Lithium ion battery

Abstract

The present invention provides a lithium ion battery which exhibits excellent discharge characteristics even below the freezing point. According to the present invention, the discharge capacity that is obtained by charging a lithium ion battery, which is provided with a positive electrode comprising a positive electrode active material, an electrolyte, and a negative electrode comprising a negative electrode active material that is composed of a carbon material, with a constant current at a charging rate of 0.1 C (wherein 1 C = 200 mA / g) until the voltage reaches 4.5 V at 25°C, then charging the lithium ion battery with a constant voltage of 4.5 V until the current value falls to 0.01 C, and subsequently discharging the lithium ion battery with a constant current at a discharging rate of 0.1 C until the voltage falls to 2.5 V at -40°C is 50% or more of the discharge capacity that is obtained by charging the lithium ion battery with a constant current at a charging rate of 0.1 C (wherein 1 C = 200 mA / g) until the voltage reaches 4.5 V at 25°C, then charging the lithium ion battery with a constant voltage of 4.5 V until the current value falls to 0.01 C, and subsequently discharging the lithium ion battery with a constant current at a discharging rate of 0.1 C until the voltage falls to 2.5 V at 25°C.

Description

Lithium-ion battery

[0001] The invention disclosed in this specification (hereinafter, sometimes referred to as "the present invention") relates to an electricity storage device, a secondary battery, etc., and particularly to a lithium ion battery.

[0002] Alternatively, the present invention relates to an object, a method, or a manufacturing method, or to a process, a machine, a manufacture, or a composition of matter, or to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, an electronic device, or a manufacturing method thereof.

[0003] In recent years, various types of power storage devices, such as lithium-ion batteries, lithium-ion capacitors, and air batteries, have been actively developed. In particular, demand for high-power, high-energy-density lithium-ion batteries has rapidly expanded in conjunction with the development of the semiconductor industry, and they are now indispensable in the modern information society as a rechargeable energy source, as they are used in portable information terminals such as mobile phones, smartphones, and notebook computers, portable music players, digital cameras, medical devices, and clean-energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs).

[0004] The charging and / or discharging characteristics of lithium-ion batteries vary depending on the charging and / or discharging environments of the batteries. For example, it is known that the discharge capacity of lithium-ion batteries varies depending on the temperature during discharging.

[0005] Therefore, there is a demand for lithium ion batteries that have excellent battery characteristics even in low-temperature environments (see, for example, Patent Document 1).

[0006] JP 2015-026608

[0007] Zhaohui Chen et al, “Staging Phase Transitions in Li▲x▼CoO▲2▼”, Journal of The Electrochemical Society, 2002, 149 (12) A1604-A1609

[0008] Patent Document 1 describes that a lithium ion battery capable of operating in a low-temperature environment has been realized by using the nonaqueous solvent described in Patent Document 1. However, even the lithium ion battery described in Patent Document 1 does not have a large discharge capacity when discharged at a temperature of 0°C or below (also called "below freezing") at the time of filing this application, and further improvement is desired.

[0009] An object of one embodiment of the present invention is to provide a lithium-ion battery that has excellent discharge characteristics even at sub-zero temperatures, or to provide a lithium-ion battery that has excellent charge characteristics even at sub-zero temperatures.

[0010] Specifically, an object of the present invention is to provide a lithium-ion battery that has a large discharge capacity and / or discharge energy density even when discharged at a sub-zero temperature (e.g., 0° C. or lower, −20° C. or lower, preferably −30° C. or lower, more preferably −40° C. or lower, even more preferably −50° C. or lower, and most preferably −60° C. or lower). Another object of the present invention is to provide a lithium-ion battery that exhibits a small rate of decrease in discharge capacity and / or discharge energy density when discharged at a sub-zero temperature (e.g., 0° C. or lower, −20° C. or lower, preferably −30° C. or lower, more preferably −40° C. or lower, even more preferably −50° C. or lower, and most preferably −60° C. or lower) compared to the discharge capacity and / or discharge energy density when discharged at 25° C. Another object of the present invention is to provide a lithium-ion battery that has a large charge capacity when charged at a sub-zero temperature (e.g., 0° C. or lower, −20° C. or lower, preferably −30° C. or lower, more preferably −40° C. or lower, even more preferably −50° C. or lower, and most preferably −60° C. or lower). Another object of the present invention is to provide a lithium-ion battery that exhibits a small rate of decrease in charge capacity when charged at a temperature below freezing (for example, 0°C or below, -20°C or below, preferably -30°C or below, more preferably -40°C or below, even more preferably -50°C or below, and most preferably -60°C or below) compared to the value when charged at 25°C.

[0011] Another object of the present invention is to provide a secondary battery with a high charging voltage, a secondary battery with high safety or reliability, a secondary battery with little deterioration, a secondary battery with a long life, or a new secondary battery.

[0012] Another object is to provide a novel substance, an active material, a power storage device, or a manufacturing method thereof.

[0013] Note that the description of these problems does not preclude the existence of other problems. Furthermore, one embodiment of the present invention does not necessarily solve all of these problems. Furthermore, problems other than these can be extracted from the description of this specification, drawings, claims, etc.

[0014] In order to solve the above problems, one embodiment of the present invention has the following configuration.

[0015] One aspect of the present invention is a lithium-ion battery including a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode having a carbonaceous negative electrode active material, wherein the electrolyte contains ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the volume ratio of the ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65), where x is 5≦x≦35 and 0<y<65, when the total content of the ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol %. The lithium ion battery is subjected to constant current charging at a charge rate of 0.1 C (where 1 C = 200 mA / g) in a 25°C environment until a voltage of 4.5 V is reached, followed by constant voltage charging at 4.5 V until the current value reaches 0.01 C, and then constant current discharging at a discharge rate of 0.1 C in a -40°C environment until a voltage of 2.5 V is reached, and the discharge capacity value obtained by this is 50% or more of the discharge capacity value obtained by constant current charging at a charge rate of 0.1 C (where 1 C = 200 mA / g) in a 25°C environment until a voltage of 4.5 V is reached, followed by constant voltage charging at 4.5 V until the current value reaches 0.01 C, and then constant current discharging at a discharge rate of 0.1 C in a 25°C environment until a voltage of 2.5 V is reached.

[0016] Alternatively, in one embodiment of the present invention, the carbon material is graphite.

[0017] Another embodiment of the present invention is a lithium-ion battery that includes a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode, and that can operate at least within a temperature range of −40° C. to 25° C.

[0018] Alternatively, one aspect of the present invention is a lithium-ion battery including a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode. When a test battery is fabricated using the positive electrode active material as a positive electrode, an electrolyte containing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, where the volume ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65) when the total content of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is taken as 100 vol%, and lithium metal is used as a negative electrode, the test battery is heated to 0.1 V at 25° C. until a voltage of 4.6 V is reached. The discharge capacity value obtained by constant current charging at a charge rate of 0.1C (where 1C = 200mA / g) at 25°C to a voltage of 4.6V, constant voltage charging at 4.6V until the current value reaches 0.01C, and then constant current discharging at a discharge rate of 0.1C at -40°C to a voltage of 2.5V is 50% or more of the discharge capacity value obtained by constant current charging at a charge rate of 0.1C (where 1C = 200mA / g) at 25°C to a voltage of 4.6V, constant voltage charging at 4.6V until the current value reaches 0.01C, and then constant current discharging at a discharge rate of 0.1C at 25°C to a voltage of 2.5V.

[0019] Alternatively, in one embodiment of the present invention, the positive electrode active material is Li ｘ CoO ２ (where 0<x≦1), and the Li ｘ CoO ２ When x is 1, the Li has a layered rock salt type crystal structure of the space group R-3m. ｘ CoO ２ When x in the charge state is greater than 0.1 and less than 0.24, the space group is P2 / m, and the lattice constant is a = 4.88 ± 0.01 (× 10 −１ nm), lattice constant b=2.82±0.01(×10 −１ nm), lattice constant c=4.84±0.01(×10 −１ nm), α=90°, β=109.58±0.01°, γ=90°.

[0020] Alternatively, in one embodiment of the present invention, the positive electrode active material is Li ｘ CoO ２ (where 0<x≦1), and the Li ｘ CoO ２ When x is 1, the Li has a layered rock salt type crystal structure of the space group R-3m. ｘ CoO ２ When the charged state of the battery is such that x is greater than 0.1 and less than 0.24, powder X-ray diffraction analysis reveals a diffraction pattern with peaks at least at 2θ = 19.37° to 19.57° and 2θ = 45.57° to 45.67°.

[0021] According to one aspect of the present invention, a lithium ion battery can be provided that has a large discharge capacity and / or discharge energy density even when discharged at a sub-zero temperature (e.g., 0°C or lower, −20°C or lower, preferably −30°C or lower, more preferably −40°C or lower, even more preferably −50°C or lower, and most preferably −60°C or lower). Alternatively, a lithium ion battery can be provided that has a small rate of decrease in discharge capacity and / or discharge energy density even when discharged at a sub-zero temperature (e.g., 0°C or lower, −20°C or lower, preferably −30°C or lower, more preferably −40°C or lower, even more preferably −50°C or lower, and most preferably −60°C or lower) compared to the discharge capacity and / or discharge energy density when discharged at 25°C. Alternatively, a lithium ion battery can be provided that has a large charge capacity even when charged at a sub-zero temperature (e.g., 0°C or lower, −20°C or lower, preferably −30°C or lower, more preferably −40°C or lower, even more preferably −50°C or lower, and most preferably −60°C or lower). Alternatively, a lithium ion battery can be provided in which, even when charged at a temperature below freezing (for example, 0°C or below, -20°C or below, preferably -30°C or below, more preferably -40°C or below, even more preferably -50°C or below, and most preferably -60°C or below), the rate of decrease in charge capacity is small compared to the value when charged at 25°C.

[0022] According to one embodiment of the present invention, a secondary battery with a high charging voltage can be provided. Alternatively, a secondary battery with high safety or reliability can be provided. Alternatively, a secondary battery with little deterioration can be provided. Alternatively, a secondary battery with a long life can be provided. Alternatively, a novel secondary battery can be provided.

[0023] According to one embodiment of the present invention, a novel substance, an active material, a power storage device, or a manufacturing method thereof can be provided.

[0024] FIGS. 1A1 and 1A2 are cross-sectional views of a positive electrode active material, and FIGS. 1B1 and 1B2 are portions of the cross-sectional views of the positive electrode active material. FIG. 2 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 3A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 3B is an FFT pattern of a region of the rock salt crystal RS, and FIG. 3C is an FFT pattern of a region of the layered rock salt crystal LRS. FIG. 4 is a diagram illustrating the crystalline structure of a positive electrode active material. FIG. 5 is a diagram illustrating the crystalline structure of a conventional positive electrode active material. FIGS. 6A1 and 6A2 are portions of the cross-sectional views of a positive electrode active material. FIGS. 6B1 to 6C show calculation results for the crystal planes and magnesium distribution of lithium cobalt oxide. FIGS. 7A and 7B are cross-sectional views of a positive electrode active material, and FIGS. 7C1 and 7C2 are portions of the cross-sectional views of the positive electrode active material. FIG. 8 is a diagram illustrating an XRD pattern calculated from the crystalline structure. FIG. 9 shows an XRD pattern calculated from the crystal structure. FIGS. 10A and 10B show XRD patterns calculated from the crystal structure. FIGS. 11A to 11C show lattice constants calculated from XRD. FIGS. 12A to 12C show lattice constants calculated from XRD. FIGS. 13A and 13B show cross-sectional views of a positive electrode active material. FIG. 14 shows a cross-sectional view of a positive electrode active material. FIGS. 15A to 15C are diagrams illustrating a method for fabricating a positive electrode active material. FIG. 16 is a diagram illustrating a method for fabricating a positive electrode active material. FIGS. 17A to 17C are diagrams illustrating a method for fabricating a positive electrode active material. FIGS. 18A to 18D are cross-sectional views illustrating an example of a positive electrode of a secondary battery. FIG. 19A is an exploded perspective view of a coin-type secondary battery, FIG. 19B is a perspective view of the coin-type secondary battery, and FIG. 19C is a cross-sectional perspective view thereof. FIG. 20A shows an example of a cylindrical secondary battery. FIG. 20B shows an example of a cylindrical secondary battery. FIG. 20C shows an example of a plurality of cylindrical secondary batteries. FIG. 20D shows an example of a power storage system having a plurality of cylindrical secondary batteries. FIGS. 21A and 21B are diagrams illustrating an example of a secondary battery, and FIG. 21C is a diagram illustrating the internal state of the secondary battery. FIGS. 22A to 22C are diagrams illustrating an example of a secondary battery. FIGS. 23A and 23B are diagrams illustrating the external appearance of a secondary battery. FIGS. 24A to 24C are diagrams illustrating a method for manufacturing a secondary battery.FIG. 25A illustrates an example of a battery pack configuration, FIG. 25B illustrates an example of a battery pack configuration, and FIG. 25C illustrates an example of a battery pack configuration. FIG. 26A is a perspective view of a battery pack illustrating one embodiment of the present invention, FIG. 26B is a block diagram of the battery pack, and FIG. 26C is a block diagram of a vehicle having a motor. FIGS. 27A to 27D are diagrams illustrating an example of a transportation vehicle. FIG. 27E is a diagram illustrating an example of an artificial satellite. FIGS. 28A and 28B are diagrams illustrating a power storage device according to one embodiment of the present invention. FIG. 29A is a diagram illustrating an electric bicycle, FIG. 29B is a diagram illustrating a secondary battery of the electric bicycle, and FIG. 29C is a diagram illustrating an electric motorcycle. FIGS. 30A to 30D are diagrams illustrating an example of an electronic device. FIG. 31A illustrates an example of a wearable device, FIG. 31B is a perspective view of a wristwatch-type device, and FIG. 31C is a diagram illustrating a side view of the wristwatch-type device. Fig. 32 is a graph showing the discharge capacity of the secondary battery at each temperature during discharge, as described in Example 1. Fig. 33 is a graph showing the charge capacity of the secondary battery at each temperature during charge, as described in Example 1. Figs. 34A and 34B are graphs showing the discharge curves of the secondary battery at each temperature, as described in Example 1. Figs. 35A and 35B are graphs showing the discharge curves of the secondary battery at each temperature, as described in Example 1. Figs. 36A and 36B are graphs showing the cycle characteristics of the secondary battery described in Example 2.

[0025] The following description will explain the embodiments of the present invention with reference to the accompanying drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes can be made in form and detail without departing from the spirit and scope of the present invention. Therefore, in the embodiments described below, the same reference numerals are used in different drawings to indicate the same objects.

[0026] Furthermore, in each of the embodiments and examples described below, unless otherwise specified, it is possible to implement the embodiments and examples described in this specification and the like in appropriate combinations.

[0027] In this specification and the like, the term "electronic device" refers to any device having a power storage device, and electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like are all electronic devices.

[0028] In this specification and the like, the term "power storage device" refers to elements and devices in general that have a power storage function, including, for example, power storage devices such as lithium ion batteries (also called "secondary batteries"), lithium ion capacitors, and electric double layer capacitors.

[0029] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. Individual planes indicating crystal planes are expressed using ( ). In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice to facilitate understanding of the structure, and Miller indices such as (hkl) and (hkil) may be used. Here, i is −(h+k).

[0030] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated. ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 275mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0031] In addition, the amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by x in the composition formula, for example, Li ｘ CoO ２In the case of a positive electrode active material of a secondary battery, x can be expressed as charge capacity / theoretical capacity. For example, LiCoO ２ When a secondary battery using as a positive electrode active material is charged at 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ The state where x is small is, for example, x≦0.24, and in consideration of the practical range when used as a lithium ion battery, it is assumed that 0.1<x≦0.24, for example.

[0032] When the lithium cobalt oxide satisfies the stoichiometric ratio, LiCoO ２ where x=1. Furthermore, the lithium cobalt oxide contained in the secondary battery after discharge is also LiCoO ２ In other words, x=1. ２ In a lithium-ion battery using a lithium cobalt oxide battery, the discharge voltage drops sharply before it reaches 2.5 V. For this reason, in this specification and the like, the state in which the voltage reaches 2.5 V (with lithium as the counter electrode) at a current of 100 mA / g or less is regarded as the end of discharge, and x = 1. Therefore, to obtain lithium cobalt oxide with x = 0.2, for example, charging to 219.2 mAh / g from the end of discharge is sufficient.

[0033] Li ｘ CoO ２ The charge capacity and / or discharge capacity used to calculate x in the above calculations should preferably be measured under conditions where there is little or no influence of short circuit and / or decomposition of the electrolyte. For example, it is not preferable to use data from a secondary battery that has experienced a sudden change in capacity that is considered to be due to a short circuit in the calculation of x.

[0034] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0035] Furthermore, if the anions have a structure in which three layers are stacked with a skew, such as ABCABC, it is called a "cubic close-packed structure." Therefore, the anions do not need to be strictly cubic lattices. At the same time, since real crystals always have defects, the analysis results do not necessarily have to be theoretical. For example, in an FFT (fast Fourier transform) pattern such as an electron diffraction pattern or a TEM image, spots may appear at positions slightly different from the theoretical positions. For example, if the orientation from the theoretical positions is 5° or less, or 2.5° or less, it can be said to have a cubic close-packed structure.

[0036] In this specification, the term "layered rock-salt type crystal structure of a composite oxide containing lithium and a transition metal" refers to a crystal structure having a rock-salt type ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. The layered rock-salt type crystal structure may have defects such as cation or anion deficiencies. Strictly speaking, the layered rock-salt type crystal structure may have a distorted rock-salt type crystal lattice.

[0037] In this specification, the term "rock salt type crystal structure" refers to a structure in which cations and anions are arranged alternately. Note that there may be deficiencies of cations or anions.

[0038] In this specification, the term "homogeneous" refers to a phenomenon in which a certain element (e.g., A) is distributed with similar characteristics in a specific region in a solid composed of multiple elements (e.g., A, B, C). Specifically, it is sufficient that the concentration of the element in each specific region is substantially the same. For example, it is sufficient that the difference in element concentration between each specific region is within 10%. Examples of specific regions include a surface layer, a surface, a convex portion, a concave portion, and an interior.

[0039] In this specification, the term "segregation" refers to a phenomenon in which a certain element (e.g., B) is spatially distributed non-uniformly in a solid composed of multiple elements (e.g., A, B, C). Alternatively, it refers to the concentration of a certain element being different from that of others. It is synonymous with uneven distribution, precipitation, non-uniformity, bias, or the presence of a mixture of areas with high concentration and areas with low concentration.

[0040] In this specification, the "surface layer" of a particle of an active material or the like refers to, for example, a region within 50 nm, more preferably within 35 nm, even more preferably within 20 nm, and most preferably within 10 nm from the surface toward the interior. Furthermore, a surface formed by a crack or a fissure can be considered the surface. Furthermore, in this specification, a region deeper than the surface layer may be referred to as the "interior." Furthermore, in this specification, the term "grain boundary" refers to, for example, a portion where particles are adhered to each other, a portion where the crystal orientation changes inside the particle (including the center), a portion containing many defects, a portion where the crystal structure is disordered, etc. The grain boundary can also be considered a type of planar defect. Furthermore, the term "vicinity of the grain boundary" refers to a region within 20 nm, preferably within 10 nm, from the grain boundary. Furthermore, in this specification, the term "particle" is not limited to a spherical shape (circular in cross section), and examples of the cross-sectional shape of each particle include elliptical, rectangular, trapezoidal, triangular, square with rounded corners, and asymmetrical shapes. Furthermore, each particle may have an irregular shape.

[0041] (Embodiment 1) [Lithium-ion battery] A lithium-ion battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. When the electrolyte contains an electrolytic solution, a separator is provided between the positive electrode and the negative electrode. The lithium-ion battery according to one embodiment of the present invention may also include an exterior body that covers at least a portion of the periphery of the positive electrode, the negative electrode, and the electrolyte.

[0042] In this embodiment, the description will focus on the configuration of a lithium ion battery that is required to realize a lithium ion battery that has excellent discharge characteristics even below freezing (for example, 0° C. or below, −20° C. or below, preferably −30° C. or below, more preferably −40° C. or below, even more preferably −50° C. or below, and most preferably −60° C. or below) and / or a lithium ion battery that has excellent charge characteristics even below freezing. Specifically, the description will focus on the positive electrode active material and electrolyte contained in the positive electrode. Details of the configuration of a lithium ion battery other than the positive electrode active material and electrolyte will be described in embodiment 3.

[0043] [Positive Electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a positive electrode active material, and may further have at least one of a conductive additive and a binder.

[0044] <Positive Electrode Active Material> The positive electrode active material has the function of incorporating and / or releasing lithium ions during charge and discharge. The positive electrode active material used in one embodiment of the present invention can be a material that exhibits little deterioration (or little increase in resistance) during charge and / or discharge (hereinafter also referred to as "charge and discharge") at sub-zero temperatures, even at high charge voltages (hereinafter also referred to as "high charge voltages"). Unless otherwise specified in this specification, "charge voltage" refers to a charge voltage of, for example, 4.6 V or higher, preferably 4.65 V or higher, 4.7 V or higher, 4.75 V or higher, or 4.8 V or higher. Two or more positive electrode active materials with different particle sizes and / or compositions can be used as long as they exhibit little deterioration during charge and discharge, even at high charge voltages. In this specification, "different compositions" refers not only to materials with different elemental compositions, but also to materials with the same elemental composition but different proportions of the elements.

[0045] As mentioned above, in this specification, a "high charging voltage" is defined as 4.6 V or higher based on the potential when the negative electrode is made of lithium metal, but when the potential when the negative electrode is made of a carbon material (e.g., graphite) is used as the reference, a "high charging voltage" is defined as 4.5 V or higher. In short, in the case of a half cell using lithium metal as the negative electrode, a charging voltage of 4.6 V or higher is defined as a high charging voltage, and in the case of a full cell using a carbon material (e.g., graphite) as the negative electrode, a charging voltage of 4.5 V or higher is defined as a high charging voltage.

[0046] By using a material that undergoes little deterioration (or little increase in resistance) during charging and discharging at any sub-zero temperature (e.g., 0°C, −20°C, preferably −30°C, more preferably −40°C, even more preferably −50°C, and most preferably −60°C) as the positive electrode active material, a lithium-ion battery with large charge and / or discharge capacities even at sub-zero temperatures can be realized. Alternatively, a lithium-ion battery can be realized in which the charge and / or discharge capacities at any sub-zero temperature (e.g., 0°C, −20°C, preferably −30°C, more preferably −40°C, even more preferably −50°C, and most preferably −60°C) are 50% or more (preferably 60% or more, more preferably 70% or more, and most preferably 80% or more) of the charge and / or discharge capacities at 25°C. The discharge capacity values ​​at any sub-zero temperature and the discharge capacity values ​​at 25°C are measured under the same conditions except for the temperature during discharge (hereinafter sometimes referred to as the “discharge temperature” in this specification).

[0047] Alternatively, a lithium ion battery having a high discharge energy density can be realized even at any sub-zero temperature (e.g., 0°C, -20°C, preferably -30°C, more preferably -40°C, even more preferably -50°C, and most preferably -60°C). Alternatively, a lithium ion battery can be realized in which the discharge energy density value at any sub-zero temperature (e.g., 0°C, -20°C, preferably -30°C, more preferably -40°C, even more preferably -50°C, and most preferably -60°C) is 50% or more (preferably 60% or more, more preferably 70% or more, and most preferably 80% or more) of the discharge energy density value at 25°C. Note that the discharge energy density value at any sub-zero temperature and the discharge energy density value at 25°C are measured under the same conditions except for the temperature during discharge.

[0048] Alternatively, a lithium-ion battery can be realized in which the charge capacity value at any sub-freezing temperature (for example, 0° C., −20° C., preferably −30° C., more preferably −40° C., even more preferably −50° C., and most preferably −60° C.) is 50% or more (preferably 60% or more, more preferably 70% or more, and most preferably 80% or more) of the charge capacity value at 25° C. Note that the charge capacity value at any sub-freezing temperature and the charge capacity value at 25° C. are measured under the same conditions except for the temperature during charging.

[0049] The temperature during charging or discharging described in this specification refers to the temperature of a lithium-ion battery. In measuring battery characteristics at various temperatures, for example, a thermostatic chamber stabilized at a desired temperature is used, and the battery to be measured (e.g., a test battery or half-cell) is placed in the thermostatic chamber. After that, measurement can be started after a sufficient time (e.g., one hour or more) has elapsed until the temperature of the test cell reaches the same level as that of the thermostatic chamber, but this method is not necessarily limited to this.

[0050] <Electrolyte> The electrolyte used in one embodiment of the present invention can be a material that has excellent lithium ion conductivity at any temperature below the freezing point (e.g., 0°C, -20°C, preferably -30°C, more preferably -40°C, even more preferably -50°C, and most preferably -60°C), even when charging and discharging at such a temperature.

[0051] An example of an electrolyte will be described below. Note that the electrolyte described in this embodiment as an example is an electrolyte (lithium salt) dissolved in an organic solvent, and can also be called an electrolytic solution. However, the electrolyte is not limited to a liquid electrolyte (electrolytic solution) that is liquid at room temperature, and a solid electrolyte can also be used. Alternatively, an electrolyte (semi-solid electrolyte) that includes both a liquid electrolyte that is liquid at room temperature and a liquid electrolyte that is solid at room temperature can also be used.

[0052] As an example, the organic solvent described in this embodiment contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and when the total content of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is 100 vol%, the volume ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is x:y:100-x-y (where 5≦x≦35 and 0<y<65). More specifically, an organic solvent containing EC, EMC, and DMC in a volume ratio of EC:EMC:DMC=30:35:35 can be used. Note that the above volume ratio may be the volume ratio before mixing with the electrolytic solution, and the outside air temperature when mixing the electrolytic solution may be room temperature (typically, 25°C).

[0053] EC is a cyclic carbonate with a high dielectric constant, which promotes the dissociation of lithium salts. However, EC has a high viscosity and a high freezing point (melting point) of 38°C, making it difficult to use EC alone as an organic solvent in low-temperature environments. Therefore, the organic solvent specifically described as one aspect of the present invention further contains EMC and DMC, rather than EC alone. EMC is a chain carbonate that reduces the viscosity of the electrolyte and has a freezing point of −54°C. DMC is also a chain carbonate that reduces the viscosity of the electrolyte. An electrolyte prepared using an organic solvent containing EC, EMC, and DMC with such physical properties, mixed in a volume ratio of x:y:100−x−y (where 5≦x≦35 and 0<y<65), assuming a total content of these three organic solvents of 100 vol%, is characterized by a freezing point of −40°C or lower.

[0054] A typical electrolyte used in a lithium ion battery freezes at a temperature of at least −20° C., making it difficult to fabricate a battery that can be charged and discharged at −40° C. The electrolyte described as an example in this embodiment has a freezing point of −40° C. or lower, making it possible to realize a lithium ion battery that can be charged and discharged even in an extremely low temperature environment of −40° C.

[0055] The electrolyte dissolved in the solvent may be a lithium salt, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (LiBOB) and at least one lithium salt thereof can be used in any combination and ratio.

[0056] Furthermore, it is preferable that the electrolyte solution be highly purified, with a low content of granular dust or elements other than the constituent elements of the electrolyte solution (hereinafter simply referred to as "impurities"). Specifically, it is preferable that the weight ratio of impurities to the electrolyte solution be 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0057] Furthermore, for the purpose of improving safety, etc., an additive such as vinylene carbonate (VC), propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), or a dinitrile compound such as succinonitrile or adiponitrile may be added to the electrolyte to form a coating (solid electrolyte interphase film) at the interface between the electrode (active material layer) and the electrolyte. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % relative to the solvent.

[0058] As described above, an example of an electrolyte that can be used in the lithium-ion battery of one embodiment of the present invention has been described, but the electrolyte that can be used in the lithium-ion battery of one embodiment of the present invention is not limited to this example. Other materials can also be used as long as they have excellent lithium-ion conductivity even when charged and discharged below freezing (for example, −20° C., preferably −40° C.).

[0059] The lithium-ion battery of one embodiment of the present invention includes at least the above-described positive electrode active material and electrolyte, and thus can achieve a lithium-ion battery having excellent discharge characteristics even below freezing point and / or a lithium-ion battery having excellent charge characteristics even below freezing point. More specifically, when a test battery is prepared by using at least the above-described positive electrode active material and electrolyte and lithium metal as the negative electrode, a lithium ion battery can be realized in which the test battery is subjected to constant current charging at a charge rate of 0.1 C or 0.2 C (where 1 C = 200 mA / g) until a voltage of 4.6 V is reached in a 25°C environment, and then discharged at a constant current rate of 0.1 C until a voltage of 2.5 V is reached in a -40°C environment, and the discharge capacity value determined by this charging is 50% or more of the discharge capacity value determined by constant current charging at a charge rate of 0.1 C or 0.2 C (where 1 C = 200 mA / g) until a voltage of 4.6 V is reached in a 25°C environment, and then discharged at a constant current rate of 0.1 C until a voltage of 2.5 V is reached in a 25°C environment. In this specification, a lithium ion battery is said to be operable at T°C (where T is an arbitrary temperature (°C)) if the battery can achieve a discharge capacity of 50% or more in a T°C environment compared to the discharge capacity in a 25°C environment.

[0060] 1 to 14 , a cathode active material that can be used in a lithium-ion battery according to one embodiment of the present invention (hereinafter, also referred to as a "cathode active material that can be used as one embodiment of the present invention") and a manufacturing method thereof will be described. As described in Embodiment 1, any material that exhibits little deterioration due to charge and discharge even at a high charge voltage (high charge voltage) can be used as the cathode active material that can be used in a lithium-ion battery according to one embodiment of the present invention. Therefore, the cathode active material that can be used in the lithium-ion battery disclosed in this specification and the like does not need to be limited to the specific materials described in this embodiment and the like. Materials that are known at the time of filing this application as materials that exhibit little deterioration due to charge and discharge even at a high charge voltage (for example, 4.6 V or higher) can also be used.

[0061] <Example of Positive Electrode Active Material> An example of a positive electrode active material that can be used in one embodiment of the present invention will be described below.

[0062] In this embodiment, a positive electrode active material 100 that can be used as one embodiment of the present invention will be described with reference to FIGS.

[0063] 1A1 and 1A2 are cross-sectional views of a positive electrode active material 100 according to one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 1A1 are shown in FIGS.

[0064] 1A1, 1B1, and 1B2, a positive electrode active material 100 has a surface layer 100a and an inner portion 100b. In these figures, a dashed line indicates the boundary between the surface layer 100a and the inner portion 100b. In addition, in FIG. 1A2, a dashed line indicates a portion of a crystal grain boundary 101.

[0065] 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 inside, more preferably within 35 nm from the surface toward the inside, even more preferably within 20 nm from the surface toward the inside, and most preferably within 10 nm from the surface toward the inside. In this specification, a surface caused by cracks and / or fissures may also be referred to as the surface. The surface layer 100a is synonymous with the near-surface, near-surface region, or shell.

[0066] The region deeper than the surface layer 100a of the positive electrode active material is referred to as the inner portion 100b, which is synonymous with the inner region or core.

[0067] The surface of the positive electrode active material 100 refers to the surface of the composite oxide including the surface layer 100a, the interior 100b, and the protrusions 103. Therefore, the positive electrode active material 100 does not contain carbonates, hydroxyl groups, or the like that are chemically adsorbed after preparation. The positive electrode active material 100 also does not contain electrolytes, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 100. The surface of the positive electrode active material 100 in a cross-sectional STEM (scanning transmission electron microscope) image, etc., is the boundary between the region where an electron beam combined image is observed and the region where it is not observed, and is the outermost region where bright spots originating from the atomic nuclei of metal elements with atomic numbers larger than that of lithium are observed. The surface in a cross-sectional STEM image, etc., may be determined in conjunction with the results of higher spatial resolution analyses, such as electron energy loss spectroscopy (EELS).

[0068] The crystal grain boundary 101 refers to, for example, a portion where particles of the positive electrode active material 100 are adhered to each other, a portion where the crystal orientation changes within the positive electrode active material 100, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. The crystal defect refers to a defect that can be observed in a cross-sectional TEM (transmission electron microscope) image, a cross-sectional STEM image, etc., that is, a structure in which other atoms have entered between the lattices, a cavity (void), etc. The crystal grain boundary 101 can be said to be one of planar defects. The vicinity of the crystal grain boundary 101 refers to a region within 20 nm (preferably within 15 nm, more preferably within 10 nm) from the crystal grain boundary 101.

[0069] <Containing Elements> The positive electrode active material 100 contains lithium, cobalt, oxygen, and an additive element. Alternatively, the positive electrode active material 100 contains lithium cobalt oxide (LiCoO ２ However, the positive electrode active material 100 described in this embodiment may have any structure as long as it has the crystal structure described below. Therefore, the composition of lithium cobalt oxide is not strictly limited to Li:Co:O=1:1:2.

[0070] The positive electrode active material of a lithium ion battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 100 of a lithium ion battery according to one embodiment of the present invention preferably contains cobalt as the transition metal responsible for the oxidation and reduction reaction. In addition to cobalt, the positive electrode active material 100 may also contain at least one of nickel and manganese. It is preferable that the positive electrode active material 100 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals contained therein, in terms of relatively easy synthesis and handling, and excellent cycle characteristics.

[0071] Furthermore, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metals in the positive electrode active material 100, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal, ｘ CoO ２ The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) accounts for the majority of the transition metal, are significantly affected by the Jahn-Teller effect, making the octahedral layers of nickel and oxygen prone to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions are larger than cobalt ions and are closer in size to lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel accounts for the majority of the transition metal, are prone to cation mixing between nickel and lithium.

[0072] The additive element contained in the positive electrode active material 100 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. When one or more transition metals are used as the additive element, the transition metals (when two or more are used, the total) are preferably less than 25 atomic %, more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0073] Specific examples of the positive electrode active material 100 include lithium cobalt oxide doped with magnesium and fluorine, lithium cobalt oxide doped with magnesium, fluorine, and titanium, lithium cobalt oxide doped with magnesium, fluorine, and aluminum, lithium cobalt oxide doped with magnesium, fluorine, and nickel, and lithium cobalt oxide doped with magnesium, fluorine, nickel, and aluminum.

[0074] As will be described later, the presence of these additional elements has the effect of further stabilizing the crystal structure of the positive electrode active material 100. In this specification and the like, the additional elements may be a mixture or a part of the raw materials.

[0075] The additive element does 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.

[0076] For example, if the cathode active material 100 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, as described above, will be even greater. The weight of manganese contained in the cathode active material 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less. In this specification, the term "substantially free" refers to a case where the manganese content is below the lower limit of detection when measured using analytical means, or a case where the manganese content is at or near the lower limit of detection but does not affect the effectiveness of the cathode active material.

[0077] <Crystal structure> <Li ｘ CoO ２ When x is 1 in the formula (I), the positive electrode active material 100 that can be used as one embodiment of the present invention is in a discharged state, that is, Li ｘ CoO ２ When x = 1 in the formula, it is preferable that the composite oxide has a layered rock-salt type crystal structure belonging to the space group R-3m. Layered rock-salt type composite oxides have a high discharge capacity, two-dimensional lithium ion diffusion paths, and are suitable for lithium ion insertion / extraction reactions, making them excellent as positive electrode active materials for secondary batteries. Therefore, it is particularly preferable that the inner portion 100b, which occupies 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, labeled R-3m(O3).

[0078] On the other hand, the surface layer 100a of the cathode active material 100 usable as one embodiment of the present invention preferably has a function of reinforcing the inner portion 100b, which is made up of octahedra of cobalt and oxygen, so that the layered structure formed by octahedra of cobalt and oxygen is not destroyed even when lithium is released from the cathode active material 100 upon charging. Alternatively, the surface layer 100a preferably functions as a barrier film for the cathode active material 100. Alternatively, the surface layer 100a, which is the outer periphery of the cathode active material 100, preferably reinforces the cathode active material 100. Here, "reinforcement" refers to suppressing structural changes in the surface layer 100a and inner portion 100b of the cathode active material 100, such as oxygen desorption, and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 100.

[0079] Therefore, it is preferable that the surface layer portion 100a has a different crystal structure from the interior portion 100b. Furthermore, it is preferable that the surface layer portion 100a has a composition and crystal structure that are more stable at room temperature (25°C) than the interior portion 100b. For example, it is preferable that at least a portion of the surface layer portion 100a of the positive electrode active material 100 that can be used as one embodiment of the present invention has a rock salt crystal structure. Alternatively, it is preferable that the surface layer portion 100a has both a layered rock salt crystal structure and a rock salt crystal structure. Alternatively, it is preferable that the surface layer portion 100a has characteristics of both a layered rock salt crystal structure and a rock salt crystal structure.

[0080] The surface layer 100a is the region from which lithium ions are first released during charging, and is a region where the lithium concentration is likely to be lower than that of the inner portion 100b. In addition, it can be said that the atoms on the surface of the positive electrode active material 100 in the surface layer 100a are in a state where some of the bonds are broken. Therefore, the surface layer 100a is likely to become unstable, and is a region where deterioration of the crystal structure is likely to begin. On the other hand, if the surface layer 100a can be made sufficiently stable, Li ｘ CoO ２ When x is small, for example, even if x is 0.24 or less, the layered structure of the inner portion 100b made of cobalt and oxygen octahedra can be made less likely to break. Furthermore, displacement of the layers made of cobalt and oxygen octahedra in the inner portion 100b can be suppressed.

[0081] In order to give the surface layer portion 100a a stable composition and crystalline structure, it is preferable that the surface layer portion 100a contains an additive element, and more preferably contains multiple additive elements. Furthermore, it is preferable that the surface layer portion 100a has a higher concentration of one or more selected from the additive elements than the interior portion 100b. Furthermore, it is preferable that one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. Furthermore, it is more preferable that the distribution of the additive elements in the positive electrode active material 100 differs depending on the additive element. For example, it is more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The concentration peak here refers to the maximum concentration value in the surface layer portion 100a or within 50 nm from the surface.

[0082] For example, as some of the additive elements, magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, calcium, etc. preferably have a concentration gradient that increases from the interior 100b toward the surface, as shown by the gradation in Fig. 1B1. In this specification, these additive elements will be referred to as additive element X.

[0083] Furthermore, it is preferable that other additive elements, such as aluminum and manganese, have a concentration gradient and / or a concentration peak in a region deeper than the additive element X, as shown by the density of the hatching in FIG. 1B2. The concentration peak may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. For example, it is preferable that the peak be present in a region of 5 nm to 50 nm from the surface toward the interior. In this specification and the like, these additive elements will be referred to as additive element Y.

[0084] For example, magnesium, which is one of the additive elements X, is divalent, and magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, so they are more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer 100a makes it easier to maintain the layered rock-salt crystal structure. This is because magnesium present at the lithium site is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. ｘ CoO ２ When x in the formula (1) is, for example, 0.24 or less, the desorption of oxygen around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the positive electrode active material 100. Furthermore, a high magnesium concentration in the surface layer portion 100a is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0085] At an appropriate concentration, magnesium does not adversely affect the intercalation and deintercalation of lithium during charging and discharging, thereby providing the above-mentioned benefits. However, excessive magnesium may adversely affect the intercalation and deintercalation of lithium. Furthermore, the effect on stabilizing the crystal structure may be reduced. This is thought to be due to magnesium occupying the cobalt site in addition to the lithium site. In addition, unnecessary magnesium compounds (oxides, fluorides, etc.) that do not substitute for either the lithium or cobalt site may segregate on the surface of the positive electrode active material and become resistance components in the secondary battery. Furthermore, as the magnesium concentration of the positive electrode active material increases, the discharge capacity of the positive electrode active material may decrease. This is thought to be due to excessive magnesium occupancy at the lithium site, reducing the amount of lithium contributing to charging and discharging.

[0086] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 100 is appropriate. For example, the number of magnesium atoms is preferably 0.001 to 0.1 times the number of cobalt atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium contained in the entire positive electrode active material 100 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 100 using, for example, glow discharge mass spectrometry (GD-MS) or inductively coupled plasma mass spectrometry (ICP-MS), or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 100.

[0087] Nickel, which is one of the additional elements X, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, its oxidation-reduction potential is lower than that of cobalt, which is preferable as it leads to an increase in discharge capacity.

[0088] Furthermore, when nickel is present at the lithium site, the layer structure consisting of octahedra of cobalt and oxygen can be prevented from shifting. Also, the volume change caused by charge and discharge is prevented. Also, the elastic modulus increases, that is, the material becomes hard. This is because nickel present at the lithium site and CoO ２This is presumably because they function as pillars supporting the layers together. This is particularly preferable because it is expected that the crystal structure will become more stable in a charged state at high temperatures, for example, 45° C. or higher.

[0089] On the other hand, an excess of nickel is undesirable because it increases the influence of strain due to the Jahn-Teller effect, and may also adversely affect the insertion and extraction of lithium.

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

[0091] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Furthermore, aluminum suppresses the elution of surrounding cobalt and improves continuous charging durability. Furthermore, because 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, the presence of aluminum as an additive element can improve safety when used in a secondary battery. Furthermore, the positive electrode active material 100 can be made to have a crystal structure that is less likely to collapse even with repeated charging and discharging. Furthermore, it is preferable that aluminum be present at a position slightly deeper than the outermost surface (specifically, the aluminum concentration peak should be located deeper than the concentration peak of the additive element X). Alternatively, it is preferable that the presence of aluminum is confirmed in a region deeper than the deepest region from the outermost surface where the presence of the additional element X is confirmed, and that the deepest region from the outermost surface is present. This is because, when aluminum is substituted for a lithium site, lithium present in the vicinity of the lithium site substituted by aluminum is also fixed, and therefore, if aluminum is present at the outermost surface, it may obstruct the diffusion path of lithium more than the additional element X.

[0092] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

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

[0094] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When a portion of the oxygen in the surface layer 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the valence of cobalt ions changes with lithium desorption, from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine, resulting in different oxidation-reduction potentials. Therefore, when a portion of the oxygen in the surface layer 100a of the positive electrode active material 100 is substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine occurs more smoothly. Therefore, when the positive electrode active material 100 is used in a secondary battery, charge / discharge characteristics, large current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer 100a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. Furthermore, as will be described in a later embodiment, when the melting point of a fluoride, such as lithium fluoride, is lower than the melting point of another additive element source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element source.

[0095] Furthermore, titanium oxide, which is one of the additive elements X, is known to have superhydrophilicity. Therefore, by providing the cathode active material 100 with titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 has good wettability with highly polar solvents. When used in a secondary battery, this improves the contact at the interface between the cathode active material 100 and a highly polar electrolyte, which may suppress an increase in internal resistance.

[0096] Furthermore, when phosphorus, which is one of the additive elements X, is contained in the surface layer portion 100a, Li ｘ CoO ２ When the value of x in the graphite layer 100 is kept small, it is possible to prevent short circuits, which is preferable. For example, it is preferable that the graphite layer 100 is present in the surface layer 100a as a compound containing phosphorus and oxygen.

[0097] When the positive electrode active material 100 contains phosphorus, the hydrogen fluoride generated by decomposition of the electrolyte reacts with the phosphorus, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0098] The electrolyte is LiPF ６ In the case where the electrolyte contains the above-mentioned compound, hydrogen fluoride may be generated by hydrolysis. Furthermore, hydrogen fluoride may be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the positive electrode, and an alkali. By reducing the hydrogen fluoride concentration in the electrolyte, corrosion of the current collector and / or peeling of the coating 104 may be suppressed. Furthermore, a decrease in adhesion due to gelation and / or insolubilization of PVDF may be suppressed.

[0099] Furthermore, when the positive electrode active material 100 contains phosphorus together with magnesium, Li ｘ CoO ２This is preferable because stability is extremely high when x is small in the positive electrode active material 100. When the positive electrode active material 100 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Alternatively, 1% to 10% is preferable. Alternatively, 1% to 8% is preferable. Alternatively, 2% to 20% is preferable. Alternatively, 2% to 8% is preferable. Alternatively, 3% to 20% is preferable. Alternatively, 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Alternatively, 0.1% to 5% is preferable. Alternatively, 0.1% to 4% is preferable. Alternatively, 0.5% to 10% is preferable. Alternatively, 0.5% to 4% is preferable. Alternatively, 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 100 using, for example, GC-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 100.

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

[0101] Furthermore, when the surface layer 100a contains both magnesium and nickel, there is a possibility that divalent magnesium can exist more stably near divalent nickel. ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 100a.

[0102] Furthermore, it is preferable to have additive elements with different distributions, such as additive element X and additive element Y, in combination, because this stabilizes the crystal structure in a wider region. For example, when the positive electrode active material 100 contains both magnesium and nickel, which are part of the additive element X, and aluminum, which is one of the additive elements Y, it can stabilize the crystal structure in a wider region than when it contains only one of the additive element X and the additive element Y. In this way, when the positive electrode active material 100 contains both additive element X and additive element Y, the surface can be sufficiently stabilized by the additive element X, such as magnesium or nickel, so that the additive element Y, such as aluminum, is not essential to the surface. Rather, it is preferable for aluminum to be widely distributed in a deep region, for example, a region from the surface to a depth of 5 nm to 50 nm, because this stabilizes the crystal structure in a wider region.

[0103] When a plurality of additive elements are contained as described above, the effects of the respective additive elements are synergistic and can contribute to further stabilization of the surface layer portion 100a. In particular, when magnesium, nickel, and aluminum are contained, the effect of providing a stable composition and crystal structure is high and is therefore preferable.

[0104] However, if the surface layer 100a is occupied only by a compound of the additive element and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 100a to be occupied only by MgO, a structure in which MgO and NiO(II) are solid-solved, and / or a structure in which MgO and CoO(II) are solid-solved. Therefore, it is preferable that the surface layer 100a contains at least cobalt and also contains lithium in a discharged state, and has a path for insertion and extraction of lithium.

[0105] Furthermore, in order to ensure sufficient paths for lithium insertion and desorption, it is preferable that the surface layer 100a has a higher cobalt concentration than magnesium. For example, it is preferable that the ratio Mg / Co of the number of magnesium atoms Mg to the number of cobalt atoms Co is 0.62 or less. It is also preferable that the surface layer 100a has a higher cobalt concentration than nickel. It is also preferable that the surface layer 100a has a higher cobalt concentration than aluminum. It is also preferable that the surface layer 100a has a higher cobalt concentration than fluorine.

[0106] Furthermore, since an excessive amount of nickel may hinder the diffusion of lithium, it is preferable that the concentration of magnesium in the surface layer 100a is higher than that of nickel. For example, it is preferable that the number of nickel atoms is 1 / 6 or less of the number of magnesium atoms.

[0107] Furthermore, although it is preferable that some of the added elements, particularly magnesium, nickel, and aluminum, have a higher concentration in the surface layer 100a than in the interior 100b, they are also preferably present randomly and dilutely in the interior 100b. When magnesium and aluminum are present at appropriate concentrations at the lithium sites in the interior 100b, it has the effect of making it easier to maintain the layered rock-salt crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior 100b, it can suppress the shift in the layered structure consisting of cobalt and oxygen octahedra, as described above. Furthermore, when magnesium and nickel are present together, a synergistic effect of suppressing magnesium elution can be expected, as described above.

[0108] Furthermore, due to the concentration gradient of the added element as described above, it is preferable that the crystal structure continuously changes 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. Alternatively, it is preferable that the surface layer 100a and the interior 100b are topotaxy.

[0109] In this specification, topotaxis refers to a three-dimensional structural similarity in which the crystal orientations roughly coincide, or to a crystallographically identical orientation, while epitaxy refers to a two-dimensional interface structural similarity.

[0110] The topotaxis between the surface layer 100a and the interior 100b can reduce distortion of the crystal structure and / or misalignment of the atomic arrangement, thereby suppressing the cause of pits. In this specification, pits refer to holes formed by the progression of defects in the positive electrode active material.

[0111] It is also preferable that the crystal structure continuously change from the interior 100b of the layered rock salt type toward the surface and surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type. Alternatively, it is preferable that the crystal orientation of the surface layer 100a, which has characteristics of the rock salt type or both the rock salt type and the layered rock salt type, and the interior 100b of the layered rock salt type are roughly the same.

[0112] In this specification, the layered rock-salt crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal such as cobalt, refers to a crystal structure having a rock-salt ion arrangement in which cations and anions are alternately arranged, and in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt crystal structure may have a structure in which the lattice of the rock-salt crystal is distorted.

[0113] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects may occur.

[0114] Furthermore, the presence of both the characteristics of the layered rock salt type and the rock salt type crystal structure can be determined by electron diffraction patterns, TEM images, cross-sectional STEM images, and the like.

[0115] In the rock salt type, there is no distinction in the cation sites, but in the layered rock salt type, there are two types of cation sites in the crystal structure, one of which is mostly occupied by lithium and the other by a transition metal. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same for both the rock salt type and the layered rock salt type. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in the ideal rock salt type, and, for example, the (003) plane in the layered rock salt type. For example, rock salt type MgO and layered rock salt type LiCoO ２ When comparing the electron diffraction patterns of LiCoO ２The bright spots on the (003) plane of MgO are observed at a distance about half the distance between the bright spots on the (111) plane of MgO. ２ In the case of a material with these two phases, the electron diffraction pattern shows a plane orientation in which bright spots with strong brightness and bright spots with weak brightness are arranged alternately. Bright spots common to both the rock salt type and the layered rock salt type have strong brightness, while bright spots occurring only in the layered rock salt type have weak brightness.

[0116] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. This characteristic is not observed in the rock-salt structure, as there is no distinction in the cation sites. In the case of a crystal structure that has the characteristics of both the rock-salt and layered rock-salt structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and furthermore, a metal with an atomic number greater than that of lithium is present in part of the low-brightness layer, i.e., the lithium layer.

[0117] Layered rock salt crystals and anions in rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that anions in the O3'-type and monoclinic O1(15) crystals described below also have a cubic close-packed structure. Therefore, when a layered rock salt crystal and a rock salt crystal come into contact, there are crystal faces where the cubic close-packed structure composed of anions is aligned.

[0118] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt has a space group R-3m and a rhombohedral structure, but to make the structure easier to understand, it is generally expressed as a compound hexagonal lattice, and the (0001) plane of the layered rock salt has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of the layered rock salt. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0119] However, the space group of the layered rock salt type crystal and the O3' type crystal is R-3m, which is different from the space group Fm-3m of the rock salt type crystal (the space group of a general rock salt type crystal), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal and the O3' type crystal and the rock salt type crystal. In this specification and the like, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt type crystal, the O3' type and the rock salt type crystal, it may be said that the crystal orientations are approximately the same.

[0120] The fact that the crystal orientations of the two regions roughly coincide can be determined from TEM (Transmission Electron Microscope) images, STEM (Scanning Transmission Electron Microscope) images, HAADF-STEM (High-angle Annular Dark Field Scanning TEM) images, ABF-STEM (Annular Bright-Field Scanning Transmission Electron Microscope) images, electron beam diffraction patterns, FFT patterns of TEM images, STEM images, etc. XRD (X-ray diffraction), electron beam diffraction, neutron beam diffraction, etc. can also be used as materials for the judgment.

[0121] 2 shows an example of a TEM image in which the orientations of the layered rock salt crystals LRS and RS are roughly the same. Images reflecting the crystal structure can be obtained in TEM images, STEM images, HAADF-STEM images, ABF-STEM images, etc.

[0122] For example, in high-resolution TEM images, contrast originating from crystal planes can be observed. When an electron beam is incident perpendicularly to the c-axis of a layered rock-salt type composite hexagonal lattice, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is observed as a repetition of bright bands (bright strips) and dark bands (dark strips). Therefore, a repetition of bright and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in Figure 2) ＲＳ and L ＬＲＳWhen the angle between the dark lines is 5° or less, or 2.5° or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5° or less, or 2.5° or less, it can be determined that the crystal orientations are roughly aligned.

[0123] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, and elements with higher atomic numbers are observed brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the position of the cobalt atoms, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate having a layered rock-salt crystal structure is observed perpendicular to the c-axis, the arrangement of the cobalt atoms perpendicular to the c-axis is observed as a bright line or an arrangement of highly bright dots, and the arrangements of lithium atoms and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0124] Therefore, when a HAADF-STEM image shows repeated bright and dark lines in two regions with different crystal structures, and the angle between the bright lines is 5° or less or 2.5° or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5° or less or 2.5° or less, it can also be determined that the crystal orientations are roughly consistent.

[0125] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but similar to HAADF-STEM, contrast according to the atomic number can be obtained, and therefore the crystal orientation can be determined in the same way as with HAADF-STEM images.

[0126] Figure 3A shows an example of an STEM image in which the orientations of the layered rock salt crystal LRS and the rock salt crystal RS are roughly the same. Figure 3B shows the FFT pattern of the region of the rock salt crystal RS, and Figure 3C shows the FFT pattern of the region of the layered rock salt crystal LRS. The left side of Figures 3B and 3C shows the composition, JCPDS card number, and the d value and angle calculated from these. The right side shows the measured values. The spot marked with O is the zeroth-order diffraction.

[0127] The spot marked A in Figure 3B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 3C is derived from the 0003 reflection of the layered rock salt type. From Figures 3B and 3C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt type roughly coincide. In other words, it can be seen that the line passing through AO in Figure 3B is roughly parallel to the line passing through AO in Figure 3C. Here, "roughly coincident" and "roughly parallel" mean that the angle is 5° or less, or 2.5° or less.

[0128] Thus, in the FFT pattern and the electron beam diffraction pattern, when the orientations of the layered rock salt type crystal and the rock salt type crystal are roughly the same, the <0003> orientation of the layered rock salt type and the <11-1> orientation of the rock salt type may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point being spot-like and not continuous with other reciprocal lattice points means high crystallinity.

[0129] Furthermore, as described above, when the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock salt crystal may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock salt crystal. For example, the spot marked B in FIG. 3C is originating from the 1014 reflection of the layered rock salt crystal. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point originating from the 0003 reflection of the layered rock salt crystal (A in FIG. 3C), and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be the same. For example, equivalent reciprocal lattice points in each may be used.

[0130] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 3B originates from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 3B) originating from the 11-1 reflection of the cubic crystal. Note that this index is merely an example and does not necessarily have to be the same. For example, equivalent reciprocal lattice points in each may be used.

[0131] It is known that layered rock-salt type positive electrode active materials, including lithium cobalt oxide, tend to have the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material using an SEM or the like, it is possible to thin-section the observation sample using an FIB or the like so that the electron beam is [12-10] incident in a TEM or the like, making the (0003) plane easier to observe. When determining whether the crystal orientation is consistent, it is preferable to thin-section the layered rock-salt type so that the (0003) plane can be easily observed.

[0132] <Li ｘ CoO ２The positive electrode active material 100 that can be used as one embodiment of the present invention has the above-described distribution of the additive element and / or the crystal structure in the discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small differs from that of conventional positive electrode active materials. In this specification and the like, "small x" means 0.1<x≦0.24.

[0133] 4 to 8, Li ｘ CoO ２ The change in the crystal structure accompanying the change in x in the positive electrode active material 100 will be described by comparing a conventional positive electrode active material with the positive electrode active material 100 usable as one embodiment of the present invention.

[0134] The change in the crystal structure of a conventional positive electrode active material is shown in FIG. 5. The conventional positive electrode active material shown in FIG. 5 is a lithium cobalt oxide (LiCoO ２ In this specification, the phrase "not containing any particular added elements" refers to a case where the added elements are below the lower limit of detection when measured using analytical means, or a case where the added elements are contained at about the lower limit of detection, but to an extent that does not affect the presence or absence of the action and effect.

[0135] In Figure 5, R-3m(O3) is added to Li ｘ CoO ２ The crystal structure of lithium cobalt oxide with x=1 in Fig. 1 shows that lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is defined as a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in the plane direction with edge sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra.

[0136] It is also known that conventional lithium cobalt oxide has a crystal structure that has high lithium symmetry when x is about 0.5 and belongs to the monoclinic space group P2 / m. This structure has CoO ２There is one layer, so it is sometimes called O1 type or monoclinic O1 type.

[0137] Furthermore, when x = 0, the positive electrode active material has a crystal structure of the trigonal space group P-3m1, and also contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, when the trigonal crystal is converted into a composite hexagonal lattice, it is sometimes called hexagonal O1 type.

[0138] Furthermore, when x is about 0.12, conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m(O3). ２ It can also be said that this crystal structure is a structure in which the structure of and the structure of are stacked alternately. For this reason, this crystal structure is sometimes called an H1-3 crystal structure. In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in Figure 5 and other parts of this specification, the c-axis of the H1-3 crystal structure is shown as half the unit cell to make it easier to compare with other crystal structures.

[0139] As an example, the coordinates of cobalt and oxygen in the unit cell of the H1-3 type crystal structure can be expressed as Co (0,0,0.42150±0.00016), O1 (0,0,0.27671±0.00045), and O2 (0,0,0.11535±0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to represent the crystal structure of the positive electrode active material can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0140] Li ｘ CoO ２ When conventional lithium cobalt oxide is repeatedly charged and discharged so that x in the formula is 0.12 or less, the crystal structure of the lithium cobalt oxide repeatedly changes (i.e., undergoes a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m(O3) structure in the discharged state.

[0141] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 5, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m(O3) in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0142] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3m(O3) crystal structure is greater than 3.5%, typically 3.9% or more.

[0143] In addition, the H1-3 type crystal structure has CoO like the trigonal O1 type. ２ A structure with continuous layers is likely to be unstable.

[0144] Therefore, when charge and discharge are repeated so that x is 0.12 or less, the crystalline structure of conventional lithium cobalt oxide collapses. The collapse of the crystalline structure causes deterioration of cycle characteristics. This is because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult to insert and extract lithium. Note that not only when charge and discharge are repeated so that x is 0.12 or less, but also when x is 0.24 or less, collapse of the crystalline structure often occurs, causing deterioration of cycle characteristics. For this reason, in practical use, conventional lithium cobalt oxide is controlled so that charge and discharge are repeated in a range where x exceeds 0.24.

[0145] On the other hand, the positive electrode active material 100 shown in FIG. 4 that can be used as one embodiment of the present invention is Li ｘ CoO ２ The change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. More specifically, the change in the crystal structure between the discharge state where x is 1 and the state where x is 0.24 or less is smaller than that of conventional positive electrode active materials. ２The layer misalignment can be reduced. Also, the volume change can be reduced when compared per cobalt atom. Therefore, the positive electrode active material 100 usable as one embodiment of the present invention is less likely to break down in crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and can achieve excellent cycle characteristics. Also, the positive electrode active material 100 usable as one embodiment of the present invention is less likely to break down in crystal structure even when repeatedly charged and discharged so that x is 0.24 or less, and can achieve excellent cycle characteristics. ｘ CoO ２ In the state where x is 0.24 or less, the positive electrode active material 100 can have a more stable crystal structure than conventional positive electrode active materials. ｘ CoO ２ When the state where x is 0.24 or less is maintained, a short circuit is unlikely to occur, and the safety of the lithium ion battery is improved.

[0146] Li ｘ CoO ２ The crystal structures of the interior 100b of the positive electrode active material 100 when x is 1, approximately 0.2, and approximately 0.15 are shown in FIG. 4. The interior 100b occupies the majority of the volume of the positive electrode active material 100 and is the portion that contributes greatly to charge and discharge. ２ The most problematic areas are layer misalignment and volume changes.

[0147] When x=1, the positive electrode active material 100 has the same crystal structure of R-3m(O3) as conventional lithium cobalt oxide.

[0148] On the other hand, when x is equal to or less than 0.24, for example, about 0.2 or 0.15, the positive electrode active material 100 has a crystal structure different from the H1-3 type crystal structure of conventional lithium cobalt oxide.

[0149] Specifically, when x=0.2, the positive electrode active material 100 has a crystal structure belonging to the trigonal space group R-3m. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. This crystal structure is shown in Figure 4 with the notation R-3m(O3)'.

[0150] In the O3' type crystal structure, the coordinates of cobalt and oxygen in the unit cell can be expressed in the range of Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25. The lattice constant of the unit cell is 2.797≦a≦2.837 (×10 −１ nm), and 2.807≦a≦2.827 (×10 −１ nm) is more preferable, and typically a=2.817 (×10 −１ The c-axis is 13.68≦c≦13.88 (×10 −１ nm), and more preferably 13.75≦c≦13.81, and typically c=13.78(×10 −１ nm).

[0151] Furthermore, when x=0.15, the positive electrode active material 100 usable as one embodiment of the present invention has a crystal structure belonging to the monoclinic space group P2 / m. This is because CoO ２ There is one layer. The amount of lithium present in the positive electrode active material 100 is approximately 15 atomic % in the discharged state. Therefore, in this specification, this crystal structure is referred to as a "monoclinic O1(15) type crystal structure." This crystal structure is shown in Figure 4, labeled P2 / m monoclinic O1(15).

[0152] The monoclinic O1(15) type crystal structure has the coordinates of cobalt and oxygen in the unit cell as follows: Co1(0.5,0,0.5), Co2(0,0.5,0.5), O1(X Ｏ１ , 0, Z Ｏ１ ), 0.23≦X Ｏ１ ≦0.24, 0.61≦Z Ｏ１ ≦0.65, O2(X Ｏ２ , 0.5, Z Ｏ２ ), 0.75≦X Ｏ２ ≦0.78, 0.68≦Z Ｏ２ The lattice constant of the unit cell is − a = 4.880 ± 0.05 (× 10 −１ nm), b=2.817±0.05(×10 −１ nm), c=4.839±0.05(×10 −１nm), α=90°, β=109.6±0.1°, γ=90°.

[0153] This crystal structure can also be fitted to the space group R-3m if a certain degree of error is allowed. In this case, the coordinates of cobalt and oxygen in the unit cell are Co(0,0,0.5), O(0,0,Z Ｏ ), 0.21≦Z Ｏ The lattice constant of the unit cell is a = 2.817 ± 0.02 (× 10 −１ nm), c=13.68±0.1(×10 −１ nm).

[0154] In both the O3' and monoclinic O1(15) crystal structures, ions of cobalt, nickel, magnesium, etc. occupy six oxygen coordination positions, although light elements such as lithium may occupy four oxygen coordination positions.

[0155] As shown by the dotted line in FIG. 4, the difference between R-3m(O3) in the discharged state and O3' and the monoclinic O1(15) type crystal structure is ２ There is almost no layer misalignment.

[0156] The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the O3'-type crystal structure is 2.5% or less, more specifically 2.2% or less, typically 1.8%.

[0157] The difference in volume per the same number of cobalt atoms between R-3m(O3) in a discharged state and the monoclinic O1(15) type crystal structure is 3.3% or less, more specifically 3.0% or less, typically 2.5%.

[0158] Table 1 shows the difference in volume per cobalt atom between discharged R-3m(O3), O3', monoclinic O1(15), H1-3 type, and trigonal O1. Regarding the lattice constants of each crystal structure used in the calculations of Table 1, reference can be made to literature values ​​(ICSD coll.code.172909 and 88721) and Non-Patent Document 1 for discharged R-3m(O3), trigonal O1, and H1-3 type. For O3' and monoclinic O1(15), the lattice constants can be calculated from experimental XRD values.

[0159]

[0160] Thus, the positive electrode active material 100 that can be used as one embodiment of the present invention is Li ｘ CoO ２ When x is small, that is, when a large amount of lithium is released, the change in crystal structure is suppressed compared to conventional positive electrode active materials. Furthermore, the change in volume is also suppressed when compared per the same number of cobalt atoms. Therefore, the positive electrode active material 100 is less likely to collapse in crystal structure even when repeatedly charged and discharged such that x is 0.24 or less, and the decrease in charge / discharge capacity during charge / discharge cycles is suppressed. Furthermore, because more lithium can be stably utilized than in conventional positive electrode active materials, the positive electrode active material 100 has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 100, a secondary battery with a high discharge capacity per weight and per volume can be fabricated.

[0161] The positive electrode active material 100 is Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, the O3' type crystal structure may be present, and it is estimated that even when x is more than 0.24 and 0.27 or less, the O3' type crystal structure is present. ｘ CoO ２ It has been confirmed that when x is greater than 0.1 and less than 0.2, typically 0.15 or more and less than 0.17, the monoclinic O1(15) type crystal structure may be present. However, the crystal structure is similar to that of Li ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0162] Therefore, the positive electrode active material 100 is Li ｘ CoO ２When x is greater than 0.1 and equal to or less than 0.24, the positive electrode active material 100 may have only the O3' type, only the monoclinic O1(15) type, or both crystal structures. Furthermore, not all of the particles in the interior 100b of the positive electrode active material 100 have the O3' type and / or the monoclinic O1(15) type crystal structure. Other crystal structures may be included, or a portion may be amorphous.

[0163] Also, Li ｘ CoO ２ To make the value of x small, it is generally necessary to charge at a high charging voltage. ｘ CoO ２ A state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when a conventional positive electrode active material is charged at a voltage of 4.6 V or more relative to the potential of lithium metal in a 25°C environment using CC / CV (constant current / constant voltage) charging, an H1-3 crystal structure appears. Therefore, a charging voltage of 4.6 V or more relative to the potential of lithium metal can be considered a high charging voltage. Furthermore, in this specification and the like, unless otherwise specified, the charging voltage will be expressed relative to the potential of lithium metal.

[0164] Therefore, the cathode active material 100 usable as one embodiment of the present invention can be said to be preferable because it can maintain a crystal structure having R-3m(O3) symmetry even when charged at a high charging voltage, for example, a voltage of 4.6 V or more at 25° C. In other words, it can be said to be preferable because it can adopt an O3'-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or more and 4.7 V or less at 25° C. In other words, it can be said to be preferable because it can adopt an even higher monoclinic O1(15)-type crystal structure when charged at a higher charging voltage, for example, a voltage of more than 4.7 V and 4.8 V or less at 25° C.

[0165] Even with the positive electrode active material 100, an H1-3 crystal structure may finally be observed when the charge voltage is further increased. As described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the temperature, the electrolyte, and the like. Therefore, when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or more and less than 4.6 V at 25° C., the positive electrode active material 100 usable as one embodiment of the present invention may be able to adopt an O3′ crystal structure. Similarly, when charged at a voltage of 4.65 V or more and 4.7 V or less at 25° C., the positive electrode active material 100 may be able to adopt a monoclinic O1(15) crystal structure.

[0166] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0167] In addition, in O3' and monoclinic O1(15) in FIG. 4, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, for example, in monoclinic O1(Li ０．５ CoO ２ The distribution of lithium can be analyzed by, for example, neutron diffraction.

[0168] The crystal structure of O3' and monoclinic O1(15) type has random lithium between layers, but CdCl ２ It can be said that this CdCl has a similar crystal structure to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２ The crystal structure is similar to that when charged to 1000V, but pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt usually have a CdCl ２ It is known that it does not have a typical crystal structure.

[0169] Furthermore, it is preferable that the concentration gradient of the additive element be similar at multiple locations on the surface layer 100a of the positive electrode active material 100. In other words, it is preferable that a barrier film derived from the additive element is uniformly present on the surface layer 100a. Even if a barrier film is present in part of the surface layer 100a, if there is a portion without the barrier film, stress may be concentrated in the absent portion. If stress is concentrated in a portion of the positive electrode active material 100, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0170] However, the additive element does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 100a of the positive electrode active material 100. Enlarged views of the vicinity of C-D in Fig. 1A1 are shown in Fig. 6A1 and Fig. 6A2. Fig. 6A1 shows an example of the distribution of additive element X near C-D in Fig. 1A1, and Fig. 6A2 shows an example of the distribution of additive element Y near C-D.

[0171] Here, the vicinity of C-D has an R-3m layered rock salt type crystal structure, and the surface has a (001) orientation. The (001)-oriented surface may have a different distribution of additive elements than the other surfaces. For example, the (001)-oriented surface and its surface layer 100a may have a distribution of concentration peaks of one or more elements selected from additive element X and additive element Y limited to a shallower portion from the surface compared to surfaces other than the (001) orientation. Alternatively, the (001)-oriented surface and its surface layer 100a may have a lower concentration of one or more elements selected from additive element X and additive element Y compared to surfaces other than the (001) orientation. Alternatively, the (001)-oriented surface and its surface layer 100a may have a concentration of one or more elements selected from additive element X and additive element Y below the lower detection limit.

[0172] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and the diffusion path of lithium ions is also parallel to the (001) plane.

[0173] CoO ２Since the layer is relatively stable, it is more stable if the surface of the positive electrode active material 100 has a (001) orientation. The main diffusion path of lithium ions during charge and discharge is not exposed on the (001) plane.

[0174] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface and the surface layer portion 100a other than the (001) orientation are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, reinforcing the surface and the surface layer portion 100a other than the (001) orientation is extremely important for maintaining the crystal structure of the entire positive electrode active material 100.

[0175] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, the distribution of the additive element in the surface other than the (001)-oriented surface and the surface layer 100a thereof is preferably as shown in Fig. 1B1 and Fig. 1B2. On the other hand, the concentration of the additive element in the (001)-oriented surface and the surface layer 100a thereof may be low or absent, as described above.

[0176] High purity LiCoO, which will be described in a later embodiment ２ In the fabrication method of fabricating the silicon nitride film, the additive element is mixed in and heated after fabrication, and the additive element spreads mainly through the diffusion path of lithium ions, so that the distribution of the additive element in the surface other than the (001) orientation and in the surface layer portion 100a thereof can be easily controlled to a preferred range.

[0177] Using Figs. 6B1 to 6C, LiCoO ２ The results of calculations of the distribution of additive elements when the additive elements are mixed and heated after the preparation of the alloy are described below.

[0178] Figure 6B1 shows the results of calculations for the (104) oriented surface and its surface layer 100a. The calculations were performed using classical molecular dynamics. LiCoO ２ (LCO) was placed in the upper part of the system with LiF and MgF as magnesium, lithium and fluorine sources. ２ The ensemble was NVT (canonical ensemble), and the density of the initial structure was 1.8 g / cm ３The system temperature was 2000 K, the elapsed time was 100 psec, the potential was optimized using the LCO crystal structure, the other atoms were mixed with UFF (Universal Force Field), the number of atoms in the system was approximately 10,000, and the charge of the system was neutral. For simplicity, only Co and Mg atoms are shown.

[0179] FIG. 6B2 shows the results of calculations up to 200 psec, and FIG. 6B3 shows the results of calculations up to 1200 psec.

[0180] From the above calculations, it is inferred that magnesium diffuses through the following process: (1) Lithium is desorbed from LCO due to heat. (2) Magnesium enters the lithium layer of LCO and diffuses inward. (3) Lithium from LiF enters the lithium layer of LCO and supplements the lithium desorbed in (1).

[0181] 6B1, after 100 psec has elapsed, clearly shows the diffusion of magnesium atoms into the LCO. The magnesium atoms diffuse along the arrangement of cobalt atoms, and in FIG. 6B3, after 1200 psec has elapsed, almost all of the magnesium atoms prepared at the top of the system are incorporated into the LCO.

[0182] Figure 6C shows the results of calculations similar to those in Figure 6B1, except for the (001) orientation. Figure 6C shows that the magnesium atoms remain on the surface of the LCO.

[0183] Such high purity LiCoO ２ By using a manufacturing method in which the additional elements are mixed and heated after the (001) plane is manufactured, the additional elements can be distributed more favorably on the surface other than the (001) orientation and on the surface layer 100a thereof than on the (001) plane.

[0184] In the manufacturing method involving initial heating, which will be described later, LiCoO ２ This is expected to remove any lithium compounds that may remain unintentionally on the surface of the material, making it easier to distribute magnesium and other additive elements in high concentrations in the surface layer.

[0185] Furthermore, it is preferable that the surface of the positive electrode active material 100 is smooth and has few irregularities, but this is not necessarily the case for the entire surface of the positive electrode active material 100. A composite oxide having a layered rock salt crystal structure of R-3m is prone to slippage on a plane parallel to the (001) plane, for example, on a plane where lithium is arranged. Here, slippage is also called stacking faults, and is a phenomenon that occurs when LiCoO is pressed. ２ This refers to a state in which the lattice fringes are deformed along the lattice fringe direction (the ab plane direction). Deformation includes the lattice fringes being displaced forward or backward. When the lattice fringes are displaced forward or backward, a step occurs on the surface in the direction perpendicular to the lattice fringes (the c-axis direction). For example, as shown in Figure 7A, when a (001) plane is present, slip may occur parallel to the (001) plane as indicated by the arrow in Figure 7B after a pressing process, resulting in deformation.

[0186] In this case, the additional element may not be present or may be below the detection limit on the surface and its surface layer 100a newly formed as a result of the slip. E-F in FIG. 7B is an example of the surface and its surface layer 100a newly formed as a result of the slip. Enlarged views of the vicinity of E-F are shown in FIGS. 7C1 and 7C2. Unlike FIGS. 1B1, 1B2, 6A1, and 6A2, the additional element X and the additional element Y are not distributed in FIGS. 7C1 and 7C2.

[0187] However, since slippage tends to occur parallel to the (001) plane, the newly formed surface and its surface layer 100a tend to have a (001) orientation. In this case, the diffusion path of lithium ions is not exposed and the surface is relatively stable, so there is almost no problem even if the added element is absent or below the detection limit.

[0188] As mentioned above, the composition is LiCoO ２ In the composite oxide having a layered rock salt type crystal structure of R-3m, the cobalt atoms are arranged parallel to the (001) plane. ２ Among these, cobalt, which has the largest atomic number, has the highest brightness. Therefore, in a HAADF-STEM image, the arrangement of bright atoms can be considered to be the arrangement of cobalt atoms. The repetition of this bright arrangement is synonymous with crystal fringes or lattice fringes.

[0189] <Grain Boundaries> In addition to the distribution described above, at least a portion of the additive element contained in the positive electrode active material 100 that can be used as one embodiment of the present invention is preferably unevenly distributed in and near the grain boundaries 101 .

[0190] For example, the magnesium concentration in and around the grain boundary 101 of the positive electrode active material 100 (for example, within a range of a region several nanometers away from the grain boundary 101) is preferably higher than that in other regions of the interior 100b. The fluorine concentration in and around the grain boundary 101 is also preferably higher than that in other regions of the interior 100b. The nickel concentration in and around the grain boundary 101 is also preferably higher than that in other regions of the interior 100b. The aluminum concentration in and around the grain boundary 101 is also preferably higher than that in other regions of the interior 100b.

[0191] Since the grain boundaries 101 are one type of planar defect, they are prone to become unstable like the surface, and changes in the crystal structure are likely to begin at these boundaries. Therefore, by increasing the concentration of the added element at and near the grain boundaries 101, such changes in the crystal structure can be more effectively suppressed.

[0192] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundaries 101, even if cracks occur along the grain boundaries 101 of the positive electrode active material 100 that can be used as one embodiment of the present invention, the magnesium concentration and fluorine concentration become high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid can be improved even in the positive electrode active material after the cracks occur.

[0193] <Particle size> If the particle size of the positive electrode active material 100 usable as one embodiment of the present invention is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector may occur. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte (electrolytic solution) may occur. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferred. Alternatively, 1 μm or more and 30 μm or less is preferred. Alternatively, 2 μm or more and 100 μm or less is preferred. Alternatively, 2 μm or more and 30 μm or less is preferred. Alternatively, 5 μm or more and 100 μm or less is preferred. Alternatively, 5 μm or more and 40 μm or less is preferred.

[0194] <Analysis method> In a certain positive electrode active material, Li ｘ CoO ２ When x in the formula (I) is small, whether the positive electrode active material 100 has an O3′ type and / or a monoclinic O1(15) type crystal structure and can be used as one embodiment of the present invention can be determined by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0195] In particular, XRD is preferable in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies the majority of the volume of the positive electrode active material 100.

[0196] The positive electrode active material 100 that can be used as one embodiment of the present invention is, as described above, ｘ CoO ２The characteristic feature is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. When charged at a high voltage (e.g., 4.6 V), a material in which the crystal structure accounts for 50% or more of a large change in the crystal structure is not preferable because it cannot withstand high-voltage charging and discharging.

[0197] However, it should be noted that simply adding an additive element may not result in an O3'-type or monoclinic O1(15)-type crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has the same point, depending on the concentration and distribution of the additive element, Li ｘ CoO ２ In some cases, x is 0.24 or less and the O3' type and / or monoclinic O1(15) type crystal structure accounts for 60% or more, and in other cases, the H1-3 type crystal structure accounts for 50% or more.

[0198] Furthermore, even in the case of a cathode active material 100 that can be used as one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated if x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the cathode active material 100 can be used as one embodiment of the present invention, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0199] Furthermore, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, the crystal structure may change from O3'-type and monoclinic O1(15)-type to H1-3-type. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0200] Furthermore, whether or not the distribution of the additive elements contained in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0201] The crystal structure of the surface layer 100 a, the grain boundaries 101 , etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 100 .

[0202] <Charging Method> An example of charging to determine whether a certain composite oxide is a cathode active material 100 that can be used as one embodiment of the present invention is a method of preparing a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a counter electrode (negative electrode in this case) made of lithium metal and charging the coin cell. Note that the charging method described below is a condition for confirming the physical properties of the cathode active material 100 that can be used as one embodiment of the present invention. Therefore, the electrolyte and other components other than the cathode active material, which will be described below, differ from the components of the lithium-ion battery that is one embodiment of the present invention.

[0203] More specifically, as an example of a positive electrode, a positive electrode current collector made of aluminum foil can be used, which is coated with a slurry in which a positive electrode active material, a conductive material, and a binder are mixed.

[0204] As an example of the negative electrode (counter electrode), lithium metal can be used. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery differs from the potential of the positive electrode. Unless otherwise specified, the voltage and potential in this specification are the potential of the positive electrode when the counter electrode is lithium metal.

[0205] As an example of the electrolyte, 1 mol / L of lithium hexafluorophosphate (LiPF) was added to an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3:7 and vinylene carbonate (VC) was added at 2 wt%. ６ ) can be used.

[0206] An example of the separator is a porous polypropylene film having a thickness of 25 μm.

[0207] As an example of the positive electrode can and the negative electrode can, those formed of stainless steel (SUS) can be used.

[0208] The coin cell fabricated under the above conditions is subjected to constant-current charging (also known as CC charging) at a current value of 10 mA / g (equivalent to 0.05 C when 1 C = 200 mA / g) to a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). To observe the phase change of the positive electrode active material, charging at such a low current value is desirable. The temperature is set to 25°C or 45°C. After charging under these conditions, the coin cell is disassembled in an argon-atmosphere glove box and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. When various analyses are performed, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container in an argon atmosphere. Furthermore, it is preferable to remove the positive electrode promptly after charging is completed and perform analysis. Specifically, within one hour after charging is completed, and more preferably within 30 minutes.

[0209] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions. For example, charging may be performed by constant current charging at a current value of 100 mA / g up to a desired voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 10 mA / g, and then constant current discharging at 2.5 V and 100 mA / g.

[0210] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at, for example, 2.5 V and a current value of 100 mA / g.

[0211] <<XRD>> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray source: CuKα １ Line output: 40 kV, 40 mA Slit width: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0212] If the measurement sample is a powder, it can be set by placing it in a glass sample holder, sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0213] CuKα calculated from the O3' type crystal structure, the monoclinic O1(15) type crystal structure, and the H1-3 type crystal structure model １ The ideal powder XRD patterns of the lines are shown in Figures 8, 9, 10A and 10B. ｘ CoO ２ LiCoO where x=1 ２ Also shown are ideal XRD patterns calculated from the crystal structure of O3 and trigonal O1 with x = 0. Figures 10A and 10B show the XRD patterns of the O3'-type crystal structure, the monoclinic O1(15)-type crystal structure, and the H1-3-type crystal structure, with Figure 10A showing an enlarged view of the region where 2θ is between 18° and 21°, and Figure 10B showing an enlarged view of the region where 2θ is between 42° and 46°. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from ICSD (Inorganic Crystal Structure Database). The 2θ range was 15° to 75°, step size = 0.01, and wavelength λ = 1.540562 × 10 −１０ m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 1. The patterns of the O3' type and monoclinic O1(15) type crystal structures were estimated from the XRD pattern of a positive electrode active material usable as one embodiment of the present invention, and fitted using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and XRD patterns were created in the same manner as the others.

[0214] As shown in Figures 8, 10A and 10B, 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°).

[0215] In addition, in the monoclinic O1(15) type crystal structure, diffraction peaks appear at 2θ=19.47±0.10° (19.37° or more and 19.57° or less) and 2θ=45.62±0.05° (45.57° or more and 45.67° or less).

[0216] On the other hand, as shown in Figures 9, 10A and 10B, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of peaks at 19.13° or more and less than 19.37° and / or 19.37° or more and 19.57° or less, and at 45.37° or more and less than 45.57° and / or 45.57° or more and 45.67° or less when x is small can be said to be a characteristic of positive electrode active material 100 that can be used as one embodiment of the present invention.

[0217] It can also be said that the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, it can be said that the difference in 2θ between the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ values ​​of 42° to 46° is 0.7° or less, more preferably 0.5° or less.

[0218] The positive electrode active material 100 that can be used in one embodiment of the present invention is Li ｘ CoO ２When x in the formula is small, the particles have an O3'-type and / or monoclinic O1(15)-type crystal structure, but not all of the particles have an O3'-type and / or monoclinic O1(15)-type crystal structure. Other crystal structures may be included, or some may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type and / or monoclinic O1(15)-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type and / or monoclinic O1(15)-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0219] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type and / or monoclinic O1(15) type crystal structure is preferably 35% or more, more preferably 40% or more, and even more preferably 43% or more.

[0220] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed at 2θ = 43° or more and 46° or less, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. Therefore, it contributes to the stabilization of the crystalline structure after charging sufficiently.

[0221] The crystallite size of the O3'-type and monoclinic O1(15) crystal structures of the positive electrode active material 100 is approximately equal to that of LiCoO in a discharged state. ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２When x in the formula is small, the peaks of the O3' type and monoclinic O1(15) crystal structure can be clearly seen. ２ In this case, even if some of the crystal structures resemble those of the O3' type and monoclinic O1(15), the crystallite size will be small and the peaks will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0222] As described above, the positive electrode active material 100 that can be used as one embodiment of the present invention preferably has a small influence of the Jahn-Teller effect. In addition to cobalt, a transition metal such as nickel or manganese may be contained as an additive element as long as the influence of the Jahn-Teller effect is small.

[0223] In the positive electrode active material, the range of the proportions of nickel and manganese and the lattice constants in which the influence of the Jahn-Teller effect is presumed to be small is considered using XRD analysis.

[0224] FIG. 11 shows the results of calculating the a-axis and c-axis lattice constants using XRD for a cathode active material 100 usable as one embodiment of the present invention, which has a layered rock-salt crystal structure and contains cobalt and nickel. FIG. 11A shows the a-axis result, and FIG. 11B shows the c-axis result. The XRD patterns used for these calculations are for powder after synthesis of the cathode active material, before incorporation into the cathode. The nickel concentration on the horizontal axis indicates the nickel concentration when the sum of the number of cobalt and nickel atoms is taken as 100%. The cathode active material was prepared according to the preparation method shown in FIGS. 15A and 15C , except that no aluminum source was used.

[0225] FIG. 12 shows the results of estimating the a-axis and c-axis lattice constants using XRD for a cathode active material usable as one embodiment of the present invention having a layered rock-salt crystal structure and containing cobalt and manganese. FIG. 12A shows the a-axis result, and FIG. 12B shows the c-axis result. The lattice constants shown in FIG. 12 were obtained by XRD measurement of the powder after synthesis of the cathode active material, prior to incorporation into the cathode. 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 cathode active material was prepared according to the preparation method shown in FIGS. 15A and 15C , except that a manganese source was used instead of the nickel source, and no aluminum source was used.

[0226] Fig. 11C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in Fig. 11A and Fig. 11B. Fig. 12C shows the value obtained by dividing the a-axis lattice constant by the c-axis lattice constant (a-axis / c-axis) for the positive electrode active materials whose lattice constant results are shown in Fig. 12A and Fig. 12B.

[0227] 11C shows that the a-axis / c-axis tends to change significantly at nickel concentrations of 5% and 7.5%, and the a-axis distortion increases at a nickel concentration of 7.5%. This distortion may be Jahn-Teller distortion. This suggests that an excellent positive electrode active material with small Jahn-Teller distortion can be obtained at nickel concentrations of less than 7.5%.

[0228] Next, Figure 12A suggests that when the manganese concentration is 5% or more, the behavior of the change in lattice constant is different and does not follow Vegard's law. Therefore, it is suggested that when the manganese concentration is 5% or more, the crystal structure is different. Therefore, the manganese concentration is preferably, for example, 4% or less.

[0229] The above ranges of nickel concentration and manganese concentration do not necessarily apply to the surface layer 100a, that is, the concentrations in the surface layer 100a may be higher than the above ranges.

[0230] From the above, a preferable range of the lattice constant was considered, and it was found that in a positive electrode active material that can be used as one embodiment of the present invention, the layered rock salt crystal structure of the positive electrode active material 100 in a state where no charge / discharge is performed or in a discharged state, which can be estimated from the XRD pattern, has an a-axis lattice constant of 2.814 × 10 −１０ m is greater than 2.817 x 10 −１０ m and the lattice constant of the c-axis is 14.05 × 10 −１０ m or larger, 14.07 x 10 −１０ It has been found that the value is preferably smaller than m. The state in which no charge and discharge are performed may be, for example, a powder state before the positive electrode of a secondary battery is produced.

[0231] Alternatively, in the layered rock-salt crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, it is preferable that the value obtained by dividing the lattice constant of the a-axis by the lattice constant of the c-axis (a-axis / c-axis) is greater than 0.20000 and smaller than 0.20049.

[0232] Alternatively, when XRD analysis is performed on the layered rock salt type crystal structure of the positive electrode active material 100 in a state where no charge or discharge is performed or in a discharged state, a first peak may be observed at 2θ of not less than 18.50° and not more than 19.30°, and a second peak may be observed at 2θ of not less than 38.00° and not more than 38.80°.

[0233] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, when monochromatic aluminum Kα rays are used as the X-ray source, it is possible to analyze a region from the surface to a depth of about 2 to 8 nm (usually 5 nm or less), and the concentration of each element can be quantitatively analyzed. Furthermore, narrow scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is about 1 atomic %, depending on the element.

[0234] In the cathode active material 100 usable as one embodiment of the present invention, the concentration of one or more selected additive elements is preferably higher in the surface layer 100a than in the interior 100b. This is equivalent to saying that the concentration of one or more selected additive elements in the surface layer 100a is preferably higher than the average concentration of the additive elements throughout the cathode active material 100. Therefore, it can be said that the concentration of one or more selected additive elements in the surface layer 100a measured by, for example, XPS is preferably higher than the average concentration of the additive elements throughout the cathode active material 100 measured by, for example, ICP-MS (inductively coupled plasma mass spectrometry) or GD-MS (glow discharge mass spectrometry). For example, the magnesium concentration in at least a portion of the surface layer 100a measured by XPS is preferably higher than the magnesium concentration throughout the cathode active material 100. Furthermore, the nickel concentration in at least a portion of the surface layer 100a is preferably higher than the nickel concentration throughout the cathode active material 100. Furthermore, it is preferable that the aluminum concentration in at least a part of the surface layer portion 100a is higher than the aluminum concentration in the entire positive electrode active material 100. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 100a is higher than the fluorine concentration in the entire positive electrode active material 100.

[0235] The surface and surface layer 100a of the cathode active material 100 usable as one embodiment of the present invention are assumed to be free of carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the cathode active material 100. The surface also is assumed to be free of the electrolyte, binder, conductive material, and compounds derived therefrom that are attached to the surface of the cathode active material 100. Therefore, when quantifying the elements contained in the cathode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0236] Furthermore, before subjecting the sample to various analyses, the sample of the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. In this case, lithium may dissolve in the solvent used for washing, but even in this case, the added element is unlikely to dissolve, and therefore the atomic ratio of the added element is not affected.

[0237] The concentration of the added element may also be compared in terms of its ratio to cobalt. This is preferable because it allows comparisons to be made while reducing the influence of carbonates and the like that are chemically adsorbed after the preparation of the positive electrode active material. For example, the ratio of magnesium to cobalt atoms (Mg / Co) determined by XPS analysis of the surface or surface layer of the positive electrode active material is preferably 0.4 to 1.5. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis of the entire positive electrode active material is preferably 0.001 to 0.06.

[0238] Similarly, in order to ensure sufficient lithium insertion / extraction paths, the positive electrode active material 100 preferably has a higher concentration of lithium and cobalt in the surface layer portion 100a than the concentrations of the respective additive elements. This means that the concentrations of lithium and cobalt in the surface layer portion 100a are preferably higher than the concentrations of one or more additive elements selected from the additive elements contained in the surface layer portion 100a as measured by XPS or the like. For example, the concentration of cobalt in at least a portion of the surface layer portion 100a as measured by XPS or the like is preferably higher than the concentration of magnesium in at least a portion of the surface layer portion 100a as measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. Furthermore, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. Furthermore, the concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. Furthermore, the concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

[0239] Furthermore, it is more preferable that the additive element Y, such as aluminum, is widely distributed in a deep region, for example, a region having a depth from the surface of 5 nm to 50 nm. Therefore, although the additive element Y, such as aluminum, is detected in an analysis of the entire positive electrode active material 100 using ICP-MS, GD-MS, or the like, it is more preferable that this is below the lower limit of detection using XPS, or the like.

[0240] Furthermore, when XPS analysis is performed on the surface or surface layer of the positive electrode active material 100 that can be used as one embodiment of the present invention, the number of magnesium atoms is preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times, relative to the number of cobalt atoms. The number of nickel atoms is preferably 0.15 to 0.15 times, more preferably 0.03 to 0.13 times, relative to the number of cobalt atoms. The number of aluminum atoms is preferably 0.12 to 0.09 times, more preferably 0.09 times, relative to the number of cobalt atoms. The number of fluorine atoms is preferably 0.3 to 0.9 times, more preferably 0.1 to 1.1 times, relative to the number of cobalt atoms.

[0241] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions: Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0242] Furthermore, when the surface or surface layer portion of the cathode active material 100 usable as one embodiment of the present invention is subjected to XPS analysis, the peak showing the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably about 684.3 eV. This value is different from both the bond energy of lithium fluoride, 685 eV, and the bond energy of magnesium fluoride, 686 eV. In other words, when the cathode active material 100 usable as one embodiment of the present invention contains fluorine, the bond is preferably other than that of lithium fluoride or magnesium fluoride.

[0243] Furthermore, when the surface or surface layer of the cathode active material 100 usable as one embodiment of the present invention is subjected to XPS analysis, the peak showing the bond energy between magnesium and other elements is preferably equal to or greater than 1302 eV and less than 1304 eV, and more preferably about 1303 eV. This value is different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide. In other words, when the cathode active material 100 usable as one embodiment of the present invention contains magnesium, the bond is preferably other than that of magnesium fluoride.

[0244] <EDX> Preferably, one or more selected from the additive elements contained in the positive electrode active material 100 have a concentration gradient. More preferably, the depth from the surface of the concentration peak varies depending on the additive element in the positive electrode active material 100. The concentration gradient of the additive element can be evaluated by exposing a cross section of the positive electrode active material 100 using, for example, a focused ion beam (FIB) or the like, and analyzing the cross section using energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0245] Among EDX measurements, EDX area analysis is performed by scanning an area and evaluating the area two-dimensionally. EDX area analysis is performed by linear scanning and evaluating the distribution of atomic concentrations within the positive electrode active material. Furthermore, linear analysis is sometimes used to extract data from a linear area obtained by EDX area analysis. Point analysis is also used to measure an area without scanning.

[0246] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer 100a, the interior 100b, and near the grain boundary 101 of the positive electrode active material 100. Furthermore, EDX ray analysis can analyze the concentration distribution and maximum value of the added element. Furthermore, analysis that thins the sample, such as STEM-EDX, is more suitable because it can analyze the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being affected by the distribution in the depth direction.

[0247] Therefore, when EDX area analysis or EDX point analysis is performed on the positive electrode active material 100 that can be used as one embodiment of the present invention, it is preferable that the concentration of each added element, particularly the added element X, in the surface layer portion 100a is higher than that in the interior portion 100b.

[0248] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 100 containing magnesium as an additive element, it is preferable that the magnesium concentration in the surface layer 100a is higher than that in the interior 100b. Furthermore, when EDX ray analysis is performed, the magnesium concentration peak in the surface layer 100a preferably exists within a depth of 3 nm from the surface of the cathode active material 100 toward the center, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm. Furthermore, it is preferable that the magnesium concentration decays to 60% or less of the peak at a depth of 1 nm from the peak top. Furthermore, it is preferable that the magnesium concentration decays to 30% or less of the peak at a depth of 2 nm from the peak top. Note that the "peak concentration" referred to here refers to the maximum concentration value.

[0249] In addition, in the positive electrode active material 100 containing magnesium and fluorine as additive elements, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference in the depth direction between the peak of the fluorine concentration and the peak of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0250] Furthermore, when EDX-ray analysis is performed, the fluorine concentration peak of the surface layer portion 100a preferably exists 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, if the fluorine concentration peak exists slightly closer to the surface than the magnesium concentration peak, resistance to hydrofluoric acid is increased, which is more preferable. For example, the fluorine concentration peak is more preferably 0.5 nm or more closer to the surface than the magnesium concentration peak, and even more preferably 1.5 nm or more closer to the surface.

[0251] Furthermore, in the positive electrode active material 100 containing nickel as an additive element, the nickel concentration peak in the surface layer 100a is preferably present 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 containing magnesium and nickel, the nickel distribution preferably overlaps with the magnesium distribution. For example, the difference in depth between the nickel concentration peak and the magnesium concentration peak is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0252] Furthermore, when the positive electrode active material 100 contains aluminum as an additive element, it is preferable that the peak of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak of the aluminum concentration in the surface layer portion 100 a when EDX-ray analysis is performed. For example, the peak of the aluminum concentration is preferably present at a depth of 0.5 nm to 50 nm, more preferably at a depth of 5 nm to 50 nm, from the surface to the center of the positive electrode active material 100.

[0253] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the surface or surface layer portion of the positive electrode active material 100, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) (Mg / Co) at the peak of the magnesium concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) (Al / Co) at the peak of the aluminum concentration is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) (Ni / Co) at the peak of the nickel concentration is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) (F / Co) at the peak of the fluorine concentration is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0254] The surface of the positive electrode active material 100 based on the EDX ray analysis results can be estimated, for example, as follows: For an element that is uniformly present in the interior 100b of the positive electrode active material 100, such as oxygen or cobalt, the point where the amount detected in the interior 100b is half that of the interior 100b is defined as the surface.

[0255] Since the positive electrode active material 100 is a composite oxide, the surface can be estimated using the detected amount of oxygen. Specifically, first, the average oxygen concentration O ａｖｅ At this time, oxygen O, which is thought to be due to chemical adsorption or background, is found in the area that can be clearly determined to be outside the surface. ｂａｃｋｇｒｏｕｎｄ If detected, O ｂａｃｋｇｒｏｕｎｄ After subtracting, the average oxygen concentration O ａｖｅ This average value O ａｖｅ Half the value of, that is, 1 / 2O ａｖｅ The measurement point showing the measurement value closest to this can be assumed to be the surface of the positive electrode active material.

[0256] The surface can also be estimated in the same manner as above using the detected amount of cobalt. Alternatively, the sum of the detected amounts of multiple transition metals can be used to similarly estimate the surface. The detected amounts of transition metals, including cobalt, are suitable for estimating the surface because they are less susceptible to chemical adsorption.

[0257] Furthermore, when a line analysis or an area analysis is performed on the surface or surface layer portion of the positive electrode active material 100, the ratio (A / Co) of the added element A to cobalt Co in the vicinity of 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, and even more preferably 0.030 or more and 0.20 or less. Note that these upper and lower limit values ​​can be freely combined unless otherwise specified in this specification.

[0258] For example, when the added element is magnesium, when a line analysis or an area analysis is performed on the surface or surface layer portion of the positive electrode active material 100, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in the vicinity of the crystal 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, and even more preferably 0.030 or more and 0.20 or less.

[0259] <EPMA> EPMA (Electron Probe Microanalysis) can also quantify elements. Area analysis can analyze the distribution of each element.

[0260] When EPMA surface analysis is performed on a cross section of the cathode active material 100 that can be used as one embodiment of the present invention, it is preferable that one or more selected from the additive elements have a concentration gradient, similar to the EDX analysis results. It is also more preferable that the depth from the surface of the concentration peak differs depending on the additive element. The preferred range of the concentration peak for each additive element is also the same as in the case of EDX.

[0261] However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from the measurement results obtained using other analytical methods. For example, when the surface of the positive electrode active material 100 is analyzed using EPMA, the concentration of each added element present in the surface layer 100 a may be lower than the result obtained using XPS.

[0262] <Charge Curve and dQ / dV Curve vs. Voltage V> The positive electrode active material 100 usable as one embodiment of the present invention may exhibit a characteristic voltage change during charging. The voltage change can be read from the dQ / dV vs. V curve obtained by differentiating (dQ / dV) the capacity (Q) of the charge curve with respect to the voltage (V). For example, it is believed that a non-equilibrium phase change occurs before and after the peak in the dQ / dV vs. V curve, causing a significant change in the crystal structure. In this specification, a non-equilibrium phase change refers to a phenomenon that causes a non-linear change in a physical quantity.

[0263] The positive electrode active material 100 that can be used as one embodiment of the present invention may have a broad peak near 4.55 V in the dQ / dV vs. V curve. The peak near 4.55 V reflects the change in voltage that occurs when the phase changes from O3 type to O3' type. Therefore, the broadness of this peak means that the change in energy required to extract lithium is smaller than when the peak is sharp, i.e., there is less change in the crystal structure. The smaller these changes are, the more likely CoO ２ This is preferable because it is less affected by layer displacement and volume change.

[0264] More specifically, when the maximum value appearing between 4.5 V and 4.6 V in the dQ / dV vs. V curve of the charging curve is defined as the first peak, it is preferable that the half width of the first peak is 0.10 V or more, since this can be said to be sufficiently broad. In this specification and the like, the half width of the first peak is defined as the average value HWHM of the first peak and the first minimum value when the minimum value of the dQ / dV value appearing between 4.3 V and 4.5 V is defined as the first minimum value. １ and the average value HWHM of the first peak and the second minimum when the minimum value of the dQ / dV value appearing between 4.6 V and 4.8 V is defined as the second minimum value. ２ The difference between and.

[0265] The charging when acquiring the dQ / dV vs. V curve can be, for example, a constant current charge of 10 mA / g up to 4.9 V. When acquiring the dQ / dV of the initial charge, it is preferable to start the charging after discharging to 2.5 V at 100 mA / g before measurement.

[0266] The data acquisition interval during charging can be set to, for example, 1 second intervals or to acquire the voltage and current when there is a voltage fluctuation of 1 mV. The value obtained by integrating the current value and time is taken as the charge capacity.

[0267] The difference between the nth and (n+1)th data of the charge capacity is defined as the nth value of the capacitance change dQ. Similarly, the difference between the nth and (n+1)th data of the voltage is defined as the nth value of the voltage change dV.

[0268] However, since the use of the above data is subject to the large influence of minute noise, dQ / dV may be calculated from the moving average of the voltage and charge capacity differences over a certain number of intervals. The number of intervals may be set to, for example, 500.

[0269] Specifically, the average value of dQ from the nth to the (n+500th) is calculated, and similarly, the average value of dV from the nth to the (n+500th) is calculated. dQ (average of 500) / dV (average of 500) can be used as dQ / dV. Similarly, the moving average value of 500 sections can be used for the voltage on the horizontal axis in the dQ / dV vs. V graph. Note that when using a moving average of 500 sections as described above, it is preferable not to use the data from the 501st section onwards in the dQ / dV vs. V graph because the data is more susceptible to noise.

[0270] When analyzing the dQ / dV vs. V curve after multiple charge / discharge cycles, the charge / discharge conditions may be different from the above-mentioned charging conditions. For example, charging may be performed at a constant current of 100 mA / g at an arbitrary voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 10 mA / g, and discharging may be performed at a constant current of 100 mA / g at 2.5 V.

[0271] In addition, the phase changes from O3 type to O3' type at around 4.55 V, and the O3 type at this time is Li ｘ CoO ２ The x in the figure is about 0.3. This has the same symmetry as the O3 type with x = 1 explained in Figure 5, but ２The distance between layers is slightly different. In this specification, when distinguishing between O3 types with different values ​​of x, O3 type with x = 1 is referred to as O3 (2θ = 18.85), and O3 type with x = 0.3 or so is referred to as O3 (2θ = 18.57). This is because the position of the peak that appears around 2θ = 19° in XRD measurement is due to the CoO ２ This is because it corresponds to the interlayer distance.

[0272] <Discharge Curve and dQ / dV vs. V Curve> Furthermore, when the positive electrode active material 100 usable as one embodiment of the present invention is charged at a high voltage and then discharged at a low current of, for example, 40 mA / g or less, 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 of voltages up to 3.5 V, lower than the peak that appears around 3.9 V, in the dQ / dV vs. V curve obtained from the discharge curve.

[0273] <<ESR>> The positive electrode active material 100 that can be used as one embodiment of the present invention preferably contains cobalt and nickel and magnesium as additive elements. ３＋ Ni ３＋ and some Li ＋ is Mg ２＋ It is preferred that Li be substituted with ＋ is Mg ２＋ With the substitution of ３＋ is reduced to Ni ２＋ In addition, some Li ＋ is Mg ２＋ is replaced by Mg ２＋ Nearby Co ３＋ is reduced to Co ２＋ In addition, some Co ３＋ is Mg ２＋ is replaced by Mg ２＋ Nearby Co ３＋ is oxidized to Co ４＋ This may occur.

[0274] Therefore, the positive electrode active material 100 is Ni ２＋ , Ni ３＋ , Co ２＋ and Co ４＋It is preferable that the positive electrode active material 100 has at least one of the following: ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ The spin density due to one or more of the above is 2.0 × 10 １７ spins / g or more 1.0×10 ２１ It is preferable that the positive electrode active material 100 has the above-mentioned spin density, because the crystal structure is stable, especially in the charged state. If the magnesium concentration is too high, the Ni ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ The spin density may be reduced due to one or more of the above.

[0275] The spin density in the positive electrode active material can be analyzed using, for example, electron spin resonance (ESR).

[0276] <Surface Roughness and Specific Surface Area> The cathode active material 100 that can be used as one embodiment of the present invention preferably has a smooth surface with few irregularities. A smooth surface with few irregularities indicates that the effect of the flux described below is fully exerted and the additive element source and the surface of the lithium cobalt oxide are fused (formed a solid solution). Therefore, this is one factor indicating that the additive element is well distributed in the surface layer portion 100 a.

[0277] Whether the surface is smooth and has few irregularities can be determined from, for example, a cross-sectional SEM image or cross-sectional TEM image of the positive electrode active material 100, the specific surface area of ​​the positive electrode active material 100, or the like.

[0278] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0279] First, the cathode active material 100 is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 100 with a protective film, protective agent, or the like. Next, an SEM image of the interface between the protective film or the like and the cathode active material 100 is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 100 is selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness at least within 400 nm of the outer periphery of the cathode active material particle.

[0280] On the particle surfaces of the positive electrode active material 100 of this embodiment, the root mean square (RMS) surface roughness, which is an index of roughness, is preferably less than 3 nm, more preferably less than 1 nm, and further preferably less than 0.5 nm.

[0281] The image processing software for noise processing, interface extraction, etc. is not particularly limited, but for example, "ImageJ" can be used. The spreadsheet software is also not particularly limited, but for example, Microsoft Office Excel can be used.

[0282] In addition, for example, the actual specific surface area S measured by a gas adsorption method using a constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 100 can also be quantified from the ratio of

[0283] Ideal specific surface area S ｉ is calculated assuming that all particles of the positive electrode active material have the same diameter D50, the same weight, and an ideal spherical shape.

[0284] The median diameter D50 can be measured by a particle size distribution analyzer using a laser diffraction / scattering method, etc. The specific surface area can be measured by a specific surface area measuring device using a gas adsorption method based on a constant volume method, for example.

[0285] The cathode active material 100 that can be used as one embodiment of the present invention has an ideal specific surface area S calculated from the median diameter D50. ｉ and the actual specific surface area S Ｒ The ratio S Ｒ / S ｉ is preferably 2.1 or less.

[0286] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 by the following method.

[0287] First, a surface SEM image of the positive electrode active material 100 is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area are the same.

[0288] Next, image processing software (for example, "ImageJ") is used to convert the SEM image into, for example, an 8-bit image (called a grayscale image). The grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 gradations. Dark areas have lower gradations, and bright areas have higher gradations. The luminance change can be quantified in relation to the number of gradations. This numerical value is called a grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material as a numerical value.

[0289] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram is a three-dimensional representation of the gradation distribution in the target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0290] In the positive electrode active material 100 usable as one embodiment of the present invention, the difference between the maximum and minimum values ​​of the grayscale value is preferably 120 or less, more preferably 115 or less, and even more preferably 70 or more and 115 or less. The standard deviation of the grayscale value is preferably 11 or less, more preferably 8 or less, and even more preferably 4 or more and 8 or less.

[0291] <Other Features> The positive electrode active material 100 may have recesses, cracks, dents, V-shaped cross sections, and the like. These are defects, and repeated charge and discharge may cause cobalt elution, collapse of the crystalline structure, cracking of the main body, oxygen desorption, and the like. Therefore, by providing an embedded portion 102 containing an additive element as shown in FIG. 1A2, cobalt elution and the like can be suppressed. As a result, the positive electrode active material 100 can have excellent reliability and cycle characteristics.

[0292] The positive electrode active material 100 may have protrusions 103 as regions where the additive element is unevenly distributed.

[0293] As described above, an excess of the additive element contained in the positive electrode active material 100 may adversely affect the insertion and desorption of lithium. Furthermore, when used as a secondary battery, this may result in an increase in internal resistance and a decrease in charge / discharge capacity. On the other hand, if the additive element is insufficient, it may not be distributed throughout the entire surface layer 100a, and the effect of suppressing deterioration of the crystal structure may be insufficient. Thus, the additive element needs to be present at an appropriate concentration in the positive electrode active material 100, but adjusting this concentration is not easy.

[0294] Therefore, when the positive electrode active material 100 has a region where the additive element is unevenly distributed, a portion of the excess additive element is removed from the interior 100b of the positive electrode active material 100, and an appropriate additive element concentration can be achieved in the interior 100b. This makes it possible to suppress an increase in internal resistance and a decrease in charge / discharge capacity when the resulting secondary battery is formed. The ability to suppress an increase in internal resistance of a secondary battery is an extremely desirable characteristic, particularly in charge / discharge at large currents, for example, at 400 mA / g or more.

[0295] Furthermore, in the positive electrode active material 100 having a region where the additive element is unevenly distributed, it is permissible to mix an excess amount of the additive element to some extent in the manufacturing process, which is preferable as it widens the margin in production.

[0296] The positive electrode active material 100 may have a coating film on at least a part of the surface thereof. Figures 13A and 13B show examples of the positive electrode active material 100 having a coating film 104.

[0297] The coating 104 is preferably formed by the accumulation of decomposition products of the electrolyte solution during charging and discharging. ｘ CoO ２ When repeated charging is performed such that x in the formula is 0.24 or less, it is expected that the charge-discharge cycle characteristics will be improved by having a coating derived from the electrolyte on the surface of the positive electrode active material 100. This is due to reasons such as suppressing an increase in impedance on the surface of the positive electrode active material or suppressing cobalt elution. The coating 104 preferably contains, for example, carbon, oxygen, and fluorine. Furthermore, when LiBOB and / or SUN (suberonitrile) are used as part of the electrolyte, a high-quality coating is easily obtained. Therefore, a coating 104 containing one or more elements selected from boron, nitrogen, sulfur, and fluorine may be a high-quality coating and is therefore preferable. Furthermore, the coating 104 does not have to cover the entire positive electrode active material 100.

[0298] Furthermore, when the positive electrode active material is charged at 4.5 V or higher, or when it is charged and discharged in a high temperature environment, for example, 45° C. or higher, progressive defects may develop that progress from the surface to the interior. The phenomenon in which defects progress to form holes in the positive electrode active material can also be called pitting corrosion, and the holes generated by this phenomenon are also called pits in this specification.

[0299] FIG. 14 shows a cross-sectional schematic diagram of a positive electrode active material 51 having pits. Crystal planes 55 parallel to the arrangement of cations are also shown. Because FIG. 14 is a cross-sectional view, pits 54 and pits 58 are shown as holes, but their opening shapes are not circular but have depth and are groove-like. Furthermore, as shown by pits 54 and pits 58, unlike recesses 52, they tend to form parallel to the arrangement of lithium ions.

[0300] Furthermore, the surface layer portions of the positive electrode active material 51 where the additive element is present are indicated by 53 and 56. The surface layer portions where the pits have occurred have less additive element than 53 and 56 or are below the detection limit, and it is expected that the function of the barrier film is reduced. It is also thought that the crystalline structure of the lithium cobalt oxide breaks down in the vicinity of where the pits are formed, resulting in a crystalline structure different from that of the layered rock salt type. When the crystalline structure breaks down, it inhibits the diffusion and release of lithium ions, which are carrier ions, and therefore the pits are thought to be a factor in the deterioration of cycle characteristics.

[0301] The source of pits may be point defects. It is thought that point defects in the positive electrode active material change with repeated charge and discharge, and are either chemically or electrochemically corroded by the surrounding electrolyte or the material deteriorates, resulting in pits. This deterioration does not occur uniformly on the surface of the positive electrode active material, but rather occurs in localized areas.

[0302] Furthermore, as shown by the crack 57 in FIG. 14 , defects such as cracks (also referred to as fissures) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In this specification and the like, cracks and pits are different. Even if cracks are present immediately after the preparation of the positive electrode active material, pits are not. Pits can be considered as holes formed by the loss of several layers of cobalt and oxygen due to charging and discharging under high voltage conditions of, for example, 4.5 V or higher or at high temperatures (45°C or higher), and can also be considered as locations where cobalt has dissolved. Cracks refer to, for example, new surfaces formed by the application of physical pressure, or cracks caused by the grain boundaries 101. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charging and discharging. Pits may also occur from cracks and / or cavities within the positive electrode active material.

[0303] 15A to 15C , an example of a method for manufacturing a positive electrode active material that can be used as one embodiment of the present invention (example 1 of a method for manufacturing a positive electrode active material) will be described. Note that the manufacturing method described here is an example of a method for manufacturing the positive electrode active material 100 having the characteristics described above in this embodiment.

[0304] <Step S11> In step S11 shown in FIG. 15A, a lithium source (Li source) and a cobalt source (Co source) are prepared as starting materials for lithium and transition metal, respectively.

[0305] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0306] As the cobalt source, a compound containing cobalt is preferably used, such as cobalt oxide or cobalt hydroxide. The cobalt source preferably has a high purity, for example, a material with a purity of 3N (99.9%) or higher, preferably 4N (99.99%) or higher, more preferably 4N (99.995%) or higher, and even more preferably 5N (99.999%) or higher. By using a high-purity material, impurities in the positive electrode active material can be controlled. As a result, the capacity of the secondary battery is increased and / or the reliability of the secondary battery is improved.

[0307] In addition, the cobalt source preferably has high crystallinity, for example, single crystal grains. The crystallinity of the transition metal source can be evaluated using a TEM (transmission electron microscope) image, a STEM (scanning transmission electron microscope) image, a HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) image, an ABF-STEM (annular bright-field scanning transmission electron microscope) image, or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, or the like. Note that the above-mentioned methods for evaluating crystallinity can be applied not only to transition metal sources but also to evaluating the crystallinity of other sources.

[0308] <Step S12> Next, in step S12 shown in FIG. 15A, the lithium source and the cobalt source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller fragments. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). However, it is 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 the transition metal source in dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0309] A ball mill, a bead mill, or the like can be used as a means for mixing, etc. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as grinding media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, it is preferable to set the peripheral speed 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).

[0310] <Step S13> Next, in step S13 shown in FIG. 15A, the mixed material is heated. Heating is preferably performed at a temperature of 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably approximately 950°C or lower (1000°C or lower). If the temperature is too low, the decomposition and melting of the lithium source and transition metal source may be insufficient. On the other hand, if the temperature is too high, lithium may evaporate from the lithium source and / or cobalt may be excessively reduced, resulting in defects. For example, cobalt may change from trivalent to divalent, causing oxygen defects.

[0311] If the heating time is too short, lithium cobalt oxide will not be synthesized, but if it is too long, productivity will decrease. Therefore, the heating time should be from 1 hour to 100 hours, and more preferably from 2 hours to 20 hours.

[0312] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0313] Heating is preferably carried out in an atmosphere with little water, such as dry air, for example, an atmosphere with a dew point of -50°C or less, more preferably an atmosphere with a dew point of -80°C or less. In this embodiment, heating is carried out in an atmosphere with a dew point of -93°C. In addition, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO ２ , and H ２ The impurity concentrations of the above should be 5 ppb (parts per billion) or less.

[0314] The heating atmosphere is preferably an atmosphere containing oxygen. For example, dry air may be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method in which oxygen is continuously introduced into the reaction chamber and flows through the reaction chamber is called "flow."

[0315] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing the oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen to prevent the oxygen from entering or leaving the reaction chamber may be used, which is called purging. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0316] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0317] The heating in this step may be carried out using a rotary kiln or a roller hearth kiln. Heating in a rotary kiln can be carried out while stirring, whether in a continuous or batch system.

[0318] The crucible used for heating is preferably made of aluminum oxide. Aluminum oxide crucibles are a material that is less likely to contain impurities. In this embodiment, an aluminum oxide crucible with a purity of 99.9% is used. It is also preferable to place a lid on the crucible before heating, as this can prevent the material from volatilizing.

[0319] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use an aluminum oxide mortar. Aluminum oxide mortars are made of a material that is less susceptible to impurities. Specifically, an aluminum oxide mortar with a purity of 90% or more, preferably 99% or more, is used. Heating conditions equivalent to those of step S13 can also be applied to heating steps other than step S13, which will be described later.

[0320] <Step S14> By the above steps, lithium cobalt oxide (LiCoO ２ ) can be synthesized.

[0321] Although the composite oxide is produced by the solid phase method in steps S11 to S14, the composite oxide may be produced by a coprecipitation method or a hydrothermal method.

[0322] <Step S15> Next, in step S15 shown in Fig. 15A, the lithium cobalt oxide is heated. Because this is the first heating of the lithium cobalt oxide, the heating in step S15 may be referred to as initial heating. Alternatively, because this heating is performed before step S20 described below, it may be referred to as preheating or pretreatment.

[0323] The initial heating causes lithium compounds and the like unintentionally remaining on the surface of the lithium cobalt oxide to be desorbed. It is also expected to have the effect of enhancing the crystallinity of the inner portion 100b. Impurities may be mixed into the lithium source and / or cobalt source prepared in step S11, etc. The initial heating can reduce impurities from the lithium cobalt oxide completed in step S14. The effect of enhancing the crystallinity of the inner portion 100b is, for example, the effect of alleviating distortion, misalignment, and the like resulting from differential shrinkage of the lithium cobalt oxide prepared in step S13.

[0324] Additionally, initial heating has the effect of smoothing the surface of the lithium cobalt oxide. A smooth surface means that there are few irregularities, the lithium cobalt oxide is rounded overall, and the corners are also rounded. Alternatively, a state in which there is little foreign matter adhering to the surface can also be called smooth. Foreign matter is thought to be a cause of irregularities, so it is preferable not to allow it to adhere to the surface.

[0325] For this initial heating, it is not necessary to separately prepare a source of lithium compound, a source of additional elements, or a material that functions as a flux.

[0326] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. The heating temperature in step S15 should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in step S15 should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0327] The heating in step S13 may cause a temperature difference between the surface and the interior of the lithium cobalt oxide. This temperature difference may induce a shrinkage difference. It is also believed that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a shrinkage difference. The energy associated with the shrinkage difference causes a difference in internal stress in the lithium cobalt oxide. This difference in internal stress is also called strain, and this energy is sometimes called strain energy. It is believed that the internal stress is removed by the initial heating in step S15; in other words, the strain energy is homogenized by the initial heating in step S15. Homogenizing the strain energy relieves the strain in the lithium cobalt oxide. This may result in a smoother surface for the lithium cobalt oxide. This is also referred to as an improved surface. In other words, it is believed that the shrinkage difference in the lithium cobalt oxide is relieved after step S15, resulting in a smoother surface for the composite oxide.

[0328] Furthermore, the shrinkage difference may cause microscopic misalignment, such as crystalline misalignment, in the lithium cobalt oxide. To reduce this misalignment, step S15 may be performed. By performing step S15, it is possible to equalize the misalignment of the composite oxide (alleviate the misalignment of the crystals, etc., that has occurred in the composite oxide, or align the crystal grains). This may result in a smoother surface of the composite oxide.

[0329] When lithium cobalt oxide with a smooth surface is used as the positive electrode active material, deterioration during charge and discharge in a secondary battery is reduced and cracking of the positive electrode active material can be prevented.

[0330] Note that pre-synthesized lithium cobalt oxide may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on pre-synthesized lithium cobalt oxide, lithium cobalt oxide with a smooth surface can be obtained.

[0331] <Step S20> Next, as shown in steps S20 to S33, it is preferable to add an additional element A as an A source to the lithium cobalt oxide that has undergone initial heating. Adding the additional element A to the lithium cobalt oxide that has undergone initial heating allows the additional element A to be added evenly. For this reason, it is preferable to add the additional element A after the initial heating (step S15), rather than adding the additional element A and then performing the initial heating (step S15). Next, details of step S20, in which the additional element A is prepared as an A source, will be described with reference to FIGS. 15B and 15C.

[0332] <Step S21> Step S20 shown in FIG. 15B includes steps S21 to S23. In step S21, an additive element A is prepared. The additive elements described in the previous embodiments, such as additive element X and additive element Y, can be used as additive element A. Specifically, one or more elements selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, and boron can be used. One or more elements selected from bromine and beryllium can also be used. FIG. 15B illustrates an example in which a magnesium source and a fluorine source are prepared. In step S21, a lithium source may be separately prepared in addition to additive element A.

[0333] When magnesium is selected as the additional element A, the source of the additional element can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Multiple magnesium sources may be used.

[0334] When fluorine is selected as the additive element A, the additive element source can be called a fluorine source. Examples of the fluorine source include lithium fluoride (LiF) and magnesium fluoride (MgF ２ ), aluminum fluoride (AlF ３ ), titanium fluoride (TiF ４ ), cobalt fluoride (CoF ２ , CoF ３ ), nickel fluoride (NiF２ ), zirconium fluoride (ZrF ４ ), vanadium fluoride (VF ５ ), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF ２ ), calcium fluoride (CaF ２ ), sodium fluoride (NaF), potassium fluoride (KF), barium fluoride (BaF ２ ), cerium fluoride (CeF ３ , CeF ４ ), lanthanum fluoride (LaF ３ ), or sodium aluminum hexafluoride (Na ３ AlF ６ Among these, lithium fluoride is preferred because it has a relatively low melting point of 848° C. and is easily melted in the heating step described below.

[0335] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Other lithium sources that can be used in step S21 include lithium carbonate.

[0336] The fluorine source may also be a gas, such as fluorine (F ２ ), fluorocarbon, sulfur fluoride, or oxygen fluoride (OF ２ , O ２ F ２ , O ３ F ２ , O ４ F ２ , O ５ F ２ , O ６ F ２ , O ２ F) or the like may be used and mixed into the atmosphere in the heating step described below. A plurality of fluorine sources may be used.

[0337] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and magnesium source. ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２The effect of lowering the melting point is greatest when the molar ratio is about 65:35. On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２ = x: 1 (x = near 0.33) is more preferable. In this specification and the like, unless otherwise specified, "near" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0338] 15B, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0339] Here, if necessary, a heating step may be performed after step S22. The heating step can be performed under heating conditions selected from those described in step S13. The heating time is preferably 2 hours or more, and the heating temperature is preferably 800°C or higher and 1100°C or lower.

[0340] 15B, the pulverized and mixed materials are collected to obtain an additive element source (A source). Note that the additive element source (A source) shown in step S23 contains a plurality of starting materials and can also be called a mixture.

[0341] The particle size of the mixture is preferably D50 (median diameter) of 600 nm to 20 μm, more preferably 1 μm to 10 μm. Even when a single material is used as the additive element source, the D50 (median diameter) is preferably 600 nm to 20 μm, more preferably 1 μm to 10 μm.

[0342] When such a finely powdered mixture (including the case where only one kind of additive element is contained) is mixed with lithium cobalt oxide in a later step, the mixture can be easily adhered uniformly to the surface of the lithium cobalt oxide. If the mixture is evenly adhered to the surface of the lithium cobalt oxide, the additive element can be easily distributed or diffused uniformly in the surface layer portion 100a of the composite oxide after heating, which is preferable.

[0343] <Step S21> A process different from that shown in Fig. 15B will be described with reference to Fig. 15C. Step S20 shown in Fig. 15C includes steps S21 to S23.

[0344] In step S21 shown in Fig. 15C, four types of additive element sources to be added to lithium cobalt oxide are prepared. That is, the types of additive element sources in Fig. 15C are different from those in Fig. 15B. In addition to the additive element sources, a lithium source may also be prepared separately.

[0345] As sources of four additive elements, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described in FIG. 15B . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0346] <Steps S22 and S23> Next, steps S22 and S23 shown in FIG. 15C are the same as the steps described with reference to FIG. 15B.

[0347] 15A , lithium cobalt oxide and an additive element source (A source) are mixed. The ratio of the number of cobalt atoms Co in the lithium cobalt oxide to the number of magnesium atoms Mg in the additive element source (A source) is preferably Co:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0348] The mixing in step S31 is preferably performed under milder conditions than those in step S12 so as not to destroy the shape of the lithium cobalt oxide. For example, it is preferable to perform the mixing under conditions of a lower rotation speed or a shorter time than those in step S12. It can also be said that dry mixing provides milder conditions than wet mixing. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the medium.

[0349] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0350] <Step S32> Next, in step S32 of Fig. 15A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0351] 15A to 15C illustrate a fabrication method in which the additive element is added only after initial heating, but the present invention is not limited to this method. The additive element may be added at other timings or multiple times. The timing may also be changed depending on the element.

[0352] For example, an additive element may be added to the lithium source and the transition metal source in step S11, i.e., in the stage of the starting material for the composite oxide. Then, in step S13, lithium cobalt oxide containing the additive element can be obtained. In this case, it is not necessary to separate steps S11 to S14 from steps S21 to S23. This method can be said to be simple and highly productive.

[0353] Alternatively, lithium cobalt oxide containing some of the additive elements may be used. For example, if lithium cobalt oxide containing magnesium and fluorine is used, steps S11 to S14 and some of step S20 can be omitted. This method is simple and has high productivity.

[0354] Alternatively, lithium cobalt oxide to which magnesium and fluorine have been added in advance may be heated in step S15, and then a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source, and an aluminum source may be added as in step S20.

[0355] <Step S33> Next, in step S33 shown in FIG. 15A, the mixture 903 is heated. This can be performed by selecting from the heating conditions described in step S13. The heating time is preferably 2 hours or more. The lower limit of the heating temperature in step S33 must be equal to or higher than the temperature at which the reaction between the lithium cobalt oxide and the additive element source proceeds. The temperature at which the reaction proceeds may be any temperature at which interdiffusion of elements contained in the lithium cobalt oxide and the additive element source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, but the melting temperature T ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 may be 500° C. or higher.

[0356] The reaction is more likely to proceed when the temperature is equal to or higher than the melting point of one or more of the materials contained in the mixture 903. For example, LiF and MgF are used as the additive element source. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0357] Also, LiCoO ２ :LiF:MgF ２ A mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830° C. in differential scanning calorimetry (DSC). Therefore, the lower limit of the heating temperature is more preferably 830° C. or higher.

[0358] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0359] The upper limit of the heating temperature is set to be lower than the decomposition temperature (1130°C) of lithium cobalt oxide. At temperatures close to the decomposition temperature, there is a concern that lithium cobalt oxide may decompose, albeit only slightly. Therefore, the upper limit of the heating temperature is preferably 1000°C or lower, more preferably 950°C or lower, and even more preferably 900°C or lower.

[0360] Taking these factors into consideration, the heating temperature in step S33 is preferably 500°C or higher and 1130°C or lower, more preferably 500°C or higher and 1000°C or lower, even more preferably 500°C or higher and 950°C or lower, and even more preferably 500°C or higher and 900°C or lower. Also, it is preferably 742°C or higher and 1130°C or lower, more preferably 742°C or higher and 1000°C or lower, even more preferably 742°C or higher and 950°C or lower, and even more preferably 742°C or higher and 900°C or lower. Also, it is preferably 800°C or higher and 1100°C or lower, or 830°C or higher and 1130°C or lower, more preferably 830°C or higher and 1000°C or lower, even more preferably 830°C or higher and 950°C or lower, and even more preferably 830°C or higher and 900°C or lower. The heating temperature in step S33 is preferably higher than that in step S13.

[0361] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0362] In the manufacturing method described in this embodiment, some materials, for example, LiF as a fluorine source, may function as a flux, which allows the heating temperature to be lowered below the decomposition temperature of lithium cobalt oxide, for example, to 742° C. or higher and 950° C. or lower, and allows additive elements such as magnesium to be distributed in the surface layer, thereby enabling the manufacture of a positive electrode active material with excellent characteristics.

[0363] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF will volatilize when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat while suppressing the volatilization of LiF. Even if LiF is not used as a fluorine source, etc., LiCoO ２There is a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may then volatilize. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization.

[0364] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization of LiF in the mixture 903.

[0365] Furthermore, the heating in this step is preferably performed so as not to cause adhesion of particles of the mixture 903. If the particles of the mixture 903 adhere to each other during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additive elements (e.g., fluorine) diffuse is blocked, which may result in poor distribution of the additive elements (e.g., magnesium and fluorine) in the surface layer portion.

[0366] Furthermore, it is believed that uniform distribution of the additive element (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with minimal irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the particles of mixture 903 do not adhere to each other.

[0367] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln during heating. For example, it is preferable to reduce the flow rate of the oxygen-containing atmosphere, or to first purge the atmosphere and then not flow the atmosphere after introducing the oxygen atmosphere into the kiln. Flowing oxygen may cause the fluorine source to evaporate, which is undesirable in terms of maintaining surface smoothness.

[0368] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 903 .

[0369] 15A, when the median diameter (D50) of the lithium cobalt oxide is about 12 μm, the heating temperature is preferably, for example, 600° C. or higher and 950° C. or lower. The heating time is preferably, for example, 3 hours or higher, more preferably 10 hours or higher, and even more preferably 60 hours or higher. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0370] When the median diameter (D50) of the lithium cobalt oxide in step S14 is about 5 μm, the heating temperature is preferably, for example, 600° C. or higher and 950° C. or lower. The heating time is preferably, for example, 1 hour or higher and 10 hours or lower, and more preferably about 2 hours. The temperature-lowering time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0371] 15A , the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. At this time, it is preferable to further sieve the recovered positive electrode active material 100. Through the above steps, the positive electrode active material 100 having the characteristics described in this embodiment can be produced.

[0372] 16 and 17 , another example of a method for manufacturing a positive electrode active material that can be used as one embodiment of the present invention (Example 2 of Method for Manufacturing a Positive Electrode Active Material) will be described. Example 2 of Method for Manufacturing a Positive Electrode Active Material differs from Example 1 of Method for Manufacturing a Positive Electrode Active Material described above in the number of times that an additive element is added and the mixing method. However, the other descriptions in Example 1 of Method for Manufacturing a Positive Electrode Active Material can be applied.

[0373] In FIG. 16, steps S11 to S15 are performed in the same manner as in FIG. 15A to prepare lithium cobalt oxide that has undergone initial heating.

[0374] <Step S20a> Next, as shown in steps S20a to S33, an additional element A1 is added to the lithium cobalt oxide that has undergone the initial heating. Step S20a is a step of preparing a first additional element source (A1 source) used to add the additional element A1, and will be described with reference to FIG. 17A.

[0375] <Step S21> In steps S21 to S23 shown in Fig. 17A, a first additive element source (A1 source) is prepared. The additive element A1 can be selected from the additive elements A described in step S21 shown in Fig. 15B and used. For example, the additive element A1 can be one or more selected from magnesium, fluorine, and calcium. Fig. 17A illustrates a case where a magnesium source (Mg source) and a fluorine source (F source) are ground and mixed and used as the A1 source.

[0376] Steps S21 to S23 shown in Fig. 17A can be performed under the same conditions as steps S21 to S23 shown in Fig. 15B. As a result, the first additive element source (A1 source) can be obtained in step S23.

[0377] Steps S31 to S33 shown in FIG. 16 can be performed under the same conditions as steps S31 to S33 shown in FIG. 15A.

[0378] <Step S34a> Next, the material heated in step S33 is recovered to produce lithium cobalt oxide containing the additional element A1. Here, this is also referred to as a second composite oxide to distinguish it from the composite oxide (first composite oxide) of step S14.

[0379] <Step S40> In steps S40 to S53 shown in Fig. 16, the additive element A2 is added to the second composite oxide. Step S40 is a step of preparing a second additive element source (A2 source) used to add the additive element A2, and will be described with reference to Figs. 17B and 17C.

[0380] <Step S41> In steps S41 to S43 shown in FIG. 17B, a second additive element source (A2 source) is prepared. The additive element A2 can be selected from the additive elements A described in step S21 shown in FIG. 15B. For example, one or more selected from nickel, titanium, boron, zirconium, and aluminum can be suitably used as the additive element A2. FIG. 17B illustrates an example in which a nickel source and an aluminum source are pulverized and mixed together and used as the A2 source.

[0381] Steps S41 to S43 shown in Fig. 17B can be performed under the same conditions as steps S21 to S23 shown in Fig. 15B. As a result, the second additive element source (A2 source) can be obtained in step S43.

[0382] Steps S41 to S43 shown in Figure 17C are a modified example of Figure 17B. In step S41 shown in Figure 17C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and in step S42a, they are independently pulverized. As a result, in step S43, a plurality of second additive element sources (A2 sources) are prepared. The steps in Figure 17C differ from those in Figure 17B in that the additive elements are independently pulverized in step S42a.

[0383] <Steps S51 to S53> Next, steps S51 to S53 shown in Fig. 16 can be performed under the same conditions as steps S31 to S33 shown in Fig. 15A. The conditions for step S53 relating to the heating step may be a lower temperature and a shorter time than those for step S33.

[0384] 16 , the heated material is recovered and crushed as necessary to obtain the positive electrode active material 100. Through the above steps, the positive electrode active material 100 having the characteristics described in this embodiment can be produced.

[0385] 16 and 17 , in the manufacturing method 2, the additive element into the lithium cobalt oxide is introduced separately as a first additive element A1 and a second additive element A2. By introducing the additive elements separately, the profile of each additive element in the depth direction can be changed. For example, it is possible to profile the first additive element so that its concentration is higher in the surface layer portion than in the interior, and profile the second additive element so that its concentration is higher in the interior than in the surface layer portion.

[0386] Embodiment 3 In this embodiment, a configuration other than the positive electrode active material and the electrolyte contained in a lithium ion battery will be described.

[0387] [Positive Electrode] The positive electrode includes a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and may further include at least one of a conductive additive and a binder. The positive electrode active material described in Embodiment 1 can be used.

[0388] FIG. 18A shows an example of a schematic cross-sectional view of a positive electrode.

[0389] The current collector 550 can be, for example, a metal foil. The positive electrode can be formed by applying a slurry to a metal foil and drying it. Note that pressing may be performed after drying. The positive electrode is formed by forming an active material layer on the current collector 550.

[0390] The slurry is a material liquid used to form an active material layer on the current collector 550, and refers to a material containing an active material, a binder, and a solvent, and preferably further mixed with a conductive additive. The slurry is also called an electrode slurry or an active material slurry, and is sometimes called a positive electrode slurry when forming a positive electrode active material layer, and a negative electrode slurry when forming a negative electrode active material layer.

[0391] The positive electrode active material 561 has the function of taking in and / or releasing lithium ions during charging and discharging. The positive electrode active material 561 used in one embodiment of the present invention can be a material that exhibits little deterioration during charging and discharging, even at high charging voltages. Unless otherwise specified, the charging voltage is expressed based on the potential of lithium metal in this specification. Furthermore, in this specification, a high charging voltage is, for example, a charging voltage of 4.6 V or higher, preferably 4.65 V or higher, more preferably 4.7 V or higher, even more preferably 4.75 V or higher, and most preferably 4.8 V or higher.

[0392] The positive electrode active material 561 used in one embodiment of the present invention can be any material that does not deteriorate much due to charging and discharging even at a high charging voltage, and can be any material described in Embodiment 1 or 2. Note that the positive electrode active material 561 can be two or more materials with different particle sizes as long as the material does not deteriorate much due to charging and discharging even at a high charging voltage.

[0393] The conductive additive is also called a conductivity-imparting agent or a conductive material, and a carbon material can be used. By attaching the conductive additive between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, the term "attachment" does not only refer to physical adhesion between the active material and the conductive additive, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, the conductive additive covers part of the surface of the active material, the conductive additive is embedded in the surface irregularities of the active material, and electrical connection even when not in contact with each other.

[0394] Specific examples of carbon materials that can be used as the conductive additive include carbon black (furnace black, acetylene black, graphite, etc.).

[0395] FIG. 18A illustrates carbon black 553 as a conductive additive.

[0396] A binder (resin) may be mixed to bond the current collector 550, such as a metal foil, and the active material to form the positive electrode of the secondary battery. The binder is also called a binding agent. The binder is a polymeric material, and adding a large amount of binder reduces the proportion of active material in the positive electrode, thereby reducing the discharge capacity of the secondary battery. Therefore, it is preferable to mix the minimum amount of binder. In FIG. 18A , the areas not filled with the positive electrode active material 561, the second active material 562, and the carbon black 553 represent voids or binders.

[0397] 18A shows an example in which the positive electrode active material 561 is illustrated as a sphere, but this is not particularly limited. For example, the cross-sectional shape of the positive electrode active material 561 may be an ellipse, a rectangle, a trapezoid, a triangle, a polygon with rounded corners, or an asymmetric shape. For example, FIG. 18B shows an example in which the positive electrode active material 561 has a polygonal shape with rounded corners.

[0398] 18B includes graphene 554 as a carbon material used as a conductive additive in the positive electrode. In FIG. 18B, a positive electrode active material layer including a positive electrode active material 561, graphene 554, and carbon black 553 is formed over a current collector 550.

[0399] In the step of mixing the graphene 554 and the carbon black 553 to obtain electrode slurry, the weight of the carbon black to be mixed is preferably 1.5 to 20 times, more preferably 2 to 9.5 times, that of the graphene.

[0400] Furthermore, when the mixture of graphene 554 and carbon black 553 is within the above range, the dispersion stability of carbon black 553 is excellent and agglomerations are less likely to occur during slurry preparation. Furthermore, when the mixture of graphene 554 and carbon black 553 is within the above range, a higher electrode density can be achieved than a positive electrode using only carbon black 553 as a conductive additive. Increasing the electrode density can increase the capacity per unit weight. Specifically, the density of the positive electrode active material layer measured gravimetrically can be 3.5 g / cc or more.

[0401] Although the electrode density is lower than that of a positive electrode using only graphene as a conductive additive, by mixing the first carbon material (graphene) and the second carbon material (acetylene black) within the above range, the positive electrode can be adapted to rapid charging, which makes the positive electrode particularly effective when used as an in-vehicle secondary battery.

[0402] 18C illustrates an example of a positive electrode in which carbon fibers 555 are used instead of graphene. Fig. 18C illustrates an example different from Fig. 18B. The use of carbon fibers 555 can prevent aggregation of carbon black 553 and improve dispersibility.

[0403] In FIG. 18C, the regions not filled with the positive electrode active material 561, the carbon fibers 555, and the carbon black 553 indicate voids or binders.

[0404] 18D shows another example of a positive electrode. In FIG. 18D, carbon fibers 555 are used in addition to graphene 554. When both graphene 554 and carbon fibers 555 are used, aggregation of carbon black such as carbon black 553 can be prevented and dispersibility can be further improved.

[0405] In FIG. 18D , regions that are not filled with the positive electrode active material 561 , the carbon fibers 555 , the graphene 554 , and the carbon black 553 indicate voids or binders.

[0406] A secondary battery can be produced by using any one of the positive electrodes shown in FIGS. 18A to 18D , placing a separator on the positive electrode, and placing the laminate obtained by placing the negative electrode on the separator in a container (such as an exterior body or a metal can) and filling the container with an electrolyte.

[0407] <Binder> As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Furthermore, as the binder, fluororubber can be used.

[0408] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0409] Alternatively, it is preferable to use, as the binder, materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (polymethyl methacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0410] The binder may be used in combination with two or more of the above.

[0411] For example, a material with a particularly excellent viscosity adjusting effect may be used in combination with other materials. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with a particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with a particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with a particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and diacetyl cellulose, cellulose derivatives such as regenerated cellulose, or starch may be used.

[0412] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials or other components when preparing electrode slurries. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0413] Water-soluble polymers stabilize viscosity by dissolving in water, allowing active materials and other materials combined as binders, such as styrene-butadiene rubber, to be stably dispersed in aqueous solutions. Furthermore, the presence of functional groups is expected to facilitate stable adsorption to the surface of active materials. Furthermore, many cellulose derivatives, such as carboxymethyl cellulose, contain functional groups such as hydroxyl or carboxyl groups, and the presence of these functional groups is expected to facilitate interactions between polymers, resulting in widespread coverage of the active material surface.

[0414] When the binder covers the surface of the active material or contacts the surface and forms a film, it is expected to function as a passive film and have the effect of suppressing decomposition of the electrolyte. Here, the "passive film" refers to a film with no electrical conductivity or a film with extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, it can suppress decomposition of the electrolyte at the battery reaction potential. Furthermore, it is more desirable that the passive film suppresses electrical conductivity while still allowing lithium ions to conduct.

[0415] <Positive Electrode Current Collector> The positive electrode current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The positive electrode current collector can be in the form of a foil, plate, sheet, mesh, punched metal, expanded metal, or the like. It is preferable that the positive electrode current collector have a thickness of 5 μm to 30 μm.

[0416] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may further have a conductive additive and a binder.

[0417] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material or a carbon material can be used.

[0418] In addition, the negative electrode active material can be an element capable of undergoing a charge-discharge reaction through alloying and dealloying reactions with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, compounds containing these elements may be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0419] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or close to 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0420] The carbon material may be graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, or the like.

[0421] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0422] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋This allows lithium-ion batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety compared to lithium metal.

[0423] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten oxide (WO ２ ), molybdenum oxide (MoO ２ ) and other oxides can be used.

[0424] In addition, as the negative electrode active material, a composite nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N ３ has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0425] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the composite nitride of lithium and a transition metal can be used as the negative electrode active material, even when a material containing lithium ions is used as the positive electrode active material, by first removing the lithium ions contained in the positive electrode active material.

[0426] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ This also occurs with fluorides such as

[0427] As the conductive additive and binder that can be contained in the negative electrode active material layer, the same materials as the conductive additive and binder that can be contained in the positive electrode active material layer can be used.

[0428] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0429] [Electrolyte] The electrolyte described in the first embodiment can be used.

[0430] [Separator] When the electrolyte contains an electrolytic solution, a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include those made of cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0431] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture thereof. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0432] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0433] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0434] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0435] [Exterior Body] The exterior body of the secondary battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0436] Embodiment Mode 4 In this embodiment mode, examples of various shapes of secondary batteries each having a positive electrode or a negative electrode manufactured by the manufacturing method described in the previous embodiment will be described.

[0437] [Coin-Type Secondary Battery] An example of a coin-type secondary battery will be described. Fig. 19A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 19B is an external view, and Fig. 19C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices.

[0438] 19A is a schematic diagram showing the overlapping of components (upper and lower positions and positional relationships) for ease of understanding, and therefore, FIGS. 19A and 19B are not completely identical corresponding views.

[0439] In Fig. 19A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. These are sealed by a negative electrode can 302 and a positive electrode can 301. Note that a gasket for sealing is not shown in Fig. 19A. The spacer 322 and the washer 312 are used to protect the inside or to fix the position inside the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 322 and the washer 312 are made of stainless steel or an insulating material.

[0440] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0441] In order to prevent short-circuiting between the positive electrode and the negative electrode, a separator 310 is disposed so as to cover the upper surface of the positive electrode 304. The separator 310 has a larger planar area than the positive electrode 304.

[0442] FIG. 19B is a perspective view of the completed coin-type secondary battery.

[0443] In the coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact with the positive electrode current collector. 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 the negative electrode current collector. The negative electrode 307 is not limited to a laminated structure, and may be formed of lithium metal foil or a lithium-aluminum alloy foil.

[0444] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0445] The positive electrode can 301 and the negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. 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.

[0446] These negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte solution, and as shown in FIG. 19C , the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303, thereby producing a coin-type secondary battery 300.

[0447] With the above configuration, the coin-type secondary battery 300 can have a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0448] [Cylindrical Secondary Battery] An example of a cylindrical secondary battery will be described with reference to Fig. 20A . As shown in Fig. 20A , a cylindrical secondary battery 616 has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0449] 20B is a schematic diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in FIG. 20B has a positive electrode cap (battery lid) 601 on the top surface and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0450] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. The battery can 602 can be made of a metal, such as nickel, aluminum, or titanium, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel), which is corrosion-resistant to the electrolyte. Furthermore, it is preferable to coat the battery can 602 with nickel, aluminum, or the like to prevent corrosion by the electrolyte. Inside the battery can 602, the wound 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. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The nonaqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0451] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector.

[0452] By using the positive electrode active material 100 obtained in Embodiments 1 and 2 or the like for the positive electrode 604, a cylindrical secondary battery 616 having a large capacity, a large discharge capacity, and excellent cycle characteristics can be obtained.

[0453] 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 a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature increases, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element is made of barium titanate (BaTiO ３ )-based semiconductor ceramics, etc. can be used.

[0454] 20C shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616. The positive electrodes of the respective secondary batteries are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of the respective secondary batteries are electrically connected to the control circuit 620 via wiring 626. The control circuit 620 may be a charge / discharge control circuit that performs charging and discharging, or a protection circuit that prevents overcharging and / or overdischarging.

[0455] 20D shows an example of a power storage system 615. The power storage system 615 has a plurality of secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of secondary batteries 616, it is possible to extract a large amount of power.

[0456] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0457] Furthermore, a temperature control device may be provided between the multiple secondary batteries 616. When the secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0458] 20D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of secondary batteries 616 via a conductive plate 614.

[0459] [Another Example of Secondary Battery Structure] An example of the structure of a secondary battery will be described with reference to FIGS. 21 and 22. FIG.

[0460] The secondary battery 913 shown in FIG. 21A has a wound body 950 provided with terminals 951 and 952 inside a housing 930. The wound body 950 is immersed in an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in FIG. 21A , for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the 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.

[0461] 21B, the housing 930 shown in Fig. 21A may be formed of a plurality of materials. For example, the secondary battery 913 shown in Fig. 21B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0462] The housing 930a can be made of an insulating material such as organic resin. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to suppress shielding of the electric field by the secondary battery 913. Note that if the shielding of the electric field by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0463] 21C shows the structure of the wound body 950. 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 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0464] Alternatively, a secondary battery 913 may be provided that has a wound body 950a as shown in Fig. 22A. The wound body 950a shown in Fig. 22A has a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 has a negative electrode active material layer 931a. The positive electrode 932 has a positive electrode active material layer 932a.

[0465] By using the positive electrode active material 100 obtained in Embodiment 1, 2, or the like for the positive electrode 932, the secondary battery 913 can have a large capacity, a high discharge capacity, and excellent cycle characteristics.

[0466] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0467] 22B , the negative electrode 931 is electrically connected to a terminal 951 by ultrasonic bonding, welding, or crimping. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952 by ultrasonic bonding, welding, or crimping. The terminal 952 is electrically connected to a terminal 911b.

[0468] 22C , the wound body 950 a and the electrolyte are covered by the housing 930 to form the secondary battery 913. It is preferable to provide the housing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of the housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0469] As shown in Fig. 22B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a can result in a secondary battery 913 with a larger discharge capacity. For other elements of the secondary battery 913 shown in Figs. 22A and 22B, the descriptions of the secondary battery 913 shown in Figs. 21A to 21C can be referred to.

[0470] 23A and 23B show examples of external views of a laminated secondary battery. The battery includes 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.

[0471] 24A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 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 a tab region). The negative electrode 506 has a negative electrode current collector 504, and a 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., a tab region. Note that the area or shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 24A .

[0472] <Method for Manufacturing Laminated Secondary Battery> An example of a method for manufacturing the laminated secondary battery whose external view is shown in FIG. 23A will be described with reference to FIGS. 24B and 24C.

[0473] First, the negative electrode 506, separator 507, and positive electrode 503 are stacked. FIG. 24B shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. This can also be called a laminate consisting of a negative electrode, a separator, and a positive electrode. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrodes 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0474] Next, the negative electrode 506 , the separator 507 , and the positive electrode 503 are arranged on the outer casing 509 .

[0475] Next, as shown in Fig. 24C, the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is joined. For example, thermocompression bonding or the like may be used for joining. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of the exterior body 509 so that an electrolyte can be introduced later.

[0476] Next, the electrolyte solution is introduced into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of the electrolyte solution is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, the laminated secondary battery 500 can be produced.

[0477] By using the positive electrode active material 100 obtained in Embodiments 1 and 2 or the like for the positive electrode 503, the secondary battery 500 can have a high capacity, a high discharge capacity, and excellent cycle characteristics.

[0478] [Example of Battery Pack] An example of a secondary battery pack according to one embodiment of the present invention, which can be wirelessly charged using an antenna, will be described with reference to FIG.

[0479] Fig. 25A is a diagram showing the appearance of secondary battery pack 531, which has a thin rectangular parallelepiped shape (also called a thick flat plate shape). Fig. 25B is a diagram illustrating the configuration of secondary battery pack 531. Secondary battery pack 531 has circuit board 540 and secondary battery 513. Label 529 is affixed to secondary battery 513. Circuit board 540 is fixed with sticker 515. Secondary battery pack 531 also has antenna 517.

[0480] The interior of the secondary battery 513 may have a structure including a wound body or a laminated body.

[0481] 25B , the secondary battery pack 531 has a control circuit 590 on a circuit board 540. The circuit board 540 is electrically connected to the terminals 514. The circuit board 540 is also electrically connected to the antenna 517, one 551 of the positive and negative leads of the secondary battery 513, and the other 552 of the positive and negative leads.

[0482] Alternatively, as shown in FIG. 25C, the device may have a circuit system 590 a provided on a circuit board 540 and a circuit system 590 b electrically connected to the circuit board 540 via a terminal 514 .

[0483] The antenna 517 is not limited to a coil shape and may be, for example, a wire shape or a plate shape. Furthermore, antennas such as a planar antenna, an aperture antenna, a traveling wave antenna, an EH antenna, a magnetic field antenna, and a dielectric antenna may also be used. Alternatively, the antenna 517 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. In other words, the antenna 517 may function as one of the two conductors of a capacitor. This allows power to be exchanged not only by electromagnetic fields and magnetic fields but also by electric fields.

[0484] The secondary battery pack 531 includes a layer 519 between the antenna 517 and the secondary battery 513. The layer 519 has a function of, for example, shielding an electromagnetic field generated by the secondary battery 513. The layer 519 can be formed using, for example, a magnetic material.

[0485] Embodiment 5 This embodiment is an example different from the cylindrical secondary battery shown in Fig. 20D. An example of application to an electric vehicle (EV) is shown using Fig. 26C.

[0486] The electric vehicle is equipped with first batteries 1301a and 1301b as main driving secondary batteries, and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0487] The internal structure of the first battery 1301a may be a wound type shown in Fig. 21C or 22A or a stacked type shown in Fig. 23A or 23B. The first battery 1301a may use the all-solid-state battery of Embodiment 6. By using the all-solid-state battery of Embodiment 6 for the first battery 1301a, a high capacity can be achieved, safety can be improved, and the battery can be made smaller and lighter.

[0488] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple secondary batteries, it is possible to extract large amounts of power. The multiple secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of secondary batteries is also called a battery pack.

[0489] In addition, in a secondary battery for vehicle use, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a in order to cut off power from multiple secondary batteries.

[0490] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0491] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0492] Next, the first battery 1301a will be described with reference to FIG. 26A.

[0493] FIG. 26A shows an example in which nine prismatic secondary batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also called a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (such as from the road surface), it is preferable to fix multiple secondary batteries using the fixing portions 1413 and 1414 or a battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0494] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS).

[0495] It is preferable to use a metal oxide that functions as an oxide semiconductor. For example, a metal oxide such as In-M-Zn oxide (wherein the element M is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, etc.) may be used as the oxide. In particular, the In-M-Zn oxide that can be used as the oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In—Ga oxide or an In—Zn oxide may be used as the oxide. The CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis oriented in a specific direction. The specific direction is the thickness direction of the CAAC-OS film, the normal direction to the surface on which the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. The crystalline regions are regions in which the atomic arrangement is periodic. When the atomic arrangement is considered as a lattice arrangement, the crystalline regions are also regions in which the lattice arrangement is aligned.

[0496] Note that "CAC-OS" has a mosaic structure in which a material is separated into a first region and a second region, and the first region is distributed throughout the film (hereinafter, also referred to as a cloud structure). That is, CAC-OS is a composite metal oxide having a structure in which the first region and the second region are mixed. However, it may be difficult to observe a clear boundary between the first region and the second region.

[0497] For example, in the case of CAC-OS in an In—Ga—Zn oxide, EDX mapping obtained using energy dispersive X-ray spectroscopy (EDX) can confirm that the CAC-OS has a structure in which a region containing In as a main component (first region) and a region containing Ga as a main component (second region) are unevenly distributed and mixed.

[0498] When a CAC-OS is used in a transistor, the conductivity due to the first region and the insulating property due to the second region act complementarily, thereby providing the CAC-OS with a switching function (a function of turning on / off). That is, a CAC-OS has a conductive function in a part of the material and an insulating function in a part of the material, and functions as a semiconductor as a whole. By separating the conductive function and the insulating function, both functions can be maximized. Therefore, by using a CAC-OS in a transistor, a high on-current (I ｏｎ ), high field-effect mobility (μ), and good switching behavior can be achieved.

[0499] Oxide semiconductors have a variety of structures, each of which has different characteristics. The oxide semiconductor of one embodiment of the present invention may include two or more of an amorphous oxide semiconductor, a polycrystalline oxide semiconductor, an a-like OS, a CAC-OS, an nc-OS, and a CAAC-OS.

[0500] Furthermore, because the control circuit unit 1320 can be used in high-temperature environments, it is preferable to use a transistor using an oxide semiconductor. To simplify the process, the control circuit unit 1320 may be formed using a unipolar transistor. Transistors using an oxide semiconductor for the semiconductor layer have a wider operating ambient temperature range than single-crystal Si, from −40° C. to 150° C., and their characteristics change less when the secondary battery is heated than single-crystal Si. The off-current of a transistor using an oxide semiconductor is below the lower limit of measurement regardless of temperature, even at 150° C., whereas the off-current characteristics of single-crystal Si transistors are highly temperature-dependent. For example, at 150° C., the off-current of a single-crystal Si transistor increases, and the current on / off ratio is not sufficiently large. The control circuit unit 1320 can improve safety. Furthermore, a synergistic effect on safety can be achieved by combining the positive electrode active material 100 obtained in Embodiments 1 and 2 with a secondary battery using the positive electrode. The secondary battery and control circuit unit 1320 using the positive electrode active material 100 obtained in the first or second embodiment or the like for the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0501] The control circuit unit 1320, which uses a memory circuit including transistors using oxide semiconductors, can also function as an automatic control device for the secondary battery to address causes of instability such as micro-short circuits. Functions for eliminating causes of secondary battery instability include overcharging prevention, overcurrent prevention, overheating control during charging, cell balancing in the battery pack, over-discharging prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of deterioration, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits, and the control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for the secondary battery can be made ultra-miniaturized.

[0502] Furthermore, a "micro-short" refers to a tiny short circuit inside a secondary battery, which is not so small that the positive and negative electrodes of the secondary battery are short-circuited and render it unable to be charged or discharged, but rather refers to a phenomenon in which a small short-circuited part allows a small amount of short-circuit current to flow. Even if the short-circuit occurs in a relatively short period of time and in a small location, a large voltage change occurs, and this abnormal voltage value may affect subsequent estimations.

[0503] One of the causes of micro-short circuits is said to be that multiple charge and discharge cycles cause uneven distribution of the positive electrode active material, resulting in localized current concentration in parts of the positive electrode and negative electrode, causing parts of the separator to stop functioning, or the generation of by-products due to side reactions, resulting in micro-short circuits.

[0504] In addition to detecting micro-shorts, the control circuit 1320 can also be said to detect the terminal voltage of the secondary battery and manage the charge / discharge state of the secondary battery. For example, to prevent overcharging, it can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0505] Next, an example of a block diagram of the battery pack 1415 shown in FIG. 26A is shown in FIG. 26B.

[0506] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the secondary battery used and limits the upper limit of the current from the outside or the upper limit of the output current to the outside. The range between the lower limit and the upper limit of the secondary battery's voltage is within the recommended voltage range, and when the secondary battery falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and / or overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. Furthermore, a PTC element may be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0507] The switch portion 1324 can be configured by combining n-channel transistors or p-channel transistors. The switch portion 1324 is not limited to a switch having a Si transistor using single crystal silicon. For example, the switch portion 1324 may be formed using a power transistor having Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), or GaOx (gallium oxide; x is a real number greater than 0). Furthermore, memory elements using OS transistors can be freely arranged by stacking them on circuits using Si transistors, and therefore integration can be easily achieved. Furthermore, OS transistors can be manufactured using the same manufacturing equipment as Si transistors, and therefore can be manufactured at low cost. That is, the control circuit portion 1320 using OS transistors can be stacked on the switch portion 1324 and integrated into a single chip. The volume occupied by the control circuit section 1320 can be reduced, which allows for miniaturization.

[0508] The first batteries 1301a and 1301b primarily supply power to 42V (high-voltage) in-vehicle devices, while the second battery 1311 supplies power to 14V (low-voltage) in-vehicle devices. Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Lead-acid batteries have the disadvantage of being more self-discharged than lithium-ion batteries and being prone to deterioration due to a phenomenon called sulfation. Using a lithium-ion battery as the second battery 1311 offers the advantage of being maintenance-free, but after prolonged use, e.g., three years or more, there is a risk of abnormalities occurring that are difficult to identify during manufacturing. In particular, if the second battery 1311, which starts the inverter, becomes inoperable, even if the first batteries 1301a and 1301b still have remaining capacity, the motor cannot be started. To prevent this, if the second battery 1311 is a lead-acid battery, power is supplied from the first battery to the second battery, and the second battery is constantly charged to maintain a fully charged state.

[0509] In this embodiment, an example in which lithium ion batteries are used for both the first battery 1301a and the second battery 1311 is shown. A lead-acid battery, an all-solid-state battery, or an electric double layer capacitor may be used for the second battery 1311. For example, the all-solid-state battery of Embodiment 6 may be used. By using the all-solid-state battery of Embodiment 6 for the second battery 1311, high capacity can be achieved, and reductions in size and weight can be achieved.

[0510] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 via the motor controller 1303 or the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0511] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the secondary battery used, and can perform rapid charging.

[0512] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a and 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a and 1301b via the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU includes a microcomputer. The ECU uses a CPU or a GPU.

[0513] External chargers installed at charging stations and the like include 100V-200V outlets, or three-phase 200V and 50kW. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0514] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0515] Furthermore, by using graphene as a conductive additive, it is possible to suppress capacity reduction and maintain high capacity even when the electrode layer is thickened and the amount of graphene supported is increased, resulting in a synergistic effect that allows for the realization of a secondary battery with significantly improved electrical properties. This is particularly effective for secondary batteries used in vehicles, and it is possible to provide a vehicle with a long driving range, specifically a driving distance of 500 km or more per charge, without increasing the ratio of the weight of the secondary battery to the total weight of the vehicle.

[0516] In particular, the secondary battery of the present embodiment described above can increase the operating voltage of the secondary battery by using the positive electrode active material 100 described in Embodiments 1, 2, etc., and can increase the usable capacity as the charging voltage increases. Furthermore, by using the positive electrode active material 100 described in Embodiments 1, 2, etc., in the positive electrode, a secondary battery for a vehicle having excellent cycle characteristics can be provided.

[0517] Next, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0518] 20D , 22C , and 26A can be installed in a vehicle to realize next-generation clean energy automobiles, such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Furthermore, the secondary battery can also be installed in agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, ships, submarines, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft. The secondary battery of one embodiment of the present invention can be a high-capacity secondary battery. Therefore, the secondary battery of one embodiment of the present invention is suitable for miniaturization and weight reduction and can be suitably used in transportation vehicles.

[0519] 27A to 27D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 27A is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. When a secondary battery is installed in a vehicle, an example of the secondary battery described in Embodiment 4 is installed in one or more locations. The automobile 2001 shown in FIG. 27A includes a battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. It is preferable that the automobile further include a charge control device electrically connected to the secondary battery module.

[0520] Furthermore, the automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging may be performed using a predetermined charging method or connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The secondary battery may be charged using a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge an electric storage device installed in the automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0521] Furthermore, although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground-based power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0522] 27B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The secondary battery module of the transport vehicle 2002 is, for example, a four-cell unit of secondary batteries with a nominal voltage of 3.0 V to 5.0 V, with 48 cells connected in series for a maximum voltage of 170 V. Apart from the number of secondary batteries constituting the secondary battery module of the battery pack 2201, the transport vehicle 2002 has the same functions as those shown in FIG. 27A, and therefore a description thereof will be omitted.

[0523] FIG. 27C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The secondary battery module of the transport vehicle 2003 has, for example, a maximum voltage of 600 V, with one hundred or more secondary batteries connected in series, each having a nominal voltage of 3.0 V or more and 5.0 V or less. Therefore, a secondary battery with minimal characteristic variation is required. By using a secondary battery that uses the positive electrode active material 100 described in embodiments 1 and 2, etc., as a positive electrode, a secondary battery with stable battery characteristics can be manufactured, enabling mass production at low cost from the perspective of yield. Furthermore, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2202, the battery pack 2202 has the same functions as those shown in FIG. 16A , and therefore a description thereof will be omitted.

[0524] Fig. 27D shows, as an example, an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 27D has wheels for takeoff and landing, it can also be considered a type of transport vehicle, and has a battery pack 2203 that includes a secondary battery module formed by connecting multiple secondary batteries and the secondary battery module and a charge control device.

[0525] The secondary battery module of the aircraft 2004 is, for example, eight 4 V secondary batteries connected in series to produce a maximum voltage of 32 V. Other than the number of secondary batteries constituting the secondary battery module of the battery pack 2203, the secondary battery module has the same functions as those shown in Fig. 27A, and therefore a description thereof will be omitted.

[0526] 27E shows, as an example, an artificial satellite 2005 equipped with a secondary battery 2204. Because the artificial satellite 2005 is used in outer space at extremely low temperatures, it is preferable that the artificial satellite 2005 be equipped with the secondary battery 2204, which is one embodiment of the present invention and has excellent low-temperature resistance. It is further preferable that the secondary battery 2204 be mounted inside the artificial satellite 2005 while being covered with a heat-insulating member.

[0527] Embodiment 6 In this embodiment, an example in which a secondary battery which is one embodiment of the present invention is mounted in a building will be described with reference to FIGS. 28A and 28B.

[0528] The house illustrated in FIG. 28A includes a power storage device 2612 including a secondary battery of one embodiment of the present invention, and a solar panel 2610. The power storage device 2612 is electrically connected to the solar panel 2610 via wiring 2611 or the like. The power storage device 2612 may also be electrically connected to a ground-mounted charging device 2604. The power obtained by the solar panel 2610 can be charged to the power storage device 2612. The power stored in the power storage device 2612 can be charged to a secondary battery included in the vehicle 2603 via the charging device 2604. The power storage device 2612 is preferably installed in an underfloor space. By installing the power storage device 2612 in the underfloor space, the space above the floor can be effectively utilized. Alternatively, the power storage device 2612 may be installed on the floor.

[0529] The power stored in the power storage device 2612 can also be supplied to other electronic devices in the house. Therefore, even when power cannot be supplied from a commercial power source due to a power outage or the like, the power storage device 2612 of one embodiment of the present invention can be used as an uninterruptible power supply, enabling the use of electronic devices.

[0530] 28B illustrates an example of a power storage device according to one embodiment of the present invention. As illustrated in FIG. 28B , a power storage device 791 according to one embodiment of the present invention is installed in an underfloor space 796 of a building 799. The control circuit described in Embodiment 7 may be provided in the power storage device 791. A synergistic effect on safety can be obtained by using a secondary battery in which the positive electrode active material 100 obtained in Embodiments 1, 2, or the like is used for the power storage device 791. The control circuit described in Embodiment 7 and the secondary battery in which the positive electrode active material 100 described in Embodiments 1, 2, or the like is used for the positive electrode can greatly contribute to preventing accidents such as fires caused by the power storage device 791 including a secondary battery.

[0531] A control device 790 is installed in the power storage device 791, and the control device 790 is electrically connected to the distribution board 703, the power storage controller 705 (also called the control device), the display 706, and the router 709 by wiring.

[0532] Electric power is sent from commercial power source 701 to distribution board 703 via service line attachment portion 710. Electric power is also sent to distribution board 703 from power storage device 791 and commercial power source 701, and distribution board 703 supplies the sent electric power to general load 707 and power storage load 708 via outlets (not shown).

[0533] The general load 707 is, for example, an electrical appliance such as a television or a personal computer, and the power storage load 708 is, for example, an electrical appliance such as a microwave oven, a refrigerator, or an air conditioner.

[0534] The power storage controller 705 includes a measurement unit 711, a prediction unit 712, and a planning unit 713. The measurement unit 711 has a function of measuring the amount of power consumed by the general load 707 and the power storage load 708 during a day (e.g., from midnight to midnight). The measurement unit 711 may also have a function of measuring the amount of power of the power storage device 791 and the amount of power supplied from the commercial power source 701. The prediction unit 712 has a function of predicting the amount of power demand to be consumed by the general load 707 and the power storage load 708 during the next day based on the amount of power consumed by the general load 707 and the power storage load 708 during the previous day. The planning unit 713 has a function of creating a plan for charging and discharging the power storage device 791 based on the amount of power demand predicted by the prediction unit 712.

[0535] The amount of power consumed by the general load 707 and the power storage load 708 measured by the measurement unit 711 can be confirmed on the display 706. It can also be confirmed on an electrical device such as a television or a personal computer via the router 709. It can also be confirmed on a portable electronic device such as a smartphone or a tablet via the router 709. The amount of power demand for each time period (or each hour) predicted by the prediction unit 712 can also be confirmed on the display 706, the electrical device, or the portable electronic device.

[0536] Embodiment 7 In this embodiment, an example in which a power storage device according to one embodiment of the present invention is mounted on a motorcycle or a bicycle will be described.

[0537] 29A illustrates an example of an electric bicycle using the power storage device of one embodiment of the present invention. The power storage device of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 29A. The power storage device of one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

[0538] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state detached from the bicycle in FIG. 29B . The power storage device 8702 includes a plurality of storage batteries 8701 included in the power storage device of one embodiment of the present invention, and a display unit 8703 can display the remaining battery charge and the like. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery, an example of which is shown in Embodiment 7. The control circuit 8704 is electrically connected to the positive electrode and the negative electrode of the storage battery 8701. A synergistic effect in terms of safety can be obtained by combining the positive electrode active material 100 obtained in Embodiments 1, 2, and the like with a secondary battery whose positive electrode is used. The secondary battery and the control circuit 8704 using the positive electrode active material 100 obtained in Embodiments 1 and 2 for the positive electrode can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0539] 29C illustrates an example of a two-wheeled vehicle using the power storage device of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 29C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. The power storage device 8602 includes a plurality of secondary batteries each using the positive electrode active material 100 obtained in Embodiments 1 and 2, for example, for a positive electrode, and thus can have a high capacity, which can contribute to miniaturization.

[0540] 29C can store a power storage device 8602 in an under-seat storage compartment 8604. The power storage device 8602 can be stored in the under-seat storage compartment 8604 even if the under-seat storage compartment 8604 is small.

[0541] Embodiment 8 In this embodiment, an example of mounting a secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0542] 30A shows an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 also includes a secondary battery 2107. By including the secondary battery 2107 using the positive electrode active material 100 described in Embodiments 1 and 2, etc. as a positive electrode, a high capacity can be achieved, and a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0543] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0544] The operation button 2103 can be provided with various functions such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0545] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0546] The mobile phone 2100 also includes an external connection port 2104, and can directly exchange data with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0547] Furthermore, the mobile phone 2100 preferably has a sensor. As the sensor, for example, a fingerprint sensor, a pulse sensor, a body temperature sensor or other human body sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like is preferably mounted.

[0548] FIG. 30B illustrates an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a secondary battery 2301 according to one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2 as a positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Thus, the secondary battery is suitable as a secondary battery to be mounted on the unmanned aerial vehicle 2300.

[0549] Fig. 30C shows an example of a robot. A robot 6400 shown in Fig. 30C includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0550] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0551] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0552] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0553] The robot 6400 includes a secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1, 2, and the like for a positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as the secondary battery 6409 to be mounted on the robot 6400.

[0554] 30D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, operation buttons 6305, a secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck up the dust from a suction port arranged on the bottom surface.

[0555] The cleaning robot 6300 can analyze an image captured by the camera 6303 and determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1, 2, and the like for its positive electrode has high energy density and high safety, and therefore can be used safely for a long period of time. Therefore, the secondary battery 6306 is suitable for use in the cleaning robot 6300.

[0556] 31A shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Furthermore, in order to improve splash-proof, water-resistant, or dust-proof performance when used at home or outdoors, there is a demand for a wearable device that can be charged wirelessly as well as via a wired connection with an exposed connector.

[0557] For example, the secondary battery of one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 31A . The eyeglasses-type device 4000 includes a frame 4000 a and a display portion 4000 b. By mounting the secondary battery on temple portions of the curved frame 4000 a, the eyeglasses-type device 4000 can be lightweight, well-balanced in weight, and has a long continuous use time. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for its positive electrode has a high energy density and can realize a configuration that can accommodate space saving associated with a miniaturized housing.

[0558] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on the headset device 4001. The headset device 4001 includes at least a microphone unit 4001a, a flexible pipe 4001b, and an earphone unit 4001c. The secondary battery can be provided in the flexible pipe 4001b or the earphone unit 4001c. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2 as a positive electrode has high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0559] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4002 that can be directly attached to the body. A secondary battery 4002b can be provided in a thin housing 4002a of the device 4002. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for a positive electrode has high energy density, and a configuration that can accommodate space saving due to miniaturization of the housing can be realized.

[0560] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided in a thin housing 4003a of the device 4003. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for its positive electrode has high energy density, and a structure that can accommodate space saving due to miniaturization of the housing can be realized.

[0561] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the belt-type device 4006. The belt-type device 4006 includes a belt portion 4006a and a wireless power receiving portion 4006b, and the secondary battery can be mounted in an inner region of the belt portion 4006a. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1 and 2, etc., for its positive electrode has high energy density and can realize a configuration that can accommodate space saving associated with miniaturization of the housing.

[0562] Furthermore, the secondary battery of one embodiment of the present invention can be mounted on the wristwatch device 4005. The wristwatch device 4005 has a display portion 4005a and a belt portion 4005b, and the secondary battery can be provided in the display portion 4005a or the belt portion 4005b. A secondary battery using the positive electrode active material 100 obtained in Embodiments 1, 2, and the like for its positive electrode has high energy density, and a structure that can accommodate space saving due to miniaturization of the housing can be realized.

[0563] The display unit 4005a can display not only the time but also various other information such as incoming emails or phone calls.

[0564] Furthermore, since the wristwatch-type device 4005 is a wearable device that is worn directly on the wrist, it may be equipped with sensors that measure the user's pulse, blood pressure, etc. Data on the user's exercise volume and health can be accumulated to manage the user's health.

[0565] FIG. 31B shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0566] 31C shows a side view of the display portion 4005a. The side view of the display portion 4005a also shows a state in which a secondary battery 913 is built in the internal region. The secondary battery 913 is the secondary battery described in Embodiment 4. The secondary battery 913 is provided in a position overlapping with the display portion 4005a, and can have high density and high capacity, and is small and lightweight.

[0567] Since the wristwatch-type device 4005 is required to be small and lightweight, by using the positive electrode active material 100 obtained in embodiments 1 and 2, etc., in the positive electrode of the secondary battery 913, it is possible to obtain a high-energy density and small-sized secondary battery 913.

[0568] In this example, the lithium ion battery described in the embodiment mode and the like was manufactured, and the results of obtaining battery characteristics are shown.

[0569] <Preparation of Positive Electrode Active Material> The positive electrode active material used in the lithium ion battery will be described. First, the steps of preparing the positive electrode active material will be described in detail with reference to the preparation method shown in Figures 16 and 17. The method of preparing the positive electrode active material used in this example conforms to the method of preparing the positive electrode active material specifically described in Embodiment 2.

[0570] Lithium cobalt oxide (LiCoO ２ As the lithium cobalt oxide, commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) containing no particular additive elements was prepared. In the heating step S15, this lithium cobalt oxide was placed in a crucible, which was then covered with a lid and heated at 850°C for 2 hours in a muffle furnace. After the muffle furnace was placed in an oxygen atmosphere, no flow (O ２ (Purge). Checking the amount recovered after the initial heating revealed that the weight had decreased slightly. This may have been due to the removal of impurities from the lithium cobalt oxide.

[0571] Next, according to steps S21 and S41 shown in Figures 17A and 17B, Mg and F, and Ni and Al were added as additive elements in two separate steps. First, according to step S21 shown in Figure 17A, LiF was prepared as the F source, and MgF was prepared as the Mg source. ２ LiF:MgF was prepared. ２ were weighed out so that the molar ratio was 1:3. Next, LiF and MgF were dissolved in dehydrated acetone. ２ The above ingredients were mixed and stirred at a rotation speed of 400 rpm for 12 hours to prepare an additive element source (Al source). A ball mill was used for mixing, and zirconium oxide balls were used as the grinding media. 20 mL of dehydrated acetone, 22 g of zirconium oxide balls (1 mm diameter), and a total of approximately 9 g of F source and Mg source were added to a 45 mL capacity container of the mixing ball mill, and mixed. The mixture was then sieved through a sieve with 300 μm openings to obtain an Al source with a uniform particle size.

[0572] Next, in step S31, the Al source was weighed so that the number of magnesium atoms in the Al source was 1 atomic % relative to the number of cobalt atoms in the lithium cobalt oxide, and the Al source was dry-mixed with the initially heated lithium cobalt oxide. The mixture was stirred at a rotation speed of 150 rpm for 1 hour, which was a gentler stirring condition than that used to obtain the Al source. Finally, the mixture was sieved with a 300 μm mesh sieve to obtain a mixture 903 with a uniform particle size (step S32).

[0573] Next, in step S33, the mixture 903 was heated. The heating conditions were 900°C and 20 hours. During heating, a lid was placed on the crucible containing the mixture 903. An oxygen-containing atmosphere was created inside the crucible, and the inflow and outflow of oxygen was blocked (purging). By heating, a composite oxide containing Mg and F was obtained (step S34a).

[0574] Next, in step S51, the composite oxide and the additive element source (A2 source) were mixed. Following step S41 shown in FIG. 17B, nickel hydroxide was prepared as the Ni source, and aluminum hydroxide was prepared as the Al source. The nickel hydroxide and aluminum hydroxide were weighed so that the number of nickel atoms relative to the number of cobalt atoms in the composite oxide was 0.5 atomic % and 0.5 atomic % relative to the number of cobalt atoms in the composite oxide, respectively, and then dry-mixed with the composite oxide. The mixture was stirred at a rotation speed of 150 rpm for 1 hour. A ball mill was used for mixing, and zirconium oxide balls were used as the grinding media. Approximately 7.5 g of the Ni source and Al source were mixed with 22 g of zirconium oxide ball...

Claims

1. A lithium-ion battery comprising a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode having a carbon material as a negative electrode active material, wherein the positive electrode active material contains lithium cobaltate represented by Li x CoO 2 (where 0 < x ≦ 1), when x in the Li x CoO 2 is 1, it has a layered rock salt type crystal structure of space group R-3m, when the charging state of x in the Li x CoO 2 exceeds 0.1 and is 0.24 or less, space group P2 / m, lattice constant a = 4.88 ± 0.01 (×10 -1 nm), lattice constant b = 2.82 ± 0.01 (×10 -1 nm), lattice constant c = 4.84 ± 0.01 (×10 -1 nm), α＝９０°、 β＝１０９．５８±０．０１°、 and has a crystal structure with γ = 90°, the electrolyte contains ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and when the total content of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 100 vol%, the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is x:y:100 - x - y (where 5 ≦ x ≦ 35 and 0 < y < 65), the value of the discharge capacity obtained by constant current charging the lithium-ion battery at a charging rate of 0.1C (where 1C = 200 mA / g) until a voltage of 4.5V is reached in a 25°C environment, then performing constant voltage charging at 4.5V until the current value becomes 0.01C, and then performing constant current discharging at a discharging rate of 0.1C until a voltage of 2.5V is reached in a -40°C environment is 50% or more compared to the value of the discharge capacity obtained by constant current charging the lithium-ion battery at a charging rate of 0.1C (where 1C = 200 mA / g) until a voltage of 4.5V is reached in a 25°C environment, then performing constant voltage charging at 4.5V until the current value becomes 0.01C, and then performing constant current discharging at a discharging rate of 0.1C until a voltage of 2.5V is reached in a 25°C environment. A lithium-ion battery.

2. The lithium-ion battery according to claim 1, wherein the carbon material is graphite.

3. A lithium-ion battery comprising a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode, and being operable in at least a temperature range of -40°C or higher and 25°C or lower.

4. A lithium-ion battery comprising a positive electrode having a positive electrode active material, an electrolyte, and a negative electrode, wherein the positive electrode active material is used as the positive electrode, The positive electrode active material has lithium cobaltate represented by Li x CoO 2 (where 0 < x ≤ 1). When x in the Li x CoO 2 is 1, it has a layered rock salt-type crystal structure with a space group of R-3m. When the charging state of x in the Li x CoO 2 exceeds 0.1 and is 0.24 or less, Space group P2 / m, Lattice constant a = 4.88 ± 0.01 (×10 -1 nm), Lattice constant b = 2.82 ± 0.01 (×10 -1 nm), Lattice constant c = 4.84 ± 0.01 (×10 -1 nm), α＝９０°、 β＝１０９．５８±０．０１°、 It has a crystal structure with γ = 90°. Using an electrolyte containing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, when the total content of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is 100 vol%, the volume ratio of the ethylene carbonate, the ethyl methyl carbonate, and the dimethyl carbonate is x:y:100 - x - y (where 5 ≤ x ≤ 35 and 0 < y < 65). When a test battery is made using lithium metal as the negative electrode, When the test battery is charged at a constant current rate of 0.1 C (where 1 C = 200 mA / g) until a voltage of 4.6 V is reached in a 25°C environment, then charged at a constant voltage of 4.6 V until the current value reaches 0.01 C, and then discharged at a constant current rate of 0.1 C until a voltage of 2.5 V is reached in a -40°C environment, the value of the discharge capacity obtained is 50% or more compared to the value of the discharge capacity obtained by charging the test battery at a constant current rate of 0.1 C (where 1 C = 200 mA / g) until a voltage of 4.6 V is reached in a 25°C environment, then charging at a constant voltage of 4.6 V until the current value reaches 0.01 C, and then discharging at a constant current rate of 0.1 C until a voltage of 2.5 V is reached in a 25°C environment. A lithium ion battery.