JP2020145061A - Electrolyte for potassium-ion battery, potassium-ion battery, electrolyte for potassium-ion capacitor, and potassium-ion capacitor - Google Patents

Abstract

To provide an electrolyte for a potassium-ion battery capable of improving the Coulomb efficiency and rate characteristics, and a potassium-ion battery including the electrolyte for the potassium-ion battery.SOLUTION: An electrolyte for a potassium-ion battery includes a solvent and a potassium salt compound, and the potassium salt compound includes potassium hexafluorophosphate, and at least one selected from the group consisting of potassium bis (fluorosulfonyl) amide and potassium bis (trifluoromethanesulfonyl) amide, and the ratio of the potassium hexafluorophosphate to the total potassium salt compound is 70 mol% to 95 mol%.SELECTED DRAWING: Figure 1

Description

The present invention relates to an electrolytic solution for a potassium ion battery, a potassium ion battery, an electrolytic solution for a potassium ion capacitor, and a potassium ion capacitor.

Currently, as a battery (secondary battery) capable of charging and discharging at high energy density, a non-aqueous electrolyte is used, and lithium ions are moved between the positive electrode and the negative electrode to perform charging and discharging. Is widely used.

Lithium, which is used as a raw material for lithium-ion batteries, has a relatively limited amount of resources and is expensive. In addition, resources are mainly unevenly distributed in South America, and Japan relies entirely on imports from overseas. Therefore, in order to reduce the cost of the battery and secure a stable supply, the development of a potassium ion secondary battery that uses potassium instead of lithium is being studied (see, for example, Patent Document 1).

Since potassium is abundant in seawater and the crust, a stable supply can be expected compared to lithium. Also, since lithium forms an alloy with many metals such as aluminum, the metal used for the current collector of the electrode is limited to copper, etc., whereas potassium does not form an alloy with aluminum, so instead of copper It becomes possible to use inexpensive aluminum.
Further, since the standard electrode potential of potassium in the carbonic acid ester solvent is about 0.1 V lower than that of lithium, it is possible to increase the voltage of the battery. Moreover, since potassium ion has a weak Coulomb interaction with a negative charge, it is possible to increase the output of the battery. Therefore, the potassium ion battery is expected as a secondary battery with high voltage and high output.

International Publication No. WO2016 / 059907

For the practical use of potassium-ion batteries, optimization of various components constituting the batteries is being studied, and as electrolytic solutions for potassium-ion batteries, those using various potassium salt compounds and solvents are being studied. ..

In view of the above circumstances, an object to be solved by the present invention is to provide an electrolytic solution for a potassium ion battery capable of improving coulombic efficiency and rate characteristics, and a potassium ion battery including the electrolytic solution for a potassium ion battery. is there.
Another problem to be solved by the present invention is to provide an electrolytic solution for a potassium ion capacitor capable of improving Coulomb efficiency and rate characteristics, and a potassium ion capacitor including the electrolytic solution for the potassium ion capacitor. ..

<1> The potassium salt compound contains a solvent and a potassium salt compound, and the potassium salt compound is at least one selected from the group consisting of potassium hexafluorophosphate, potassium bis (fluorosulfonyl) amide and potassium bis (trifluoromethanesulfonyl) amide. An electrolytic solution for a potassium ion battery, wherein the ratio of the potassium hexafluorophosphate to the total potassium salt compound is 70 mol% to 95 mol%.
<2> A potassium-ion battery comprising the electrolytic solution for a potassium-ion battery according to <1>.
<3> The potassium ion battery according to <2>, further comprising an electrode containing graphite.
<4> The potassium ion battery according to <2> or <3>, further comprising a member containing aluminum.
<5> The potassium salt compound contains a solvent and a potassium salt compound, and the potassium salt compound is at least one selected from the group consisting of potassium hexafluorophosphate, potassium bis (fluorosulfonyl) amide and potassium bis (trifluoromethanesulfonyl) amide. An electrolytic solution for a potassium ion capacitor, wherein the ratio of the potassium hexafluorophosphate to the whole potassium salt compound is 70 mol% to 95 mol%.
<6> A potassium ion capacitor comprising the electrolytic solution for a potassium ion capacitor according to <5>.

According to the present invention, there is provided an electrolytic solution for a potassium ion battery capable of improving coulombic efficiency and rate characteristics, and a potassium ion battery including the electrolytic solution for a potassium ion battery. Further, according to the present invention, there is provided an electrolytic solution for a potassium ion capacitor capable of improving Coulomb efficiency and rate characteristics, and a potassium ion capacitor including the electrolytic solution for the potassium ion capacitor.

It is a schematic diagram which shows an example of the potassium ion battery 10 which concerns on this embodiment. It is a graph which shows the charge / discharge measurement result of a graphite electrode. It is a graph which shows the charge / discharge measurement result of a graphite electrode. It is a graph which shows the evaluation result of the rate characteristic of a graphite electrode. It is a graph which shows the evaluation result of the solvent dependence of a graphite electrode. It is a graph which shows the evaluation result of the solvent dependence performed in an Example. It is a graph which shows the evaluation result of the aluminum corrosion inhibitory property. It is an electron micrograph which shows the evaluation result of the aluminum corrosion inhibitory property. It is a graph which shows the charge / discharge measurement result of the positive electrode. It is a graph which shows the charge / discharge measurement result of the positive electrode. It is a graph which shows the measurement result of the ionic conductivity of an electrolytic solution. It is a graph which shows the charge / discharge measurement result of the potassium ion battery. It is a graph which shows the charge / discharge measurement result of the potassium ion battery.

The contents of the present invention will be described in detail below. The description of the constituent elements described below may be based on a typical embodiment of the present invention, but the present invention is not limited to such an embodiment. In the specification of the present application, "~" is used to mean that the numerical values described before and after it are included as the lower limit value and the upper limit value.
The combination of two or more preferred embodiments in this embodiment is a more preferred embodiment.

<Potassium-ion battery electrolyte and potassium-ion capacitor electrolyte>
The electrolytic solution for a potassium ion battery and the electrolytic solution for a potassium ion capacitor (hereinafter, also simply referred to as an electrolytic solution) according to the present embodiment contains a solvent and a potassium salt compound, and the potassium salt compound is potassium hexafluorophosphate (hereinafter, hereinafter). , KPF ₆ ) and at least one selected from the group consisting of potassium bis (fluorosulfonyl) amide (hereinafter, also referred to as KFSA) and potassium bis (trifluoromethanesulfonyl) amide (hereinafter, also referred to as KTFSA). The proportion of KPF _{6 in} the total potassium salt compound is 70 mol% to 95 mol%.

At present, since the aluminum foil used as the positive electrode current collector can be passivated, an electrolytic solution containing KPF ₆ as a potassium salt compound is mainly used for potassium ion batteries. However, there is room for improvement in the Coulomb efficiency (particularly when a graphite electrode is used as the negative electrode) of the potassium ion battery using the electrolytic solution containing KPF ₆ .

As a method for improving the Coulomb efficiency of the electrolytic solution containing KPF ₆ , it is conceivable to use another potassium salt compound having excellent Coulomb efficiency in combination. However, considering that the amount of potassium salt compound soluble in a solvent is limited, using both the KPF ₆ and another potassium salt compound leads to a decrease in the amount of KPF _6, of the use of KPF ₆ The benefits may be diminished.

Results of studies of the present inventors, the use of at least one selected from the group consisting of KFSA and KTFSA as potassium salt compound in combination with KPF _6, effectively a coulomb efficiency while suppressing the reduction of the amount of KPF ₆ It turned out that it could be improved.
Furthermore, as a result of the studies by the present inventors, it was found that the rate characteristics are also improved by using at least one selected from the group consisting of KFSA and KTFSA as the potassium salt compound to be used in combination with KPF ₆ .

It is not always clear why an electrolytic solution containing at least one selected from the group consisting of KFSA and KTFSA in addition to KPF ₆ has excellent coulombic efficiency, but as confirmed in Examples described later, KPF ₆ It is conceivable that a more stable passivation film is formed on the surface of the graphite electrode as compared with the case where the above is used alone.
It is not always clear why an electrolytic solution containing at least one selected from the group consisting of KFSA and KTFSA in addition to KPF ₆ has excellent rate characteristics, but as described above, when KPF ₆ is used alone. It is considered that a more stable passivation film is formed on the surface of the graphite electrode.

Further, according to the electrolytic solution of the present invention, as compared with the electrolytic solution containing only at least one selected from the group consisting of KFSA and KTFSA as the potassium salt compound, aluminum corrosion occurs when aluminum is used as a current collector or the like. The effect of being suppressed is obtained.

In the present embodiment, KPF ₆ is a potassium salt compound represented by the following structural formula (a), KFSA is a potassium salt compound represented by the following structural formula (b), and KTFSA is the following structural formula (KTFSA). It is a potassium salt compound represented by c).

The amount of KPF ₆ contained in the electrolytic solution is not particularly limited as long as the ratio of KPF _{6 to} the total potassium salt compound is 70 mol% to 95 mol%. From the viewpoint of sufficiently securing the benefits of KPF _6, it is preferable that the ratio of KPF ₆ that the total potassium salt compounds is 75 mol% or more, more preferably 80 mol% or more, 85 mol% or more It is more preferable to have.

The amount of at least one selected from the group consisting of KFSA and KTFSA contained in the electrolytic solution is not particularly limited as long as the ratio of KPF _{6 to} the total potassium salt compound is 70 mol% to 95 mol%. From the viewpoint of suppressing corrosion of members containing aluminum, the proportion of at least one selected from the group consisting of KFSA and KTFSA in the total potassium salt compound is preferably 25 mol% or less, preferably 20 mol% or less. More preferably, it is more preferably 15 mol% or less.

The electrolytic solution may contain a potassium salt compound other than KPF ₆ , KFSA and KTFSA, if necessary. Examples of potassium salt compounds other than KPF ₆ , KFSA and KTFSA include potassium fluoroborate (KBF ₄ ), potassium perchlorate (KClO ₄ ), KCF ₃ O ₃ , KN (SO ₂ C ₂ F ₅ ) _{2 and the} like. ..

When the electrolytic solution contains a potassium salt compound other than KPF ₆ , KFSA and KTFSA, the amount thereof is preferably 20 mol% or less, more preferably 10 mol% or less, and 5 mol of the total potassium salt compound. It is more preferably% or less.

The concentration of the entire potassium salt compound contained in the electrolytic solution is not particularly limited, and can be adjusted according to the ionic conductivity, viscosity, solubility of the potassium salt compound, and the like of the electrolytic solution. For example, it may be in the range of 0.5 mol / kg to 1.5 mol / kg.

The type of solvent contained in the electrolytic solution is not particularly limited, and can be selected from those generally used as a non-aqueous electrolytic solution. At least one solvent selected from the group consisting of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate and propylene carbonate. Preferably, from the viewpoint of the potential window and ionic conductivity, a carbonate ester solvent such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate and propylene carbonate is more preferable. The solvent contained in the electrolytic solution may be only one type or a combination of two or more types.

The ratio of the carbonic acid ester solvent to the total mass of the solvent contained in the electrolytic solution is preferably 50% by mass or more, more preferably 80% by mass or more, and more preferably 90% by mass or more. More preferably, it is 95% by mass or more, and most preferably 99% by mass or more.

As the solvent, a solvent that has been dehydrated with a dehydrating agent and then rectified may be used. Examples of the dehydrating agent include molecular sieves, mirabilite, magnesium sulfate, calcium hydride, sodium hydride, potassium hydride, lithium aluminum hydride and the like. Further, a solvent obtained by only dehydrating with a dehydrating agent without rectification may be used.

The content of the solvent contained in the electrolytic solution is not particularly limited, and is preferably an amount that satisfies the above-mentioned preferable concentration range of the potassium salt compound.

The electrolytic solution may contain other components, if necessary, in addition to the potassium salt compound and the solvent. As other components, known additives can be used, and examples thereof include fluoroethylene carbonate (FEC), vinylene carbonate (VC), and ethylene sulfide (ES). Alternatively, an overcharge inhibitor, a dehydrating agent, a deoxidizing agent and the like can be mentioned.

<Potassium-ion battery>
The potassium-ion battery according to the present embodiment is a potassium-ion battery including the electrolytic solution for the potassium-ion battery according to the present embodiment.
The potassium ion battery according to the present embodiment can be suitably used as a potassium ion secondary battery.
The potassium-ion battery according to the present embodiment preferably includes a positive electrode and a negative electrode in addition to the electrolytic solution, and more preferably includes a positive electrode, a negative electrode, and a separator in addition to the electrolytic solution.

The potassium ion battery of the present embodiment may include an electrode containing graphite. Examples of the graphite-containing electrode include a graphite-containing negative electrode.
The potassium ion battery of the present embodiment may include a member containing aluminum. Examples of the member containing aluminum include a current collector of electrodes, a case, and the like.

The potassium ion battery according to the present embodiment may include various known materials used in the lithium ion battery such as a battery case, a spacer, a gasket, and a spring without particular limitation.

The method for producing the potassium-ion battery according to the present embodiment is not particularly limited, and can be carried out according to a known method. The shape and size of the manufactured battery are not particularly limited, and various shapes such as a cylindrical shape, a square shape, a coin shape, and any size can be adopted.

(Positive electrode)
When the potassium ion battery according to the present embodiment includes a positive electrode, the positive electrode preferably contains a positive electrode active material for a potassium ion battery (hereinafter, also referred to as a positive electrode active material).

-Positive electrode active material-
The type of the positive electrode active material is not particularly limited, and a known positive electrode active material for a potassium ion battery can be used.
Specifically as the positive electrode active _{material, K x M y [Fe (} CN) 6] z potassium salt of (M = Fe, Mn, Co , Ni, represents Cr or Cu, x is a number of 0 to 2 represents, y represents a number of 0.5 to 1.5, z represents a number of 0.5 to 1.5.), KFeSO ₄ F, potassium iron phosphate compound, vanadium phosphate potassium compound, Examples thereof include activated carbon, α-FePO ₄ , K _0.3 MnO ₂ , and anhydrous perylene.

The shape of the positive electrode active material is not particularly limited and may be any desired shape, but a particulate positive electrode active material is preferable from the viewpoint of dispersibility at the time of forming the positive electrode.
When the shape of the positive electrode active material is particulate, the arithmetic mean particle size of the positive electrode active material is preferably 10 nm to 200 μm, preferably 50 nm to 100 μm, from the viewpoint of dispersibility and durability of the positive electrode. It is more preferably 100 nm to 80 μm, and particularly preferably 200 nm to 50 μm.

The arithmetic mean particle size of the positive electrode active material is measured by the laser diffraction / scattering method. Specifically, using a laser diffraction / scattering type particle size distribution measuring device (for example, LA-950 manufactured by Horiba Seisakusho Co., Ltd.), the measurement is preferably performed under the conditions of dispersion medium: water and laser wavelengths: 650 nm and 405 nm. can do. The arithmetic mean particle size of the positive electrode active material contained in the positive electrode can be measured by using a solvent or the like or by physically separating the positive electrode active material contained in the positive electrode.

The positive electrode may contain components other than the positive electrode active material. The other components are not particularly limited, and known additives used for producing the positive electrode of the battery can be used. Specific examples thereof include a conductive auxiliary agent and a binder.
Further, the positive electrode preferably contains a positive electrode active material, a conductive auxiliary agent and a binder from the viewpoint of durability and moldability.
The shape and size of the positive electrode are not particularly limited, and can be a desired shape and size according to the shape and size of the battery to be used.
From the viewpoint of the output and charge / discharge capacity of the potassium ion battery, the positive electrode preferably contains 10% by mass or more of the positive electrode active material for the potassium ion battery with respect to the total mass of the positive electrode (excluding the current collector described later). It is more preferably 20% by mass or more, further preferably 50% by mass or more, and particularly preferably 70% by mass or more.

-Conductive aid-
The positive electrode may be formed by molding the positive electrode active material into a desired shape and used as it is as the positive electrode, but it is preferable to further contain a conductive auxiliary agent in order to improve the rate characteristics (output) of the positive electrode.

Preferred examples of the conductive auxiliary agent used in the present embodiment include carbons such as carbon blacks, graphites, carbon nanotubes (CNTs), and vapor-grown carbon fibers (VGCF).
Examples of carbon blacks include acetylene black, oil furnace, and Ketjen black. Among them, from the viewpoint of conductivity, at least one conductive auxiliary agent selected from the group consisting of acetylene black and Ketjen black is preferable, and acetylene black or Ketjen black is more preferable. The conductive auxiliary agent may be used alone or in combination of two or more.

When the positive electrode contains a conductive auxiliary material, 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 based on the total mass of the positive electrode active material contained in the positive electrode. It is preferably 1% by mass to 80% by mass, more preferably 2% by mass to 60% by mass, further preferably 5% by mass to 50% by mass, and 5% by mass to 25% by mass. It is particularly preferable to have. Within the above range, a positive electrode having a higher output can be 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 coated with the conductive auxiliary agent by mixing the positive electrode active material with the conductive auxiliary agent in an inert gas atmosphere. 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 auxiliary agent and the positive electrode active material are mixed, a pulverization / dispersion treatment such as a dry ball mill or a bead mill to which a dispersion medium such as a small amount of water is added may be performed. By performing the pulverization and dispersion treatment, the adhesion and dispersibility between the conductive auxiliary agent and the positive electrode active material can be improved, and the electrode density can be increased.

-Binder-
From the viewpoint of moldability, the positive electrode preferably further contains a binder. The binder is not particularly limited, and a known binder can be used, and a polymer compound is preferable. Specifically, as the polymer compound, a fluororesin such as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene Fluororubber such as fluororubber (VDF-HFP-TFE fluororubber), polyolefin resin such as polyethylene, polyamide resin such as aromatic polyamide, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber, styrene- Butadiene-styrene block copolymer, its hydrogen additive, styrene-ethylene-butadiene-styrene copolymer, styrene-isoprene-styrene block copolymer, its hydrogen additive, syndiotactic-1,2-polybutadiene, etc. Rubber-like polymer, polyimide resin such as polyamideimide, ethylene-vinyl acetate copolymer, propylene-α-olefin (2 to 12 carbon atoms) copolymer, polyglutamic acid, starch, cellulose, methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose , Hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl hydroxyethyl cellulose, nitrocellulose and other cellulose-based polymerized compounds, and polyacrylic acid, sodium polyacrylate, polyacrylonitrile and other acrylic-based polymer compounds. The binder may be used alone or in combination of two or more.

From the viewpoint of increasing the electrode density, the density of the material used as the binder is preferably larger than 1.2 g / cm ³ .
Further, from the viewpoint of increasing the electrode density and the adhesive strength, 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. Is more preferable. The upper limit of the weight average molecular weight is not particularly limited, but is preferably 2 million or less.

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 even more preferably 2% by mass to 15% by mass. Within the above range, the moldability and durability are excellent.

The method for producing a positive electrode containing a positive electrode active material, a conductive auxiliary agent, and a binder is not particularly limited. For example, a positive electrode active material, a conductive auxiliary agent, and a binder are mixed and pressure-molded. Alternatively, it may be a method of preparing a slurry described later to form a positive electrode.

-Current collector-
The positive electrode used in this embodiment preferably further contains a current collector.
Examples of the current collector include those containing a conductive material such as nickel, aluminum, and stainless steel (SUS). Of these, a current collector containing aluminum is preferable.
The shape of the current collector is not particularly limited, and examples thereof include foil, plate, mesh, expanded grid (expanded metal), and punched metal. The mesh opening, wire diameter, number of meshes, etc. are not particularly limited, and conventionally known ones can be used.

The method for producing a positive electrode provided with a current collector is not particularly limited, but a positive electrode active material slurry is prepared by mixing a positive electrode active material and, if necessary, a conductive auxiliary agent and a binder with an organic solvent or water. An example is a method of applying this to a current collector.
As the organic solvent used for preparing the positive electrode active material slurry, amine-based solvents such as N, N-dimethylaminopropyriamine and diethyltriamine; ether-based solvents such as ethylene oxide and tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester-based solvents such as methyl acetate , Dimethylacetamide, aprotic polar solvents such as N-methyl-2-pyrrolidone and the like.

Examples of the method of coating the positive electrode active material 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. ..

(Negative electrode)
When the potassium ion battery includes a negative electrode, the negative electrode preferably contains a negative electrode active material.
Examples of the negative electrode active material include graphite, coke, hard carbon, carbon black, pyrolytic carbon, carbon fiber, carbon material such as calcined organic polymer compound, KTi ₂ (PO ₄ ) ₃ , P, Sn, Sb, etc. Examples thereof include MXene (composite atomic layer material). Among these, graphite and hard carbon are preferable, and graphite is more preferable. Potassium metal can also be preferably used as the negative electrode active material. Further, as the negative electrode, the negative electrode described in International Publication No. 2016/059907 can also be preferably used. As the negative electrode active material, one type may be used alone, or two or more types may be used in combination.
Examples of the shape of the negative electrode active material include flakes such as natural graphite, spheres such as mesocarbon microbeads, fibrous aggregates such as graphitized carbon fibers, and particulate aggregates, and are not particularly limited. The carbon material as the negative electrode active material may also serve as a conductive auxiliary agent.

The graphite in the present embodiment means a graphite-based carbon material. Examples of the graphite-based carbon material include natural graphite, artificial graphite, expanded graphite and the like. Examples of natural graphite include scaly graphite and lump graphite, and examples of artificial graphite include lump graphite, vapor-grown graphite, scaly graphite and fibrous graphite. Among these, scaly graphite and lump graphite are preferable because of high packing density and the like. Further, two or more kinds of graphite may be used in combination.
The average particle size of graphite is preferably 30 μm as an upper limit value, more preferably 15 μm, further preferably 10 μm, further preferably 0.5 μm as a lower limit value, further preferably 1 μm, further preferably 2 μm. The average particle size 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.
Hard carbon is a carbon material in which the stacking order hardly changes even when heat-treated at a high temperature of 2,000 ° C. or higher, and is also called graphitized carbon. As hard carbon, carbon fiber obtained by carbonizing infusible yarn, which is an intermediate product of the carbon fiber manufacturing process, at about 1,000 ° C to 1,400 ° C, and an organic compound after air oxidation at about 150 ° C to 300 ° C. , A carbon material carbonized at about 1,000 ° C to 1,400 ° C can be exemplified. The method for producing hard carbon is not particularly limited, and hard carbon produced by a conventionally known method can be used. The average particle size, true density, surface spacing of the (002) plane, and the like of the hard carbon are not particularly limited, and a preferable one can be appropriately selected and carried out.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably 80% by mass to 95% by mass of the entire negative electrode (excluding the current collector described later).

Examples of the form of the negative electrode include those composed of only the negative electrode active material and those containing the negative electrode active material and the binder in addition to the negative electrode active material. Further, a current collector and a negative electrode active material layer formed on the surface of the current collector can be mentioned. The current collector used for the negative electrode is not particularly limited, and the current collector used for the positive electrode described above can be preferably used. Of these, a current collector containing aluminum is preferable.
The shape and size of the negative electrode are not particularly limited, and can be a desired shape and size according to the shape and size of the battery to be used.

(Separator)
The potassium ion battery may further include a separator.
The separator plays a role of physically separating the positive electrode and the negative electrode to prevent an internal short circuit. Specifically, it is made of a porous material, and its voids are impregnated with an electrolyte, and has ion permeability (particularly, at least potassium ion permeability) in order to secure a battery reaction.

As the separator, for example, a porous membrane, a non-woven fabric, or the like can be used. The separator may be formed only of a layer of a porous membrane or a layer of a non-woven fabric, or may be a laminate in which a plurality of layers having different compositions, forms, etc. are combined. Examples of the laminate include a laminate having a plurality of resin porous layers having different compositions, a laminate having a porous film layer and a non-woven fabric layer, and the like.

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, and organic materials such as resin and inorganic materials such as glass are preferable.
Resins used for the separator include polyolefin resins such as polyethylene, polypropylene and ethylene-propylene copolymers; polyphenylene sulfide resins such as polyphenylene sulfide and polyphenylene sulfide ketones; polyamide resins such as aromatic polyamide resins (aramid resins); polyimide resins. Etc. can be exemplified. One of these resins may be used alone, or two or more of these resins may be used in combination.
The separator preferably contains at least one selected from the group consisting of glass (preferably glass fiber), polyolefin resin, polyamide resin and polyphenylene sulfide resin. Of these, a glass filter (glass filter paper) is more preferable.
The separator may contain an inorganic filler. Examples of the inorganic filler include ceramics such as silica, alumina, zeolite, and titania, talc, mica, and wollastonite. 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, more preferably 20% by mass to 80% by mass.
The shape or size of the separator is not particularly limited, and can be selected according to a desired battery shape or the like.

FIG. 1 is a schematic view showing an example of the configuration of the potassium ion battery 10 according to the present embodiment. However, it goes without saying that the potassium ion battery according to the present embodiment is not limited to this. The potassium ion battery 10 shown in FIG. 1 is a coin-type battery, and includes a battery case 12, a gasket 14, a negative electrode 16, a separator 18, a positive electrode 20, a spacer 22, a spring 24, and a battery case 26 on the positive electrode side. .. The separator 18 is impregnated with the electrolytic solution (not shown) according to the present embodiment.

<Potassium ion capacitor>
The potassium ion capacitor according to the present embodiment includes an electrolytic solution for a potassium ion capacitor according to the present embodiment.
Further, the potassium ion capacitor according to the present embodiment is the same as the conventional lithium ion capacitor, except that the electrolytic solution for the potassium ion capacitor according to the present embodiment is used as the electrolytic solution and potassium ions are used instead of lithium ions. It can be basically manufactured with the configuration of.
Further, each of the above-mentioned constituent members in the potassium-ion battery can be suitably used for the potassium-ion capacitor according to the present embodiment.

Hereinafter, the present invention will be described in more detail with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be appropriately changed as long as they do not deviate from the gist of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.

<Preparation of electrolyte>
Each electrolytic solution was prepared by mixing the following potassium salt compounds and solvents so as to have the concentrations of the following potassium salt compounds. Details of the compounds used are shown below.

Potassium bis (fluorosulfonyl) amide (KFSA): SOLVIONIC SA potassium hexafluorophosphate (KPF ₆ ): Tokyo Chemical Industry Co., Ltd. ethylene carbonate (EC): Kishida Chemical Co., Ltd. propylene carbonate (PC): Made by Kishida Chemical Co., Ltd. Diethyl carbonate (DEC): Made by Kishida Chemical Co., Ltd. Ethylmethyl carbonate (EMC): Made by Kishida Chemical Co., Ltd.

Example 1: KPF ₆ : KFSA (molar ratio 3: 1) EC: DEC (volume ratio 1: 1) solution (potassium salt compound concentration: 1 mol / kg)

Example 2: KPF ₆ : KFSA (molar ratio 9: 1) EC: DEC (volume ratio 1: 1) solution (potassium salt compound concentration: 0.75 mol / kg)

Comparative Example 1: EC: DEC (volume ratio 1: 1) solution of KPF ₆ (potassium salt compound concentration: 0.75 mol / kg)

Comparative Example 2: EC: DEC (volume ratio 1: 1) solution of KFSA (potassium salt compound concentration: 1 mol / kg)

Comparative Example 3: KPF ₆ : KFSA (molar ratio 1: 1) EC: DEC (volume ratio 1: 1) solution (potassium salt compound concentration: 1 mol / kg)

<Charging / discharging characteristics of graphite electrodes>
(1) Preparation of Graphite Electrode 10 parts by mass of sodium polyacrylate (PANa, manufactured by Kishida Chemical Co., Ltd., molecular weight 2 million to 6 million) as a binder was added to water for adjusting the viscosity, and further. 90 parts by mass of graphite (SNO3 manufactured by SEC Carbon Co., Ltd., particle diameter of about 3 μm) as a negative electrode active material was added, and the mixture was mixed and stirred in a dairy pot to obtain a negative electrode mixture slurry. The obtained negative electrode mixture slurry was applied onto an aluminum foil which is a negative electrode current collector, and dried in a vacuum dryer at 150 ° C. to obtain an electrode sheet. A graphite electrode was obtained by punching this electrode sheet into a circle having a diameter of 10 mm with an electrode punching machine. The thickness of the portion of the obtained graphite electrode not including the current collector was 0.02 mm to 0.03 mm, and the mass was 0.5 mg to 1 mg.

(2) Preparation of coin cell The graphite electrode produced above, potassium metal (manufactured by Aldrich) as counter electrode, separator (glass filter, manufactured by ADVANTEC Co., Ltd.), SUS-Al clad 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. A coin cell for charge / discharge measurement was prepared using a 25 mm washer manufactured by Hosen Co., Ltd., and the separator was impregnated with the electrolytic solutions prepared in Examples 1 and 2 and Comparative Examples 1 and 2. The amount of the electrolytic solution used was such that the separator was sufficiently filled with the electrolytic solution (0.15 mL to 0.3 mL).

(3) Charge / discharge measurement Using the produced coin cell, the charge current density and the discharge current density were set to the constant current mode, and the charge / discharge measurement was performed at room temperature (25 ° C.). The current density was set to 25 mA / g, and the charging voltage was constantly charged to 0.0 V. After charging, constant current discharge was repeated until the charging voltage became 0.0 V and the discharge end voltage became 2.0 V. The Coulomb efficiency was calculated by the following formula. The results are shown in FIGS. 2 and 3.
Coulomb efficiency (%) = discharge capacity (mAh) / charge capacity (mAh) x 100

In this embodiment, the negative electrode is charged with potassium and the discharge is depotassified, the positive electrode is charged with depotassium, and the discharge is potassiumized.

As shown in FIGS. 2 and 3, the coin cell using the electrolytic solution of Examples 1 and 2 containing KPF ₆ and KFSA as the potassium salt compound contains the electrolytic solution of Comparative Example 1 containing only KPF ₆ as the potassium salt compound. It showed higher Coulomb efficiency (%) than the coin cell used. Further, in the coin cell using the electrolytic solution of Examples 1 and 2, the decrease in discharge capacity (mAh / g) due to the increase in the charge / discharge cycle was suppressed as compared with the coin cell using the electrolytic solution of Comparative Example 1. ..

<Rate characteristics of graphite electrodes>
The charge / discharge current density was set to the constant current mode, and the charge / discharge measurement was performed at room temperature (25 ° C.). The charging current density was fixed at 0.1C (1C = 279mA / g), and constant current charging was performed until the charging voltage became 0.0V. Discharge current density Change in the order of 0.1C, 0.2C, 0.5C, 1C, 2C, 4C, 6C, 8C, 10C, 15C, 0.1C every 3 cycles, and set the discharge voltage until it reaches 2.0V. Current discharge was performed. The results are shown in FIG.

As shown in FIG. 4, the coin cell using the electrolytic solution of Examples 1 and 2 containing KPF ₆ and KFSA as the potassium salt compound is a coin cell using the electrolytic solution of Comparative Example 1 containing only KPF ₆ as the potassium salt compound. It showed higher rate characteristics than the above.

<Solvent dependence of graphite electrodes>
In the electrolytic solution of Example 2, the above except that the composition of the solvent was changed from EC: DEC (volume ratio 1: 1) to EC: PC (volume ratio 1: 1) and EC: EMC (volume ratio 1: 1). The charge / discharge measurement of the coin cell was performed under the same conditions as above. The results are shown in FIGS. 5 and 6.

As shown in FIGS. 5 and 6, the coin cell using the electrolytic solution containing KPF ₆ and KFSA as the potassium salt compound showed high Coulomb efficiency even when the composition of the solvent was changed.

<Surface analysis of graphite electrodes>
The surface analysis of the graphite electrode was performed after charging and discharging the coin cell 10 times under the same conditions as the charge / discharge test described above. Specifically, analysis was performed by hard X-ray photoelectron spectroscopy (HAXPES) at the beamline BL46XU of the large-scale radiation facility SPring-8. After charging and discharging, the electrodes were washed with DEC in a glove box under an argon atmosphere and sufficiently dried in the glove box. A laminate pack and a transfer vessel were used to move the measurement sample, and the sample was introduced into the device so as not to be exposed to air. The excitation X-ray energy was 7.94 keV, the photoelectron detection angle was 80 °, and the path energy of the analyzer was 200 eV. The binding energy of the obtained spectrum was calibrated with the binding energy of sp ² carbon as 284.6 eV, and the peak intensity was normalized by the area intensity of sp ² carbon. The peak area ratio from each element to sp ² carbon peaks are shown in Table 1.

As shown in Table 1, when the electrolytic solution of Example 2 containing KPF ₆ and KFSA was used, the peak intensity derived from C was increased as compared with the case of using the electrolytic solution of Comparative Example 1 containing only KPF _6. Since it is small, it is considered that organic substances derived from the solvent are less deposited on the surface of the graphite electrode.
Further, when the electrolytic solution of Example 2 containing KPF ₆ and KFSA was used, the increase in peak intensity derived from F and P was small as compared with the case of using the electrolytic solution of Comparative Example 1 containing only KPF _6. Therefore, it is considered that the decomposition of KPF ₆ is suppressed by the addition of KFSA.

From the above results, the reason why the Coulomb efficiency is improved when the electrolytic solution containing KPF ₆ and KFSA is used as compared with the case where the electrolytic solution containing only KPF ₆ is used is that the solvent in the electrolytic solution or KPF _{6 is used.} It is considered that a more stable passivation film is formed with less deposition of the decomposition products on the surface of the graphite electrode. Further, as seen in the peak intensity derived from S, it is considered that the inclusion of the decomposition product of KFSA contained in the electrolytic solution of Example 2 in the passivation film also contributes to the stabilization of the film.

<Aluminum corrosion inhibitory>
Cyclic voltammetry (CV) measurement was performed using each of the electrolytic solutions obtained in Examples 1 and 2 and Comparative Examples 2 and 3. The CV measurement was carried out using an aluminum foil for the working electrode and potassium metal (manufactured by Aldrich) for the counter electrode and the reference electrode, with a scan rate of 0.5 mV / s and a voltage sweep range of 2.0 V to 6.0 V. In CV measurement, an oxidation current is generated as the aluminum corrodes. Therefore, it is considered that the smaller the measured current density, the better the corrosion inhibitory property of aluminum. The results are shown in FIG.

As shown in FIG. _7, KPF 6: KFSA molar ratio of 3: 1 in an exemplary 1 and _KPF 6: the molar ratio of KFSA 9: 1 is the electrolytic solution of Example 2, compared to including KFSA only It is considered that the current density measured by CV measurement is smaller than that of the electrolytic solution of Comparative Example 3 in which the molar ratio of Example 2 and KPF ₆ : KFSA is 1: 1 and aluminum is less likely to corrode.

FIG. 8 shows an electron microscope image of the surface of the aluminum foil after CV measurement. As shown in FIG. _8, KPF 6: the molar ratio of KFSA is 3: 1 in an exemplary 1 and _KPF 6: the molar ratio of KFSA 9: in the electrolytic solution of EXAMPLE 2 is 1, the corrosion of the aluminum foil ( The low-brightness region in the figure) was hardly observed.

<Charging / discharging characteristics of positive electrode>
(1) Preparation of positive electrode K ₂ Mn [Fe (CN) ₆ ], Ketjen Black (KB, manufactured by Lion Specialty Chemicals Co., Ltd.), PVdF (polyvinylidene fluoride resin, manufactured by Kureha Corporation # 1100) ) At a mass ratio of 70:20:10, and then coated on an aluminum foil (manufactured by Hosen Co., Ltd., thickness 0.017 mm) as a positive electrode current collector, and a circular shape with a diameter of 10 mm using an electrode punching machine. The one punched out was used as the positive electrode. The thickness of the portion of the obtained positive electrode not including the current collector was 0.03 mm to 0.04 mm, and the mass was 0.5 mg to 1 mg.

(2) Preparation of Coin Cell A coin cell was produced in the same manner as in the charge / discharge test of graphite electrodes except that the positive electrode produced above was used.

(3) Charge / discharge measurement Using the produced coin cell, the charge current density and the discharge current density were set to the constant current mode, and the charge / discharge measurement was performed at room temperature (25 ° C.). The current density was set to 15 mA / g, and the charging voltage was constant current charging up to 4.3 V. After charging, constant current discharge was repeated until the discharge voltage reached 2.5 V, and the Coulomb efficiency was determined. The results are shown in FIGS. 9 and 10.

As shown in FIGS. 9 and 10, the coin cell using the electrolytic solutions of Examples 1 and 2 containing KPF ₆ and KFSA as the potassium salt compound used the electrolytic solution of Comparative Example 1 containing only KPF ₆ as the potassium salt compound. shows the same high coulombic efficiency and coin cells using, it has been suggested _{K 2 Mn [Fe (CN)} 6] can be applied to the potassium ion battery that a positive electrode.

<Measurement of ionic conductivity>
For each of the electrolytic solutions obtained in Examples and Comparative Examples, the ionic conductivity (mS / cm) was measured while changing the concentration (mol / kg) of the potassium salt compound.
The ionic conductivity was measured at room temperature (25 ° C.) using Eutech CON2700 manufactured by Nikko Hansen Co., Ltd. The measurement results are shown in FIG.

As shown in FIG. 11, the electrolytic solutions of Example 1 and Example 2 containing KPF ₆ and KFSA showed higher ionic conductivity than the electrolytic solution of Comparative Example 1 containing only KPF ₆ . The electrolytic solutions of Comparative Example 1 and Example 2 did not dissolve when the concentration of the potassium salt compound was 1.0 mol / kg.

<Charging / discharging characteristics of potassium-ion batteries>
(1) Preparation of Coin Cell A coin cell was produced in the same manner as described above except that the positive electrode produced above and the graphite electrode produced above were used as the negative electrode.

(2) Charge / discharge measurement Using the produced coin cell, the charge current density and the discharge current density were set to the constant current mode, and the charge / discharge measurement was performed at room temperature (25 ° C.). The current density was set to 15 mA / g, and the charging voltage was constant current charging up to 4.3 V. After charging, constant current discharge was repeated until the discharge voltage reached 1.5 V, and the Coulomb efficiency was determined. The results are shown in FIGS. 12 and 13.

As shown in FIGS. 12 and 13, the potassium ion batteries of Examples 1 and 2 using the electrolytic solution containing KPF ₆ and KFSA as the potassium salt compound are the electrolysis of Comparative Example 1 containing only KPF ₆ as the potassium salt compound. It showed excellent Coulomb efficiency as compared with the potassium ion battery using the liquid.

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 (6)

It contains a solvent and a potassium salt compound, and the potassium salt compound contains potassium hexafluorophosphate and at least one selected from the group consisting of potassium bis (fluorosulfonyl) amide and potassium bis (trifluoromethanesulfonyl) amide. , An electrolytic solution for a potassium ion battery, wherein the ratio of the potassium hexafluorophosphate to the total potassium salt compound is 70 mol% to 95 mol%. A potassium-ion battery comprising the electrolytic solution for a potassium-ion battery according to claim 1. The potassium ion battery according to claim 2, further comprising an electrode containing graphite. The potassium-ion battery according to claim 2 or 3, further comprising a member containing aluminum. It contains a solvent and a potassium salt compound, and the potassium salt compound contains potassium hexafluorophosphate and at least one selected from the group consisting of potassium bis (fluorosulfonyl) amide and potassium bis (trifluoromethanesulfonyl) amide. , An electrolytic solution for a potassium ion capacitor in which the ratio of the potassium hexafluorophosphate to the total potassium salt compound is 70 mol% to 95 mol%. A potassium ion capacitor comprising the electrolytic solution for a potassium ion capacitor according to claim 5.