Water-based potassium-ion battery

Abstract

To disclose a technology capable of inhibiting elution of Al from a current collector containing Al into an aqueous electrolytic solution when the current collector is used in an aqueous potassium ion battery.SOLUTION: An aqueous potassium ion battery of the present disclosure includes a positive electrode, an aqueous electrolytic solution and a negative electrode. One or both of the positive electrode and the negative electrode have a current collector containing Al. The current collector is in contact with the aqueous electrolytic solution. The aqueous electrolytic solution contains water and potassium pyrophosphate dissolved in the water. The aqueous electrolytic solution has no freezing point at -60°C or higher.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This application discloses an aqueous potassium ion battery. [Background technology]

[0002] Patent Document 1 discloses an aqueous electrolyte used in an aqueous potassium ion battery, which contains water and potassium pyrophosphate dissolved at a concentration of 2 mol or more per kg of water. When an aqueous potassium ion battery is constructed using the aqueous electrolyte disclosed in Patent Document 1, the aqueous electrolyte has a wide reduction potential window, so that decomposition of the aqueous electrolyte on the negative electrode surface is easily suppressed even when the aqueous potassium ion battery is charged and discharged. In addition, Patent Document 1 uses Ti foil or Au foil as an electrode current collector when evaluating the aqueous electrolyte. [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] Conventional aqueous potassium ion batteries have room for improvement in terms of corrosion prevention measures for current collectors. For example, when an aqueous potassium ion battery uses a current collector containing Al, Al is likely to dissolve from the current collector into the aqueous electrolyte during charging and discharging of the battery. When an aqueous potassium ion battery uses a current collector containing Al, a new technology is required to suppress the dissolution of Al from the current collector into the aqueous electrolyte. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> An aqueous potassium ion battery having a positive electrode, an aqueous electrolyte, and a negative electrode, one or both of the positive electrode and the negative electrode has a current collector containing Al; The current collector is in contact with the aqueous electrolyte solution, the aqueous electrolyte solution comprises water and potassium pyrophosphate dissolved in the water, The aqueous electrolyte does not have a freezing point at −60° C. or higher. Water-based potassium-ion battery. <Aspect 2> The aqueous electrolyte does not precipitate salt when cooled from 0°C to -60°C. 2. The aqueous potassium ion battery of embodiment 1. <Aspect 3> At least the positive electrode has the current collector, 3. The aqueous potassium ion battery of embodiment 1 or 2. <Aspect 4> the aqueous electrolyte solution contains the water and the potassium pyrophosphate dissolved in a concentration of 4 mol or more and 7 mol or less per 1 kg of the water; The aqueous potassium ion battery according to any one of embodiments 1 to 3. <Aspect 5> The aqueous electrolyte has a viscosity of 40 mPa·s or more and 350 mPa·s or less at 20°C. The aqueous potassium ion battery according to any one of embodiments 1 to 4. <Aspect 6> A bipolar structure is formed, and a positive electrode active material layer is formed on one surface of the current collector, and a negative electrode active material layer is formed on the other surface of the current collector. The aqueous potassium ion battery according to any one of embodiments 1 to 5. Effect of the Invention

[0006] According to the aqueous potassium ion battery of the present disclosure, elution of Al from the current collector into the aqueous electrolyte is easily suppressed. [Brief description of the drawings]

[0007] [Figure 1]1 shows a schematic diagram of an example of the configuration of an aqueous potassium ion battery. [Diagram 2] 1 shows a schematic diagram of an example of the configuration of an aqueous potassium ion battery. [Diagram 3] 1 shows a time-potential curve when a 0.5 ml aqueous electrolyte solution is used in the electrochemical cell of the embodiment and a constant current is applied to the oxidation side. [Figure 4] 1 shows a time-potential curve when a 5 ml aqueous electrolyte solution is used in the electrochemical cell of the embodiment and a constant current is applied to the oxidation side. [Diagram 5] 1 shows a time-potential curve when 1 ml of an aqueous electrolyte is used in the electrochemical cell of the embodiment and a constant current is applied on the reduction side. [Figure 6] 1 shows a time-potential curve when a 5 ml aqueous electrolyte solution is used and a constant current is applied on the reduction side in the electrochemical cell of the embodiment. [Figure 7A] This shows the charge / discharge curves of the evaluation cell, in which Al foil was used as the current collector. [Figure 7B] This shows the charge / discharge curves of the evaluation cell, in which Ni foil was used as the current collector. [Figure 8A] 1 is a graph showing the relationship between the concentration of an aqueous electrolyte solution and the crystallization peak temperature (freezing point). [Figure 8B] 1 is a graph showing the relationship between the concentration of an aqueous electrolyte solution and crystallization peak intensity. [Figure 9] 1 is a graph showing the relationship between the concentration of an aqueous electrolyte solution and the glass transition temperature. [Figure 10] 1 is a graph showing the relationship between the concentration and viscosity of an aqueous electrolyte solution. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] 1. Water-based potassium-ion battery Hereinafter, a potassium ion battery according to an embodiment will be described with reference to the drawings, but the technology of the present disclosure is not limited to the following embodiment. FIG. 1 shows a schematic configuration of an aqueous potassium ion battery 100 according to an embodiment. As shown in FIG. 1, the aqueous potassium ion battery 100 has a positive electrode 10, an aqueous electrolyte 20, and an anode 30. One or both of the positive electrode 10 and the anode 30 have a current collector containing Al. The current collector is in contact with the aqueous electrolyte 20. The aqueous electrolyte 20 has water and potassium pyrophosphate dissolved in the water. The aqueous electrolyte 20 does not have a freezing point at -60°C or higher.

[0009] 1.1 Positive electrode Any known positive electrode for an aqueous potassium ion battery can be used as the positive electrode 10. As shown in FIG.

[0010] 1.1.1 Cathode active material layer The positive electrode active material layer 11 includes a positive electrode active material. The positive electrode active material layer 11 is impregnated with an aqueous electrolyte solution 20. The positive electrode active material layer 11 may include a conductive assistant, a binder, and the like in addition to the positive electrode active material. The positive electrode active material layer 11 may also include various other additives. The content of each component in the positive electrode active material layer 11 may be appropriately determined according to the intended battery performance. For example, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more, or may be 100% by mass or less, or 90% by mass or less, assuming that the entire positive electrode active material layer 11 (total solid content) is 100% by mass. The shape of the positive electrode active material layer 11 is not particularly limited, and may be, for example, a sheet-like positive electrode active material layer having a substantially flat surface. The thickness of the positive electrode active material layer 11 is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may be 2 mm or less, 1 mm or less, or 500 μm or less.

[0011] The positive electrode active material may be any material capable of functioning as a positive electrode active material for an aqueous potassium ion battery. The positive electrode active material has a higher charge / discharge potential than the negative electrode active material described below, and may be appropriately selected in consideration of the potential window of the aqueous electrolyte 20 described below. For example, the positive electrode active material may contain K element. Specific examples include oxides and polyanions containing K element. More specifically, the positive electrode active material may be a composite oxide of potassium and a transition metal. The composite oxide may be a potassium cobalt composite oxide (KCoO2, etc.), a potassium nickel composite oxide (KNiO2, etc.), a potassium nickel titanium composite oxide (KNi 1 / 2 Ti 1 / 2 O2, etc.), potassium nickel manganese composite oxide (KNi 1 / 2 Mn 1 / 2 O2, KNi 1 / 3 Mn 2 / 3 O2, etc.), potassium manganese oxide (KMnO2, KMn2O4, etc.), potassium iron manganese oxide (K 2 / 3 Fe 1 / 3 Mn 2 / 3 O2, etc.), potassium nickel cobalt manganese composite oxide (KNi 1 / 3 Co 1 / 3 Mn 1 / 3 The positive electrode active material may be at least one selected from potassium titanate, TiO2, sulfur (S), etc., which exhibit a charge / discharge potential higher than that of the negative electrode active material described below. The positive electrode active material may be one that deintercalates K by intercalation, or one that deintercalates K by a conversion reaction, an alloying reaction, or the like. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination.

[0012] The shape of the positive electrode active material may be any shape that can function as a positive electrode active material of a battery. The positive electrode active material may be, for example, particulate. The positive electrode active material may be solid, hollow, voided, or porous. The positive electrode active material may be primary particles, or secondary particles formed by agglomeration of a plurality of primary particles. The average particle diameter D50 of the positive electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. The average particle diameter D50 in the present application is the particle diameter (median diameter) at 50% cumulative value in the volume-based particle size distribution obtained by the laser diffraction / scattering method.

[0013] Examples of the conductive assistant that may be included in the positive electrode active material layer 11 include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metal materials that are poorly soluble in the electrolyte, including nickel, titanium, aluminum, and stainless steel. The conductive assistant may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive assistant may be used alone, or two or more types may be used in combination.

[0014] Examples of binders that can be included in the positive electrode active material layer 11 include butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0015] 1.1.2 Positive electrode current collector As shown in FIG. 1, the positive electrode 10 may include a positive electrode current collector 12 in contact with the positive electrode active material layer 11. The positive electrode current collector 12 is in contact with the aqueous electrolyte 20. Any positive electrode current collector that can function as a positive electrode current collector of an aqueous potassium ion battery can be used as the positive electrode current collector 12. When the negative electrode current collector 32 described later contains Al, the positive electrode current collector 12 may contain Al or may not contain Al. When the negative electrode current collector 32 described later does not contain Al, the positive electrode current collector 12 contains Al. As described later, the elution of Al from the current collector into the aqueous electrolyte is particularly likely to occur on the positive electrode side, which is at an oxidation potential. According to the potassium ion battery 100 of the present disclosure, even if the positive electrode current collector 12 contains Al, the elution of Al from the positive electrode current collector 12 into the aqueous electrolyte 20 can be suppressed. That is, in the potassium ion battery 100, at least the positive electrode 10 may have a current collector containing Al.

[0016] When the positive electrode current collector 12 contains Al, the positive electrode current collector 12 may be entirely made of Al, or at least a part of the surface of the positive electrode current collector 12 may be made of Al. For example, the positive electrode current collector 12 may be made of Al foil, or may be a metal foil or a substrate whose surface is coated with Al. The positive electrode current collector 12 may have Al present on at least a part of the surface that contacts the aqueous electrolyte solution 20, or may have Al present over the entire surface that contacts the aqueous electrolyte solution 20.

[0017] The positive electrode collector 12 may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The positive electrode collector 12 may be made of a metal foil or a metal mesh. In particular, a metal foil is excellent in terms of ease of handling. The positive electrode collector 12 may be made of a plurality of foils. Examples of the metal material constituting the positive electrode collector 12 include those 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, as described above, the positive electrode collector 12 preferably contains Al. The positive electrode collector 12 may be a metal foil or a substrate on which the above metal is plated or vapor-deposited. In addition, when the positive electrode collector 12 is made of a plurality of metal foils, some layer may be present between the plurality of metal foils. The thickness of the positive electrode collector 12 is not particularly limited. For example, it may be 0.1 μm or more, or 1 μm or more, and may be 1 mm or less, or 100 μm or less.

[0018] 1.2 Aqueous electrolyte The aqueous electrolyte solution 20 includes water and potassium pyrophosphate dissolved in the water. The aqueous electrolyte solution 20 is in contact with the above-mentioned positive electrode current collector 12, is contained in the above-mentioned positive electrode active material layer 11, is in contact with the below-described negative electrode current collector 32, is contained in the below-described negative electrode active material layer 31, and can be held by the separator 40 between the positive electrode 10 and the negative electrode 30.

[0019] 1.2.1 Solvents The aqueous electrolyte 20 contains water as a solvent. The solvent contains water as a main component. That is, based on the total amount of the solvent constituting the aqueous electrolyte (100 mol%), water accounts for 50 mol% or more and 100 mol% or less. Water may account for 70 mol% or more, 90 mol% or more, or 95 mol% or more of the total amount of the solvent. Meanwhile, there is no particular upper limit to the proportion of water in the solvent. The solvent may consist of only water (100 mol% water).

[0020] The solvent may contain a solvent other than water in addition to water, within a range in which the above-mentioned problems can be solved, for example, from the viewpoint of forming a solid electrolyte interphase (SEI) on the surface of the active material. Examples of the solvent other than water include 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] 1.2.2 Electrolytes An electrolyte is dissolved in the aqueous electrolyte solution 20, and the electrolyte can be dissociated into cations and anions in the aqueous electrolyte solution 20. In the aqueous electrolyte solution 20, the cations and anions may be in close proximity to each other to form an association.

[0022] 1.2.2.1 Potassium pyrophosphate The aqueous electrolyte 20 contains potassium pyrophosphate dissolved in the water as an electrolyte. + , P2O7 4- , KP2O7 3- , K2P2O7 2- , K3P2O7 - In the aqueous electrolyte solution 20, the ions and associations contained in the aqueous electrolyte solution 20 are converted into potassium pyrophosphate to determine the "concentration of potassium pyrophosphate dissolved in water."

[0023] In the aqueous potassium ion battery 100 of the present disclosure, the components and concentrations in the aqueous electrolyte 20 are determined so that "the aqueous electrolyte 20 does not have a freezing point at -60°C or higher". According to the new knowledge of the present inventor, in order to achieve the condition that "the aqueous electrolyte 20 does not have a freezing point at -60°C or higher", the concentration of potassium pyrophosphate in the aqueous electrolyte 20 should be higher than that of the conventional solution. According to the new knowledge of the present inventor, when the freezing point of the aqueous electrolyte is adjusted by dissolving only potassium pyrophosphate in water, the freezing point of the aqueous electrolyte decreases as the concentration of potassium pyrophosphate relative to water increases, and when the concentration of potassium pyrophosphate in the aqueous electrolyte becomes a certain level or higher, the freezing point essentially disappears. That is, when the concentration of potassium pyrophosphate in the aqueous electrolyte is a certain level or higher, the aqueous electrolyte does not have a freezing point at -60°C or higher. For example, when the aqueous electrolyte 20 contains the water and the potassium pyrophosphate dissolved at a concentration of 4 mol or more per kg of water, the freezing point of the aqueous electrolyte 20 disappears, and the aqueous electrolyte 20 does not have a freezing point at −60° C. or higher. However, when other components are dissolved in the water in addition to potassium pyrophosphate in the aqueous electrolyte 20, the minimum concentration of potassium pyrophosphate at which the freezing point of the aqueous electrolyte disappears may be lower than 4 mol. In this regard, the concentration of potassium pyrophosphate per kg of water in the aqueous electrolyte 20 may be, for example, 2.5 mol or more, 3.0 mol or more, 3.5 mol or more, or 4.0 mol or more.

[0024] As described above, in order to achieve the condition that "the aqueous electrolyte 20 does not have a freezing point at -60°C or higher", it is effective to adjust the concentration of potassium pyrophosphate in the aqueous electrolyte 20 to a higher concentration than before. Specifically, when the aqueous electrolyte 20 contains the water and the potassium pyrophosphate dissolved at a concentration of 4 mol or more per 1 kg of the water, the condition is easily met. Although the upper limit of the concentration of potassium pyrophosphate in the aqueous electrolyte 20 is not particularly limited, if the concentration is excessively high, the viscosity of the aqueous electrolyte 20 increases excessively, and the ionic conductivity of the aqueous electrolyte 20 may decrease. Considering the achievement of the condition that "the aqueous electrolyte 20 does not have a freezing point at -60°C or higher" and the viscosity of the aqueous electrolyte 20 being in an appropriate range, the aqueous electrolyte 20 may have the water and the potassium pyrophosphate dissolved at a concentration of 4 mol to 7 mol per 1 kg of the water. The concentration may be 6 mol or less.

[0025] The aqueous electrolyte solution 20 may contain potassium ions as cations. In the aqueous electrolyte solution 20, the entire amount of potassium ions contained in the aqueous electrolyte solution 20 does not have to be calculated as "dissolved potassium pyrophosphate". In other words, the aqueous electrolyte solution 20 may contain more potassium ions than the concentration that can be calculated as potassium pyrophosphate. For example, when producing the aqueous electrolyte solution 20, a potassium ion source other than K4P2O7 (e.g., KOH, CH3COOK, K3PO4, K5P3O7) may be added to water together with K4P2O7. 10 , K6P4O 13 , K7P5O 16 When potassium ions such as (KPO3)n, (KPO3)n, etc. are added and dissolved in the aqueous electrolyte 20, the aqueous electrolyte 20 contains more potassium ions than the concentration that can be converted into potassium pyrophosphate. The aqueous electrolyte 20 may contain other cations as long as the above-mentioned problem can be solved. For example, the aqueous electrolyte 20 may contain alkali metal ions, alkaline earth metal ions, transition metal ions, etc. other than potassium ions.

[0026] The aqueous electrolyte 20 contains pyrophosphate ions (as described above, P2O74- In addition, KP2O7 3- , K2P2O7 2- , K3P2O7 - The aqueous electrolyte 20 may contain pyrophosphate ions, which may be present in a state of being bound to a cation, for example. In the aqueous electrolyte 20, the entire pyrophosphate ions contained in the aqueous electrolyte 20 may not be converted into "dissolved potassium pyrophosphate". That is, the aqueous electrolyte 20 may contain more pyrophosphate ions than the concentration that can be converted into potassium pyrophosphate. For example, when the aqueous electrolyte 20 is produced, if a pyrophosphate ion source other than K4P2O7 (e.g., H4P2O7, etc.) is added to water together with K4P2O7 and dissolved therein, the aqueous electrolyte 20 will contain more pyrophosphate ions than the concentration that can be converted into potassium pyrophosphate. The aqueous electrolyte 20 may contain other anions to the extent that the above problem can be solved. For example, it may contain anions derived from other electrolytes, which will be described later.

[0027] 1.2.2.2 Other components that may be contained in aqueous electrolytes The aqueous electrolyte 20 may contain other electrolytes, such as KPF6, KBF4, K2SO4, KNO3, CH3COOK, (CF3SO2)2NK, KCF3SO3, (FSO2)2NK, K2HPO4, KH2PO4, KPO3, and K5P3O. 10 , K6P4O 13 , K7P5O 16 , (KPO3)n, etc. The other electrolytes may account for 50 mol % or less, 30 mol % or less, or 10 mol % or less of the total amount of electrolytes dissolved in the electrolytic solution (100 mol %).

[0028] In addition to the electrolytes described above, the aqueous electrolyte solution 20 may contain an acid, a hydroxide, or the like for adjusting the pH of the aqueous electrolyte solution 20. In addition, various additives may be included.

[0029] 1.2.3 Freezing point As described above, the aqueous electrolyte 20 does not have a freezing point at -60°C or higher. Here, the presence or absence of the "freezing point" of the aqueous electrolyte 20 is confirmed by differential scanning calorimetry (DSC). The sweep rate by DSC is 5°C / min for both the temperature drop and temperature rise, and the sweep range is from room temperature to -120°C, and then to 40°C. The atmosphere in the DSC is an inert gas atmosphere such as Ar, and the pressure is equivalent to atmospheric pressure. However, since a sealed aluminum container is used for the evaluation, the atmosphere inside the container is air sealed under atmospheric pressure. When the aqueous electrolyte is measured under the above conditions, and the crystallization peak temperature (freezing point temperature) is not confirmed at -60°C or higher, the aqueous electrolyte is deemed to "not have a freezing point at -60°C or higher." The aqueous electrolyte 20 may not have a freezing point at -80°C or higher, may not have a freezing point at -100°C or higher, or may not have a freezing point at -120°C or higher. In the aqueous potassium ion battery 100 of the present disclosure, in order to achieve the condition that "the aqueous electrolyte 20 does not have a freezing point at -60°C or higher", it is effective to set the pyrophosphoric acid concentration in the aqueous electrolyte 20 to a high concentration (for example, a high concentration of 4 mol or more per kg of water) as described above. Since the aqueous electrolyte 20 does not have a freezing point at -60°C, Al contained in the current collector is less likely to dissolve into the aqueous electrolyte 20, as described later. Furthermore, since the aqueous electrolyte 20 does not have a freezing point at -60°C, the aqueous potassium ion battery 100 can be used even at extremely low temperatures. That is, the aqueous potassium ion battery 100 of the present disclosure operates appropriately even in cold regions.

[0030] 1.2.4 Other properties As long as the aqueous electrolyte solution 20 contains the above-mentioned solvent and electrolyte and satisfies the above-mentioned requirements regarding the freezing point, there are no particular limitations on other properties of the aqueous electrolyte solution 20. An example of other properties of the aqueous electrolyte solution 20 will be described below.

[0031] 1.2.4.1 Presence or absence of salt precipitation It is preferable that the aqueous electrolyte 20 does not cause salt precipitation when cooled from 0°C to -60°C. In this way, since the aqueous electrolyte 20 does not cause salt precipitation due to temperature change, stable ion conduction is possible even at low temperatures. For example, the aqueous potassium ion battery 100 can be used even at extremely low temperatures such as in cold regions. As described above, the aqueous electrolyte 20 contains water and potassium pyrophosphate dissolved in the water. According to the findings of the present inventor, the saturated solubility of potassium pyrophosphate in water has little temperature dependency and hardly changes at low temperatures below 0°C. In this regard, even if potassium pyrophosphate is dissolved at a high concentration (for example, a high concentration of 4 mol or more per kg of water) in the aqueous electrolyte 20 at 0°C and the aqueous electrolyte 20 is cooled from 0°C to -60°C, precipitation of potassium pyrophosphate in the aqueous electrolyte 20 does not substantially occur.

[0032] 1.2.4.2 Viscosity If the viscosity of the aqueous electrolyte solution 20 is too high, the ionic conductivity of the aqueous electrolyte solution 20 may decrease. On the other hand, if potassium pyrophosphate is dissolved in the aqueous electrolyte solution 20 at a high concentration, the aqueous electrolyte solution 20 may have a viscosity of a certain level or higher. From the above viewpoints, the aqueous electrolyte solution 20 may have a viscosity of 40 mPa·s or more and 350 mPa·s or less at 20° C. The viscosity may be 300 mPa·s or less, 250 mPa·s or less, or 200 mPa·s or less.

[0033] 1.2.4.3 pH The pH of the aqueous electrolyte 20 is not particularly limited. However, if the pH is too high, the oxidation side potential window of the aqueous electrolyte may be narrowed. In this respect, the pH of the aqueous electrolyte may be 4 or more and 13 or less. The pH may be 5 or more, 6 or more, or 7 or more, or may be 12 or less.

[0034] 1.3 Negative electrode Any known negative electrode for an aqueous potassium ion battery can be used as the negative electrode 30. As shown in FIG.

[0035] 1.3.1 Negative electrode active material layer The negative electrode active material layer 31 includes a negative electrode active material. The negative electrode active material layer 31 is impregnated with the aqueous electrolyte solution 20. The negative electrode active material layer 31 may include a conductive assistant, a binder, and the like in addition to the negative electrode active material. The negative electrode active material layer 31 may also include various other additives. The content of each component in the negative electrode active material layer 31 may be appropriately determined according to the intended battery performance. For example, the content of the negative electrode active material may be 40 mass% or more, 50 mass% or more, 60 mass% or more, or 70 mass% or more, or may be 100 mass% or less, or 90 mass% or less, assuming that the entire negative electrode active material layer 31 (total solid content) is 100 mass%. The shape of the negative electrode active material layer 31 is not particularly limited, and may be, for example, a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer 31 is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may be 2 mm or less, 1 mm or less, or 500 μm or less.

[0036] The negative electrode active material may be any material capable of functioning as a negative electrode active material for an aqueous potassium ion battery. The negative electrode active material has a lower charge / discharge potential than the above-mentioned positive electrode active material, and may be appropriately selected in consideration of the potential window of the above-mentioned aqueous electrolyte 20. For example, the negative electrode active material may be a potassium-transition metal composite oxide; titanium oxide; metal sulfide such as Mo6S8; elemental sulfur; KTi2(PO4)3; NASICON-type compound, etc. The negative electrode active material may be one that de-inserts K by intercalation, or one that de-inserts K by a conversion reaction, an alloying reaction, or the like. Only one type of negative electrode active material may be used alone, or two or more types may be used in combination.

[0037] The shape of the negative electrode active material may be any shape that can function as a negative electrode active material of a battery. The negative electrode active material may be, for example, particulate. The negative electrode active material may be solid, hollow, voided, or porous. The negative electrode active material may be primary particles, or may be secondary particles formed by agglomeration of a plurality of primary particles. The average particle diameter D50 of the negative electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

[0038] Examples of the conductive assistant that may be included in the negative electrode active material layer 31 include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metal materials that are poorly soluble in the electrolyte, including nickel, titanium, aluminum, and stainless steel. The conductive assistant may be, for example, particulate or fibrous, and the size thereof is not particularly limited. Only one type of conductive assistant may be used alone, or two or more types may be used in combination.

[0039] Examples of binders that can be included in the negative electrode active material layer 31 include butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0040] 1.3.2 Negative electrode current collector As shown in FIG. 1, the negative electrode 30 may include a negative electrode current collector 32 in contact with the negative electrode active material layer 31. The negative electrode current collector 32 is in contact with the aqueous electrolyte solution 20. Any negative electrode current collector that can function as a negative electrode current collector for an aqueous potassium ion battery can be used as the negative electrode current collector 32. When the above-mentioned positive electrode current collector 12 contains Al, the negative electrode current collector 32 may contain Al or may not contain Al. When the above-mentioned positive electrode current collector 12 does not contain Al, the negative electrode current collector 32 contains Al.

[0041] When the negative electrode current collector 32 contains Al, the negative electrode current collector 32 may be entirely made of Al, or at least a part of its surface may be made of Al. For example, the negative electrode current collector 32 may be made of Al foil, or may be a metal foil or a substrate whose surface is coated with Al. The negative electrode current collector 32 may have Al present on at least a part of its surface in contact with the aqueous electrolyte solution 20, or may have Al present over the entire surface in contact with the aqueous electrolyte solution 20.

[0042] The negative electrode current collector 32 may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The negative electrode current collector 32 may be made of a metal foil or a metal mesh. In particular, a metal foil is excellent in terms of ease of handling. The negative electrode current collector 32 may be made of a plurality of foils. Examples of the metal material constituting the negative electrode current collector 32 include those 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, the negative electrode current collector 32 preferably contains at least one element selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, and as described above, it is preferable that the negative electrode current collector 32 contains Al. Al, Ti, Pb, Zn, Sn, Mg, Zr and In all have low work functions, and it is considered that even if the negative electrode collector 32 contacts the aqueous electrolyte solution 20 at a reduction potential, electrolysis of the aqueous electrolyte solution 20 is unlikely to occur. The negative electrode collector 32 may be a metal foil or a substrate on which the above metal is plated or vapor-deposited. In addition, when the negative electrode collector 32 is made of a plurality of metal foils, some layer may be present between the plurality of metal foils. The thickness of the negative electrode collector 32 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0043] 1.4 Other configurations In addition to the basic configuration described above, the aqueous potassium ion battery 100 may have other configurations, for example, as described below.

[0044] 1.4.1 Separators As described above, in the aqueous potassium ion battery 100, the separator 40 may be present between the positive electrode 10 and the negative electrode 30. The separator 40 may be a separator used in conventional aqueous electrolyte batteries (nickel-metal hydride batteries, zinc-air batteries, etc.). For example, the separator 40 may be a hydrophilic separator such as a nonwoven fabric made of cellulose. The thickness of the separator 40 is not particularly limited, and may be, for example, 5 μm or more and 1 mm or less.

[0045] 1.4.2 Bipolar structure As described above, in the aqueous potassium ion battery 100, both the positive electrode current collector 12 and the negative electrode current collector 32 may contain Al. In this regard, in the aqueous potassium ion battery 100, a current collector containing Al may be used as a bipolar current collector that serves both the positive electrode current collector 12 and the negative electrode current collector 32. That is, the positive electrode 10 and the negative electrode 30 may share one current collector. FIG. 2 shows an example of a bipolar structure. As shown in FIG. 2, the aqueous potassium ion battery 100 may have a bipolar structure, and the positive electrode active material layer 11 may be formed on one surface of a current collector 50 (a bipolar current collector that functions as both the positive electrode current collector 12 and the negative electrode current collector 32) containing Al, and the negative electrode active material layer 31 may be formed on the other surface of the current collector 50. In this case, the Al-containing current collector 50 may be one that does not have liquid permeability, that is, one that does not allow the aqueous electrolyte solution 20 to pass from the positive electrode active material layer 11 through the current collector 50 to the negative electrode active material layer 31, and vice versa.

[0046] 1.4.3 Terminals etc. In addition to the above configuration, the aqueous potassium ion battery 100 may include terminals, a battery case, etc. Other configurations will be obvious to those skilled in the art who have read this application, and therefore will not be described here.

[0047] 1.5 Manufacturing method of aqueous potassium ion battery The aqueous potassium ion battery 100 of the present disclosure can be manufactured, for example, as follows.

[0048] 1.5.1 Manufacturing method of aqueous electrolyte The aqueous electrolyte solution 20 can be produced, for example, by mixing water with K4P2O7. Alternatively, the aqueous electrolyte solution 20 can be produced by mixing water with a potassium ion source and a pyrophosphate ion source. There is no particular limitation on the mixing means, and any known mixing means can be used. Simply filling a container with water and potassium pyrophosphate and leaving it to stand will cause them to mix with each other, and the aqueous electrolyte solution 20 will ultimately be obtained.

[0049] 1.5.2 Positive electrode fabrication The positive electrode 10 is manufactured, for example, as follows. The positive electrode active material constituting the positive electrode active material layer 11 is dispersed in a solvent to obtain a positive electrode mixture paste (slurry). The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The positive electrode mixture paste (slurry) is applied to the surface of the positive electrode current collector 12 using a doctor blade or the like, and then dried to form the positive electrode active material layer 11 on the surface of the positive electrode current collector 12, thereby forming the positive electrode 10. As the application method, in addition to the doctor blade method, an electrostatic application method, a dip coating method, a spray coating method, or the like can also be adopted.

[0050] 1.5.3 Negative electrode fabrication The negative electrode 30 is manufactured, for example, as follows. The negative electrode active material constituting the negative electrode active material layer 31 is dispersed in a solvent to obtain a negative electrode mixture paste (slurry). The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The negative electrode mixture paste (slurry) is applied to the surface of the negative electrode current collector 32 using a doctor blade or the like, and then dried to form the negative electrode active material layer 31 on the surface of the negative electrode current collector 32, thereby forming the negative electrode 30. As the application method, in addition to the doctor blade method, an electrostatic application method, a dip coating method, a spray coating method, or the like can also be adopted.

[0051] 1.5.4 Storage in a battery case, etc. The aqueous electrolyte 20, the positive electrode 10, and the negative electrode 30 are housed in a battery case to form an aqueous potassium ion battery 100. For example, a separator 40 is sandwiched between the positive electrode 10 and the negative electrode 30 to obtain a laminate having a positive electrode current collector 12, a positive electrode active material layer 11, the separator 40, a negative electrode active material layer 31, and a negative electrode current collector 32 in this order. Other members such as terminals are attached to the laminate as necessary. The laminate is housed in a battery case, and the battery case is filled with the aqueous electrolyte 20. The laminate is immersed in the aqueous electrolyte 20, and the laminate and the electrolyte are sealed in the battery case to obtain an aqueous potassium ion battery 100.

[0052] 1.6 Effects of aqueous potassium ion batteries According to the aqueous potassium ion battery 100 of the present disclosure, the elution of Al from the current collector into the aqueous electrolyte solution 20 can be suppressed by the following advantageous effects.

[0053] 1.6.1 Effects of Al-containing positive electrode current collector During charging and discharging of the battery, the potential of the positive electrode becomes the oxidation potential. Therefore, the Al contained in the positive electrode current collector is in a state where it is easy to release electrons and dissolve. Specifically, the Al contained in the positive electrode current collector dissolves into the aqueous electrolyte while coordinating with the anions and water molecules contained in the aqueous electrolyte.

[0054] In the potassium ion battery 100 of the present disclosure, as described above, the aqueous electrolyte 20 does not have a freezing point at −60° C. or higher. That is, in the aqueous electrolyte 20, H2O molecules cannot form a crystallization network with each other, and there are few free H2O molecules that can be solidified, and there are also few free hydroxide ions. Therefore, Al that has dissolved from the current collector into the aqueous electrolyte 20 is converted into H2O or OH. - Diffusion through the aqueous electrolyte solution 20 is unlikely to occur, and Al dissolved into the aqueous electrolyte solution 20 is likely to remain in the vicinity of the current collector. On the other hand, as described above, potassium pyrophosphate is dissolved in the aqueous electrolyte solution 20, that is, anions derived from potassium pyrophosphate, such as pyrophosphate ions, may be present in the aqueous electrolyte solution 20. Therefore, in the potassium ion battery 100 of the present disclosure, Al contained in the positive electrode current collector 12 is likely to be coordinated with anions derived from potassium pyrophosphate and dissolved out during charging and discharging of the battery. Here, for example, aluminum pyrophosphate has an extremely low solubility in the aqueous electrolyte solution 20. Therefore, Al coordinated to anions derived from potassium pyrophosphate is quickly precipitated as a solid. In other words, an insoluble or poorly soluble Al compound is precipitated near the surface of the positive electrode current collector 12, and the Al compound adheres to the surface of the positive electrode current collector 12, forming a protective film (passive film) on the surface. As a result, in the potassium ion battery 100 of the present disclosure, the protective film can suppress the elution of Al from the positive electrode current collector 12 into the aqueous electrolyte solution 20.

[0055] 1.6.2 Effects of containing Al in the negative electrode current collector During charging and discharging of the battery, the potential of the negative electrode becomes the reduction potential. Therefore, when the aqueous electrolyte in contact with the negative electrode is electrolyzed, hydroxide ions are generated, and the pH of the aqueous electrolyte near the negative electrode tends to increase. When the pH of the aqueous electrolyte near the negative electrode increases, the solubility of Al in the aqueous electrolyte increases, and the Al contained in the negative electrode current collector becomes more likely to dissolve into the aqueous electrolyte.

[0056] In contrast, in the potassium ion battery 100 of the present disclosure, as described above, the aqueous electrolyte 20 does not have a freezing point at −60° C. or higher. That is, in the aqueous electrolyte 20, there is little free water that can be solidified, in other words, there is little water that can be electrolyzed in the first place. Therefore, the elution of Al contained in the negative electrode current collector 32 is difficult to proceed. In addition, as described above, potassium pyrophosphate is dissolved in the aqueous electrolyte 20, that is, anions derived from potassium pyrophosphate, such as pyrophosphate ions, may be present in the aqueous electrolyte 20. Therefore, in the potassium ion battery 100 of the present disclosure, even if Al contained in the negative electrode current collector 32 is eluted into the aqueous electrolyte 20 during charging and discharging of the battery, it quickly coordinates with anions derived from potassium pyrophosphate to become an Al compound and precipitate as a solid. In other words, an insoluble or poorly soluble Al compound is precipitated near the surface of the negative electrode current collector 32, and the Al compound is attached to the surface of the negative electrode current collector 32, forming a protective film (passive film) on the surface. As a result, in the potassium ion battery 100 of the present disclosure, the elution of Al from the negative electrode current collector 32 into the aqueous electrolyte solution 20 can be further suppressed by the protective film.

[0057] 1.6.3 Other Effects In aqueous electrolyte batteries, a current collector containing Ti or Ni is used to prevent corrosion of the current collector (for example, Patent Document 1). It has been thought that it is difficult to use metals other than these because they dissolve at the positive electrode potential, for example. However, since Ti and Ni are expensive, alternative technologies using cheaper metals are required to widely disseminate aqueous electrolyte batteries. In this regard, in the aqueous potassium ion battery 100 disclosed herein, a current collector containing Al is used to reduce the cost of the entire battery, while the aqueous electrolyte 20 described above is used to suppress the dissolution of Al from the current collector into the aqueous electrolyte.

[0058] 2. Method to suppress the elution of Al from the current collector into the aqueous electrolyte of aqueous potassium ion batteries The technology of the present disclosure also has an aspect of being a method for suppressing the elution of Al from a current collector of an aqueous potassium ion battery into an aqueous electrolyte. That is, the method of the present disclosure is characterized in that, in an aqueous potassium ion battery, a current collector containing Al is used, and an aqueous electrolyte that satisfies the following requirements (1) and (2) is used. Details of the aqueous electrolyte and the battery configuration are as described above. (1) The aqueous electrolyte solution contains water and potassium pyrophosphate dissolved in the water. (2) The aqueous electrolyte does not have a freezing point at or above -60°C.

[0059] 3. Vehicles with aqueous potassium-ion batteries As described above, the aqueous potassium ion battery of the present disclosure can suppress the elution of Al from the current collector into the aqueous electrolyte. That is, the deterioration of the battery is easily suppressed. Such an aqueous potassium ion battery can be suitably used in at least one vehicle selected from, for example, a hybrid vehicle (HEV), a plug-in hybrid vehicle (PHEV), and an electric vehicle (BEV). That is, the technology of the present disclosure also has an aspect of a vehicle having an aqueous potassium ion battery, the aqueous potassium ion battery having a positive electrode, an aqueous electrolyte, and a negative electrode, one or both of the positive electrode and the negative electrode having an Al-containing current collector, the aqueous electrolyte having water and potassium pyrophosphate dissolved in the water, and the aqueous electrolyte having no freezing point at -60°C or higher. Details of the aqueous electrolyte and the battery configuration are as described above. EXAMPLES

[0060] The technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0061] 1. Preparation of Aqueous Electrolyte K4P2O7 was dissolved in 1 kg of pure water to a predetermined concentration (0.5 to 5 mol) to obtain an aqueous electrolyte for evaluation.

[0062] 2. Electrochemical Cell Fabrication In an electrochemical cell (VM4, manufactured by EC Frontier), an Al foil was used as the working electrode, a Pt mesh was used as the counter electrode, Ag / AgCl was used as the reference electrode, and the above aqueous electrolyte was used as the electrolyte.

[0063] 3. Evaluation conditions A constant current was passed through the electrochemical cell for 30 minutes at 25°C under the following conditions (1-1) and (1-2). In addition, a constant potential was applied for 30 minutes at 25°C under the following conditions (2-1) and (2-2). Experiments were performed for both the oxidation side current and potential and the reduction side current and potential. (Oxidation side) (1-1) Constant current: 0.1→0.1→0.2→0.3→0.5mA / cm 2 , 30 minutes each (The evaluation interval is 30 minutes on the OCP) (2-1) Constant voltage: 1.0→1.1→1.2→1.4→1.6V (vs. Ag / AgCl), 30 minutes each (The evaluation interval is 30 minutes on the OCP) (Return side) (1-2) Constant current: -0.1→-0.1→-0.2→-0.3→-0.5mA / cm 2 , 30 minutes each (The evaluation interval is 30 minutes on the OCP) (2-2) Constant voltage: -1.3→-1.4→-1.5→-1.7→-1.9V (vs. Ag / AgCl), 30 minutes each (The evaluation interval is 30 minutes on the OCP)

[0064] 4. Evaluation results (constant current) 4.1 In the case of above condition 1-1 and 0.5m electrolyte When an oxidation current was applied to an electrochemical cell using a 0.5 mol / kg K4P2O7 aqueous electrolyte, the Al foil on the oxidizing electrode (positive electrode) disappeared, and the electrolyte turned into a white gel. According to the potential-pH diagram, the standard electrode potential of Al at pH 11 of the aqueous electrolyte is about -2.3 V (vs. SHE), so it is possible that Al may dissolve (corrode) simply by immersing the Al foil in the aqueous electrolyte, and it is considered that the dissolution proceeds at an accelerated rate with the application of a small oxidation current. In fact, when an oxidation current was applied using the Al foil, the evaluation surface that had been in contact with the electrolyte completely disappeared. It can be seen that the protective layer of the oxide film that Al originally has did not function at all, and the dissolution proceeded. It is highly likely that the electrolyte turned into a white gel after evaluation because the dissolved Al ions reacted with the P2O7 ions in the electrolyte to precipitate as Al4(P2O7)3. This suggests that when Al ions react with P2O7 ions, a compound with very low solubility is produced and precipitates as a solid. Figure 3 shows the time-potential curve when a 0.5m aqueous electrolyte is used and a constant current is applied to the oxidation side.

[0065] 4.2 In the case of above condition 1-1 and 5m electrolyte When an oxidation current was applied to an electrochemical cell using a 5 mol / kg K4P2O7 aqueous electrolyte, the potential continued to rise over time, reaching the cutoff voltage of the device and the evaluation was interrupted. It is believed that the overvoltage increased as the Al foil surface became passivated and the solid-liquid interface was gradually insulated electrochemically. Looking at the Al foil after the evaluation, the evaluation surface maintained its metallic luster, and the aqueous electrolyte showed no change from the initial state, confirming that the condition was significantly different from that of the 0.5 m electrolyte mentioned above. As mentioned above, it has been confirmed that when Al ions and P2O7 ions interact, a solid precipitate with very low solubility is formed. In this regard, in a dilute solution such as the 0.5 m electrolyte mentioned above, once dissolved, the Al ions are converted into H2O or OH. - It is believed that Al dissolves and diffuses as a ligand, and then gradually solidifies by interacting with the P2O7 ions. As a result, solids precipitate and precipitate in the aqueous electrolyte, and Al dissolution into the electrolyte proceeds. In contrast, in the case of the 5m electrolyte, the P2O7 ion concentration in the electrolyte is very high, and the color change of the pH test paper immersed in the electrolyte after the evaluation of the reduction side described later was also slow, so it is believed that Al dissolves in the electrolyte due to the presence of H2O or OH. - It is believed that there is almost no diffusion or interaction through the electrolyte, and the Al ions dissolved from the Al foil react with the P2O7 ions quickly before diffusing far away. Therefore, a solid precipitates near the surface of the Al foil, and the solid adheres to the surface of the Al foil, forming a passive film (protective film) such as Al4(P2O7)3, which is thought to suppress the continuous dissolution of Al. Figure 4 shows the time-potential curve when a 5 ml aqueous electrolyte is used and a constant current is applied to the oxidation side.

[0066] 4.3 In the case of above condition 1-2 and 1m electrolyte In a general aqueous electrolyte, when a reduction current flows, hydrogen gas is generated as shown in the following formula, and the pH of the electrolyte rises. Therefore, when an amphoteric metal, Al foil, is used for the electrode (negative electrode) on the reduction side, as the pH rises and the electrolyte becomes strongly alkaline, Al dissolution from the Al foil into the electrolyte proceeds, and the Al foil eventually disappears. In fact, when a reduction current is passed through an electrochemical cell using a 1 mol / kg K4P2O7 aqueous electrolyte, the Al foil on the electrode (negative electrode) on the reduction side disappears, and the electrolyte turns into a white gel. This is thought to be due to the dissolution of Al from the Al foil into the electrolyte, and the precipitation and precipitation of Al4(P2O7)3 and other substances in the electrolyte, as in the case of passing an oxidation current in a 0.5 m electrolyte. Figure 5 shows the time-potential curve when a 1 m aqueous electrolyte is used and a constant current is passed on the reduction side. 2H2O+2e- → H2+2OH -

[0067] 4.4 In the case of above condition 1-2 and 5m electrolyte In the 5 mol / kg K4P2O7 aqueous electrolyte, Al did not dissolve even after a current was applied, and the shape was maintained, and no changes were observed in the electrolyte. When a pH test paper was immersed in the electrolyte after this evaluation, there was no particular change in color immediately after immersion, but after a few minutes the color gradually changed to purple, indicating a pH of 14. This suggests that when the electrolyte of an aqueous electrolyte is mainly composed of K4P2O7, it has a different nature from a general aqueous solution, and the H2O and OH in the solution - It was found that the diffusion of was very slow, and the reason why the Al foil did not dissolve even in the presence of such a strong base is that a passive film such as Al4(P2O7)3 was formed on the surface of the Al foil, as in the evaluation results of the oxidation side described above. Figure 6 shows the time-potential curve when a constant current was applied to the reduction side using 5 ml of aqueous electrolyte.

[0068] 4.5 Supplementary Information When comparing the presence or absence of dissolution of Al foil in K4P2O7 aqueous electrolytes of various concentrations, in electrochemical cells using aqueous electrolytes of 4 mol / kg or more, where the overvoltage clearly increased on the oxidation side, holes due to dissolution of Al were no longer observed. On the reduction side, a slight expansion of the potential (expansion of the potential window) was observed with increasing concentration, but no extreme increase in overvoltage like on the oxidation side occurred. On the reduction side, holes due to dissolution of Al were confirmed in electrolytes of 3 mol / kg or less, but not in electrolytes of 4 mol / kg or more. In addition, while on the oxidation side, the Al dissolution reaction occurs in a neutral to weakly basic solution of pH 11, on the reduction side, the Al dissolution reaction occurs after the pH increases locally after the hydrogen generation reaction, resulting in a reaction system with a different pH at the interface between the Al foil and the electrolyte. It is thought that the solubility of the passive film is higher in the strong basic region than in the neutral region, but if the passive film is dense, the dissolution of the passive film is suppressed. It is considered that a dense passive film was formed on the surface of the Al foil in an electrolyte of 4 mol / kg or more on both the oxidizing and reducing sides.

[0069] 5. Evaluation results (constant voltage) 5.1 Condition 2-1 When a constant voltage was applied to the oxidation side in each electrochemical cell, in the electrochemical cell using a low-concentration aqueous electrolyte of 3.0 mol / kg or less, the current increased linearly as the applied potential increased, whereas in the electrochemical cell using a high-concentration aqueous electrolyte of 4.0 mol / kg or more, the current value was observed to saturate at 1.1 V (vs. Ag / AgCl). This indicates that, while the current changes according to the normal Ohm's law on the low-concentration side, on the high-concentration side, a resistive layer (the above-mentioned passive film) is formed on the surface of the Al foil.

[0070] 5.2 Condition 2-2 When a constant voltage was applied to the reduction side of each electrochemical cell, the current value increased as the applied potential became more noble, but in electrochemical cells using electrolytes of less than 5 mol / kg, the evaluation limit was -1.7 V (vs. Ag / AgCl). In electrochemical cells using 5 mol / kg electrolytes, evaluation was possible up to -1.9 V (vs. Ag / AgCl), but the current value was 300 mA / cm. 2 Despite this, the surface of the Al foil only slightly discolored, and no signs of holes or deterioration due to corrosion were observed. Unlike the oxidizing side, no extreme increase in resistance was observed on the Al foil surface on the reducing side, suggesting that the passive film does not thicken. However, when a high-concentration electrolyte (e.g., a high-concentration electrolyte of 4 mol / kg or more) is used, P2O7 ions are present with sufficient activity when Al ions are eluted, so it is thought that a passive film sufficient to suppress the elution of Al ions is formed in the equilibrium state when a potential is applied.

[0071] 6. Consideration of metal type Instead of Al foil, Ti foil, Ni foil, Sn foil, Cu foil, Zn foil, Pt foil or W foil was used, and a 5 mol / kg K4P2O7 aqueous solution was used as the aqueous electrolyte to construct the same electrochemical cell as above, and evaluation was performed under the same constant current conditions as above (condition 1-1 or condition 1-2 above). The results are shown in Table 1 below.

[0072] [Table 1]

[0073] As is clear from the results shown in Table 1, only Ti foil was found to have a rapid increase in electrode resistance due to passivation, as with Al foil. Ni foil and Pt foil are known to be stable even at oxidizing potentials in aqueous systems with an acidic pH or higher, and showed a general current-potential relationship. In contrast, Sn foil, Cu foil, Zn foil, and W foil, which are known to be unstable at oxidizing potentials, continued to dissolve metal ions even in a 5 mol / kg K4P2O7 aqueous electrolyte, unlike Al foil. This suggests that unless the solubility of the compound between the eluted metal ion and the coordinated P2O7 compound is low, a passive film due to the adhesion and accumulation of the compound will not be formed, and the dissolution reaction cannot be stopped. In other words, it can be said that the effect of the passive film in suppressing the dissolution of metal foil is particularly pronounced when a metal foil containing Al is used.

[0074] 7. Evaluation assuming aqueous potassium ion battery A 5mol / kg K2P2O7 aqueous solution was prepared as an aqueous electrolyte. In addition, an Al foil or Ni foil was prepared as a positive electrode current collector, and a spinel type Li-Mn oxide (LiMn2O4, during charging and discharging, part of Li can be replaced by K by ion exchange) was mixed as a positive electrode active material with acetylene black as a conductive assistant, and PVDF and CMC as binders in a mass ratio of 85:10:4.5:0.5 to prepare a positive electrode active material mixture, and the positive electrode active material mixture was uniformly coated on the surface of the Al foil or Ni foil using a doctor blade, and dried to obtain a positive electrode for evaluation. That is, the positive electrode is a positive electrode active material layer containing a positive electrode active material and the like formed on the surface of the Al foil or Ni foil as a positive electrode current collector. In an electrochemical cell (VM4, manufactured by EC Frontier), an evaluation cell was prepared using the above positive electrode as the working electrode, the above aqueous electrolyte as the electrolyte, a Pt mesh as the counter electrode, and Ag / AgCl as the reference electrode. The evaluation cell thus constructed was charged at 0.1 mA for 1 hour, discharged at -0.1 mA with a cut potential of -0.4 V vs. Ag / AgCl, and a charge / discharge curve was obtained. The results are shown in Figures 7A and 7B. Figure 7A shows the results when Al foil was used, and Figure 7B shows the results when Ni foil was used. As is clear from the results shown in Figures 7A and B, there was no difference in the charge / discharge results of the LiMn2O4 electrode between the case where Al foil that forms a passive film was used and the case where Ni foil that does not form a passive film but is insoluble was used. From this, it was confirmed that when charging and discharging were performed after coating and forming the electrode, sufficient conduction was ensured between the active material and the Al foil, and a passive film was unlikely to form between solids such as between the active material and the Al foil. In other words, it is considered that in the aqueous potassium ion battery, the adverse effects due to the formation of the above-mentioned passive film do not substantially occur.

[0075] 8.Summary From the above results, it can be seen that when an aqueous electrolyte solution in which K2P2O7 is dissolved is used as an electrolyte, if the concentration of K2P2O7 is a high concentration of 4 mol / kg or more, it is possible to charge and discharge the active material while suppressing the dissolution of Al foil at oxidation potential or reduction potential. That is, it can be said that an aqueous potassium ion battery having a positive electrode, an aqueous electrolyte solution, and a negative electrode, one or both of the positive electrode and the negative electrode have a current collector containing Al, the current collector is in contact with the aqueous electrolyte solution, and the aqueous electrolyte solution contains water and potassium pyrophosphate dissolved at a concentration of 4 mol or more per kg of water, can suppress the dissolution of Al from the current collector into the aqueous electrolyte solution.

[0076] 9. Further investigation into aqueous electrolytes In the above, we investigated the case where only K2P2O7 was dissolved in pure water as an electrolyte in an aqueous electrolyte. As a result, we found that when the concentration of K2P2O7 is high, such as 4 mol / kg or more, a passive film is formed on the surface of the Al foil, suppressing the dissolution of Al at oxidation potential or reduction potential. However, this concentration of "4 mol / kg" is not necessarily the minimum concentration required to suppress the dissolution of Al. For example, when other electrolytes or some additives are dissolved together with K2P2O7 in an aqueous electrolyte, it is possible that the dissolution of Al foil at oxidation potential or reduction potential can be suppressed even if the concentration of K2P2O7 is less than 4 mol / kg. That is, (1) Al ions dissolved from the Al foil into the aqueous electrolyte react with anions derived from K2P2O7 (e.g., P2O7 ions) to precipitate as insoluble compounds; and (2) The insoluble compounds precipitate near the aluminum foil and adhere to the surface of the aluminum foil to form a passive film (the dissolved aluminum dissolves in water and dissolves in water). - (It does not diffuse far from the aluminum foil via the If these two conditions can be satisfied, it is believed that even if the concentration of K2P2O7 in the aqueous electrolyte is less than 4 mol / kg, the dissolution of Al at an oxidation potential or a reduction potential is suppressed. The present inventors conducted various experiments to examine the difference between the physical properties of an aqueous electrolyte that satisfies the conditions (1) and (2) and thereby suppresses the dissolution of Al, and the physical properties of an aqueous electrolyte that does not satisfy the conditions (1) and (2) and thus does not suppress the dissolution of Al.

[0077] 9.1 Preparation of aqueous electrolyte K4P2O7 was dissolved in 1 kg of pure water to a predetermined concentration (0.5 to 7 mol) to obtain an aqueous electrolyte for evaluation.

[0078] 9.2 Verification by DSC The crystallization peak temperature (freezing point temperature) and glass transition temperature of each aqueous electrolyte were confirmed by differential scanning calorimetry (DSC). The sweep rate by DSC was 5°C / min for both temperature drop and temperature rise. The sweep range was from room temperature to -120°C, and then to 40°C. The atmosphere in the DSC was an Ar atmosphere, and the pressure was the same as atmospheric pressure. However, since a sealed aluminum container was used for the evaluation, the atmosphere inside the container was air sealed under atmospheric pressure. Figure 8A shows the relationship between the concentration of the aqueous electrolyte and the crystallization peak temperature (freezing point), and Figure 8B shows the relationship between the concentration of the aqueous electrolyte and the crystallization peak intensity. Figure 9 shows the relationship between the concentration of the aqueous electrolyte and the glass transition temperature.

[0079] 8A and 8B, it can be seen that when the concentration of K4P2O7 in the aqueous electrolyte is 4 mol / kg or more, there is no crystallization peak in the DSC, that is, the aqueous electrolyte does not have a freezing point at or above −60° C. Furthermore, it can be seen from the results shown in FIG 9 that when the concentration of K4P2O7 in the aqueous electrolyte is around 4 mol / kg, the glass transition temperature changes specifically.

[0080] These results suggest that when the concentration of K4P2O7 in an aqueous electrolyte is 4 mol / kg or higher, the solution structure (clusters, networks, etc.) changes significantly due to the disappearance of interactions between H2O molecules, etc. For example, it is thought that the H2O molecules are surrounded by ions and associations derived from K4P2O7, the network derived from K4P2O7 becomes stronger, and the H2O molecules are no longer able to form a network for crystallization.

[0081] In other words, if it is an aqueous electrolyte in which K4P2O7 is dissolved and does not have a freezing point at -60°C or higher, the dissolved Al can react with the anions derived from K4P2O7, and the dissolved Al can diffuse in the solution without reacting with the H2O molecules and OH molecules. - It can be said that the network does not exist, and the above two conditions are met, i.e. (1) Al ions dissolved from the Al foil into the aqueous electrolyte react with anions derived from K2P2O7 (e.g., P2O7 ions) to precipitate as insoluble compounds; and (2) The insoluble compounds precipitate near the aluminum foil and adhere to the surface of the aluminum foil to form a passive film (the dissolved aluminum dissolves in water and dissolves in water). - (It does not diffuse far from the aluminum foil via the For example, even if the concentration of K4P2O7 in the aqueous electrolyte is less than 4 mol / kg, it is considered that the above conditions (1) and (2) are satisfied by dissolving other electrolytes, additives, etc. so that the aqueous electrolyte does not have a freezing point at -60°C or higher.

[0082] 9.3 Presence or absence of salt precipitation after cooling Each aqueous electrolyte was cooled from 0°C to -60°C, and the presence or absence of salt precipitation was confirmed by DSC. As a result, no peaks due to salt precipitation were confirmed, regardless of the concentration of the aqueous electrolyte.

[0083] Among conventional high-concentration aqueous electrolytes, there are some in which the amount of electrolyte that can be dissolved varies greatly depending on the temperature, and salts are easily precipitated by temperature changes. For example, in a high-concentration aqueous lithium imide salt solution known as an aqueous electrolyte for aqueous lithium ion batteries, and a high-concentration aqueous sodium imide salt solution known as an aqueous electrolyte for aqueous sodium ion batteries, salts are precipitated when the temperature drops by a few degrees from room temperature, and there is a concern that the precipitated salts may inhibit the battery reaction. In contrast, the aqueous electrolyte for aqueous potassium ion batteries according to this embodiment, as described above, is a stable electrolyte that does not freeze even at extremely low temperatures and does not precipitate salts, and can be used in various environments.

[0084] 9.4 Checking Viscosity The viscosity of each aqueous electrolyte at 20°C was confirmed. The viscosity was measured using a laboratory vibration viscometer (MODEL VM-10A-L, VM-10A-M, SEKONIC). Figure 10 shows the relationship between the concentration and viscosity of the aqueous electrolyte. As shown in Figure 10, when the concentration of K4P2O7 in the aqueous electrolyte is 4 mol / kg or more, the viscosity increases rapidly. From the viewpoint of suppressing the decrease in ion conductivity, it is better for the viscosity of the aqueous electrolyte to be small. For example, if the concentration of K4P2O7 is 7 mol / kg or less, the viscosity does not increase excessively, which is preferable.

[0085] 10. Summary From the above results, it can be said that when constructing an aqueous potassium ion battery, if the aqueous electrolyte satisfies the following requirements (A) and (B), it is possible to suppress the elution of Al from an Al-containing current collector into the aqueous electrolyte at an oxidation potential or a reduction potential. (A) The aqueous electrolyte has water and potassium pyrophosphate dissolved in the water. (B) The aqueous electrolyte does not have a freezing point above -60°C. [Explanation of symbols]

[0086] 10 positive electrode 11 Cathode active material layer 12 Positive electrode current collector 20 Aqueous electrolyte 30 negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 40 Separator 100 Water-based potassium ion battery

Claims

1. An aqueous potassium ion battery having a positive electrode, an aqueous electrolyte, and a negative electrode, one or both of the positive electrode and the negative electrode has a current collector containing Al; The current collector is in contact with the aqueous electrolyte solution, the aqueous electrolyte solution comprises water and potassium pyrophosphate dissolved in the water at a concentration of 4 mol to 7 mol per kg of the water, The aqueous electrolyte does not have a freezing point at −60° C. or higher. Water-based potassium-ion battery.

2. When the aqueous electrolyte is cooled from 0° C. to −60° C., no salt is precipitated.

2. The aqueous potassium ion battery of claim 1.

3. At least the positive electrode has the current collector, 2. The aqueous potassium ion battery of claim 1.

4. The aqueous electrolyte has a viscosity of 40 mPa·s or more and 350 mPa·s or less at 20° C.

2. The aqueous potassium ion battery of claim 1.

5. A bipolar structure is formed, and a positive electrode active material layer is formed on one surface of the current collector, and a negative electrode active material layer is formed on the other surface of the current collector. The aqueous potassium ion battery according to any one of claims 1 to 4.

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