Potassium ion secondary battery

Abstract

To provide a novel combination of cathode active material capable of charging / discharging and water-based electrolyte as a potassium ion secondary battery.SOLUTION: A potassium ion secondary battery contains a positive electrode active material and a water-based electrolyte. The positive electrode active material contains β-type Ni(OH)2. The water-based electrolyte contains: a solvent with a pH of 7.0 to 13.0 and containing water; and a potassium salt dissolved in the solvent. The potassium salt may be potassium pyrophosphate. Preferably, the potassium salt is dissolved in the solvent at a concentration of 2.0 mol or more per 1.0 kg of the solvent.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to potassium ion secondary batteries. [Background technology]

[0002] Patent Document 1 discloses an aqueous electrolyte solution containing water and potassium pyrophosphate dissolved in water as an electrolyte solution for use in an aqueous potassium ion battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-220294 A Summary of the Invention [Problem to be solved by the invention]

[0004] For example, a cyanide-based compound called Prussian blue has been used as a positive electrode active material for potassium ion secondary batteries. Cyanide-based compounds used as positive electrode active materials have the risk of generating cyanide when the battery malfunctions, and their large molecular structure means that their energy density is insufficient.

[0005] An object of the present disclosure is to provide a novel combination of a positive electrode active material and an aqueous electrolyte solution that can be charged and discharged as a potassium ion secondary battery. [Means for solving the problem]

[0006] The present inventors have found that the above object can be achieved by the following means: Aspect 1 A potassium ion secondary battery, The battery includes a positive electrode active material and an aqueous electrolyte solution, The positive electrode active material is β-type Ni(OH) 2 It contains The aqueous electrolyte has a pH of 7.0 to 13.0 and contains a solvent containing water and a potassium salt dissolved in the solvent. Potassium ion secondary battery. Aspect 2 2. The potassium ion secondary battery of claim 1, wherein the potassium salt is potassium pyrophosphate. Aspect 3 3. The potassium ion secondary battery according to claim 2, wherein the potassium salt is dissolved in the solvent at a concentration of 2.0 mol or more per 1.0 kg of the solvent. Aspect 4 3. The potassium ion secondary battery according to claim 2, wherein the potassium salt is dissolved in the solvent at a concentration of 5.0 mol or more per 1.0 kg of the solvent. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a novel combination of a positive electrode active material and an aqueous electrolyte solution that can be charged and discharged as a potassium ion secondary battery. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing a potassium ion secondary battery 100 according to a first embodiment of the present disclosure. [Diagram 2] FIG. 2 is a graph showing the charge / discharge curves of Example 1. [Diagram 3] FIG. 3 is a graph showing the charge / discharge curves of Example 2. [Figure 4] FIG. 4 is a graph showing the charge / discharge curves of Comparative Example 1. [Diagram 5] FIG. 5 is a graph showing the relationship between potassium pyrophosphate concentration and pH. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0010] The potassium ion secondary battery of the present disclosure has a positive electrode active material and an aqueous electrolyte solution, and the positive electrode active material is β-type Ni(OH) 2 The aqueous electrolyte solution has a pH of 7.0 to 13.0 and contains a solvent containing water, and a potassium salt dissolved in the solvent.

[0011] The potassium ion secondary battery of the present disclosure is a combination of a positive electrode active material and an aqueous electrolyte solution, and has an aspect of being a novel combination that allows charging and discharging of the positive electrode active material.

[0012] Conventionally, almost no positive electrode active material is known that can be charged and discharged in an electrolyte solution in which a potassium compound is dissolved, regardless of whether it is a non-aqueous or aqueous system. The optimal combination of an electrolyte solution and a positive electrode active material that can be charged and discharged is limited to only a very small part of the infinite number of combinations of electrolyte solutions and positive electrode active materials, and even a slight change in the crystal structure of the active material, the components of the electrolyte solution, the potential window, etc. makes it impossible to charge and discharge. In other words, the optimal combination cannot be found unless the electrolyte solution and the positive electrode active material are actually combined and evaluated.

[0013] The technology disclosed herein was developed by finding, through trial and error, a new combination that enables charging and discharging from the infinite number of combinations of electrolytes and positive electrode active materials, and is not something that could easily be conceived of using conventional technology.

[0014] 《Cathode active material》 The potassium ion secondary battery of the present disclosure uses β-type Ni(OH) as a positive electrode active material. 2 Contains:

[0015] β-type Ni(OH) 2 Ni(OH) has a β-type crystal structure. 2 In addition, Ni(OH) has a β-type crystal structure. 2 As long as the β-type crystal structure is maintained, other elements may be dissolved in the material or may be deposited thereon.

[0016] β-type Ni(OH) 2 The shape of the positive electrode active material in a battery may be any shape that is common to the positive electrode active material in a battery. 2 The β-type Ni(OH) may be, for example, particulate. In this case, the particle size is not particularly limited, and an appropriate size may be selected depending on the design of the battery. 2 The primary particle size of β-type Ni(OH) may be 1 nm or more, 5 nm or more, 10 nm or more, or 50 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 10 μm or less. 2 The primary particles may aggregate to form secondary particles. In this case, the particle diameter of the secondary particles is not particularly limited, and may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, or 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less.

[0017] 《Aqueous electrolyte》 The aqueous electrolyte solution in the potassium ion secondary battery of the present disclosure has a pH of 7.0 to 13.0, and contains a solvent containing water, and a potassium salt dissolved in the solvent.

[0018] The pH of the aqueous electrolyte may be 7.0 or more, 8.0 or more, 9.0 or more, or 10.0 or more, and may be 13.0 or less, 12.5 or less, 12.0 or less, or 11.5 or less.

[0019] <solvent> The solvent used in the potassium ion secondary battery of the present disclosure contains water. The solvent may contain water as a main component. That is, based on the total amount of the solvent constituting the electrolyte (100 mol%), water may account for 50 mol% or more, 70 mol% or more, 90 mol% or more, or 95 mol% or more. There is no particular upper limit to the proportion of water in the solvent, and the solvent may be 100 mol%, that is, the entire amount may be water.

[0020] The solvent may consist of only water, but may further contain other components, such as one or more organic solvents selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. The solvent other than water may account for 50 mol % or less, 30 mol % or less, 10 mol % or less, or 5 mol % or less based on the total amount of the solvents constituting the electrolyte (100 mol %).

[0021] <Potassium salt> In the potassium ion secondary battery of the present disclosure, a potassium salt serving as an electrolyte is dissolved in the solvent.

[0022] The potassium salt may be potassium pyrophosphate.

[0023] Here, potassium pyrophosphate being "dissolved" in the solvent does not necessarily mean that it is not completely ionized into potassium ions and pyrophosphate ions in the aqueous electrolyte. + , P 2 O 7 4- , K.P. 2 O 7 3- , K 2 P 2 O 7 2- , K 3 P 2 O 7 - In addition, in the aqueous electrolyte, the "dissolved potassium pyrophosphate" may be a salt of potassium and pyrophosphate (K 4 P 2 O 7 ) derived from (K in water 4 P 2 O 7 For example, the water may not be treated with a potassium ion source (KOH or CH 3 COOK et al.) and a pyrophosphate ion source (H 4 P 2 O 7The aqueous electrolyte solution also includes those in which the above-mentioned ions or associations are formed in water as a result of adding and dissolving the above-mentioned elements separately.

[0024] The concentration of potassium pyrophosphate in the aqueous electrolyte is not particularly limited and may be appropriately selected depending on the intended performance of the battery.

[0025] In the aqueous electrolyte, potassium pyrophosphate may be dissolved in water at a concentration of 2.0 mol or more, or 5.0 mol or more per 1.0 kg of water.

[0026] According to the new findings of the present inventor, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the smaller the hysteresis of the positive electrode active material during charging and discharging, and the more likely it is to obtain high performance as a potassium ion secondary battery. In addition, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the lower the overvoltage, and the more likely it is to develop a good charge and discharge plateau. Furthermore, it is considered that the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the more likely it is that pyrophosphate ions and potassium ions approach each other to form an association. Therefore, for example, it is considered that pyrophosphate ions are easily dragged by potassium ions to move to the negative electrode side when charging a potassium ion secondary battery. It is considered that the pyrophosphate ions that reach the negative electrode are decomposed at the high work function site on the surface of the negative electrode, and a coating is formed on the surface of the negative electrode, and as a result, direct contact between the aqueous electrolyte and the high work function site on the surface of the negative electrode is suppressed, and electrolysis of the aqueous electrolyte is easily suppressed.

[0027] The concentration of "dissolved potassium pyrophosphate" in the aqueous electrolyte can be determined as follows. For example, elements and ions contained in the aqueous electrolyte are determined by elemental analysis or ion analysis, and the potassium ion concentration and pyrophosphate ion concentration in the aqueous electrolyte are determined, and the determined ion concentration is converted into the potassium pyrophosphate concentration. Alternatively, the solvent is removed from the aqueous electrolyte, and the solid content is chemically analyzed and converted into the potassium pyrophosphate concentration.

[0028] In the case of an aqueous electrolyte, the entire amount of potassium ions contained in the electrolyte does not have to be calculated as "dissolved potassium pyrophosphate". That is, the aqueous electrolyte may contain more potassium ions than can be calculated as potassium pyrophosphate. For example, when producing an aqueous electrolyte, a potassium ion source other than the potassium pyrophosphate source (e.g., KOH or CH) is added to water together with the potassium pyrophosphate source. 3 COOK and K 3 PO 4 etc.) is added and dissolved, the aqueous electrolyte contains more potassium ions than can be converted into potassium pyrophosphate.

[0029] The aqueous electrolyte may contain cations other than potassium ions. For example, alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc. may be contained. In addition, the aqueous electrolyte may contain pyrophosphate ions (as described above, P 2 O 7 4- In addition, KP 2 O 7 3- , K 2 P 2 O 7 2- , K 3 P 2 O 7 - The anion may contain anions other than those mentioned above (which may be present in a state combined with a cation, such as anions derived from other electrolytes, which will be described later).

[0030] The aqueous electrolyte solution of the present disclosure may contain other electrolytes dissolved therein. For example, KPF 6 , K.B.F. 4 , K 2 SO 4 , KNO 3 , C.H. 3 COOK, (CF 3 SO 2 ) 2 N.K., K.C.F. 3 SO 3 , (FSO 2 )2 N.K., K. 2 HPO 4 , K.H. 2 PO 4 The other electrolytes may account for 50 mol % or less, 30 mol % or less, 10 mol % or less, 5 mol % or less, or 1 mol % or less based on the total amount of electrolytes dissolved in the electrolytic solution (100 mol %).

[0031] <Other Ingredients> The aqueous electrolyte may contain, in addition to the above-mentioned solvent and electrolyte, an acid or hydroxide for adjusting the pH of the aqueous electrolyte, and may also contain various additives.

[0032] Other configurations The potassium ion secondary battery of the present disclosure is not particularly limited as long as it has the above-mentioned positive electrode active material and aqueous electrolyte solution. The potassium ion secondary battery of the present disclosure is configured so that the above-mentioned positive electrode active material is in contact with the aqueous electrolyte solution.

[0033] FIG. 1 is a schematic diagram showing a configuration of a potassium ion secondary battery 100 according to a first embodiment of the present disclosure. As shown in FIG. 1, the potassium ion secondary battery 100 may include a positive electrode 10, an electrolyte layer 20, and a negative electrode 30. The positive electrode 10 may include a positive electrode active material layer 11 and a positive electrode current collector 12, and the negative electrode 30 may include a negative electrode active material layer 31 and a negative electrode current collector 32. In this case, the positive electrode active material layer 11 may contain the above positive electrode active material. The positive electrode 10, the electrolyte layer 20, and the negative electrode 30 may all contain the above aqueous electrolyte solution.

[0034] <Positive electrode> The positive electrode may have a known configuration, except that it contains the above-mentioned positive electrode active material and aqueous electrolyte solution. For example, the positive electrode may include a positive electrode active material layer and a positive electrode current collector.

[0035] (Cathode active material layer) The positive electrode active material layer contains the positive electrode active material described above, and may further contain an arbitrary conductive assistant, a binder, etc. The thickness of the positive electrode active material layer is not particularly limited, and may be, for example, 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0036] The type of the positive electrode active material contained in the positive electrode active material layer is as described above. The amount of the positive electrode active material contained in the positive electrode active material layer is not particularly limited. For example, based on the entire positive electrode active material layer (100 mass%), the positive electrode active material may be contained in an amount of 20 mass% or more, 40 mass% or more, 60 mass% or more, or 70 mass% or more, or 99 mass% or less, 97 mass% or less, or 95 mass% or less.

[0037] The conductive assistant optionally contained in the positive electrode active material layer can be any of those known as conductive assistants used in potassium ion secondary batteries. For example, carbon materials can be used. Specifically, they are ketjen black (KB), vapor grown carbon fiber (VGCF), acetylene black (AB), carbon nanotube (CNT), carbon nanofiber (CNF), carbon black, coke, graphite, etc. Alternatively, they may be metal materials that can withstand the environment during use of the battery. Only one type of conductive assistant may be used alone, or two or more types may be used in combination. The conductive assistant may be in various shapes, such as powder and fiber. The amount of the conductive assistant contained in the positive electrode active material layer is not particularly limited.

[0038] The binder optionally contained in the positive electrode active material layer may be any binder known to be used in potassium ion secondary batteries. For example, styrene butadiene rubber (SBR)-based binders, carboxymethyl cellulose (CMC)-based binders, acrylonitrile butadiene rubber (ABR)-based binders, butadiene rubber (BR)-based binders, polyvinylidene fluoride (PVDF)-based binders, polytetrafluoroethylene (PTFE)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination. The amount of binder contained in the positive electrode active material layer is not particularly limited.

[0039] (Positive electrode current collector) The positive electrode current collector may be made of a known metal that can be used as a positive electrode current collector for a potassium ion secondary battery. Examples of such metals include metal materials containing at least one element selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. The form of the positive electrode current collector is not particularly limited. It may take various forms such as a foil, a mesh, or a porous form. The above metal may be vapor-deposited or plated on the surface of a substrate.

[0040] <Electrolyte layer> In the potassium ion secondary battery, for example, an electrolyte layer may be disposed between the positive electrode active material layer and the negative electrode active material layer. The electrolyte layer may be composed of a separator and the above-mentioned aqueous electrolyte solution. As the separator, a separator known to be used in secondary batteries (for example, nickel-metal hydride batteries, zinc-air batteries, etc.) may be adopted. For example, the separator may be a hydrophilic one such as a nonwoven fabric made of cellulose. The thickness of the separator is not particularly limited, and may be, for example, 5 μm or more and 1 mm or less.

[0041] <Negative electrode> The negative electrode may have a known structure as a negative electrode for a potassium ion secondary battery, for example, the negative electrode may include a negative electrode active material layer and a negative electrode current collector.

[0042] (Negative electrode active material layer) The negative electrode active material layer includes a negative electrode active material. The negative electrode active material layer may include a conductive assistant and a binder in addition to the negative electrode active material. The thickness of the negative electrode active material layer is not particularly limited, but may be, for example, 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0043] The negative electrode active material contained in the negative electrode active material layer may be selected from active materials having carrier ion charge / discharge potentials lower than those of the positive electrode active material, taking into consideration the potential window of the aqueous electrolyte. For example, potassium-transition metal complex oxide, titanium oxide, Mo 6 S 8 Metal sulfides such as; Elemental sulfur; KTi 2 (PO 4 ) 3 NASICON type compounds, etc. The negative electrode active material may be used alone or in combination of two or more kinds.

[0044] The shape of the negative electrode active material is not particularly limited, and may be, for example, particulate. In this case, the particle size is not particularly limited, and an appropriate size may be selected according to the design of the battery. The negative electrode active material may have a primary particle size of 1 nm or more, 5 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 10 μm or less. In addition, the negative electrode active material may be formed by aggregation of primary particles. In this case, the particle size of the secondary particles is not particularly limited, and may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, and may be 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less.

[0045] The amount of the negative electrode active material contained in the negative electrode active material layer is not particularly limited. For example, the negative electrode active material may be contained in an amount of 20 mass% or more, 40 mass% or more, 60 mass% or more, or 70 mass% or more, or 99 mass% or less, 97 mass% or less, or 95 mass% or less, based on the entire negative electrode active material layer (100 mass%).

[0046] The type of conductive assistant and binder optionally contained in the negative electrode active material layer is not particularly limited, and may be appropriately selected from those exemplified as the conductive assistant and binder optionally contained in the positive electrode active material layer. The amount of conductive assistant and binder contained in the negative electrode active material layer is not particularly limited.

[0047] (Negative electrode current collector) The negative electrode current collector may be made of a known metal that can be used as a negative electrode current collector for a potassium ion secondary battery. Examples of such metals include metal materials containing at least one element selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. In particular, when considering stability in an aqueous electrolyte, the negative electrode current collector may contain at least one element selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, may contain at least one element selected from the group consisting of Ti, Pb, Zn, Sn, Mg, Zr, and In, or may contain Ti. All of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In have low work functions, and it is considered that electrolysis of the aqueous electrolyte is unlikely to occur even if they come into contact with the aqueous electrolyte.

[0048] The form of the negative electrode current collector is not particularly limited, and may be in various forms such as foil, mesh, porous, etc. The above-mentioned metal may be plated or vapor-deposited on the surface of a substrate.

[0049] The surface of the negative electrode current collector may be coated with a carbon material. That is, the negative electrode may further include a negative electrode current collector and a coating layer provided on the surface of the negative electrode current collector on which the aqueous electrolyte is disposed (between the negative electrode current collector and the negative electrode active material layer), and the coating layer may contain a carbon material. Examples of the carbon material include Ketjen Black (KB), vapor grown carbon fiber (VGCF), acetylene black (AB), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon black, coke, graphite, etc.

[0050] The thickness of the coating layer is not particularly limited. The coating layer may be provided on the entire surface of the negative electrode current collector, or may be provided on only a part of the surface.

[0051] The coating layer may contain a binder for binding the carbon materials together and between the carbon materials and the negative electrode current collector.

[0052] When a coating layer containing a carbon material is provided on the surface of a negative electrode current collector, the withstand voltage of the reduction side of the aqueous electrolyte is likely to be improved. Since the edge portion of the carbon material has high reaction activity, it is considered that the adsorption and decomposition of anions contained in the aqueous electrolyte, such as pyrophosphate ions, are likely to occur, and a coating is likely to accumulate. Therefore, when the above-mentioned aqueous electrolyte is used in a potassium ion secondary battery, the edge portion of the carbon material is inactivated, and the electrolysis of the aqueous electrolyte at the edge portion can be suppressed, and as a result, the reduction side potential window of the aqueous electrolyte is considered to be expanded.

[0053] In addition to the above configuration, the potassium ion secondary battery may have obvious configurations for a battery, such as terminals and a battery case.

[0054] A potassium ion secondary battery having the above configuration can be manufactured, for example, by forming a positive electrode active material layer on the surface of a positive electrode current collector in a dry or wet manner to obtain a positive electrode, forming a negative electrode active material layer on the surface of a negative electrode current collector in a dry or wet manner to obtain a negative electrode, and disposing a separator between the positive electrode and the negative electrode and impregnating them with an aqueous electrolyte solution. EXAMPLES

[0055] Examples 1 and 2, and Comparative Example 1 Example 1 β-type Ni(OH) as a positive electrode active material 2 The positive electrode active material mixture was prepared by mixing the conductive additive, acetylene black, and the binder, PVDF, and CMC, in a mass ratio of 85:10:4.5:0.5. The positive electrode active material mixture was uniformly coated on the surface of Ni foil using a doctor blade, and dried to obtain a positive electrode for evaluation. That is, the positive electrode was a positive electrode active material layer containing a positive electrode active material and the like formed on the surface of Ni foil as a positive electrode current collector.

[0056] (Preparation of evaluation cells) An evaluation cell having the following configuration was fabricated. Cell: VM5 (manufactured by EC Frontier) Working electrode: the above positive electrode, opening area 1 cm 2 Counter electrode: Pt mesh Reference pole:Ag / AgCl Aqueous electrolyte: 5.0 mol / kg potassium pyrophosphate aqueous solution

[0057] (Cell evaluation) The evaluation cell was charged and discharged under the following conditions to evaluate the charge and discharge characteristics. Charge / discharge current value: 0.2mA / cm 2 Cutoff voltage: 0.25~0.75V vs. Ag / AgCl Number of cycles: 20 Measurement temperature: 25℃

[0058] Example 2 and Comparative Example 1 The test of Example 2 was carried out in the same manner as in Example 1, except that the aqueous electrolyte solution was a 2.0 mol / kg aqueous solution of potassium pyrophosphate.

[0059] Similarly, the aqueous electrolyte was 28.0 mol / kg CH 3The test of Comparative Example 1 was carried out in the same manner as in Example 1, except that the COOK solution was used as the aqueous solution.

[0060] <result> As is clear from the results shown in Figures 2 to 4, the β-type Ni(OH) 2 It can be seen that when an aqueous electrolyte solution having potassium pyrophosphate dissolved therein is combined with an aqueous electrolyte solution having potassium pyrophosphate dissolved therein (Examples 1 and 2), charging and discharging of the positive electrode active material is possible.

[0061] On the other hand, the electrolyte of the aqueous electrolyte is CH 3 It can also be seen that when the material was changed to COOK (Comparative Example 1), it became difficult to charge and discharge the positive electrode active material.

[0062] 《Reference example》 The relationship between the concentration and pH of the aqueous potassium pyrophosphate solution is shown in FIG.

[0063] When the potassium pyrophosphate concentration in the aqueous potassium pyrophosphate solution was more than 0 mol / kg and not more than 8 mol / kg, the pH of the aqueous potassium pyrophosphate solution was 10.0 to 12.0. [Explanation of symbols]

[0064] 10 positive electrode 11 Cathode active material layer 12 Positive electrode current collector 20 Electrolyte layer 30 negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 100 Potassium ion secondary battery

Claims

1. A potassium ion secondary battery, The battery includes a positive electrode active material and an aqueous electrolyte solution, The positive electrode active material is β-type Ni(OH) ２ It contains The aqueous electrolyte has a pH of 7.0 to 13.0 and contains a solvent containing water and a potassium salt dissolved in the solvent; the potassium salt is potassium pyrophosphate; The potassium salt is dissolved in the solvent at a concentration of 2.0 mol or more per 1.0 kg of the solvent. Potassium ion secondary battery.

2. The potassium ion secondary battery according to claim 1 , wherein the potassium salt is dissolved in the solvent at a concentration of 5.0 mol or more per 1.0 kg of the solvent.

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