Positive electrode active material for potassium ion battery, positive electrode for potassium ion battery, and potassium ion battery

Abstract

To provide a positive electrode active material for a potassium-ion cell capable of obtaining a potassium-ion cell having a large discharge capacitance.SOLUTION: A positive electrode active material for a potassium-ion cell containing a potassium vanadium fluoride compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a potassium ion battery, a positive electrode for a potassium ion battery, and a potassium ion battery. [Background technology]

[0002] Currently, non-aqueous electrolyte secondary batteries are widely used as high energy density secondary batteries, which use a non-aqueous electrolyte and perform charging and discharging by moving, for example, lithium ions between a positive electrode and a negative electrode.

[0003] In such non-aqueous electrolyte secondary batteries, lithium transition metal composite oxides having a layered structure, such as lithium nickel oxide (LiNiO) and lithium cobalt oxide (LiCoO), are generally used as the positive electrode, and carbon materials capable of absorbing and releasing lithium, lithium metal, lithium alloys, etc. are used as the negative electrode (see, for example, Patent Document 1). Furthermore, the positive electrode of a non-aqueous electrolyte secondary battery described in Patent Document 2 is known.

[0004] Lithium-ion secondary batteries, which can achieve high voltage and high energy density, have been the predominant type of rechargeable secondary battery used to date. However, lithium resources are relatively limited and expensive. Furthermore, lithium resources are concentrated in South America, and Japan relies entirely on imports. Therefore, to reduce battery costs and ensure stable supply, development is currently underway for sodium-ion secondary batteries as an alternative to lithium-ion secondary batteries. However, sodium-ion secondary batteries have a larger atomic weight than lithium, and their standard electrode potential is about 0.33 V higher than that of lithium, resulting in lower cell voltages, making it difficult to achieve high capacity.

[0005] Recently, research has begun on non-aqueous electrolyte secondary batteries that utilize potassium ions instead of lithium ions and sodium ions. The electrode active material, particularly the positive electrode active material, constituting a potassium ion secondary battery must be a potassium compound containing potassium as a constituent element because it must be a source of potassium ions. Currently, the positive electrode active material for potassium ion secondary batteries is, for example, a crystalline potassium ion compound having a layered rock salt structure. 0.3 Known examples include those made of MnO2 (see Non-Patent Document 1) and those made of Prussian blue-based material crystals (see Non-Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-151549 [Patent Document 2] Special Publication No. 2015-515081 [Non-patent literature]

[0007] [Non-Patent Document 1] Christoph Vaalma, et al., Journal of The Electrochemical Society, 163(7), A1295-A1299 (2016) [Non-patent document 2] Ali Eftekhari, Journal of Power Sources, 126, 221-228 (2004) Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a positive electrode active material for a potassium ion battery, which enables a potassium ion battery with a large discharge capacity to be obtained, a positive electrode for a potassium ion battery including the positive electrode active material for a potassium ion battery, or a potassium ion battery including the positive electrode for a potassium ion battery. [Means for solving the problem]

[0009] The present invention provides the following <1> ~ <9> The method includes the steps described in <1> A positive electrode active material for potassium ion batteries comprising a potassium vanadium fluoride compound. <2> The average oxidation number of vanadium contained in the potassium vanadium fluoride compound is 2 or more and 4 or less. <1> The positive electrode active material for a potassium ion battery according to claim 1. <3> The potassium vanadium fluoride compound is a compound represented by the following formula (1): <1> or <2> The positive electrode active material for a potassium ion battery according to claim 1. K x V y F z Formula (1) (In formula (1), x, y, and z satisfy 0.05≦x / (x+y+z)≦0.5 and 3≦z / y≦6.) <4> The potassium vanadium fluoride compounds are KVF3, KVF4, K3VF6, and K5V3F. 14 At least one selected from the group consisting of <1> ~ <3> 10. The positive electrode active material for a potassium ion battery according to claim 9, wherein the positive electrode active material is a positive electrode active material for a potassium ion battery. <5> The potassium vanadium fluoride compounds are K3VF6 and K5V3F 14 At least one selected from the group consisting of <1> ~ <4> 10. The positive electrode active material for a potassium ion battery according to claim 9, wherein the positive electrode active material is a positive electrode active material for a potassium ion battery. <6> Contains carbon black <1> ~ <5> 10. The positive electrode active material for a potassium ion battery according to claim 9, wherein the positive electrode active material is a positive electrode active material for a potassium ion battery. <7> The content of the carbon black is 5% by mass or more and 25% by mass or less with respect to the entire positive electrode active material for a potassium ion battery. <6> The positive electrode active material for a potassium ion battery according to claim 1. <8> <1> ~ <7> 10. A positive electrode for a potassium ion battery, comprising the positive electrode active material for a potassium ion battery according to any one of claims 1 to 9. <9> <8> A potassium ion battery comprising the positive electrode for a potassium ion battery according to claim 1. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a positive electrode active material for a potassium ion battery that enables a potassium ion battery with a large discharge capacity to be obtained, a positive electrode for a potassium ion battery that includes the positive electrode active material for a potassium ion battery, or a potassium ion battery that includes the positive electrode for a potassium ion battery. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram illustrating an example of a potassium ion battery 10 according to an embodiment of the present invention. [Figure 2] 1 shows charge / discharge profiles up to the 15th cycle when the positive electrode active material for a potassium ion battery obtained in Example 1 is used. [Figure 3] 1 shows charge / discharge profiles up to the 15th cycle when the positive electrode active material for a potassium ion battery obtained in Example 2 is used. [Figure 4] 10 shows charge / discharge profiles up to the 15th cycle when the positive electrode active material for a potassium ion battery obtained in Example 3 is used. [Figure 5] 10 shows charge / discharge profiles up to the 15th cycle when the positive electrode active material for a potassium ion battery obtained in Example 4 is used. [Figure 6] 10 shows charge / discharge profiles up to the 15th cycle when the positive electrode active material for a potassium ion battery obtained in Example 5 is used. [Figure 7] 10 shows charge / discharge profiles up to the 15th cycle when the positive electrode active material for a potassium ion battery obtained in Example 7 is used. [Figure 8] 1 shows a charge-discharge profile for the first cycle when the positive electrode active material for a potassium ion battery obtained in Comparative Example 1 is used. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail below. The following description of the constituent elements may be based on a representative embodiment of the present invention, but the present invention is not limited to such an embodiment. In this specification, the symbol "to" is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit. In one embodiment of the present invention, "mass %" and "weight %" are synonymous, and "parts by mass" and "parts by weight" are synonymous. In one embodiment of the present invention, a combination of two or more preferred aspects is a more preferred aspect.

[0013] (Potassium ion battery cathode active material) A positive electrode active material for a potassium ion battery according to one embodiment of the present invention includes a potassium vanadium fluoride compound. Here, the potassium vanadium fluoride compound is a compound composed of potassium atoms, vanadium atoms, and fluorine atoms.

[0014] As mentioned above, lithium resources are relatively limited and expensive. In addition, resources are concentrated in South America, and Japan, for example, relies entirely on imports from overseas. On the other hand, potassium is abundant in both seawater and the earth's crust, making it a stable resource and enabling cost reductions. Specifically, the amount of potassium produced in 2012 was 27,146 tons on a net basis. Furthermore, in the case of lithium-ion batteries, lithium forms alloys with many metals, including aluminum, so expensive copper had to be used for the negative electrode substrate. However, potassium does not form alloys with aluminum, so cheaper aluminum can be used for the negative electrode substrate instead of copper, which is a major cost-saving advantage. The electrode active material constituting the potassium ion secondary battery, particularly the positive electrode active material, must be a potassium compound containing potassium as a constituent element because it must serve as a supply source of potassium ions. At present, the positive electrode active materials for potassium ion batteries described in the above-mentioned Non-Patent Documents 1 and 2 are known, but no positive electrode active material for potassium ion batteries has been found that can provide a sufficient output for practical use. Potassium vanadium fluoride compounds include K3VF6 (see, for example, K. Koyama and Y. Hashimoto, BCSJ, 1977, 50, 1333-1336, and R. Nagarajan, N. Tyagi, S. Lofland and KV Ramanujachary, Polyhedron, 2011, 30, 1425-1429), K5V3F 14 (See, for example, DW Aldous and P. Lightfoot, Solid State Sciences, 2009, 11, 315-319.) However, their use as positive electrode active materials for potassium ion batteries has not been fully investigated.

[0015] In one embodiment of the present invention, the positive electrode active material for a potassium ion battery includes a potassium vanadium fluoride compound. Potassium vanadium fluoride compounds exhibit a wide range of valence changes, from divalent to pentavalent, allowing many potassium ions to be inserted and removed per transition metal (vanadium). Furthermore, only fluoride ions and vanadium ions constitute the crystalline framework, and the mass ratio of electrochemically inactive atoms is smaller than that of previously reported oxoacid-based materials such as KVPO4F and KFePO4. This results in a positive electrode active material for a potassium ion battery that can provide a potassium ion battery with a large discharge capacity.

[0016] From the viewpoint of discharge capacity in a potassium ion battery, the positive electrode active material for a potassium ion battery according to one embodiment of the present invention preferably contains a potassium vanadium fluoride compound in an amount of 50 mass % or more, more preferably 80 mass % or more, and even more preferably 90 mass % or more, relative to the total mass of the positive electrode active material for a potassium ion battery, and is particularly preferably made of a potassium vanadium fluoride compound. The positive electrode active material for a potassium ion battery according to one embodiment of the present invention may contain, as an impurity, a compound in which potassium in a potassium vanadium fluoride compound is substituted with lithium or sodium.

[0017] From the viewpoint of discharge capacity in a potassium ion battery, the average oxidation number of vanadium contained in the potassium vanadium fluoride compound is preferably 2 or more and 4 or less, more preferably 2 or more and 3 or less, and even more preferably 3.

[0018] The average oxidation number of vanadium is the arithmetic mean value of the oxidation number of each vanadium atom contained in the potassium vanadium fluoride compound.

[0019] From the viewpoint of the discharge capacity of a potassium ion battery, the potassium vanadium fluoride compound is preferably a compound represented by the following formula (1). K x V y F z Formula (1) In formula (1), x, y, and z satisfy the conditions 0.05≦x / (x+y+z)≦0.5 and 3≦z / y≦6.

[0020] In terms of discharge capacity of a potassium ion battery, x / (x+y+z) in formula (1) is preferably 0.1 or more and 0.45 or less, more preferably 0.2 or more and 0.4 or less, even more preferably 0.2 or more and 0.35 or less, and particularly preferably 0.2 or more and 0.3 or less. In terms of the discharge capacity of the potassium ion battery, z / y in the formula (1) is preferably 4 or more and 6 or less.

[0021] Specific examples of the compound represented by formula (1) include KVF3, KVF4, K3VF6, and K5V3F. 14 , K2V5F 17 , K2VF6, etc.

[0022] As the compound represented by formula (1), from the viewpoint of discharge capacity in a potassium ion battery, KVF3, KVF4, K3VF6, and K5V3F are preferred. 14 Preferably, the antibody is at least one selected from the group consisting of KVF3, K3VF6, and K5V3F. 14 More preferably, it is at least one selected from the group consisting of K3VF6 and K5V3F 14 It is more preferable that the material is at least one selected from the group consisting of: The potassium vanadium fluoride compounds may be used alone or in combination of two or more.

[0023] From the viewpoint of the discharge capacity of a potassium ion battery, the positive electrode active material for a potassium ion battery preferably contains carbon black. It is presumed that the inclusion of carbon black in the positive electrode active material for a potassium ion battery improves the electronic conductivity of the positive electrode active material for a potassium ion battery, thereby improving the discharge capacity of the potassium ion battery.

[0024] Examples of carbon black include acetylene black, oil furnace black, ketjen black, etc. Among them, from the viewpoint of conductivity, at least one conductive aid selected from the group consisting of acetylene black and ketjen black is preferred, and ketjen black is more preferred. The carbon black may be used alone or in combination of two or more types.

[0025] From the viewpoint of the discharge capacity of a potassium ion battery, the content of carbon black is preferably 5% by mass or more and 25% by mass or less, more preferably 7% by mass or more and 20% by mass or less, and even more preferably 10% by mass or more and 15% by mass or less, based on the entire positive electrode active material for a potassium ion battery.

[0026] Furthermore, the shape of the positive electrode active material for a potassium ion battery according to one embodiment of the present invention is not particularly limited as long as it has a desired shape, but from the viewpoint of dispersibility during positive electrode formation, it is preferable that the positive electrode active material be in the form of particles. When the positive electrode active material for a potassium ion battery according to one embodiment of the present invention is in the form of particles, the arithmetic mean particle size of the positive electrode active material for a potassium ion battery according to one embodiment of the present invention is, from the viewpoints of dispersibility and durability of the positive electrode, preferably 10 nm to 200 μm, more preferably 50 nm to 100 μm, even more preferably 75 nm to 75 μm, and particularly preferably 100 nm to 50 μm. In one embodiment of the present invention, the arithmetic mean particle size can be suitably measured using, for example, a HORIBA Laser Scattering Particle Size Distribution Analyzer LA-950 manufactured by HORIBA, Ltd., with water as the dispersion medium and laser wavelengths of 650 nm and 405 nm. In addition, in the case of a positive electrode described later, the positive electrode active material inside the positive electrode can be separated using a solvent or the like, or physically separated, and then measured.

[0027] A method for producing the potassium vanadium fluoride compound will be described below, but the present invention is not limited thereto. The potassium vanadium fluoride compound can be produced, for example, by mixing predetermined amounts of KF and VF3 to form a mixture, and then calcining the mixture.

[0028] The method for mixing KF and VF3 is not particularly limited, and examples thereof include methods using a stirring rod, a ball mill, a bead mill, a mixer, a blender, or the like. The mixture is preferably fired, for example, in an inert gas (for example, nitrogen, argon) atmosphere at a temperature of 300° C. or higher and 600° C. or lower. The firing time of the mixture is preferably, for example, 10 hours or more and 14 hours or less. The mixture is preferably fired while wrapped in metal foil.

[0029] There are no particular limitations on the method for incorporating carbon black into the positive electrode active material for a potassium ion battery. For example, there is a method in which, when producing a potassium vanadium fluoride compound, a predetermined amount of carbon black is mixed with KF and VF3 to form a mixture, and the mixture is then fired.

[0030] (Potassium ion battery cathode) A positive electrode for a potassium ion battery according to one embodiment of the present invention includes the positive electrode active material for a potassium ion battery according to one embodiment of the present invention. The positive electrode for a potassium ion battery according to one embodiment of the present invention may contain compounds other than the positive electrode active material for a potassium ion battery according to one embodiment of the present invention. The other compounds are not particularly limited, and known additives used in the production of battery positive electrodes can be used, such as conductive additives, binders, and current collectors. Moreover, from the viewpoint of durability and formability, the positive electrode for a potassium ion battery according to one embodiment of the present invention preferably contains the positive electrode active material for a potassium ion battery according to one embodiment of the present invention, a conductive additive, and a binder. The shape and size of the positive electrode for a potassium ion battery according to one embodiment of the present invention are not particularly limited, and can be made to a desired shape and size in accordance with the shape and size of the battery to be used. From the viewpoint of discharge capacity in a potassium ion battery, a positive electrode for a potassium ion battery according to one embodiment of the present invention preferably contains a potassium vanadium fluoride compound in an amount of 10 mass % or more, more preferably 20 mass % or more, even more preferably 50 mass % or more, and particularly preferably 70 mass % or more, based on the total mass of the positive electrode for a potassium ion battery.

[0031] <Conductive additive> In the positive electrode for a potassium ion battery according to one embodiment of the present invention, the positive electrode active material for a potassium ion battery according to one embodiment of the present invention may be formed into a desired shape and used as is as a positive electrode; however, in order to improve the rate characteristics (output) of the positive electrode, it is preferable that the positive electrode for a potassium ion battery according to one embodiment of the present invention further contains a conductive additive. Preferred examples of the conductive additive used in one embodiment of the present invention include carbon such as carbon black, graphite, carbon nanotubes (CNT), and vapor grown carbon fiber (VGCF). Examples of carbon black include acetylene black, oil furnace black, ketjen black, etc. Among them, from the viewpoint of conductivity, at least one conductive aid selected from the group consisting of acetylene black and ketjen black is preferred, and acetylene black or ketjen black is more preferred. The conductive additives may be used alone or in combination of two or more. The mixing ratio of the positive electrode active material and the conductive additive is not particularly limited, but the content of the conductive additive in the positive electrode is preferably 1% by mass to 80% by mass, more preferably 2% by mass to 60% by mass, even more preferably 5% by mass to 50% by mass, and particularly preferably 5% by mass to 25% by mass, relative to the total mass of the positive electrode active material contained in the positive electrode. Within the above range, a positive electrode with higher output and excellent durability can be obtained.

[0032] The conductive additive and the positive electrode active material can be mixed in an inert gas atmosphere to increase the electrical conductivity of the positive electrode. Examples of the inert gas include nitrogen gas and argon gas, and argon gas is preferred. Furthermore, when mixing the conductive additive and the positive electrode active material, a pulverization / dispersion treatment may be performed using a dry ball mill or a bead mill to which a small amount of a dispersing medium such as water has been added. The pulverization / dispersion treatment can improve the adhesion and dispersibility between the conductive additive and the positive electrode active material, thereby increasing the electrode density.

[0033] <Binder> From the viewpoint of formability, the positive electrode for a potassium ion battery according to one embodiment of the present invention preferably further contains a binder. The binder is not particularly limited, and known binders can be used, including polymer compounds, such as fluororesins, polyolefin resins, rubbery polymers, polyamide resins, polyimide resins (such as polyamideimides), and cellulose ethers. Specific examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), polyethylene, aromatic polyamide, cellulose, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymers, hydrogenated products thereof, styrene-ethylene-butadiene-styrene copolymers, styrene-isoprene-styrene block copolymers, hydrogenated products thereof, syndiotactic 1,2-polybutadiene, ethylene-vinyl acetate copolymers, propylene-α-olefin (having 2 to 12 carbon atoms) copolymers, starch, methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethylhydroxyethyl cellulose, nitrocellulose, polyacrylic acid, sodium polyacrylate, and polyacrylonitrile.

[0034] From the viewpoint of increasing the electrode density, the specific gravity of the compound used as a binder is 1.2 g / cm 3 Larger is preferred. In order to increase the electrode density and adhesive strength, the weight-average molecular weight of the binder is preferably 1,000 or more, more preferably 5,000 or more, and even more preferably 10,000 or more. There is no particular upper limit, but it is preferably 2,000,000 or less.

[0035] The binder may be used alone or in combination of two or more kinds. The mixing ratio of the positive electrode active material and the binder is not particularly limited, but the content of the binder in the positive electrode is preferably 0.5% by mass to 30% by mass, more preferably 1% by mass to 20% by mass, and even more preferably 2% by mass to 15% by mass, relative to the total mass of the positive electrode active material contained in the positive electrode. When the content is within the above range, excellent moldability and durability are achieved.

[0036] The method for manufacturing a positive electrode containing a positive electrode active material, a conductive additive, and a binder is not particularly limited. For example, the positive electrode active material, the conductive additive, and the binder may be mixed and pressure-molded, or a method may be used in which a slurry described below is prepared to form a positive electrode.

[0037] <Current collector> The positive electrode for a potassium ion battery according to one embodiment of the present invention may further include a current collector. Examples of the current collector include foil, mesh, expanded grid (expanded metal), punched metal, etc., made of a conductive material such as nickel, aluminum, stainless steel (SUS), etc. The mesh opening, wire diameter, mesh number, etc. are not particularly limited, and conventionally known current collectors can be used. The shape of the current collector is not particularly limited and may be selected according to the desired shape of the positive electrode, for example, a foil shape, a plate shape, or the like.

[0038] The method for forming a positive electrode on a current collector is not particularly limited, but an example is a method in which a positive electrode active material, a conductive additive, a binder, and an organic solvent or water are mixed to prepare a positive electrode active material slurry, and the slurry is then applied to the current collector. Examples of organic solvents include amines such as N,N-dimethylaminopropylamine and diethyltriamine, ethers such as ethylene oxide and tetrahydrofuran, ketones such as methyl ethyl ketone, esters such as methyl acetate, and aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone. The prepared slurry is applied to a current collector, dried, and then fixed by pressing, etc., to produce a positive electrode. Examples of methods for applying the slurry to a current collector include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.

[0039] (potassium ion battery) A potassium ion battery according to one embodiment of the present invention includes a positive electrode for a potassium ion battery according to one embodiment of the present invention. Moreover, the potassium ion battery according to one embodiment of the present invention can be suitably used as a potassium ion secondary battery. A potassium ion battery according to one embodiment of the present invention preferably includes the positive electrode for a potassium ion battery according to one embodiment of the present invention, a negative electrode, and an electrolyte.

[0040] <Negative electrode> The negative electrode used in one embodiment of the present invention may be any negative electrode containing a negative electrode active material, and examples thereof include a negative electrode made of a negative electrode active material, and a negative electrode having a current collector and a negative electrode active material layer formed on the surface of the current collector, the negative electrode active material layer containing a negative electrode active material and a binder. The current collector is not particularly limited, and the current collector described above for the positive electrode can be suitably used. The shape and size of the negative electrode are not particularly limited, and can be made to a desired shape and size in accordance with the shape and size of the battery to be used.

[0041] Examples of the negative electrode active material include carbon materials such as natural graphite, artificial graphite, cokes, hard carbon, carbon black, pyrolytic carbons, carbon fiber, and baked organic polymer compounds. The carbon material may be in the form of, for example, flakes such as natural graphite, spheres such as mesocarbon microbeads, fibers such as graphitized carbon fiber, or particulate aggregates. Here, the carbon material may also function as a conductive additive. Among these, graphite or hard carbon is preferred, and graphite is more preferred. Furthermore, potassium metal can also be suitably used as the negative electrode active material. Furthermore, the negative electrode described in International Publication No. 2016 / 059907 can also be suitably used as the negative electrode.

[0042] Graphite in one embodiment of the present invention refers to a graphite-based carbon material. Examples of graphite-based carbon materials include natural graphite, artificial graphite, and expanded graphite. Examples of natural graphite that can be used include flake graphite and block graphite. Examples of artificial graphite that can be used include block graphite, vapor-grown graphite, flake graphite, and fibrous graphite. Among these, flake graphite and block graphite are preferred because of their high packing density. Two or more types of graphite may also be used in combination. The upper limit of the average particle size of graphite is preferably 30 μm, more preferably 15 μm, and even more preferably 10 μm, and the lower limit is preferably 0.5 μm, more preferably 1 μm, and even more preferably 2 μm. The average particle size of graphite is a value measured by electron microscope observation. Further, examples of graphite include graphite having a lattice spacing d(002) of 3.354 to 3.370 Å (angstroms, 1 Å=0.1 nm) and a crystallite size Lc of 150 Å or more. Furthermore, hard carbon in one embodiment of the present invention is a carbon material that does not graphitize even when heat-treated at high temperatures of 2,000°C or higher, and is also called non-graphitizable carbon. Examples of hard carbon include carbon fiber obtained by carbonizing infusible yarn, an intermediate product in the carbon fiber manufacturing process, at about 1,000°C to 1,400°C, and carbon materials obtained by air-oxidizing organic compounds at about 150°C to 300°C and then carbonizing them at about 1,000°C to 1,400°C. The method for manufacturing hard carbon is not particularly limited, and hard carbon manufactured by a conventional method can be used. The average particle size, true density, and interplanar spacing of the (002) plane of the hard carbon are not particularly limited, and any suitable ones can be selected and used.

[0043] The negative electrode active materials may be used alone or in combination of two or more. The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably 80 to 95 mass %.

[0044] <Electrolytes> As the electrolyte used in one embodiment of the present invention, either an electrolytic solution or a solid electrolyte can be used. The electrolyte is not particularly limited as long as it contains a potassium salt as the main electrolyte. In the case of an aqueous electrolyte, examples of potassium salts include KClO, KPF, KNO, KOH, KCl, KSO, and KS. These potassium salts can be used singly or in combination of two or more. In the case of a non-aqueous electrolyte, for example, an electrolyte (e.g., KPF6, KBF4, CF3SO3K, KAsF6, KB(C6H5)4, CH3SO3K, KN(SO2CF3)2, KN(SO2C2F5)2, KC(SO2CF3)3, KN(SO3CF3)2, etc.) can be used as an electrolyte containing a solvent, for example, propylene carbonate (PC).In addition, a solution obtained by dissolving the electrolyte in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), or a solution obtained by dissolving the electrolyte in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC), etc., can also be used as an electrolyte. Among these, KPF6 is preferred as the potassium salt.

[0045] In addition, examples of the solvent for the electrolyte include carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, fluoroethylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, and 2,2,3,3-tetrafluoropropyl Ethers such as difluoromethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, etc.; esters such as methyl formate, methyl acetate, γ-butyrolactone, etc.; nitriles such as acetonitrile, butyronitrile, etc.; amides such as N,N-dimethylformamide, N,N-dimethylacetamide, etc.; carbamates such as 3-methyl-2-oxazolidone, etc.; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, 1,3-propane sultone, etc.; or the above-mentioned solvents further having a fluoro group introduced as a substituent of a hydrogen atom can be used. The solvent for the electrolytic solution may be used alone or in combination of two or more, but it is preferable to use a mixture of two or more. Among these, at least one solvent selected from the group consisting of propylene carbonate, ethylene carbonate, and diethyl carbonate is preferred, and a mixed solvent of at least two solvents selected from the group consisting of propylene carbonate, ethylene carbonate, and diethyl carbonate is more preferred. The concentration of the potassium salt in the electrolyte is not particularly limited, but is preferably 0.1 mol / L or more and 2 mol / L or less, and more preferably 0.5 mol / L or more and 1.5 mol / L or less.

[0046] The solid electrolyte may be any known solid electrolyte. For example, an organic solid electrolyte such as a polyethylene oxide polymer compound, or a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain may be used. A so-called gel-type solid electrolyte, in which a nonaqueous electrolyte solution is held in a polymer compound, may also be used.

[0047] <Separator> The potassium ion battery according to one embodiment of the present invention preferably further includes a separator. The separator serves to physically separate the positive electrode and the negative electrode and to prevent internal short circuits. The separator is made of a porous material, the pores of which are impregnated with an electrolyte, and has ion permeability (particularly, permeability to at least potassium ions) to ensure the battery reaction. As the separator, for example, a resin porous membrane or a nonwoven fabric can be used. The separator may be formed of only a porous membrane layer or a nonwoven fabric layer, or may be formed as a laminate of multiple layers with different compositions and forms. Examples of the laminate include a laminate having multiple resin porous layers with different compositions, and a laminate having a porous membrane layer and a nonwoven fabric layer.

[0048] The material of the separator can be selected taking into consideration the operating temperature of the battery, the composition of the electrolyte, and the like. Examples of resins contained in the fibers forming the porous membrane and nonwoven fabric include polyolefin resins such as polyethylene, polypropylene, and ethylene-propylene copolymer; polyphenylene sulfide resins such as polyphenylene sulfide and polyphenylene sulfide ketone; polyamide resins such as aromatic polyamide resins (e.g., aramid resins); and polyimide resins. These resins may be used alone or in combination of two or more. The fibers forming the nonwoven fabric may also be inorganic fibers such as glass fibers. The separator is preferably a separator containing at least one material selected from the group consisting of glass, polyolefin resin, polyamide resin, and polyphenylene sulfide resin, and among these, a glass filter is more preferred as the separator. The separator may also contain an inorganic filler. Examples of inorganic fillers include ceramics (silica, alumina, zeolite, titania, etc.), talc, mica, wollastonite, etc. The inorganic filler is preferably in the form of particles or fibers. The content of the inorganic filler in the separator is preferably 10% by mass to 90% by mass, and more preferably 20% by mass to 80% by mass. The shape and size of the separator are not particularly limited and may be selected appropriately according to the desired shape of the battery.

[0049] In the potassium ion battery according to one embodiment of the present invention, various known materials used in conventional lithium ion batteries and sodium ion batteries can be used for components such as the battery case, spacers, gaskets, leaf springs, and other structural materials, and there are no particular limitations. The potassium ion battery according to one embodiment of the present invention may be assembled using the battery elements according to a known method. In this case, the shape of the battery is not particularly limited, and various shapes and sizes, such as a cylindrical, prismatic, or coin-shaped battery, may be appropriately adopted.

[0050] An example of a potassium ion battery according to one embodiment of the present invention is the potassium ion battery shown in FIG. 1, but it goes without saying that the invention is not limited to this. FIG. 1 is a schematic diagram showing an example of a potassium ion battery 10 according to one embodiment of the present invention. The potassium ion battery 10 shown in FIG. 1 is a coin-type battery, and is formed by stacking, in order from the negative electrode side, a negative electrode side battery case 12, a gasket 14, a negative electrode 16, a separator 18, a positive electrode (cathode) 20 for a potassium ion battery according to one embodiment of the present invention, a spacer 22, a leaf spring 24, and a positive electrode side battery case 26, and then fitting the battery case 12 and the battery case 26 together. The separator 18 is impregnated with an electrolyte (not shown). [Example]

[0051] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, processing details, processing procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.

[0052] Example 1: Synthesis of K3VF6 Mixture A1 was obtained by mixing 0.3705 g (0.0064 mol) of KF (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.; hereinafter the same) and 0.2295 g (0.0021 mol) of VF3 (manufactured by Kojundo Chemical Laboratory Co., Ltd.; hereinafter the same). Mixture A1 was then crushed in a ball mill at 600 rpm (rotation per minute) for 12 hours to obtain mixture A2. Mixture A2 was formed into pellets (disk-shaped, 10 mm in diameter and 1.5 mm thick), and the entire pellet was wrapped in gold foil. The gold-foil-wrapped pellets were then calcined at 300°C for 12 hours in an argon atmosphere to obtain a positive electrode active material for potassium-ion batteries consisting of K3VF6.

[0053] Example 2: Synthesis of K3VF6 / C-5 Mixture B1 was obtained by mixing 0.2779 g (0.0048 mol) of KF, 0.1721 g (0.0016 mol) of VF3, and 0.024 g of Ketjen Black (Lion Specialty Chemicals Co., Ltd.). Mixture B1 was then crushed in a ball mill at 600 rpm (rotation per minute) for 12 hours to obtain mixture B2. Mixture B2 was then formed into pellets (disks with a diameter of 10 mm and a thickness of 1.5 mm) and wrapped in gold foil. The pellets were then calcined at 300°C for 12 hours under an argon atmosphere to obtain a cathode active material for potassium-ion batteries consisting of K3VF6 containing carbon black (K3VF6 / C-5).

[0054] Example 3: Synthesis of K3VF6 / C-10 A positive electrode active material for a potassium ion battery made of K3VF6 containing carbon black (K3VF6 / C-10) was obtained in the same manner as in Example 2, except that the amount of Ketjen black added was changed to 0.050 g.

[0055] Example 4: Synthesis of K3VF6 / C-15 A positive electrode active material for a potassium ion battery made of K3VF6 containing carbon black (K3VF6 / C-15) was obtained in the same manner as in Example 2, except that the amount of Ketjen black added was changed to 0.079 g.

[0056] Example 5: Synthesis of K3VF6 / C-20 A positive electrode active material for a potassium ion battery made of K3VF6 containing carbon black (K3VF6 / C-20) was obtained in the same manner as in Example 2, except that the amount of Ketjen black added was changed to 0.113 g.

[0057] Example 6: K5V3F 14 Synthesis of> Mixture C1 was obtained by mixing 0.2837 g (0.0049 mol) of KF and 0.3163 g (0.0029 mol) of VF. The resulting mixture C1 was formed into a pellet (disk-shaped, 10 mm in diameter and 1.5 mm thick), and the entire pellet was wrapped in gold foil. The gold foil-wrapped pellet was then fired at 600 °C for 12 hours in an argon atmosphere to obtain K5V3F. 14 A positive electrode active material for a potassium ion battery consisting of the above was obtained.

[0058] Example 7: K5V3F 14 / C-15 Synthesis> Mixture D1 was obtained by mixing 0.2412 g (0.0042 mol) of KF, 0.2688 g (0.0025 mol) of VF, and 0.09 g of Ketjen black. The obtained mixture D1 was formed into a pellet (disk-shaped, 10 mm in diameter, 1.5 mm thick), and the entire pellet was wrapped in gold foil. The gold foil-wrapped pellet was then fired at 600 °C for 12 hours in an argon atmosphere to obtain K5V3F 14 (K5V3F 14 / C-15) was used as a positive electrode active material for potassium ion batteries.

[0059] Comparative Example 1: Synthesis of KFePO4 / C-12.5 Mixture E1 was obtained by mixing K2CO3, Fe2C2O4·2H2O, and (NH4)2HPO4 in a stoichiometric ratio. The resulting mixture E1 was pelletized and calcined at 300 °C for 10 hours under an argon atmosphere. The resulting powder was pelletized again and calcined at 800 °C for 8 hours under an argon atmosphere. Ketjen black was added to the calcined powder so that the mass of the calcined powder was 12.5% ​​by mass relative to the positive electrode active material for potassium ion batteries. The mixture was then mixed in a ball mill to obtain a positive electrode active material for potassium ion batteries consisting of KFePO4 containing carbon black (KFePO4 / C-12.5).

[0060] Each of the potassium vanadium fluoride compounds obtained above was subjected to X-ray diffraction structural analysis to identify its chemical structure.

[0061] <Preparation of positive electrodes for potassium ion batteries> Each of the obtained potassium vanadium fluoride compounds was used as a positive electrode active material for potassium ion batteries to fabricate a positive electrode. The resulting potassium vanadium fluoride compound (i.e., the cathode active material for potassium ion batteries) was mixed with Ketjen Black (KB, manufactured by Lion Specialty Chemicals Co., Ltd.) and PVDF (polyvinylidene fluoride, manufactured by Kureha Corporation) in a mass ratio of 8:1:1, and then coated onto aluminum foil (manufactured by Hosen Co., Ltd., thickness 0.017 mm) to prepare a cathode. The shape of the cathode without the aluminum foil was cylindrical, with a diameter of 10 mm and a thickness of 0.03 to 0.04 mm. The mass of the cathode without the aluminum foil was 3 to 5 mg.

[0062] <Charge / discharge measurement> Charge-discharge measurements were performed using a coin cell prepared using a 1M KPF6 / ethylene carbonate (EC)-propylene carbonate (PC) (mass ratio EC:PC = 1:1) mixed solution as the electrolyte, the positive electrode prepared according to the procedure described above as the positive electrode, potassium metal (manufactured by Aldrich) as the negative electrode, a separator (glass filter, manufactured by Housen Co., Ltd.), a stainless steel battery case and a polypropylene gasket (CR2032, manufactured by Housen Co., Ltd.), a spacer (material: stainless steel, diameter 16 mm × height 0.5 mm, manufactured by Housen Co., Ltd.), and a leaf spring (material: stainless steel, inner diameter 10 mm, height 2.0 mm, thickness 0.25 mm, washer, manufactured by Housen Co., Ltd.). The amount of the electrolyte used was such that the separator was sufficiently filled with the electrolyte (0.15 mL to 0.3 mL). The electrolyte solution was prepared using KPF6 manufactured by Tokyo Chemical Industry Co., Ltd., ethylene carbonate manufactured by Kishida Chemical Co., Ltd., and propylene carbonate manufactured by Kishida Chemical Co., Ltd.

[0063] The charge / discharge conditions were set to a constant current mode for the charge / discharge current density, and measurements were performed at room temperature (25°C). Using the obtained positive electrode, the current density was set as follows, and constant current charging was performed up to a charge voltage of 4.5 V. After charging, constant current discharging was repeatedly performed until the charge voltage reached 4.5 V and the discharge cut-off voltage reached 1.5 V. One charge / discharge is considered one cycle, and the reversible capacity (Capacity, unit: mAh / g, where h represents hour) measured in a specific cycle is shown in Figures 2 to 8.

[0064] The current density conditions are as follows: When the positive electrode active material for potassium ion batteries obtained in Example 1 was used: 3.798 mA / g When the positive electrode active materials for potassium ion batteries obtained in Examples 2 to 5 were used: 18.99 mA / g When the positive electrode active material for potassium ion batteries obtained in Example 7 was used: 65.43 mA / g When the positive electrode active material for a potassium ion battery obtained in Comparative Example 1 was used: 7.1 mA / g

[0065] 2 to 7 show charge / discharge profiles up to the 15th cycle when the positive electrode active materials for potassium ion batteries obtained in Examples 1 to 5 and 7 were used. FIG. 8 shows the charge-discharge profile for the first cycle when the positive electrode active material for a potassium ion battery obtained in Comparative Example 1 was used. The vertical axis of the charge / discharge profiles in Figures 2 to 8 represents the potential (Voltage, unit: V (V vs. K)) based on the standard single-electrode potential of potassium, and the horizontal axis represents the capacity (Capacity, unit: mAhg -1 )

[0066] Table 1 also shows the discharge capacity at the first cycle and the capacity retention rate at the 15th cycle (discharge capacity at the 15th cycle / discharge capacity at the first cycle×100). In Table 1, when the results of the discharge capacity at the first cycle and the capacity retention rate at the 15th cycle are marked with "-", this means that no evaluation was performed.

[0067] [Table 1]

[0068] As described above, by using the positive electrode active material for a potassium ion battery according to one embodiment of the present invention, a potassium ion battery with a large discharge capacity was obtained. The positive electrode active material for potassium ion batteries obtained in Example 6 (K5V3F 14 Although no charge / discharge measurements were performed using the positive electrode active material for potassium ion batteries (K5V3F 14 From this, it will be understood by those skilled in the art with reference to the present disclosure that the positive electrode active material for a potassium ion battery obtained in Example 6 also exhibits a large discharge capacity. [Explanation of symbols]

[0069] 10: Potassium ion battery, 12: Battery case (negative electrode side), 14: Gasket, 16: Negative electrode, 18: Separator, 20: Positive electrode, 22: Spacer, 24: Leaf spring, 26: Battery case (positive electrode side)

Claims

1. A positive electrode active material for a potassium ion battery, comprising a potassium vanadium fluoride compound represented by the following formula (1): K x V y F z Formula (1) (In formula (1), x, y, and z satisfy 0.05≦x / (x+y+z)≦0.5 and 3≦z / y≦6.)

2. 2. The positive electrode active material for a potassium ion battery according to claim 1, wherein the average oxidation number of vanadium contained in the potassium vanadium fluoride compound is 2 or more and 4 or less.

3. The potassium vanadium fluoride compound is KVF ３ , KVF ４ , K. ３ VF ６ , and K ５ V ３ F １４ The positive electrode active material for a potassium ion battery according to claim 1 or 2, which is at least one selected from the group consisting of:

4. The potassium vanadium fluoride compound is K ３ VF ６ , and K ５ V ３ F １４ The positive electrode active material for a potassium ion battery according to any one of claims 1 to 3, which is at least one selected from the group consisting of:

5. The positive electrode active material for a potassium ion battery according to any one of claims 1 to 4, comprising carbon black.

6. 6. The positive electrode active material for a potassium ion battery according to claim 5, wherein the content of the carbon black is 5% by mass or more and 25% by mass or less with respect to the entire positive electrode active material for a potassium ion battery.

7. A positive electrode for a potassium ion battery comprising the positive electrode active material for a potassium ion battery according to any one of claims 1 to 6.

8. A potassium ion battery comprising the positive electrode for a potassium ion battery according to claim 7.

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