Aqueous battery

Abstract

To provide a novel inorganic compound capable of functioning as a negative electrode active material for an aqueous battery and an aqueous battery using the same.SOLUTION: An aqueous battery disclosed herein includes a positive electrode, an aqueous electrolyte, and a negative electrode, the aqueous electrolyte contains water and an alkali metal ion or a zinc ion, the negative electrode contains an inorganic compound as a negative electrode active material, and the inorganic compound has a crystal structure belonging to space group I23.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present application discloses an aqueous battery having an aqueous electrolyte. [Background technology]

[0002] Various types of aqueous batteries, such as aqueous potassium ion batteries and aqueous zinc ion batteries, are known. An aqueous battery is a battery having an aqueous electrolyte (for example, Patent Document 1). Organic compounds such as Prussian blue and perylene are known as active materials for aqueous batteries. For example, perylene derivatives have been studied as negative electrode active materials for aqueous potassium ion batteries, but are currently poorly practical. [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] Active materials made of organic compounds have a large space within the lattice, which is disadvantageous in terms of energy density, etc. In this regard, new active materials that can be used in various aqueous batteries and that are made of inorganic compounds are needed. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> An aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, the aqueous electrolyte contains water and an alkali metal ion or a zinc ion; The negative electrode contains an inorganic compound as a negative electrode active material, The inorganic compound has a crystal structure belonging to space group I23. water-based battery. <Aspect 2> the inorganic compound comprises element A, element M and element O, The element A is at least one of Bi and La, The element M is at least one of Bi, Mn, Fe, Co, Ti and Ni; The aqueous battery of embodiment 1. <Aspect 3> The element A is Bi; The aqueous battery of embodiment 2. <Aspect 4> The element M is at least one of Mn, Fe, Co, and Ni; The aqueous battery of embodiment 2 or 3. <Aspect 5> The element A is Bi, The element M is at least one of Mn, Fe, and Ni; The aqueous battery according to any one of embodiments 2 to 4. <Aspect 6> The aqueous electrolyte contains water and alkali metal ions. The aqueous battery of any one of embodiments 1 to 5. <Aspect 7> The aqueous electrolyte contains water and potassium ions. The aqueous battery of any one of embodiments 1 to 6. <Aspect 8> The aqueous electrolyte solution contains water and potassium pyrophosphate dissolved in the water. The aqueous battery of any one of embodiments 1 to 7. <Aspect 9> the aqueous electrolyte solution contains water and potassium ions; a molar ratio of the water to the potassium ions (HO / K) is 3.5 or less; The aqueous electrolyte solution contains potassium pyrophosphate dissolved in the water. The aqueous battery of any one of embodiments 1 to 8. <Aspect 10> The pH of the aqueous electrolyte is 3 or more and 13 or less. The aqueous battery of any one of embodiments 1 to 9. <Aspect 11> The aqueous electrolyte does not have a freezing point at −60° C. or higher. The aqueous battery of any one of embodiments 1 to 10. <Aspect 12> The aqueous electrolyte does not precipitate salt when cooled from 0°C to -60°C. The aqueous battery according to any one of embodiments 1 to 11. <Aspect 13> The aqueous electrolyte has a viscosity of 10 mPa·s or more and 350 mPa·s or less at 20°C. The aqueous battery of any one of embodiments 1 to 12. Effect of the Invention

[0006] The inorganic compound of the present disclosure can be used as a negative electrode active material in various aqueous batteries such as aqueous alkali metal ion batteries and aqueous zinc ion batteries. When the inorganic compound of the present disclosure is used as a negative electrode active material in an aqueous battery, the energy density of the aqueous battery is more likely to be improved than when an organic compound is used. [Brief description of the drawings]

[0007] [Figure 1] 1 shows a schematic diagram of an example of the configuration of an aqueous battery. [Diagram 2] An example of a crystal structure belonging to space group I23 is shown. [Diagram 3] 2 shows charge / discharge curves of an evaluation cell of Example 1. [Figure 4] 4 shows charge / discharge curves of an evaluation cell of Example 2. [Diagram 5] 4 shows charge / discharge curves of an evaluation cell of Example 3. [Figure 6] 1 shows charge / discharge curves of an evaluation cell of Example 4. [Figure 7] 1 shows charge / discharge curves of an evaluation cell of Example 5. [Figure 8] 1 shows X-ray diffraction peaks for each of the negative electrode active materials of Examples 1 to 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] 1. Water-based potassium-ion battery Hereinafter, an aqueous 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 battery 100 according to an embodiment. As shown in FIG. 1, the aqueous battery 100 has a positive electrode 10, an aqueous electrolyte solution 20, and an anode 30. The aqueous electrolyte solution 20 contains water and alkali metal ions or zinc ions. The anode 30 contains an inorganic compound as an anode active material. The inorganic compound has a crystal structure belonging to space group I23.

[0009] 1.1 Positive electrode Any known positive electrode for aqueous batteries 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 may be impregnated with an aqueous electrolyte solution 20. The positive electrode active material layer 11 may include a conductive assistant, a binder, various additives, and the like in addition to the positive electrode active material. 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 mass% or more, 50 mass% or more, 60 mass% or more, or 70 mass% or more, or 100 mass% or less, or 90 mass% or less, assuming that the entire positive electrode active material layer 11 (total solid content) is 100 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 battery. The charge / discharge potential of the positive electrode active material is higher than that of the negative electrode active material described below. The type of positive electrode active material may be appropriately selected in consideration of the type of carrier ion, the potential window of the aqueous electrolyte 20, and the like. The positive electrode active material may be one that de-inserts and de-inserts carrier ions by intercalation, or one that de-inserts and de-inserts carrier ions 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] When potassium ions are used as the carrier ions, the positive electrode active material may contain K element. Specifically, oxides and polyanions containing K element may be mentioned. 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 / 3The positive electrode active material may be at least one selected from potassium iron complex oxide (KFeO2, etc.), potassium chromium complex oxide (KCrO2, etc.), potassium iron phosphate compound (KFePO4, etc.), potassium manganese phosphate compound (KMnPO4, etc.), and potassium cobalt phosphate compound (KCoPO4). Even if the K site in the complex oxide is composed of another alkali metal element, it is possible to insert and remove K ions by ion exchange or the like. Alternatively, the positive electrode active material may be an organic active material such as Prussian blue. As will be described later, in the water-based battery 100 of the present disclosure, even if an organic compound is used as the positive electrode active material, the energy density can be improved by using an inorganic compound as the negative electrode active material. Alternatively, the positive electrode active material may be at least one selected from potassium titanate, TiO2, sulfur (S), etc., which show a charge / discharge potential that is higher than that of the negative electrode active material described later.

[0013] When lithium ions are used as the carrier ions, the positive electrode active material can be any of those known as positive electrode active materials for lithium ion batteries. The positive electrode active material for lithium ion batteries may be an organic material or an inorganic material. The positive electrode active material for lithium ion batteries may be, for example, at least one selected from various lithium-containing compounds, elemental sulfur, and sulfur compounds. The lithium-containing compound as the positive electrode active material may be a lithium-containing oxide containing at least one element M', Li, and O. The element M' may be, for example, at least one selected from Mn, Ni, Co, Al, Mg, Ca, Sc, V, Cr, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, Bi, Fe, and Ti, or at least one selected from the group consisting of Mn, Ni, Co, Al, Fe, and Ti. More specifically, the lithium-containing oxide may be lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium nickel manganese oxide, lithium cobalt manganese oxide, lithium nickel cobalt manganese oxide (Li 1±α Ni x Co y Mn z O2±δ (For example, 0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1)), spinel lithium compounds (Li 1+x Mn 2-x-y M y O4 (where M is one or more selected from Al, Mg, Co, Fe, Ni, and Zn), such as heteroatom-substituted Li-Mn spinel with a composition represented thereby), lithium nickel cobalt aluminate (for example, Li 1±α Ni p Co q Al r O 2±δ (For example, p + q + r = 1)), lithium titanate, lithium metal phosphate (such as LiMPO4, where M is one or more selected from Fe, Mn, Co, and Ni), etc., and at least one selected therefrom may be used.

[0014] When sodium ions are employed as carrier ions, any of the cathode active materials known as cathode active materials for sodium-ion batteries can be adopted. The cathode active material for a sodium-ion battery may be an organic substance or an inorganic substance. The cathode active material for a sodium-ion battery may be, for example, at least one selected from various sodium-containing compounds, elemental sulfur, sulfur compounds, etc. The sodium-containing compound as the cathode active material may be a lithium-containing oxide containing at least one element M', Na, and O. The element M' may be, for example, at least one selected from Mn, Ni, Co, Al, Mg, Ca, Sc, V, Cr, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, Bi, Fe, and Ti, and may also be at least one selected from the group consisting of Mn, Ni, Co, Al, Fe, and Ti. For example, it may have a chemical composition such that Li in the above lithium-containing oxide is replaced by Na (however, the crystal structure may change by replacing Li with Na).

[0015] When zinc ions are used as carrier ions, the positive electrode active material may be a material capable of inserting and removing zinc ions. Such a material may be, for example, at least one selected from various inorganic composite oxides (e.g., vanadium-containing oxides such as V2O5-based oxides, manganese-containing oxides such as MnO2-based oxides, spinel-type oxides such as ZnMn2O4, or composite oxides of alkali metals and transition metals disclosed in JP 2020-27703 A) and organic materials (e.g., fullerenes disclosed in JP 2020-30997 A, compounds having a pentacene skeleton or compounds having a rubrene skeleton disclosed in JP 2020-43029 A).

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 1.1.2 Positive electrode current collector As shown in FIG. 1, the positive electrode 10 may include a positive electrode collector 12 in contact with the positive electrode active material layer 11. The positive electrode collector 12 may be in contact with the aqueous electrolyte 20. Any positive electrode collector that can function as a positive electrode collector for an aqueous battery can be used as the positive electrode collector 12. 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. The positive electrode current collector 12 may be a metal foil or a substrate plated or vapor-deposited with the above metal. When the positive electrode current 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 current 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.

[0020] 1.2 Aqueous electrolyte The aqueous electrolyte solution 20 contains water and alkali metal ions or zinc ions. The alkali metal ions and zinc ions each function as carrier ions in the aqueous battery 100. The aqueous electrolyte solution 20 may be in contact with the above-mentioned positive electrode current collector 12, may be contained in the above-mentioned positive electrode active material layer 11, may be in contact with the later-described negative electrode current collector 32, may be contained in the later-described negative electrode active material layer 31, or may be held by the separator 40 between the positive electrode 10 and the negative electrode 30.

[0021] 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).

[0022] From the viewpoint of forming a solid electrolyte interphase (SEI) on the surface of the active material, the aqueous electrolyte solution 20 may contain a solvent other than water in addition to water. 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 aqueous electrolyte solution 20 (100 mol%).

[0023] 1.2.2 Electrolytes An electrolyte may be dissolved in the aqueous electrolyte solution 20. The electrolyte may be dissociated into cations and anions in the aqueous electrolyte solution 20. In the aqueous electrolyte solution 20, the cations may be completely dissociated from the anions, or may form associations by coming close to the anions. According to the findings of the present inventors, when the aqueous electrolyte solution 20 contains water and alkali metal ions, and particularly when the aqueous electrolyte solution 20 contains water and potassium ions, the capacity of the aqueous battery 100 is more likely to be increased.

[0024] The aqueous electrolyte solution 20 may contain multiple types of cations. For example, when the aqueous electrolyte solution 20 contains potassium ions as carrier ions, other cations may be contained in addition to the potassium ions. For example, alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc. may be contained. The aqueous electrolyte solution 20 may also contain one or more types of anions as counter ions of the carrier ions. There is no particular limitation on the type of anion.

[0025] The concentration of the electrolyte in the aqueous electrolyte solution 20 is not particularly limited. The concentration of the electrolyte in the aqueous electrolyte solution 20 can be adjusted depending on the target performance. According to the knowledge of the present inventor, the higher the concentration of the electrolyte in the aqueous electrolyte solution 20, the easier it is to expand the potential window of the aqueous electrolyte solution 20. In addition, the higher the concentration, the easier it is to increase the capacity of the aqueous battery 100. Furthermore, the higher the concentration, the easier it is to suppress the dissolution of the current collector into the aqueous electrolyte solution 20. On the other hand, the lower the concentration, the easier it is to reduce the viscosity of the aqueous electrolyte solution 20.

[0026] 1.2.2.1 Potassium ion source The electrolyte that can be a potassium ion source may be any of various potassium compounds that can dissolve in water to generate potassium ions. The electrolytes contained in the aqueous electrolyte solution 20 include KPF6, KBF4, K2SO4, KNO3, CH3COOK, (CF3SO2)2NK, KCF3SO3, (FSO2)2NK, K2HPO4, KH2PO4, KPO3, K4P2O7, K5P3O 10 , K6P4O 13, K7P5O 16 , (KPO3)n, etc. According to the findings of the present inventors, when the aqueous electrolyte solution 20 contains potassium phosphate or acetate as an electrolyte, especially when it contains phosphate, particularly when it contains potassium pyrophosphate (K4P2O7), the capacity of the aqueous battery 100 is likely to be large. When the aqueous electrolyte solution 20 contains potassium pyrophosphate as an electrolyte, it may contain other electrolytes together with the potassium pyrophosphate. In this case, based on the total amount of electrolytes dissolved in the aqueous electrolyte solution 20 (100 mol%), potassium pyrophosphate may account for 50 mol% to 100 mol%, 70 mol% to 100 mol%, or 90 mol% to 100 mol%, and other electrolytes may account for 0 mol% to 50 mol%, 0 mol% to 30 mol%, or 0 mol% to 10 mol%.

[0027] As described above, the aqueous electrolyte 20 may contain water and potassium pyrophosphate dissolved in the water. The aqueous electrolyte 20 may also contain water, potassium pyrophosphate dissolved in the water, and other potassium ion sources dissolved in the water. In the aqueous electrolyte 20, the "potassium pyrophosphate dissolved in water" may be K + , 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." In addition, the aqueous electrolyte solution 20 may contain pyrophosphate ions (as described above, P2O7 4- In addition, KP2O7 3- , K2P2O7 2- , K3P2O7 -In the case where the aqueous electrolyte solution 20 contains pyrophosphate ions (which may be present in a state bound to a cation, such as K4P2O7), the entire amount of pyrophosphate 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 pyrophosphate ions than the concentration that can be converted into potassium pyrophosphate. For example, when the aqueous electrolyte solution 20 is produced, if a pyrophosphate ion source other than K4P2O7 (e.g., H4P2O7) is added to water together with K4P2O7 and dissolved therein, the aqueous electrolyte solution 20 will contain more pyrophosphate ions than the concentration that can be converted into potassium pyrophosphate.

[0028] The aqueous electrolyte 20 may contain water and potassium pyrophosphate dissolved at a certain concentration or more per 1 kg of water. The concentration of potassium pyrophosphate 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 per 1 kg of water. The upper limit of the concentration of potassium pyrophosphate in the aqueous electrolyte 20 is not particularly limited, but if the concentration is too high, the viscosity of the aqueous electrolyte 20 may be excessively increased. In this regard, the concentration of potassium pyrophosphate may be 7 mol or less or 6 mol or less per 1 kg of water. According to the knowledge of the present inventor, when only potassium pyrophosphate is dissolved in water in the aqueous electrolyte 20, the freezing point of the aqueous electrolyte decreases as the concentration of potassium pyrophosphate in water increases, and when the concentration of potassium pyrophosphate in the aqueous electrolyte becomes a certain concentration or more, the freezing point substantially disappears. That is, when the concentration of potassium pyrophosphate in the aqueous electrolyte is equal to or higher than a certain level, the aqueous electrolyte does not have a freezing point at −60° C. or higher. Specifically, 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 capacity of the aqueous battery 100 increases, and the aqueous electrolyte 20 loses its freezing point, and the aqueous electrolyte 20 loses its freezing point at −60° C. or higher. However, when other components are dissolved in the aqueous electrolyte 20 together with potassium pyrophosphate, the minimum concentration of potassium pyrophosphate at which the aqueous electrolyte loses its freezing point may be lower than 4 mol. According to the knowledge of the present inventor, the aqueous electrolyte 20 contains water and potassium ions, and the molar ratio (H2O / K) of the water to the potassium ions is 3.5 or less, and the aqueous electrolyte 20 contains potassium pyrophosphate dissolved in the water, etc., the above-mentioned freezing point disappearance effect is easily obtained. In this case, the "potassium ion" is at least partially derived from potassium pyrophosphate. As described above, the "potassium ion" may form an association with an anion by being in close proximity. The molar ratio of water to potassium ion in the aqueous electrolyte 20 can be easily determined by measuring the amount of water and the amount of K dissolved in the water.

[0029] 1.2.2.2 Lithium-ion Source The electrolyte that can be a lithium ion source may be any of various lithium compounds that can dissolve in water to generate lithium ions. The electrolytes contained in the aqueous electrolyte solution 20 include LiPF6, LiBF4, Li2SO4, LiNO3, CH3COOLi, (CF3SO2)2NLi, LiCF3SO3, (FSO2)2NLi, Li2HPO4, LiH2PO4, LiPO3, Li4P2O7, Li5P3O 10 , Li6P4O 13 , Li7PO 16 , (LiPO3)n, etc. In particular, when the aqueous battery 100 contains at least one imide salt of lithium bistrifluoromethanesulfonylimide ((CF3SO2)2NLi, LiTFSI) and lithium bisfluorosulfonylimide ((FSO2)2NLi, LiFSI), the performance of the aqueous battery 100 is likely to be improved. That is, the aqueous electrolyte 20 may contain water and a lithium imide salt dissolved in the water. The aqueous electrolyte 20 may also contain water, a lithium imide salt dissolved in the water, and another lithium ion source dissolved in the water. For example, the aqueous electrolyte 20 may contain water and a lithium imide salt dissolved at a concentration of 1 mol or more per kg of water. The concentration of the lithium imide salt may be 5 mol or more, 10 mol or more, 15 mol or more, or 20 mol or more per kg of water, and may be 40 mol or less, 35 mol or less, 30 mol or less, or 25 mol or less.

[0030] 1.2.2.3 Sodium ion source The electrolyte that can be a sodium ion source may be any of various sodium compounds that can dissolve in water to generate sodium ions. The electrolytes contained in the aqueous electrolyte solution 20 include NaPF6, NaBF4, Na2SO4, NaNO3, CH3COONa, (CF3SO2)2NNa, NaCF3SO3, (FSO2)2NNa, Na2HPO4, NaH2PO4, NaPO3, Na4P2O7, Na5P3O 10 , Na6P4O 13 , Na7PO 16, (NaPO3)n, etc. In particular, when NaCF3SO3 is contained, the performance of the aqueous battery 100 is likely to be improved. That is, the aqueous electrolyte 20 may contain water and NaCF3SO3 dissolved in the water. The aqueous electrolyte 20 may also contain water, NaCF3SO3 dissolved in the water, and other sodium ion sources dissolved in the water. For example, the aqueous electrolyte 20 may contain water and NaCF3SO3 dissolved at a concentration of 1 mol or more per kg of water. The concentration of NaCF3SO3 may be 3 mol or more, 5 mol or more, 7 mol or more, or 9 mol or more per kg of water, and may be 20 mol or less, 18 mol or less, 16 mol or less, or 14 mol or less.

[0031] 1.2.2.4 Zinc ion source The electrolyte that can be a zinc ion source may be various zinc compounds that can dissolve in water to generate zinc ions. The electrolyte contained in the aqueous electrolyte 20 may be at least one selected from ZnSO4, Zn(CF3SO3)2, Zn(NO3)2, ZnCl2, ZnI2, ZnO, ZnCO3, etc. In particular, when ZnSO4 is contained, the performance of the aqueous battery 100 is likely to be improved. That is, the aqueous electrolyte 20 may contain water and ZnSO4 dissolved in the water. The aqueous electrolyte 20 may also contain water, ZnSO4 dissolved in the water, and other zinc ion sources dissolved in the water. For example, the aqueous electrolyte 20 may contain water and ZnSO4 dissolved at a concentration of 1 mol or more per kg of water. The concentration of ZnSO4 may be 2 mol or more per kg of water, and may be 20 mol or less, 15 mol or less, 10 mol or less, or 5 mol or less.

[0032] 1.2.3 Other ingredients In addition to the above-mentioned solvent and electrolyte, 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.

[0033] 1.2.4 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 regard, the pH of the aqueous electrolyte may be 3 or more and 13 or less. The pH may be 4 or more, 5 or more, 6 or more, or 7 or more, or may be 12 or less.

[0034] 1.2.5 Freezing point The aqueous electrolyte 20 may 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 considered to have "no 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 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 make the concentration of the electrolyte in the aqueous electrolyte 20 high (for example, dissolving potassium pyrophosphate at a high concentration of 4 mol or more per kg of water, etc.) as described above. Since the aqueous electrolyte 20 does not have a freezing point at -60°C, metals contained in the current collector are less likely to dissolve into the aqueous electrolyte 20. Furthermore, since the aqueous electrolyte 20 does not have a freezing point at -60°C, the aqueous battery 100 can be used even at extremely low temperatures. That is, the aqueous battery 100 can operate appropriately even in cold regions.

[0035] 1.2.6 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 battery 100 can be used even at extremely low temperatures such as in cold regions. As described above, the aqueous electrolyte 20 may contain water and potassium pyrophosphate dissolved in the water. Here, according to the knowledge 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.

[0036] 1.2.7 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 an electrolyte 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. The aqueous electrolyte solution 20 may have a viscosity of 10 mPa·s or more and 350 mPa·s or less at 20°C. The viscosity may be 15 mPa·s or more, 20 mPa·s or more, 30 mPa·s or more, or 40 mPa·s or more, or may be 300 mPa·s or less, 250 mPa·s or less, or 200 mPa·s or less.

[0037] 1.3 Negative electrode The negative electrode 30 contains an inorganic compound as a negative electrode active material, and the inorganic compound has a crystal structure belonging to space group I23. The negative electrode 30 may have a known configuration as a negative electrode for a water-based battery, except for containing such a specific negative electrode active material. As shown in FIG. 1, the negative electrode 30 may include a negative electrode active material layer 31 and a negative electrode current collector 32. In this case, the negative electrode active material layer 31 contains the specific negative electrode active material.

[0038] 1.3.1 Negative electrode active material layer The negative electrode active material layer 31 contains an inorganic compound having a crystal structure belonging to space group I23 as the negative electrode active material. The negative electrode active material layer 31 may contain a conductive assistant, a binder, various additives, and the like in addition to the negative electrode active material. The negative electrode active material layer 31 may be impregnated with the aqueous electrolyte solution 20. 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.

[0039] FIG. 2 shows an example of a crystal structure belonging to space group I23. The crystal structure shown in FIG. 12 MnO 20 However, other chemical compositions are also acceptable. As shown in FIG. 2, the crystal structure belonging to space group I23 has spaces or gaps in a part of the structure, and it is considered that alkali metal ions and zinc ions are inserted and removed by intercalation through such spaces or gaps. As far as the present inventors have confirmed, inorganic compounds having a crystal structure belonging to space group I23 have a charge / discharge potential lower than the charge / discharge potential of the above-mentioned positive electrode active material, and are capable of charging and discharging various carrier ions within the range of the potential window of the above-mentioned aqueous electrolyte 20.

[0040] The inorganic compound having a crystal structure belonging to space group I23 may contain, for example, element A, element M, and O. Here, the element A is at least one of Bi and La, and the element M is at least one of Bi, Mn, Fe, Co, Ti, and Ni. Both element A and element M may be Bi. When the element A is at least one of Bi and La, and the element M is at least one of Bi, Mn, Fe, Co, Ti, and Ni, it is considered that alkali metal ions and zinc ions are more easily inserted and removed from the above crystal structure. Among them, when the element A is Bi, a higher effect can be expected. Also, when the element M is at least one of Mn, Fe, Co, and Ni, a higher effect can be expected. Furthermore, when the element A is Bi, and the element M is at least one of Mn, Fe, and Ni, a particularly high effect can be expected. The inorganic compound having a crystal structure belonging to space group I23 may be, for example, La4Re6O 19 , La4Os6O 19 , Nd4Os6O 19 , La4Ru6O 19 , Ba4Os6ClO 18 , Sr4Ru6ClO 18 ,Pr4Re6O 19 , Nd4Re6O 19 , Ce4Re6O 19 , Ca4Rh6Pb 19 , and Yb4Rh6Pb 19 Like, "A4M6O 19 ” (or “A4M6ClO 18 "), and Bi 12 GeO 20 , Bi 12 SiO 20 , Bi 12 TiO 20 , Bi 12 AsO 20 , Bi 12 PbO 20 , Bi 12 Chief of Staff 20 , Bi 12 PO 20 , Bi 12 GaO20 , Bi 13 O 20 , Bi 12 AlO 20 , Bi 12 SbO 20 , Bi 12 MnO 20 , Bi 12 RuO 20 , Bi 12 BO 20 , Bi 12 DyO 20 , Bi 12 YO 20 , Bi 12 HoO 20 , Bi 12 ErO 20 , Bi 12 YbO 20 , Bi 12 ZnO 20 , Bi 12 NiO 20 , Bi 12 NaO 20 , Bi 12 BeO 20 , Bi 12 LiO 20 , Bi 12 IrO 20 , and Bi 12 FeO 20 Like, "A 12 MO 20 ", and may be La2Ti3O9, Ba2Ir3O9, Bi2O3, YbCd6, K7Be 12 P 12 O 48 The above-mentioned A4M6O 19 (or A4M6ClO 18 ) or A 12 MO 20 In the formula, A may be composed of two or more elements, and M may be composed of two or more elements. For example, Bi 25 FeO 40 Is Bi 12 Bi 0.5 Fe 0.5 O 20 It is also possible to write it as A above. 12 MO20 It can be said to be a type of chemical composition represented by the following formula.

[0041] The inorganic compound having a crystal structure belonging to space group I23 can be produced, for example, by reacting various raw materials in a solid or liquid phase. Specifically, the compound containing the element A (various salts such as carbonates, oxides, hydroxides, etc.) and the compound containing the element M (various salts such as carbonates, oxides, hydroxides, etc.) are mixed and fired at a predetermined temperature in an oxygen-containing atmosphere. Here, the charge ratio of the various raw materials before firing does not necessarily have to match the target chemical composition ratio. As shown in the examples described later, a crystal structure belonging to the above-mentioned space group I23 may be obtained as one of multiple types of crystal structures together with other crystal structures.

[0042] The negative electrode active material may contain other compounds in addition to the inorganic compound having a crystal structure belonging to the space group I23. For example, the negative electrode active material may contain at least one compound selected from potassium-transition metal complex oxides, titanium oxide, metal sulfides such as Mo6S8, elemental sulfur, KTi2(PO4)3, NASICON type compounds, etc., as other compounds. The other compounds may be compounds that deintercalate carrier ions by intercalation, or compounds that deintercalate carrier ions by conversion reaction, alloying reaction, etc. Only one type of negative electrode active material may be used alone, or two or more types may be used in combination.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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 may be in contact with the aqueous electrolyte 20. Any negative electrode current collector that can function as a negative electrode current collector for an aqueous battery can be used as the negative electrode current collector 32. 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 selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, and preferably 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 current 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 current 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 current collector 32 is made of multiple sheets of metal foil, some layer may be present between the multiple sheets of metal foil. The thickness of the negative electrode current 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.

[0047] 1.4 Other configurations In addition to the basic configuration described above, the aqueous battery 100 may have other configurations, for example, as described below.

[0048] 1.4.1 Separators As described above, in the aqueous 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.

[0049] 1.4.2 Terminals etc. In addition to the above configuration, the aqueous battery 100 may also include terminals, a battery case, etc. Other configurations will be obvious to those skilled in the art after reading this application, and therefore will not be described here.

[0050] 1.5 Manufacturing method of aqueous batteries The aqueous battery 100 of the present disclosure can be manufactured, for example, as follows. (1) 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. (2) The aqueous electrolyte solution 20 can be produced, for example, by mixing water and an electrolyte. There is no particular limitation on the mixing means, and any known mixing means can be used. Even if the water and the electrolyte are simply filled into a container and left to stand, they will mix with each other, and the aqueous electrolyte solution 20 will finally be obtained. (3) 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. (4) The positive electrode 10, the aqueous electrolyte 20, and the negative electrode 30 are housed in a battery case to form an aqueous 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 the aqueous battery 100.

[0051] 1.6 Effects of water-based batteries As for potassium ions and zinc ions, there are few reports of inorganic active materials due to the large ionic radius. That is, there is little knowledge about what inorganic compounds can be used as active materials, and the activity of a number of inorganic compounds is confirmed one by one. In particular, research on aqueous batteries is still in its infancy, and there are even fewer reports on them compared to non-aqueous batteries, and only organic compounds such as Prussian blue have been considered. Organic compounds have large spaces in the lattice, which is disadvantageous in terms of energy density, etc. In contrast, in the aqueous battery 100 disclosed herein, a specific inorganic compound is used as the negative electrode active material. When a negative electrode active material made of an inorganic compound is used, the energy density of the aqueous battery is more likely to be improved than when a negative electrode active material made of an organic compound is used. In addition, the inorganic compound used in the aqueous battery disclosed herein has excellent stability of the crystal structure, and from this perspective, the battery performance is also more likely to be improved. Furthermore, the inorganic compound used in the aqueous battery disclosed herein can be manufactured from inexpensive raw materials.

[0052] 2.Negative electrode active material for water-based batteries The technology of the present disclosure also has an aspect of being a negative electrode active material for aqueous batteries. That is, the negative electrode active material of the present disclosure is a negative electrode active material used in aqueous batteries having alkali metal ions or zinc ions as carrier ions, and is characterized in that it contains an inorganic compound, and the inorganic compound has a crystal structure belonging to space group I23. Details of the aqueous battery are as described above.

[0053] 3. Methods for improving the energy density of aqueous batteries The technology of the present disclosure also has an aspect of being a method for improving the energy density of an aqueous battery. That is, the method of the present disclosure is characterized in that, in an aqueous battery, an aqueous electrolyte that satisfies the following requirement (1) is used, and an anode active material that satisfies the following requirement (2) is used. Details of the aqueous battery are as described above. (1) The aqueous electrolyte contains water and alkali metal ions or zinc ions. (2) The negative electrode active material is an inorganic compound having a crystal structure belonging to space group I23.

[0054] 4. Vehicles with water-based batteries As described above, the technology of the present disclosure can improve the energy density of an aqueous battery. Such an aqueous battery can be suitably used in at least one vehicle selected from, for example, a hybrid electric vehicle (HEV), a plug-in hybrid electric 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 battery, the aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, the aqueous electrolyte containing water and an alkali metal ion or a zinc ion, the negative electrode containing an inorganic compound as a negative electrode active material, and the inorganic compound having a crystal structure belonging to space group I23. Details of the aqueous battery are as described above. EXAMPLES

[0055] 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.

[0056] 1. Preparation of negative electrode active material 1.1 Example 1 Bi2O3 and Fe2O3 were mixed in a molar ratio of Bi:Fe=1:1, formed into a pellet shape, and then heated to 650°C at a heating rate of 5°C / min and fired at 650°C for 4 hours to obtain an intermediate material. The obtained intermediate material was heated again to 650°C at a heating rate of 5°C / min and fired at 650°C for 4 hours to obtain the negative electrode active material according to Example 1.

[0057] 1.2 Example 2 Bi2O3 and MnO were mixed in a molar ratio of Bi:Mn=1:1, formed into a pellet shape, and then heated to 800°C at a heating rate of 5°C / min and fired at 800°C for 4 hours to obtain an intermediate material. The obtained intermediate material was heated again to 800°C at a heating rate of 5°C / min and fired at 800°C for 4 hours to obtain the negative electrode active material according to Example 2.

[0058] 1.3 Example 3 Bi2O3 and NiO were mixed in a molar ratio of Bi:Ni=1:1, formed into a pellet shape, and then heated to 850°C at a heating rate of 5°C / min and fired at 850°C for 4 hours to obtain an intermediate material. The obtained intermediate material was heated again to 900°C at a heating rate of 5°C / min and fired at 900°C for 4 hours to obtain a negative electrode active material according to Example 3.

[0059] 1.4 Example 4 Bi2O3, Fe2O3, MnO, and CoO were mixed in a molar ratio of Bi:Fe:Mn:Co=1:0.33:0.33:0.33, formed into a pellet shape, and then heated to 600°C at a heating rate of 5°C / min and fired at 600°C for 4 hours to obtain a negative electrode active material of Example 4.

[0060] 1.5 Example 5 Bi2O3, Fe2O3, and MnO were mixed in a molar ratio of Bi:Fe:Mn=1:0.7:0.3, formed into a pellet shape, and then heated to 800°C at a heating rate of 5°C / min and fired at 800°C for 10 minutes to obtain a negative electrode active material according to Example 5.

[0061] 2. Preparation of the Negative Electrode The negative electrode active material of any one of Examples 1 to 5, Ti as a conductive assistant, PTFE as a binder, and 2wt% CMC as a thickener were weighed out so that the mass ratio of the negative electrode active material:Ti:PTFE:2wt%CMC=25.0:70.0:4.5:0.5, and water was added thereto to prepare an ink. The ink obtained was applied to a Ti foil with a doctor blade and dried to obtain a negative electrode for evaluation. The weight of the negative electrode active material of Example 1 was 1.627 mg, the weight of the negative electrode active material of Example 2 was 1.541 mg, the weight of the negative electrode active material of Example 3 was 1.528 mg, the weight of the negative electrode active material of Example 4 was 1.901 mg, and the weight of the negative electrode active material of Example 5 was 6.654 mg.

[0062] 3. Preparation of Aqueous Electrolyte An aqueous electrolyte for evaluation was prepared by dissolving 5 mol of K4P2O7 in 1 kg of pure water. The aqueous electrolyte had a pH in the range of 3 to 13, no freezing point above -60°C, no salt precipitation when cooled from 0°C to -60°C, and a viscosity at 20°C in the range of 10 mPa s to 350 mPa s.

[0063] 4. Preparation of Evaluation Cell Using the negative electrode of any one of Examples 1 to 5 and the above aqueous electrolyte solution, an evaluation cell having the following configuration was produced. Cell: VM5 (manufactured by EC Frontier) Working electrode: the negative electrode mentioned above Counter electrode: Pt mesh Reference electrode:Ag / AgCl

[0064] 5. Cell Evaluation The evaluation cell was charged and discharged under the following conditions to obtain a charge-discharge curve. Charge / discharge current value: ±0.1mA / cm 2 Cutoff voltage: -1.20~0.85V vs. Ag / AgCl Measurement temperature: 25℃

[0065] 6. Evaluation Results 6.1 Charge-discharge curve Figures 3 to 7 show the results of charge / discharge evaluation of the evaluation cells of Examples 1 to 5. As shown in Figures 3 to 7, the negative electrode active materials of Examples 1 to 5 all absorb or release potassium ions as carrier ions at a predetermined potential in an aqueous electrolyte, and are therefore usable as negative electrode active materials for aqueous potassium ion batteries.

[0066] 6.2 Crystal structure of the negative electrode active material X-ray diffraction was used to identify the crystalline phase contained in each of the negative electrode active materials of Examples 1 to 5. The results are shown in the following Table 1. Furthermore, Fig. 8 shows the X-ray diffraction peaks of each of the negative electrode active materials of Examples 1 to 4.

[0067] [Table 1]

[0068] The results shown in Table 1 reveal the following: (1) The negative electrode active material according to Example 1 has a crystal structure belonging to space group I23, a crystal structure belonging to space group R3c, and a crystal structure belonging to space group R-3c. Here, it is known that Fe2O3 belonging to space group R-3c cannot absorb and release potassium ions in an aqueous electrolyte (inactive phase). Therefore, in Example 1, it is considered that at least one of the crystal structure belonging to space group I23 and the crystal structure belonging to space group R3c enables the absorption and release of potassium ions in an aqueous electrolyte. (2) The negative electrode active material according to Example 2 has a crystal structure belonging to the space group Pbam and a crystal structure belonging to the space group I23. It has not been confirmed whether these are inactive phases. That is, in Example 2, it is considered that at least one of the crystal structure belonging to the space group Pbam and the crystal structure belonging to the space group I23 enables the absorption and release of potassium ions in the aqueous electrolyte. (3) The negative electrode active material according to Example 3 has a crystal structure belonging to space group P21 / c, a crystal structure belonging to space group Fm-3m, and a crystal structure belonging to space group I23. Here, it is known that Bi2O belonging to space group P21 / c and NiO belonging to space group Fm-3m cannot absorb and release potassium ions in an aqueous electrolyte (inactive phase). Therefore, in Example 3, it is considered that the crystal structure belonging to space group I23 enables the absorption and release of potassium ions in an aqueous electrolyte. (4) The negative electrode active material according to Example 4 has a crystal structure belonging to space group I23, a crystal structure belonging to space group R-3c, and a crystal structure belonging to space group Fd-3m. Here, it is known that Fe2O3 belonging to space group R-3c and Co3O4 belonging to space group Fd-3m cannot absorb and release potassium ions in an aqueous electrolyte (inactive phase). Therefore, in Example 4, Bi25 FeO 40 It is believed that this enables the absorption and release of potassium ions in the aqueous electrolyte. (5) The negative electrode active material according to Example 5 has two types of crystal structures belonging to space group R3c and a crystal structure belonging to space group I23. It has not been confirmed whether these are inactive phases. That is, in Example 5, it is considered that at least one of the two types of crystal structures belonging to space group R3c and the crystal structure belonging to space group I23 enables the absorption and release of potassium ions in the aqueous electrolyte.

[0069] In Examples 1 to 5, the common crystal phase, except for the inactive phase, belongs to space group I23. That is, the above results suggest that an inorganic compound having a crystal structure belonging to space group I23 can desorb and insert potassium ions in an aqueous electrolyte at a predetermined potential, and can be used as a negative electrode active material for an aqueous potassium ion battery. The validity of the suggestion was confirmed from a specific crystal structure. FIG. 2 shows an example of a crystal structure belonging to space group I23. As shown in FIG. 2, the crystal structure belonging to space group I23 has spaces and gaps in a part of the structure, and it is considered that potassium ions are desorbed and inserted by intercalation through such spaces and gaps.

[0070] 7. Evaluation of electrolyte type change Except for changing the type of the aqueous electrolyte solution to that shown in Table 2 below, evaluation cells were prepared and charge / discharge evaluation was performed in the same manner as in Example 5. Table 2 below shows the discharge capacity of each evaluation cell when the discharge capacity of the evaluation cell according to Example 5 is taken as 100%.

[0071] [Table 2]

[0072] From the results shown in Table 2, it can be seen that inorganic compounds having a crystal structure belonging to space group I23 can insert and remove alkali metal ions such as potassium ions, sodium ions, and lithium ions, and zinc ions in an aqueous electrolyte, and can be used as negative electrode active materials for aqueous batteries using alkali metal ions or zinc ions as carrier ions. In particular, when the aqueous electrolyte contains water and alkali metal ions, especially when the aqueous electrolyte contains water and potassium ions, and even more especially when the aqueous electrolyte contains water and potassium pyrophosphate dissolved in the water, the capacity of the aqueous battery is likely to be large. In addition, when the aqueous electrolyte contains water and potassium pyrophosphate dissolved in the water, the capacity of the aqueous battery is likely to be large when the concentration of potassium pyrophosphate in the aqueous electrolyte is high. As far as the present inventors have confirmed, the concentration of potassium pyrophosphate in the aqueous electrolyte is preferably more than 2.0 mol / kg, 3.0 mol / kg or more, 4.0 mol / kg or more, or 5.0 mol / kg or more.

[0073] 8. Supplementary Information In the above examples, the performance of the active material contained in the negative electrode was evaluated simply using an evaluation cell having a specific aqueous electrolyte, a specific negative electrode as the working electrode, a Pt mesh as the counter electrode, and Ag / AgCl as the reference electrode, but when actually configuring an aqueous battery, for example, a specific aqueous electrolyte and a specific negative electrode may be combined with an appropriate positive electrode. Since the positive electrode is not the essence of the technology disclosed herein, no specific example is shown in this example, but any known positive electrode for an aqueous battery may be used.

[0074] 9. Summary From the above results, it can be said that an aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, which satisfies the following (1) and (2), has a specific inorganic compound that functions as the negative electrode active material, and therefore the energy density of the aqueous battery is more likely to be improved than when an organic compound is used as the negative electrode active material. (1) The aqueous electrolyte contains water and alkali metal ions or zinc ions. (2) The negative electrode contains an inorganic compound as a negative electrode active material, and the inorganic compound has a crystal structure belonging to space group I23. [Explanation of symbols]

[0075] 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 batteries

Claims

1. An aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, the aqueous electrolyte contains water and an alkali metal ion or a zinc ion; the negative electrode contains an inorganic compound as a negative electrode active material, The inorganic compound has a crystal structure belonging to space group I23. water-based battery.

2. the inorganic compound comprises an element A, an element M, and an element O; The element A is at least one of Bi and La, The element M is at least one of Bi, Mn, Fe, Co, Ti, and Ni; The aqueous battery of claim 1.

3. The element A is Bi; The aqueous battery of claim 2.

4. The element M is at least one of Mn, Fe, Co, and Ni; The aqueous battery of claim 2.

5. The element A is Bi, The element M is at least one of Mn, Fe, and Ni; The aqueous battery of claim 2.

6. The aqueous electrolyte contains water and alkali metal ions. The aqueous battery of claim 1.

7. The aqueous electrolyte contains water and potassium ions. The aqueous battery of claim 1.

8. The aqueous electrolyte solution contains water and potassium pyrophosphate dissolved in the water. The aqueous battery of claim 1.

9. the aqueous electrolyte solution contains water and potassium ions; The molar ratio of the water to the potassium ions (H ２ O / K) is 3.5 or less, The aqueous electrolyte solution contains potassium pyrophosphate dissolved in the water. The aqueous battery of claim 1.

10. The pH of the aqueous electrolyte is 3 or more and 13 or less. The aqueous battery according to any one of claims 1 to 9.

11. The aqueous electrolyte does not have a freezing point at −60° C. or higher. The aqueous battery according to any one of claims 1 to 9.

12. When the aqueous electrolyte is cooled from 0° C. to −60° C., no salt is precipitated. The aqueous battery according to any one of claims 1 to 9.

13. The aqueous electrolyte has a viscosity of 10 mPa·s or more and 350 mPa·s or less at 20° C. The aqueous battery according to any one of claims 1 to 9.

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