Positive electrode active material

Abstract

To provide a positive electrode active material in which a decrease in discharge capacity during charge-discharge cycles is suppressed, and a secondary battery including the positive electrode active material.SOLUTION: A positive electrode active material in which a change in a crystal structure, e.g., a shift in CoO2 layers is small between a discharged state and a high-voltage charged state is provided. For example, a positive electrode active material that has a layered rock-salt crystal structure belonging to the space group R-3m in a discharged state and a crystal structure belonging to the space group P2 / m in a charged state where x in LixCoO2 is greater than 0.1 and less than or equal to 0.24 is provided. When the positive electrode active material is analyzed by powder X-ray diffraction, a diffraction pattern has at least diffraction peaks at 2θ of 19.47±0.10° and 2θ of 45.62±0.05°.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] One aspect of the present invention relates to a product, method, or method of manufacture; or to a process, machine, manufacture, or composition of matter. Another aspect of the present invention relates to an energy storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a method of manufacturing the same.

[0002] In this specification, "electronic equipment" refers to all devices that have an energy storage device, and all electro-optical devices with an energy storage device, information terminal devices with an energy storage device, etc., are considered electronic equipment. [Background technology]

[0003] In recent years, there has been a great deal of development on various energy storage devices, including lithium-ion secondary batteries, lithium-ion capacitors, air batteries, and all-solid-state batteries. In particular, the demand for lithium-ion secondary batteries, which offer high output and high capacity, has expanded rapidly in line with the development of the semiconductor industry, and they have become indispensable as a source of rechargeable energy in today's information society.

[0004] In particular, there is a high demand for secondary batteries for mobile electronic devices that have a large discharge capacity per unit weight and excellent cycle characteristics. To meet these demands, there is a great deal of research being done on improving the positive electrode active material of secondary batteries (for example, Patent Documents 1 to 3). Research is also being conducted on the crystal structure of positive electrode active materials (Non-Patent Documents 1 to 3).

[0005] X-ray diffraction (XRD) is one of the methods used to analyze the crystal structure of positive electrode active materials. XRD data can be analyzed using ICSD (Inorganic Crystal Structure Database), which is introduced in Non-Patent Literature 4. For Rietveld method analysis, for example, the analysis program RIETAN-FP (Non-Patent Literature 5) can be used. [Prior art documents] [Chartered documents]

[0006]

Patent Document 1

Patent document 2

Patent document 3

Non-licensed literature

[0007] [Non-licensed document 1] Toyoki Okumura et al, "Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3-and O2-lithium cobalt oxides from first-principle calculation", Journal of Materials Chemistry, 2012, 22, p.17340-17348 [Non-licensed document 2] Motohashi, T. et al, "Electronic phase diagram of the layered cobalt oxide system LixCoO2 (0.0≦x≦1.0) ", Physical Review B, 80(16);165114 [Non-licensed document 3] Zhaohui Chen et al, “Staging Phase Transitions in LixCoO2”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609

Non-licensed Document 4

[0008] Lithium-ion rechargeable batteries still have room for improvement in various aspects, including discharge capacity, cycle characteristics, reliability, safety, and cost.

[0009] Therefore, the positive electrode active material used in this system also needs to be a material that can improve issues such as discharge capacity, cycle characteristics, reliability, safety, and cost when used in a secondary battery.

[0010] One aspect of the present invention aims to provide a positive electrode active material or composite oxide that can be used in lithium-ion secondary batteries and in which the decrease in discharge capacity during charge-discharge cycles is suppressed. Alternatively, one aspect of the present invention aims to provide a positive electrode active material or composite oxide that does not easily collapse in crystal structure even after repeated charge-discharge cycles. Alternatively, one aspect of the present invention aims to provide a positive electrode active material or composite oxide with a large discharge capacity. Alternatively, one aspect of the present invention aims to provide a secondary battery that is safe or highly reliable.

[0011] Furthermore, one aspect of the present invention aims to provide a positive electrode active material, a composite oxide, an energy storage device, or a method for producing the same.

[0012] Furthermore, the description of these problems does not preclude the existence of other problems. Moreover, one aspect of the present invention does not need to solve all of these problems. It is possible to extract other problems from the description, drawings, and claims. [Means for solving the problem]

[0013] To solve the above problems, one aspect of the present invention involves charging with a high voltage, Li x The objective is to provide a positive electrode active material or composite oxide that exhibits little change in crystal structure even when x in CoO2 is reduced.

[0014] Alternatively, in one aspect of the present invention, the charging voltage is 4.6V or more and 4.8V or less, or Li x The objective is to provide a positive electrode active material having a crystal structure in which the displacement of the CoO2 layer is suppressed, unlike the H1-3 type crystal structure, even when x in CoO2 is greater than 0.1 and less than or equal to 0.24, and typically when x is between 0.15 and 0.17.

[0015] More specifically, one aspect of the present invention has a layered rock salt type crystal structure of space group R-3m when in a discharge state, Li x When x in CoO2 is in a charged state greater than 0.1 and less than or equal to 0.24, it is a positive electrode active material having a crystal structure with space group P2 / m, lattice constants a=4.88±0.01Å, b=2.82±0.01Å, c=4.84±0.01Å, α=90°, β=109.58±0.01°, and γ=90°.

[0016] Furthermore, in the above, Li x In the charged state where x in CoO2 is greater than 0.1 and less than or equal to 0.24, the crystal structure is preferably such that the coordinates of cobalt and oxygen in the unit cell are Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(0.232,0,0.645), and O2(0.781,0.5,0.679).

[0017] Another aspect of the present invention is that, in the discharge state, it has a layered rock salt type crystal structure of space group R-3m, Li x When the x in CoO2 is in a charged state between 0.1 and 0.24, analysis by powder X-ray diffraction reveals that the diffraction pattern has peaks at least at 2θ between 19.37° and 19.57°, and between 45.57° and 45.67°, indicating that it is a positive electrode active material.

[0018] Another aspect of the present invention is that, in the discharge state, it has a layered rock salt type crystal structure of space group R-3m, Li x When the x in CoO2 is in a charged state greater than 0.1 and less than or equal to 0.24, analysis by powder X-ray diffraction reveals that the diffraction pattern has peaks at least at 2θ of 19.13° to less than 19.37°, 19.37° to 19.57°, 45.37° to less than 45.57°, and 45.57° to 45.67°, indicating that it is a positive electrode active material.

[0019] Another aspect of the present invention is a positive electrode active material having lithium cobaltate. When the positive electrode active material is used for the positive electrode and lithium metal is used for the negative electrode to fabricate a battery, after the battery is charged multiple times by CCCV at a voltage of 4.7 V or higher, when the positive electrode of the battery is analyzed by powder X-ray diffraction using CuKα1 radiation in an argon atmosphere, the XRD pattern has diffraction peaks at least at 2θ = 19.47 ± 0.10° and 2θ = 45.62 ± 0.05°.

[0020] Another aspect of the present invention is a positive electrode active material having lithium cobaltate. When the positive electrode active material is used for the positive electrode and lithium metal is used for the negative electrode, and a mixture containing 1 mol / L lithium hexafluorophosphate, ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of EC:DEC = 3:7 and 2 wt% vinylene carbonate (VC) is used as the electrolyte to fabricate a battery, when the battery is charged at a constant current of 10 mA / g to a voltage of 4.75 V in a 45°C environment and then the positive electrode is analyzed by powder X-ray diffraction using CuKα1 radiation in an argon atmosphere, the XRD pattern has diffraction peaks at least at 2θ = 19.47 ± 0.10° and 2θ = 45.62 ± 0.05°.

[0021] Another aspect of the present invention is a positive electrode active material having lithium cobaltate. The positive electrode active material is analyzed by Raman spectroscopy with a laser wavelength of 532 nm and an output of 2.5 mW. Regarding the integrated intensity of each peak, for 580 cm -1 to 600 cm -1 as I2, and for 665 cm -1 to 685 cm -1 as I3, the value of I3 / I2 is 1% or more and 10% or less.

[0022] Also, in the above, it is preferable that 90 atomic% or more of the transition metal M contained in the positive electrode active material is cobalt.

[0023] Also, in the above, it is preferable that the H1-3 type and O1 type crystal structures are 50% or less of the positive electrode active material.

[0024] Furthermore, in the above, it is preferable that the positive electrode active material has magnesium, nickel, and aluminum in its surface layer.

[0025] Furthermore, in the above, it is preferable that, in the linear analysis results by energy-dispersive X-ray spectroscopy, the peaks of magnesium and nickel concentrations are located closer to the surface of the positive electrode active material than the peak of aluminum concentration. [Effects of the Invention]

[0026] According to one aspect of the present invention, it is possible to provide a positive electrode active material or composite oxide that can be used in lithium-ion secondary batteries and in which the decrease in discharge capacity during charge-discharge cycles is suppressed. Alternatively, it is possible to provide a positive electrode active material or composite oxide that is resistant to crystal structure collapse even after repeated charge-discharge cycles. Alternatively, it is possible to provide a positive electrode active material or composite oxide with a large discharge capacity. Alternatively, it is possible to provide a secondary battery with high safety or reliability.

[0027] Furthermore, according to one aspect of the present invention, a positive electrode active material, a composite oxide, an energy storage device, or a method for producing the same can be provided.

[0028] Furthermore, the description of these effects does not preclude the existence of other effects. Moreover, one aspect of the present invention does not necessarily have to possess all of these effects. Other effects will naturally become apparent from the description in the specification, drawings, and claims, and it is possible to extract other effects from the description in the specification, drawings, and claims. [Brief explanation of the drawing]

[0029] [Figure 1] Figure 1(A) is a cross-sectional view of the positive electrode active material, and Figures 1(B1) and 1(B2) are partial cross-sectional views of the positive electrode active material. [Figure 2] Figure 2 shows an example of a TEM image where the crystal orientation is roughly consistent. [Figure 3]Figure 3(A) is an example of a STEM image where the crystal orientation is roughly consistent. Figure 3(B) is the FFT pattern of the region of the rock salt crystal RS, and Figure 3(C) is the FFT pattern of the region of the layered rock salt crystal LRS. [Figure 4] Figure 4 illustrates the crystal structure of the positive electrode active material. [Figure 5] Figure 5 illustrates the crystal structure of a conventional positive electrode active material. [Figure 6] Figures 6(A1) and 6(A2) are partial cross-sectional views of the positive electrode active material. Figures 6(B1) through 6(C) show the calculated results regarding the crystal planes of lithium cobalt oxide and the distribution of magnesium. [Figure 7] Figures 7(A) and 7(B) are cross-sectional views of the positive electrode active material, and Figures 7(C1) and 7(C2) are partial cross-sectional views of the positive electrode active material. [Figure 8] Figure 8 shows the XRD pattern calculated from the crystal structure. [Figure 9] Figure 9 shows the XRD pattern calculated from the crystal structure. [Figure 10] Figures 10(A) and 10(B) show the XRD patterns calculated from the crystal structure. [Figure 11] Figures 11(A) through 11(C) show the lattice constants calculated from XRD. [Figure 12] Figures 12(A) through 12(C) show the lattice constants calculated from XRD. [Figure 13] Figure 13 is a cross-sectional view of the positive electrode active material. [Figure 14] Figure 14 is a cross-sectional view of the positive electrode active material. [Figure 15] Figures 15(A) to 15(C) illustrate the method for preparing the positive electrode active material. [Figure 16] Figure 16 is a diagram illustrating the method for preparing the positive electrode active material. [Figure 17] Figures 17(A) to 17(C) illustrate the method for preparing the positive electrode active material. [Figure 18]Figures 18(A) and 18(B) are cross-sectional views of the active material layer when graphene or a graphene compound is used as the conductive material. [Figure 19] Figures 19(A) and 19(B) illustrate examples of secondary batteries. [Figure 20] Figures 20(A) through 20(C) illustrate examples of secondary batteries. [Figure 21] Figures 21(A) and 21(B) illustrate examples of secondary batteries. [Figure 22] Figures 22(A) and 22(B) illustrate a coin-type rechargeable battery. Figure 22(C) illustrates the charging and discharging of a rechargeable battery. [Figure 23] Figures 23(A) through 23(D) illustrate a cylindrical secondary battery. [Figure 24] Figures 24(A) and 24(B) illustrate examples of energy storage devices. [Figure 25] Figures 25(A) through 25(D) illustrate examples of energy storage devices. [Figure 26] Figures 26(A) and 26(B) illustrate examples of secondary batteries. [Figure 27] Figure 27 illustrates an example of a secondary battery. [Figure 28] Figures 28(A) through 28(C) illustrate laminate-type secondary batteries. [Figure 29] Figures 29(A) and 29(B) illustrate laminate-type secondary batteries. [Figure 30] Figure 30 shows the external appearance of a secondary battery. [Figure 31] Figure 31 shows the external appearance of a secondary battery. [Figure 32] Figures 32(A) to 32(C) illustrate the method for manufacturing a secondary battery. [Figure 33] Figures 33(A) through 33(H) illustrate an example of an electronic device. [Figure 34] Figures 34(A) to 34(C) illustrate an example of an electronic device. [Figure 35] Figure 35 illustrates an example of an electronic device. [Figure 36] Figures 36(A) to 36(D) illustrate an example of an electronic device. [Figure 37] Figures 37(A) to 37(C) show examples of electronic devices. [Figure 38] Figures 38(A) to 38(C) illustrate an example of a vehicle. [Figure 39] Figures 39(A) to 39(F) are SEM images of the surface of the positive electrode active material. [Figure 40] Figures 40(A) to 40(H) are SEM images of the surface of the positive electrode active material. [Figure 41] Figures 41(A) and 41(B) are HAADF-STEM images of the cathode active material. [Figure 42] Figures 42(A) and 42(B) are HAADF-STEM images of the cathode active material. [Figure 43] Figures 43(A) and 43(B) are HAADF-STEM images of the cathode active material. [Figure 44] Figures 44(A) and 44(B) show the micro-electron diffraction patterns. [Figure 45] Figures 45(A) and 45(B) show the micro-electron diffraction patterns. [Figure 46] Figures 46(A) and 46(B) show the micro-electron diffraction patterns. [Figure 47] Figure 47(A) is a HAADF-STEM image of the cathode active material, Figure 47(B) is a cobalt mapping image, Figure 47(C) is an oxygen mapping image, Figure 47(D) is a magnesium mapping image, Figure 47(E) is an aluminum mapping image, and Figure 47(F) is a silicon mapping image. [Figure 48] Figure 48(A) shows the scanning method for STEM-EDX analysis, and Figure 48(B) shows the profile of the STEM-EDX analysis. [Figure 49] Figure 49 is an enlarged view of a portion of Figure 48(B). [Figure 50]Figures 50(A) and 50(B) are HAADF-STEM images of the cathode active material. [Figure 51] Figures 51(A) and 51(B) show the micro-electron diffraction patterns. [Figure 52] Figures 52(A) and 52(B) show the micro-electron diffraction patterns. [Figure 53] Figures 53(A) and 53(B) show the micro-electron diffraction patterns. [Figure 54] Figure 54(A) is a HAADF-STEM image of the cathode active material, Figure 54(B) is a silicon mapping image, Figure 54(C) is an oxygen mapping image, Figure 54(D) is a magnesium mapping image, Figure 54(E) is an aluminum mapping image, and Figure 54(F) is a nickel mapping image. [Figure 55] Figure 55(A) shows the scanning method for STEM-EDX radiation analysis, and Figure 55(B) shows the profile of the STEM-EDX radiation analysis. [Figure 56] Figure 56 is an enlarged view of a portion of Figure 55(B). [Figure 57] Figures 57(A) and 57(B) are HAADF-STEM images of the cathode active material. [Figure 58] Figures 58(A) and 58(B) show the measured particle size distribution of the positive electrode active material. [Figure 59] Figures 59(A) to 59(C) are SEM images of the surface of the positive electrode active material. [Figure 60] Figures 60(A) to 60(C) are graphs showing the distribution of grayscale values ​​for the positive electrode active material. [Figure 61] Figures 61(A) to 61(C) are luminance histograms of the positive electrode active material. [Figure 62] Figures 62(A) to 62(D) are graphs showing the cycle characteristics of a secondary battery. [Figure 63] Figures 63(A) to 63(D) are graphs showing the cycle characteristics of a secondary battery. [Figure 64] Figures 64(A) to 64(D) are graphs showing the cycle characteristics of a secondary battery. [Figure 65] Figures 65(A) to 65(D) are graphs showing the cycle characteristics of a secondary battery. [Figure 66] Figures 66(A) and 66(B) are graphs showing the cycle characteristics of a secondary battery. [Figure 67] Figure 67(A) is a photograph of the pellet. Figures 67(B) and 67(C) are SEM images of the surface of the cathode active material. [Figure 68] Figure 68(A) is a surface STEM image of the positive electrode active material, and Figure 68(B) is a cross-sectional STEM image of the positive electrode active material. [Figure 69] Figures 69(A1) and 69(B1) are cross-sectional HAADF-STEM images of the positive electrode active material. Figures 69(A2) to 69(A4) and 69(B2) to 69(B4) are EDX mapping images. [Figure 70] Figure 70 shows the dQ / dV vs. sV curve for a secondary battery. [Figure 71] Figure 71 shows the dQ / dV vs V curve for a secondary battery. [Figure 72] Figure 72 shows the dQ / dV vs V curve for a secondary battery. [Figure 73] Figure 73 shows the dQ / dV vs V curve for a secondary battery. [Figure 74] Figure 74 shows the XRD pattern of the positive electrode. [Figure 75] Figures 75(A) and 75(B) are enlarged XRD patterns of a portion of Figure 74. [Figure 76] Figure 76 shows the XRD pattern of the positive electrode. [Figure 77] Figures 77(A) and 77(B) are enlarged XRD patterns of a portion of Figure 76. [Figure 78] Figure 78 shows the XRD pattern of the positive electrode. [Figure 79] Figures 79(A) and 79(B) are enlarged XRD patterns of a portion of Figure 78. [Figure 80] Figure 80 shows the XRD pattern of the positive electrode. [Figure 81] Figures 81(A) and 81(B) are enlarged XRD patterns of a portion of Figure 80. [Figure 82] Figure 82 shows the XRD pattern of the positive electrode. [Figure 83] Figures 83(A) and 83(B) are enlarged XRD patterns of a portion of Figure 82. [Figure 84] Figure 84 shows the XRD pattern of the positive electrode. [Figure 85] Figures 85(A) and 85(B) are enlarged XRD patterns of a portion of Figure 84. [Figure 86] Figure 86 shows the XRD pattern of the positive electrode. [Figure 87] Figures 87(A) and 87(B) are enlarged XRD patterns of a portion of Figure 86. [Figure 88] Figure 88 shows the XRD pattern of the positive electrode. [Figure 89] Figures 89(A) and 89(B) are enlarged XRD patterns of a portion of Figure 88. [Figure 90] Figure 90 shows the XRD pattern of the positive electrode. [Figure 91] Figures 91(A) and 91(B) are enlarged XRD patterns of a portion of Figure 90. [Figure 92] Figure 92 is a diagram relating to powder resistance measurement. [Figure 93] Figure 93 is a graph showing the discharge curve obtained by the current pause method measurement. [Figure 94] Figure 94 shows the analysis method for current pause measurement. [Figure 95] Figures 95(A) and 95(B) show the analysis results of the current pause method measurement. [Figure 96] Figure 96 shows the analysis results of the current pause method measurement. [Figure 97] Figures 97(A) and 97(B) show the Raman spectra of the positive electrode active material. [Figure 98] Figure 98(A) shows the Raman spectrum of the positive electrode active material, and Figure 98(B) shows the Raman spectrum of the positive electrode. [Modes for carrying out the invention]

[0030] The following describes embodiments for carrying out the present invention with reference to drawings and other illustrations. However, the present invention is not limited to the following embodiments. It is possible to modify the embodiments for carrying out the invention without departing from the spirit of the present invention.

[0031] In this specification, space groups are expressed using international notation (or Hermann-Mauguin notation) in short notation. Crystal planes and crystal directions are expressed using Miller indices. Individual planes are indicated using ( ). In crystallography, space groups, crystal planes, and crystal directions are expressed by superscripting numbers, but in this specification, due to formatting constraints, a minus sign (-) may be placed before the number instead of a superscript. Individual orientations within a crystal are indicated by [ ], collective orientations indicating all equivalent directions are indicated by < >, individual planes indicating crystal planes are indicated by ( ), and collective planes with equivalent symmetry are indicated by {}. Furthermore, for ease of understanding the structure, a trigonal crystal represented by the space group R-3m is generally represented as a composite hexagonal lattice of a hexagonal crystal, and (hkil) may be used as Miller indices in addition to (hkl). Here, i is -(h+k).

[0032] In this specification, the term "particle" is not limited to spherical shapes (circular cross-sections), but may also refer to individual particles with elliptical, rectangular, trapezoidal, triangular, rounded quadrilateral, or asymmetrical cross-sections, and individual particles may also have irregular shapes.

[0033] Furthermore, the theoretical capacity of the positive electrode active material refers to the amount of electric charge when all of the insertable and detachable lithium present in the positive electrode active material is detached. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, the theoretical capacity of LiNiO2 is 274 mAh / g, and the theoretical capacity of LiMn2O4 is 148 mAh / g.

[0034] Furthermore, the extent to which insertable and detachable lithium remains in the positive electrode active material is determined by x in the composition formula, for example, Li xThis is denoted by x in CoO2. In the case of the positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, when a secondary battery using LiCoO2 as the positive electrode active material is charged at 219.2 mAh / g, Li 0.2 It can be expressed as CoO2 or x=0.2. x A small x in CoO2 means, for example, 0.1 <x≦0.24をいう。

[0035] When properly synthesized lithium cobalt oxide, before being used as the positive electrode, approximately satisfies the stoichiometric ratio, it is LiCoO2 and x=1. Similarly, lithium cobalt oxide contained in a secondary battery after discharge can also be said to be LiCoO2 and x=1. Here, "discharge completed" refers to a state where, for example, the current is 100mAh or less and the voltage is 3.0V or 2.5V or less.

[0036] Li x The charging and / or discharging capacities used to calculate x in CoO2 should preferably be measured under conditions with little or no influence from short circuits and / or decomposition of the electrolyte. For example, data from secondary batteries that have experienced a sudden change in capacity, which may be attributed to a short circuit, should not be used to calculate x.

[0037] Furthermore, the space group of a crystal structure is identified by methods such as XRD, electron diffraction, and neutron diffraction. Therefore, in this specification, "belonging to a certain space group," "being part of a certain space group," or "being a certain space group" can be rephrased as "being identified to a certain space group."

[0038] Furthermore, if the anion has a structure where three layers are stacked with a slight offset from each other, such as ABCABC, it will be called a cubic close-packed structure. Therefore, the anion does not have to be strictly a cubic lattice. At the same time, since real crystals always have defects, the analytical results do not necessarily conform to theory. For example, in FFT (Fast Fourier Transform) patterns such as electron diffraction patterns or TEM images, spots may appear at positions slightly different from the theoretical positions. For example, if the orientation from the theoretical position is 5 degrees or less, or 2.5 degrees or less, it can be said that it has a cubic close-packed structure.

[0039] Homogeneity, in a solid composed of multiple elements (e.g., A, B, C), refers to the phenomenon where a certain element (e.g., A) is distributed in specific regions with similar characteristics. It is sufficient that the elemental concentrations in these specific regions are substantially the same. For example, the difference in elemental concentrations between specific regions should be within 10%. Examples of specific regions include the surface, surface layer, convex areas, concave areas, and interior.

[0040] Furthermore, positive electrode active materials to which additive elements have been added may be expressed as composite oxides, positive electrode materials, positive electrode materials, positive electrode materials for secondary batteries, etc. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention has a compound. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention has a composition. In this specification, it is preferable that the positive electrode active material of one aspect of the present invention has a composite.

[0041] Furthermore, when describing the characteristics of individual particles of the positive electrode active material in the following embodiments, it is not necessarily required that all particles possess those characteristics. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of the positive electrode active material possess those characteristics, it can be said that this is sufficient to improve the properties of the positive electrode active material and the secondary battery having it.

[0042] As the charging voltage of a secondary battery increases, the voltage at the positive electrode generally rises. The positive electrode active material according to one aspect of the present invention has a stable crystal structure even at high voltages. The stability of the crystal structure of the positive electrode active material in the charged state suppresses the decrease in charge / discharge capacity that occurs with repeated charging and discharging.

[0043] Furthermore, a short circuit in a secondary battery not only causes malfunctions in the charging and / or discharging operations of the secondary battery, but can also lead to overheating and ignition. To realize a safe secondary battery, it is preferable that the short-circuit current is suppressed even at high charging voltages. The positive electrode active material of one aspect of the present invention suppresses the short-circuit current even at high charging voltages. Therefore, it is possible to create a secondary battery that achieves both high discharge capacity and safety.

[0044] Unless otherwise specified, the materials of a secondary battery (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) will be described in their state before degradation. Note that a decrease in discharge capacity due to aging and burn-in treatments during the secondary battery manufacturing process is not considered degradation. For example, a lithium-ion secondary single cell and lithium-ion secondary battery pack (hereinafter referred to as "lithium-ion secondary battery") can be considered to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. The rated capacity for lithium-ion secondary batteries for portable devices conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to various JIS and IEC standards, including those for electric vehicle propulsion and industrial use, in addition to the above JIS standard.

[0045] In this specification, the state of a secondary battery before material degradation may be referred to as the initial product or initial condition, and the state after degradation (the state in which the secondary battery has a discharge capacity of less than 97% of its rated capacity) may be referred to as a used product or in use, or a used product or a used condition.

[0046] (Embodiment 1) In this embodiment, a positive electrode active material 100 according to one aspect of the present invention will be described with reference to Figures 1 to 14.

[0047] Figure 1(A) is a cross-sectional view of a positive electrode active material 100 according to one embodiment of the present invention. Figures 1(B1) and 1(B2) show enlarged views of the area around AB in Figure 1(A).

[0048] As shown in Figures 1(A) to 1(B2), the positive electrode active material 100 has a surface layer 100a and an interior layer 100b. In these figures, the boundary between the surface layer 100a and the interior layer 100b is indicated by a dashed line. Also, in Figure 1(A), a portion of the grain boundary 101 is shown by a dashed line.

[0049] In this specification, the surface layer 100a of the positive electrode active material 100 refers to, for example, a region within 50 nm from the surface toward the interior, more preferably within 35 nm from the surface toward the interior, even more preferably within 20 nm from the surface toward the interior, and most preferably within 10 nm perpendicular or substantially perpendicular from the surface toward the interior. 'Subject to perpendicular' means an angle of 80° to 100°. Surfaces formed by cracks and / or fissures may also be considered the surface. The surface layer 100a is synonymous with the vicinity of the surface, the vicinity of the surface region, or the shell.

[0050] Furthermore, the region deeper than the surface layer 100a of the positive electrode active material is called the interior 100b. Interior 100b is synonymous with the interior region or core.

[0051] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer 100a and the interior 100b. Therefore, the positive electrode active material 100 does not contain metal oxides that do not have lithium sites that can contribute to charging and discharging, such as aluminum oxide (Al2O3), nor does it contain carbonates, hydroxyl groups, etc. that have been chemically adsorbed after the positive electrode active material was manufactured. The adhering metal oxide refers to, for example, a metal oxide whose crystal structure does not match that of the interior 100b.

[0052] Furthermore, the product does not contain electrolytes, organic solvents, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 100.

[0053] Since the positive electrode active material 100 is a compound containing a transition metal and oxygen that allows for lithium insertion and removal, the interface between the region where the transition metal M (e.g., Co, Ni, Mn, Fe, etc.) and oxygen, which undergo oxidation and reduction during lithium insertion and removal, are present and the region where they are absent, is considered the surface of the positive electrode active material. Surfaces created by slip, cracks, and / or fissures may also be considered the surface of the positive electrode active material. When the positive electrode active material is subjected to analysis, a protective film may be applied to the surface, but the protective film is not included in the positive electrode active material. As a protective film, single-layer or multi-layer films of carbon, metal, oxide, resin, etc. may be used.

[0054] Therefore, the surface of the positive electrode active material in STEM-EDX radiation analysis, etc., is defined as the average value of the internally detected amount of the transition metal M. AVE And the background mean M BG The point where the sum of the two is 50%, and oxygen is the average value of the internally detected amount O AVE And the background mean O BG This point is defined as the point where the sum of the internal and background levels is 50%. Note that if the point where the sum of the internal and background levels is 50% differs for the transition metal M and oxygen, this is thought to be due to the influence of oxygen-containing metal oxides, carbonates, etc., adhering to the surface. Therefore, the average value of the detected amount of the internal transition metal M is used. AVE And the background mean M BG The point that is 50% of the sum of these can be adopted. Also, in the case of a positive electrode active material having multiple transition metals M, the element M with the highest count in internal 100b is adopted. AVE and M BG The surface can be determined using this method.

[0055] The average background value M of the above transition metal M BG For example, the average value of the internally detected amount M can be determined by averaging the outer range of 2 nm or more, preferably 3 nm or more, while avoiding the vicinity where the detected amount of transition metal M begins to increase. AVEThis can be determined by averaging a range of 2 nm or more, preferably 3 nm or more, from a depth of 30 nm or more, preferably more than 50 nm, in the region where the counts of transition metal M and oxygen are saturated and stable, for example, from the region where the detected amount of transition metal M begins to increase. Average value of oxygen background O BG and the average value of the amount of oxygen detected inside O AVE This can be calculated in a similar manner.

[0056] Furthermore, the surface of the positive electrode active material 100 in cross-sectional STEM (scanning transmission electron microscope) images, etc., is defined as the boundary between the region where an image originating from the crystal structure of the positive electrode active material is observed and the region where it is not observed, and is the outermost region where atomic columns originating from the nuclei of metal elements with atomic numbers greater than lithium among the metal elements constituting the positive electrode active material are confirmed. Alternatively, it is defined as the intersection of the tangent line drawn to the brightness profile from the surface toward the bulk in the STEM image and the depth axis. The surface in STEM images, etc., may also be determined in conjunction with analyses that have higher spatial resolution.

[0057] Furthermore, the spatial resolution of STEM-EDX is approximately 1 nm. Therefore, the maximum value of the additive element profile may be off by about 1 nm. For example, even if the maximum value of the additive element profile, such as magnesium, lies outside the surface determined above, if the difference between the maximum value and the surface is less than 1 nm, it can be considered an error.

[0058] Furthermore, in STEM-EDX analysis, the peak refers to the detection intensity in each element profile, or the maximum value of the characteristic X-ray for each element. Noise in STEM-EDX analysis can include measurements with a width at half maximum (FWHM) below the spatial resolution (R), for example, R / 2 or less.

[0059] Scanning the same location multiple times under the same conditions can reduce the impact of noise. For example, the cumulative values ​​from six scans can be used as the profile for each element. The number of scans is not limited to six; more scans can be performed, and the average of these values ​​can be used as the profile for each element.

[0060] STEM-EDX radiation analysis can be performed, for example, as follows: First, a protective film is deposited on the surface of the positive electrode active material. For example, carbon can be deposited using an ion sputtering system (Hitachi High-Tech MC1000).

[0061] Next, the positive electrode active material is thinned to prepare a STEM cross-sectional sample. For example, thinning can be performed using a FIB-SEM device (Hitachi High-Tech XVision200TBS). In this case, pickup is performed using an MPS (microprobing system), and the finishing conditions can be set to, for example, an acceleration voltage of 10kV.

[0062] STEM-EDX radiation analysis can be performed using, for example, a STEM instrument (Hitachi High-Tech HD-2700) and an EDAX Octane T Ultra W (two-pronged) EDX detector. During EDX radiation analysis, the emission current of the STEM instrument should be set to between 6 μA and 10 μA, and the areas of the thinned sample with little depth and surface irregularity should be measured. The magnification should be, for example, around 150,000x. The conditions for EDX radiation analysis can be drift-corrected, with a line width of 42 nm, a pitch of 0.2 nm, and 6 or more frames.

[0063] Furthermore, the grain boundary 101 refers to areas where particles of the positive electrode active material 100 are fixed together, areas where the crystal orientation changes within the positive electrode active material 100, i.e., areas where the repetition of bright and dark lines in STEM images, etc., becomes discontinuous, areas containing many crystal defects, areas where the crystal structure is disordered, etc. Crystal defects refer to defects observable by cross-sectional TEM (transmission electron microscope), cross-sectional STEM images, etc., i.e., structures where other atoms are inserted between lattice spaces, cavities, etc. The grain boundary 101 can be considered a type of surface defect. Furthermore, the vicinity of the grain boundary 101 refers to the region within 10 nm of the grain boundary 101.

[0064] <Contained elements> The positive electrode active material 100 comprises lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 may be lithium cobalt oxide (LiCoO2) with the additive element added. However, the positive electrode active material 100 in one embodiment of the present invention may have the crystal structure described later. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0065] The positive electrode active material of a lithium-ion secondary battery needs to contain a redox-capable transition metal in order to maintain charge neutrality even when lithium ions are inserted and removed. In one embodiment of the present invention, the positive electrode active material 100 preferably uses cobalt as the transition metal responsible for the redox reaction. In addition to cobalt, at least one or two selected from nickel and manganese may be used. Among the transition metals contained in the positive electrode active material 100, if cobalt is 75 atomic% or more, preferably 90 atomic% or more, and more preferably 95 atomic% or more, it is preferable as it offers many advantages, such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics.

[0066] Furthermore, if the cobalt content among the transition metals of the positive electrode active material 100 is 75 atomic percent or more, preferably 90 atomic percent or more, then compared to composite oxides such as lithium nickelate (LiNiO2) in which nickel accounts for the majority of the transition metal, Li xThe stability of CoO2 is better when x is small. This is thought to be because cobalt is less affected by strain due to the Jahn-Teller effect than nickel. In transition metal compounds, the strength of the Jahn-Teller effect varies depending on the number of electrons in the d orbitals of the transition metal. In layered rock salt type composite oxides in which octahedral low-spin nickel(III) constitutes the majority of the transition metal, such as lithium nickelate, the Jahn-Teller effect is significant, and strain is likely to occur in the layers consisting of octahedra of nickel and oxygen. Therefore, there is a growing concern that the crystal structure may collapse during charge-discharge cycles. Also, nickel ions are larger than cobalt ions and are close in size to lithium ions. Therefore, in layered rock salt type composite oxides in which nickel constitutes the majority of the transition metal, such as lithium nickelate, there is a problem that cation mixing of nickel and lithium is likely to occur.

[0067] The additive elements in the positive electrode active material 100 are preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. Furthermore, the sum of the transition metals among the additive elements is preferably less than 25 atomic%, more preferably less than 10 atomic%, and even more preferably less than 5 atomic%.

[0068] In other words, the positive electrode active material 100 can include lithium cobalt oxide with magnesium and fluorine added, lithium cobalt oxide with magnesium, fluorine and titanium added, lithium cobalt oxide with magnesium, fluorine and aluminum added, lithium cobalt oxide with magnesium, fluorine and nickel added, lithium cobalt oxide with magnesium, fluorine, nickel and aluminum added, and so on.

[0069] It is preferable that the additive elements are solid-dissolved in the positive electrode active material 100. Therefore, for example, when performing STEM-EDX line analysis, it is preferable that the depth at which the amount of additive elements detected increases is deeper than the depth at which the amount of transition metal M detected increases, i.e., located on the interior side of the positive electrode active material 100.

[0070] In this specification, the depth at which the amount of a certain element detected increases in STEM-EDX line analysis refers to the depth at which measurements that can be judged as not being noise in terms of intensity and spatial resolution can be obtained continuously.

[0071] These additive elements further stabilize the crystalline structure of the positive electrode active material 100, as will be described later. In this specification, the term "additive elements" is synonymous with "mixture" or "part of the raw materials."

[0072] Furthermore, the additive elements do not necessarily have to include magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0073] For example, if the positive electrode active material 100 is substantially free of manganese, the above advantages such as being relatively easy to synthesize, easy to handle, and having excellent cycle characteristics become even greater. The weight of manganese contained in the positive electrode active material 100 is preferably 600 ppm or less, more preferably 100 ppm or less.

[0074] <Crystal structure> ≪Li x When x in CoO2 is 1 >> In one aspect of the present invention, the positive electrode active material 100 is in a discharge state, i.e., Li xIn the case of x=1 in CoO2, it is preferable to have a layered rock salt type crystal structure belonging to space group R-3m. Layered rock salt type composite oxides have high discharge capacity, possess a two-dimensional lithium ion diffusion pathway, and are suitable for lithium ion insertion / desorption reactions, making them excellent as positive electrode active materials for secondary batteries. For this reason, it is particularly preferable that the interior 100b, which accounts for most of the volume of the positive electrode active material 100, has a layered rock salt type crystal structure. Figure 4 shows the layered rock salt type crystal structure with R-3m O3 attached.

[0075] On the other hand, in one embodiment of the present invention, it is preferable that the surface layer 100a of the positive electrode active material 100 has a function to reinforce the layered structure of the interior 100b, which consists of octahedrons of cobalt and oxygen, so that it does not break down even if lithium is removed from the positive electrode active material 100 due to charging. Alternatively, it is preferable that the surface layer 100a functions as a barrier film for the positive electrode active material 100. Alternatively, it is preferable that the surface layer 100a, which is the outer periphery of the positive electrode active material 100, reinforces the positive electrode active material 100. Reinforcement as used here means suppressing structural changes in the surface layer 100a and interior 100b of the positive electrode active material 100, including oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the positive electrode active material 100.

[0076] Therefore, it is preferable that the surface layer 100a has a different crystal structure from the interior 100b. It is also preferable that the surface layer 100a has a composition and crystal structure that is more stable at room temperature (25°C) than the interior 100b. For example, it is preferable that at least a part of the surface layer 100a of the positive electrode active material 100 in one aspect of the present invention has a rock salt type crystal structure. Alternatively, it is preferable that the surface layer 100a has both layered rock salt type and rock salt type crystal structures. Alternatively, it is preferable that the surface layer 100a has characteristics of both layered rock salt type and rock salt type crystal structures.

[0077] The surface layer 100a is the region where lithium ions first desorb during charging, and it is a region where the lithium concentration tends to be lower than in the interior 100b. Furthermore, the atoms on the surface of the positive electrode active material 100 particles in the surface layer 100a can be described as having some of their bonds broken. Therefore, the surface layer 100a is prone to instability, and it is a region where degradation of the crystal structure is likely to begin. On the other hand, if the surface layer 100a can be made sufficiently stable, Li x Even when x in CoO2 is small, for example, when x is 0.24 or less, the layered structure consisting of octahedrons of cobalt and oxygen in the interior 100b can be made less prone to breaking. Furthermore, the displacement of the layers consisting of octahedrons of cobalt and oxygen in the interior 100b can be suppressed.

[0078] To ensure a stable composition and crystal structure for the surface layer 100a, it is preferable that the surface layer 100a contains additive elements, and more preferably that it contains multiple additive elements. Furthermore, it is preferable that the surface layer 100a has a higher concentration of one or more selected additive elements than the interior layer 100b. It is also preferable that one or more selected additive elements in the positive electrode active material 100 have a concentration gradient. Moreover, it is more preferable that the distribution of the additive elements in the positive electrode active material 100 differs. For example, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element. Here, "concentration peak" refers to the maximum concentration value in the surface layer 100a or below 50 nm from the surface.

[0079] For example, some of the additive elements, such as magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, and calcium, preferably have a concentration gradient that increases from the interior 100b towards the surface, as shown by the gradient in Figure 1(B1). Additive elements having such a concentration gradient will be called additive elements X.

[0080] It is preferable that other additive elements, such as aluminum and manganese, have a concentration gradient and a concentration peak in a region deeper than that of additive element X, as shown by the density of the hatches in Figure 1(B2). The concentration peak may be located in the surface layer 100a or deeper than the surface layer 100a. For example, it is preferable that the peak is in a region of 5 nm to 30 nm from the surface inward. Additive elements having such a concentration gradient will be called additive element Y.

[0081] 〔magnesium〕 For example, magnesium, one of the additive elements X, is divalent, and magnesium ions are more stable in lithium sites than in cobalt sites in the layered rock salt crystal structure, so they readily enter the lithium sites. The presence of magnesium at an appropriate concentration in the lithium sites of the surface layer 100a makes it easier to maintain the layered rock salt crystal structure. This is presumed to be because the magnesium present in the lithium sites functions as pillars that support the CoO2 layers. Furthermore, the presence of magnesium allows Li x When x in CoO2 is, for example, 0.24 or less, the desorption of oxygen around magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. In addition, a high magnesium concentration in the surface layer 100a is expected to improve corrosion resistance to hydrofluoric acid produced by the decomposition of the electrolyte.

[0082] At appropriate concentrations, magnesium does not adversely affect lithium insertion and removal during charging and discharging, and the above benefits can be enjoyed. However, excessive magnesium may adversely affect lithium insertion and removal. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium enters not only lithium sites but also cobalt sites. In addition, unwanted magnesium compounds (oxides and fluorides, etc.) that do not substitute for lithium or cobalt sites may segregate on the surface of the positive electrode active material, potentially becoming a resistive component of the secondary battery. Moreover, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be because too much magnesium enters the lithium sites, reducing the amount of lithium that contributes to charging and discharging.

[0083] Therefore, it is preferable that the total amount of magnesium in the positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.002 to 0.06 times the number of cobalt atoms, more preferably 0.005 to 0.03 times, and even more preferably about 0.01 times. The amount of magnesium in the total positive electrode active material 100 referred to here may be the value obtained by performing an elemental analysis of the entire positive electrode active material 100 using, for example, GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material 100.

[0084] 〔nickel〕 Furthermore, nickel, one of the additive elements X, can be present in both cobalt sites and lithium sites. When present in cobalt sites, its oxidation-reduction potential is lower compared to cobalt, which leads to an increase in discharge capacity and is therefore preferable.

[0085] Furthermore, the presence of nickel at the lithium sites can suppress the displacement of the layered structure consisting of octahedra of cobalt and oxygen. Volume changes associated with charging and discharging are also suppressed. Additionally, the elastic modulus increases, meaning it becomes harder. This is presumed to be because the nickel present at the lithium sites also functions as pillars supporting the CoO2 layers. Therefore, it is desirable that the crystal structure becomes more stable, especially in the charged state at high temperatures, such as 45°C or above.

[0086] Furthermore, the distance between the cations and anions in nickel oxide (NiO) is closer to the average distance between the cations and anions in LiCoO2 than that of MgO and CoO, making it easier for their orientations to match those of LiCoO2.

[0087] Furthermore, the ionization tendencies are lowest for magnesium, aluminum, cobalt, and nickel, in that order. Therefore, nickel is thought to be less likely to dissolve into the electrolyte during charging than the other elements mentioned above. For this reason, it is thought to have a high effect in stabilizing the crystal structure of the surface layer in the charged state.

[0088] Furthermore, nickel is Ni 2+ Ni 3+ Ni 4+ Ni 2+ Nickel is the most stable, and its trivalent ionization energy is higher than that of cobalt. Therefore, it is known that nickel and oxygen alone do not form a spinel-type crystal structure. Thus, nickel is thought to have the effect of suppressing the phase change from layered rock salt type to spinel-type crystal structure.

[0089] On the other hand, an excess of nickel is undesirable because it intensifies the distortion caused by the Jahn-Teller effect. Furthermore, an excess of nickel may negatively affect lithium insertion and removal.

[0090] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 100 is appropriate. For example, the number of nickel atoms in the positive electrode active material 100 is preferably more than 0% and 7.5% or less of the number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferable that it is more than 0% and 4% or less. Alternatively, it is preferable that it is more than 0% and 2% or less. Alternatively, it is preferable that it is more than 0% and 7.5% or less. Alternatively, it is preferable that it is more than 0.05% to 2%. Alternatively, it is preferable that it is more than 0.1% to 7.5%. Alternatively, it is preferable that it is more than 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition in the process of manufacturing the positive electrode active material.

[0091] 〔aluminum〕 Furthermore, aluminum, one of the additive elements Y, can be present in the cobalt sites of the layered rock salt crystal structure. Since aluminum is a trivalent typical element and its valency does not change, lithium around the aluminum does not easily move during charging and discharging. Therefore, the aluminum and the surrounding lithium can function as pillars, suppressing changes in the crystal structure. In addition, aluminum suppresses the leaching of surrounding cobalt, improving continuous charging endurance. Moreover, since the Al-O bond is stronger than the Co-O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, including aluminum as an additive element can improve the safety when using positive electrode active material 100 in a secondary battery. Furthermore, it is possible to create positive electrode active material 100 whose crystal structure is less likely to collapse even after repeated charging and discharging.

[0092] On the other hand, an excess of aluminum may negatively affect the insertion and removal of lithium.

[0093] Therefore, it is preferable that the total amount of aluminum in the positive electrode active material 100 is appropriate. For example, the total number of aluminum atoms in the positive electrode active material 100 is preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.3% to 1.5% of the total number of cobalt atoms. Alternatively, 0.05% to 2% is preferred. Alternatively, 0.1% to 4% is preferred. The total amount of aluminum in the positive electrode active material 100 referred to here may be, for example, the value obtained by elemental analysis of the entire positive electrode active material 100 using GD-MS, ICP-MS, etc., or it may be based on the value of the raw material composition during the manufacturing process of the positive electrode active material 100.

[0094] [Fluorine] Furthermore, fluorine, one of the additive elements X, is a monovalent anion, and if some of the oxygen in the surface layer 100a is replaced by fluorine, the lithium desorption energy decreases. This is because the oxidation-reduction potential of cobalt ions associated with lithium desorption differs depending on the presence or absence of fluorine. In other words, if fluorine is absent, the cobalt ions change from trivalent to tetravalent with lithium desorption. On the other hand, if fluorine is present, the cobalt ions change from divalent to trivalent with lithium desorption. The oxidation-reduction potential of cobalt ions differs in these two cases. Therefore, if some of the oxygen in the surface layer 100a of the positive electrode active material 100 is replaced by fluorine, the desorption and insertion of lithium ions near the fluorine can occur more smoothly. As a result, when the positive electrode active material 100 is used in a secondary battery, the charge-discharge characteristics, high-current characteristics, etc. can be improved. In addition, the presence of fluorine in the surface layer 100a, which is the part that comes into contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid. As will be described in later embodiments, if the melting point of fluorides, including lithium fluoride, is lower than that of other additive element sources, they can function as a flux (also called a melting agent) to lower the melting point of the other additive element sources.

[0095] Furthermore, titanium oxide, one of the additive elements X, is known to be superhydrophilic. Therefore, by using a positive electrode active material 100 having titanium oxide in its surface layer 100a, it is possible that wettability with highly polar solvents will be improved. When used in a secondary battery, good contact at the interface between the positive electrode active material 100 and the highly polar electrolyte may be achieved, potentially suppressing an increase in internal resistance.

[0096] Furthermore, if phosphorus, one of the additive elements X, is present in the surface layer 100a, Li x It is preferable that the state in which x in CoO2 is kept small may suppress short circuits. For example, it is preferable that it exists in the surface layer 100a as a compound containing phosphorus and oxygen.

[0097] If the positive electrode active material 100 contains phosphorus, it is preferable that the phosphorus reacts with hydrogen fluoride generated by the decomposition of the electrolyte or electrolyte solution, potentially reducing the concentration of hydrogen fluoride in the electrolyte.

[0098] If the electrolyte contains LiPF6, hydrolysis may generate hydrogen fluoride. Furthermore, the reaction between polyvinylidene fluoride (PVDF), used as a component of the positive electrode, and alkali may also generate hydrogen fluoride. Reducing the hydrogen fluoride concentration in the electrolyte may suppress corrosion of the current collector and / or peeling of the coating portion 104. It may also suppress the decrease in adhesion due to gelation and / or insolubilization of the PVDF.

[0099] If the positive electrode active material 100 contains phosphorus along with magnesium, Li xThe stability is extremely high and preferable when x in CoO2 is small. 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%. Or 1% to 10% is preferable. Or 1% to 8% is preferable. Or 2% to 20% is preferable. Or 2% to 8% is preferable. Or 3% to 20% is preferable. Or 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and even more preferably 0.7% to 4%. Or 0.1% to 5% is preferable. Or 0.1% to 4% is preferable. Or 0.5% to 10% is preferable. Or 0.5% to 4% is preferable. Or 0.7% to 10% is preferable. Alternatively, a concentration of 0.7% to 5% is preferred. The concentrations of phosphorus and magnesium shown herein may be, for example, values ​​obtained by performing an overall elemental analysis of the positive electrode active material 100 using GC-MS, ICP-MS, etc., or they may be based on the values ​​of the raw material formulation during the manufacturing process of the positive electrode active material 100.

[0100] Furthermore, if the positive electrode active material 100 has cracks, the propagation of the cracks can be suppressed by the presence of phosphorus, or more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the cracks on its surface, for example, in the embedded portion 102.

[0101] [Synergistic effect of multiple elements] Furthermore, if the surface layer 100a contains both magnesium and nickel, divalent nickel may be able to exist more stably near divalent magnesium. Therefore, Li x Even when x in CoO2 is small, the elution of magnesium can be suppressed. Therefore, this can contribute to the stabilization of the surface layer 100a.

[0102] For similar reasons, in the manufacturing process, when adding additive elements to lithium cobalt oxide, it is preferable that magnesium be added before nickel. Alternatively, it is preferable that magnesium and nickel be added in the same process. Magnesium has a large ionic radius and tends to remain on the surface of lithium cobalt oxide regardless of the process in which it is added, whereas nickel can diffuse widely into the interior of lithium cobalt oxide if magnesium is not present. Therefore, if nickel is added before magnesium, there is a concern that the nickel will diffuse into the interior of the lithium cobalt oxide and will not remain on the surface in a desirable amount.

[0103] Furthermore, it is preferable to have additive elements with different distributions, such as additive element X and additive element Y, as this allows for stabilization of the crystal structure over a wider area. For example, if the positive electrode active material 100 contains magnesium and nickel, which are part of additive element X, and aluminum, which is part of additive element Y, it can stabilize the crystal structure over a wider area than when it contains only one of additive element X or Y. In this way, when the positive electrode active material 100 contains both additive element X and additive element Y, surface stabilization can be sufficiently achieved by additive element X such as magnesium and nickel, so additive element Y such as aluminum is not essential for the surface. Rather, it is preferable for aluminum to be widely distributed in deeper regions. For example, it is preferable for aluminum to be continuously detected in the region from 1 nm to 25 nm in depth from the surface. It is preferable for aluminum to be widely distributed in the region from 0 nm to 100 nm from the surface, preferably in the region from 0.5 nm to 50 nm from the surface, as this allows for stabilization of the crystal structure over a wider area.

[0104] As described above, having multiple additive elements allows the effects of each element to synergistically contribute to further stabilization of the surface layer 100a. In particular, the presence of magnesium, nickel, and aluminum is highly preferable as it is more effective in achieving a stable composition and crystal structure.

[0105] However, it is undesirable if the surface layer 100a is occupied only by compounds of additive elements and oxygen, as this makes lithium insertion and removal difficult. For example, it is undesirable for the surface layer 100a to be occupied only by MgO, a structure in which MgO and NiO(II) are in solid solution, and / or a structure in which MgO and CoO(II) are in solid solution. Therefore, the surface layer 100a must contain at least cobalt, and in the discharge state, it must also contain lithium and have pathways for lithium insertion and removal.

[0106] To ensure sufficient pathways for lithium insertion and removal, it is preferable that the surface layer 100a has a higher cobalt concentration than magnesium. For example, the ratio of the number of magnesium atoms (Mg) to the number of cobalt atoms (Co), Mg / Co, is preferably 0.62 or less. It is also preferable that the surface layer 100a has a higher cobalt concentration than nickel. Furthermore, it is preferable that the surface layer 100a has a higher cobalt concentration than aluminum. Furthermore, it is preferable that the surface layer 100a has a higher cobalt concentration than fluorine.

[0107] Furthermore, since too much nickel may inhibit lithium diffusion, it is preferable that the surface layer 100a has a higher magnesium concentration than nickel. For example, it is preferable that the number of nickel atoms be 1 / 6 or less of the number of magnesium atoms.

[0108] Furthermore, while it is preferable that some of the additive elements, particularly magnesium, nickel, and aluminum, are present at higher concentrations in the surface layer 100a than in the interior 100b, it is also preferable that they be present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations in the lithium sites of the interior 100b, it has the effect of making it easier to maintain a layered rock salt-type crystal structure, as described above. Also, when nickel is present at an appropriate concentration in the interior 100b, the displacement of the layered structure consisting of octahedra of cobalt and oxygen can be suppressed, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing the elution of magnesium can be expected, as described above.

[0109] Furthermore, due to the concentration gradient of the added elements as described above, it is preferable that the crystal structure changes continuously from the interior 100b toward the surface. Alternatively, it is preferable that the crystal orientations of the surface layer 100a and the interior 100b are roughly the same.

[0110] For example, it is preferable that the crystal structure changes continuously from the interior 100b of the layered rock salt type toward the surface and surface layer 100a which has characteristics of both the rock salt type and the layered rock salt type. Alternatively, it is preferable that the orientation of the surface layer 100a which has characteristics of both the rock salt type and the layered rock salt type and the interior 100b of the layered rock salt type is roughly the same.

[0111] In this specification, the layered rock salt crystal structure belonging to space group R-3m, which is found in composite oxides containing lithium and transition metals such as cobalt, refers to a crystal structure that has a rock salt-type ionic arrangement in which cations and anions are arranged alternately, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, thereby enabling two-dimensional diffusion of lithium. Defects such as vacancies in cations or anions may be present. Furthermore, strictly speaking, the layered rock salt crystal structure may have a distorted lattice structure of the rock salt crystal.

[0112] Furthermore, a rock salt-type crystal structure refers to a cubic crystal structure, including the space group Fm-3m, in which cations and anions are arranged alternately. It is also acceptable for there to be vacancies in either the cation or anion.

[0113] Furthermore, the presence of characteristics of both layered rock salt and rock salt crystal structures can be determined by electron diffraction, TEM images, cross-sectional STEM images, etc.

[0114] In rock salt-type MgO, there is no distinction in the sites of cations, but in layered rock salt-type MgO, there are two types of cation sites in the crystal structure: one is mostly occupied by lithium, and the other by transition metals. The layered structure, in which two-dimensional planes of cations and two-dimensional planes of anions are arranged alternately, is the same for both rock salt-type and layered rock salt-type MgO. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmission spot) is taken as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in ideal rock salt-type MgO and the (003) plane in layered rock salt-type MgO. For example, when comparing the electron diffraction patterns of rock salt-type MgO and layered rock salt-type LiCoO2, the distance between bright spots on the (003) plane of LiCoO2 is observed to be about half the distance between bright spots on the (111) plane of MgO. Therefore, when the analysis region contains two phases, for example, rock salt type MgO and layered rock salt type LiCoO2, the electron diffraction pattern will show plane orientations in which bright spots of high and low brightness are arranged alternately. Bright spots common to both rock salt type and layered rock salt type will have high brightness, while bright spots occurring only in the layered rock salt type will have low brightness.

[0115] Furthermore, in cross-sectional STEM images, when a layered rock salt crystal structure is observed from a direction perpendicular to the c-axis, layers with high brightness and layers with low brightness are observed alternately. This feature is not seen in rock salt crystals because there is no distinction in the sites of cations. In the case of a crystal structure that possesses characteristics of both rock salt and layered rock salt crystals, when observed from a specific crystal orientation, layers with high brightness and layers with low brightness are observed alternately in cross-sectional STEM images, and furthermore, a metal with an atomic number greater than lithium is present in a part of the low-brightness layer, i.e., the lithium layer.

[0116] Layered rock salt crystals and the anions in rock salt crystals adopt a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in the O3' type and monoclinic O1(15) crystals, which will be discussed later, also adopt a cubic close-packed structure. Therefore, when layered rock salt crystals and rock salt crystals are in contact, there exists a crystal plane in which the orientation of the cubic close-packed structure composed of anions is aligned.

[0117] Alternatively, it can be explained as follows: The anions in the {111} plane of the cubic crystal structure have a triangular lattice. The layered rock salt type has a space group R-3m and a rhombohedral structure, but to facilitate understanding of the structure, it is generally represented as a composite hexagonal lattice, and the (0001) plane of the layered rock salt type has a hexagonal lattice. The triangular lattice of the {111} plane of the cubic crystal has a similar atomic arrangement to the hexagonal lattice of the (0001) plane of the layered rock salt type. The consistency between the two lattices can be described as the orientation of the cubic close-packed structure being aligned.

[0118] However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space group Fm-3m of rock salt crystals (the space group of typical rock salt crystals). Therefore, the Miller indices of crystal planes that satisfy the above conditions differ between layered rock salt crystals, O3'-type crystals, and rock salt crystals. In this specification, when the orientation of the cubic close-packed structure composed of anions is aligned in layered rock salt crystals, O3'-type crystals, and rock salt crystals, it is sometimes said that the crystal orientations are roughly the same. Furthermore, having a three-dimensional structural similarity such that the crystal orientations are roughly the same, or having the same crystallographic orientation, is called topotaxy.

[0119] The approximate agreement of crystal orientation in two regions can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron diffraction patterns, FFT patterns of TEM and STEM images, etc. XRD (X-ray Diffraction), electron diffraction, neutron diffraction, etc., can also be used as a basis for determination.

[0120] Figure 2 shows an example of a TEM image in which the orientations of layered rock salt crystals (LRS) and rock salt crystals (RS) are roughly consistent. TEM, STEM, HAADF-STEM, ABF-STEM, etc., yield images that reflect the crystal structure.

[0121] For example, in high-resolution TEM images, contrast originating from crystal planes can be obtained. Due to the diffraction and interference of electron beams, for example, when an electron beam is incident perpendicular to the c-axis of a layered rock salt composite hexagonal lattice, contrast originating from the (0003) plane is obtained as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, in TEM images, a repetition of bright and dark lines is observed, and the bright lines interact with each other (for example, L as shown in Figure 2). RS and L LRS If the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes roughly coincide, i.e., the crystal orientations roughly coincide. Similarly, if the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations roughly coincide.

[0122] Furthermore, HAADF-STEM images yield contrast proportional to the atomic number, with elements having higher atomic numbers appearing brighter. For example, in the case of layered rock salt lithium cobalt oxide belonging to space group R-3m, cobalt (atomic number 27) has the highest atomic number, so electron beams are strongly scattered at the positions of cobalt atoms, and the arrangement of cobalt atoms is observed as bright lines or a sequence of bright points. Therefore, when lithium cobalt oxide with a layered rock salt crystal structure is observed perpendicular to the c-axis, the arrangement of cobalt atoms perpendicular to the c-axis is observed as bright lines or a sequence of bright points, while the arrangement of lithium atoms and oxygen atoms is observed as dark lines or low-brightness regions. The same applies when lithium cobalt oxide contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0123] Therefore, in HAADF-STEM images, if the repetition of bright and dark lines is observed in two regions with different crystal structures, and the angle between the bright lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the atomic arrangement is roughly consistent, i.e., the crystal orientation is roughly consistent. Similarly, if the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can also be determined that the crystal orientation is roughly consistent.

[0124] In ABF-STEM, elements with smaller atomic numbers appear brighter, but since contrast corresponding to atomic number is obtained, similar to HAADF-STEM, the orientation of the crystal can be determined in the same way as with HAADF-STEM images.

[0125] Figure 3(A) shows an example of a STEM image where the orientations of layered rock salt crystals (LRS) and rock salt crystals (RS) are roughly consistent. The FFT pattern of the rock salt crystal region is shown in Figure 3(B), and the FFT pattern of the layered rock salt crystal region (LRS) is shown in Figure 3(C). The left side of Figures 3(B) and 3(C) shows the composition, JCPDS card number, and the d-value and angle calculated from these. The right side shows the measured values. Spots marked with O represent zero-order diffraction.

[0126] The spot labeled A in Figure 3(B) originates from the 11-1 reflection of the cubic crystal. The spot labeled A in Figure 3(C) originates from the 0003 reflection of the layered rock salt type. From Figures 3(B) and 3(C), it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type are roughly coincide. That is, the line passing through AO in Figure 3(B) and the line passing through AO in Figure 3(C) are roughly parallel. Here, roughly coincidental and roughly parallel means that the angle is 5 degrees or less, or 2.5 degrees or less.

[0127] Thus, in the FFT pattern and electron diffraction pattern, if the orientations of the layered rock salt crystal and the rock salt crystal are roughly the same, the <0003> orientation of the layered rock salt crystal and the <11-1> orientation of the rock salt crystal may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. The fact that the reciprocal lattice points are spot-like and not continuous with other reciprocal lattice points means that the crystallization is high.

[0128] Furthermore, as mentioned above, if the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt type may be observed in a reciprocal space different from the orientation of the 0003 reflection of the layered rock salt type. For example, the spot labeled B in Figure 3(C) originates from the 1014 reflection of the layered rock salt type. This spot may be observed at an angle of 52° to 56° from the orientation of the reciprocal point originating from the 0003 reflection of the layered rock salt type (A in Figure 3(C)) (i.e., ∠AOB is between 52° and 56°), and where d is between 0.19 nm and 0.21 nm. Note that this index is just an example and does not necessarily have to match. For example, a reciprocal point equivalent to 0003 and 1014 would also be acceptable.

[0129] Similarly, spots not originating from the cubic 11-1 reflection may be observed in reciprocal space at a different orientation than the orientation where the cubic 11-1 reflection was observed. For example, the spot labeled B in Figure 3(B) originates from the cubic 200 reflection. This is because diffraction spots may be observed at an angle between 54° and 56° from the orientation of the cubic 11-1 reflection (A in Figure 3(B)) (i.e., ∠AOB is between 54° and 56°). Note that this index is just an example and does not necessarily have to match. For example, it could be a reciprocal lattice point equivalent to 11-1 and 200.

[0130] It is known that layered rock salt type cathode active materials, including lithium cobalt oxide, tend to exhibit (0003) planes and equivalent planes, as well as (10-14) planes and equivalent planes, as crystal planes. Therefore, by carefully observing the shape of the cathode active material with an SEM or similar device, it is possible to thin-section the observation sample using a FIB or similar device so that the electron beam is incident at [12-10] in a TEM or similar device, making the (0003) plane easier to observe. When it is necessary to determine the consistency of the crystal orientation, it is preferable to thin-section the layered rock salt type so that the (0003) plane is easily observable.

[0131] ≪Li x When x is small during CoO2≫ In one aspect of the present invention, the positive electrode active material 100 has the above-described distribution of additive elements and / or crystal structure in the discharge state, resulting in Li x The crystal structure of CoO2 when x is small differs from that of conventional cathode active materials. Here, x is small when it is 0.1 <x≦0.24をいうこととする。

[0132] Using Figures 4 to 8, Li x The change in crystal structure associated with the change in x in CoO2 will be explained by comparing a conventional positive electrode active material with a positive electrode active material 100 according to one embodiment of the present invention.

[0133] Figure 5 shows the changes in the crystal structure of a conventional positive electrode active material. The conventional positive electrode active material shown in Figure 5 is lithium cobalt oxide (LiCoO2) without any additive elements. Changes in the crystal structure of lithium cobalt oxide without any additive elements are described in Non-Patent Documents 1 to 3, etc.

[0134] Figure 5 shows Li with R-3m O3 attached. xIt shows the crystal structure of lithium cobaltate with x = 1 in CoO₂. In this crystal structure, lithium occupies octahedral sites, and there are three CoO₂ layers in the unit cell. Therefore, this crystal structure is sometimes called the O3-type crystal structure. Here, the CoO₂ layer refers to a structure in which an octahedral structure with cobalt coordinated by six oxygens is continuous in a plane in a state of edge sharing. It is also sometimes called a layer composed of octahedra of cobalt and oxygen.

[0135] Conventionally known lithium cobaltate also has a crystal structure that belongs to the monoclinic space group P2 / m with enhanced symmetry of lithium when x is about 0.5. This structure has one CoO₂ layer in the unit cell. Therefore, it is sometimes called the O1-type or monoclinic O1-type.

[0136] The cathode active material when x = 0 has a crystal structure of the trigonal space group P-3m1, and also has one CoO₂ layer in the unit cell. Therefore, this crystal structure is sometimes called the O1-type or trigonal O1-type. Also, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called the hexagonal O1-type.

[0137] Conventionally known lithium cobaltate when x is about 0.12 has a crystal structure of the space group R-3m. This structure can also be said to be a structure in which a CoO₂ structure like the trigonal O1-type and a LiCoO₂ structure like the R-3m O3-type are alternately stacked. Therefore, this crystal structure is sometimes called the H1-3-type crystal structure. In fact, the insertion and extraction of lithium do not always occur uniformly within the cathode active material, and the lithium concentration can be uneven, so experimentally, the H1-3-type crystal structure is observed from about x = 0.25. Also, in reality, the H1-3-type crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in this specification including Figure 5, for easier comparison with other crystal structures, it will be shown in a figure where the c-axis of the H1-3-type crystal structure is halved of the unit cell.

[0138] As an example, the H1-3 type crystal structure can be represented by the coordinates of cobalt and oxygen in the unit cell as Co(0,0,0.42150±0.00016), O1(0,0,0.27671±0.00045), and O2(0,0,0.11535±0.00045), as described in Non-Patent Document 3. O1 and O2 are oxygen atoms respectively. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, a unit cell with a smaller GOF (goodness of fit) value can be adopted.

[0139] Li x When charging and discharging are repeated such that x in CoO2 is 0.24 or less, conventional lithium cobaltate repeats a change in crystal structure (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m O3 structure in the discharged state.

[0140] However, the deviation of the CoO2 layers between these two crystal structures is large. As shown by the dotted lines and arrows in FIG. 5, in the H1-3 type crystal structure, the CoO2 layer is significantly deviated from the R-3m O3 structure in the discharged state. Such dynamic structural changes can have an adverse effect on the stability of the crystal structure.

[0141] Furthermore, the volume difference between these two crystal structures is also large. When compared per the same number of cobalt atoms, the volume difference between the H1-3 type crystal structure and the R-3m O3 type crystal structure in the discharged state exceeds 3.5%, typically 3.9% or more.

[0142] In addition, the structure in which the CoO2 layers are continuous, such as the trigonal O1 type, which the H1-3 type crystal structure has, is likely to be unstable.

[0143] Therefore, repeated charging and discharging cycles that result in x being 0.24 or less cause the conventional lithium cobalt oxide crystal structure to break down. This breakdown of the crystal structure leads to a deterioration of the cycle characteristics. This is because the breakdown of the crystal structure reduces the number of sites where lithium can exist stably, and also makes it more difficult for lithium to be inserted and removed.

[0144] On the other hand, in the positive electrode active material 100 of one embodiment of the present invention shown in Figure 4, Li x The change in crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less in CoO2 is less than that of conventional positive electrode active materials. More specifically, the displacement of the CoO2 layer between the state where x is 1 and the state where x is 0.24 or less can be reduced. Furthermore, the change in volume when compared per cobalt atom can be reduced. Therefore, the positive electrode active material 100 of one aspect of the present invention is less prone to crystal structure collapse even when repeated charging and discharging where x is 0.24 or less, and can achieve excellent cycle characteristics. In addition, the positive electrode active material 100 of one aspect of the present invention is Li x When x in CoO2 is 0.24 or less, it can adopt a more stable crystal structure than conventional positive electrode active materials. Therefore, the positive electrode active material 100 in one aspect of the present invention is Li x When the value of x in CoO2 remains below 0.24, short circuits are less likely to occur. In such cases, the safety of the secondary battery is further improved, which is preferable.

[0145] Li x Figure 4 shows the crystal structure of the interior 100b of the positive electrode active material 100 when x in CoO2 is approximately 1, 0.2, and 0.15. The interior 100b occupies most of the volume of the positive electrode active material 100 and is a part that greatly contributes to charging and discharging, so it can be said that this is the part where shifting and volume changes of the CoO2 layer are most problematic.

[0146] When x=1, the positive electrode active material 100 has the same R-3m O3 crystal structure as conventional lithium cobalt oxide.

[0147] However, when x is 0.24 or less, for example, around 0.2 and 0.15, the positive electrode active material 100 has a crystal structure different from that of conventional lithium cobalt oxide, which has an H1-3 type crystal structure.

[0148] When x = approximately 0.2, the positive electrode active material 100 of one embodiment of the present invention has a crystal structure that belongs to the trigonal space group R-3m. This is because the symmetry of the CoO2 layer is the same as that of O3. Therefore, this crystal structure will be called the O3' type crystal structure. Figure 4 shows this crystal structure with R-3m O3'.

[0149] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co(0,0,0.5), O(0,0,x), and 0.20≦x≦0.25. The lattice constant of the unit cell is preferably 2.797≦a≦2.837(Å) for the a-axis, more preferably 2.807≦a≦2.827(Å), and typically a=2.817(Å). For the c-axis, it is preferably 13.681≦c≦13.881(Å), more preferably 13.751≦c≦13.811(Å), and typically c=13.781(Å).

[0150] Furthermore, when x = approximately 0.15, the positive electrode active material 100 of one embodiment of the present invention has a crystal structure that belongs to the monoclinic space group P2 / m. In this case, there is one CoO2 layer in the unit cell. Also, the lithium present in the positive electrode active material 100 at this time is approximately 15 atomic percent, which is the discharge state. Therefore, this crystal structure will be called a monoclinic O1(15) type crystal structure. Figure 4 shows this crystal structure with P2 / m monoclinic O1(15) labeled.

[0151] The monoclinic O1(15) crystal structure is such that the coordinates of cobalt and oxygen in the unit cell are 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, can be shown within the range. Also, the lattice constants of the unit cell are a = 4.880 ± 0.05 Å, b = 2.817 ± 0.05 Å, c = 4.839 ± 0.05 Å, α = 90°, β = 109.6 ± 0.1°, γ = 90°.

[0152] Note that this crystal structure can also show the lattice constants in the space group R - 3m if a certain degree of error is tolerated. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z O ), 0.21 ≦ Z O ≦0.23, can be shown within the range. Also, the lattice constants of the unit cell are a = 2.817 ± 0.02 Å, c = 13.68 ± 0.1 Å.

[0153] In both the O3’ - type and monoclinic O1(15) - type crystal structures, ions such as cobalt, nickel, and magnesium occupy the oxygen six - coordination positions. Note that light elements such as lithium and magnesium may occupy the oxygen four - coordination positions.

[0154] As shown by the dotted line in Figure 4, there is almost no displacement of the CoO2 layer between the R - 3m O3 in the discharged state and the O3’ and monoclinic O1(15) - type crystal structures.

[0155] Also, the volume difference per the same number of cobalt atoms between the R - 3m O3 in the discharged state and the O3’ - type crystal structure is 2.5% or less, more specifically 2.2% or less, and typically 1.8%.

[0156] Furthermore, the difference in volume per unit number of cobalt atoms between R-3m O3 in the discharged state and the monoclinic O1(15) type crystal structure is 3.3% or less, more specifically 3.0% or less, and typically 2.5%.

[0157] Table 1 shows the difference in volume per cobalt atom between R-3m O3 in the discharge state and O3', monoclinic O1(15), H1-3 type, and trigonal O1. The lattice constants of each crystal structure used in the calculations in Table 1 can be found in literature for R-3m O3 in the discharge state and trigonal O1 (ICSD coll.code.172909 and 88721). For H1-3, refer to Non-Patent Literature 3. For O3' and monoclinic O1(15), the values ​​can be calculated from experimental XRD data.

[0158] [Table 1]

[0159] Thus, in the positive electrode active material 100 of one aspect of the present invention, Li x When x in CoO2 is small, that is, when a large amount of lithium is desorbed, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume per unit of the same number of cobalt atoms is also suppressed. Therefore, the crystal structure of positive electrode active material 100 is less likely to collapse even when repeated charging and discharging cycles occur where x is 0.24 or less. As a result, the decrease in charge / discharge capacity during charge / discharge cycles is suppressed in positive electrode active material 100. Also, because it can stably utilize more lithium than conventional positive electrode active materials, positive electrode active material 100 has a high discharge capacity per unit weight and per unit volume. Therefore, by using positive electrode active material 100, secondary batteries with high discharge capacity per unit weight and per unit volume can be manufactured.

[0160] The positive electrode active material 100 is Li x It has been confirmed that when x in CoO2 is between 0.15 and 0.24, it may have an O3' type crystal structure, and it is estimated that it also has an O3' type crystal structure when x is greater than 0.24 and less than or equal to 0.27. xIt has been confirmed that when x in CoO2 is greater than 0.1 and less than or equal to 0.2, typically when x is between 0.15 and 0.17, it may have a monoclinic O1(15) type crystal structure. However, the crystal structure is Li x Because x in CoO2 is affected not only by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc., it is not necessarily limited to the range of x mentioned above.

[0161] Therefore, the positive electrode active material 100 is Li x When x in CoO2 is greater than 0.1 and less than or equal to 0.24, it may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have to have the O3' type and / or monoclinic O1(15) type crystal structure. It may contain other crystal structures, or a portion may be amorphous.

[0162] Also Li x To make x in CoO2 small, it is generally necessary to charge with a high charging voltage. x A state where x in CoO2 is small can be rephrased as a state where it has been charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6V or higher relative to the potential of lithium metal in an environment of 25°C, the H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6V or higher relative to the potential of lithium metal can be said to be a high charging voltage. Furthermore, unless otherwise specified in this specification, the charging voltage is expressed relative to the potential of lithium metal.

[0163] Therefore, the positive electrode active material 100 according to one aspect of the present invention is preferable because it can maintain a crystal structure having the symmetry of R-3m O3 even when charged at a high charging voltage, for example, a voltage of 4.6V or higher at 25°C. It is also preferable because it can adopt an O3' type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65V or higher and 4.7V or lower at 25°C. Furthermore, it is preferable because it can adopt a monoclinic O1(15) type crystal structure when charged at an even higher charging voltage, for example, a voltage exceeding 4.7V and 4.8V or lower at 25°C.

[0164] Even with the positive electrode active material 100, an H1-3 type crystal may only be observed 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., so even at lower charging voltages, for example, when the charging voltage is between 4.5V and 4.6V at 25°C, the positive electrode active material 100 in one embodiment of the present invention may take on an O3' type crystal structure. Similarly, when charged at a voltage between 4.65V and 4.7V at 25°C, it may take on a monoclinic O1(15) type crystal structure.

[0165] Furthermore, in the case of a secondary battery, if graphite is used as the negative electrode active material, for example, the voltage of the secondary battery will decrease by the amount of the graphite's potential compared to the above. The potential of graphite is approximately 0.05V to 0.2V relative to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is observed when the voltage obtained by subtracting the graphite's potential from the above voltage is obtained.

[0166] Furthermore, while Figure 4 shows O3' and monoclinic O1(15) with lithium present at all lithium sites with equal probability, this is not the only case. Lithium may be concentrated at some lithium sites, or, for example, as shown in Figure 5, monoclinic O1(Li 0.5 It may have symmetry similar to that of CoO2. The distribution of lithium can be analyzed, for example, by neutron diffraction.

[0167] Furthermore, the O3' and monoclinic O1(15) type crystal structures can be said to be similar to the CdCl2 type crystal structure, although they have lithium randomly placed between the layers. This CdCl2 type-like crystal structure is formed when lithium nickelate is used as Li 0.06 Although the crystal structure is similar to that of NiO2 when charged to this level, it is known that pure lithium cobalt oxide, or layered rock salt-type cathode active materials containing a large amount of cobalt, do not usually adopt a CdCl2-type crystal structure.

[0168] Furthermore, it is preferable that the concentration gradient of the added elements be similar at multiple locations on the surface layer 100a of the positive electrode active material 100. In other words, it is preferable that the reinforcement derived from the added elements is uniformly present on the surface layer 100a. Even if there is reinforcement in a part of the surface layer 100a, if there are parts without reinforcement, stress may concentrate in those parts. If stress concentrates in a part of the positive electrode active material 100, defects such as cracks may occur from that point, which may lead to cracking of the positive electrode active material and a decrease in discharge capacity.

[0169] However, the added elements do not necessarily have to have the same concentration gradient across the entire surface layer 100a of the positive electrode active material 100. Enlarged views of the area around CD in Figure 1(A) are shown in Figures 6(A1) and 6(A2). An example of the distribution of added element X near CD in Figure 1(A) is shown in Figure 6(A1), and an example of the distribution of added element Y near CD is shown in Figure 6(A2).

[0170] Here, the region near CD has a layered rock salt crystal structure of R-3m, and the surface is (001) oriented. The (001) oriented surface may have a different distribution of additive elements than other surfaces. For example, the distribution of one or more concentration peaks selected from additive elements X and Y on the (001) oriented surface and its surface layer 100a may be limited to a shallower portion from the surface compared to surfaces other than the (001) oriented surface. Alternatively, the concentration of one or more selected from additive elements X and Y on the (001) oriented surface and its surface layer 100a may be lower compared to surfaces other than the (001) oriented surface. Alternatively, the concentration of one or more selected from additive elements X and Y on the (001) oriented surface and its surface layer 100a may be below the detection limit.

[0171] In the layered rock salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This indicates a structure in which CoO2 layers and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion pathways for lithium ions also exist parallel to the (001) plane.

[0172] Since the CoO2 layer is relatively stable, the surface of the positive electrode active material 100 is more stable when it is oriented in the (001) direction. The main diffusion pathway of lithium ions during charging and discharging is not exposed on the (001) plane.

[0173] On the other hand, the lithium ion diffusion pathways are exposed on surfaces other than those oriented in the (001) direction. Therefore, the surfaces and surface layer 100a other than those oriented in the (001) direction are important regions for maintaining the lithium ion diffusion pathways, but at the same time, they are prone to instability because they are the regions where lithium ions first desorb. For this reason, reinforcing the surfaces and surface layer 100a other than those oriented in the (001) direction is extremely important for maintaining the overall crystal structure of the positive electrode active material 100.

[0174] Therefore, in another embodiment of the positive electrode active material 100 of the present invention, it is important that the distribution of additive elements on surfaces other than the (001) oriented surface and its surface layer 100a is as shown in Figure 1(B1) or Figure 1(B2). Among the additive elements, it is particularly preferable that nickel be detected on surfaces other than the (001) oriented surface and its surface layer 100a. On the other hand, on the (001) oriented surface and its surface layer 100a, the concentration of additive elements may be low or absent, as described above.

[0175] For example, the distribution of magnesium in the (001)-oriented surface and its surface layer 100a preferably has a full width at half maximum of 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 distribution of magnesium in the non-(001)-oriented surface and its surface layer 100a preferably has a full width at half maximum of more than 200 nm and 500 nm or less, more preferably more than 200 nm and 300 nm or less, and even more preferably 230 nm or more and 270 nm or less.

[0176] Furthermore, the nickel distribution on the non-oriented surface and its surface layer 100a is preferably such that its full width at half maximum is 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.

[0177] In the manufacturing method described in a later embodiment, in which high-purity LiCoO2 is produced and then additive elements are mixed in and heated, the additive elements spread mainly through the diffusion pathway of lithium ions. Therefore, it is easy to bring the distribution of additive elements on surfaces other than the (001) orientation and its surface layer 100a within a desirable range.

[0178] Using Figures 6(B1) to 6(C), we will explain the results of calculations regarding the distribution of additive elements when high-purity LiCoO2 is prepared, mixed with additive elements, and then heated.

[0179] Figure 6(B1) shows the calculation results for the (104) oriented surface and its surface layer 100a. The calculation was performed using classical molecular dynamics. LiCoO2 (LCO) was placed at the bottom of the system, and LiF and MgF2 were placed at the top of the system as magnesium, lithium, and fluorine sources, respectively. The ensemble was NVT, and the density of the initial structure was 1.8 g / cm³. 3 The system temperature was 2000K, the elapsed time was 100 psec, the potential was optimized for the LCO crystal structure, other atoms were mixed with UFF, the number of atoms in the system was approximately 10,000, and the system charge was neutral. To simplify the diagram, only Co and Mg atoms are shown.

[0180] Figure 6(B2) shows the results calculated up to 200 psec, and Figure 6(B3) shows the results calculated up to 1200 psec.

[0181] From the above calculations, it can be inferred that magnesium diffuses through the following process: (1) Lithium is detached from LCO by heat. (2) Magnesium enters the lithium layer of LCO and diffuses into the interior. (3) Lithium from LiF enters the lithium layer of LCO, replenishing the lithium detached in (1).

[0182] Figure 6(B1), taken after 100 psec, clearly shows the diffusion of magnesium atoms into the LCO. The magnesium atoms diffuse along the arrangement of cobalt atoms, and in Figure 6(B3), taken after 1200 psec, almost all of the magnesium atoms prepared at the top of the system are incorporated into the LCO.

[0183] Figure 6(C) shows the results calculated in the same way as Figure 6(B1), except that the orientation is (001). In Figure 6(C), it can be seen that the magnesium atoms remain on the surface of the LCO. However, Figure 6(C) is the calculation result after 100 psec has elapsed. In actual positive electrode active material 100, heating is performed for, for example, 2 hours or more, so it is thought that the magnesium atoms slowly diffuse into the interior of the LCO.

[0184] By producing high-purity LiCoO2 in this manner, and then mixing in additive elements and heating, the additive elements on surfaces other than the (001) orientation and their surface layer 100a can be distributed more favorably than on the (001) plane.

[0185] Furthermore, in the manufacturing method that involves initial heating, as described later, it is expected that lithium in the surface layer 100a will be detached from LiCoO2 by the initial heating, making it easier to distribute added elements, such as magnesium, at a higher concentration in the surface layer.

[0186] Furthermore, while it is preferable that the surface of the positive electrode active material 100 be smooth and have few irregularities, it is not necessarily required that the entire surface of the positive electrode active material 100 be so. In composite oxides having a layered rock salt type crystal structure of R-3m, slip is likely to occur on planes parallel to the (001) plane, for example, on planes where lithium is arranged. For example, as shown in Figure 7(A), if a (001) plane is present, slip may occur parallel to the (001) plane and deformation may occur as shown by the arrow in Figure 7(B) after going through processes such as pressing.

[0187] In this case, the newly formed surface and its surface layer 100a resulting from the slip may either not contain the added element or its concentration may be below the detection limit. EF in Figure 7(B) is an example of the newly formed surface and its surface layer 100a resulting from the slip. Enlarged views of the area around EF are shown in Figures 7(C1) and 7(C2). Unlike Figures 1(B1) and 1(B2), added elements X and Y are not distributed in Figures 7(C1) and 7(C2).

[0188] However, since slip tends to occur parallel to the (001) plane, the newly formed surface and its surface layer 100a tend to be (001) oriented. In this case, the diffusion pathway of lithium ions is not exposed and it is relatively stable, so there is little problem even if the additive element is absent or its concentration is below the detection limit.

[0189] As mentioned above, in composite oxides with the composition LiCoO2 and the layered rock salt type crystal structure R-3m, cobalt atoms are arranged parallel to the (001) plane. Furthermore, in HAADF-STEM images, the brightness of cobalt, which has the highest atomic number in LiCoO2, is the highest. Therefore, in HAADF-STEM images, the arrangement of atoms with high brightness can be considered to be the arrangement of cobalt atoms. The repetition of this high-brightness arrangement is synonymous with crystal fringes or lattice fringes.

[0190] ≪Grain Boundaries≫ In addition to the distribution described above, it is more preferable that the additive elements in the positive electrode active material 100 of one aspect of the present invention are concentrated in the grain boundaries 101 and their vicinity.

[0191] In this specification, "non-uniformity" refers to a situation where the concentration of an element in one region differs from that in other regions. It is synonymous with segregation, precipitation, heterogeneity, bias, or a mixture of areas with high and low concentrations.

[0192] For example, it is preferable that the magnesium concentration at and near the grain boundary 101 of the positive electrode active material 100 is higher than that in other regions of the interior 100b. It is also preferable that the fluorine concentration at and near the grain boundary 101 is higher than that in other regions of the interior 100b. Furthermore, it is preferable that the nickel concentration at and near the grain boundary 101 is higher than that in other regions of the interior 100b. It is also preferable that the aluminum concentration at and near the grain boundary 101 is higher than that in other regions of the interior 100b.

[0193] The grain boundary 101 is a type of surface defect. Therefore, like the grain surface, it is prone to instability and easily initiates changes in the crystal structure. For this reason, if the concentration of the added element at and near the grain boundary 101 is high, changes in the crystal structure can be suppressed more effectively.

[0194] Furthermore, if the magnesium and fluorine concentrations are high at and near the grain boundary 101, even if a crack occurs along the grain boundary 101 of the positive electrode active material 100 according to one embodiment of the present invention, the magnesium and fluorine concentrations will be high near the surface created by the crack. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the crack has occurred.

[0195] <Particle size> In one embodiment of the present invention, if the particle size of the positive electrode active material 100 is too large, problems arise such as difficulty in lithium diffusion and excessive roughness of the surface of the active material layer when coated onto the current collector. On the other hand, if it is too small, problems arise such as difficulty in supporting the active material layer when coating onto the current collector and excessive reaction with the electrolyte. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Or preferably 1 μm or more and 40 μm or less. Or preferably 1 μm or more and 30 μm or less. Or preferably 2 μm or more and 100 μm or less. Or preferably 2 μm or more and 30 μm or less. Or preferably 5 μm or more and 100 μm or less. Or preferably 5 μm or more and 40 μm or less.

[0196] <Analysis method> A certain positive electrode active material is Li x Whether or not the positive electrode active material 100 of one embodiment of the present invention has an O3' type and / or monoclinic O1(15) type crystal structure when x in CoO2 is small depends on Li x The presence of a positive electrode active material with a small x value in CoO2 can be determined by analyzing it using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc.

[0197] XRD is particularly favored because it can analyze the symmetry of transition metals such as cobalt in the positive electrode active material with high resolution, compare the crystallinity and crystal orientation, analyze the periodic strain of the lattice and the crystallite size, and obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling a secondary battery. Among XRD methods, powder XRD provides diffraction peaks that reflect the crystal structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100.

[0198] When analyzing crystallite size using powder XRD, it is preferable to measure while excluding the influence of orientation due to pressure, etc. For example, it is preferable to extract the positive electrode active material from the positive electrode obtained by disassembling a secondary battery, and measure it after obtaining a powder sample.

[0199] In one aspect of the present invention, the positive electrode active material 100 is Li x A characteristic feature is that there is little change in the crystal structure when x in CoO2 is 1 and when it is 0.24 or less. When charged at high voltage, materials in which the crystal structure changes significantly and where the crystal structure accounts for more than 50% are undesirable because they cannot withstand high-voltage charging and discharging.

[0200] It is also important to note that simply adding alloying elements may not result in the formation of an O3'-type or monoclinic O1(15)-type crystal structure. For example, even if lithium cobalt oxide contains magnesium and fluorine, or lithium cobalt oxide contains magnesium and aluminum, the structure may vary depending on the concentration and distribution of the alloying elements. x In CoO2, there are two cases: one where x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more; and another where the H1-3 type crystal structure accounts for 50% or more.

[0201] Furthermore, even with the positive electrode active material 100 according to one aspect of the present invention, if x is too small, such as 0.1 or less, or under conditions where the charging voltage exceeds 4.9V, an H1-3 type or trigonal O1 type crystal structure may occur. Therefore, in order to determine whether or not it is the positive electrode active material 100 according to one aspect of the present invention, analysis of the crystal structure, including XRD, and information such as charging capacity or charging voltage are necessary.

[0202] However, positive electrode active materials with a small x value may undergo changes in their crystal structure when exposed to air. For example, they may change from O3'-type and monoclinic O1(15)-type crystal structures to H1-3-type crystal structures. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0203] Furthermore, whether the distribution of additive elements in a particular positive electrode active material is in the state described above can be determined by analyzing it using methods such as XPS, energy-dispersive X-ray spectroscopy (EDX), and EPMA (electron probe microanalysis).

[0204] Furthermore, the crystal structure of the surface layer 100a, grain boundaries 101, etc., can be analyzed by electron diffraction of the cross-section of the positive electrode active material 100.

[0205] ≪Charging method≫ To determine whether a certain composite oxide is the positive electrode active material 100 according to one aspect of the present invention, charging can be performed, for example, by fabricating a coin cell (CR2032 type, 20 mm in diameter and 3.2 mm in height) with a lithium counter electrode and then charging it.

[0206] More specifically, the positive electrode can be made by coating an aluminum foil positive electrode current collector with a slurry of a mixture of positive electrode active material, conductive material, and binder.

[0207] Lithium metal can be used for the counter electrode. However, if a material other than lithium metal is used for the counter electrode, the potential of the secondary battery and the potential of the positive electrode will differ. Unless otherwise specified, the voltage and potential in this specification refer to the potential of the positive electrode.

[0208] The electrolyte in the electrolyte solution may be 1 mol / L lithium hexafluoride phosphate (LiPF6), and the electrolyte solution may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7, and vinylene carbonate (VC) at 2 wt%.

[0209] A 25 μm thick porous polypropylene film can be used as the separator.

[0210] The positive electrode and negative electrode cans can be made of stainless steel (SUS).

[0211] The coin cell prepared under the above conditions is charged with an arbitrary voltage (e.g., 4.5V, 4.55V, 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V). The charging method is not particularly limited as long as it can be charged at any voltage for a sufficient amount of time. For example, when charging with CCCV, the current in CC charging can be 20mA / g or more and 100mA / g or less. CV charging can be completed at 2mA / g or more and 10mA / g or less. It is desirable to charge with such small current values ​​in order to observe the phase change of the positive electrode active material. The temperature should be 25°C or 45°C. After charging in this way, the coin cell can be disassembled in a glove box with an argon atmosphere and the positive electrode removed to obtain a positive electrode active material with an arbitrary charge capacity. When performing various analyses thereafter, it is preferable to seal it in an argon atmosphere to suppress reactions with external components. For example, XRD can be performed by sealing it in a sealed container with an argon atmosphere. It is also preferable to remove the positive electrode quickly after charging is complete and subject it to analysis. Specifically, it is preferable to do this within one hour of the charging completion, and more preferably within 30 minutes.

[0212] Furthermore, when analyzing the crystal structure of the charged state after multiple charge-discharge cycles, the conditions for these multiple charge-discharge cycles may differ from the charging conditions described above. For example, charging can be performed by constant current charging at a current value of 20 mA / g to 100 mA / g up to any voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V), then by constant voltage charging until the current value becomes 2 mA / g to 10 mA / g, and discharging can be performed by constant current discharge at 2.5V, 20 mA / g to 100 mA / g.

[0213] Furthermore, when analyzing the crystal structure of the discharged state after multiple charge-discharge cycles, a constant current discharge can be performed, for example, at 2.5V and a current value of 20mA / g to 100mA / g.

[0214] ≪XRD≫ The equipment and conditions for XRD measurement are not particularly limited. For example, measurements can be taken using the following equipment and conditions. XRD system: Bruker AXS D8 ADVANCE X-ray source:CuKα1 ray Output: 40kV, 40mA Divergence angle: Div.Slit, 0.5° Detector: LynxEye Scanning method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0215] If the sample to be measured is a powder, it can be set up by placing it in a glass sample holder or by sprinkling the sample onto a grease-coated silicone anti-reflective plate. If the sample to be measured is a positive electrode, the positive electrode can be attached to a substrate with double-sided tape, and the positive electrode active material layer can be set up to match the measurement surface required by the device.

[0216] Figures 8, 9, 10(A), and 10(B) show ideal powder XRD patterns calculated using CuKα1 lines from models of the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure. Li is also shown for comparison. x The ideal XRD patterns calculated from the crystal structures of LiCoO2O3 at x=1 and trigonal O1 at x=0 in CoO2 are also shown. Figures 10(A) and 10(B) show the XRD patterns of the O3' type crystal structure, monoclinic O1(15) type crystal structure, and H1-3 type crystal structure side by side. Figure 10(A) is an enlarged view of the region where the 2θ range is 18° to 21°, and Figure 10(B) is an enlarged view of the region where the 2θ range is 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), from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database) (see Non-Patent Literature 4). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength λ1 of 1.540562 × 10⁻¹⁰. -10 m and λ2 were not set, and the Monochromator was set to single. The H1-3 type crystal structure pattern was similarly created from the crystal structure information described in Non-Patent Literature 3. For the O3' type and monoclinic O1(15) type crystal structure patterns, the crystal structure was estimated from the XRD pattern of the cathode active material of one embodiment of the present invention, fitted using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker), and the XRD pattern was created in the same manner as the others.

[0217] As shown in Figures 8, 10(A), and 10(B), in the O3' type crystal structure, diffraction peaks appear at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°).

[0218] Furthermore, in the monoclinic O1(15) crystal structure, diffraction peaks appear at 2θ = 19.47 ± 0.10° (19.37° to 19.57°) and 2θ = 45.62 ± 0.05° (45.57° to 45.67°).

[0219] However, as shown in Figures 9, 10(A), and 10(B), peaks do not appear at these positions in the H1-3 type crystal structure and trigonal O1. Therefore, Li x The appearance of peaks at 19.13° to less than 19.37° and / or 19.37° to 19.57°, and at 45.37° to less than 45.57° and / or 45.57° to 45.67°, when x in CoO2 is small, can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0220] This can also be described as the positions where XRD diffraction peaks appear being close together in the crystal structure at x=1 and x≦0.24. More specifically, for the main diffraction peaks in the crystal structure at x=1 and x≦0.24 where 2θ is between 42° and 46°, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0221] In one embodiment of the present invention, the positive electrode active material 100 is Li x When x in CoO2 is small, it has an O3' type and / or monoclinic O1(15) type crystal structure, but not all particles have to have an O3' type and / or monoclinic O1(15) type crystal structure. Other crystal structures may be present, and some may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, it is preferable that the O3' type and / or monoclinic O1(15) type crystal structure accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3' type and / or monoclinic O1(15) type crystal structure accounts for 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be made into a cathode active material with sufficiently excellent cycle characteristics.

[0222] Furthermore, even after more than 100 charge-discharge cycles from the start of measurement, it is preferable that the O3' type and / or monoclinic O1(15) type crystal structure accounts for 35% or more, more preferably 40% or more, and even more preferably 43% or more when Rietveld analysis is performed.

[0223] Furthermore, when Rietveld analysis is performed in a similar manner, it is preferable that the H1-3 type and O1 type crystal structures account for 50% or less.

[0224] Furthermore, the sharpness of diffraction peaks in the XRD pattern indicates high crystallinity. Therefore, it is preferable for each diffraction peak after charging to be sharp, i.e., have a narrow full width at half maximum (FMAX). The FMAX varies depending on the XRD measurement conditions and the value of 2θ, even for peaks arising from the same crystalline phase. Under the measurement conditions described above, for peaks observed between 2θ = 43° and 46°, the FMAX is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. It is not necessary for all peaks to satisfy this requirement. If some peaks satisfy this requirement, it can be said that the crystal phase has high crystallinity. Such high crystallinity contributes to the stabilization of the crystal structure after sufficient charging.

[0225] Furthermore, the crystallite size of the O3'-type and monoclinic O1(15) crystal structures of the positive electrode active material 100 decreases to only about 1 / 20th of that of LiCoO2(O3) in the discharged state. Therefore, even under the same XRD measurement conditions as the positive electrode before charging and discharging, Li x When x is small in CoO2, clear peaks of the O3'-type and / or monoclinic O1(15) crystal structure can be observed. On the other hand, in conventional LiCoO2, even if some parts can adopt a structure similar to the O3'-type and / or monoclinic O1(15) crystal structure, the crystallite size becomes small, and the peak becomes broad and small. The crystallite size can be determined from the full width at half maximum of the XRD peak.

[0226] In one embodiment of the present invention, the positive electrode active material 100 is preferably less affected by the Jahn-Teller effect, as described above. Within a range where the influence of the Jahn-Teller effect is small, transition metals such as nickel and manganese may be included as additive elements in addition to cobalt.

[0227] In the cathode active material, we will use XRD analysis to investigate the proportions of nickel and manganese and the range of lattice constants in which the Jahn-Teller effect is presumed to be small.

[0228] Figure 11 shows the results of calculating the lattice constants of the a-axis and c-axis using XRD 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 11(A) shows the results for the a-axis, and Figure 11(B) shows the results for the c-axis. The XRD patterns used for these calculations are from the powder after synthesis of the positive electrode active material, before it is incorporated into the positive electrode. The nickel concentration on the horizontal axis represents the nickel concentration when the sum of the number of cobalt and nickel atoms is taken as 100%. The positive electrode active material was prepared according to the manufacturing method in Figure 15, except that an aluminum source was not used.

[0229] Figure 12 shows the results of estimating the lattice constants on the a-axis and c-axis using XRD when the positive electrode active material 100 according to one embodiment of the present invention has a layered rock salt type crystal structure and contains cobalt and manganese. Figure 12(A) shows the results for the a-axis, and Figure 12(B) shows the results for the c-axis. Note that the lattice constants shown in Figure 12 are for the powder after synthesis of the positive electrode active material and were measured by XRD before being incorporated into the positive electrode. The manganese concentration on the horizontal axis represents the manganese concentration when the sum of the number of cobalt and manganese atoms is taken as 100%. The positive electrode active material was prepared according to the manufacturing method in Figure 15, except that a manganese source was used instead of a nickel source, and an aluminum source was not used.

[0230] Figure 11(C) shows the a-axis lattice constant divided by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active material whose lattice constant results are shown in Figures 11(A) and 11(B). Figure 12(C) shows the a-axis lattice constant divided by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active material whose lattice constant results are shown in Figures 12(A) and 12(B).

[0231] Figure 11(C) shows a significant change in the a-axis / c-axis at nickel concentrations of 5% and 7.5%, with the a-axis distortion being larger at a nickel concentration of 7.5%. This distortion may be due to the Jahn-Teller distortion of trivalent nickel. This suggests that a superior cathode active material with low Jahn-Teller distortion can be obtained at nickel concentrations below 7.5%.

[0232] Next, Figure 12(A) suggests that when the manganese concentration is 5% or higher, the behavior of the lattice constant changes is different and does not follow Vegard's law. Therefore, it is suggested that the crystal structure is different when the manganese concentration is 5% or higher. Thus, a manganese concentration of, for example, 4% or less is preferable.

[0233] The above-mentioned ranges for nickel and manganese concentrations do not necessarily apply to the surface layer 100a. In other words, the concentrations in the surface layer 100a may be higher than those mentioned above.

[0234] Based on the above considerations regarding the preferred range of lattice constants, in a positive electrode active material according to one embodiment of the present invention, the layered rock salt type crystal structure of the positive electrode active material 100 in a non-charged or discharged state, as estimated from the XRD pattern, has a lattice constant of 2.814 × 10¹⁴ in the a-axis. -10 Larger than m, 2.817 × 10 -10 It is smaller than m, and the lattice constant of the c axis is 14.05 × 10⁻¹⁰. -10 Larger than m, 14.07 × 10 -10 It was found that a value smaller than m is preferable. The state without charging and discharging may, for example, be the powder state before the positive electrode of the secondary battery is manufactured.

[0235] Alternatively, in the layered rock salt type crystal structure of the positive electrode active material 100 in a non-charging / discharging state or in a discharged state, it is preferable that the value obtained by dividing the lattice constant of the a axis by the lattice constant of the c axis (a axis / c axis) is greater than 0.20000 and less than 0.20049.

[0236] Alternatively, when XRD analysis is performed on the layered rock salt-type crystal structure of the positive electrode active material 100 in a non-charging / discharging state or in a discharged state, a first peak may be observed when 2θ is between 18.50° and 19.30°, and a second peak may be observed when 2θ is between 38.00° and 38.80°.

[0237] ≪XPS≫ In X-ray photoelectron spectroscopy (XPS), when using monochromatic aluminum Kα as the X-ray source for inorganic oxides, it is possible to analyze regions from the surface to a depth of approximately 2 to 8 nm (usually less than 5 nm). Therefore, the concentration of each element can be quantitatively analyzed in a region that is about half the depth of the surface layer 100a. Furthermore, the bonding state of elements can be analyzed using narrow-scan analysis. The quantitative accuracy of XPS is usually around ±1 atomic percent, and the detection limit is also around 1 atomic percent, although this varies depending on the element.

[0238] In one embodiment of the present invention, it is preferable that the concentration of one or more selected additive elements in the positive electrode active material 100 is higher in the surface layer 100a than in the interior 100b. This is equivalent to saying that it is preferable that the concentration of one or more selected additive elements in the surface layer 100a is higher than the average concentration of the positive electrode active material 100 as a whole. For example, it can be said that it is preferable that the concentration of one or more selected additive elements from the surface layer 100a, measured by XPS, etc., is higher than the average concentration of additive elements of the positive electrode active material 100 as a whole, measured by ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry), etc. For example, it is preferable that the concentration of magnesium in at least a portion of the surface layer 100a, measured by XPS, etc., is higher than the magnesium concentration of the positive electrode active material 100 as a whole. It is also preferable that the concentration of nickel in at least a portion of the surface layer 100a is higher than the nickel concentration of the positive electrode active material 100 as a whole. Furthermore, it is preferable that the concentration of aluminum in at least a portion of the surface layer 100a is higher than the total aluminum concentration of the positive electrode active material 100. Also, it is preferable that the concentration of fluorine in at least a portion of the surface layer 100a is higher than the total fluorine concentration of the positive electrode active material 100.

[0239] In one embodiment of the present invention, the surface and surface layer 100a of the positive electrode active material 100 do not contain carbonates, hydroxyl groups, etc., that have been chemically adsorbed after the positive electrode active material 100 was manufactured. Furthermore, electrolyte, binder, conductive material, or compounds derived therefrom that are not included on the surface of the positive electrode active material 100. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, etc., which can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds through analysis, and corrections may be made to exclude CF bonds derived from the binder.

[0240] Furthermore, before subjecting the sample to various analyses, the positive electrode active material and positive electrode active material layer may be washed to remove electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve into the solvent used for washing, but even if this occurs, the added elements are unlikely to dissolve, so it will not affect the atomic ratio of the added elements.

[0241] Furthermore, the concentrations of the added elements may be compared in ratio to cobalt. Using the ratio to cobalt is preferable because it reduces the influence of carbonates and other substances chemically adsorbed after the production of the positive electrode active material. For example, the ratio of magnesium to cobalt atoms, Mg / Co, determined by XPS analysis is preferably 0.4 or more and 1.5 or less. On the other hand, the Mg / Co determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0242] Similarly, in order to ensure sufficient pathways for lithium insertion and deinsertion, it is preferable that the concentrations of lithium and cobalt in the surface layer 100a of the positive electrode active material 100 are higher than those of each of the other additive elements. This means that it is preferable that the concentrations of lithium and cobalt in the surface layer 100a are higher than the concentrations of one or more of the other additive elements selected from the additive elements present in the surface layer 100a as measured by XPS, etc. For example, it is preferable that the concentration of cobalt in at least a portion of the surface layer 100a as measured by XPS, etc. is higher than the concentration of magnesium in at least a portion of the surface layer 100a as measured by XPS, etc. Similarly, it is preferable that the concentration of lithium is higher than the concentration of magnesium. It is also preferable that the concentration of cobalt is higher than the concentration of nickel. Similarly, it is preferable that the concentration of lithium is higher than the concentration of nickel. It is also preferable that the concentration of cobalt is higher than that of aluminum. Similarly, it is preferable that the concentration of lithium is higher than that of aluminum. It is also preferable that the concentration of cobalt is higher than that of fluorine. Similarly, it is preferable that the concentration of lithium is higher than that of fluorine.

[0243] Furthermore, it is more preferable that the additive element Y, including aluminum, is widely distributed in deeper regions, for example, in the region between 5 nm and 50 nm from the surface. Therefore, it is more preferable that, although the additive element Y, including aluminum, is detected in the analysis of the entire cathode active material 100 using ICP-MS, GD-MS, etc., its concentration is below the detection limit in XPS, etc.

[0244] Furthermore, when XPS analysis was performed on the positive electrode active material 100 according to one embodiment of the present invention, the number of magnesium atoms is preferably 0.4 to 1.2 times the number of cobalt atoms, and more preferably 0.65 to 1.0 times. Also, the number of nickel atoms is preferably 0.15 times or less, and more preferably 0.03 to 0.13 times the number of cobalt atoms. Also, the number of aluminum atoms is preferably 0.12 times or less, and more preferably 0.09 times or less. Also, the number of fluorine atoms is preferably 0.3 to 0.9 times the number of cobalt atoms, and more preferably 0.1 to 1.1 times. Being within these ranges indicates that these additive elements are not attached to a narrow area on the surface of the positive electrode active material 100, but are widely distributed at a preferred concentration in the surface layer 100a of the positive electrode active material 100.

[0245] For XPS analysis, for example, monochromatic aluminum Kα radiation can be used as the X-ray source. The extraction angle can be set to, for example, 45°. Measurements can be performed using, for example, the following equipment and conditions. Measurement device: PHI QuanteraII X-ray source: Monochromatic Al Kα (1486.6eV) Detection area: 100 μmφ Detection depth: Approximately 4-5 nm (extraction angle 45°) Measurement spectrum: Wide scan, narrow scan of each detected element

[0246] Furthermore, when the positive electrode active material 100 according to one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably around 684.3 eV. This value is different from both the bond energy of lithium fluoride, which is 685 eV, and the bond energy of magnesium fluoride, which is 686 eV. In other words, when the positive electrode active material 100 according to one embodiment of the present invention contains fluorine, it is preferable that the bond is with something other than lithium fluoride and magnesium fluoride.

[0247] Furthermore, when the positive electrode active material 100 according to one embodiment of the present invention is subjected to XPS analysis, the peak indicating the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably around 1303 eV. This value is different from the bond energy of magnesium fluoride, which is 1305 eV, and is close to the bond energy of magnesium oxide. In other words, when the positive electrode active material 100 according to one embodiment of the present invention contains magnesium, it is preferable that the bond is with an element other than magnesium fluoride.

[0248] ≪EDX≫ It is preferable that one or more of the additive elements in the positive electrode active material 100 have a concentration gradient. More preferably, the depth of the concentration peaks from the surface differs 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 the cross-section of the positive electrode active material 100 using a FIB (Focused Ion Beam) and analyzing the cross-section using energy dispersive X-ray spectroscopy (EDX), EPMA (Electron Probe Microanalysis), etc.

[0249] Among EDX measurements, EDX surface analysis refers to a method of scanning within a region to evaluate it in two dimensions. Line analysis refers to a method of scanning linearly to evaluate the distribution of atomic concentrations within the positive electrode active material. Furthermore, line analysis is sometimes used to describe the extraction of linear region data from EDX surface analysis. Finally, point analysis refers to a method of measuring a region without scanning.

[0250] EDX surface analysis (e.g., elemental mapping) allows for quantitative analysis of the concentrations of additive elements in the surface layer 100a, interior 100b, and near grain boundaries 101 of the positive electrode active material 100. EDX radiation analysis can also analyze the concentration distribution and maximum value of the additive elements. Furthermore, analysis methods that thin the sample, such as STEM-EDX, are preferable because they allow for the analysis of the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region, without being affected by the distribution in the depth direction.

[0251] Therefore, when EDX surface analysis or EDX point analysis is performed on the positive electrode active material 100 according to one embodiment of the present invention, it is preferable that the concentration of each additive element in the surface layer 100a, particularly additive element X, is higher than that of the interior 100b.

[0252] For example, when EDX surface analysis or EDX point analysis is performed on a positive electrode active material 100 having magnesium as an additive element, it is preferable that the magnesium concentration in the surface layer 100a is higher than the magnesium concentration in the interior 100b. Furthermore, when EDX radiation analysis is performed, it is preferable that the magnesium concentration peak in the surface layer 100a is located within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. It is also preferable that the magnesium concentration attenuates to 60% or less of the peak at a depth of 1 nm from the peak top, and more preferably to 30% or less of the peak at a depth of 2 nm from the peak top. Here, the concentration peak refers to the maximum value of the concentration.

[0253] Furthermore, in a positive electrode active material 100 having magnesium and fluorine as additive elements, it is preferable that the distribution of fluorine overlaps with the distribution of magnesium. For example, it is preferable that the difference in depth between the peak of fluorine concentration and the peak of magnesium concentration is within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0254] Furthermore, when EDX radiation analysis is performed, the fluorine concentration peak in the surface layer 100a is preferably located within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. It is also preferable that the fluorine concentration peak is located slightly closer to the surface than the magnesium concentration peak, as this increases resistance to hydrofluoric acid. For example, it is more preferable that the fluorine concentration peak is located 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferable that it is located 1.5 nm or more closer to the surface.

[0255] Furthermore, in the positive electrode active material 100 having nickel as an additive element, the nickel concentration peak in the surface layer 100a is preferably located at a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, in the positive electrode active material 100 having magnesium and nickel, the distribution of nickel is preferably superimposed on the distribution of magnesium. For example, the difference in depth 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.

[0256] Furthermore, if the positive electrode active material 100 contains aluminum as an additive element, when EDX radiation analysis is performed, it is preferable that the peaks of magnesium, nickel, or fluorine concentrations are closer to the surface than the peak of aluminum concentration in the surface layer 100a. For example, the aluminum concentration peak is preferably located at a depth of 0.5 nm to 50 nm from the surface toward the center of the positive electrode active material 100, and more preferably at a depth of 5 nm to 50 nm.

[0257] Furthermore, when EDX radiation analysis, surface analysis, or point analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of magnesium Mg to cobalt Co (Mg / Co) at the magnesium concentration peak is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum Al to cobalt Co (Al / Co) at the aluminum concentration peak is preferably 0.05 or more and 0.6 or less, and more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel Ni to cobalt Co (Ni / Co) at the nickel concentration peak is preferably 0 or more and 0.2 or less, and more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine F to cobalt Co (F / Co) at the fluorine concentration peak is preferably 0 or more and 1.6 or less, and more preferably 0.1 or more and 1.4 or less.

[0258] Furthermore, the surface of the positive electrode active material 100 in the EDX radiation analysis results can be estimated, for example, as follows: For elements uniformly present in the interior 100b of the positive electrode active material 100, such as oxygen or cobalt, the point where the detected amount in the interior 100b becomes half is defined as the surface.

[0259] Since the positive electrode active material 100 is a composite oxide, the surface can be estimated using the amount of oxygen detected. Specifically, first, the average value of the oxygen concentration is obtained from the region where the amount of oxygen detected in the interior 100b is stable. ave We determine the amount of oxygen O in the region that is clearly outside the surface, which is thought to be due to chemiadsorption or background. bg If detected, O bg Subtracting this gives the average oxygen concentration O ave This can be done. This average value O ave The value of 1 / 2, that is, 1 / 2O ave The measurement point that shows the closest measurement value can be estimated to be the surface of the positive electrode active material.

[0260] The surface can also be estimated in the same way as described above using the amount of cobalt detected. Alternatively, it can be estimated similarly using the sum of the detected amounts of multiple transition metals. The detected amounts of transition metals, including cobalt, are suitable for surface estimation because they are less affected by chemiadsorption.

[0261] Furthermore, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the number of atoms of the added element A to cobalt Co (A / Co) near the grain boundary 101 is preferably 0.020 or more and 0.50 or less. More preferably 0.025 or more and 0.30 or less. More preferably 0.030 or more and 0.20 or less. Or preferably 0.020 or more and 0.30 or less. Or preferably 0.020 or more and 0.20 or less. Or preferably 0.025 or more and 0.50 or less. Or preferably 0.025 or more and 0.20 or less. Or preferably 0.030 or more and 0.50 or less. Or preferably 0.030 or more and 0.30 or less.

[0262] For example, when the additive element is magnesium, when line analysis or surface analysis is performed on the positive electrode active material 100, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) near the grain boundary 101 is preferably 0.020 or more and 0.50 or less. Furthermore, 0.025 or more and 0.30 or less is preferable. Furthermore, 0.030 or more and 0.20 or less is preferable. Or 0.020 or more and 0.30 or less is preferable. Or 0.020 or more and 0.20 or less is preferable. Or 0.025 or more and 0.50 or less is preferable. Or 0.030 or more and 0.50 or less is preferable. Or 0.030 or more and 0.30 or less is preferable. Furthermore, if the above range is observed at multiple locations on the positive electrode active material 100, for example, three or more locations, it indicates that the added element is not adhering to a narrow area on the surface of the positive electrode active material 100, but is widely distributed at a favorable concentration in the surface layer 100a of the positive electrode active material 100.

[0263] ≪EPMA≫ EPMA (Electron Probe Microanalysis) can also quantify elements. Surface analysis allows for the analysis of the distribution of each element.

[0264] When EPMA surface analysis is performed on a cross-section of the positive electrode active material 100 according to one embodiment of the present invention, it is preferable that one or more selected additive elements have a concentration gradient, similar to the results of EDX analysis. Furthermore, it is more preferable that the depth of the concentration peak from the surface differs depending on the additive element. The preferred range of the concentration peak for each additive element is also the same as in the case of EDX.

[0265] However, EPMA analyzes only the area from the surface down to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from those obtained using other analytical methods. For example, when surface analysis of the positive electrode active material 100 is performed using EPMA, the concentration of each additive element present in the surface layer 100a may be lower than that obtained with XPS.

[0266] ≪Charging curve and dQ / dV vs sV curve≫ In one embodiment of the present invention, the positive electrode active material 100 may exhibit characteristic voltage changes during charging. These voltage changes can be read from the dQ / dVvsV curve, obtained by differentiating the capacitance (Q) of the charging curve with respect to voltage (V) (dQ / dV). For example, around the peak in the dQ / dVvsV curve, a non-equilibrium phase change is thought to occur, resulting in a significant change in the crystal structure. In this specification, a non-equilibrium phase change refers to a phenomenon that causes a nonlinear change in a physical quantity.

[0267] In one embodiment of the present invention, the positive electrode active material 100 may have a broad peak around 4.55V in the dQ / dVvsV curve. The peak around 4.55V reflects the voltage change during the phase transition from the O3 type crystal structure to the O3' type crystal structure. Therefore, a broad peak means that the change in energy required to extract lithium is smaller than when the peak is sharp, i.e., the change in crystal structure is smaller. Smaller changes are preferable because they reduce the influence of shift and volume changes in the CoO2 layer.

[0268] More specifically, in the dQ / dV vs V curve of the charging curve, when the maximum value appearing between 4.5V and 4.6V is defined as the first peak, a full width at half maximum (FMAX) of the first peak of 0.10V or more is considered sufficiently broad and preferable. In this specification, the full width at half maximum (FMAX) of the first peak is defined as the difference between HWHM1, which is the average value of the first peak and the first minimum value when the minimum value of the dQ / dV value appearing between 4.3V and 4.5V is defined as the first minimum value, and HWHM2, which is the average value of the first peak and the second minimum value when the minimum value of the dQ / dV value appearing between 4.6V and 4.8V is defined as the second minimum value.

[0269] When acquiring the dQ / dV vs V curve, charging can be performed using a constant current charge of 10mA / g up to, for example, 4.9V. Furthermore, when acquiring the dQ / dV for the initial charge, it is preferable to discharge the battery to 2.5V at a rate of 20mA / g to 100mA / g before starting the above-mentioned charging.

[0270] The data acquisition interval during charging can be set to, for example, every second or to acquire voltage and current when there is a voltage fluctuation of 1 mV. The integrated value of the current and time is used as the charging capacity.

[0271] The difference between the nth and (n+1)th data points of the above charging capacity data is taken as the nth value of the capacity change dQ. Similarly, the difference between the nth and (n+1)th data points of the above voltage data is taken as the nth value of the voltage change dV.

[0272] However, since the above data is susceptible to the effects of minute noise, the dQ / dV value may be calculated from a moving average of a certain number of intervals for the difference between voltage and charging capacity. The number of intervals can be, for example, 500.

[0273] Specifically, the average value of dQ from the nth to the n+500th value is calculated, and similarly, the average value of dV from the nth to the n+500th value is calculated. The dQ / dV value can be obtained by dividing dQ(average of 500 values) by dV(average of 500 values). Similarly, the voltage on the horizontal axis of the dQ / dV vs V graph can also be calculated using a moving average of 500 intervals. However, when using a moving average of 500 intervals as described above, it is preferable not to use the data from the 501st to the last data point to the last data point in the dQ / dV vs V graph because the noise has a significant effect on this data.

[0274] Furthermore, when analyzing the dQ / dV vs. V curve after multiple charge-discharge cycles, the conditions for these multiple charge-discharge cycles may differ from the charging conditions described above. For example, charging can be performed at an arbitrary voltage (e.g., 4.6V, 4.65V, 4.7V, 4.75V, or 4.8V) with a constant current of 20mA / g to 100mA / g, followed by constant voltage charging until the current value is between 2mA / g and 10mA / g, and discharging can be performed at 2.5V with a constant current of 20mA / g to 100mA / g.

[0275] Furthermore, at around 4.55V, a phase change occurs from the O3 type crystal structure to the O3' type crystal structure, and at this time the O3 type crystal structure is Li x The x value in CoO2 is approximately 0.3. This has the same symmetry as the O3-type crystal structure with x=1 explained in Figure 5, but the distance between CoO2 layers is slightly different. In this specification, when distinguishing between O3-type crystal structures with different x values, the O3-type crystal structure with x=1 is referred to as O3 (2θ=18.85), and the O3-type crystal structure with x=0.3 is referred to as O3 (2θ=18.57). This is because the position of the peak that appears around 2θ=19° in XRD measurements corresponds to the interlayer distance of CoO2 layers.

[0276] ≪Discharge curves and dQ / dV vs. sV curves≫ Further, when the positive electrode active material 100 of one embodiment of the present invention is discharged at a low current of, for example, 40 mA / g or less after charging at a high voltage, a characteristic voltage change may appear near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the range up to 3.5 V at a voltage lower than the peak appearing around 3.9 V in the dQ / dV vs V curve obtained from the discharge curve.

[0277] ≪ESR≫ The positive electrode active material 100 of one embodiment of the present invention preferably contains cobalt and has nickel and magnesium as additive elements. As a result, a part of Co 3+ is replaced by Ni 3+ and a part of Li + is preferably replaced by Mg 2+ . As Li + is replaced by Mg 2+ , the Ni 3+ may be reduced to Ni 2+ . Also, a part of Li + is replaced by Mg 2+ , and accordingly, Co 2+ near Mg 3+ may be reduced to Co 2+ . Further, a part of Co 3+ is replaced by Mg 2+ , and accordingly, Co 2+ near Mg 3+ may be oxidized to Co 4+ .

[0278] Therefore, the positive electrode active material 100 preferably contains any one or more of Ni 2+ , Ni 3+ , Co 2+ and Co 4+ . Also, the spin density attributable to any one or more of Ni 2+ , Ni 3+ , Co 2+ and Co 4+ per unit weight of the positive electrode active material 100 is 2.0×10 17 spins / g or more and 1.0×10 21It is preferable that the spin density is less than or equal to spins / g. Using the positive electrode active material 100 having the aforementioned spin density is preferable because it stabilizes the crystal structure, especially in the charged state. However, if the magnesium concentration is too high, Ni 2+ Ni 3+ Co 2+ and Co 4+ A decrease in spin density may occur due to one or more of the following factors.

[0279] The spin density in the positive electrode active material can be analyzed using methods such as electron spin resonance (ESR).

[0280] ≪Surface roughness and specific surface area≫ In one embodiment of the present invention, the positive electrode active material 100 preferably has a smooth surface with few irregularities. A smooth surface with few irregularities indicates that the effect of the flux described later has been fully exerted, and that the surface of the additive element source and lithium cobaltate has melted. Therefore, it is one factor indicating that the distribution of additive elements in the surface layer 100a is good.

[0281] The smoothness and minimal irregularities of the surface can be determined, for example, from a cross-sectional SEM image or TEM image of the positive electrode active material 100, or from the specific surface area of ​​the positive electrode active material 100.

[0282] For example, the surface smoothness of the positive electrode active material 100 can be quantified from a cross-sectional SEM image, as shown below.

[0283] First, the positive electrode active material 100 is processed using FIB or the like to expose its cross-section. At this time, it is preferable to cover the positive electrode active material 100 with a protective film, protective agent, etc. Next, an SEM image of the interface between the protective film, etc. and the positive electrode active material 100 is taken. Noise processing is performed on the SEM image using image processing software. For example, Gaussian blurring (σ=2) is performed, followed by binarization. Interface extraction is then performed using image processing software. Furthermore, the interface line between the protective film, etc. and the positive electrode active material 100 is selected using an automatic selection tool, etc., and the data is extracted into spreadsheet software, etc. Using the functions of the spreadsheet software, correction is performed from the regression curve (quadratic regression), parameters for roughness calculation are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness of the positive electrode active material at least at 400 nm from the outer circumference of the particle.

[0284] In this embodiment, the root mean square (RMS) surface roughness of the positive electrode active material 100 is preferably less than 3 nm, more preferably less than 1 nm, and more preferably less than 0.5 nm.

[0285] The image processing software used for noise reduction, interface extraction, etc., is not particularly limited, but for example, "ImageJ" described in Non-Patent Documents 6 to 8 can be used. Similarly, the spreadsheet software is not particularly limited, but for example, Microsoft Office Excel can be used.

[0286] For example, the actual specific surface area S measured by the gas adsorption method using the constant-volume method. R And the ideal specific surface area S i The surface smoothness of the positive electrode active material 100 can also be quantified from this ratio.

[0287] Ideal specific surface area S i This is calculated by assuming that all particles have the same diameter as D50, the same weight, and are ideal spheres.

[0288] The median diameter D50 can be measured using a particle size analyzer that employs laser diffraction and scattering methods. The specific surface area can be measured using a specific surface area measuring device that employs a gas adsorption method based on constant volume, for example.

[0289] In one aspect of the present invention, the positive electrode active material 100 has an ideal specific surface area S determined from the median diameter D50. i And the actual specific surface area S R Ratio S R / S i It is preferable that the value is 2.1 or less.

[0290] Alternatively, the surface smoothness of the cathode active material 100 can be quantified from a cross-sectional SEM image using the following method.

[0291] First, a surface SEM image of the positive electrode active material 100 is obtained. A conductive coating may be applied as a pretreatment before observation. It is preferable that the observation surface is perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area should be the same.

[0292] Next, using image processing software (for example, "ImageJ"), the above SEM image is converted to, for example, an 8-bit image (this is called a grayscale image). A grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be represented by 2 to the power of 8 = 256 gradations. Darker areas have a lower number of gradations, and brighter areas have a higher number of gradations. The change in luminance can be quantified in relation to the number of gradations. This numerical value is called the grayscale value. By obtaining the grayscale value, it becomes possible to numerically evaluate the unevenness of the positive electrode active material.

[0293] Furthermore, it becomes possible to represent the brightness changes of the target area using a histogram. A histogram is a three-dimensional representation of the grayscale distribution in a target area, and is also called a brightness histogram. By obtaining a brightness histogram, it becomes possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0294] In one embodiment of the present invention, the positive electrode active material 100 preferably has a difference of 120 or less between the maximum and minimum values ​​of the grayscale values, more preferably 115 or less, and even more preferably 70 or more and 115 or less. Furthermore, the standard deviation of the grayscale values ​​is preferably 11 or less, more preferably 8 or less, and even more preferably 4 or more and 8 or less.

[0295] ≪Current pause method≫ The magnesium and other additive elements present in the surface layer of the positive electrode active material 100 according to one aspect of the present invention may undergo slight changes in their distribution during repeated charging and discharging cycles. For example, the distribution of the additive elements may improve, leading to a decrease in the electron conduction resistance. As a result, the electrical resistance, i.e., the fast-responding resistance component R(0.1s) measured by the current pause method, may decrease in the initial stages of the charge-discharge cycle.

[0296] For example, when comparing the nth charge (where n is a natural number greater than 1) with the (n+1)th charge, the fast-responding resistance component R(0.1s), measured by the current pause method, may be lower in the (n+1)th charge than in the nth charge. Consequently, the discharge capacity in the (n+1)th charge may be higher than that in the nth charge. When n is 1, that is, when comparing the first charge with the second charge, the second charge capacity may be larger, which can occur even with positive electrode active materials that do not contain additive elements. Therefore, it is preferable that n is, for example, between 2 and 10. However, this is not limited to the initial stages of the charge-discharge cycle. A charge-discharge capacity that is approximately equal to the rated capacity, for example, 97% or more of the rated capacity, can be considered the initial stage of the charge-discharge cycle.

[0297] ≪Raman spectroscopy≫ As described above, it is preferable that at least a part of the surface layer portion 100a of the positive electrode active material 100 of one aspect of the present invention has a rock salt-type crystal structure. Therefore, when the positive electrode active material 100 and the positive electrode having the same are analyzed by Raman spectroscopy, it is preferable that a cubic crystal structure including a rock salt type as well as the crystal structure of layered rock salt is observed. In the STEM image and the selected area electron diffraction pattern described later, cobalt substituted at the lithium position with a certain frequency in the depth direction at the time of observation, and cobalt present at the oxygen four-coordination position, etc. cannot be detected as bright spots in the STEM image and the selected area electron diffraction pattern. On the other hand, since Raman spectroscopy is an analysis that captures the vibration mode of bonds such as Co-O, even if the abundance of the corresponding Co-O bond is small, a peak at the wave number of the corresponding vibration mode may be observed. Furthermore, Raman spectroscopy can measure a range of about several μm in area and about 1 μm in depth of the surface layer portion, so it can sensitively capture the state existing only on the particle surface. 2 For example, when the laser wavelength is 532 nm, in layered rock salt-type LiCoO2, peaks (vibration modes: E, A) are observed at 470 cm to 490 cm and 580 cm to 600 cm. On the other hand, in cubic CoO (0 <x <1) (rock salt-type CoO (0 <y <1) or spinel-type Co3O4), a peak (vibration mode: A) is observed at 665 cm to 685 cm.

[0298] For example, when the laser wavelength is 532 nm, in layered rock salt-type LiCoO2, peaks are observed at 470 cm to 490 cm and 580 cm to 600 cm. -1 to 490 cm -1 , 580 cm -1 to 600 cm -1 (vibration mode: E g , A 1g ). On the other hand, in cubic CoO x (0 <x <1) (rock salt-type Co 1-y O (0 <y <1) or spinel-type Co3O4), a peak (vibration mode: A -1 ) is observed at 665 cm -1 to 685 cm 1g .

[0299] Therefore, the integrated intensity of each peak is defined as I1 for 470 cm to 490 cm, I2 for 580 cm to 600 cm, and I3 for 665 cm to 685 cm. -1 to 490 cm -1 , I2 for 580 cm -1 to 600 cm -1 , and I3 for 665 cm -1 to 685 cm -1When I3 is defined as I3, the value of I3 / I2 is preferably between 1% and 10%, and more preferably between 3% and 9%.

[0300] If cubic crystal structures, including rock salt type, are observed within the range described above, it can be said that the surface layer 100a of the positive electrode active material 100 has a rock salt type crystal structure within a favorable range.

[0301] ≪Micro-electron beam diffraction pattern≫ Similar to Raman spectroscopy, it is preferable that the characteristics of the rock salt type crystal structure be observed in the micro-electron diffraction pattern along with the crystal structure of the layered rock salt. However, in STEM images and micro-electron diffraction patterns, taking into account the differences in sensitivity mentioned above, it is preferable that the characteristics of the rock salt type crystal structure do not become too strong in the surface layer 100a, especially at the outermost surface (for example, at a depth of 1 nm from the surface). This is because having additive elements such as magnesium in the lithium layer while maintaining the layered rock salt type crystal structure is preferable to having the outermost surface covered with a rock salt type crystal structure, as this ensures a diffusion pathway for lithium and provides a stronger function in stabilizing the crystal structure.

[0302] Therefore, when obtaining, for example, an ultra-micro electron diffraction pattern from a region with a depth of 1 nm or less from the surface and an ultra-micro electron diffraction pattern from a region with a depth of 3 nm to 10 nm, it is preferable that the difference in lattice constants calculated from these is small.

[0303] For example, the difference in lattice constants calculated from a measurement point at a depth of 1 nm or less from the surface and a measurement point at a depth of 3 nm to 10 nm is preferably 0.1 Å or less for the a-axis and 1.0 Å or less for the c-axis. It is even more preferably 0.05 Å or less for the a-axis and 0.6 Å or less for the c-axis. It is even more preferably 0.04 Å or less for the a-axis and 0.3 Å or less for the c-axis.

[0304] <Additional features> The positive electrode active material 100 may have depressions, cracks, pits, or V-shaped cross-sections. These are defects, and repeated charging and discharging may lead to cobalt leaching, collapse of the crystal structure, cracking of the positive electrode active material 100, and desorption of oxygen. However, if there are embedded portions 102 as shown in Figure 1(A) to fill these defects, cobalt leaching and other issues can be suppressed. Therefore, a positive electrode active material 100 with excellent reliability and cycle characteristics can be obtained.

[0305] As mentioned above, if the additive elements in the positive electrode active material 100 are in excess, they may adversely affect the insertion and removal of lithium. Furthermore, when the positive electrode active material 100 is used in a secondary battery, it may lead to an increase in internal resistance and a decrease in charge / discharge capacity. On the other hand, if there are insufficient elements, they may not be distributed throughout the entire surface layer 100a, and the effect of suppressing the deterioration of the crystal structure may be insufficient. Thus, the additive elements in the positive electrode active material 100 need to be at an appropriate concentration, but adjusting this is not easy.

[0306] Therefore, if the positive electrode active material 100 has regions where the additive elements are unevenly distributed, some of the excess atoms of the additive elements are removed from the interior 100b of the positive electrode active material 100, allowing for an appropriate concentration of the additive elements in the interior 100b. This suppresses the increase in internal resistance and the decrease in charge / discharge capacity when used as a secondary battery. The ability to suppress the increase in the internal resistance of a secondary battery is an extremely desirable characteristic, especially when charging and discharging at high currents, such as 400 mA / g or higher.

[0307] Furthermore, in positive electrode active material 100 having regions where the additive elements are unevenly distributed, it is permissible to mix the additive elements in excess to some extent during the manufacturing process. This is preferable because it widens the margin in production.

[0308] Furthermore, the coating may be attached to at least a portion of the surface of the positive electrode active material 100. Figure 13 shows an example of the positive electrode active material 100 with the coating 104 attached.

[0309] Preferably, the coating portion 104 is formed by the accumulation of decomposition products of the electrolyte and organic electrolyte during charging and discharging. In particular, Li x When charging is repeated such that x in CoO2 is 0.24 or less, the charge-discharge cycle characteristics are expected to improve by having a coating portion derived from the 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 elution of cobalt. The coating portion 104 preferably has carbon, oxygen, and fluorine, for example. Furthermore, when LiBOB and / or SUN (sveronitrile) is used as the electrolyte, a good quality coating portion is more likely to be obtained. Therefore, a coating portion 104 having one or more selected from boron, nitrogen, sulfur, and fluorine may be a good quality coating portion and is therefore preferable. Also, the coating portion 104 does not have to cover the entire surface of the positive electrode active material 100. For example, it is sufficient if it covers 50% or more of the surface of the positive electrode active material 100, more preferably 70% or more, and even more preferably 90% or more.

[0310] Furthermore, the positive electrode active material may develop progressive defects that penetrate deep from the surface into the interior when charged under conditions such as charging at 4.5V or higher, or when charged and discharged in high-temperature environments, such as 45°C or higher. The phenomenon in which defects in the positive electrode active material progress to form holes can also be called pitting corrosion, and the holes generated by this phenomenon are referred to as pits in this specification.

[0311] Figure 14 shows a schematic cross-sectional view of the positive electrode active material 51 having pits. A crystal plane 55 parallel to the arrangement of cations is also shown. Since Figure 14 is a cross-sectional view, pits 54 and 58 are shown as holes, but the shape of these openings is not circular but has depth and a groove-like shape. Also, as shown in pits 54 and 58, unlike recesses 52, they tend to occur parallel to the arrangement of lithium ions.

[0312] Furthermore, the surface layers of the positive electrode active material 51 where the added elements are present are shown as 53 and 56. In the surface layers where pits form, the amount of added elements is less than in 53 and 56, or the concentration is below the detection limit, and it is expected that the function of the barrier film is reduced. In addition, it is thought that the crystal structure of lithium cobalt oxide collapses near where the pits form, resulting in a crystal structure different from that of the layered rock salt type. Since the collapse of the crystal structure inhibits the diffusion and release of lithium ions, which are carrier ions, the pits are considered to be a factor in the deterioration of cycle characteristics.

[0313] The source of the pits may be point defects. It is thought that point defects in the positive electrode active material change with repeated charging and discharging, and are chemically or electrochemically eroded by the surrounding electrolyte, or the material deteriorates, leading to the formation of pits. This deterioration does not occur uniformly on the surface of the positive electrode active material, but rather occurs locally and concentrated.

[0314] Furthermore, as shown in crack 57 in Figure 14, defects such as cracks (also called fissures) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In this specification, cracks and pits are different. Cracks may exist immediately after the positive electrode active material is manufactured, but pits do not. A pit can be described as a hole where several layers of cobalt and oxygen have been removed due to charging and discharging under high voltage conditions of 4.5V or higher or high temperature (45°C or higher), and can also be described as a location where cobalt has dissolved. A crack refers to a new surface created by, for example, the application of physical pressure, or a fissure caused by a grain boundary 101. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In addition, pits may occur from cracks and / or cavities inside the positive electrode active material.

[0315] This embodiment can be used in combination with other embodiments.

[0316] (Embodiment 2) In this embodiment, an example of a method for producing a positive electrode active material 100, which is one embodiment of the present invention, will be described.

[0317] In order to produce a positive electrode active material 100 having the distribution, composition, and / or crystal structure of the added elements as described in the previous embodiment, the method of adding the added elements is important. At the same time, it is also important that the crystallinity of the interior 100b is good.

[0318] Therefore, in the process of preparing the positive electrode active material 100, it is preferable to first synthesize lithium cobalt oxide, and then mix it with the additive element source and perform a heat treatment.

[0319] In a method of synthesizing lithium cobalt oxide containing additive elements by mixing a cobalt source, a lithium source, and an additive element source simultaneously, it is difficult to increase the concentration of the additive elements in the surface layer 100a. Furthermore, if the additive element source is only mixed after the synthesis of lithium cobalt oxide without heating, the additive elements will only adhere to the lithium cobalt oxide without solid dissolving. Without sufficient heating, it is also difficult to properly distribute the additive elements. Therefore, it is preferable to synthesize lithium cobalt oxide, then mix in the additive element source, and then perform a heat treatment. This heat treatment after mixing in the additive element source is sometimes called annealing.

[0320] However, if the annealing temperature is too high, cation mixing occurs, increasing the likelihood that doped elements, such as magnesium, will enter the cobalt site. Magnesium present in the cobalt site is Li x When x in CoO2 is small, it does not maintain the layered rock salt crystal structure of R-3m. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as the reduction of cobalt to divalent cobalt and the evaporation of lithium.

[0321] Therefore, it is preferable to mix a material that functions as a flux with the elemental additive source. Any material that has a lower melting point than lithium cobalt oxide can be considered a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux causes a decrease in the melting points of both the elemental additive source and lithium cobalt oxide. By lowering the melting points, it becomes easier to distribute the elemental additive well at a temperature where cation mixing is less likely to occur.

[0322] [Initial heating] Furthermore, it is preferable to heat the lithium cobalt oxide after synthesis and before mixing in the additive elements. This heating is sometimes referred to as initial heating.

[0323] Initial heating causes lithium to desorb from a portion of the surface layer 100a of lithium cobalt oxide, resulting in an even better distribution of the added elements.

[0324] More specifically, the following mechanism is thought to make it easier to differentiate the distribution of added elements through initial heating. First, lithium is desorbed from a portion of the surface layer 100a by initial heating. Next, lithium cobalt oxide having this lithium-deficient surface layer 100a is mixed with a source of added elements, including a nickel source, an aluminum source, and a magnesium source, and heated. Of the added elements, magnesium is a divalent typical element, and nickel is a transition metal but easily forms a divalent ion. Therefore, in a portion of the surface layer 100a, Mg 2+ and Ni 2+ And, due to lithium deficiency, Co 2+ A rock salt-type phase is formed, which has the following characteristics. However, since this phase is formed only in a part of the surface layer 100a, it may not be clearly visible in electron microscope images such as STEM and electron diffraction patterns.

[0325] Of the additive elements, nickel readily dissolves in the surface layer 100a of layered rock salt-type lithium cobalt oxide and diffuses to the interior 100b, but tends to remain in the surface layer 100a if a portion of the surface layer 100a is rock salt-type. Therefore, initial heating can make it easier for divalent additive elements, including nickel, to remain in the surface layer 100a. This effect of initial heating is particularly significant on surfaces other than the (001) orientation of the positive electrode active material 100 and on its surface layer 100a.

[0326] Furthermore, in these rock salt types, the bond distance between metallic Me and oxygen (Me-O distance) tends to be longer than in layered rock salt types.

[0327] For example, rock salt mold mayonnaise0.5 Mg 0.5 The Me-O distance in O is 2.09 Å, and the Me-O distance in rock salt type MgO is 2.11 Å. Furthermore, even if a spinel-type phase is formed in a part of the surface layer 100a, the Me-O distance of spinel-type NiAl2O4 is 2.0125 Å, and the Me-O distance of spinel-type MgAl2O4 is 2.02 Å. In both cases, the Me-O distance exceeds 2 Å. Note that 1 Å = 10⁻¹⁰ -10 It is m.

[0328] On the other hand, in layered rock salt, the bond distance between metals other than lithium and oxygen is shorter than described above. For example, the Al-O distance in layered rock salt LiAlO2 is 1.905 Å (Li-O distance is 2.11 Å). Also, the Co-O distance in layered rock salt LiCoO2 is 1.9224 Å (Li-O distance is 2.0916 Å).

[0329] According to Shannon et al., Acta A 32 (1976) 751, the ionic radius of 6-coordinate aluminum is 0.535 Å, and the ionic radius of 6-coordinate oxygen is 1.4 Å, with their sum being 1.935 Å.

[0330] From the above, it is considered that aluminum exists more stably at sites other than lithium in the layered rock salt type than in the rock salt type. Therefore, aluminum is more likely to be distributed in deeper regions with the layered rock salt type and / or in the interior 100b of the surface layer 100a than in the region closer to the surface with the rock salt type phase.

[0331] Furthermore, initial heating is expected to enhance the crystallinity of the layered rock salt-type crystalline structure within the internal 100b layer.

[0332] Therefore, especially Li x When x in CoO2 is, for example, between 0.15 and 0.17, it is preferable to perform this initial heating to produce a positive electrode active material 100 having a monoclinic O1(15) type crystal structure.

[0333] However, initial heating is not always necessary. By controlling the atmosphere, temperature, time, etc., in other heating processes, such as annealing, Li x When x in CoO2 is small, it may be possible to produce a positive electrode active material 100 having the O3' type and / or monoclinic O1(15) type.

[0334] Method for preparing positive electrode active material 1 Method 1 for producing the positive electrode active material 100, which undergoes annealing and initial heating, will be explained using Figures 15(A) to 15(C).

[0335] <Step S11> In step S11 shown in Figure 15(A), lithium sources (Li sources) and cobalt sources (Co sources) are prepared as the starting materials, lithium and transition metals, respectively.

[0336] As a lithium source, it is preferable to use a lithium-containing compound, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. The lithium source should preferably have high purity; for example, a material with a purity of 99.99% or higher is preferable.

[0337] As a cobalt source, it is preferable to use a compound containing cobalt, such as cobalt oxide or cobalt hydroxide.

[0338] The cobalt source should preferably have high purity; for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N5 (99.995%) or higher, and even more preferably 5N (99.999%) or higher should be used. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased and / or the reliability of the secondary battery is improved.

[0339] In addition, it is preferable that the cobalt source has high crystallinity, for example, that it has single crystal grains. The crystallinity of the cobalt source can be evaluated by TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle scattering annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, etc., or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. The above methods for evaluating crystallinity can be applied not only to cobalt sources but also to the evaluation of other materials.

[0340] <Step S12> Next, as step S12 shown in Figure 15(A), the lithium source and cobalt source are crushed and mixed to prepare a mixed material. Crushing and mixing can be done dry or wet. Wet crushing is preferred because it allows for finer crushing. If wet crushing is used, a solvent is prepared. Suitable solvents include ketones such as acetone, alcohols such as ethanol and isopropanol, ethers, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. It is more preferable to use an aprotic solvent that does not react easily with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and cobalt source with dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then crush and mix them. By using dehydrated acetone of such purity as described above, the amount of impurities that may be introduced can be reduced.

[0341] For grinding and mixing, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use aluminum oxide balls or zirconium oxide balls as the grinding media. Zirconium oxide balls are preferable because they produce less impurity. Also, when using a ball mill or a bead mill, the peripheral speed should be set to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0342] <Step S13> Next, as step S13 shown in Figure 15(A), the mixed material is heated. The heating is preferably carried out at 800°C to 1100°C, more preferably at 900°C to 1000°C, and even more preferably at around 950°C. If the temperature is too low, the decomposition and melting of the lithium source and cobalt source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to the evaporation of lithium from the lithium source and / or the excessive reduction of cobalt. For example, cobalt may change from trivalent to divalent, inducing oxygen defects, etc.

[0343] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. For example, the heating time should ideally be between 1 hour and 100 hours, and even more preferably between 2 hours and 20 hours.

[0344] The heating rate depends on the target temperature, but a rate between 80°C / h and 250°C / h is generally recommended. For example, when heating to 1000°C for 10 hours, a heating rate of 200°C / h is appropriate.

[0345] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or lower, more preferably an atmosphere with a dew point of -80°C or lower. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In order to suppress impurities that may be mixed into the material, the concentrations of impurities such as CH4, CO, CO2, and H2 in the heating atmosphere should be kept below 5 ppb (parts per billion) each.

[0346] An atmosphere containing oxygen is preferred as the heating atmosphere. For example, one method is to continuously introduce dry air into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flowing through the chamber is called flow.

[0347] When the heating atmosphere is an oxygen-containing atmosphere, a method that does not involve flowing oxygen is also acceptable. For example, the reaction chamber can be depressurized and then filled with oxygen (which can also be called purging), preventing the oxygen from entering or leaving the reaction chamber. For instance, the reaction chamber can be depressurized to -970 hPa and then filled with oxygen up to 50 hPa.

[0348] After heating, natural cooling is acceptable, but it is preferable that the cooling time from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary; it is sufficient if it cools to a temperature acceptable for the next step.

[0349] Heating in this process may be carried out using a rotary kiln or a roller hearth kiln. When using a rotary kiln, heating can be performed while stirring, whether in a continuous or batch system.

[0350] For heating, an aluminum oxide crucible is preferred. Aluminum oxide crucibles are made of a material that does not easily release impurities. In this embodiment, an aluminum oxide crucible with a purity of 99.9% is used. It is preferable to place a lid on the crucible before heating. This prevents the material from volatilizing.

[0351] Furthermore, it is preferable to use a used crucible rather than a new one. In this specification, a new crucible is defined as one that has undergone the heating process of adding lithium, transition metal M, and / or additive elements two or fewer times. A used crucible is defined as one that has undergone the heating process of adding lithium, transition metal M, and / or additive elements three or more times. This is because, when a new crucible is used, there is a risk that some of the material, including lithium fluoride, may be absorbed, diffused, migrated, and / or adhered to the casing during heating. If some of the material is lost in this way, there is a growing concern that the distribution of elements on the surface of the positive electrode active material will not be within a desirable range. On the other hand, this risk is less with a used crucible.

[0352] After heating is complete, the material may be crushed and sieved as needed. When collecting the heated material, it may be transferred from the crucible to a mortar before collection. It is preferable to use an aluminum oxide mortar. Aluminum oxide mortars are made of a material that does not easily release impurities. Specifically, an aluminum oxide mortar with a purity of 90% or higher, preferably 99% or higher, should be used. In addition, the same heating conditions as in step S13 can be applied to the heating processes described later, other than step S13.

[0353] <Step S14> Through the above process, lithium cobalt oxide (LiCoO2) shown in step S14 of Figure 15(A) can be synthesized.

[0354] Although examples of producing composite oxides by solid-phase methods have been shown as in steps S11 to S14, composite oxides may also be produced by coprecipitation or by hydrothermal methods.

[0355] <Step S15> Next, as shown in step S15 in Figure 15(A), the lithium cobalt oxide is heated. Because this is the first heating of the lithium cobalt oxide, the heating in step S15 is sometimes called initial heating. Alternatively, because it is heated before step S20 shown below, it may be called preheating or pretreatment.

[0356] As described above, initial heating causes lithium to be released from a portion of the surface layer 100a of the lithium cobalt oxide. It is also expected to improve the crystallinity of the interior 100b. Furthermore, the lithium source and / or cobalt source prepared in step S11, etc., may contain impurities. Initial heating makes it possible to reduce impurities from the lithium cobalt oxide completed in step S14.

[0357] Furthermore, initial heating has the effect of smoothing the surface of lithium cobalt oxide. A smooth surface means that there are few irregularities, the composite oxide is generally rounded, and the corners are also rounded. In addition, a smooth surface is defined as having few foreign substances adhering to it. Foreign substances are thought to be a cause of irregularities, so it is preferable that they do not adhere to the surface.

[0358] This initial heating does not require a lithium compound source, nor a source of added elements, nor a material that functions as a flux.

[0359] If the heating time in this process is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, it can be performed by selecting from the heating conditions described in step S13. To add to those heating conditions, the heating temperature in this process should be lower than the temperature in step S13 in order to maintain the crystal structure of the composite oxide. Also, the heating time in this process should be shorter than the time in step S13 in order to maintain the crystal structure of the composite oxide. For example, heating at a temperature of 700°C to 1000°C for 2 to 20 hours is recommended.

[0360] Furthermore, the effect of increasing the crystallinity of internal 100b is, for example, the effect of mitigating strain, displacement, etc., that originate from the difference in shrinkage, etc., of the lithium cobalt oxide prepared in step S13.

[0361] In lithium cobalt oxide, heating in step S13 may create a temperature difference between the surface and the interior. This temperature difference can induce a difference in shrinkage. It is thought that the difference in shrinkage occurs because the fluidity of the surface and the interior differs due to the temperature difference. The energy associated with the difference in shrinkage creates a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and the energy associated with it is sometimes called strain energy. The internal stress is removed by the initial heating in step S15, or in other words, the strain energy is considered to be homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in lithium cobalt oxide is relieved. Consequently, the surface of the lithium cobalt oxide may become smoother. This is also referred to as the surface being improved. In other words, it is thought that after step S15, the difference in shrinkage that occurred in the lithium cobalt oxide is relieved, and the surface of the composite oxide becomes smoother.

[0362] Furthermore, the difference in shrinkage can cause microscopic displacements in the lithium cobalt oxide, such as crystal displacements. This process is also recommended to reduce such displacements. This process makes it possible to homogenize the displacements of the composite oxide. When the displacements are homogenized, the surface of the composite oxide may become smoother. This can also be described as the crystal grains being aligned. In other words, it is believed that step S15 alleviates the crystal displacements that have occurred in the composite oxide, resulting in a smoother surface.

[0363] Using lithium cobalt oxide, which has a smooth surface, as the positive electrode active material reduces degradation during charging and discharging in a secondary battery and prevents cracking of the positive electrode active material.

[0364] Alternatively, pre-synthesized lithium cobaltate may be used as step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on the pre-synthesized lithium cobaltate, lithium cobaltate with a smooth surface can be obtained.

[0365] <Step S20> Next, as shown in step S20, it is preferable to add element A to the lithium cobalt oxide that has undergone initial heating. Adding element A to lithium cobalt oxide that has undergone initial heating allows for even addition of element A. Therefore, it is preferable to add element A after initial heating. The step of adding element A will be explained using Figures 15(B) and 15(C).

[0366] <Step S21> In step S21 shown in Figure 15(B), a source of additive element A (source A) to be added to lithium cobalt oxide is prepared. A lithium source may also be prepared together with the additive element source A.

[0367] As additive element A, the additive elements described in the previous embodiment, such as additive element X and additive element Y, can be used. Specifically, one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. Alternatively, one or two selected from bromine and beryllium can also be used.

[0368] When magnesium is selected as the additive element, the additive element source can be called a magnesium source. Suitable magnesium sources include magnesium fluoride, magnesium oxide, magnesium hydroxide, or magnesium carbonate. Multiple magnesium sources may also be used.

[0369] When fluorine is selected as the additive element, the additive element source can be called a fluorine source. Examples of such 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). Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the heating process described later.

[0370] Magnesium fluoride can be used as both a fluorine source and a magnesium source. Lithium fluoride can also be used as a lithium source. Another lithium source used in step S21 is lithium carbonate.

[0371] The fluorine source may also be a gas, such as fluorine (F2), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF2, O2F2, O3F2, O4F2, O5F2, O6F2, O2F), which may be mixed into the atmosphere during the heating process described later. Multiple fluorine sources may also be used.

[0372] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF2) is prepared as both the fluorine source and the magnesium source. The effect of lowering the melting point is greatest when lithium fluoride and magnesium fluoride are mixed in a molar ratio of approximately LiF:MgF2 = 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be in excess and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably 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=approximately 0.33). In this specification, "approximately" means a value greater than 0.9 times and less than 1.1 times the value.

[0373] <Step S22> Next, in step S22 shown in Figure 15(B), the magnesium source and the fluorine source are crushed and mixed. This step can be performed by selecting from the crushing and mixing conditions described in step S12.

[0374] <Step S23> Next, in step S23 shown in Figure 15(B), the material that has been crushed and mixed above is recovered to obtain the additive element A source (A source). The additive element A source shown in step S23 has multiple starting materials and can be called a mixture.

[0375] The particle size of the above mixture is preferably such that the D50 (median diameter) is 600 nm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less. Even when one material is used as the source of the added element, the D50 (median diameter) is preferably such that it is 600 nm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less.

[0376] Such a finely powdered mixture (including cases where only one additive element is present) makes it easier to uniformly adhere the mixture to the surface of the lithium cobalt oxide particles when mixed with lithium cobalt oxide in a later process. When the mixture is uniformly adhered to the surface of the lithium cobalt oxide particles, it is preferable because it makes it easier to uniformly distribute or diffuse the additive element onto the surface layer 100a of the composite oxide after heating.

[0377] <Step S21> A different process from that shown in Figure 15(B) will be explained using Figure 15(C). In step S21 shown in Figure 15(C), four types of additive element sources are prepared to be added to lithium cobalt oxide. In other words, the types of additive element sources in Figure 15(C) are different from those in Figure 15(B). A lithium source may also be prepared along with the additive element sources.

[0378] Four types of additive element sources are prepared: a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source). The magnesium and fluorine sources can be selected from the compounds described in Figure 15(B). Nickel sources such as nickel oxide and nickel hydroxide can be used. Aluminum sources such as aluminum oxide and aluminum hydroxide can be used.

[0379] <Steps S22 and S23> Steps S22 and S23 shown in Figure 15(C) are the same as the steps described in Figure 15(B).

[0380] <Step S31> Next, in step S31 shown in Figure 15(A), lithium cobalt oxide is mixed with the additive element A source (A source). The ratio of the number of cobalt atoms Co in lithium cobalt oxide to the number of magnesium atoms Mg in the additive element A source is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0381] The mixing in step S31 is preferably carried out under milder conditions than the mixing in step S12 in order to avoid destroying the shape of the lithium cobalt oxide particles. For example, it is preferable to use conditions with a lower rotation speed or shorter time than the mixing in step S12. Also, dry mixing is generally milder than wet mixing. For mixing, for example, a ball mill or a bead mill can be used. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0382] In this embodiment, the mixing is performed dry using a ball mill with zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour. The mixing is carried out in a dry room with a dew point of -100°C or higher and -10°C or lower.

[0383] <Step S32> Next, in step S32 of Figure 15(A), the mixed materials are recovered to obtain mixture 903. During recovery, if necessary, the materials may be crushed and then sieved.

[0384] Although Figures 15(A) to 15(C) illustrate a manufacturing method in which additive elements are added only after initial heating, the present invention is not limited to the above method. Additive elements may be added at other times, or added in multiple steps. The timing may also be varied depending on the element.

[0385] For example, the additive elements may be added to the lithium source and cobalt source at step S11, that is, at the stage of the starting materials for the composite oxide. Then, lithium cobalt oxide containing the additive elements can be obtained in step S13. In this case, it is not necessary to separate the processes of steps S11 to S14 from the processes of steps S21 to S23. This can be said to be a simple and highly productive method.

[0386] Alternatively, lithium cobalt oxide containing some of the additive elements may be used. For example, if lithium cobalt oxide with magnesium and fluorine added is used, steps S11 to S14 and some steps in S20 can be omitted. This can be considered a simple and highly productive method.

[0387] Alternatively, after heating in step S15 with lithium cobalt oxide to which magnesium and fluorine have been added beforehand, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added as in step S20.

[0388] <Step S33> Next, in step S33 shown in Figure 15(A), the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. A heating time of 2 hours or more is preferable.

[0389] Let me add some information about the heating temperature. The lower limit of the heating temperature in step S33 must be above the temperature at which the reaction between lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds is any temperature at which interdiffusion of the elements present in lithium cobalt oxide and the additive element source occurs, and it may be lower than the melting temperature of these materials. Let me explain using oxides as an example, but the melting temperature T m 0.757 times (Tammann temperature T) d It is known that solid-phase diffusion occurs from ). Therefore, the heating temperature in step S33 should be 650°C or higher.

[0390] Of course, the reaction proceeds more easily if the heating temperature is above the melting point of one or more of the materials selected from the mixture 903. For example, if LiF and MgF2 are used as the additive element sources, the eutectic point of LiF and MgF2 is around 742°C, so it is preferable to set the lower limit of the heating temperature in step S33 to 742°C or higher.

[0391] Furthermore, when mixture 903 is obtained by mixing LiCoO2:LiF:MgF2 in a molar ratio of 100:0.33:1, an endothermic peak is observed around 830°C in differential scanning calorimetry (DSC measurement). Therefore, a lower limit of the heating temperature of 830°C or higher is more preferable.

[0392] A higher heating temperature is preferable because it facilitates the reaction, shortens the heating time, and increases productivity.

[0393] The upper limit of the heating temperature should be below the decomposition temperature of lithium cobalt oxide (1130°C). At temperatures near the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit in small amounts. Therefore, a temperature of 1000°C or lower is more preferable, 950°C or lower is even more preferable, and 900°C or lower is even more preferable.

[0394] Based on these considerations, the heating temperature in step S33 is preferably 650°C to 1130°C, more preferably 650°C to 1000°C, even more preferably 650°C to 950°C, and even more preferably 650°C to 900°C. Furthermore, 742°C to 1130°C is preferred, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, 800°C to 1100°C, 830°C to 1130°C are preferred, more preferably 830°C to 1000°C, even more preferably 830°C to 950°C, and even more preferably 830°C to 900°C. The heating temperature in step S33 should be higher than that in step S13.

[0395] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride, which may be caused by the fluorine source, to an appropriate range.

[0396] In the manufacturing method described in this embodiment, some materials, such as LiF (a fluorine source), may function as a flux. This function allows the heating temperature to be lowered to below the decomposition temperature of lithium cobalt oxide, for example, between 742°C and 950°C, enabling the distribution of additive elements, including magnesium, to the surface layer and producing a positive electrode active material with good properties.

[0397] However, since LiF is less dense than oxygen in its gaseous state, it may volatilize when heated, and if it volatilizes, the amount of LiF in mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, Li on the surface of LiCoO2 may react with F from the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, the same need to suppress volatilization is necessary.

[0398] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 while the partial pressure of LiF in the heating furnace is high. Such heating can suppress the volatilization of LiF in the mixture 903.

[0399] In this process, it is preferable to heat the mixture 903 particles in such a way that they do not stick together. If the mixture 903 particles stick together during heating, the contact area with oxygen in the atmosphere decreases, and the diffusion pathway of the added elements (e.g., fluorine) is obstructed, which may worsen the distribution of added elements (e.g., magnesium and fluorine) to the surface layer.

[0400] Furthermore, it is believed that if the additive elements (e.g., fluorine) are uniformly distributed on the surface, a smooth positive electrode active material with few irregularities can be obtained. Therefore, in order to maintain or further improve the smooth surface after heating in step S15 of this process, it is preferable that the particles of mixture 903 do not stick together.

[0401] Furthermore, when heating with a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere inside the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to purge the atmosphere initially and then not allow the atmosphere to flow after introducing the oxygen atmosphere into the kiln. If oxygen flows, the fluorine source may evaporate, which is undesirable for maintaining surface smoothness.

[0402] When heating by roller hearth kiln, for example, the mixture 903 can be heated in an atmosphere containing LiF by placing a lid on the container containing the mixture 903.

[0403] A note regarding heating time: The heating time varies depending on conditions such as the heating temperature, the size of the lithium cobalt oxide in step S14, and its composition. When the lithium cobalt oxide is small, a lower temperature or shorter time may be preferable than when it is large.

[0404] In step S14 of Figure 15(A), if the median diameter (D50) of the lithium cobalt oxide is approximately 12 μm, the heating temperature is preferably, for example, 650°C to 950°C. The heating time is preferably, for example, 3 hours to 60 hours, more preferably 10 hours to 30 hours, and even more preferably around 20 hours. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0405] On the other hand, if the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 650°C to 950°C. The heating time is preferably, for example, 1 hour to 10 hours, and more preferably about 5 hours. The cooling time after heating is preferably, for example, 10 hours to 50 hours.

[0406] <Step S34> Next, in step S34 shown in Figure 15(A), the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sift the recovered particles. Through the above steps, one embodiment of the positive electrode active material 100 of the present invention can be produced. The positive electrode active material of one embodiment of the present invention has a smooth surface.

[0407] Method for preparing positive electrode active material (2) Next, a method 2 for producing a positive electrode active material, which is an embodiment of the present invention and differs from method 1 for producing a positive electrode active material, will be described using Figures 16 to 17(C). Method 2 for producing a positive electrode active material differs mainly from method 1 in the number of times the additive elements are added and the mixing method. Other details can be referenced from the description of method 1.

[0408] In Figure 16, steps S11 to S15 are performed in the same manner as in Figure 15(A) to prepare lithium cobalt oxide that has undergone initial heating.

[0409] <Step S20a> Next, as shown in step S20a, it is preferable to add element A1 to the lithium cobalt oxide that has undergone initial heating.

[0410] <Step S21> In step S21 shown in Figure 17(A), a first additive element source is prepared. The first additive element source can be selected from additive element A described in step S21 shown in Figure 15(B). For example, as additive element A1, one or more selected from magnesium, fluorine, and calcium can be suitably used. Figure 17(A) illustrates the case where a magnesium source (Mg source) and a fluorine source (F source) are used as the first additive element source.

[0411] Steps S21 to S23 shown in Figure 17(A) can be carried out under the same conditions as steps S21 to S23 shown in Figure 15(B). As a result, the additive element source (A1 source) can be obtained in step S23.

[0412] Furthermore, steps S31 to S33 shown in Figure 16 can be carried out in the same manner as steps S31 to S33 shown in Figure 15(A).

[0413] <Step S34a> Next, the material heated in step S33 is recovered to produce lithium cobalt oxide containing additive element A1. This is also called the second composite oxide to distinguish it from the composite oxide of step S14.

[0414] <Step S40> In step S40 shown in Figure 16, additive element A2 is added. This will be explained with reference to Figures 17(B) and 17(C).

[0415] <Step S41> In step S41 shown in Figure 17(B), a second additive element source is prepared. The second additive element source can be selected from additive element A described in step S21 shown in Figure 15(B). For example, as additive element A2, one or more can be suitably selected from nickel, titanium, boron, zirconium, and aluminum. Figure 17(B) illustrates the case where a nickel source (Ni source) and an aluminum source (Al source) are used as the second additive element source.

[0416] Steps S41 to S43 shown in Figure 17(B) can be carried out under the same conditions as steps S21 to S23 shown in Figure 15(B). As a result, an additive element source (A2 source) can be obtained in step S43.

[0417] Furthermore, Figure 17(C) shows a modified version of the steps described using Figure 17(B). In step S41 shown in Figure 17(C), a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are each pulverized independently. As a result, in step S43, multiple second additive element sources (A2 sources) are prepared. The steps in Figure 17(C) differ from those in Figure 17(B) in that the additive elements are pulverized independently in step S42a.

[0418] <Steps S51 to S53> Next, steps S51 to S53 shown in Figure 16 can be carried out under the same conditions as steps S31 to S34 shown in Figure 15(A). The conditions for step S53, which relates to the heating process, can be at a lower temperature and for a shorter time than in step S33. Through these steps, a positive electrode active material 100 according to one embodiment of the present invention can be produced in step S54. The positive electrode active material according to one embodiment of the present invention has a smooth surface.

[0419] As shown in Figures 16 and 17, in manufacturing method 2, the additive elements to lithium cobalt oxide are introduced separately as additive element A1 and additive element A2. By introducing them separately, the depth profile of each additive element can be changed. For example, it is possible to profile additive element A1 so that its concentration is higher in the surface layer than in the interior, and additive element A2 so that its concentration is higher in the interior than in the surface layer.

[0420] As demonstrated in this embodiment, a positive electrode active material with a smooth surface can be obtained through the initial heating process.

[0421] The initial heating described in this embodiment is performed on lithium cobalt oxide. Therefore, it is preferable that the initial heating is performed under conditions that are lower than the heating temperature required to obtain lithium cobalt oxide and shorter than the heating time required to obtain lithium cobalt oxide. The step of adding the additive element to the lithium cobalt oxide is preferably performed after the initial heating. This addition step can be divided into two or more steps. Following this order of steps is preferable because it maintains the surface smoothness obtained during the initial heating.

[0422] This embodiment can be used in combination with other embodiments.

[0423] (Embodiment 3) In this embodiment, an example of a secondary battery according to one aspect of the present invention will be described using Figures 18 to 21.

[0424] <Example of a secondary battery configuration 1> The following explanation uses a secondary battery, in which the positive electrode, negative electrode, and electrolyte are enclosed in an outer casing, as an example.

[0425] [Positive electrode] The positive electrode comprises 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 additive) and a binder. The positive electrode active material used is a positive electrode active material manufactured using the manufacturing method described in the previous embodiment.

[0426] Alternatively, the positive electrode active material described in the previous embodiment may be used in combination with other positive electrode active materials.

[0427] Other positive electrode active materials include composite oxides having olivine-type crystal structures, layered rock salt-type crystal structures, or spinel-type crystal structures. Examples include compounds such as LiFePO4, LiFeO2, LiNiO2, LiMn2O4, V2O5, Cr2O5, and MnO2.

[0428] In addition, it is preferable to mix a lithium-containing material having a spinel-type crystal structure containing manganese such as LiMn2O4 with lithium nickelate (LiNiO2 or LiNi 1-x M x O2 (0 < x < 1) (M = Co, Al, etc.)). By adopting such a configuration, the characteristics of the secondary battery can be improved.

[0429] In addition, as another positive electrode active material, a lithium manganese composite oxide represented by the composition formula Li a Mn b M c O d can be used. Here, the element M is preferably a metal element selected from elements other than lithium and manganese, or silicon or phosphorus, and more preferably nickel. Further, when measuring the entire particles of the lithium manganese composite oxide, it is preferable to satisfy 0 < a / (b + c) < 2, c > 0, and 0.26 ≦ (b + c) / d < 0.5 during discharge. The composition of metals, silicon, phosphorus, etc. in the entire particles of the lithium manganese composite oxide can be measured using, for example, ICP-MS (inductively coupled plasma mass spectrometer). Also, the oxygen composition of the entire particles of the lithium manganese composite oxide can be measured using, for example, EDX (energy dispersive X-ray analysis method). Further, it can be determined by using valence evaluation of melting gas analysis and XAFS (X-ray absorption fine structure) analysis in combination with ICP-MS analysis. The lithium manganese composite oxide refers to an oxide containing at least lithium and manganese, and may 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.

[0430] Hereinafter, as an example, a cross-sectional configuration example when graphene or a graphene compound is used as the conductive material in the active material layer 200 will be described.

[0431] Figure 18(A) shows a longitudinal cross-sectional view of the active material layer 200. The active material layer 200 includes granular positive electrode active material 100, graphene or graphene compound 201 as a conductive material, and a binder (not shown).

[0432] In this specification, graphene compound 201 includes multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multigraphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multigraphene oxide, graphene quantum dots, etc. A graphene compound is a material having carbon, having a plate-like or sheet-like shape, and having a two-dimensional structure formed by a six-membered carbon ring. This two-dimensional structure formed by a six-membered carbon ring may be called a carbon sheet. Graphene compounds may have functional groups. It is also preferable that graphene compounds have a bent shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0433] In this specification, graphene oxide refers to a material having carbon and oxygen, having a sheet-like structure, and possessing functional groups, particularly epoxy groups, carboxyl groups, or hydroxyl groups.

[0434] In this specification, reduced graphene oxide refers to a material having carbon and oxygen, having a sheet-like shape, and possessing a two-dimensional structure formed by a six-membered carbon ring. Reduced graphene oxide can function as a single sheet, but multiple sheets may be stacked. Preferably, reduced graphene oxide has a portion where the carbon concentration is greater than 80 atomic%, and the oxygen concentration is between 2 atomic% and 15 atomic%. With such carbon and oxygen concentrations, it can function as a highly conductive material even in small quantities. Furthermore, preferably, reduced graphene oxide has a G / D intensity ratio of 1 or more in the Raman spectrum. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small quantities.

[0435] Graphene compounds may possess excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Furthermore, graphene compounds may have a sheet-like shape. Graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. They may also exhibit very high conductivity even when thin, allowing for the efficient formation of conductive paths within the active material layer with a small amount. Therefore, using graphene compounds as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene compound covers 80% or more of the active material's surface area. It is also preferable that the graphene compound adheres to at least a portion of the active material particles. Furthermore, it is preferable that the graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the irregularities of a single active material particle or the irregularities formed by multiple active material particles. It is also preferable that the graphene compound surrounds at least a portion of the active material particles. Furthermore, the graphene compound may have holes.

[0436] When using active material particles with small particle sizes, such as 1 μm or less, the specific surface area of ​​the active material particles is large, and more conductive paths are required to connect the active material particles. In such cases, it is preferable to use a graphene compound that can efficiently form conductive paths even in small amounts.

[0437] Due to the properties described above, graphene compounds are particularly effective as conductive materials in secondary batteries that require rapid charging and rapid discharging. For example, secondary batteries for two-wheeled or four-wheeled vehicles, and secondary batteries for drones, may require rapid charging and rapid discharging characteristics. Mobile electronic devices may also require rapid charging characteristics. Rapid charging and discharging refers to charging and discharging at, for example, 200 mA / g, 400 mA / g, or 1000 mA / g or higher.

[0438] In the longitudinal section of the active material layer 200, as shown in Figure 18(B), sheet-like graphene or graphene compound 201 is dispersed approximately uniformly within the active material layer 200. In Figure 18(B), graphene or graphene compound 201 is schematically represented by thick lines, but in reality, it is a thin film having a single or multilayer thickness of carbon molecules. Multiple layers of graphene or graphene compound 201 are formed to partially cover multiple granular positive electrode active materials 100, or to adhere to the surface of multiple granular positive electrode active materials 100, and are therefore in surface contact with each other.

[0439] Here, multiple graphenes or graphene compounds can bond together to form a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net). When the active material is coated with the graphene net, the graphene net can also function as a binder that binds the active materials together. Therefore, the amount of binder can be reduced or eliminated, thereby improving the ratio of active material to electrode volume and electrode weight. In other words, the discharge capacity of the secondary battery can be increased.

[0440] Here, it is preferable to use graphene oxide as graphene or graphene compound 201, mix it with the active material to form a layer that becomes the active material layer 200, and then reduce it. In other words, it is preferable that the completed active material layer has reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in polar solvents, for the formation of graphene or graphene compound 201, the graphene or graphene compound 201 can be dispersed approximately uniformly within the active material layer 200. By volatilizing and removing the solvent from the dispersion medium containing the uniformly dispersed graphene oxide and reducing the graphene oxide, the graphene or graphene compound 201 remaining in the active material layer 200 partially overlaps and is dispersed to the extent that it is in surface contact with each other, thereby forming a three-dimensional conductive path. The reduction of graphene oxide may be carried out, for example, by heat treatment or by using a reducing agent.

[0441] Therefore, unlike granular conductive materials such as acetylene black, which make point contact with the active material, graphene or graphene compound 201 enables surface contact with low contact resistance. As a result, the electrical conductivity between the granular positive electrode active material 100 and graphene or graphene compound 201 can be improved with a smaller amount than with conventional conductive materials. Consequently, the ratio of positive electrode active material 100 in the active material layer 200 can be increased. This allows for an increase in the discharge capacity of the secondary battery.

[0442] Furthermore, by using a spray-drying device beforehand, the entire surface of the active material can be covered with a graphene compound, which is a conductive material, forming a coating, and conductive paths can also be formed between the active material particles using the graphene compound.

[0443] Alternatively, the active material layer 200 may be made by mixing the graphene compound with the material used to form the graphene compound. For example, particles used as a catalyst when forming the graphene compound may be mixed together with the graphene compound. Examples of catalysts used when forming the graphene compound include silicon dioxide (SiO2, SiO2). x Examples of particles include those having (x<2), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The median diameter (D50) of the particles is preferably 1 μm or less, and more preferably 100 nm or less.

[0444] [Binder] As a binder, it is preferable to use rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer. Fluororubber can also be used as a binder.

[0445] Furthermore, it is preferable to use a water-soluble polymer as the binder. Examples of water-soluble polymers include polysaccharides. One or more of the following polysaccharides can be used: carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, diacetylcellulose, cellulose derivatives such as regenerated cellulose, and starch. It is even more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0446] Alternatively, it is preferable to use materials such as polystyrene, methyl polyacrylate, 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, or nitrocellulose as the binder.

[0447] You may use a combination of several of the binders mentioned above.

[0448] For example, a material with particularly excellent viscosity-modifying properties may be used in combination with other materials. For instance, rubber materials have excellent adhesive and / or elastic properties, but their viscosity can be difficult to adjust when mixed with a solvent. In such cases, it is preferable to mix them with a material with particularly excellent viscosity-modifying properties. As a material with particularly excellent viscosity-modifying properties, a water-soluble polymer may be used. As a water-soluble polymer with particularly excellent viscosity-modifying properties, the aforementioned polysaccharides, such as carboxymethylcellulose (CMC), methylcellulose, ethylcellulose, hydroxypropylcellulose, and diacetylcellulose, cellulose derivatives such as regenerated cellulose, and starch can be used.

[0449] Furthermore, cellulose derivatives such as carboxymethylcellulose can be made more effective as viscosity modifiers by increasing their solubility, for example, by using salts such as sodium salts and ammonium salts of carboxymethylcellulose. Increased solubility also improves the dispersibility of the active material and other components when preparing electrode slurries. In this specification, cellulose and cellulose derivatives used as electrode binders include their salts.

[0450] Water-soluble polymers stabilize viscosity by dissolving in water and can stably disperse active materials and other materials used as binders, such as styrene-butadiene rubber, in aqueous solutions. Furthermore, because they possess functional groups, they are expected to be easily and stably adsorbed onto the surface of the active material. Additionally, cellulose derivatives such as carboxymethylcellulose often contain functional groups such as hydroxyl and carboxyl groups, and because of these functional groups, the polymers interact with each other, allowing them to broadly cover the surface of the active material.

[0451] When a binder covers or is in contact with the surface of the active material, it is expected to act as a passivation film, suppressing the decomposition of the electrolyte. Here, a passivation film is a film that does not conduct electricity, or has extremely low electrical conductivity. For example, when a passivation film is formed on the surface of the active material, the decomposition of the electrolyte can be suppressed at the battery reaction potential. Furthermore, it is even more desirable for the passivation film to suppress electrical conductivity while still allowing lithium ions to conduct.

[0452] [Current collector] As the current collector, highly conductive materials such as stainless steel, gold, platinum, aluminum, titanium, and alloys thereof can be used. Furthermore, it is preferable that the material used for the positive electrode current collector does not dissolve at the positive electrode potential. Aluminum alloys with added elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, and molybdenum, can also be used. Alternatively, it may be formed from a metallic element that reacts with silicon to form a silicide. Examples of metallic elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in various shapes, such as foil, plate, sheet, mesh, perforated metal, or expanded metal. The current collector should preferably have a thickness of 5 μm to 30 μm.

[0453] [Negative electrode] The negative electrode comprises a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also contain a conductive material and a binder.

[0454] [Negative electrode active material] For example, alloy-based materials and / or carbon-based materials can be used as the negative electrode active material.

[0455] As the negative electrode active material, elements capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium can be used. For example, materials containing one or more selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc., can be used. Such elements have a larger charge-discharge capacity compared to carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Compounds containing these elements may 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, SbSn, etc. In this context, elements capable of undergoing charge-discharge reactions through alloying and de-alloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloying materials.

[0456] In this specification, SiO refers to silicon monoxide, for example. Alternatively, SiO refers to SiO x It can also be expressed as follows: Here, it is preferable that x has one neighboring value. For example, x is preferably between 0.2 and 1.5, and more preferably between 0.3 and 1.2. Or preferably between 0.2 and 1.2. Or preferably between 0.3 and 1.5.

[0457] Suitable carbon-based materials include graphite, easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0458] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spheroidal graphite having a spherical shape can be used as artificial graphite. For example, MCMB may have a spherical shape and is therefore preferable. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, which may also be preferable. Examples of natural graphite include flake graphite and spheroidized natural graphite.

[0459] When lithium ions are inserted into graphite (during the formation of lithium-graphite intercalation compounds), graphite exhibits a potential as low as that of lithium metal (0.05V to 0.3V vs. Li / Li). + This allows lithium-ion secondary batteries to exhibit a high operating voltage. Furthermore, graphite is preferred because it has advantages such as a relatively high charge / discharge capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0460] Furthermore, titanium dioxide (TiO2) and lithium titanium oxide (Li4Ti5O2) are used as negative electrode active materials. 12 ), lithium-graphite intercalation compound (Li x Oxides such as C6, niobium pentoxide (Nb2O5), tungsten oxide (WO2), and molybdenum oxide (MoO2) can be used.

[0461] Furthermore, as the negative electrode active material, a Li3N type structure is used, which is a lithium and transition metal binitride. 3-x M x N (M = Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 The N3 has a large charge / discharge capacity (900mAh / g, 1890mAh / cm²). 3 ) indicates a preference.

[0462] Using a lithium-transition metal complex nitride is preferable because it contains lithium ions in the negative electrode active material, allowing it to be combined with lithium-ion-free materials such as V2O5 and Cr3O8 as the positive electrode active material. Even when using a lithium-ion-containing material as the positive electrode active material, the lithium-transition metal complex nitride can be used as the negative electrode active material by pre-desorbing the lithium ions contained in the positive electrode active material.

[0463] Furthermore, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used as the negative electrode active material. Other materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, Cr2O3, and CoS 0.89 This can also occur with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N, and Ge3N4, phosphides such as NiP2, FeP2, and CoP3, and fluorides such as FeF3 and BiF3.

[0464] The conductive material and binder that the negative electrode active material layer can have can be the same materials as the conductive material and binder that the positive electrode active material layer can have.

[0465] [Negative electrode current collector] The negative electrode current collector can be made from the same material as the positive electrode current collector. However, it is preferable to use a material for the negative electrode current collector that does not alloy with carrier ions such as lithium.

[0466] [Electrolyte] The electrolyte contains a solvent and an electrolyte. The solvent for the electrolyte is preferably an aprotic organic solvent, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or two or more of these can be used in any combination and ratio.

[0467] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from rupturing and / or igniting even if the internal temperature rises due to an internal short circuit or overcharging. Ionic liquids consist of cations and anions, and include 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, as well as aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonic acid anions, perfluoroalkyl sulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, or perfluoroalkyl phosphate anions.

[0468] Furthermore, examples of electrolytes to be dissolved in the above solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B. 10 Cl 10 Li2B12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), and LiN(C2F5SO2)2 can be used individually or in any combination and ratio of two or more of these salts.

[0469] For secondary batteries, it is preferable to use a highly purified electrolyte with a low content of particulate matter or elements other than the constituent elements of the electrolyte (hereinafter also simply referred to as "impurities"). Specifically, it is preferable that the weight ratio of impurities to the electrolyte be 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0470] Furthermore, additives such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), and dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolyte. The concentration of the added material should be, for example, 0.1 wt% to 5 wt% relative to the total solvent. VC or LiBOB are particularly preferred because they easily form a good coating.

[0471] Alternatively, a polymer gel electrolyte, obtained by swelling a polymer with an electrolyte solution, may be used.

[0472] Using polymer gel electrolytes enhances safety against leakage and other issues. Furthermore, it enables the secondary battery to be made thinner and lighter.

[0473] As the polymer to be gelled, silicone gels, acrylic gels, acrylonitrile gels, polyethylene oxide-based gels, polypropylene oxide-based gels, fluorine-based polymer gels, and the like can be used.

[0474] Examples of polymers that can be used include polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The resulting polymer may also have a porous structure.

[0475] Furthermore, instead of an electrolyte, a solid electrolyte containing inorganic materials such as sulfides or oxides, or a solid electrolyte containing polymeric materials such as PEO (polyethylene oxide), can be used. When a solid electrolyte is used, the installation of separators and / or spacers becomes unnecessary. In addition, since the entire battery can be solidified, the risk of leakage is eliminated, dramatically improving safety.

[0476] [Separator] Furthermore, secondary batteries preferably have a separator. As the separator, for example, paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers made of nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane can be used. It is preferable that the separator is processed into an envelope shape and arranged to enclose either the positive or negative electrode.

[0477] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine material, a polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0478] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging, and thus improving the reliability of secondary batteries. Coating with fluorine-based materials improves adhesion between the separator and electrodes, thereby improving output characteristics. Coating with polyamide materials, particularly aramid, improves heat resistance, thus enhancing the safety of secondary batteries.

[0479] For example, a polypropylene film may be coated on both sides with a mixture of aluminum oxide and aramid. Alternatively, the side of the polypropylene film in contact with the positive electrode may be coated with a mixture of aluminum oxide and aramid, and the side in contact with the negative electrode may be coated with a fluorine-based material.

[0480] By using a multilayer separator, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, thus increasing the discharge capacity per unit volume of the secondary battery.

[0481] [Exterior] For the outer casing of a secondary battery, metal materials such as aluminum and / or resin materials can be used. Alternatively, a film-like outer casing can be used. As a film, for example, a three-layer film can be used, in which a highly flexible metal thin film such as aluminum, stainless steel, copper, or nickel is provided on a film made of materials such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is provided on the metal thin film as the outer surface of the outer casing.

[0482] <Example of a secondary battery configuration 2> The following describes the configuration of a secondary battery using a solid electrolyte layer as an example of a secondary battery configuration.

[0483] As shown in Figure 19(A), a secondary battery 400 according to one embodiment of the present invention has a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430.

[0484] The positive electrode 410 has a positive electrode current collector 413 and a positive electrode active material layer 414. The positive electrode active material layer 414 has a positive electrode active material 411 and a solid electrolyte 421. The positive electrode active material 411 is a positive electrode active material manufactured using the manufacturing method described in the previous embodiment. The positive electrode active material layer 414 may also have a conductive material and a binder.

[0485] The solid electrolyte layer 420 has a solid electrolyte 421. The solid electrolyte layer 420 is located between the positive electrode 410 and the negative electrode 430 and is a region that does not contain either the positive electrode active material 411 or the negative electrode active material 431.

[0486] The negative electrode 430 has a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 has a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also have a conductive material and a binder. When metallic lithium is used for the negative electrode 430, the negative electrode 430 can be made without the solid electrolyte 421, as shown in Figure 19(B). Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery 400.

[0487] As the solid electrolyte 421 in the solid electrolyte layer 420, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halogen-based solid electrolyte, etc., can be used.

[0488] Sulfide-based solid electrolytes include thiolysicone-based (Li 10 GeP2S 12 Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glass (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), sulfide crystallized glass (Li7P3S 11 Li 3.25 P 0.95It contains S4, etc. Sulfide-based solid electrolytes have advantages such as the availability of materials with high conductivity, the ability to be synthesized at low temperatures, and their relatively soft nature which helps maintain conductive paths even after charging and discharging.

[0489] Oxide-based solid electrolytes include materials having a perovskite-type crystal structure (La 2 / 3-x Li 3x Materials having a NASICON-type crystal structure (Li 1-x Al x Ti 2-x (PO4)3 etc.) Materials having a garnet-type crystal structure (Li7La3Zr2O 12 Materials having a LISICON-type crystal structure (Li 14 ZnGe4O 16 etc.), LLZO(Li7La3Zr2O 12 ), oxide glass (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3, etc.), oxide crystallized glass (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, etc. are included. Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0490] Halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, and LiI. Furthermore, composite materials in which these halide-based solid electrolytes are packed into the pores of porous aluminum oxide and / or porous silica can also be used as solid electrolytes.

[0491] Alternatively, different solid electrolytes may be mixed and used.

[0492] In particular, Li has a NASICON-type crystal structure. 1+x Al x Ti 2-x(PO4)3(0 < x < 1) (hereinafter referred to as LATP) contains elements such as aluminum and titanium that the cathode active material used in the secondary battery 400 of one aspect of the present invention may have. Therefore, a synergistic effect can be expected for improving the cycle characteristics, which is preferable. In addition, an improvement in productivity due to the reduction of processes can also be expected. In this specification and the like, the NASICON-type crystal structure is a compound represented by M2(XO4)3 (M: transition metal, X: S, P, As, Mo, W, etc.), and has a structure in which MO6 octahedra and XO4 tetrahedra share vertices and are three-dimensionally arranged.

[0493] 〔Shape of the exterior body and the secondary battery〕 For the exterior body of the secondary battery 400 of one aspect of the present invention, various materials and shapes can be used, but it is preferable to have a function of pressing the cathode, the solid electrolyte layer, and the anode.

[0494] For example, FIG. 20 is an example of a cell for evaluating the materials of an all-solid-state battery.

[0495] FIG. 20(A) is a schematic cross-sectional view of the evaluation cell. The evaluation cell has a lower member 761, an upper member 762, and fixing screws or wing nuts 764 for fixing them. By rotating the pressing screw 763, the electrode plate 753 is pressed to fix the evaluation material. An insulator 766 is provided between the lower member 761 and the upper member 762 made of a stainless material. Also, an O-ring 765 for sealing is provided between the upper member 762 and the pressing screw 763.

[0496] The evaluation material is placed on the electrode plate 751, surrounded by an insulating tube 752 around it, and is in a state of being pressed by the electrode plate 753 from above. A perspective view of the periphery of this evaluation material enlarged is shown in FIG. 20(B).

[0497] As an example of the evaluation material, a lamination of a cathode 750a, a solid electrolyte layer 750b, and an anode 750c is shown, and a cross-sectional view is shown in FIG. 20(C). In FIGS. 20(A) to 20(C), the same reference numerals are used for the same parts.

[0498] The electrode plate 751 and lower member 761, which are electrically connected to the positive electrode 750a, can be considered to correspond to the positive electrode terminal. The electrode plate 753 and upper member 762, which are electrically connected to the negative electrode 750c, can be considered to correspond to the negative electrode terminal. Electrical resistance and other parameters can be measured while applying pressure to the evaluation material via the electrode plates 751 and 753.

[0499] Furthermore, it is preferable to use a package with excellent airtightness for the outer casing of a secondary battery according to one embodiment of the present invention. For example, a ceramic package and / or a resin package can be used. Also, when sealing the outer casing, it is preferable to block out the outside air and perform the sealing in a sealed atmosphere, for example, inside a glove box.

[0500] Figure 21(A) shows a perspective view of a secondary battery according to one embodiment of the present invention, having a different exterior and shape from that of Figure 20. The secondary battery in Figure 21(A) has external electrodes 771 and 772 and is sealed with an exterior having a plurality of package members.

[0501] Figure 21(B) shows an example of a cross-section cut by the dashed line in Figure 21(A). The laminate having a positive electrode 750a, a solid electrolyte layer 750b, and a negative electrode 750c is enclosed and sealed by a package member 770a having an electrode layer 773a on a flat plate, a frame-shaped package member 770b, and a package member 770c having an electrode layer 773b on a flat plate. Insulating materials, such as resin materials and / or ceramics, can be used for the package members 770a, 770b, and 770c.

[0502] The external electrode 771 is electrically connected to the positive electrode 750a via the electrode layer 773a and functions as a positive electrode terminal. The external electrode 772 is electrically connected to the negative electrode 750c via the electrode layer 773b and functions as a negative electrode terminal.

[0503] This embodiment can be used in appropriate combination with other embodiments.

[0504] (Embodiment 4) This embodiment describes an example of the shape of a secondary battery having a positive electrode, as described in the previous embodiment. The materials used in the secondary battery described in this embodiment can be referenced to those described in the previous embodiment.

[0505] <Coin-type rechargeable battery> First, let's describe an example of a coin-type rechargeable battery. Figure 22(A) is an external view of a coin-type (single-layer flat type) rechargeable battery, and Figure 22(B) is a cross-sectional view thereof. Coin-type rechargeable batteries are mainly used in small electronic devices. In this specification, the term "coin-type battery" includes button-type batteries.

[0506] The coin-type secondary battery 300 has a positive electrode casing 301, which also serves as the positive electrode terminal, and a negative electrode casing 302, which also serves as the negative electrode terminal, both 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 provided 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 provided in contact with it.

[0507] Furthermore, for the positive electrode 304 and negative electrode 307 used in the coin-type secondary battery 300, the active material layer only needs to be formed on one side.

[0508] The positive electrode can 301 and the negative electrode can 302 can be made of metals such as nickel, aluminum, or titanium, which are corrosion-resistant to the electrolyte, or alloys thereof and / or alloys of these with other metals (e.g., stainless steel). Furthermore, it is preferable to coat them with nickel and / or aluminum, etc., to prevent corrosion by the electrolyte. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0509] The negative electrode 307, positive electrode 304, and separator 310 are impregnated with an electrolyte, and as shown in Figure 22(B), the positive electrode 304, separator 310, negative electrode 307, and negative electrode 302 are stacked in this order with the positive electrode 301 at the bottom, and the positive electrode 301 and negative electrode 302 are pressed together via a gasket 303 to manufacture a coin-type secondary battery 300.

[0510] By using the positive electrode active material described in the previous embodiment for the positive electrode 304, a coin-type secondary battery 300 with high discharge capacity and excellent cycle characteristics can be obtained.

[0511] Here, Figure 22(C) is used to explain the current flow during charging of a secondary battery. When a lithium-ion secondary battery is considered as a closed circuit, the movement of lithium ions and the flow of current are in the same direction. In a lithium-ion secondary battery, the anode and cathode are reversed during charging and discharging, and the oxidation and reduction reactions are reversed. Therefore, 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. Accordingly, in this specification, whether charging or discharging, whether a reverse pulse current is flowing or a charging current is flowing, the positive electrode will be called the "positive electrode" or "+ electrode (positive electrode)," and the negative electrode will be called the "negative electrode" or "- electrode (negative electrode)." Using the terms anode (positive electrode) and cathode (negative electrode) in relation to oxidation and reduction reactions could lead to confusion as they are reversed during charging and discharging. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode and cathode are used, it should be specified whether they refer to the charging or discharging phase, and whether they correspond to the positive or negative electrode.

[0512] A charger is connected to the two terminals shown in Figure 22(C), and the secondary battery 300 is charged. As the secondary battery 300 charges, the potential difference between the electrodes increases.

[0513] <Cylindrical rechargeable battery> Next, an example of a cylindrical secondary battery will be described with reference to Figure 23. Figure 23(A) shows an external view of a cylindrical secondary battery 600. Figure 23(B) is a schematic cross-section of the cylindrical secondary battery 600. As shown in Figure 23(B), the cylindrical secondary battery 600 has a positive electrode cap (battery cover) 601 on the top surface and a battery casing (outer casing) 602 on the sides and bottom. The positive electrode cap 601 and the battery casing (outer casing) 602 are insulated from each other by a gasket (insulating packing) 610.

[0514] Inside the hollow cylindrical battery casing 602, a battery element is provided, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 in between. Although not shown, the battery element is wound around a center pin. The battery casing 602 is closed at one end and open at the other. The battery casing 602 can be made of a metal such as nickel, aluminum, or titanium, or an alloy of these and / or an alloy of these with other metals (e.g., stainless steel), which is corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat the battery casing 602 with nickel and / or aluminum to prevent corrosion by the electrolyte. Inside the battery casing 602, the battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. In addition, a non-aqueous electrolyte (not shown) is injected into the inside of the battery casing 602 in which the battery element is provided. The non-aqueous electrolyte can be the same as that used in coin-type secondary batteries.

[0515] Since the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form active material on both sides of the current collector. A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can be made of metal materials such as aluminum. The positive electrode terminal 603 is resistance-welded to the safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC element (Positive Temperature Coefficient) 611. The safety valve mechanism 612 disconnects the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the rise in the internal pressure of the battery exceeds a predetermined threshold. Furthermore, the PTC element 611 is a thermal resistance element whose resistance increases when the temperature rises, and it prevents abnormal heat generation by limiting the current amount through the increase in resistance. Barium titanate (BaTiO3) based semiconductor ceramics can be used for the PTC element.

[0516] Alternatively, as shown in Figure 23(C), a module 615 may be constructed by sandwiching multiple secondary batteries 600 between conductive plates 613 and 614. The multiple secondary batteries 600 may be connected in parallel, in series, or connected in parallel and then in series. By constructing a module 615 with multiple secondary batteries 600, a large amount of power can be extracted.

[0517] Figure 23(D) is a top view of module 615. The conductive plate 613 is shown with a dotted line for clarity. As shown in Figure 23(D), module 615 may have conductors 616 that electrically connect a plurality of secondary batteries 600. A conductive plate can be superimposed on the conductors 616. A temperature control device 617 may also be provided between the plurality of secondary batteries 600. When a secondary battery 600 is overheated, it can be cooled by the temperature control device 617, and when a secondary battery 600 is too cold, it can be heated by the temperature control device 617. This makes the performance of module 615 less susceptible to the influence of ambient temperature. The heat transfer medium in the temperature control device 617 is preferably insulating and non-flammable.

[0518] By using the positive electrode active material described in the previous embodiment for the positive electrode 604, a cylindrical secondary battery 600 with high discharge capacity and excellent cycle characteristics can be obtained.

[0519] <Example of the structure of an energy storage device including a secondary battery> Another example of the structure of an energy storage device including a secondary battery will be explained using Figures 24 to 28.

[0520] Figures 24(A) and 24(B) show the external view of the battery pack. The battery pack includes a secondary battery 913 and a circuit board 900. The secondary battery 913 is connected to the antenna 914 via the circuit board 900. A label 910 is attached to the secondary battery 913. Furthermore, as shown in Figure 24(B), the secondary battery 913 is connected to terminals 951 and 952. The circuit board 900 is secured with a seal 915.

[0521] The circuit board 900 has terminal 911 and circuit 912. Terminal 911 is connected to terminal 951, terminal 952, antenna 914, and circuit 912. Multiple terminals 911 may be provided, and each of the multiple terminals 911 may be used as a control signal input terminal, a power supply terminal, etc.

[0522] The circuit 912 may be provided on the back surface of the circuit board 900. The antenna 914 is not limited to a coil shape, but may be, for example, linear or plate-shaped. Alternatively, antennas such as a planar antenna, aperture antenna, traveling wave antenna, EH antenna, magnetic field antenna, or dielectric antenna may be used. Or, the antenna 914 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. In other words, the antenna 914 may function as one of the two conductors of the capacitor. This makes it possible to exchange power not only through electromagnetic and magnetic fields, but also through electric fields.

[0523] The battery pack has a layer 916 between the antenna 914 and the secondary battery 913. The layer 916 has the function of shielding, for example, the electromagnetic field from the secondary battery 913. For the layer 916, a magnetic material can be used, for example.

[0524] Note that the structure of the battery pack is not limited to that shown in Figure 24.

[0525] For example, as shown in Figures 25(A) and 25(B), antennas may be provided on each of the opposing pairs of faces of the secondary battery 913 shown in Figures 24(A) and 24(B). Figure 25(A) is an external view showing one of the pair of faces, and Figure 25(B) is an external view showing the other of the pair of faces. For parts that are the same as those of the secondary battery shown in Figures 24(A) and 24(B), the explanation of the secondary battery shown in Figures 24(A) and 24(B) can be appropriately referred to.

[0526] As shown in Figure 25(A), an antenna 914 is provided on one of the pair of surfaces of the secondary battery 913 with a layer 916 in between, and as shown in Figure 25(B), an antenna 918 is provided on the other of the pair of surfaces of the secondary battery 913 with a layer 917 in between. The layer 917 has the function of shielding the electromagnetic field from, for example, the secondary battery 913. For the layer 917, a magnetic material can be used, for example.

[0527] By adopting the above structure, the size of both antenna 914 and antenna 918 can be increased. Antenna 918 has a function that allows for data communication with external devices, for example. Antenna 918 can be fitted with an antenna of a shape that is applicable to antenna 914, for example. As a communication method between the secondary battery and other devices via antenna 918, response methods that can be used between the secondary battery and other devices, such as NFC (Near Field Communication), can be applied.

[0528] Alternatively, as shown in Figure 25(C), a display device 920 may be provided on the secondary battery 913 shown in Figures 24(A) and 24(B). The display device 920 is electrically connected to terminal 911. Note that a label 910 does not need to be provided on the part where the display device 920 is provided. Note that for the same parts as the secondary battery shown in Figures 24(A) and 24(B), the explanation of the secondary battery shown in Figures 24(A) and 24(B) can be appropriately applied.

[0529] The display device 920 may display, for example, an image indicating whether or not it is charging, or an image indicating the amount of stored power. The display device 920 can be, for example, electronic paper, liquid crystal display, or electroluminescent (EL) display. For example, using electronic paper can reduce the power consumption of the display device 920.

[0530] Alternatively, as shown in Figure 25(D), a sensor 921 may be provided on the secondary battery 913 shown in Figures 24(A) and 24(B). The sensor 921 is electrically connected to terminal 911 via terminal 922. For parts that are the same as those of the secondary battery shown in Figures 24(A) and 24(B), the explanation of the secondary battery shown in Figures 24(A) and 24(B) can be appropriately applied.

[0531] The sensor 921 may have the function of measuring, for example, displacement, position, velocity, acceleration, angular velocity, rotational speed, distance, light, liquid, magnetism, temperature, chemical substances, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, gradient, vibration, odor, or infrared radiation. By providing the sensor 921, for example, data indicating the environment in which the secondary battery is placed (such as temperature) can be detected and stored in the memory of the circuit 912.

[0532] Furthermore, an example of the structure of the secondary battery 913 will be explained using Figures 26 and 27.

[0533] The secondary battery 913 shown in Figure 26(A) has a wound body 950 with terminals 951 and 952 inside a housing 930. The wound body 950 is impregnated with an electrolyte inside the housing 930. Terminal 952 is in contact with the housing 930, while terminal 951 is not in contact with the housing 930 due to the use of an insulating material or the like. In Figure 26(A), the housing 930 is shown separated for convenience, but in reality, the wound body 950 is covered by the housing 930, and terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (e.g., aluminum) or a resin material.

[0534] Furthermore, as shown in Figure 26(B), the housing 930 shown in Figure 26(A) may be formed from multiple materials. For example, in the secondary battery 913 shown in Figure 26(B), housing 930a and housing 930b are bonded together, and the winding body 950 is provided in the area surrounded by housing 930a and housing 930b.

[0535] For the housing 930a, an insulating material such as organic resin can be used. In particular, by using a material such as organic resin on the surface where the antenna is formed, shielding of the electric field by the secondary battery 913 can be suppressed. If the shielding of the electric field by the housing 930a is small, an antenna such as antenna 914 may be provided inside the housing 930a. For the housing 930b, for example, a metal material can be used.

[0536] Furthermore, the structure of the wound body 950 is shown in Figure 27. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the separator 933 in between, and the stacked sheets are wound up. Note that multiple stacks of the negative electrode 931, positive electrode 932, and separator 933 may be stacked.

[0537] The negative terminal 931 is connected to terminal 911 shown in Figure 24 via one of terminals 951 and 952. The positive terminal 932 is connected to terminal 911 shown in Figure 24 via the other of terminals 951 and 952.

[0538] By using the positive electrode active material described in the previous embodiment for the positive electrode 932, a secondary battery 913 with high discharge capacity and excellent cycle characteristics can be obtained.

[0539] <Laminated rechargeable battery> Next, an example of a laminated secondary battery will be described with reference to Figures 28 to 32(A). If a laminated secondary battery has a flexible structure, it can be bent in accordance with the deformation of the electronic device if it is mounted on an electronic device that has at least a part of a flexible component.

[0540] A laminated secondary battery 980 will be described using Figure 28. The laminated secondary battery 980 has a wound body 993 as shown in Figure 28(A). The wound body 993 has a negative electrode 994, a positive electrode 995, and a separator 996. Similar to the wound body 950 described in Figure 27, the wound body 993 is formed by stacking the negative electrode 994 and the positive electrode 995 with the separator 996 in between, and then winding the stacked sheet.

[0541] The number of layers in the stack consisting of the negative electrode 994, positive electrode 995, and separator 996 can be appropriately designed according to the required charge / discharge capacity and element volume. The negative electrode 994 is connected to a negative electrode current collector (not shown) via one of the lead electrodes 997 and 998, and the positive electrode 995 is connected to a positive electrode current collector (not shown) via the other of the lead electrodes 997 and 998.

[0542] As shown in Figure 28(B), a secondary battery 980 can be manufactured as shown in Figure 28(C) by housing the aforementioned wound body 993 in a space formed by bonding a film 981, which serves as the outer casing, and a film 982, which has a recess, by thermocompression or the like. The wound body 993 has lead electrodes 997 and 998 and is impregnated with an electrolyte solution inside the film 981 and the film 982, which has a recess.

[0543] The film 981 and the film 982 having recesses can be made of metal materials such as aluminum and / or resin materials. If resin materials are used for the film 981 and the film 982 having recesses, the film 981 and the film 982 having recesses can be deformed when an external force is applied, making it possible to create a flexible storage battery.

[0544] Furthermore, although Figures 28(B) and 28(C) show examples using two films, a space may be formed by folding a single film, and the aforementioned wound body 993 may be housed in that space.

[0545] By using the positive electrode active material described in the previous embodiment for the positive electrode 995, a secondary battery 980 with high discharge capacity and excellent cycle characteristics can be obtained.

[0546] Furthermore, while Figure 28 illustrates an example of a secondary battery 980 having a wound body in a space formed by a film that serves as the outer casing, a secondary battery having multiple strip-shaped positive electrodes, separators, and negative electrodes in a space formed by a film that serves as the outer casing may also be used, for example, as shown in Figure 29.

[0547] The laminate-type secondary battery 500 shown in Figure 29(A) comprises a positive electrode 503 having a positive electrode current collector 501 and a positive electrode active material layer 502, a negative electrode 506 having a negative electrode current collector 504 and a negative electrode active material layer 505, a separator 507, an electrolyte 508, and an outer casing 509. The separator 507 is installed between the positive electrode 503 and the negative electrode 506, which are located inside the outer casing 509. The outer casing 509 is filled with the electrolyte 508. The electrolyte 508 can be the electrolyte shown in Embodiment 3.

[0548] In the laminate-type secondary battery 500 shown in Figure 29(A), the positive electrode current collector 501 and the negative electrode current collector 504 also serve as terminals for obtaining electrical contact with the outside. Therefore, parts of the positive electrode current collector 501 and the negative electrode current collector 504 may be arranged to be exposed to the outside from the outer casing 509. Alternatively, the positive electrode current collector 501 and the negative electrode current collector 504 may not be exposed to the outside from the outer casing 509, and instead, lead electrodes may be used to ultrasonically bond the lead electrodes to the positive electrode current collector 501 or the negative electrode current collector 504, thereby exposing the lead electrodes to the outside.

[0549] In a laminate-type secondary battery 500, the outer casing 509 can be made of a laminate film with a three-layer structure, in which a highly flexible metal thin film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is provided on the metal thin film as the outer surface of the outer casing.

[0550] Furthermore, an example of the cross-sectional structure of the laminate-type secondary battery 500 is shown in Figure 29(B). For simplicity, Figure 29(A) shows an example consisting of two current collectors, but in reality, it is composed of multiple electrode layers as shown in Figure 29(B).

[0551] Figure 29(B) shows an example where the number of electrode layers is 16. Even with 16 electrode layers, the secondary battery 500 retains its flexibility. Figure 29(B) shows a structure with 8 layers of negative electrode current collectors 504 and 8 layers of positive electrode current collectors 501, for a total of 16 layers. Figure 29(B) also shows a cross-section of the negative electrode extraction section, where the 8 layers of negative electrode current collectors 504 are ultrasonically bonded. Of course, the number of electrode layers is not limited to 16; it can be more or fewer. A larger number of electrode layers allows for a secondary battery with a greater discharge capacity. Conversely, a smaller number of electrode layers allows for a thinner design and a secondary battery with superior flexibility.

[0552] Here, an example of the external view of a laminate-type secondary battery 500 is shown in Figures 30 and 31. Figures 30 and 31 show a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0553] Figure 32(A) shows the external view of the positive electrode 503 and the negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and the positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as the tab region). The negative electrode 506 has a negative electrode current collector 504, and the negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., the tab region. The area and shape of the tab regions of the positive and negative electrodes are not limited to the example shown in Figure 32(A).

[0554] <Method for manufacturing laminated rechargeable batteries> Here, an example of a method for manufacturing a laminate-type secondary battery, whose external view is shown in Figure 30, will be explained using Figures 32(B) and (C).

[0555] First, the negative electrode 506, separator 507, and positive electrode 503 are stacked. Figure 32(B) shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example using 5 sets of negative electrodes and 4 sets of positive electrodes is shown. Next, the tab regions of the positive electrode 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For joining, ultrasonic welding, for example, can be used. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0556] Next, the negative electrode 506, separator 507, and positive electrode 503 are placed on the outer casing 509.

[0557] Next, as shown in Figure 32(C), the outer casing 509 is bent at the portion indicated by the dashed line. Then, the outer periphery of the outer casing 509 is joined. For joining, for example, heat sealing may be used. At this time, a region that is not joined (hereinafter referred to as the inlet) is provided on a part (or one side) of the outer casing 509 so that the electrolyte 508 can be added later.

[0558] Next, the electrolyte 508 (not shown) is introduced into the inside of the outer casing 509 through an inlet provided in the outer casing 509. It is preferable to introduce the electrolyte 508 under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is sealed. In this way, a laminate-type secondary battery 500 can be manufactured.

[0559] By using the positive electrode active material described in the previous embodiment for the positive electrode 503, a secondary battery 500 with high discharge capacity and excellent cycle characteristics can be obtained.

[0560] In all-solid-state batteries, applying a predetermined pressure in the stacking direction of the stacked positive and negative electrodes can maintain good contact conditions at the internal interfaces. Applying a predetermined pressure in the stacking direction of the positive and negative electrodes can suppress expansion in the stacking direction due to charging and discharging of the all-solid-state battery, thereby improving the reliability of the all-solid-state battery.

[0561] This embodiment can be used in appropriate combination with other embodiments.

[0562] (Embodiment 5) This embodiment describes an example of mounting a secondary battery, which is one aspect of the present invention, into an electronic device.

[0563] First, Figures 33(A) to 33(G) show examples of mounting the bendable secondary battery described in the previous embodiment into an electronic device. Examples of electronic devices to which the bendable secondary battery is applied include television equipment (also called televisions or television receivers), monitors for computers, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game consoles, personal information terminals, sound playback devices, and large game machines such as pachinko machines.

[0564] Furthermore, rechargeable batteries with flexible shapes can be incorporated along curved surfaces such as the interior or exterior walls of houses and buildings, or the interior or exterior of automobiles.

[0565] Figure 33(A) shows an example of a mobile phone. The mobile phone 7400 includes a display unit 7402 built into the housing 7401, as well as operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. The mobile phone 7400 also has a secondary battery 7407. By using a secondary battery according to one embodiment of the present invention for the above secondary battery 7407, a lightweight and long-lasting mobile phone can be provided.

[0566] Figure 33(B) shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and the entire device is bent, the secondary battery 7407 located inside is also bent. Figure 33(C) shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in the bent state. The secondary battery 7407 has lead electrodes that are electrically connected to a current collector. For example, the current collector is made of copper foil, and a portion of it is alloyed with gallium to improve the adhesion with the active material layer that is in contact with the current collector, resulting in a configuration that ensures high reliability of the secondary battery 7407 when it is bent.

[0567] Figure 33(D) shows an example of a bangle-type display device. The portable display device 7100 comprises a housing 7101, a display unit 7102, operation buttons 7103, and a secondary battery 7104. Figure 33(E) shows the state of the bent secondary battery 7104. When the secondary battery 7104 is worn on the user's arm in a bent state, the housing deforms, and the curvature of part or all of the secondary battery 7104 changes. The degree of curvature at any point in the curve is expressed as the radius of the corresponding circle, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or all of the main surface of the housing or secondary battery 7104 changes within the range of a radius of curvature of 40 mm to 150 mm. High reliability can be maintained as long as the radius of curvature on the main surface of the secondary battery 7104 is within the range of 40 mm to 150 mm. By using a secondary battery according to one embodiment of the present invention for the above secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0568] Figure 33(F) shows an example of a wristwatch-type personal information terminal. The personal information terminal 7200 includes a housing 7201, a display unit 7202, a band 7203, a buckle 7204, operation buttons 7205, input / output terminals 7206, and the like.

[0569] The 7200 personal digital assistant (PDCA) can run various applications such as mobile phone calls, email, document viewing and creation, music playback, internet communication, and computer games.

[0570] The display unit 7202 has a curved display surface, allowing it to display information along the curved surface. The display unit 7202 also features a touch sensor, allowing it to be operated by touching the screen with a finger or stylus. For example, touching the icon 7207 displayed on the display unit 7202 can launch an application.

[0571] The operation button 7205 can be assigned various functions, including time setting, power on / off, wireless communication on / off, silent mode activation / deactivation, and power saving mode activation / deactivation. For example, the functions of the operation button 7205 can be freely configured by the operating system built into the personal digital assistant 7200.

[0572] Furthermore, the 7200 portable information terminal is capable of performing standardized short-range wireless communication. For example, it can communicate with a wireless-enabled headset to enable hands-free calling.

[0573] Furthermore, the portable information terminal 7200 is equipped with an input / output terminal 7206, allowing it to directly exchange data with other information terminals via a connector. It can also be charged via the input / output terminal 7206. Note that charging may also be performed wirelessly without using the input / output terminal 7206.

[0574] The display unit 7202 of the portable information terminal 7200 has a secondary battery according to one aspect of the present invention. By using a secondary battery according to one aspect of the present invention, a lightweight and long-lasting portable information terminal can be provided. For example, the secondary battery 7104 shown in Figure 33(E) can be incorporated inside the housing 7201 in a curved state, or inside the band 7203 in a bendable state.

[0575] The portable information terminal 7200 preferably has sensors. Preferably, the sensors include, for example, human body sensors such as fingerprint sensors, pulse sensors, and body temperature sensors, as well as touch sensors, pressure sensors, acceleration sensors, and the like.

[0576] Figure 33(G) shows an example of an armband-type display device. The display device 7300 has a display unit 7304 and a secondary battery according to one embodiment of the present invention. The display device 7300 may also be equipped with a touch sensor on the display unit 7304 and may function as a portable information terminal.

[0577] The display unit 7304 has a curved display surface, allowing it to display information along the curved surface. Furthermore, the display device 7300 can change its display status via standardized short-range wireless communication.

[0578] Furthermore, the display device 7300 is equipped with input / output terminals, allowing it to directly exchange data with other information terminals via connectors. It can also be charged via the input / output terminals. Note that charging may also be performed wirelessly without using the input / output terminals.

[0579] By using a secondary battery according to one aspect of the present invention as the secondary battery of the display device 7300, a lightweight and long-life display device can be provided.

[0580] Furthermore, an example of mounting the secondary battery with good cycle characteristics shown in the previous embodiment into an electronic device will be explained using Figures 33(H), 34, and 35.

[0581] By using a secondary battery according to one embodiment of the present invention as a secondary battery in everyday electronic devices, it is possible to provide lightweight and long-lasting products. Examples of everyday electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. For the secondary batteries in these products, there is a demand for batteries that are stick-shaped for ease of handling by the user, are small, lightweight, and have a large discharge capacity.

[0582] Figure 33(H) is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In Figure 33(H), the electronic cigarette 7500 consists of an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 including a liquid supply bottle and a sensor. To enhance safety, a protection circuit to prevent overcharging and / or over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in Figure 33(H) has external terminals so that it can be connected to a charging device. Since the secondary battery 7504 is the tip when held, it is desirable that its total length is short and its weight is light. A secondary battery according to one aspect of the present invention has a high discharge capacity and good cycle characteristics, so it is possible to provide a small and lightweight electronic cigarette 7500 that can be used for a long time over a long period of time.

[0583] Next, Figures 34(A) and 34(B) show an example of a foldable tablet terminal. The tablet terminal 9600 shown in Figures 34(A) and 34(B) includes a housing 9630a, a housing 9630b, a movable part 9640 connecting housings 9630a and 9630b, a display unit 9631 having display units 9631a and 9631b, switches 9625 to 9627, a fastener 9629, and an operation switch 9628. By using a flexible panel for the display unit 9631, a tablet terminal with a larger display area can be created. Figure 34(A) shows the tablet terminal 9600 in an open state, and Figure 34(B) shows the tablet terminal 9600 in a closed state.

[0584] Furthermore, the tablet terminal 9600 has a power storage unit 9635 inside the housing 9630a and housing 9630b. The power storage unit 9635 is provided across housing 9630a and housing 9630b, passing through the movable part 9640.

[0585] The display unit 9631 can have all or part of its area designated as a touch panel area, and data can be entered by touching images, characters, input forms, etc., including icons, displayed in that area. For example, keyboard buttons may be displayed on the entire surface of the display unit 9631a on the housing 9630a side, and information such as characters and images may be displayed on the display unit 9631b on the housing 9630b side.

[0586] Alternatively, the display unit 9631b on the housing 9630b may be used to display a keyboard, while the display unit 9631a on the housing 9630a may be used to display information such as characters and images. Alternatively, the display unit 9631 may be used to display a touch panel keyboard display switching button, so that the keyboard is displayed on the display unit 9631 when the button is touched with a finger or stylus.

[0587] Furthermore, it is possible to simultaneously input touch input to the touch panel area of ​​the display unit 9631a on the housing 9630a and the touch panel area of ​​the display unit 9631b on the housing 9630b.

[0588] Furthermore, switches 9625 to 9627 may not only serve as an interface for operating the tablet terminal 9600, but also as an interface for switching various functions. For example, one or more switches selected from switches 9625 to 9627 may function as switches to turn the tablet terminal 9600 on and off. Also, for example, one or more switches selected from switches 9625 to 9627 may have a function to switch the display orientation, such as portrait or landscape, or a function to switch between monochrome and color display. Also, for example, one or more switches selected from switches 9625 to 9627 may have a function to adjust the brightness of the display unit 9631. The brightness of the display unit 9631 can be optimized according to the amount of ambient light during use, as detected by the light sensor built into the tablet terminal 9600. Note that the tablet terminal may incorporate other detection devices in addition to the light sensor, such as a gyroscope, an accelerometer, or other sensors that detect tilt.

[0589] Furthermore, while Figure 34(A) shows an example where the display area of ​​the display unit 9631a on the housing 9630a side and the display unit 9631b on the housing 9630b side are approximately the same, the display areas of the display units 9631a and 9631b are not particularly limited, and the size of one may differ from the other, as may the display quality. For example, one display panel may be capable of dis...

Claims

1. Li ｘ CoO ２ When x is 1, it has a layered rock salt type crystal structure of space group R-3m, Li ｘ CoO ２ When x in the battery is greater than 0.1 and less than or equal to 0.24, space group P2 / m, Lattice constant a = 4.88 ± 0.01 Å, b=2.82±0.01 Å, c=4.84±0.01Å, α＝９０°、 β＝１０９．５８±０．０１°、 A positive electrode active material having a crystal structure of γ=90°.

2. In claim 1, Li ｘ CoO ２ In the case where x is more than 0.1 and not more than 0.24 in the charged state, the crystal structure is The coordinates of cobalt and oxygen in the unit cell are Co1(0.5,0,0.5), Co2 (0, 0.5, 0.5), O1 (0.232, 0, 0.645), O2 (0.781, 0.5, 0.679), the positive electrode active material.

3. Li ｘ CoO ２ When x is 1, it has a layered rock salt type crystal structure of space group R-3m, Li ｘ CoO ２ When x in the battery is greater than 0.1 and less than or equal to 0.24, When analyzed by powder X-ray diffraction, the diffraction pattern shows at least 19.37° or more and 19.57° or less, A positive electrode active material having a peak at 45.57° or more and 45.67° or less.

4. Li ｘ CoO ２ When x is 1, it has a layered rock salt type crystal structure of space group R-3m, Li ｘ CoO ２ In the charged state, x is greater than 0.1 and not greater than 0.24, when analyzed by powder X-ray diffraction, the diffraction pattern has at least 19.13 or more and less than 19.37, 19.37° or more and 19.57° or less, 45.37° or more and less than 45.57°; A positive electrode active material having a peak at 45.57° or more and 45.67° or less.

5. A positive electrode active material having lithium cobalt oxide, When a battery is produced using the positive electrode active material as a positive electrode and lithium metal as a negative electrode, After the battery is CCCV-charged multiple times at a voltage of 4.7 V or higher, The positive electrode of the battery was treated with CuKα １ A positive electrode active material, wherein when analyzed by powder X-ray diffraction using a XRD pattern having diffraction peaks at least at 2θ=19.47±0.10° and 2θ=45.62±0.05°.

6. In claim 5, The battery contains 1 mol / L lithium hexafluorophosphate (LiPF ６ ) and a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC = 3:7, containing 2 wt % vinylene carbonate (VC) as an electrolyte.

7. A positive electrode active material having lithium cobalt oxide, The positive electrode active material is used as a positive electrode, and lithium metal is used as a negative electrode, 1 mol / L lithium hexafluorophosphate (LiPF ６ ) and a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC = 3:7 (volume ratio) and 2 wt% vinylene carbonate (VC) were used as the electrolyte to prepare a battery. The battery was charged at a constant current of 10 mA / g up to a voltage of 4.75 V in a 45° C. environment, and then The positive electrode of the battery was treated with CuKα １ A positive electrode active material, wherein when analyzed by powder X-ray diffraction using a XRD pattern having diffraction peaks at least at 2θ=19.47±0.10° and 2θ=45.62±0.05°.

8. A positive electrode active material having lithium cobalt oxide, The positive electrode active material was analyzed by Raman spectroscopy with a laser wavelength of 532 nm and an output of 2.5 mW. For the integrated intensity of each peak, 580cm －１ ~600cm －１ I2, 665cm －１ ~685cm －１ When I3 is taken as the A positive electrode active material having a value of I3 / I2 of 1% or more and 10% or less.

9. In any one of claims 1 to 8, The positive electrode active material has a transition metal M, The positive electrode active material, wherein the transition metal M is 90 atomic % or more of cobalt.

10. In claim 9, A positive electrode active material in which the H1-3 type and O1 type crystal structures account for 50% or less of the positive electrode active material.

11. In claim 10, The positive electrode active material has magnesium, nickel, and aluminum in a surface layer portion.

12. In claim 11, The positive electrode active material has the following properties as a result of line analysis by energy dispersive X-ray analysis: The magnesium and nickel concentration peaks are The positive electrode active material is located on the surface side of the aluminum concentration peak of the positive electrode active material.

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