a later embodiment, the additive elements are mainly lithium. The added elements may spread through the diffusion path of the ions. In order to set the distribution of the additive elements in the outer surface and the surface layer 100a thereof within a preferred range, It is better to mix the additive elements after preparing the lithium cobalt oxide.

[0224] 〔magnesium〕 Magnesium is divalent and in the layered rock salt crystal structure, it is added with aluminum or Since nickel is stable at the cobalt site, magnesium ions are stable at the cobalt site. It is easy for magnesium to exist on the lithium site rather than on the sulphur site, that is, it is easy for magnesium to enter the lithium site. The presence of lithium at an appropriate concentration at the lithium site of the surface layer 100a allows the formation of layered rock salt crystals. This is because magnesium present at the lithium site can be easily transferred to CoO It is speculated that this is because it acts as a pillar supporting the two layers. So, Li x When x in CoO2 is, for example, 0.24 or less, the surroundings of magnesium This can suppress the release of oxygen from the magnesium oxide, thereby suppressing the thermal decomposition reaction. The presence of ammonium is expected to increase the density of the positive electrode active material 100. If the magnesium concentration in the portion 100a is high, the hydrofluoric acid generated by the decomposition of the organic electrolyte solution, etc. It is also expected to improve corrosion resistance.

[0225] At an appropriate concentration, magnesium does not adversely affect the intercalation and deintercalation of lithium during charging and discharging. However, excess magnesium can reduce the effects of lithium. Furthermore, the effect of stabilizing the crystal structure may be reduced. This is because magnesium may occupy the cobalt site in addition to the lithium site. In addition, it is thought that the Unnecessary magnesium compounds (oxides and fluorides, etc.) that are not replaced remain on the surface of the positive electrode active material, etc. In addition, the magnesium concentration in the positive electrode active material may segregate and become a resistance component in the secondary battery. As the charge increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the battery, reducing the amount of lithium that contributes to charging and discharging. do.

[0226] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is between 0.002 and 0.06 times the number of cobalt atoms. A value of 0.005 to 0.03 times is preferable, and a value of about 0.01 times is even more preferable. The amount of magnesium contained in the entire positive electrode active material 100 is preferably, for example, G Using D-MS (glow discharge mass spectrometry), ICP-MS (inductively coupled plasma mass spectrometry), etc. The value may be a value obtained by performing elemental analysis on the entire positive electrode active material 100, or a value obtained by performing elemental analysis on the positive electrode active material 100. It may be based on the value of the composition of raw materials in the manufacturing process.

[0227] [Fluorine] Fluorine is a monovalent anion, and when fluorine is adsorbed on the surface of the positive electrode active material 100, the positive electrode The energy required for lithium to be released from the electrode active material 100 is reduced. As long as the amount of fluorine in the surface layer 100a is sufficient, fluorine may be substituted for a portion of the oxygen in the surface layer 100a. The redox potential of the cobalt ion accompanying lithium desorption differs depending on whether or not fluorine is present. In other words, if there is no fluorine, the cobalt ions become trivalent as lithium is released. On the other hand, when fluorine is present, the cobalt ion changes to tetravalent as lithium is removed. The cobalt ion changes from divalent to trivalent. The redox potential of the cobalt ion is different in both cases. The desorption and insertion of lithium ions in the vicinity of the fluorine atoms occurs smoothly, and the positive electrode active material 100 It is preferable that the surface or surface layer of the positive electrode active material 100 contains fluorine. When used in this way, it is possible to improve the charge / discharge characteristics, large current characteristics, etc. The presence of fluorine on the surface or surface layer, or the adsorption or non-adsorption of fluoride on the surface By attaching the positive electrode active material 100, excessive reaction between the positive electrode active material 100 and the electrolyte can be suppressed. In addition, the corrosion resistance to hydrofluoric acid can be effectively improved.

[0228] In addition, the melting point of fluorine compounds (sometimes called fluorides), including lithium fluoride, is If the melting point of the other additive element source is lower than that of the other additive element source, the fluorine compound may be used to lower the melting point of the other additive element source. Fluorine compounds such as LiF and MgF2 can act as fluxes (also called fluxing agents). As shown in Figure 10 (quoted and added from Figure 5 in Non-Patent Document 12), Since the eutectic point P of gF2 is around 742°C (T1), the heating process after mixing the additive elements In this case, the heating temperature is preferably 742°C or higher.

[0229] Here, the differential scanning calorimetry (DSC) test for fluorine compounds and mixtures is shown in Figure 1. The mixture in FIG. 11 contains lithium cobalt oxide as the lithium oxide. The fluorine compounds are LiF and MgF2. The mixture was LiCoO2:LiF:MgF2 = 100:0.33:1 (molar ratio). The fluorine compound in Figure 11 is a mixture of LiF and MgF2. Specifically, the mixture is LiF:MgF2 = 1:3 (molar ratio). It is a mixture of

[0230] As shown in Figure 11, an endothermic peak is observed around 735°C for fluorine compounds. An endothermic peak was observed around 830°C in the mixture of lithium valence oxide, LiF, and MgF2. Therefore, the heating temperature after mixing the additive elements is preferably 742°C or higher. 830°C or higher is more preferable. Also, 800°C (T2 in FIG. 10) or higher, which is between these, is preferable. Good too.

[0231] 〔nickel〕 Nickel can reside on both the cobalt site and the lithium site. When present in lithium, the redox potential is lower than that of cobalt, so for example, This means that the lithium is easily released, which is expected to result in faster charging and discharging speeds. .

[0232] When nickel is present at the lithium site, a layer structure consisting of cobalt and oxygen octahedra is formed. The change in volume caused by charging and discharging can be suppressed. This is because the nickel present in the lithium site and the CoO2 layer It is thought that this is because the structure functions as a pillar supporting the building. In this charged state, the crystal structure is expected to be more stable, which is preferable.

[0233] In addition, the distance between the cations and anions in nickel oxide (NiO) is smaller than that in MgO and CoO. , which is close to the average distance between cations and anions in LiCoO2, and the orientation matches that of LiCoO2. Easy to do.

[0234] The ionization tendency is lowest in the order of magnesium, aluminum, cobalt, and nickel. Therefore, nickel is thought to be less likely to dissolve into the electrolyte than the other elements mentioned above during charging. Therefore, it is considered that the effect of stabilizing the crystalline structure of the surface layer is high in the charged state.

[0235] Furthermore, nickel is Ni 2+ , Ni 3+ , Ni 4+ Of which Ni 2+ is the most stable, Nickel has a higher trivalent ionization energy than cobalt. Therefore, nickel and oxygen It is known that nickel does not take a spinel-type crystal structure by itself. It is thought that this has the effect of suppressing the phase change from a rock salt type to a spinel type crystal structure.

[0236] On the other hand, if nickel is excessive, the influence of strain due to the Jahn-Teller effect becomes stronger, which is undesirable. Also, excessive nickel may adversely affect lithium insertion and extraction. .

[0237] Therefore, it is preferable that the amount of nickel contained in the entire positive electrode active material 100 is appropriate. For example, the number of nickel atoms in the positive electrode active material 100 is less than the number of cobalt atoms. It is preferably more than 0% and not more than 7.5% of the number of atoms in the base, and more preferably 0.05% or more and not more than 4%. It is preferably 0.1% or more and 2% or less, and more preferably 0.2% or more and 1% or less. More than 4% or less is preferable, or more than 0% or less than 2% is preferable, or 0.05% or less is preferable. Preferably, it is between 0.05% and 2%. Or, it is preferably between 0.1% and 7.5%. The nickel content is preferably 7.5% or less, or 0.1% to 4%. The amount was determined by performing elemental analysis of the entire positive electrode active material using, for example, GD-MS, ICP-MS, etc. It may be a value based on the composition of raw materials in the process of producing the positive electrode active material. .

[0238] 〔aluminum〕 Aluminum can also exist in the cobalt sites of layered rock salt crystal structures. Aluminum is a trivalent typical element and its valence does not change, so even when charging and discharging, The lithium in the aluminum alloy is difficult to move, so the aluminum and the surrounding lithium act as pillars. Therefore, as will be described later, the positive electrode active material 100 can be made of lithium. Even if the force of expansion and contraction in the c-axis direction is exerted by the insertion and desorption of ions, that is, Even if a force that expands or contracts in the c-axis direction is applied by changing the charging rate, the positive electrode active material 100 Deterioration can be suppressed.

[0239] Aluminum also suppresses the elution of surrounding cobalt, improving the battery's durability against continuous charging. Also, since the Al-O bond is stronger than the Co-O bond, the oxygen around the aluminum is released. These effects improve the thermal stability. When the cathode active material 100 contains aluminum as a component, safety is improved when the cathode active material 100 is used in a secondary battery. Furthermore, the positive electrode active material 100 has a crystalline structure that is resistant to breakdown even after repeated charge and discharge. This can be done.

[0240] On the other hand, excessive aluminum may adversely affect lithium insertion and extraction. .

[0241] Therefore, it is preferable that the amount of aluminum contained in the entire positive electrode active material 100 is appropriate. For example, the number of aluminum atoms in the entire positive electrode active material 100 is 1 / 1 the number of cobalt atoms. Preferably, the content is 0.05% or more and 4% or less, more preferably 0.1% or more and 2% or less, and even more preferably 0.3% or more and 1% or less. 0.5% or less is more preferable, or 0.05% to 2% or less is more preferable, or 0.1% or more The amount contained in the entire positive electrode active material 100 is preferably 4% or less. S may be a value obtained by performing elemental analysis on the entire positive electrode active material 100 using ICP-MS or the like. Alternatively, it may be based on the values ​​of the blending of raw materials in the process of producing the positive electrode active material 100 .

[0242] Titanium oxide is known to have superhydrophilic properties. By using a cathode active material 100 containing titanium oxide in a, it is possible to prevent wettability with highly polar solvents. When a secondary battery is formed, the cathode active material 100 and the highly polar electrolyte This may improve the contact at the interface with the liquid and suppress an increase in internal resistance.

[0243] Furthermore, when phosphorus is present in the surface layer portion 100a, Li x When x in CoO2 is kept small, In this case, it is preferable because short circuits can be suppressed. For example, a compound containing phosphorus and oxygen It is preferable that the surface layer portion 100a is provided with the fluorine-containing compound.

[0244] When the positive electrode active material 100 contains phosphorus, the phosphorus is generated by decomposition of the electrolyte or lithium salt. This is preferable because hydrogen fluoride reacts with phosphorus, potentially reducing the hydrogen fluoride concentration in the electrolyte. stomach.

[0245] If the lithium salt contains LiPF6, there is a risk of hydrogen fluoride being generated by hydrolysis. In addition, polyvinylidene fluoride (PVDF) and alkali metal fluoride, which are used as components of the positive electrode, There is also a risk of hydrogen fluoride being generated by the reaction with lithium. This reduces the corrosion of the current collector and / or peeling of the coating 104 (see FIG. 19). It may also be possible to prevent the loss of adhesiveness due to gelation and / or insolubilization of PVDF. It may be possible to control it.

[0246] When the positive electrode active material 100 contains phosphorus together with magnesium, Li x x in CoO2 is small This is preferable because the stability of the crystal structure in this state is extremely high. In this case, the number of phosphorus atoms is preferably 1% or more and 20% or less of the number of cobalt atoms, and more preferably 2% or less. More preferably, the ratio is 3% to 8%. Or, the ratio is 1% to 10%. Preferably, it is 1% or more and 8% or less, or preferably, it is 2% or more and 20% or less. Or, 2% or more and 8% or less is preferable. Or, 3% or more and 20% or less is preferable. Or, 3% or more In addition, the number of magnesium atoms should be 0.1% or less of the number of cobalt atoms. Preferably, the ratio is 0.5% to 5% or less, more preferably 0.7% to 4%. More preferably, the range is 0.1% or more and 5% or less. Alternatively, the range is 0.1% or more and 4% or less. Preferably, the range is 0.5% or more and 10% or less. Preferably, the range is 0.5% or more and 4% or less. Alternatively, 0.7% or more and 10% or less is preferable. Alternatively, 0.7% or more and 5% or less is preferable. The concentrations of phosphorus and magnesium indicated by are measured using, for example, GD-MS, ICP-MS, etc. The value may be obtained by performing elemental analysis on the entire active material 100, or may be obtained by performing elemental analysis on the entire active material 100 during the preparation of the positive electrode active material 100. It may be based on the value of the raw material composition in the process.

[0247] When a crack is formed on a part of the surface of the positive electrode active material 100, the part of the surface that is in contact with the crack may be The buried portion 102 (see FIG. 7(H)) contains phosphorus, more specifically, phosphorus and oxygen. The presence of the compound containing hydroxybenzoates can inhibit the progression of cracks.

[0248] [Synergistic effect of multiple elements] Furthermore, when the surface layer 100a contains both magnesium and nickel, divalent magnesium It is possible that divalent nickel can exist more stably near the Li x C Even when x in oO2 is small, the elution of magnesium can be suppressed. This may contribute to the stabilization of 00a.

[0249] For the same reason, in the manufacturing process, when adding additive elements to lithium cobalt oxide, It is preferable that magnesium is added in a process before nickel is added. It is preferable to add magnesium in the same process. Even if added during the manufacturing process, nickel tends to remain on the surface of the lithium cobalt oxide. In the absence of magnesium, it can diffuse widely within the lithium cobalt oxide. When nickel is added before magnesium, nickel diffuses into the lithium cobalt oxide. There is a concern that the desired amount of the material will not remain on the surface.

[0250] Furthermore, when additive elements with different distributions are added, the crystal structure in a wider region can be stabilized, which is preferable. For example, the positive electrode active material 100 may contain magnesium, nickel, and aluminum. When the crystalline structure is present, it is possible to stabilize a wider area of ​​the crystalline structure than when only one of the two is present. When the positive electrode active material 100 contains additive elements with different distributions, the surface is stabilized. Aluminum is not essential for the surface, as this can be achieved by magnesium, nickel, etc. It is preferable that aluminum be distributed widely in a deeper region. It is preferable that aluminum is continuously detected in the depth region of 1 nm to 25 nm. A region of 0 nm to 100 nm from the surface, preferably 0.5 nm or more from the surface A wide distribution within a region of 50 nm is preferable because it stabilizes the crystal structure over a wider region. .

[0251] When multiple additive elements are included as described above, the effects of each additive element are synergistic, and the surface layer 1 In particular, magnesium, nickel, and aluminum can contribute to further stabilization of 00a. If it is present, it is highly effective in providing a stable composition and crystal structure, and is therefore preferable.

[0252] However, if the surface layer 100a is occupied only by a compound of the added element and oxygen, the intercalation and deintercalation of lithium will be difficult. For example, if the surface layer 100a is made of MgO, MgO and Only NiO(II) solid solution structures and / or MgO and CoO(II) solid solution structures Therefore, the surface layer portion 100a contains at least cobalt, In the discharged state, it must also contain lithium and have a path for lithium insertion and desorption. .

[0253] In order to ensure sufficient paths for lithium intercalation and deintercalation, the surface layer 100a is made of magnesium. For example, when XPS (X-ray photoelectron spectroscopy) is used, the positive electrode active material When measured from the surface of 100, the ratio of the number of magnesium atoms Mg to the number of cobalt atoms Co The Mg / Co ratio is preferably 0.62 or less. It is preferable that the concentration of cobalt is high. It is preferable that the concentration of cobalt is higher than that of fluorine in the surface layer portion 100a. It is preferable that

[0254] Furthermore, if there is too much nickel, there is a risk of impeding the diffusion of lithium. For example, it is preferable that the concentration of magnesium is higher than that of nickel in the positive electrode active material. When measured from the surface of 100, the number of nickel atoms is less than 1 / 6 of the number of magnesium atoms. It is preferable that:

[0255] In addition, some of the added elements, especially magnesium, nickel and aluminum, are more resistant to oxidation than the internal 100b. It is preferable that the concentration of the surface layer 100a is higher than that of the inner layer 100b. It is preferable that magnesium and aluminum are present in the lithium samarium oxide layer 100b. When present in the rock salt at an appropriate concentration, it is possible to easily maintain the layered rock salt crystal structure as described above. In addition, when nickel is present in the interior 100b at an appropriate concentration, Similarly, the deviation of the layer structure consisting of the transition metal M and oxygen octahedra can be suppressed. When magnesium and nickel are present together, a synergistic effect of suppressing the elution of magnesium is observed. Results can be expected.

[0256] <Li x MO2 with small x≫ The positive electrode active material 100 according to one embodiment of the present invention has the above-described distribution of the additive element and / or crystal structure. Due to the presence of Li x When x in MO2 is small, that is, when the voltage is high, the charging state The crystal structure at this point is different from that of conventional positive electrode active materials. <x≦ 0.24. Also, high voltage in a charged state means 4.5V or more, preferably means 4.6V or more, and more preferably 4.8V or more.

[0257] Using Figs. 12 and 13, Li x Regarding the change in crystal structure with the change in x in MO2, The description will be made while comparing a conventional positive electrode active material with the positive electrode active material 100 of one embodiment of the present invention.

[0258] The change in the crystal structure of a conventional positive electrode active material is shown in Figure 13. The conventional positive electrode active material shown in Figure 13 has the following characteristics: In particular, lithium cobalt oxide (LiCoO2) without any additive elements is used. The change in the crystal structure of lithium cobalt oxide that does not occur is described in Non-Patent Documents 1 to 3, etc. It is being done.

[0259] In Figure 13, R-3m O3 is added to show the discharge state, i.e., Li x Cobalt in CoO2 with x=1 In the discharged state, conventional lithium cobalt oxide has a crystal structure similar to that of the present invention. The crystal structure is the same as that of the positive electrode active material 100 of one embodiment.

[0260] In addition, in conventional lithium cobalt oxide, the symmetry of lithium increases when x is about 0.5, and the It is known that the crystal structure belongs to the clinic space group P2 / m. There is one CoO2 layer in the unit cell. Therefore, it is an O1 type or monoclinic crystal (mono in the figure). It is sometimes called type O1 (written as "clinic").

[0261] When x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1. Therefore, this crystal structure is called O1 type, or Trigonal crystal (marked as trigonal in the figure) is sometimes called O1 type. It is sometimes converted into a lattice and called hexagonal O1 type.

[0262] In addition, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is different from the trigonal O1 type CoO2 structure and the R-3m O3 type The structure of LiCoO2 and the structure of LiCoO2 are stacked alternately. This is sometimes called the H1-3 type crystal structure (H1-3 in the figure). The insertion and desorption of lithium does not necessarily occur uniformly within the positive electrode active material, and the lithium concentration may become uneven. Therefore, the H1-3 type crystal structure is experimentally observed from about x = 0.25. The H1-3 crystal structure has twice the number of cobalt atoms per unit cell as other structures. However, in this specification, including Figure 13, in order to make it easier to compare with other crystal structures, The c-axis of the H1-3 type crystal structure is shown as half of the unit cell.

[0263] As an example, the H1-3 type crystal structure is described in Non-Patent Document 3, in the unit cell The coordinates of cobalt and oxygen in the (0,0,0.27671±0.00045), O2(0,0,0.11535±0.0 0045), where O1 and O2 are oxygen atoms. Which unit cell should be used to represent the crystal structure of a compound? This can be determined by Rietveld analysis. For example, GOF (goodness of field) It is sufficient to use a unit cell with a value of f fit close to 1.

[0264] Li, for which the H1-3 type crystal structure can be experimentally confirmed x x in CoO2 is 0.24 or less When conventional lithium cobalt oxide is repeatedly charged and discharged, the H1-3 crystal structure The change in the crystal structure between the structure of R-3m O3 in the discharge state and that of R-3m O3 in the discharge state (i.e., a non-equilibrium phase change) ) is repeated.

[0265] These two crystal structures have a large offset of the CoO2 layers. As shown by the dotted line in Figure 13, In the 1-3 type crystal structure, the CoO2 layer is significantly deviated from the R-3m O3 in the discharged state. 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. In this case, the difference in volume between the H1-3 type crystal structure and the R-3m O3 type crystal structure in the discharged state is 3.5 %, typically 3.9% or more.

[0267] In addition, the H1-3 type crystal structure has a continuous CoO2 layer structure like the trigonal O1 type. is likely to be unstable.

[0268] Therefore, if charging and discharging are repeated so that x becomes 0.24 or less, the conventional cobalt The crystalline structure of lithium oxide is gradually broken down. This break in the crystalline structure causes a deterioration in cycle characteristics. This is because the crystal structure collapses, reducing the number of sites where lithium can exist stably. Also, it becomes difficult to insert and extract lithium.

[0269] On the other hand, in the positive electrode active material 100 according to one embodiment of the present invention shown in FIG. x x in MO2 is 1 The change in the crystal structure during discharge and when x is 0.24 or less is greater than that of conventional positive electrode active materials. More specifically, the MO2 layer in the state where x is 1 and the state where x is 0.24 or less The difference can be reduced. When compared with conventional positive electrode active materials, the volume change of the positive electrode active material 100 is Therefore, in the positive electrode active material 100 according to one embodiment of the present invention, x is 0.24 or less. Even with repeated charging and discharging, the crystalline structure is not easily broken down, and lithium exists stably. Therefore, the sites that can be utilized are maintained, and excellent cycle characteristics can be achieved.

[0270] The positive electrode active material 100 according to one embodiment of the present invention is Li x x in MO2 is 0.24 or less In this case, a more stable crystal structure than that of a conventional positive electrode active material can be obtained. The positive electrode active material 100 is Li x When x in MO2 was kept below 0.24, Even if the temperature is high, oxygen is less likely to be released, and the thermal decomposition reaction can be suppressed. A secondary battery using such a positive electrode active material 100 is preferable because it has improved safety.

[0271] Li x When x in MO2 is 1, approximately 0.2, or approximately 0.15, the inside of the positive electrode active material 100 The crystal structure of the inner portion 100b is shown in FIG. This accounts for half of the total volume and contributes greatly to charging and discharging. This can also be considered a problematic area.

[0272] As described above, when x=1, the positive electrode active material 100 has the same R- However, the positive electrode active material 100 has a crystal structure of 3m O3. The H1-3 type crystal structure is obtained when x is 0.24 or less, for example, about 0.2 or about 0.15. At this temperature, the crystals have a different structure.

[0273] When x=approximately 0.2, the positive electrode active material 100 according to one embodiment of the present invention is a trigonal space group R-3 The crystal structure is attributed to m. This is because the symmetry of the MO2 layer is the same as that of O3. This crystal structure is called the O3' type crystal structure. In the XRD pattern, a pattern similar to a spinel structure may appear, and this crystal structure This structure is sometimes called a pseudo-spinel structure. As will be described later, the crystal structure may change to the H1-3 type via the O3' type crystal structure. The positive electrode active material 100 that has undergone the O3' type crystal structure may have, for example, an H1-3 type crystal structure. Therefore, the lithium battery using the positive electrode active material 100 can effectively suppress oxygen release even when the positive electrode active material 100 is used. It is estimated that even if a nail penetration test is conducted on an ion secondary battery, ignition is suppressed.

[0274] In the O3' type crystal structure, when M is cobalt, the cobalt and oxygen sites in the unit cell are The target is Co(0,0,0.5), O(0,0,x), within the range of 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797×10 -10 ≦a ≦2.837×10 -10 (m) is preferred, 2.807 × 10 -10 ≦a≦2.827 x10 -10 (m) is more preferable, and typically a=2.817×10 -10 (m) The c-axis is 13.681×10 -10 ≦c≦13.881×10 -10 (m) is preferred , 13.751×10 -10 ≦c≦13.811×10 -10 (m) is more preferred, Explicitly, c = 13.781 × 10 -10 (m).

[0275] Furthermore, when x=approximately 0.15, the positive electrode active material 100 according to one embodiment of the present invention is a monoclinic space group It has a crystal structure attributed to P2 / m, which is a structure in which one CoO2 layer exists in the unit cell. In addition, when x=0.15, the lithium present in the positive electrode active material 100 is in a discharged state. It can be considered that the ratio is about 15 atomic %. Therefore, this crystal structure is called a monoclinic O1(15) type crystal structure. In Figure 12, P2 / m monoclinic (denoted as monoclinic in the figure) O1(1 5) indicates this crystal structure.

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

[0277] This crystal structure can be calculated in the Rietveld method by allowing for a certain degree of error. It is also possible to show the lattice constants for the group R-3m. The coordinates of the root and oxygen are Co(0,0,0.5), O(0,0,Z O ), 0.21≦Z O ≦0.23. The lattice constant of the unit cell is a = 0.2817 ± 0.002 nm, c=1.368±0.01 nm.

[0278] In both the O3' and monoclinic O1(15) crystal structures, cobalt, nickel, Ions such as magnesium occupy the oxygen hexacoordinate position. Light elements may occupy the oxygen tetracoordination positions.

[0279] As shown by the dotted line in Figure 12, the R-3m O3 in the discharge state, O3' and monoclinic O1(1 5) The MO2 layer is hardly shifted from the crystalline structure.

[0280] In addition, the volume per cobalt atom of the same number of R-3m O3 and O3' type crystal structure in the discharged state is The difference is not more than 2.5%, more particularly not more than 2.2%, typically not more than 1.8%.

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

[0282] Table 1 shows the discharged R-3m O3, O3', monoclinic O1(15), H1-3 type and triclinic O1(15). The difference in volume per cobalt atom in cubic O1 is shown. The lattice constants of the -3m O3 and trigonal O1 crystal structures are given in ICSD coll.code For H1-3, see non-patent literature. 3 can be referred to. For O3' and monoclinic O1(15), the experimental values ​​of XRD are It can be calculated.

[0283] [Table 1]

[0284] In this way, in the positive electrode active material 100 according to one embodiment of the present invention, Li x When x in CoO2 is small In other words, the change in the crystal structure when a large amount of lithium is released is suppressed compared to conventional positive electrode active materials. Compared with a positive electrode active material 100 with the same number of cobalt atoms, a conventional positive electrode active material In comparison, the volume change of the positive electrode active material 100 is smaller than that of conventional positive electrode active materials. Therefore, the positive electrode active material 100 remains crystalline even after repeated charge and discharge such that x becomes 0.24 or less. Therefore, the positive electrode active material 100 has a low charge / discharge capacity during charge / discharge cycles. Furthermore, since more lithium can be used stably than with conventional positive electrode active materials, Therefore, the positive electrode active material 100 has a large discharge capacity per weight and per volume. By using material 100, a secondary battery with high discharge capacity per weight and per volume can be produced. can.

[0285] The positive electrode active material 100 is Li x When x in MO2 is 0.15 or more and 0.24 or less, O3 It has been confirmed that the crystal structure may be of the ' type, and x is more than 0.24 and 0.27 or less. It is also believed that Li has an O3' type crystal structure. x x in MO2 is 0.1 When x is between 0.15 and 0.17, the monoclinic crystal is formed. It has been confirmed that some of them have a crystalline structure of the Li type. x MO In addition to x in 2, the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc. Therefore, the range of x is not necessarily limited to the above range.

[0286] Therefore, the positive electrode active material 100 is Li x When x in MO2 is greater than 0.1 and less than or equal to 0.24 , O3' type only, monoclinic O1(15) type only, or both All of the particles in the interior 100b of the positive electrode active material 100 may have a crystalline structure. The crystal structure may not necessarily be monoclinic O1(15) type and / or monoclinic O1(15) type. Alternatively, a part of the amorphous material may be amorphous.

[0287] Also Li x To make the x in MO2 small, it is generally necessary to charge it at a high charging voltage. Therefore, Li x The state where x in MO2 is small is considered to be the state where it is charged at a high charging voltage. In other words, for example, at a voltage of 4.6 V or more based on the potential of lithium metal, When CC / CV charging is performed in a 25°C environment, the H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6V or more is considered a high charging voltage based on the potential of lithium metal. In this specification, unless otherwise specified, the charging voltage is the lithium Let us express the potential of aluminum metal as the reference.

[0288] Therefore, the positive electrode active material 100 according to one embodiment of the present invention can be used at a high charging voltage, for example, 4 Even when charged at a voltage of 0.6V or higher, the crystal structure with the symmetry of R-3m O3 can be maintained. In other words, it is preferable to use a higher charging voltage, for example, at 25°C. When charged at a voltage between 4.65V and 4.7V, the O3'-type crystal structure can be formed. In other words, it is preferable to use a higher charging voltage, for example, at 25°C. When charged at a voltage above 4.7V but below 4.8V, the monoclinic O1(15) type crystal structure This can be rephrased as being preferable because it is possible to obtain

[0289] Even with the positive electrode active material 100 according to one embodiment of the present invention, when the charging voltage is further increased, the charge current finally reaches H1-3 As mentioned above, the crystal structure changes with the number of charge / discharge cycles. Because it is affected by the charge / discharge current, temperature, electrolyte, etc., when the charging voltage is lower, for example, Even if the applied voltage is 4.5 V or more and less than 4.6 V at 25° C., the positive electrode active material 1 according to one embodiment of the present invention 00 may take the O3' type crystal structure. When charged at a voltage below 0.7V, it may take on a monoclinic O1(15) type crystal structure. be.

[0290] In addition, when graphite is used as the negative electrode active material in a secondary battery, the graphite The voltage of the secondary battery drops by the amount of the potential. The potential of graphite is based on the potential of lithium metal. Therefore, the negative electrode active material is a secondary battery using graphite. In the case of a pond, when the voltage obtained by subtracting the potential of graphite from the voltage above, the same crystalline structure as above is formed. It has.

[0291] In addition, in O3' and monoclinic O1(15) in Fig. 12, lithium occupies all lithium sites equally. However, this is not limited to the case where the lithium atoms are present at a certain lithium site. For example, the monoclinic O1 (Li 0.5 CoO2) The lithium distribution can be analyzed, for example, by neutron diffraction. It is possible.

[0292] The O3' and monoclinic O1(15) crystal structures have random lithium interlayers. It can be said that the crystal structure is similar to that of CdCl2. A crystal structure similar to the l2 type is Lithium nickel oxide (Li 0.06 Charged up to NiO2 The crystal structure is similar to that of pure lithium cobaltate, or layered rock salt containing a large amount of cobalt. It is known that positive electrode active materials of this type do not usually have a CdCl2 type crystal structure.

[0293] As described above, the conventional lithium cobalt oxide and the positive electrode active material 100 according to one embodiment of the present invention are different in charging. Change in electrical depth, i.e., Li x The crystal structure changes depending on the change in x in CoO2. The change in the c-axis length of conventional lithium cobalt oxide described in Patent Document 12 is shown in FIG. The round markers represent the hexagonal phase, and the diamond markers represent the monoclinic phase.

[0294] The change in the c-axis length of lithium cobalt oxide is due to the change in the lithium cobalt oxide in the XRD pattern. For example, this corresponds to the change in the angle at which the peak of the (003) plane appears. In RD, the peak of the (003) plane of lithium cobalt oxide is at 2θ around 19° to 20°. It is known to occur in

[0295] <Grain boundary> The additive elements contained in the positive electrode active material 100 of one embodiment of the present invention may be distributed as described above. It is more preferable that at least a part of the crystal grains is unevenly distributed in the grain boundaries 101 and their vicinity.

[0296] In this specification, uneven distribution means that the concentration of an element in a certain region is different from that in other regions. Segregation, precipitation, unevenness, bias, or a mixture of high concentration and low concentration areas , is synonymous with.

[0297] For example, the magnesium concentration at and near the grain boundary 101 of the positive electrode active material 100 is It is preferable that the fluorine concentration in the grain boundary 101 and its vicinity is higher than that in the other regions of 0b. The degree of the crystal grain boundary 101 and its vicinity is also preferably higher than that of the other regions of the interior 100b. The nickel concentration in the grain boundary 100b is also preferably higher than in other regions of the interior 100b. The aluminum concentration in and around the interior 100b is also preferably higher than in other regions of the interior 100b. stomach.

[0298] The grain boundary 101 is one of the planar defects. Therefore, it is easily unstable like the grain surface. Therefore, the concentration of the added element at the grain boundary 101 and its vicinity is If the concentration is high, the change in the crystal structure can be more effectively suppressed.

[0299] In addition, when the magnesium concentration and fluorine concentration at the grain boundary 101 and its vicinity are high, Even if a crack occurs along the grain boundary 101 of the positive electrode active material 100 according to one embodiment of the present invention, the crack The magnesium and fluorine concentrations are high near the surface created by the rack. Even after cracks have occurred in the positive electrode active material, the corrosion resistance to hydrofluoric acid can be improved. do.

[0300] <Particle size> If the particle size of the positive electrode active material 100 that enables high voltage charging is too large, it becomes difficult for lithium to diffuse. This causes problems such as the surface of the active material layer becoming too rough when coated on the current collector. On the other hand, if the size is too small, problems such as excessive reaction with the electrolyte may occur. The diameter (D50) is preferably 1 μm or more and 100 μm or less, and more preferably 2 μm or more and 40 μm or less. It is more preferable that the thickness is 5 μm or more and 30 μm or less, and further more preferable that the thickness is 1 μm or more and 40 μm or less. Preferably, it is 1 μm or more and 30 μm or less. Or, it is 2 μm or more and 100 μm or less. Preferably, the thickness is 2 μm or less and 30 μm or less, or preferably, 5 μm or more and 100 μm or less. It is preferably 5 μm or less, or 5 μm or more and 40 μm or less.

[0301] In addition, by mixing particles of different particle sizes and using them in the positive electrode, the electrode density can be increased, It is preferable that the secondary battery has a high energy density. Material 100 is expected to have high charge / discharge rate characteristics. The secondary battery using the substance 100 has high charge / discharge cycle characteristics and can maintain a high discharge capacity. is expected.

[0302] In addition, when particles with different median diameters (D50) are mixed and used in the positive electrode, Assuming that lithium is desorbed from the surface, Li x The rate at which x in CoO2 decreases is The positive electrode active material 100 having a relatively small particle size is more effective than the positive electrode active material 100 having a relatively large particle size. Therefore, powder XRD measurement is performed on a positive electrode active material that is a mixture of particles with different particle sizes. In some cases, both the O3'-type crystal structure and the monoclinic O1(15)-type crystal structure are detected. do.

[0303] <Analysis method> A certain positive electrode active material is Li x When x in CoO2 is small, O3' type and / or monoclinic O1 Whether or not the positive electrode active material 100 of one embodiment of the present invention has a (15) type crystal structure can be determined by Li x The positive electrode with a positive electrode active material with a small x in CoO2 was analyzed by XRD, electron diffraction, and neutron diffraction. In this case, it can be determined by analyzing using electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. Cut.

[0304] In particular, XRD can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution. The degree of crystallinity and the orientation of the crystals can be compared. Analysis is possible, and sufficient accuracy can be obtained even when measuring the positive electrode obtained by disassembling the secondary battery. Among XRD methods, powder XRD is preferred in terms of the following points. Diffraction peaks reflecting the crystal structure of the interior 100b of the positive electrode active material 100 are obtained.

[0305] When analyzing the crystallite size by powder XRD, the influence of pressure on the orientation of the positive electrode active material particles can be considered. For example, the positive electrode active material is removed from the positive electrode obtained by disassembling the secondary battery. It is preferable to take out the powder sample and measure it.

[0306] As described above, the positive electrode active material 100 according to one embodiment of the present invention is Lix x in CoO2 is 1 The characteristic of this material is that there is little change in the crystal structure when the charge voltage is 0.24 or less. When charged, the crystal structure changes significantly. Materials with a crystal structure of 50% or more are prone to high voltage charging. This is not preferable because it cannot withstand repeated charging and discharging.

[0307] In addition, the addition of an additional element alone does not result in the formation of an O3' or monoclinic O1(15) type crystal structure. It should be noted that there are cases where the cobalt with magnesium and fluorine is not lithium cobalt oxide, or lithium cobalt oxide with magnesium and aluminum, Even if the points are the same, depending on the concentration and distribution of the added elements, x x in CoO2 is 0.24 When the O3' type and / or monoclinic O1(15) type crystal structure is 60% or more, In some cases, the H1-3 type crystal structure accounts for 50% or more.

[0308] In addition, even in the positive electrode active material 100 of one embodiment of the present invention, when x is too small, such as 0.1 or less, Under conditions where the charging voltage exceeds 4.9 V, the H1-3 type or trigonal O1 type crystal structure is formed. Therefore, when determining whether or not the positive electrode active material 100 is one embodiment of the present invention, To do this, analysis of the crystal structure, including XRD, and information such as charge capacity or charge voltage are required. Information is needed.

[0309] However, when the positive electrode active material has a small x, the crystal structure may change when it is exposed to the air. For example, the crystal structure of the O3' type and monoclinic O1(15) type can be converted to the H1-3 type crystal structure. Therefore, all samples used for crystal structure analysis were stored in an argon atmosphere. It is preferable to handle the material in an inert atmosphere such as air.

[0310] In addition, whether the distribution of the additive elements contained in a certain positive electrode active material is in the state described above or not can be analyzed using, for example, XPS, EDX, EPMA (electron probe microanalysis), etc. This can be determined.

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

[0312] <Powder Resistivity Measurement> The volume resistivity of the powder of the positive electrode active material 100 according to one embodiment of the present invention will be described.

[0313] In one embodiment of the present invention, the volume resistivity of the powder of the positive electrode active material 100 is 64 MPa. 1.0×10 in pressure 4 It is preferable that the resistance is Ω·cm or more, and 1.0×10 5 Ω·c m or more, and 1.0 × 10 6 It is more preferable that the resistance is Ω·cm or more. In addition, at a pressure of 64 MPa, the 9 Preferably it is Ω·cm or less , 1.0×10 8 It is more preferable that the resistance is Ω·cm or less, and 1.0×10 7 Ω cm or less It is more preferable that fluorine is adsorbed on the surface of the positive electrode active material 100, and a small amount of the fluorine is At least a part of the positive electrode active material 100 is bonded to the lithium contained therein, This can suppress the migration of ions. For example, the particle size of the powder of the positive electrode active material 100 can be set to the above value. The resistivity is 1.0 × 10 at a pressure of 64 MPa. 5 Can be made to 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 voltages. Therefore, the volume resistivity of the powder of the positive electrode active material 100 is within the above range. The surface layer 10 is important for the crystalline structure of the positive electrode active material to be stable in a charged state. 0a can be used as an index showing that the positive electrode active material 10 In this case, it is preferable that at least the shell 100s has a high resistance.

[0315] However, when the high resistance region is thick from the surface toward the inside of the positive electrode active material 100, Therefore, the thin area near the surface of the surface layer portion 100a is It is more preferable that only the surface layer 100a has high resistance. It is preferable that the region is thin from the surface toward the inside. For example, the surface layer portion 100a In this case, the region where Mg is present at a high concentration can become a high resistance region. It is preferable that Mg is located at 00a.

[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. do.

[0317] As shown in FIG. 20(A), the device for measuring the volume resistivity of powder has a terminal for measuring resistance. A first mechanism 10 and a second mechanism 1 that applies pressure to a powder sample S (sample) to be measured. The second mechanism 11 preferably has a cylinder into which the powder sample S is poured. The piston may have a spring and may move up and down within the cylinder. etc. are connected to each other, and pressure can be applied to the sample in the cylinder. It is preferable to have a measuring electrode in contact with the bottom surface of the cylinder. As an example of a measuring device having a mechanism for applying pressure to the target powder, Mitsubishi Chemical Analytical The MCP-PD51 manufactured by Tech can be used. The resistance meter is Loresta-GP. Alternatively, the Hiresta UP can be used. The Loresta GP can be used as shown in Figure 20(B). It can be used to measure low resistance samples using the four-point probe method. It can be used to measure high resistance samples using the two-terminal method shown in (C). The measurement environment is preferably a stable environment such as a dry room. The environment may be a laboratory environment. For example, a dry room environment may be between 20°C and 25°C. A temperature environment with a dew point of -40°C or less is preferable. The environment was a temperature environment of 15°C to 30°C and a humidity environment of 30% to 70%. That's fine.

[0318] The measurement of the volume resistivity of powder using the measuring device shown above will be explained. The pull is set in the second mechanism 11. The second mechanism 11 has a measuring section, and in the measuring section, The sample is placed in the cylinder, and the bottom of the cylinder comes into contact with the measuring electrode. It also has a structure that includes a piston that can apply pressure to the sample. 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 is measured while applying pressure to the powder. Measure the thickness of the powder. The pressure applied to the powder can be measured under multiple conditions. Example: For example, 13 MPa, 25 MPa, 38 MPa, 51 MPa, and 64 MPa, respectively. Under pressure conditions, the electrical resistance of the powder and the thickness of the powder can be measured. The volume resistivity of the powder can be calculated from the electrical resistance of the powder and the thickness of the powder.

[0320] This section explains how to calculate volume resistivity. Measure it using the two-terminal method with Hirestar UP. In this case, multiply the electrical resistance of the powder by the area of ​​the measuring electrode that is in contact with the powder, and divide by the thickness of the powder. When measuring with the four-probe method using Loresta GP, the volume resistivity can be calculated. The volume resistivity can be calculated by multiplying the electrical resistance by the correction factor and then by the thickness of the powder. is a value that changes depending on the sample shape, dimensions, and measurement position. This can be calculated using the calculation software available.

[0321] When the above-described measurement is performed, the powder of the positive electrode active material 100 according to one embodiment of the present invention is The resistivity is 1.0×10 at a pressure of 64 MPa. 4 It is preferable that the resistance is Ω·cm or more. 1.0×10 5 It is more preferable that the resistance is Ω·cm or more, and 1.0×10 6 Ω cm or more It is more preferable that the viscosity is 1.0×10 or more at a pressure of 64 MPa. 9 Ω cm Preferably, it is 1.0 x 10 or less. 8 It is more preferable that the resistance is Ω·cm or less, and 1 .0×10 7 It is more preferable that the volume resistivity is Ω·cm or less. The battery containing the active material 100 exhibits favorable cycles in a charge-discharge cycle test under high voltage conditions. In addition, the battery is designed to be less likely to ignite during internal short-circuit tests such as nail penetration tests. It is possible.

[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, This composite oxide was used to produce a coin cell (CR2032 type, This can be determined by creating a 20mm diameter, 3.2mm height carp and charging it. The cell includes an electrolyte, a separator, a positive electrode can, and a negative electrode can.

[0323] More specifically, the positive electrode contains the composite oxide as a positive electrode active material, a conductive material, and a binder. The mixed slurry 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 can 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. Unless otherwise specified, voltages and potentials in this specification and the like are those of the positive electrode.

[0325] The lithium salt in the electrolyte is 1 mol / L lithium hexafluorophosphate (LiPF6) The electrolyte is ethylene carbonate (EC) and diethyl carbonate (DEC). The volume ratio of EC:DEC was 3:7, and vinylene carbonate (VC) was 2 wt%. The above-mentioned materials can be used.

[0326] The separator can be a 25 μm thick porous polypropylene film.

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

[0328] The coin cell made under the above conditions was tested at an arbitrary voltage (e.g., 4.5V, 4.55V, 4.6V Charges up to any voltage (4.65V, 4.7V, 4.75V or 4.8V). There is no particular restriction on the charging method as long as it can be charged over a long period of time. For example, charging with CCCV In this case, the current for CC charging can be 20mA / g or more and 100mA / g or less. CV charging can be completed at 2mA / g or more and 10mA / g or less. To observe the phase change, it is desirable to charge at such a small current value. Since it is difficult to measure D below 0°C, the temperature is set to 25°C. After charging in this way, the coin cell was placed in an argon atmosphere in a glove box. By disassembling the battery and taking out the positive electrode, you can obtain a positive electrode active material with any desired charging capacity. When performing analysis, it is preferable to seal the container in an argon atmosphere to prevent reactions with external components. For example, XRD measurement is performed by sealing the positive electrode active material in a sealed container in an argon atmosphere. After the charging is completed, it is preferable to quickly remove the positive electrode and subject it to analysis. Specifically, it is preferable to do so within one hour after the completion of charging, and more preferably within 30 minutes.

[0329] In addition, when analyzing the crystal structure in the charged state after multiple charge / discharge cycles, The charging conditions may differ from those stated above. For example, charging can be performed at any voltage (e.g., 4.6V, 4 4.65V, 4.7V, 4.75V or 4.8V), current value 20mA / g or more 100m Charge at a constant current of 2 mA / g or less, and then charge until the current value becomes 2 mA / g or more and 10 mA / g or less. Charge at a constant voltage, discharge at a constant current of 2.5V, 20mA / g or more and 100mA / g or less. It is possible.

[0330] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, The discharge conditions are, for example, 2.5 V and a current value of 20 mA / g or more and 100 mA / g or less. It may be a current discharge.

[0331] <XRD> As long as proper adjustment and calibration are performed, the apparatus and conditions for XRD measurement are not particularly limited. For example, The measurement can be performed using the following equipment and conditions. XRD equipment: Bruker AXS, D8 ADVANCE X-ray: Cu Kα1 Output: 40kV, 40mA Divergence angle: Div.Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm Standard samples used for adjustment and calibration include, for example, NIST (National Institute of Standards and Technology) standards. Sintered aluminum quasi-oxide plate SRM 1976 or the like can be used.

[0332] If the sample to be measured is a powder, place it on a glass sample holder or apply grease. The sample can be set by sprinkling it on a silicon non-reflective plate. If the measurement sample is a positive electrode, attach the positive electrode to the substrate with double-sided tape and then attach the positive electrode active material layer. It can be set to suit the measurement surface required by the device.

[0333] A filter or the like may be used to monochromatize the characteristic X-rays, or the XRD pattern may be obtained and then analyzed by XRD. This may be done using data analysis software, such as DIFFRAC.EVA (Bruk Using XRD data analysis software manufactured by ER, the peak due to CuKα2 radiation was removed and the C It is possible to extract only the peak due to uKα1 line. Ground removal can also be performed.

[0334] In this specification, data processing when referring to the 2θ value of a certain diffraction peak is explained. First, we used crystal structure analysis software to compare the calculation model with the XRD pattern. In the calculated pattern, the diffraction peak The value of 2θ at which the peak top of a diffraction peak appears is called the value of 2θ of that diffraction peak. The crystal structure analysis software used for this analysis is not particularly limited, but for example, TOPASver. .3 (crystal structure analysis software manufactured by Bruker) can be used.

[0335] Figure 15 shows the crystal structure of the O3 type and the O3' type when CuKα1 is used for X-rays. The structure and the XRD pattern corresponding to the monoclinic O1(15) type crystal structure are shown in Fig. 16. is an ideal powder X-ray diffraction pattern calculated from a model of the H1-3 crystal structure using CuKα1 radiation. D pattern and the ideal CuKα1 radiation calculated from the trigonal O1 crystal structure at x=0 17(A) and 17(B) show typical XRD patterns of the above-mentioned XRD patterns. However, the range of 2θ is 18° or more and 21° or less, and the range of 2θ is The angle is between 42° and 46°. The line is ICSD (Inorganic Crystal Structure Data) Based on the crystal structure information obtained from Materials Science (see Non-Patent Document 5), Reflex Powder, one of the modules in Studio (BIOVIA) The 2θ range was 15° to 75°. Step size = 0.01, wavelength λ = 1.54 × 10 -10 m, Monochr The H1-3 crystal structure pattern is shown in Non-Patent Document 3. The crystal structure of the O3' and monoclinic O1(15) forms was similarly created from the crystal structure information described in the previous section. The pattern is a crystal structure estimated from the XRD pattern of the positive electrode active material 100 according to one embodiment of the present invention. Then, the crystal structure was analyzed using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker). It is a fitting.

[0336] As shown in Figures 15, 17(A) and 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 A diffraction peak appears at ±0.10° (45.37° or more and less than 45.57°).

[0337] In addition, in the monoclinic O1(15) crystal structure, 2θ=19.47±0.10° (19.37 ° or more and 19.57° or less), and 2θ=45.62±0.05° (45.57° or more and 45 Diffraction peaks appear at angles below 0.67°.

[0338] However, as shown in Figures 16, 17(A) and 17(B), the H1-3 type crystal structure and In cubic O1, no peaks appear at these positions. x x in CoO2 In a small state, 2θ is 19.13° or more and less than 19.37° and / or 19.37° or more and 9.57° or less, and 45.37° or more but less than 45.57° and / or 45.57° or more The appearance of a peak at a position of 45.67° or less indicates that the positive electrode active material 100 according to one embodiment of the present invention This can be said to be a characteristic of

[0339] This is because the positions where the XRD diffraction peaks appear are close to each other in the crystal structures of x=1 and x≦0.24. More specifically, the main diffraction peaks of the crystal structure for x=1 and x≦0.24 are For peaks that appear at 2θ between 42° and 46°, the difference in 2θ is 0 It can be said that the angle is less than 0.7°, and more preferably less than 0.5°.

[0340] The positive electrode active material 100 according to one embodiment of the present invention is Li x When x in CoO2 is small, O3 The crystal structure of the monoclinic O1(15) type is O3' type and / or monoclinic O1(15) type, but all of the particles are O3' type and It may not be a monoclinic O1(15) type crystal structure, and may contain other crystal structures. However, the Rietveld analysis of the XRD pattern is not required. When the above procedure is carried out, the crystal structure of O3' type and / or monoclinic O1(15) type is 50% or more. It is preferable that the ratio is 60% or more, more preferable that the ratio is 66% or more. It is preferable that the crystal structure of O3' type and / or monoclinic O1(15) type is 50% or more, more preferably If the ratio is 60% or more, and more preferably 66% or more, the positive electrode exhibits excellent cycle characteristics. It can be used as an electrode active material.

[0341] In addition, even after more than 100 charge / discharge cycles from the start of measurements, Rietveld analysis showed that It is preferred that the crystal structure of the monoclinic O3' type and / or monoclinic O1(15) type is 35% or more. It is more preferable that the ratio is 40% or more, and even more preferable that the ratio is 43% or more.

[0342] In addition, when Rietveld analysis was performed in the same manner, the H1-3 and O1 crystal structures were more than 50%. Preferably, it is less than 34%. More preferably, it is less than 34%. It is more preferable that this not be the case.

[0343] The sharpness of the diffraction peaks in the XRD pattern indicates high crystallinity. Each of the subsequent diffraction peaks is preferably sharp, i.e., the half width, for example, the full width at half maximum, is preferably narrow. Even if the peaks originate from the same crystalline phase, they differ depending on the XRD measurement conditions and the 2θ value. Under the above measurement conditions, the peak observed at 2θ = 43° or more and 46° or less The full width at half maximum is preferably 0.2° or less, more preferably 0.15° or less, and more preferably 0.12° or less. It is more preferable that the peaks are 0°C or less. However, not all peaks necessarily meet this requirement. If some peaks satisfy this requirement, it can be said that the crystallinity of the crystalline phase is high. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0344] In addition, the crystallite size of the O3' type and monoclinic O1(15) crystal structures of the positive electrode active material 100 The noise is reduced to only about 1 / 20 of that of LiCoO2(O3) in the discharged state. Even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, x When x in CoO2 is small, The peaks of the crystal structure of O3' and / or monoclinic O1(15) are clearly visible. In conventional LiCoO2, some of the crystal structures resemble O3' and / or monoclinic O1(15). Even if a structure like this can be obtained, 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 according to one embodiment of the present invention, as described above, the influence of the Jahn-Teller effect As long as the influence of the Jahn-Teller effect is small, the Other additive elements may include transition metals such as nickel and manganese.

[0346] In the positive electrode active material, it was estimated using XRD analysis that the influence of the Jahn-Teller effect is small. The ratio of nickel and manganese used and the range of lattice constants are considered.

[0347] FIG. 18 shows a positive electrode active material 100 according to one embodiment of the present invention having a layered rock salt crystal structure and containing cobalt. The lattice constants of the a-axis and c-axis were calculated using XRD for the case of SiO2 and Ni. The results are shown in Fig. 18(A) for the a-axis and Fig. 18(B) for the c-axis. The XRD pattern used is the powder after the synthesis of the positive electrode active material, and The nickel concentration on the horizontal axis is calculated by taking the sum of the number of cobalt and nickel atoms as 100%. The nickel concentration is shown in the figure.

[0348] FIG. 18(C) shows the results of the lattice constants of the positive electrode active materials shown in FIGS. 18(A) and 18(B). The value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is shown.

[0349] From Figure 18(C), it can be seen that the a-axis / c-axis tends to change significantly when the nickel concentration is 5% and 7.5%. At a nickel concentration of 7.5%, the distortion of the a-axis is large. This may be due to the Jahn-Teller distortion of nickel at a nickel concentration of less than 7.5%. It is suggested that an excellent cathode active material with small Jahn-Teller distortion can be obtained at do.

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

[0351] From the above, a preferable range of the lattice constant was considered, and it was found that the positive electrode In the active material, the state without charging or discharging, which can be estimated from the XRD pattern, In the layered rock salt type crystal structure of the positive electrode active material 100 in this state, the lattice constant of the a-axis is 2. 814×10 -10 m, 2.817×10 -10 Smaller than m and c-axis lattice The constant is 14.05 x 10 -10 m, 14.07 × 10 -10 It is smaller than m The state in which no charge and discharge is performed is, for example, when preparing a positive electrode for a secondary battery. It may be in the form of a powder.

[0352] Alternatively, the layered structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state may be In the salt-type crystal structure, the lattice constant of the a-axis divided by the lattice constant of the c-axis (a-axis / c-axis) is It is preferably greater than 0.20000 and less than 0.20049.

[0353] Alternatively, the layered structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state may be In the salt-type crystalline structure, when XRD analysis was performed, 2θ was 18.50° or more and 19.30° The first peak is observed below 38.00° and the second peak is observed between 38.00° and 38.80°. Peaks may be observed.

[0354] XPS In the case of inorganic oxides, when monochromatic aluminum Kα rays are used as X-rays, the surface It is possible to analyze areas up to a depth of about 2 to 8 nm (usually 5 nm or less) from the surface. The concentration of each element is quantitatively analyzed in a region that is approximately half the depth of the surface layer 100a. Furthermore, narrow scan analysis allows us to analyze the bonding state of elements. Cut.

[0355] The positive electrode active material 100 according to one embodiment of the present invention has an internal concentration of one or more selected from the group consisting of additive elements. It is preferable that the surface layer 100a has a higher temperature than the surface layer 100b. The concentration of one or more additive elements selected from the additive elements in the positive electrode active material 100 is higher than the average of the entire positive electrode active material 100. Therefore, for example, the surface layer 100 measured by XPS or the like is preferably The concentration of one or more added elements selected from a is measured by ICP-MS, GD-MS, etc. is preferably higher than the average concentration of the added element in the entire positive electrode active material 100 measured by For example, it can be said that the microstructure of at least a part of the surface layer portion 100a measured by XPS or the like is The magnesium concentration is preferably higher than the magnesium concentration in the entire positive electrode active material 100. Furthermore, the nickel concentration in at least a part of the surface layer 100a is higher than that in the entire positive electrode active material 100. The concentration of aluminum in at least a part of the surface layer 100a is preferably higher than the concentration of nickel. It is preferable that the concentration of ammonium is higher than the aluminum concentration in the entire positive electrode active material 100. In addition, the fluorine concentration in at least a part of the surface layer portion 100a is higher than that in the entire positive electrode active material 100. It is preferable that the concentration is higher than that of the

[0356] The surface and surface layer 100a of the positive electrode active material 100 according to one embodiment of the present invention are Carbonates, hydroxyl groups, etc. that are chemically adsorbed after the preparation of the positive electrode active material 10 are not included. This does not include electrolyte, binder, conductive material, or compounds derived from these materials attached to the surface of the Therefore, when quantifying the elements contained in the positive electrode active material, surface analysis methods such as XPS are used. Corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc. that may be detected in the above. For example, XPS can analyze and separate the types of bonds, and C- Correction to exclude F bonds may be performed.

[0357] Furthermore, before being subjected to various analyses, the electrolyte, binder, conductive material, and Or, to remove compounds derived from these, samples of the positive electrode active material and the positive electrode active material layer are washed. In this case, lithium may be dissolved in the solvent used for washing, Even in this case, the added elements are difficult to dissolve, so the atomic ratio of the added elements is not affected. There is no such thing.

[0358] The concentration of the added element may also be compared in terms of its ratio to cobalt. This allows comparison by reducing the influence of carbonates and other substances chemically adsorbed after the positive electrode active material is produced. For example, the ratio of magnesium to cobalt atoms, Mg On the other hand, the ratio / Co is preferably 0.4 or more and 1.5 or less. The Mg / Co ratio is preferably 0.001 or more and 0.06 or less.

[0359] Similarly, the positive electrode active material 100 has a surface layer portion in order to secure a sufficient path for lithium insertion and desorption. In 100a, it is preferable that the concentrations of lithium and cobalt are higher than those of the additional elements. This is one or more additive elements selected from the additive elements contained in the surface layer portion 100a measured by XPS or the like. The concentrations of lithium and cobalt in the surface layer 100a are higher than the concentrations of the other additive elements. For example, it can be said that it is preferable that at least the surface layer portion 100a measured by XPS or the like The magnesium concentration of at least the surface layer 100a measured by XPS or the like is higher than that of the most part of the surface layer 100a. A high concentration of some cobalt is preferred. Similarly, a high concentration of lithium is preferred over a high concentration of magnesium. It is preferable that the concentration of cobalt is higher than that of nickel. Similarly, it is preferable that the concentration of lithium is higher than the concentration of nickel. It is preferable that the concentration of cobalt is higher than the concentration of aluminum. It is preferable that the concentration of lithium is higher than that of fluorine. Similarly, it is preferable that the concentration of lithium is higher than the concentration of fluorine. is preferred.

[0360] Furthermore, added elements such as aluminum are used in deep regions, for example, at a depth of 5 nm from the surface. It is more preferable that the diameter is widely distributed within a range of 50 nm or less. In the analysis of the entire positive electrode active material 100 using D-MS, etc., additives such as aluminum were Although elements are detected, the concentration does not match the results of analysis using XPS etc., which targets the surface at about 5 nm. They may be different.

[0361] Furthermore, when the positive electrode active material 100 according to one embodiment of the present invention was subjected to XPS analysis, the cobalt The number of magnesium atoms is preferably 0.4 to 1.2 times the number of electrons, and 0.65 It is more preferable that the number of nickel atoms is 1.0 times or more and 1.0 times or less. is preferably 0.15 times or less, and more preferably 0.03 times or more and 0.13 times or less. The number of aluminum atoms is preferably 0.12 times or less, and 0.09 times or less, relative to the number of aluminum atoms. The number of fluorine atoms is preferably 0.1 to 1.0 times the number of cobalt atoms. It is preferably 0.1 times or less, and more preferably 0.3 times or more and 0.9 times or less. The reason is that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100. It shows that the positive electrode active material 100 is widely distributed at a preferred concentration in the surface layer 100a. Yes.

[0362] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as X-rays. The take-off angle can be set to, for example, 45°. For example, measurements were taken using the following equipment and conditions: It is possible. Measurement equipment: PHI Quantera II X-ray: Monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: Approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: Wide scan, narrow scan for each detected element

[0363] Furthermore, when the positive electrode active material 100 according to one embodiment of the present invention was analyzed by XPS, fluorine and other elements were The peak showing the binding energy of is preferably 682 eV or more and less than 685 eV, It is more preferable that the bond energy of the lithium fluoride is about 684.3 eV. The binding energy of magnesium fluoride is 685 eV, and the binding energy of magnesium fluoride is 686 eV. The values ​​are different from each other.

[0364] Furthermore, when the positive electrode active material 100 according to one embodiment of the present invention was analyzed by XPS, magnesium The peak showing the binding energy of other elements is between 1302 eV and 1304 eV. It is preferable that the energy is about 1303 eV, and more preferable that the energy is about 1303 eV. The bond energy of magnesium oxide is 1305 eV, which is different from the bond energy of sodium. This is a value close to the energy.

[0365] EDX One or more additive elements selected from the positive electrode active material 100 have a concentration gradient. In addition, the positive electrode active material 100 has a concentration peak at a depth from the surface due to the added element. It is more preferable that the concentration gradient of the added element is positively measured by FIB or the like. A cross section of the electrode active material 100 is exposed and analyzed using EDX, EPMA, etc. It can be evaluated as follows.

[0366] Among EDX measurements, ED is a method of measuring while scanning an area and evaluating the area two-dimensionally. This is called X-plane analysis. It is also measured by scanning linearly, and the distribution of atomic concentration within the positive electrode active material is Evaluating this is called line analysis. Furthermore, data on linear areas can be extracted from EDX area analysis. Measurements made over an area without scanning are sometimes called line analysis. This is called point analysis.

[0367] By EDX surface analysis (for example, element mapping), the surface layer 100a and the inner layer 100b of the positive electrode active material 100 are The concentration of the added element in the portion 100b and the vicinity of the grain boundary 101 is quantitatively analyzed. In addition, EDX analysis can be used to analyze the concentration distribution and maximum value of added elements. In addition, analysis using thin-sectioned samples, such as STEM-EDX, allows for the observation of the depth direction. The depth from the surface to the center of the positive electrode active material in a specific region is not affected by the distribution of This is more preferable because it allows for analysis of the density distribution in the direction.

[0368] Therefore, the positive electrode active material 100 according to one embodiment of the present invention was subjected to EDX area analysis or EDX point analysis. When the surface layer 100a is heated, the concentration of the additive element such as magnesium becomes higher than that of the inner layer 100b. It is preferable that the temperature is also high.

[0369] For example, the positive electrode active material 100 containing magnesium as an additive element was subjected to EDX surface analysis or When EDX point analysis was performed, the magnesium concentration in the surface layer 100a was higher than that in the inner layer 100b. Furthermore, when EDX analysis was performed, the concentration of the surface layer 100a was The peak of magnesium concentration is located at a depth of 3 nm from the surface of the positive electrode active material 100 toward the center. It is preferable that the surface is present at a depth of 1 nm, and more preferable that the surface is present at a depth of 0. It is more preferable that the magnesium concentration is present within 5 nm. It is preferable that the peak is attenuated to 60% or less at a point from the peak top to a depth of 1 nm. It is preferable that the concentration be reduced to 30% or less of the peak at a point from the point of the concentration gradient to a depth of 2 nm. The peak refers to the maximum value of the concentration.

[0370] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the amount of fluorine The distribution preferably overlaps with the magnesium distribution. For example, the fluorine concentration peak and The difference in the depth direction of the magnesium concentration peak is preferably within 10 nm, and more preferably within 3 nm. It is more preferable that the thickness is within 1 nm, and it is even more preferable that the thickness is within 1 nm.

[0371] In addition, when EDX analysis was performed, the peak of the fluorine concentration in the surface layer 100a was It is preferable that the surface of the pores is located at a depth of 3 nm from the surface to the center, and the depth of 1 nm from the surface to the center. It is more preferable that the surface roughness is present in the range of 0.5 nm to 100 nm, and it is even more preferable that the surface roughness is present in the range of 0.5 nm to 100 nm. In addition, the fluorine concentration peak is located slightly closer to the surface than the magnesium concentration peak. For example, the peak of the fluorine concentration is It is more preferable that the thickness is 0.5 nm or more on the surface side from the peak of the concentration of the silicon, and 1.5 nm or more on the surface side. It is more preferable that the surface side be the surface side.

[0372] In addition, in the positive electrode active material 100 containing nickel as an additive element, the nickel in the surface layer portion 100a The peak of the concentration of ions is present within a depth of 3 nm from the surface of the positive electrode active material 100 toward the center. It is preferable that the thickness of the pores is within 1 nm, and more preferable that the thickness of the pores is within 0.5 nm. In addition, it is more preferable that the positive electrode active material containing magnesium and nickel is present in the range of 100 to 1500 nm. In 100, the distribution of nickel preferably overlaps with the distribution of magnesium. The difference in depth between the nickel concentration peak and the magnesium concentration peak is within 10 nm. It is preferable that the thickness is within 3 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0373] In addition, when the positive electrode active material 100 contains aluminum as an added element, EDX analysis is performed. When the aluminum concentration peak is higher in the surface layer 100a than in the magnesium and nickel Alternatively, it is preferable that the peak of the fluorine concentration is close to the surface. The cracks are present at a depth of 0.5 nm to 50 nm from the surface to the center of the positive electrode active material 100. It is preferable that the pores are present at a depth of 5 nm or more and 50 nm or less.

[0374] In addition, when EDX analysis, area analysis, or point analysis was performed on the positive electrode active material 100, magnesium The ratio of magnesium Mg to cobalt Co atoms at the peak of magnesium concentration (Mg / Co) is less than 1, and is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.4 or less. It is more preferable that the aluminum concentration peak is aluminum Al and the cobalt Co. The atomic ratio (Al / Co) is less than 1, and is preferably 0.05 or more and 0.6 or less, More preferably, the ratio of nickel Ni at the peak of nickel concentration is 0.1 or more and 0.45 or less. The ratio of the number of atoms to the number of atoms of cobalt (Ni / Co) is less than 1, and is between 0 and 0.2. Preferably, the Ni / Co ratio is 0.01 or more and 0.1 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of cobalt Co to nickel Ni is preferably 0.5 or less. Co:Ni=90:10, Co:Ni=80:20, Co:Ni=70:30, or these It is preferable that the ratio of fluorine F at the peak of the fluorine concentration is between The ratio of the number of atoms to the number of atoms (F / Co) is less than 1, preferably 0 or more and 1.6 or less, and more preferably 0.1 More preferably, it is 1.4 or less.

[0375] The surface of the positive electrode active material 100 in the EDX analysis results is estimated as follows, for example: The elements that are uniformly present in the interior 100b of the positive electrode active material 100, such as acids, For silicon or cobalt, the point where the detected amount is half that of the inside 100b 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, the oxygen concentration is measured from the region where the amount of detected oxygen in the interior 100b is stable. Average value of O ave At this time, chemical adsorption or Oxygen O is thought to be due to background bg If O is detected, the measurement b g The average oxygen concentration is calculated by subtracting ave This average value O ave 1 / 2 of The value of O ave The measurement point that showed the measurement value closest to / 2 was selected as the surface of the positive electrode active material. It can be estimated that:

[0377] The surface can also be estimated in the same way as above using the amount of cobalt detected. It can also be estimated in the same way by using the sum of the detected amounts of the transition metals. The amount of detected transition metals is suitable for estimating the surface because it is less susceptible to chemical adsorption.

[0378] Furthermore, when the positive electrode active material 100 is subjected to a line analysis or an area analysis, the positive electrode active material 100 is The ratio of the number of atoms of the added element A to the number of atoms of cobalt Co (A / Co) is 0.020 or more and 0. It is preferably 0.50 or less. It is more preferably 0.025 or more and 0.30 or less. It is further preferably 0.0 Preferably, it is 0.30 or more and 0.20 or less. Or, it is 0.020 or more and 0.30 or less. Or, it is 0. Preferably, it is 0.020 or more and 0.20 or less, or 0.025 or more and 0.50 or less. is preferably 0.025 or more and 0.20 or less, or 0.030 or more and 0.50 or less. Alternatively, it is preferably 0.030 or more and 0.30 or less.

[0379] For example, when the added element is magnesium, the positive electrode active material 100 is subjected to line analysis or area analysis. When the grain boundary is 101, the ratio of the number of magnesium atoms to the number of cobalt atoms (M g / Co) is preferably 0.020 or more and 0.50 or less, and more preferably 0.025 or more and 0.3 It is preferably 0 or less. It is further preferably 0.030 or more and 0.20 or less. Preferably, it is 0.020 or more and 0.20 or less. Preferably, it is 0.5 or more and 0.50 or less, or 0.025 or more and 0.20 or less, or 0. Preferably, the ratio is 0.030 or more and 0.50 or less, or 0.030 or more and 0.30 or less. When the positive electrode active material 100 has a plurality of portions, for example, three or more portions, within the above range, the additive element The positive electrode active material 100 is not attached to a narrow area on the surface thereof, but is attached to the surface layer of the positive electrode active material 100. This indicates that the concentration is widely distributed in the range of 100a.

[0380] EPMA EPMA can also quantify elements. Area analysis allows for the distribution of each element to be analyzed. .

[0381] When the cross section of the positive electrode active material 100 according to one embodiment of the present invention was analyzed by EPMA, As with the analysis results, one or more selected from the added elements have a concentration gradient. It is also 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 added 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. The quantitative values ​​of the positive electrode active material 10 may differ from the measurement results obtained using other analytical methods. When the surface of the sample was analyzed by EPMA, the concentrations of the additive elements present in the surface layer 100a were as follows: May be lower than the XPS results.

[0383] <Raman spectroscopy> As described above, the positive electrode active material 100 according to one embodiment of the present invention has at least one surface layer 100a. It is preferable that the positive electrode active material 100 and the When the positive electrode having the layered rock salt was analyzed by Raman spectroscopy, the crystal structure of the layered rock salt was also found. It is preferable that the cubic crystal structure, which is the target of the crystal structure, is also observed. In the M image and the ultrafine electron diffraction pattern, lithium is observed with a certain frequency in the depth direction during observation. Without the cobalt substituted at the position and the cobalt at the oxygen tetracoordinate position, HAAD It cannot be detected as a bright spot in the F-STEM image or the electron microbeam diffraction pattern. Since Raman spectroscopy is an analysis that captures the vibrational modes of bonds such as Co-O, Even if the amount of Co-O bonds present is small, the wavenumber peak of the corresponding vibrational mode can be observed. Furthermore, Raman spectroscopy can measure the surface area of ​​a few μm 2 , within a depth of about 1 μm Since it is possible to measure the amount of carbon that exists only on the particle surface, it is possible to capture the state of carbon that exists only on the particle surface with high sensitivity.

[0384] For example, when the laser wavelength is 532 nm, the layered rock salt LiCoO2 has a wavelength of 470 cm -1 No To 490 cm -1 , 580 cm -1 and even up to 600 cm -1 has a peak (vibration mode: E g , A 1g ). On the other hand, in cubic CoO x (0 < x < 1) (rock-salt type Co 1-y O(0 < y < 1) or spinel type Co3O4), a peak is observed at 665 cm -1 and even up to 685 cm -1 at a peak (vibration mode: A 1g ).

[0385] Therefore, among the integrated intensities of each peak, 470 cm[[ID=__]]<__> -1 to 490 cm -1 [[ID=__]]<__>is defined as I1, 58 0 cm -1 to 600 cm -1 is defined as I2, 665 cm -1 to 685 cm -1 is defined as I3. When this is the case, it is preferable that the value of I3 / I2 is 1% or more and 10% or less, and more preferably 3% or more and 9% or less.

[0386] If a cubic crystal structure including a rock-salt type is observed within the above range, it can be said that the surface layer portion 100a of 100 of the positive electrode active material has a rock-salt type crystal structure within a preferable range. <__><__>≪Ultra-micro electron beam diffraction pattern≫ Similar to Raman spectroscopy, it is preferable that the characteristics of the rock-salt type crystal structure are also observed in the ultra-micro electron beam diffraction pattern together with the crystal structure of the layered rock-salt. However, in the STEM image and the ultra-micro electron beam diffraction pattern , taking into account the above-mentioned difference in sensitivity, it is preferable that the characteristics of the rock-salt type crystal structure do not become too strong at the surface layer portion 100a, especially at the outermost surface (for example a depth of 1 nm from the surface). ​Rather than the outermost surface being covered with a rock salt type crystal structure, it is preferable to keep the layered rock salt type crystal structure and then apply it to the surface. The presence of an additive element such as magnesium in the lithium layer ensures a diffusion path for lithium, In addition, the function of stabilizing the crystal structure becomes stronger.

[0388] Therefore, for example, the ultrafine electron diffraction pattern of the region below 1 nm in depth from the surface and the region below 3 nm in depth are When the ultrafine electron diffraction pattern of the region from 1000 nm to 1000 nm is obtained, It is preferable that the difference in the calculated lattice constants is small.

[0389] For example, there are measurement points with a depth of 1 nm or less from the surface and measurement points with a depth of 3 nm to 10 nm. The difference in lattice constant calculated from the above is preferably 0.01 nm or less for the a-axis, and It is preferable that the thickness is 0.1 nm or less for the a-axis. It is more preferable that the c-axis is 0.06 nm or less. It is more preferable that the thickness is 0.004 nm or less for the c-axis, and 0.03 nm or less for the c-axis. and even more preferable.

[0390] <Additional Features> The positive electrode active material 100 may have recesses, cracks, dents, V-shaped cross sections, etc. These are defects, and repeated charging and discharging causes cobalt to leach out of them, causing the crystal structure to collapse. However, cracks in the positive electrode active material 100 and oxygen desorption may occur. If the buried portion 102 shown in FIG. 7(H) exists, the elution of cobalt and the like may occur. Therefore, the reliability and durability of the secondary battery using the positive electrode active material 100 can be improved. The cycle characteristics can be improved.

[0391] As described above, if the additive element contained in the positive electrode active material 100 is in excess, lithium insertion and Furthermore, when the positive electrode active material 100 is used in a secondary battery, On the other hand, if the surface layer 100 is insufficient, it may cause an increase in the resistance of the surface layer 100 and a decrease in the charge / discharge capacity. If the crystal structure is not distributed throughout the entire crystal, the effect of suppressing deterioration of the crystal structure may be insufficient. In addition, the additive element must be at an appropriate concentration in the positive electrode active material 100, but this adjustment is easy. It's not easy.

[0392] Therefore, if the positive electrode active material 100 has a region where the additive element is unevenly distributed, excessive additive element A part of the atoms of the element is removed from the interior 100b of the positive electrode active material 100, and the atoms of the element are properly removed from the interior 100b. This allows for an appropriate concentration of added elements, which reduces the increase in internal resistance when used as a secondary battery. Therefore, it is possible to suppress a decrease in charge / discharge capacity, etc. It is possible to suppress an increase in the internal resistance of the secondary battery. This is particularly advantageous in charging and discharging at a large current, for example, 400 mA / g or more. This is a desirable characteristic.

[0393] In addition, in the positive electrode active material 100 having a region where the additive element is unevenly distributed, It is permissible to add some excess elements to the alloy. This is preferable as it becomes wider.

[0394] Furthermore, a coating portion may be attached to at least a portion of the surface of the positive electrode active material 100. 9(A) and 19(B) show the positive electrode active materials shown in FIGS. 7(G) and 7(H), respectively. 100 shows the structure to which the covering portion 104 is attached.

[0395] Decomposition products of lithium salt, organic electrolyte solution, etc. accumulate on the coating portion 104 during charging and discharging, for example. It is preferable that the Li x x in CoO2 is 0.24 or less When such charging is repeated, the surface of the positive electrode active material 100 has a coating portion derived from the organic electrolyte solution. This is expected to improve the charge-discharge cycle characteristics. This is because it is necessary to suppress the increase in impedance or to suppress the elution of cobalt. 104 preferably contains, for example, carbon, oxygen, and fluorine. When OB and / or SUN (suberonitrile) are used, a high-quality coating can be obtained. Therefore, the substrate containing one or more selected from boron, nitrogen, sulfur and fluorine is The covering portion 104 is preferably a high-quality covering portion. It is not necessary to cover the entire surface of the positive electrode active material 100. For example, 50% or more of the surface of the positive electrode active material 100 may be covered. It is sufficient if the surface is covered by 70% or more, and more preferably by 90% or more. In the area where the coating portion 104 is not present, even if fluorine is adsorbed on the surface of the positive electrode active material 100, good.

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

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

[0398] Positive alloys having the distribution, composition, and / or crystal structure of the additive elements described in the previous embodiments In order to prepare the electrode active material 100, the method of adding the additive element is important. It is also important that the crystallinity of b is good.

[0399] In the process of producing the positive electrode active material 100, lithium cobalt oxide is synthesized, and then the additive element source is added. There is also a method in which the cobalt source and the lithium source are mixed together and then subjected to heat treatment. A method of synthesizing lithium cobalt oxide containing an additive element by mixing the sources may also be used. In addition to mixing the lithium cobalt oxide and the additive element source, the additive element is added by heating. It is preferable that the additive element is well distributed in the lithium cobalt oxide. Therefore, the heat treatment after mixing the additive element source is The heat treatment after mixing the additive element sources is sometimes called firing or annealing. .

[0400] However, if the heating temperature is too high, cation mixing occurs, and the added elements, e.g., magnesium, The possibility of magnesium entering the cobalt site increases. Li x When x in CoO2 is small, the effect of maintaining the layered rocksalt crystal structure of R-3m Furthermore, if the heat treatment temperature is too high, cobalt will be reduced to divalent. There are also concerns about adverse effects such as lithium evaporating.

[0401] Therefore, a material that functions as a flux may be mixed together with or as a source of additive elements. As a material that functions as a flux, a material having a melting point lower than that of lithium cobalt oxide is preferable. The flux may be a fluorine compound such as lithium fluoride. By adding a flux, the melting point of the added element source and lithium cobalt oxide is lowered. By lowering the melting point, the added elements can be mixed well at a temperature where cation mixing is unlikely to occur. It becomes easy to distribute.

[0402] [Initial heating] Furthermore, it is more preferable to heat the lithium cobalt oxide after synthesis and before mixing it with the additive elements. This heating is sometimes called initial heating.

[0403] The initial heating causes lithium to be desorbed from a part of the surface layer 100a of the lithium cobalt oxide. This results in a better distribution of the added elements.

[0404] More specifically, the initial heating makes it easier for the distribution to differ depending on the added elements through the following mechanism: First, lithium is released from a part of the surface layer portion 100a due to the initial heating. Next, the lithium cobalt oxide having the lithium-deficient surface layer 100a and the nickel Sources of additive elements, including a Kel source, an aluminum source, and a magnesium source, are mixed and heated. Of the added elements, magnesium is a divalent typical element, and nickel is a transition metal but is divalent. Therefore, Mg ions are easily formed in a part of the surface layer 100a. 2+ and Ni 2+ And, Co reduced by deficiency of lithium 2+ A rock salt phase is formed having the following structure: This phase is formed in a part of the surface layer 100a, so it is difficult to see it in an electron microscope image such as a STEM. In some cases, it may not be clearly visible in the electron diffraction pattern.

[0405] Among the additive elements, nickel is used when the surface layer 100a is a layered rock salt type lithium cobalt oxide. However, if a part of the surface layer 100a is of the rock salt type, Therefore, by performing initial heating, nickel and other metals tend to remain in the area 100a. The divalent additive element can be easily retained in the surface layer portion 100a. In particular, the surface other than the (001) oriented surface of the positive electrode active material 100 and the surface layer 100a thereof big.

[0406] In addition, in these rock salt structures, the bond distance between metal Me and oxygen (Me-O distance) is longer than in the layered rock salt structure. also tends to be longer.

[0407] For example, rock salt Ni 0.5 Mg 0.5 The Me-O distance in O is 2.09 × 10 -10 m , the Me-O distance in rocksalt MgO is 2.11×10 -10 m. Also, if the surface Even if a spinel-type phase is formed in part of 100a, the M of spinel-type NiAl2O4 The eO distance is 2.0125×10 -10 m, and the Me-O distance in spinel-type MgAl2O4 is 2.02×10 -10 m. In both cases, the Me-O distance is 2×10 -10 More than m.

[0408] On the other hand, in the layered rock salt structure, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, The Al-O distance in layered rocksalt LiAlO2 is 1.905×10 -10 m(Li-O distance The distance is 2.11 x 10 -10 m), and the Co-O distance in layered rock salt LiCoO2 The distance is 1.9224 x 10 -10 m (Li-O distance is 2.0916 × 10 -10 m) .

[0409] According to the Shannon ionic radius described in Non-Patent Document 15, the hexacoordinated aluminum The ionic radius of is 0.535×10 -10 m, the ionic radius of hexacoordinated oxygen is 1.4 × 10 - 10m, and the sum of these is 1.935×10 -10 m.

[0410] From the above, aluminum is more stable at the non-lithium sites in the layered rocksalt structure than in the rocksalt structure. Therefore, aluminum is considered to be present in the rock salt phase even in the surface layer portion 100a. The deeper regions with layered rock salt type and / or the inner 10 It tends to be distributed in 0b.

[0411] It is also expected that the initial heating will enhance the crystallinity of the layered rock salt-type crystal structure of the inner 100b. can.

[0412] Therefore, especially Li x For example, when x in CoO2 is between 0.15 and 0.17, monoclinic To produce a positive electrode active material 100 having an O1(15) type crystal structure, this initial heating is performed. It is preferable to do so.

[0413] However, initial heating is not necessarily required. In other heating steps, the atmosphere, temperature, By controlling the time, etc., Li x When x in CoO2 is small, O3' type and / or simple There are cases where it is possible to produce a positive electrode active material 100 having a clinic O1(15) type.

[0414] <<Method 1 for preparing positive electrode active material>> Regarding the manufacturing method 1 of the positive electrode active material 100 that undergoes initial heating, FIGS. 21(A) to 22(C) are shown. This will be explained using:

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

[0416] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate. Lithium hydroxide, lithium nitrate, lithium fluoride, etc. can be used. The hydrogen source preferably has a high purity, for example, a material with a purity of 99.99% or higher.

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

[0418] The cobalt source preferably has a high purity, for example, a purity of 3N (99.9%) or more, preferably 4N (99.99%) or more, more preferably 4N5 (99.995%) or more, and even more preferably It is advisable to use materials with a purity of 5N (99.999%) or higher. As a result, the capacity of the secondary battery is increased, and the amount of impurities in the positive electrode active material can be controlled. and / or the reliability of the secondary battery is improved.

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

[0420] <Step S12> Next, in step S12 shown in FIG. 21(A), the lithium source and the cobalt source are crushed and The mixture is mixed to produce a mixed material. The grinding and mixing can be carried out in a dry or wet manner. The wet method is preferable because it can be crushed into smaller particles. The solvents are ketones such as acetone, alcohols such as ethanol and isopropanol. , ether, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. It is possible to use an aprotic solvent that does not react easily with lithium. In this embodiment, it is more preferable to use dehydrated acetone with a purity of 99.5% or more. The water content is reduced to 10 ppm or less, and the purity is 99.5% or more. It is preferable to mix the lithium source and the cobalt source, and then grind and mix them. By using high-purity dehydrated acetone, it is possible to reduce impurities that may be present.

[0421] A ball mill, a bead mill, or the like can be used as a means for pulverizing and mixing. When using a mill, use aluminum oxide balls or zirconium oxide balls as grinding media. It is recommended to use balls. Zirconium oxide balls are preferable because they emit less impurities. When using a ball mill or bead mill, contamination from the media can be suppressed. To control this, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less. In this case, the peripheral speed was 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm) and implement it.

[0422] <Step S13> Next, in step S13 shown in FIG. 21(A), the mixed material is heated. It is preferable to carry out the heating at a temperature of 00°C or higher and 1100°C or lower, and it is preferable to carry out the heating at a temperature of 900°C or higher and 1000°C or lower. If the temperature is too low, the lithium source and If the temperature is too high, the decomposition and melting of the cobalt source may be insufficient. This is due to lithium evaporation from the lithium source and / or excessive reduction of cobalt. For example, cobalt may change from trivalent to divalent, resulting in oxygen defects. It may be induced.

[0423] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time should be between 1 hour and 100 hours, and between 2 hours and 20 hours. It is more preferable to set the following.

[0424] The temperature rise rate depends on the heating temperature reached, but is preferably between 80°C / h and 250°C / h. For example, when heating at 1000°C for 10 hours, the temperature rise rate should be 200°C / h.

[0425] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, with a dew point of -50°C or less. An atmosphere with a dew point of -80°C or less is preferable. Heating is carried out in an atmosphere of -93°C. In addition, to suppress impurities that may be mixed into the material, In order to achieve this, the impurity concentrations of CH4, CO, CO2, and H2 in the heated atmosphere must be It is recommended that each be kept below 5 ppb (parts per billion).

[0426] The heating atmosphere is preferably an oxygen-containing atmosphere. For example, dry air is introduced into the reaction chamber. In this case, the flow rate of dry air is preferably 10 L / min. The oxygen is continuously introduced into the reaction chamber and the way in which the oxygen flows through 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, The reaction chamber is depressurized and then filled with oxygen (or purged), and the oxygen is removed from the reaction chamber. For example, the reaction chamber is depressurized to -970 hPa, and then 5 Just fill with oxygen up to 0 hPa.

[0428] After heating, the product can be cooled naturally, but the time required to cool the product from the specified temperature to room temperature must be 10 hours or more. For example, the temperature drop rate (hereinafter also referred to as the cooling rate) is 80°C / h The temperature is preferably 180°C / h or higher and 210°C / h or lower, and more preferably 180°C / h or higher and 210°C / h or lower. However, cooling to room temperature is not necessary, as long as it is cooled to a temperature acceptable for the next step. stomach.

[0429] The heating in this step may be performed using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln involves stirring the material, whether it is a continuous or batch type. It is possible.

[0430] The crucible used for heating is preferably an aluminum oxide crucible. The crucible is made of a material that does not easily release impurities. In this embodiment, the purity is 99.9%. Use an aluminum oxide crucible. It is preferable to heat the crucible with a lid on. The lid is placed to prevent evaporation or sublimation of the solvent during the temperature increase and decrease in this step. In this case, it is sufficient to prevent the evaporation or sublimation of the material, and it is not necessary to seal the crucible with a lid. For example, as described above, the reaction chamber can be filled with oxygen to seal the crucible. It is also possible to perform this step without closing the

[0431] It is also preferable to use a second-hand crucible rather than a new one. A new crucible is one that contains materials containing lithium, transition metals M, and / or additive elements. A used crucible is one that has been heated twice or less. The process of adding materials containing tungsten, transition metal M and / or additive elements and heating them was carried out three or more times. This means that when a new crucible is used, lithium fluoride is This is because some of the materials, including the sheath, may be absorbed, diffused, migrated, and / or adhere to the sheath. When a part of the material is lost due to these reasons, the distribution of elements in the surface layer of the positive electrode active material becomes unfavorable. This raises concerns that the temperature may not be within the correct range. However, this risk is less with used crucibles.

[0432] After heating, the material may be crushed and sieved as necessary. When recovering the powder, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use an aluminum oxide mortar or a zirconium oxide mortar. The mortar for aluminum is made of a material that does not easily release impurities. Specifically, the purity is preferably 90% or more. A mortar made of aluminum oxide with a purity of 99% or more is used. In the heating step described below, the same heating conditions as in step S13 can be applied.

[0433] <Step S14> By the above steps, lithium cobalt oxide (LiCo O2) can be synthesized. The particle size of lithium cobalt oxide is the median diameter (D50 ) to obtain a positive electrode active material 100 with a relatively small median diameter (D50) , it is advisable to crush lithium cobalt oxide.

[0434] As shown in steps S11 to S14, an example of producing a composite oxide by a solid phase method is shown. However, the composite oxide may be prepared by a coprecipitation method or a hydrothermal method.

[0435] <Step S15> Next, in step S15 shown in FIG. 21(A), the lithium cobalt oxide is heated. Since this is the first heating for lithium phosphate, the heating in step S15 is called initial heating. Or, since it is heated before step S20 described below, it is not necessary to preheat or preheat. The crucible and / or lid used in this step are removed in step S13. The following effects are expected from the initial heating, which is one aspect of the present invention. Initial heating is not essential to obtain a positive electrode active material.

[0436] As described above, the initial heating causes lithium to be released from a part of the surface layer 100a of the lithium cobalt oxide. In addition, the crystallinity of the inner portion 100b can be improved. The lithium and / or cobalt sources prepared in step 1 may contain impurities. The reduction of impurities from the lithium cobalt oxide completed in step S14 is the initial process. This is possible with heat.

[0437] Furthermore, the initial heating has the effect of smoothing the surface of the lithium cobalt oxide. The surface of the composite oxide is smooth when it has few irregularities, is rounded overall, and is Furthermore, the condition where there is little foreign matter adhering to the surface is called smooth. Foreign matter is considered to be a cause of unevenness, so it is preferable that it does not adhere to the surface.

[0438] For this initial heating, it is not necessary to prepare a lithium compound source or an additive element source. Alternatively, it is not necessary to provide a material that functions as a flux.

[0439] If the heating time in this process is too short, sufficient effect will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. Regarding the heat conditions, the heating temperature in this process is set to maintain the crystal structure of the composite oxide. The heating time in this step is preferably set lower than that in Step S13. To maintain the temperature, the time should be shorter than that of step S13. It is recommended to heat at a temperature of 0°C or below for 2 to 20 hours.

[0440] The effect of increasing the crystallinity of the inner portion 100b is, for example, the effect of increasing the crystallinity of the inner portion 100b by using cobalt produced in step S13. This is the effect of alleviating distortion, displacement, etc. resulting from differential shrinkage, etc., which lithium oxide has.

[0441] The lithium cobalt oxide is heated in step S13 to form a layer on the surface and inside of the lithium cobalt oxide. Temperature differences may occur in the area. Temperature differences may induce differential shrinkage. It is thought that the difference in fluidity between the surface and the interior may cause the difference in shrinkage. The associated energy gives the lithium cobalt oxide a difference in internal stress. The difference is also called strain, and the energy is sometimes called strain energy. In other words, the strain energy is removed by the initial heating in step S15. It is thought that the initial heating causes homogenization. When the strain energy is homogenized, the cobalt The distortion of the lithium cobalt oxide is alleviated. As a result, the surface of the lithium cobalt oxide becomes smooth. This may be referred to as an improved surface. In other words, after step S15, It is thought that this will alleviate the difference in contraction that occurs in the lithium cobalt oxide, making the surface of the composite oxide smooth. can be obtained.

[0442] The difference in shrinkage may cause microscopic deviations in the lithium cobalt oxide, such as deviations in crystal structure. In order to reduce this deviation, it is advisable to carry out this process. It is possible to equalize the deviation of the composite oxide. The surface of the object may become smooth. This is also called grain alignment. After step S15, the deviation of the crystals in the composite oxide is alleviated, and the composite oxide The surface is expected to be smooth.

[0443] When lithium cobalt oxide, which has a smooth surface, is used as the positive electrode active material, it can be used as a secondary battery. This reduces deterioration during charging and prevents cracking of the positive electrode active material.

[0444] In step S14, pre-synthesized lithium cobalt oxide may be used. In this case, steps S11 to S13 can be omitted. By performing step S15 on the lithium cobalt oxide thus formed, the surface becomes smooth. Therefore, lithium cobalt oxide can be obtained.

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

[0446] <Steps S21 to S23> 21(B) and 21(C), the step of preparing the additive element A source (A source) will be described. A lithium source may be prepared together with the additive element A source.

[0447] The additive element A is the additive element described in the previous embodiment, for example, magnesium, fluorine, etc. , nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium , niobium, arsenic, zinc, silicon, sulfur, phosphorus and boron. One or two selected from bromine and beryllium can also be used. .

[0448] <Step S21> Step S21 shown in Fig. 21(B) will be described. When magnesium is selected as the additive element, In this case, the source of the added element can be called a magnesium source (Mg source). Examples include magnesium fluoride, magnesium oxide, magnesium hydroxide, and magnesium carbonate. In addition, 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). The fluorine source may be, for example, lithium fluoride (LiF), magnesium fluoride (MgF ), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (C oF2, CoF3), nickel fluoride (NiF2), zirconium fluoride (ZrF4), Vanadium fluoride (VF5), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride , zinc fluoride (ZnF2), calcium fluoride (CaF2), sodium fluoride (NaF ), potassium fluoride (KF), barium fluoride (BaF2), cerium fluoride (CeF3 , CeF4), lanthanum fluoride (LaF3), or sodium aluminum hexafluoride (N Among them, lithium fluoride has a melting point of 848°C, This is preferable because it is relatively low and easily melts in the heating step described below.

[0450] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can be used as both a fluorine source and a lithium source. Another lithium source used in S21 is lithium carbonate.

[0451] The fluorine source may also be a gas, such as fluorine (F2), fluorocarbon, sulfur fluoride, or fluoroacid. Using elements (OF2, O2F2, O3F2, O4F2, O5F2, O6F2, O2F), etc. The fluorine sources may be mixed in the atmosphere during the heating step described below. It's fine.

[0452] In this embodiment, lithium fluoride (LiF) is prepared as a fluorine source. Prepare magnesium fluoride (MgF2) as a magnesium source. When magnesium chloride is mixed in a ratio of LiF:MgF2 = 65:35 (molar ratio), the melting point On the other hand, if the amount of lithium fluoride is too high, the lithium will be excessive. There is a concern that the cycle characteristics may deteriorate over time. The molar ratio of MgF to LiF is preferably LiF:MgF = x:1 (0 ≤ x ≤ 1.9), LiF:MgF2=x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF2= More preferably, x:1 (x=0.33 or its vicinity). is a value greater than 0.9 times and less than 1.1 times that value.

[0453] <Step S22> Next, in step S22 shown in FIG. 21(B), the magnesium source and the fluorine source are crushed and This step is carried out under the conditions selected from the grinding and mixing conditions described in step S12. It is possible.

[0454] <Step S23> Next, in step S23 shown in FIG. 21(B), the crushed and mixed material is collected and The additive element A source (A source) can be obtained. The additive element A source shown in step S23 is , which have multiple starting materials and can be called a mixture.

[0455] The particle size of the above mixture must be a median diameter (D50) of 600 nm or more and 10 μm or less. The thickness is preferably 1 μm or more and more preferably 5 μm or less. Even when using the above material, the median diameter (D50) is 600 nm or more and 10 μm or less. It is preferable that the thickness is 1 μm or more and 5 μm or less, and more preferable that the thickness is 1 μm or more and 5 μm or less.

[0456] Such a finely powdered mixture (including the case where only one type of added element is added) can be easily processed in the subsequent process. When mixed with lithium cobalt oxide, the mixture is evenly distributed on the surface of the lithium cobalt oxide particles. If the mixture is evenly adhered to the surface of the lithium cobalt oxide particles, the This is preferred because it is easy to distribute or diffuse the additive elements uniformly in the surface layer 100a of the composite oxide after heating. It's nice.

[0457] <Step S21> A process different from that shown in FIG. 21(B) will be explained using FIG. 21(C). In step S21 shown in FIG. 1, four kinds of additive element sources to be added to lithium cobalt oxide are prepared. That is, Fig. 21(C) differs from Fig. 21(B) in the type of additive element source. The lithium source may be prepared in this manner.

[0458] The four additive element sources are magnesium source (Mg source), fluorine source (F source), nickel source ( Ni source, and aluminum source (Al source). The nickel source can be selected from the compounds described in FIG. 21(B). Nickel oxide, nickel hydroxide, etc. can be used as the aluminum source. Aluminum, aluminum hydroxide, etc. can be used.

[0459] <Steps S22 and S23> Steps S22 and S23 shown in FIG. 21(C) are the same as the steps described in FIG. 21(B). It is similar to the top.

[0460] <Step S31> Next, in step S31 shown in FIG. 21(A), lithium cobalt oxide and a source of additive element A ( The number of cobalt atoms in lithium cobalt oxide (Co) and the number of added elements (A source) are mixed. The ratio of the number of magnesium atoms to Mg is Co:Mg=100:y (0.1≦y≦6) It is preferable that Co:Mg=100:y (0.3≦y≦3). I wish.

[0461] The mixing in step S31 is carried out in order not to destroy the shape of the lithium cobalt oxide particles. It is preferable to use milder conditions than those in step S12. For example, It is preferable to use conditions of a lower rotation speed or a shorter time than in the case of the wet method. It can be said that the formula is a milder condition. For mixing, a ball mill, a bead mill, etc. When using a ball mill, for example, zirconium oxide ball mills can be used as the media. It is preferable to use a fluorine-containing ...

[0462] In this embodiment, a ball mill using zirconium oxide balls with a diameter of 1 mm is used. The mixture shall be mixed at 100°C for 1 hour at a dew point of -100°C or higher. This should be done in a dry room at a temperature below 10°C.

[0463] <Step S32> Next, in step S32 of FIG. 21(A), the mixed materials are collected and mixed 9 03 is obtained. When recovering, the powder may be crushed and then sieved if necessary.

[0464] 21(A) to 21(C) show a manufacturing method in which an additive element is added after initial heating. However, the present invention is not limited to the above method. The added element may be added at other timings. The timing may be changed depending on the additive element. That's fine.

[0465] For example, as shown in FIG. 22(A) to FIG. 22(C), in step S11, that is, The additive elements may be added to the lithium source and the cobalt source at the stage of the oxide starting material. 22(A) shows the flow of adding a magnesium source to a lithium source and a cobalt source. In FIG. 22(B), the magnesium source and aluminum source are replaced with the lithium source and cobalt source. In FIG. 22(C), a magnesium source and a nickel source are added to a lithium The flow of adding the aluminum source and the cobalt source shown in Figures 22(A) to 22(C) is shown. The sources of additional elements are exemplary.

[0466] Then, the process proceeds to step S12, and then step S13, and in step S14, the additive element is added. Depending on the timing of adding the additive element, lithium cobalt oxide can be obtained. It is also possible to control the distribution of the added elements. The added element is expected to be located inside the positive electrode active material 100. 22(C), the above-mentioned steps S11 to S14 Since there is no need to separate the steps S21 to S23, production is simple. Of course, the flow shown in Fig. 22(A) to Fig. 22(C) Alternatively, a new additive element may be added in step S20.

[0467] Alternatively, lithium cobalt oxide containing some of the additive elements may be used. If magnesium and fluorine-doped lithium cobalt oxide is used, steps S11 to S16 can be performed. It is possible to omit some of the steps S14 and S20. It can be said that this is a highly effective method.

[0468] In addition, lithium cobalt oxide to which magnesium and fluorine have been added in advance was After the heating in step S15, a magnesium source and a fluorine source are added in step S20. Alternatively, a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added.

[0469] <Step S33> Next, in step S33 shown in Figure 21(A), the mixture 903 is heated. The heating conditions can be selected from those described in step S13. The heating time is 2 hours. At this time, in order to increase the oxygen partial pressure in the heating atmosphere, the pressure inside the furnace must be higher than atmospheric pressure. If the oxygen partial pressure in the heating atmosphere is insufficient, cobalt and other elements will be reduced, resulting in cobalt This is because there is a risk that lithium valence oxide or the like will no longer be able to maintain its layered rock salt type crystal structure.

[0470] Here, we will add a supplementary note about the heating temperature. The lower limit of the heating temperature in step S33 is The temperature must be higher than the temperature at which the reaction between the aluminum and the added element source proceeds. The temperature may be any temperature at which mutual diffusion between lithium phosphate and the element contained in the additive element source occurs. The melting temperature of the oxide may be lower than the melting temperature T m of 0.757 times (Tanman temperature T d ) solid-state diffusion occurs. The heating temperature in step S33 may be 650° C. or higher.

[0471] Of course, if the temperature is higher than the melting point of one or more of the materials contained in the mixture 903, For example, when LiF and MgF2 are used as additive element sources, Since the eutectic point of LiF and MgF2 is around 742°C (see eutectic point P in Figure 10), The lower limit of the heating temperature of the cup S33 is preferably 742°C or higher.

[0472] In addition, the molar ratio of LiCoO2:LiF:MgF2 was 100:0.33:1. The mixture 903 obtained by mixing the above showed an endothermic peak around 830°C in the 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 proceeds more easily, the heating time is shorter, and productivity is higher.

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

[0475] Considering these, the heating temperature in step S33 is set to 650°C or higher, 1130°C or higher. ° C. or less, more preferably 650° C. or more and 1000° C. or less, and more preferably 650° C. or more and 950° C. More preferably, the temperature is 650°C or higher and 900°C or lower, and even more preferably, the temperature is 742°C or higher. The temperature is preferably 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, and more preferably 742°C or higher and 1000°C or lower. It is more preferable that the temperature is between 742°C and 900°C, and more preferably between 742°C and 900°C. Preferably, the temperature is 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, A temperature of 830°C or higher and 950°C or lower is more preferable, and a temperature of 830°C or higher and 900°C or lower is even more preferable. The heating temperature in step S33 is preferably lower than the heating temperature in step S13. stomach.

[0476] An example of the heating furnace used in step S33 will now be described with reference to FIG.

[0477] The heating furnace 220 shown in FIG. 25 includes a heating furnace space 202, a hot plate 204, a pressure gauge 221, a heater, and a The container 216, which corresponds to a crucible or sheath, has a lid 218. By using this configuration, the container 216 and the lid 218 can be heated. The space 219 can be filled with a fluoride-containing atmosphere. The concentration of gasified fluoride in 219 is kept constant or does not decrease by covering it. By maintaining this, it is possible to include fluorine and magnesium near the particle surface of the mixture 903. Since the volume of the space 219 is smaller than that of the space 202 in the heating furnace, a small amount of fluoride is volatilized. By evaporating the fluoride, the atmosphere containing the fluoride can be obtained. The reaction atmosphere can be changed to a fluoride-containing atmosphere without significantly affecting the amount of fluoride contained. Furthermore, by using the lid 218, it is possible to easily and inexpensively The mixture 903 can be heated in an atmosphere containing the oxygen.

[0478] Before heating in the heating furnace space 202, the heating furnace space 202 is placed in an atmosphere containing oxygen. A process of placing the mixture 903 in the container 216 in the space 202 inside the heating furnace. By performing the steps in this order, the mixture 903 is heated in an atmosphere containing oxygen and fluoride. For example, heating can be performed while gas is flowing (flow). Gas can be introduced from the bottom of the heating furnace space 202 and exhausted to the top. The space inside the heating furnace 202 can be sealed to form a closed space so that gases are not transported to the outside. Purge.

[0479] There is no particular limitation on the method for creating an oxygen-containing atmosphere in the space 202 inside the heating furnace. After evacuating the space 202 inside the thermal furnace, oxygen gas or a gas containing oxygen such as dry air is introduced. and a method in which oxygen gas or a gas containing oxygen such as dry air is introduced for a certain period of time. However, it is preferable to introduce oxygen gas (oxygen substitution) after evacuating the space 202 inside the heating furnace. The atmosphere in the heating furnace space 202 may be regarded as an atmosphere containing oxygen.

[0480] In addition, fluoride and the like adhering to the inner walls of the container 216 and the lid 218 are re-emitted by heating. It can also be attached to the mixture 903.

[0481] There is no particular limitation on the process for heating the heating furnace 220. Heating may be performed using

[0482] There is no particular limitation on how the mixture 903 is arranged when placed in the container 216. In other words, the upper surface of the mixture 903 is flat with respect to the bottom surface of the container 216. 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 carried out while controlling the pressure inside the furnace with a pressure gauge 221. The inside of the furnace is preferably under atmospheric pressure or under pressure. When the lithium cobalt oxide is heated, the surface of the lithium cobalt oxide melts. The surface of lithium cobalt oxide heated with F and MgF2 can be melted by applying pressure. .

[0484] The cooling after heating in step S33 may be done by natural cooling. The time is preferably 10 hours or more and 50 hours or less, and for example, the temperature drop rate is 80°C / h or more and 2 The temperature is preferably 50°C / h or less, and more preferably 180°C / h or more and 210°C / h or less. The cooling rate in step S33 is preferably faster than that in step S13. The shell can be properly formed by performing rapid cooling after the melting. Specifically, it becomes possible to produce a narrow shell. The reaction does not need to be at room temperature, but rather it is sufficient to cool to a temperature acceptable for the next step.

[0485] Furthermore, when the mixture 903 is heated, the partial pressure of fluorine or fluorine compounds resulting from the fluorine source, etc. It is preferable to control the temperature within an appropriate range. As mentioned above, the lid can prevent the material from volatilizing or In other words, the volatilization of the material can be prevented from occurring during the temperature rise and fall in this step. Alternatively, it is sufficient to prevent sublimation, and it is not necessary to seal the crucible with a lid. For example, by filling the reaction chamber in which the crucible is placed with oxygen, this step can be carried out without sealing the crucible. The positive electrode active material having an appropriate amount of fluorine or a fluorine compound can also be used. This is preferable because it can suppress heat generation and smoke generation even in the event of a partial short circuit.

[0486] In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, are used as a flux. This function may function as a catalyst to keep the heating temperature below the decomposition temperature of lithium cobalt oxide. For example, the temperature can be lowered to between 742°C and 950°C, and magnesium and other materials can be added to the surface layer. By distributing the additive elements, a positive electrode active material with good characteristics can be produced.

[0487] However, since LiF has a lower specific gravity in gaseous state than oxygen, heating causes LiF to volatilize or There is a possibility that the LiF in the mixture 903 will sublime, and if it volatilizes, the LiF in the mixture 903 will decrease. Therefore, it is necessary to heat the LiF while suppressing its volatilization. Even if LiF is not used as a fluorine source, the LiCoO2 surface There is also a possibility that LiF may react with the fluorine source F to produce LiF, which may then volatilize. Even if a fluorine compound with a higher melting point is used, it is still necessary to suppress volatilization.

[0488] Therefore, the mixture 903 is heated in an atmosphere containing LiF, that is, the Li It is preferable to heat the mixture 903 under a high partial pressure of F. It is possible to suppress the volatilization of LiF in the mixture 903. It is advisable to place a lid on the crucible.

[0489] The heating in this step is preferably performed so that the particles of the mixture 903 do not stick together. When the particles of the mixture 903 adhere to each other during the heating, the contact area with the oxygen in the atmosphere decreases. By blocking the route for the diffusion of additional elements (e.g. fluorine), the additional elements (e.g. Reaction with atmospheric oxygen can lead to poor distribution of elements such as magnesium and fluorine. To promote this, the crucible does not need to be sealed with a lid.

[0490] Furthermore, if the added element (e.g., fluorine) is uniformly distributed in the surface layer, the surface will be smooth and have few irregularities. It is believed that a positive electrode active material can be obtained. To maintain the smoothness of the surface after treatment, or to make it even smoother, It is preferable that the particles of the mixture 903 do not stick together.

[0491] In addition, when heating using a rotary kiln, the flow rate of the atmosphere containing oxygen in the kiln should be It is preferable to heat the material in a controlled manner. For example, the flow rate of the oxygen-containing atmosphere is reduced. It is preferable to purge the atmosphere and not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln. Flowing oxygen may evaporate the fluorine source, which may cause the surface to become smooth. It is not desirable for this purpose.

[0492] When heating by a roller hearth kiln, for example, the container containing the mixture 903 is covered with a lid. By disposing the mixture 903 in a crucible, it is possible to heat the mixture 903 in an atmosphere containing LiF. It is similar to a lid.

[0493] Regarding the heating time, the heating time depends on the heating temperature, the cobalt content obtained in step S14, The amount of lithium cobalt oxide varies depending on the size and composition of the lithium cobalt oxide. In this case, a lower temperature or shorter time may be more preferable than in the case where the temperature is large.

[0494] The median diameter (D50) of the lithium cobalt oxide obtained in step S14 of FIG. When the thickness is about 12 μm, the heating temperature is preferably, for example, 650° C. or more and 950° C. or less. The time is preferably, for example, 3 hours or more and 60 hours or less, and more preferably 10 hours or more and 30 hours or less. The temperature-lowering time after heating is preferably 10 hours or more, and more preferably about 20 hours. It is preferable to set the temperature at 50 hours or less.

[0495] On the other hand, the median diameter (D50) of the lithium cobalt oxide obtained in step S14 is 5 μm. In this case, the heating temperature is preferably, for example, 650°C or more and 950°C or less. For example, the time is preferably from 1 hour to 10 hours, and more preferably about 5 hours. The heating time 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, if necessary, decomposed. The particles are then crushed to obtain the positive electrode active material 100. At this time, the collected particles are 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] <<Method 2 for preparing positive electrode active material>> Next, a positive electrode active material different from the positive electrode active material production method 1 according to one embodiment of the present invention is described. The manufacturing method 2 of the positive electrode active material will be described with reference to FIGS. Method 2 differs from Method 1 mainly in the number of times the additive elements are added and the mixing method. For details, please refer to the description of Preparation Method 1.

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

[0499] <Step S20a> Next, in step S20a, an additive element A1 is added to the lithium cobalt oxide that has undergone the initial heating. The additive element A1 source is prepared using FIG. 24(A). The process will be described below.

[0500] <Step S21> Step S21 shown in Fig. 24(A) will be described. As the additional element A1, B) is selected from the elements exemplified as the additive element A described in step S21. For example, the additive element A1 can be magnesium, fluorine, and calcium. In FIG. 24(A), one or more selected from the following can be suitably used. In the case where magnesium and fluorine are selected as the additive element A1, in step S21, An example of preparing a magnesium source and a fluorine source will be described.

[0501] Steps S21 to S23 shown in FIG. 24(A) are the same as those shown in FIG. 21(B). This can be done under the same conditions as steps S21 to S23. In step S23, the source of the additive element A1 (A1 source) can be obtained.

[0502] 21(A) as for steps S31 to S33 shown in FIG. 23. This can be done in the same manner as steps S31 to S33.

[0503] <Step S34a> Next, in step S33, the heated material is recovered and a lithium cobalt oxide having the additive element A1 is obtained. To distinguish it from the composite oxide in step S14, it is also called the second composite oxide. .

[0504] <Step S40> In step S40 shown in FIG. 23, a second composite oxide is added with an additive element A2. 24(B) and 24(C), the source of the additive element A2 is prepared. The steps for preparing the source will be described below.

[0505] <Step S41> Step S41 shown in FIG. 24(B) will be described. The additional element A2 is C) is selected from the elements exemplified as the additive element A described in step S21. For example, the additive element A2 can be nickel, titanium, boron, zirconium, or the like. One or more materials selected from the group consisting of aluminum and silicon can be suitably used. In FIG. 24(B), nickel and aluminum are selected as the additive element A2. An example of preparing a nickel source (Ni source) and an aluminum source (Al source) will be described.

[0506] Steps S41 to S43 shown in FIG. 24B are the same as those shown in FIG. 21B. This can be done under the same conditions as steps S21 to S23. In step S43, a source of the additional element A2 (A2 source) can be obtained.

[0507] Also, FIG. 24(C) shows the step of preparing the source of the additive element A2 described with reference to FIG. 24(B). In step S41 shown in FIG. 24(C), a nickel source (Ni source), and Aluminum sources (Al sources) are prepared, and in step S42a, they are individually pulverized. As a result, in step S43, a plurality of additive element A2 sources (A2 sources) are prepared. In the step of FIG. 24(C), the additive elements are individually crushed in step S42a. This is different from FIG. 24(B).

[0508] <Steps S51 to S54> Next, steps S51 to S54 shown in FIG. 23 are the same as steps S51 to S54 shown in FIG. 21(A). Steps S31 to S34 can be carried out under the same conditions as in Step S52. 904. The conditions for the heating step in step S53 are lower than those in step S33. By the above process, in step S54, the method of the present invention can be carried out at a low temperature for a short time. The positive electrode active material 100 according to one embodiment of the present invention has a smooth surface. It is.

[0509] As shown in FIGS. 23 to 24C, in the manufacturing method 2, the additive to the lithium cobalt oxide is The element is introduced separately as an additional element A1 and an additional element A2. The position of the additive element in the depth direction can be changed. For example, the additive element A1 can be The added element A2 is positioned so that the concentration is higher in the surface layer than in the surface layer. It is also possible to position it so that the concentration is low.

[0510] After the initial heating described in this embodiment, a positive electrode active material with a smooth surface can be obtained. do.

[0511] The initial heating shown in this embodiment is performed on lithium cobalt oxide. The heat is lower than the heating temperature for obtaining lithium cobalt oxide, and The heating time is preferably shorter than that required to obtain the desired lithium cobaltate. The step of adding the hydroxybenzoate is preferably performed after the initial heating. The adding step can be performed in two or more steps. This process sequence is preferred because it maintains the smooth surface obtained by the initial heating. I wish.

[0512] The positive electrode active material 100 having a smooth surface is more resistant to physical damage such as pressure than a positive electrode active material having a smooth surface. For example, in tests involving pressure such as nail penetration tests, the positive electrode is The substance 100 may be less prone to destruction, resulting in increased safety.

[0513] This embodiment can be used in combination with other embodiments.

[0514] (Embodiment 3) In this embodiment, an example of a secondary battery of one embodiment of the present invention will be described with reference to FIGS.

[0515] <Example of secondary battery configuration> The following description will be given taking as an example a secondary battery in which a positive electrode, a negative electrode, and an electrolyte are housed in an exterior body.

[0516] [Positive electrode] FIG. 26A shows an example of a cross-sectional view of a positive electrode 503 used in a secondary battery 1004 or the like. The electrode 503 has a positive electrode active material layer 502 on a positive electrode current collector 501. The positive electrode active material layer 502 is , a positive electrode active material 100, a positive electrode active material 562, a conductive material 553, a conductive material 554, and an electrolyte solution 53 The positive electrode active material layer 502 also contains a binder (not shown). A configuration including either the conductive material 553 or the conductive material 554 may be used.

[0517] The median diameter (D50) of the positive electrode active material 100 is 1 μm or more and 50 μ...

Claims

1. A secondary battery including an exterior body and a laminate having a plurality of positive electrodes housed in the exterior body, the positive electrode has a positive electrode active material, the positive electrode active material has a first region and a second region, the first region comprises lithium, cobalt, aluminum, magnesium, nickel, and oxygen; the second region comprises lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region, the first region has a first portion having a (001) oriented surface and a second portion having a surface other than a (001) oriented surface; a peak position of the detected amount of magnesium in the depth direction of the first portion is shallower than a peak position of the detected amount of magnesium in the depth direction of the second portion; a peak position of the detected amount of aluminum in the depth direction of the first portion is shallower than a peak position of the detected amount of aluminum in the depth direction of the second portion; Nickel is detected in the second portion; A secondary battery, wherein when a nail penetration test is conducted on the secondary battery under the conditions of a voltage of the secondary battery of 4.5 V, a nail diameter of 3 mm, and a nail penetration speed of 5 mm / sec, the temperature rise ΔT of the secondary battery is 50°C or less.

2. A secondary battery including an exterior body and a laminate having a plurality of positive electrodes housed in the exterior body, the positive electrode has a positive electrode active material, the positive electrode active material has a first region and a second region, the first region comprises lithium, cobalt, aluminum, magnesium, nickel, and oxygen; the second region comprises lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region, the first region has a first portion having a (001) oriented surface and a second portion having a surface other than a (001) oriented surface; a peak position of the magnesium concentration in the depth direction of the first portion is shallower than a peak position of the magnesium concentration in the depth direction of the second portion; a peak position of the aluminum concentration in the depth direction of the first portion is shallower than a peak position of the aluminum concentration in the depth direction of the second portion; Nickel is detected in the second portion; A secondary battery, wherein when a nail penetration test is conducted on the secondary battery under the conditions of a voltage of the secondary battery of 4.5 V, a nail diameter of 3 mm, and a nail penetration speed of 5 mm / sec, the temperature rise ΔT of the secondary battery is 50°C or less.

3. A secondary battery including an exterior body and a laminate having a plurality of positive electrodes housed in the exterior body, the positive electrode has a positive electrode active material and a conductive material, the conductive material comprises a graphene compound; the graphene compound has a shape that corresponds to at least a part of the shape of the positive electrode active material, the positive electrode active material has a first region and a second region, the first region comprises lithium, cobalt, aluminum, magnesium, nickel, and oxygen; the second region comprises lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region, the first region has a first portion having a (001) oriented surface and a second portion having a surface other than a (001) oriented surface; a peak position of the magnesium concentration in the depth direction of the first portion is shallower than a peak position of the magnesium concentration in the depth direction of the second portion; a peak position of the aluminum concentration in the depth direction of the first portion is shallower than a peak position of the aluminum concentration in the depth direction of the second portion; Nickel is detected in the second portion; A secondary battery, wherein when a nail penetration test is conducted on the secondary battery under the conditions of a voltage of the secondary battery of 4.5 V, a nail diameter of 3 mm, and a nail penetration speed of 5 mm / sec, the temperature rise ΔT of the secondary battery is 50°C or less.

4. A secondary battery including an exterior body and a laminate having a plurality of positive electrodes housed in the exterior body, the positive electrode has a positive electrode active material, the positive electrode active material has a first region and a second region, the first region comprises lithium, cobalt, aluminum, magnesium, nickel, and oxygen; the second region comprises lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region, the first region has a first portion having a (001) oriented surface and a second portion having a surface other than a (001) oriented surface; a peak position of the detected amount of magnesium in the depth direction of the first portion is shallower than a peak position of the detected amount of magnesium in the depth direction of the second portion; a peak position of the detected amount of aluminum in the depth direction of the first portion is shallower than a peak position of the detected amount of aluminum in the depth direction of the second portion; Nickel is detected in the second portion.

5. A secondary battery including an exterior body and a laminate having a plurality of positive electrodes housed in the exterior body, the positive electrode has a positive electrode active material, the positive electrode active material has a first region and a second region, the first region comprises lithium, cobalt, aluminum, magnesium, nickel, and oxygen; the second region comprises lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region, the first region has a first portion having a (001) oriented surface and a second portion having a surface other than a (001) oriented surface; a peak position of the magnesium concentration in the depth direction of the first portion is shallower than a peak position of the magnesium concentration in the depth direction of the second portion; a peak position of the aluminum concentration in the depth direction of the first portion is shallower than a peak position of the aluminum concentration in the depth direction of the second portion; Nickel is detected in the second portion.

6. A secondary battery including an exterior body and a laminate having a plurality of positive electrodes housed in the exterior body, the positive electrode has a positive electrode active material and a conductive material, the conductive material comprises a graphene compound; the graphene compound has a shape that corresponds to at least a part of the shape of the positive electrode active material, the positive electrode active material has a first region and a second region, the first region comprises lithium, cobalt, aluminum, magnesium, nickel, and oxygen; the second region comprises lithium, cobalt, and oxygen; the first region is located closer to the surface of the positive electrode active material than the second region, the first region has a first portion having a (001) oriented surface and a second portion having a surface other than a (001) oriented surface; a peak position of the magnesium concentration in the depth direction of the first portion is shallower than a peak position of the magnesium concentration in the depth direction of the second portion; a peak position of the aluminum concentration in the depth direction of the first portion is shallower than a peak position of the aluminum concentration in the depth direction of the second portion; Nickel is detected in the second portion.

7. In any one of claims 1 to 6, A secondary battery, wherein the thickness of the first region is 2 nm or more and 20 nm or less.

8. In any one of claims 1 to 6, A secondary battery, wherein the concentration of the magnesium in the first region is greater than 0 and not greater than 10 atomic %.

9. In any one of claims 1 to 6, The positive electrode active material has a volume resistivity of 1.0×10 5 Ω·cm or more in powder form at a pressure of 64 MPa.

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