WO2018123487A1

WO2018123487A1 - Positive electrode active material for potassium-ion cell, positive electrode for potassium-ion cell, and potassium-ion cell

Info

Publication number: WO2018123487A1
Application number: PCT/JP2017/043860
Authority: WO
Inventors: 慎一駒場; 久保田　圭; ▲暁▼非別
Original assignee: 学校法人東京理科大学
Priority date: 2016-12-26
Filing date: 2017-12-06
Publication date: 2018-07-05
Also published as: JP2018106911A

Abstract

The positive electrode active material for a potassium-ion cell according to the present embodiment contains a compound represented by formula (1). In formula (1), m represents a number from 0.5 to 2, x represents a number from 0.5 to 1.5, y represents a number from 0.5 to 1.5, and z represents zero or a positive number. Also provided is a positive electrode for a potassium-ion cell containing the positive electrode active material for a potassium-ion cell according to the present embodiment, or a potassium-ion cell provided with the positive electrode for a potassium-ion cell. Formula (1): K_mFe_xMn_y(CN)₆•zH₂O

Description

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

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.

Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte as a secondary battery with a high energy density, for example, charge and discharge by moving lithium ions between the positive electrode and the negative electrode are widely used. Yes.

In such a non-aqueous electrolyte secondary battery, a lithium transition metal composite oxide having a layered structure such as lithium nickelate (LiNiO ₂ ) or lithium cobaltate (LiCoO ₂ ) is generally used as a positive electrode, and lithium is occluded as a negative electrode. Further, carbon materials that can be released, lithium metal, lithium alloys, and the like are used (see, for example, JP-A-2003-151549).
In addition, as a positive electrode of a non-aqueous electrolyte secondary battery, those described in JP-T-2015-515081 are known.

Lithium ion secondary batteries that can achieve high energy density at a high voltage have been mainly used so far as secondary batteries that can be charged and discharged, but lithium has a relatively limited amount of resources. And expensive. In addition, resources are unevenly distributed in South America, and Japan relies entirely on imports from overseas. Therefore, in order to reduce the cost of the battery and provide a stable supply, a sodium ion secondary battery replacing the lithium ion secondary battery is currently being developed. However, since the atomic weight is larger than that of lithium, the standard electrode potential is about 0.33 V higher than that of lithium, and the cell voltage is lowered, there is a problem that it is difficult to increase the capacity.

Recently, research on non-aqueous electrolyte secondary batteries using potassium ions instead of lithium ions and sodium ions has been started.
Since the electrode active material which comprises a potassium ion secondary battery, especially a positive electrode active material must become a supply source of potassium ion, it needs to be a potassium compound which contains potassium as a structural element. At present, as a positive electrode active material for a potassium ion secondary battery, for example, a material composed of crystal K _0.3 MnO ₂ having a layered rock salt structure (Christoph Vaalma, et al., Journal of The Electrochemical Society, 163 (7), A1295-A1299 (2016)) and Prussian blue material crystals (see Ali Eftekhari, Journal of Power Souces, 126, 221-228 (2004)) are known.

Problems to be solved by the present invention include a positive electrode active material for a potassium ion battery from which a potassium ion battery having a high energy density is obtained, and a positive electrode for a potassium ion battery containing the positive electrode active material for a potassium ion battery, or the potassium It is providing the potassium ion battery provided with the positive electrode for ion batteries.

The above problems have been solved by the means described in <1>, <2> or <3> below.
<1> A positive electrode active material for a potassium ion battery, comprising a compound represented by the following formula (1).
_{_{_{_{K m Fe x Mn y (CN}}}} ) 6 · zH 2 O (1)
In formula (1), m represents a number from 0.5 to 2, x represents a number from 0.5 to 1.5, y represents a number from 0.5 to 1.5, z Represents 0 or a positive number.
<2> A positive electrode for a potassium ion battery comprising the positive electrode active material for a potassium ion battery according to <1>.
<3> A potassium ion battery comprising the positive electrode for a potassium ion battery according to <2>.

According to the present invention, a positive electrode active material for a potassium ion battery from which a potassium ion battery having a high energy density is obtained, and a positive electrode for a potassium ion battery comprising the positive electrode active material for a potassium ion battery, or the positive electrode for a potassium ion battery Can be provided.

It is a mimetic diagram showing an example of potassium ion battery 10 concerning this embodiment. It is a charge / discharge profile up to the third cycle when K—Mn [Fe (CN) ₆ ] is used. It is a charge / discharge profile up to the third cycle when Na—Mn [Fe (CN) ₆ ] is used. It is a charge / discharge profile up to the third cycle when Li—Mn [Fe (CN) ₆ ] is used. This is the charge / discharge profile at the second cycle when K—Mn [Fe (CN) ₆ ], Na—Mn [Fe (CN) ₆ ] or Li—Mn [Fe (CN) ₆ ] is used. It is a figure showing the change of the discharge capacity in the cycle progress when using K-Mn [Fe (CN) ₆ ], Na-Mn [Fe (CN) ₆ ] or Li-Mn [Fe (CN) ₆ ]. It is a charge / discharge profile at the first cycle when K—Mn [Fe (CN) ₆ ] is used. It is a charge / discharge profile at the first cycle when K—Fe [Fe (CN) ₆ ] is used. It is a charge / discharge profile at the first cycle when K—Co [Fe (CN) ₆ ] is used. It is a charge / discharge profile at the first cycle when K—Ni [Fe (CN) ₆ ] is used. It is a charge / discharge profile at the first cycle when K-Cu [Fe (CN) ₆ ] is used. FIG. 5 is a diagram summarizing one cycle of charge / discharge profiles in a case where KM [Fe (CN) ₆ ] (M = Mn, Fe, Co, Ni, or Cu) is used. It is a figure showing the change of the discharge capacity in cycle progress at the time of using K-Mn [Fe (CN) ₆ ] or K-Fe [Fe (CN) ₆ ].

Hereinafter, the contents of the present invention will be described in detail. The description of the constituent elements described below may be made based on typical embodiments of the present invention, but the present invention is not limited to such embodiments. In the present specification, “to” is used to mean that the numerical values described before and after it are included as a lower limit value and an upper limit value.
In the present embodiment, “mass%” and “wt%” are synonymous, and “part by mass” and “part by weight” are synonymous.
Moreover, in this embodiment, the combination of 2 or more preferable aspects is a more preferable aspect.

(Positive electrode active material for potassium ion battery)
The positive electrode active material for a potassium ion battery according to this embodiment includes a compound represented by the following formula (1).
_{_{_{_{K m Fe x Mn y (CN}}}} ) 6 · zH 2 O (1)
In formula (1), m represents a number from 0.5 to 2, x represents a number from 0.5 to 1.5, y represents a number from 0.5 to 1.5, z Represents 0 or a positive number.
Moreover, the positive electrode active material for potassium ion batteries which concerns on this embodiment is used suitably as a positive electrode active material for potassium ion secondary batteries.

As described above, lithium has a relatively limited amount of resources and is expensive. Also, resources are unevenly distributed in South America. For example, in Japan, the entire amount depends on imports from overseas.
On the other hand, potassium is abundant in seawater and the earth's crust, so it becomes a stable resource and can be reduced in cost.
Specifically, global lithium production in 2012 is 34,970 t in terms of pure content, and potassium production is 27,146 t in terms of pure content.
In addition, in the case of lithium ion batteries, lithium forms an alloy with many metals such as aluminum, so it was necessary to use expensive copper for the negative electrode substrate. Alternatively, inexpensive aluminum can be used for the negative electrode substrate, which is a great cost reduction advantage.
Since the electrode active material which comprises a potassium ion secondary battery, especially a positive electrode active material must become a supply source of potassium ion, it needs to be a potassium compound which contains potassium as a structural element.
Currently described in Christoph Vaalma, et al., Journal of The Electrochemical Society, 163 (7), A1295-A1299 (2016) or Ali Eftekhari, Journal of Power Souces, 126, 221-228 (2004). In addition, positive electrode active materials for potassium ion batteries are known, but no positive electrode active material for potassium ion batteries has been found that can provide a sufficient output for practical use.

In this embodiment, the positive electrode active material for a potassium ion battery has an energy density by using a compound represented by the above formula (1), that is, a hexacyano metal acid potassium salt having Fe and Mn as a central metal. A high potassium ion battery is obtained.
Moreover, the positive electrode active material for potassium ion batteries which concerns on this embodiment uses the compound represented by said Formula (1), and in addition to the above, charging / discharging capacity is high, it is high output, and charging / discharging is repeated. However, a potassium ion battery in which the charge / discharge capacity is hardly deteriorated can be obtained.
In the compound represented by the formula (1), Fe and Mn are presumed to be divalent or trivalent, and the potassium ion in the compound represented by the formula (1) is increased when the valence is increased. Estimated to be released.

In the positive electrode active material for a potassium ion battery according to the present embodiment, the compound represented by the formula (1) is added to the total mass of the positive electrode active material for a potassium ion battery from the viewpoint of output and charge / discharge capacity in the potassium ion battery. On the other hand, it is preferable to contain 50% by mass or more, more preferably 80% by mass or more of the compound represented by the formula (1), and 90% by mass or more of the compound represented by the formula (1). More preferably, it consists of a compound represented by the formula (1).
In addition, the positive electrode active material for a potassium ion battery according to this embodiment may include a compound in which potassium of the compound represented by the formula (1) is replaced with lithium or sodium as an impurity.

M in the formula (1) is preferably 1.0 or more and 2.0 or less, more preferably 1.2 or more and 2.0 or less, from the viewpoint of energy density in the potassium ion battery. It is particularly preferably 5 or more and 2.0 or less. Further, from the viewpoint of synthesis, it is preferably 1.2 or more and 2.0 or less, and particularly preferably 1.5 or more and 2.0 or less.

X in the formula (1) is preferably 0.7 or more and 1.3 or less, more preferably 0.8 or more and 1.2 or less, from the viewpoint of energy density in the potassium ion battery. It is especially preferable that it is 85 or more and 1.15 or less.
Y in the formula (1) is preferably 0.7 or more and 1.3 or less, more preferably 0.8 or more and 1.2 or less, from the viewpoint of energy density in the potassium ion battery. It is especially preferable that it is 85 or more and 1.15 or less.
Further, x / y in the formula (1) is preferably 0.5 or more and 1.5 or less, more preferably 0.7 or more and 1.3 or less, from the viewpoint of energy density in the potassium ion battery. Preferably, it is 0.8 or more and 1.2 or less, more preferably 0.85 or more and 1.15 or less.
The compound represented by the formula (1) preferably has at least one structure selected from the group consisting of a hexacyanoiron structure and a hexacyanomanganese structure.
Z in the formula (1) represents the amount of water of hydration, preferably crystallization water, and is not particularly limited as long as it is 0 or a positive number, but is 0 from the viewpoint of energy density and charge / discharge capacity in a potassium ion battery. It is preferably 10 or less, more preferably 0 or more and 5 or less, still more preferably 0 or more and 2 or less, and particularly preferably 0 or more and 1 or less.

Specific examples of the compound represented by the formula _{_{_{(1), K 2 FeMn (}}} CN) 6 · zH 2 O, K 2 Fe 0.5 Mn 1.5 (CN) 6 · zH 2 O, K 2 Fe 1 _.5 Mn _0.5 (CN) ₆ · zH ₂ O, K _0.5 FeMn (CN) ₆ · zH ₂ O, K _1.0 FeMn (CN) ₆ · zH ₂ O, K _1.5 FeMn (CN _{_{_{_{) 6 · zH 2 O, K}}}} 1.88 Fe 1.00 Mn 1.08 (CN) 6 · 0.62H 2 O , and the like. In the specific examples, z represents 0 or a positive number, and is preferably 0 or more and 2 or less.

In addition, the shape of the positive electrode active material for a potassium ion battery according to the present embodiment is not particularly limited, and may be any desired shape, but is a particulate positive electrode active material from the viewpoint of dispersibility during positive electrode formation. It is preferable.
When the shape of the positive electrode active material for a potassium ion battery according to this embodiment is particulate, the arithmetic average particle size of the positive electrode active material for a potassium ion battery according to this embodiment is from the viewpoint of dispersibility and positive electrode durability. The thickness is preferably 10 nm to 200 μm, more preferably 50 nm to 100 μm, still more preferably 75 nm to 75 μm, and particularly preferably 100 nm to 50 μm.
The method for measuring the arithmetic average particle diameter in the present embodiment is preferably, for example, using HORIBA Laser Scattering Particle Size Distribution Analyzer LA-950 manufactured by Horiba, Ltd., with dispersion medium: water, and laser wavelengths used: 650 nm and 405 nm. Can be measured.
Moreover, in the positive electrode mentioned later, the positive electrode active material inside a positive electrode can be measured using a solvent etc. or physically separating.

There is no restriction | limiting in particular in the manufacturing method of the compound represented by said Formula (1), For example, a liquid phase method is mentioned.
Specifically, for example, it can be produced by reacting K ₄ Fe (CN) ₆ (potassium ferrocyanide), MnCl ₂ , and KCl in a predetermined amount in an aqueous solution.

(Positive electrode for potassium ion battery)
The positive electrode for potassium ion batteries according to the present embodiment includes the positive electrode active material for potassium ion batteries according to the present embodiment.
The positive electrode for a potassium ion battery according to this embodiment may contain a compound other than the positive electrode active material for a potassium ion battery according to this embodiment.
There is no restriction | limiting in particular as another compound, The well-known additive used for preparation of the positive electrode of a battery can be used. Specifically, a conductive assistant, a binder, a current collector, and the like can be given.
In addition, the positive electrode for a potassium ion battery according to the present embodiment preferably includes the positive electrode active material for a potassium ion battery, a conductive additive, and a binder according to the present embodiment from the viewpoint of durability and moldability. .
There is no restriction | limiting in particular in the shape and magnitude | size of the positive electrode for potassium ion batteries which concerns on this embodiment, According to the shape and magnitude | size of the battery to be used, it can be set as a desired shape and magnitude | size.
The positive electrode for potassium ion batteries which concerns on this embodiment is 10 masses of compounds represented by said Formula (1) with respect to the total mass of the positive electrode for potassium ion batteries from a viewpoint of the output in a potassium ion battery, and charging / discharging capacity. %, Preferably 20% by mass or more, more preferably 50% by mass or more, and particularly preferably 70% by mass or more.

<Conductive aid>
In the positive electrode for a potassium ion battery according to this embodiment, the positive electrode active material for a potassium ion battery according to this embodiment may be formed into a desired shape and used as it is as the positive electrode, but the positive electrode rate characteristics (output) In order to improve this, it is preferable that the positive electrode for potassium ion batteries which concerns on this embodiment further contains a conductive support agent.
Preferred examples of the conductive aid used in the present embodiment include carbons such as carbon blacks, graphites, carbon nanotubes (CNT), and vapor grown carbon fibers (VGCF).
Examples of carbon blacks include acetylene black, oil furnace, and ketjen black. Especially, it is preferable that it is at least 1 sort (s) of conductive support agent chosen from the group which consists of acetylene black and Ketjen black from an electroconductive viewpoint, and it is more preferable that they are acetylene black or Ketjen black.
A conductive support agent may be used individually by 1 type, or may use 2 or more types together.
The mixing ratio of the positive electrode active material and the conductive auxiliary agent is not particularly limited, but the content of the conductive auxiliary agent in the positive electrode is 1% by mass to 80% by mass with respect to the total mass of the positive electrode active material contained in the positive electrode. It is preferably 2% by mass to 60% by mass, more preferably 5% by mass to 50% by mass, and particularly preferably 5% by mass to 25% by mass. Within the above range, a higher output positive electrode is obtained, and the durability of the positive electrode is excellent.

As a method of mixing the conductive auxiliary agent and the positive electrode active material, the positive electrode active material can be mixed with the conductive auxiliary agent in an inert gas atmosphere to increase the electric conductivity of the positive electrode. As the inert gas, nitrogen gas, argon gas, or the like can be used, and argon gas can be preferably used.
Further, when the conductive additive and the positive electrode active material are mixed, pulverization and dispersion treatment such as a dry ball mill or a bead mill to which a small amount of a dispersion medium such as water is added may be performed. By performing the pulverization / dispersion treatment, the adhesion and dispersibility between the conductive additive and the positive electrode active material can be increased, and the electrode density can be increased.

<Binder>
It is preferable that the positive electrode for potassium ion batteries which concerns on this embodiment further contains a binder from a viewpoint of a moldability.
The binder is not particularly limited, and known binders can be used, and examples thereof include polymer compounds such as fluororesins, polyolefin resins, rubber-like polymers, polyamide resins, polyimide resins (polyamideimide, etc.). And cellulose ether are preferred.
Specific examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber ( VDF-HFP-TFE fluorine rubber), polyethylene, aromatic polyamide, cellulose, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer, its hydrogenated product, styrene -Ethylene-butadiene-styrene copolymer, styrene-isoprene-styrene block copolymer, hydrogenated product thereof, syndiotactic-1,2-polybutadiene, ethylene-vinyl acetate copolymer, propylene- -Olefin (2 to 12 carbon atoms) copolymer, starch, methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylhydroxyethylcellulose, nitrocellulose, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, etc. Is mentioned.

From the viewpoint of increasing the electrode density, the specific gravity of the compound used as the binder is preferably greater than 1.2 g / cm ³ .
Further, from the viewpoint of increasing the electrode density and increasing the adhesive force, the weight average molecular weight of the binder is preferably 1,000 or more, more preferably 5,000 or more, and 10,000 or more. More preferably. There is no particular upper limit, but it is preferably 2 million or less.

A binder may be used individually by 1 type, or may use 2 or more types together.
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 0.5% by mass to 30% by mass with respect to the total mass of the positive electrode active material contained in the positive electrode. %, More preferably 1% by mass to 20% by mass, and still more preferably 2% by mass to 15% by mass. It is excellent in a moldability and durability as it is the said range.

The method for producing the positive electrode including the positive electrode active material, the conductive auxiliary agent, and the binder is not particularly limited. For example, the positive electrode active material, the conductive auxiliary agent, and the binder are mixed and subjected to pressure molding. Moreover, the method of preparing the slurry mentioned later and forming a positive electrode may be sufficient.

<Current collector>
The positive electrode for a potassium ion battery according to this embodiment may further include a current collector.
Examples of the current collector include a foil, a mesh, an expanded grid (expanded metal), a punched metal, and the like using a conductive material such as nickel, aluminum, and stainless steel (SUS). The mesh opening, wire diameter, number of meshes, etc. are not particularly limited, and conventionally known ones can be used.
The shape of the current collector is not particularly limited, and may be selected according to a desired shape of the positive electrode. For example, foil shape, plate shape, etc. are mentioned.

The method for forming the positive electrode on the current collector is not particularly limited, but a positive electrode active material slurry is prepared by mixing a positive electrode active material, a conductive additive, a binder, an organic solvent or water, and a current collector. Examples of the method of coating are shown in FIG. Examples of the organic solvent 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; dimethylacetamide and N-methyl- Examples include aprotic polar solvents such as 2-pyrrolidone.
The prepared slurry is applied, for example, on a current collector, dried, pressed, and fixed to produce a positive electrode. Examples of the method of coating the slurry on the current collector include a slit die coating method, a screen coating method, a curtain coating method, a knife coating method, a gravure coating method, and an electrostatic spray method.

(Potassium ion battery)
The potassium ion battery according to the present embodiment includes the positive electrode for a potassium ion battery according to the present embodiment.
Moreover, the potassium ion battery which concerns on this embodiment can be used suitably as a potassium ion secondary battery.
The potassium ion battery according to the present embodiment preferably includes the positive electrode for potassium ion battery, the negative electrode, and the electrolyte according to the present embodiment.

<Negative electrode>
The negative electrode used in the present embodiment only needs to contain a negative electrode active material. For example, a negative electrode active material, a current collector and a negative electrode active material layer formed on the surface of the current collector are used. And the negative electrode active material layer includes a negative electrode active material and a binder.
There is no restriction | limiting in particular as said electrical power collector, The electrical power collector mentioned above in the positive electrode can be used suitably.
There is no restriction | limiting in particular in the shape and magnitude | size of a negative electrode, According to the shape and magnitude | size of the battery to be used, it can be set as a desired shape and magnitude | size.

Examples of the negative electrode active material include carbon materials such as natural graphite, artificial graphite, coke, hard carbon, carbon black, pyrolytic carbon, carbon fiber, and fired organic polymer compound. The shape of the carbon material may be, for example, a flake shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or a particulate aggregate. Here, the carbon material may play a role as a conductive additive.
Among these, graphite or hard carbon is preferable, and graphite is more preferable.
Moreover, potassium metal can also be used suitably as a negative electrode active material.
Furthermore, as the negative electrode, the negative electrode described in International Publication No. 2016/059907 can also be suitably used.

The graphite in the present embodiment refers to a graphite-based carbon material.
Examples of the graphite-based carbon material include natural graphite, artificial graphite, and expanded graphite. As natural graphite, for example, scaly graphite, massive graphite and the like can be used. As the artificial graphite, for example, massive graphite, vapor-grown graphite, flaky graphite, fibrous graphite and the like can be used. Among these, flaky graphite and lump graphite are preferable for reasons such as high packing density. Two or more types of graphite may be used in combination.
The average particle diameter of graphite is preferably 30 μm, more preferably 15 μm, further preferably 10 μm, more preferably 0.5 μm, more preferably 1 μm, and even more preferably 2 μm as the upper limit. The average particle diameter of graphite is a value measured by an electron microscope observation method.
Examples of graphite include those having an interplanar spacing d (002) of 3.354 to 3.370 Å (angstrom, 1 Å = 0.1 nm) and a crystallite size Lc of 150 Å or more.
The hard carbon in the present embodiment is a carbon material that does not graphitize even when heat-treated at a high temperature of 2,000 ° C. or higher, and is also called non-graphitizable carbon. As hard carbon, carbon fiber obtained by carbonizing an infusible yarn, which is an intermediate product in the production process of carbon fiber, at about 1,000 ° C to 1,400 ° C, and an organic compound after air oxidation at about 150 ° C to 300 ° C Examples thereof include carbon materials carbonized at about 1,000 ° C. to 1,400 ° C. The method for producing hard carbon is not particularly limited, and hard carbon produced by a conventionally known method can be used.
There are no particular restrictions on the average particle size, true density, and (002) plane spacing of the hard carbon, and it can be carried out by selecting preferred ones as appropriate.

A negative electrode active material may be used individually by 1 type, or may use 2 or more types together.
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% by mass.

<Electrolyte>
As the electrolyte used in the present embodiment, both an electrolytic solution and a solid electrolyte can be used.
The electrolyte solution is not particularly limited as long as it has a potassium salt as a main electrolyte.
Examples of the potassium salt include KClO ₄ , KPF ₆ , KNO ₃ , KOH, KCl, K ₂ SO ₄ , and K ₂ S in the case of an aqueous electrolyte. These potassium salts can be used alone or in combination of two or more.
In the case of a non-aqueous electrolyte, for example, an electrolyte (for example, KPF ₆ , KBF ₄ , CF ₃ SO ₃ K, KAsF ₆ , KB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ K, KN (SO ₂ CF ₃ ) ₂ , KN (SO ₂ C ₂ F ₅ ) ₂ , KC (SO ₂ CF ₃ ) ₃ , KN (SO ₃ CF ₃ ) _2, etc.) and an electrolysis containing propylene carbonate (PC) It can be used as a liquid, but besides this, it can be dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), or a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). What was melt | dissolved in the solvent etc. can be used as electrolyte solution.
Among these, as the potassium salt, KPF ₆ is preferable.

In addition, as a solvent for the electrolytic solution, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, fluoroethylene carbonate, 4-trifluoromethyl-1,3-dioxolane-2- ON, carbonates such as 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyldifluoro Ethers such as methyl ether, tetrahydrofuran and 2-methyltetrahydrofuran; Esters such as methyl formate, methyl acetate and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile Amides such as N, N-dimethylformamide and N, N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; Or what introduce | transduced the fluoro group into the said solvent further as a substituent of a hydrogen atom can be used.
The solvent of the electrolytic solution may be used singly or as a mixture 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 preferable, and at least two mixed solvents selected from the group consisting of propylene carbonate, ethylene carbonate and diethyl carbonate are more preferable. preferable.
Further, the concentration of the potassium salt in the electrolytic solution 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. preferable.

As the solid electrolyte, a known solid electrolyte can be used. For example, an organic solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution to the high molecular compound can also be used.

<Separator>
The potassium ion battery according to this embodiment preferably further includes a separator.
A separator plays the role which isolates a positive electrode and a negative electrode physically, and prevents an internal short circuit.
The separator is made of a porous material, and the voids are impregnated with an electrolyte, and have ion permeability (particularly at least potassium ion permeability) in order to ensure a battery reaction.
As the separator, for example, a nonwoven fabric other than a resin porous film can be used. The separator may be formed of only a porous membrane layer or a non-woven fabric layer, or may be formed of a laminate of a plurality of layers having different compositions and forms. Examples of the laminate include a laminate having a plurality of resin porous layers having different compositions, and a laminate having a porous membrane layer and a nonwoven fabric layer.

The material of the separator can be selected in consideration of the operating temperature of the battery, the composition of the electrolyte, and the like.
Examples of the resin contained in the fibers forming the porous film and the nonwoven fabric include polyolefin resins such as polyethylene, polypropylene, and ethylene-propylene copolymer; polyphenylene sulfide resins such as polyphenylene sulfide and polyphenylene sulfide ketone; and aromatic polyamide resins (aramid). Examples thereof include polyamide resins such as resins) and polyimide resins. One of these resins may be used alone, or two or more thereof may be used in combination. The fibers forming the nonwoven fabric may 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. Among these, a glass filter is more preferable as the separator.
Further, the separator may include an inorganic filler.
Examples of the inorganic filler include ceramics (silica, alumina, zeolite, titania, etc.), talc, mica, wollastonite and the like. The inorganic filler is preferably particulate or fibrous.
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 appropriately selected according to a desired battery shape and the like.

In the potassium ion battery according to the present embodiment, various known materials used in conventional lithium ion batteries and sodium ion batteries are used for elements such as battery cases, spacers, gaskets, leaf springs, and other structural materials. There are no particular restrictions.
What is necessary is just to assemble the potassium ion battery which concerns on this embodiment according to a well-known method using the said battery element. In this case, the shape of the battery is not particularly limited, and various shapes and sizes such as a cylindrical shape, a square shape, and a coin shape can be appropriately employed.

An example of the potassium ion battery according to the present embodiment is the potassium ion battery shown in FIG. 1, but it goes without saying that the potassium ion battery is not limited to this.
FIG. 1 is a schematic diagram illustrating an example of a potassium ion battery 10 according to the present embodiment.
A potassium ion battery 10 shown in FIG. 1 is a coin-type battery, and in order from the negative electrode side, a battery case 12 on the negative electrode side, a gasket 14, a negative electrode 16, a separator 18, and a positive electrode (positive electrode) for a potassium ion battery according to this embodiment. 20, the spacer 22, the leaf spring 24, and the battery case 26 on the positive electrode side are stacked, and the battery case 12 and the battery case 26 are fitted together.
The separator 18 is impregnated with an electrolytic solution (not shown).

Hereinafter, the present invention will be described more specifically with reference to examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples can be changed as appropriate 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.

<Synthesis of alkali metal salt of each hexacyano acid metal>
Details of the compounds used are shown below.
K ₄ Fe (CN) ₆ : manufactured by Kanto Chemical Co., Ltd. MCl ₂ (M = Mn, Fe, Co, Ni or Cu): manufactured by Kanto Chemical Co., Inc. ACl (A = K, Na or Li): Wako Pure Chemical Industries Made by Kogyo Co., Ltd.

At room temperature (25 ° C., the same applies hereinafter), 45 g of ACl was mixed with 120 mL of deionized water to obtain an ACl aqueous solution.
Next, at room temperature, K ₄ Fe (CN) ₆ (2 mmol) was mixed with 10 mL of deionized water, and 40 mL of an ACl aqueous solution was further mixed to obtain a solution A.
Further, at room temperature, MCl ₂ (4 mmol) was mixed with 10 mL of deionized water, and further 80 mL of an ACl aqueous solution was mixed to obtain a solution B.
Solution A placed in a reaction vessel was placed in a 60 ° C. hot water bath, nitrogen gas was bubbled, and solution B was slowly added dropwise at a rate of 40 mL / hour or less while stirring at a speed of 500 rpm (rotation per minute). As soon as it was dropped, a precipitate was formed.
After completion of the dropwise addition, the liquid temperature was kept at 60 ° C., and stirring was performed for 4 hours at a speed of 500 rpm while bubbling nitrogen gas.
Thereafter, the reaction solution was centrifuged (MX-301 manufactured by Tommy Seiko Co., Ltd., 8,000 rpm to 10,000 rpm) to obtain a mixture of an alkali metal salt of hexacyano acid metal and ACl.
The resulting mixture was washed with 1 L of deionized water, and centrifuged (8,000 rpm to 10,000 rpm) several times to obtain a metal potassium hexacyanoate containing a small amount of water.
The obtained alkali metal salt of hexacyano acid metal containing a small amount of water was dried at 80 ° C. for 12 hours using a constant temperature dryer to obtain an alkali metal salt of hexacyano acid metal.

The composition of the obtained alkali metal salt of each hexacyano acid metal is shown below together with abbreviations.
K—Mn [Fe (CN) ₆ ] (M = Mn, A = K): K _1.75 Mn [Fe (CN) ₆ ] _0.93 · 0.58H ₂ O (= K _1.88 Fe _{1. _{_{00 Mn 1.08 (CN) 6 ·}}} 0.62H 2 O)
Na—Mn [Fe (CN) ₆ ] (M = Mn, A = Na): Na _{1.5 to 2.0} Mn [Fe (CN) ₆ ] · 0.1 to 2.0H ₂ O
Li—Mn [Fe (CN) ₆ ] (M = Mn, A = Li): Li _{1.5 to 2.0} Mn [Fe (CN) ₆ ] · 0.1 to 2.0H ₂ O
K—Fe [Fe (CN) ₆ ] (M = Fe, A = K): K _{1.5 to 2.0} Fe [Fe (CN) ₆ ] · 0.1 to 2.0H ₂ O
K—Co [Fe (CN) ₆ ] (M = Co, A = K): K _{1.5 to 2.0} Co [Fe (CN) ₆ ] · 0.1 to 2.0H ₂ O
K—Ni [Fe (CN) ₆ ] (M = Ni, A = K): K _{1.5 to 2.0} Ni [Fe (CN) ₆ ] · 0.1 to 2.0H ₂ O
K—Cu [Fe (CN) ₆ ] (M = Cu, A = K): K _{1.5 to 2.0} Cu [Fe (CN) ₆ ] · 0.1 to 2.0H ₂ O

In addition, the alkali metal salt of each hexacyano acid metal obtained above was measured by elemental analysis and X-ray diffraction structure analysis, respectively, and the chemical structure was specified.

<Preparation of positive electrode for potassium ion battery>
Each of the obtained alkali metal salts of hexacyano acid metal was used to prepare positive electrodes.
The obtained alkali metal salt of hexacyano acid metal, ketjen black (KB, manufactured by Lion Specialty Chemicals) and PTFE (polytetrafluoroethylene, manufactured by Daikin Industries, Ltd.) 7: 2: After mixing at a mass ratio of 1, a coating was made on an aluminum foil (manufactured by Hosen Co., Ltd., thickness: 0.017 mm) to form a positive electrode. The shape of the positive electrode not including the aluminum foil was a cylindrical shape having a diameter of 10 mm and a thickness of 0.03 mm to 0.04 mm. The mass of the positive electrode not containing the aluminum foil was 3 mg to 5 mg.

<Charge / discharge measurement using positive electrode using K salt, Na salt or Li salt>
-Measurement of charge / discharge using positive electrode using K salt (K-Mn [Fe (CN) ₆ ])-
Charging / discharging measurement is performed by using 0.8M KPF ₆ / ethylene carbonate (EC) -diethyl carbonate (DEC) (mass ratio EC: DEC = 1: 1) mixed solution as an electrolyte, potassium metal (manufactured by Aldrich) as a negative electrode, separator (Glass filter, manufactured by Hosen Co., Ltd.), SUS battery case and polypropylene gasket (CR2032 manufactured by Hosen Co., Ltd.), spacer (material: SUS, diameter 16 mm x height 0.5 mm, Hosen Co., Ltd.) Made) and a leaf spring (material: SUS, inner diameter 10 mm, height 2.0 mm, thickness 0.25 mm, Hosen Co., Ltd. washer).
The amount of the electrolyte used was an amount sufficient to fill the separator with the electrolyte (0.15 mL to 0.3 mL).
The electrolytic solution was prepared using KPF ₆ manufactured by Tokyo Chemical Industry Co., Ltd., ethylene carbonate manufactured by Kishida Chemical Co., and diethyl carbonate manufactured by Kishida Chemical Co., Ltd.

The charge / discharge conditions were measured at room temperature (25 ° C.) with the charge / discharge current density set to the constant current mode. Using the obtained positive electrode, the current density was set to 30 mA / g, and the charge voltage was constant current charged to 4.5V. After charging, constant current discharge was repeated until the charge voltage was 4.5V and the end-of-discharge voltage was 2.0V.

-Measurement of charge / discharge using positive electrode using Na salt (Na-Mn [Fe (CN) ₆ ])-
Charging / discharging measurement was carried out using 1.0M NaPF ₆ / ethylene carbonate (EC) -diethyl carbonate (DEC) (mass ratio EC: DEC = 1: 1) mixed solution (made by Kishida Chemical Co., Ltd.) as the electrolyte and sodium as the negative electrode. Metal (manufactured by Kanto Chemical Co., Inc.), separator (glass filter, manufactured by Hosen Co., Ltd.), SUS battery case and polypropylene gasket (CR2032 manufactured by Hosen Co., Ltd.), spacer (material: SUS, diameter 16 mm × 0.5 mm in height, manufactured by Hosen Co., Ltd.) and a leaf spring (material: SUS, inner diameter 10 mm, height 2.0 mm, thickness 0.25 mm, manufactured by Hosen Co., Ltd.) I went in the coin cell.
The amount of the electrolyte used was an amount sufficient to fill the separator with the electrolyte (0.15 mL to 0.3 mL).

The charge / discharge conditions were measured at room temperature (25 ° C.) with the charge / discharge current density set to the constant current mode. Using the obtained positive electrode, the current density was set to 30 mA / g, and constant current charging was performed up to a charging voltage of 4.0V. After charging, constant current discharging was repeated until the charging voltage was 4.0 V and the final discharge voltage was 2.0 V.

-Measurement of charge / discharge using positive electrode using Li salt (Li-Mn [Fe (CN) ₆ ])-
Charging / discharging measurement was performed by using 1.0M LiPF ₆ / ethylene carbonate (EC) -diethyl carbonate (DEC) (mass ratio EC: DEC = 1: 1) mixed solution (manufactured by Kishida Chemical Co., Ltd.) as the electrolyte and lithium as the negative electrode. Metal (manufactured by Honjo Chemical Co., Ltd.), separator (glass filter, manufactured by Hosen Co., Ltd.), SUS battery case and polypropylene gasket (CR2032 manufactured by Hosen Co., Ltd.), spacer (material: SUS, diameter 16 mm × 0.5 mm in height, manufactured by Hosen Co., Ltd.) and a leaf spring (material: SUS, inner diameter 10 mm, height 2.0 mm, thickness 0.25 mm, manufactured by Hosen Co., Ltd.) I went in the coin cell.
The amount of the electrolyte used was an amount sufficient to fill the separator with the electrolyte (0.15 mL to 0.3 mL).

The charge / discharge conditions were measured at room temperature (25 ° C.) with the charge / discharge current density set to the constant current mode. Using the obtained positive electrode, the current density was set to 30 mA / g, and constant current charging was performed up to a charging voltage of 4.8V. After charging, constant current discharge was repeated until the charge voltage was 4.8 V and the end-of-discharge voltage was 2.0 V.

In the above charge / discharge measurement using the positive electrode using K salt, Na salt or Li salt, one charge / discharge is defined as one cycle, and the measurement results are shown in the following Table 1 and FIGS.

The energy density in Table 1 was calculated from the average working potential x discharge capacity.

FIGS. 2 to 4 show charge / discharge profiles up to the third cycle in the case of using an alkali metal salt of each hexacyano metal acid.
FIG. 5 shows a charge / discharge profile at the second cycle when an alkali metal salt of each hexacyano metal acid is used.
The vertical axis of the charge / discharge profiles in FIGS. 2 to 5 represents the potential (Voltage, unit: V (V vs. A / V) based on the standard unipolar potential of the alkali metal of the alkali metal salt of each hexacyano metal acid used. A ⁺ )), and the horizontal axis represents capacity (capacity, unit: mAh / g).
Further, FIG. 6 is a diagram showing a change in discharge capacity over the course of the cycle. The vertical axis of FIG. 6 represents the discharge capacity (Discharge Capacity, unit: mAh / g), and the horizontal axis represents the cycle number (Cycle Number).

As described above, a potassium ion battery having a high energy density was obtained by using the positive electrode active material for a potassium ion battery according to this embodiment.

<Charge / discharge measurement using positive electrode using potassium salt of each hexacyano metal acid>
Charge / discharge measurement using a positive electrode using a potassium salt of each hexacyano metal acid was carried out by using 0.8 M KPF ₆ / ethylene carbonate (EC) -diethyl carbonate (DEC) (mass ratio EC: DEC = 1: 1) in the electrolyte solution. Mixed solution, negative electrode potassium metal (Aldrich), separator (glass filter, manufactured by Hosen Co., Ltd.), SUS battery case and polypropylene gasket (Hosen Co., Ltd. CR2032), spacer (material: SUS, Diameter 16 mm x height 0.5 mm, manufactured by Hosen Co., Ltd.) and leaf spring (material: SUS, inner diameter 10 mm, height 2.0 mm, thickness 0.25 mm, washer manufactured by Hosen Co., Ltd.) It was performed in the coin cell produced by using.
The amount of the electrolyte used was an amount sufficient to fill the separator with the electrolyte (0.15 mL to 0.3 mL).
The electrolytic solution was prepared using KPF ₆ manufactured by Tokyo Chemical Industry Co., Ltd., ethylene carbonate manufactured by Kishida Chemical Co., and diethyl carbonate manufactured by Kishida Chemical Co., Ltd.

In the above charge / discharge measurement using the positive electrode using the potassium salt of each hexacyano metal acid, one charge / discharge is defined as one cycle, and the measurement results are shown in the following Table 2 and FIGS.

The coulombic efficiency in Table 2 is calculated from the initial charge capacity / initial discharge capacity × 100, the energy density is calculated from the average operating potential × discharge capacity, and the average potential is a value obtained by integrating the measured voltage by the discharge capacity / It was calculated from the discharge capacity.

7 to 11 show charge / discharge profiles in the first cycle when potassium salts of hexacyano metal acids are used.
Further, FIG. 12 shows a diagram in which charge / discharge profiles of one cycle in the case where potassium salts of hexacyano metal acids are used are combined into one figure.
7 to 12, the vertical axis represents the potential (Voltage, unit: V (V vs. K / K ⁺ )) based on the standard unipolar potential of potassium, and the horizontal axis represents the capacity ( Capacity, unit: mAh / g).
Further, FIG. 13 is a diagram showing a change in discharge capacity over the course of a cycle when K—Mn [Fe (CN) ₆ ] or K—Fe [Fe (CN) ₆ ] is used. The vertical axis in FIG. 13 represents the discharge capacity (Discharge Capacity, unit: mAh / g), and the horizontal axis represents the cycle number (Cycle Number).

The disclosure of Japanese Patent Application No. 2016-251910 filed on Dec. 26, 2016 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards described in this specification are the same as if each document, patent application, and technical standard were specifically and individually stated to be incorporated by reference. Which is incorporated herein by reference.

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

The positive electrode active material for potassium ion batteries containing the compound represented by following formula (1).
K m Fe x Mn y (CN ) 6 · zH 2 O (1)
In formula (1), m represents a number from 0.5 to 2, x represents a number from 0.5 to 1.5, y represents a number from 0.5 to 1.5, z Represents 0 or a positive number.
A positive electrode for a potassium ion battery comprising the positive electrode active material for a potassium ion battery according to claim 1.
A potassium ion battery comprising the positive electrode for a potassium ion battery according to claim 2.