Secondary battery

Abstract

Provided is a highly safe secondary battery. The present invention is a secondary battery comprising a positive electrode and a negative electrode, the positive electrode containing a positive electrode active material, the positive electrode active material containing lithium cobalt oxide containing magnesium, the magnesium concentration in a surface layer portion of the positive electrode active material being higher than the magnesium concentration in the positive electrode active material, and the negative electrode containing a negative electrode active material containing a carbon material, when AC impedance measurement is performed on the secondary battery charged to a voltage of 4.5 V, an AC impedance value at a frequency of 1 kHz satisfies less than 90 m [Omega].

Description

Technical Field

[0001] One aspect of the present invention relates to a secondary battery. Furthermore, the present invention is not limited to the above-described fields, but relates to semiconductor devices, display devices, light-emitting devices, energy storage devices, lighting devices, electronic devices, vehicles, and methods for manufacturing the same. The aforementioned semiconductor devices, display devices, light-emitting devices, energy storage devices, lighting devices, electronic devices, and vehicles can use the secondary battery of the present invention as a power source as needed. For example, the aforementioned electronic devices include information terminal devices, etc., in which a secondary battery is installed. Furthermore, the aforementioned energy storage devices include stationary energy storage devices, etc. Background Technology

[0002] In recent years, the demand for high-output, high-capacity rechargeable batteries (also known as batteries) has surged, becoming a necessity in modern society as a reusable energy source.

[0003] It is said that it is difficult to simultaneously ensure both high capacity and safety in secondary batteries. For example, in positive electrode active materials with a layered rock-salt crystal structure used in lithium-ion secondary batteries, high capacity is expected because the diffusion path of lithium ions exists in two dimensions within this crystal structure. However, it is believed that in positive electrode active materials with a layered rock-salt crystal structure, excessive lithium ion release during charging can easily lead to thermal runaway due to crystal structure collapse, thus posing a safety concern. Safety tests include needle penetration tests, and to suppress battery temperature rise under abnormal conditions such as needle penetration, for example, Patent Document 1 discloses a structure in which a protective layer is provided between the positive electrode binder layer and the positive electrode current collector.

[0004] Lithium cobalt oxide (LiCoO2) is known as a positive electrode active material with a layered rock-salt crystal structure. Lithium cobalt oxide possesses a layered rock-salt crystal structure, in which lithium ions can migrate two-dimensionally between the layers composed of CoO6 octahedra, thus exhibiting excellent cycle characteristics. However, lithium cobalt oxide suffers from a phase transition during charge and discharge. For example, during charging, a phase transition occurs from hexagonal to monoclinic crystals when a certain amount of lithium ions are released. Therefore, to utilize lithium cobalt oxide with excellent cycle characteristics, the amount of lithium ion release must be limited. To address these issues, patent documents 2 to 4 disclose structures in which additive elements are added to lithium cobalt oxide.

[0005] Furthermore, studies on the crystal structure of positive electrode active materials have been conducted (Non-Patent Documents 1 to 4). XRD (X-ray Diffraction) is one of the methods used to analyze the crystal structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) described in Non-Patent Document 5. For example, the lattice constant of lithium cobalt oxide described in Non-Patent Document 6 can be used with reference to the ICSD. Alternatively, analysis using the Rietwald method can be performed, for example, using the analysis program RIETAN-FP (Non-Patent Document 7).

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

[0007] Nanobeam electron diffraction is also very effective in identifying the crystal structure of positive electrode active materials, especially the crystal structure of the surface layer. For example, the analysis program ReciPro (non-patent document 11) can be used in the analysis of electron diffraction patterns.

[0008] Since ancient times, fluorides such as fluorite (calcium fluoride) have been used as fluxes in iron smelting and other processes, and their physical properties have been studied (Non-Patent Literature 12).

[0009] It is known that in lithium-ion secondary batteries, thermal runaway occurs through several states when the temperature rises during charging (Non-Patent Document 13).

[0010] Various studies and developments have been conducted on the reliability and safety of lithium-ion secondary batteries. For example, non-patent literature 14 describes the thermal stability of the positive electrode active material and the electrolyte. [Prior technical literature] [Patent Literature]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2019-129009 [Patent Document 2] Japanese Patent Application Publication No. 2019-179758 [Patent Document 3] No. WO2020 / 026078 [Patent Document 4] Japanese Patent Application Publication No. 2020-140954 [Non-patent literature]

[0012] [Non-patent document 1] Toyoki Okumura et al., "Correlation of lithium iondistribution and X-ray absorption near-edge structure in O3-and O2-lithiumcobalt oxides from first-principle calculation", Journal of MaterialsChemistry, 2012, 22, p.17340-17348 [Non-Patent Literature 2] Motohashi, T. et al., “Electronic phase diagram of the layered cobalt oxide system Li x CoO2(0.0≤x≤1.0)”, Physical Review B, 80(16); 165114 [Non-Patent Literature 3] Zhaohui Chen et al., “Staging Phase Transitions in Li x CoO2", Journal of The Electrochemical Society, 2002, 149(12)A1604-A1609 [Non-Patent Literature 4] GGAmatucci et al., "CoO2, The End Member of the Li x CoO2Solid Solution” J.Electrochem.Soc.143(3)1114(1996). [Non-patent document 5] Belsky, A. et al., "New developments in the InorganicCrystal 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 structurerefinement of LiCoO2 single crystals" Journal of Solid State Chemistry (1998) 141, p. 298-302. [Non-Patent Literature 7] F. Izumi and K. Momma, Solid State Phenom., 130, 15-20 (2007) [Non-patent document 8] Rasband, WS, ImageJ, USNational Institutes of Health, Bethesda, Maryland, USA, http: / / rsb.info.nih.gov / ij / , 1997-2012. [Non-Patent Literature 9] Schneider, CA, Rasband, WS, Eliceiri, KW, “NIH Image to Image J: 25 years of image analysis”, Nature Methods 9, 671-675, 2012. [Non-Patent Literature 10] Abramoff, MD, Magelhaes, PJ, Ram, SJ, “Image Processing with ImageJ”, Biophotonics International, volume 11, issue 7, pp. 36-42, 2004. [Non-patent document 11] Seto, Y. & Ohtsuka, M., "ReciPro: free and open-source multipurpose crystallographic software integrating a crystal model database and viewer, diffraction and microscopy simulators, and diffraction data analysis tools" (2022) J.Appl.Cryst., 55. [Non-Patent Document 12] W.E. Counts, R.Roy, and E.F. Osborn, "Fluoride Model Systems: II, The Binary Systems CaF2-BeF2, MgF2-BeF2, and LiF-MgF2", Journal of the American Ceramic Society, 36[1] 12-17 (1953). [Non-Patent Document 13] Nobuo Eda, Learning about the Charge and Discharge Technology of Li-Ion Batteries from the Mechanism Data of 2-4 Exothermic Reactions, published by Nippon CQ Publishing on April 4, 2020, pp. 68-72 [Non-Patent Document 14] Shinya Kitano et al., GSYuasa Technical Report, Volume 2, Issue 2 (GSYuasa Technical Report, Volume 2, Issue 2), December 2015, pp. 18-24 Summary of the Invention Technical Problem to be Solved by the Invention

[0013] It is said that lithium cobaltate (sometimes denoted as LiCoO2, LCO) disclosed in Patent Documents 2 to 4 has low thermal stability. In addition, when an internal short circuit occurs due to a nail penetration test, which is one of the safety tests for lithium-ion secondary batteries, in a secondary battery, Joule heat is generated, sometimes leading to thermal runaway. Therefore, when using the above-mentioned lithium cobaltate, safety measures for the secondary battery are indispensable. Patent Document 1 discloses a structure in which a protective layer is provided between the positive electrode current collector and the positive electrode mixture layer to suppress the increase in battery temperature during nail penetration.

[0014] In view of the above description, one of the objectives of one aspect of the present invention is to provide a battery with high safety. And one of the objectives of one aspect of the present invention is to provide a battery with a large capacity and high safety.

[0015] Note that the description of these objectives does not preclude the existence of other objectives. One aspect of the present invention does not need to achieve all of the above objectives. In addition, objectives other than the above can be extracted from the descriptions in the specification, drawings, and claims. Means for Solving the Technical Problem

[0016] One aspect of the present invention is a secondary battery including a positive electrode and a negative electrode, wherein the positive electrode contains a positive electrode active material, the positive electrode active material contains lithium cobaltate containing magnesium, the magnesium concentration in the surface layer portion of the positive electrode active material is higher than that in the interior of the positive electrode active material, the negative electrode contains a negative electrode active material, the negative electrode active material contains a carbon material, and when measuring the AC impedance of the secondary battery in a state charged to a voltage of 4.5V, the AC impedance value at a frequency of 1 kHz satisfies less than 90 mΩ.

[0017] Another aspect of the present invention is a secondary battery comprising a positive electrode, a negative electrode, an separator, and an electrolyte, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprises lithium cobalt oxide containing magnesium, the magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material, the negative electrode comprises a negative electrode active material, the negative electrode active material comprises a carbon material, the electrolyte solvent comprises ethylene carbonate and diethyl carbonate, the separator comprises polypropylene, and when the AC impedance of the secondary battery charged to a voltage of 4.5V is measured, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

[0018] Another aspect of the present invention is a secondary battery comprising a positive electrode and a negative electrode, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprises lithium cobalt oxide containing magnesium, the magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material, the negative electrode comprises a negative electrode active material, the negative electrode active material comprises carbon material, and when the AC impedance of the secondary battery charged to a voltage of 4.5V and a capacity of 2000mAh or more is measured, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

[0019] Another aspect of the present invention is a secondary battery comprising a positive electrode, a negative electrode, an separator, and an electrolyte, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprises lithium cobalt oxide containing magnesium, the magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material, the negative electrode comprises a negative electrode active material, the negative electrode active material comprises a carbon material, the electrolyte solvent comprises ethylene carbonate and diethyl carbonate, the separator comprises polypropylene, and when the AC impedance of the secondary battery charged to a voltage of 4.5V and a capacity of 2000mAh or more is measured, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

[0020] In the present invention described above, it is preferred that when a secondary battery is subjected to a needle penetration test under the conditions that the diameter of the steel needle is 3 mm and the needle penetration rate is 5 mm / sec, the temperature rise ΔT is less than 50°C.

[0021] In the present invention described above, lithium cobalt oxide preferably further comprises aluminum.

[0022] In the present invention described above, lithium cobalt oxide preferably further comprises nickel.

[0023] In the present invention described above, lithium cobalt oxide preferably further contains fluorine. Effects of the Invention

[0024] According to one aspect of the present invention, a highly safe rechargeable battery can be provided. Additionally, according to another aspect of the present invention, a rechargeable battery with both large capacity and high safety can be provided.

[0025] Note that the description of these effects does not preclude the existence of other effects. One embodiment of the invention does not necessarily require all of the aforementioned effects. Furthermore, effects other than those described above are clearly present in the specification, drawings, and claims, and these effects can be obtained from the description in the specification, drawings, and claims. Brief description of the attached figures

[0026] Figure 1A and Figure 1B This is a diagram illustrating the acupuncture test. Figure 2A and Figure 2B It is a diagram illustrating the acupuncture procedure. Figure 3 It is a graph showing the change in internal temperature as a secondary battery experiences an internal short circuit. Figure 4 This is a graph showing the changes in the internal temperature of a secondary battery as it rises. Figures 5A to 5C This is an example of the cross-sectional structure of a positive electrode active material. Figure 6 This is an example of a TEM image where the crystal orientation is roughly consistent. Figure 7A This is an example of a STEM image with roughly consistent crystal orientation. Figure 7B It is the FFT pattern of the region of the rock salt type crystal RS. Figure 7C It is the FFT pattern of the region of layered rock salt type crystal LRS. Figure 8 It is a diagram illustrating the crystal structure of the positive electrode active material. Figure 9 It is a diagram illustrating the crystal structure of existing positive electrode active materials. Figure 10 This is a diagram illustrating the depth of charge and c-axis length of the positive electrode active material. Figure 11 This is a diagram showing the XRD pattern calculated from the crystal structure. Figure 12 This is a diagram showing the XRD pattern calculated from the crystal structure. Figure 13A and Figure 13B This is a diagram showing the XRD pattern calculated from the crystal structure. Figures 14A to 14C It is the lattice constant calculated from XRD. Figure 15A and Figure 15B This is a cross-sectional view of the positive electrode active material. Figure 16A and Figure 16B This is a diagram illustrating a laminated secondary battery. Figure 17This is a phase diagram showing the relationship between the composition and temperature of lithium fluoride and magnesium fluoride. Figure 18 This is a graph illustrating the DSC measurement results. Figures 19A to 19C This is a diagram illustrating the manufacturing method of the positive electrode active material. Figures 20A to 20C This is a diagram illustrating the manufacturing method of the positive electrode active material. Figure 21 This is a diagram illustrating the manufacturing method of the positive electrode active material. Figures 22A to 22C This is a diagram illustrating the manufacturing method of the positive electrode active material. Figure 23 It is a diagram illustrating the heating furnace and heating conditions. Figure 24A and Figure 24B This is a diagram illustrating the positive electrode. Figures 25A to 25C This is a diagram illustrating a coin-type secondary battery. Figures 26A to 26D This is a diagram illustrating a cylindrical secondary battery. Figure 27A and Figure 27B This is a diagram illustrating a wound secondary battery. Figure 28 This is a diagram illustrating a wound secondary battery. Figures 29A to 29D It is a diagram illustrating an electronic device. Figures 30A to 30C This is a diagram showing an electronic device. Figures 31A to 31C It's a diagram illustrating the vehicle. Figures 32A to 32C It is a graph showing the charge and discharge curves. Figure 33A It is a graph showing the results of the impedance test. Figure 33B This is a diagram showing the equivalent circuit used for analysis. Figures 34A to 34C It is a photo illustrating the results of the acupuncture test. Figures 35A to 35C It is a chart illustrating the results of the acupuncture test. Figures 36A to 36C It is a chart illustrating the results of the acupuncture test. Figures 37A to 37C It is a chart illustrating the results of the acupuncture test. Figures 38A to 38C It is a chart illustrating the results of the acupuncture test. Methods of implementing the invention

[0027] Hereinafter, examples of embodiments of the present invention will be described using accompanying drawings and other means. Note that the present invention should not be construed as being limited to the examples described below. Embodiments of the invention may be modified without departing from the spirit of the invention.

[0028] In this specification, space groups are represented using the shortnotation of the International Notation (or Hermann-Mauguin notation). Furthermore, Miller indices are used to represent crystal planes and crystal orientations. In crystallography, a superscript is added to numbers to indicate space groups, crystal planes, and crystal orientations. However, in this specification, due to the notation limitations in the patent application, sometimes a - (negative number) is added before numbers to indicate space groups, crystal planes, and crystal orientations instead of a superscript. Additionally, "[]" indicates the individual orientation within a crystal, "<>" indicates the collective orientation of all equivalent orientations, "()" indicates the individual facet of a crystal plane, and "{}" indicates a collective facet with equivalent symmetry. Furthermore, generally, for ease of understanding of the structure, the trigonal crystal system represented by space group R-3m is represented by a composite hexagonal lattice; in this specification, unless otherwise specified, space group R-3m is also represented by a composite hexagonal lattice. Sometimes, in addition to (hkl), (hkil) is used as the Miller index. Here, i is -(h+k).

[0029] In this specification, the space group of positive electrode active materials, etc., is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification, belonging to a certain space group or space group means being identified as a certain space group.

[0030] Furthermore, the structure of anions stacked in three layers, such as ABCABC, with each layer offset from the others, is called a cubic close-packed structure. Therefore, anions can also be non-strictly cubic lattices. At the same time, real crystals always have defects, so analytical results may not be based on theory. For example, spots may appear at slightly different positions in the FFT (Fast Fourier Transform) patterns of electron diffraction images or TEM (Transmission Electron Microscope) images. For instance, a cubic close-packed structure can be said to exist when the orientation deviation from the theoretical position is less than 5 degrees or less than 2.5 degrees.

[0031] Furthermore, in this specification, particles are not limited to spherical shapes (with circular cross-sectional shapes). The cross-sectional shape of each particle can also be elliptical, rectangular, trapezoidal, triangular, quadrilateral with arc-shaped corners, asymmetrical shapes, etc., and each particle can also be amorphous.

[0032] In addition, in the present specification and the like, the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the lithium that can be intercalated and deintercalated in the positive electrode active material is deintercalated. For example, the theoretical capacity of LiCoO2 per unit weight of the positive electrode active material is 274 mAh / g, the theoretical capacity of LiNiO2 per unit weight of the positive electrode active material is 275 mAh / g, and the theoretical capacity of LiMn2O4 per unit weight of the positive electrode active material is 148 mAh / g.

[0033] In the present specification and the like, for x in the compositional formula, for example, Li x in MO2, x represents the amount of lithium remaining in the positive electrode active material that can be intercalated and deintercalated. M represents a transition metal. In the present specification and the like, without special explanation, M is cobalt and / or nickel. In the positive electrode active material of a lithium-ion secondary battery, x = (theoretical capacity - charging capacity) / theoretical capacity can hold. For example, when a lithium-ion secondary battery using LiMO2 as the positive electrode active material is charged to 219.2 mAh / g per unit weight of the positive electrode active material, it can be said that the positive electrode active material is Li 0.2 MO2 or it can be said that x = 0.2. A smaller x in Li x MO2, for example, means a case where 0.1 < x ≤ 0.24. When lithium cobaltate approximately satisfies the stoichiometric ratio, the lithium cobaltate is LiCoO2 and x = 1. In addition, when lithium cobaltate is used for the positive electrode, the lithium cobaltate in the lithium-ion secondary battery at the end of discharge can also be said to be LiCoO2 and x = 1. Here, "at the end of discharge" means, for example, a state where the current becomes 100 mA / g or less per unit weight of the positive electrode active material and the voltage becomes 3.0 V or 2.5 V or less.

[0034] For calculating the charging capacity and / or discharging capacity for x in Li x MO2, it is preferable to measure under conditions where there is no influence of decomposition of short circuit and / or electrolyte, or the influence of decomposition of short circuit and / or electrolyte is small. For example, data of a lithium-ion secondary battery in which a capacity change regarded as a short circuit occurs suddenly cannot be used for calculating x.

[0035] In the present specification and the like, the segregation of a certain element refers to a state in which a certain element (for example, A) is unevenly distributed in a solid containing multiple elements (for example, A, B, C).

[0036] In this specification, the distribution of an element refers to a state where the element is continuously present, provided it is detected by any analytical method within a range where it is not noise. Additionally, the continuous change in the element's concentration is sometimes referred to as a concentration gradient. The maximum value in the distribution is sometimes called a peak. Furthermore, when specifying a peak, a distribution limited to a certain region can be used as the object. The above distribution is not limited to a normal distribution. In cases equivalent to a normal distribution, the half-width of the distribution can also be set.

[0037] In this specification, concentrated distribution refers to a situation where the concentration of an element in any region differs from that in other regions. Concentrated distribution is synonymous with deviation or the mixing of regions with high and low concentrations. The state of concentrated distribution in solid solution is called segregation.

[0038] In this specification, the surface portion of the positive electrode active material refers to a region within 20 nm or 50 nm in a direction perpendicular or substantially perpendicular to the surface, extending inward from the surface. The surface portion is synonymous with the vicinity of the surface or the region near the surface. "Perpendicular or substantially perpendicular" specifically refers to an angle of 80° or more but less than 100° with respect to the surface. Furthermore, the region deeper than the surface portion of the positive electrode active material is called the interior. The interior is synonymous with bulk or core.

[0039] In this specification and other materials, the positive electrode active material is sometimes referred to as a composite oxide, positive electrode material, positive electrode material for secondary batteries, positive electrode material for lithium-ion secondary batteries, etc. Furthermore, in this specification and other materials, the positive electrode active material of one embodiment of the present invention preferably comprises a compound. Furthermore, in this specification and other materials, the positive electrode active material of one embodiment of the present invention preferably comprises a composition. Furthermore, in this specification and other materials, the positive electrode active material of one embodiment of the present invention preferably comprises a complex.

[0040] In this specification, etc., characteristics of positive electrode active materials are sometimes described, but it is not necessary for all positive electrode active materials included in the positive electrode to have that characteristic. For example, as long as three or more of five or more positive electrode active material particles are randomly selected to have that characteristic, it can be said that it has the effect of sufficiently improving the characteristics of the positive electrode active material and the secondary battery including the positive electrode active material.

[0041] In this instruction manual, an internal short circuit in a secondary battery refers to the phenomenon where the positive and negative terminals inside the battery come into contact. Furthermore, the term "external short circuit" in a secondary battery is a misnomer and refers to the phenomenon where the positive and negative terminals outside the battery come into contact. An internal short circuit in a secondary battery can be intentionally induced through a nail penetration test.

[0042] In this specification, ignition in a needle penetration test refers to the observation of open flame or thermal runaway of the secondary battery on the exterior of the outer packaging within one minute after the needle is punctured. For example, if thermal decomposition products of the positive and / or negative electrodes are observed at a distance of more than 2 cm from the needle after the needle penetration test, this situation is referred to as thermal runaway. Thermal decomposition products of the positive and / or negative electrodes include, for example, alumina formed from the oxidation of aluminum foil used for the positive electrode current collector, and copper oxide formed from the oxidation of copper foil used for the negative electrode current collector. Even if open flame, sparks, and / or smoke are observed during the needle penetration test, if they occur only at the needle location without fire spread and no thermal runaway of the secondary battery occurs, this is not considered ignition. For example, if no ignition occurs even after a needle penetration test on the secondary battery, it can be said that the secondary battery is a non-ignitable secondary battery.

[0043] In this specification, unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) included in a secondary battery in a state prior to degradation are described. Furthermore, degradation is not considered to occur when the charging capacity and / or discharging capacity decreases due to aging and burn-in processes during the manufacturing of the secondary battery. For example, a state prior to degradation may be considered to occur when the discharge capacity of a secondary battery consisting of a single cell or a battery pack is 97% or more of its rated capacity. When using secondary batteries for portable devices, the rated capacity is based on JIS C 8711:2019. When using secondary batteries other than those described above, the JIS standard is not limited to the above-mentioned JIS standard; various JIS and IEC standards, such as those for electric vehicle propulsion and industrial applications, are used.

[0044] In this specification and the like, a lithium-ion secondary battery refers to a battery that uses lithium ions as carrier ions; however, the carrier ions of the present invention are not limited to lithium ions. For example, alkali metal ions or alkaline earth metal ions can be used as carrier ions in the present invention, specifically sodium ions, etc. In this case, lithium ions can be referred to as sodium ions, etc., to understand the present invention. Furthermore, when there are no restrictions on the carrier ions, it is sometimes referred to as a secondary battery.

[0045] In this specification, the (001) surface and the (003) surface are sometimes collectively referred to as the (00l) surface. Note that the (00l) surface is sometimes referred to as the C surface, the base surface, etc. In addition, lithium ions in lithium cobalt oxide have a two-dimensional diffusion path. That is, it can be said that the diffusion path of lithium ions exists along the surface. In this specification, the surface other than the surface where the diffusion path of lithium ions is exposed, i.e., the surface where lithium is inserted and removed (specifically, the (001) surface), is sometimes referred to as the edge surface.

[0046] In this specification, the loading capacity refers to the weight of the active material per unit area of ​​the current collector surface. The loading capacity of the negative electrode active material can be adjusted according to the capacity of the positive electrode. In the two-sided coating of the current collector with a slurry containing the active material, the weight of one surface is considered as the loading capacity.

[0047] In this specification, secondary particles refer to particles formed by the aggregation of primary particles. In this specification, single particles refer to particles that do not exhibit grain boundaries. In this specification, single-crystal particles refer to crystalline particles that do not have grain boundaries within them, while polycrystalline particles refer to crystalline particles that have grain boundaries within them. Polycrystalline particles may also be referred to as aggregates of multiple grains, and grain boundaries may be referred to as interfaces existing between two or more grains.

[0048] In this specification, etc., it is sometimes written as "A and / or B", which is an example of a statement that includes "A only", "B only" and "A and B".

[0049] (Implementation Method 1) The needle penetration test is a test in which a secondary battery is brought to a fully charged state and punctured at a certain rate using a steel needle of a specified diameter selected from 2 mm to 20 mm. A full charge refers to a state where the charge rate, expressed as State of Charge (SOC), is 100%. In this embodiment, the needle penetration test apparatus will first be described.

[0050] <Needle Puncture Testing Device> Figure 1A A cross-sectional view of a needle penetration testing apparatus 1000 is shown. The needle penetration testing apparatus 1000 includes a stage 1001, a drive unit 1002, a steel needle 1003, a voltage measuring instrument 1015, a temperature measuring instrument 1016, and a control unit 1018. The drive unit 1002 includes a drive mechanism 1012 that moves the steel needle 1003 in the direction of the arrow in the drawing. The drive mechanism 1012 moves the steel needle 1003 through a secondary battery 1004 disposed on the stage 1001. At this time, the secondary battery 1004 is fully charged, and this action is referred to as the needle penetration action. Note that... Figure 1A The dotted line in the figure represents the recess of the stage 1001 provided to accommodate the steel needle 1003 that passes through the secondary battery 1004 during the needle-piercing action.

[0051] The voltage measuring instrument 1015 transmits information about the voltage of the secondary battery during the needle's movement to the control unit 1018. Specifically, it transmits information such as voltage changes to the control unit 1018. Furthermore, the temperature measuring instrument 1016 transmits information about the temperature during the needle's movement to the control unit 1018. The control unit 1018 can transmit control signals to the drive unit 1002 when controlling the operating conditions of the steel needle 1003.

[0052] Figure 1B This is a perspective view illustrating the upper part of the stage 1001 of the needle penetration testing apparatus 1000. A secondary battery 1004, mounted on the stage 1001, is electrically connected to wiring 1005a and wiring 1005b. Note that wiring 1005a and wiring 1005b are included in a voltage measuring instrument 1015, and are electrically connected to the positive and negative terminals of the secondary battery 1004, respectively, enabling voltage measurement of the secondary battery 1004. The voltage of the secondary battery 1004 is referred to as voltage, voltage between the positive and negative terminals, battery voltage, or open-circuit voltage. When used as a temperature measuring instrument 1016, the temperature sensor is positioned in contact with the surface of the outer casing of the secondary battery 1004.

[0053] Figure 1B This shows that a first temperature sensor 1006a and a second temperature sensor 1006b are configured in the secondary battery 1004. Figure 1A The example shown is of a steel needle 1003 equipped with a third temperature sensor 1006c, but one or more temperature sensors can also be configured in the secondary battery 1004. In the secondary battery 1004, a first temperature sensor 1006a is configured on the side without wiring 1005a and 1005b, and a second temperature sensor 1006b is configured on the side with wiring 1005a and 1005b. By configuring two or more temperature sensors in this way, even if one temperature sensor cannot be used due to expansion of the outer packaging, other temperature sensors can be used, which is preferred.

[0054] Furthermore, there is an adhesive area along the edge where wiring 1005a and wiring 1005b are provided, but since the outer packaging is folded along the edge where wiring 1005a and wiring 1005b are not provided, there is no such adhesive area. Therefore, even if the outer packaging expands, the expansion on the side where wiring 1005a and wiring 1005b are not provided is suppressed, and the first temperature sensor 1006a is not easily peeled off compared to the second temperature sensor 1006b, so it is preferred.

[0055] Figure 1BThe dashed ellipse in the figure represents the area where the steel needle 1003 penetrates the secondary battery 1004 during the puncture action. Preferably, the first temperature sensor 1006a and the second temperature sensor 1006b, located in the secondary battery 1004, are positioned in an area equidistant from the area penetrated by the steel needle 1003. Typically, it is preferred to position the first temperature sensor 1006a and the second temperature sensor 1006b within 5 cm, and more preferably within 2 cm, of the area penetrated by the steel needle 1003. This sensor placement allows for monitoring of temperature changes in the area penetrated by the steel needle 1003 and its vicinity, which is therefore preferred. Here, "vicinity" refers to the area within 1 cm of the penetration area. Furthermore, when two or more temperature sensors are configured, it is preferable to begin the puncture action only after confirming that the temperature difference displayed by the two temperature sensors is within ±5°C, preferably within ±2°C.

[0056] Secondary batteries in the needle penetration test Next, use Figure 2A and Figure 2B Let's reiterate the state of the secondary battery in the nail penetration test. The nail penetration test is a test in which the secondary battery 1004 is fully charged, and a steel needle 1003 is used to puncture the secondary battery 1004 at a certain rate. The diameter of the steel needle 1003 is more than 2 mm and less than 10 mm. Figure 2A This is a cross-sectional view showing the secondary battery 1004 pierced by a steel needle 1003. The secondary battery 1004 has a structure in which a positive electrode 503, a separator 508, a negative electrode 506, and an electrolyte 530 are contained in an outer packaging 531. The positive electrode 503 includes a positive current collector 501 and positive active material layers 502 formed on both surfaces of the positive current collector 501. The negative electrode 506 includes a negative current collector 511 and negative active material layers 512 formed on both surfaces of the negative current collector 511. The positive active material layer is a layer that contains at least positive active material, such as conductive material and / or adhesive. The negative active material layer is a layer that contains at least negative active material, such as conductive material and / or adhesive. Figure 2B An enlarged view of the steel needle 1003 and the positive current collector 501 and its vicinity is shown, as well as the positive active material 100 and the conductive material 553 in the positive active material layer 502.

[0057] like Figure 2A and Figure 2B As shown, when the secondary battery 1004 is pierced by a steel needle 1003, and the steel needle 1003 penetrates both the positive electrode 503 and the negative electrode 506, an internal short circuit occurs. Consequently, the potential of the steel needle 1003 becomes equal to the potential of the negative electrode 506, and electrons (e...)... -The material flows through the steel needle 1003, as indicated by the black arrow, to the positive electrode 503, generating Joule heating in and around the internal short-circuit section. Furthermore, carrier ions, typically lithium ions (Li- ions), detach from the negative electrode 506 due to the internal short circuit. + As indicated by the white arrow, the lithium ions are released into the electrolyte. However, due to the Joule heating generated by the internal short circuit, the temperature of the secondary battery rises sharply, causing the electrolyte to begin reducing and decomposing on the surface of the negative electrode before the lithium ions are completely released. This is one of the electrochemical reactions known as the electrolyte reduction reaction caused by the negative electrode.

[0058] When the temperature of the Joule-heated secondary battery 1004 rises, lithium cobalt oxide, used as the positive electrode active material, undergoes a phase change (i.e., structural change) from an H1-3 type structure to an O1 type structure, sometimes accompanied by heat generation. Note that the H1-3 type structure and the O1 type structure will be explained later. Heat generation occurs continuously when an internal short circuit occurs.

[0059] And, as Figure 2A and Figure 2B As shown, due to the electrons (e) flowing to the positive electrode 503 - In lithium cobalt oxide under charging conditions, tetravalent Co is reduced to trivalent or divalent Co. This reduction reaction releases oxygen from the lithium cobalt oxide, and the electrolyte 530 is decomposed due to oxidation caused by this oxygen. This is one of the electrochemical reactions known as the oxidation reaction of the electrolyte caused by the positive electrode. The current flow rate into the positive electrode active material 100 is considered to affect the above-mentioned electrochemical reaction, and the current flow rate can be reduced according to the insulation properties of the positive electrode active material.

[0060] As mentioned above, when an internal short circuit occurs in a secondary battery, the temperature is considered to be as follows: Figure 3 The chart shown changes as it does. Figure 3 This is a modified version of the graph shown on page 70 of Non-Patent Document 13 [Figure 2-12], which is a graph showing the temperature (specifically, the internal temperature) of a secondary battery over time. When an internal short circuit occurs at (P0), the temperature of the secondary battery increases over time. As shown in (P1), the heating caused by Joule heating continues until the temperature of the secondary battery rises to or near 100°C, exceeding the battery's reference temperature (Ts). Consequently, in (P2), reduction and heating of the electrolyte occur due to the negative electrode (C6Li when using graphite); in (P3), oxidation and heating of the electrolyte occur due to the positive electrode; and in (P4), heating occurs due to the thermal decomposition of the electrolyte. Then, the secondary battery experiences thermal runaway and ignition.

[0061] Thermal runaway of secondary batteries The principle of thermal runaway in secondary batteries Figure 4The diagram shown is a modified version of the diagram cited on page 69 of Non-Patent Document 13 [Figure 2-11]. Such secondary batteries, for example, experience thermal runaway through several states when the temperature (specifically, the internal temperature) rises during charging. Figure 4 This is a graph of the temperature of a secondary battery relative to time. For example, when the temperature of the secondary battery reaches 100°C or near, (1) the SEI (Solid Electrolyte Interphase) of the negative electrode is destroyed and heat is generated. In addition, when the temperature of the secondary battery exceeds 100°C, (2) the reduction and heat generation of the electrolyte caused by the negative electrode (when using graphite, the negative electrode is C6Li) occurs, and (3) the oxidation and heat generation of the electrolyte caused by the positive electrode occurs at 150°C or near. Furthermore, when the temperature of the secondary battery reaches 180°C or near, (4) the thermal decomposition of the electrolyte occurs, (5) oxygen is released from the positive electrode and the thermal decomposition of the positive electrode occurs (this thermal decomposition includes structural changes of the positive electrode active material). Then, when the temperature of the secondary battery exceeds 200°C, (6) the negative electrode decomposes, and finally (7) the positive and negative electrodes come into direct contact. After such states, especially after states (5), (6), or (7), the secondary battery leads to thermal runaway.

[0062] It is considered that, in order to prevent thermal runaway, it is preferable to suppress the temperature rise of the secondary battery and to ensure the stability of the negative electrode, positive electrode, and / or electrolyte at high temperatures. Examples of structures that suppress temperature rise include those that provide a stable current flow rate to the positive electrode active material. Furthermore, as for structures that exhibit stable positive electrode properties at high temperatures, specifically, structures that prevent the positive electrode active material from releasing oxygen at high temperatures are examples. In such cases, a significant effect can be expected in secondary batteries that are less prone to thermal runaway and thus less likely to catch fire. One embodiment of the present invention provides a positive electrode active material that simultaneously possesses both the aforementioned stable structure and the structure that provides a stable current flow rate.

[0063] [Positive electrode active material] This invention describes a positive electrode active material 100 according to one aspect of the present invention. As the positive electrode active material 100, a material that can support ions (typically lithium ions, Li) is used. + Compounds containing transition metals and oxygen that can be inserted and released are acceptable. One or more transition metals selected from cobalt (Co), nickel (Ni), manganese (Mn), and iron (Fe) can be used. Figure 5A An example of the cross-sectional structure of the positive electrode active material 100 is shown. The positive electrode active material 100 includes a surface portion 100a and an interior portion 100b, with the (00l) surface of the lithium ion diffusion surface indicated by dashed lines.

[0064] <Main Ingredients> Preferably, in one embodiment of the present invention, the positive electrode active material 100 uses cobalt as the main component of the transition metal M that carries out the redox reaction. Note that the main component of the transition metal M in this specification refers to the component with the highest atomic number ratio in the transition metal M. For example, lithium cobalt oxide can be used in the positive electrode active material 100 as a compound in which Co is used as the transition metal. Alternatively, the positive electrode active material 100 preferably contains lithium cobalt oxide (LiCoO2) with added elements described later. Note that the positive electrode active material 100 in one embodiment of the present invention preferably has the crystal structure described later. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O = 1:1:2.

[0065] In one embodiment of the present invention, the positive electrode active material 100 preferably includes an insulating region or a region with high resistance. This region preferably has a narrow width of 1 nm or more and 20 nm or less, more preferably 2 nm or more and 10 nm or less, and more preferably 2 nm or more and 5 nm or less when viewed in cross-section. In this specification, this narrow region is sometimes referred to as a "shell". A cross-sectional STEM (Scanning Transmission Electron Microscope) image can be used as an example of a cross-sectional view.

[0066] During a needle penetration test, in the positive electrode active material, due to the rapid influx of electrons, cobalt is reduced from tetravalent to divalent, thereby releasing oxygen from the positive electrode active material. Because this reaction is exothermic, it accelerates thermal runaway. To suppress this reaction, for example, the positive electrode active material 100 of the present invention has a structure, described later, of a shell 100s, that makes oxygen release difficult. When oxygen is not released from the positive electrode active material, the aforementioned reduction reaction (e.g., Co...) is... 4+ to become Co 2+ The reaction was also inhibited.

[0067] Figure 5B The diagram shows a positive electrode active material 100 including a shell 100s. The positive electrode active material 100 including the shell 100s has increased resistance, therefore, even during a needle penetration test, the current flow rate into the positive electrode active material 100 can be kept stable, thus suppressing fires, etc., making it preferable. To ensure a stable current flow rate into the positive electrode active material 100, the high-resistance shell 100s is more preferably located in the surface layer 100a.

[0068] <Add element> The positive electrode active material 100 preferably contains additive elements. Examples of additive elements include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium, and it is preferred to use one or more additive elements selected from the above elements.

[0069] As additive elements, it is not necessary to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium. For example, by manufacturing a positive electrode active material 100 that substantially does not contain manganese, advantages such as easier synthesis, easier processing, and good cycle characteristics can be improved. The weight of manganese contained in the positive electrode active material 100 is preferably 600 ppm or less, and more preferably 100 ppm or less.

[0070] Magnesium is one of the suitable elements for the formation of the shell 100s. Specifically, magnesium is preferred as an additive element that does not easily release oxygen because the closer oxygen is to magnesium, the greater the energy required for its removal. Magnesium also has the function of stabilizing the crystal structure of the positive electrode active material, so it is suitable as an additive element. While stabilizing the current flow rate into the positive electrode active material 100, magnesium can maintain a stable crystal structure.

[0071] When viewed in cross-section, the magnesium in the positive electrode active material 100 preferably exists with a relatively small width, such as 1 nm or more but less than 20 nm, more preferably 2 nm or more but less than 10 nm, and more preferably 2 nm or more but less than 5 nm, at a distance from the surface. Magnesium is preferably present in the shell 100s on the edge surface at a higher concentration than the shell 100s on the base surface.

[0072] The shell 100s contains at least magnesium, thus suppressing oxygen release from this region and improving thermal stability. Therefore, it can be considered a structure less prone to thermal runaway. Thus, as... Figure 5A As shown, the shell 100s is preferably provided in a manner that fully covers the entire positive electrode active material 100, and when viewed in cross-section, the shell 100s is preferably formed in a region of 60% or more, more preferably in a region of 80% or more. In addition, it is preferable to form a uniform shell 100s, but it is also possible to form a shell 100s that is thicker in specific regions of the positive electrode active material 100, such as the surface other than the (00l) surface.

[0073] Note that the positive electrode active material 100 can be encased in any manner as long as it does not ignite during the needle penetration test, and as long as lithium ions (Li) can be ignited. + The insertion and detachment can simultaneously stabilize the rate of current inflow caused by internal short circuits, allowing magnesium to be located outside the shell 100s, or even inside the shell 100b.

[0074] Furthermore, considering rapid charging and discharging, the positive electrode active material 100 may preferably have a region without a shell 100s.

[0075] When magnesium is added to the positive electrode active material 100, magnesium fluoride is preferably used as the magnesium source. Fluorine can also be added to the positive electrode active material 100 by using magnesium fluoride. That is, the positive electrode active material 100 sometimes preferably contains both magnesium and fluorine as additive elements. In tests such as nail penetration, lithium and fluorine sometimes react, and the heat generated in this reaction is suppressed compared to when lithium and oxygen react, thereby suppressing the temperature rise of the secondary battery.

[0076] Furthermore, in one embodiment of the present invention, the positive electrode active material 100 preferably includes magnesium and nickel as additive elements. In other words, nickel is preferably also present in the shell 100s. Nickel is preferably present in the shell 100s at a higher concentration than that at the base surface of the shell 100s, i.e., at a higher concentration than that at the base surface of the shell 100s. By employing this structure, oxygen detachment from the positive electrode active material or structural changes in the positive electrode active material can be suppressed.

[0077] Note that the aforementioned added elements are preferably present at least in the shell 100s. In other words, the aforementioned added elements are preferably present at least in the surface layer 100a. Added elements that contribute to the stability of the crystal structure of the positive electrode active material 100 are preferably present in the surface layer 100a, which is prone to degradation. The added elements are preferably present in the shell 100s at the edge surface in a manner that is greater than that at the base surface of the shell 100s, that is, at a higher concentration than that at the base surface of the shell 100s.

[0078] Here, as a method to confirm whether a shell 100s has formed in the positive electrode active material 100, a resistivity measurement (also called powder resistance) of the powder used as the positive electrode active material is employed. The powder resistance value when a shell 100s is formed in the positive electrode active material 100 is higher than that when no shell 100s is formed. Typically, when the powder resistance of the positive electrode active material containing added elements is higher than that of the positive electrode active material without added elements, it is possible that a shell 100s has formed in the positive electrode active material 100.

[0079] The shell 100s preferably contains cobalt in addition to added elements. When at least the shell 100s contains cobalt, it can facilitate the production of lithium ions (Li... + Simultaneous insertion and deintercalation can stabilize the rate of current inflow caused by internal short circuits. This allows for the smoothing of lithium-ion (Li) insertion and deintercalation. + From the perspective of embedding and detachment, the surface layer 100a preferably also contains added elements and cobalt.

[0080] The magnesium concentration in the lithium cobalt oxide casing 100s preferably satisfies a value greater than 0 atomic% and less than 10 atomic%; more preferably greater than 0 atomic% and less than 5 atomic%; and even more preferably greater than 0 atomic% and less than 2 atomic%. This magnesium concentration can be identified using methods such as line analysis with energy dispersive X-ray spectroscopy (EDX). When magnesium is present at a high concentration throughout the surface layer 100a, the insulation is high, making it difficult to obtain optimal battery characteristics in charge-discharge cycle tests. On the other hand, when magnesium is present at an appropriate concentration in the surface layer 100a, especially in a suitable area of ​​the casing 100s, it can stabilize the lithium cobalt oxide and suppress fires in tests such as the aforementioned needle penetration test, which is preferred. Furthermore, when magnesium is present at an appropriate concentration in the casing 100s, it is expected to improve the hardness of the lithium cobalt oxide.

[0081] in addition, Figure 5C The diagram shows a positive electrode active material 100 containing grain boundaries 101. Grain boundaries 101 can refer to, for example, portions where particles of the positive electrode active material 100 are bonded together; portions where the crystal orientation changes within the positive electrode active material 100, i.e., portions with repeated discontinuities of bright and dark lines in STEM images, portions containing multiple crystal defects, portions with disordered crystal structures, etc. Furthermore, crystal defects refer to defects observable through cross-sectional TEM, cross-sectional STEM images, etc., i.e., structures where other atoms have entered the inter-lattice, voids, etc. Grain boundaries 101 can be considered a type of planar defect. Additionally, the region near grain boundaries 101 refers to the area within 10 nm of the grain boundaries 101.

[0082] in addition, Figure 5C A positive electrode active material 100 with crack 102 is shown. The crack includes a region where the crystal plane deviates or the crystal plane is broken, and in many cases it is generated along the (00l) plane. It is preferable that such crack 102 is not observed, but it is acceptable to have crack 102 as long as no fire occurs during a needle penetration test. As a structure that does not fire during a needle penetration test, it is preferable that a shell is also formed in the surface portion that is reformed when crack 102 is generated. In addition, the vicinity of crack 102 refers to the region within 10 nm from crack 102.

[0083] <Crystallization> The positive electrode active material 100 preferably has high crystallinity, more preferably it is monocrystalline or polycrystalline. In particular, when the positive electrode active material 100 is monocrystalline, cracks are less likely to form even if the positive electrode active material 100 undergoes volume changes due to charging and discharging, so it is preferred. In other words, when the positive electrode active material 100 is monocrystalline, secondary batteries using the positive electrode active material 100 are considered less likely to catch fire, thus improving safety. The crystallinity of the positive electrode active material 100 is improved during the initial heating described later, so it is preferred. The positive electrode active material 100 is preferably a single particle (also called a primary particle) rather than a secondary particle.

[0084] <Median particle size (D50) of positive electrode active material> This section describes the median particle size (D50) of the positive electrode active material in a high-safety secondary battery. When the positive electrode active material is too small, coating during positive electrode manufacturing can be difficult. Furthermore, a small positive electrode active material has a large surface area, which may lead to excessive reaction between the surface of the positive electrode active material and the electrolyte. Additionally, a small positive electrode active material sometimes requires mixing with a large amount of conductive material, which may result in a decrease in capacity. Therefore, the median particle size (D50) of the positive electrode active material is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 9 μm or more. A small median particle size (D50) in the positive electrode active material reduces the likelihood of misalignment regions, making it preferred. A small median particle size (D50) in the positive electrode active material also reduces the likelihood of cracks forming during the pressing process, making it preferred as well.

[0085] On the other hand, when most of the active material is too small, there are concerns such as a decrease in the density of the positive electrode active material layer and an increase in side reactions with the electrolyte. In view of this, the median particle size (D50) of the positive electrode active material is preferably 20 μm or less, more preferably 18 μm or less, and even more preferably 15 μm or less.

[0086] In other words, the median particle size (D50) of the positive electrode active material can be arbitrarily combined with the above-mentioned upper and lower limits. For example, the median particle size is 1 μm or more and 20 μm or less, preferably 1 μm or more and 18 μm or less, and more preferably 1 μm or more and 15 μm or less.

[0087] The aforementioned median particle size (D50) can be measured, for example, by observation using SEM or TEM, or by a particle size analyzer using laser diffraction and scattering. When measured using a particle size analyzer using laser diffraction and scattering, the median particle size (D50) refers to the particle size at which the cumulative amount in the cumulative curve of the particle size distribution measurement results accounts for 50%. Furthermore, as a method for measuring the median particle size (D50) based on analysis using SEM or TEM, for example, a cumulative curve can be generated by measuring 20 or more particles, and the particle size at which the cumulative amount accounts for 50% can be set as the median particle size (D50).

[0088] <Surface of positive electrode active material> The surface of the positive electrode active material 100 refers to the surface of the composite oxide, including the aforementioned surface layer 100a and the interior layer 100b. This surface can be confirmed when viewed in cross-section. Therefore, the surface of the positive electrode active material 100 does not include: substances such as aluminum oxide (Al2O3) that do not contain lithium sites contributing to charging and discharging; or carbonates and hydroxyl groups that are chemically adsorbed after the manufacture of the positive electrode active material. Furthermore, the attached metal oxide refers, for example, to a metal oxide whose crystal structure differs from that of the interior layer 100b.

[0089] The positive electrode active material 100 is a compound containing transition metals and oxygen that allows lithium to intercalate and deintercalate. Therefore, the interface between the region containing transition metal M (e.g., Co, Ni, Mn, Fe, etc.) and oxygen that are oxidized or reduced during lithium intercalation and deintercalation, and the region without transition metal M and oxygen, is called the surface of the positive electrode active material. When analyzing the positive electrode active material, a protective film is sometimes used to cover the surface, but the protective film is not included in the positive electrode active material itself. As a protective film, single-layer or multi-layer films of carbon, metals, oxides, resins, etc., are sometimes used.

[0090] Therefore, the benchmark in STEM-EDX line analysis, etc., is the average value of the detected amount of characteristic X-rays of the transition metal M inside the positive electrode active material. AVE The point is 50% of the reference point. When describing the positive electrode active material with the left side of the reference point as the exterior and the right side as the interior, this reference point is sometimes referred to as the position of the surface of the positive electrode active material. Furthermore, in STEM-EDX line analysis, if the characteristic X-ray detection amount of the transition metal M to the left of the reference point is not sufficiently reduced, the characteristic X-ray detection amount of the transition metal M to the left of the reference point is sometimes referred to as the background, and the average value M of the characteristic X-ray detection amount of the transition metal M in this background is calculated. BG The average value of the detection amount of the internal transition metal M AVEThe point representing 50% of the sum is used as the reference point. Alternatively, the characteristic X-ray detection value of oxygen inside the positive electrode active material can be used instead of the transition metal M, and the reference point can be determined by substituting oxygen for the aforementioned transition metal M. Note that since oxygen is easily affected by external factors of the positive electrode active material, it is preferable to use the average value M of the characteristic X-ray detection value of the transition metal M. AVE The baseline point is calculated as 50%. Additionally, the average value of the X-ray detection quantity of the internal transition metal M is... AVE The location of 50% of the reference point and the average value M of the internal oxygen characteristics X-ray detection. AVE When the positions of 50% of the reference points are different, this difference may be due to the influence of oxygen-containing metal oxides, carbonates, etc., attached to the surface. Therefore, it is preferable to use the average value of the internal characteristic X-ray detection of the aforementioned transition metal M. AVE 50% of the points. Additionally, when using positive electrode active materials containing multiple transition metals M, the element with the highest detection amount of characteristic X-rays within the material can be used. AVE Or M AVE and M BG To obtain the reference point.

[0091] The average value M of the internal characteristic X-ray detection quantity AVE The value can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, in the region where the detection amount of characteristic X-rays of transition metal M and oxygen is stable or saturated, such as the region where the detection amount of characteristic X-rays of transition metal M begins to increase or the region where the detection amount of characteristic X-rays of oxygen begins to increase, up to a depth of 30 nm or more, preferably up to a depth of more than 50 nm. Additionally, the average background value M... BG For example, it can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, in the stable region of the detection quantity near the part where the detection quantity of the characteristic X-rays of the transition metal M begins to increase.

[0092] In addition, the surface of the positive electrode active material 100 in cross-sectional STEM images, etc., refers to the boundary between the region where an image of the crystal structure of the positive electrode active material is observed and the region where the image is not observed, and refers to the outermost side of the region where the atomic column originating from the atomic nuclei of a metal element with an atomic number greater than lithium in the metal elements constituting the positive electrode active material is confirmed.

[0093] Furthermore, the minimum spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the peak position (also known as the maximum value) of the characteristic X-rays corresponding to the added element may deviate by about 1 nm. For example, if the peak of the detected amount of characteristic X-rays corresponding to added elements such as magnesium is located to the left of the surface calculated above, an error can be considered as long as the difference between the peak and the surface is less than 1 nm.

[0094] In STEM-EDX line analysis, a peak refers to the maximum or maximum value of the characteristic X-ray corresponding to each element. Noise in STEM-EDX line analysis is considered in terms of measurements of half-width below the spatial resolution (R), such as R / 2.

[0095] By performing multiple scans on the same portion under identical conditions, the effects of noise can be reduced. For example, the cumulative values ​​from six scans can be used as a graph of the characteristic X-rays of each element. The number of scans is not limited to six; more than six scans can be performed and averaged to create a graph of the characteristic X-rays of each element.

[0096] STEM-EDX line analysis can be performed, for example, by the following steps. First, a protective film is deposited on the surface of the positive electrode active material under atmospheric conditions. For example, carbon can be deposited using the carbon coating unit of an ion sputtering device (MC1000 manufactured by Hitachi High Technology Corporation).

[0097] Next, the positive electrode active material is thinned to create a STEM cross-sectional sample. For example, thinning can be performed using a FIB-SEM apparatus (Hitachi High Technology Corporation's XVision200TBS). At this point, an MPS (microprobe system) is used for pickup, and the final processing conditions can be, for example, an accelerating voltage of 10 kV.

[0098] For example, the EDX detector installed in a STEM setup can be the Octane T Ultra W (Dual EDS) manufactured by EDAX Corporation. During EDX line analysis, the accelerating voltage of the STEM setup is set to 200 kV and the emission current is set to 6 μA or more and 10 μA or less. Measurements are taken on shallow, low-impact portions of a thinned sample. The magnification is, for example, around 150,000x. The EDX line analysis conditions are as follows: drift correction; linewidth of 42 nm; spacing of 0.2 nm; and at least six frames.

[0099] <Continuous Changes in Crystal Structure> Furthermore, it is preferable that the crystal structure changes continuously from the interior 100b to the surface due to the concentration gradient of the aforementioned added elements. Alternatively, it is preferable that the crystal orientation of the surface layer 100a and the interior 100b is substantially the same.

[0100] For example, preferably, the crystal structure continuously changes from the layered rock salt type interior 100b to the surface and outer layer 100a, which are rock salt type or have characteristics of both rock salt type and layered rock salt type. Alternatively, preferably, the crystal orientation of the outer layer 100a, which is rock salt type or has characteristics of both rock salt type and layered rock salt type, is substantially consistent with that of the layered rock salt type interior 100b.

[0101] Furthermore, in this specification, the layered rock-salt type crystal structure belonging to space group R-3m in composite oxides containing transition metals such as lithium and cobalt refers to a crystal structure with a rock-salt type ionic arrangement of alternating cations and anions, where the transition metals and lithium are regularly arranged to form a two-dimensional plane, thus allowing lithium to diffuse in two dimensions. Additionally, it may include defects such as vacancies of cations or anions. Strictly speaking, the layered rock-salt type crystal structure is sometimes a structure formed by lattice deformation of rock-salt type crystals, and the symmetry of the layered rock-salt type crystal structure is sometimes lower than that of the rock-salt type crystal structure.

[0102] Furthermore, rock salt-type crystal structures possess cubic crystal structures belonging to the space group Fm-3m, in which cations and anions are arranged alternately. Additionally, they may include vacancies of either cations or anions.

[0103] In addition, electron diffraction, TEM images, and cross-sectional STEM images can be used to determine whether a layered rock salt crystal structure or both characteristics of a rock salt crystal structure are present.

[0104] The cation sites in the rock salt crystal structure are indistinguishable, but in the layered rock salt crystal structure, there are two types of cation sites: one dominated by lithium and the other dominated by transition metals. Both rock salt and layered rock salt crystals have a stacked structure in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately. In the bright spots of the electron diffraction pattern forming the crystal planes of the above two-dimensional planes, with the origin 000 of the central spot (through the spot), the bright spot closest to the central spot is, for example, the bright spot of the (111) plane of the ideal rock salt crystal structure or the bright spot of the (003) plane of the ideal layered rock salt crystal structure. For example, when comparing the electron diffraction patterns of rock salt MgO and layered rock salt LiCoO2, the distance between the bright spots of the (003) plane of LiCoO2 is observed to be about half the distance between the bright spots of the (111) plane of MgO. Therefore, in cases where the analytical region includes two phases, such as MgO (rock salt type) and LiCoO2 (layered rock salt type), the electron diffraction pattern exhibits an alternating arrangement of bright spots with higher and lower brightness. Bright spots common to both rock salt and layered rock salt types are brighter, while bright spots occurring only in the layered rock salt type are dimmer.

[0105] Furthermore, in cross-sectional STEM images, when observing a layered rock-salt crystal structure from a direction perpendicular to the c-axis, alternating layers of higher and lower brightness are observed. Since the cation sites in the rock-salt type are indistinguishable, these characteristics are not observed. When employing a crystal structure exhibiting characteristics of both rock-salt and layered rock-salt types, when observed from a specific crystal orientation, alternating layers of higher and lower brightness are observed in cross-sectional STEM images, and a metal with an atomic number greater than lithium is present in the lower-brightness layer, i.e., a portion of the lithium layer.

[0106] The anions of layered rock salt crystals and rock salt crystals respectively form a cubic close-packed structure (face-centered cubic lattice structure). It can be assumed that the anions of O3' type and monoclinic O1(15) crystals, which will be described later, also form a cubic close-packed structure. Therefore, when layered rock salt crystals come into contact with rock salt crystals, there are crystal planes with consistent orientation of the cubic close-packed structure formed by the anions.

[0107] Furthermore, it can be explained as follows: The anions on the {111} facets of a cubic crystal have a triangular lattice. Layered rock salt has a rhombic structure belonging to space group R-3m, but for ease of understanding, it is usually represented by a composite hexagonal lattice; the (0001) facets of layered rock salt have a hexagonal lattice. The triangular lattice of the {111} facets of a cubic crystal has the same atomic arrangement as the hexagonal lattice of the (0001) facets of the layered rock salt. The lattice integration of both can be described as a state of consistent orientation in the closest packing of cubic structures.

[0108] Note that the space group for layered rock salt crystals and O3'-type crystals is R-3m, which differs from the space group Fm-3m (the general space group for rock salt crystals). Therefore, the Miller indices of crystal faces satisfying the above conditions differ between layered rock salt crystals, O3'-type crystals, and rock salt crystals. In this specification, the consistent orientation of the cubic closest packing structure formed by anions in layered rock salt crystals, O3'-type crystals, and rock salt crystals sometimes refers to a state where the crystal orientations are approximately consistent. Furthermore, a state where the crystal orientations are approximately consistent and have three-dimensional structural similarity or the same crystallographic orientation is called topological derivation.

[0109] The crystal orientation of two regions can be roughly determined using TEM images, STEM images, HAADF-STEM (High-angle Annular Dark Field STEM) images, ABF-STEM (Annular Bright Field STEM) images, electron diffraction patterns, and FFT patterns from TEM and STEM images. Additionally, XRD, electron diffraction, and neutron diffraction can be used as diagnostic criteria.

[0110] Figure 6 This example shows a TEM image where the orientations of layered rock salt crystals (LRS) and rock salt crystals (RS) are roughly the same. TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc., can all provide images that reflect the crystal structure.

[0111] For example, contrast originating from crystal planes can be obtained from high-resolution TEM images. Due to electron beam diffraction and interference, for example, when the electron beam is incident on the c-axis perpendicular to the layered rock-salt composite hexagonal lattice, repetitions of high-contrast bands (bright bands) and dark bands (dark bands) originating from the (0003) plane can be obtained. Therefore, repetitions of bright and dark lines are observed in TEM images, with bright lines (e.g., Figure 6 L in RS and L LRS When the angle between dark lines is less than 5 degrees or less than 2.5 degrees, it can be determined that the crystal planes are roughly the same, that is, the crystal orientation is roughly the same. Similarly, when the angle between dark lines is less than 5 degrees or less than 2.5 degrees, it can also be determined that the crystal orientation is roughly the same.

[0112] Furthermore, in HAADF-STEM images, contrast is obtained proportional to atomic number; the higher the atomic number of an element, the brighter it appears. For example, when using layered rock-salt type lithium cobalt oxide belonging to space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is scattered more strongly at the positions of cobalt atoms, resulting in the arrangement of cobalt atoms being observed as bright lines or high-brightness dots. Therefore, when observing lithium cobalt oxide with a layered rock-salt crystal structure in a direction perpendicular to the c-axis, the arrangement of cobalt atoms is observed as bright lines or high-brightness dots in this direction, while the arrangement of lithium and oxygen atoms is observed as dark lines or lower-brightness regions in the same direction. The same applies when fluorine (atomic number 9) and magnesium (atomic number 12) are included as additive elements in lithium cobalt oxide.

[0113] Therefore, in HAADF-STEM images, the repetition of bright and dark lines is observed in two regions with different crystal structures. When the angle between the bright lines is less than 5 degrees or less than 2.5 degrees, it can be determined that the atomic arrangement is roughly the same, that is, the crystal orientation is roughly the same. Similarly, when the angle between the dark lines is less than 5 degrees or less than 2.5 degrees, it can also be determined that the crystal orientation is roughly the same.

[0114] In addition, in ABF-STEM, elements with smaller atomic numbers appear brighter, but the contrast corresponding to the atomic number can be obtained in the same way as in HAADF-STEM, so the crystal orientation can be determined in the same way as in HAADF-STEM images.

[0115] Figure 7A An example of a STEM image showing that the orientations of layered rock salt crystals (LRS) and rock salt crystals (RS) are roughly the same. Figure 7B The FFT pattern of the region of rock salt-type crystal RS is shown. Figure 7C The FFT pattern of the region of layered rock salt type crystallized LRS is shown. Figure 7B and Figure 7C The left side shows the composition, the JCPDS card number, and the d-value and angle calculated from the data of the JCPDS card number. The right side shows the measured values. The spots marked with 'O' indicate zero-order diffraction.

[0116] exist Figure 7B The spot with A attached to it originates from the 11-1 reflection of the cubic crystal. Figure 7C The spot marked A in the image originates from the 0003 reflection of layered rock salt. From... Figure 7B and Figure 7C It can be seen that the orientation of the 11-1 reflection of the cubic crystal is roughly the same as the orientation of the 0003 reflection of the layered rock salt type. That is to say, it can be seen that after... Figure 7B The straight line of AO and passing through Figure 7C The lines AO are approximately parallel. Here, "approximately consistent" and "approximately parallel" refer to situations where the angle formed by the aforementioned lines is less than 5 degrees or less than 2.5 degrees.

[0117] As mentioned above, sometimes in FFT patterns and electron diffraction patterns, when the orientations of layered rock salt crystals and rock salt crystals are roughly the same, sometimes the layered rock salt crystals... <0003> The orientation is roughly the same as that of the <11-1> type of rock salt. In this case, their reciprocal lattice points are preferably speckled, that is, not continuous with other reciprocal lattice points. The speckled nature of the reciprocal lattice points and their non-continuity with other reciprocal lattice points indicates high crystallinity.

[0118] Furthermore, as mentioned above, when the orientation of the 11-1 reflection of the cubic crystal is approximately the same as the orientation of the 0003 reflection of the layered rock salt type, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt type are sometimes observed in reciprocal space, which is different from the orientation of the 0003 reflection of the layered rock salt type. For example, in Figure 7C The spot marked B originates from the 1014 reflection of the layered rock salt type. This spot sometimes appears at the inverted lattice point (0003 reflection) originating from the layered rock salt type. Figure 7C The observation is performed at locations where the azimuth difference between points A and B is greater than 52° and less than 56° (i.e., ∠AOB is greater than 52° and less than 56°) and the d value is greater than 0.19nm and less than 0.21nm. Note that the above index is just an example and does not need to be consistent with this index. For example, inverse grid points equivalent to 0003 and 1014 can also be used.

[0119] Similarly, sometimes spots not originating from cubic crystal 11-1 reflections are observed in reciprocal lattice space, in orientations different from those observed in cubic crystal 11-1 reflections. For example, in Figure 7B The spot marked with B originates from the 20° reflection of the cubic crystal. This diffraction spot sometimes coincides with the reciprocal lattice point originating from the 11-1 reflection of the cubic crystal. Figure 7B The observation is performed at locations where the azimuth difference between A and B is greater than 54° and less than 56° (i.e., ∠AOB is greater than 54° and less than 56°). Note that the above index is just an example and does not need to be consistent with it. For example, inverted grid points equivalent to 11-1 and 200 can also be used.

[0120] Note that it is known that layered rock salt-type cathode active materials such as lithium cobalt oxide tend to exhibit crystalline surfaces on the (0003) plane and its equivalent planes, as well as the (10-14) plane and its equivalent planes. Therefore, for example, when observing the (0003) plane using TEM, it is preferable to first select cathode active material particles whose crystalline surfaces are estimated to be (0003) using SEM, and then perform thin-film processing on these cathode active particles using FIB (Focused Ion Beam) or similar methods, so that the (0003) plane can be observed, for example, by electron beam incident at [12-10] in TEM. When it is necessary to determine the consistency of crystal orientation, thin-film processing is preferred to facilitate the observation of the layered rock salt-type (0003) plane.

[0121] <Characteristics of Secondary Batteries in Needle Penetration Test 1> When measuring the AC impedance of a fully charged secondary battery, the secondary battery preferably has an AC impedance value of 100 mΩ or less, and more preferably less than 90 mΩ, at a frequency of 1 kHz. It can be said that a secondary battery with such internal resistance has high safety. In a fully charged state, the charging capacity of the secondary battery preferably is 2000 mAh or more, more preferably 2400 mAh or more. Since a fully charged secondary battery may sometimes discharge over time, it is preferable to perform the AC impedance measurement within 24 hours, more preferably within 12 hours, and more preferably within 6 hours after reaching a fully charged state.

[0122] <Characteristics of Secondary Batteries in Needle Penetration Test 2> The temperature rise of the secondary battery during the needle penetration test, i.e., the difference between the temperature before the needle penetration test and the highest temperature reached after the needle penetration (also referred to as the temperature rise ΔT), is preferably below 100°C, more preferably below 70°C, and even more preferably below 50°C. The temperature is the temperature within 5 cm of the needle hole, preferably within 2 cm; specifically, it is the value output by a temperature sensor positioned within 5 cm of the needle hole, preferably within 2 cm. When the temperature sensor is positioned in contact with the outer packaging of the secondary battery, this temperature is the same as the temperature of the outer packaging.

[0123] Furthermore, the temperature of the steel needle 1003 is preferably below 100°C, more preferably below 80°C, and even more preferably below 60°C. This value is the output of a temperature sensor disposed on the steel needle 1003.

[0124] Furthermore, the maximum temperature of the secondary battery in the needle penetration test is preferably below 150°C, more preferably below 100°C, and even more preferably below 80°C. More preferably, the maximum temperature is below the temperature at which the electrolyte oxidizes due to the positive electrode. Even more preferably, this maximum temperature is below the flash point of the mixed organic solvent used in the electrolyte. When the flash point of the mixed organic solvent is unknown, the flash point of each organic solvent can be used as a reference.

[0125] <Characteristics of Secondary Batteries in Needle Penetration Test 3> Preferably, the loading of the positive electrode active material in the positive electrode of the secondary battery is set to 15 mg / cm³. 2 Above and 25mg / cm 2 The following is more preferably 18 mg / cm³ 2 Above and 23 mg / cm 2 The following is a further preferred value: 20 mg / cm³ 2 Above and 22mg / cm 2 The following describes how a highly safe rechargeable battery can be provided when such a load capacity is achieved.

[0126] The positive and negative electrode capacity ratio in a secondary battery is preferably 75% or more and 100% or less, more preferably 80% or more and 90% or less. Having such a positive and negative electrode capacity ratio provides a secondary battery with high safety. The positive and negative electrode capacity ratio will be explained in detail in the embodiments.

[0127] <Characteristics of Secondary Batteries in Needle Penetration Test 4> Preferably, the particles of the positive electrode active material in the secondary battery have very few cracks. For example, when observing the positive electrode active material particles by surface SEM or cross-sectional SEM, the number of cracks that can be observed in a single positive electrode active material particle is preferably 0 or more and 5 or less.

[0128] Cracks sometimes originate from the 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, it is preferable to set the pressure of the press to, for example, a linear pressure of 500 kN / m or less, more preferably a linear pressure of 300 kN / m or less, and even more preferably a linear pressure of 250 kN / m or less.

[0129] After the above pressing, the thickness of the positive electrode active material layer is preferably 70 μm or more and 80 μm or less, more preferably 75 μm or more and 78 μm or less.

[0130] <Crystal Structure of Positive Electrode Active Material> A positive electrode active material 100 of the present invention is illustrated by comparison with existing positive electrode active materials.

[0131] < <Li x The case where x is 1 in MO2 >> Figure 8 The crystal structure of a positive electrode active material 100 according to one embodiment of the present invention is shown. Preferably, the positive electrode active material 100 according to one embodiment of the present invention is in a discharged state, i.e., in Li x In MO2, when x = 1 (M is a transition metal, specifically cobalt and / or nickel), it exhibits a layered rock-salt type crystal structure belonging to space group R-3m. Layered rock-salt type composite oxides have large discharge capacity and a two-dimensional lithium-ion diffusion path, suitable for lithium-ion insertion and deintercalation, making them excellent positive electrode active materials for secondary batteries. Therefore, the interior 100b, which occupies a large portion of the volume of the positive electrode active material 100, preferably has a layered rock-salt type crystal structure. Figure 8 In this context, R-3m O3 represents a layered rock salt crystal structure. In R-3m O3, the lattice constant is a = 2.81610 × 10⁻⁶. -10 (m), b = 2.81610 × 10 -10 (m), c = 14.05360 × 10 -10(m), α = 90.0000, β = 90.0000, γ = 120.0000, and the coordinates of lithium, cobalt, and oxygen in the unit cell are Li(0, 0, 0), Co(0, 0, 0.5), and O(0, 0, 0.23951), respectively (Non-Patent Literature 10). In Figure 8 In this crystal structure, with O3 attached below the space group, lithium occupies octahedral sites and includes three MO2 layers in the unit cell; therefore, this crystal structure is sometimes called the O3-type structure. The MO2 layer refers to a continuous structure in which the transition metal M, with six oxygen atoms aligned to octahedral sites, shares edges on a single plane. Sometimes this structure is referred to as a layer composed of octahedral transition metal M and oxygen atoms. Furthermore, although in... Figure 8 Lithium ions are present in all lithium sites, but as mentioned above, ions of added elements, such as magnesium ions, are sometimes located in lithium sites.

[0132] On the other hand, in one embodiment of the present invention, the surface portion 100a of the positive electrode active material 100 preferably strengthens the layered structure of the inner 100b, composed of transition metal M and oxygen octahedra, even when lithium is detached from the positive electrode active material 100 due to charging, to prevent the collapse of the layered structure. Alternatively, the surface portion 100a is preferably used as a barrier film for the positive electrode active material 100. Alternatively, the surface portion 100a of the outer periphery of the positive electrode active material 100 preferably strengthens the positive electrode active material 100. Here, strengthening refers to suppressing structural changes in the surface portion 100a and inner 100b of the positive electrode active material 100, such as oxygen detachment and / or deviation of the layered structure composed of transition metal M and oxygen octahedra, and / or suppressing the decomposition of organic electrolytes on the surface of the positive electrode active material 100.

[0133] Therefore, the surface layer 100a preferably has a different crystal structure than the interior layer 100b. Furthermore, the composition and crystal structure of the surface layer 100a at room temperature (25°C) are preferably more stable than those of the interior layer 100b. For example, at least a portion of the surface layer 100a of the positive electrode active material 100 of one embodiment of the present invention preferably has a rock salt-type crystal structure. Alternatively, the surface layer 100a preferably has both a layered rock salt-type crystal structure and a rock salt-type crystal structure. Alternatively, the surface layer 100a preferably exhibits characteristics of both a layered rock salt-type and a rock salt-type crystal structure.

[0134] The surface layer 100a is the region where lithium ions initially detach during charging, and it is also the region where the lithium concentration tends to decrease more easily than that of the inner layer 100b. Furthermore, it can be said that some of the atomic bonds on the surface of the positive electrode active material 100 particles contained in the surface layer 100a are broken. Therefore, it can also be said that the surface layer 100a is prone to becoming an unstable region where crystal structure degradation easily begins. For example, it is believed that when the crystal structure of the layered structure composed of transition metal M and oxygen octahedra in the surface layer 100a deviates, its effect extends to the inner layer 100b, and the crystal structure of the layered structure in the inner layer 100b also deviates, leading to the degradation of the overall crystal structure of the positive electrode active material 100. On the other hand, as long as the surface layer 100a can be sufficiently stabilized, in Li x When the x-value in CoO2 is relatively small, such as below 0.24, the layered structure of the inner 100b composed of transition metal M and oxygen octahedra is less prone to collapse. Furthermore, it can suppress the deviation of the inner 100b layers composed of transition metal M and oxygen octahedra.

[0135] The density of defects such as dislocations in the interior 100b of the positive electrode active material 100 is preferably low. Furthermore, the grain size of the positive electrode active material 100, as measured by XRD, is preferably large. In other words, the crystallinity of the interior 100b is preferably high. Additionally, the surface of the positive electrode active material 100 is preferably smooth. These characteristics are important factors supporting the reliability of the positive electrode active material 100 when used in a secondary battery. Higher reliability of the positive electrode active material allows for an increase in the upper limit of the charging voltage of the secondary battery, enabling the realization of a secondary battery with a large discharge capacity.

[0136] Internal 100b dislocations can be observed using TEM, for example. When the density of defects such as dislocations is sufficiently low, it is sometimes possible to observe them within a specific 1μm area of ​​the sample. 2 Defects such as dislocations cannot be observed in crystals. Dislocations are a type of crystal defect, distinct from point defects.

[0137] The grain size measured by XRD is preferably 300 nm or larger. A larger grain size, as explained later, is desirable in Li... x In CoO2, the smaller the x-value, the easier it is to maintain the O3'-type structure, and the reduction along the c-axis length is more easily suppressed.

[0138] It is believed that the fewer defects such as dislocations observed using TEM, the larger the grain size measured using XRD.

[0139] The XRD diffraction pattern used to calculate the grain size is preferably obtained in the state containing only the positive electrode active material, but it can also be obtained in the state where the positive electrode contains current collectors, binders, and conductive materials in addition to the positive electrode active material. Note that in the positive electrode state, the particles of the positive electrode active material may be oriented in a manner where the crystal planes of the particles are aligned in one direction due to the influence of pressure or other factors during the manufacturing process. When the degree of orientation is large, the grain size may not be accurately calculated. Therefore, it is more preferable to obtain the XRD diffraction pattern by removing the positive electrode active material layer from the positive electrode, removing the binder or other components in the positive electrode active material layer to some extent using a solvent or the like, and filling it into a sample holder or the like. In addition, there is a method in which grease is applied to a silicon non-reflective plate, and the powder sample of the positive electrode active material is attached to the silicon non-reflective plate.

[0140] The grain size can be determined, for example, from the Scherrer formula below.

[0141] [Formula 1]

[0142] In grain size calculations, for example, diffraction patterns obtained using Bruker D8 ADVANCE, CuKα X-rays as X-rays, 2θ between 15° and 90°, increment of 0.005, and a LYNXEYE XE-T detector can be used; and ICSD coll.code.172909 can be used as the literature value for lithium cobalt oxide. Analysis can be performed using DIFFRAC.TOPAS ver.6 crystal structure analysis software, for example, with the following settings. Emission Profile: CuKa5.lam Background: Chebychev polynomial, 5 times Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2nd angular range: 2.9 FDS angle: 0.3 Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5 Secondary Sollers: 2.5 Corrections Specimen displacement:Refine LP Factor: 0

[0143] Preferably, the LVol-IB value of the grain size calculated by the above method is used as the grain size. Note that when the calculated Preferred Orientation is less than 0.8, the orientation of the sample is too large, and therefore the sample is sometimes unsuitable for grain size calculation.

[0144] 〔distributed〕 Taking the discharge state as an example, the distribution of added elements in the positive electrode active material 100 is explained. To ensure a stable composition and crystal structure in the surface layer 100a, the surface layer 100a preferably contains added elements, and more preferably contains multiple added elements. Furthermore, the concentration of one or more added elements selected from the surface layer 100a is preferably higher than that in the interior 100b. Additionally, one or more added elements selected from the positive electrode active material 100 preferably have a concentration gradient. Furthermore, more preferably, the distribution of added elements in the positive electrode active material 100 is different. For example, more preferably, the depth of the peak of the added element detection amount from the surface is different. Here, the peak of the detection amount refers to the maximum value of the detection amount; the peak of the detection amount in the surface layer 100a is the maximum value of the detection amount in the surface layer 100a. Sometimes, for example, in EDX line analysis, when the vertical axis is count, the detection amount is called intensity, or when the vertical axis is atomic%, the detection amount is called concentration.

[0145] The detection amount of magnesium in the surface layer 100a is preferably greater than that in the interior layer 100b. Furthermore, it is preferable that the magnesium detection amount peaks are located in the region closer to the surface of the surface layer 100a.

[0146] Similar to magnesium, the detection amount of fluorine in the surface layer 100a is preferably greater than that in the interior layer 100b. Furthermore, it is preferable that a peak in the fluorine detection amount is present in a region closer to the surface within the surface layer 100a.

[0147] The detection amount of nickel in the surface layer 100a is preferably greater than that in the interior layer 100b. Furthermore, it is preferable that a peak in the nickel detection amount is present in the region closer to the surface of the surface layer 100a. For example, it is preferable that the nickel detection amount in the shell of the surface layer 100a is greater than the nickel detection amount in the region closer to the interior compared to the shell, and a peak in the nickel detection amount is present in the shell. Here, the ratio of the number of nickel (Ni) atoms to the number of cobalt (Co) atoms (Ni / Co) in the shell of the surface layer 100a is less than 1. That is, the number of nickel (Ni) atoms in the shell of the surface layer 100a is less than the number of cobalt (Co) atoms. Additionally, the ratio of the number of nickel (Ni) atoms to the number of cobalt (Co) atoms in the nickel detection peak is less than 1. The ratio of the number of nickel (Ni) atoms to the number of cobalt (Co) atoms in the region closer to the interior compared to the shell is less than the ratio of the number of nickel (Ni) atoms to the number of cobalt (Co) atoms in the shell. Note that the amount of nickel detected in the interior 100b is sometimes much smaller than that in the surface 100a. Therefore, in both the surface 100a and the interior 100b, the number of nickel (Ni) atoms is less than the number of cobalt (Co) atoms. Furthermore, the number of nickel (Ni) atoms in the positive electrode active material 100 is less than the number of cobalt (Co) atoms.

[0148] Furthermore, when both magnesium and nickel are present, the distributions of magnesium and nickel preferably overlap. Note that overlapping distributions include cases where the peaks of magnesium and nickel detection amounts are identical or the difference between their peaks is less than 3 nm; the peaks may also not overlap overall. For example, the peak of magnesium detection amounts may be closer to one side of the surface, and the peak of nickel detection amounts may also be closer to one side of the surface.

[0149] The detection amount of titanium in the surface layer 100a is preferably greater than that in the interior layer 100b. Furthermore, it is preferable that the detection amount of titanium has a peak in the region closer to the surface in the surface layer 100a.

[0150] The detection amount of silicon, phosphorus, boron and / or calcium added to the surface layer 100a is preferably greater than that to the interior layer 100b. Furthermore, it is preferable that peaks of silicon, phosphorus, boron and / or calcium are present in the surface layer 100a in regions closer to the surface.

[0151] The added element aluminum preferably has a peak of detectable quantity closer to the interior than magnesium. The distributions of magnesium and aluminum can overlap, or they can have almost no overlap. Note that overlapping distributions include cases where the peaks of magnesium and aluminum are identical or their peak differences are less than 3 nm; the peaks may also not overlap overall. The peak of aluminum detection can be present in the surface layer 100a or at a depth deeper than the surface layer 100a. When the peak of aluminum detection is deeper than the surface layer 100a, sometimes the distributions of magnesium and aluminum have almost no overlap. For example, aluminum preferably has a peak in the region from the surface to the interior, between 5 nm and 30 nm. This is considered because aluminum diffuses more easily than magnesium, thus diffusing deeper into the interior compared to magnesium.

[0152] The distribution of aluminum is sometimes not normally distributed. For example, in the case of aluminum with a maximum value Max... Al When segmenting the aluminum distribution curve, the tail length sometimes differs between the surface side and the interior side. When a vertical line is drawn from the maximum value to the horizontal axis, the maximum aluminum detection value (Max) is... Al ) of the height of 1 / 5 of the maximum height (1 / 5 Max) Al When the peak width at point (W) is divided into two, the peak width on the inner side (W) c Sometimes the peak width is greater than that on one side of the surface (W) s )big.

[0153] On the other hand, it is speculated that aluminum is more stable in regions where it is not in high concentrations of dissolved magnesium compared to regions where magnesium and other materials are dissolved in high concentrations, thus resulting in lower aluminum detection levels in areas closest to the surface. More specifically, compared to layered rock salt type LiAlO2, the cation-oxygen distance is longer in regions where magnesium is in high concentrations of dissolved magnesium in space group R-3m or cubic rock salt type regions, making it difficult for aluminum to exist stably. Furthermore, the presence of Co around cobalt... 3+ to become Co 2+ To make up for Li + Replaced with Mg 2+ The oxidation state of Al changes, thus maintaining cation balance. However, Al only takes the trivalent oxidation state, so it is believed that Al and magnesium are difficult to coexist in rock salt or layered rock salt structures.

[0154] Similar to aluminum, manganese, when added, preferably has a detectable peak closer to the interior than magnesium.

[0155] Note that it is not necessary for all added elements in the surface layer 100a of the positive electrode active material 100 to have the same distribution.

[0156] For example, the distribution of added elements in the region having the (001) orientation of the positive electrode active material 100 may differ from that in the region having a surface other than the (001) orientation. For example, the detection amount of one or more added elements selected from the (001) orientation surface and its surface layer 100a may be lower than that of the surface other than the (001) orientation. Specifically, the detection amount of nickel may also be lower. In particular, in analytical methods such as EDX that detect characteristic X-rays, the energies of Kβ for cobalt and Kα for nickel are close, making it difficult to detect trace amounts of nickel in materials where cobalt is the main element. Therefore, it is preferable to use not only line analysis but also point analysis to confirm the presence of nickel. Alternatively, the peaks selected from the (001) orientation surface region may be located at a shallower position from the surface compared to the region having a surface other than the (001) orientation. Specifically, the peaks of magnesium and aluminum may also be located at a shallower position from the surface. The region having the aforementioned surfaces is included in the surface layer 100a.

[0157] The reason why the peaks of magnesium and aluminum are located shallower than the surface in regions with a (001) orientation is examined. In the layered rock-salt type crystal structure belonging to R-3m, the cations are arranged parallel to the (001) plane. It can be said that this is a structure in which CoO2 layers and lithium layers are stacked alternately parallel to the (001) plane. Therefore, the diffusion path of lithium ions is also parallel to the (001) plane. The CoO2 layer is more stable, so it is more stable when the surface of the positive electrode active material 100 is (001) oriented. Therefore, magnesium and aluminum do not diffuse easily, and their peaks are located shallower than the surface.

[0158] Furthermore, the main diffusion path of lithium ions during charging and discharging is not exposed on the (001) surface. On the other hand, the diffusion path of lithium ions is exposed on surfaces other than the (001) orientation. Therefore, the surfaces other than the (001) orientation and their surface layer 100a are regions that are important for maintaining the diffusion path of lithium ions, and are also regions where lithium ions first detach, thus tending to be unstable. Therefore, in order to maintain the overall crystal structure of the positive electrode active material 100, it is preferable to strengthen the surfaces other than the (001) orientation and their surface layer 100a.

[0159] Therefore, regarding the positive electrode active material 100, sometimes the focus is on the concentration distribution of added elements on the surface and surface layer 100a other than the (001) orientation. It is preferable that added elements, especially magnesium, are detected on the surface and surface layer 100a other than the (001) orientation. On the other hand, in the (001) oriented surface and surface layer 100a, the concentration of added elements may be low, or the added elements may not be present. Nickel may also be absent from the (001) oriented surface and surface layer 100a.

[0160] For example, the half-width of the magnesium distribution on the (001) oriented surface and its surface layer 100a is preferably 10 nm or more and 200 nm or less, more preferably 50 nm or more and 150 nm or less, and even more preferably 80 nm or more and 120 nm or less. Furthermore, the half-width of the magnesium distribution on surfaces other than the (001) oriented surface and its surface layer 100a is preferably 200 nm or more and 500 nm or less, more preferably 200 nm or more and 300 nm or less, and even more preferably 230 nm or more and 270 nm or less. In other words, sometimes the half-width of the magnesium distribution on surfaces other than the (001) oriented surface and its surface layer 100a is greater than that on the (001) oriented surface and its surface layer 100a.

[0161] In addition, the half-width of the nickel distribution on the surface other than the orientation (001) and its surface layer 100a is preferably 30 nm or more and 150 nm or less, more preferably 50 nm or more and 130 nm or less, and even more preferably 70 nm or more and 110 nm or less.

[0162] In the manufacturing method described in the following embodiments, which involves heating after mixing the added elements, the added elements sometimes diffuse primarily through the diffusion path of lithium ions. Therefore, in order to set the distribution of the added elements in the surface other than the (001) orientation and its surface layer 100a within a preferred range, it is preferable to mix the added elements and heat them after manufacturing high-purity lithium cobalt oxide or after manufacturing lithium cobalt oxide with a smooth surface.

[0163] 〔magnesium〕 Magnesium is divalent. In layered rock-salt crystal structures, the added elements aluminum or nickel are stably present at cobalt sites. Therefore, magnesium ions readily reside at lithium sites rather than cobalt sites, i.e., they readily enter lithium sites. When magnesium is present at an appropriate concentration at lithium sites in the surface layer (100a), the layered rock-salt crystal structure can be easily maintained. This is because magnesium present at lithium sites acts as a support between CoO2 layers. Furthermore, in the presence of magnesium, for example in Li... x When the x value in CoO2 is below 0.24, the removal of oxygen from the magnesium can be suppressed, thereby inhibiting the thermal decomposition reaction. Furthermore, the density of the positive electrode active material 100 can be expected to increase in the presence of magnesium. Additionally, a high magnesium concentration in the surface layer 100a can be expected to improve resistance to corrosion from hydrofluoric acid produced by the decomposition of organic electrolytes, etc.

[0164] If magnesium has an appropriate concentration, it has no negative impact on lithium insertion and extraction during charging and discharging, thus enjoying the aforementioned advantages. However, excess magnesium may negatively affect lithium insertion and extraction. Furthermore, its contribution to crystal structure stabilization sometimes diminishes. This is because magnesium enters not only lithium sites but also cobalt sites. There are also concerns that unwanted magnesium compounds (e.g., oxides and fluorides) that do not substitute for lithium or cobalt sites may segregate onto the surface of the positive electrode active material, becoming a resistive component of the secondary battery. Additionally, an increase in the magnesium concentration of the positive electrode active material sometimes reduces its discharge capacity. This is because excess magnesium entering lithium sites reduces the amount of lithium contributing to charging and discharging.

[0165] Therefore, it is preferable that the positive electrode active material 100 contains an appropriate amount of magnesium. For example, the number of magnesium atoms is preferably 0.002 times or more and 0.06 times or less than the number of cobalt atoms, more preferably 0.005 times or more and 0.03 times or less, and even more preferably about 0.01 times. Here, the amount of magnesium in the positive electrode active material 100 can be, for example, a value obtained by analyzing all elements of the positive electrode active material 100 using GD-MS (glow discharge mass spectrometry), ICP-MS (inductively coupled plasma mass spectrometry), or a value based on the proportion of raw materials used in the manufacturing process of the positive electrode active material 100.

[0166] 〔fluorine〕 Fluorine is a monovalent anion. When fluorine is adsorbed on the surface of the positive electrode active material 100, the energy required for lithium to desorb from the positive electrode active material 100 decreases. Only when this energy decreases can fluorine be used to remove a portion of the oxygen in the representative layer 100a. This is because the redox potential of cobalt ions accompanying lithium desorption differs depending on the presence or absence of fluorine. That is, for example, without fluorine, cobalt ions change from trivalent to tetravalent as lithium desorption occurs. On the other hand, with fluorine, cobalt ions change from divalent to trivalent as lithium desorption occurs. The redox potential of cobalt ions differs in these two cases. Therefore, the desorption and insertion of lithium ions near fluorine readily occur, and it is preferable to include fluorine on the surface or surface layer of the positive electrode active material 100. When the fluorine-containing positive electrode active material 100 is used in a secondary battery, charge-discharge characteristics, high-current characteristics, etc., can be improved. Furthermore, by having fluorine present in the surface or surface layer of the portion in contact with the electrolyte, or by adsorbing or attaching fluorides to the surface, excessive reaction between the positive electrode active material 100 and the electrolyte can be suppressed. Furthermore, it can effectively improve resistance to hydrofluoric acid corrosion.

[0167] 〔nickel〕 Nickel can be present at either cobalt or lithium sites. When nickel is present at cobalt sites, its redox potential is lower than that of cobalt, so it can be said that it is more likely to lose lithium during charging. Therefore, an increase in charging speed can be expected.

[0168] Furthermore, the presence of nickel at lithium sites suppresses deviations from the layered structure formed by the octahedrons of cobalt and oxygen. Volume changes occurring during charging and discharging are also suppressed. Additionally, the elastic modulus increases, i.e., the material becomes harder. This is because the nickel present at the lithium sites also acts as a support between the CoO2 layers. Therefore, a more stable crystal structure can be expected, especially when charged at temperatures above 45°C, making it preferable.

[0169] Nickel can be present at either cobalt or lithium sites. When nickel is present at cobalt sites, its redox potential is lower than that of cobalt, thus it can be said that it readily loses lithium during discharge. Therefore, an increase in charge-discharge rate can be expected, i.e., an improvement in charge-discharge rate characteristics.

[0170] Furthermore, compared to the distance between cations and anions of MgO and CoO, the distance between cations and anions of nickel oxide (NiO) is closer to the average distance between cations and anions of LiCoO2, and its orientation is more likely to be consistent with that of LiCoO2.

[0171] Furthermore, the ionization tendency decreases in the order of magnesium, aluminum, cobalt, and nickel (Mg>Al>Co>Ni). Therefore, it can be considered that nickel is less likely to dissolve in the electrolyte during charging than the other elements mentioned above. Thus, it can be concluded that the effect of stabilizing the crystal structure of the surface layer is higher during charging.

[0172] Furthermore, nickel in Ni 2+ Ni 3+ Ni 4+ Chinese Ni 2+ Nickel is the most stable alloy, and its trivalent ionization energy is greater than that of cobalt. Therefore, it can be concluded that nickel and oxygen alone are insufficient to form a spinel-type crystal structure. Consequently, it can be considered that nickel has the effect of suppressing the phase transition from a layered rock salt-type crystal structure to a spinel-type crystal structure.

[0173] On the other hand, excessive nickel content amplifies the distortion caused by the Jameer-Taylor effect, making it less desirable. Furthermore, excessive nickel content can sometimes negatively impact lithium insertion and extraction.

[0174] Therefore, it is preferable that the positive electrode active material 100 contains an appropriate amount of nickel. For example, the number of nickel atoms contained in the positive electrode active material 100 is less than the number of cobalt atoms, preferably more than 0% and less than 7.5% of the number of cobalt atoms, more preferably more than 0.05% and less than 4%, further preferably more than 0.1% and less than 2%, and even more preferably more than 0.2% and less than 1%. Alternatively, it is preferable to exceed 0% and less than 4%. Alternatively, it is preferable to exceed 0% and less than 2%. Alternatively, it is preferable to exceed 0.05% and less than 7.5%. Alternatively, it is preferable to exceed 0.05% and less than 2%. Alternatively, it is preferable to exceed 0.1% and less than 7.5%. Alternatively, it is preferable to exceed 0.1% and less than 4%. The amount of nickel shown here can be, for example, a value obtained by elemental analysis of the positive electrode active material as a whole using GD-MS, ICP-MS, etc., or a value obtained based on the mixing of raw materials during the manufacturing process of the positive electrode active material.

[0175] 〔aluminum〕 Furthermore, aluminum may exist at cobalt sites in layered rock-salt-type crystal structures. Aluminum is a typical trivalent element with a constant valence, so lithium around aluminum does not easily migrate during charging and discharging. Therefore, aluminum and the surrounding lithium may act as pillars to suppress changes in the crystal structure. Thus, as explained later, even if the force of stretching and contracting in the c-axis direction of the positive electrode active material 100 due to lithium ion insertion and deintercalation—that is, the force of stretching and contracting in the c-axis direction due to changes in charging depth or charging rate—acts, the degradation of the positive electrode active material 100 can be suppressed.

[0176] Furthermore, aluminum has the effect of suppressing the dissolution of surrounding cobalt, thereby improving continuous charging tolerance. Additionally, the Al-O bond is stronger than the Co-O bond, thus suppressing oxygen desorption around the aluminum. Through these effects, thermal stability is improved. Therefore, including aluminum as an additive element can improve the safety of using the positive electrode active material 100 in secondary batteries. Furthermore, it is possible to achieve a positive electrode active material 100 that is not prone to crystal structure collapse even after repeated charge and discharge cycles.

[0177] On the other hand, excessive aluminum content may negatively affect lithium insertion and extraction.

[0178] Therefore, it is preferable that the positive electrode active material 100 contains an appropriate amount of aluminum. For example, the number of aluminum atoms in the positive electrode active material 100 is preferably 0.05% or more and 4% or less of the number of cobalt atoms, more preferably 0.1% or more and 2% or less, and even more preferably 0.3% or more and 1.5% or less. Alternatively, it is preferably 0.05% or more and 2% or less. Alternatively, it is preferably 0.1% or more and 4% or less. The amount in the positive electrode active material 100 shown here can be, for example, a value obtained by elemental analysis of the positive electrode active material 100 using GD-MS, ICP-MS, etc., or a value obtained based on the mixing of raw materials during the manufacturing process of the positive electrode active material 100.

[0179] Furthermore, titanium oxide is known to be superhydrophilic. Therefore, by manufacturing a positive electrode active material 100 containing titanium oxide in the surface layer 100a, it sometimes exhibits good wettability to highly polar solvents. During the manufacture of secondary batteries, the positive electrode active material 100 maintains good contact with the highly polar electrolyte interface, and can sometimes suppress the increase in internal resistance.

[0180] Furthermore, by including phosphorus in the surface layer 100a, it is possible to maintain Li x When the x value in CoO2 is relatively small, short circuits can sometimes be suppressed, so it is preferred. For example, it is preferred to be present in the surface layer 100a as a compound containing phosphorus and oxygen.

[0181] When the positive electrode active material 100 contains phosphorus, the hydrogen fluoride produced by the decomposition of the electrolyte or lithium salt may react with phosphorus, thereby potentially reducing the concentration of hydrogen fluoride in the electrolyte, which is therefore preferred.

[0182] In the presence of LiPF6 in lithium salts, hydrogen fluoride is sometimes generated due to hydrolysis. Additionally, hydrogen fluoride is sometimes generated due to the reaction of polyvinylidene fluoride (PVDF), a component of the positive electrode, with an alkali. Reducing the concentration of hydrogen fluoride in the organic electrolyte can sometimes suppress corrosion of the current collector and / or peeling of the cover 104 (see Figure 15). Furthermore, it can sometimes suppress the reduction in adhesion caused by the gelation and / or insolubility of PVDF.

[0183] When the positive electrode active material 100 contains phosphorus in addition to magnesium, it in Li xThe crystal structure of CoO2 with a small x-value is extremely stable, and is therefore preferred. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, it is preferably 1% to 10%. Alternatively, it is preferably 1% to 8%. Alternatively, it is preferably 2% to 20%. Alternatively, it is preferably 2% to 8%. Alternatively, it is preferably 3% to 20%. Alternatively, it is preferably 3% to 10%. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and even more preferably 0.7% to 4%. Alternatively, it is preferably 0.1% to 5%. Alternatively, it is preferably 0.1% to 4%. Alternatively, it is preferably 0.5% to 10%. Alternatively, it is preferably 0.5% to 4%. Alternatively, it is preferred to be 0.7% or more and 10% or less. Alternatively, it is preferred to be 0.7% or more and 5% or less. The phosphorus and magnesium concentrations shown here can be values ​​obtained, for example, from elemental analysis of the positive electrode active material 100 as a whole using GD-MS, ICP-MS, etc., or values ​​obtained from the mixing of raw materials during the manufacturing process of the positive electrode active material 100.

[0184] Furthermore, when crack 102 is formed on a portion of the surface of positive electrode active material 100, the propagation of crack may be suppressed when phosphorus is present in the embedded portion in contact with that portion of the surface, or more specifically, when a compound containing phosphorus and oxygen is present.

[0185] [Synergistic effect of multiple elements] Furthermore, when both magnesium and nickel are present in the surface layer 100a, divalent nickel may exist more stably near divalent magnesium. Therefore, in Li x The dissolution of magnesium in CoO2 with a lower x value may also be suppressed. Thus, magnesium and nickel contribute to the stabilization of the surface layer 100a.

[0186] For the same reason, in the manufacturing process, it is preferable to add magnesium before adding nickel when adding additive elements to lithium cobalt oxide. Alternatively, it is preferable to add magnesium and nickel in the same step. Magnesium has a large ionic radius and tends to remain in the surface layer of lithium cobalt oxide regardless of which step it is added in. In contrast, in the absence of magnesium, nickel is likely to diffuse extensively into the interior of lithium cobalt oxide. Therefore, when nickel is added before magnesium, nickel diffuses into the interior of lithium cobalt oxide and may not remain in an appropriate amount in the surface layer.

[0187] Furthermore, using additive elements with different distributions simultaneously can stabilize the crystal structure over a wider area, which is preferable. For example, when the positive electrode active material 100 uses magnesium, nickel, and aluminum, it can stabilize the crystal structure over a wider area compared to including only one of these elements. Thus, when the positive electrode active material 100 contains additive elements with different distributions, magnesium, nickel, etc., can sufficiently stabilize the surface, so aluminum is not needed on the surface. Instead, aluminum is preferably widely distributed in a deeper area. For example, aluminum is preferably continuously detected in a region at a depth of 1 nm or more and 25 nm or less from the surface. When it is widely distributed in a region at a depth of 0 nm or more and 100 nm or less from the surface, preferably at a depth of 0.5 nm or more and 50 nm or less from the surface, it can stabilize the crystal structure over a wider area, which is preferable.

[0188] When multiple additive elements are included as described above, each additive element has a synergistic effect, contributing to further stabilization of the surface layer 100a. In particular, the inclusion of magnesium, nickel, and aluminum is highly effective in achieving stable composition and crystal structure, and is therefore preferred.

[0189] Note that lithium is not readily intercalated and deintercalated when only compounds containing elements and oxygen are added to the surface layer 100a, so this is not preferred. For example, structures containing only MgO, MgO and NiO(II) in solid solution, and / or MgO and CoO(II) in solid solution are not preferred. Therefore, the surface layer 100a needs to contain at least cobalt, lithium in the discharged state, and have pathways for lithium intercalation and deintercalation.

[0190] To ensure adequate pathways for lithium insertion and extraction, the cobalt concentration in the surface layer 100a is preferably higher than the magnesium concentration. For example, when measured using XPS (X-ray photoelectron spectroscopy) on the surface of the positive electrode active material 100, the ratio of magnesium atoms (Mg) to cobalt atoms (Co) (Mg / Co) is preferably 0.62 or less. Furthermore, the cobalt concentration in the surface layer 100a is preferably higher than the nickel concentration. Additionally, the cobalt concentration in the surface layer 100a is preferably higher than the aluminum concentration. Furthermore, the cobalt concentration in the surface layer 100a is preferably higher than the fluorine concentration.

[0191] Furthermore, excessive nickel may hinder lithium diffusion, so the magnesium concentration in the surface layer 100a is preferably higher than the nickel concentration. For example, when measuring the surface of the positive electrode active material 100 using XPS, the number of nickel atoms is preferably less than 1 / 6 of the number of magnesium atoms.

[0192] In addition, for a part of the added elements, particularly the surface layer part 100a of magnesium, nickel, and aluminum, the concentration is preferably higher than that in the interior 100b, but they are preferably also present irregularly and in small amounts in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, effects such as being prone to maintaining a layered rock salt-type crystal structure are achieved in the same manner as described above. In addition, when nickel is present at an appropriate concentration in the interior 100b, similarly to the above, the deviation of the layered structure composed of octahedra of transition metal M and oxygen is suppressed. In addition, when magnesium and nickel are used simultaneously, a synergistic effect of suppressing the dissolution of magnesium can also be expected in the same manner as described above.

[0193] <<Li x MO2 in a state where x is relatively small>> Due to having the above-described distribution of added elements and / or crystal structure, the cathode active material 100 of one embodiment of the present invention has a different crystal structure from that of conventional cathode active materials in the state where x in Li x MO2 is relatively small, that is, in the high-voltage charging state. Note that here, a relatively small x means a case where 0.1 < x ≤ 0.24. The high voltage in the charging state means 4.5 V or more, preferably 4.6 V or more, and more preferably 4.8 V or more.

[0194] Use Figure 8 And Figure 9 To compare a conventional cathode active material and the cathode active material 100 of one embodiment of the present invention to illustrate the change in crystal structure accompanying the change in x in Li x MO2.

[0195] Figure 9 Show the change in the crystal structure of a conventional cathode active material. Figure 9 The conventional cathode active material shown is lithium cobaltate (LiCoO2) that does not contain added elements. Non-Patent Documents 1 to Non-Patent Document 3, etc., describe the change in the crystal structure of lithium cobaltate that does not contain added elements.

[0196] In Figure 9 It is attached that R-3m O3 represents the discharged state, that is, the crystal structure of lithium cobaltate with x = 1 in Li x CoO2. In the discharged state, the crystal structure of the conventional lithium cobaltate is the same as that of the cathode active material 100 of one embodiment of the present invention.

[0197] In addition, it is known that: for conventional lithium cobaltate, the lithium symmetry increases at around x = 0.5 and it has a crystal structure belonging to the monoclinic space group P2 / m. In this structure, the unit cell includes one CoO2 layer. Therefore, it is sometimes referred to as the O1-type structure or monoclinic (denoted as monoclinic in the drawings) O1-type structure.

[0198] The positive electrode active material at x=0 has a trigonal crystal system belonging to space group P-3m1, and the unit cell also includes a CoO2 layer. Therefore, this crystal structure is sometimes called the O1 type structure or the trigonal (denoted as trigonal in the attached diagram) O1 type structure. Alternatively, sometimes the trigonal crystal is transformed into a composite hexagonal lattice and called the hexagonal O1 type.

[0199] Furthermore, existing lithium cobalt oxide at x = 0.12 has a crystal structure belonging to space group R-3m. This structure can also be described as an alternating stacking of CoO2 structures (like the trigonal O1 type structure) and LiCoO2 structures (like R-3m O3). Therefore, this crystal structure is sometimes referred to as the H1-3 type structure (denoted as H1-3 in the attached figure). In reality, lithium insertion and deintercalation do not necessarily occur uniformly in the positive electrode active material, and the lithium concentration may be uneven. Therefore, the H1-3 type structure is experimentally observed from around x = 0.25. In addition, the number of cobalt atoms per unit cell in the H1-3 type structure is actually twice that of other structures.

[0200] As an example of the H1-3 type structure, as shown in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell can be represented by Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). Both O1 and O2 are oxygen atoms. For example, Rietwald analysis of the XRD pattern can determine which unit cell represents the crystal structure of the positive electrode active material. For instance, a unit cell with a GOF (goodness of fit) value close to 1 can be used.

[0201] When repeated experiments were conducted, the H1-3 type structure of Li could be confirmed. x When the x in CoO2 is below 0.24, the existing crystal structure of lithium cobalt oxide changes repeatedly between the H1-3 type structure and the R-3m O3 structure in the discharge state (i.e., non-equilibrium phase transition).

[0202] The CoO2 layers in the two crystal structures described above deviate significantly. For example... Figure 9 As shown by the dashed line, in the H1-3 type structure, the CoO2 layer deviates significantly from the R-3m O3 type structure under discharge conditions. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0203] Furthermore, the volume difference between the two crystal structures is also significant. When comparing the same number of cobalt atoms, the volume difference between the H1-3 type structure and the R-3m O3 type structure in the discharge state exceeds 3.5%, typically more than 3.9%.

[0204] In addition to the above, the H1-3 type structure, which has a continuous CoO2 layer like the trigonal O1 type, is more likely to be unstable.

[0205] Therefore, the existing lithium cobalt oxide crystal structure collapses when repeatedly charged and discharged at x below 0.24. This collapse leads to a deterioration in cycle performance. This is because the collapse reduces the number of stable lithium sites and makes lithium insertion and extraction more difficult.

[0206] On the other hand, Figure 8 In the positive electrode active material 100 of one embodiment of the present invention shown, Li x The crystal structure change in MO2 between the discharge state with x = 1 and the state with x = 0.24 or less is less than that of existing positive electrode active materials. More specifically, the deviation of the MO2 layer between the state with x = 1 and the state with x = 0.24 or less can be reduced. In addition, when comparing positive electrode active material 100 containing the same number of transition metal M atoms with existing positive electrode active materials, the volume change of positive electrode active material 100 is smaller than that of existing positive electrode active materials. Therefore, the positive electrode active material 100 of one aspect of the present invention is less prone to crystal structure collapse even when repeatedly charged and discharged with x = 0.24 or less, maintaining sites where lithium can be stably present and achieving good cycle characteristics.

[0207] Additionally, in one embodiment of the present invention, the positive electrode active material 100 is in Li x In MO2, a state where x is 0.24 or less can exhibit a more stable crystal structure than existing positive electrode active materials. Therefore, in the positive electrode active material 100 of one embodiment of the present invention, even with Li... x In MO2, the x-value is below 0.24, and oxygen is not easily released, thus suppressing thermal decomposition reactions. This means that the safety of secondary batteries using the positive electrode active material 100 according to one aspect of the present invention is further improved, and therefore it is preferred.

[0208] Figure 8 Shown in Li x The crystal structure of the internal 100b of the positive electrode active material 100 under various conditions where x in MO2 is around 1, 0.2, and 0.15. The internal 100b occupies most of the volume of the positive electrode active material 100 and is the part that greatly facilitates charging and discharging. Therefore, it can be said to be the part with the greatest impact from the deviation and volume change of the MO2 layer.

[0209] As described above, the positive electrode active material 100 has the same R-3m O3 type structure as conventional lithium cobalt oxide when x = 1. However, when the x value of conventional lithium cobalt oxide with an H1-3 type structure is 0.24 or less, for example, around 0.2 or 0.15, the positive electrode active material 100 has a crystal structure different from the above-described structure.

[0210] In one embodiment of the present invention, the positive electrode active material 100 at x = approximately 0.2 has a crystal structure belonging to the trigonal crystal system and space group R-3m. The symmetry of the MO2 layer in this structure is the same as that of O3. Therefore, this crystal structure is referred to as an O3' type structure. Alternatively, in XRD patterns, patterns resembling spinel structures, although not spinel structures, sometimes appear; this crystal structure is sometimes referred to as a pseudo-spinel structure. Figure 8 The attached R-3m O3' indicates this crystal structure. Although it will be explained later, the positive electrode active material 100 sometimes has an H1-3 type structure after the O3' type structure. It is speculated that the positive electrode active material 100 with the O3' type structure, for example, even with the H1-3 type structure, has the effect of suppressing oxygen release. Therefore, even if a nail penetration test is performed on a lithium-ion secondary battery using the positive electrode active material 100, fire is suppressed.

[0211] When M is cobalt, the coordinates of cobalt and oxygen in the unit cell of the O3' type structure can be represented by Co(0, 0, 0.5) and O(0, 0, x), respectively, within the range of 0.20 ≤ x ≤ 0.25. Furthermore, the lattice constants of the unit cell are as follows: the a-axis is preferably 2.797 × 10⁻⁶. -10 ≤a≤2.837×10 -10 (m), more preferably 2.807×10 -10 ≤a≤2.827×10 -10 (m), typically a = 2.817 × 10 -10 (m). The c-axis is preferably 13.681 × 10 -10 ≤c≤13.881×10 -10 (m), more preferably 13.751×10 -10 ≤c≤13.811×10 -10 (m), typically c = 13.781 × 10 -10 (m).

[0212] Furthermore, the positive electrode active material 100 of one embodiment of the present invention, when x = approximately 0.15, has a monoclinic crystal structure belonging to space group P2 / m. A CoO2 layer exists within the unit cell of this structure. Additionally, x = approximately 0.15 can be considered to mean that the amount of lithium present in the positive electrode active material 100 is approximately 15 atomic% in the discharge state. Therefore, this crystal structure is referred to as a monoclinic O1(15) type structure. Figure 9 The crystal structure is represented by P2 / m monoclinic crystal O1(15).

[0213] When M is cobalt, in a monoclinic O1(15) type structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as follows: Co1(0.5, 0, 0.5), Co2(0, 0.5, 0.5), O1(X O1 ,0,Z O1 ), 0.23≤X O1 ≤0.24, 0.61≤Z O1 ≤0.65、 O2(X O2 0.5, Z O2 ), 0.75≤X O2 ≤0.78, 0.68≤Z O2 ≤0.71. In addition, the lattice constant of the unit cell is as follows: a = 0.4880 ± 0.005 nm b = 0.2817 ± 0.005 nm c = 0.4839 ± 0.005 nm α = 90° β=109.6±0.1° γ = 90°.

[0214] This crystal structure, within the space group R⁻³m, can still represent the lattice constant under conditions allowing for certain errors, such as in the Rietwald method. The coordinates of cobalt and oxygen in the unit cell can then be expressed as follows: Co(0, 0, 0.5), O(0, 0, Z) O ), 0.21≤Z O ≤0.23. In addition, the lattice constant of the unit cell is as follows: a = 0.2817 ± 0.002 nm c = 1.368 ± 0.01 nm.

[0215] In both the O3' type structure and the monoclinic O1(15) type structure, ions of cobalt, nickel, magnesium, etc., occupy six oxygen sites. In addition, sometimes light elements such as lithium and magnesium occupy four oxygen sites.

[0216] The MO2 layers in the R-3m O3 type structure, O3' type structure and monoclinic O1(15) type structure under the discharge state are almost completely deviated.

[0217] In addition, the volume difference of cobalt atoms in the R-3m O3 type structure and the O3' type structure under the same number of discharge conditions is less than 2.5%, more specifically less than 2.2%, and typically 1.8%.

[0218] In addition, the volume difference of cobalt atoms in the R-3m O3 type structure and the monoclinic O1(15) type structure under the same number of discharge conditions is less than 3.3%, more specifically less than 3.0%, and typically 2.5%.

[0219] The table below shows the volume difference of a single cobalt atom in the R-3m O3 type structure, O3' type structure, monoclinic O1(15) type structure, H1-3 type structure, and trigonal O1 type structure under discharge conditions. The lattice constants of the R-3m O3 type structure and trigonal O1 type structure used in the calculations in the table below can be found in ICSD coll.code.172909 and 88721. The H1-3 type structure can be found in Non-Patent Literature 3. The O3' type structure and monoclinic O1(15) type structure can be calculated from experimental values ​​obtained by XRD.

[0220] [Table 1]

[0221] Thus, in the positive electrode active material 100 of one embodiment of the present invention, in Li x When x in CoO2 is small, i.e., when more lithium is removed, the change in crystal structure is suppressed compared to existing positive electrode active materials. When comparing positive electrode active material 100 containing the same number of cobalt atoms with existing positive electrode active materials, the volume change of positive electrode active material 100 is smaller than that of existing positive electrode active materials. Therefore, the crystal structure of positive electrode active material 100 is not easily collapsed even after repeated charging and discharging with x below 0.24. Thus, the decrease in charge-discharge capacity of positive electrode active material 100 due to charge-discharge cycles is suppressed. Furthermore, a larger amount of lithium can be stably used compared to existing positive electrode active materials, resulting in a larger discharge capacity per unit weight and per unit volume for positive electrode active material 100. Therefore, by using positive electrode active material 100, a secondary battery with a larger discharge capacity per unit weight and per unit volume can be manufactured.

[0222] In addition, it was confirmed that the positive electrode active material 100 was in Li x MO2 sometimes exhibits an O3'-type structure when x is above 0.15 and below 0.24, and it can be considered that it also exhibits an O3'-type structure when x exceeds 0.24 and is below 0.27. Additionally, it was confirmed that Li... x In MO2, x is greater than 0.1 and less than 0.2, typically 0.15 and less than 0.17, sometimes exhibiting a monoclinic O1(15) structure. However, crystal structures other than Li... x In addition to the x in MO2, it is also affected by the number of charge-discharge cycles, charge-discharge current, temperature, electrolyte, etc., so it is not limited to the range of x mentioned above.

[0223] Therefore, positive electrode active material 100 in Li x When x in MO2 exceeds 0.1 and is below 0.24, it may only have an O3' type structure, a monoclinic O1(15) type structure, or both. Furthermore, the particles 100b inside the positive electrode active material 100 do not necessarily all have an O3' type structure and / or a monoclinic O1(15) type structure. Alternatively, it may have other crystal structures or be partially amorphous.

[0224] In addition, in order to achieve Li x The smaller x-states in MO2 generally require high charging voltages. Therefore, Li can also be used... x The state with a smaller 'x' in MO2 is called the state of charging at a high charging voltage. For example, when CC / CV charging is performed at 25°C with a voltage of 4.6V or higher based on the lithium metal potential, an H1-3 type structure is exhibited in existing positive electrode active materials. Therefore, a charging voltage of 4.6V or higher based on the lithium metal potential can be considered a high charging voltage. Furthermore, unless otherwise specified in this specification, the charging voltage is expressed based on the lithium metal potential.

[0225] Therefore, it can also be said that the positive electrode active material 100 of one aspect of the present invention can maintain a crystal structure with R-3m O3 symmetry when charged, for example, at a high charging voltage of 25°C and 4.6V or higher, and is therefore preferred. Furthermore, it can also be said that it can have an O3' type structure when charged, for example, at a higher charging voltage of 25°C and 4.65V or higher and 4.7V or lower, and is therefore preferred. Additionally, it can also be said that it can have a monoclinic O1(15) type structure when charged, for example, at a further high charging voltage of 25°C and above 4.7V and below 4.8V, and is therefore preferred.

[0226] In one embodiment of the present invention, the positive electrode active material 100 sometimes exhibits an H1-3 type structure only when the charging voltage is further increased. Furthermore, as mentioned above, the crystal structure is affected by the number of charge-discharge cycles, charge-discharge current, temperature, electrolyte, etc. Therefore, even at lower charging voltages, for example, even at 25°C and a charging voltage of 4.5V or higher but lower than 4.6V, the positive electrode active material 100 of one embodiment of the present invention sometimes exhibits an O3' type structure. Similarly, when charged at 25°C and a voltage of 4.65V or higher but lower than 4.7V, it sometimes exhibits a monoclinic O1(15) type structure.

[0227] Furthermore, when graphite is used as the negative electrode active material in a secondary battery, for example, the voltage drop of the aforementioned secondary battery is equivalent to the potential of graphite. The potential of graphite is approximately 0.05V to 0.2V based on the potential of lithium metal. Therefore, in a secondary battery using graphite as the negative electrode active material, the same crystal structure as described above is present at the voltage obtained by subtracting the potential of graphite from the aforementioned voltage.

[0228] In addition, Figure 8 In the O3' type structure and the monoclinic O1(15) type structure, lithium exists with equal probability at all lithium sites, but the present invention is not limited thereto. Lithium can also be concentrated at a subset of lithium sites. For example, it can also have structures like... Figure 9 The monoclinic O1-type structure shown (Li) 0.5 Lithium exhibits symmetry similar to that of CoO2. The distribution of lithium can be analyzed, for example, through neutron diffraction.

[0229] Furthermore, although the O3' type structure and the monoclinic O1(15) type structure contain lithium irregularly in the interlayer, they can also have a crystal structure similar to the CdCl2 type crystal structure. This crystal structure similar to the CdCl2 type crystal structure is approximated by charging lithium nickelate to become Li 0.06 The crystal structure of NiO2 is known, but pure lithium cobalt oxide or layered rock salt type positive electrode active materials containing a large amount of cobalt usually do not have the CdCl2 type crystal structure.

[0230] As described above, the crystal structure of the existing lithium cobalt oxide and the positive electrode active material 100 of one embodiment of the present invention varies with the depth of charge, i.e., Li x The change is due to the variation of x in CoO2. Figure 10 The c-axis length variation of conventional lithium cobalt oxide described in Non-Patent Document 12 is shown. Circles indicate hexagonal phases, and rhombuses indicate monoclinic phases.

[0231] Note that the change in the c-axis length of lithium cobalt oxide corresponds to a change in the angle of the peak of the (003) plane of lithium cobalt oxide in the XRD pattern, for example. It is known that in XRD using CuKα1 rays (CuKα2 rays removed from CuKα rays), the 2θ of the peak of the (003) plane of lithium cobalt oxide appears around 19° to 20°.

[0232] <<Crystal Boundaries>> Preferably, in addition to the distribution described above, at least a portion of the added elements of the positive electrode active material 100 of one embodiment of the present invention are concentrated in and around the grain boundary 101. As described above, "around the grain boundary 101" refers to the region within 10 nm from the grain boundary 101.

[0233] For example, the magnesium concentration at and near the grain boundary 101 of the positive electrode active material 100 is preferably higher than that in other regions of the interior 100b. Additionally, the fluorine concentration at and near the grain boundary 101 is preferably higher than that in other regions of the interior 100b. Furthermore, the nickel concentration at and near the grain boundary 101 is also preferably higher than that in other regions of the interior 100b. Additionally, the aluminum concentration at and near the grain boundary 101 is also preferably higher than that in other regions of the interior 100b.

[0234] Grain boundary 101 is a type of planar defect. Therefore, like particle surfaces, it tends to be unstable and prone to crystal structure changes. Consequently, the higher the concentration of added elements at and near grain boundary 101, the more effectively changes in crystal structure can be suppressed.

[0235] Furthermore, when the magnesium and fluorine concentrations at and near the grain boundary 101 are high, even if cracks occur at the grain boundary 101 of the positive electrode active material 100 along one aspect of the present invention, the magnesium and fluorine concentrations on the surface and near the cracked area also increase. Therefore, the corrosion resistance of the positive electrode active material to hydrofluoric acid after the cracks have formed can also be improved.

[0236] <Analytical Methods> To determine whether a certain positive electrode active material is in Li x When the x content in CoO2 is small, it exhibits an O3'-type structure and / or a monoclinic O1(15)-type structure. One aspect of the positive electrode active material 100 of this invention can contain Li... x The positive electrode of the positive electrode active material with a smaller x in CoO2 is determined by analysis using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0237] In particular, XRD is preferred because it offers the following advantages: it allows for high-resolution analysis of the symmetry of transition metals such as cobalt contained in the positive electrode active material; it enables comparison of the degree of crystallinity and the orientation of crystals; it allows analysis of periodic distortions of the crystal lattice and grain size; and it provides sufficient accuracy when directly measuring the positive electrode obtained by disassembling a secondary battery. XRD, especially powder XRD, can obtain diffraction peaks that reflect the internal crystal structure 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0238] When analyzing grain size using powder XRD, it is preferable to perform the measurement without considering the influence of factors such as pressure on the orientation of the positive electrode active material particles. For example, it is preferable to measure the positive electrode active material layer obtained from the disassembled secondary battery. Alternatively, the positive electrode active material can be measured by removing it from the positive electrode active material layer.

[0239] As described above, the positive electrode active material 100 of one embodiment of the present invention is characterized by: Li x The crystal structure of CoO2 changes little between x=1 and below 0.24. Materials with a crystal structure where the significant changes occur (more than 50%) during high-voltage charging cannot withstand repeated high-voltage charging and discharging, and are therefore not preferred.

[0240] Note that sometimes simply adding an additive element is insufficient to achieve an O3'-type structure or a monoclinic O1(15)-type structure. For example, even under the same conditions, lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum, the structure varies depending on the concentration and distribution of the added element in Li. x When x in CoO2 is below 0.24, the O3' type structure and / or monoclinic O1(15) type structure sometimes account for more than 60%, or the H1-3 type structure sometimes accounts for more than 50%.

[0241] Furthermore, when x is too small (below 0.1) or when the charging voltage exceeds 4.9V, an H1-3 type structure or a trigonal O1 type structure may also be generated in the positive electrode active material 100 of one embodiment of the present invention. Therefore, in order to determine whether it is the positive electrode active material 100 of one embodiment of the present invention, analysis of crystal structure such as XRD and information such as charging capacity or charging voltage are required.

[0242] Note that sometimes the crystal structure of positive electrode active materials in the state of low x changes when exposed to air. For example, sometimes it changes from the O3' type structure and the monoclinic O1(15) type structure to the H1-3 type structure. Therefore, all samples used in the analysis of crystal structure are preferably treated in an inert atmosphere such as argon.

[0243] In addition, by using methods such as XPS, EDX, and EPMA (electron probe microanalysis), it can be determined whether the distribution of added elements in a positive electrode active material is in the state described above.

[0244] Furthermore, the crystal structure of the surface layer 100a, grain boundary 101, etc., can be analyzed by electron diffraction or other methods on the cross-section of the positive electrode active material 100.

[0245] <<Charging Methods>> By manufacturing, for example, a coin-shaped battery (CR2032 type, 20 mm in diameter and 3.2 mm in height) using a composite oxide as the positive electrode and lithium metal as the counter electrode, and charging it, it can be determined whether the composite oxide is the positive electrode active material 100 of an embodiment of the present invention. The coin-shaped battery includes an electrolyte, a separator, a positive electrode container, and a negative electrode container.

[0246] More specifically, the positive electrode can be a positive electrode formed by coating an aluminum foil positive current collector with a slurry made by mixing the composite oxide used as the positive electrode active material, a conductive material and a binder.

[0247] As mentioned above, lithium metal can be used as the counter electrode, but materials other than lithium metal can also be used. When materials other than lithium metal are used, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, the voltages and potentials in this specification refer to the potential of the positive electrode.

[0248] As the lithium salt contained in the electrolyte, 1 mol / L lithium hexafluorophosphate (LiPF6) is used. As the electrolyte, a solution prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 and 2 wt% vinylene carbonate (VC) can be used.

[0249] A 25 μm thick porous polypropylene membrane can be used as an insulator.

[0250] The positive and negative electrode containers can be made of stainless steel (SUS).

[0251] The coin-shaped battery manufactured under the above conditions is charged to any voltage (e.g., 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V). There are no restrictions on the charging method as long as sufficient time can be given to charge to any voltage. For example, when charging with CCCV, the current in CC charging can be set to 20 mA / g or more and 100 mA / g or less. CV charging can be performed up to 2 mA / g or more and 10 mA / g or less. Charging with the aforementioned lower current values ​​is preferred for observing the phase transition of the positive electrode active material. Since XRD measurements are difficult to perform below 0°C, the temperature is set to 25°C. Note that 25°C is just an example. After charging in this manner, the coin-shaped battery is disassembled and the positive electrode is removed in an argon-atmosphere glove box, thereby obtaining a positive electrode active material with any charge capacity. For subsequent analyses, it is preferable to seal the battery under an argon atmosphere to prevent reactions with external components. For example, XRD measurements can be performed with the positive electrode active material sealed in a container under an argon atmosphere. Furthermore, it is preferable to remove the positive electrode immediately after charging to begin post-process XRD analysis. Specifically, it is preferable to perform the analysis within one hour of charging completion, and more preferably within 30 minutes.

[0252] Furthermore, when analyzing the crystal structure after multiple charge-discharge cycles, the conditions for these multiple charge-discharge cycles can differ from the charging conditions described above. For example, charging can be performed with a constant current of 20 mA / g or higher and 100 mA / g or lower to any voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V), followed by constant voltage charging until the current reaches 2 mA / g or higher and 10 mA / g or lower. Discharging can be performed with a constant current of 20 mA / g or higher and 100 mA / g or lower to 2.5V.

[0253] In addition, when analyzing the crystal structure in the discharge state after multiple charge-discharge cycles, the discharge conditions for these multiple charge-discharge cycles can be, for example, constant current discharge to 2.5V with a current value of 20 mA / g or more and 100 mA / g or less.

[0254] < <xrd>> With proper adjustment and calibration, there are no restrictions on the apparatus and conditions for XRD measurements. For example, measurements can be performed using the following apparatus under the following conditions. XRD unit: Bruker AXS D8 ADVANCE X-rays: Cu Kα rays, Cu Kα1 rays Output: 40kV, 40mA Launch angle: Div.Slit, 0.5° Detector: LynxEye Scanning method: 2θ / θ continuous scan Measurement range (2θ): 15° and below 90° Step width (2θ): Set to 0.01° Counting time: 1 second / step Sample stage rotation: 15 rpm For example, standard samples used for adjustment and calibration can be the NIST (National Institute of Standards and Technology) standard sintered alumina plate SRM 1976, etc.

[0255] When measuring a powder sample, the sample can be mounted by placing it in a glass sample holder; or by sprinkling the sample onto a silicon non-reflective plate coated with grease; etc. When measuring a positive electrode sample, the positive electrode active material layer can be mounted according to the measurement surface required by the device by attaching the positive electrode to the substrate with double-sided tape.

[0256] Monochromatic X-rays can be performed using filters or XRD data analysis software after the XRD pattern is obtained. For example, DIFFRAC.EVA (an XRD data analysis software manufactured by Bruker) can be used to remove peaks passing through CuKα2 rays, extracting only peaks passing through CuKα1 rays. This software can also be used for background removal.

[0257] This manual, etc., describes the data processing involved in determining the 2θ value of a diffraction peak. First, a computational model is fitted to an XRD pattern using crystal structure analysis software to obtain the calculated pattern. In the calculated pattern, the 2θ value appearing at the peak apex of the diffraction peak is referred to as the 2θ value of that diffraction peak. There are no particular limitations on the crystal structure analysis software used for fitting; for example, TOPASver.3 (a crystal structure analysis software manufactured by Bruker) can be used.

[0258] Figure 11 The XRD patterns corresponding to the O3 type structure, O3' type structure and monoclinic O1(15) type structure when using CuKα1 are shown. Figure 12 The ideal powder XRD pattern obtained by CuKα1 rays is shown from the model of the H1-3 type structure and the ideal XRD pattern obtained by CuKα1 rays is shown from the trigonal O1 type structure with x=0. Figure 13A and Figure 13B All of the above-described XRD patterns are shown. Note that in the figures, 2θ ranges from 18° to 21° and from 42° to 46°. The patterns of LiCoO2(O3) and CoO2(O1) were created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), based on crystal structure information obtained from ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). Here, the range of 2θ was set to 15° to 75°, Step size = 0.01, and wavelength λ = 1.54 × 10⁻⁶. -10 m, Monochromator is set to single. The pattern of the H1-3 type structure is made in the same way as the crystal structure information described in Non-Patent Document 3. The patterns of the O3' type structure and the monoclinic O1(15) type structure are made by the following method: the crystal structure is deduced from the XRD pattern of the positive electrode active material 100 of one embodiment of the present invention and fitted using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker).

[0259] like Figure 11 , Figure 13A and Figure 13B As shown, in the O3' type structure, diffraction peaks appear at 2θ of 19.25±0.12° (above 19.13° and less than 19.37°) and at 2θ of 45.47±0.10° (above 45.37° and less than 45.57°).

[0260] In addition, in the monoclinic O1(15) type structure, diffraction peaks appear at 2θ of 19.47±0.10° (above 19.37° and below 19.57°) and 2θ of 45.62±0.05° (above 45.57° and below 45.67°).

[0261] However, as Figure 12 , Figure 13A and Figure 13B As shown, in the H1-3 type structure and the trigonal O1 type structure, the peak does not appear at the aforementioned positions. Therefore, it can be said that in Li... x The presence of peaks in the CoO2 at positions where 2θ is greater than or equal to 19.13° and less than 19.37° and / or greater than or equal to 19.37° and less than 19.57°, and at positions where 2θ is greater than or equal to 45.37° and less than 45.57° and / or greater than or equal to 45.57° and less than 45.67°, in a state where x is relatively small, is a characteristic of the positive electrode active material 100 of one aspect of the present invention.

[0262] This also indicates that the position of the XRD diffraction peak in the crystal structure when x=1 is close to the position of the XRD diffraction peak in the crystal structure when x≤0.24; more specifically, for the main diffraction peaks of the crystal structure when x=1 and the crystal structure when x≤0.24, the peaks with 2θ above 42° and below 46° have a difference of 2θ of less than 0.7°, preferably less than 0.5°.

[0263] Additionally, in one embodiment of the present invention, the positive electrode active material 100 is in Li x When the x-value in CoO2 is small, it has an O3' type structure and / or a monoclinic O1(15) type structure, but it is not necessary for all particles to have an O3' type structure and / or a monoclinic O1(15) type structure. It can have other crystal structures or be partially amorphous. Note that when performing Rietwald analysis on the XRD pattern, the O3' type structure and / or the monoclinic O1(15) type structure preferably accounts for more than 50%, more preferably more than 60%, and even more preferably more than 66%. When the O3' type structure and / or the monoclinic O1(15) type structure accounts for more than 50%, more preferably more than 60%, and even more preferably more than 66%, a positive electrode active material with sufficiently excellent cycle characteristics can be achieved.

[0264] Furthermore, even after more than 100 charge-discharge cycles from the start of the measurement, the O3' type structure and / or monoclinic O1(15) type structure in the Ritwald analysis preferably account for more than 35%, more preferably more than 40%, and even more preferably more than 43%.

[0265] Furthermore, when performing the same Rietwald analysis, the H1-3 type structure and the O1 type structure preferably account for 50% or less. Alternatively, the crystal structure is more preferably 34% or less. Or, more preferably, the structure is substantially unobservable.

[0266] Furthermore, the sharpness of diffraction peaks in an XRD pattern indicates the degree of crystallinity. Therefore, each diffraction peak after charging is preferably sharp, i.e., the half-width at half maximum (HWHM) is preferably narrow. The HWHM of peaks for the same crystalline phase varies depending on the XRD measurement conditions and the value of 2θ. When using the above measurement conditions, for example, the HWHM of peaks observed at 2θ of 43° or higher and 46° or lower is preferably 0.2° or lower, more preferably 0.15° or lower, and even more preferably 0.12° or lower. Note that not all peaks need to meet the above conditions. As long as some peaks meet the above conditions, it can be said that the crystalline phase has high crystallinity. This higher crystallinity contributes significantly to the stabilization of the crystal structure after charging.

[0267] Furthermore, the grain size of the O3'-type structure and the monoclinic O1(15)-type structure of the positive electrode active material 100 is reduced to only about 1 / 20 of that of LiCoO2(O3) in the discharge state. Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, the grain size of LiCoO2(O3) in the discharge state can be reduced to only about 1 / 20 of that of LiCoO2(O3). x When the x-value in CoO2 is small, distinct peaks of the O3' type structure and / or monoclinic O1(15) type structure are confirmed. On the other hand, even if some existing LiCoO2 may have a structure similar to the O3' type structure and / or monoclinic O1(15) type structure, the grain size will be smaller, and its peaks will be wider and smaller. The grain size can be determined from the half-width at half-maximum (WHM) of the XRD peaks.

[0268] As described above, the influence of the Jameer-Taylor effect in the positive electrode active material 100 of one embodiment of the present invention is preferably small. As long as the influence of the Jameer-Taylor effect is small, transition metals such as nickel and manganese can be included as additive elements in addition to cobalt.

[0269] XRD analysis was conducted to examine the proportions and lattice constants of nickel and manganese, which are presumed to be less affected by the Jamie-Taylor effect in the positive electrode active material.

[0270] Figures 14A to 14C The results of XRD calculation of the lattice constants of the a-axis and c-axis are shown when the positive electrode active material 100 of one embodiment of the present invention has a layered rock salt type crystal structure and contains cobalt and nickel. Figure 14A The results for the a-axis are shown, while Figure 14B The results for the c-axis are shown. The XRD patterns used for these calculations are of the powder after the synthesis of the positive electrode active material and before it is assembled onto the positive electrode. The nickel concentration on the horizontal axis represents the nickel concentration when the sum of the number of cobalt and nickel atoms is 100%.

[0271] Figure 14C The results showing its lattice constant are shown in Figure 14A and Figure 14B The value of the lattice constant of the positive electrode active material divided by the lattice constant of the c-axis (a-axis / c-axis).

[0272] from Figure 14C It can be seen that the a-axis / c-axis tends to change significantly between nickel concentrations of 5% and 7.5%, with the a-axis skew becoming larger at a nickel concentration of 7.5%. This skewness may be due to the Jameer-Taylor skewness of trivalent nickel. Excellent positive electrode active materials with small Jameer-Taylor skewness can be obtained at nickel concentrations below 7.5%.

[0273] Furthermore, the aforementioned range of nickel concentrations does not necessarily apply to the surface layer 100a. In other words, the nickel concentration in the surface layer 100a can be higher than the aforementioned concentration.

[0274] In summary, when considering the preferred range of lattice constants, it can be seen that in the positive electrode active material of one aspect of the present invention, the lattice constant of the a-axis in the layered rock-salt type crystal structure of the positive electrode active material 100 in the uncharged or discharged state, which can be inferred from the XRD pattern, is preferably greater than 2.814 × 10⁻⁶. -10 m and less than 2.817 × 10 -10 m, and preferably a lattice constant greater than 14.05 × 10⁻⁶ for the c-axis. -10 m and less than 14.07 × 10 -10 m. The state without charging or discharging can refer to, for example, the powder state of a secondary battery before the positive electrode is formed.

[0275] Alternatively, the value of the lattice constant of the a-axis divided by the lattice constant of the c-axis in the layered rock salt-type crystal structure of the positive electrode active material 100 in the non-charged or discharged state (a-axis / c-axis) is preferably greater than 0.20000 and less than 0.20049.

[0276] Alternatively, in the layered rock salt-type crystal structure of the positive electrode active material 100 in the uncharged or discharged state, when XRD analysis is performed, sometimes a first peak is observed with 2θ above 18.50° and below 19.30°, and a second peak is observed with 2θ above 38.00° and below 38.80°.

[0277] < <xps>> XPS, when analyzing inorganic oxides and using monochromatic aluminum Kα X-rays, can perform analysis at depths ranging from approximately 2 nm to 8 nm (generally less than 5 nm) from the surface. Therefore, it can quantitatively analyze the concentration of each element in about half of the depth region of the surface layer 100a. Furthermore, by performing narrow-scan analysis, the bonding states of elements can be analyzed.

[0278] In one aspect of the positive electrode active material 100 of the present invention, the concentration of one or more additive elements selected from the surface layer 100a is preferably higher than the concentration in the interior layer 100b. This means that the concentration of one or more additive elements selected from the surface layer 100a is preferably higher than the average concentration of the positive electrode active material 100 as a whole. Therefore, for example, it can be said that the concentration of one or more additive elements selected from the surface layer 100a, as measured by XPS or the like, is preferably higher than the average concentration of the additive elements in the positive electrode active material 100 as measured by ICP-MS or GD-MS or the like. For example, the magnesium concentration of at least a portion of the surface layer 100a, as measured by XPS or the like, is preferably higher than the average magnesium concentration in the positive electrode active material 100 as a whole. In addition, the nickel concentration of at least a portion of the surface layer 100a is preferably higher than the average nickel concentration in the positive electrode active material 100 as a whole. In addition, the aluminum concentration of at least a portion of the surface layer 100a is preferably higher than the average aluminum concentration in the positive electrode active material 100 as a whole. In addition, the fluorine concentration of at least a portion of the surface layer 100a is preferably higher than the average fluorine concentration of the entire positive electrode active material 100.

[0279] Note that the surface and surface layer 100a of the positive electrode active material 100 in one embodiment of the present invention do not contain carbonates, hydroxyl groups, etc., that are chemically adsorbed after the manufacture of the positive electrode active material 100. Furthermore, it does not contain electrolytes, binders, conductive materials, or compounds derived from them that adhere to the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections can be made to remove carbon, hydrogen, excess oxygen, excess fluorine, etc., which may be detected by surface analysis such as XPS. For example, XPS can identify the type of bonding and can also be used to remove CF bonds derived from the binder.

[0280] Alternatively, samples of the positive electrode active material and its layer can be washed before various analyses to remove electrolyte, binders, conductive materials, or compounds derived from them adhering to the surface of the positive electrode active material. In this case, lithium may sometimes dissolve in the solvent used for washing, but added elements do not readily dissolve, so the atomic ratio of added elements is not affected.

[0281] Furthermore, the concentration of the added element can also be compared using its ratio to cobalt. Using the ratio to cobalt allows for comparison while minimizing the influence of chemisorbed carbonates and other substances after the manufacture of the positive electrode active material, making it preferable. For example, the Mg / Co atomic ratio obtained by XPS analysis is preferably 0.4 or more and 1.5 or less. On the other hand, the Mg / Co ratio obtained by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0282] Similarly, to ensure sufficient pathways for lithium insertion and extraction, the concentrations of lithium and cobalt in the surface layer 100a of the positive electrode active material 100 are preferably higher than the concentrations of each added element. This means that the concentrations of lithium and cobalt in the surface layer 100a are preferably higher than the concentrations of one or more added elements selected from those included in the surface layer 100a, as measured by XPS or similar methods. For example, the cobalt concentration in at least a portion of the surface layer 100a, as measured by XPS or similar methods, is preferably higher than the magnesium concentration in at least a portion of the surface layer 100a, as measured by XPS or similar methods. Similarly, the lithium concentration is preferably higher than the magnesium concentration. Furthermore, the cobalt concentration is preferably higher than the nickel concentration. Similarly, the lithium concentration is preferably higher than the nickel concentration. Furthermore, the cobalt concentration is preferably higher than the aluminum concentration. Similarly, the lithium concentration is preferably higher than the aluminum concentration. Furthermore, the cobalt concentration is preferably higher than the fluorine concentration. Similarly, the lithium concentration is preferably higher than the fluorine concentration.

[0283] Furthermore, added elements such as aluminum are more preferably widely distributed in deeper regions, for example, in regions with a depth of 5 nm or more but less than 50 nm from the surface. Therefore, when added elements such as aluminum are detected in the overall analysis of the positive electrode active material 100 using ICP-MS, GD-MS, etc., their concentrations may differ from the analysis results using XPS or other methods that target a depth of approximately 5 nm from the surface.

[0284] Furthermore, when performing XPS analysis on the positive electrode active material 100 according to one aspect of the present invention, the number of magnesium atoms relative to the number of cobalt atoms is preferably 0.4 times or more and 1.2 times or less, more preferably 0.65 times or more and 1.0 times or less. Additionally, the number of nickel atoms relative to the number of cobalt atoms is preferably 0.15 times or less, more preferably 0.03 times or more and 0.13 times or less. Furthermore, the number of aluminum atoms relative to the number of cobalt atoms is preferably 0.12 times or less, more preferably 0.09 times or less. Additionally, the number of fluorine atoms relative to the number of cobalt atoms is preferably 0.1 times or more and 1.1 times or less, more preferably 0.3 times or more and 0.9 times or less. In other words, the above ranges mean that the added elements are not attached to a narrow range on the surface of the positive electrode active material 100, but are widely distributed in the surface layer 100a of the positive electrode active material 100 at a preferred concentration.

[0285] When performing XPS analysis, monochromatic aluminum Kα rays are used, for example, as X-rays. Furthermore, the extraction angle can be, for example, 45°. Measurements can be performed, for example, with the following apparatus and conditions. Measuring device: Quantera II manufactured by PHI Corporation X-ray: Monochromatic aluminum Kα (1486.6 eV) Detection area: Detection depth: approximately 4nm to 5nm (extraction angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element.

[0286] Furthermore, when analyzing the positive electrode active material 100 of one embodiment of the present invention using XPS, it is preferred to show that the peak of the bond energy between fluorine and other elements is 682 eV or higher and less than 685 eV, more preferably around 684.3 eV. This value is different from the bond energy of lithium fluoride (685 eV) and magnesium fluoride (686 eV).

[0287] Furthermore, when XPS analysis was performed on the positive electrode active material 100 of one embodiment of the present invention, the peak of the bond energy between magnesium and other elements was preferably 1302 eV or higher and less than 1304 eV, more preferably around 1303 eV. This value differs from the 1305 eV bond energy of magnesium fluoride and is close to the bond energy of magnesium oxide.

[0288] < <edx>> The additive elements selected from one or more of the additives included in the positive electrode active material 100 preferably have a concentration gradient. Furthermore, it is more preferable that the depth of the concentration peak from the surface varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive elements can be evaluated, for example, by exposing a cross-section of the positive electrode active material 100 using methods such as FIB and analyzing that cross-section using EDX, EPMA, etc.

[0289] In EDX measurements, a method that involves scanning and measuring within a region to perform a two-dimensional evaluation is called EDX surface analysis. Conversely, a method that involves scanning and measuring along a line to evaluate the atomic concentration distribution within the positive electrode active material is called line analysis. Sometimes, the method of extracting data from a linear region from EDX surface analysis is also called line analysis. Furthermore, a method that involves measuring without scanning a certain region is called point analysis.

[0290] EDX surface analysis (e.g., elemental mapping) allows for the quantitative analysis of the concentration of added elements in the surface layer 100a, interior layer 100b, grain boundaries 101, and their vicinity of the positive electrode active material 100. Furthermore, EDX line analysis allows for the analysis of the concentration distribution and maximum values ​​of added elements. Additionally, in analyses using thin-film samples, such as STEM-EDX, the concentration distribution from the surface of the positive electrode active material in a specific region towards the center in a depth direction can be analyzed without being affected by the distribution in the longitudinal direction, making this method preferred.

[0291] Therefore, when performing EDX surface analysis or EDX spot analysis on the positive electrode active material 100 of one embodiment of the present invention, the concentration of added elements such as magnesium in the surface layer 100a is preferably higher than that in the interior 100b.

[0292] For example, when performing EDX surface analysis or EDX spot analysis on the positive electrode active material 100, which contains magnesium as an added element, the magnesium concentration in the surface layer 100a is preferably higher than that in the interior layer 100b. Furthermore, when performing EDX line analysis, the magnesium concentration peak in the surface layer 100a preferably appears within a depth of 3 nm from the surface of the positive electrode active material 100 towards the center, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Additionally, the magnesium concentration preferably decreases to less than 60% of the peak concentration at a point from the peak top to a depth of 1 nm. Furthermore, it is preferable to decrease to less than 30% of the peak concentration at a point from the peak top to a depth of 2 nm. The concentration peaks described here refer to the maximum concentration values.

[0293] Furthermore, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in depth between the fluorine concentration peak and the magnesium concentration peak is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0294] Furthermore, during EDX line analysis, the fluorine concentration peak of the surface layer 100a preferably appears within a depth of 3 nm from the surface of the positive electrode active material 100 towards the center, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Additionally, the fluorine concentration peak is preferably slightly closer to the surface than the magnesium concentration peak, as this improves resistance to hydrofluoric acid, and is therefore preferred. For example, the fluorine concentration peak is preferably at least 0.5 nm closer to the surface than the magnesium concentration peak, more preferably at least 1.5 nm closer to the surface than the magnesium concentration peak.

[0295] Furthermore, in the positive electrode active material 100 containing nickel as an added element, the nickel concentration peak of the surface layer 100a preferably appears within a depth of 3 nm from the surface of the positive electrode active material 100 towards the center, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Additionally, in the positive electrode active material 100 containing both magnesium and nickel, the distribution of nickel preferably overlaps with the distribution of magnesium. For example, the depth difference between the nickel concentration peak and the magnesium concentration peak is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0296] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, during EDX line analysis, the concentration peaks of magnesium, nickel, or fluorine are closer to the surface than the aluminum concentration peak in the surface layer 100a. For example, the aluminum concentration peak preferably appears in a depth of 0.5 nm or more and 50 nm or less from the surface of the positive electrode active material 100 towards the center, and more preferably in a depth of 5 nm or more and 50 nm or less.

[0297] Furthermore, when performing EDX line analysis, area analysis, or spot analysis on the positive electrode active material 100, the atomic ratio of magnesium (Mg) to cobalt (Co) in the magnesium concentration peak (Mg / Co) is less than 1, preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The atomic ratio of aluminum (Al) to cobalt (Co) in the aluminum concentration peak (Al / Co) is less than 1, preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The atomic ratio of nickel (Ni) to cobalt (Co) in the nickel concentration peak (Ni / Co) is less than 1, preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. Alternatively, Ni / Co is preferably 0.1 or more and 0.5 or less. Additionally, the atomic ratio of cobalt (Co) to nickel (Ni) is preferably Co:Ni = 90:10, Co:Ni = 80:20, Co:Ni = 70:30, or a ratio between these. The ratio of the number of fluorine (F) to the number of cobalt (Co) atoms in the fluorine concentration peak is less than 1, preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0298] Based on the EDX line analysis results, the surface of the positive electrode active material 100 can be inferred as follows: The point where the amount of an element, such as oxygen or cobalt, which is uniformly present in the interior 100b of the positive electrode active material 100 is half the amount detected in the interior 100b is the surface.

[0299] The positive electrode active material 100 is a composite oxide, so the surface concentration can be inferred from the oxygen detection level. Specifically, the average oxygen concentration (O) is first calculated from the region where the oxygen detection level is stable within the internal 100b. ave At this point, when oxygen O2, caused by chemisorption or background, is detected in areas that can be clearly determined to be located outside the surface... bg When, subtract O from the measured value bg To calculate the average oxygen concentration O ave It can be inferred that the value closest to this average value O is... ave The value of 1 / 2, that is, O ave The measurement point for / 2 is the surface of the positive electrode active material.

[0300] Alternatively, the surface area can be inferred using the detection amount of cobalt in the same way as described above. Or, it can be inferred by summing the detection amounts of multiple transition metals. The detection amounts of transition metals such as cobalt are not easily affected by chemisorption, which is suitable for surface inference.

[0301] When performing line or surface analysis on the positive electrode active material 100, the ratio of the number of atoms of added element A to cobalt Co at or near the grain boundary 101 (A / Co) is preferably 0.020 or more and 0.50 or less. More preferably, it is 0.025 or more and 0.30 or less. Further preferably, it is 0.030 or more and 0.20 or less. Alternatively, it is preferably 0.020 or more and 0.30 or less. Alternatively, it is preferably 0.020 or more and 0.20 or less. Alternatively, it is preferably 0.025 or more and 0.50 or less. Alternatively, it is preferably 0.025 or more and 0.20 or less. Alternatively, it is preferably 0.030 or more and 0.50 or less. Alternatively, it is preferably 0.030 or more and 0.30 or less.

[0302] For example, when magnesium is added, when performing line or surface analysis on the positive electrode active material 100, the ratio of magnesium to cobalt atoms (Mg / Co) at or near the grain boundary 101 is preferably 0.020 or more and 0.50 or less. More preferably, it is 0.025 or more and 0.30 or less. Further preferably, it is 0.030 or more and 0.20 or less. Alternatively, it is preferably 0.020 or more and 0.30 or less. Alternatively, it is preferably 0.020 or more and 0.20 or less. Alternatively, it is preferably 0.025 or more and 0.50 or less. Alternatively, it is preferably 0.025 or more and 0.20 or less. Alternatively, it is preferably 0.030 or more and 0.50 or less. Alternatively, it is preferably 0.030 or more and 0.30 or less. Furthermore, when multiple portions of the positive electrode active material 100, such as three or more portions, fall within the aforementioned range, it can be said that the added elements are not attached to a narrow range on the surface of the positive electrode active material 100, but are widely distributed in the surface portion 100a of the positive electrode active material 100 at a preferred concentration.

[0303] Alternatively, at least a portion of the surface of the positive electrode active material 100 may be covered with a coating. Figure 15A and Figure 15B The cover 104 is attached to the following: Figure 5A and Figure 5C The structure of the positive electrode active material 100 is shown.

[0304] For example, the cover 104 is preferably formed by the deposition of decomposition products of lithium salt and organic electrolyte during charging and discharging. In particular, during repeated Li... x When the x in CoO2 is less than 0.24 during charging, improved charge-discharge cycle characteristics can be expected by having a covering portion derived from the organic electrolyte on the surface of the positive electrode active material 100. This is because it suppresses the increase in impedance on the surface of the positive electrode active material; or suppresses the dissolution of cobalt; etc. The covering portion 104 preferably contains, for example, carbon, oxygen, and fluorine. In addition, high-quality covering portions are easily obtained when LiBOB and / or SUN (suberonitrile) are used as electrolytes. Therefore, covering portions 104 containing at least one or more selected from boron, nitrogen, sulfur, and fluorine are sometimes high-quality covering portions and are therefore preferred. Alternatively, the covering portion 104 may not cover the entire positive electrode active material 100. For example, covering 50% or more of the surface of the positive electrode active material 100 is sufficient, preferably 70% or more, and more preferably 90% or more. In the portion without the covering portion 104, fluorine can also be adsorbed onto the surface of the positive electrode active material 100.

[0305] <Laminated secondary battery> This invention describes a secondary battery according to one aspect of the present invention. Firstly, using... Figure 16A and Figure 16B These illustrate typical laminated secondary batteries.

[0306] like Figure 16A As shown, the secondary battery 1004 includes multiple positive electrodes 503, multiple negative electrodes 506, and multiple separators 508. The separators 508 are disposed between the positive electrodes 503 and the negative electrodes 506. For convenience, [the separators are omitted here]. Figure 16A The separator 508 is shown in dashed lines. The separator 508 sometimes contains an electrolyte, specifically a liquid electrolyte (also called an electrolyte solution). Note that the secondary battery 1004 may not include the separator 508 when a solid or semi-solid electrolyte is used as the electrolyte.

[0307] Both the positive electrode 503 and the negative electrode 506 have protruding tab portions and portions other than the tab portions. The tab portions can be electrically connected to wiring 1005a and wiring 1005b in a needle penetration testing device. The positive electrode 503 includes a positive current collector and a positive active material layer formed on the positive current collector, preferably formed on both surfaces of the positive current collector. The negative electrode 506 includes a negative current collector and a negative active material layer formed on the negative current collector, preferably formed on both surfaces of the negative current collector.

[0308] like Figure 16B As shown, a plurality of positive electrodes 503, a plurality of negative electrodes 506, and a plurality of insulators 508 are stacked, sometimes referred to as a laminate in this specification. The tab portions of the plurality of negative electrodes 506 are joined to wires 512b at junction 515b and electrically connected to each other. Furthermore, the tab portions of the plurality of positive electrodes 503 are joined to wires 512a at junction 515a and electrically connected to each other. The positive and negative active material layers (sometimes simply referred to as active material layers) have higher insulation properties than the positive and negative current collectors (sometimes simply referred to as current collectors), therefore it is preferable that no active material layer is formed at the tab portions. Wires 512a and 512b can be made of materials selected from aluminum, nickel, copper, titanium, and their alloys. Ultrasonic welding can be used for the joining at the junctions. Note that in the needle penetration test, it is not necessary to set wires 512a and 512b, but when setting them, wires 1005a and 1005b should be connected to wires 512a and 512b respectively.

[0309] Furthermore, the secondary battery 1004 includes an outer packaging (not shown). Figure 16A The stacked structure shown is housed within an outer packaging unit. Then, an electrolyte solution containing dissolved lithium salts is injected into the outer packaging unit. That is, the electrolyte contains carrier ions, typically lithium ions. A secondary battery containing these lithium ions is called a lithium-ion secondary battery.

[0310] From a lightweight perspective, the outer packaging is preferably film-shaped, and a secondary battery including a film-shaped outer packaging is called a laminated secondary battery. Furthermore, from the viewpoint of excellent cooling performance, a laminated structure of a polymer and a metal with high thermal conductivity can also be used as the outer packaging. Specifically, polypropylene and aluminum are preferably used as the polymer and metal, respectively, and nylon or the like can also be placed on the outside of the outer packaging. Additionally, a metal can can also be used as the outer packaging; when a round can is used, the battery is called a coin-type secondary battery.

[0311] This implementation method can be used in combination with other implementation methods.

[0312] (Implementation Method 2) In this embodiment, an example of a method for manufacturing the positive electrode active material 100 according to one aspect of the present invention is described.

[0313] In order to manufacture a positive electrode active material 100 having the distribution, composition, and / or crystal structure of the added elements as described in the above embodiments, the method of adding the added elements is important. Furthermore, good crystallinity of the internal structure 100b is also important.

[0314] One method for manufacturing the positive electrode active material 100 involves synthesizing lithium cobalt oxide and then heating it with an additive element source. Alternatively, a method can be used to synthesize lithium cobalt oxide containing the additive element by mixing it with a cobalt source and a lithium source. Furthermore, when heating is performed while mixing not only lithium cobalt oxide and the additive element source, the additive element can dissolve in the lithium cobalt oxide, which is preferred. Moreover, sufficient heating is preferable to ensure good distribution of the additive element. Therefore, the heat treatment after mixing the additive element source is important. This heat treatment after mixing the additive element source is sometimes referred to as calcination or annealing.

[0315] However, at excessively high heating temperatures, cation mixing occurs, increasing the likelihood of added elements such as magnesium entering cobalt sites. Magnesium present at cobalt sites does not possess the properties required for Li... x When the x-value in CoO2 is relatively small, it maintains the effect of belonging to the R-3m layered rock salt crystal structure. Furthermore, there are concerns that excessively high heat treatment temperatures may lead to adverse effects such as cobalt being reduced to divalent form and lithium evaporation.

[0316] Therefore, it is preferable to use a material as a flux, either together with or mixed with an additive element source. A substance with a melting point lower than that of lithium cobalt oxide can be used as the flux. Fluorine compounds, such as lithium fluoride, are preferred as solvents. When a flux is added, the melting point of the additive element source and lithium cobalt oxide decreases. By lowering the melting point, the additive element can be readily distributed at temperatures where cation mixing is less likely.

[0317] [Initial heating] Furthermore, more preferably, heating is performed after the synthesis of lithium cobalt oxide and before the addition of elements. This heating is sometimes referred to as initial heating.

[0318] By performing initial heating, the distribution of added elements is better because lithium is removed from a portion of the surface layer 100a of the lithium cobalt oxide.

[0319] More specifically, the distribution of each added element can be considered to differ easily through initial heating via the following mechanism. First, lithium is desorbed from a portion of the surface layer 100a through initial heating. Then, heating is performed while mixing lithium cobalt oxide (including the lithium-deficient surface layer 100a) with added element sources such as nickel, aluminum, and magnesium. Magnesium, among the added elements, is a typical divalent element, and nickel, while a transition metal, readily becomes a divalent ion. Therefore, a portion of the surface layer 100a contains Mg. 2+ and Ni 2+ And Co reduced due to lithium deficiency 2+ It is a rock salt type phase. Note that this phase forms in a portion of the surface layer 100a, so it is sometimes not clearly identified in electron microscopes such as STEM and electron diffraction patterns.

[0320] When the surface layer 100a of the lithium cobalt oxide is of the layered rock salt type, nickel is easily dissolved and diffuses into the interior 100b. However, when a portion of the surface layer 100a is of the rock salt type, it tends to remain in the surface layer 100a. Therefore, by performing initial heating, divalent additives such as nickel can be easily retained in the surface layer 100a. The effect of this initial heating is particularly significant on the surface of the positive electrode active material 100 other than the (001) orientation and in its surface layer 100a.

[0321] In addition, compared with layered rock salt, the bonding distance between metal Me and oxygen (Me-O distance) tends to be longer in these rock salt types.

[0322] For example, rock salt type Ni 0.5 Mg 0.5 The Me-O distance in O is 2.09 × 10⁻⁶. -10 m, the Me-O distance in rock salt MgO is 2.11 × 10 m. -10 m. Furthermore, if a spinel-type phase is formed in a portion of the surface layer 100a, the Me-O distance of the spinel-type NiAl₂O₄ is 2.0125 × 10⁻⁶ m. -10 The Me-O distance of spinel-type MgAl₂O₄ is 2.02 × 10⁻⁶ m. -10 m. The Me-O distance exceeds 2×10 m. -10 m.

[0323] On the other hand, the bonding distances between metals other than lithium and oxygen in layered rock salt formations are shorter than the distances mentioned above. For example, the Al-O distance in layered rock salt LiAlO2 is 1.905 × 10⁻⁶. -10 m(Li-O distance is 2.11×10 -10 (m). Additionally, the Co-O distance in layered rock salt type LiCoO2 is 1.9224 × 10⁻⁶ m. -10 m(Li-O distance is 2.0916×10) -10 m).

[0324] Furthermore, based on the Shannon ionic radius, the ionic radius of six-coordinate aluminum is 0.535 × 10⁻⁶. -10 m, the ionic radius of the six-coordinate oxygen is 1.4 × 10⁻⁶. -10 m, their sum is 1.935 × 10 -10 m.

[0325] Therefore, it can be concluded that aluminum exists more stably in sites other than lithium in the layered rock salt type compared to the rock salt type. Thus, aluminum is more likely to be distributed in the deeper regions and / or interior 100b of the layered rock salt type than in the near-surface region of the surface layer 100a, which has the rock salt phase.

[0326] In addition, the following effect can be expected through initial heating: the crystallinity of the internal 100b layered rock salt-type crystal structure is improved.

[0327] Therefore, especially in order to manufacture in Li x When the x in CoO2 is, for example, 0.15 or more and 0.17 or less, a positive electrode active material 100 with a monoclinic O1(15) type structure is preferably subjected to this initial heating.

[0328] However, initial heating is not always necessary. By controlling the atmosphere, temperature, and time in other heating processes, it is sometimes possible to manufacture products in Li... x When the x in CoO2 is small, it has a positive electrode active material 100 with an O3' type structure and / or a monoclinic O1(15) type structure.

[0329] Eutectic point Here, when the melting point of fluorine compounds (sometimes referred to as fluorides) such as lithium fluoride is lower than that of other additive element sources, the fluorine compound can be used as a flux (also called a solvent) to lower the melting point of those other additive element sources. When the fluorine compound contains LiF and MgF₂, such as... Figure 17 (Referencing and modifying non-patent document 12) Figure 6 As shown in the figure, the eutectic point P of LiF and MgF2 is around 742℃ (T1), so the heating temperature in the heating process after mixing and adding elements is preferably above 742℃.

[0330] Here, use Figure 18 This describes the DSC test performed on fluorine compounds and mixtures. Figure 18 The mixture is prepared by mixing lithium cobalt oxide as a lithium oxide and LiF and MgF2 as fluorine compounds. More specifically, the mixture is prepared by mixing LiCoO2:LiF:MgF2 in a molar ratio of 100:0.33:1. Figure 18 The fluorine compound in the mixture is a mixture of LiF and MgF2. Specifically, the mixture is prepared in a molar ratio of LiF:MgF2 = 1:3.

[0331] like Figure 18 As shown, an endothermic peak was observed near 735°C in fluorine compounds. Furthermore, an endothermic peak was observed near 830°C in a mixture of lithium cobalt oxide, LiF, and MgF₂. Therefore, the heating temperature after adding the mixed elements is preferably 742°C or higher, more preferably 830°C or higher. Alternatively, a temperature between the above-mentioned temperatures and 800°C may also be used. Figure 17 (T2 and above)

[0332] <<Method 1 for Manufacturing Positive Electrode Active Materials>> Next, refer to Figures 19A to 19C Method 1 for manufacturing positive electrode active material 100 after initial heating.

[0333] <Step S11> exist Figure 19A In step S11 shown, lithium and transition metal materials, which serve as starting materials, are prepared as lithium source (Li source) and cobalt source (Co source), respectively.

[0334] Lithium-containing compounds are preferably used as lithium sources, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. High purity of the lithium source is preferred; for example, materials with a purity of 99.99% or higher are preferred.

[0335] The preferred cobalt source is a cobalt-containing compound, such as cobalt oxide (e.g., cobalt tetroxide) or cobalt hydroxide.

[0336] The cobalt source preferably has high purity, for example, materials with a purity of 3N (99.9%) or higher are preferred, 4N (99.99%) or higher are more preferred, 4N5 (99.995%) or higher are even more preferred, and 5N (99.999%) or higher are even more preferred. By using high-purity materials, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is improved and / or the reliability of the secondary battery is improved.

[0337] Furthermore, the cobalt source preferably has high crystallinity, for example, it is preferred to have single crystal particles. Methods for evaluating the crystallinity of a cobalt source include: evaluation using TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.; or evaluation using XRD, electron diffraction, neutron diffraction, etc. These methods for evaluating crystallinity can also be used to evaluate other crystallinities besides cobalt sources.

[0338] <Step S12> Next, as Figure 19A In step S12, the lithium source and cobalt source are pulverized and mixed to produce a mixed material. Pulverization and mixing can be performed using a dry or wet method. Wet methods are preferred because they allow for finer grinding. When performing wet pulverization and mixing, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, diethyl ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). A non-protic solvent that does not readily react with lithium is preferred. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. Preferably, the lithium source and cobalt source are pulverized and mixed using dehydrated acetone with a moisture content suppressed to below 10 ppm and a purity of 99.5% or higher. Using dehydrated acetone of this purity reduces the potential for impurities to be introduced.

[0339] As the unit for crushing and mixing, a ball mill or a sand mill can be used. When using a ball mill, alumina balls or zirconia balls are preferably used as the crushing medium. Zirconia balls have less impurity discharge, so they are preferred. In addition, when using a ball mill or sand mill, in order to suppress contamination from the medium, the circumferential speed is preferably set to 100 mm / s or more and 2000 mm / s or less. In this embodiment, the circumferential speed is preferably set to 838 mm / s (400 rpm, ball mill diameter 40 mm) for crushing and mixing.

[0340] <Step S13> Next, as Figure 19A In step S13, the above-mentioned mixed material is heated. Heating is preferably performed at 800°C or higher and 1100°C or lower, more preferably at 900°C or higher and 1000°C or lower, and even more preferably at around 950°C. At excessively low temperatures, there is concern about insufficient decomposition and melting of the lithium and cobalt sources. On the other hand, at excessively high temperatures, defects may occur due to the following reasons: lithium evaporation from the lithium source; and / or excessive reduction of cobalt; etc. For example, cobalt may change from trivalent to divalent, causing oxygen defects, etc.

[0341] Lithium cobalt oxide will not be synthesized if the heating time is too short, but the productivity will decrease if the heating time is too long. For example, the heating time is preferably more than 1 hour and less than 100 hours, and more preferably more than 2 hours and less than 20 hours.

[0342] Although the heating rate varies depending on the temperature reached, it is preferably 80°C / h or higher and 250°C / h or lower. For example, in the case of heating at 1000°C for 10 hours, the heating rate is preferably 200°C / h.

[0343] Heating is preferably performed in an atmosphere with low moisture content, such as dry air, for example, preferably in an atmosphere with a dew point of -50°C or lower, and more preferably in an atmosphere with a dew point of -80°C or lower. In this embodiment, heating is performed in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the concentration of impurities such as CH4, CO, CO2, and H2 in the heating atmosphere is preferably 5 ppb (parts per billion) or lower.

[0344] The heating atmosphere is preferably an oxygen-containing atmosphere. For example, a method of continuously introducing dry air into the reaction chamber can be cited. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber so that oxygen flows through the reaction chamber is called "flow".

[0345] When using an oxygen-containing atmosphere as a heating atmosphere, a non-flowing method is preferred. One such method is to first depressurize the reaction chamber and then fill it with oxygen (also known as "purging") to prevent oxygen leakage from the reaction chamber. For example, it is preferable to depressurize the reaction chamber to -970 hPa as indicated by a differential pressure gauge, and then continue filling with oxygen up to 50 hPa.

[0346] After heating, the material can be allowed to cool naturally. The cooling time from the specified temperature to room temperature is preferably between 10 and 50 hours. For example, the cooling rate (hereinafter also referred to as the cooling speed) is preferably between 80°C / h and 250°C / h, more preferably between 180°C / h and 210°C / h. Note that it is not necessary to cool to room temperature; cooling to the temperature allowed for the next step is sufficient.

[0347] In this process, heating can be performed using a rotary kiln or a roller kiln. Heating using a continuous or batch-type rotary kiln allows for simultaneous stirring and heating.

[0348] An alumina crucible is preferably used for heating. Alumina crucibles are made of a material that does not easily release impurities. In this embodiment, an alumina crucible with a purity of 99.9% is used. It is preferable to heat the crucible lid. This prevents the material from volatilizing or sublimating. As for the lid, it is sufficient to prevent the material from volatilizing or sublimating during the heating and cooling process in this step; it is not necessary to seal the crucible with the lid. For example, as described above, this step can also be performed by filling the reaction chamber with oxygen without sealing the crucible.

[0349] Furthermore, a semi-new crucible is preferred over a new one. In this specification, a new crucible refers to one that has undergone heating with materials containing lithium, transition metal M, and / or additive elements less than twice. A semi-new crucible refers to one that has undergone heating with materials containing lithium, transition metal M, and / or additive elements three or more times. This is because when using a new crucible, there is a concern that some of the material, such as lithium fluoride, may be absorbed, diffused, moved, or / or adhered to the crucible during heating. Therefore, if some material is lost due to the above reasons, the elemental distribution, especially on the surface of the positive electrode active material, may be outside the preferred range, increasing the likelihood that this possibility is low in a semi-new crucible.

[0350] After heating, the material can be crushed and screened as needed. When recovering the heated material, it can be moved from the crucible to the mortar first, and then the heated material can be recovered. Furthermore, an alumina mortar or a zirconium oxide mortar is preferably used. Alumina mortars are less likely to release impurities. Specifically, an alumina mortar with a purity of 90% or higher, preferably 99% or higher, is used. Additionally, the same heating conditions as in step S13 can be used in subsequent heating processes other than step S13.

[0351] <Step S14> Through the above processes, it is possible to synthesize Figure 19A The lithium cobalt oxide (LiCoO2) shown in step S14. When the median particle size (D50) is used as the particle size of lithium cobalt oxide, it is preferable to pulverize the lithium cobalt oxide in order to obtain a positive electrode active material 100 with a relatively small median particle size (D50).

[0352] As shown in steps S11 to S14, an example of manufacturing composite oxides by a solid-state method is illustrated, but composite oxides can also be manufactured by a co-precipitation method. Additionally, composite oxides can also be manufactured by a hydrothermal method.

[0353] <Step S15> Next, as Figure 19A Step S15, as shown, involves heating the lithium cobalt oxide. This heating is the first heating of the lithium cobalt oxide, so it is sometimes referred to as the initial heating. Alternatively, this heating is performed before step S20, as shown below, and is therefore sometimes referred to as preheating or pretreatment. The crucible and / or lid used in this step are the same as those used in step S13. Although the effects of initial heating are expected, it is not necessary to obtain the positive electrode active material according to one aspect of the invention.

[0354] As described above, lithium is detached from a portion of the surface layer 100a of the lithium cobalt oxide by initial heating. Furthermore, it is expected to improve the crystallinity of the interior 100b. Additionally, the lithium source and / or cobalt source prepared in step S11, etc., sometimes contain impurities. Initial heating can reduce impurities in the lithium cobalt oxide produced in step S14.

[0355] Initial heating also has the effect of smoothing the surface of lithium cobalt oxide. Surface smoothness of the composite oxide refers to a state with fewer bumps and irregularities, and the composite oxide as a whole has a rounded shape, including rounded corners. Furthermore, a state with fewer foreign matter adhering to the surface is also described as "smooth." Foreign matter can be considered the cause of bumps and irregularities, and it is preferable that it does not adhere to the surface.

[0356] In the initial heating described above, a lithium source may not be required. Alternatively, an element source may not be required. Alternatively, a material to be used as a flux may not be required.

[0357] If the heating time in this process is too short, sufficient results cannot be obtained; however, if the heating time is too long, productivity decreases. For example, the heating conditions described in step S13 can be selected and implemented. Further explanation of these heating conditions: To maintain the crystal structure of the composite oxide, the heating temperature in this process is preferably lower than the temperature in step S13. Additionally, to maintain the crystal structure of the composite oxide, the heating time in this process is preferably shorter than the heating time in step S13. For example, it is preferable to heat at a temperature of 700°C or higher and 1000°C or lower for 2 hours or more and 20 hours or less.

[0358] In addition, the effect of improving the crystallinity of the internal 100b refers to the effect of mitigating the skewing and deviation caused by the shrinkage difference of the lithium cobalt oxide produced in step S13.

[0359] In lithium cobalt oxide, a temperature difference sometimes occurs between the surface and interior of the lithium cobalt oxide due to heating in step S13. This temperature difference sometimes leads to a shrinkage difference. It can also be considered that the shrinkage difference occurs because the fluidity of the surface and interior differs according to the temperature difference. A difference in internal stress occurs in the lithium cobalt oxide due to the energy associated with this shrinkage difference. This difference in internal stress is also called distortion, and this energy is sometimes called distortion energy. It can be considered that the internal stress is removed by the initial heating in step S15; in other words, the distortion energy is homogenized by the initial heating in step S15. When the distortion energy is homogenized, the distortion of the lithium cobalt oxide is mitigated. Therefore, the surface of the lithium cobalt oxide may become smoother. It can also be said that the surface is improved. In other words, it can be considered that the shrinkage difference generated in the lithium cobalt oxide by step S15 is mitigated, thereby smoothing the surface of the composite oxide.

[0360] Furthermore, shrinkage differences can sometimes lead to minute deviations in the aforementioned lithium cobalt oxide, such as deviations in crystallization. This process is preferable to reduce these deviations. Through this process, it is possible to homogenize the deviations in the aforementioned composite oxide. When the deviations are homogenized, the surface of the composite oxide may become smoother. In other words, the crystalline particles are aligned. In other words, it can be considered that step S15 mitigates the deviations such as crystallization that occur in the composite oxide, thereby smoothing the surface of the composite oxide.

[0361] By using smooth-surfaced lithium cobalt oxide as the positive electrode active material, the degradation during charging and discharging of the secondary battery is reduced, thereby preventing the cracking of the positive electrode active material.

[0362] Alternatively, pre-synthesized lithium cobalt oxide can be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on the pre-synthesized lithium cobalt oxide, a lithium cobalt oxide with a smooth surface can be obtained.

[0363] <Step S20> Next, as shown in step S20, additive element A is preferably added to the initially heated lithium cobalt oxide. When adding additive element A to the initially heated lithium cobalt oxide, additive element A can be added uniformly. Therefore, it is preferable to perform initial heating first and then add additive element A. Figure 19B and Figure 19C Explain the steps for adding element A.

[0364] <Steps S21 to S23> use Figure 19B and Figure 19C The steps for preparing to add element A source (A source) are described separately. In addition to adding element A source, a lithium source can also be prepared.

[0365] As additive element A, one or more of the additive elements described in the above embodiments, such as magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron, may be used. Alternatively, one or two of bromine and beryllium may also be used.

[0366] <Step S21> right Figure 19B Step S21 will be explained below. When magnesium is selected as the added element, the source of the added element can be referred to as a magnesium source (Mg source). Magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate can be used as this magnesium source. Alternatively, multiple magnesium sources mentioned above can be used.

[0367] When fluorine is selected as the added element, the source of the added element can be referred to as the fluorine source (F source). Examples of suitable fluorine sources include lithium fluoride (LiF), magnesium fluoride (MgF2), aluminum fluoride (AlF3), titanium fluoride (TiF4), cobalt fluoride (CoF2, 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 (Na3AlF6). Lithium fluoride is preferred due to its low melting point (848°C) and its ease of melting during the heating process described later.

[0368] 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. Lithium carbonate is another lithium source used in step S21.

[0369] In addition, the fluorine source can also be a gas, and fluorine (F2), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF2, O2F2, O3F2, O4F2, O5F2, O6F2, O2F) can be mixed in an atmosphere during the heating process described later. Alternatively, multiple of the above-mentioned fluorine sources can be used.

[0370] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as both the fluorine and magnesium sources. A molar ratio of approximately LiF:MgF2 = 65:35 is most effective in lowering the melting point. On the other hand, when there is a large amount of lithium fluoride, the lithium content becomes excessive, which may lead to a deterioration in cycling characteristics. Therefore, the preferred molar ratio of lithium fluoride to magnesium fluoride is LiF:MgF2 = x:1 (0 ≤ x ≤ 1.9), more preferably LiF:MgF2 = x:1 (0.1 ≤ x ≤ 0.5), and even more preferably LiF:MgF2 = x:1 (x = 0.33 or nearby). Furthermore, in this specification, "nearly" refers to a value greater than 0.9 times and less than 1.1 times the specified value.

[0371] <Step S22> Next, in Figure 19B Step S22, as shown, involves pulverizing and mixing the magnesium source and the fluorine source. This step can be performed using the pulverizing and mixing conditions described in step S12.

[0372] <Step S23> Next, in Figure 19B Step S23, as shown, can recover the pulverized and mixed materials to obtain an additive element A source (A source). The additive element A source shown in step S23 contains multiple starting materials and can be referred to as a mixture.

[0373] The median particle size (D50) of the above mixture is preferably 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less. When a material is used as the source of the added element, the median particle size (D50) is also preferably 600 nm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less.

[0374] When using the above-mentioned micronized mixture (including cases where only one element is added), it is easy to uniformly adhere the mixture to the surface of the lithium cobalt oxide particles during subsequent mixing processes. Since the mixture is uniformly adhered to the surface of the lithium cobalt oxide particles, it is easier to uniformly distribute or diffuse the added element in the surface layer 100a of the composite oxide after heating, which is therefore preferred.

[0375] <Step S21> use Figure 19C Explanation and Figure 19B Different processes. Figure 19C Step S21, as shown, prepares four sources of additive elements to be added to lithium cobalt oxide. That is to say, Figure 19C The types of added element sources and Figure 19B Different. In addition to adding an element source, a lithium source can also be prepared.

[0376] As four sources of additive elements, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium and fluorine sources can be obtained from... Figure 19B The compounds described are selected from those listed. Nickel oxide, nickel hydroxide, etc., can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc., can be used as the aluminum source.

[0377] <Steps S22 and S23> Figure 19C Steps S22 and S23 shown are Figure 19B The steps described in the text are the same.

[0378] <Step S31> Next, in Figure 19A In step S31, lithium cobalt oxide and element source A (source A) are mixed. The ratio of the number of cobalt atoms Co in lithium cobalt oxide to the number of magnesium atoms Mg in element source A is preferably Co:Mg = 100:y (0.1 ≤ y ≤ 6), more preferably Co:Mg = 100:y (0.3 ≤ y ≤ 3).

[0379] To avoid damaging the shape of the lithium cobalt oxide particles, the mixing in step S31 is preferably carried out under milder conditions than the mixing in step S12. For example, it is preferable to carry out the mixing under conditions with fewer rotations or shorter time than the mixing in step S12. Furthermore, dry mixing is arguably a milder condition than wet mixing. For example, a ball mill or sand mill can be used for mixing. When using a ball mill, zirconia balls are preferably used as the mixing medium.

[0380] In this embodiment, a ball mill using zirconia balls with a diameter of 1 mm was used to dry mix the mixture at 150 rpm for 1 hour. Furthermore, this mixing was carried out in a drying chamber with a dew point of -100°C or higher and -10°C or lower.

[0381] <Step S32> Next, in Figure 19A In step S32, the above-mentioned mixed materials are recovered to obtain mixture 903. During recovery, screening may also be performed after grinding if necessary.

[0382] Note that in Figures 19A to 19C The invention describes a manufacturing method in which additives are added after initial heating, but the invention is not limited to the above method. Additives can be added at other times or added in multiple stages. Furthermore, the timing can be changed depending on the additives.

[0383] For example, such as Figures 20A to 20C As shown, the added elements can also be added to the lithium source and cobalt source in the stage of step S11, that is, the stage of the starting material of the composite oxide. Figure 20A The process of adding a magnesium source to a lithium source and a cobalt source is shown. Figure 20B The process of adding magnesium and aluminum sources to lithium and cobalt sources is shown. Figure 20C The process of adding magnesium and nickel sources to lithium and cobalt sources is shown. Figures 20A to 20C The example shown is just one example of adding an element source.

[0384] Then, proceeding to step S12 and passing through step S13, lithium cobalt oxide containing the added elements can be obtained in step S14. The distribution of the added elements can be controlled according to the timing of their addition. For example... Figures 20A to 20C The added elements are expected to be located inside the positive electrode active material 100. Figures 20A to 20C In the illustrated process, it is not necessary to separate the operations from steps S11 to S14 and steps S21 to S23. Therefore, it can be said that the above method is simple and highly productive. Of course, even in Figures 20A to 20C New elements can also be added in step S20 of the process shown.

[0385] Alternatively, lithium cobalt oxide with some added elements can be used. For example, when using lithium cobalt oxide with added magnesium and fluorine, steps S11 to S14 and part of step S20 can be omitted. The above method can be considered simple and highly productive.

[0386] Alternatively, lithium cobalt oxide pre-added with magnesium and fluorine can be heated in step S15, and then magnesium source and fluorine source or magnesium source, fluorine source, nickel source and aluminum source can be added as in step S20.

[0387] <Step S33> Next, in Figure 19A In step S33, mixture 903 is heated. This heating can be performed from the heating conditions described in step S13. The heating time is preferably 2 hours or more. At this time, in order to increase the oxygen partial pressure of the heating atmosphere, the pressure inside the furnace can also be greater than atmospheric pressure. This is because when the oxygen partial pressure of the heating atmosphere is insufficient, cobalt and the like are reduced, and lithium cobalt oxide and the like may not be able to maintain their layered rock salt-type crystal structure.

[0388] Here, further explanation of the heating temperature is provided. The lower limit of the heating temperature in step S33 needs to be above the temperature at which the reaction between lithium cobalt oxide and the added element source progresses. The reaction progress temperature is preferably set to the temperature at which interdiffusion occurs between lithium cobalt oxide and the elements contained in the added element source, but it can also be lower than the melting temperature of the aforementioned materials. Taking an oxide as an example, it is known that from the melting temperature T... m 0.757 times (Taman temperature T) d Solid-phase diffusion occurs. Therefore, the heating temperature in step S33 is preferably set to 650°C or higher.

[0389] Of course, the reaction proceeds more easily when the melting temperature of one or more of the materials selected from mixture 903 is set above that of the mixture. For example, when LiF and MgF2 are included as the source of added elements, the eutectic point of LiF and MgF2 is around 742°C (see reference). Figure 17 Since the eutectic point P is the eutectic point, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0390] Furthermore, the mixture 903 obtained by mixing LiCoO2:LiF:MgF2 in a molar ratio of 100:0.33:1 showed an endothermic peak at around 830°C in DSC testing. Therefore, the lower limit of the heating temperature is more preferably set to 830°C or higher.

[0391] Higher heating temperatures facilitate reaction progress, shorten heating time, and increase productivity, making them the preferred option.

[0392] The upper limit of the heating temperature is set below the decomposition temperature of lithium cobalt oxide (melting point 1130°C). At temperatures near the decomposition temperature, minor decomposition of lithium cobalt oxide may occur. Therefore, the upper limit of the heating temperature is more preferably below 1000°C, further preferably below 950°C, and even more preferably below 900°C.

[0393] In summary, the heating temperature for step S33 is preferably 650°C or higher and 1130°C or lower, more preferably 650°C or higher and 1000°C or lower, even more preferably 650°C or higher and 950°C or lower, and even more preferably 650°C or higher and 900°C or lower. Furthermore, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Additionally, it is preferably 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, and even more preferably 830°C or higher and 900°C or lower. Furthermore, the heating temperature for step S33 is preferably lower than the heating temperature for step S13.

[0394] Here, use Figure 23 Here is an example of a heating furnace used in this step S33.

[0395] Figure 23 The furnace 220 shown includes a furnace interior space 202, a hot plate 204, a pressure gauge 221, a heater section 206, and a thermal insulator 208. It is preferable to cover the container 216, which is equivalent to a crucible or sagger, with a lid 218 before heating. This structure allows the atmosphere within the space 219 formed by the container 216 and the lid 218 to be a fluoride-containing atmosphere. By covering the space during heating to maintain a constant concentration of gaseous fluoride within the space 219 or to prevent a decrease in the concentration of fluoride, fluorine and magnesium can be contained near the particle surface of the mixture 903. The volume of the space 219 is smaller than the furnace interior space 202, so even when a small amount of fluoride is volatilized, the atmosphere within the space 219 can be a fluoride-containing atmosphere. That is, the atmosphere of the reaction system can be made fluoride-containing, preventing a significant reduction in the amount of fluoride contained in the mixture 903. Furthermore, by using the lid 218, the mixture 903 can be heated simply and inexpensively in a fluoride-containing atmosphere.

[0396] Furthermore, before heating in the furnace interior space 202, a process is performed to create an oxygen-containing atmosphere in the furnace interior space 202, and a process is performed to place the container 216 containing the mixture 903 in the furnace interior space 202. By employing this sequence of steps, the mixture 903 can be heated in an atmosphere containing oxygen and fluorides. For example, gas can be circulated during heating. Gas can be introduced from the bottom surface of the furnace interior space 202 and discharged to the top surface. Alternatively, during heating, the furnace interior space 202 can be sealed to become a closed space to prevent gas from being transported to the outside (purging).

[0397] There are no restrictions on the method for making the atmosphere inside the furnace space 202 an oxygen-containing atmosphere. For example, methods include: venting the air inside the furnace space 202 and then introducing an oxygen-containing gas such as oxygen gas or dry air; or allowing the oxygen-containing gas such as oxygen gas or dry air to flow through for a certain period of time. Preferably, oxygen gas is introduced after venting the air inside the furnace space 202 (oxygen replacement). Alternatively, the air inside the furnace space 202 can also be considered as an oxygen-containing atmosphere.

[0398] Alternatively, heating can cause fluorides and other substances adhering to the inner walls of container 216 and lid 218 to re-fly and adhere to mixture 903.

[0399] There are no restrictions on the process of heating the heating furnace 220. It is preferable to use a heating mechanism installed in the heating furnace 220 for heating.

[0400] Furthermore, there are no particular restrictions on the preparation method of the mixture 903 when placed in container 216, but as... Figure 23 As shown, preferably, the top surface of the mixture 903 is flat with respect to the bottom surface of the container 216, that is, the mixture 903 is arranged in such a way that the height of the top surface of the mixture 903 is uniform.

[0401] The heating in step S33 described above is preferably performed while the furnace pressure is controlled using pressure gauge 221. The furnace is preferably under atmospheric pressure or a pressurized state. For example, it is believed that the surface of lithium cobalt oxide melts when exposed to pressure. Therefore, the surface of lithium cobalt oxide heated together with LiF and MgF2 may be melted by pressurization.

[0402] After heating in step S33, the material can be allowed to cool naturally. The cooling time from the specified temperature to room temperature is preferably between 10 and 50 hours. For example, the cooling rate is preferably between 80°C / h and 250°C / h, more preferably between 180°C / h and 210°C / h. The cooling rate in step S33 is preferably faster than in step S13. This rapid cooling rate is referred to as quenching. By quenching the material after melting, a shell can be appropriately manufactured. Specifically, a shell with a narrow width can be manufactured. Note that the temperature at the end of cooling does not necessarily need to be room temperature; cooling to the temperature allowed for the next step is sufficient.

[0403] Furthermore, when heating mixture 903, it is preferable to control the partial pressure of fluorine or fluorine compounds originating from fluorine sources within an appropriate range. This partial pressure can also be controlled by covering the crucible used in this step with a lid and then heating it. As explained above, the lid prevents the material from volatilizing or sublimating. That is, as long as volatilization or sublimation is prevented during the heating and cooling process in this step, it is not necessary to seal the crucible with a lid. For example, this step can be performed by filling the reaction chamber containing the crucible with oxygen, without sealing the crucible. A suitable positive electrode active material containing fluorine or fluorine compounds is preferred because it prevents overheating and fumes even in the event of an internal short circuit.

[0404] In the manufacturing method described in this embodiment, sometimes a portion of the material, such as LiF, which serves as a fluorine source, is used as a flux. Through the aforementioned function, the heating temperature can be lowered to below the decomposition temperature of lithium cobalt oxide, for example, between 742°C and 950°C, allowing additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent properties.

[0405] However, gaseous LiF is lighter than oxygen, so LiF may volatilize or sublimate upon heating, reducing the amount of LiF in mixture 903. This reduces LiF's function as a flux. Therefore, heating is necessary to suppress LiF volatilization. Furthermore, even without using LiF as a fluorine source, Li on the LiCoO2 surface may react with F (as a fluorine source) to form LiF, which then volatilizes. Therefore, even when using fluorine compounds with melting points higher than LiF, volatilization must still be suppressed.

[0406] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 under conditions of high partial pressure of LiF in the heating furnace. This heating method can suppress the volatilization of LiF in the mixture 903. To suppress LiF volatilization, it is preferable to cover the crucible with a lid.

[0407] The heating in this process is preferably carried out in a manner that prevents the particles of mixture 903 from sticking together. When the particles of mixture 903 stick together during heating, the area of ​​contact between the particles and oxygen in the atmosphere is reduced, and the diffusion path of added elements (e.g., fluorine) is blocked, which may make it difficult for added elements (e.g., magnesium and fluorine) to be distributed in the surface layer. To promote reaction with oxygen in the atmosphere, the crucible may not be sealed with a lid.

[0408] Furthermore, it is considered that when the added element (e.g., fluorine) is uniformly distributed in the surface layer, a smooth positive electrode active material with fewer irregularities can be obtained. Therefore, in order to maintain the smooth surface state after heating in step S15 or to make it even smoother in this process, it is preferable not to allow the particles of mixture 903 to stick together.

[0409] Furthermore, when using a rotary kiln for heating, it is preferable to control the flow rate of the oxygen-containing atmosphere within the kiln. For example, it is preferable to: reduce the flow rate of the oxygen-containing atmosphere; first purge the atmosphere to introduce oxygen into the kiln, and then stop the atmosphere flow; etc. When oxygen is allowed to flow, the fluorine source may evaporate, which is not preferable in order to maintain surface smoothness.

[0410] When heating is performed using a roller kiln, the mixture 903 can be heated in a LiF-containing atmosphere, for example, by covering the container containing the mixture 903. The container is covered in the same manner as the crucible.

[0411] Additional information on heating time. The heating time varies depending on the heating temperature, the size and composition of the lithium cobalt oxide obtained in step S14, and other conditions. When the lithium cobalt oxide is small, it is sometimes more preferable to heat at a lower temperature or for a shorter time compared to when the lithium cobalt oxide is larger.

[0412] exist Figure 19A When the median particle size (D50) of the lithium cobalt oxide obtained in step S14 is about 12 μm, the heating temperature is preferably set to 650°C or higher and 950°C or lower. The heating time is preferably set to 3 hours or higher and 60 hours or lower, more preferably 10 hours or higher and 30 hours or lower, and even more preferably about 20 hours. In addition, the cooling time after heating is preferably set to 10 hours or higher and 50 hours or lower.

[0413] On the other hand, when the median particle size (D50) of the lithium cobalt oxide obtained in step S14 is about 5 μm, the heating temperature is preferably set to 650°C or higher and 950°C or lower. The heating time is preferably set to 1 hour or higher and 10 hours or lower, more preferably about 5 hours. In addition, the cooling time after heating is preferably set to 10 hours or higher and 50 hours or lower.

[0414] <Step S34> Next, in Figure 19A In step S34, the heated material is recovered to obtain the positive electrode active material 100. At this time, the recovered particles may also be screened. Through the above process, 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.

[0415] <<Method 2 for Manufacturing Positive Electrode Active Materials>> Next, use Figures 21 to 22C This invention describes a method 2 for manufacturing a positive electrode active material, which differs from method 1. The main difference between method 2 and method 1 lies in the number of times the added elements are added and the mixing method. Other details can be found in the description of method 1.

[0416] exist Figure 21 In, with Figure 19A Similarly, steps S11 to S15 are performed to prepare lithium cobalt oxide after initial heating.

[0417] <Step S20a> Next, in step S20a, a source of additive element A1 is prepared for adding additive element A1 to the initially heated lithium cobalt oxide. Using... Figure 22A This describes the process of preparing to add source A1.

[0418] <Step S21> illustrate Figure 22A The step S21 is shown. As an added element A1, it can be obtained from... Figure 19B The element used in step S21 is selected from the elements exemplified as additive element A. For example, one or more elements selected from magnesium, fluorine, and calcium may be suitable as additive element A1. Figure 22A An example is shown where magnesium and fluorine are selected as added elements A1, and magnesium source (Mg source) and fluorine source (F source) are prepared in step S21.

[0419] Figure 22A Steps S21 to S23 shown can be combined with Figure 19B The same conditions are applied to steps S21 to S23. As a result, the added element A1 source (A1 source) can be obtained in step S23.

[0420] in addition, Figure 21 Steps S31 to S33 shown can be achieved by... Figure 19A The same procedures are performed for steps S31 to S33 shown.

[0421] <Step S34a> Next, the material heated in step S33 is recycled to produce lithium cobalt oxide containing added element Al. To distinguish it from the composite oxide of step S14, this composite oxide is also referred to as the second composite oxide.

[0422] <Step S40> exist Figure 21 Step S40, as shown, prepares the source of additive element A2 for adding additive element A2 to the second composite oxide. Using... Figure 22B and Figure 22C Each step describes the process of preparing to add element A2 source.

[0423] <Step S41> illustrate Figure 22B The step S41 is shown. As an added element A2, it can be obtained from... Figure 19C The element used in step S21 is selected from the elements exemplified as additive element A. For example, one or more of nickel, titanium, boron, zirconium, and aluminum can be used as additive element A2. Figure 22B Examples of preparing nickel (Ni source) and aluminum (Al source) by selecting nickel and aluminum as the added elements A2 are shown.

[0424] Figure 22B Steps S41 to S43 shown can be combined with Figure 19B The same conditions are applied to steps S21 to S23. As a result, the added element A2 source (A2 source) can be obtained in step S43.

[0425] in addition, Figure 22C Showing the use Figure 22B This illustrates a variation of the process for preparing to add element A2 source. Figure 22C In step S41, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and they are pulverized independently in step S42a. As a result, multiple A2 source materials (A2 source) are prepared in step S43. Figure 22C Steps and Figure 22B The difference lies in the fact that the added elements are crushed independently in step S42a.

[0426] <Steps S51 to S54> then, Figure 21 Steps S51 to S54 shown can be combined with... Figure 19A Steps S31 to S34 are performed under the same conditions, and mixture 904 is obtained in step S52. Furthermore, the conditions for step S53, which relates to the heating process, can be as follows: the heating temperature is lower than that of step S33, and the heating time is shorter than that of step S33. Through the above processes, a positive electrode active material 100 of one aspect of the present invention can be manufactured in step S54. The surface of the positive electrode active material of one aspect of the present invention is smooth.

[0427] like Figures 21 to 22C As shown, in manufacturing method 2, the added elements are introduced into lithium cobalt oxide separately as added element A1 and added element A2. By introducing the added elements separately, the presence position of each added element in the depth direction can be changed. For example, added element A1 can be present in such a way that its concentration in the surface layer is higher than that in the interior layer, and added element A2 can be present in such a way that its concentration in the interior layer is higher than that in the surface layer.

[0428] After the initial heating shown in this embodiment, a positive electrode active material with a smooth surface can be obtained.

[0429] The initial heating shown in this embodiment is performed on lithium cobalt oxide. Therefore, the initial heating is preferably performed under the following conditions: the heating temperature is lower than the heating temperature used to obtain lithium cobalt oxide, and the heating time is shorter than the heating time used to obtain lithium cobalt oxide. The process of adding additive elements to lithium cobalt oxide is preferably performed after the initial heating. This addition process can be performed in two or more steps. By following the above process sequence, the smoothness of the surface obtained by the initial heating can be maintained, which is therefore preferred.

[0430] The positive electrode active material 100 with a smooth surface may have higher resistance to physical damage caused by pressure or other factors than a positive electrode active material with an uneven surface. For example, the positive electrode active material 100 is less likely to be damaged in tests accompanied by pressure, such as nail penetration tests, and the safety of the results may be improved.

[0431] This implementation method can be used in combination with other implementation methods.

[0432] (Implementation Method 3) In this embodiment, the positive electrode, negative electrode, etc. of a secondary battery according to one aspect of the present invention will be described.

[0433] [positive electrode] Figure 24A An example cross-sectional view of a positive electrode 503 for a secondary battery 1004, etc., is shown. The positive electrode 503 includes a positive electrode active material layer 502 on a positive electrode current collector 501. The positive electrode active material layer 502 includes positive electrode active material 100, positive electrode active material 562, conductive material 553, conductive material 554, and electrolyte 530. The positive electrode active material layer 502 also includes a binder (not shown). The secondary battery may also have a structure having either conductive material 553 or conductive material 554.

[0434] The median particle size (D50) of the positive electrode active material 100 is 1 μm or more and 50 μm or less, preferably 5 μm or more and 30 μm or less. To improve the filling density, it is preferable to add a positive electrode active material 562 with a different median particle size (D50). The median particle size (D50) of the positive electrode active material 562 is preferably 1 / 10 or more and 1 / 6 or less of the median particle size (D50) of the positive electrode active material 100. When the particle size distribution of the active material mixed with the positive electrode active material 100 and the positive electrode active material 562 is measured, two peaks with different maxima are identified. Of course, more than two peaks can also be identified. Note that the filling density can be improved even without the positive electrode active material 562.

[0435] Both positive electrode active material 100 and positive electrode active material 562 preferably include a shell. The inclusion of a shell in the positive electrode active material improves insulation and reduces the likelihood of thermal runaway. Figure 24A The boundaries between the surface and interior are marked with dashed lines, but the boundaries are not necessarily as shown. Figure 24A That's so clear. Furthermore, it can be used as a shell. Figure 5B The structure shown is not limited to the positive electrode active material. Figure 24A For example, either positive electrode active material 100 or positive electrode active material 562 may also include a shell.

[0436] The compositions of positive electrode active material 100 and positive electrode active material 562 may be the same or different. Cases with the same composition include those where the main components of the positive electrode active materials are the same, but the presence or absence of added elements, etc., differ. Cases with different compositions include those where the main components of the positive electrode active materials are different.

[0437] Although already stated, positive electrode active material 100 and positive electrode active material 562 preferably contain additive elements, and more preferably, the shell contains additive elements. The additive elements can be concentrated in the shell or distributed at a low concentration inside. Concentrated distribution means that the additive elements are unevenly distributed or exist in a biased manner. Therefore, sometimes the state in which the concentration of the additive elements increases from the inside to the shell is referred to as the additive elements being concentrated in the shell.

[0438] The added element may also be included in the surface layer portion. Preferably, there is a concentration difference between the concentration of the added element in the surface layer portion and the concentration of the added element in the interior, and the concentration of the added element in the surface layer portion is preferably higher than the concentration in the interior. Sometimes this state is referred to as the added element being concentratedly distributed in the surface layer portion.

[0439] The positive electrode active material 100 and the positive electrode active material 562 are sometimes referred to as positive electrode active material particles, but the positive electrode active material has various shapes other than particulate. Figure 24B Shows a positive electrode 503 including a positive electrode active material having a shape other than particulate different from Figure 24A . In Figure 24B , other than the shape of the positive electrode active material is the same as Figure 24A , so the description is omitted. In Figure 24A and Figure 24B , the positive electrode active material 100 and the positive electrode active material 562 are shown as primary particles, but they may also be secondary particles.

[0440] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and may also contain a conductive material (synonymous with a conductive aid) and a binder. The positive electrode active material uses the positive electrode active material formed by the manufacturing method described in the above embodiments. For example, a positive electrode active material with a relatively small median particle diameter (D50) and a positive electrode active material with a relatively large median particle diameter (D50) can be mixed and used.

[0441] In addition, the positive electrode active material described in the above embodiments and other positive electrode active materials may be mixed and used.

[0442] As other positive electrode active materials, for example, there are composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, compounds such as LiFePO4, LiFeO2, LiNiO2, LiMn2O4, V2O5, Cr2O5, and MnO2 can be cited.

[0443] In addition, as other positive electrode active materials, it is preferable to mix a lithium-containing material containing manganese such as LiMn2O4 having a spinel-type crystal structure with lithium nickelate (LiNiO2 or LiNi 1-x M x O2 (0 < x < 1) (M = Co, Al, etc.)) and use them. By adopting this structure, the characteristics of the secondary battery can be improved.

[0444] In addition, as other positive electrode active materials, those that can be represented by the composition formula Li a Mn b M c O d The lithium manganese composite oxide is represented here. Element M is preferably a metal element selected from elements other than lithium and manganese, or silicon and phosphorus, and more preferably nickel. Furthermore, when measuring the overall particle composition of the lithium manganese composite oxide, it is preferable that the discharge condition satisfies 0 <a / (b+c)<2、c>0 and 0.26 ≤ (b+c) / d < 0.5. The composition of the overall lithium manganese composite oxide particles, including metals, silicon, and phosphorus, can be measured, for example, using ICP-MS. Additionally, the oxygen composition of the overall lithium manganese composite oxide particles can be measured, for example, using EDX. Furthermore, the valence can be evaluated using fusion gas analysis and XAFS (X-ray absorption fine structure) analysis in conjunction with ICP-MS analysis. Note that lithium manganese composite oxide refers to an oxide containing at least lithium and manganese, and may also contain one or more elements selected from the group consisting of chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, and phosphorus.

[0445] [Conductive Materials] Conductive materials serve to facilitate current paths between active materials and current collectors, or between multiple active materials. To achieve this function, conductive materials preferably contain materials with lower resistivity than the active materials. Based on their function, conductive materials are sometimes referred to as conductive aids or conductive promoters.

[0446] Conductive materials typically utilize carbon or metallic materials. These materials are often in particulate form, with examples including carbon black (furnace black, acetylene black, graphite, etc.). Carbon black with particle sizes smaller than the positive electrode active material is commonly found. Fibrous conductive materials also exist, with carbon nanotubes (CNTs) and VGCF (registered trademark) serving as conductive additives in this form. Sheet-like conductive materials are also available, with multilayer graphene serving as an example of a sheet-like conductive additive. In the cross-section of the positive electrode, sheet-like conductive additives are sometimes observed to appear as linear structures.

[0447] Particulate conductive materials can enter the gaps between positive electrode active materials and easily aggregate. Therefore, particulate conductive materials can assist in the conductive path between positive electrode active materials disposed nearby. Although they have bending regions, fibrous conductive materials are larger than positive electrode active materials. Therefore, fibrous conductive materials can assist in the conductive path between separated positive electrode active materials, in addition to the conductive paths between adjacent positive electrode active materials. Thus, it is preferable that the mixed conductive additives have two or more shapes.

[0448] When multilayer graphene is used as a sheet-like conductive material and carbon black is used as a particle-like conductive material, the weight of carbon black in the slurry containing them is preferably 1.5 times or more and 20 times or less than that of multilayer graphene, more preferably 2 times or more and 9.5 times or less. ​

[0449] When the mixing ratio of multilayer graphene to carbon black is set within the above range, the carbon black does not aggregate and is easily dispersed. Furthermore, when the mixing ratio of multilayer graphene to carbon black is set within the above range, the electrode density can be increased compared to using carbon black alone as a conductive additive. By increasing the electrode density, the capacity per unit weight can be increased.

[0450] Furthermore, by setting the mixing ratio of multilayer graphene and carbon black to the above range, rapid charging can be achieved.

[0451] In this specification, graphene includes multilayer graphene and multigraphene. In other words, graphene refers to a compound containing carbon and having a two-dimensional structure formed by six-membered rings composed of carbon atoms, with shapes such as flat sheets or thin sheets. Sometimes, this two-dimensional structure formed by six-membered rings composed of carbon atoms is also called a carbon sheet. Graphene compounds include graphene oxide, multilayer graphene oxide, multi-element graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-element graphene oxide, graphene quantum dots, etc. In other words, graphene compounds may also have functional groups. Furthermore, graphene or graphene compounds preferably have a curved shape. Additionally, graphene or graphene compounds may also be coiled; sometimes, coiled graphene is called carbon nanofiber.

[0452] In this specification and the like, graphene oxide refers to a graphene compound containing carbon and oxygen, having a sheet-like shape, and including functional groups, especially epoxy, carboxyl, or hydroxyl groups.

[0453] In this specification, the reduced graphene oxide contains carbon and oxygen, has a sheet-like shape, and possesses a two-dimensional structure formed by six-membered rings composed of carbon atoms. A single layer of reduced graphene oxide can function, but a multilayer structure is also possible. Preferably, the reduced graphene oxide has a carbon concentration greater than 80 atomic% and an oxygen concentration of 2 atomic% or more and less than 15 atomic%. With these carbon and oxygen concentrations, even a small amount of reduced graphene oxide can function as a highly conductive material. Furthermore, it is preferable that the intensity ratio of the G band to the D band in the Raman spectrum of the reduced graphene oxide, G / D, is 1 or more. Even a small amount of reduced graphene oxide with this intensity ratio can function as a highly conductive material.

[0454] Fluorinated graphene can also be used as the graphene compound. Fluorine in the graphene compound is preferably adsorbed onto the surface. Fluorinated graphene can be manufactured by contacting graphene with a fluorine compound (a process known as fluorination). Fluorine (F2) or a fluorine compound is preferably used in the fluorination process. Hydrogen fluoride, fluorinated halides (ClF3, IF5, etc.), gaseous fluorides (BF3, NF3, PF5, SiF4, SF6, etc.), and metal fluorides (LiF, NiF2, AlF3, MgF2, etc.) are preferred as fluorination compounds. Gaseous fluorides are preferred in the fluorination process, but they can also be diluted with an inert gas. The fluorination temperature is preferably room temperature, and more preferably 0°C or higher and 250°C or lower. By performing the fluorination process at a temperature above 0°C, fluorine can be adsorbed onto the surface of the graphene.

[0455] Graphene compounds sometimes possess excellent electrical properties such as high conductivity and excellent physical properties such as high flexibility and high mechanical strength. Furthermore, graphene compounds have a sheet-like shape. Graphene compounds sometimes have curved surfaces, enabling surface contacts with low contact resistance. Graphene compounds sometimes exhibit very high conductivity even when thin, thus allowing conductive pathways to be formed efficiently in a small amount within the active material layer. Therefore, by using graphene compounds as conductive materials, the contact area between the active material and the conductive material can be increased. Preferably, the graphene compound covers more than 80% of the area of ​​the active material. Note that it is preferable that the graphene compound clings to at least a portion of the active material. Preferably, the graphene compound covers at least a portion of the active material. Preferably, the shape of the graphene compound conforms to at least a portion of the shape of the active material. The shape of the active material refers to the irregularities and irregularities present in a single active material or formed by multiple active materials. Preferably, the graphene compound surrounds at least a portion of the active material. The graphene compound may have pores.

[0456] When using active materials with small particle sizes, such as those with a particle size of less than 1 μm, more conductive paths are needed to connect the active materials to each other. In this case, graphene compounds that can efficiently form conductive paths even in small quantities are preferred.

[0457] Due to the aforementioned properties, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and discharging. For example, secondary batteries for two- or four-wheeled vehicles, and for drones, sometimes require rapid charging and discharging. Mobile electronic devices also sometimes need rapid charging characteristics. Rapid charging and discharging, for example, refers to charging and discharging at 200 mA / g, 400 mA / g, or 1000 mA / g or higher.

[0458] In the longitudinal section of the positive electrode active material layer 502, such as Figure 24B As shown, sheet-like graphene or graphene compounds are dispersed relatively uniformly within the positive electrode active material layer 502. Figure 24B Although graphene or graphene compounds are schematically shown in thick lines, in reality, graphene or graphene compounds are thin films with a single or multiple layers of carbon molecules. Since multiple graphene or graphene compounds are formed either by covering a portion of multiple particulate positive electrode active materials 100 or by attaching to the surface of multiple particulate positive electrode active materials 100, multiple graphene or graphene compounds form surface contacts with multiple particulate positive electrode active materials 100.

[0459] Here, by bonding multiple graphene or graphene compounds together, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound mesh or graphene mesh) can be formed. When the graphene mesh covers the active material, it can be used as a binder to bind the active material together. Therefore, the amount of binder can be reduced or eliminated, thereby increasing the proportion of active material in the electrode volume and weight. In other words, the discharge capacity of the secondary battery can be increased.

[0460] Preferably, graphene oxide is used as graphene or a graphene compound, and is reduced after being mixed with it to form a layer of positive electrode active material layer 502. That is, the completed active material layer preferably contains reduced graphene oxide. By using highly dispersible graphene oxide in a polar solvent in the formation of graphene or graphene compound, graphene or graphene compound can be dispersed substantially uniformly inside the positive electrode active material layer 502. By removing the solvent from the dispersion medium containing the uniformly dispersed graphene oxide through evaporation, the graphene oxide is reduced, and thus the graphene or graphene compound remaining in the positive electrode active material layer 502 partially overlaps each other, dispersing in a surface contact manner, thereby forming a three-dimensional conductive path. Furthermore, the reduction of graphene oxide can be carried out by heat treatment or by using a reducing agent.

[0461] Therefore, unlike granular conductive materials such as acetylene black that form point contacts with the active material, graphene or graphene compounds can form surface contacts with low contact resistance. This allows for improved conductivity between the granular positive electrode active material 100 and the graphene or graphene compound with less conductive material compared to conventional conductive materials. Consequently, the proportion of the positive electrode active material 100 in the positive electrode active material layer 502 can be increased. This, in turn, increases the discharge capacity of the secondary battery.

[0462] Furthermore, by using a spray drying device beforehand, a graphene compound serving as a conductive material for covering the entire active material can be formed, thereby creating conductive pathways between the active materials using the graphene compound.

[0463] Alternatively, materials used in forming the graphene compound, other than the graphene compound itself, can be mixed into the positive electrode active material layer 502. For example, particles used as catalysts in forming the graphene compound can be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon dioxide (SiO2, SiO2). x (x<2)), particles of aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The median particle size (D50) of these particles is preferably less than 1 μm, and more preferably less than 100 nm.

[0464] As a conductive material, acetylene black (referred to as AB) can also be used in addition to graphene. Furthermore, fluorinated acetylene black can also be used. The fluorine in fluorinated acetylene black is preferably adsorbed on the surface. In addition, fluorinated acetylene black can be manufactured by contacting acetylene black with a fluorine compound (a process known as fluorination). Regarding fluorination, the methods described in the section on graphene can be applied to acetylene black.

[0465] In addition to graphene and acetylene black, carbon fiber materials (referred to as carbon nanotubes or CNTs) can also be used as conductive materials. Furthermore, fluorocarbon nanotubes can also be used. The fluorine in the fluorocarbon nanotubes is preferably adsorbed on the surface. Fluorocarbon nanotubes can be manufactured by contacting carbon nanotubes with a fluorine compound (a process known as fluorination). Regarding fluorination, the methods described for graphene can be applied to carbon nanotubes.

[0466] [Adhesive] An adhesive is needed to strengthen the adhesion of powdered active material without obscuring its surface. Furthermore, the adhesive needs to exhibit adhesiveness to the current collector. That is, materials with adhesive components are preferably used as the adhesive. Moreover, considering the expansion of the active material, the adhesive preferably exhibits sufficient flexibility to accommodate changes in the state of the active material. The adhesive also needs to be compatible with the electrolyte. Furthermore, since vigorous oxidation and reduction reactions occur in secondary batteries, an adhesive that does not deteriorate or has low reactivity relative to these reactions is required.

[0467] Preferred adhesives include styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluororubber can also be used as an adhesive.

[0468] Furthermore, water-soluble polymers are preferably used as adhesives. Polysaccharides, for example, can also be used as water-soluble polymers. Among the polysaccharides, one or more of the following can be used: carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, and starch. More preferably, these water-soluble polymers and the aforementioned rubber material are used in combination.

[0469] Alternatively, as an adhesive, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose are preferred.

[0470] As an adhesive, multiple of the above materials can also be used in combination.

[0471] For example, materials with particularly good viscosity-modifying effects can be used in combination with other materials. For instance, while materials like rubber may have high adhesive strength and / or high elasticity, viscosity can sometimes be difficult to adjust when mixed in a solvent. In such cases, it is preferable to mix them with materials that have particularly good viscosity-modifying effects. Water-soluble polymers can be used as examples of materials with particularly good viscosity-modifying effects. Furthermore, polysaccharides mentioned above can be used as water-soluble polymers with particularly good viscosity-modifying properties; for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, cellulose derivatives such as regenerated cellulose, and starch can be used.

[0472] Note that cellulose derivatives such as carboxymethyl cellulose, for example, by being converted into sodium and ammonium salts of carboxymethyl cellulose, have increased solubility and thus readily function as viscosity modifiers. Due to the increased solubility, the dispersibility of the active material with other components can be improved when forming the electrode slurry. In this specification, cellulose and cellulose derivatives used as binders for electrodes include their salts.

[0473] By dissolving water-soluble polymers in water to stabilize their viscosity, active materials and other materials used as binders, such as styrene-butadiene rubber, can be stably dispersed in aqueous solutions. Because water-soluble polymers possess functional groups, they are expected to readily and stably adhere to the surface of active materials. Cellulose derivatives, such as carboxymethyl cellulose, often possess functional groups such as hydroxyl and carboxyl groups. Due to these functional groups, the polymers are expected to interact and extensively cover the surface of active materials.

[0474] When an adhesive forms a film covering or contacting the surface of the active material, it is also desirable for it to function as a passivation film to suppress electrolyte decomposition. Here, the passivation film is a film with no conductivity or extremely low conductivity; for example, when the passivation film is formed on the surface of the active material, it suppresses electrolyte decomposition at the battery reaction potential. More preferably, the passivation film can transport lithium ions while suppressing conductivity.

[0475] [current collector] As the current collector, highly conductive materials such as stainless steel, gold, platinum, aluminum, titanium, and their alloys can be used. Furthermore, the material used for the positive electrode current collector is preferably one that does not dissolve due to the positive electrode's potential. Additionally, aluminum alloys with added elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can be used. Furthermore, metallic elements that react with silicon to form silicides can also be used. Examples of metallic elements that react with silicon to form silicides include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be appropriately shaped as foil, plate, sheet, mesh, perforated metal mesh, or drawn metal mesh. The thickness of the current collector is preferably 5 μm or more and 30 μm or less.

[0476] 〔negative electrode〕 The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also contain conductive materials and binders.

[0477] [Negative Electrode Active Material] As a negative electrode active material, alloy materials and / or carbon materials can be used, for example.

[0478] As the negative electrode active material, elements capable of charge-discharge reactions through alloying / dealloying with lithium can be used. For example, materials containing one or more of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium can be used. The charge-discharge capacity of such elements is greater than that of carbon, especially silicon, which has a theoretical capacity of 4200 mAh / g. Therefore, silicon is preferred as the negative electrode active material. Furthermore, compounds containing these elements can also be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Elements that can undergo charge-discharge reactions through alloying / dealloying with lithium, and compounds containing such elements, are sometimes referred to as alloy materials.

[0479] In this specification and other materials, SiO refers to silicon monoxide, for example. Alternatively, SiO may also be represented as SiO₂. x For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. Alternatively, it is preferably 0.2 or more and 1.2 or less. Alternatively, it is preferably 0.3 or more and 1.5 or less.

[0480] As carbon-based materials, graphite, easily graphitized carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), carbon nanotubes, graphene, carbon black, etc. can be used.

[0481] Examples of graphite include synthetic graphite and natural graphite. Examples of synthetic graphite include mesophase carbon microspheres (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Spherical graphite with a spherical shape can be used as the synthetic graphite. For example, MCMB sometimes has a spherical shape, which is preferred. Furthermore, MCMB is relatively easy to reduce its surface area, so it is sometimes preferred. Examples of natural graphite include flake graphite and spheroidized natural graphite.

[0482] When lithium ions are intercalated in graphite (during the formation of lithium-graphite intercalation compounds), graphite exhibits a low potential similar to that of lithium metal (above 0.05V and below 0.3V vs. Li / Li). + Therefore, lithium-ion secondary batteries can exhibit high operating voltages. Graphite also has the following advantages: larger charge / discharge capacity per unit volume; smaller volume expansion; lower cost; and higher safety compared to lithium metal, making it a preferred choice.

[0483] In addition, oxides such as titanium dioxide (TiO2) and lithium titanium oxide (Li4Ti5O) can be used as negative electrode active materials. 12 ), lithium-graphite intercalation compounds (Li x C6), niobium pentoxide (Nb2O5), tungsten dioxide (WO2), molybdenum dioxide (MoO2), etc.

[0484] Furthermore, Li3N-type structures containing lithium and transition metal nitrides can be used as negative electrode active materials. 3-x M x N (M = Co, Ni, Cu). For example, Li 2.6 Co 0.4 N indicates a large charge / discharge capacity (900 mAh / g, 1890 mAh / cm³). 3 Therefore, it is the preferred option.

[0485] When lithium- and transition metal-containing nitrides are used as negative electrode active materials, lithium ions are present in the negative electrode active material. Therefore, this negative electrode active material can be combined with materials that do not contain lithium ions, such as V₂O₅ and Cr₃O₈, which are used as positive electrode active materials, making this a preferred method. Note that when lithium-ion-containing materials are used as positive electrode active materials, lithium ions can be pre-desorbed from the positive electrode active material, and lithium- and transition metal-containing nitrides can also be used as negative electrode active materials.

[0486] Furthermore, materials that induce the conversion reaction can also be used as negative electrode active materials. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as negative electrode active materials. Other examples of materials that induce the conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS. 0.89 Sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N, and Ge3N4, phosphides such as NiP2, FeP2, and CoP3, and fluorine compounds such as FeF3 and BiF3.

[0487] The conductive materials and adhesives that may be included in the negative electrode active material layer can be the same materials that may be included in the positive electrode active material layer.

[0488] [Negative electrode current collector] The same material as the positive electrode current collector can be used as the negative electrode current collector. Furthermore, it is preferable to use a material that does not alloy with carrier ions such as lithium as the negative electrode current collector.

[0489] [Electrolyte] Electrolytes can be used as a form of electrolyte. Electrolytes contain a solvent and a lithium salt. As the solvent mentioned above (also referred to as the solvent for the electrolyte), aprotic organic solvents are preferred, such as ethylene carbonate (EC), propylene carbonate (PC), butenyl carbonate, vinyl chloride carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethyl glycol ether (DME), dimethyl sulfoxide, diethyl ether, methyl diethylene glycol dimethyl ether, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sulfonyl ether, etc., or two or more of the above can be used in any combination and ratio. Two or more solvents are referred to as mixed organic solvents.

[0490] When the organic solvent in the electrolyte contains ethylene carbonate (EC) and diethyl carbonate (DEC), a mixed organic solvent with a volume ratio of x:100 - x (note that 20 ≤ x ≤ 40) of ethylene carbonate and diethyl carbonate can be used in the state where ethylene carbonate and diethyl carbonate are 100 vol%. More specifically, a mixed organic solvent containing EC and DEC in a volume ratio of EC:DEC = 30:70 (volume ratio) can be used.

[0491] In addition, when the organic solvent in the electrolyte contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), a mixed organic solvent with a volume ratio of x:y:100 - x - y (note that 5 ≤ x ≤ 35 and 0 < y < 65) of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate can be used in the state where ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are 100 vol%. More specifically, a mixed organic solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC = 30:35:35 (volume ratio) can be used.

[0492] In addition, as the organic solvent in the electrolyte, a mixed organic solvent containing a fluorinated cyclic carbonate or a fluorinated chain carbonate can be used. And it is preferable that the above-mentioned mixed organic solvent contains both a fluorinated cyclic carbonate and a fluorinated chain carbonate at the same time. Both the fluorinated cyclic carbonate and the fluorinated chain carbonate have substituents with electron-withdrawing properties, so the solvation energy of lithium ions becomes low, which is why they are preferable. Therefore, both the fluorinated cyclic carbonate and the fluorinated chain carbonate are suitable for use in the electrolyte, and it is preferable to use these mixed organic solvents.

[0493] As the fluorinated cyclic carbonate, for example, fluorinated ethylene carbonate (fluorinated ethylene carbonate, FEC, F1EC), difluorinated ethylene carbonate (DFEC, F2EC), trifluorinated ethylene carbonate (F3EC), or tetrafluorinated ethylene carbonate (F4EC) etc. can be used. In addition, as DFEC, isomers such as cis-4,5 and trans-4,5 can be cited. Any fluorinated cyclic carbonate has substituents with electron-withdrawing properties, so it is considered that the solvation energy of lithium ions is low.

[0494] The following structural formula (H10) is the structural formula of FEC. In FEC, the electron-withdrawing substituent is the F group.

[0495] [Chemical formula 1]

[0496] As the fluorinated chain carbonate, there is methyl 3,3,3-trifluoropropionate. The following structural formula (H22) is the structural formula of methyl 3,3,3-trifluoropropionate. The abbreviation of methyl 3,3,3-trifluoropropionate is "MTFP". In MTFP, the electron-withdrawing substituent is the CF3 group.

[0497] [Chemical Formula 2]

[0498] FEC is a type of cyclic carbonate. Due to its high relative permittivity, it promotes the dissociation of lithium salts when used in organic solvents. Furthermore, because FEC contains electron-withdrawing substituents, it is easier to generate and desolvate lithium ions compared to ethylene carbonate (EC). Specifically, FEC... Therefore, lithium ions easily leave the surfaces of both the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Moreover, FEC is considered to have a deep Highest Occupied Molecular Orbital (HOMO) level, making it less susceptible to oxidation and improving its antioxidant properties. On the other hand, the high viscosity of FEC is a concern. Therefore, it is preferable to use an organic solvent containing MTFP in addition to FEC in the electrolyte. MTFP is a type of chain carbonate that can reduce the viscosity of the electrolyte or maintain the viscosity at room temperature (typically 25°C) even at low temperatures (typically 0°C). Furthermore, MTFP has a lower solvation energy than methyl propionate (MP) which does not have electron-withdrawing substituents, and therefore can sometimes be solvated with lithium ions when used in electrolytes.

[0499] Preferably, the mixture is used with 100 vol% of a mixed organic solvent containing FEC and MTFP having such physical properties, in a volume ratio of x:100-x (note that 5 ≤ x ≤ 30, preferably 10 ≤ x ≤ 20). That is, it is preferable to mix the mixture with more MTFP than FEC in the mixed organic solvent.

[0500] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room temperature molten salts) as the solvent for the electrolyte, the secondary battery can be prevented from ruptured and / or ignited even if the internal temperature rises due to internal short circuits or overcharging. Ionic liquids consist of cations and anions, including organic cations and anions. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, or aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methylide anions, fluorosulfonic acid anions, perfluoroalkyl sulfonic acid anions, tetrafluoroborate anions, perfluoroalkyl borate anions, hexafluorophosphate anions, or perfluoroalkyl phosphate anions.

[0501] Preferably, the solvent of the electrolyte contains low levels of particulate matter or molecules other than the constituent molecules of the organic solvent (hereinafter referred to as "impurities," including oxygen (O2), water (H2O), or moisture) and is highly purified. Specifically, the amount of such impurities in the solvent of the electrolyte is 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. For example, moisture can be detected using Karl Fischer titration.

[0502] Furthermore, the aforementioned organic solvent preferably has peaks originating from impurities that are almost impossible to confirm by NMR measurements. "Almost impossible to confirm" means that the ratio of the integrated area of ​​the peak originating from the main component to the integrated area of ​​the peak originating from the impurities (referred to as the integration ratio) is 0.005 or less, preferably 0.002 or less. There are no particular limitations on the apparatus used for NMR measurements; for example, the "AVANCE III 400" manufactured by Bruker can be used. Additionally, in 1H-NMR measurements, the central peak among the five acetonitrile peaks originating from acetonitrile-d3 used as the solvent can be set to 1.94 ppm.

[0503] For example, it is known that in MTFP, when measuring 1H-NMR using acetonitrile-d3 solvent, four peaks are generated at δ values ​​above 3.29 ppm and below 3.43 ppm. However, when other peaks are generated in the vicinity, such as when a peak is generated at δ values ​​above 3.24 ppm and below 3.29 ppm, this peak is considered to originate from impurities. Therefore, when the ratio (integral ratio) of the peak area above 3.29 ppm to the peak area below 3.43 ppm to the peak area above 3.24 ppm to the peak area below 3.29 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks originating from impurities are almost impossible to identify.

[0504] [Lithium salts] As lithium salts (also known as electrolytes) dissolved in the above solvents, for example, LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B can be used. 10 Cl 10 Li2B 12 Cl 12 One of the following lithium salts can be used: LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), and LiN(C2F5SO2)2, or two or more of the above can be used in any combination and ratio. The lithium salt concentration in the solvent should be set between 0.5 mol / L and 3.0 mol / L. The safety of lithium-ion secondary batteries is improved when using fluorides such as LiPF6 and LiBF4.

[0505] [additive] In addition, additives such as vinylene carbonate (VC), propanesulfonate lactone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium dioxoborate (LiBOB), or dinitrile compounds such as succinate and adiponitrile can be added to the electrolyte. The concentration of the added material can be set, for example, to be more than 0.1 wt% and less than 5 wt% of the total solvent. VC or LiBOB readily forms a good coating and is therefore particularly preferred.

[0506] [Gel Electrolyte] Polymer gel electrolytes, in which the polymer is swollen by the electrolyte solution, can also be used as gel electrolytes. Furthermore, by using polymer gel electrolytes, a semi-solid electrolyte layer can be provided, improving safety against leakage and other issues. Moreover, it allows for the thinning and weight reduction of secondary batteries.

[0507] As a gelling polymer, silicone gels, acrylic gels, acrylonitrile gels, polyethylene oxide gels, polyoxypropylene gels, fluoropolymer gels, etc. can be used.

[0508] As polymers, polymers with a polyoxyalkylene structure, such as polyethylene oxide (PEO), PVDF, and polyacrylonitrile, as well as copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. Furthermore, the resulting polymer can also have a porous shape.

[0509] [Solid electrolyte] Solid electrolytes containing inorganic materials such as sulfides or oxides, or solid electrolytes containing polymeric materials such as PEO (polyethylene oxide), can be used instead of liquid electrolytes. When using solid electrolytes, there is no need to install separators and / or spacers. Furthermore, since the entire battery can be solidified, there is no concern about leakage, significantly improving safety.

[0510] [Isolation] The secondary battery preferably includes a separator. Materials that can be used as the separator include, for example, paper, nonwoven fabric, glass fiber, ceramic, or synthetic fibers containing nylon (polyamide), vinylon (polyvinyl alcohol fibers), polyester, acrylic resin, polyolefin, polyurethane, etc. Preferably, the separator is processed into a bag shape and configured to surround either the positive or negative electrode.

[0511] The separator can have a multilayer structure. For example, ceramic materials, fluorinated materials, polyamide materials, or mixtures thereof can be coated onto organic materials such as polypropylene and polyethylene films. Examples of ceramic materials include alumina particles and silicon oxide particles. Examples of fluorinated materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aromatic polyamides (meta-aromatic polyamides and para-aromatic polyamides).

[0512] Coating with ceramic materials improves oxidation resistance, thereby suppressing separator degradation during high-voltage charging and enhancing the reliability of the secondary battery. Coating with fluorine-based materials facilitates a tighter connection between the separator and the electrodes, improving output characteristics. Coating with polyamide materials (especially aromatic polyamides) improves heat resistance, thus enhancing the safety of the secondary battery.

[0513] For example, a mixture of alumina and aromatic polyamide can be coated on both sides of the polypropylene film. Alternatively, the side of the polypropylene film in contact with the positive electrode can be coated with a mixture of alumina and aromatic polyamide, while the side in contact with the negative electrode can be coated with a fluorinated material.

[0514] By employing a multi-layered separator, the safety of the secondary battery can be ensured even if the total thickness of the separator is small, thus increasing the discharge capacity per unit volume of the secondary battery.

[0515] [Outer Packaging] The outer packaging of a secondary battery can be made of materials such as aluminum (metal) and / or resin. Alternatively, a thin-film outer packaging can be used. For example, a three-layer film can be used as the outer packaging: a flexible metal film (such as aluminum, stainless steel, copper, or nickel) is formed on a film made of materials such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide; a fibrous film of a synthetic resin such as polyamide or polyester resin can also be formed on this metal film as the outer surface of the outer packaging. The fiber formed from polyamide resin is called nylon. Sometimes, a three-layer film containing aluminum is called an aluminum laminate film.

[0516] This embodiment can be used in combination with other embodiments as appropriate.

[0517] (Implementation Method 4) In this embodiment, examples of secondary battery shapes other than the laminated secondary batteries described in the above embodiments are explained in detail.

[0518] <Coin-type secondary battery> Here is an example illustrating a coin-type secondary battery. Figure 25A This is an image of a coin-shaped (single-layer flat) secondary battery. Figure 25B This is a cross-sectional view. Coin-type secondary batteries are mainly used in small electronic devices. In this instruction manual, etc., coin-type secondary batteries include button batteries.

[0519] In the coin-type secondary battery 300, the positive electrode container 301, which also serves as the positive terminal, and the negative electrode container 302, which also serves as the negative terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 disposed in contact with it. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 disposed in contact with it. An separator 310 is provided between the positive electrode 304 and the negative electrode 307.

[0520] The active material layers included in the positive electrode 304 and negative electrode 307 of the coin-type secondary battery 300 can be formed on only one surface of the current collector.

[0521] As the positive electrode container 301 and the negative electrode container 302, metals such as nickel, aluminum, and titanium, their alloys, and / or alloys of them with other metals (e.g., stainless steel) that are resistant to electrolyte corrosion can be used. Furthermore, to prevent corrosion caused by the electrolyte, the positive electrode container 301 and the negative electrode container 302 are preferably covered with nickel and / or aluminum. The positive electrode container 301 is electrically connected to the positive electrode 304, and the negative electrode container 302 is electrically connected to the negative electrode 307.

[0522] By immersing these negative electrode 307, positive electrode 304, and separator 310 in the electrolyte, such as Figure 25B As shown, a coin-shaped secondary battery 300 is manufactured by stacking a positive electrode 304, an insulator 310, a negative electrode 307, and a negative electrode can 302 in sequence below a positive electrode can 301, and pressing the positive electrode can 301 and the negative electrode can 302 together with a gasket 303.

[0523] By using the positive electrode active material described in the above embodiments in the positive electrode 304, a coin-type secondary battery 300 with high discharge capacity and excellent cycle characteristics can be realized.

[0524] Here, refer to Figure 25C This describes how current flows when charging a secondary battery. When a lithium-ion secondary battery is considered as a closed circuit, the direction of lithium-ion migration is the same as the direction of current flow. Note that in lithium-ion secondary batteries, since the anode and cathode, oxidation and reduction reactions are reversed depending on whether the battery is charging or discharging, the electrode with the higher reaction potential is called the positive electrode, and the electrode with the lower reaction potential is called the negative electrode. Therefore, in this specification, even when charging, discharging, supplying a reverse pulse current, and supplying a charging current, the positive electrode is referred to as the "positive electrode" or "+ electrode," and the negative electrode is referred to as the "negative electrode" or "- electrode." If the terms anode and cathode related to oxidation and reduction reactions are used, the anode and cathode would be reversed during charging and discharging, which could cause confusion. Therefore, the terms anode and cathode are not used in this specification. When the terms anode and cathode are used, it is clearly indicated whether it is during charging or discharging, and it is shown whether it corresponds to the positive electrode (+ electrode) or the negative electrode (- electrode).

[0525] Figure 25C The two terminals shown are connected to a charger to charge the secondary battery 300. As the secondary battery 300 is charged, the potential difference between the electrodes increases.

[0526] Cylindrical secondary battery Next, an example of a cylindrical secondary battery will be described with reference to FIG26. Figure 26A The diagram shows the external appearance of a cylindrical secondary battery 600. Figure 26B This is a schematic cross-sectional view of a cylindrical secondary battery 600. (Example) Figure 26B As shown, the cylindrical secondary battery 600 has a positive electrode cover (battery ...

Claims

1. A secondary battery comprising a positive electrode and a negative electrode, in, The positive electrode contains a positive electrode active material. The positive electrode active material includes lithium cobalt oxide containing magnesium. The magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material. The negative electrode contains a negative electrode active material. The negative electrode active material contains carbon materials. Furthermore, when the AC impedance of the secondary battery is measured at a voltage of 4.5V, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

2. A secondary battery comprising a positive electrode, a negative electrode, an insulator, and an electrolyte. in, The positive electrode contains a positive electrode active material. The positive electrode active material includes lithium cobalt oxide containing magnesium. The magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material. The negative electrode contains a negative electrode active material. The negative electrode active material contains carbon materials. The solvent for the electrolyte includes ethylene carbonate and diethyl carbonate. The separator contains polypropylene. Furthermore, when the AC impedance of the secondary battery is measured at a voltage of 4.5V, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

3. A secondary battery comprising a positive electrode and a negative electrode, in, The positive electrode contains a positive electrode active material. The positive electrode active material includes lithium cobalt oxide containing magnesium. The magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material. The negative electrode contains a negative electrode active material. The negative electrode active material contains carbon materials. Furthermore, when the AC impedance of the secondary battery is measured at a voltage of 4.5V and a capacity of 2000mAh or more, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

4. A secondary battery comprising a positive electrode, a negative electrode, an insulator, and an electrolyte. in, The positive electrode contains a positive electrode active material. The positive electrode active material includes lithium cobalt oxide containing magnesium. The magnesium concentration on the surface of the positive electrode active material is higher than the magnesium concentration inside the positive electrode active material. The negative electrode contains a negative electrode active material. The negative electrode active material contains carbon materials. The solvent for the electrolyte includes ethylene carbonate and diethyl carbonate. The separator contains polypropylene. Furthermore, when the AC impedance of the secondary battery is measured at a voltage of 4.5V and a capacity of 2000mAh or more, the AC impedance value at a frequency of 1kHz is less than 90mΩ.

5. The secondary battery according to any one of claims 1 to 4, When the secondary battery is subjected to a needle penetration test with a steel needle diameter of 3 mm and a needle penetration rate of 5 mm / sec, the temperature rise ΔT is below 50°C.

6. The secondary battery according to any one of claims 1 to 4, The lithium cobalt oxide therein also contains aluminum.

7. The secondary battery according to any one of claims 1 to 4, The lithium cobalt oxide therein also contains nickel.

8. The secondary battery according to any one of claims 1 to 4, The lithium cobalt oxide therein also contains fluorine.

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