Lithium ion secondary battery

Abstract

To provide a novel positive electrode active material, a positive electrode, and a lithium ion secondary battery.SOLUTION: A lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode active material including a complex oxide containing lithium and cobalt. The positive electrode active material contains barium, magnesium, and aluminum in a surface layer part. In the analysis of the surface layer part, it is preferable to include a region where a first point at which the concentration of barium is the maximum and a second point at which the concentration of magnesium is the maximum exist on a surface side of the positive electrode active material relative to a third point at which the concentration of aluminum is the maximum.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] BACKGROUND OF THE INVENTION 1. Field of the Invention One embodiment of the present invention relates to a method for manufacturing a positive electrode active material, a method for manufacturing a positive electrode, a method for manufacturing a secondary battery, or a portable information terminal, a power storage system, a vehicle, or the like including a secondary battery.

[0002] One embodiment of the present invention relates to an object, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. Another embodiment of the present invention relates 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. Note that one embodiment of the present invention particularly relates to a method for manufacturing a positive electrode active material or a positive electrode active material. Another embodiment of the present invention particularly relates to a method for manufacturing a positive electrode or a positive electrode. Another embodiment of the present invention particularly relates to a method for manufacturing a secondary battery or a secondary battery.

[0003] In this specification, the term "semiconductor device" refers to any device that can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic devices are all included in the category of semiconductor devices.

[0004] In this specification, an electronic device refers to a device in general that has a positive electrode active material, a secondary battery, or a power storage device, and electro-optical devices that have a positive electrode active material, a positive electrode, a secondary battery, or a power storage device, and information terminal devices that have a power storage device are all electronic devices.

[0005] In this specification, 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 secondary batteries (also called secondary batteries), lithium ion capacitors, and electric double layer capacitors. [Background technology]

[0006] In recent years, there has been active development of various energy storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. In particular, lithium-ion secondary batteries, which have high output and high energy density, are applied to mobile information terminals such as mobile phones, smartphones, and laptop computers, portable music players, digital cameras, medical devices, home energy storage systems, industrial energy storage systems, and next-generation clean energy vehicles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs). Demand for lithium-ion secondary batteries has expanded rapidly alongside the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0007] Among these, composite oxides such as lithium cobalt oxide and lithium nickel-cobalt-manganese oxide, which have a layered rock salt structure, are widely used. These materials have useful properties as active materials for power storage devices, such as high capacity and high discharge voltage. However, to achieve high capacity, the positive electrode must be exposed to a high potential relative to lithium during charging. Under such high potential conditions, large amounts of lithium are desorbed, which can reduce the stability of the crystal structure and lead to significant deterioration during charge-discharge cycles. Against this background, efforts have been made to improve the positive electrode active materials of secondary battery positive electrodes in order to develop high-capacity, highly stable secondary batteries (e.g., Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2018-088400 A [Patent Document 2] WO2018 / 203168 Brochure [Patent Document 3] Japanese Patent Publication No. 2020-140954 Summary of the Invention [Problem to be solved by the invention]

[0009] Although improvements to positive electrode active materials have been actively carried out in the above Patent Documents 1 to 3, there remains room for improvement in various aspects of lithium ion secondary batteries and positive electrode active materials used therein, such as charge / discharge capacity, cycle characteristics, reliability, safety, and cost.

[0010] In view of the above, an object of one embodiment of the present invention is to provide a method for manufacturing a positive electrode active material that is stable in a high-potential state (also referred to as a high-voltage charging state) and / or a high-temperature state. Another object is to provide a method for manufacturing a positive electrode active material whose crystal structure is not easily destroyed even after repeated charging and discharging. Another object is to provide a method for manufacturing a positive electrode active material that has excellent charge-discharge cycle characteristics. Another object is to provide a method for manufacturing a positive electrode active material that has large charge-discharge capacity. Another object is to provide a method for manufacturing a highly reliable or safe secondary battery.

[0011] Another object of one embodiment of the present invention is to provide a method for manufacturing a positive electrode that is stable in a high potential state and / or a high temperature state, a method for manufacturing a positive electrode that has excellent charge-discharge cycle characteristics, or a method for manufacturing a positive electrode that has large charge-discharge capacity.

[0012] Another object of one embodiment of the present invention is to provide a novel substance, active material particles, an electrode, a secondary battery, a power storage device, or a manufacturing method thereof.Another object of one embodiment of the present invention is to provide a manufacturing method of a secondary battery or a secondary battery having one or more characteristics selected from high purity, high performance, and high reliability.

[0013] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims. [Means for solving the problem]

[0014] One embodiment of the present invention is a positive electrode including a positive electrode active material having a composite oxide containing lithium and cobalt, and the positive electrode active material has barium, magnesium, and aluminum in a surface layer portion.

[0015] Another embodiment of the present invention is a positive electrode including a positive electrode active material having a composite oxide containing lithium and cobalt, the positive electrode active material including barium, magnesium, and aluminum in a surface layer portion, and including a region in which the barium and magnesium are present closer to the surface of the positive electrode active material than the aluminum in the surface layer portion.

[0016] Another embodiment of the present invention is a positive electrode including a positive electrode active material having a composite oxide containing lithium and cobalt, the positive electrode active material having barium, magnesium, and aluminum in a surface layer portion, and in which, when the surface layer portion is analyzed in cross-sectional STEM-EDX ray analysis, a first point at which a characteristic X-ray detection value of barium is maximized and a second point at which a characteristic X-ray detection value of magnesium is maximized are present on a surface side of the positive electrode active material relative to a third point at which a characteristic X-ray detection value of aluminum is maximized.

[0017] Another embodiment of the present invention is a positive electrode including a positive electrode active material having a composite oxide containing lithium and cobalt, the positive electrode active material including barium, magnesium, and aluminum in a surface layer portion, and in which lithium is uniformly distributed in the positive electrode active material in a charged state of 0.8 or more.

[0018] Another embodiment of the present invention is a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode active material including a composite oxide containing lithium and cobalt, and the positive electrode active material includes barium, magnesium, and aluminum in a surface layer portion.

[0019] Another embodiment of the present invention is a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode active material including a composite oxide containing lithium and cobalt. The positive electrode active material includes barium, magnesium, and aluminum in a surface layer portion, and the barium and magnesium have a region located closer to the surface of the positive electrode active material than the aluminum.

[0020] Another embodiment of the present invention is a lithium-ion secondary battery including a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode active material including a composite oxide containing lithium and cobalt. The positive electrode active material includes barium, magnesium, and aluminum in a surface layer portion. When the surface layer portion is analyzed in cross-sectional STEM-EDX ray analysis, a first point at which a characteristic X-ray detection value of barium is maximized and a second point at which a characteristic X-ray detection value of magnesium is maximized are located closer to the surface of the positive electrode active material than a third point at which a characteristic X-ray detection value of aluminum is maximized.

[0021] Another embodiment of the present invention is a lithium-ion secondary battery including a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes, in a surface layer portion, a positive electrode active material including a composite oxide containing lithium and cobalt. The positive electrode active material includes barium, magnesium, and aluminum. The lithium distribution in the positive electrode active material is uniform in a charged state of 0.8 or more depth of charge.

[0022] In any one of the lithium ion secondary batteries described above, the negative electrode preferably contains a carbon-based material.

[0023] In any one of the lithium ion secondary batteries described above, the electrolyte preferably includes a solid electrolyte.

[0024] One aspect of the present invention is a mobile object including any one of the lithium ion secondary batteries described above.

[0025] One embodiment of the present invention is a power storage system including any one of the above lithium-ion secondary batteries.

[0026] One embodiment of the present invention is an electronic device including any one of the above lithium-ion secondary batteries.

[0027] Another embodiment of the present invention is a method for preparing a composite oxide, the method including: heating a composite oxide containing lithium and cobalt at a temperature of 700°C to 1000°C for two hours or more; adding a first mixture containing a barium source and a second mixture containing a magnesium source to the composite oxide to prepare a third mixture; heating the third mixture at a temperature of 800°C to 1100°C for two hours or more; adding a nickel source and an aluminum source to the third mixture to prepare a fourth mixture; and heating the fourth mixture at a temperature of 800°C to 1100°C for two hours or more.

[0028] In the method for producing a lithium ion secondary battery described above, when the number of barium atoms in the barium source is represented by atBa and the number of magnesium atoms in the magnesium source is represented by atMg, it is preferable that atBa / (atBa+atMg) is 0.1 or more and 0.5 or less.

[0029] In any one of the methods for producing a lithium ion secondary battery described above, when the barium source is barium fluoride, the magnesium source is magnesium fluoride, and the number of moles of barium fluoride is expressed as mBaF2 and the number of moles of magnesium fluoride is expressed as mMgF2, it is preferable that mBaF2 / (mBaF2+mMgF2) is 0.1 or more and 0.5 or less. [Effects of the Invention]

[0030] According to one embodiment of the present invention, a method for producing a positive electrode active material having a large discharge capacity can be provided. Alternatively, according to one embodiment of the present invention, a method for producing a positive electrode active material that can withstand high charge / discharge voltages can be provided. Alternatively, according to one embodiment of the present invention, a method for producing a positive electrode active material that is less likely to deteriorate can be provided. Alternatively, according to one embodiment of the present invention, a novel positive electrode active material can be provided.

[0031] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 is a flow diagram showing a manufacturing process for a positive electrode active material according to one embodiment of the present invention. [Figure 2] 2A to 2C are flow charts showing a manufacturing process of a positive electrode active material according to one embodiment of the present invention. [Figure 3] FIG. 3A is a cross-sectional view of the positive electrode active material, and FIGS. 3B1 to 3C2 are partial cross-sectional views of the positive electrode active material. [Figure 4] 4A to 4C are diagrams showing calculation models of the positive electrode active material. [Figure 5] 5A and 5B are diagrams showing the calculation results for the positive electrode active material. [Figure 6] FIG. 6 illustrates a crystal structure of a positive electrode active material of one embodiment of the present invention. [Figure 7] Figure 7 shows the XRD pattern calculated from the crystal structure. [Figure 8] FIG. 8 is a diagram illustrating the crystal structure of a positive electrode active material of a comparative example. [Figure 9] FIG. 9 shows the XRD pattern calculated from the crystal structure. [Figure 10] 10A to 10C are images of the positive electrode active material observed after the cycle test. [Figure 11] 11A to 11C are images of the positive electrode active material observed after the cycle test. [Figure 12] 12A to 12C are images of the positive electrode active material observed after the cycle test. [Figure 13] 13A to 13E are diagrams showing the observation results of the positive electrode active material after the cycle test. [Figure 14] FIG. 14A is a diagram showing a calculation model of the positive electrode active material, and FIGS. 14B and 14C are diagrams showing the calculation results of the positive electrode active material. [Figure 15] 15A1 and 15B1 are diagrams showing a calculation model of the positive electrode active material, and FIGS. 15A2 and 15B2 are diagrams showing the calculation results of the positive electrode active material. [Figure 16] 16A is an exploded perspective view of the coin-type secondary battery, FIG. 16B is a perspective view of the coin-type secondary battery, and FIG. 16C is a cross-sectional perspective view thereof. [Figure 17] Fig. 17A shows an example of a cylindrical secondary battery. Fig. 17B shows an example of a cylindrical secondary battery. Fig. 17C shows an example of multiple cylindrical secondary batteries. Fig. 17D shows an example of a power storage system having multiple cylindrical secondary batteries. [Figure 18] 18A and 18B are diagrams illustrating an example of a secondary battery, and FIG. 18C is a diagram showing the inside of the secondary battery. [Figure 19] 19A to 19C are diagrams illustrating an example of a secondary battery. [Figure 20] 20A and 20B are diagrams showing the external appearance of a secondary battery. [Figure 21] 21A to 21C are diagrams illustrating a method for manufacturing a secondary battery. [Figure 22] 22A to 22C are diagrams showing examples of the configuration of a battery pack. [Figure 23] 23A and 23B are cross-sectional views of an active material layer in which a graphene compound is used as the conductive material. [Figure 24] 24A and 24B are diagrams illustrating an example of a secondary battery. [Figure 25]25A to 25C are diagrams illustrating an example of a secondary battery. [Figure 26] 26A and 26B are diagrams illustrating an example of a secondary battery. [Figure 27] FIG. 27A is a perspective view of a battery pack showing one embodiment of the present invention, FIG. 27B is a block diagram of the battery pack, and FIG. 27C is a block diagram of a vehicle having a motor. [Figure 28] 28A to 28D are diagrams illustrating an example of a transportation vehicle. [Figure 29] 29A and 29B illustrate a power storage device according to one embodiment of the present invention. [Figure 30] FIG. 30A is a diagram showing an electric bicycle, FIG. 30B is a diagram showing a secondary battery of the electric bicycle, and FIG. 30C is a diagram explaining an electric motorcycle. [Figure 31] 31A to 31D are diagrams illustrating an example of an electronic device. [Figure 32] Fig. 32A shows an example of a wearable device, Fig. 32B shows a perspective view of a wristwatch-type device, Fig. 32C is a diagram illustrating a side view of the wristwatch-type device, and Fig. 32D is a diagram illustrating an example of a wireless earphone. [Figure 33] 33A to 33C are SEM images of the surface of the positive electrode active material. [Figure 34] FIG. 34A is a cross-sectional STEM image of the positive electrode active material, FIGS. 34B1 to 34B4 are EDX mapping images, and FIG. 34C is a graph showing the results of EDX line analysis. [Figure 35] 35A to 35C are graphs showing the results of EDX line analysis of the positive electrode active material. [Figure 36] 36A and 36B are graphs showing the cycling characteristics of the half-cell. [Figure 37] 37A and 37B are graphs showing the cycling characteristics of the half-cell. [Figure 38] 38A and 38B are graphs showing the cycling characteristics of the half-cell. [Figure 39]39A and 39B are graphs showing charge-discharge characteristics in a cycle test of a half cell. [Figure 40] 40A and 40B are graphs showing charge-discharge characteristics in a cycle test of a half cell. [Figure 41] 41A and 41B are graphs showing charge-discharge characteristics in a cycle test of a half cell. [Figure 42] 42A to 42C are cross-sectional STEM images of the positive electrode active material after the cycle test. DETAILED DESCRIPTION OF THE INVENTION

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the 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 modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0034] In this specification and the like, the term "composite oxide" refers to an oxide containing multiple types of metal elements in its structure.

[0035] Furthermore, in this specification and elsewhere, crystal planes and directions are indicated using Miller indices. In crystallography, crystal planes and directions are indicated by a superscript bar after the number; however, due to limitations in application notation, in this specification and elsewhere, instead of a bar above the number, a minus sign (-) may be placed before the number. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal faces are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Miller indices for trigonal and hexagonal crystals, including R-3m, may use (hkil) instead of just (hkl). Here, i is -(h+k).

[0036] In this specification, the 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. Defects such as cation or anion deficiencies may also be present. Strictly speaking, the layered rock-salt type crystal structure may have a distorted rock-salt type crystal lattice.

[0037] In this specification and the like, the rock salt type crystal structure refers to a structure in which cations and anions are arranged alternately. Note that the crystal structure may have a deficiency of cations or anions in part.

[0038] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable lithium contained in the positive electrode active material is deintercalated. For example, the theoretical capacity of LiFePO4 is 170 mAh / g, that of LiCoO2 is 274 mAh / g, that of LiNiO2 is 275 mAh / g, and that of LiMn2O4 is 148 mAh / g.

[0039] The amount of lithium remaining in the positive electrode active material that can be inserted or removed can be determined by the x in the composition formula, for example, Li x x in CoO2, or Li x The x in MO2 (M is a transition metal element) is used. x CoO2 is appropriately Li x This can be read as MO (where M is a transition metal element). x can be considered as an occupancy rate, and in the case of a positive electrode active material in a secondary battery, x can be expressed as x = (theoretical capacity - charging capacity) / theoretical capacity. For example, when a secondary battery using LiCoO as the positive electrode active material is charged at 219.2 mAh / g, Li 0.2 CoO2 or x=0.2. x A small value of x in CoO2 is, for example, 0.1 <x≦0.24をいう。

[0040] When lithium cobalt oxide approximately satisfies the stoichiometric ratio, it is LiCoO2, and the occupancy rate of Li on the lithium sites is x = 1. A secondary battery that has completed discharge is also LiCoO2, and x = 1. The completion of discharge here refers to a state where, for example, the voltage is 2.5 V (lithium counter electrode) or less at a current of 100 mA / g. In a lithium-ion secondary battery, when the occupancy rate of lithium on the lithium sites reaches x = 1 and no more lithium can enter, the voltage drops rapidly. At this point, discharge can be said to have completed. Generally, in lithium-ion secondary batteries that use LiCoO2, the discharge voltage drops rapidly before it reaches 2.5 V, so discharge is considered to have completed under the above conditions.

[0041] In this specification and the like, the state of charge when all intercalable and deintercalable lithium in the positive electrode active material is intercalated may be referred to as 0, and the state of charge when all intercalable and deintercalable lithium in the positive electrode active material is deintercalated may be referred to as 1. For example, Li x When x in MO2 is 1, the state of charge is 0; when x is 0, the state of charge is 1; and when x is 0.2, the state of charge is 0.8.

[0042] In this specification, the active material may be referred to as an active material particle, but the shape may vary and is not limited to a particle shape. For example, the shape of the active material (active material particle) may be, in addition to a circle, an ellipse, a rectangle, a trapezoid, a triangle, a square with rounded corners, or an asymmetric shape in one cross section.

[0043] In this specification and the like, a smooth surface of an active material can be said to have a surface roughness of at least 10 nm or less when surface irregularity information is quantified from measurement data on a cross section of the active material.

[0044] In this specification and the like, a cross section is a cross section obtained when observing with, for example, a scanning transmission electron microscope (STEM).

[0045] (Embodiment 1) In this embodiment, a method for manufacturing a positive electrode active material, which is one embodiment of the present invention, will be described.

[0046] <<Method 1 for preparing positive electrode active material>> <Step S11> In step S11 shown in FIG. 1, a lithium source (Li source) and a transition metal source (M source) are prepared as starting materials for lithium and transition metal, respectively.

[0047] 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 for example, it is recommended to use a material with a purity of 99.99% or higher.

[0048] The transition metal can be selected from elements in Groups 3 to 11 of the periodic table, and for example, at least one of manganese, cobalt, and nickel is used. The transition metal may be only cobalt, only nickel, two elements (cobalt and manganese), two elements (cobalt and nickel), or three elements (cobalt, manganese, and nickel). When only cobalt is used, the resulting positive electrode active material has lithium cobalt oxide (LCO). When cobalt, manganese, and nickel are used, the resulting positive electrode active material has lithium nickel-cobalt-manganese oxide (NCM).

[0049] As the transition metal source, it is preferable to use a compound containing the above transition metal, and for example, an oxide or hydroxide of the metal exemplified as the above transition metal can be used. As a cobalt source, cobalt oxide, cobalt hydroxide, etc. can be used. As a manganese source, manganese oxide, manganese hydroxide, etc. can be used. As a nickel source, nickel oxide, nickel hydroxide, etc. can be used. As an aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0050] The transition metal 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.

[0051] In addition, it is preferable that the transition metal source has high crystallinity, for example, single crystal grains. The crystallinity of the transition metal source can be evaluated using TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, etc., or by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. 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.

[0052] When two or more transition metal sources are used, the two or more transition metal sources are preferably prepared in a ratio (mixing ratio) that allows the resulting composite oxide to have a layered rock salt type crystal structure.

[0053] <Step S12> Next, in step S12 shown in FIG. 1, the lithium source and the transition metal source are crushed and mixed to prepare a mixed material. The crushing and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller crushing. When performing 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). It is more preferable to use an aprotic solvent that does not react easily with lithium. In this embodiment, dehydrated acetone with a purity of 99.5% or higher is used. It is preferable to mix the lithium source and 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 crush and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0054] A ball mill, a bead mill, or the like can be used as a means for mixing, etc. When using a ball mill, it is preferable to use alumina balls or zirconia balls as grinding media. Zirconia balls are preferable 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 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).

[0055] <Step S13> Next, in step S13 shown in FIG. 1, the mixed material is heated. The heating temperature is preferably 800°C or higher and 1100°C or lower, more preferably 900°C or higher and 1000°C or lower, and even more preferably around 950°C. 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, defects may occur due to lithium evaporation or sublimation from the lithium source and / or excessive reduction of the metal used as the transition metal source. For example, when cobalt is used as the transition metal, excessive reduction can cause cobalt to change from trivalent to divalent, which can induce oxygen defects.

[0056] The heating time is preferably from 1 hour to 100 hours, more preferably from 2 hours to 20 hours.

[0057] The temperature rise rate depends on the heating temperature reached, but should be between 80°C / h and 250°C / h. For example, if heating at 1000°C for 10 hours, the temperature should be raised at 200°C / h.

[0058] The heating atmosphere is preferably 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 performed in an atmosphere with a dew point of -93°C. In addition, to suppress impurities that may be mixed into the material, the impurity concentrations of CH4, CO, CO2, and H2 in the heating atmosphere should each be 5 ppb (parts per billion) or less.

[0059] An oxygen-containing atmosphere is preferred as the heating atmosphere. For example, dry air can be continuously introduced into the reaction chamber. In this case, the flow rate of the dry air is preferably 10 L / min. The method of continuously introducing oxygen into the reaction chamber and having oxygen flow through the reaction chamber is called flow.

[0060] When the heating atmosphere is an oxygen-containing atmosphere, a non-flow method is also possible. For example, the reaction chamber can be depressurized and then filled with oxygen to prevent the oxygen from entering or leaving the reaction chamber. This method is called purging. For example, the reaction chamber can be depressurized to -970 hPa and then filled with oxygen to 50 hPa.

[0061] After heating, the material can be cooled naturally, but it is preferable that the time required to cool the material from the specified temperature to room temperature is between 10 and 50 hours. However, cooling to room temperature is not always necessary, as long as the material is cooled to a temperature acceptable for the next step.

[0062] 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.

[0063] A crucible can be used as a container for heating, and alumina is the preferred material for the container. Alumina crucibles are a material that does not easily release impurities. In this embodiment, an alumina crucible with a purity of 99.9% is used. It is preferable to place a lid on the crucible when heating, as this prevents evaporation or sublimation of the material. Alternatively, a flat-bottomed container called a sheath or setter may be used instead of a crucible. Mullite (Al2O3-SiO2 ceramics) may also be used as the container material.

[0064] 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. An alumina mortar is preferably used as the mortar. An alumina mortar is a material that does not easily release impurities. Specifically, an alumina mortar with a purity of 90% or more, preferably 99% or more, is used. Note that heating conditions equivalent to those of step S13 can be applied to heating steps other than step S13, which will be described later.

[0065] <Step S14> Through the above steps, a composite oxide (LiMO2) containing a transition metal can be obtained in step S14 shown in Figure 1. The composite oxide only needs to have the crystal structure of a lithium composite oxide represented by LiMO2, and its composition is not strictly limited to Li:M:O = 1:1:2. When cobalt is used as the transition metal, for example, lithium cobalt oxide can be obtained, represented by LiCoO2. The composition is not strictly limited to Li:Co:O = 1:1:2.

[0066] Although the composite oxide is produced by the solid phase method in steps S11 to S14, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0067] <Step S15> Next, in step S15 shown in FIG. 1, the composite oxide is heated. Because this is the first heating of the composite oxide, the heating in step S15 is sometimes called initial heating. After initial heating, the surface of the composite oxide becomes smooth. A smooth surface refers to a state in which there are few irregularities, the composite oxide is rounded overall, and the corners are also rounded. Furthermore, a state in which there is little foreign matter adhering to the surface is called smooth. Foreign matter is thought to be a cause of irregularities, so it is preferable that it does not adhere to the surface.

[0068] The initial heating is performed after the composite oxide is completed, and the inventors have found that initial heating for the purpose of smoothing the surface can reduce deterioration after charge and discharge. The initial heating for smoothing the surface does not require the preparation of a lithium compound source.

[0069] Alternatively, the initial heating to smooth the surface does not require the provision of a source of the additive element.

[0070] Alternatively, initial heating to smooth the surface does not require the use of a fluxing agent.

[0071] The initial heating is performed before step S20 described below, and may be called preheating or pretreatment.

[0072] The lithium source and the transition metal source prepared in step S11 etc. may contain impurities. The initial heating can reduce the amount of impurities in the composite oxide completed in step S14.

[0073] The heating conditions for this step may be any conditions that result in a smooth surface of the composite oxide. For example, the heating conditions may be selected from those described for step S13. Regarding the heating conditions, the heating temperature for this step should be lower than the temperature for step S13 in order to maintain the crystalline structure of the composite oxide. Furthermore, the heating time for this step should be shorter than the time for step S13 in order to maintain the crystalline structure of the composite oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer is recommended.

[0074] The heating in step S13 may cause a temperature difference between the surface and interior of the composite oxide. This temperature difference may induce a contraction difference. It is thought that the temperature difference causes a difference in fluidity between the surface and interior, resulting in a contraction difference. The energy associated with the contraction difference causes an internal stress difference in the composite oxide. The internal stress difference is also called strain, and this energy is sometimes called strain energy. The internal stress is removed by the initial heating in step S15; in other words, the strain energy is thought to be homogenized by the initial heating in step S15. When the strain energy is homogenized, the strain in the composite oxide is alleviated. Therefore, the surface of the composite oxide may become smoother after step S15. This is also called an improved surface. In other words, it is thought that the contraction difference that occurred in the composite oxide is alleviated after step S15, resulting in a smoother surface of the composite oxide.

[0075] Furthermore, the difference in shrinkage may cause microscopic misalignment in the composite oxide, such as misalignment of crystals. This step is preferably carried out in order to reduce such misalignment. This step makes it possible to equalize the misalignment of the composite oxide. When the misalignment is equalized, the surface of the composite oxide may become smooth. This is also referred to as the alignment of crystal grains. In other words, it is believed that the misalignment of crystals and the like that has occurred in the composite oxide is alleviated after step S15, and the surface of the composite oxide becomes smooth.

[0076] When a composite oxide with a smooth surface is used as a 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.

[0077] A smooth surface of a complex oxide can be defined as a surface roughness of at least 10 nm or less when surface irregularity information is quantified from measurement data on a cross section of the complex oxide. The cross section is, for example, a cross section obtained when observing with a scanning transmission electron microscope (STEM).

[0078] Alternatively, a composite oxide containing lithium, a transition metal, and oxygen that has been synthesized in advance may be used in step S14. In this case, steps S11 to S13 can be omitted. By performing step S15 on a composite oxide that has been synthesized in advance, a composite oxide with a smooth surface can be obtained.

[0079] It is conceivable that the initial heating may reduce lithium in the composite oxide. The reduced lithium, which will be explained in the next step S20, etc., may facilitate incorporation of the additive element X and the additive element Y into the composite oxide. Note that the method for producing a positive electrode active material according to one embodiment of the present invention is not necessarily limited to a production method that includes initial heating.

[0080] <Steps S20 and S21> The additive element X and the additive element Y may be added to a composite oxide having a smooth surface, as long as the composite oxide can have a layered rock salt type crystal structure. When the additive element X and the additive element Y are added to a composite oxide having a smooth surface, the additive element X and the additive element Y can be added evenly. Therefore, the order of adding the additive element X and the additive element Y after the initial heating is preferable. The step of adding the additive element X and the additive element Y will be described with reference to FIGS. 2A and 2B.

[0081] <Step S22> In step S22 shown in Fig. 2A, a source of an additive element X to be added to the composite oxide is prepared. The additive element X is barium (Ba). It is desirable to further include a lithium source (Li source) as the additive element X source. Fig. 2A shows an example in which a barium source (Ba source) and a lithium source (Li source) are prepared in step S22.

[0082] Examples of barium sources that can be used include barium fluoride (BaF2), barium oxide (BaO), barium hydroxide (Ba(OH)2), barium nitrate (Ba(NO3)2), and barium sulfate (BaSO4).

[0083] As the Li source, for example, lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride can be used.

[0084] When a barium source and a Li source are used as sources of the additional element X, it is preferable to use barium fluoride as the barium source and lithium fluoride as the lithium source, since they have a eutectic point.

[0085] <Step S23> Next, in step S23, the Ba source and Li source prepared in step S22 are pulverized and mixed. The pulverization and mixing can be performed under conditions selected from the pulverization and mixing conditions described in step S12 of FIG.

[0086] <Step S24> 2A, the pulverized and mixed materials are collected to obtain an additive element X source (X source). The additive element X source shown in step S24 contains a plurality of starting materials and can be called a mixture.

[0087] <Step S25> In step S25 shown in Fig. 2B, a source of an additive element Y to be added to the composite oxide is prepared. It is desirable that the source of the additive element Y also includes a lithium source (Li source). Fig. 2B shows an example in which a magnesium source (Mg source) and a lithium source (Li source) are prepared in step S25.

[0088] The additional element Y can be one or more selected from magnesium, calcium, fluorine, aluminum, nickel, cobalt, manganese, titanium, zirconium, yttrium, vanadium, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, and boron.

[0089] When magnesium is selected as the additional element Y, the source of the additional element Y can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. may be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0090] When fluorine is selected as the additive element Y, the source of the additive element Y can be called a fluorine source. Examples of the fluorine source that can be used include lithium fluoride (LiF), magnesium fluoride (MgF), aluminum fluoride (AlF), titanium fluoride (TiF), cobalt fluoride (CoF, CoF), nickel fluoride (NiF), zirconium fluoride (ZrF), vanadium fluoride (VF), manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride (ZnF), calcium fluoride (CaF), sodium fluoride (NaF), potassium fluoride (KF), cerium fluoride (CeF), lanthanum fluoride (LaF), and sodium aluminum hexafluoride (NaAlF). 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.

[0091] Magnesium fluoride can be used as both a fluorine source and a magnesium source, and lithium fluoride can be used as a lithium source. Another lithium source that can be used in step S25 is lithium carbonate.

[0092] The fluorine source may be a gas, such as fluorine (F), carbon fluoride, sulfur fluoride, or oxygen fluoride (OF, OF, OF, OF, OF), which may be mixed into the atmosphere during the heating step described below. A plurality of the above-mentioned fluorine sources may also be used.

[0093] In one example of the Y source, lithium fluoride (LiF) is prepared as the lithium source (and fluorine source), and magnesium fluoride (MgF2) is prepared as the magnesium source (and fluorine source). Mixing lithium fluoride and magnesium fluoride at a molar ratio of approximately LiF:MgF2 = 65:35 maximizes the effect of lowering the melting point. On the other hand, excessive lithium fluoride can lead to excessive lithium, potentially deteriorating cycle performance. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is preferably LiF:MgF2 = x:1 (0 ≦ x ≦ 1.9), more preferably LiF:MgF2 = x:1 (0.1 ≦ x ≦ 0.5), and even more preferably LiF:MgF2 = x:1 (x = 0.33 or near 0.33). In this specification, "near" refers to a value greater than 0.9 times but less than 1.1 times the value.

[0094] <Step S26> 2B, the magnesium source and the lithium source are crushed and mixed. This step can be performed under crushing and mixing conditions selected from those described in step S12.

[0095] If necessary, a heating step may be performed after step S26. 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.

[0096] <Step S27> 2B, the pulverized and mixed materials are collected to obtain a source of the additive element Y (Y source). The source of the additive element Y shown in step S27 includes a plurality of starting materials and can be called a mixture.

[0097] The particle size D50 (median diameter) of the mixture in steps S24 and S27 is preferably 50 nm to 10 μm, more preferably 100 nm to 3 μm. Even when a single material is used as the additive element source, the D50 (median diameter) is preferably 50 nm to 10 μm, more preferably 100 nm to 3 μm.

[0098] Such a finely pulverized mixture (including the case where only one additive element is added) makes it easier to uniformly adhere the mixture to the surface of the composite oxide particles when mixed with the composite oxide in a later step. Uniform adhesion of the mixture to the surface of the composite oxide is preferable because it makes it easier to distribute or diffuse barium and magnesium uniformly in the surface layer of the composite oxide after heating. The region where barium and magnesium are distributed can also be called the surface layer. If there is an area in the surface layer that does not contain barium and magnesium, it may be difficult to obtain the O3'-type crystal structure described below in the charged state.

[0099] <Step S31> Next, in step S31 shown in FIG. 1 , the composite oxide, the source of the additive element X (X source), and the source of the additive element Y (Y source) are mixed. The ratio of the number of transition metal atoms (atM) in the composite oxide containing lithium, transition metal, and oxygen to the number of barium atoms (atBa) in the additive element X and the number of magnesium atoms (atMg) in the additive element Y is preferably atM:(atBa+atMg)=100:y (0.1≦y≦6), more preferably atM:(atBa+atMg)=100:y (0.3≦y≦3). Furthermore, when the number of barium atoms (atBa) in the additive element X is 1, the number of magnesium atoms (atMg) in the additive element Y is preferably 1 or more and 9 or less, more preferably 1 or more and 4 or less, and even more preferably 1. By satisfying the quantitative relationship between the additive element X and the additive element Y shown above, it is possible to prepare a positive electrode active material that is stable at high potential and / or high temperature.

[0100] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the composite oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than those in step S12. It can also be said that dry mixing conditions are milder than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media.

[0101] In this embodiment, dry mixing is performed in a ball mill using zirconia 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.

[0102] <Step S32> 1, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0103] <Step S33> 1, the mixture 903 is heated. The heating conditions can be selected from those described in step S13. The heating time is preferably 2 hours or more.

[0104] Here, a supplementary note about the heating temperature will be provided. 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 composite oxide (LiMO2) and the additive element X source and the additive element Y source proceeds. The temperature at which the reaction proceeds may be any temperature at which mutual diffusion of elements contained in LiMO2 and the additive element X source and the additive element Y source occurs, and may be lower than the melting temperature of these materials. An oxide will be used as an example for explanation, and the melting temperature T m 0.757 times (Tanman temperature T d ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 500° C. or higher.

[0105] Of course, the reaction proceeds more easily at a temperature equal to or higher than the temperature at which at least a portion of the mixture 903 melts. For example, when LiF and BaF2 are used as the additive element X source, the eutectic point of LiF and BaF2 is around 765°C, so the lower limit of the heating temperature in step S33 is preferably 765°C or higher. For example, when LiF and MgF2 are used as the additive element Y source, the eutectic point of LiF and MgF2 is around 742°C, so the lower limit of the heating temperature in step S33 is preferably 742°C or higher. For example, when LiF, BaF2, and MgF2 are used as the additive element X source and the additive element Y source, the eutectic point of LiF, BaF2, and MgF2 is around 654°C, so the lower limit of the heating temperature in step S33 is preferably 654°C or higher. Therefore, the heating temperature in step S33 is preferably 654°C or higher, more preferably 742°C or higher, and even more preferably 775°C or higher.

[0106] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0107] The upper limit of the heating temperature is below the decomposition temperature of LiMO2 (the decomposition temperature of LiCoO2 is 1130°C). At temperatures close to the decomposition temperature, there is a concern that LiMO2 may decompose, albeit in a small amount. Therefore, a temperature of 1000°C or less is more preferable, a temperature of 950°C or less is even more preferable, and a temperature of 900°C or less is even more preferable.

[0108] In light of these, the heating temperature in step S33 is preferably 500°C to 1130°C, more preferably 500°C to 1000°C, even more preferably 500°C to 950°C, and even more preferably 500°C to 900°C. Also, it is preferably 654°C to 1130°C, more preferably 654°C to 1000°C, even more preferably 654°C to 950°C, and even more preferably 654°C to 900°C. Also, it is preferably 742°C to 1130°C, more preferably 742°C to 1000°C, even more preferably 742°C to 950°C, and even more preferably 742°C to 900°C. Also, it is preferably 765°C to 1130°C, more preferably 765°C to 1000°C, even more preferably 765°C to 950°C, and even more preferably 765°C to 900°C. The heating temperature in step S33 is preferably higher than that in step S13.

[0109] Furthermore, when heating the mixture 903, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range in a furnace or a heating vessel such as a crucible where heating is performed.

[0110] In the fabrication method described in this embodiment, some materials, such as LiF, which is a lithium source, may function as a flux. This function allows the heating temperature to be lowered below the decomposition temperature of the composite oxide (LiMO2), for example, to a temperature between 654°C and 950°C, and allows additive elements such as barium and magnesium to be distributed in the surface layer, resulting in the fabrication of a positive electrode active material with excellent characteristics.

[0111] However, because LiF has a lower specific gravity in its gaseous state than oxygen, it may evaporate or sublime upon heating. If this occurs, the amount of LiF in the mixture 903 will decrease. This weakens its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the evaporation or sublimation of LiF. Even if LiF is not used as the lithium source, the Li on the LiMO2 surface may react with the fluorine source F, generating LiF, which may then evaporate or sublime. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress the evaporation or sublimation.

[0112] 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 evaporation or sublimation of LiF in the mixture 903.

[0113] The heating in this step is preferably performed so as not to cause the particles of mixture 903 to stick together. If the particles of mixture 903 stick together during heating, the contact area with oxygen in the atmosphere will decrease, and the diffusion paths of the additional elements X and Y (e.g., barium, magnesium, and fluorine) will be blocked, which may result in a poor distribution of the additional elements X and Y (e.g., barium, magnesium, and fluorine) in the surface layer.

[0114] It is also believed that uniform distribution of the additive element X and the additive element Y (e.g., barium, magnesium, and fluorine) in the surface layer portion results in a smooth, less uneven cathode active material. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is better for the particles not to stick together.

[0115] 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 or sublimate, which is undesirable in terms of maintaining surface smoothness.

[0116] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by placing a lid on a container containing the mixture 903, for example.

[0117] Regarding the heating time, the heating time varies depending on conditions such as the heating temperature, the size and composition of the LiMO2 particles in step S14. When the particles are small, a lower temperature or shorter heating time may be preferable than when the particles are large.

[0118] When the median diameter (D50) of the composite oxide (LiMO2) in step S14 of Fig. 1 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 reduction time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0119] On the other hand, when the median diameter (D50) of the composite oxide (LiMO2) 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 reduction time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0120] <Step S34> Next, in step S33, the heated material is recovered to obtain a composite oxide having the additional element X and the additional element Y. This is also called a second composite oxide to distinguish it from the composite oxide in step S14.

[0121] <Step S40> 1, a source of the additional element Z is added. An example in which nickel and aluminum are used as the additional element Z will be described with reference to FIG. 2C.

[0122] <Step S41> 2C, a nickel source (Ni source) and an aluminum source (Al source) are prepared, and then pulverized independently in step S42. As a result, in step S43, a source of additional element Z (Z source) is prepared.

[0123] The additional element Z can be one or more selected from magnesium, calcium, fluorine, aluminum, nickel, cobalt, manganese, titanium, zirconium, yttrium, vanadium, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, and boron.

[0124] When nickel and aluminum are selected as the additional element Z, nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0125] <Steps S51 to S53> Next, steps S51 to S53 shown in FIG. 1 can be prepared under the same conditions as steps S31 to S34. The mixture 904 prepared in step S52 is heated in step S53. At this time, the heating conditions in step S53 may be a lower temperature and a shorter time than those in step S33. Through the above steps, a cathode active material 100 according to one embodiment of the present invention can be obtained in step S54. The cathode active material according to one embodiment of the present invention has a smooth surface.

[0126] After the initial heating described in this embodiment, a positive electrode active material with a smooth surface can be obtained.

[0127] The initial heating shown in this embodiment is performed on the composite oxide. Therefore, the initial heating is preferably performed under conditions that are lower than the heating temperature required to obtain the composite oxide and shorter than the heating time required to obtain the composite oxide. When an additional element is added to the composite oxide, it is preferable to perform the addition step after the initial heating. The addition step can be divided into two or more steps. Following this order of steps is preferable because it maintains the surface smoothness obtained by the initial heating. When the composite oxide contains cobalt as a transition metal, it can be interpreted as a composite oxide containing cobalt.

[0128] This embodiment can be used in combination with other embodiments.

[0129] (Embodiment 2) In this embodiment, a positive electrode active material of one embodiment of the present invention will be described with reference to FIGS.

[0130] Fig. 3A is a cross-sectional view of a positive electrode active material 100 according to one embodiment of the present invention. Enlarged views of the vicinity of AB in Fig. 3A are shown in Fig. 3B1 and Fig. 3B2. Enlarged views of the vicinity of CD in Fig. 3A are shown in Fig. 3C1 and Fig. 3C2.

[0131] 3A to 3C2, the positive electrode active material 100 has a surface layer 100a and an inner portion 100b. In these figures, the boundary between the surface layer 100a and the inner portion 100b is indicated by a dashed line. FIG. 3A also shows a case where the positive electrode active material 100 has a crystal grain boundary (indicated by a dashed line).

[0132] In this specification, the region extending from the surface of the positive electrode active material to a depth of about 50 nm toward the inside is referred to as the surface layer 100a. Surfaces formed by cracks may also be referred to as the surface. The surface layer 100a may also be referred to as the vicinity of the surface, the vicinity of the surface region, or the shell. The region deeper than the surface layer 100a of the positive electrode active material is referred to as the interior 100b. The interior 100b may also be referred to as the interior region or the core.

[0133] The surface layer 100a preferably has a higher concentration of the additive elements (additive element X, additive element Y, and additive element Z) described below than the inner portion 100b. The additive elements (additive element X, additive element Y, and additive element Z) preferably have a concentration gradient. When there are multiple additive elements (additive element X, additive element Y, and additive element Z), it is preferable that the depth from the surface of the concentration peak differs depending on the additive element.

[0134] For example, the additional element X and the additional element Y preferably have a concentration gradient that increases from the interior 100b toward the surface, as shown by the gradation in Fig. 3B1. Examples of the additional element X and the additional element Y that preferably have such a concentration gradient include barium, magnesium, fluorine, titanium, silicon, phosphorus, boron, and calcium.

[0135] The additional element Z preferably has a concentration gradient as shown by the gradation in FIG. 3B2 and a concentration peak in a region deeper than that in FIG. 3B1. The concentration peak may be present in the surface layer portion 100a or may be deeper than the surface layer portion 100a. For example, the additional element Z preferably has a peak in a region from 5 nm to 50 nm from the surface. Examples of additional elements Z that preferably have such a concentration gradient include aluminum and manganese.

[0136] Furthermore, due to the concentration gradient of the additional elements (additive element X, additional element Y, and additional element Z) contained in positive electrode active material 100, the crystal structure preferably changes continuously from interior 100b toward the surface.

[0137] <Contained elements> The positive electrode active material 100 contains lithium, a transition metal M, oxygen, an additive element X, an additive element Y, and an additive element Z. The positive electrode active material 100 may be said to be a composite oxide represented by LiMO2 to which an additive element has been added. However, the positive electrode active material of one embodiment of the present invention is only required to have the crystal structure of a lithium composite oxide represented by LiMO2, and its composition is not strictly limited to Li:M:O=1:1:2. Furthermore, a positive electrode active material to which an additive element has been added may also be referred to as a composite oxide.

[0138] The transition metal M contained in the positive electrode active material 100 is preferably a metal capable of forming a layered rock-salt composite oxide belonging to the space group R-3m with lithium. For example, at least one of manganese, cobalt, and nickel can be used. That is, the transition metal contained in the positive electrode active material 100 may be cobalt alone, nickel alone, a combination of cobalt and manganese, or a combination of cobalt and nickel, or three of cobalt, manganese, and nickel. That is, the positive electrode active material 100 may contain a composite oxide containing lithium and the transition metal M, such as lithium cobalt oxide, lithium nickel oxide, lithium cobalt oxide in which some of the cobalt is substituted with manganese, lithium cobalt oxide in which some of the cobalt is substituted with nickel, or nickel-manganese-lithium cobalt oxide.

[0139] In particular, using 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt as the transition metal M in the positive electrode active material 100 has many advantages, such as relatively easy synthesis and handling, and excellent cycle characteristics. Furthermore, containing nickel in addition to the cobalt in the above range as the transition metal M can sometimes suppress the deviation of the layered structure consisting of cobalt and oxygen octahedra. This is preferable because it can make the crystal structure more stable, especially in the charged state at high temperatures.

[0140] The transition metal M does not necessarily have to contain manganese. By making the cathode active material 100 substantially free of manganese, the above-mentioned advantages of relatively easy synthesis and handling, and excellent cycle characteristics may be enhanced. 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.

[0141] On the other hand, when nickel is used as the transition metal M in the positive electrode active material 100 at 33 atomic % or more, preferably at 60 atomic % or more, and more preferably at 80 atomic % or more, the raw material may be cheaper than when cobalt is used in large amounts, and the charge / discharge capacity per weight may increase, which is preferable.

[0142] The transition metal M does not necessarily have to contain nickel. By making the positive electrode active material 100 substantially free of nickel, the above-mentioned advantages of relatively easy synthesis and handling, and excellent cycle characteristics may be enhanced. The weight of nickel contained in the positive electrode active material 100 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less.

[0143] The additive elements (additive element X, additive element Y, and additive element Z) contained in the positive electrode active material 100 are preferably at least one of barium, magnesium, fluorine, aluminum, titanium, zirconium, vanadium, chromium, niobium, cobalt, zinc, silicon, sulfur, phosphorus, and boron. These additive elements may further stabilize the crystalline structure of the positive electrode active material 100, as described below. Specifically, the positive electrode active material 100 may include lithium cobalt oxide doped with barium and magnesium, lithium cobalt oxide doped with barium, magnesium, and aluminum, lithium nickel-cobalt oxide doped with barium and magnesium, lithium cobalt-aluminate doped with barium, magnesium, and nickel, lithium nickel-cobalt-aluminate, lithium nickel-cobalt-aluminate doped with barium and magnesium, or lithium nickel-manganese-cobalt oxide doped with barium and magnesium. In this specification and the like, the additive elements may also be referred to as a mixture, a part of a raw material, an impurity element, or the like.

[0144] The additive element does not necessarily have to include magnesium, fluorine, aluminum, titanium, zirconium, vanadium, iron, chromium, niobium, cobalt, arsenic, zinc, silicon, sulfur, phosphorus, or boron.

[0145] In the positive electrode active material 100 according to one embodiment of the present invention, the surface layer 100a, i.e., the outer periphery of the particle, is reinforced by the additive element so that the layered structure of cobalt and oxygen octahedra is not destroyed even when lithium is released from the positive electrode active material 100 upon charging. Therefore, it is preferable that the concentration of the additive element is high in the surface layer 100a.

[0146] Furthermore, it is preferable that the concentration gradient of the additive elements (additive element X, additive element Y, and additive element Z) contained in the positive electrode active material 100 be the same throughout the entire surface layer 100a of the positive electrode active material 100. It can be said that it is preferable that the reinforcement resulting from the high concentration of the additive elements is uniformly present in the surface layer 100a. Even if a portion of the surface layer 100a is reinforced, if there is a portion without reinforcement, stress may be concentrated in the unreinforced portion. If stress is concentrated in a portion of the particle, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in charge / discharge capacity.

[0147] In this specification, "homogeneous" refers to the 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). 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.

[0148] However, the concentration gradient of the additional elements (additive element X, additional element Y, and additional element Z) contained in the positive electrode active material 100 does not necessarily have to be uniform throughout the entire surface layer portion 100a of the positive electrode active material 100. An example of the distribution of the additional element X and the additional element Y near the CD in FIG. 3A is shown in FIG. 3C1. An example of the distribution of the additional element Z near the CD is shown in FIG. 3C2.

[0149] Here, the CD vicinity has an R-3m layered rock-salt crystal structure, and the surface has a (001) orientation. The (001)-oriented surface may have a different distribution of additive elements (additive element X, additive element Y, and additive element Z) than the other surfaces. For example, as shown in FIGS. 3C1 and 3C2, the (001)-oriented surface and its surface layer 100a may have a distribution of at least one of additive element X, additive element Y, and additive element Z that is shallower from the surface than the other orientations. Alternatively, the (001)-oriented surface and its surface layer 100a may have a lower concentration of at least one of additive element X, additive element Y, and additive element Z than the other orientations. Alternatively, the (001)-oriented surface and its surface layer 100a may have at least one of additive element X and additive element Y below the lower detection limit.

[0150] In the layered rock-salt crystal structure of R-3m, cations are arranged parallel to the (001) plane. This can be described as a structure in which CoO2 layers, consisting of octahedra of cobalt and oxygen, and lithium layers are alternately stacked parallel to the (001) plane. Therefore, the diffusion path for lithium ions also exists parallel to the (001) plane.

[0151] The CoO2 layer, which is made up of octahedra of cobalt and oxygen, is relatively stable, so the (001) surface on which the CoO2 layer exists is relatively stable. The (001) surface does not expose a diffusion path for lithium ions.

[0152] On the other hand, the diffusion paths of lithium ions are exposed on surfaces other than those with the (001) orientation. Therefore, the surfaces and the surface layer portion 100a other than those with the (001) orientation are important regions for maintaining the diffusion paths 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 surfaces and the surface layer portion 100a other than those with the (001) orientation is preferable for maintaining the crystal structure of the entire positive electrode active material 100.

[0153] Therefore, in the positive electrode active material 100 according to another embodiment of the present invention, the distribution of the additive elements (additive elements X, Y, and Z) in the plane other than the (001) plane and in the surface layer portion 100a thereof is preferably as shown in Fig. 3B1 and Fig. 3B2. On the other hand, in the (001) plane and in the surface layer portion 100a thereof, the additive elements may be shallow, small, or absent, as described above.

[0154] In the manufacturing method shown in the previous embodiment, in which high-purity LiMO2 is manufactured, and then additive elements are mixed and heated, the additive elements (additive element X, additive element Y, and additive element Z) spread mainly through the diffusion path of lithium ions, so that the distribution of the additive elements (additive element X, additive element Y, and additive element Z) in the planes other than (001) and in the surface layer portion 100a thereof can be easily controlled to a preferred range.

[0155] When the positive electrode active material 100 of one embodiment of the present invention has crystal grain boundaries 101, it is more preferable that the additional elements (additive element X, additive element Y, and additive element Z) are partly segregated at the crystal grain boundaries 101 shown in FIG. 3A in addition to the distribution described above.

[0156] More specifically, the barium concentration, magnesium concentration, and / or aluminum concentration at and near the grain boundaries 101 of the positive electrode active material 100 is preferably higher than that in other regions of the interior 100b. Also, the fluorine concentration at and near the grain boundaries 101 is preferably higher than that in other regions of the interior 100b.

[0157] The grain boundaries 101 are one type of planar defect. Therefore, like the grain surfaces, they are prone to instability and are prone to initiating changes in the crystal structure. Therefore, if the barium concentration, magnesium concentration, and / or aluminum concentration at and near the grain boundaries 101 are high, the changes in the crystal structure can be more effectively suppressed.

[0158] Furthermore, when the barium concentration, magnesium concentration, and / or aluminum concentration is high at and near the grain boundary, even if cracks occur along the grain boundary 101 of the particles of the positive electrode active material 100 of one embodiment of the present invention, the barium concentration, magnesium concentration, and / or aluminum concentration is high near the surface where the cracks occur. Therefore, the corrosion resistance to hydrofluoric acid and the like can be improved even in the positive electrode active material after the cracks occur.

[0159] In this specification, the vicinity of the grain boundary 101 refers to a region up to about 50 nm from the grain boundary. The grain boundary refers to a surface where there is a change in the atomic arrangement, and can be observed in an electron microscope image. Specifically, the vicinity of the grain boundary refers to a location where the angle between repeated bright and dark lines in an electron microscope image changes by more than 5 degrees, or a location where the crystal structure can no longer be observed.

[0160] The positive electrode active material 100 may have recesses, cracks, dents, V-shaped cross sections, etc. These are defects, and repeated charge and discharge may cause elution of the transition metal M, collapse of the crystalline structure, cracks in the main body, and oxygen desorption. However, if the embedded portion 102 is present to embed these defects, elution of the transition metal M, etc., can be suppressed. This allows the positive electrode active material 100 to have excellent reliability and cycle characteristics.

[0161] Furthermore, the positive electrode active material 100 may have uneven distribution portions 103 as regions where the additional elements (additive element X, additional element Y, and / or additional element Z) are unevenly distributed. The uneven distribution portions 103 may have a convex shape.

[0162] As described above, if the additive elements (additive element X, additive element Y, and additive element Z) contained in the positive electrode active material 100 are present in excess, they may adversely affect the insertion and extraction of lithium. Furthermore, when used in 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 elements are insufficient, they may not be distributed throughout the entire surface layer portion 100a, and the effect of suppressing deterioration of the crystal structure may be insufficient. Thus, the additive elements (also called impurity elements) must be present in an appropriate concentration in the positive electrode active material 100, but adjusting this concentration is not easy.

[0163] Therefore, if the positive electrode active material 100 has a region where impurity elements are unevenly distributed, some of the excess impurities can be removed from the interior 100b of the positive electrode active material 100, allowing for an appropriate impurity concentration in the interior 100b. This can suppress an increase in internal resistance and a decrease in charge / discharge capacity when used as a secondary battery. Suppressing an increase in internal resistance of a secondary battery is an extremely desirable characteristic, particularly during high-rate charge / discharge, such as charge / discharge at 2C or higher. Furthermore, when barium, magnesium, and fluorine are used as the additive element X and the additive element Y, BaMgF, LiBaF, BaO, MgO, BaF, MgF, and the like may be detected in the region where the impurity elements are unevenly distributed.

[0164] Furthermore, in the positive electrode active material 100 having a region where the impurity element is unevenly distributed, it is permissible to mix an excessive amount of impurity to some extent in the manufacturing process, which is preferable because it widens the margin in production.

[0165] In this specification and the like, uneven distribution refers to the concentration of an element in a certain region being different from that in other regions, and may also be referred to as segregation, precipitation, non-uniformity, bias, high concentration, low concentration, etc.

[0166] Furthermore, in a positive electrode active material 100 according to one embodiment of the present invention, having surface layer portion 100a containing additional element X, additional element Y, and / or additional element Z, the surface layer portion 100a containing a high concentration of additional element X, additional element Y, and / or additional element Z, i.e., the outer periphery of the particle, is reinforced so that the layered structure consisting of octahedra of cobalt and oxygen is not destroyed even when lithium is released from positive electrode active material 100 upon charging. It is desirable that surface layer portion 100a containing a high concentration of additional element X, additional element Y, and / or additional element Z is provided on at least a portion of the surface layer portion of the particle, preferably on at least half of the surface layer region of the particle, and more preferably on the entire surface layer region of the particle.

[0167] In addition, in the positive electrode active material 100 according to one embodiment of the present invention, the concentration gradient region of the additive element X, the additive element Y, and / or the additive element Z is preferably provided in at least a portion of the surface layer of the particle, preferably more than half of the surface layer of the particle, and more preferably the entire surface layer of the particle. This is because even if a portion of the surface layer 100a is reinforced, if there is an unreinforced portion, stress may be concentrated in the unreinforced portion, which is undesirable. If stress is concentrated in a portion within the particle, defects such as closed cracks and fissures may occur, leading to a decrease in charge / discharge capacity.

[0168] [Calculations for barium] 4A to 4C show the structural model used in the calculation, and Fig. 5A and Fig. 5B show the results of the structural calculation when barium is added as the added element X to lithium cobalt oxide.

[0169] Figures 4A to 4C show three crystalline structures that are assumed to be formed when barium is dissolved in lithium cobalt oxide having a crystal structure of the space group R-3m. Figure 4A shows a structure in which Li in the Li layer is replaced with Ba, Figure 4B shows a structure in which Co in the Co layer is replaced with Ba, and Figure 4C shows a structure in which Ba occupies both the Li and Co layers and is present at 12 coordination positions. The Co layer refers to a layer made of cobalt in a CoO layer made of a cobalt-oxygen octahedron.

[0170] The calculations were performed using the first-principles calculation software VASP (The Vienna Ab initio simulation package).

[0171] FIG. 5A shows the c-axis length after the structural stabilization calculation for the structure shown in FIGS. 4A to 4C as a result of the calculation. As a result of the structural stabilization calculation, the c-axis length was 13.83930×10 -1 nm)> Co layer with Ba substituted for Co ("Co layer" in Figure 5A: 13.68914 × 10 -1 nm) > Structure with Ba at 12-coordinated positions ("12-coordinated" in Figure 5A: 13.67994 × 10 -1 nm) > Structure without Ba ("Undoped" in Figure 5A: 13.64023 × 10 -1 The calculated results showed that the c-axis length was longer in all of the structures containing Ba compared to those without Ba.

[0172] Figure 5B shows the calculation results for the stabilization effect (difference in energy from the undoped structure) and the energy required to vacate a site for the structures shown in Figures 4A to 4C. A negative stabilization effect value indicates that the structure is more stable than the undoped structure, and the larger the negative value, the more stable the structure.

[0173] As shown in Figure 5B, the stabilizing effect of Ba was greatest (smallest stabilizing effect value) in the structure where Ba was present at the 12-coordination position, confirming stabilization. On the other hand, the energy required to vacate the 12-coordination site (the energy required to secure space for Ba doping) was greatest in the structure where Ba was present at the 12-coordination position. Therefore, although it is not easy to insert Ba into the 12-coordination site, it is predicted that when Ba is present at the 12-coordination position, the stabilizing effect is greatest among the structures calculated in this study.

[0174] Aluminum, gallium, boron, and indium are trivalent and can reside at the transition metal site in the layered rock salt crystal structure. Gallium, aluminum, boron, and indium can suppress the dissolution of surrounding cobalt. Gallium, aluminum, boron, and indium can also suppress cation mixing of surrounding cobalt (cobalt moving to the lithium site). Gallium, aluminum, boron, and indium have strong oxygen-binding properties, which can suppress the desorption of oxygen from the surrounding gallium, aluminum, boron, and indium. Therefore, the addition of one or more of gallium, aluminum, boron, and indium as the additive element Z can result in a positive electrode active material 100 whose crystal structure is resistant to breakdown even with repeated charge and discharge.

[0175] Magnesium is divalent and is more stable at the lithium site than at the transition metal site in the layered rock-salt crystal structure, and therefore more likely to enter the lithium site. The presence of magnesium at an appropriate concentration at the lithium site in the surface layer portion 100a facilitates the maintenance of the layered rock-salt crystal structure. Furthermore, magnesium has a strong bond with oxygen, which can prevent oxygen from being released from the surrounding area of ​​magnesium. At an appropriate concentration, magnesium is preferable because it does not adversely affect the intercalation and deintercalation of lithium during charge and discharge. However, excessive magnesium may adversely affect the intercalation and deintercalation of lithium.

[0176] Fluorine is a monovalent anion, and when some of the oxygen in the surface layer portion 100a is substituted with fluorine, the lithium desorption energy decreases. This is because the change in valence of cobalt ions accompanying lithium desorption differs depending on whether or not fluorine is present. For example, the redox potential of cobalt ions changes from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine. Therefore, when some of the oxygen in the surface layer portion 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 easily occurs. Therefore, when used in secondary batteries, charge / discharge characteristics, rate characteristics, etc. are improved, which is preferable.

[0177] Titanium oxide is known to have superhydrophilic properties. Therefore, by providing a cathode active material 100 having titanium oxide in the surface layer portion 100a, it is possible that the cathode active material 100 may have improved wettability with highly polar solvents. When used in a secondary battery, this may improve the contact at the interface between the cathode active material 100 and a highly polar electrolyte solution, thereby suppressing an increase in resistance. In this specification and the like, the term "electrolyte solution" may be used interchangeably with "electrolyte."

[0178] As the charging voltage of a secondary battery increases, the voltage of the positive electrode generally increases. The positive electrode active material of one embodiment of the present invention has a stable crystal structure even at high voltages. The stable crystal structure of the positive electrode active material in a charged state can suppress a decrease in capacity due to repeated charge and discharge.

[0179] Furthermore, a short circuit in a secondary battery not only causes problems in the charging and / or discharging operations of the secondary battery, but may also lead to heat generation and fire. To achieve a safe secondary battery, it is preferable that the short circuit current be suppressed even at a high charging voltage. The positive electrode active material 100 of one embodiment of the present invention suppresses the short circuit current even at a high charging voltage. Therefore, a secondary battery that achieves both high capacity and safety can be obtained.

[0180] A secondary battery using the positive electrode active material 100 of one embodiment of the present invention preferably simultaneously satisfies high capacity, excellent charge / discharge cycle characteristics, and safety.

[0181] The concentration gradient of the added element can be evaluated, for example, using energy dispersive X-ray spectroscopy (EDX). EDX can be used in combination with SEM or STEM. Among EDX measurements, evaluation along a line segment connecting two points is sometimes called EDX line analysis. Among EDX measurements, measurement while scanning a rectangular or other area and evaluation of the area in two dimensions is sometimes called EDX area analysis. Furthermore, extraction of data from a linear area from EDX area analysis and evaluation of the distribution of atomic concentration within the positive electrode active material are also sometimes called EDX line analysis. In EDX area analysis and EDX line analysis, the point at which the characteristic X-ray detection value of a certain element is maximum is sometimes called the concentration peak.

[0182] EDX area analysis (e.g., element mapping) can quantitatively analyze the concentration of the added element in the surface layer portion 100a, the interior portion 100b, and the vicinity of the grain boundaries of the positive electrode active material 100. Furthermore, EDX line analysis can analyze the concentration peak of the added element.

[0183] When EDX ray analysis is performed on the positive electrode active material 100, the point at which the characteristic X-ray detection value of barium and / or magnesium in the surface layer portion 100a is maximum is preferably located within a depth of 50 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 30 nm, and even more preferably within a depth of 20 nm.

[0184] Furthermore, the distribution of aluminum in the positive electrode active material 100 preferably overlaps with the distribution of barium and / or magnesium. Therefore, when EDX analysis is performed, the point at which the characteristic X-ray detection value of aluminum in the surface layer portion 100a is maximum is preferably located within a depth of 50 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 40 nm, and even more preferably within a depth of 30 nm.

[0185] Furthermore, the distributions of barium, magnesium, and aluminum in the positive electrode active material 100 preferably have overlapping regions where the concentration peaks are different. For example, as shown in Figures 3A to 3C2, the concentration peaks of barium and magnesium are preferably located closer to the surface of the positive electrode active material 100 than the concentration peak of aluminum, and the distributions of barium, magnesium, and aluminum preferably have overlapping regions. In other words, in the surface layer portion 100a, the point where the barium characteristic X-ray detection value is maximum and the point where the magnesium characteristic X-ray detection value is maximum are preferably located closer to the surface of the positive electrode active material 100 than the point where the aluminum characteristic X-ray detection value is maximum, and preferably have regions containing the characteristic X-rays of barium, magnesium, and aluminum.

[0186] [Crystal structure] Materials with a layered rock-salt crystal structure, such as lithium cobalt oxide (LiCoO2), are known to have high discharge capacity and are excellent as positive electrode active materials for secondary batteries. An example of a material with a layered rock-salt crystal structure is the composite oxide represented by LiMO2.

[0187] It is known that 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.

[0188] In compounds containing nickel, distortion may occur due to the Jahn-Teller effect. Therefore, when LiNiO2 is charged and discharged at high voltages, there is a concern that the crystal structure may collapse due to distortion. In LiCoO2, the influence of the Jahn-Teller effect is suggested to be small, and LiCoO2 may have better durability against charge and discharge at high voltages, which is preferable.

[0189] The positive electrode active material will be described with reference to Fig. 6 to Fig. 9. Fig. 6 to Fig. 9 describe the case where cobalt is used as the transition metal contained in the positive electrode active material.

[0190] The positive electrode active material shown in FIG. 8 is lithium cobalt oxide (LiCoO2) that is substantially free of the additional element X, the additional element Y, and the additional element Z. The crystal structure of the lithium cobalt oxide shown in FIG. 8 changes depending on the depth of charge. In other words, Li x When written as CoO2, the crystal structure changes depending on the occupancy rate x of lithium on the lithium site.

[0191] In this specification, the depth of charge is a value that indicates the amount of capacity that has been charged based on the theoretical capacity of the positive electrode active material, in other words, the amount of lithium that has been released from the positive electrode. For example, lithium cobalt oxide (LiCoO2) and lithium nickel-cobalt-manganese oxide (LiNi x Co y Mn z In the case of a positive electrode active material with a layered rock salt structure such as O2 (x+y+z=1), the theoretical capacity is based on 274mAh / g. When the charge depth is 0, it means that no Li has been released from the positive electrode active material. When the charge depth is 0.5, it means that 137mAh / g of lithium has been released from the positive electrode. When the charge depth is 0.8, it means that 219.2mAh / g of lithium has been released from the positive electrode. x When expressing CoO2 (0≦x≦1), if the charge depth is 0, it is expressed as LiCoO2 with x being 1, and if the charge depth is 0.5, it is expressed as LiCoO2 with x being 0.5. 0.5 If the charge depth is 0.8, then x is 0.2. 0.2 It is written as CoO2.

[0192] As shown in Figure 8, lithium cobalt oxide at a depth of charge of 0 (discharged state, x = 1) has a region with a crystal structure of space group R-3m, with three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is six-coordinated with oxygen, and the layers are connected in an edge-sharing manner to form a plane.

[0193] When the charge depth is 1 (x = 0), the crystal structure has the space group P-3m1, and there is one CoO2 layer in the unit cell. Therefore, this crystal structure is sometimes called the O1-type crystal structure.

[0194] Furthermore, when x is approximately 0.12, lithium cobalt oxide has a crystal structure of the space group R-3m. This structure can be described as a structure in which a CoO2 structure such as P-3m1(O1) and a LiCoO2 structure such as R-3m(O3) are alternately stacked. Therefore, this crystal structure is sometimes called the H1-3 crystal structure. However, because lithium insertion and desorption can be uneven, the H1-3 crystal structure is experimentally observed from approximately x = 0.25. In practice, the H1-3 crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in Figure 8 and other parts of this specification, for ease of comparison with other structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0195] As an example, the coordinates of cobalt and oxygen in the unit cell of the H1-3 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 an oxygen atom. Thus, the H1-3 crystal structure is expressed by a unit cell using one cobalt and two oxygen atoms. On the other hand, as described below, the O3' crystal structure of one embodiment of the present invention is preferably expressed by a unit cell using one cobalt and one oxygen atom. This indicates that the symmetry between cobalt and oxygen differs between the O3' crystal structure and the H1-3 structure, and that the O3' crystal structure exhibits smaller changes from the O3 structure than the H1-3 structure. The unit cell that is more preferably used to represent the crystal structure of the positive electrode active material may be selected, for example, so that the GOF (goodness of fit) value is smaller in Rietveld analysis of the XRD pattern.

[0196] When lithium cobalt oxide is repeatedly charged and discharged at a high voltage of 4.6 V or higher, based on the redox potential of lithium metal, or at a deep charge depth of 0.8 or higher (x less than 0.2), the crystal structure of the lithium cobalt oxide changes repeatedly between the H1-3 crystal structure and the R-3m(O3) structure in the discharged state (i.e., a non-equilibrium phase change).

[0197] However, these two crystal structures have a large deviation in the CoO2 layers. As shown by the dotted lines and arrows in Figure 8, in the H1-3 type crystal structure, the CoO2 layers are significantly deviated from the R-3m(O3) structure. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0198] Furthermore, the difference in volume is large; when comparing regions with the same number of cobalt atoms, the difference in volume between the H1-3 type crystal structure and the O3 type crystal structure in the discharged state is 3.0% or more.

[0199] In addition, the H1-3 type crystal structure, which has continuous CoO2 layers such as P-3m1(O1), is likely to be unstable.

[0200] Therefore, repeated high-voltage charging and discharging causes the crystalline structure of lithium cobalt oxide to collapse, which leads to a deterioration in cycle characteristics. This is thought to be because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0201] The positive electrode active material 100 of one embodiment of the present invention can reduce the displacement of the CoO2 layer during repeated high-voltage charge and discharge. Furthermore, the change in volume can be reduced. Therefore, the positive electrode active material of one embodiment of the present invention can achieve excellent cycle characteristics. Furthermore, the positive electrode active material of one embodiment of the present invention can have a stable crystal structure in a high-voltage charged state. Therefore, the positive electrode active material of one embodiment of the present invention may be less likely to cause a short circuit when maintained in a high-voltage charged state. In such cases, safety is further improved, which is preferable.

[0202] In the positive electrode active material of one embodiment of the present invention, the change in crystal structure and the difference in volume per the same number of transition metal atoms between a fully discharged state and a high-voltage charged state are small.

[0203] The crystal structure of the positive electrode active material 100 before and after charge and discharge is shown in Figure 6. The positive electrode active material 100 is a composite oxide containing lithium, cobalt as a transition metal, and oxygen. In addition to the above, it is preferable to contain barium as the additional element X and magnesium as the additional element Y. It is also preferable to contain fluorine as the additional element Y.

[0204] The crystal structure at a charge depth of 0 (discharged state, x = 1) in Figure 6 is the same as that in Figure 8, R-3m(O3). On the other hand, when the cathode active material 100 is fully charged, it has a crystal structure different from the H1-3 crystal structure. This structure is in the space group R-3m and is not a spinel crystal structure. However, ions such as cobalt and magnesium occupy six oxygen coordination positions, and the cation arrangement has a symmetry similar to that of the spinel structure. Furthermore, the periodicity of the CoO2 layers in this structure is the same as that of the O3 type. Therefore, this structure is referred to herein as an O3' type crystal structure or a pseudo-spinel type crystal structure. Therefore, the O3' type crystal structure may also be referred to as a pseudo-spinel type crystal structure. Note that in the diagram of the O3' type crystal structure shown in Figure 6, lithium is omitted to explain the symmetry of the cobalt atoms and the oxygen atoms. However, in reality, for example, 20 atomic % or less of lithium is present relative to the cobalt between the CoO2 layers. In both the O3-type and O3'-type crystal structures, magnesium is preferably present in a dilute form between the CoO2 layers, i.e., at the lithium sites, and halogens such as fluorine are preferably present randomly and dilutely at the oxygen sites.

[0205] In the O3'-type crystal structure, light elements such as lithium may occupy the oxygen tetracoordination site, and in this case too, the ion arrangement has a symmetry similar to that of the spinel type.

[0206] It can also be said that the O3' type crystal structure is similar to the CdCl2 type crystal structure, although it has random Li between the layers. This CdCl2 type-like crystal structure was observed when lithium nickel oxide was charged to a charge depth of 0.94 (x = 0.06) (Li 0.06 The crystal structure is similar to that of lithium cobaltate (NiO2), but it is known that pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt do not usually adopt this crystal structure.

[0207] In the positive electrode active material 100 according to one embodiment of the present invention, when a large amount of lithium is released during charging at a high voltage, the change in the crystal structure is suppressed more than in conventional positive electrode active materials. For example, as shown by the dotted line in Figure 6, there is almost no displacement of the CoO2 layers in these crystal structures.

[0208] More specifically, the cathode active material 100 of one embodiment of the present invention exhibits high structural stability even at high charge voltages. For example, in conventional cathode active materials, even at charge voltages where the H1-3 crystal structure is obtained, for example, at a voltage of about 4.6 V relative to the potential of lithium metal, there exists a charge voltage range in which the R-3m(O3) crystal structure can be maintained. Furthermore, even at higher charge voltages, for example, at voltages of about 4.65 V to 4.7 V relative to the potential of lithium metal, there exists a region in which the O3' crystal structure can be formed. Furthermore, at higher charge voltages, for example, at voltages of about 4.65 V to 4.7 V relative to the potential of lithium metal, there exists a region in which the H1-3 crystal structure can be formed. Furthermore, when graphite is used as the anode active material in a secondary battery, there exists a charge voltage range in which the R-3m(O3) crystal structure can be maintained even at secondary battery voltages of 4.3 V to 4.5 V, and there exists a region in which the O3' crystal structure can be formed at even higher charge voltages, for example, at voltages of 4.35 V to 4.55 V relative to the potential of lithium metal.

[0209] Therefore, in the positive electrode active material 100 of one embodiment of the present invention, the crystal structure is not easily broken even when charge and discharge are repeated at a high voltage.

[0210] In addition, in the positive electrode active material 100, the difference in volume per unit cell between the O3 type crystal structure at a charge depth of 0 (x=1) and the O3' type crystal structure at a charge depth of 0.8 (x=0.2) is 2.5% or less, more specifically 2.2% or less.

[0211] In addition, the O3' type crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), 0.20≦x≦0.25.

[0212] An additive element Y, such as magnesium, randomly and dilutely present between CoO2 layers, i.e., at the lithium sites, has the effect of suppressing the misalignment of the CoO2 layers. Therefore, the presence of magnesium between CoO2 layers facilitates the formation of an O3'-type crystal structure. Therefore, magnesium is preferably present in at least a portion of the surface layer of the particles of the positive electrode active material 100 of one embodiment of the present invention, preferably in more than half of the surface layer of the particles, and more preferably in the entire surface layer of the particles. Furthermore, in order to distribute magnesium throughout the entire surface layer of the particles, heat treatment is preferably performed during the manufacturing process of the positive electrode active material 100 of one embodiment of the present invention.

[0213] However, if the heat treatment temperature is too high, cation mixing occurs, increasing the possibility that the additive element Y, such as magnesium, will enter the cobalt site. Magnesium present in the cobalt site is ineffective in maintaining the R-3m structure during high-voltage charging. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as cobalt being reduced to a divalent state and lithium evaporating.

[0214] Therefore, it is preferable to add a halogen compound such as a fluorine compound to the lithium cobalt oxide before the heat treatment to distribute magnesium throughout the entire surface layer. Adding the halogen compound lowers the melting point of the lithium cobalt oxide. Lowering the melting point makes it easier to distribute magnesium throughout the particles at a temperature where cation mixing is unlikely to occur. Furthermore, the presence of a fluorine compound is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0215] However, if the magnesium concentration is increased beyond a desired value, the effect on stabilizing the crystal structure may be reduced. This is thought to be because magnesium occupies not only the lithium site but also the cobalt site. The number of magnesium atoms in the positive electrode active material of one embodiment of the present invention is preferably 0.001 to 0.1 times the number of transition metal atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably approximately 0.02 times. The magnesium concentration shown here may be a value obtained by performing elemental analysis of the entire particles of the positive electrode active material using, for example, ICP-MS (inductively coupled plasma mass spectrometry), or may be based on the value of the raw material composition during the production process of the positive electrode active material.

[0216] As shown in the legend in Figure 6, transition metals such as nickel and manganese, as well as gallium, aluminum, boron, and indium, are preferably present at the cobalt site, and some may be present at the lithium site, but the less the better. Magnesium is preferably present at the lithium site. Oxygen may be partially substituted with fluorine.

[0217] Furthermore, the presence of barium as the additive element X is expected to stabilize the structure of the surface layer portion 100a of the positive electrode active material 100, as shown in the above-described calculation regarding barium, and to enhance stability in a high-voltage charged state. Thus, due to the synergistic effect of the additive elements X, Y, and Z, the positive electrode active material of one embodiment of the present invention becomes a positive electrode active material that is less likely to deteriorate at high charge / discharge voltages.

[0218] As the contents of the additive elements X, Y, and Z in the positive electrode active material 100 of one embodiment of the present invention increase, the capacity of the positive electrode active material may decrease. This may be due to, for example, the incorporation of gallium, aluminum, boron, or indium into the transition metal site, which may prevent nearby lithium ions from contributing to charge and discharge. Furthermore, the incorporation of barium or magnesium into the lithium site may reduce the amount of lithium contributing to charge and discharge. Furthermore, excessive barium may produce barium compounds that do not contribute to charge and discharge, or excessive magnesium may produce magnesium compounds that do not contribute to charge and discharge.

[0219] In Figure 6, the symmetry of the oxygen atoms is slightly different between the O3 crystal structure and the O3' crystal structure. Specifically, the oxygen atoms in the O3 crystal structure are aligned along the dotted line, whereas the oxygen atoms in the O3' crystal structure are not strictly aligned. This is because, in the O3' crystal structure, as lithium decreases, tetravalent cobalt increases, increasing Jahn-Teller distortion and distorting the octahedral structure of CoO6. Another factor is that as lithium decreases, the repulsion between oxygen atoms in the CoO2 layer becomes stronger.

[0220] As described above, the surface layer portion 100a of the cathode active material 100 of one embodiment of the present invention preferably has a different composition from the interior portion 100b, i.e., a higher concentration of the additive element Y, such as magnesium and fluorine, than the interior portion 100b. Furthermore, the composition preferably has a stable crystal structure at room temperature. Therefore, the surface layer portion 100a may have a different crystal structure from the interior portion 100b. For example, at least a portion of the surface layer portion 100a of the cathode active material 100 of one embodiment of the present invention may have a rock-salt crystal structure. Furthermore, when the surface layer portion 100a and the interior portion 100b have different crystal structures, it is preferable that the crystal orientations of the surface layer portion 100a and the interior portion 100b roughly match.

[0221] The anions in layered rock salt crystals and rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in O3'-type crystals also have a cubic close-packed structure. When they contact, there is a crystal plane where the cubic close-packed structure formed by the anions is oriented in the same direction. However, the space group of layered rock salt crystals and O3'-type crystals is R-3m, which is different from the space groups Fm-3m (the space group of general rock salt crystals) and Fd-3m (the space group of rock salt crystals with the simplest symmetry) of rock salt crystals. Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and O3'-type crystals and rock salt crystals. In this specification, when the cubic close-packed structure formed by the anions is oriented in the same direction in layered rock salt crystals, O3'-type crystals, and rock salt crystals, it may be said that the crystal orientations are approximately the same.

[0222] The general alignment of the crystal orientations of the two regions can be determined from TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, and ABF-STEM (annular bright-field scanning transmission electron microscope) images. X-ray diffraction (XRD), electron diffraction, and neutron diffraction can also be used for this determination. If the crystal orientations are generally aligned, the difference in the orientation of the alternating linear array of cations and anions can be observed in TEM images, etc., to be 5 degrees or less, more preferably 2.5 degrees or less. Light elements such as oxygen and fluorine may not be clearly visible in TEM images, but in such cases, the alignment of the orientations can be determined from the arrangement of metal elements.

[0223] However, if the surface layer 100a is composed of only MgO or only a solid solution of MgO and CoO(II), it becomes difficult to insert and extract lithium. Therefore, the surface layer 100a must contain at least cobalt, and in a discharged state, it must also contain lithium, providing a path for lithium insertion and extraction. It is also preferable that the concentration of cobalt be higher than that of magnesium.

[0224] Additionally, the additional element X, the additional element Y, and the additional element Z are preferably located in the surface layer portion 100a of the particle of the positive electrode active material 100 of one embodiment of the present invention. For example, the positive electrode active material 100 of one embodiment of the present invention may be covered with a coating containing the additional element X, the additional element Y, and the additional element Z.

[0225] Grain boundaries are also planar defects. Therefore, like particle surfaces, they are prone to become unstable and changes in the crystal structure are likely to occur. Therefore, if the concentration of the additional element X and / or the additional element Y at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0226] Furthermore, when the concentrations of the additional element X, the additional element Y, and / or the additional element Z are high at and near the grain boundaries, even if cracks occur along the grain boundaries of particles of the positive electrode active material 100 of one embodiment of the present invention, the concentrations of the additional element X, the additional element Y, and / or the additional element Z 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.

[0227] [High-voltage charged state of positive electrode active material] Whether or not a positive electrode active material is a positive electrode active material 100 of one embodiment of the present invention that exhibits an O3'-type crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. XRD is particularly preferred because 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, and it can obtain sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery.

[0228] As described above, the cathode active material 100 of one embodiment of the present invention is characterized by minimal change in crystal structure between a high-voltage charged state and a discharged state. Materials in which 50 wt% or more of a crystal structure exhibiting a significant change from a high-voltage charged state to a discharged state is present in a high-voltage charged state are undesirable because they cannot withstand high-voltage charging and discharging. It should be noted that the desired crystal structure may not be achieved simply by adding an additive. For example, even if both materials share the common feature of being lithium cobalt oxide containing magnesium and fluorine, there are cases in which the O3'-type crystal structure accounts for 60 wt% or more of the O3'-type crystal structure and cases in which the H1-3-type crystal structure accounts for 50 wt% or more of the H1-3-type crystal structure when charged at a high voltage. Furthermore, at a certain voltage, the O3'-type crystal structure may be nearly 100 wt%, and further increasing the voltage may result in the H1-3-type crystal structure. Therefore, crystal structure analysis, such as XRD, is required to determine whether a material is the cathode active material 100 of one embodiment of the present invention.

[0229] However, when positive electrode active materials are charged or discharged at high voltage, their crystal structure may change when exposed to air. For example, they may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is recommended that all samples be handled in an inert atmosphere such as an argon atmosphere.

[0230] <Charging method 1> As a high-voltage charging method for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention, for example, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) can be fabricated using a lithium counter electrode and charged.

[0231] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive material, and a binder.

[0232] Lithium metal can be used for the counter electrode. 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, voltages and potentials in this specification refer to the potential of the positive electrode.

[0233] The electrolyte used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution can be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC).

[0234] The separator can be made of polypropylene with a thickness of 25 μm.

[0235] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0236] The coin cell prepared under the above conditions was charged at a constant current of 4.6 V and 0.5 C, followed by constant voltage charging until the current reached 0.01 C. Here, 1 C corresponds to 137 mA / g. The temperature was 25°C. After charging in this manner, the coin cell was disassembled in an argon-filled glove box and the positive electrode was removed to obtain a positive electrode active material charged at high voltage. When various analyses were performed, it was preferable to seal the cell in an argon-filled atmosphere to prevent reactions with external components. For example, XRD could be performed by sealing the cell in an airtight container in an argon atmosphere.

[0237] <xrd> Figures 7 and 9 show ideal powder XRD patterns calculated from the O3'-type crystal structure and H1-3-type crystal structure models using CuKα1 radiation. For comparison, ideal XRD patterns calculated from the crystal structures of LiCoO2(O3) at a charge depth of 0 (x = 1) and CoO2(O1) at a charge depth of 1 (x = 0) are also shown. The LiCoO2(O3) and CoO2(O1) patterns were created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database). The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 = 1.540562 × 10. -10 m and λ2 were not set, and the monochromator was set to single. The pattern of the H1-3 type crystal structure was similarly created from crystal structure information (WE Counts et al., Journal of the American Ceramic Society, 1953, 36[1] pp.12-17. Fig.01471). The pattern of the O3' type crystal structure was estimated from the XRD pattern of a positive electrode active material according to one embodiment of the present invention, and fitted using TOPAS ver.3 (crystal structure analysis software manufactured by Bruker). The XRD pattern was created in the same manner as the others.

[0238] As shown in Figure 7, the O3'-type crystal structure exhibits diffraction peaks at 2θ = 19.30 ± 0.20° (19.10° to 19.50°) and 2θ = 45.55 ± 0.10° (45.45° to 45.65°). More specifically, sharp diffraction peaks appear at 2θ = 19.30 ± 0.10° (19.20° to 19.40°) and 2θ = 45.55 ± 0.05° (45.50° to 45.60°). However, as shown in Figure 9, the H1-3-type crystal structure and CoO2(P-3m1, O1) do not exhibit peaks at these positions. Therefore, the appearance of peaks at 2θ=19.30±0.20° and 2θ=45.55±0.10° in a state charged at a high voltage can be said to be a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0239] This means that the positions at which the XRD diffraction peaks appear are close between the crystal structure at a charge depth of 0 (x=1) and the crystal structure in a high-voltage charging state. More specifically, the difference in the positions at which two or more, preferably three or more, of the main diffraction peaks of both structures appear is 2θ=0.7° or less, more preferably 2θ=0.5° or less.

[0240] Although the positive electrode active material 100 of one embodiment of the present invention has an O3'-type crystal structure when charged at a high voltage, not all of the particles need to have the O3'-type crystal structure. Other crystal structures may be included, or some particles may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more. When the O3'-type crystal structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, the positive electrode active material can have sufficiently excellent cycle characteristics.

[0241] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the O3' type crystal structure is preferably 35 wt% or more, more preferably 40 wt% or more, and even more preferably 43 wt% or more.

[0242] Furthermore, the crystallite size of the O3'-type crystal structure possessed by the particles of the positive electrode active material only decreases to about one-tenth of that of LiCoO2(O3) in the discharged state. Therefore, even under the same XRD measurement conditions as for the positive electrode before charging and discharging, a clear peak of the O3'-type crystal structure can be confirmed in the high-voltage charged state. On the other hand, with simple LiCoO2, even if some of the material adopts a structure similar to the O3'-type crystal structure, the crystallite size becomes smaller and the peak becomes broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0243] As described above, the positive electrode active material of one embodiment of the present invention preferably has a small influence of the Jahn-Teller effect. The positive electrode active material of one embodiment of the present invention preferably has a layered rock-salt crystal structure and contains cobalt as a transition metal. Furthermore, the positive electrode active material of one embodiment of the present invention may contain the aforementioned additional element X and / or additional element Y in addition to cobalt, as long as the influence of the Jahn-Teller effect is small.

[0244] In addition, in a positive electrode active material of one embodiment of the present invention, when particles of the positive electrode active material in a state where no charging or discharging is performed or in a discharged state are subjected to XRD analysis, a first peak may be observed at 2θ of 18.50° to 19.30°, and a second peak may be observed at 2θ of 38.00° to 38.80°.

[0245] The peaks appearing in the powder XRD pattern reflect the crystalline structure of the interior 100b of the positive electrode active material 100, which occupies most of the volume of the positive electrode active material 100. The crystalline structure of the surface layer 100a and the like can be analyzed by electron beam diffraction or the like of a cross section of the positive electrode active material 100.

[0246] [Defects in the positive electrode active material] Examples of defects that may occur in the positive electrode active material are shown in Figures 10 to 13. The positive electrode active material of one embodiment of the present invention is expected to have the effect of suppressing the occurrence of the following defects.

[0247] When charging and discharging under high voltage charging conditions of 4.5 V or higher or at high temperature (45°C or higher), closed cracks (also called crack closure), which are one of the progressive defects, may occur inside the positive electrode active material.

[0248] To show an example of the defect, a positive electrode active material without the additive element X was prepared, and a slurry mixed with the positive electrode active material, a conductive material, and a binder was applied to an aluminum foil positive electrode current collector to fabricate a positive electrode sample. Using the positive electrode sample as the positive electrode and a lithium foil as the negative electrode, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) was fabricated, and charging and discharging were repeated 50 times. Charging was performed at a constant current of 0.5 C up to 4.7 V, and then at a constant voltage until the current value reached 0.05 C. Discharging was performed at a constant current of 0.5 C up to 2.5 V. Here, 1 C was set to 200 mA / g. The temperature was set to three conditions: 25°C, 45°C, and 60°C. After repeating charging and discharging 50 times in this way, the coin cell was disassembled in a glove box under an argon atmosphere to take out the positive electrode. The obtained deteriorated positive electrode samples were designated as sample C1, sample C2, and sample C3. Here, the positive electrode after the test under the 25°C condition is called sample C1, the positive electrode after the test under the 45°C condition is called sample C2, and the positive electrode after the test under the 60°C condition is called sample C3.

[0249] <STEM Observation> Next, the cross sections of the positive electrodes of the secondary batteries after 50 cycles were observed using a scanning transmission electron microscope (STEM). Samples were processed using a focused ion beam (FIB) for cross-sectional observation. The results of cross-sectional STEM observation of sample C1 are shown in FIGS. 10A to 10C, those of sample C2 are shown in FIGS. 11A to 11C, and those of sample C3 are shown in FIGS. 12A to 12C. FIG. 10C is an enlarged image of the region surrounded by the solid line in FIG. 10B, and FIG. 10B is an enlarged image of the region surrounded by the solid line in FIG. 10A. FIG. 11C is an enlarged image of the region surrounded by the solid line in FIG. 11B, and FIG. 11B is an enlarged image of the region surrounded by the solid line in FIG. 11A. FIG. 12C is an enlarged image of the region surrounded by the solid line in FIG. 12B, and FIG. 12B is an enlarged image of the region surrounded by the solid line in FIG. 12A. To obtain cross-sectional STEM images, a Hitachi High-Tech HD-2700 was used, and the accelerating voltage was set to 200 kV.

[0250] In sample C1, shown in Figures 10A to 10C, where the cycle test was performed at 25°C, no closed cracks were observed within the positive electrode active material. In contrast, in sample C2, shown in Figures 11A to 11C, where the cycle test was performed at 45°C, closed cracks were observed within the positive electrode active material. Similarly, in sample C3, shown in Figures 12A to 12C, where the cycle test was performed at 60°C, closed cracks were also observed within the positive electrode active material. In Figures 10 to 12, the locations of the closed cracks are indicated by arrows. The observed closed cracks extend in a direction parallel to the lattice fringes. The lattice fringes shown in Figures 10C, 11C, and 12C are image contrasts resulting from the atomic arrangement (crystal planes) of the positive electrode active material. In this case, they are considered to be lattice fringes resulting from crystal planes perpendicular to the c-axis.

[0251] The results of a detailed analysis of sample C2 shown in Figures 11A to 11C are shown in Figures 13A to 13E. Figure 13B is an enlarged image of the area surrounded by the solid line in Figure 13A. Nanobeam electron diffraction (NBED) was performed at a point near the closed crack (Point 1 in Figure 13B). A diffraction pattern derived from the spinel structure (white arrow in Figure 13C) and diffraction patterns identified as the O1 structure (1, 2, and 3 in Figure 13C) were found. Analysis of diffraction spots 1, 2, 3, and transmission spot O in Figure 13C revealed that the interplanar spacing calculated from the distance between diffraction spot 1 and transmission spot O was d = 0.239 nm, the interplanar spacing calculated from the distance between diffraction spot 2 and transmission spot O was d = 0.208 nm, and the interplanar spacing calculated from the distance between diffraction spot 3 and transmission spot O was d = 0.429 nm. Furthermore, the angle between diffraction spot 1, transmission spot O, and diffraction spot 2 was 29°, the angle between diffraction spot 1, transmission spot O, and diffraction spot 3 was 89°, and the angle between diffraction spot 2, transmission spot O, and diffraction spot 3 was 60°. Therefore, there is a possibility that the O1 structure may have a region existing as CoO2, for example. Note that FIG. 13D is a diagram showing a CoO2 structure as an example of the O1 structure, and FIG. 13E is a diagram showing a LiCo2O4 structure as an example of the spinel structure.

[0252] [Calculation for closed cracks 1] Based on the analysis results shown in Figures 13A to 13E, molecular dynamics calculations for the closed crack were performed using the SCIGRESS program. The atomic charges used in the molecular dynamics calculations were CoO2 and Li 0.5 The structure of CoO2 was optimized using the VASP program, and the average value obtained by Bader charge analysis of the charge density distribution was used. The calculation conditions are shown in Tables 1 to 3. Table 1 shows the conditions for VASP, Table 2 shows the conditions for the atomic charges obtained from Bader charge analysis, and Table 3 shows the conditions for SCIGRESS. Figure 14A shows the structural model used in the molecular dynamics calculation. Figure 14B shows the structure after calculation. Figure 14C is an enlarged view of a portion of Figure 14B.

[0253] [Table 1]

[0254] [Table 2]

[0255] [Table 3]

[0256] As shown in the analysis results in Figure 13, a region with an O1 structure (CoO2) was confirmed near the closed crack. Furthermore, the discharge capacity of this sample had decreased to about 50% due to the charge-discharge cycle test before the cross-sectional STEM analysis. Therefore, the CoO2 and Li 0.5 CoO2 is a fully discharged LiCoO2 with all Li removed, and Li 0.5 CoO2 is in a state of charge depth 0.5 (x=0.5), which is a state where half of the Li has been removed from LiCoO2 in a fully discharged state. 0.5 When comparing CoO2 and Li, the c-axis length of the unit cell is different. 0.5 It is thought that stress occurs in the area where CoO2 comes into contact with Li. 0.5 A structural model of the region where CoO2 and are in contact was created, and molecular dynamics calculations were performed. The calculation conditions were NTV ensemble, temperature was room temperature (298K), time step width was 1fs, and simulation time was 30ps. The calculation program used was SCIGRESS.

[0257] Figure 14B shows the calculation results of the structural model shown in Figure 14A, which was relaxed at room temperature. A closed crack was formed in the center of the figure. Therefore, CoO2 and Li 0.5 In the contact area between CoO2 and Li, stress occurs due to the difference in c-axis length, and CoO2(Li 0.5 The stretching of the SiO2 (which is more easily deformed than CoO2) may result in the formation of closed cracks. Furthermore, as shown in Figure 14C, a part of the spinel structure was formed near the closed cracks.

[0258] [Calculation for closed cracks 2] A molecular dynamics calculation for a closed crack was performed using a different structural model than that used in calculation 1 for a closed crack. The atomic charges used were 0.8964 for Li, 1.5073 for Co, and -0.7910 for O. The calculation conditions were an NTV ensemble, room temperature (298 K), a time step size of 1 fs, and a simulation time of 10 ps. The calculation program used was SCIGRESS. Figures 15A1 and 15B1 show the structural model used in the calculation. The structure after calculation is shown in Figures 15A2 and 15B2.

[0259] In calculation 1 for closed cracks, the region with a CoO2 structure was used, and in calculation 2 for closed cracks, the region with a Li 0.083 CoO2. Furthermore, Li 0.5 The region with the CoO2 structure is omitted, and Co and O at the right end are fixed. 0.083 The structure model and the calculated structure are shown for the case where the Li distribution in the CoO2 region is uniform. 0.083 This is a structural model and calculated structure when the Li distribution in the CoO2 region is non-uniform.

[0260] The calculation results shown in Figures 15A2 and 15B2 indicate that when the Li distribution was nonuniform, closed cracks with a width of approximately 1 nm were formed, as shown in Figure 15B2. However, when the Li distribution was uniform, the width of the closed cracks that were formed was found to be less than 0.5 nm, as shown in Figure 15A2. Therefore, if the Li distribution within the positive electrode active material is uniform in the charged state, it is possible to suppress the formation of closed cracks. For example, in high-voltage charging at a charge depth of 0.8 or more (x = less than 0.2), it is preferable that the Li distribution within the positive electrode active material is uniform. In the positive electrode active material of one embodiment of the present invention, it is expected that the Li distribution within the positive electrode active material is uniform in the high-voltage charging state at a charge depth of 0.8 or more (x = less than 0.2).

[0261] [Surface roughness and specific surface area] The positive electrode active material 100 according to one embodiment of the present invention preferably has a smooth surface with few irregularities. A smooth surface with few irregularities is one factor indicating that the distribution of the additive element Y in the surface layer portion 100a is uniform.

[0262] 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.

[0263] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 100 as follows.

[0264] First, the cathode active material 100 is processed using FIB or the like to expose its 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. The interface is then extracted using image processing software. The interface line between the protective film or the like and the cathode active material 100 is selected using a magic hand tool or the like, and the data is extracted to a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed using a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction. The root mean square (RMS) surface roughness is calculated by calculating the standard deviation. This surface roughness is the surface roughness within at least 400 nm of the outer periphery of the cathode active material particles.

[0265] The particle surfaces of the positive electrode active material 100 of this embodiment preferably have a root mean square (RMS) surface roughness, which is an index of roughness, of less than 3 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

[0266] The image processing software used for noise processing, boundary extraction, etc. is not particularly limited, but for example, "ImageJ" can be used. Similarly, the spreadsheet software is not particularly limited, but for example, Microsoft Office Excel can be used.

[0267] For example, the actual specific surface area A measured by the gas adsorption method using the constant volume method R and the ideal specific surface area A i The smoothness of the surface of the positive electrode active material 100 can also be quantified from the ratio of the surface roughness to the surface smoothness.

[0268] 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.

[0269] Ideal specific surface area A i is calculated assuming that all particles have the same diameter as D50, the same weight, and an ideal spherical shape.

[0270] The positive electrode active material 100 of one embodiment of the present invention has an ideal specific surface area A (when considered as a perfect sphere) calculated from the median diameter D50. i and the actual specific surface area A R Ratio A R / A i is preferably 1 or more and 2 or less.

[0271] If the particle size of the positive electrode active material 100 of 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 arise. 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 occur. Therefore, the average particle size (D50: also referred to as median diameter) 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.

[0272] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0273] (Embodiment 3) In this embodiment, examples of a plurality of shapes of secondary batteries including the positive electrode active material 100 manufactured by the manufacturing method described in the previous embodiment will be described.

[0274] [Coin-type secondary battery] An example of a coin-type secondary battery will be described. Fig. 16A is an exploded perspective view of a coin-type (single-layer flat) secondary battery, Fig. 16B is an external view, and Fig. 16C is a cross-sectional view thereof. Coin-type secondary batteries are mainly used in small electronic devices. In this specification, coin-type batteries include button-type batteries.

[0275] 16A is a schematic diagram that shows the overlapping of components (upper and lower relationships and positional relationships) for ease of understanding, and therefore, FIGS. 16A and 16B are not completely identical corresponding views.

[0276] In Fig. 16A, a positive electrode 304, a separator 310, a negative electrode 307, a spacer 322, and a washer 312 are stacked. These are sealed between a negative electrode can 302 and a positive electrode can 301. Note that a gasket for sealing is not shown in Fig. 16A. The spacer 322 and the washer 312 are used to protect the interior 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.

[0277] 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 .

[0278] To prevent short-circuiting between the positive electrode and the negative electrode, a separator 310 and a ring-shaped insulator 313 are arranged so as to cover the side and top surfaces of the positive electrode 304. The separator 310 has a larger planar area than the positive electrode 304.

[0279] FIG. 16B is a perspective view of the completed coin-type secondary battery.

[0280] 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 made of lithium metal foil or a lithium-aluminum alloy foil.

[0281] In the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300, the active material layer may be formed only on one surface of the current collector.

[0282] Positive electrode can 301 and negative electrode can 302 can be made of metals such as nickel, aluminum, titanium, or alloys thereof, or alloys of these with other metals (e.g., stainless steel), which are corrosion-resistant to the electrolyte. Furthermore, to prevent corrosion by the electrolyte, etc., they are preferably coated with nickel, aluminum, or the like. Positive electrode can 301 is electrically connected to positive electrode 304, and negative electrode can 302 is electrically connected to negative electrode 307.

[0283] These negative electrode 307, positive electrode 304, and separator 310 are immersed in an electrolyte, and as shown in FIG. 16C, positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order with positive electrode can 301 facing downwards, and positive electrode can 301 and negative electrode can 302 are crimped together via gasket 303 to produce coin-type secondary battery 300.

[0284] The above-described configuration allows for a coin-type secondary battery 300 with a high capacity, a high charge / discharge capacity, and excellent cycle characteristics. Note that when a secondary battery has a solid electrolyte layer between the negative electrode 307 and the positive electrode 304, the separator 310 may be unnecessary.

[0285] [Cylindrical secondary battery] An example of a cylindrical secondary battery will be described with reference to Fig. 17A. As shown in Fig. 17A, 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.

[0286] Fig. 17B is a diagram showing a cross section of a cylindrical secondary battery. The cylindrical secondary battery shown in Fig. 17B 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 and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0287] 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 alloys of these metals with other metals (e.g., stainless steel), which are corrosion-resistant to the electrolyte. To prevent corrosion by the electrolyte, the battery can 602 is preferably coated with nickel, aluminum, or the like. 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.

[0288] 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. While Figures 17A to 17D illustrate a secondary battery 616 in which the height of the cylinder is greater than the diameter, this is not limiting. A secondary battery in which the diameter of the cylinder is greater than the height of the cylinder may also be used. This configuration, for example, can contribute to miniaturization of the secondary battery.

[0289] By using the positive electrode active material 100 obtained in the above-described embodiment for the positive electrode 604, a cylindrical secondary battery 616 can be obtained that has a high capacity, a high charge / discharge capacity, and excellent cycle characteristics.

[0290] 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 with an increase in temperature. This increase in resistance limits the amount of current and prevents abnormal heat generation. The PTC element can be made of a barium titanate (BaTiO3)-based semiconductor ceramic or the like.

[0291] 17C 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. A protection circuit or the like that prevents overcharging or overdischarging can be used as the control circuit 620.

[0292] 17D 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.

[0293] A plurality of secondary batteries 616 may be connected in parallel and then further connected in series.

[0294] 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 the performance of the power storage system 615 less susceptible to the outside temperature.

[0295] 17D, 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.

[0296] [Other examples of secondary battery structures] An example of the structure of the secondary battery will be described with reference to FIGS.

[0297] A secondary battery 913 shown in Fig. 18A 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. 18A, 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 (such as aluminum) or a resin material.

[0298] 18B, the housing 930 shown in Fig. 18A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 18B has housings 930a and 930b bonded together, and a wound body 950 is provided in the area surrounded by the housings 930a and 930b.

[0299] 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 prevent the secondary battery 913 from blocking the electric field. Note that if the electric field blocking effect of 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.

[0300] 18C shows the structure of wound body 950. Winding body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. Winding body 950 is a wound body in which negative electrode 931 and positive electrode 932 are stacked on top of each other with separator 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple stacks of negative electrode 931, positive electrode 932, and separator 933 may be stacked.

[0301] 19A to 19C, a secondary battery 913 may be provided having a wound body 950a. The wound body 950a shown in Fig. 19A 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.

[0302] By using the positive electrode active material 100 obtained in the above-described embodiment for the positive electrode 932, the secondary battery 913 can be made to have a high capacity, a high charge / discharge capacity, and excellent cycle characteristics.

[0303] The separator 933 has a width wider than the negative electrode active material layer 931a and the positive electrode active material layer 932a, and is wound so as to overlap the negative electrode active material layer 931a and the positive electrode active material layer 932a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931a be wider than the positive electrode active material layer 932a. A wound body 950a having such a shape is preferable because of its high safety and productivity.

[0304] 19B, negative electrode 931 is electrically connected to terminal 951. Terminal 951 is electrically connected to terminal 911a. Positive electrode 932 is electrically connected to terminal 952. Terminal 952 is electrically connected to terminal 911b.

[0305] 19C, wound body 950a and the electrolyte are covered by casing 930 to form secondary battery 913. It is preferable to provide casing 930 with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of casing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0306] As shown in Fig. 19B, the secondary battery 913 may have multiple wound bodies 950a. Using multiple wound bodies 950a allows the secondary battery 913 to have a larger charge / discharge capacity. For other elements of the secondary battery 913 shown in Figs. 19A and 19B, the descriptions of the secondary battery 913 shown in Figs. 18A to 18C can be referred to.

[0307] <Laminated secondary battery> 20A and 20B show examples of external views of a laminated secondary battery, which 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.

[0308] FIG. 21A 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. The area and shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 21A.

[0309] <Method for manufacturing laminated secondary batteries> Here, an example of a method for manufacturing the laminated secondary battery whose external view is shown in FIG. 20A will be described with reference to FIGS. 21B and 21C.

[0310] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 21B shows the stacked negative electrode 506, the separator 507, and the positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. The stacked negative electrodes, separators, and positive electrodes can be called a laminate. 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.

[0311] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are placed on the exterior body 509 .

[0312] Next, as shown in Fig. 21C, exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of exterior body 509 is joined. For the joining, for example, thermocompression bonding or the like may be used. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of exterior body 509 so that an electrolyte can be introduced later.

[0313] 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.

[0314] By using the positive electrode active material 100 obtained in the above-described embodiment for the positive electrode 503, it is possible to obtain a secondary battery 500 having a high capacity, a high charge / discharge capacity, and excellent cycle characteristics.

[0315] [Example of a battery pack] An example of a secondary battery pack according to one embodiment of the present invention, which is capable of wireless charging using an antenna, will be described with reference to FIGS. 22A to 22C.

[0316] FIG. 22A is a diagram showing the appearance of secondary battery pack 531, which has a thin rectangular parallelepiped shape (which can also be called a thick flat plate shape). FIG. 22B 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 attached to secondary battery 513. Circuit board 540 is fixed with sticker 515. Secondary battery pack 531 also has antenna 517.

[0317] The inside of the secondary battery 513 may have a structure including a wound body or a laminated body.

[0318] 22B, 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 electrode lead and the negative electrode lead of the secondary battery 513, and the other 552 of the positive electrode lead and the negative electrode lead of the secondary battery 513.

[0319] Alternatively, as shown in FIG. 22C, the device may have a circuit system 590a provided on the circuit board 540 and a circuit system 590b electrically connected to the circuit board 540 via the terminals 514.

[0320] The antenna 517 is not limited to a coil shape, and may be, for example, a wire or plate shape. Also, 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 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.

[0321] The secondary battery pack 531 has 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 caused by the secondary battery 513. The layer 519 can be made of, for example, a magnetic material.

[0322] [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 include a conductive material and a binder. The positive electrode active material 100 manufactured by the manufacturing method described in the above embodiment is used as the positive electrode active material.

[0323] Hereinafter, as an example, a cross-sectional structure example will be described in which graphene or a graphene compound is used as a conductive material in a positive electrode active material layer. The graphene compound will be described later.

[0324] 23A shows a vertical cross-sectional view of a positive electrode active material layer 200. The positive electrode active material layer 200 includes granular positive electrode active material 100, graphene or a graphene compound 201 as a conductive material, and a binder (not shown).

[0325] For secondary batteries that require rapid charging and rapid discharging, it is particularly effective to use graphene compounds as conductive materials. For example, rapid charging and rapid discharging characteristics may be required for secondary batteries for two-wheeled or four-wheeled vehicles and secondary batteries for drones. Rapid charging and rapid discharging may also be referred to as high-rate charging and high-rate discharging. For example, rapid charging and rapid discharging refer to charging and discharging at 1C, 2C, or 5C or higher.

[0326] 23B , in a longitudinal cross section of the positive electrode active material layer 200, sheet-like graphene or graphene compounds 201 are dispersed approximately uniformly within the positive electrode active material layer 200. In FIG. 23B , the graphene or graphene compounds 201 are schematically represented by thick lines, but in reality, they are thin films having a thickness corresponding to a single layer or multiple layers of carbon molecules. The plurality of graphene or graphene compounds 201 are formed so as to partially cover the plurality of granular positive electrode active material 100 or to be attached to the surfaces of the plurality of granular positive electrode active material 100, and are therefore in surface contact with each other.

[0327] Here, a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net) can be formed by bonding multiple graphenes or graphene compounds together. When an active material is covered with a graphene net, the graphene net can also function as a binder that bonds the active materials together. Therefore, the amount of binder can be reduced or can be eliminated, thereby improving the ratio of the active material to the electrode volume and electrode weight. In other words, the charge / discharge capacity of the secondary battery can be increased.

[0328] Here, it is preferable to use graphene oxide as the graphene or graphene compound 201, mix it with an active material to form a layer that becomes the positive electrode active material layer 200, and then reduce it. That is, the completed active material layer preferably has reduced graphene oxide. By using graphene oxide, which has extremely high dispersibility in a polar solvent, to form the graphene or graphene compound 201, it is possible to substantially uniformly disperse the graphene or graphene compound 201 inside the positive electrode active material layer 200. The solvent is volatilized and removed from the dispersion medium containing the uniformly dispersed graphene oxide, and the graphene oxide is reduced. Therefore, the graphene or graphene compound 201 remaining in the positive electrode active material layer 200 is dispersed to the extent that it partially overlaps and is in surface contact with each other, thereby forming a three-dimensional conductive path. Note that the reduction of the graphene oxide may be performed, for example, by heat treatment or using a reducing agent.

[0329] Therefore, unlike granular conductive materials such as acetylene black that make point contact with the active material, graphene or graphene compound 201 enables surface contact with low contact resistance. Thus, the electrical conductivity between the granular positive electrode active material 100 and graphene or graphene compound 201 can be improved with a smaller amount than that of ordinary conductive materials. Therefore, the ratio of the positive electrode active material 100 in the positive electrode active material layer 200 can be increased. Thereby, the discharge capacity of the secondary battery can be increased.

[0330] Also, by using a spray drying apparatus in advance, the entire surface of the active material can be covered to form a graphene compound, which is a conductive material, as a film, and a conductive path can also be formed between the active materials with the graphene compound

[0331] The positive electrode active material layer 200 may also be used by mixing the positive electrode active material described in the previous embodiment with other positive electrode active materials

[0332] Examples of other positive electrode active materials include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, compounds such as LiFePO₄, LiFeO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, and MnO₂ can be mentioned

[0333] Also, as another positive electrode active material, a lithium-containing material having a spinel-type crystal structure containing manganese such as LiMn₂O₄ is mixed with lithium nickelate (denoted as LiNiO₂ or LiNi 1-x M x O₂(0 < x < 1)(M = Co, Al, etc.)) is preferably used. By adopting such a configuration, the characteristics of the secondary battery can be improved

[0334] Also, as another positive electrode active material, the composition formula is Li a Mn b M c O d A lithium-manganese composite oxide that can be expressed by the formula (1) can be used. Here, element M is preferably a metal element selected from among lithium and manganese, or silicon or phosphorus, and more preferably nickel. When measuring the entire lithium-manganese composite oxide particle, <a / (b+c)<2、かつc>it is preferable that the composition be 0 0 during discharge and satisfy the relationship 0.26≦(b+c) / d<0.5. The composition of metals, silicon, phosphorus, etc. in the entire lithium-manganese composite oxide particle can be measured, for example, using ICP-MS (inductively coupled plasma mass spectrometry). The oxygen composition in the entire lithium-manganese composite oxide particle can be measured, for example, using EDX (energy dispersive X-ray spectrometry). In addition, the composition can be determined by valence evaluation using fusion gas analysis and XAFS (X-ray absorption fine structure) analysis in combination with ICP-MS analysis. The lithium manganese composite oxide refers to an oxide containing at least lithium and manganese, and may contain at least one element selected from the group consisting of chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, phosphorus, and the like.

[0335] <Conductive material> The conductive material, also called a conductive additive or a conductive agent, is made of a carbon material. By attaching the conductive material between multiple active materials, the active materials are electrically connected to each other, increasing their conductivity. Note that "attachment" does not only refer to physical adhesion between the active material and the conductive material, but also encompasses cases where a covalent bond is formed, bonding due to van der Waals forces, the conductive material covering part of the surface of the active material, the conductive material fitting into the surface irregularities of the active material, and electrical connection even when not in contact with each other.

[0336] Carbon black (furnace black, acetylene black, graphite, etc.) is a typical carbon material used as a conductive material.

[0337] It is more preferable to use graphene or a graphene compound as the conductive material.​

[0338] In this specification and the like, graphene compounds include multilayer graphene, multigraphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, reduced graphene oxide, reduced multilayer graphene oxide, reduced multi-graphene oxide, graphene quantum dots, etc. Graphene compounds contain carbon, have a shape such as a plate or sheet, and have a two-dimensional structure formed by six-membered carbon rings. The two-dimensional structure formed by six-membered carbon rings may also be called a carbon sheet. Graphene compounds may have functional groups. Furthermore, graphene compounds preferably have a curved shape. Furthermore, graphene compounds may be rolled up to resemble carbon nanofibers.

[0339] In this specification and the like, graphene oxide refers to a material that contains carbon and oxygen, has a sheet shape, and has a functional group, in particular, an epoxy group, a carboxy group, or a hydroxy group.

[0340] In this specification, reduced graphene oxide refers to a material containing carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by six-membered carbon rings. It may also be called a carbon sheet. Although a single sheet of reduced graphene oxide can function, multiple sheets may also be stacked. Reduced graphene oxide preferably has a portion where the carbon concentration is greater than 80 atomic % and the oxygen concentration is between 2 atomic % and 15 atomic %. By achieving these carbon and oxygen concentrations, reduced graphene oxide can function as a highly conductive material even in small amounts. Furthermore, reduced graphene oxide preferably has an intensity ratio G / D between the G band and the D band in a Raman spectrum of 1 or more. Reduced graphene oxide with such an intensity ratio can function as a highly conductive material even in small amounts.

[0341] Graphene and graphene compounds may have excellent electrical properties, such as high conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene and graphene compounds may also have a sheet-like shape. Graphene and graphene compounds may have curved surfaces, enabling surface contact with low contact resistance. Even when thin, they may have high conductivity, allowing a small amount to efficiently form a conductive path within an active material layer. Therefore, using graphene or graphene compounds as a conductive material can increase the contact area between the active material and the conductive material. It is preferable that the graphene or graphene compound covers 80% or more of the active material. It is preferable that the graphene or graphene compound clings to at least a portion of the active material particles. It is also preferable that the graphene or graphene compound overlaps at least a portion of the active material particles. It is also preferable that the shape of the graphene or graphene compound matches at least a portion of the shape of the active material particles. The shape of the active material particles refers, for example, to the unevenness of a single active material particle or the unevenness formed by multiple active material particles. Preferably, the graphene or graphene compound surrounds at least a portion of the active material particles, and the graphene or graphene compound may have holes.

[0342] When using active material particles with a small particle size, for example, 1 μm or less, the specific surface area of ​​the active material particles is large, and more conductive paths connecting the active material particles are required. In such cases, it is preferable to use graphene or a graphene compound, which can efficiently form conductive paths even in a small amount.

[0343] Because of the properties described above, graphene compounds are particularly effective as conductive materials for secondary batteries that require rapid charging and rapid discharging. For example, rapid charging and rapid discharging characteristics may be required for secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. Rapid charging and rapid discharging may also be referred to as high-rate charging and high-rate discharging. For example, rapid charging and rapid discharging refer to charging and discharging at 1C, 2C, or 5C or higher.

[0344] Furthermore, a material used in forming graphene or a graphene compound may be mixed with the graphene or graphene compound and used in the active material layer. For example, particles used as a catalyst in forming the graphene compound may be mixed with the graphene compound. Examples of catalysts used in forming the graphene compound include silicon oxide (SiO2, SiO x (x<2)), aluminum oxide, iron, nickel, ruthenium, iridium, platinum, copper, germanium, etc. The particles preferably have a median diameter (D50) of 1 μm or less, more preferably 100 nm or less.

[0345] <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. Also, fluororubber can be used as the binder.

[0346] 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.

[0347] Alternatively, it is preferable to use 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 as the binder.

[0348] The binder may be used in combination with two or more of the above.

[0349] 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, and starch may be used.

[0350] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium salts and ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with the active material and other components when preparing electrode slurry. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0351] Water-soluble polymers stabilize viscosity by dissolving in water, and can stably disperse active materials and other materials combined as binders, such as styrene-butadiene rubber, 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 groups and carboxyl groups. Because of these functional groups, the polymers are expected to interact with each other and widely cover the surface of the active material.

[0352] When the binder covering or contacting the surface of the active material 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 is 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.

[0353] <Positive electrode current collector> The 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 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 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 current collector have a thickness of 5 μm to 30 μm.

[0354] [Negative electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also include a conductive material and a binder.

[0355] The negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, materials containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium 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, silicon is preferred as the negative electrode active material. Compounds containing these elements can also be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Here, elements that can undergo charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes called alloy-based materials.

[0356] In this specification and the like, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO 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.

[0357] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0358] 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 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.

[0359] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a low potential similar to that of metallic lithium (0.05V to 0.3V vs. Li / Li + ) This allows lithium-ion secondary batteries using graphite to exhibit high operating voltages. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0360] In addition, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), molybdenum oxide (MoO2), and other oxides can be used.

[0361] In addition, the negative electrode active material is a composite nitride of lithium and transition metals, Li3N-type 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N3 has a large charge / discharge capacity (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0362] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, and therefore it can be preferably combined with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0363] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 , sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N and Ge3N4, phosphides such as NiP2, FeP2 and CoP3, and fluorides such as FeF3 and BiF3.

[0364] Lithium can also be used as the negative electrode active material. When lithium is used as the negative electrode active material, foil-shaped lithium can be provided on the negative electrode current collector. Lithium can also be provided on the negative electrode current collector by a gas phase method such as vapor deposition or sputtering. Lithium can also be electrochemically deposited on the negative electrode current collector in a solution containing lithium ions.

[0365] The conductive agent and binder that can be contained in the negative electrode active material layer can be the same materials as the conductive agent and binder that can be contained in the positive electrode active material layer.

[0366] The negative electrode current collector may be made of copper or the like in addition to the same materials as the positive electrode current collector. It is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0367] As another embodiment of the negative electrode of the present invention, a negative electrode having no negative electrode active material can be used. In a secondary battery using a negative electrode having no negative electrode active material, lithium is deposited on the negative electrode current collector during charging, and the lithium on the negative electrode current collector can be eluted during discharging. Therefore, except in a fully discharged state, lithium is present on the negative electrode current collector.

[0368] When a negative electrode having no negative electrode active material is used, a film for uniformly depositing lithium may be provided on the negative electrode current collector. For example, a solid electrolyte having lithium ion conductivity can be used as the film for uniformly depositing lithium. Examples of solid electrolytes that can be used include sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Among these, polymer-based solid electrolytes are suitable as a film for uniformly depositing lithium because they can be formed uniformly on the negative electrode current collector with relative ease.

[0369] Furthermore, when a negative electrode that does not have a negative electrode active material is used, a negative electrode current collector having projections and recesses can be used. When a negative electrode current collector having projections and recesses is used, the recesses of the negative electrode current collector become cavities into which lithium contained in the negative electrode current collector can be easily deposited, and therefore, when lithium is deposited, it is possible to prevent it from forming a dendritic shape.

[0370] [Electrolyte] As one form of electrolyte, an electrolytic solution having a solvent and an electrolyte dissolved in the solvent can be used. The solvent for the electrolytic solution is preferably an aprotic organic solvent, and for example, one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., or two or more of these can be used in any combination and ratio.

[0371] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the electricity storage device from exploding or catching fire even if the internal temperature of the electricity storage device rises due to an internal short circuit or overcharging. Ionic liquids are composed of cations and anions, including organic cations and anions. Examples of organic cations used in the electrolyte include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0372] Examples of the electrolyte to be dissolved in the solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B 12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, lithium bis(oxalato)borate (Li(C2O4)2, LiBOB), etc. can be used alone or in any combination and ratio of two or more of these.

[0373] The electrolyte used in the electricity storage device is preferably a highly purified electrolyte with a low content of granular waste or elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0374] The electrolyte may also contain additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % relative to the solvent in which the electrolyte is dissolved.

[0375] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0376] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0377] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel. For example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0378] [Separator] The separator may be made of, for example, cellulose-containing fibers such as paper, nonwoven fabric, glass fiber, ceramics, or synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane.

[0379] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine-based material, a polyamide-based material, or a mixture of these. Examples of ceramic materials that can be used include aluminum oxide particles and silicon oxide particles. Note that glass-like materials can also be used as ceramic materials, but unlike the glass used in electrodes, they preferably have low electronic conductivity. Examples of fluorine-based materials that can be used include PVDF and polytetrafluoroethylene. Examples of polyamide-based materials that can be used include nylon and aramid (meta-aramid, para-aramid).

[0380] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging 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.

[0381] 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.

[0382] The content of this embodiment mode can be freely combined with the content of other embodiment modes.

[0383] (Fourth embodiment) This embodiment shows an example of fabricating an all-solid-state battery using the positive electrode active material 100 obtained in the above-described embodiment.

[0384] As shown in FIG. 24A, a secondary battery 400 according to one embodiment of the present invention includes a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430.

[0385] Positive electrode 410 has positive electrode current collector 413 and positive electrode active material layer 414. Positive electrode active material layer 414 has positive electrode active material 411 and solid electrolyte 421. Positive electrode active material 411 is made of positive electrode active material 100 obtained in the above-described embodiment. Positive electrode active material layer 414 may also contain a conductive agent and a binder.

[0386] Solid electrolyte layer 420 has solid electrolyte 421. Solid electrolyte layer 420 is located between positive electrode 410 and negative electrode 430, and is a region that has neither positive electrode active material 411 nor negative electrode active material 431.

[0387] The negative electrode 430 has a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 has a negative electrode active material 431 and a solid electrolyte 421. The negative electrode active material layer 434 may also have a conductive agent and a binder. When metallic lithium is used as the negative electrode active material 431, it is not necessary to form the material into particles, and therefore, as shown in FIG. 24B , the negative electrode 430 can be one that does not have a solid electrolyte 421. Using metallic lithium for the negative electrode 430 is preferable because it can improve the energy density of the secondary battery 400.

[0388] As the solid electrolyte 421 of the solid electrolyte layer 420, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, or the like can be used.

[0389] Sulfide-based solid electrolytes include thiolithium-based (Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glasses (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), sulfide crystallized glasses (Li7P3S 11 , Li 3.25 P 0.95 Sulfide-based solid electrolytes have the advantages of being highly conductive, being able to be synthesized at low temperatures, and being relatively soft, which makes it easy to maintain conductive paths even after charging and discharging.

[0390] Oxide-based solid electrolytes include materials with a perovskite crystal structure (La 2 / 3-x Li 3x TiO3, etc.), materials with NASICON-type crystal structure (Li 1-Y Al Y Ti 2-Y (PO4)3, etc.), materials with garnet-type crystal structure (Li7La3Zr2O 12 etc.), materials with LISICON-type crystal structure (Li 14 ZnGeO 16 etc.), LLZO (Li7La3Zr2O 12 ), oxide glasses (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3, etc.), oxide crystallized glasses (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, etc.) are included. Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0391] Halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, LiI, etc. Also, composite materials in which these halide-based solid electrolytes are filled in the pores of porous aluminum oxide or porous silica can be used as solid electrolytes.

[0392] Also, different solid electrolytes may be mixed and used.

[0393] Among them, Li 1+x Al x Ti 2-x (PO4)3 (0 < x < 1) (hereinafter, LATP) contains aluminum and titanium, which are elements that the positive electrode active material used in the secondary battery 400 of one aspect of the present invention may have. Therefore, a synergistic effect can be expected for improving the cycle characteristics, which is preferable. Also, an improvement in productivity due to reduction of processes can be expected. In this specification, etc., the NASICON-type crystal structure refers to a compound represented by M2(XO4)3 (M: transition metal, X: S, P, As, Mo, W, etc.), and has a structure in which MO6 octahedra and XO4 tetrahedra share vertices and are three-dimensionally arranged.

[0394] 〔Shape of the exterior body and the secondary battery〕 For the exterior body of the secondary battery 400 of one aspect of the present invention, various materials and shapes can be used, but it is preferable to have a function of pressing the positive electrode, the solid electrolyte layer, and the negative electrode.

[0395] For example, Figure 25 shows an example of a cell for evaluating materials for all-solid-state batteries.

[0396] 25A is a cross-sectional schematic diagram of the evaluation cell, which has a lower member 761, an upper member 762, and a fixing screw or wing nut 764 that fixes them together, and electrode plate 753 is pressed to fix the evaluation material by rotating a holding screw 763. An insulator 766 is provided between lower member 761 and upper member 762, which are made of stainless steel. An O-ring 765 is also provided between upper member 762 and holding screw 763 to provide a tight seal.

[0397] The evaluation material is placed on electrode plate 751, surrounded by insulating tube 752, and pressed from above by electrode plate 753. An enlarged perspective view of the evaluation material and its surroundings is shown in Figure 25B.

[0398] The evaluation material is an example of a laminate of a positive electrode 750a, a solid electrolyte layer 750b, and a negative electrode 750c, and its cross-sectional view is shown in Fig. 25C. Note that the same reference numerals are used for the same parts in Figs. 25A to 25C.

[0399] The electrode plate 751 and lower member 761, which are electrically connected to the positive electrode 750a, can be said to correspond to a positive electrode terminal. The electrode plate 753 and upper member 762, which are electrically connected to the negative electrode 750c, can be said to correspond to a negative electrode terminal. Electrical resistance and the like can be measured by applying pressure to the evaluation material via the electrode plate 751 and the electrode plate 753.

[0400] The secondary battery of one embodiment of the present invention preferably uses an airtight package for its exterior. For example, a ceramic package or a resin package can be used. Furthermore, the exterior is preferably sealed in a sealed atmosphere, such as in a glove box, while blocking external air.

[0401] Fig. 26A shows a perspective view of a secondary battery of one embodiment of the present invention, which has an exterior body and a shape different from those in Fig. 25. The secondary battery in Fig. 26A has external electrodes 771 and 772 and is sealed in an exterior body having multiple package members.

[0402] An example of a cross section taken along the dashed line in Figure 26A is shown in Figure 26B. A laminate including positive electrode 750a, solid electrolyte layer 750b, and negative electrode 750c is enclosed and sealed within package member 770a, which is a flat plate with electrode layer 773a provided thereon, frame-shaped package member 770b, and package member 770c, which is a flat plate with electrode layer 773b provided thereon. Package members 770a, 770b, and 770c can be made of an insulating material, such as a resin material or ceramic.

[0403] The external electrode 771 is electrically connected to the positive electrode 750a via the electrode layer 773a and functions as a positive electrode terminal, while the external electrode 772 is electrically connected to the negative electrode 750c via the electrode layer 773b and functions as a negative electrode terminal.

[0404] By using the positive electrode active material 100 obtained in the above-described embodiment, an all-solid-state secondary battery having high energy density and good output characteristics can be realized.

[0405] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0406] (Embodiment 5) In this embodiment, an example in which a secondary battery different from the cylindrical secondary battery shown in FIG. 17D is applied to an electric vehicle (EV) is shown with reference to FIG. 27C.

[0407] 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.

[0408] The internal structure of the first battery 1301a may be a wound type as shown in Fig. 18A or 19C, or may be a stacked type as shown in Fig. 20A or 20B. The first battery 1301a may use the all-solid-state battery of Embodiment 4. Using the all-solid-state battery of Embodiment 4 for the first battery 1301a allows for a high capacity, improved safety, and reductions in size and weight.

[0409] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more may be connected in parallel. Also, if the first battery 1301a can store sufficient power, the first battery 1301b may not be necessary. 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.

[0410] In addition, in order to cut off power from a plurality of secondary batteries in a vehicle, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0411] 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 (power steering) 1307, a heater 1308, and a defogger 1309) via a DC-DC circuit 1306. When a rear motor 1317 is provided on the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0412] Furthermore, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0413] The first battery 1301a will be described with reference to FIG. 27A.

[0414] FIG. 27A shows an example in which nine prismatic secondary batteries 1300 are used as one battery pack 1415. Furthermore, nine prismatic secondary batteries 1300 are connected in series, with one electrode fixed by fixing portion 1413 made of an insulator and the other electrode fixed by fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by 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 fixing portions 1413 and 1414 and a battery housing box. Furthermore, one electrode is electrically connected to control circuit unit 1320 by wiring 1421. Furthermore, the other electrode is electrically connected to control circuit unit 1320 by wiring 1422.

[0415] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit unit 1320. A charge control circuit or a battery control system having 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.

[0416] 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 M is one or more elements selected from aluminum, gallium, tin, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium) can 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 Crystalline 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. A CAAC-OS is an oxide semiconductor having multiple crystalline regions, each of which has a c-axis aligned in a specific direction. The specific direction refers to the thickness direction of the CAAC-OS film, the normal direction to the surface where the CAAC-OS film is formed, or the normal direction to the surface of the CAAC-OS film. A crystalline region is a region with periodic atomic arrangement. If the atomic arrangement is considered as a lattice arrangement, a crystalline region is also a region with a uniform lattice arrangement. Furthermore, the CAAC-OS has a region where multiple crystalline regions are connected in the ab-plane direction, and the region may have distortion. The distortion refers to a location where the lattice arrangement changes between a region with a uniform lattice arrangement and a region with a different uniform lattice arrangement in the region where multiple crystalline regions are connected. In other words, the CAAC-OS is an oxide semiconductor that is c-axis oriented but not clearly oriented in the ab-plane direction. The CAC-OS is, for example, a material in which elements constituting a metal oxide are unevenly distributed within a size range of 0.5 nm to 10 nm, preferably 1 nm to 3 nm, or in the vicinity thereof. In the following, a state in which one or more metal elements are unevenly distributed in a metal oxide and regions containing the metal elements are mixed in a size of 0.5 nm to 10 nm, preferably 1 nm to 3 nm, or a size close to this size, is also referred to as a mosaic or patch state.

[0417] Furthermore, CAC-OS has a mosaic structure in which the material is separated into first and second regions, and the first regions are 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 and second regions are mixed.

[0418] Here, the atomic ratios of In, Ga, and Zn to the metal elements constituting the CAC-OS in the In-Ga-Zn oxide are denoted as [In], [Ga], and [Zn], respectively. For example, in the CAC-OS in the In-Ga-Zn oxide, the first region is a region where [In] is larger than [In] in the composition of the CAC-OS film. The second region is a region where [Ga] is larger than [Ga] in the composition of the CAC-OS film. Alternatively, for example, the first region is a region where [In] is larger than [In] in the second region and [Ga] is smaller than [Ga] in the second region. The second region is a region where [Ga] is larger than [Ga] in the first region and [In] is smaller than [In] in the first region.

[0419] Specifically, the first region is a region whose main component is indium oxide, indium zinc oxide, or the like. The second region is a region whose main component is gallium oxide, gallium zinc oxide, or the like. In other words, the first region can be rephrased as a region whose main component is In. The second region can be rephrased as a region whose main component is Ga.

[0420] It should be noted that there are cases where a clear boundary between the first region and the second region cannot be observed.

[0421] For example, in the case of CAC-OS in In-Ga-Zn oxide, EDX mapping obtained using EDX (Energy Dispersive X-ray spectroscopy) confirms that the CAC-OS has a structure in which a region mainly composed of In (first region) and a region mainly composed of Ga (second region) are unevenly distributed and mixed.

[0422] When CAC-OS is used in a transistor, the conductivity due to the first region and the insulating property due to the second region act in a complementary manner, thereby providing the CAC-OS with a switching function (the ability to turn on and off). In other words, CAC-OS has a conductive function in part of the material and an insulating function in part of the material, and the material as a whole functions as a semiconductor. By separating the conductive function from the insulating function, both functions can be maximized. Therefore, by using CAC-OS in a transistor, a high on-current (I on ), high field-effect mobility (μ), and good switching behavior can be achieved.

[0423] Oxide semiconductors have a variety of structures, each with 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, a nanocrystalline oxide semiconductor (nc-OS), and a CAAC-OS.

[0424] Furthermore, because the control circuit unit 1320 can be used in high-temperature environments, it is preferable to use transistors using oxide semiconductors. To simplify the process, the control circuit unit 1320 may be formed using unipolar transistors. Transistors using oxide semiconductors in the semiconductor layer have a wider operating ambient temperature range than single-crystal Si transistors, from -40°C to 150°C, and their characteristics change less when the secondary battery is heated than single-crystal Si transistors. The off-current of transistors using oxide semiconductors 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 single-crystal Si transistors increases, and the current on / off ratio is not sufficiently large. The control circuit unit 1320 can improve safety. Furthermore, combining the cathode active material 100 obtained in the above-described embodiment with a secondary battery using the cathode as its cathode can provide a synergistic effect in terms of safety.

[0425] 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 that eliminate causes of secondary battery instability include overcharging prevention, overcurrent prevention, overheating control during charging, maintaining cell balance 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. The control circuit unit 1320 has at least one of these functions. Furthermore, the automatic control device for the secondary battery can be ultra-miniaturized.

[0426] A micro-short circuit refers to a tiny short circuit within a secondary battery, which is not so severe that the positive and negative electrodes of the secondary battery are short-circuited and render it unable to be charged or discharged, but rather a small short-circuit current flows through the tiny short-circuited area. Even a small area can cause a large voltage change over a relatively short period of time, and this abnormal voltage value may affect the estimation of the subsequent charge / discharge state of the secondary battery.

[0427] 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.

[0428] 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.

[0429] FIG. 27B shows an example of a block diagram of the battery pack 1415 shown in FIG. 27A.

[0430] The control circuit unit 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 unit 1320 sets upper and lower voltage limits for the secondary battery used and limits the upper limit of the external current and the upper limit of the output current. The range between the lower and upper voltage limits of the secondary battery is within the recommended voltage range. If the voltage falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit unit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and 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. A PTC element may also be provided in the charge / discharge path to provide a function for cutting off the current in response to a rise in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0431] The switch unit 1324 can be configured by combining n-channel transistors and p-channel transistors. The switch unit 1324 is not limited to a switch having a Si transistor using single crystal silicon, and may be configured using, for example, Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), GaO x The switch unit 1324 may be formed using a power transistor having gallium oxide (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, which facilitates integration. Furthermore, OS transistors can be manufactured using the same manufacturing equipment as Si transistors, which allows for low-cost manufacturing. Specifically, a control circuit unit 1320 using OS transistors can be stacked on the switch unit 1324 and integrated into a single chip. The volume occupied by the control circuit unit 1320 can be reduced, enabling miniaturization.

[0432] The first batteries 1301a and 1301b mainly supply power to 42V (high voltage) in-vehicle devices, and the second battery 1311 supplies power to 14V (low voltage) in-vehicle devices.

[0433] In this embodiment, an example in which lithium ion secondary batteries are used for both the first battery 1301a and the second battery 1311 is shown. The second battery 1311 may be a lead storage battery, an all-solid-state battery, or an electric double layer capacitor. For example, the all-solid-state battery of Embodiment 4 may be used. By using the all-solid-state battery of Embodiment 4 for the second battery 1311, high capacity can be achieved, and miniaturization and weight reduction can be achieved.

[0434] Furthermore, regenerated 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 from the motor controller 1303 and 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 regenerated energy, it is desirable that the first batteries 1301a and 1301b be capable of being rapidly charged.

[0435] 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 according to the charging characteristics of the secondary battery used, and can perform rapid charging.

[0436] Although not shown, when the electric vehicle is connected to an external charger, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. The power supplied from the external charger is charged to the first batteries 1301a, 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the function of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a, 1301b via a control circuit unit 1320 to prevent overcharging. In some cases, the charger's outlet or the charger's connection cable is provided with a control circuit. The control circuit unit 1320 is also 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.

[0437] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging is also possible by receiving power from external charging equipment using methods such as contactless power supply.

[0438] When rapid charging is performed, a secondary battery that can withstand high voltage charging is desired in order to charge in a short time.

[0439] The secondary battery of the present embodiment described above uses the positive electrode active material 100 obtained in the above-described embodiment. Furthermore, by using graphene as a conductive agent, a secondary battery with significantly improved electrical characteristics can be realized, as a synergistic effect of suppressing capacity reduction and maintaining high capacity even when the electrode layer is thickened and the loading amount is increased. This is particularly effective for secondary batteries used in vehicles, and can provide vehicles with a long driving range, specifically a driving range 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.

[0440] In particular, the secondary battery of the present embodiment described above can increase the operating voltage of the secondary battery by using the cathode active material 100 described in the above embodiment, and the usable capacity can be increased as the charging voltage increases. Furthermore, by using the cathode active material 100 described in the above embodiment for the cathode, a secondary battery for vehicles with excellent cycle characteristics can be provided.

[0441] 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.

[0442] 17D, 19C, and 27A, next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs) can be realized. Secondary batteries can also be installed in transportation vehicles such as agricultural machinery, motorized bicycles including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing 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.

[0443] 28A to 28D illustrate a transportation vehicle as an example of a moving object using one embodiment of the present invention. An automobile 2001 illustrated in FIG. 28A is an electric automobile that uses an electric motor as a power source for traveling. Alternatively, the automobile 2001 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 3 is installed in one or more locations. The automobile 2001 illustrated in FIG. 28A includes a battery pack 2200, which includes a secondary battery module to which multiple secondary batteries are connected. It is preferable that the automobile 2001 further includes a charge control device electrically connected to the secondary battery module.

[0444] Furthermore, automobile 2001 can charge its secondary battery by receiving power supply from an external charging facility using a plug-in method, a contactless power supply method, or the like. Charging can be performed using a predetermined charging method and connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The charging device may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge an electricity storage device installed in 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.

[0445] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground power transmitting device for charging. In the case of this contactless power supply method, by incorporating a power transmitting device into the 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, and the secondary battery can be charged while the vehicle is stopped and while moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0446] 28B 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 3.0V to 5.0V, with 48 cells connected in series for a maximum voltage of 170V. 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. 28A, and therefore a description thereof will be omitted.

[0447] FIG. 28C 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, one hundred or more secondary batteries with a nominal voltage of 3.0 V to 5.0 V connected in series, with a maximum voltage of 600 V. By using a secondary battery whose positive electrode is the positive electrode active material 100 described in the above embodiment, a secondary battery with excellent rate characteristics and charge / discharge cycle characteristics can be manufactured, contributing to improved performance and a longer life of the transport vehicle 2003. Furthermore, except for the number of secondary batteries constituting the secondary battery module of the battery pack 2202, the same functions as those shown in FIG. 28A are provided, and therefore a description thereof will be omitted.

[0448] As an example, Fig. 28D shows an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 28D has wheels for takeoff and landing, it can also be considered a type of transportation vehicle, and has a battery pack 2203 that includes a secondary battery module formed by connecting multiple secondary batteries and includes the secondary battery module and a charge control device.

[0449] The secondary battery module of the aircraft 2004 has, for example, eight 4V secondary batteries connected in series to produce a maximum voltage of 32V. Other than the number of secondary batteries constituting the secondary battery module of the battery pack 2203, it has the same functions as those in Fig. 28A, and therefore a description thereof will be omitted.

[0450] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0451] (Sixth embodiment) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted in a building will be described with reference to FIGS. 29A and 29B.

[0452] 29A 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 through a wiring 2611 or the like. The power storage device 2612 may be electrically connected to a ground-mounted charging device 2604. The power storage device 2612 can be charged with power obtained by the solar panel 2610. The power stored in the power storage device 2612 can be charged to a secondary battery included in a 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.

[0453] 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.

[0454] 29B illustrates an example of a power storage device according to one embodiment of the present invention. As illustrated in FIG. 29B, 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 5 may be provided in the power storage device 791. The power storage device 791 can have a long lifetime by using a secondary battery in which the positive electrode active material 100 obtained in the above embodiment is used for its positive electrode.

[0455] A control device 790 is installed in the power storage device 791, and the control device 790 is electrically connected to a distribution board 703, a power storage controller 705 (also called a control device), a display 706, and a router 709 by wiring.

[0456] Electric power is sent from commercial power source 701 to distribution board 703 via service line attachment section 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 an outlet (not shown).

[0457] 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.

[0458] The power storage controller 705 has 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 one day (for example, 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.

[0459] 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 electrical appliances such as televisions and personal computers via the router 709. It can also be confirmed on portable electronic devices such as smartphones and tablets 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 appliances, and the portable electronic devices.

[0460] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0461] (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.

[0462] 30A is 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 shown in FIG. 30A. The power storage device of one embodiment of the present invention includes, for example, a plurality of storage batteries and a protection circuit.

[0463] 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. 30B . The power storage device 8702 includes a plurality of built-in storage batteries 8701, which are included in the power storage device of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit 8703. The power storage device 8702 also includes a control circuit 8704 that can control charging or detect an abnormality of the secondary battery, as shown as an example in Embodiment 5. The control circuit 8704 is electrically connected to the positive and negative electrodes of the storage battery 8701. The control circuit 8704 may be provided with the small solid-state secondary battery shown in FIGS. 26A and 26B. By providing the small solid-state secondary battery shown in FIGS. 26A and 26B in the control circuit 8704, power can be supplied to retain data in a memory circuit included in the control circuit 8704 for a long period of time. Furthermore, a synergistic effect in terms of safety can be obtained by combining the positive electrode active material 100 obtained in the above-described embodiment with a secondary battery using the positive electrode active material 100 obtained in the above-described embodiment. The secondary battery using the positive electrode active material 100 obtained in the above-described embodiment and the control circuit 8704 can greatly contribute to eliminating accidents such as fires caused by secondary batteries.

[0464] 30C is an example of a two-wheeled vehicle including a power storage device of one embodiment of the present invention. A scooter 8600 shown in FIG. 30C 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 the above embodiment for its positive electrode, and thus can have a high capacity, which can contribute to miniaturization.

[0465] 30C, the power storage device 8602 can be stored in the under-seat storage 8604. The power storage device 8602 can be stored in the under-seat storage 8604 even if the under-seat storage 8604 is small.

[0466] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0467] (Embodiment 8) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted in an electronic device will be described. Examples of electronic devices in which a secondary battery is mounted 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.

[0468] 31A shows an example of a mobile phone. Mobile phone 2100 includes a display unit 2102 built into housing 2101, operation buttons 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that mobile phone 2100 also includes secondary battery 2107. By including secondary battery 2107 using positive electrode active material 100 described in the above embodiment as a positive electrode, high capacity can be achieved, and a configuration that can accommodate space savings associated with miniaturization of housings can be realized.

[0469] 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.

[0470] 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 built into the mobile phone 2100.

[0471] 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.

[0472] The mobile phone 2100 also has an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Charging may also be performed by wireless power supply without using the external connection port 2104.

[0473] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0474] FIG. 31B shows an unmanned aerial vehicle 2300 having multiple 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 the above-described embodiment as a positive electrode has a high energy density and is highly safe, and therefore can be used safely for a long period of time. Therefore, the secondary battery is suitable as a secondary battery to be installed in the unmanned aerial vehicle 2300.

[0475] Fig. 31C shows an example of a robot. A robot 6400 shown in Fig. 31C 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.

[0476] The microphone 6402 has a function of detecting the user's 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.

[0477] 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.

[0478] 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.

[0479] 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 the above embodiment 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 6409 is suitable for the robot 6400.

[0480] 31D shows an example of a cleaning robot. The cleaning robot 6300 has a display unit 6302 arranged on the top surface of a housing 6301, multiple cameras 6303 arranged on the side, a brush 6304, an operation button 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.

[0481] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 to determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, if 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 the above embodiment 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.

[0482] Figure 32A 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.

[0483] For example, a secondary battery according to one embodiment of the present invention can be mounted on an eyeglasses-type device 4000 as shown in FIG. 32A . The eyeglasses-type device 4000 includes a frame 4000a and a display unit 4000b. Mounting a secondary battery on temples of the curved frame 4000a makes it possible to provide an eyeglasses-type device 4000 that is lightweight, has a good weight balance, and can be used for a long time. A secondary battery using the positive electrode active material 100 obtained in the above embodiment for its positive electrode has a high energy density and can realize a configuration that can accommodate space saving associated with a smaller housing.

[0484] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a 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 the above embodiment as a positive electrode has a high energy density and can realize a configuration that can accommodate space saving associated with a miniaturized housing.

[0485] 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 the above embodiment for its positive electrode has high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0486] 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 the above embodiment for its positive electrode has high energy density, and a configuration that can accommodate space saving associated with a miniaturized housing can be realized.

[0487] Furthermore, the secondary battery according to 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. The secondary battery using the positive electrode active material 100 obtained in the above embodiment as a positive electrode has high energy density, and can realize a configuration that can accommodate space saving associated with miniaturization of the housing.

[0488] Furthermore, the secondary battery according to one embodiment of the present invention can be mounted on a wristwatch device 4005. The wristwatch device 4005 includes 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 the above embodiment as its positive electrode has high energy density and can achieve a space-saving configuration that can be achieved by miniaturizing the housing.

[0489] The display unit 4005a can display not only the time but also various other information such as incoming emails and phone calls.

[0490] Furthermore, since the wristwatch 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.

[0491] FIG. 32B shows a perspective view of the wristwatch type device 4005 removed from the wrist.

[0492] 32C shows a side view of the display portion 4005a. FIG. 32C 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 3. The secondary battery 913 is provided at a position overlapping with the display portion 4005a, and can have high density and high capacity, and is small and lightweight.

[0493] Since the wristwatch-type device 4005 is required to be small and lightweight, by using the positive electrode active material 100 obtained in the above-described embodiment for the positive electrode of the secondary battery 913, it is possible to obtain a high-energy density and small-sized secondary battery 913.

[0494] 32D shows an example of a wireless earphone, which is shown here as having a pair of bodies 4100a and 4100b, but this does not necessarily have to be a pair.

[0495] The main bodies 4100a and 4100b each have a driver unit 4101, an antenna 4102, and a secondary battery 4103. They may also have a display unit 4104. They also preferably have a substrate on which a circuit such as a wireless IC is mounted, a charging terminal, and the like. They may also have a microphone.

[0496] The case 4110 has a secondary battery 4111. It also preferably has a board on which circuits such as a wireless IC and a charge control IC are mounted, and a charging terminal. It may also have a display unit, buttons, etc.

[0497] The main units 4100a and 4100b can wirelessly communicate with other electronic devices such as smartphones. This allows sound data and the like sent from other electronic devices to be played back on the main units 4100a and 4100b. If the main units 4100a and 4100b have microphones, they can send sounds picked up by the microphones to the other electronic devices, and the sound data after processing by the electronic devices can be sent back to the main units 4100a and 4100b for playback. This allows them to be used as, for example, translation devices.

[0498] Furthermore, the secondary battery 4111 in the case 4110 can charge the secondary battery 4103 in the main body 4100a. The coin-type secondary battery, the cylindrical secondary battery, or the like described in the above embodiment can be used as the secondary battery 4111 and the secondary battery 4103. A secondary battery using the positive electrode active material 100 obtained in the above embodiment as a positive electrode has high energy density, and by using the secondary battery 4103 and the secondary battery 4111, a configuration that can accommodate space saving associated with miniaturization of wireless earphones can be realized.

[0499] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Example]

[0500] In this example, a positive electrode active material 100 according to one embodiment of the present invention was fabricated and its characteristics were analyzed.

[0501] Samples A to C fabricated in this example will be described with reference to the fabrication method shown in FIG. 1 and FIGS. 2A to 2C.

[0502] <Sample A> For the LiMO2 in step S14 in Figure 1, commercially available lithium cobalt oxide (Cellseed C-10N, manufactured by Nippon Chemical Industry Co., Ltd.) was prepared, which had cobalt as the transition metal M and no additional elements. For the heating in step S15, this lithium cobalt oxide was placed in a sheath, covered, and heated in a muffle furnace at 850°C for 2 hours. After the muffle furnace was conditioned to an oxygen atmosphere, no oxygen flow occurred (O2 purging). Checking the amount recovered after the initial heating revealed a slight weight loss. This weight loss may have been due to the removal of impurities from the LCO.

[0503] An additive element X source was prepared according to steps S22, S23, and S24 shown in FIG. 2A. BaF2 and LiF were prepared as additive element X sources, and were weighed out so that BaF2:LiF was 3:1 (molar ratio), in accordance with the compounding ratio of Sample A shown in Table 4. Next, BaF2 and LiF were mixed in dehydrated acetone and stirred at a rotation speed of 400 rpm for 12 hours to obtain additive element X source. A The powder was then sieved through a sieve with 300 μm openings to obtain additive element source X with a uniform particle size.

[0504] An additive element Y source was prepared according to steps S25, S26, and S27 shown in FIG. 2B. For the additive element Y source, LiF was prepared as the Li source, and MgF2 was prepared as the Mg source. These were weighed out so that MgF2:LiF was 3:1 (molar ratio), according to the compounding ratio of Sample A shown in Table 4. Next, LiF and MgF2 were mixed in dehydrated acetone and stirred at a rotation speed of 400 rpm for 12 hours to prepare additive element source Y. After that, the mixture was sieved with a sieve having 300 μm openings to obtain additive element source Y with a uniform particle size.

[0505] Next, according to step S31 shown in FIG. 1, BaF2 contained in the additive element X source and MgF2 contained in the additive element Y source were weighed out so that the total amount was 1 at% relative to the cobalt contained in the LCO, and this was dry-mixed with the LCO after the initial heating according to the compounding ratio of Sample A shown in Table 4. In Sample A, the molar ratio of BaF2 to MgF2 was 1:1. The mixture was stirred at a rotation speed of 150 rpm for 1 hour, which is a gentler stirring condition than when obtaining the additive element X source or the additive element Y source. Finally, the mixture was sieved with a 300 μm mesh to obtain a first mixture A with a uniform particle size.

[0506] [Table 4]

[0507] Next, the first mixture A was heated. The heating conditions were 900°C and 20 hours. During heating, a lid was placed on the sheath containing the first mixture A. The inside of the sheath was filled with an oxygen-containing atmosphere, and the inflow and outflow of the oxygen was blocked (purged). By heating, an LCO (composite oxide A) containing Ba and Mg was obtained.

[0508] Next, the source of additive element Z is added to composite oxide A. Following step S41 shown in FIG. 2C, Ni(OH)2 was prepared as the Ni source and Al(OH)3 was prepared as the Al source. Ni(OH)2 and Al(OH)3 were weighed out so that they accounted for 0.5 at% of the transition metal M, and 0.5 at% of the transition metal M, respectively, and then dry-mixed with composite oxide A. The mixture was stirred at a rotation speed of 150 rpm for 1 hour. This was a gentler stirring condition than when obtaining the source of additive element X or the source of additive element Y. Finally, the mixture was sieved through a sieve with 300 μm openings to obtain a second mixture A with a uniform particle size.

[0509] Next, the second mixture A was heated. The heating conditions were 850°C and 10 hours. During heating, a lid was placed on the sheath containing the second mixture A. The inside of the sheath was filled with an oxygen-containing atmosphere, and the inflow and outflow of the oxygen was blocked (purged). By heating, an LCO containing Ba, Mg, F, Ni, and Al was obtained. The positive electrode active material obtained in this manner was designated Sample A.

[0510] <Sample B> Sample B was prepared in the same manner as Sample A, except that the additive element X source and the additive element Y source were mixed in the blending ratios shown in Table 4 for Sample B when mixed with LCO after the initial heating.

[0511] <Sample C> Sample C was prepared in the same manner as Sample A, except that the additive element X source and additive element Y source were mixed in the same proportions as Sample D shown in Table 4 when mixed with LCO after the initial heating.

[0512] <sem> Figures 33A to 33C show the SEM (Scanning Electron Microscope) observation results of Samples A to C. The SEM observation in this example was performed using a scanning electron microscope S4800 manufactured by Hitachi High-Tech Corporation, and the measurement conditions were an acceleration voltage of 5 kV and a magnification of 10,000 times.

[0513] From the SEM observation results, it was found that the surfaces of Sample A and Sample B prepared using an additive element X source containing Ba were very smooth. Since the surfaces are very smooth, when the positive electrode is fabricated using Sample A and Sample B in a later process, it is expected that when pressing the positive electrode active material layer, slippage between particles is easy, and thus the occurrence of cracks or slips in the positive electrode active material particles can be suppressed.

[0514] <STEM and EDX (Energy Dispersive X-ray Analysis)> Next, STEM observation, line analysis, and surface analysis by STEM-EDX were performed on the surface layer of Sample A. As the STEM apparatus and STEM-EDX apparatus, HD-2700 manufactured by Hitachi High-Technologies Corporation was used, and the measurement conditions were an acceleration voltage of 200 kV and a magnification of 100,000 times. Figure 34A shows a cross-sectional STEM image (ZC image) of Sample A (LCO having Ba, Mg, and Al as additive elements).

[0515] Figures 34B1 to 34B4 show the results of element mapping in STEM-EDX surface analysis in the observation region shown in Figure 34A. Figure 34B1 shows Co, Figure 34B2 shows Mg, Figure 34B3 shows Al, and Figure 34B4 shows Ba. It can be seen that a large amount of Ba, Mg, and Al are present in the surface layer of LCO, which is Sample A. In Figures 34B1 to 34B4, the brightness and darkness of the element mapping images are normalized according to the detected amount of characteristic X-rays for each element.

[0516] FIG. 34C shows the results of STEM-EDX analysis of the area between A and B shown in FIG. 34A. FIG. 34C shows the distributions of Co, Ba, Mg, and Al. Similar to the results shown in FIGS. 34B1 to 34B4, it can be seen that large amounts of Ba, Mg, and Al are present in the surface layer of LCO sample A. FIG. 35 shows the distribution of Co and each of Ba, Mg, and Al individually from the results of STEM-EDX analysis shown in FIG. 34C. FIG. 35A shows the distributions of Co and Ba, FIG. 35B shows the distributions of Co and Mg, and FIG. 35C shows the distributions of Co and Al. A closer look at the distributions of Ba, Mg, and Al inside the particle reveals that the points where the characteristic X-ray detection value of Ba is maximum and the points where the characteristic X-ray detection value of Mg is maximum are closer to the surface than the points where the characteristic X-ray detection value of Al is maximum. In other words, the peaks of Ba and Mg concentrations are closer to the surface than the peak of Al concentration. It can also be seen that the distributions of Ba, Mg, and Al have overlapping regions. The Ba and Mg concentration peaks were found within 10 nm from the LCO surface, and the Al concentration peak was found within 20 nm from the LCO surface. Therefore, it can be said that Ba and Mg are preferably present closer to the surface of the positive electrode active material than Al in the surface layer portion of the positive electrode active material. In other words, it can be said that Al is preferably present closer to the interior of the positive electrode active material than Ba ​​and Mg in the surface layer portion of the positive electrode active material. It can also be said that the distributions of Ba, Mg, and Al preferably have overlapping regions in the surface layer portion of the positive electrode active material. It can also be said that the surface layer portion of the positive electrode active material preferably has, from the surface to the interior, a region where the distributions of Ba, Mg, and Al overlap, a region where the distributions of Mg and Al overlap, and a region containing Al, in the above order. [Example]

[0517] <Half-cell charge / discharge cycle characteristics> In this example, a half cell was assembled using a positive electrode active material according to one embodiment of the present invention, and cycle characteristics were evaluated. The performance of the positive electrode alone was determined by evaluating the cycle characteristics of the half cell.

[0518] First, half cells were assembled using the positive electrode active materials of Samples A to C shown in Example 1. The conditions for the half cells are explained below.

[0519] The positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 95:3:2 to prepare a slurry, which was then applied to an aluminum current collector using NMP as the solvent.

[0520] After the slurry was applied to the current collector, the solvent was evaporated. A positive electrode was obtained by the above process. The amount of active material carried on the positive electrode was approximately 7 mg / cm. 2 It was decided.

[0521] The electrolyte used was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC) added as an additive. The electrolyte contained 1 mol / L lithium hexafluorophosphate (LiPF6). Polypropylene was used for the separator.

[0522] Metallic lithium was prepared as the counter electrode, and a coin-shaped half cell equipped with the above positive electrode and other components was formed, and the cycle characteristics were measured.

[0523] We will now explain the discharge rate and charge rate of the cycle conditions. The discharge rate is the relative ratio of the current during discharge to the battery capacity, and is expressed in units of C. For a battery with a rated capacity of X (Ah), the current equivalent to 1C is X (A). When a battery is discharged at a current of 2X (A), it is said to have been discharged at 2C, and when a battery is discharged at a current of X / 5 (A), it is said to have been discharged at 0.2C. The same applies to the charge rate; when a battery is charged at a current of 2X (A), it is said to have been charged at 2C, and when a battery is charged at a current of X / 5 (A), it is said to have been charged at 0.2C.

[0524] The cycle characteristics are shown in Figures 36A to 38B. Charging was performed at a constant current of 0.5 C up to 4.60 V, 4.65 V, or 4.7 V, followed by constant voltage charging until the current value reached 0.05 C. Discharging was performed at a constant current of 0.5 C down to 2.5 V. Here, 1 C was set to 200 mA / g. Two temperature conditions were used: 25°C and 45°C. Charge and discharge were repeated 50 times in this manner.

[0525] Figures 36A to 38B show the results of charge-discharge cycle tests. Figure 36A shows the results for a charge voltage of 4.60 V and a measurement temperature of 25°C. Figure 36B shows the results for a charge voltage of 4.60 V and a measurement temperature of 45°C. Figure 37A shows the results for a charge-discharge voltage of 4.65 V and a measurement temperature of 25°C. Figure 37B shows the results for a charge-discharge voltage of 4.65 V and a measurement temperature of 45°C. Figure 38A shows the results for a charge-discharge voltage of 4.70 V and a measurement temperature of 25°C. Figure 38B shows the results for a charge-discharge voltage of 4.70 V and a measurement temperature of 45°C. All results are graphs showing the change in discharge capacity versus the number of cycles. The horizontal axis of the graph represents the number of cycles, and the vertical axis represents the discharge capacity retention rate (%; the maximum discharge capacity after 50 cycles is set to 100%). Evaluation results for coin cells using Samples A to C are shown in Table 5, Table 6, and Table 7, respectively.

[0526] [Table 5]

[0527] [Table 6]

[0528] [Table 7]

[0529] Looking at the discharge capacity retention rates after 50 cycles shown in Table 7, it was confirmed that Samples A and B exhibited good characteristics in terms of resistance to deterioration in the harsh environment of high temperature (45°C) and high charging voltage, and that Sample A in particular exhibited excellent characteristics.

[0530] Figures 39A to 41B are graphs related to the cycle characteristics shown in Figures 36A to 38B. Each of Figures 39A to 41B shows the charge and discharge curves of Sample A from cycle 1 to cycle 50, overlapping each other. Figure 39A shows the results for a charge voltage of 4.60 V and a measurement temperature of 25°C. Figure 39B shows the results for a charge voltage of 4.60 V and a measurement temperature of 45°C. Figure 40A shows the results for a charge / discharge voltage of 4.65 V and a measurement temperature of 25°C. Figure 40B shows the results for a charge / discharge voltage of 4.65 V and a measurement temperature of 45°C. Figure 41A shows the results for a charge / discharge voltage of 4.70 V and a measurement temperature of 25°C. Figure 41B shows the results for a charge / discharge voltage of 4.70 V and a measurement temperature of 45°C. The arrows in the figures indicate the direction of change in the charge and discharge curves as the number of charge / discharge cycles increases. Focusing on the change in the discharge curve, the tendency of change is that under the five conditions other than 45°C and 4.70V, no significant change in the shape of the discharge curve was observed while the discharge capacity decreased, but under the condition of 45°C and 4.70V, the shape of the discharge curve became significantly flattened while the discharge capacity decreased, and the discharge voltage decreased overall. From this, it is inferred that there is a possibility that the internal resistance of the secondary battery will increase significantly when charge-discharge cycles are performed under the conditions of 45°C and 4.70V.

[0531] <Defects in the positive electrode active material after cycle testing> The cross section of the positive electrode after 50 cycles was observed using a scanning transmission electron microscope (STEM). The sample was processed using an FIB for cross-sectional observation. The results of cross-sectional STEM observation of Sample A after 50 cycles at 45°C and 4.70V are shown in Figures 42A to 42C. Figure 42C is an enlarged image of the area surrounded by the solid line in Figure 42B, and Figure 42B is an enlarged image of the area surrounded by the solid line in Figure 42A. To obtain the cross-sectional STEM image, a Hitachi High-Tech HD-2700 was used, and the accelerating voltage was set to 200 kV.

[0532] 42A to 42C, no closed cracks were observed inside the positive electrode active material of Sample A after 50 cycles at 45°C and 4.70V, demonstrating a more stable structure than the Ba-free positive electrode active material shown in Figures 10 to 12. It is known that Ba is present in large amounts in the surface layer after the production of the positive electrode active material. However, considering that the occurrence of closed cracks inside the positive electrode active material after cycle degradation at 45°C and 4.70V was suppressed as shown in Figures 42A to 42C, it is possible that Ba was dispersed in the LCO bulk in an amount undetectable at the STEM-EDX level (1 atomic % or less), and that the structural stabilization effect of Ba suppressed the occurrence of closed cracks inside the active material. [Explanation of symbols]

[0533] 100: positive electrode active material, 100a: surface layer portion, 100b: interior portion, 101: grain boundary, 102: embedded portion, 103: unevenly distributed portion, 200: positive electrode active material layer, 201: graphene compound, 903: mixture, 904: mixture< / sem> < / xrd>

Claims

1. A lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, the positive electrode has a positive electrode active material including lithium and cobalt, the positive electrode active material has a layered rock salt crystal structure, In STEM-EDX ray analysis of the positive electrode active material, barium and magnesium concentration peaks have a region located closer to the surface of the positive electrode active material than an aluminum concentration peak; In the lithium ion secondary battery, in the STEM-EDX analysis of the positive electrode active material, the distributions of barium, magnesium, and aluminum have overlapping regions.

2. In claim 1, the positive electrode active material has a grain boundary, The lithium-ion secondary battery, wherein the concentration of barium, the concentration of magnesium, and the concentration of aluminum at and near the grain boundary are higher than those in other regions inside the positive electrode active material.

3. In claim 1, the positive electrode active material has a grain boundary, The lithium-ion secondary battery, wherein the concentration of fluorine at and near the grain boundaries is higher than that in other regions inside the positive electrode active material.

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