Secondary battery

Abstract

Provided is a secondary battery that has good interfacial contact between an active material and an electrolyte. The secondary battery has a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer. The positive electrode layer has a positive-electrode active material and a first solid electrolyte. The negative electrode layer has a negative-electrode active material and a second solid electrolyte. The electrolyte layer has a third solid electrolyte and an ionic liquid. The ionic liquid is impregnated in gaps in the third solid electrolyte. The secondary battery is bendable.

Description

Secondary batteries, electronic devices and aircraft

[0001] One aspect of the present invention relates to a secondary battery, an electronic device, and an aircraft.

[0002] 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.

[0003] In this specification, electronic equipment refers to devices in general that have secondary batteries, and includes electro-optical devices that have secondary batteries, information terminal devices that have secondary batteries, and the like.

[0004] In recent years, there has been active development of various types of electricity storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. In particular, demand for high-power, high-capacity lithium-ion secondary batteries has rapidly expanded in line with the development of the semiconductor industry, and they have become indispensable in today's information society as a rechargeable energy source.

[0005] Most lithium-ion batteries currently in use use an electrolyte (also called an organic electrolyte) in which lithium salts are dissolved in polar organic solvents. However, because these organic solvents are flammable, secondary batteries using these electrolytes pose a risk of fire or ignition.

[0006] Large-scale secondary batteries used in automobiles and other applications require high reliability, especially safety. Therefore, solid-state batteries, which have a solid electrolyte between the positive and negative electrodes instead of a liquid electrolyte, are being considered. Solid electrolytes can be broadly divided into organic and inorganic types.

[0007] As an inorganic solid electrolyte, for example, Patent Document 1 discloses a secondary battery having a sulfide-based solid electrolyte or an oxide-based solid electrolyte. Also, Non-Patent Documents 1 to 3 describe changes in the crystal structure of lithium cobalt oxide.

[0008] JP 2012-14892 A

[0009] Motohashi, T. et al, “Electronic phase diagram of the layered cobalt oxide system Li▲x▼CoO▲2▼(0.0≦x≦1.0)”, Physical Review B, 80 (16); 165114 Zhaohui Chen et al, “Staging Phase Transitions in Li▲x▼CoO▲2▼”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609Belsky, A. et al. , “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Crystal. , (2002) B58 364-369.

[0010] In Patent Literature 1, a nonaqueous electrolyte battery is proposed in which an intervening layer is provided between a negative electrode active material layer and a solid electrolyte layer, in consideration of the fact that the volume change (expansion / contraction) of the negative electrode active material layer is large, which reduces the bonding strength between the negative electrode active material layer and the solid electrolyte layer and increases the resistance to lithium ion migration at the interface between the two layers. Patent Literature 1 describes that the intervening layer is made of a polymer containing a lithium salt or an ionic liquid.

[0011] However, when an intermediate layer is provided, there arises a problem of interfacial contact between the negative electrode active material layer and the intermediate layer, or between the solid electrolyte layer and the intermediate layer. Interfacial contact is sometimes referred to as interfacial resistance.

[0012] Therefore, an object of one embodiment of the present invention is to improve contact at an interface in a secondary battery, for example, at an interface between an active material and an electrolyte.Another object of one embodiment of the present invention is to provide a secondary battery with improved safety.Another object of one embodiment of the present invention is to provide a flexible secondary battery.

[0013] Note that the description of these problems does not preclude the existence of other problems. Furthermore, these problems are considered to be independent of each other, and one embodiment of the present invention does not necessarily solve all of these problems. Furthermore, problems other than these can be extracted from the description of the specification, drawings, and claims in this specification and the like.

[0014] In order to solve the above problems, one aspect of the present invention is a secondary battery including a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer includes a positive electrode active material and a first solid electrolyte, the negative electrode layer includes a negative electrode active material and a second solid electrolyte, and the electrolyte layer includes a third solid electrolyte and an ionic liquid, and the ionic liquid is impregnated into voids in the electrolyte layer, specifically, voids in the third solid electrolyte.

[0015] Another aspect of the present invention is a secondary battery comprising a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer comprises a positive electrode active material and a first solid electrolyte, the negative electrode layer comprises a negative electrode active material and a second solid electrolyte, the electrolyte layer comprises a third solid electrolyte, and the positive electrode layer, the negative electrode layer, and the electrolyte layer comprise an ionic liquid, and the ionic liquid is impregnated into voids in the electrolyte layer, specifically, voids in the third solid electrolyte.

[0016] Another aspect of the present invention is a secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has first to third electrolyte layers, the first to third electrolyte layers each having an ionic liquid, and the ionic liquid is impregnated into voids in the second electrolyte layer, specifically, into voids in the third solid electrolyte of the second electrolyte layer.

[0017] Another aspect of the present invention is a secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer located between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has the first electrolyte layer and the second electrolyte layer, the first electrolyte layer and the second electrolyte layer have an ionic liquid, and the ionic liquid is impregnated into voids in the second electrolyte layer, specifically, into voids in a third solid electrolyte included in the second electrolyte layer.

[0018] In any one of the aspects of the present invention, the positive electrode active material preferably has a composite oxide having a layered rock salt type crystal structure, a spinel type crystal structure, or an olivine type crystal structure.

[0019] In any one of the aspects of the present invention, the positive electrode active material having a layered rock salt crystal structure preferably includes lithium cobalt oxide or lithium nickel-manganese-cobalt oxide.

[0020] In any one of the aspects of the present invention, the negative electrode active material preferably contains silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, or indium.

[0021] In any one of the aspects of the present invention, the negative electrode active material preferably contains a carbon material.

[0022] An electronic device, a wristwatch-type electronic device, or an aircraft including a secondary battery according to one embodiment of the present invention.

[0023] According to one embodiment of the present invention, a secondary battery having favorable interface resistance can be provided. Alternatively, according to one embodiment of the present invention, a secondary battery having improved safety can be provided. Alternatively, according to one embodiment of the present invention, a flexible secondary battery can be provided.

[0024] Note that the description of these effects does not preclude the existence of other effects. Furthermore, these effects are considered to be independent of each other, and one embodiment of the present invention does not necessarily exhibit all of these effects. Furthermore, effects other than these can be extracted from the description in this specification, etc.

[0025] FIGS. 1A and 1B are diagrams illustrating a secondary battery of one embodiment of the present invention. FIGS. 2A and 2B are diagrams illustrating a secondary battery of one embodiment of the present invention. FIG. 3 is a diagram illustrating a secondary battery of one embodiment of the present invention. FIGS. 4A and 4B are diagrams illustrating a secondary battery of one embodiment of the present invention. FIGS. 5A to 5C are diagrams illustrating a method for manufacturing a secondary battery of one embodiment of the present invention. FIGS. 6A to 6D are diagrams illustrating a method for manufacturing a secondary battery of one embodiment of the present invention. FIGS. 7A to 7D are diagrams illustrating a method for manufacturing a secondary battery of one embodiment of the present invention. FIGS. 8A and 8B are diagrams illustrating a method for manufacturing a secondary battery of one embodiment of the present invention. FIGS. 9A and 9B are diagrams illustrating a method for manufacturing a secondary battery of one embodiment of the present invention. FIG. 10 is a diagram illustrating a manufacturing apparatus for a secondary battery of one embodiment of the present invention. FIG. 11 is a flow chart illustrating a method for manufacturing an electrolyte layer of a secondary battery of one embodiment of the present invention. FIGS. 12A and 12B are diagrams illustrating a heating process for an electrolyte layer of a secondary battery of one embodiment of the present invention. FIGS. 13A and 13B are cross-sectional views of a positive electrode active material, and FIGS. 13C to 13F are partial cross-sectional views of the positive electrode active material. FIG. 14 is an example of a TEM image in which the crystal orientations are roughly consistent. FIG. 15A is an example of an STEM image in which the crystal orientations are roughly consistent. FIG. 15B is an FFT pattern of a region with a rock-salt crystal structure RS. FIG. 15C is an FFT pattern of a region with a layered rock-salt crystal structure LRS. FIG. 16 is a diagram illustrating the crystal structure of a positive electrode active material. FIG. 17 is a diagram illustrating the crystal structure of a conventional positive electrode active material. FIGS. 18A and 18B are cross-sectional views of a positive electrode active material, and FIGS. 18C1 and 18C2 are partial cross-sectional views of the positive electrode active material. FIG. 19 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 20 is a diagram illustrating an XRD pattern calculated from the crystal structure. FIG. 21 is a cross-sectional view of a positive electrode active material. FIGS. 22A to 22C are diagrams illustrating a method for producing a positive electrode active material. 23A and 23B are diagrams illustrating a laminated secondary battery of one embodiment of the present invention. 24A to 24C are diagrams illustrating a method for manufacturing a laminated secondary battery of one embodiment of the present invention. 25A and 25B are diagrams illustrating a bent secondary battery of one embodiment of the present invention. 26A and 26B are diagrams illustrating a secondary battery of one embodiment of the present invention.FIGS. 27A and 27B are diagrams illustrating a bent secondary battery according to one embodiment of the present invention. FIGS. 28A to 28C are diagrams illustrating a wristwatch-type electronic device according to one embodiment of the present invention. FIGS. 29A to 29G are diagrams illustrating a wristwatch-type electronic device according to one embodiment of the present invention. FIGS. 30A to 30C are diagrams illustrating a wristwatch-type electronic device according to one embodiment of the present invention. FIG. 31 is a diagram illustrating a wristwatch-type electronic device according to one embodiment of the present invention. FIGS. 32A and 32B are perspective views illustrating an example of an aircraft according to one embodiment of the present invention. FIG. 32C is a cross-sectional view illustrating an example of an aircraft according to one embodiment of the present invention. FIGS. 33A and 33B are perspective views illustrating an example of an aircraft according to one embodiment of the present invention. FIGS. 34A to 34C are diagrams illustrating a coin-type secondary battery according to one embodiment of the present invention. FIGS. 35A to 35D are diagrams illustrating a cylindrical secondary battery according to one embodiment of the present invention. FIGS. 36A to 36C are diagrams illustrating an external appearance of a secondary battery pack according to one embodiment of the present invention. FIGS. 37A to 37C are diagrams illustrating an external appearance of a secondary battery pack according to one embodiment of the present invention. Figures 38A to 38C are diagrams illustrating an example of application to an electric vehicle (EV). Figures 39A to 39D are diagrams illustrating an example of a vehicle. Figures 40A to 40C are diagrams illustrating an example of a vehicle. Figures 41A to 41E are diagrams illustrating an example of an electronic device. Figure 42 is a diagram illustrating an example of an electronic device. Figures 43A and 43B are planar SEM images of a first sheet-shaped electrolyte layer. Figures 44A and 44B are planar SEM images of a second sheet-shaped electrolyte layer. Figures 45A and 45B are planar SEM images of the second sheet-shaped electrolyte layer in a state where the voids in the second sheet-shaped electrolyte layer are impregnated with an ionic liquid.

[0026] The embodiments 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 in form and detail can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of the embodiments shown below. In the configuration of the invention described below, the same parts or parts having similar functions will be denoted by the same reference numerals in different drawings, and their repeated description may be omitted.

[0027] Furthermore, in order to facilitate understanding of the invention, the position, size, range, etc. of each component shown in the drawings, etc. may not represent the actual position, size, range, etc. Therefore, the invention disclosed in this specification, etc. is not necessarily limited to the position, size, range, etc. disclosed in the drawings, etc.

[0028] In this specification and the like, the terms "above" and "below" do not limit the positional relationship of components to directly above or directly below. Furthermore, the terms "above" and "below" do not limit the relationship to contact. For example, in the expression "active material layer B on current collector A," active material layer B does not need to be formed on current collector A in contact with each other, and other components may be included between current collector A and active material B.

[0029] Ordinal numbers such as "first" and "second" used in this specification are used to avoid confusion between components, and do not indicate the order or ranking of processes or stacking order. Furthermore, even if a term is not used in this specification, an ordinal number may be used in the claims to avoid confusion between components. Furthermore, even if a term is used in this specification, a different ordinal number may be used in the claims. Furthermore, even if a term is used in this specification, the ordinal number may be omitted in the claims.

[0030] In this specification and the like, as a secondary battery using a positive electrode and a positive electrode active material, an example in which lithium metal is used for the negative electrode is sometimes shown, but the secondary battery of one embodiment of the present invention is not limited thereto. Other materials, such as graphite and lithium titanate, may also be used for the negative electrode. As long as one embodiment of the present invention is a positive electrode and a positive electrode active material, the material for the negative electrode is not limited in any way.

[0031] In this specification, the term "electrolyte layer" refers to a region that electrically insulates the positive electrode and the negative electrode and has lithium ion conductivity. The electrolyte layer sandwiched between the positive electrode and the negative electrode may be identified as a layer.

[0032] In this specification, a semi-solid battery is a battery having a semi-solid material in at least one of the electrolyte layer, positive electrode, and negative electrode, and preferably has an electrolyte layer made of a semi-solid material. Semi-solid means that the battery has solid properties, such as small volume change, while also possessing properties similar to those of a fluid liquid. It does not mean that the solid material content is 50%. The term "semi-solid material" refers to a battery that exhibits the above properties with a single material or multiple materials. For example, a gel-like material is considered a semi-solid material because it can exhibit the above properties with a single material. Furthermore, if a porous solid material is impregnated (also referred to as infiltration) with a liquid material as multiple materials, and exhibits the above properties, it may also be called a semi-solid material.

[0033] In this specification and the like, the positive electrode and the negative electrode may be collectively referred to as electrodes.

[0034] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). Crystal planes and crystal directions are expressed using Miller indices. Individual planes indicating crystal planes are expressed using ( ). In crystallography, space groups, crystal planes, and crystal directions are expressed with a bar above the number. However, due to formatting constraints, in this specification, instead of a bar above the number, a minus sign (-) may be added 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 planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Trigonal crystals represented by the space group R-3m are generally expressed as a hexagonal composite hexagonal lattice for ease of understanding the structure, and Miller indices such as (hkl) and (hkil) are sometimes used. Here, i is -(h+k).

[0035] In this specification and the like, particles are not limited to spherical particles having a circular cross-sectional shape, but include particle cross-sectional shapes such as ellipsoids, rectangles, trapezoids, triangles, quadrilaterals with rounded corners, asymmetric shapes, etc. Furthermore, the shapes of multiple particles do not have to be uniform, and individual particles may have irregular shapes.

[0036] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all intercalable and deintercalable lithium is deintercalated from the positive electrode active material. ２ The theoretical capacity of LiNiO is 274 mAh / g. ２ The theoretical capacity of LiMn is 275mAh / g. ２ O ４ The theoretical capacity of the battery is 148 mAh / g.

[0037] In this specification, the amount of intercalable and deintercalable lithium remaining in the positive electrode active material is determined by the x in the composition formula of the positive electrode active material, for example, Li ｘ CoO ２ x in, or Li ｘ MO ２ The value of x is Li ｘ CoO ２ , or Li ｘ MO ２ In this specification, Li ｘ CoO ２ Co is an example of a transition metal, and Li ｘ MO ２ (M represents a transition metal). 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, LiCoO ２ When a secondary battery using as a positive electrode active material is charged to 219.2 mAh / g, Li ０．２ CoO ２ Or we can say x = 0.2. ｘ CoO ２ The small value of x in the formula means, for example, 0.1<x≦0.24.

[0038] When the synthesized lithium cobalt oxide satisfies the stoichiometric ratio, LiCoO ２ and x=1. Also, LiCoO ２ Even when discharge of a secondary battery using LiCoO as a positive electrode is completed, ２ Alternatively, it may be said that x = 1. The completion of discharge here refers to a state in which the voltage becomes 3.0 V or 2.5 V or less at a current of 100 mA / g or less, for example.

[0039] Li ｘ CoO２ The charge capacity and / or discharge capacity used to calculate x in the above should preferably be measured under conditions where there is no or little influence of short circuit and / or electrolyte decomposition. For example, the charge capacity and / or discharge capacity when a sudden change in capacity that is considered to be a short circuit occurs should not be used to calculate x.

[0040] In this specification, etc., the space group of a crystal structure is identified by X-ray diffraction (XRD), electron diffraction, neutron diffraction, etc. Therefore, in this specification, etc., "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0041] In this specification, if the anions have a structure in which three layers are stacked with a skew, such as ABCABC, it is called a cubic close-packed structure. Therefore, the anions do not need to be in a strict cubic lattice. At the same time, since real crystals always have defects, the analysis results do not necessarily match the theoretical results. For example, in fast Fourier transform (FFT) of electron diffraction or transmission electron microscope (TEM) images, spots may appear at positions slightly different from the theoretical positions.

[0042] In this specification, "homogeneous" refers to a phenomenon in which a certain element (e.g., A) is distributed with similar characteristics in a specific region in a solid composed of multiple elements (e.g., A, B, C). It can be said that the material is homogeneous if the concentration of the element (e.g., A) in each specific region is substantially the same. For example, it can be said that the material is homogeneous if the difference in the concentration of the element (e.g., A) in each specific region is within 10%. Examples of specific regions in an active material include a surface layer, a surface, a convex portion, a concave portion, and an interior.

[0043] In this specification and the like, the positive electrode active material may be referred to as a composite oxide, a positive electrode material, a positive electrode material, a positive electrode material for a secondary battery, etc. In addition, in this specification and the like, the positive electrode active material of one embodiment of the present invention preferably contains an additive element, and the positive electrode active material containing the additive element may be referred to as a compound, a composition, or a composite.

[0044] 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 discharge capacity due to repeated charge and discharge.

[0045] 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 a short circuit be suppressed even at a high charging voltage. The positive electrode active material of one embodiment of the present invention suppresses short-circuit current even at a high charging voltage. Therefore, a secondary battery that achieves both high discharge capacity and safety can be obtained.

[0046] In one embodiment of the present invention, an electrolyte layer includes a solid material and a liquid material. In this embodiment, an example of an electrolyte layer according to one embodiment of the present invention and a secondary battery or the like including the electrolyte layer will be described.

[0047] 1A is a schematic cross-sectional view of a secondary battery 100 of one embodiment of the present invention. The secondary battery 100 includes a positive electrode layer 106, an electrolyte layer 103, and a negative electrode layer 107. The positive electrode layer 106 includes a positive electrode current collector 101 and a positive electrode active material layer 102. The negative electrode layer 107 includes a negative electrode current collector 105 and a negative electrode active material layer 104.

[0048] 1B is a schematic cross-sectional view of a secondary battery 100 of one embodiment of the present invention, and unlike FIG. 1A , illustrates a configuration in which the negative electrode active material layer 104 is not required. For example, when a metal foil containing lithium is used for the negative electrode current collector 105, the negative electrode active material layer 104 can be eliminated. The electrolyte layer 103 may be disposed at a certain distance from the negative electrode current collector 105. This is to ensure a region in which lithium is deposited on the negative electrode current collector 105.

[0049] In Figures 1A and 1B, the electrolyte layer 103 includes a solid material and a liquid material.

[0050] The electrolyte layer 103 has a function of moving carrier ions. Examples of the carrier ions that can be used include lithium ions, sodium ions, and the like. The electrolyte layer 103 has carrier ion conductivity and exhibits the function of moving carrier ions. Specifically, the electrolyte layer 103 may be made of a solid material with high carrier ion conductivity. For example, the lithium ion conductivity of the solid material used in the electrolyte layer 103 may be 0.1 mS / cm or more and 20 mS / cm or less at room temperature. The electrolyte layer 103 may also include a liquid material with high carrier ion conductivity in addition to the solid material. For example, the lithium ion conductivity of the liquid material used in the electrolyte layer 103 may be 0.1 mS / cm or more and 20 mS / cm or less at room temperature. If the lithium ion conductivity of the solid material is higher than that of the liquid material, the electrolyte layer 103 may contain more solid material than liquid material.

[0051] In order to satisfy the above-mentioned lithium ion conductivity, it is preferable to use a solid electrolyte as the solid material. Specific examples of the solid electrolyte will be described later.

[0052] In order to satisfy the above-mentioned lithium ion conductivity, it is preferable to use an ionic liquid as the liquid material, specific examples of which will be described later.

[0053] Furthermore, it is preferable that the electrolyte layer 103 contains a lithium salt. For example, when the electrolyte layer 103 contains an ionic liquid as a solvent, it is preferable that the electrolyte layer 103 contains a lithium salt as a solute.

[0054] In the electrolyte layer 103 having a solid material and a liquid material, the solid material is preferably capable of retaining the liquid material. For example, voids may be formed in a sintered solid material, and the liquid material is easily impregnated into the voids and can be easily retained in the voids.

[0055] Furthermore, by controlling the viscosity of the liquid material, it becomes easier for the liquid material to be impregnated into the solid material and retained therein. For example, a low viscosity is preferable when the liquid material is impregnated into the solid material. Also, once retained in the solid material, a high viscosity is preferable for the liquid material. A state of high viscosity is sometimes referred to as a gel state. The gel state can be said to be a material form intermediate between a solid and a liquid.

[0056] By having a configuration in which the above-mentioned solid material holds the liquid material, i.e., the solid material is impregnated with the liquid material, it is possible to prevent the liquid material from seeping out of the electrolyte layer 103 compared to a configuration in which the solid material and the liquid material are simply mixed.

[0057] With this configuration, it is possible to provide a secondary battery in which the liquid material in the electrolyte layer etc. is less likely to leak, thereby improving safety.

[0058] As long as the above-described retention configuration is satisfied, the ratio of solid material to liquid material in the electrolyte layer 103 is not particularly limited; however, a ratio of solid material greater than the ratio of liquid material is preferable for easier retention. The ratio of solid material to liquid material can be determined in terms of volume percent. For example, the solid material preferably accounts for 70% by volume or more and 95% by volume or less, and preferably 80% by volume or more and 93% by volume or less, in the electrolyte layer 103. The liquid material accounts for the remainder in the electrolyte layer 103, and therefore is preferably 5% by volume or more and 30% by volume or less, and preferably 7% by volume or more and 20% by volume or less. Note that lithium salt may be dissolved in the liquid material, and the above ratio may be satisfied as long as the lithium salt is dissolved in the liquid material.

[0059] The electrolyte layer 103 that satisfies the above ratio can be said to contain a semi-solid material. The electrolyte layer 103 containing a semi-solid material may be referred to as a semi-solid electrolyte layer. A secondary battery containing such a semi-solid electrolyte layer is preferable because it is easy to bend.

[0060] When a solid electrolyte is used as the solid material and an ionic liquid is used as the liquid material, the transference number of lithium ions is often higher in the solid electrolyte than in the ionic liquid. Therefore, it is preferable that the proportion of the solid electrolyte is higher than that of the ionic liquid, and if this condition is met, the stagnation of lithium ion migration in the electrolyte layer 103 is suppressed. The transference number of lithium ions is an index similar to lithium ion conductivity and indicates the ease of migration of lithium ions.

[0061] In a structure in which a solid material is impregnated with a liquid material, carrier ions, such as lithium ions, can move through both the solid material and the liquid material. In a structure in which a solid material is impregnated with a liquid material, lithium ions can also move only through the solid material. In a structure in which a solid material is impregnated with a liquid material, lithium ions can also move only through the liquid material.

[0062] Although the case where a solid material and a liquid material are present in the electrolyte layer has been described, one or both of the solid material and the liquid material may also be present in the positive electrode layer. The presence of one or both of the solid material and the liquid material in the electrolyte layer and the positive electrode layer can reduce the interfacial resistance between the electrolyte layer and the positive electrode layer compared to when an intervening layer is present. One or both of the solid material and the liquid material may also be present in the negative electrode layer. The presence of one or both of the solid material and the liquid material in the electrolyte layer and the negative electrode layer can reduce the interfacial resistance between the electrolyte layer and the negative electrode layer compared to when an intervening layer is present. When one or both of the solid material and the liquid material are present in the positive electrode layer or the negative electrode layer, one or both of the solid material and the liquid material may be mixed with the positive electrode slurry or the negative electrode slurry. If one or both of the solid material and the liquid material can hold the active material, the binder for the positive electrode layer or the negative electrode layer can be unnecessary or reduced. Furthermore, if the conductivity can be ensured by the solid material, the conductive additive for the positive electrode layer or the negative electrode layer can be unnecessary or reduced.

[0063] The solid material of the positive electrode layer or the negative electrode layer may have a different shape from the solid material of the electrolyte layer. The electrolyte layer is preferably a solid material having voids, but the positive electrode layer or the negative electrode layer may be a particulate solid material. When the positive electrode layer or the negative electrode layer does not contain a liquid material, a particulate solid material is particularly preferable. The solid material of the positive electrode layer or the negative electrode layer may be a material different from the solid material of the electrolyte layer, but the same material is preferable in terms of suppressing interfacial resistance.

[0064] The liquid material of the positive electrode layer or the negative electrode layer may be in a different state from the liquid material of the electrolyte layer. The electrolyte layer is preferably in a gel state, but the positive electrode layer or the negative electrode layer may be a liquid material. The liquid material of the positive electrode layer or the negative electrode layer may be a different material from the liquid material of the electrolyte layer, but in consideration of suppressing interfacial resistance, it is preferable to use the same material, and only the state may be different.

[0065] When the liquid material contained in the positive electrode layer or the negative electrode layer is the same as the liquid material of the electrolyte layer and is in a liquid state, the secondary battery can be completed by assembling the secondary battery and then injecting the liquid material.

[0066] Furthermore, due to a pressing process or the like during assembly of the secondary battery, the solid material and liquid material present in the electrolyte layer may migrate to the positive electrode layer or the negative electrode layer. In such a case, the solid material and liquid material may be said to have seeped out of the electrolyte layer, and the solid material and liquid material of the positive electrode layer or the negative electrode layer may be the same as the solid material and liquid material of the electrolyte layer. In the case of the solid material and liquid material used in the electrolyte layer of one embodiment of the present invention, the solid material retains the liquid material, thereby preventing the solid material and liquid material from leaking out of the secondary battery.

[0067] Furthermore, the condition that the proportion of the solid electrolyte is higher than the ionic liquid is limited to the electrolyte layer, and if one or both of the solid electrolyte and the ionic liquid are present in the positive electrode layer or the negative electrode layer, the above condition does not need to be met. For example, the proportion of the solid electrolyte may be higher than the ionic liquid in the electrolyte layer, but the proportion of the ionic liquid may be higher than the proportion of the solid electrolyte in the positive electrode layer or the negative electrode layer. Alternatively, the ionic liquid may be present in the positive electrode layer or the negative electrode layer, but the solid electrolyte may not be present. Alternatively, the solid electrolyte may be present in the positive electrode layer or the negative electrode layer, but the ionic liquid may not be present.

[0068] When the positive electrode layer or the negative electrode layer contains a solid material and a liquid material, the liquid material may be held by the solid material, and this state may be referred to as a semi-solid state.

[0069] In any of the electrolyte layer, the positive electrode layer, and the negative electrode layer, in order to retain a liquid material by a solid material, the solid material is preferably an inorganic material, but an organic material can also be used as the solid material. If an inert gel-like material is used as the organic material, it will be possible to retain the liquid material and exhibit a semi-solid state.

[0070] The liquid material should have a viscosity sufficient to be held by the solid material, and for example, a highly viscous gel-like material can be used. When an ionic liquid is used as the liquid material, a gel-like ionic liquid can be used.

[0071] Furthermore, it is preferable that the viscosity of the liquid material is sufficient at least when the electrolyte layer 103 is completed, since this makes it less likely to seep out of the electrolyte layer 103, etc.; it is not necessary for the liquid material to be sufficient when it is a starting material. In other words, the viscosity of the liquid material may be changed. For example, if a liquid material with low viscosity is used as a starting material, it is easier to impregnate the voids. Thereafter, in order to maintain the state of being held in the solid material, it is advisable to increase the viscosity of the liquid material at least when the electrolyte layer 103 is formed or when the secondary battery is completed. Specifically, a gelation process may be performed on the liquid material using a heating process, which is a process for producing the electrolyte layer, etc., so that the viscosity of the liquid material increases after the heating process. The viscosity of the liquid material may also be reduced by using a heating process when mixing the solid material and the liquid material.

[0072] In addition, another material may be added to the starting material to adjust the viscosity of the liquid material. For example, the viscosity of the liquid material can be controlled by mixing an organic solvent with the ionic liquid. The organic solvent may be one or more selected from 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.

[0073] In addition, the viscosity of the liquid material can be controlled by controlling the amount of lithium salt added to the ionic liquid. ６ , LiClO ４ , LiBF ４ , Li(C ２ F ５ SO ２ ) ２ N, Li(CF ３ SO ２ ) ２ N, Li(SO ２ F) ２ N, lithium bis(oxalato)borate (Li(C) ２ O ４ ) ２ , LiBOB), and the like can be used.

[0074] When a solid electrolyte is used as the solid material, the starting material for the solid electrolyte is often in a particulate shape. A particulate shape includes a shape that is circular or perfectly circular when viewed from a cross section. However, the shape of the solid electrolyte changes when the starting material is subjected to a firing process, i.e., a heating process or a mixing process. That is, the solid electrolyte may have a shape different from the particulate shape. A different shape includes an uneven shape or an elliptical shape when viewed from a cross section. That is, the solid material in the electrolyte layer 103 of the secondary battery 100 is not limited to a particulate shape, and the effects of the present application can be achieved even with a variety of shapes.

[0075] The above-described shape change can also occur in active material. For example, the starting material of the active material is often in a particulate form. However, the shape of the starting material changes when it undergoes a baking process, i.e., a heating process or a mixing process. That is, the active material may have a shape different from the particle shape. That is, the active material in the positive electrode active material layer 102 is not limited to a particle shape. Furthermore, the active material in the negative electrode active material layer 104 is not limited to a particle shape. The effects of the present application can be achieved even when the active material has a variety of shapes.

[0076] 2A is a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. As shown in FIG. 1A, FIG. 2A illustrates a configuration including the negative electrode active material layer 104. Of course, the negative electrode active material layer 104 may be omitted in FIG. 2A, as shown in FIG. 1B.

[0077] In FIG. 2A , the positive electrode active material layer 102 includes at least a positive electrode active material 111 and a solid electrolyte 113. In FIG. 2A , the positive electrode active material 111 and the solid electrolyte 113 are shown in particle form, but are not limited to particle form. Because the positive electrode active material layer 102 includes the solid electrolyte 113, the secondary battery 100 can operate even if the positive electrode active material layer 102 does not include an ionic liquid. Furthermore, as shown in FIG. 2A , the solid electrolyte 113 is continuously present from the electrolyte layer 103 to the positive electrode active material layer 102, thereby reducing the interfacial resistance between the layers. The solid electrolyte 113 included in the positive electrode active material layer 102 is preferably made of the same material as the solid electrolyte 113 included in the electrolyte layer 103, and it is preferable to have different shapes, with one being particulate and the other being a sintered body. A sintered body includes a state in which particles are bonded together, and voids may occur between the particles.

[0078] The positive electrode active material layer 102 may contain a conductive additive, but the conductive additive is omitted in FIG. 2A . Because the positive electrode active material layer 102 contains the solid electrolyte 113, it is possible to eliminate the need for a conductive additive. The positive electrode active material layer 102 may also contain a binder, but the binder is omitted in FIG. 2A . Because the positive electrode active material layer 102 contains the solid electrolyte 113, it is possible to eliminate the need for a binder.

[0079] The positive electrode active material layer 102 may have an ionic liquid instead of the solid electrolyte 113, or may have an ionic liquid in addition to the solid electrolyte 113. The ionic liquid in the positive electrode active material layer 102 is preferably made of the same material as the ionic liquid in the electrolyte layer 103, and the two may be in different states, with one being in a gel state and the other being in a liquid state.

[0080] In FIG. 2A , the negative electrode active material layer 104 includes at least a negative electrode active material 117 and a solid electrolyte 113. In FIG. 2A , the negative electrode active material 117 and the solid electrolyte 113 are shown in particle form, but are not limited to particle form. Because the negative electrode active material layer 104 includes the solid electrolyte 113, the negative electrode active material layer 104 can operate as a secondary battery 100 even if it does not include an ionic liquid. Furthermore, as shown in FIG. 2A , the solid electrolyte 113 is continuously present from the electrolyte layer 103 to the negative electrode active material layer 104, thereby reducing the interfacial resistance between the layers. The solid electrolyte 113 in the negative electrode active material layer 104 is preferably made of the same material as the solid electrolyte 113 in the electrolyte layer 103, and it is preferable to have different shapes, with one being particulate and the other being a sintered body. A sintered body includes a state in which particles are bonded together, and voids may occur between the particles.

[0081] The negative electrode active material layer 104 may contain a conductive additive, but the conductive additive is omitted in FIG. 2A . Because the negative electrode active material layer 104 contains the solid electrolyte 113, it is possible to eliminate the need for a conductive additive. The negative electrode active material layer 104 may also contain a binder, but the binder is omitted in FIG. 2A . Because the negative electrode active material layer 104 contains the solid electrolyte 113, it is possible to eliminate the need for a binder.

[0082] The negative electrode active material layer 104 may contain an ionic liquid instead of the solid electrolyte 113, or may contain an ionic liquid in addition to the solid electrolyte 113. The ionic liquid contained in the negative electrode active material layer 104 is preferably made of the same material as the ionic liquid contained in the electrolyte layer 103, and the two ionic liquids may be in different states, with one being in a gel state and the other being in a liquid state.

[0083] In Fig. 2A, the electrolyte layer 103 has a solid electrolyte 113 as a solid material and an ionic liquid 118 as a liquid material. Fig. 2B shows an enlarged schematic view of a partial region 114 of the electrolyte layer 103. As shown in Fig. 2B, the solid electrolyte 113 is preferably a sintered body. Furthermore, in the electrolyte layer 103, a portion of the solid electrolyte 113 may have a particle shape.

[0084] As shown in FIG. 2B , the sintered solid electrolyte 113 also has voids. The voids can be formed by adjusting the firing conditions of the solid electrolyte. For example, in order to prevent short circuits between the positive and negative electrodes, firing conditions can be set for the electrolyte layer 103 to reduce the voids. However, in the present invention, the voids are not necessarily reduced in order to fill them with the ionic liquid 118. To prevent short circuits in the voids, for example, the viscosity of the ionic liquid 118 can be controlled.

[0085] Increasing the proportion of the ionic liquid 118 in the electrolyte layer 103 also increases the voids. To increase the voids, the firing process may be performed in a state where an organic material having a melting point lower than the sintering temperature is mixed. In the firing process, voids can be formed corresponding to the organic material that has melted and disappeared. In this way, increasing the voids while controlling them rather than reducing them is also included in one embodiment of the present invention.

[0086] 2B, the region where the ionic liquid 118 is present corresponds to the void. In order to prevent a short circuit between the positive electrode and the negative electrode, it is preferable that the filled ionic liquid 118 is in a gel state.

[0087] The electrolyte layer 103 may be processed into a sheet shape through a process such as pressing. The electrolyte layer 103 may have a plurality of particulate solid electrolyte particles. That is, even when the electrolyte layer 103 has a plurality of particulate solid electrolyte particles without being a sintered body, the electrolyte layer 103 can have voids between the particles.

[0088] The thickness of the sheet-like electrolyte layer 103 is 1 μm or more and 100 μm or less, preferably 1 μm or more and 50 μm or less, and more preferably 1 μm or more and 20 μm or less.

[0089] The positions of voids may be controlled in the electrolyte layer 103. If voids are connected from the positive electrode layer to the negative electrode layer and formed like holes, the secondary battery may be more likely to short-circuit due to dendrites or the like formed in the negative electrode layer. Therefore, it is preferable to control the positions of the voids in the electrolyte layer 103 so that the positions of the voids are offset from each other to prevent such holes from being formed.

[0090] As mentioned above, since the voids are filled with a liquid material, it is also possible to suppress short circuits in the secondary battery caused by dendrites and the like by increasing the viscosity of the liquid material.

[0091] The proportion of voids in the electrolyte layer 103 may be controlled. For example, the proportion of voids in the center of the electrolyte layer 103 may be made higher than the proportion of voids in the electrolyte layer 103 closer to the positive electrode layer or the negative electrode layer. To suppress dendrites from forming in the negative electrode layer, the proportion of voids in the electrolyte layer 103 may be made lower as it gets closer to the negative electrode layer.

[0092] The electrolyte layer 103 may have a laminated structure, and two or more, preferably three or more, electrolyte layers may be laminated. In the case of a three-layer structure, the void ratio of the central electrolyte layer can be made different from the void ratios of the electrolyte layers disposed above and below it. Such an electrolyte layer 103 can suppress the aforementioned short circuit of the secondary battery. Furthermore, a separator may be disposed in place of the central electrolyte layer.

[0093] Again, it is preferable to prepare a sheet-shaped electrolyte layer 103. A sheet-shaped electrolyte layer is also suitable for the above-mentioned laminated structure. To prepare the sheet-shaped electrolyte layer 103, it is preferable to use a gel-state ionic liquid rather than a liquid material, because this allows the electrolyte layer 103 to maintain its shape.

[0094] In order to prevent the above-mentioned short circuit of the secondary battery, a separator may be disposed in addition to the electrolyte layer 103 .

[0095] 2A and 2B , in the electrolyte layer 103, the solid electrolyte 113 holds the ionic liquid 118. In other words, in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118 may be a liquid material.

[0096] The electrolyte layer 103 may also contain a binder, which makes it easier for the solid electrolyte 113 to hold the ionic liquid 118. However, the binder is omitted in Figures 2A and 2B.

[0097] 2A shows the boundaries between the layers as if they exist, the boundaries may not be clearly visible in the secondary battery 100. For example, when pressing the sheet-like electrolyte layer 103 to be bonded to the positive electrode layer 106, the boundary between the electrolyte layer 103 and the positive electrode layer 106 becomes unclear. This is because some of the positive electrode active material 111 penetrates into the electrolyte layer 103, and some of the solid electrolyte 113 penetrates into the positive electrode layer 106. Similarly, some of the negative electrode active material 117 may penetrate into the electrolyte layer 103, and some of the solid electrolyte 113 may penetrate into the negative electrode layer 107, making the boundary between the electrolyte layer 103 and the negative electrode layer 107 unclear.

[0098] 3 is a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. As shown in FIG. 1A, FIG. 3 illustrates a configuration including the negative electrode active material layer 104. Of course, in FIG. 3, the negative electrode active material layer 104 can be omitted as shown in FIG. 1B.

[0099] 2A, the secondary battery 100 shown in Fig. 3 has the ionic liquid 118 located throughout the entire secondary battery 100. When the secondary battery 100 is formed through a process of injecting the ionic liquid 118 after laminating the positive electrode layer 106, the electrolyte layer 103, and the negative electrode layer 107, the ionic liquid 118 can be located throughout the entire secondary battery 100 as shown in Fig. 3. In this case, it is preferable that the ionic liquid does not gel, or that a gelling treatment is performed after injection.

[0100] The other configurations are the same as those in FIGS. 2A and 2B.

[0101] 2A and 3 have in common that, at least in the electrolyte layer 103, the solid electrolyte 113 holds the ionic liquid 118. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118 may be a liquid material.

[0102] In FIG. 3, similarly to FIG. 2A and the like, a clear boundary line may not be visible in the secondary battery 100.

[0103] 4A is a schematic cross-sectional view of a secondary battery 100 according to one embodiment of the present invention. As shown in FIG. 1A, FIG. 4 illustrates a configuration including the negative electrode active material layer 104. Of course, in FIG. 4, the negative electrode active material layer 104 can be omitted as shown in FIG. 1B.

[0104] 2A and the like, the secondary battery 100 shown in Fig. 4A has a solid electrolyte 113 located only at the center of the electrolyte layer 103, with regions on the positive electrode layer side and the negative electrode layer side that do not have the solid electrolyte 113. The electrolyte layer 103 having such a structure can be divided into a first electrolyte layer 103a, a second electrolyte layer 103b, and a third electrolyte layer 103c depending on the content of the solid electrolyte 113. This structure is sometimes referred to as a laminated structure, and Fig. 4A illustrates an example in which the electrolyte layer 103 has a three-layer laminated structure.

[0105] A laminated structure of two or more layers can be applied to the electrolyte layer 103. Fig. 4B shows a secondary battery 100 having an electrolyte layer 103 with a laminated structure of two layers.

[0106] 4A, a gel-like ionic liquid or the like may be used for the first electrolyte layer 103a located in the region not including the solid electrolyte 113 and the third electrolyte layer 103c located in the region not including the solid electrolyte 113. In FIG. 4B, a gel-like ionic liquid or the like may be used for the first electrolyte layer 103a not including the solid electrolyte 113.

[0107] The other configurations are the same as those in FIGS. 2A, 2B, and 3.

[0108] 2A, 3, 4A, and 4B all show that at least in the electrolyte layer 103, the solid electrolyte 113 holds the ionic liquid 118. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118 may be a liquid material.

[0109] Although FIGS. 4A and 4B show boundary lines between the layers, as in FIG. 2A and the like, clear boundary lines may not be visible in the secondary battery 100.

[0110] Next, each component of the secondary battery 100 shown in FIGS. 1 to 4 will be described.

[0111] <Current Collector> The positive electrode current collector 101 and the negative electrode current collector 105 can each be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, copper, or titanium, or an alloy thereof. The material used for the positive electrode current collector is preferably one that does not dissolve at the potential of the positive electrode. An aluminum alloy containing an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector may be in a foil, plate, sheet, mesh, punched metal, expanded metal, or other shape. A carbon black or graphene layer may also be included as an undercoat. The current collector preferably has a thickness of 5 μm to 30 μm. The term "foil-like" refers to a thickness of 1 μm or more and 100 μm or less, preferably 5 μm or more and 30 μm or less.

[0112] In particular, when LiFSI (FSI is an abbreviation for bis(fluorosulfonyl)imide anion) is used as the lithium salt, the positive electrode current collector 101 and the negative electrode current collector 105 are preferably made of a material that is resistant to corrosion by LiFSI. For example, titanium and titanium compounds are preferred because they are resistant to corrosion. Carbon-coated titanium, titanium compounds, or aluminum are also preferred.

[0113] <Active Material> The positive electrode active material 111 of the positive electrode layer 106 may be, for example, a composite oxide having a layered rock salt crystal structure, a spinel crystal structure, or an olivine crystal structure. For example, a composite oxide containing lithium and a transition metal, such as lithium cobalt oxide, lithium nickel oxide, lithium cobalt oxide in which part of the cobalt is substituted with manganese, lithium cobalt oxide in which part of the cobalt is substituted with nickel, lithium nickel-manganese-cobalt oxide, lithium iron phosphate, lithium ferrate, or lithium manganate, may be used. Furthermore, as long as the material functions as a positive electrode active material, it does not necessarily have to contain lithium, and V ２ O ５ , Cr ２ O ５ , MnO ２ etc. may also be used.

[0114] Other positive electrode active materials will be described later.

[0115] The negative electrode active material 117 contained in the negative electrode layer 107 can be an element capable of undergoing a charge-discharge reaction by alloying / de-alloying with lithium. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. These elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. For this reason, it is preferable to use silicon as the negative electrode active material. Alternatively, a compound containing these elements can be used. For example, SiO (silicon monoxide, SiO Ｘ where x is preferably 0.2 or more and 1.5 or less), Mg ２ Si, Mg ２ Ge, SnO, SnO２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements capable of undergoing charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials.

[0116] Silicon nanoparticles can be used as the silicon-containing negative electrode active material. The median diameter (D50) of the silicon nanoparticles is 5 nm or more and less than 1 μm, preferably 10 nm or more and 300 nm or less, and more preferably 10 nm or more and 100 nm or less. The silicon nanoparticles may be crystalline. Furthermore, the silicon nanoparticles may have a crystalline region and an amorphous region.

[0117] The silicon-containing negative electrode active material may be in the form of silicon monoxide particles containing one or more silicon crystal grains. The silicon monoxide may be amorphous. The silicon monoxide particles may be carbon-coated. The carbon-coated particles may be mixed with graphite to form the negative electrode active material.

[0118] Examples of carbon materials that can be used include graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black. It is preferable to incorporate fluorine into these carbon materials. A carbon material containing fluorine can also be called a particulate or fibrous fluorinated carbon material. When measuring the carbon material by X-ray photoelectron spectroscopy, the fluorine concentration is preferably 1 atomic % (sometimes referred to as at %) or more relative to the total concentration of fluorine, oxygen, lithium, and carbon.

[0119] Furthermore, although negative electrode active materials may undergo volume changes during charge and discharge, disposing an organic compound containing fluorine, such as a fluorinated carbonate, between the negative electrode active materials makes them slippery even when volume changes occur during charge and discharge, suppressing cracks and improving cycle characteristics. It is important that an organic compound containing fluorine is present between multiple negative electrode active materials.

[0120] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is therefore preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0121] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a low potential similar to that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion secondary batteries to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and higher safety compared to lithium metal.

[0122] Titanium dioxide (TiO ２ ), lithium titanium oxide (Li ４ Ti５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten oxide (WO ２ ), molybdenum oxide (MoO ２ ) and other oxides can be used.

[0123] In addition, as the negative electrode active material, a composite nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N ３ has a large charge / discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0124] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, can be used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the composite nitride of lithium and a transition metal can be used as the negative electrode active material, even when a material containing lithium ions is used as the positive electrode active material, by first removing the lithium ions contained in the positive electrode active material.

[0125] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may be used as the negative electrode active material. The conversion reaction can also occur when Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９ , sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ This also occurs with fluorides such as

[0126] 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.

[0127] As the conductive additive and binder that can be contained in the negative electrode active material layer, the same materials as the conductive additive and binder that can be contained in the positive electrode active material layer can be used.

[0128] In addition to the same materials as the positive electrode current collector, copper etc. can also be used as the current collector. It is preferable to use a material that does not alloy with carrier ions such as lithium for the negative electrode current collector.

[0129] As another form of the negative electrode, 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 dissolved during discharging. Therefore, except in a fully discharged state, lithium is present on the negative electrode current collector.

[0130] 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, and an electrolyte layer can be disposed on the negative electrode current collector.

[0131] As the solid electrolyte, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, a polymer-based solid electrolyte, or the like can be used. Among them, a polymer-based solid electrolyte is suitable as a film for uniformly depositing lithium because it is relatively easy to form a uniform film on the negative electrode current collector. Sulfide-based solid electrolytes, oxide-based solid electrolytes, halide-based solid electrolytes, and polymer-based solid electrolytes will be described later.

[0132] 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.

[0133] <Solid Electrolyte> A solid electrolyte can be used as a solid material for the electrolyte layer, etc., of one embodiment of the present invention. Solid electrolytes include oxide-based, sulfide-based, and halide-based solid electrolytes, and a solid electrolyte obtained by mixing these may also be used for the electrolyte layer, etc.

[0134] As an oxide-based solid electrolyte, a material having a perovskite crystal structure (La ２／３−ｘ Li ３ｘ TiO ３ etc.), materials having a NASICON type crystal structure (Li １＋Ｘ Al Ｘ Ti ２−Ｘ (P.O. ４ ) ３ etc.), materials having a garnet-type crystal structure (Li ７ La ３ Zr ２ O １２ (LLZO), or Li ６．２５ La ３ Zr ２ Al ０．２５ O １２ (LLZAO), materials having a LISICON type crystal structure (Li １４ ZnGe ４ O １６ etc.), oxide glass (Li ３ P.O. ４ -Li ４ SiO ４ , 50Li４ SiO ４ ・50Li ３ BO ３ etc.), or oxide crystallized glass (Li １．０７ Al ０．６９ Ti １．４６ (P.O. ４ ) ３ , Li １．５ Al ０．５ Ge １．５ (P.O. ４ ) ３ Oxide-based solid electrolytes have the advantage of being heat-resistant and more stable in the atmosphere than sulfide-based solid electrolytes, which will be described later.

[0135] As a sulfide-based solid electrolyte, thiolithium-based (Li １０ GeP ２ S １２ , Li ３．２５ Ge ０．２５ P ０．７５ S ４ etc.), sulfide glass (70Li ２ S・30P ２ S ５ , 30Li ２ S・26B ２ S ３ ・44LiI, 63Li ２ S・36SiS ２ ・1Li ３ P.O. ４ , 57Li ２ S・38SiS ２ ・5Li ４ SiO ４ , 50Li ２ S・50GeS ２ etc.), or sulfide crystallized glass (Li ７ P ３ S １１ , Li ３．２５ P ０．９５ S ４ Sulfide-based solid electrolytes have the advantages of being highly conductive, being able to be synthesized at low temperatures, and being relatively soft, which means that the conductive path is easily maintained even after charging and discharging.

[0136] As a halide-based solid electrolyte, LiAlCl ４ , Li ３InBr ６ , LiF, LiCl, LiBr, LiI, etc.

[0137] As the solid material used for the electrolyte layer or the like of one embodiment of the present invention, a mixed material in which a solid electrolyte is filled into the pores of porous aluminum oxide or porous silica can also be used. In other words, a mixture of a solid electrolyte and a ceramic material may be used for the electrolyte layer or the like.

[0138] Although the use of a solid electrolyte has been described as being solid, any material that does not have fluidity and can hold the ionic liquid may be used, such as a polymer material. A configuration in which the ionic liquid is held by a polymer material is also sometimes called a semi-solid. In this case, the electrolyte layer 103 of the secondary battery 100 is sometimes called a semi-solid electrolyte layer.

[0139] An example of a polymer material used in the electrolyte layer or the like of one embodiment of the present invention is a lithium ion conductive polymer. The lithium ion conductive polymer may also be referred to as a polymer solid electrolyte. Examples of the lithium ion conductive polymer include polyethylene oxide (PEO), derivatives having polyethylene oxide as a main chain, polypropylene oxide, polyacrylic acid ester, polymethacrylic acid ester, polysiloxane, and polyphosphazene.

[0140] In the electrolyte layer or the like of one embodiment of the present invention, the solid electrolyte may be mixed with a graphene compound or graphene. A graphene compound has excellent physical properties, such as high flexibility and high mechanical strength, and therefore can impart high flexibility and high mechanical strength to the solid electrolyte.

[0141] Graphene compounds include multilayer graphene, graphene oxide, multilayer graphene oxide, multi-graphene oxide, 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 is sometimes called a carbon sheet. Graphene compounds may have functional groups. Graphene compounds preferably have a curved shape. Graphene compounds may also be rolled up to resemble carbon nanofibers.

[0142] 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.

[0143] <Ionic Liquid> An ionic liquid can be used as a liquid material for the electrolyte layer or the like of one embodiment of the present invention. The ionic liquid will be described.

[0144] Ionic liquids, sometimes referred to as room-temperature molten salts, contain cations and anions. The cations include imidazolium-based, ammonium-based, pyrrolidinium-based, piperidinium-based, pyridinium-based, or phosphonium-based basic skeletons. Cations with imidazolium-based basic skeletons can provide ionic liquids with lower viscosity than cations with ammonium-based basic skeletons. Low viscosity tends to increase the conductivity of carrier ions. Furthermore, the viscosity of the ionic liquid can be controlled by the alkyl groups on the side chains of the cations.

[0145] <General Formula of Cation> The cation of the ionic liquid of one embodiment of the present invention will be described.

[0146] An ionic liquid of one embodiment of the present invention includes an imidazolium cation represented by General Formula (G1).

[0147]

[0148] In the above general formula (G1), R １ represents an alkyl group having 1 to 10 carbon atoms, and R ２ ~R ４ each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; R５ represents an alkyl group having 1 to 6 carbon atoms, or an ether group, a thioether group, or a siloxane having a main chain composed of two or more atoms selected from C, O, Si, N, S, and P atoms. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0149] An ionic liquid of one embodiment of the present invention has a pyridinium-based cation represented by General Formula (G2).

[0150]

[0151] In the above general formula (G2), R ６ has a main chain composed of two or more atoms selected from an alkyl group having 1 to 6 carbon atoms or C, O, Si, N, S, and P atoms. ７ ~R １１ each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. ８ or R ９ In the general formula (G2), A may represent a hydroxyl group. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0152] The ionic liquid of one embodiment of the present invention may have a quaternary ammonium cation, for example, a quaternary ammonium cation represented by General Formula (G3).

[0153]

[0154] In the above general formula (G3), R ２８ ~R ３１ each independently represents an alkyl group having 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethyl group, or a hydrogen atom. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0155] An ionic liquid of one embodiment of the present invention has a cation represented by General Formula (G4).

[0156]

[0157] In the above general formula (G4), R １２ and R １７ R each independently represents an alkyl group having 1 to 3 carbon atoms. １３ ~R １６ each independently represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0158] An ionic liquid of one embodiment of the present invention has a cation represented by General Formula (G5).

[0159]

[0160] In the above general formula (G5), R １８ and R ２４ R each independently represents an alkyl group having 1 to 3 carbon atoms. １９ ~R ２３ each independently represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0161] An ionic liquid of one embodiment of the present invention has a cation represented by General Formula (G6).

[0162]

[0163] In the general formula (G6), n and m are 1 or more and 3 or less, α is 0 or more and 6 or less, β is 0 or more and 6 or less, and X or Y represents, as a substituent, a linear or side chain alkyl group having 1 to 4 carbon atoms, a linear or side chain alkoxy group having 1 to 4 carbon atoms, or a linear or side chain alkoxyalkyl group having 1 to 4 carbon atoms. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0164] An ionic liquid of one embodiment of the present invention has a tertiary sulfonium cation represented by General Formula (G7).

[0165]

[0166] In the above general formula (G7), R ２５ ~R ２７ each independently represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. ２５ ~R ２７ each independently has a main chain composed of two or more atoms selected from C, O, Si, N, S, and P atoms. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0167] An ionic liquid of one embodiment of the present invention has a quaternary phosphonium cation represented by General Formula (G8) below.

[0168]

[0169] In the above general formula (G8), R ３２ ~R ３５ each independently represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. ３２ ~R ３５ each independently has a main chain composed of two or more atoms selected from C, O, Si, N, S, and P atoms. − represents an anion, and is preferably FSI or TFSI, which will be described later.

[0170] <Cations> Specific examples of the cation of General Formula (G1) include structural formulas (111) to (174). Structural formula (111) is a 1-ethyl-3-methylimidazolium cation, abbreviated as EMI. Structural formula (113) is a 1-butyl-3-methylimidazolium cation, abbreviated as BMI.

[0171]

[0172]

[0173]

[0174]

[0175]

[0176]

[0177] Specific examples of the cation of the above general formula (G2) include structural formulae (701) to (719).

[0178]

[0179]

[0180] Specific examples of the cation of the general formula (G4) include structural formulae (501) to (520).

[0181]

[0182] Specific examples of the cation of the general formula (G5) include structural formulae (601) to (630).

[0183]

[0184]

[0185] Specific examples of the cation of the general formula (G6) include structural formulae (301) to (309) and structural formulae (401) to (419).

[0186]

[0187]

[0188] In addition, structural formulas (301) to (309) and structural formulas (401) to (419) show examples in which m is 1 in general formula (G6), but m may be changed to 2 or 3 in structural formulas (301) to (309) and structural formulas (401) to (419).

[0189] Specific examples of the cation of the general formula (G7) include structural formulae (201) to (215).

[0190]

[0191] <Anion> The anion of the ionic liquid of one embodiment of the present invention will be described. Examples of the anion include a halide ion, tetrafluoroborate, hexafluorophosphate, bis(trifluoromethylsulfonyl)amide, and bis(fluorosulfonyl)imide.

[0192] Specific anions that can be used include one or more selected from monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, perfluoroalkylphosphate anions, and tetrafluoroborate anions.

[0193] The monovalent amide anion is represented by the general formula (C ｎ F ２ｎ＋１ SO ２ ) ２ N − (n is 0 or more and 3 or less).

[0194] When n is 0, the above general formula is called a bis(fluorosulfonyl)imide anion, and is represented by the following structural formula (H11): The abbreviation for bis(fluorosulfonyl)imide anion is FSI or FSA.

[0195]

[0196] When n is 1, the above general formula is called a bis(trifluoromethanesulfonyl)imide anion, and is represented by the following structural formula (H12): The abbreviation for bis(trifluoromethanesulfonyl)imide anion is TFSI or TFSA.

[0197]

[0198] One of the monovalent cyclic amide anions is called 4,4,5,5-tetrafluoro-1,3,2-dithiazolidinetetraoxide anion, and is represented by the following structural formula (H13).

[0199]

[0200] The monovalent methide anion is represented by the general formula (C ｎ F２ｎ＋１ SO ２ ) ３ C − (n is 0 or more and 3 or less).

[0201] One of the monovalent cyclic methide anions is called 4,4,5,5-tetrafluoro-2-[(trifluoromethyl)sulfonyl]-1,3-dithiolane tetraoxide anion, and is represented by the following structural formula (H14).

[0202]

[0203] The fluoroalkylsulfonate anion is represented by the general formula (C ｍ F ２ｍ＋１ SO ３ ) − (m is 0 or more and 4 or less).

[0204] When m is 0, the above general formula is a fluorosulfonate anion, and when m is 1, 2, 3, or 4, the above general formula is a perfluoroalkylsulfonate anion.

[0205] Fluoroalkylborate anions have the general formula {BF ｎ (C ｍ H ｋ F ２ｍ＋１−ｋ ) ４−ｎ} − (n is 0 or more and 3 or less, m is 1 or more and 4 or less, and k is 0 or more and 2m or less).

[0206] The fluoroalkyl phosphate anion has the general formula {PF ｎ (C ｍ H ｋ F ２ｍ＋１−ｋ ) ６−ｎ} − (n is 0 or more and 5 or less, m is 1 or more and 4 or less, and k is 0 or more and 2m or less).

[0207] The ionic liquid of one embodiment of the present invention can have one or more anions selected from the above-described anions.

[0208] Since such ionic liquids are composed solely of ions, they exhibit strong electrostatic interactions, are nonvolatile, thermally stable, and highly heat-resistant, and secondary batteries using such ionic liquids are highly safe and do not ignite within the operating temperature range.

[0209] <Organic Solvent> An organic solvent can be used as a liquid material for the electrolyte layer or the like of one embodiment of the present invention. A mixed material of an organic solvent and an ionic liquid can be used as a liquid material for the electrolyte layer or the like of one embodiment of the present invention. The organic solvent will be described.

[0210] The organic solvent in one embodiment of the present invention may be an aprotic organic solvent, and may be, for example, one or more selected from 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.

[0211] Furthermore, the organic solvent may contain a fluorinated carbonate or a cyclic carbonate. An example of a fluorinated carbonate is a fluorinated cyclic carbonate. Fluorinated cyclic carbonates have a high flash point and can improve the safety of secondary batteries.

[0212] Examples of fluorinated cyclic carbonates that can be used include fluorinated ethylene carbonates such as monofluoroethylene carbonate (fluoroethylene carbonate, FEC, F1EC), difluoroethylene carbonate (DFEC, F2EC), trifluoroethylene carbonate (F3EC), and tetrafluoroethylene carbonate (F4EC). DFEC includes isomers such as cis-4,5 and trans-4,5 isomers.

[0213] One of the fluorinated cyclic carbonates according to one embodiment of the present invention is monofluoroethylene carbonate, abbreviated as FEC.

[0214] One of the fluorinated cyclic carbonates according to one embodiment of the present invention is tetrafluoroethylene carbonate, abbreviated as F4EC.

[0215] One of the fluorinated cyclic carbonates according to one embodiment of the present invention is difluoroethylene carbonate, abbreviated as F2EC.

[0216] Although a fluorinated cyclic carbonate has been described, a cyclic carbonate having a cyano group can also be used as the organic solvent of one embodiment of the present invention.

[0217] <Gelling Agent> The ionic liquid or organic solvent described above may be gelled. Gelling can suppress seepage from the electrolyte layer 103. The gelling agent can be selected according to a method such as chemical gelling treatment or physical gelling treatment. The gelling agent used in the chemical gelling treatment preferably contains a polymer and a crosslinking agent.

[0218] A gelling agent is added to the ionic liquid or organic solvent and mixed. At this time, the mixture is heated to 75° C. or higher and 100° C. or lower, preferably 85° C. or higher and 95° C. or lower, thereby obtaining a gelled ionic liquid or a gelled organic solvent.

[0219] As a specific gelling agent, poly(dimethylaminoethyl methacrylate) can be used as the polymer, and N,N,N',N'-tetra(trifluoromethanesulfonyl)-dodecane-1,12-diamine can be used as the crosslinking agent. The crosslinking agent causes the polymer to have a crosslinked structure, and the ionic liquid or organic solvent is held in this crosslinked structure, resulting in a gel state.

[0220] <Lithium Salt> The lithium salt used in the electrolyte layer or the like according to one embodiment of the present invention is preferably a lithium salt having a halogen. More preferably, it is a fluorine-containing imide lithium salt. As the fluorine-containing imide lithium salt, Li(CF ３ SO ２ ) ２ N (hereinafter, sometimes referred to as "LiTFSI" or "LiTFSA"), Li(C ２ F ５ SO ２ ) ２ N (hereinafter, sometimes referred to as "LiBETI"), or Li(SO２ F) ２ N (hereinafter, sometimes referred to as "LiFSI" or "LiFSA"), etc. can be used.

[0221] Another example of a halogen-containing lithium salt is LiPF ６ , LiBF ４ , LiClO ４ etc. can be used.

[0222] Furthermore, LiBOB may be used as a halogen-free lithium salt.

[0223] These lithium salts may be used alone or in combination.

[0224] <Exterior Body> The exterior body of the secondary battery according to one embodiment of the present invention will be described. For example, a metal material such as aluminum or a resin material can be used as the exterior body of the secondary battery. Examples of resin materials include rubber materials. Examples of rubber include natural rubber and synthetic rubber. Examples of synthetic rubber include rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer.

[0225] The secondary battery preferably has an exterior body in the form of a film. The exterior body that can be in the form of a film may be made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc. The exterior body that can be in the form of a film may be made of a thin metal film with excellent flexibility such as aluminum, stainless steel, copper, or nickel.

[0226] Furthermore, the exterior body that can be in a film shape may have a laminated structure, in which the first layer preferably includes a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and the second layer preferably includes a thin metal film with excellent flexibility such as aluminum, stainless steel, copper, or nickel.

[0227] Furthermore, the outer surface of the exterior body may be provided with an insulating synthetic resin film such as a polyamide resin, a polyester resin, etc. When the configuration of the outer surface is applied to the laminated structure of the first layer and the second layer, a three-layer film can be used.

[0228] A secondary battery having an electrolyte layer according to one embodiment of the present invention is preferable because it is easily bendable. The exterior body using the insulating synthetic resin film described above is suitable for a bent secondary battery or a secondary battery that changes between a bent state and an extended state. Because the ionic liquid is retained in the solid electrolyte, it is prevented from seeping out even when the secondary battery is bent. Even if the ionic liquid seeps out, the exterior body described above, particularly an exterior body having a laminated structure, can prevent the ionic liquid from leaking out of the secondary battery.

[0229] 1 to 4 , the positive electrode layer 106 and the negative electrode layer 107 may contain a binder. Furthermore, the electrolyte layer 103 may also contain a 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, or ethylene-propylene-diene copolymer. Furthermore, fluororubber can be used as the binder.

[0230] 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.

[0231] Alternatively, it is preferable to use, as the binder, one or more selected from polystyrene, polyvinyl butyral (PVB), 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, nitrocellulose, and the like.

[0232] The binder may be used in combination with two or more of the above.

[0233] 1 to 4 , the positive electrode layer 106 and the negative electrode layer 107 may contain a conductive additive. As the conductive additive, a carbon material such as acetylene black (AB), carbon nanotube, graphene, or fullerene can be used.

[0234] Graphene is a thin flake with excellent electrical properties, such as high conductivity, and excellent physical properties, such as mechanical strength, and therefore, by using graphene as a conductive additive, it is possible to increase the contact points or contact area between active materials.

[0235] Graphene includes single-layer graphene and multi-layer graphene having from 2 to 100 layers. Single-layer graphene refers to a sheet of carbon molecules having a single atomic layer with π bonds.

[0236] <Fabrication Process 1> An example of a fabrication process for the secondary battery 100 shown in Figures 1 to 4 will be described. The ionic liquid used in describing the fabrication process may contain a lithium salt. An ionic liquid containing a lithium salt may be referred to as a lithium liquid electrolyte or a lithium ion electrolyte.

[0237] As shown in FIG. 5A , a positive electrode layer 106 is prepared. The positive electrode layer 106 is formed by coating a positive electrode current collector 101 with a slurry containing a dispersion medium, a positive electrode active material 111, a solid electrolyte 113, and the like. The dispersion medium and the like are removed from the slurry to form a positive electrode active material layer 102. If a firing step is not performed, the solid electrolyte 113 is in a particulate state, and if a firing step is performed, the solid electrolyte 113 may form a sintered body. In FIG. 5A , the positive electrode layer 106 contains the particulate solid electrolyte 113.

[0238] As shown in FIG. 5B, an electrolyte layer 103 is prepared. The electrolyte layer 103 includes a solid electrolyte 113 and an ionic liquid 118. The solid electrolyte 113 is preferably sintered through a firing process, which makes it easier to retain the ionic liquid 118. Furthermore, it is preferable to use an electrolyte layer 103 processed into a sheet shape. This may be referred to as a sheet-shaped electrolyte layer. The sheet-shaped electrolyte layer is placed on the positive electrode layer 106, and a pressing process is carried out. Note that the pressing process may be carried out after the negative electrode layer 107, which will be described later, is placed on the electrolyte layer 103. Heat may be applied during the pressing process. While FIG. 5B shows the boundaries of each layer, the pressing process may make the boundaries unclear.

[0239] As shown in Fig. 5C, the negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, a solid electrolyte 113, and the like on the negative electrode current collector 105. The dispersion medium and the like are removed from the slurry to form the negative electrode active material layer 104. In Fig. 5C, the negative electrode layer 107 contains particulate solid electrolyte 113.

[0240] The negative electrode layer 107 is placed on the electrolyte layer 103, and a pressing process is performed. Heat may be applied during the pressing process. Although FIG. 5C shows the boundary lines between the layers, the boundary lines may not be clearly visible after the pressing process.

[0241] The pressing step described with reference to FIG. 5C can be combined with the pressing step described with reference to FIG. 5B, and therefore the pressing step described with reference to FIG. 5B can be omitted.

[0242] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 retains the ionic liquid 118 in the electrolyte layer 103. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118 may be a liquid material.

[0243] <Fabrication Process 2> An example of a fabrication process for the secondary battery 100 that is different from Fabrication Process 1 will be described.

[0244] As shown in FIG. 6A , a positive electrode layer 106 is prepared. The positive electrode layer 106 is formed by coating a positive electrode current collector 101 with a slurry containing a dispersion medium, a positive electrode active material 111, a solid electrolyte 113, and the like. The dispersion medium and the like are removed from the slurry to form a positive electrode active material layer 102. If a firing step is not performed, the solid electrolyte 113 is in a particulate state, and if a firing step is performed, the solid electrolyte 113 may form a sintered body. In FIG. 6A , the positive electrode layer 106 contains the particulate solid electrolyte 113.

[0245] As shown in FIG. 6B, an electrolyte layer 103 is prepared. At this stage, the electrolyte layer 103 has a solid electrolyte 113 and is processed into a sheet. If the solid electrolyte 113 is sintered through a firing process, this is preferable as it makes it easier to retain the ionic liquid 118 described below. The sheet-like electrolyte layer is placed on the positive electrode layer 106, and a pressing process is then carried out. The pressing process may also be carried out after the negative electrode layer 107 described below is placed on the electrolyte layer 103. Heat may also be applied during the pressing process. Although FIG. 6B shows the boundary lines between the layers, the pressing process may make it difficult to clearly see the boundaries.

[0246] As shown in Fig. 6C , the negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, a solid electrolyte 113, and the like on the negative electrode current collector 105. The dispersion medium and the like are removed from the slurry to form the negative electrode active material layer 104. In Fig. 6C , the negative electrode layer 107 contains particulate solid electrolyte 113.

[0247] The negative electrode layer 107 is placed on the electrolyte layer 103, and a pressing process is performed. Heat may be applied during the pressing process. Although FIG. 6C shows the boundary lines between the layers, the boundaries may not be clearly visible after the pressing process.

[0248] The pressing step described with reference to FIG. 6C can be combined with the pressing step described with reference to FIG. 6B, and therefore the pressing step described with reference to FIG. 6B can be omitted.

[0249] 6D, the ionic liquid 118 is injected. The ionic liquid 118 is preferably injected in a vacuum atmosphere or a reduced pressure atmosphere.

[0250] The injected ionic liquid 118 may be subjected to a gelling treatment. When heating is used for the gelling treatment, the heating performed in the pressing step may be used. That is, the gelling treatment may be carried out while pressing.

[0251] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 retains the ionic liquid 118 in the electrolyte layer 103. In other words, at least in the electrolyte layer 103, the ionic liquid 118 is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118 does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118 may be a liquid material.

[0252] <Manufacturing Process 3> An example of a manufacturing process of the secondary battery 100 that is different from Manufacturing Process 1 and Manufacturing Process 2 will be described.

[0253] As shown in FIG. 7A , a positive electrode layer 106 is prepared. The positive electrode layer 106 is formed by coating a positive electrode current collector 101 with a slurry containing a dispersion medium, a positive electrode active material 111, a solid electrolyte 113, and the like. The dispersion medium and the like are removed from the slurry to form a positive electrode active material layer 102. If a firing step is not performed, the solid electrolyte 113 is in a particulate state, and if a firing step is performed, the solid electrolyte 113 may form a sintered body. In FIG. 7A , the positive electrode layer 106 contains the particulate solid electrolyte 113.

[0254] As shown in FIG. 7B, a first electrolyte layer 103a is prepared. The first electrolyte layer 103a preferably contains a gelled ionic liquid 118a and does not contain a solid electrolyte. The first electrolyte layer 103a is placed on the positive electrode layer 106. If the first electrolyte layer 103a contains a gelled ionic liquid, it may be adhesive, making the pressing step unnecessary. Of course, a pressing step may also be performed. The pressing step may also be performed after laminating up to the third electrolyte layer 103c (described below) or after placing the negative electrode layer 107 on the third electrolyte layer 103c. Heat may also be applied during the pressing step. While FIG. 7B shows the boundaries of each layer, the pressing step may make the boundaries unclear.

[0255] As shown in FIG. 7C, a second electrolyte layer 103b is prepared. The second electrolyte layer 103b has a solid electrolyte 113, and a sheet-shaped electrolyte layer may be used. Sintering the solid electrolyte 113 through a firing process facilitates processing into a sheet. A firing process may also be performed when processing into a sheet to form a sintered body. The sheet-shaped electrolyte layer is placed on the first electrolyte layer 103a. If the first electrolyte layer 103a is a layer containing a gelled ionic liquid, it may have adhesive properties, making the pressing process unnecessary. Of course, a pressing process may be performed, and heat may be applied during the pressing process. While FIG. 7C shows the boundaries of each layer, the pressing process may obscure the clear boundaries.

[0256] Furthermore, a third electrolyte layer 103c is prepared as shown in FIG. 7C. The third electrolyte layer 103c can be similar to the first electrolyte layer 103a, and preferably a layer containing gelled ionic liquid 118b. The third electrolyte layer 103c is placed on the second electrolyte layer 103b. If the third electrolyte layer 103c is a layer containing gelled ionic liquid, it may have adhesive properties, making the pressing step unnecessary. Of course, a pressing step may be performed, and heat may be applied during the pressing step. While FIG. 7C shows the boundaries of each layer, the pressing step may make the boundaries unclear.

[0257] As shown in Fig. 7D , the negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, a solid electrolyte 113, and the like on the negative electrode current collector 105. The dispersion medium and the like are removed from the slurry to form the negative electrode active material layer 104. In Fig. 7D , the negative electrode layer 107 contains particulate solid electrolyte 113.

[0258] The negative electrode layer 107 is placed on the third electrolyte layer 103c, and a pressing process is performed. Heat may be applied during the pressing process. Although FIG. 7D shows the boundary lines between the layers, the pressing process may make the boundaries unclear. Furthermore, the process shown in FIG. 7D and other figures may cause some of the ionic liquid 118a to penetrate into the voids of the solid electrolyte 113 in the second electrolyte layer 103b. Furthermore, some of the ionic liquid 118b may penetrate into the voids of the solid electrolyte 113 in the second electrolyte layer 103b.

[0259] In the secondary battery obtained through this fabrication process, the solid electrolyte 113 retains the ionic liquid 118a or ionic liquid 118b in the electrolyte layer 103. In other words, at least in the electrolyte layer 103, the ionic liquid 118a or ionic liquid 118b is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118a or ionic liquid 118b does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118a or ionic liquid 118b may be a liquid material.

[0260] <Manufacturing Step 4> An example of a manufacturing step of the secondary battery 100 that is different from the manufacturing steps 1 to 3 will be described.

[0261] As shown in FIG. 8A, in accordance with the above-described secondary battery manufacturing process 3, layers up to the second electrolyte layer 103b are prepared.

[0262] As shown in Fig. 8B , the negative electrode layer 107 is prepared. The negative electrode layer 107 is formed by coating a slurry containing a dispersion medium, a negative electrode active material 117, a solid electrolyte 113, and the like on the negative electrode current collector 105. The dispersion medium and the like are removed from the slurry to form the negative electrode active material layer 104. In Fig. 8B , the negative electrode layer 107 contains particulate solid electrolyte 113.

[0263] The negative electrode layer 107 is placed on the second electrolyte layer 103b, and a pressing process is performed. Heat may be applied during the pressing process. Although FIG. 8B shows the boundary lines between the layers, the pressing process may make the boundaries unclear. Furthermore, the process shown in FIG. 8B and other figures allows a portion of the ionic liquid 118a to penetrate into the voids of the solid electrolyte 113 in the second electrolyte layer 103b.

[0264] In the secondary battery obtained through this manufacturing process, the solid electrolyte 113 retains a portion of the ionic liquid 118a in the second electrolyte layer 103b. In other words, at least in the electrolyte layer 103, a portion of the ionic liquid 118a is impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118a does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118 may be a liquid material.

[0265] <Manufacturing Step 5> An example of a manufacturing step of the secondary battery 100 that is different from the manufacturing steps 1 to 4 will be described.

[0266] 9A, structure A is prepared by laminating layers up to second electrolyte layer 103b according to the above-described secondary battery fabrication process 3. Also, as shown in FIG. 9A, structure B is prepared by laminating negative electrode layer 107 and third electrolyte layer 103c. Structure A and structure B are then bonded together as indicated by the outline arrow.

[0267] As shown in FIG. 9B , a pressing step is performed after bonding. Heat may be applied during the pressing step. While FIG. 9B shows the boundaries of each layer, the pressing step may make the boundaries less clear. Furthermore, the step shown in FIG. 9B and other figures allows a portion of the ionic liquid 118a or a portion of the ionic liquid 118b to be impregnated into the voids of the solid electrolyte 113 of the first electrolyte layer 103a.

[0268] In the secondary battery obtained through this fabrication process, the solid electrolyte 113 retains a portion of the ionic liquid 118a and a portion of the ionic liquid 118b in the electrolyte layer 103. In other words, at least in the electrolyte layer 103, a portion of the ionic liquid 118a and a portion of the ionic liquid 118b are impregnated into the solid electrolyte 113. Such an electrolyte layer 103 is preferable because the ionic liquid 118a or the ionic liquid 118b does not seep out. This electrolyte layer 103 may be referred to as a semi-solid electrolyte layer. As described above, the solid electrolyte 113 may be a solid material, and the ionic liquid 118a or the ionic liquid 118b may be a liquid material.

[0269] This embodiment can be used in combination with other embodiments.

[0270] The above-described manufacturing steps are preferably performed continuously using a roll-to-roll manufacturing apparatus or the like. The roll-to-roll method can be applied to the above-described manufacturing steps 1 to 5. In this embodiment, the manufacturing step shown in manufacturing step 5 will be described using the manufacturing apparatus shown in FIG.

[0271] Using the manufacturing apparatus shown in FIG. 10 , it is possible to carry out at least step 310 of applying a slurry onto a positive electrode current collector 101, step 320 of drying the slurry to form a positive electrode active material layer 102, step 330 of superposing an electrolyte layer 103 on the positive electrode active material layer 102, and step 340 of passing the positive electrode current collector 101 on which the positive electrode active material layer 102 has been formed, together with the electrolyte layer 103, between a pair of pressure rolls (a first pressure roll 325 and a second pressure roll 326).

[0272] The above step 310 will now be described. As shown in FIG. 10 , the manufacturing apparatus has a delivery mechanism 311 (also referred to as an unwinder), and a first bobbin 312 around which a positive electrode current collector 101 is wound is placed on the delivery mechanism 311. The positive electrode current collector 101 is moved by utilizing the rotation of rollers 313, and a slurry is applied to one surface of the positive electrode current collector 101 by first slurry application means 314a. The slurry contains at least a dispersant, a positive electrode active material, and a solid electrolyte. The rollers 313 are paired, and pressing is also possible when passing between them.

[0273] As the first slurry applying means 314a, for example, a slot die coater, a lip coater, a blade coater, a reverse coater, a gravure coater, or the like can be used. Note that, depending on the type of coater used, the number of rollers for reversing the positive electrode current collector 101 may be increased. Also, techniques such as a dipping method or a spraying method can be used as the first slurry applying means 314a. Depending on the material used, the slurry is applied while the first slurry applying means 314a is heated. It is preferable that the slurry be applied in a heated state.

[0274] In step 320, the slurry applied to the positive electrode current collector 101 is dried in a heating chamber 321a having an air inlet 322, an exhaust outlet 323, and drying means 324. By drying the slurry, the positive electrode active material layer 102 can be formed on the positive electrode current collector 101. The air inlet 322 and the exhaust outlet 323 are preferably installed on the ceiling (also referred to as the top surface) of the heating chamber 321a, but they may also be installed on the wall (also referred to as the side surface) or floor (also referred to as the bottom surface) of the heating chamber 321a. The drying means 324 can be one or a combination of two or more methods selected from hot air heating, lamp heating, induction heating, air blowing, etc.

[0275] In step 320, an example is shown in which the slurry is allowed to cool naturally after drying, and no cooling means is provided, but a cooling means may be provided in or near the heating chamber 321a to forcibly cool the slurry.

[0276] In step 330, the first electrolyte layer 103a and the second electrolyte layer 103b are formed on the positive electrode active material layer 102 by the second slurry applying means 314b. The first electrolyte layer 103a and the second electrolyte layer 103b may be prepared as a laminate and then formed on the positive electrode active material layer 102. Alternatively, a plurality of applying means equivalent to the second slurry applying means 314b may be provided, and the first electrolyte layer 103a and the second electrolyte layer 103b may be formed on the positive electrode active material layer 102 in this order.

[0277] In parallel with the treatment of the positive electrode current collector 101, the negative electrode current collector 105 is also treated. A second bobbin 405 around which the negative electrode current collector 105 is wound is placed on a delivery mechanism 315, and using the rotation of a roller 316, a third slurry application means 314c applies a slurry to one surface of the negative electrode current collector 105. The slurry contains at least a dispersion medium, a negative electrode active material, and a solid electrolyte. The rollers 316 are paired, and pressing can also be performed as the slurry passes between them.

[0278] As the third slurry applying means 314c, for example, a slot die coater, a lip coater, a blade coater, a reverse coater, a gravure coater, or the like can be used. Note that, depending on the type of coater used, the number of rollers for reversing the negative electrode current collector 105 may be increased. Furthermore, as the third slurry applying means 314c, a method such as a dipping method or a spraying method can also be used. Furthermore, depending on the material used, the third slurry applying means 314c is heated while applying the slurry. It is preferable that the slurry be applied in a heated state.

[0279] Next, in the heating chamber 321b, the slurry applied onto the negative electrode current collector 105 is dried. The heating chamber 321b may have the same configuration as the heating chamber 321a. By drying the slurry, the negative electrode active material layer 104 can be formed on the negative electrode current collector 105. After drying, the slurry may be allowed to cool naturally, or may be forcibly cooled by installing a cooling means in or near the heating chamber 321b.

[0280] Next, the slurry is applied onto the negative electrode active material layer 104 by the fourth slurry applying means 314d, and the slurry is passed through the heating chamber 321c to form the third electrolyte layer 103c. After passing through the roller 406, the process proceeds to step 340. The heating chamber 321c may have the same configuration as the heating chamber 321a.

[0281] The fourth slurry applying means 314d may be, for example, a slot die coater, a lip coater, a blade coater, a reverse coater, or a gravure coater. Depending on the type of coater used, the number of rollers for reversing the negative electrode current collector 105 may be increased. Alternatively, the fourth slurry applying means 314d may be a dipping method or a spraying method. Depending on the material used, the fourth slurry applying means 314d may be heated while the slurry is applied. The slurry may be applied in a heated state.

[0282] In step 340, the positive electrode current collector 101 is overlapped with the negative electrode current collector 105 and pressed together using the rotation of a pair of pressure rolls (first pressure roll 325 and second pressure roll 326). Heat may be applied during pressing. This step may temporarily melt (solate) the ionic liquid contained in the electrolyte layer. The melted ionic liquid can impregnate the adjacent positive electrode layer or negative electrode layer.

[0283] Finally, the laminate is wound around a second bobbin 328 installed in a winding mechanism 327 (also called a winder), and then cut into a desired shape by cutting means such as a laser cutter or a cutter (not shown).

[0284] Although FIG. 10 shows an example in which the laminate is wound up, it may be cut into a desired shape by cutting means such as a laser cutter or a cutter (not shown) without winding up.

[0285] Through the above steps, a secondary battery can be manufactured.

[0286] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0287] In this embodiment, a process of processing an electrolyte layer of one embodiment of the present invention into a sheet shape will be described. The electrolyte layer processed into a sheet shape is easy to handle and can improve productivity, which is preferable.

[0288] As shown in step S50 of Fig. 11, an electrolyte source, a binder, a plasticizer, and a solvent are prepared. LLZAO powder is prepared as the electrolyte source. Polyvinyl butyral (PVB) is prepared as the binder. Dioctyl phthalate (DOP) is prepared as the plasticizer. N-methyl-2-pyrrolidone (NMP) is prepared as the solvent.

[0289] In addition to PVB, the aforementioned materials such as polyvinyl alcohol (PVA) may be used as the binder, and acrylic resin may also be used as the binder. In addition to DOP, phthalate esters may be used as the plasticizer, and for example, one or more selected from dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), etc. may be used. In addition to NMP, one or more selected from water, dimethylformamide (DMF), etc. may be used as the solvent.

[0290] As shown in step S52 of FIG. 11, the above-mentioned materials are mixed, and as shown in step S54, a slurry is obtained. Before the mixing shown in step S52, the materials shown in step S50 may be mixed independently. Step S52 can be performed using, for example, a planetary centrifugal mixer. The rotation speed can be 1000 rpm or more and 3000 rpm or less. The rotation time can be 1 minute or more and 10 minutes or less. Mixing using the mixer may be performed not once but two or more times.

[0291] As shown in step S54 of Fig. 11, the slurry is applied to a substrate for coating. The substrate for coating may be made of a material from which the sheet-like electrolyte layer can be easily peeled, such as a silicone substrate. A release agent or the like may be applied to the surface of the substrate for coating to facilitate peeling.

[0292] As shown in step S55 of Fig. 11, the slurry is dried using a drying oven or the like. The temperature during drying may be 25°C or higher and 200°C or lower, preferably 45°C or higher and 85°C or lower. The solvent and the like contained in the slurry are removed by drying.

[0293] As shown in step S56 of Figure 11, the electrolyte layer sheet is peeled off from the coating substrate. The peeling in step S56 may also be referred to as separating the electrolyte layer sheet from the coating substrate. Note that it is preferable to peel off the unpressed electrolyte layer, i.e., the electrolyte layer before pressing, from the coating substrate. The unpressed electrolyte layer may be referred to as the unpressed electrolyte layer.

[0294] As shown in step S58 of FIG. 11 , pressing is performed after drying. A roll press can be used for pressing. The gap of the roll press is set to, for example, 50% to 70% of the film thickness of the unpressed electrolyte layer. For example, when the film thickness of the unpressed electrolyte layer is 140 μm, the gap of the roll press is set to 60 μm to 100 μm, preferably 70 μm to 85 μm.

[0295] As shown in step S59 of FIG. 11 , a first sheet-shaped electrolyte layer can be obtained. The first sheet-shaped electrolyte layer may have a thickness of 100 μm to 150 μm, preferably 120 μm to 140 μm. From SEM (scanning electron microscope) images and the like, voids can be confirmed in the first sheet-shaped electrolyte layer. Furthermore, from SEM images and the like, it can be confirmed that the powder of LLZAO, which is the solid electrolyte, is connected to each other via a binder in the first sheet-shaped electrolyte layer.

[0296] 11, the first sheet-shaped electrolyte layer is heated to a heating temperature of 1000° C. to 1300° C., preferably 1100° C. to 1250° C. The heating atmosphere is preferably an oxygen-containing atmosphere, but may also be an atmosphere containing oxygen and an inert gas, or an atmosphere containing an inert gas.

[0297] 12A and 12B show the state of the first sheet-shaped electrolyte layer during heating. The first sheet-shaped electrolyte layer 125 is heated in a circular punched state. FIG. 12A is a schematic top view showing the first sheet-shaped electrolyte layer 125 placed on an alumina substrate 126. Between the alumina substrate 126 and the first sheet-shaped electrolyte layer 125, there is a region 128 where LLZAO powder is sprayed. It is preferable to spray the LLZAO powder to prevent the alumina substrate 126 and the first sheet-shaped electrolyte layer 125 from adhering to each other.

[0298] 12B is a schematic cross-sectional view showing a region 128 where LLZAO powder is dispersed between the alumina substrate 126 and the first sheet-like electrolyte layer 125. A substrate 129 facing the alumina substrate 126 is disposed so as to cover the region using a gap preservation material 130. An alumina substrate is preferably used for the substrate 129 as well. Furthermore, LLZAO powder may be dispersed on the upper surface of the first sheet-like electrolyte layer 125, and this dispersed region is designated region 128b.

[0299] As shown in step S61 of FIG. 11 , a second sheet-shaped electrolyte layer is obtained. The second sheet-shaped electrolyte layer may shrink more than the first sheet-shaped electrolyte layer due to the heating process. For example, if the first sheet-shaped electrolyte layer is punched into a circular shape with a diameter of 12 mm, the second sheet-shaped electrolyte layer will shrink to a circular shape with a diameter of 10 mm. The second sheet-shaped electrolyte layer may have a thickness of 80 μm to 120 μm, preferably 90 μm to 110 μm, and its thickness is also smaller than that of the first sheet-shaped electrolyte layer.

[0300] SEM observation of the second sheet-shaped electrolyte layer, etc., shows that the solid electrolyte LLZAO has become a sintered body, and voids are confirmed in the second sheet-shaped electrolyte layer. Furthermore, SEM observations, etc., sometimes show that the binder is not visible in the second sheet-shaped electrolyte layer. For example, it is believed that the binder, etc. is removed by the heating in step S60.

[0301] The second sheet-shaped electrolyte layer thus obtained can be used as the solid material for the electrolyte layer 103 shown in the above-described embodiment and the like.

[0302] This embodiment can be used in combination with other embodiments.

[0303] Embodiment 4 In this embodiment, a positive electrode active material that can be used in a secondary battery of one embodiment of the present invention and a manufacturing method thereof will be described.

[0304] 13A and 13B are cross-sectional views of a cathode active material 200 that can be used in a secondary battery of one embodiment of the present invention. Enlarged views of the vicinity of A-B in FIG. 13A are shown in FIGS. 13C and 13D. Enlarged views of the vicinity of C-D in FIG. 13A are shown in FIGS. 13E and 13F.

[0305] 13A to 13F, positive electrode active material 200 has surface layer 200a and interior 200b. In these figures, the boundary between surface layer 200a and interior 200b is indicated by a dashed line. In addition, FIG. 13B shows an example of a crystal grain boundary 201 by a dashed-dotted line.

[0306] In this specification, the surface layer 200a of the positive electrode active material 200 refers to, for example, a region within 50 nm from the surface toward the inside, more preferably within 35 nm from the surface toward the inside, even more preferably within 20 nm from the surface toward the inside, and most preferably within 10 nm from the surface toward the inside. Surfaces caused by cracks and / or fissures may also be referred to as the surface. The surface layer 200a is synonymous with the near-surface, near-surface region, or shell.

[0307] The region deeper than the surface layer 200a of the positive electrode active material is referred to as the inner portion 200b, which is synonymous with the inner region or core.

[0308] The surface of the positive electrode active material 200 refers to the surface of the composite oxide including the surface layer 200a, the interior 200b, and the protrusions 203. The positive electrode active material 200 does not contain carbonates, hydroxyl groups, or the like that are chemically adsorbed after preparation. The positive electrode active material 200 also does not contain electrolytes, binders, conductive materials, or compounds derived therefrom that are attached to the positive electrode active material 200. The surface of the positive electrode active material 200 in a cross-sectional STEM (scanning transmission electron microscope) image or the like is the boundary between an area where an electron beam combined image is observed and an area where it is not observed, and is the outermost area where bright spots originating from the atomic nuclei of metal elements with atomic numbers larger than that of lithium are observed. The surface in a cross-sectional STEM image or the like may be determined in conjunction with the results of an analysis with higher spatial resolution, such as electron energy loss spectroscopy (EELS).

[0309] The crystal grain boundary 201 refers to, for example, a portion where the positive electrode active material 200 adheres to itself, a portion where the crystal orientation changes within the positive electrode active material 200, i.e., a portion where the repetition of bright and dark lines in an STEM image or the like becomes discontinuous, a portion containing many crystal defects, a portion where the crystal structure is disordered, etc. The crystal defect refers to a defect that can be observed in a cross-sectional TEM (transmission electron microscope), a cross-sectional STEM image, etc., that is, a structure in which other elements have entered between the lattices, a cavity, etc. The crystal grain boundary 201 can be said to be one type of planar defect. The vicinity of the crystal grain boundary 201 refers to a region within 10 nm of the crystal grain boundary 201.

[0310] <Containing Elements> The positive electrode active material 200 contains lithium, a transition metal M, oxygen, and an additive element A. Alternatively, the positive electrode active material 200 may contain a composite oxide containing lithium and a transition metal M (LiMO). ２ ) to which an additive element A is added. However, the composition of the composite oxide is not strictly limited to Li:M:O=1:1:2. In addition, a positive electrode active material to which an additive element A is added is also sometimes called a composite oxide.

[0311] The positive electrode active material of a lithium-ion secondary battery must contain a transition metal capable of oxidation and reduction in order to maintain charge neutrality even when lithium ions are inserted and removed. The positive electrode active material 200 of one embodiment of the present invention preferably uses cobalt as the transition metal M responsible for the oxidation and reduction reaction. In addition to cobalt, one or more selected from nickel and manganese may also be used. It is preferable that the positive electrode active material 200 contains 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of cobalt, among the transition metals M, because of its many advantages, such as relatively easy synthesis, ease of handling, and excellent cycle characteristics.

[0312] Furthermore, when cobalt is 75 atomic % or more, preferably 90 atomic % or more, and more preferably 95 atomic % or more of the transition metal M of the positive electrode active material 200, lithium nickel oxide (LiNiO ２ ) and other composite oxides in which nickel accounts for the majority of the transition metal M, ｘ CoO ２ The stability is superior when x in the formula is small. This is thought to be because cobalt is less susceptible to distortion due to the Jahn-Teller effect than nickel. The strength of the Jahn-Teller effect in transition metal compounds varies depending on the number of electrons in the d orbital of the transition metal. Layered rock-salt composite oxides, such as lithium nickel oxide, in which octahedral low-spin nickel(III) dominates, are significantly affected by the Jahn-Teller effect, making the nickel-oxygen octahedral layers prone to distortion. This increases the risk of crystal structure collapse during charge-discharge cycles. Furthermore, nickel ions have a larger ionic radius than cobalt ions, closer to the size of lithium ions. Therefore, layered rock-salt composite oxides, such as lithium nickel oxide, in which nickel dominates, have the problem of nickel and lithium cation mixing being more likely to occur.

[0313] On the other hand, when nickel is used as the transition metal M in the positive electrode active material 200 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 discharge capacity per weight may increase, which is preferable.

[0314] The additive element A contained in the positive electrode active material 200 is preferably one or more selected from magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, and beryllium. The additive element A is preferably less than 25 atomic % of the transition metals (or the sum of the transition metals when there are two or more transition metals), more preferably less than 10 atomic %, and even more preferably less than 5 atomic %.

[0315] That is, the positive electrode active material 200 may include lithium cobalt oxide doped with magnesium and fluorine, lithium cobalt oxide doped with magnesium, fluorine, and titanium, lithium cobalt oxide doped with magnesium, fluorine, and aluminum, lithium cobalt oxide doped with magnesium, fluorine, and nickel, lithium cobalt oxide doped with magnesium, fluorine, nickel, and aluminum, etc.

[0316] As will be described later, these additional elements A further stabilize the crystal structure of the positive electrode active material 200. In this specification and the like, the additional elements A are part of the raw materials of the positive electrode active material, but are referred to as the additional elements because their concentration is lower than that of the main component.

[0317] The additional element A does not necessarily have to contain magnesium, fluorine, nickel, aluminum, titanium, zirconium, vanadium, iron, manganese, chromium, niobium, arsenic, zinc, silicon, sulfur, phosphorus, boron, bromine, or beryllium.

[0318] For example, if the cathode active material 200 is substantially free of manganese, the advantages of relatively easy synthesis and handling, and excellent cycle characteristics, are further enhanced. The weight of manganese contained in the cathode active material 200 is preferably, for example, 600 ppm or less, more preferably 100 ppm or less. The weight of manganese can be analyzed using, for example, glow discharge mass spectrometry (GD-MS).

[0319] <Crystal Structure> Using Figs. 14 to 20, Li ｘ CoO ２The change in the crystal structure due to x in the positive electrode active material will be described by comparing a conventional positive electrode active material with the positive electrode active material 200 of one embodiment of the present invention. The value of x indicates the amount of intercalable and deintercalable lithium remaining in the lithium cobalt oxide, and Li ｘ CoO ２ It can be said that this is the lithium occupancy rate in the above range. Note that Co is an example of a transition metal, and cobalt may be appropriately read as the transition metal M, and the cobalt site may be appropriately read as the transition metal M site.

[0320] In this specification, the layered rock-salt type crystal structure belonging to the space group R-3m, which is possessed by a composite oxide containing lithium and a transition metal M such as cobalt, 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 M and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt type crystal structure may have a distorted lattice structure of the rock-salt type crystal structure.

[0321] The rock salt crystal structure refers to a cubic crystal structure, such as that of the space group Fm-3m, in which cations and anions are arranged alternately. Note that cation or anion defects are also acceptable.

[0322] The fact that the material has both the characteristics of the layered rock salt type crystal structure and the rock salt type crystal structure can be determined by electron diffraction, TEM images, cross-sectional STEM images, and the like.

[0323] In the rock salt crystal structure, there is no distinction in the cation sites, but in the layered rock salt crystal structure, there are two types of cation sites, one of which is mostly occupied by lithium and the other by a transition metal M. The layered structure in which two-dimensional planes of cations and two-dimensional planes of anions are alternately arranged is the same in both the rock salt crystal structure and the layered rock salt crystal structure. Among the bright spots in the electron diffraction pattern corresponding to the crystal planes that form this two-dimensional plane, when the central spot (transmitted spot) is set as the origin 000, the bright spot closest to the central spot is, for example, the (111) plane in an ideal rock salt crystal structure, and, for example, the (003) plane in a layered rock salt crystal structure. For example, MgO with a rock salt crystal structure and LiCoO with a layered rock salt crystal structure ２ When comparing the electron diffraction patterns of LiCoO ２ The bright spots on the (003) plane of MgO are observed at a distance about half the distance of the bright spots on the (111) plane of MgO. ２ In the case of a material having these two phases, the electron diffraction pattern shows the presence of crystal planes in which bright spots of high brightness and bright spots of low brightness are arranged alternately. Bright spots common to both the rock salt type crystal structure and the layered rock salt type crystal structure have high brightness, while bright spots occurring only in the layered rock salt type crystal structure have low brightness.

[0324] Furthermore, when a layered rock-salt crystal structure is observed from a direction perpendicular to the c-axis in cross-sectional STEM images, layers observed with high brightness and layers observed with low brightness are observed alternately. Rock-salt crystal structures do not exhibit such characteristics because there is no distinction between cation sites. In the case of a crystal structure that has the characteristics of both rock-salt and layered rock-salt crystal structures, when observed from a specific crystal orientation, layers observed with high brightness and layers observed with low brightness are observed alternately in cross-sectional STEM images, and the layers observed with low brightness correspond to lithium layers, with metals with atomic numbers higher than that of lithium present in some of the lithium layers.

[0325] The layered rock salt crystal structure and the anions in the rock salt crystal structure form a cubic close-packed structure (face-centered cubic lattice structure). It is estimated that the anions in the O3' crystal structure, which will be described later, also form a cubic close-packed structure. Therefore, when a layered rock salt crystal structure and a rock salt crystal structure come into contact, there are crystal planes where the cubic close-packed structure formed by the anions is oriented in the same direction.

[0326] Alternatively, it can be explained as follows: Anions on the {111} plane of a cubic crystal structure have a triangular lattice. Layered rock salt crystal structures are in the space group R-3m and are rhombohedral structures, but are generally expressed as a compound hexagonal lattice to facilitate understanding of the structure, and the (0001) plane of a layered rock salt crystal structure has a hexagonal lattice. The triangular lattice on the cubic {111} plane has the same atomic arrangement as the hexagonal lattice on the (0001) plane of a layered rock salt crystal structure. The compatibility of the two lattices can be said to be the alignment of the cubic close-packed structures.

[0327] However, the space group of the layered rock salt type crystal structure and the O3' type crystal structure described below is R-3m, which is different from the space group Fm-3m of the rock salt type crystal structure (space group Fm-3m is the space group of a general rock salt type crystal structure), and therefore the Miller indices of the crystal planes that satisfy the above conditions are different between the layered rock salt type crystal structure and the O3' type crystal structure and the rock salt type crystal structure. In this specification, when the orientations of the cubic close-packed structures formed by anions are aligned in the layered rock salt type crystal structure, the O3' type crystal structure and the rock salt type crystal structure, it may be said that the crystal orientations are approximately the same.

[0328] The fact that the crystal orientation in the two regions roughly coincides can be determined from a TEM image, a STEM (scanning transmission electron microscope) image, a HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) image, an ABF-STEM (annular bright-field scanning transmission electron microscope) image, an electron diffraction pattern, an FFT pattern of a TEM image, a STEM image, etc. XRD, electron diffraction, neutron diffraction, etc. can also be used as materials for determination.

[0329] 14 shows an example of a TEM image in which the orientation of the circled layered rock-salt-type crystal structure LRS and the circled rock-salt-type crystal structure RS generally coincides. Images reflecting the crystal structure can be obtained from such a TEM image as well as STEM images, HAADF-STEM images, and ABF-STEM images.

[0330] For example, in a high-resolution TEM image, a contrast originating from a crystal plane can be obtained. When an electron beam is incident in a direction perpendicular to the c-axis of a composite hexagonal lattice of a layered rock-salt crystal structure, for example, due to the diffraction and interference of the electron beam, the contrast originating from the (0003) plane is obtained as a repetition of bright bands (bright strips or bright lines) and dark bands (dark strips or dark lines). Therefore, a repetition of bright lines and dark lines is observed in the TEM image, and the bright lines (for example, the L shown in FIG. 14) are not clearly distinguishable from each other. ＲＳ and L ＬＲＳ When the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal planes are roughly aligned, i.e., the crystal orientations are roughly aligned. Similarly, when the angle between the dark lines is 5 degrees or less, or 2.5 degrees or less, it can be determined that the crystal orientations are roughly aligned.

[0331] Furthermore, in HAADF-STEM images, contrast proportional to atomic number is obtained, with elements with higher atomic numbers being observed brighter. For example, in the case of layered rock-salt lithium cobaltate belonging to the space group R-3m, cobalt (atomic number 27) has the highest atomic number, so the electron beam is strongly scattered at the position of the cobalt atoms, and the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots. Therefore, when lithium cobaltate having a layered rock-salt crystal structure is observed in the direction perpendicular to the c-axis, the arrangement of the cobalt atoms is observed as a bright line or an arrangement of highly bright dots in the direction perpendicular to the c-axis, while the arrangements of lithium and oxygen atoms are observed as dark lines or low-brightness regions. The same is true when lithium cobaltate contains fluorine (atomic number 9) and magnesium (atomic number 12) as additive elements.

[0332] Therefore, in an HAADF-STEM image, when repetitions of bright and dark lines are observed in two regions with different crystal structures and the angle between the bright lines is 5 degrees or less or 2.5 degrees or less, it can be determined that the atomic arrangements are roughly consistent, i.e., the crystal orientations are roughly consistent. Similarly, when the angle between the dark lines is 5 degrees or less or 2.5 degrees or less, it can also be determined that the crystal orientations are roughly consistent.

[0333] In ABF-STEM, elements with smaller atomic numbers are observed brighter, but like HAADF-STEM, contrast according to atomic number is obtained, and therefore ABF-STEM images can be used to determine the crystal orientation in the same way as HAADF-STEM images.

[0334] Figure 15A shows an example of an STEM image in which the orientation of the layered rock-salt-type crystal structure LRS marked with a square and the orientation of the rock-salt-type crystal structure RS marked with a square are roughly the same. Figure 15B shows the FFT of the region of the rock-salt-type crystal structure RS, and Figure 15C shows the FFT of the region of the layered rock-salt-type crystal structure LRS. The composition is shown on the left of Figures 15B and 15C, and the JCPDS card number is shown to the right of the composition, along with the d value and angle calculated from this. Measured values ​​are shown on the right. The spot marked with O is the zeroth-order diffraction.

[0335] The spot marked A in Figure 15B is derived from the 11-1 reflection of the cubic crystal. The spot marked A in Figure 15C is derived from the 0003 reflection of the layered rock salt crystal structure. From Figures 15B and 15C, it can be seen that the orientation of the 11-1 reflection of the cubic crystal and the orientation of the 0003 reflection of the layered rock salt crystal structure roughly coincide. In other words, it can be seen that the line passing through AO in Figure 15B and the line passing through AO in Figure 15C are roughly parallel. Here, "roughly coincident" and "roughly parallel" mean that the angle between the lines is 5 degrees or less, or 2.5 degrees or less.

[0336] In this way, in FFT and electron beam diffraction, when the orientations of the layered rock salt type crystal structure and the rock salt type crystal structure roughly coincide, the <0003> orientation of the layered rock salt type crystal structure and the <11-1> orientation of the rock salt type crystal structure may roughly coincide. In this case, it is preferable that these reciprocal lattice points are spot-like, that is, not continuous with other reciprocal lattice points. A reciprocal lattice point that is spot-like and not continuous with other reciprocal lattice points indicates high crystallinity.

[0337] Furthermore, as described above, when the orientation of the 11-1 reflection of a cubic crystal and the orientation of the 0003 reflection of a layered rock-salt crystal structure are approximately the same, depending on the incident orientation of the electron beam, spots not originating from the 0003 reflection of the layered rock-salt crystal structure may be observed in a reciprocal lattice space different from the orientation of the 0003 reflection of the layered rock-salt crystal structure. For example, the spot marked B in FIG. 15C originates from the 1014 reflection of the layered rock-salt crystal structure. This spot may be observed at an angle of 52° to 56° (i.e., ∠AOB is 52° to 56°) from the orientation of the reciprocal lattice point (A in FIG. 15C) originating from the 0003 reflection of the layered rock-salt crystal structure, and at a point where d is 0.19 nm to 0.21 nm. Note that this index is merely an example and does not necessarily have to be the same. For example, equivalent reciprocal lattice points in each may be used.

[0338] Similarly, spots not originating from the 11-1 reflection of the cubic crystal may be observed in a reciprocal lattice space other than the orientation where the 11-1 reflection of the cubic crystal is observed. For example, the spot marked B in FIG. 15B is originating from the 200 reflection of the cubic crystal. This is because a diffraction spot may be observed at an angle of 54° or more and 56° or less (i.e., ∠AOB is 54° or more and 56° or less) from the orientation of the reflection (A in FIG. 15B) originating from the 11-1 reflection of the cubic crystal. Note that this index is just an example and does not necessarily have to match. For example, equivalent reciprocal lattice points in each may be used.

[0339] It is known that positive electrode active materials with a layered rock-salt crystal structure, such as lithium cobalt oxide, tend to exhibit the (0003) plane and its equivalents, as well as the (10-14) plane and its equivalents, as crystal planes. Therefore, by carefully observing the shape of the positive electrode active material using an SEM or the like, it is possible to thin-section the observation sample using an FIB or the like so that the electron beam is [12-10] incident in a TEM or the like, making the (0003) plane easier to observe. When determining whether the crystal orientation is consistent, it is preferable to thin-section the sample so that the (0003) plane of the layered rock-salt crystal structure is easier to observe.

[0340] <Li ｘ CoO ２ When x is 1 in the figure, the discharge state, that is, Li ｘ CoO ２ 17 shows the crystal structure of conventional lithium cobalt oxide when x is 1 (x=1). The crystal structure has a layered rock salt type crystal structure belonging to the space group R-3m. The conventional positive electrode active material shown in FIG. 17 is a lithium cobalt oxide (LiCoO ２ In particular, the change in the crystal structure of lithium cobalt oxide not containing the additive element A is described in Non-Patent Documents 1 to 3, etc.

[0341] Furthermore, in this crystal structure, lithium occupies octahedral sites and CoO ２ There are three layers. Therefore, this crystal structure is sometimes called an O3 type crystal structure. ２ The layer is a layer in which octahedral structures in which oxygen is six-coordinated to cobalt are connected in a planar structure with edge-sharing. This is sometimes called a layer consisting of cobalt and oxygen octahedra. In Figure 17, the crystal structure when x = 1 is labeled R-3m(O3).

[0342] FIG. 16 shows the discharge state, i.e., Li ｘ CoO ２1 shows a crystal structure of lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention when x is 1 (x=1). The crystal structure has a layered rock-salt crystal structure belonging to the space group R-3m. In this crystal structure, lithium occupies octahedral sites and CoO ２ In Fig. 16, the crystal structure when x = 1 is denoted by R-3m(O3).

[0343] Lithium cobalt oxide having a layered rock-salt crystal structure has a high discharge capacity, two-dimensional lithium ion diffusion paths, and is suitable for lithium ion insertion / extraction reactions, making it an excellent positive electrode active material for secondary batteries. Therefore, in the positive electrode active material 200 of one embodiment of the present invention, the inner portion 200b, which occupies the majority of the volume, is preferably lithium cobalt oxide having a layered rock-salt crystal structure.

[0344] The surface layer 200 a of the lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention remains unchanged even when lithium is removed from the positive electrode active material 200 by charging, and the layer (e.g., CoO ２ It is preferable that the surface layer portion 200a has a function of reinforcing the layer structure of the cathode active material 200 (the inner layer 200b) so that it is not broken. That is, it is preferable that the surface layer portion 200a functions as a barrier film for the cathode active material 200. Alternatively, it is preferable that the surface layer portion 200a reinforces the cathode active material 200. Reinforcing includes suppressing structural changes in the surface layer portion 200a and the inner portion 200b of the cathode active material 200 and / or suppressing oxidative decomposition of the electrolyte on the surface of the cathode active material 200.

[0345] Therefore, in the lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention, the surface layer portion 200a preferably has a different crystal structure from the inner portion 200b. Specifically, the surface layer portion 200a preferably has a composition and crystal structure that are more stable at room temperature (25°C) than the inner portion 200b. For example, the surface layer portion 200a preferably has at least a rock-salt crystal structure. It is more preferable that the entire surface layer portion 200a has a rock-salt crystal structure, but this is not limited thereto. For example, the surface layer portion 200a may have both a rock-salt crystal structure and a layered rock-salt crystal structure.

[0346] Here, the surface layer 200a will be described. The surface layer 200a is the region from which lithium ions are first desorbed during charging, and is a region where the lithium concentration is likely to be lower than that of the interior 200b. In addition, it can be said that, on the surface of the surface layer 200a, due to the desorption of lithium ions, atoms (e.g., oxygen) that make up lithium cobalt oxide are present in a state where their bonds are broken. In other words, the surface layer 200a is more likely to become unstable than the interior 200b, and is a region where deterioration of the crystal structure is likely to begin. Therefore, if at least the surface layer 200a can be made sufficiently stable, Li ｘ CoO ２ Even when x is small (for example, x is 0.24 or less), the layer structure of the inner portion 200b made of cobalt and oxygen octahedra can be made less likely to break. Note that breaking of the layer structure includes displacement of the edges of the layer structure made of cobalt and oxygen octahedra, and if the surface layer portion 200a is sufficiently stable, this displacement can be suppressed.

[0347] To stabilize the surface layer portion 200a, it is sufficient for the surface layer portion 200a to have a stable composition or stable crystalline structure. For this purpose, the surface layer portion 200a preferably contains an additive element A. It is more preferable that the additive element A contains two or more elements with different concentration distributions, such as additive element X and additive element Y, which will be described later. Furthermore, the surface layer portion 200a containing the additive element A includes a situation in which the concentration of the additive element A present in the surface layer portion 200a is higher than the concentration of the additive element A present in the interior portion 200b. The situation in which the concentration of the additive element varies includes a situation in which the additive element A has a concentration gradient in the surface layer portion 200a, or a situation in which the additive element A has a concentration gradient from the surface layer portion 200a to the interior portion 200b. Furthermore, in the case of additive element X having a concentration gradient and additive element Y having a concentration gradient, it is preferable that the concentration distributions indicating the concentration gradients are different from each other. It is even more preferable that the peak position indicating the maximum concentration of additive element X and the peak position indicating the maximum concentration of additive element Y are different from each other. The maximum concentration value is sometimes referred to as a peak top, and the maximum concentration value is sometimes referred to as a peak.

[0348] For example, the additional element X selected from the additional elements A preferably has a concentration distribution that increases from the interior 200b toward the surface, as shown by the gradation in Fig. 13C, and specifically is one or more elements selected from magnesium, fluorine, nickel, titanium, silicon, phosphorus, boron, calcium, etc. The peak top of the additional element X is preferably present in the surface layer portion 200a. For example, the additional element X preferably has a concentration distribution in which the peak top is located in a region of 0.5 nm to 10 nm from the surface toward the interior.

[0349] The additive element Y selected from the additive elements A preferably has a concentration gradient as shown by the density of the hatching in FIG. 13D and a peak top in a region deeper than the peak top in FIG. 13C , and specifically is one or more elements selected from aluminum, manganese, etc. The peak top of the additive element Y may be present in the surface layer portion 200a or may be present deeper than the surface layer portion 200a. For example, the additive element Y preferably has a concentration distribution in which the peak top is located in a region of 5 nm to 30 nm from the surface toward the interior. The position of the peak top of the additive element Y is preferably different from the position of the peak top of the additive element X. Furthermore, the concentration distribution of the additive element Y is preferably different from the concentration distribution of the additive element X.

[0350] For example, magnesium ions, which are one of the additive elements X, are divalent, and since the magnesium ions are more stable at the lithium site than at the cobalt site in the layered rock-salt crystal structure, they are more likely to enter the lithium site. In other words, when magnesium is present at an appropriate concentration at the lithium site in the surface layer 200a, the layered rock-salt crystal structure in the interior 200b is more easily maintained. This is because magnesium present at the lithium site in the surface layer 200a is easily absorbed by CoO ２ It is presumed that this is because it functions as a pillar supporting the layers. In addition, the presence of magnesium in lithium cobalt oxide allows Li ｘ CoO ２Even when x in the graph is, for example, 0.24 or less, the desorption of oxygen around the magnesium can be suppressed. Furthermore, the presence of magnesium is expected to increase the density of the lithium cobalt oxide. Furthermore, if the magnesium concentration in the surface layer portion 200a is higher than that in the interior portion 200b, it is expected that the corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte will be improved.

[0351] At an appropriate concentration, magnesium does not adversely affect the intercalation and deintercalation of lithium during charging and discharging, providing the above benefits. However, excessive magnesium may adversely affect the intercalation and deintercalation of lithium. Furthermore, its effect on stabilizing the crystal structure may be reduced. This is thought to be because, as the magnesium concentration increases, magnesium occupies both the lithium and cobalt sites. Furthermore, magnesium may not substitute for either the lithium or cobalt sites, but may form magnesium compounds (e.g., oxides or fluorides) and segregate on the surface of the positive electrode active material, potentially becoming a resistive component in secondary batteries. Furthermore, as the magnesium concentration of the positive electrode active material increases, the discharge capacity may decrease. This is thought to be due to excessive magnesium occupancy at the lithium sites, reducing the amount of lithium contributing to charging and discharging.

[0352] Therefore, it is preferable that the amount of magnesium contained in the entire positive electrode active material 200 is appropriate. For example, the number of magnesium atoms is preferably 0.001 to 0.1 times the number of cobalt atoms, more preferably more than 0.01 to less than 0.04 times, and even more preferably about 0.02 times. The amount of magnesium contained in the entire positive electrode active material 200 referred to here may be a value obtained by performing elemental analysis of the entire positive electrode active material 200 using, for example, GD-MS or ICP-MS (inductively coupled plasma mass spectrometry), or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material 200.

[0353] Nickel, which is one of the additive elements X, can exist on either the cobalt site or the lithium site. When nickel exists on the cobalt site, it has a lower oxidation-reduction potential than cobalt, which leads to an increase in discharge capacity, which is preferable.

[0354] Furthermore, when nickel is present at the lithium site, a layer consisting of an octahedron of cobalt and oxygen (e.g., CoO ２ The displacement of the layer (layer) is suppressed. Also, the change in volume caused by charging and discharging is suppressed. Also, the elastic modulus increases, that is, the material becomes hard. This is because the nickel present in the lithium site and the CoO ２ This is presumably because the layers function as pillars to support each other. Therefore, it is expected that the crystal structure will be more stable, particularly in a charged state under a high temperature environment, for example, 45° C. or higher, which is preferable.

[0355] On the other hand, excessive nickel may increase the influence of strain due to the Jahn-Teller effect, and may also adversely affect lithium insertion and extraction.

[0356] Therefore, it is preferable that the total amount of nickel in the positive electrode active material 200 is an appropriate amount. For example, the number of nickel atoms in the positive electrode active material 200 is preferably more than 0% but not more than 7.5% of the total number of cobalt atoms, preferably 0.05% to 4%, preferably 0.1% to 2%, and more preferably 0.2% to 1%. Alternatively, it is preferably more than 0% but not more than 4%. Alternatively, it is preferably more than 0% but not more than 2%. Alternatively, it is preferably 0.05% to 7.5%. Alternatively, it is preferably 0.05% to 2%. Alternatively, it is preferably 0.1% to 7.5%. Alternatively, it is preferably 0.1% to 4%. The amount of nickel shown here may be, for example, a value obtained by performing elemental analysis of the entire positive electrode active material using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the positive electrode active material.

[0357] Furthermore, aluminum, one of the additive elements Y, can exist at the cobalt site in the layered rock salt crystal structure. Because aluminum is a trivalent typical element and its valence does not change, lithium around the aluminum is less likely to move during charging and discharging. Therefore, the aluminum and its surrounding lithium function as pillars, suppressing changes in the crystal structure. Furthermore, aluminum has the effect of suppressing the elution of the surrounding transition metal M and improving continuous charging durability. Furthermore, because the Al—O bond is stronger than the Co—O bond, it can suppress the desorption of oxygen around the aluminum. These effects improve thermal stability. Therefore, the presence of aluminum as the additive element Y can improve safety when used in secondary batteries. Furthermore, a positive electrode active material 200 can be obtained whose crystal structure is less likely to collapse even after repeated charging and discharging.

[0358] On the other hand, an excess of aluminum may adversely affect the intercalation and deintercalation of lithium.

[0359] Therefore, it is preferable that the total amount of aluminum contained in the cathode active material 200 is appropriate. For example, the number of aluminum atoms contained in the entire cathode active material 200 is preferably 0.05% to 4% of the number of cobalt atoms, preferably 0.1% to 2% and more preferably 0.3% to 1.5%. Alternatively, 0.05% to 2% is preferable. Alternatively, 0.1% to 4% is preferable. The amount contained in the entire cathode active material 200 referred to here may be, for example, a value obtained by performing elemental analysis of the entire cathode active material 200 using GD-MS, ICP-MS, or the like, or may be based on the value of the composition of raw materials in the process of producing the cathode active material 200.

[0360] Furthermore, fluorine, one of the additive elements X, is a monovalent anion. When some of the oxygen atoms in the surface layer 200a are substituted with fluorine, the lithium desorption energy decreases. This is because the valence of cobalt ions changes with lithium desorption (from trivalent to tetravalent in the absence of fluorine, and from divalent to trivalent in the presence of fluorine), resulting in different redox potentials. Therefore, when some of the oxygen atoms in the surface layer 200a are substituted with fluorine, it can be said that desorption and insertion of lithium ions near the fluorine atoms occurs more smoothly. Therefore, when fluorine-containing lithium cobalt oxide is used in a secondary battery, charge / discharge characteristics, current characteristics, etc. can be improved. Furthermore, the presence of fluorine in the surface layer 200a, which has a surface that contacts the electrolyte, can effectively improve corrosion resistance against hydrofluoric acid. As will be described later, when the melting point of a fluoride, such as lithium fluoride, is lower than that of the other additive element A source, it can function as a flux (also called a fluxing agent) that lowers the melting point of the other additive element A source.

[0361] Furthermore, titanium oxide, which is one of the additive elements X, is known to have superhydrophilicity. Therefore, by providing the cathode active material 200 with titanium oxide in the surface layer portion 200a, it is possible that the cathode active material 200 has good wettability with a highly polar solvent. When used in a secondary battery, this improves the contact at the interface between the cathode active material 200 and a highly polar electrolyte, which may suppress an increase in internal resistance.

[0362] Furthermore, when phosphorus, which is one of the additive elements X, is contained in the surface layer portion 200a, Li ｘ CoO ２ When the value of x in the graph is kept small, short circuits can be prevented, which is preferable. For example, it is preferable that the graphite oxide is present in the surface layer portion 200a as a compound containing phosphorus and oxygen.

[0363] When the positive electrode active material 200 contains phosphorus, the hydrogen fluoride generated by decomposition of the electrolyte reacts with the phosphorus, which may reduce the concentration of hydrogen fluoride in the electrolyte, which is preferable.

[0364] LiPF as a lithium salt ６In the case of a cathode containing a fluoride-containing electrolyte, hydrogen fluoride may be generated by hydrolysis. Hydrogen fluoride may also be generated by a reaction between polyvinylidene fluoride (PVDF), which is used as a component of the cathode, and an alkali. A reduction in the hydrogen fluoride concentration in the electrolyte may suppress corrosion of the current collector. Furthermore, a decrease in adhesion due to insolubilization of PVDF may be suppressed.

[0365] When the positive electrode active material 200 contains phosphorus together with magnesium, Li ｘ CoO ２ This is preferable because stability is extremely high when x is small in the positive electrode active material 200. When the positive electrode active material 200 contains phosphorus, the number of phosphorus atoms is preferably 1% to 20% of the number of cobalt atoms, more preferably 2% to 10%, and even more preferably 3% to 8%. Or 1% to 10% is preferable. Or 1% to 8% is preferable. Or 2% to 20% is preferable. Or 2% to 8% is preferable. Or 3% to 20% is preferable. Or 3% to 10% is preferable. In addition, the number of magnesium atoms is preferably 0.1% to 10% of the number of cobalt atoms, more preferably 0.5% to 5%, and more preferably 0.7% to 4%. Or 0.1% to 5% is preferable. Or 0.1% to 4% is preferable. Or 0.5% to 10% is preferable. Or 0.5% to 4% is preferable. Or 0.7% to 10% is preferable. The concentrations of phosphorus and magnesium shown here may be values ​​obtained by performing elemental analysis of the entire cathode active material 200 using, for example, GC-MS, ICP-MS, or the like, or may be based on values ​​of the composition of raw materials in the process of producing the cathode active material 200.

[0366] Furthermore, when the positive electrode active material 200 has a crack, the progression of the crack can be suppressed by the presence of phosphorus, more specifically, a compound containing phosphorus and oxygen, inside the positive electrode active material with the crack on the surface, for example, in the embedded portion 202 shown in FIG. 13B .

[0367] Furthermore, when the surface layer 200a contains both magnesium and nickel, there is a possibility that divalent magnesium can exist more stably near divalent nickel.ｘ CoO ２ Even when the value of x in the formula is small, the elution of magnesium can be suppressed, which can contribute to the stabilization of the surface layer portion 200a.

[0368] Furthermore, when the additive element A is a combination of additive element X and additive element Y, the concentration distribution of additive element X differs from the concentration distribution of additive element Y, which is preferable because it stabilizes the crystal structure in a wider region. For example, when the positive electrode active material 200 contains both magnesium and nickel, which are part of the additive element X, and aluminum, which is one of the additive elements Y, it can stabilize the crystal structure in a wider region than when it contains only one of the additive element X and the additive element Y. In this way, when the positive electrode active material 200 contains both additive element X and additive element Y, the surface can be sufficiently stabilized by additive element X, such as magnesium, so that additive element Y, such as aluminum, is not essential to the surface. Rather, it is preferable for aluminum to be widely distributed in a deep region, for example, a region from the surface to a depth of 5 nm to 50 nm, because this stabilizes the crystal structure in a wider region.

[0369] As described above, when a plurality of additional elements A are contained, the effects of the respective additional elements A are synergistic and can contribute to further stabilization of the surface layer portion 200 a and the inner portion 200 b. In particular, when magnesium, nickel, and aluminum are contained, the effect of forming a stable crystal structure is high and it is preferable.

[0370] However, if the surface layer 200a is occupied only by a compound of the additional element A and oxygen, it is not preferable because it makes it difficult to insert and extract lithium. For example, it is not preferable for the surface layer 200a to be occupied only by MgO, a structure in which MgO and NiO(II) are solid-solved, and / or a structure in which MgO and CoO(II) are solid-solved. Therefore, the surface layer 200a must contain at least cobalt, and in a discharged state, it must also contain lithium, and must have a path for the insertion and extraction of lithium.

[0371] In order to ensure a path for lithium insertion / desorption, it is preferable that the concentration of cobalt in the surface layer 200a is higher than that of magnesium. For example, when the atomic number A of magnesium is Ｍｇ and the number of cobalt atoms A Ｃｏ Ratio A Ｍｇ / A Ｃｏis preferably 0.62 or more. The surface layer 200a preferably has a higher cobalt concentration than nickel. The surface layer 200a preferably has a higher cobalt concentration than aluminum. The surface layer 200a preferably has a higher cobalt concentration than fluorine.

[0372] Furthermore, since too much nickel may inhibit the diffusion of lithium, it is preferable that the concentration of magnesium is higher than that of nickel in the surface layer portion 200a. For example, it is preferable that the number of nickel atoms is 1 / 6 or less of the number of magnesium atoms.

[0373] Furthermore, although it is preferable that a part of the additive element A, particularly magnesium, nickel, and aluminum, has a higher concentration in the surface layer portion 200a than in the interior portion 200b, it is also preferable that it is present randomly and in a sparse manner in the interior portion 200b. When magnesium and aluminum are present at an appropriate concentration in the lithium sites in the interior portion 200b, it has the effect of easily maintaining the layered rock-salt type crystal structure, as described above. Furthermore, when nickel is present at an appropriate concentration in the interior portion 200b, it is possible to prevent the formation of layers consisting of octahedra of cobalt and oxygen (for example, CoO ２ Furthermore, when magnesium and nickel are present together, divalent magnesium may be able to exist more stably near divalent nickel, so a synergistic effect of suppressing the elution of magnesium can be expected.

[0374] It is also preferable that the crystal structure continuously changes from the interior 200b toward the surface due to the concentration gradient of the added element A. Alternatively, it is preferable that the crystal orientation of the surface layer 200a and the interior 200b roughly coincide.

[0375] For example, it is preferable that the crystal structure continuously change from the interior 200b having a layered rock salt type crystal structure to the surface layer 200a (i.e., the surface) having a rock salt type crystal structure or characteristics of both a rock salt type crystal structure and a layered rock salt type crystal structure. Alternatively, it is preferable that the orientation of the rock salt type crystal structure or the surface layer 200a having characteristics of both a rock salt type crystal structure and a layered rock salt type crystal structure is approximately the same as that of the interior 200b having the layered rock salt type crystal structure.

[0376] <Li ｘ CoO ２ State in which x is small in the positive electrode active material 200 of one embodiment of the present invention has the above-described distribution of the additional element A and / or the crystal structure in the discharged state, and therefore, ｘ CoO ２ The crystal structure when x is small is different from that of conventional positive electrode active materials. Here, "small x" means 0.1<x≦0.24.

[0377] First, the change in the crystal structure of the conventional positive electrode active material is shown in Figure 17. Conventional lithium cobalt oxide has a structure similar to Li ｘ CoO ２ It is known that when x is about 0.5 (when x = 0.5), the symmetry of lithium increases and the crystal structure belongs to the monoclinic space group P2 / m. This structure has CoO ２ There is one layer. Therefore, it is sometimes called O1 type or monoclinic O1 type. In Figure 17, the crystal structure when x = 0.5 is labeled P2 / m (monoclinic O1).

[0378] Also Li ｘ CoO ２ When x is 0 (x=0), conventional lithium cobalt oxide has a crystal structure of the trigonal space group P-3m1, and the unit cell contains CoO ２ There is one layer. Therefore, this crystal structure is sometimes called O1 type or trigonal O1 type. In addition, the trigonal crystal is sometimes converted into a composite hexagonal lattice and called hexagonal O1 type. In Figure 17, the crystal structure when x = 0 is labeled P-3m1 (trigonal O2).

[0379] Also Li ｘ CoO ２ When x is about 0.24 (when x=about 0.24), conventional lithium cobalt oxide has a crystal structure of the space group R-3m. This structure is similar to CoO, such as trigonal O1 type. ２ and LiCoO such as R-3m(O3). ２It can be said that the structure of and are stacked alternately. Therefore, this crystal structure is sometimes called an H1-3 type crystal structure. In Figure 17, the crystal structure when x = 0.12 is labeled R-3m (H1-3).

[0380] In reality, the number of cobalt atoms per unit cell in the H1-3 crystal structure is twice that of other structures. However, in this specification, including Figure 17, the c-axis of the H1-3 crystal structure is shown as half the unit cell to facilitate comparison with other crystal structures.

[0381] As an example of the H1-3 type crystal structure, as described in Non-Patent Document 3, the coordinates of cobalt and oxygen in the unit cell can be expressed as Co (0, 0, 0.42150 ± 0.00016), O1 (0, 0, 0.27671 ± 0.00045), and O2 (0, 0, 0.11535 ± 0.00045). O1 and O2 are each oxygen atoms. Which unit cell should be used to express the crystal structure of lithium cobalt oxide can be determined, for example, by Rietveld analysis of the XRD pattern. In this case, it is sufficient to adopt a unit cell that results in a small GOF (goodness of fit) value.

[0382] Li ｘ CoO ２ When conventional lithium cobalt oxide is repeatedly charged so that x becomes 0.24 or less and discharged so that x becomes 1, the crystal structure changes repeatedly (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m(O3) structure in the discharged state.

[0383] However, these two crystal structures are different from CoO ２ As shown by the dotted lines and arrows in FIG. 17, in the H1-3 type crystal structure, CoO ２ The layer is significantly different from the R-3m(O3) in the discharged state. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0384] Furthermore, the difference in volume between these two crystal structures is large: per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the discharged R-3m(O3) crystal structure is greater than 3.5%, typically 3.9% or more.

[0385] In addition, the H1-3 type crystal structure has a trigonal O1 type-like CoO ２ A structure with continuous layers is likely to be unstable.

[0386] Therefore, when conventional lithium cobalt oxide is repeatedly charged so that x is 0.24 or less and discharged so that x is 1, the crystal structure of the battery collapses. This collapse of the crystal structure causes a deterioration in cycle characteristics. This is because the collapse of the crystal structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0387] On the other hand, the positive electrode active material 200 of one embodiment of the present invention shown in FIG. 16 has a crystal structure belonging to the trigonal space group R-3m when x is about 0.2. ２ The symmetry of the layers is the same as that of O3. Therefore, this crystal structure is called an O3'-type crystal structure. In Figure 16, the crystal structure when x = 0.2 is labeled R-3m(O3').

[0388] The lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention is Li ｘ CoO ２ The change in the crystal structure during discharge when x is 1 and during charge when x is 0.24 or less is smaller than that of conventional lithium cobalt oxide. Specifically, as shown by the dotted line in Figure 16, the difference between the R-3m(O3) in the discharged state and the O3'-type crystal structure is smaller than that of conventional lithium cobalt oxide. ２ There is almost no layer misalignment. Furthermore, the change in volume per cobalt atom can be reduced. Specifically, the difference in volume per the same number of cobalt atoms between the R-3m(O3) and O3'-type crystal structures in a discharged state is 2.5% or less, more specifically 2.2% or less, and typically 1.8%. Therefore, the positive electrode active material 200 of one embodiment of the present invention is less likely to lose its crystal structure even when repeatedly charged so that x is 0.24 or less and discharged so that x is 1, and can achieve excellent cycle characteristics.

[0389] As described above, in the positive electrode active material 200 according to one embodiment of the present invention, Li ｘ CoO ２ The crystal structure in which x is 0.24 or less differs from that of conventional positive electrode active materials, and changes from the crystal structure in a discharged state in which x is 1 are suppressed. Furthermore, the change in volume when compared per the same number of cobalt atoms is also suppressed in the positive electrode active material 200 of one embodiment of the present invention. Therefore, the crystal structure of the positive electrode active material 200 is not easily broken even when charging and discharging are repeated so that x is 0.24 or less. Therefore, the decrease in discharge capacity of the positive electrode active material 200 during charge-discharge cycles is suppressed. Furthermore, the lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention is Li ｘ CoO ２ When x is 0.24 or less, the positive electrode active material 200 according to one embodiment of the present invention can have a more stable crystal structure than conventional lithium cobalt oxide. ｘ CoO ２ When the value of x in the positive electrode active material 200 is kept at 0.24 or less, a short circuit is unlikely to occur, and the safety of the secondary battery is improved. Furthermore, since more lithium can be stably used than in conventional positive electrode active materials, the positive electrode active material 200 of one embodiment of the present invention has a large discharge capacity per weight and per volume. Therefore, by using the positive electrode active material 200 of one embodiment of the present invention, a secondary battery with a high discharge capacity per weight and per volume can be manufactured.

[0390] The lithium cobalt oxide used in the positive electrode active material 200 of one embodiment of the present invention has an O3'-type crystal structure, and the coordinates of cobalt and oxygen in the unit cell can be expressed as Co(0,0,0.5), O(0,0,x), within the range of 0.20≦x≦0.25. Regarding the lattice constant of the unit cell, the a-axis is preferably 0.2797≦a≦0.2837 (nm), more preferably 0.2807≦a≦0.2827 (nm), and typically a=0.2817 (nm). The c-axis is preferably 1.3681≦c≦1.3881 (nm), more preferably 1.3751≦c≦1.3811 (nm), and typically c=1.3781 (nm).

[0391] In the O3' type crystal structure, ions of cobalt, nickel, magnesium, etc. occupy the hexacoordinated oxygen positions. Light elements such as lithium may occupy the tetracoordinated oxygen positions.

[0392] In the positive electrode active material 200 of one embodiment of the present invention, Li ｘ CoO ２ It has been confirmed that when x is 0.15 or more and 0.24 or less, it may have an O3' type crystal structure, and it is presumed that even when x is more than 0.24 and 0.27 or less, it has an O3' type crystal structure. However, the crystal structure is ｘ CoO ２ The range of x is not necessarily limited to the above range, since it is affected not only by the x in the formula but also by the number of charge / discharge cycles, charge / discharge current, temperature, electrolyte, etc.

[0393] Therefore, the positive electrode active material 200 is Li ｘ CoO ２ When x is more than 0.1 and not more than 0.24, the entire interior 200b of the positive electrode active material 200 does not have to have an O3′-type crystal structure, but may contain other crystal structures, or may be partially amorphous.

[0394] Also Li ｘ CoO ２ To make the value of x smaller, it is generally necessary to charge at a high charging voltage. ｘ CoO ２ A state where x is small can be rephrased as a state where the battery is charged at a high charging voltage. For example, when CC / CV charging is performed at a voltage of 4.6 V or higher relative to the potential of lithium metal in an environment of 25°C, a H1-3 type crystal structure appears in conventional positive electrode active materials. Therefore, a charging voltage of 4.6 V or higher relative to the potential of lithium metal can be said to be a high charging voltage. Furthermore, in this specification and the like, unless otherwise specified, charging voltages are expressed relative to the potential of lithium metal.

[0395] Therefore, in other words, the positive electrode active material 200 of one embodiment of the present invention is preferable because it can maintain its crystal structure even when charged at a high charging voltage, for example, a voltage of 4.6 V or higher in an environment at 25° C. In other words, the positive electrode active material 200 of one embodiment of the present invention is preferable because it can adopt an O3′-type crystal structure when charged at a higher charging voltage, for example, a voltage of 4.65 V or higher and 4.7 V or lower in an environment at 25° C.

[0396] In the positive electrode active material 200 of one embodiment of the present invention, when the charge voltage is further increased, H1-3 type crystals may be observed. Furthermore, as described above, the crystal structure is affected by the number of charge / discharge cycles, the charge / discharge current, the electrolyte, and the like. Therefore, even when the charge voltage is lower, for example, even when the charge voltage is 4.5 V or higher but lower than 4.6 V in a 25° C. environment, the positive electrode active material 200 of one embodiment of the present invention may have an O3′ type crystal structure.

[0397] In addition, when graphite is used as the negative electrode active material in a secondary battery, the voltage of the secondary battery is lower than the above by the potential of the graphite. The potential of graphite is about 0.05 V to 0.2 V with respect to the potential of lithium metal. Therefore, in the case of a secondary battery using graphite as the negative electrode active material, the same crystal structure is maintained at a voltage obtained by subtracting the potential of graphite from the above voltage.

[0398] In addition, in the O3' type crystal structure of FIG. 16, lithium is shown to exist at all lithium sites with equal probability, but this is not limited to this. It may exist unevenly at some lithium sites, or, for example, a monoclinic O1 (Li ０．５ CoO ２ The lithium distribution can be analyzed, for example, by neutron diffraction.

[0399] The O3' type crystal structure has random lithium between layers, but CdCl ２ It can be said that this crystal structure is similar to that of the CdCl type. ２ A similar crystal structure to the Li-type is lithium nickel oxide. ０．０６ NiO ２However, pure lithium cobaltate or layered rock salt cathode active materials containing a large amount of cobalt usually have a crystal structure similar to that of CdCl ２ It is known that it does not have a typical crystal structure.

[0400] Furthermore, it is preferable that the concentration gradient of the additive element A is similar at multiple locations in the surface layer portion 200a of the positive electrode active material 200. In other words, it is preferable that the reinforcement derived from the additive element A is uniformly present in the surface layer portion 200a. Even if a portion of the surface layer portion 200a 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 positive electrode active material 200, defects such as cracks may occur there, which may lead to breakage of the positive electrode active material and a decrease in discharge capacity.

[0401] However, the additional element A does not necessarily have to have the same concentration gradient throughout the entire surface layer portion 200a of the positive electrode active material 200. An example of the distribution of the additional element X near C-D in FIG. 13A is shown in FIG. 13E, and an example of the distribution of the additional element Y near C-D is shown in FIG. 13F.

[0402] Here, the area around C-D has an R-3m layered rock salt type crystal structure, and the surface is (001) oriented. The (001) oriented surface may have a different distribution of the additional element A than the other surfaces. For example, the (001) oriented surface and its surface layer 200a may have a concentration distribution or peak top of one or more elements selected from the additional element X and the additional element Y located shallower from the surface than the other oriented surfaces. Alternatively, the (001) oriented surface and its surface layer 200a may have a lower concentration of one or more elements selected from the additional element X and the additional element Y than the other oriented surfaces. Alternatively, the (001) oriented surface and its surface layer 200a may have one or more elements selected from the additional element X and the additional element Y below the lower detection limit.

[0403] In the layered rock salt type crystal structure of R-3m, cations are arranged parallel to the (001) plane. ２ The structure is composed of alternately stacked layers and lithium layers parallel to the (001) plane, and therefore the diffusion path of lithium ions is also parallel to the (001) plane.

[0404] CoO ２ The layer is relatively stable, so CoO ２ The (001) plane, on which the layer exists, is relatively stable. The (001) plane does not expose the main diffusion path of lithium ions during charge and discharge.

[0405] On the other hand, the diffusion path of lithium ions is exposed on the surface other than the (001) orientation. Therefore, the surface other than the (001) orientation and the surface layer portion 200a are important regions for maintaining the diffusion path of lithium ions, and at the same time, they are regions from which lithium ions are first desorbed and are therefore prone to instability. Therefore, it is important to reinforce the surface other than the (001) orientation and the surface layer portion 200a in order to maintain the crystal structure of the entire positive electrode active material 200.

[0406] Therefore, in the positive electrode active material 200 according to another embodiment of the present invention, the distribution of the additional element A in the plane other than the (001) plane and in the surface layer portion 200a thereof is preferably as shown in Fig. 13C and Fig. 13D . On the other hand, the concentration of the additional element A in the (001) plane and in the surface layer portion 200a thereof may be low or absent, as described above.

[0407] High purity LiCoO, which will be described in a later embodiment ２ In the manufacturing method of manufacturing the silicon nitride film, the additional element A is mixed in and heated after the manufacturing process, and the additional element A spreads mainly through the diffusion path of lithium ions. Therefore, it is easy to make the distribution of the additional element A in the planes other than the (001) plane and the surface layer portion 200 a thereof fall within a preferred range.

[0408] Furthermore, it is preferable that the surface of the positive electrode active material 200 is smooth and has few irregularities, but this is not necessarily the case for the entire positive electrode active material 200. A composite oxide having an R-3m layered rock salt crystal structure is prone to slippage in a plane parallel to the (001) plane, for example, in a plane where lithium is arranged. For example, when a (001) plane is present as shown in FIG. 18A , slippage parallel to the (001) plane may occur as a result of a pressing process or the like, as indicated by the arrow in FIG. 18B , resulting in deformation.

[0409] In this case, the additional element A may not be present or may be below the detection limit on the surface and its surface layer 200a newly formed as a result of the slip. E-F in FIG. 18B is an example of the surface and its surface layer 200a newly formed as a result of the slip. Enlarged views of the vicinity of E-F are shown in FIGS. 18C1 and 18C2. Unlike FIGS. 13C to 13F, the additional element X and the additional element Y are not distributed in FIGS. 18C1 and 18C2.

[0410] However, since slippage tends to occur parallel to the (001) plane, the newly formed surface and its surface layer 200a tend to have a (001) orientation. In this case, the diffusion path of lithium ions is not exposed and the surface is relatively stable, so there is almost no problem even if the additive element A is absent or is below the detection limit.

[0411] As mentioned above, the composition is LiCoO ２ In the composite oxide having a layered rock salt type crystal structure of R-3m, cobalt is arranged parallel to the (001) plane. ２ Among these, cobalt, which has the largest atomic number, has the highest brightness. Therefore, in a HAADF-STEM image, the arrangement of bright atoms can be considered to be the arrangement of cobalt. The repetition of this arrangement of bright atoms is synonymous with crystal fringes or lattice fringes.

[0412] <Grain Boundary> In addition to the above distribution, at least a portion of the additional element A contained in the positive electrode active material 200 of one embodiment of the present invention is preferably unevenly distributed in and near the grain boundary 201 .

[0413] 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 is synonymous with segregation, precipitation, non-uniformity, bias, or the mixture of areas with high concentration and areas with low concentration.

[0414] For example, the magnesium concentration at and near the grain boundary 201 of the positive electrode active material 200 is preferably higher than that in other regions of the interior 200b. The fluorine concentration at and near the grain boundary 201 is also preferably higher than that in other regions of the interior 200b. The nickel concentration at and near the grain boundary 201 is also preferably higher than that in other regions of the interior 200b. The aluminum concentration at and near the grain boundary 201 is also preferably higher than that in other regions of the interior 200b.

[0415] The grain boundaries 201 are one type of planar defect. Therefore, like the surface, they tend to become unstable and are prone to change in the crystal structure. Therefore, if the concentration of the added element A at and near the grain boundaries 201 is high, the change in the crystal structure can be more effectively suppressed.

[0416] Furthermore, when the magnesium concentration and fluorine concentration are high at and near the grain boundary 201, even if cracks occur along the grain boundary 201 in the positive electrode active material 200 of one embodiment of the present invention, the magnesium concentration and fluorine concentration are 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.

[0417] <Particle size> If the particle size of the positive electrode active material 200 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 may occur. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte may occur. Therefore, the median diameter (D50) is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less. Alternatively, 1 μm or more and 40 μm or less is preferable. Alternatively, 1 μm or more and 30 μm or less is preferable. Alternatively, 2 μm or more and 100 μm or less is preferable. Alternatively, 2 μm or more and 30 μm or less is preferable. Alternatively, 5 μm or more and 100 μm or less is preferable. Alternatively, 5 μm or more and 40 μm or less is preferable.

[0418] <Analysis method> A certain positive electrode active material is ｘ CoO ２When x in the formula (I) is small, it can be determined whether the positive electrode active material 200 of one embodiment of the present invention has an O3′-type crystal structure by Li ｘ CoO ２ This can be determined by analyzing a positive electrode having a positive electrode active material with a small x using XRD, electron beam diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), or the like.

[0419] In particular, XRD is preferable in that it can analyze the symmetry of the transition metal M such as cobalt contained in the positive electrode active material with high resolution, it can compare the level of crystallinity and the orientation of the crystals, it can analyze the periodic distortion of the lattice and the crystallite size, it can obtain sufficient accuracy even when measuring the positive electrode obtained by disassembling the secondary battery as it is, etc. Among XRD methods, powder XRD can obtain diffraction peaks that reflect the crystalline structure of the interior 200b of the positive electrode active material 200, which occupies the majority of the volume of the positive electrode active material 200.

[0420] As described above, the positive electrode active material 200 according to one embodiment of the present invention is Li ｘ CoO ２ The characteristic of this material is that there is little change in the crystal structure when x is 1 and when it is 0.24 or less. If the crystal structure with large changes accounts for 50% or more of the total, it is not preferable because it cannot withstand high-voltage charging and discharging.

[0421] It should also be noted that simply adding an additional element A may not result in an O3'-type crystal structure. For example, even if lithium cobalt oxide containing magnesium and fluorine or lithium cobalt oxide containing magnesium and aluminum has something in common, depending on the concentration and distribution of the additional element A, Li ｘ CoO ２ In this case, x is 0.24 or less and the O3' type crystal structure accounts for 60% or more, and in other cases the H1-3 type crystal structure accounts for 50% or more.

[0422] Furthermore, even in the positive electrode active material 200 of one embodiment of the present invention, an H1-3 type or trigonal O1 type crystal structure may be generated when x is too small, such as 0.1 or less, or under conditions where the charge voltage exceeds 4.9 V. Therefore, to determine whether or not the positive electrode active material 200 of one embodiment of the present invention is a positive electrode active material, analysis of the crystal structure, such as XRD, and information such as the charge capacity or the charge voltage are required.

[0423] Furthermore, when a positive electrode active material with a small x is exposed to the air, its crystal structure may change. For example, it may change from an O3'-type crystal structure to an H1-3-type crystal structure. Therefore, it is preferable to handle all samples used for crystal structure analysis in an inert atmosphere such as an argon atmosphere.

[0424] Furthermore, whether or not the distribution of the additive element A in a certain positive electrode active material is in the state described above can be determined by analysis using, for example, XPS, energy dispersive X-ray spectroscopy (EDX), electron probe microanalysis (EPMA), or the like.

[0425] The crystal structure of the surface layer 200 a, the grain boundaries 201 , etc. can be analyzed by electron beam diffraction of a cross section of the positive electrode active material 200 .

[0426] <Charging Method> Whether a certain composite oxide is the positive electrode active material 200 of one embodiment of the present invention can be determined by high-voltage charging. For example, a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) may be fabricated using the composite oxide for a positive electrode and lithium metal for a negative electrode (also referred to as a counter electrode), and high-voltage charging may be performed.

[0427] 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.

[0428] 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.

[0429] The electrolyte contained in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF ６ ) is used, and the electrolyte may be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7 with 2 wt % vinylene carbonate (VC).

[0430] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0431] The coin cell prepared under the above conditions is charged at a constant current of 10 mA / g to a desired voltage (e.g., 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V). To observe the phase change of the positive electrode active material, charging at such a low current is desirable. The ambient temperature in which the coin cell is placed is 25°C or 45°C. After charging in this manner, the coin cell is disassembled in a glove box under an argon atmosphere and the positive electrode is removed to obtain a positive electrode active material with the desired charge capacity. When various analyses are performed after this, it is preferable to seal the cell in an argon atmosphere to prevent reactions with external components. For example, XRD can be performed by sealing the cell in a sealed container under an argon atmosphere. Furthermore, it is preferable to remove the positive electrode promptly after charging is completed and perform the analysis. Specifically, within one hour after charging is completed, and more preferably within 30 minutes.

[0432] When analyzing the crystal structure in the charged state after multiple charge / discharge cycles, the conditions for the multiple charge / discharge cycles may be different from the above-mentioned conditions for charge / discharge. For example, charging may be performed by constant current charging at a current value of 100 mA / g up to a desired voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current value reaches 10 mA / g, and then constant current discharging at 2.5 V and 100 mA / g.

[0433] Furthermore, when analyzing the crystal structure in the discharged state after multiple charge / discharge cycles, constant current discharge can be performed at, for example, 2.5 V and a current value of 100 mA / g.

[0434] <<XRD>> The apparatus and conditions for XRD measurement are not particularly limited. For example, the measurement can be performed using the following apparatus and conditions. XRD apparatus: D8 ADVANCE manufactured by Bruker AXS X-ray source: Cu Output: 40 KV, 40 mA Slit width: Div. Slit, 0.5° Detector: LynxEye Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 90° Step width (2θ): 0.01° setting Counting time: 1 second / step Sample stage rotation: 15 rpm

[0435] If the measurement sample is a powder, it can be set by placing it in a glass sample holder, or by sprinkling the sample on a greased silicone anti-reflective plate, etc. If the measurement sample is a positive electrode, the positive electrode can be attached to the substrate with double-sided tape, and the positive electrode active material layer can be set to match the measurement surface required by the device.

[0436] The ideal powder XRD patterns calculated from the O3' type crystal structure and the H1-3 type crystal structure model using CuKα1 radiation are shown in Figures 19 and 20. For comparison, Li ｘ CoO ２ LiCoO where x=1 ２ The ideal XRD patterns calculated from the crystal structures of O3, H1-3 type, and trigonal O1 with x = 0 are also shown. ２ (O3) and CoO ２ The pattern of (O1) was created using Reflex Powder Diffraction, one of the modules of Materials Studio (BIOVIA), from crystal structure information obtained from the Inorganic Crystal Structure Database (ICSD) (see Non-Patent Document 3). The 2θ range was set to 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562 × 10. −１０m and λ2 were not set, and the monochromator was single. The pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. The pattern of the O3' type crystal structure was created by estimating the crystal structure from the XRD pattern of the positive electrode active material of one embodiment of the present invention and fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and an XRD pattern was created in the same manner as the others.

[0437] As shown in FIG. 19, in the O3' type crystal structure, diffraction peaks appear at 2θ=19.25±0.12° (19.13° or more and less than 19.37°) and 2θ=45.47±0.10° (45.37° or more and less than 45.57°).

[0438] However, as shown in FIG. 20, no peaks appear at these positions in the H1-3 type crystal structure and trigonal O1. ｘ CoO ２ The appearance of diffraction peaks at 2θ = 19.25 ± 0.12° (19.13° or more and less than 19.37°) and 2θ = 45.47 ± 0.10° (45.37° or more and less than 45.57°) when x is small can be said to be a characteristic of the positive electrode active material 200 of one embodiment of the present invention.

[0439] This can also be said to be because the positions at which XRD diffraction peaks appear are close between the crystal structures of x = 1 and x ≦ 0.24. More specifically, for the main diffraction peaks of the crystal structures of x = 1 and x ≦ 0.24 that appear at 2θ of 42° or more and 46° or less, the difference in 2θ is 0.7° or less, more preferably 0.5° or less.

[0440] The positive electrode active material 200 according to one embodiment of the present invention is Li ｘ CoO ２When x in the formula is small, the material has an O3'-type crystal structure, but not all of it needs to be an O3'-type crystal structure. It may contain other crystal structures, or a portion may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the O3'-type crystal structure is preferably 50% or more, more preferably 60% or more, and even more preferably 66% or more. If the O3'-type crystal structure is 50% or more, more preferably 60% or more, and even more preferably 66% or more, it can be a positive electrode active material with sufficiently excellent cycle characteristics.

[0441] 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% or more, more preferably 40% or more, and even more preferably 43% or more.

[0442] Furthermore, the sharpness of the diffraction peaks in the XRD pattern indicates the degree of crystallinity. Therefore, it is preferable that each diffraction peak after charging is sharp, i.e., the half-width is narrow. The half-width varies depending on the XRD measurement conditions or the value of 2θ, even for peaks arising from the same crystalline phase. Under the above-mentioned measurement conditions, for peaks observed between 2θ = 43° and 46°, the half-width is preferably 0.2° or less, more preferably 0.15° or less, and even more preferably 0.12° or less. Note that not all peaks necessarily meet this requirement. If some peaks meet this requirement, it can be said that the crystallinity of the crystalline phase is high. High crystallinity contributes to the stabilization of the crystal structure after charging.

[0443] The crystallite size of the O3'-type crystal structure of the positive electrode active material 200 is ２ Therefore, even under the same XRD measurement conditions as the positive electrode before and after charging and discharging, the ｘ CoO ２ When x in the graph is small, a clear peak of the O3' type crystal structure can be confirmed. ２ In this case, even if a part of the crystal structure resembles the O3'-type crystal structure, the crystallite size will be small and the peak will be broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0444] <XPS> In X-ray photoelectron spectroscopy (XPS), in the case of inorganic oxides, using monochromated aluminum Kα rays as the X-ray source allows analysis of a region from the surface to a depth of approximately 2 to 8 nm (typically 5 nm or less), allowing quantitative analysis of the concentration of each element in a region approximately half the depth of the surface layer 200a. Furthermore, narrow scan analysis allows analysis of the bonding state of elements. The quantitative accuracy of XPS is often approximately ±1 atomic %, and the lower detection limit is approximately 1 atomic %, depending on the element.

[0445] In the cathode active material 200 of one embodiment of the present invention, the concentration of one or more selected from the additive elements A is preferably higher in the surface layer portion 200a than in the interior portion 200b. This is equivalent to saying that the concentration of one or more selected from the additive elements A in the surface layer portion 200a is preferably higher than the average concentration throughout the cathode active material 200. Therefore, for example, it can be said that the concentration of one or more selected from the additive elements A in the surface layer portion 200a measured by XPS or the like is preferably higher than the average concentration of the additive elements A throughout the cathode active material 200 measured by ICP-MS, GD-MS, or the like. For example, the magnesium concentration in at least a portion of the surface layer portion 200a measured by XPS or the like is preferably higher than the magnesium concentration throughout the cathode active material 200. Furthermore, the nickel concentration in at least a portion of the surface layer portion 200a is preferably higher than the nickel concentration throughout the cathode active material 200. Furthermore, the aluminum concentration in at least a portion of the surface layer portion 200a is preferably higher than the aluminum concentration throughout the cathode active material 200. It is also preferable that the fluorine concentration in at least a part of the surface layer portion 200 a is higher than the fluorine concentration in the entire positive electrode active material 200 .

[0446] Note that the surface and surface layer portion 200a of the positive electrode active material 200 according to one embodiment of the present invention do not contain carbonates, hydroxyl groups, and the like that are chemically adsorbed after the preparation of the positive electrode active material 200. Furthermore, the surface of the positive electrode active material 200 also does not contain an electrolyte, a binder, a conductive material, or compounds derived therefrom that are attached to the surface of the positive electrode active material 200. Therefore, when quantifying the elements contained in the positive electrode active material, corrections may be made to exclude carbon, hydrogen, excess oxygen, excess fluorine, and the like that can be detected by surface analysis such as XPS. For example, XPS can separate the types of bonds by analysis, and corrections may be made to exclude C—F bonds derived from the binder.

[0447] Furthermore, before being subjected to various analyses, samples such as the positive electrode active material and the positive electrode active material layer may be washed to remove the electrolyte, binder, conductive material, or compounds derived therefrom that adhere to the surface of the positive electrode active material. At this time, lithium may dissolve in the solvent used for washing, but even in this case, the additional element A is unlikely to dissolve, and therefore the atomic ratio of the additional element A is not affected.

[0448] The concentration of the additive element A may also be compared in terms of its ratio to cobalt. Using the ratio to cobalt is preferable because it allows comparisons to be made while reducing the influence of carbonates and other substances that are chemically adsorbed after the preparation of the positive electrode active material. For example, the ratio of the number of magnesium atoms to cobalt atoms (Mg / Co) determined by XPS analysis is preferably 0.4 or more and 1.5 or less. Meanwhile, the ratio of Mg / Co determined by ICP-MS analysis is preferably 0.001 or more and 0.06 or less.

[0449] Similarly, in order to ensure sufficient paths for lithium insertion and desorption, the positive electrode active material 200 preferably has a higher concentration of lithium and cobalt in the surface layer portion 200a than the concentrations of one or more additional elements A selected from the additional elements A contained in the surface layer portion 200a, as measured by XPS or the like. This means that the concentrations of lithium and cobalt in the surface layer portion 200a are preferably higher than the concentrations of one or more additional elements A selected from the additional elements A contained in the surface layer portion 200a, as measured by XPS or the like. For example, the concentration of cobalt in at least a portion of the surface layer portion 200a, as measured by XPS or the like, is preferably higher than the concentration of magnesium in at least a portion of the surface layer portion 200a, as measured by XPS or the like. Similarly, the concentration of lithium is preferably higher than the concentration of magnesium. Furthermore, the concentration of cobalt is preferably higher than the concentration of nickel. Similarly, the concentration of lithium is preferably higher than the concentration of nickel. Furthermore, the concentration of cobalt is preferably higher than the concentration of aluminum. Similarly, the concentration of lithium is preferably higher than the concentration of aluminum. Furthermore, the concentration of cobalt is preferably higher than the concentration of fluorine. Similarly, the concentration of lithium is preferably higher than the concentration of fluorine.

[0450] Furthermore, it is more preferable that the additive element Y, such as aluminum, is widely distributed in a deep region, for example, a region having a depth from the surface of 5 nm to 50 nm. Therefore, although the additive element Y, such as aluminum, is detected in an analysis of the entire positive electrode active material 200 using ICP-MS, GD-MS, or the like, it is more preferable that this is below the lower limit of detection using XPS, or the like.

[0451] Furthermore, when the positive electrode active material 200 of one embodiment of the present invention was analyzed by XPS, the number of magnesium atoms relative to the number of cobalt atoms was preferably 0.4 to 1.2 times, more preferably 0.65 to 1.0 times. The number of nickel atoms relative to the number of cobalt atoms was preferably 0.15 times, more preferably 0.03 to 0.13 times. The number of aluminum atoms relative to the number of cobalt atoms was preferably 0.12 times, more preferably 0.09 times. The number of fluorine atoms relative to the number of cobalt atoms was preferably 0.3 to 0.9 times, more preferably 0.1 to 1.1 times.

[0452] When performing XPS analysis, for example, monochromated aluminum Kα rays can be used as the X-ray source. The take-off angle can be set to, for example, 45°. Measurement can be performed, for example, using the following equipment and conditions. Measurement equipment: PHI Quantera II X-ray source: monochromated Al Kα (1486.6 eV) Detection area: 100 μmφ Detection depth: approximately 4 to 5 nm (take-off angle 45°) Measurement spectrum: wide scan, narrow scan for each detected element

[0453] Furthermore, when the positive electrode active material 200 of one embodiment of the present invention is subjected to XPS analysis, the peak representing the bond energy between fluorine and another element is preferably equal to or greater than 682 eV and less than 685 eV, and more preferably approximately 684.3 eV. This value is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV). In other words, when the positive electrode active material 200 of one embodiment of the present invention contains fluorine, the bond is preferably other than that of lithium fluoride or magnesium fluoride.

[0454] Furthermore, when the positive electrode active material 200 of one embodiment of the present invention is subjected to XPS analysis, the peak representing the bond energy between magnesium and another element is preferably greater than or equal to 1302 eV and less than 1304 eV, and more preferably about 1303 eV. This value is different from the bond energy of magnesium fluoride, 1305 eV, and is close to the bond energy of magnesium oxide. In other words, when the positive electrode active material 200 of one embodiment of the present invention contains magnesium, the bond is preferably other than that of magnesium fluoride.

[0455] <EDX> The additive element A contained in the positive electrode active material 200 preferably has a concentration gradient. It is more preferable that the concentration distribution or the position of the peak top differs depending on the additive element A. The concentration distribution includes a concentration gradient. The concentration distribution of the additive element A can be evaluated, for example, by exposing a cross section of the positive electrode active material 200 using a focused ion beam (FIB) or the like and analyzing the cross section using EDX, electron probe microanalysis (EPMA), or the like.

[0456] In EDX measurements, scanning an area and evaluating the area two-dimensionally is called area analysis. Linear analysis is used to measure linearly and evaluate the distribution of atomic concentrations within the positive electrode active material. Linear analysis can also be used to extract data from a linear area of ​​EDX area analysis. Point analysis is used to measure an area without scanning.

[0457] EDX area analysis (e.g., element mapping) allows for semi-quantitative analysis of the concentration of the additive element A in the surface layer 200a, the interior 200b, and near the grain boundary 201 of the positive electrode active material 200. Furthermore, EDX ray analysis allows for analysis of the concentration distribution or peak top of the additive element A. Furthermore, analysis that thins the sample, such as STEM-EDX, is more suitable because it allows analysis of the concentration distribution in the depth direction from the surface to the center of the positive electrode active material in a specific region without being significantly affected by the distribution in the depth direction.

[0458] Therefore, when EDX area analysis or EDX point analysis is performed on the positive electrode active material 200 of one embodiment of the present invention, the concentration of each additional element A, particularly the additional element X, in the surface layer portion 200a is preferably higher than that in the interior portion 200b.

[0459] For example, when EDX area analysis or EDX point analysis is performed on a cathode active material 200 having magnesium as the additive element X, the magnesium concentration in the surface layer 200a is preferably higher than the magnesium concentration in the interior 200b. Furthermore, when EDX ray analysis is performed, the peak top of the magnesium concentration in the surface layer 200a preferably exists at a depth of 3 nm from the surface toward the center of the cathode active material 200, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, the magnesium concentration distribution preferably has a concentration gradient that attenuates to 60% or less of the peak top at a position shifted 1 nm from the peak top. Furthermore, it is preferable that the concentration gradient attenuates to 30% or less of the peak top at a position shifted 2 nm from the peak top. The shifted position may be shifted toward the surface or toward the interior from the peak top. The concentration gradient may exist at a position shifted toward either the surface or the interior.

[0460] Furthermore, in the positive electrode active material 200 having magnesium and fluorine as the added element X, the distribution of fluorine preferably overlaps with the distribution of magnesium. For example, the difference between the peak top positions of the fluorine concentration and the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0461] Furthermore, when EDX-ray analysis is performed, the peak top of the fluorine concentration in the surface layer portion 200a is preferably present at a depth of 3 nm from the surface toward the center of the positive electrode active material 200, more preferably at a depth of 1 nm, and even more preferably at a depth of 0.5 nm. Furthermore, if the peak top of the fluorine concentration is present slightly closer to the surface than the peak top of the magnesium concentration, resistance to hydrofluoric acid is increased, which is more preferable. For example, the peak top of the fluorine concentration is more preferably 0.5 nm or more closer to the surface than the peak top of the magnesium concentration, and even more preferably 1.5 nm or more closer to the surface.

[0462] Furthermore, in the positive electrode active material 200 having nickel as the additive element X, the peak top of the nickel concentration in the surface layer 200a is preferably present at a depth of up to 3 nm from the surface toward the center of the positive electrode active material 200, more preferably at a depth of up to 1 nm, and even more preferably at a depth of up to 0.5 nm. Furthermore, in the positive electrode active material 200 containing magnesium and nickel, the distribution of nickel preferably overlaps with the distribution of magnesium. For example, the difference between the positions of the peak tops of the magnesium concentration and the peak tops of the magnesium concentration is preferably within 10 nm, more preferably within 3 nm, and even more preferably within 1 nm.

[0463] Furthermore, when the positive electrode active material 200 contains aluminum as the added element Y, it is preferable that, when EDX-ray analysis is performed, the peak top of the magnesium, nickel, or fluorine concentration is closer to the surface than the peak top of the aluminum concentration in the surface layer portion 200 a. For example, the peak top of the aluminum concentration is preferably present at a depth of 0.5 nm to 50 nm, more preferably 5 nm to 50 nm, from the surface to the center of the positive electrode active material 200.

[0464] Furthermore, when EDX-ray analysis, area analysis, or point analysis is performed on the positive electrode active material 200, the ratio of the number of atoms of magnesium (Mg) to cobalt (Co) at the peak top of the magnesium concentration (Mg / Co) is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.4 or less. The ratio of the number of atoms of aluminum (Al) to cobalt (Co) at the peak top of the aluminum concentration (Al / Co) is preferably 0.05 or more and 0.6 or less, more preferably 0.1 or more and 0.45 or less. The ratio of the number of atoms of nickel (Ni) to cobalt (Co) at the peak top of the nickel concentration (Ni / Co) is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.1 or less. The ratio of the number of atoms of fluorine (F) to cobalt (Co) at the peak top of the fluorine concentration (F / Co) is preferably 0 or more and 1.6 or less, more preferably 0.1 or more and 1.4 or less.

[0465] The surface of the positive electrode active material 200 in the EDX analysis results can be estimated, for example, as follows: For an element that is uniformly present in the interior 200b of the positive electrode active material 200, such as oxygen or cobalt, the point where the detected amount is half that of the interior 200b is defined as the surface.

[0466] Since the positive electrode active material 200 is a composite oxide, the surface can be estimated using the detected amount of oxygen. Specifically, first, the average oxygen concentration O ａｖｅ At this time, oxygen O, which is thought to be due to chemical adsorption or background, is found in the area that can be clearly determined to be outside the surface. ｂａｃｋｇｒｏｕｎｄ If detected, O ｂａｃｋｇｒｏｕｎｄ The average oxygen concentration O ａｖｅ This average value Oａｖｅ Half the value of, that is, 1 / 2O ａｖｅ The measurement point showing the measurement value closest to this can be assumed to be the surface of the positive electrode active material.

[0467] The surface can also be estimated in the same way as above using the detected amount of cobalt. Alternatively, the sum of the detected amounts of multiple transition metals can be used to estimate the surface. The detected amounts of transition metals, including cobalt, are less susceptible to the influence of chemical adsorption, making them suitable for estimating the surface.

[0468] Furthermore, when the positive electrode active material 200 is subjected to linear analysis or area analysis, the ratio (A / Co) of the added element A to cobalt Co in the vicinity of the grain boundary 201 is preferably 0.020 or more and 0.50 or less. It is further preferably 0.025 or more and 0.30 or less. It is further preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0469] For example, when the added element X is magnesium, when the positive electrode active material 200 is subjected to a linear analysis or an area analysis, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in the vicinity of the grain boundary 201 is preferably 0.020 or more and 0.50 or less. It is further preferably 0.025 or more and 0.30 or less. It is further preferably 0.030 or more and 0.20 or less. It is also preferably 0.020 or more and 0.30 or less. It is also preferably 0.020 or more and 0.20 or less. It is also preferably 0.025 or more and 0.50 or less. It is also preferably 0.025 or more and 0.20 or less. It is also preferably 0.030 or more and 0.50 or less. It is also preferably 0.030 or more and 0.30 or less.

[0470] <EPMA> EPMA (Electron Probe Microanalysis) can also quantify elements. Area analysis can analyze the distribution of each element.

[0471] When EPMA surface analysis is performed on a cross section of the positive electrode active material 200 according to one embodiment of the present invention, it is preferable that one or more selected from the additive elements A have a concentration gradient, similar to the EDX analysis results. It is also more preferable that the depth from the surface of the concentration peak differs depending on the additive element A. The preferred range of the concentration peak of each additive element A is also the same as in the case of EDX.

[0472] However, EPMA analyzes a region from the surface to a depth of about 1 μm. Therefore, the quantitative values ​​of each element may differ from the measurement results obtained using other analytical methods. For example, when the surface of the positive electrode active material 200 is analyzed using EPMA, the concentration of each added element A present in the surface layer 200 a may be lower than the result obtained using XPS.

[0473] <Charge Curve and dQ / dV Curve> The positive electrode active material 200 of one embodiment of the present invention may exhibit a characteristic voltage change during charging. The voltage change can be read from the dQ / dV curve obtained by differentiating (dQ / dV) the capacity (Q) with respect to the voltage (V) from the charge curve. For example, it is believed that a non-equilibrium phase change occurs around the peak in the dQ / dV curve, causing a significant change in the crystal structure. Note that in this specification, a non-equilibrium phase change refers to a phenomenon that causes a non-linear change in a physical quantity.

[0474] The positive electrode active material 200 according to one embodiment of the present invention may have a broad peak near 4.55 V in the dQ / dV curve. The peak near 4.55 V reflects the change in voltage that occurs when the phase changes from O3 type to O3' type. Therefore, the broadness of this peak means that the change in energy required for lithium extraction is smaller than when the peak is sharp, i.e., the change in the crystal structure is smaller. The smaller these changes, the more likely CoO ２ This is preferable because it is less affected by layer displacement and volume change.

[0475] More specifically, when the maximum value appearing between 4.5 V and 4.6 V in the dQ / dV curve of the charging curve is defined as the first peak, it is considered to be sufficiently broad and is preferable if the half-width of the first peak is 0.10 V or more. In this specification, the half-width of the first peak is defined as the average value HWHM of the first peak and the first minimum value when the minimum value of the dQ / dV value appearing between 4.3 V and 4.5 V is defined as the first minimum value. １ and the average value HWHM of the first peak and the second minimum when the minimum value of the dQ / dV value appearing between 4.6 V and 4.8 V is defined as the second minimum value. ２ The difference between and.

[0476] The charging when acquiring the dQ / dV curve can be, for example, a constant current charge of 10 mA / g up to 4.9 V. When acquiring the dQ / dV of the initial charge, it is preferable to start the charging after discharging to 2.5 V at 100 mA / g before measurement.

[0477] The data acquisition interval during charging can be set to, for example, 1 second intervals or to acquire the voltage and current when there is a voltage fluctuation of 1 mV. The value obtained by integrating the current value and time is taken as the charge capacity.

[0478] The difference between the nth and (n+1)th data of the charge capacity is defined as the nth value of the capacitance change dQ. Similarly, the difference between the nth and (n+1)th data of the voltage is defined as the nth value of the voltage change dV.

[0479] However, since the use of the above data is subject to the large influence of minute noise, dQ / dV may be calculated from the moving average of the voltage and charge capacity differences over a certain number of intervals. The number of intervals may be set to, for example, 500.

[0480] Specifically, the average value of dQ from the nth to the (n+500th) data is calculated, and similarly, the average value of dV from the nth to the (n+500th) data is calculated. dQ (average of 500 data) / dV (average of 500 data) can be used as dQ / dV. Similarly, the moving average value of 500 sections can be used for the voltage on the horizontal axis of the dQ / dV curve. Note that when using a moving average of 500 sections as described above, it is preferable not to use the data from the 501st data from the end to the last data in the dQ / dV curve because the data is significantly affected by noise.

[0481] When analyzing the dQ / dV curve after multiple charge / discharge cycles, the charge / discharge conditions may be different from the above-mentioned conditions. For example, the charge may be performed at a constant current of 100 mA / g at a given voltage (e.g., 4.6 V, 4.65 V, 4.7 V, 4.75 V, or 4.8 V), followed by constant voltage charging until the current reaches 10 mA / g, and then discharge at a constant current of 100 mA / g at 2.5 V.

[0482] In addition, the phase changes from O3 type to O3' type at around 4.55 V, and the O3 type at this time is Li ｘ CoO ２ The x in the figure is about 0.3. This has the same symmetry as the O3 type with x=1 explained in FIG. 17, but ２ The distance between layers is slightly different. In this specification, when distinguishing between O3 types with different values ​​of x, O3 type with x = 1 is referred to as O3 (2θ = 18.85), and O3 type with x = 0.3 or so is referred to as O3 (2θ = 18.57). This is because the position of the peak that appears around 2θ = 19° in XRD measurement is due to the CoO ２ This is because it corresponds to the interlayer distance.

[0483] <Discharge Curve and dQ / dV Curve> Furthermore, when the positive electrode active material 200 of one embodiment of the present invention is charged at a high voltage and then discharged at a low current of, for example, 40 mA / g or less, a characteristic voltage change may appear near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the voltage range up to 3.5 V, which is lower than the peak that appears around 3.9 V, in the dQ / dV curve obtained from the discharge curve.

[0484] <<ESR>> The positive electrode active material 200 of one embodiment of the present invention preferably contains cobalt and nickel and magnesium as the additional element A. As a result, a portion of Co ３＋ Ni ３＋ and some Li ＋ is Mg ２＋ It is preferred that Li be substituted with ＋ is Mg ２＋ With the substitution of ３＋ is reduced to Ni ２＋ In addition, some Li ＋ is Mg ２＋ is replaced by Mg ２＋ Nearby Co ３＋ is reduced to Co ２＋ In addition, some Co ３＋ is Mg ２＋ is replaced by Mg ２＋ Nearby Co ３＋ is oxidized to Co ４＋ This may occur.

[0485] Therefore, the positive electrode active material 200 is Ni ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ It is preferable that the positive electrode active material 200 contains at least one of the following: ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ The spin density due to one or more of the above is 2.0 × 10 １７ spins / g or more 1.0×10 ２１ It is preferable that the positive electrode active material 200 has the above-mentioned spin density, because the crystal structure is stable, especially in the charged state. If the magnesium concentration is too high, the Ni ２＋ , Ni ３＋ , Co ２＋ and Co ４＋ The spin density may be reduced due to one or more of the above.

[0486] The spin density in the positive electrode active material can be analyzed using, for example, electron spin resonance (ESR).

[0487] <Surface Roughness and Specific Surface Area> The positive electrode active material 200 of one embodiment of the present invention preferably has a smooth surface with little unevenness. A smooth surface with little unevenness indicates that the effect of the flux described below is fully exerted, and the surface of the additive element A source and the composite oxide are melted. Therefore, this is one factor indicating that the distribution of the additive element A in the surface layer portion 200a is good. Good distribution means, for example, that the concentration distribution of the additive element A in the surface layer portion 200a is uniform.

[0488] 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 200, the specific surface area of ​​the positive electrode active material 200, or the like.

[0489] For example, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 200 as follows.

[0490] First, the cathode active material 200 is processed using an FIB or the like to expose a cross section. At this time, it is preferable to cover the cathode active material 200 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 200 is taken. The SEM image is subjected to noise processing using image processing software. For example, Gaussian blurring (σ = 2) is performed, followed by binarization. Interface extraction is then performed using image processing software. The interface line between the protective film or the like and the cathode active material 200 is selected using an automatic selection tool or the like, and the data is extracted into a spreadsheet or the like. Using the functions of the spreadsheet or the like, correction is performed from a regression curve (quadratic regression), and parameters for calculating roughness are obtained from the data after slope correction, and the root mean square surface roughness (RMS) is calculated by calculating the standard deviation. This surface roughness is the surface roughness of at least the outer 400 nm of the cathode active material.

[0491] On the surface of the positive electrode active material 200 of the present embodiment, the root mean square (RMS) surface roughness, which is an index of roughness, is preferably less than 3 nm, more preferably less than 1 nm, and further preferably less than 0.5 nm.

[0492] The image processing software used for noise processing, interface extraction, etc. is not particularly limited, but for example, "ImageJ" can be used. The spreadsheet software is also not particularly limited, but for example, Microsoft Office Excel can be used.

[0493] For example, the actual specific surface area S measured by the gas adsorption method using the constant volume method Ｒ and the ideal specific surface area S ｉ The smoothness of the surface of the positive electrode active material 200 can also be quantified from the ratio of

[0494] Ideal specific surface area S ｉ is calculated assuming that all the positive electrode active materials have the same diameter D50, the same weight, and an ideal spherical shape.

[0495] 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.

[0496] The positive electrode active material 200 according to one embodiment of the present invention has an ideal specific surface area A calculated from the median diameter (D50). ｉ and the actual specific surface area S Ｒ The ratio S Ｒ / S ｉ is preferably 2.1 or less.

[0497] Alternatively, the surface smoothness can be quantified from a cross-sectional SEM image of the positive electrode active material 200 by the following method.

[0498] First, a surface SEM image of the positive electrode active material 200 is obtained. At this time, a conductive coating may be applied as a pretreatment for observation. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area are the same.

[0499] Next, image processing software (for example, "ImageJ") is used to convert the SEM image to, for example, 8 bits to obtain an image (called a grayscale image). The grayscale image contains luminance (brightness information). For example, in an 8-bit grayscale image, luminance can be expressed in 2 to the power of 8 = 256 gradations. Dark areas have lower gradations, and bright areas have higher gradations. The luminance change can be quantified in relation to the number of gradations. This numerical value is called a grayscale value. By obtaining the grayscale value, it is possible to evaluate the unevenness of the positive electrode active material as a numerical value.

[0500] Furthermore, it is possible to display the brightness change of the target area as a histogram. A histogram is a three-dimensional representation of the gradation distribution in the target area, and is also called a brightness histogram. Obtaining a brightness histogram makes it possible to visually evaluate the unevenness of the positive electrode active material in an easy-to-understand manner.

[0501] In the positive electrode active material 200 of one embodiment of the present invention, the difference between the maximum and minimum values ​​of the grayscale value is preferably 120 or less, more preferably 115 or less, and even more preferably 70 or more and 115 or less. The standard deviation of the grayscale value is preferably 11 or less, more preferably 8 or less, and even more preferably 4 or more and 8 or less.

[0502] <Current-Rest Method> The distribution of the additive element A, such as magnesium, contained in the surface layer portion of the positive electrode active material 200 of one embodiment of the present invention may change slightly during repeated charge and discharge. For example, the distribution of the additive element A may become better, resulting in a decrease in electronic conduction resistance. As a result, the electrical resistance at the beginning of the charge and discharge cycle, i.e., the fast-response resistance component R(0.1 s) measured by the current-rest method, may decrease.

[0503] For example, when comparing the nth charge (n is an integer greater than 1) with the n+1th charge, the fast-response resistance component R(0.1 s) measured by the current-rest method may be lower at the n+1th charge than at the nth charge. Accordingly, the n+1th discharge capacity may be higher than the nth discharge capacity. When n is 1, that is, when comparing the first charge with the second charge, the second charge capacity may be larger, especially for a positive electrode active material that does not contain an additive element. Therefore, n is preferably, for example, 2 or more and 10 or less. However, this is not limited to the initial stage of the charge-discharge cycle. A charge-discharge capacity that is comparable to the rated capacity, for example, 97% or more of the rated capacity, can be considered to be at the initial stage of the charge-discharge cycle.

[0504] <Additional Features> The positive electrode active material 200 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, cracking of the main body, desorption of oxygen, etc. However, if there is an embedded portion 202 as shown in FIG. 13B that embeds these defects, elution of the transition metal M, etc., can be suppressed. Therefore, the positive electrode active material 200 can have excellent reliability and cycle characteristics.

[0505] Furthermore, the positive electrode active material 200 may have a convex portion 203 as a region where the additional element A is unevenly distributed, as shown in FIG. 13B.

[0506] As described above, an excess of the additive element A in the positive electrode active material 200 may adversely affect the insertion and desorption 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, an insufficient amount of the additive element A may result in the additive element A not being distributed throughout the entire surface layer 200a, resulting in an insufficient effect of suppressing deterioration of the crystal structure. Thus, the additive element A needs to be present at an appropriate concentration in the positive electrode active material 200, but adjusting this concentration is not easy.

[0507] Therefore, when the positive electrode active material 200 has a region where the additive element A is unevenly distributed, a portion of the excess additive element A is removed from the interior 200b of the positive electrode active material 200, and an appropriate concentration of the additive element A can be achieved in the interior 200b. This makes it possible to suppress an increase in internal resistance and a decrease in charge / discharge capacity when the positive electrode active material 200 is used as a secondary battery. The ability to suppress an increase in internal resistance of a secondary battery is an extremely desirable characteristic, particularly in charge / discharge at large currents, for example, at 400 mA / g or more.

[0508] Furthermore, in the positive electrode active material 200 having a region where the additive element A is unevenly distributed, it is permissible to mix a certain amount of excess additive element A in the manufacturing process. This is therefore preferable as it widens the margin in production.

[0509] Furthermore, when the positive electrode active material is charged at 4.5 V or higher, or when the positive electrode active material is charged and discharged at a high temperature, for example, an ambient temperature of 45° C. or higher, progressive defects may develop that progress from the surface to the interior. The phenomenon in which defects progress to form holes in the positive electrode active material can also be called pitting corrosion, and the holes generated by this phenomenon are also called pits in this specification.

[0510] Figure 21 shows a cross-sectional schematic diagram of a positive electrode active material 51 having pits. Crystal planes 55 parallel to the arrangement of cations are also shown. Because Figure 21 is a cross-sectional view, pits 54 and pits 58 are shown as holes, but their opening shapes are not circular but have depth and are groove-like. Furthermore, as shown by pits 54 and pits 58, unlike recesses 52, they tend to form parallel to the arrangement of lithium ions.

[0511] Furthermore, the surface layer portions of the positive electrode active material 51 where the additive element A is present are indicated by 53 and 56. The surface layer portions where the pits have occurred have less additive element A than 53 and 56 or below the lower detection limit, and it is expected that the function of the barrier film is reduced. It is also thought that the crystalline structure of the composite oxide breaks down in the vicinity of where the pits are formed, resulting in a crystalline structure different from that of the layered rock salt type. When the crystalline structure breaks down, it inhibits the diffusion and release of lithium ions, which are carrier ions, and therefore the pits are thought to be a factor in the deterioration of cycle characteristics.

[0512] The source of pits may be point defects. It is thought that point defects in the positive electrode active material change with repeated charge and discharge, and are either chemically or electrochemically corroded by the surrounding electrolyte or the like, or that the material deteriorates. This deterioration does not occur uniformly on the surface of the positive electrode active material, but rather occurs in localized areas.

[0513] Furthermore, as shown by the crack 57 in FIG. 21 , defects such as cracks (also referred to as fissures) may occur due to the expansion and contraction of the positive electrode active material during charging and discharging. In this specification, cracks and pits are different. Even if cracks are present immediately after the preparation of the positive electrode active material, pits are not. Pits can be considered as holes formed by the loss of several layers of transition metal M and oxygen due to charging and discharging under high voltage conditions of, for example, 4.5 V or higher or at high temperatures (45°C or higher), and can also be considered as locations where the transition metal M has dissolved. Cracks refer to, for example, new surfaces formed by the application of physical pressure, or fissures caused by the grain boundaries 201. Cracks may also occur due to the expansion and contraction of the positive electrode active material during charging and discharging. Pits may also occur from cracks and / or cavities within the positive electrode active material.

[0514] [Method for Producing Positive Electrode Active Material] A method for producing the positive electrode active material 200 having the distribution, composition, and / or crystal structure of the additive element A as described in the previous embodiment will be described.

[0515] In the process of producing the positive electrode active material 200, it is preferable to first synthesize a composite oxide containing lithium and a transition metal, and then mix in a source of the additive element A and perform a heat treatment.

[0516] In a method of synthesizing a composite oxide containing the additive element A, lithium, and the transition metal M by mixing the additive element A source with the transition metal M source and the lithium source at the same time, it is difficult to increase the concentration of the additive element A in the surface layer portion 200a. Furthermore, if the additive element A source is simply mixed without heating after synthesizing the composite oxide containing lithium and the transition metal M, the additive element will simply adhere to the composite oxide without dissolving in the composite oxide. Without sufficient heating, it is also difficult to achieve a good distribution of the additive element A. For this reason, it is preferable to mix the additive element A source after synthesizing the composite oxide and then perform a heat treatment. This heat treatment after mixing the additive element A source is sometimes called annealing.

[0517] However, if the annealing temperature is too high, cation mixing occurs, increasing the possibility that the additive element A, for example, magnesium, enters the transition metal M site. Magnesium present in the transition metal M site is Li ｘ CoO ２ When x in the formula is small, the layered rock salt crystal structure of R-3m cannot be maintained. Furthermore, if the heat treatment temperature is too high, there are concerns about adverse effects such as cobalt being reduced to divalent cobalt and lithium being evaporated or sublimated.

[0518] Therefore, it is preferable to mix a material that functions as a flux with the source of the additional element A. If the melting point is lower than that of the composite oxide containing lithium and the transition metal M, the material can be said to function as a flux. For example, fluorine compounds such as lithium fluoride are suitable. Adding a flux lowers the melting points of the source of the additional element A and the composite oxide containing lithium and the transition metal M. Lowering the melting point makes it easier to distribute the additional element A well at a temperature where cation mixing is unlikely to occur.

[0519] Furthermore, it is more preferable to carry out heating after synthesizing the composite oxide containing lithium and the transition metal M and before mixing with the additional element A. This heating is sometimes called initial heating.

[0520] The initial heating causes lithium to be released from a part of the surface layer 200a of the composite oxide containing lithium and the transition metal M, which results in a more favorable distribution of the additive element A.

[0521] More specifically, it is believed that initial heating facilitates the differentiation of distribution depending on the additive element A through the following mechanism. First, lithium is eliminated from a portion of the surface layer 200a by initial heating. Next, a composite oxide containing lithium and a transition metal M, which has the lithium-deficient surface layer 200a, is mixed with an additive element A source, such as a nickel source, an aluminum source, or a magnesium source, and heated. Of the additive elements A, magnesium is a divalent typical element, and nickel is a transition metal, but is prone to becoming a divalent ion. Therefore, Mg is released from a portion of the surface layer 200a. ２＋ and Ni ２＋ and Co reduced by lithium deficiency. ２＋ A rock salt type phase having

[0522] Of the added elements A, nickel is easily dissolved and diffuses to the interior 200b when the surface layer 200a is a layered rock-salt type composite oxide containing lithium and a transition metal M, but when part of the surface layer 200a is rock-salt type, nickel tends to remain in the surface layer 200a.

[0523] Furthermore, in these rock salt types, the bond distance between the metal Me and oxygen (Me-O distance) tends to be longer than in the layered rock salt type.

[0524] For example, rock salt type Ni ０．５ Mg ０．５ The Me-O distance in the rock salt MgO is 0.209 nm, and the Me-O distance in the rock salt MgO is 0.211 nm. ２ O ４ The Me-O distance is 0.20125 nm, and the spinel type MgAl ２ O ４ The Me-O distance of is 0.202 nm. In all cases, the Me-O distance exceeds 0.2 nm.

[0525] On the other hand, in the layered rock salt type, the bond distance between metals other than lithium and oxygen is shorter than the above. For example, layered rock salt type LiAlO ２ The Al-O distance in the layered rock salt LiCoO is 0.1905 nm (the Li-O distance is 0.211 nm). ２ The Co—O distance in this case is 0.1.9224 nm (the Li—O distance is 0.20916 nm).

[0526] According to Shannon's ionic radii (Shannon et al., Acta A 32 (1976) 751.), the ionic radius of hexacoordinated aluminum is 0.0535 nm, the ionic radius of hexacoordinated oxygen is 0.14 nm, and the sum of these is 0.1935 nm.

[0527] From the above, it is considered that aluminum exists more stably in non-lithium sites in the layered rock-salt type than in the rock-salt type, and therefore aluminum is more likely to be distributed in the deeper region having the layered rock-salt type and / or the interior 200b than in the region close to the surface having the rock-salt type phase in the surface layer portion 200a.

[0528] Furthermore, the initial heating is expected to have the effect of increasing the crystallinity of the layered rock salt type crystal structure in the interior 200b.

[0529] However, initial heating is not necessarily required. In other heating steps, such as annealing, the atmosphere, temperature, time, etc. can be controlled to prevent Li ｘ CoO ２ When x in the formula is small, a positive electrode active material 200 having an O3' type can sometimes be produced.

[0530] An example of a manufacturing flow of the positive electrode active material 200 that undergoes initial heating will be described with reference to FIGS. 22A to 22C.

[0531] <Step S11> In step S11 shown in FIG. 22A, a lithium source (Li source) and a transition metal M source (M source) are prepared as starting materials for lithium and transition metal M, respectively.

[0532] As the lithium source, it is preferable to use a compound containing lithium, such as lithium carbonate, lithium hydroxide, lithium nitrate, or lithium fluoride. It is preferable that the lithium source has high purity, and it is preferable to use a material with a purity of, for example, 99.99% or higher.

[0533] The transition metal M can be selected from elements in Groups 4 to 13 of the periodic table, and for example, at least one of manganese, cobalt, and nickel is used. As the transition metal M, only cobalt may be used, only nickel may be used, two elements of cobalt and manganese may be used, two elements of cobalt and nickel may be used, or three elements of cobalt, manganese, and nickel may be used. When only cobalt is used, the resulting positive electrode active material has lithium cobalt oxide (LCO), and when three elements of cobalt, manganese, and nickel are used, the resulting positive electrode active material has lithium nickel-cobalt-manganese oxide (NCM).

[0534] As the transition metal M source, it is preferable to use a compound containing the transition metal M, and for example, an oxide or hydroxide of a metal exemplified as the transition metal M 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. Although not a transition metal, an aluminum source can also be used, and as an aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0535] The transition metal M 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.

[0536] In addition, the transition metal M source preferably has high crystallinity, for example, single crystal grains. The crystallinity of the transition metal M 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 M sources but also to evaluating the crystallinity of other sources.

[0537] When two or more transition metal M sources are used, the two or more transition metal M sources are preferably prepared in a ratio (mixing ratio) that allows the two or more transition metal M sources to form a layered rock salt type crystal structure.

[0538] <Step S12> Next, in step S12 shown in FIG. 22A , the lithium source and the transition metal M source are pulverized and mixed to prepare a mixed material. The pulverization and mixing can be performed by either a dry or wet method. The wet method is preferred because it allows for smaller pulverization. When using the wet method, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react 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 M source in dehydrated acetone with a purity of 99.5% or higher, with a water content reduced to 10 ppm or less, and then pulverize and mix them. Using dehydrated acetone with the above purity can reduce potential impurities.

[0539] A ball mill, a bead mill, or the like can be used as a means for mixing, etc. When using a ball mill, aluminum oxide balls or zirconium oxide balls are preferably used as grinding media. Zirconium oxide balls are preferred because they emit less impurities. Furthermore, when using a ball mill, a bead mill, or the like, the peripheral speed is preferably set to 100 mm / s or more and 2000 mm / s or less in order to suppress contamination from the media. In this embodiment, the peripheral speed is set to 838 mm / s (rotation speed 400 rpm, ball mill diameter 40 mm).

[0540] <Step S13> Next, in step S13 shown in FIG. 22A , 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 about 950°C. If the temperature is too low, the decomposition and melting of the lithium source and the transition metal M source may be insufficient. On the other hand, if the temperature is too high, defects may occur due to lithium evaporation from the lithium source and / or excessive reduction of the metal used as the transition metal M source. For example, when cobalt is used as the transition metal M, excessive reduction can cause cobalt to change from trivalent to divalent, which can induce oxygen defects.

[0541] If the heating time is too short, LiMO ２ However, if the heating time is too long, productivity decreases. For example, the heating time is preferably 1 hour or more and 100 hours or less, and more preferably 2 hours or more and 20 hours or less.

[0542] The temperature rise rate depends on the heating temperature reached, but is preferably 80° C. / h to 250° C. / h. For example, when heating at 1000° C. for 10 hours, the temperature rise rate should be 200° C. / h.

[0543] The heating atmosphere is preferably an atmosphere with little water, such as dry air, and 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, in order to suppress impurities that may be mixed into the material, the CH ４ , CO, CO２ , and H ２ The impurity concentrations of the above should be set to 5 ppb (parts per billion) or less.

[0544] 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 allowing oxygen to flow through the reaction chamber is called flow.

[0545] When the heating atmosphere is an atmosphere containing oxygen, a method of not allowing the oxygen to flow may be used. For example, a method of reducing the pressure of the reaction chamber and then filling it with oxygen to prevent the oxygen from entering or leaving the reaction chamber may be used, which is called purging. For example, the reaction chamber may be reduced in pressure to -970 hPa and then filled with oxygen to 50 hPa.

[0546] After heating, the material may be cooled naturally, but it is preferable that the time required for the temperature to drop from the specified temperature to room temperature is within a range of 10 to 50 hours. However, cooling to room temperature is not necessarily required, as long as the material is cooled to a temperature acceptable for the next step.

[0547] 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.

[0548] The crucible or sheath used during heating is preferably made of a highly heat-resistant material such as alumina (aluminum oxide), mullite / cordierite, magnesia, or zirconia. Furthermore, since aluminum oxide is a material that is less susceptible to impurities, the purity of the alumina crucible or sheath is 99% or higher, preferably 99.5% or higher. In this embodiment, a crucible made of aluminum oxide with a purity of 99.9% is used. It is preferable to heat the crucible or sheath with a lid. This can prevent the material from volatilizing or sublimating.

[0549] After heating, the material may be crushed and sieved as necessary. When recovering the heated material, it may be transferred from the crucible to a mortar and then recovered. It is preferable to use a mortar made of aluminum oxide. A mortar made of aluminum oxide is a material that does not easily release impurities. Specifically, a mortar made of aluminum oxide 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.

[0550] <Step S14> By the above steps, a composite oxide having a transition metal M (LiMO ２ The composite oxide can be obtained by ２ The composition is not strictly limited to Li:M:O=1:1:2, as long as it has a crystalline structure of a lithium composite oxide represented by the formula: ２ The composition is not strictly limited to Li:Co:O=1:1:2.

[0551] Although the example of producing the composite oxide by the solid phase method in steps S11 to S14 has been shown, the composite oxide may also be produced by a coprecipitation method or a hydrothermal method.

[0552] <Step S15> Next, in step S15 shown in Fig. 22A, the composite oxide is heated. Because this is the first heating of the composite oxide, the heating in step S15 may be called initial heating. Alternatively, because this heating is performed before step S20 described below, it may be called preheating or pretreatment.

[0553] As described above, the initial heating causes lithium to be desorbed from a portion of the surface layer 200a of the composite oxide. It is also expected to have the effect of increasing the crystallinity of the inner portion 200b. Furthermore, the lithium source and / or transition metal M prepared in step S11 or the like may contain impurities. The initial heating can reduce the amount of impurities in the composite oxide completed in step S14.

[0554] Furthermore, initial heating has the effect of smoothing the surface of the composite oxide. A smooth surface means that there are few irregularities, the composite oxide is rounded overall, and the corners are rounded. Furthermore, a smooth surface means that there is little foreign matter adhering to the surface. Foreign matter is thought to be a cause of irregularities, so it is preferable that it does not adhere to the surface.

[0555] For this initial heating, it is not necessary to prepare a lithium compound source, a source of the additional element A, or a material that functions as a flux.

[0556] If the heating time in this step is too short, sufficient effects will not be obtained, but if it is too long, productivity will decrease. For example, the heating conditions can be selected from those described in step S13. In addition to the heating conditions, the heating temperature in this step should be lower than the temperature in step S13 in order to maintain the crystalline structure of the complex oxide. Furthermore, the heating time in this step should be shorter than the time in step S13 in order to maintain the crystalline structure of the complex oxide. For example, heating at a temperature of 700°C or higher and 1000°C or lower for 2 hours or longer and 20 hours or shorter is recommended.

[0557] The effect of increasing the crystallinity of the inner portion 200b is, for example, the effect of alleviating distortion, displacement, etc. resulting from the difference in shrinkage of the composite oxide produced in step S13.

[0558] The heating in step S13 may cause a temperature difference between the surface and the interior of the composite oxide. The temperature difference may induce a contraction difference. It is also thought that the temperature difference causes a difference in fluidity between the surface and the interior, resulting in a contraction difference. The energy associated with the contraction difference causes a difference in internal stress 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.

[0559] Furthermore, the difference in shrinkage may cause microscopic deviations in the composite oxide, such as deviations in crystals. This step is preferably carried out in order to reduce such deviations. This step makes it possible to equalize the deviations in the composite oxide. When the deviations are 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 step S15 reduces the deviations of crystals and the like that have occurred in the composite oxide, resulting in a smooth surface of the composite oxide.

[0560] 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.

[0561] The smooth surface of a composite oxide can be said to have a surface roughness of at least 10 nm or less when surface irregularity information is quantified from measurement data at a cross section of the composite oxide. The cross section is, for example, a cross section obtained during STEM observation.

[0562] Alternatively, a composite oxide containing lithium, a transition metal M, 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.

[0563] It is possible that the lithium in the composite oxide is reduced by the initial heating, and the added element A, which will be explained in the next step S20, etc., may be more likely to enter the composite oxide due to the reduced lithium.

[0564] <Step S20> The additive element A may be added to a composite oxide having a smooth surface, as long as it can form a layered rock salt crystal structure. Adding the additive element A to a composite oxide having a smooth surface allows the additive element A to be added evenly. Therefore, it is preferable to add the additive element A after the initial heating. The step of adding the additive element A will be described with reference to FIGS. 22B and 22C.

[0565] 22B, a source of an additive element A (A source) to be added to the composite oxide is prepared. A lithium source may be prepared together with the additive element A source.

[0566] As the additive element A, one or more selected from nickel, cobalt, magnesium, calcium, chlorine, fluorine, aluminum, manganese, titanium, zirconium, yttrium, vanadium, iron, chromium, niobium, lanthanum, hafnium, zinc, silicon, sulfur, phosphorus, boron, and arsenic can be used. Furthermore, as the additive element, one or more selected from bromine and beryllium can also be used. However, since bromine and beryllium are elements that are toxic to living organisms, it is more preferable to use the additive elements described above.

[0567] When magnesium is selected as the additional element A, the source of the additional element A can be called a magnesium source. As the magnesium source, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, or the like can be used. Furthermore, a plurality of the above-mentioned magnesium sources may be used.

[0568] When fluorine is selected as the additional element A, the source of the additional element A can be called a fluorine source. Examples of the fluorine source include lithium fluoride, magnesium fluoride, aluminum fluoride, titanium fluoride, cobalt fluoride, nickel fluoride, zirconium fluoride, vanadium fluoride, manganese fluoride, iron fluoride, chromium fluoride, niobium fluoride, zinc fluoride, calcium fluoride, sodium fluoride, potassium fluoride, barium fluoride, cerium fluoride, and lanthanum fluoride (LaF ３ 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.

[0569] 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 S21 is lithium carbonate.

[0570] The fluorine source may also be gaseous, such as fluorine (F ２ ), fluorine carbon, sulfur fluoride, oxygen fluoride, or the like may be used and mixed into the atmosphere in the heating step described below. Also, a plurality of the above-mentioned fluorine sources may be used.

[0571] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source, and magnesium fluoride (MgF) is prepared as the fluorine source and magnesium source. ２ Lithium fluoride and magnesium fluoride are prepared as LiF:MgF ２ When the molar ratio of lithium fluoride to magnesium fluoride is 65:35 or around that, the melting point is lowered and the gel effect is maximized. On the other hand, if the amount of lithium fluoride is too large, there is a concern that the lithium will be excessive and the cycle characteristics will deteriorate. Therefore, the molar ratio of lithium fluoride to magnesium fluoride is set to LiF:MgF ２ =x:1 (0≦x≦1.9), and LiF:MgF ２ =x:1 (0.1≦x≦0.5) is more preferable, and LiF:MgF ２= x:1 (x = 0.33 or its vicinity) is more preferable. In this specification, "or its vicinity" refers to a value that is greater than 0.9 times and smaller than 1.1 times the value.

[0572] At the same time, the amount of magnesium added is ２ Based on the standard, the content is preferably more than 0.1 atomic % and not more than 3 atomic %, more preferably 0.5 atomic % to 2 atomic %, and even more preferably 0.5 atomic % to 1 atomic %. When the amount of magnesium added is 0.1 atomic % or less, the initial discharge capacity is high, but the discharge capacity rapidly decreases with repeated charge and discharge to increase the depth of charge. When the amount of magnesium added is more than 0.1 atomic % and not more than 3 atomic %, both the initial discharge characteristics and the charge and discharge cycle characteristics are good even when repeated charge and discharge to increase the depth of charge. On the other hand, when the amount of magnesium added exceeds 3 atomic %, both the initial discharge capacity and the charge and discharge cycle characteristics tend to gradually deteriorate.

[0573] 22B, the magnesium source and the fluorine source are pulverized and mixed. This step can be performed under pulverization and mixing conditions selected from those described in step S12.

[0574] If necessary, a heating step may be performed after step S22. The heating step can be performed under heating conditions selected from those described in step S13. The heating time is preferably 2 hours or more, and the heating temperature is preferably 800°C or higher and 1100°C or lower.

[0575] 22B, the pulverized and mixed materials are collected to obtain a source of the additional element A (A source). The source of the additional element A shown in step S23 includes a plurality of starting materials and can be called a mixture.

[0576] The particle size of the mixture is preferably such that the median diameter (D50) is 600 nm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less. Even when a single material is used as the source of the additional element A, the median diameter (D50) is preferably 600 nm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less.

[0577] Such a finely powdered mixture (including the case where only one type of additive element A is contained) makes it easier to uniformly adhere the mixture to the surface of the composite oxide when it is mixed with the composite oxide in a later step. Uniformly adhering a mixture of the additive element A source and the like to the surface of the composite oxide is preferable because it makes it easier to uniformly distribute or diffuse fluorine and magnesium in the surface layer portion of the composite oxide after heating. The region where fluorine and magnesium are distributed can also be referred to as the surface layer portion. If there is a region in the surface layer that does not contain fluorine and magnesium, it may be difficult to obtain an O3'-type crystal structure in the charged state. Although fluorine has been used in the description, fluorine may also be chlorine, and chlorine can be read as including these and so is halogen.

[0578] <Step S21> A step different from that shown in Fig. 22B will be described with reference to Fig. 22C. In step S21 shown in Fig. 22C, four types of additive element A sources to be added to the composite oxide are prepared. That is, Fig. 22C differs from Fig. 22B in the type of additive element A source. A lithium source may be prepared together with the additive element A source.

[0579] As sources of four types of additive element A, a magnesium source (Mg source), a fluorine source (F source), a nickel source (Ni source), and an aluminum source (Al source) are prepared. The magnesium source and the fluorine source can be selected from the compounds described in FIG. 22B . Nickel oxide, nickel hydroxide, etc. can be used as the nickel source. Aluminum oxide, aluminum hydroxide, etc. can be used as the aluminum source.

[0580] <Step S22> and <Step S23> Next, step S22 and step S23 shown in FIG. 22C are the same as the steps described in FIG. 22B.

[0581] 22A , the composite oxide is mixed with a source of the additional element A. The ratio of the number of atoms M of the transition metal M in the composite oxide containing lithium, the transition metal M, and oxygen to the number of atoms Mg of magnesium in the additional element A is preferably M:Mg=100:y (0.1≦y≦6), and more preferably M:Mg=100:y (0.3≦y≦3).

[0582] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the composite oxide. 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 the dry method provides milder conditions than the wet method. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconium oxide balls as the media.

[0583] In this embodiment, dry mixing is performed in a ball mill using zirconium oxide balls with a diameter of 1 mm at 150 rpm for 1 hour in a dry room with a dew point of −100° C. or higher and −10° C. or lower.

[0584] <Step S32> Next, in step S32 of Fig. 22A, the mixed materials are collected to obtain a mixture 903. When collecting the materials, they may be crushed and then sieved, if necessary.

[0585] In this embodiment, a method is described in which lithium fluoride as a fluorine source and magnesium fluoride as a magnesium source are added to the composite oxide after the initial heating. However, the present invention is not limited to the above method. In step S11, that is, in the stage of the starting materials for the composite oxide, a magnesium source, a fluorine source, etc. can be added to the lithium source and the transition metal M source. Then, in step S13, heating is performed to obtain LiMO with added magnesium and fluorine. ２ In this case, it is not necessary to separate the steps S11 to S14 from the steps S21 to S23. This method is simple and has high productivity.

[0586] Alternatively, a composite oxide to which magnesium and fluorine have been added in advance may be used. If a composite oxide to which magnesium and fluorine have been added is used, steps S11 to S32 and step S20 can be omitted. This method can be said to be simple and highly productive.

[0587] Alternatively, a magnesium source and a fluorine source, or a magnesium source, a fluorine source, a nickel source and an aluminum source may be further added to a composite oxide to which magnesium and fluorine have been added in advance in step S20.

[0588] 22A, 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.

[0589] Here, a supplementary note about the heating temperature will be given. The lower limit of the heating temperature in step S33 is ２ The temperature at which the reaction between the LiMO and the additive element A source proceeds must be equal to or higher than the temperature at which the reaction between the LiMO and the additive element A source proceeds. ２ The temperature may be lower than the melting point of these materials as long as it is a temperature at which mutual diffusion of elements contained in the source of the additive element A occurs. ｍ 0.757 times (Tammann temperature T ｄ ) solid-phase diffusion occurs. Therefore, the heating temperature in step S33 should be 500° C. or higher.

[0590] Of course, the reaction proceeds more easily when the temperature is equal to or higher than the temperature at which at least a part of the mixture 903 melts. ２ When LiF and MgF ２ Since the eutectic point of is around 742°C, the lower limit of the heating temperature in step S33 is preferably set to 742°C or higher.

[0591] Also, LiCoO ２ :LiF:MgF ２A mixture 903 obtained by mixing the components so that the molar ratio was 100:0.33:1 exhibits an endothermic peak at around 830° C. in differential scanning calorimetry (DSC). Therefore, the lower limit of the heating temperature is more preferably 830° C. or higher.

[0592] A higher heating temperature is preferable because the reaction proceeds more easily, the heating time is shorter, and productivity is higher.

[0593] The upper limit of heating temperature is LiMO ２ Decomposition temperature of LiCoO ２ The decomposition temperature of LiMO is less than 1130°C. At temperatures close to the decomposition temperature, a small amount of LiMO ２ Therefore, the temperature is more preferably 1000°C or lower, even more preferably 950°C or lower, and even more preferably 900°C or lower.

[0594] Taking these factors into consideration, 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 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 800°C to 1100°C, or 830°C to 1130°C, more preferably 830°C to 1000°C, even more preferably 830°C to 950°C, and even more preferably 830°C to 900°C. The heating temperature in step S33 is preferably lower than that in step S13.

[0595] Furthermore, when the mixture 903 is heated, it is preferable to control the partial pressure of fluorine or fluoride resulting from the fluorine source or the like within an appropriate range.

[0596] In the manufacturing method described in this embodiment, some materials, for example, LiF, which is a fluorine source, may function as a flux. This function allows the heating temperature to be controlled to the temperature of the composite oxide (LiMO). ２) can be lowered to a temperature lower than the decomposition temperature, for example, 742°C or higher and 950°C or lower, and additive element A including magnesium can be distributed in the surface layer portion, thereby producing a positive electrode active material with good characteristics.

[0597] However, since LiF has a lower specific gravity in a gaseous state than oxygen, there is a possibility that LiF may volatilize or sublime when heated, and if it volatilizes, the amount of LiF in the mixture 903 will decrease. This will weaken its function as a flux. Therefore, it is necessary to heat the mixture while suppressing the volatilization or sublimation of LiF. Even if LiF is not used as a fluorine source, etc., LiMO ２ There is a possibility that Li on the surface reacts with F in the fluorine source to produce LiF, which may volatilize or sublime. Therefore, even if a fluoride with a higher melting point than LiF is used, it is still necessary to suppress volatilization or sublimation.

[0598] Therefore, it is preferable to heat the mixture 903 in an atmosphere containing LiF, that is, to heat the mixture 903 in a state where the partial pressure of LiF is high in the heating furnace. By heating in this manner, it is possible to suppress the volatilization or sublimation of LiF in the mixture 903.

[0599] The heating in this step is preferably performed so as not to stick together the mixture 903. If the mixture 903 sticks together during heating, the contact area with oxygen in the atmosphere decreases, and the route along which the additional element A (for example, fluorine) diffuses is blocked, which may result in a poor distribution of the additional element A (for example, magnesium and fluorine) in the surface layer portion.

[0600] It is also believed that uniform distribution of the additive element A (e.g., fluorine) in the surface layer portion results in a smooth cathode active material with few irregularities. Therefore, in order to maintain or further smooth the surface after the heating in step S15 in this process, it is preferable that the mixture 903 does not stick to itself.

[0601] Furthermore, when heating in a rotary kiln, it is preferable to control the flow rate of the oxygen-containing atmosphere in the kiln. 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 volatilize or sublimate, which is undesirable in terms of maintaining surface smoothness.

[0602] When heating is performed using a roller hearth kiln, the mixture 903 can be heated in an atmosphere containing LiF by, for example, placing a lid on a container containing the mixture 903 .

[0603] The heating time is determined by the heating temperature, the LiMO ２ It changes depending on the size and composition of LiMO. ２ When is small, a lower temperature or shorter time may be more preferable than when is large.

[0604] The composite oxide (LiMO) in step S14 of FIG. ２ When the median diameter (D50) of the powder is about 12 μm, the heating temperature is preferably, for example, 600° C. or higher and 950° C. or lower. The heating time is, for example, preferably 3 hours or higher, more preferably 10 hours or higher, and even more preferably 60 hours or higher. The cooling time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0605] On the other hand, the composite oxide (LiMO) ２ When the median diameter (D50) of the powder 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 cooling time after heating is preferably, for example, 10 hours or higher and 50 hours or lower.

[0606] <Step S34> Next, in step S34 shown in Fig. 22A , the heated material is recovered and crushed as necessary to obtain positive electrode active material 200. At this time, it is preferable to further sieve the recovered positive electrode active material 200. Through the above steps, positive electrode active material 200 of one embodiment of the present invention can be produced. The positive electrode active material of one embodiment of the present invention has a smooth surface.

[0607] This embodiment can be used in combination with other embodiments.

[0608] Embodiment Mode 5 In this embodiment mode, an example of the secondary battery described in the previous embodiment mode will be described.

[0609] 23A and 23B show examples of the external appearance of a laminated secondary battery 100. The laminated secondary battery 100 includes a positive electrode layer 106, a negative electrode layer 107, an electrolyte layer 103, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0610] <Method for Manufacturing Laminated Secondary Battery> An example of a method for manufacturing the laminated secondary battery whose external view is shown in FIG. 23A will be described with reference to FIGS. 24B and 24C.

[0611] As shown in FIG. 24A , a positive electrode layer 106 and a negative electrode layer 107 are prepared. In the positive electrode layer 106, a positive electrode active material layer 102 is formed on one side of a positive electrode current collector. The positive electrode active material layer 102 may be formed on the other side of the positive electrode current collector. The positive electrode layer 106 has a region where the positive electrode current collector is partially exposed (hereinafter referred to as a tab region), and this tab region is referred to as a positive electrode tab 501. In the negative electrode layer 107, a negative electrode active material layer 104 is formed on one side of a negative electrode current collector. The negative electrode active material layer 104 may be formed on the other side of the negative electrode current collector. The negative electrode layer 107 has a region where the negative electrode current collector is partially exposed, i.e., a tab region, and this tab region is referred to as a negative electrode tab 504. The area and shape of each tab region are not limited to the example shown in FIG. 24A .

[0612] Next, the negative electrode layer 107, the electrolyte layer 103, and the positive electrode layer 106 are laminated. FIG. 24B shows the laminated negative electrode layer 107, the electrolyte layer 103, and the positive electrode layer 106. The area of ​​the electrolyte layer 103 is preferably larger than the area of ​​the negative electrode layer 107 and the positive electrode layer 106. FIG. 24B shows an example configuration in which five pairs of negative electrodes and four pairs of positive electrodes are laminated. Next, the positive electrode tabs 501 are bonded to each other, and the positive electrode lead electrode 510 is bonded to the outermost tab region. For example, ultrasonic welding or the like may be used for bonding. Similarly, the negative electrode tabs 504 are bonded to each other, and the negative electrode lead electrode 511 is bonded to the outermost tab region.

[0613] Next, as shown in Fig. 24C, the negative electrode layer 107, the electrolyte layer 103, and the positive electrode layer 106 are disposed on the exterior body 509, and the exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of the exterior body 509 is bonded. The area used for bonding is referred to as the bonding area. For example, thermocompression bonding or the like may be used for bonding.

[0614] Next, an ionic liquid can be injected into the inside of the exterior body 509 through an inlet provided in the exterior body 509. The introduction of a liquid material such as an ionic liquid 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 100 can be produced.

[0615] As described in the above embodiment, in the secondary battery of one embodiment of the present invention, the solid electrolyte holds an ionic liquid in the electrolyte layer 103. In other words, the ionic liquid is impregnated into the solid electrolyte at least in the electrolyte layer 103. Such an electrolyte layer 103 is preferable because the ionic liquid does not seep out.

[0616] This embodiment can be used in combination with other embodiments.

[0617] Embodiment Mode 6 In this embodiment mode, an example of the secondary battery described in the previous embodiment mode will be described.

[0618] For example, the laminated secondary battery 100 can be bent after being manufactured. That is, the secondary battery 100 has flexibility.

[0619] Fig. 25A shows a secondary battery 100 in a bent shape. Fig. 25A shows a secondary battery 100 having a positive electrode layer 106, an electrolyte layer 103, and a negative electrode layer 107 bent toward the positive electrode layer 106. Of course, the secondary battery 100 can also be bent toward the negative electrode layer 107. The electrolyte layer 103, which is one aspect of the present invention, is suitable for a bent secondary battery 100 because it prevents leakage of an electrolytic solution such as an ionic liquid. Note that a bent shape includes a shape having an arc-shaped portion in one cross section of the secondary battery 100.

[0620] The smallest unit constituting a secondary battery is referred to as a battery unit, and the battery unit includes a positive electrode layer 106, an electrolyte layer 103, and a negative electrode layer 107. The secondary battery 100 of one embodiment of the present invention may include a plurality of battery units. That is, the secondary battery 100 may have a configuration in which a plurality of battery units are stacked. The electrolyte layer 103 of one embodiment of the present invention is suitable for stacking units because it prevents leakage of an electrolyte solution such as an ionic liquid.

[0621] Although one battery unit is illustrated in FIG. 25A, a configuration in which multiple battery units are stacked may also be used.

[0622] The secondary battery 100 also has an exterior body, but the exterior body described in the above embodiment can conform to the curved battery unit. Therefore, the exterior body is not shown in FIG. 25A.

[0623] 25A , in the secondary battery 100, the radius of curvature 1802 of the layer closer to the center of curvature 1800, for example, the positive electrode layer 106, is smaller than the radius of curvature 1804 of the layer farther from the center of curvature 1800, for example, the negative electrode layer 107. To facilitate bending, it is preferable that the layer with the smaller radius of curvature, for example, the positive electrode layer 106, be thinner than the negative electrode layer 107.

[0624] 25B, when the secondary battery 100 is bent as shown in Fig. 25A, as indicated by the arrows, a compressive stress is applied to the surface of the positive electrode layer 106, and a tensile stress is applied to the surface of the negative electrode layer 107. In order to alleviate the compressive stress, the layer with a smaller radius of curvature, for example, the positive electrode layer 106, may be thicker than the negative electrode layer 107.

[0625] As one mode for alleviating the above-mentioned compressive stress and tensile stress, a configuration in which recesses and protrusions are provided on the exterior body will be described with reference to FIGS. 26A and 26B.

[0626] The recesses and protrusions are formed on the surface of exterior body 1805, and form a pattern. As can be seen in a cross section of exterior body 1805, when a protrusion is provided on the exterior body, the recesses are also formed at the same time, and when a recess is provided on the exterior body, the protrusions are also formed at the same time. In other words, it is not necessary to form both recesses and protrusions on the exterior body; by providing one, the other is formed at the same time.

[0627] The above-mentioned compressive stress and tensile stress can be alleviated by the exterior body 1805. That is, the secondary battery 100 can be deformed in a range in which the radius of curvature of the exterior body on the side closer to the center of curvature is 30 mm or more, preferably 10 mm or more.

[0628] 26A and 26B has an adhesive region 1807 at the end. The adhesive region 1807 is a region where the exterior body 1805 is adhered by thermocompression or the like. In the adhesive region 1807, an adhesive layer 1803 may be located between the exterior body 1805.

[0629] In the adhesive region 1807, it is preferable that the recessed portions or protruding portions provided on the top and bottom of the exterior body 1805 overlap each other. Since the recessed portions or protruding portions overlap each other, the recessed portions or protruding portions may be newly formed on the exterior body 1805 when the exterior body is adhered. Such a configuration can increase adhesive strength.

[0630] FIG. 26A shows secondary battery 100 having space 1810 in region 1808 at the end of exterior body 1805 and not in adhesive region 1807 .

[0631] 26B shows a secondary battery 100 in which an area 1808 at the end of the exterior body 1805, which is not the adhesive area 1807, has ionic liquid 118. The ionic liquid 118 is held in the electrolyte layer 103, but in the case of a secondary battery with a curved shape, it is conceivable that the ionic liquid 118 may leak out. Note that if the area 1808 in FIG. 26B is not filled with the ionic liquid 118, there may be a configuration in which the area 1808 in FIG. 26B has a space between the ionic liquid 118. Because the adhesive strength of the exterior body 1805 is high, the ionic liquid 118 does not leak out of the exterior body 1805.

[0632] The shape of the curved secondary battery 100 is not limited to a simple arc shape in cross section, and may be a shape that includes a partial arc. For example, it may be the shape shown in Fig. 27A, or the wave shape shown in Fig. 27B, or an S-shape. The exterior body having the above-described recessed or protruding portions can also be applied to the secondary battery 100 shown in Fig. 27A and Fig. 27B, and a battery unit in which multiple layers are stacked can also be applied.

[0633] As shown in Figure 27A or 27B, when the curved surface of the secondary battery 100 has a shape with multiple centers of curvature, the secondary battery can be bent within a range in which the radius of curvature of the outer casing closest to the center of curvature is 10 mm or more, preferably 30 mm or more.

[0634] In the secondary battery of one embodiment of the present invention, the solid electrolyte holds an ionic liquid in the electrolyte layer 103. In other words, the ionic liquid is impregnated into the solid electrolyte at least in the electrolyte layer 103. Such an electrolyte layer 103 is preferable because the ionic liquid does not seep out.

[0635] This embodiment can be used in combination with other embodiments.

[0636] Embodiment Mode 7 In this embodiment mode, an electronic device having a secondary battery will be described.

[0637] As described above, the secondary battery of one embodiment of the present invention is bendable (may also be referred to as having flexibility). That is, the secondary battery of one embodiment of the present invention can be made flexible. The secondary battery of one embodiment of the present invention can be fixed in a bent state. Furthermore, the secondary battery of one embodiment of the present invention can be deformed from the bent state.

[0638] Configuration Example 1 As one embodiment of the present invention, a case where a secondary battery is mounted in a wristwatch-type electronic device will be described.

[0639] 28A shows a wristwatch-type electronic device 70. The wristwatch-type electronic device 70 includes a frame 71 (the frame is also referred to as a case), a display unit 72, a belt 21, a buckle 27, a sensor 74, and operation buttons 77. The wristwatch-type electronic device 70 can execute various applications such as mobile phone calls, e-mail, browsing and composing text, music playback, internet communication, or computer games.

[0640] The belt 21 is a part for attaching the watch to the wrist, and is also called a band, strap, or bracelet.

[0641] The display unit 72 may have a curved display surface. A display can be made along the curved display surface. The display unit 72 may also include a touch sensor, which may be arranged along the curved display surface. The applications can be operated by touching the touch sensor with a finger or a stylus. For example, touching an icon 73 displayed on the display unit 72 can launch an application associated with the icon.

[0642] The operation button 77 can have various functions, such as turning the power on and off, turning wireless communication on and off, enabling and disabling silent mode, and enabling and disabling power saving mode. The functions of the operation button 77 can be freely set by an operating system built into the wristwatch-type electronic device 70.

[0643] The wristwatch-type electronic device 70 is capable of performing short-range wireless communication according to a communication standard. For example, it can communicate with a wirelessly enabled headset for hands-free calling. The wristwatch-type electronic d...

Claims

1. A secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer positioned between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has a third solid electrolyte which is a sintered body and an ionic liquid, and the ionic liquid is impregnated into voids of the sintered body.

2. A secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer positioned between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has a third solid electrolyte which is a sintered body, the positive electrode layer, the negative electrode layer, and the electrolyte layer have an ionic liquid, and the ionic liquid is impregnated into voids of the sintered body.

3. A secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer positioned between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has a first electrolyte layer to a third electrolyte layer, the first electrolyte layer to the third electrolyte layer have an ionic liquid, the second electrolyte layer has a third solid electrolyte which is a sintered body, and the ionic liquid is impregnated into voids of the sintered body.

4. A secondary battery having a positive electrode layer, a negative electrode layer, and an electrolyte layer positioned between the positive electrode layer and the negative electrode layer, wherein the positive electrode layer has a positive electrode active material and a first solid electrolyte, the negative electrode layer has a negative electrode active material and a second solid electrolyte, the electrolyte layer has a first electrolyte layer and a second electrolyte layer, the first electrolyte layer and the second electrolyte layer have an ionic liquid, the second electrolyte layer has a third solid electrolyte which is a sintered body, and the ionic liquid is impregnated into voids of the sintered body.

5. In any one of Claims 1 to 4, the secondary battery, wherein the positive electrode active material has a composite oxide having a layered rock salt-type crystal structure, a spinel-type crystal structure, or an olivine-type crystal structure.

6. In Claim 5, the secondary battery, wherein the positive electrode active material having the layered rock salt-type crystal structure has lithium cobaltate or lithium nickel-manganese-cobaltate.

7. In any one of Claims 1 to 4, The secondary battery, wherein the negative electrode active material contains silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, or indium.

8. In any one of Claims 1 to 4, The secondary battery, wherein the negative electrode active material contains a carbon material.