Aqueous battery

Abstract

Disclosed is technology that, when a collector containing Al is employed in an aqueous battery, makes it possible to suppress elution of Al from the collector to an aqueous electrolyte. An aqueous battery according to the present disclosure comprises a positive electrode, an aqueous electrolyte, and a negative electrode. The positive electrode and / or the negative electrode has a collector containing Al. The collector is in contact with the aqueous electrolyte. The aqueous electrolyte contains: water; potassium polyphosphate which is dissolved in the water; and at least substance which is dissolved in the water and which is selected from among potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and carboxylic acid.

Description

water-based battery

[0001] This application discloses an aqueous battery.

[0002] Patent Document 1 discloses an aqueous electrolyte solution containing water and potassium pyrophosphate dissolved at a concentration of 2 mol or more per kg of water. When an aqueous battery is constructed using the aqueous electrolyte solution disclosed in Patent Document 1, the aqueous electrolyte solution has a wide reduction-side potential window, so decomposition of the aqueous electrolyte solution on the electrode surface is easily suppressed even when the aqueous battery is charged and discharged. Furthermore, Patent Document 1 uses Ti foil or Au foil as the electrode current collector when evaluating the aqueous electrolyte solution.

[0003] Japanese Patent Application Laid-Open No. 2019-220294

[0004] Conventional aqueous batteries have room for improvement in terms of measures against corrosion of current collectors. For example, when an aqueous battery uses a current collector containing Al, Al is likely to leach from the current collector into the aqueous electrolyte during charging and discharging of the battery. When an aqueous battery uses a current collector containing Al, a new technology is needed to suppress the leach- ing of Al from the current collector into the aqueous electrolyte.

[0005] The present application discloses the following multiple aspects as means for solving the above problems. <Aspect 1> An aqueous battery comprising a positive electrode, an aqueous electrolyte, and a negative electrode, wherein one or both of the positive electrode and the negative electrode have a current collector containing Al, the current collector being in contact with the aqueous electrolyte, the aqueous electrolyte comprising: water, potassium polyphosphate dissolved in the water, and at least one of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and a carboxylic acid dissolved in the water. <Aspect 2> The aqueous battery of Aspect 1, wherein the aqueous electrolyte comprises the potassium polyphosphate dissolved at a concentration of 3 mol or more per kg of water. <Aspect 3> The aqueous battery of Aspect 1, wherein the aqueous electrolyte comprises the potassium polyphosphate dissolved at a concentration of 3 mol or more and 6 mol or less per kg of water. <Aspect 4> The aqueous battery of Aspect 1, wherein the aqueous electrolyte solution contains the potassium polyphosphate dissolved at a concentration of 4 mol to 5 mol per kg of water. <Aspect 5> The aqueous battery of any of Aspects 1 to 4, wherein the aqueous electrolyte solution contains potassium dihydrogen phosphate dissolved in the water, and wherein the K concentration in the aqueous electrolyte solution is 11.16 mol / L to 14.94 mol / L. <Aspect 6> The aqueous battery of any of Aspects 1 to 5, wherein the aqueous electrolyte solution does not have a freezing point at -40°C or higher. <Aspect 7> The aqueous battery of any of Aspects 1 to 6, wherein the aqueous electrolyte solution does not precipitate salt when cooled from 0°C to -40°C. <Aspect 8> The aqueous battery of any one of Aspects 1 to 7, wherein the aqueous electrolyte has a viscosity of 20 mPa·s or more and 400 mPa·s or less at 20° C. <Aspect 9> The aqueous battery of any one of Aspects 1 to 8, wherein the aqueous electrolyte has a pH of 3 or more and 12 or less. <Aspect 10> The aqueous battery of any one of Aspects 1 to 9, wherein at least the positive electrode has the current collector.Aspect 11 The aqueous battery of any one of Aspects 1 to 10, having a bipolar structure, in which 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.

[0006] According to the aqueous battery of the present disclosure, elution of Al from the current collector into the aqueous electrolyte solution is easily suppressed.

[0007] 1A and 1B show a schematic diagram of an example of the configuration of an aqueous battery, a schematic diagram of an example of the configuration of an aqueous battery, a cyclic voltammogram for Comparative Example 5, a cyclic voltammogram for Example 17, and a cyclic voltammogram for Example 47.

[0008] Hereinafter, one embodiment of the aqueous battery of the present disclosure will be described with reference to the drawings, but the technology of the present disclosure is not limited to the following embodiment.

[0009] As shown in Figure 1, an aqueous battery 100 according to one embodiment includes a positive electrode 10, an aqueous electrolyte solution 20, and an anode 30. One or both of the positive electrode 10 and the anode 30 include a current collector containing Al. The current collector is in contact with the aqueous electrolyte solution 20. The aqueous electrolyte solution 20 includes water, potassium polyphosphate dissolved in the water, and at least one of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and a carboxylic acid dissolved in the water.

[0010] 1. Positive Electrode Any known positive electrode for aqueous batteries can be used as the positive electrode 10. As shown in FIG. 1 , the positive electrode 10 can include a positive electrode active material layer 11 and a positive electrode current collector 12.

[0011] 1.1 Positive Electrode Active Material Layer The positive electrode active material layer 11 contains 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 contain, in addition to the positive electrode active material, a conductive additive, a binder, or the like. The positive electrode active material layer 11 may also contain various other additives. The content of each component in the positive electrode active material layer 11 may be appropriately determined depending on the desired 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 100% by mass or less, or 90% by mass or less, where the total weight of the positive electrode active material layer 11 (total solid content) is taken as 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.

[0012] The positive electrode active material can be any material that can function as a positive electrode active material for an aqueous battery (e.g., an aqueous battery using an aqueous electrolyte solution containing potassium ions). The positive electrode active material has a higher charge / discharge potential than the negative electrode active material described below, and can be appropriately selected in consideration of the potential window of the aqueous electrolyte solution 20 described below. In this embodiment, various ions derived from the aqueous electrolyte solution can serve as charge compensation ions. Specifically, the charge compensation ions that are inserted and removed by the positive electrode active material can be one or more of potassium ions, protons, anions derived from the electrolyte, and hydroxide ions. The positive electrode active material can be, for example, Ni(OH) ２ (For example, JP 2023-154313 A) and A ｘ K ｙ Ni １－ｚ M ｚ O ２±δ ・nH ２O (A is at least one of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, and Sc, M is at least one of a transition metal element, a group 2A element, a group 3A element, a group 2B element, and a group 3B element, and the relationships 0≦x<0.5, 0<y≦0.5, 0≦z≦0.5, 0<n≦2, and (α.x)+y≦0.5 are satisfied, and α is the valence of the cation of A. For example, see JP 2023-132287 A), and manganese spinel (for example, LiMn ２ O ４ The positive electrode active material may be various hydroxides or oxides such as nickel-manganese-cobalt composite oxide (NMC). The positive electrode active material may also be an oxide or polyanion containing an alkali metal element. More specifically, the positive electrode active material may be a composite oxide of an alkali metal element and a transition metal. The composite oxide may be an alkali metal-cobalt composite oxide (AmCoO ２ "Am" is an alkali metal element. The same applies below.), alkali metal nickel composite oxide (AmNiO ２ etc.), alkali metal nickel titanium composite oxide (AmNi １／２ Ti １／２ O ２ etc.), alkali metal nickel manganese composite oxide (AmNi １／２ Mn １／２ O ２ , AmNi １／３ Mn ２／３ O ２ etc.), alkali metal manganese composite oxides (AmMnO ２ , AmMn ２ O ４ etc.), alkali metal iron manganese composite oxide (Am ２／３ Fe １／３ Mn ２／３ O ２ etc.), alkali metal nickel cobalt manganese composite oxide (AmNi １／３ Co １／３ Mn １／３ O ２ etc.), alkali metal iron composite oxide (AmFeO ２ etc.), alkali metal chromium composite oxides (AmCrO ２ etc.), alkali metal iron phosphate compounds (AmFePO ４etc.), alkali metal manganese phosphate compounds (AmMnPO ４ etc.), alkali metal cobalt phosphate compounds (AmCoPO ４ Alternatively, the positive electrode active material may be an active material such as Prussian blue. Alternatively, the positive electrode active material may be an alkali metal titanium composite oxide, TiO, which exhibits a charge / discharge potential higher than that of the negative electrode active material described below. ２ , sulfur (S), etc. The positive electrode active material may be one that deintercalates and inserts charge compensation ions by intercalation, or one that deintercalates and inserts charge compensation 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.

[0013] The shape of the positive electrode active material may be any shape that can function as a positive electrode active material for a battery. The positive electrode active material may be, for example, particulate. The positive electrode active material may be solid, hollow, void-containing, or porous. The positive electrode active material may be primary particles or secondary particles formed by aggregation of multiple 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, or 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Note that the average particle diameter D50 referred to in this application is the particle diameter (median diameter) at 50% of the integrated value in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0014] Examples of conductive additives that can be contained 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, stainless steel, etc. The conductive additive may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive additive may be used alone, or two or more types may be used in combination.

[0015] Examples of binders that can be contained 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.

[0016] 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 solution 20. Any positive electrode current collector 12 that can function as a positive electrode current collector for an aqueous battery can be used. When the negative electrode current collector 32 described below contains Al, the positive electrode current collector 12 may or may not contain Al. Furthermore, when the negative electrode current collector 32 described below does not contain Al, the positive electrode current collector 12 contains Al. As described below, Al elution from the current collector into the aqueous electrolyte solution is particularly likely to occur on the positive electrode side, which has an oxidizing potential. However, according to the aqueous battery 100 of the present disclosure, even if the positive electrode current collector 12 contains Al, elution of Al from the positive electrode current collector 12 into the aqueous electrolyte solution 20 can be suppressed. That is, in the aqueous battery 100, at least the positive electrode 10 may have a current collector containing Al.

[0017] When the positive electrode current collector 12 contains Al, the positive electrode current collector 12 may be made entirely of Al, or at least a portion of its surface 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 portion of its surface that comes into contact with the aqueous electrolyte solution 20, or may have Al present over the entire surface that comes into contact with the aqueous electrolyte solution 20.

[0018] The positive electrode current collector 12 may be in the form of a foil, plate, mesh, punched metal, foam, or the like. The positive electrode current collector 12 may be composed of a metal foil or metal mesh. Metal foil is particularly easy to handle. The positive electrode current collector 12 may be composed of multiple foils. Examples of metal materials constituting the positive electrode current 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. As described above, the positive electrode current collector 12 preferably contains Al. The positive electrode current collector 12 may be a metal foil or a substrate plated or vapor-deposited with the above metal. Furthermore, when the positive electrode current collector 12 is composed of multiple metal foils, some layer may be present between the multiple 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.

[0019] 2. Aqueous Electrolyte The aqueous electrolyte solution 20 contains water, potassium polyphosphate dissolved in the water, and at least one of potassium hydrogen phosphate, phosphoric acid, and polyphosphoric acid dissolved in the water. The aqueous electrolyte solution 20 is in contact with the positive electrode current collector 12, is contained in the positive electrode active material layer 11, is in contact with the negative electrode current collector 32 described below, is contained in the negative electrode active material layer 31 described below, and can be held by the separator 40 between the positive electrode 10 and the negative electrode 30.

[0020] 2.1 Solvent The aqueous electrolyte solution 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 solution (100 mol%), water accounts for 50 mol% to 100 mol%. Water may account for 70 mol% or more, 90 mol% or more, or 95 mol% or more of the total amount of the solvent. On the other hand, there is no particular upper limit to the proportion of water in the solvent. The solvent may consist of only water (100 mol% water).

[0021] The solvent may contain a solvent other than water in addition to water, as long as the above-mentioned problem 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 solution (100 mol%).

[0022] 2.2 Electrolyte 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 close to each other to form an association complex.

[0023] 2.2.1 Potassium Polyphosphate The aqueous electrolyte solution 20 contains potassium polyphosphate dissolved in the water. "Potassium polyphosphate" refers to a salt in which at least a portion of the hydrogen atoms in polyphosphate are substituted with potassium. In other words, "potassium polyphosphate" is a concept that includes potassium hydrogen polyphosphate. Specific examples of potassium polyphosphate include potassium pyrophosphate (K ４－ｘ H ｘ P ２ O ７ ), potassium tripolyphosphate (K ５－ｘ H ｘ P ３ O １０ Among them, potassium polyphosphates include potassium pyrophosphate (K ４－ｘ H ｘ P ２ O ７) is employed, even higher performance is likely to be ensured. In the aqueous electrolyte solution 20, "potassium polyphosphate dissolved in water" may exist as potassium ions, polyphosphate ions, associations of these ions, or associations with ions derived from potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and carboxylic acid, as described below. In the aqueous electrolyte solution 20, the "concentration of potassium polyphosphate dissolved in water" can be determined by converting the ions and associations contained in the aqueous electrolyte solution 20 into potassium polyphosphate. Note that, in the present application, "potassium polyphosphate dissolved in water" may refer to the aqueous electrolyte solution 20 in which a cation source (e.g., a potassium compound) and an anion source (e.g., polyphosphoric acid) are separately added, resulting in the formation of the above ions and associations thereof in the aqueous electrolyte solution 20.

[0024] The concentration of potassium polyphosphate in the aqueous electrolyte solution 20 is not particularly limited. According to the inventor's new findings, when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 3 mol or more per 1 kg of water, particularly when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 3 mol or more to 6 mol or less per 1 kg of water, particularly when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 4 mol or more to 6 mol or less per 1 kg of water, and particularly when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 4 mol or more to 5 mol or less per 1 kg of water, it is expected that the aqueous electrolyte solution 20 will not only suppress Al elution from the current collector but also improve other properties such as the electrochemical stability of the electrolyte. Furthermore, when the concentration of potassium pyrophosphate in the aqueous electrolyte solution 20 is such a concentration, it is easy to obtain an aqueous electrolyte solution 20 that does not have a freezing point at or above −40° C.

[0025] 2.2.2 Potassium hydrogen phosphate and phosphoric acid (K ３－ｘ H ｘ P.O. ４ (1≦x≦3)), polyphosphoric acid, and carboxylic acid. The aqueous electrolyte solution 20 contains at least one of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and carboxylic acid dissolved in the water. "Potassium hydrogen phosphate" refers to potassium monohydrogen phosphate (K ２ HPO４ ) and potassium dihydrogen phosphate (KH ２ P.O. ４ ) may be one or both of them. According to the knowledge of the present inventors, when one or both of potassium monohydrogen phosphate and potassium dihydrogen phosphate are dissolved in the aqueous electrolyte solution 20, a better effect of suppressing Al elution is likely to be obtained. The "carboxylic acid" may be a monocarboxylic acid or a polycarboxylic acid, for example, acetic acid. In the aqueous electrolyte solution 20, "potassium hydrogen phosphate dissolved in water" and "phosphoric acid dissolved in water" are not limited to K ＋ , H ＋ , P.O. ４ ３－ , K.P.O. ４ ２－ , H.P.O. ４ ２－ , K. ２ P.O. ４ － , H ２ P.O. ４ － , KHPO ４ － The "polyphosphoric acid dissolved in water" may exist as an ion such as H ＋ , polyphosphate anion, or the above-mentioned potassium polyphosphate-derived ion, and the "carboxylic acid dissolved in water" may be present as an associated compound with the ion. ＋ , carboxylate anions, or may exist as an associated compound with ions derived from the potassium polyphosphate. In the aqueous electrolyte solution 20, the ions and associated compounds contained in the aqueous electrolyte solution 20 can be converted into potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, or carboxylic acid, thereby specifying the "concentration of potassium hydrogen phosphate dissolved in water," the "concentration of phosphoric acid dissolved in water," the "concentration of polyphosphoric acid dissolved in water," and the "concentration of carboxylic acid dissolved in water." In the present application, "potassium hydrogen phosphate dissolved in water" refers to the aqueous electrolyte solution 20 in which a cation source (e.g., a potassium compound) and an anion source (e.g., phosphoric acid) are separately added, resulting in the aqueous electrolyte solution 20 containing potassium hydrogen phosphate. ＋ , H ＋, P.O. ４ ３－ , K.P.O. ４ ２－ , H.P.O. ４ ２－ , K. ２ P.O. ４ － , H ２ P.O. ４ － , KHPO ４ － The same applies to "carboxylic acid dissolved in water."

[0026] The concentrations of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and carboxylic acid in the aqueous electrolyte solution 20 are not particularly limited. According to the new findings of the present inventors, an excellent effect of suppressing Al elution can be obtained by dissolving potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and carboxylic acid together with potassium pyrophosphate in the aqueous electrolyte solution 20. In the aqueous electrolyte solution 20, the molar ratio of the total of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and carboxylic acid to potassium polyphosphate ([potassium hydrogen phosphate + phosphoric acid + polyphosphoric acid + carboxylic acid] / potassium polyphosphate) may be, for example, greater than 0 and 20.00 or less, greater than 0 and 15.00 or less, greater than 0 and 10.00 or less, greater than 0 and 8.50 or less, greater than 0 and 7.00 or less, greater than 0 and 5.00 or less, greater than 0 and 3.00 or less, or greater than 0 and 1.00 or less.

[0027] According to the new findings of the present inventors, the Al elution suppression effect becomes more pronounced when the aqueous electrolyte solution 20 contains potassium dihydrogen phosphate dissolved in the water and the K concentration in the aqueous electrolyte solution 20 is 11.16 mol / L or more and 14.94 mol / L or less, particularly 11.18 mol / L or more and 13.56 mol / L or less, and further particularly 11.46 mol / L or more and 12.62 mol / L or less.

[0028] 2.2.3 Cations The aqueous electrolyte solution 20 may contain potassium ions as cations. In the aqueous electrolyte solution 20, some of the potassium ions contained in the aqueous electrolyte solution 20 may be converted into "dissolved potassium polyphosphate" and some may be converted into "dissolved potassium hydrogen phosphate." However, the aqueous electrolyte solution 20 may contain more potassium ions than can be converted into potassium polyphosphate and potassium hydrogen phosphate. For example, when producing the aqueous electrolyte solution 20, potassium polyphosphate and potassium hydrogen phosphate may be mixed with water, and other potassium ion sources (e.g., KOH, CH ３ COOK, K. ３ P.O. ４ , K. ５ P ３ O １０ , K. ６ P ４ O １３ , K. ７ P ５ O １６ , (KPO ３ )n, etc.) is added and dissolved, the aqueous electrolyte solution 20 will contain more potassium ions than can be converted into potassium polyphosphate and potassium hydrogen phosphate. The aqueous electrolyte solution 20 may contain other cations to the extent that the above-mentioned problems can be solved. For example, the aqueous electrolyte solution 20 may contain alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc. Furthermore, the aqueous electrolyte solution 20 naturally contains protons.

[0029] 2.2.4 Anions The aqueous electrolyte solution 20 may contain polyphosphate ions (which may exist in a state bound to a cation, as described above) or phosphate ions (which may exist in a state bound to a cation, as described above) as anions. The aqueous electrolyte solution 20 may also contain other anions to the extent that the above-described problems can be solved. For example, the aqueous electrolyte solution 20 may contain anions derived from other electrolytes, as described below. The aqueous electrolyte solution 20 also naturally contains hydroxide ions.

[0030] 2.2.5 Other Components That May Be Included in the Aqueous Electrolyte The aqueous electrolyte 20 may contain other electrolytes. For example, KPF６ , K.B.F. ４ , K. ２ SO ４ , KNO ３ , (CF ３ SO ２ ) ２ NK, KCF ３ SO ３ , (FSO ２ ) ２ N.K., K. ２ HPO ４ , K.H. ２ P.O. ４ , K.P.O. ３ The aqueous electrolyte solution 20 may contain at least one selected from the following. 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 electrolyte solution (100 mol %). In addition to the above electrolytes, the aqueous electrolyte solution 20 may contain various additives.

[0031] 2.3 Other Properties As long as the aqueous electrolyte solution 20 contains the above-described solvent and electrolyte, 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.

[0032] 2.3.1 Freezing Point The aqueous electrolyte 20 may not have a freezing point above −40°C. The presence or absence of a “freezing point” of the aqueous electrolyte 20 is confirmed by differential scanning calorimetry (DSC). The DSC sweep rate is 5°C / min for both temperature decreases and increases, and the sweep range is from room temperature to −120°C and then to 40°C. The DSC atmosphere 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 evaluation, the atmosphere inside the container is air sealed under atmospheric pressure. When the aqueous electrolyte is measured under the above conditions, if a crystallization peak temperature (freezing point temperature) is not confirmed above −40°C, the aqueous electrolyte is deemed to “not have a freezing point above −40°C.” The aqueous electrolyte solution 20 may have no freezing point at −60° C. or higher, may have no freezing point at −80° C. or higher, may have no freezing point at −100° C. or higher, or may have no 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 solution 20 has no freezing point at −40° C. or higher,” the concentration of potassium polyphosphate, potassium hydrogen phosphate, and phosphate (K ３－ｘ H ｘ P.O. ４ It is effective to increase the concentration of the electrolyte 20 and the concentration of the polyphosphoric acid. Since the aqueous electrolyte solution 20 does not have a freezing point at −40° C., the effect of suppressing Al elution is more likely to be enhanced. Furthermore, since the aqueous electrolyte solution 20 does not have a freezing point at −40° C., the aqueous battery 100 can be used even at extremely low temperatures. In other words, the aqueous battery 100 can operate appropriately even in cold climates.

[0033] 2.3.2 Presence or Absence of Salt Precipitation The aqueous electrolyte solution 20 may be one that does not precipitate salt when cooled from 0°C to −40°C. If the aqueous electrolyte solution 20 does not precipitate salt due to temperature changes, 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 solution 20 contains water and potassium polyphosphate and the like dissolved in the water. According to the inventor's findings, the saturated solubility of potassium polyphosphate, potassium hydrogen phosphate, and the like in water has little temperature dependency and changes little at low temperatures below 0°C. In this regard, even when the aqueous electrolyte solution 20 is cooled from 0°C to −40°C, salt precipitation in the aqueous electrolyte solution 20 is unlikely to occur.

[0034] 2.3.3 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 or the like is dissolved in the aqueous electrolyte solution 20 at a high concentration, the aqueous electrolyte solution 20 may have a viscosity equal to or higher than a certain level. From the above perspective, the aqueous electrolyte solution 20 may have a viscosity of 20 mPa·s or more and 400 mPa·s or less at 20°C. The viscosity may be 350 mPa·s or less, 300 mPa·s or less, 250 mPa·s or less, or 200 mPa·s or less.

[0035] 2.3.4 pH The pH of the aqueous electrolyte solution 20 is not particularly limited. However, if the pH is too high, there is a risk that the oxidation-side potential window of the aqueous electrolyte solution will be narrowed. In this regard, the pH of the aqueous electrolyte solution 20 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, and may be 12 or less, 11 or less, 10 or less, or 9 or less. In particular, when the pH of the aqueous electrolyte solution 20 is 3 or more and 12 or less, particularly 4 or more and 11 or less, and even more particularly 5 or more and 10 or less, a more excellent effect of suppressing Al elution is likely to be obtained.

[0036] 2.4 Modifications In one embodiment, the aqueous electrolyte solution 20 may have the following configuration: That is, the aqueous electrolyte solution 20 according to one embodiment is characterized by containing water, polyphosphate ions, at least one of hydrogen phosphate ions, phosphate ions, and carboxylate ions, and potassium ions.

[0037] 3. Negative Electrode Any known negative electrode for aqueous batteries can be used as the negative electrode 30. As shown in FIG. 1 , the negative electrode 30 can include a negative electrode active material layer 31 and a negative electrode current collector 32.

[0038] 3.1 Negative Electrode Active Material Layer The negative electrode active material layer 31 contains 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 contain, in addition to the negative electrode active material, a conductive additive, a binder, or the like. The negative electrode active material layer 31 may also contain various other additives. The content of each component in the negative electrode active material layer 31 may be appropriately determined depending on the desired battery performance. For example, the content of the negative 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 100% by mass or less, or 90% by mass or less, where the total weight of the negative electrode active material layer 31 (total solid content) is taken as 100% by 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] The negative electrode active material can be any material that can function as a negative electrode active material for an aqueous battery. The negative electrode active material has a lower charge / discharge potential than the above-described positive electrode active material, and can be appropriately selected in consideration of the potential window of the above-described aqueous electrolyte solution 20, etc. In this embodiment, various ions derived from the aqueous electrolyte solution can serve as charge compensation ions. Specifically, the charge compensation ions that are inserted and removed by the negative electrode active material can be one or more of potassium ions, protons, anions derived from the electrolyte, and hydroxide ions, for example. The negative electrode active material can be, for example, an alkali metal-transition metal composite oxide; titanium oxide; Mo６ S ８ Metal sulfides such as; elemental sulfur; alkali metal-titanium complex phosphate compounds; NASICON type compounds; WO ３ The negative electrode active material may be a hydrogen storage alloy. Alternatively, the negative electrode active material may be an inorganic compound having a crystal structure belonging to space group I23. 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, and Ni, and both element A and element M may be Bi. The negative electrode active material may de-insert and de-insert charge compensation ions by intercalation, or may de-insert and de-insert charge compensation ions 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.

[0040] The shape of the negative electrode active material may be any shape that can function as a negative electrode active material for a battery. The negative electrode active material may be, for example, particulate. The negative electrode active material may be solid, hollow, void-containing, or porous. The negative electrode active material may be primary particles or secondary particles formed by aggregation of multiple 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.

[0041] Examples of conductive additives that can be contained 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, stainless steel, and the like. The conductive additive may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive additive may be used alone, or two or more types may be used in combination.

[0042] Examples of binders that can be contained 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.

[0043] 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 above-mentioned 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 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 or may not contain Al. Furthermore, when the above-mentioned positive electrode current collector 12 does not contain Al, the negative electrode current collector 32 contains Al.

[0044] When the negative electrode current collector 32 contains Al, the negative electrode current collector 32 may be made entirely of Al, or at least a portion 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 portion of its surface that comes into contact with the aqueous electrolyte solution 20, or may have Al present over the entire surface that comes into contact with the aqueous electrolyte solution 20.

[0045] The negative electrode current collector 32 may be in the form of a foil, plate, mesh, punched metal, foam, or the like. The negative electrode current collector 32 may be made of a metal foil or metal mesh. Metal foil is particularly easy to handle. The negative electrode current collector 32 may be made of multiple foils. Examples of metal materials 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, preferably contains Al. Al, Ti, Pb, Zn, Sn, Mg, Zr, and In all have low work functions, and are therefore thought to make it difficult for electrolysis of the aqueous electrolyte solution 20 to occur even when the negative electrode current collector 32 comes into contact with the aqueous electrolyte solution 20 at a reduction potential. The negative electrode current collector 32 may be a metal foil or a substrate plated or vapor-deposited with any of the above metals. Furthermore, 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, or 1 mm or less or 100 μm or less.

[0046] 4. Other Configurations In the aqueous battery 100, various ions derived from the aqueous electrolyte solution 20 can function as carrier ions and charge compensation ions. The aqueous battery 100 may be an aqueous battery (aqueous proton battery) in which protons function as carrier ions and charge compensation ions, an aqueous battery (aqueous potassium ion battery) in which potassium ions function as carrier ions and charge compensation ions, an aqueous battery (aqueous hydroxide ion battery) in which hydroxide ions function as carrier ions and charge compensation ions, an aqueous battery (aqueous polyphosphate anion battery) in which polyphosphate anions function as carrier ions and charge compensation ions, or an aqueous battery in which other ions derived from the aqueous electrolyte solution 20 function as carrier ions and charge compensation ions. In the aqueous battery 100, multiple types of ions derived from the aqueous electrolyte solution 20 may function as carrier ions and charge compensation ions. In addition to the basic configuration described above, the aqueous battery 100 may also have other configurations, such as those described below.

[0047] 4.1 Separator As described above, in the aqueous battery 100, a 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, it 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.

[0048] 4.2 Bipolar Structure As described above, in the aqueous 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 battery 100, a current collector containing Al may be used as a bipolar current collector that serves both as 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 a single current collector. FIG. 2 shows an example of a bipolar structure. As shown in FIG. 2, the aqueous battery 100 may have a bipolar structure, in which a positive electrode active material layer 11 is formed on one surface of an Al-containing current collector 50 (a bipolar current collector that functions as both the positive electrode current collector 12 and the negative electrode current collector 32), and a negative electrode active material layer 31 is formed on the other surface of the current collector 50. In this case, the current collector 50 containing Al may be impermeable to liquid, that is, may be impermeable to the aqueous electrolyte solution 20 from the positive electrode active material layer 11 through the current collector 50 to the negative electrode active material layer 31, and vice versa.

[0049] 4.3 Terminals, etc. In addition to the above components, the aqueous battery 100 may also include terminals, a battery case, etc. Other components will be obvious to those skilled in the art after reading this application, and therefore will not be described here.

[0050] 5. Method for Manufacturing Aqueous Battery The aqueous battery 100 of the present disclosure can be manufactured, for example, as follows.

[0051] 5.1 Method for Producing Aqueous Electrolyte Solution The aqueous electrolyte solution 20 can be produced, for example, by mixing water, potassium polyphosphate, and at least one of potassium hydrogen phosphate, phosphoric acid, and polyphosphoric acid. Alternatively, the aqueous electrolyte solution 20 can be produced by mixing water, a potassium ion source, a polyphosphate ion source, and a phosphate ion source. The mixing means is not particularly limited, and known mixing means can be used. Simply filling a container with water, potassium polyphosphate, and at least one of potassium hydrogen phosphate, phosphoric acid, and polyphosphoric acid and leaving it alone will allow the components to mix together, ultimately producing the aqueous electrolyte solution 20.

[0052] 5.2 Manufacturing of Positive Electrode The positive electrode 10 is manufactured, for example, as follows. The positive electrode active material and other components that constitute the positive electrode active material layer 11 are 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 coating method, in addition to the doctor blade method, electrostatic coating, dip coating, spray coating, and the like can also be used.

[0053] 5.3 Manufacturing of Negative Electrode The negative electrode 30 is manufactured, for example, as follows. The negative electrode active material and other components that constitute the negative electrode active material layer 31 are 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 coating method, in addition to the doctor blade method, electrostatic coating, dip coating, spray coating, and the like can also be used.

[0054] 5.4 Placement in a Battery Case, etc. The aqueous electrolyte solution 20, positive electrode 10, and negative electrode 30 are placed 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, a 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 needed. The laminate is placed in a battery case, and the battery case is filled with the aqueous electrolyte solution 20. The laminate and the electrolyte solution are sealed in the battery case so that the laminate is immersed in the aqueous electrolyte solution 20, and the aqueous battery 100 can be obtained.

[0055] 6. Effects of the Aqueous Battery of the Present Disclosure The aqueous battery 100 of the present disclosure has the following effects, making it possible to suppress the elution of Al from the current collector into the aqueous electrolyte solution 20.

[0056] 6.1 Effects of Al Contained in the Positive Electrode Current Collector During battery charge and discharge, the positive electrode potential becomes an oxidizing potential. Therefore, the Al contained in the positive electrode current collector is prone to release electrons and dissolve. Specifically, the Al contained in the positive electrode current collector dissolves into the aqueous electrolyte while coordinating with anions and water molecules contained in the aqueous electrolyte. Potassium polyphosphate is dissolved in the aqueous electrolyte solution 20 of the aqueous battery 100 of the present disclosure. In other words, anions derived from potassium polyphosphate, such as polyphosphate ions, may be present in the aqueous electrolyte solution 20. Therefore, in the aqueous battery 100 of the present disclosure, the Al contained in the positive electrode current collector 12 is prone to coordinate with anions derived from potassium polyphosphate during battery charge and discharge. Here, for example, aluminum polyphosphate has extremely low solubility in the aqueous electrolyte solution 20. Therefore, the Al coordinated with anions derived from potassium polyphosphate quickly precipitates as a solid. In other words, an insoluble or poorly soluble Al compound precipitates 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 aqueous 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.

[0057] The aqueous electrolyte solution 20 of the aqueous battery 100 of the present disclosure contains potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and / or carboxylic acid dissolved therein. According to the inventor's new findings, dissolving potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and / or carboxylic acid together with potassium polyphosphate in the aqueous electrolyte solution 20 significantly increases the onset potential of passivation, thereby significantly improving the effect of suppressing Al elution. Dissolving potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and / or carboxylic acid together with potassium polyphosphate in the aqueous electrolyte solution 20 can lower the pH of the aqueous electrolyte solution 20 while still achieving the passivation effect of potassium polyphosphate. It is also believed that the effects of mixing polyphosphate anions with other anions in the aqueous electrolyte solution and the effects of increasing the anion concentration are exerted. That is, it is believed that the Al passivation film formed on the surface of the current collector is more stabilized while maintaining the performance of the aqueous electrolyte solution 20.

[0058] 6.2 Effects of Al Contained in the Negative Electrode Current Collector During charging and discharging of the battery, the potential of the negative electrode becomes a 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, making it easier for Al contained in the negative electrode current collector to dissolve into the aqueous electrolyte. In contrast, in the aqueous battery 100 disclosed herein, as described above, potassium polyphosphate is dissolved in the aqueous electrolyte 20. Therefore, in the aqueous battery 100 disclosed herein, even if Al contained in the negative electrode current collector 32 dissolves into the aqueous electrolyte 20 during charging and discharging of the battery, it quickly coordinates with anions derived from potassium polyphosphate to form an Al compound and precipitate as a solid. In other words, an insoluble or poorly soluble Al compound precipitates near the surface of the negative electrode current collector 32, and the Al compound adheres to the surface of the negative electrode current collector 32, forming a protective film (passive film) on the surface. As a result, in the aqueous battery 100 of the present disclosure, the protective film can suppress the elution of Al from the negative electrode current collector 32 into the aqueous electrolyte solution 20.

[0059] Furthermore, in the aqueous battery 100 of the present disclosure, as described above, potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and / or carboxylic acid are dissolved in the aqueous electrolyte solution 20. This makes it easier to maintain a low pH in the aqueous electrolyte solution 20 compared to when potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and / or carboxylic acid are not dissolved. That is, in the aqueous battery 100 of the present disclosure, an increase in pH near the negative electrode is easily suppressed, and the solubility of Al near the negative electrode is easily reduced. It is also believed that the effects of mixing polyphosphate anions with other anions in the aqueous electrolyte solution and the effects of increasing the anion concentration are exerted. This is also believed to contribute to the reduced elution of Al contained in the negative electrode current collector into the aqueous electrolyte solution 20.

[0060] 6.3 Other Effects In aqueous electrolyte batteries, current collectors containing Ti or Ni are used to prevent corrosion of the current collectors (see, for example, Patent Document 1). It has been considered difficult to use metals other than these because they leach out at the positive electrode potential, for example. However, because Ti and Ni are expensive, alternative technologies using cheaper metals are needed to widely popularize aqueous electrolyte batteries. In this regard, the aqueous battery 100 disclosed herein uses a current collector containing Al, thereby reducing the overall cost of the battery, while the use of the aqueous electrolyte 20 described above can suppress the leaching of Al from the current collector into the aqueous electrolyte.

[0061] 7. Uses of Aqueous Batteries As described above, the aqueous battery of the present disclosure can suppress the elution of Al from the current collector into the aqueous electrolyte. In other words, battery degradation is easily suppressed. Such aqueous batteries can be suitably used in at least one type of vehicle selected from, for example, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (BEVs). That is, the technology of the present disclosure also has an aspect of a vehicle having an aqueous battery, the aqueous battery including a positive electrode, an aqueous electrolyte, and a negative electrode, one or both of the positive electrode and the negative electrode including a current collector containing Al, and the aqueous electrolyte including water, potassium polyphosphate dissolved in the water, and at least one of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and a carboxylic acid dissolved in the water. Details of the aqueous electrolyte and the battery configuration are as described above.

[0062] 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.

[0063] 1. Comparative Examples 1 to 6 1.1 Preparation of aqueous electrolyte solution Potassium pyrophosphate (K ４ P ２ O ７ ) was dissolved at a predetermined concentration to obtain aqueous electrolyte solutions according to Comparative Examples 1 to 6. ４ P ２ O ７ The concentrations of , K and pH are as shown in Table 1. The K concentration was measured by ICP, and the pH was measured by a pH meter (Seven Multi, manufactured by Mettler Trade).

[0064] 1.2 Electrochemical Measurement An electrochemical cell (SB1A, manufactured by InterChem) was prepared using Al foil as the working electrode, Ni foil as the counter electrode, Ag / AgCl as the reference electrode, and the above aqueous electrolyte as the electrolyte. The electrochemical cell was evaluated by cyclic voltammetry under the following conditions: Sweep rate: 1 mV / s Sweep range: OCP to 1.0 V (vs. Ag / AgCl) Number of cycles: 2 cycles each

[0065] After the electrochemical measurements, the degree of Al foil elution in the aqueous electrolyte was evaluated using the cyclic voltammogram from the second cycle, using three levels: A, B, and C. The results are shown in Table 1. A: When the potential was increased from OCP, no current flowed until the potential reached a certain level (i.e., the Al foil surface was passivated, and Al foil elution did not occur until the potential reached a certain level). B: When the potential was increased from OCP, current flowed immediately after the potential increase, but the current gradually converged as the potential was further increased (i.e., the Al foil elution reaction proceeded immediately after the potential increase, but further increase in potential resulted in an equilibrium between passivation of the Al foil surface and elution, suppressing the increase in the Al foil elution rate and achieving a suppression effect similar to that of Comparative Example 4). C: When the potential was increased from OCP, current flowed immediately after the potential increase, but the current did not converge even when the potential was further increased, resulting in a Faradaic current (i.e., Al foil elution continued regardless of the potential).

[0066] 1.3 Evaluation of Low-Temperature Stability After the aqueous electrolyte solution was kept in a thermostatic chamber at -60°C for 8 hours or more, the state of the aqueous electrolyte solution was visually observed to confirm whether it had frozen. If the aqueous electrolyte solution did not freeze, it can be said that the aqueous electrolyte solution does not have a freezing point at -60°C or higher. The results are shown in Table 1 below.

[0067] 1.4 Evaluation results

[0068] As is clear from the results shown in Table 1, it is difficult to suppress the dissolution of Al foil when the potassium pyrophosphate concentration in the aqueous electrolyte is less than 4 mol / kg (Comparative Examples 1 to 3), whereas it is possible to suppress the dissolution of Al foil when the potassium pyrophosphate concentration in the aqueous electrolyte is 4 mol / kg or more (Comparative Examples 4 to 6). However, in Comparative Examples 4 to 6, the aqueous electrolyte has a weak alkaline pH of about 12, so there is a possibility that the effect of passivating Al in the aqueous electrolyte is not fully exerted.

[0069] Figure 3 shows the cyclic voltammogram for Comparative Example 5. As shown in Figure 3, Comparative Example 5 exhibits a behavior in which a current flows due to the dissolution of the Al foil with a slight increase in potential from the OCP, and then the current converges (passivation occurs). A small amount of current continues to flow even after the sweep is turned back, and no clear change in the shape of the CV curve is observed even after two cycles have passed. This is thought to be because the state of the passive film formed on the surface of the Al foil is reset by returning the potential to the OCP. It is possible that a strong passive film exists only when a potential is applied, suppressing the dissolution of Al. In other words, it can be said that Comparative Example 5 has room for improvement in terms of the stability of the Al-containing current collector.

[0070] 2. Examples 1 to 20 2.1 Preparation of aqueous electrolyte solution Potassium pyrophosphate (K ４ P ２ O ７ ) and potassium dihydrogen phosphate (KH ２ P.O. ４ ) were dissolved at predetermined concentrations to obtain aqueous electrolyte solutions according to Examples 1 to 20. ４ P ２ O ７ Concentration of K ４ P ２ O ７ The molar ratio of additive to K ４ P ２ O ７ ), K concentration, and pH are as shown in Table 2 below.

[0071] 2.2 Electrochemical Measurement Electrochemical cells were prepared using the aqueous electrolyte solutions of Examples 1 to 20 in the same manner as in Comparative Examples 1 to 6, and were evaluated by cyclic voltammetry, with the evaluation being performed on the three levels A, B, and C. The results are shown in Table 2 below.

[0072] 2.3 Evaluation of Low-Temperature Stability For the aqueous electrolyte solutions of Examples 1 to 20, whether or not the aqueous electrolyte solutions were frozen was confirmed in the same manner as in Comparative Examples 1 to 6. The results are shown in Table 2 below.

[0073] 2.4 Evaluation results

[0074] As is clear from the results shown in Tables 1 and 2, dissolving potassium dihydrogen phosphate together with potassium pyrophosphate in the aqueous electrolyte enhances the effect of suppressing elution of Al foil. In particular, when the aqueous electrolyte contains potassium dihydrogen phosphate together with potassium pyrophosphate at a concentration of 3 mol / kg or more, and particularly when the aqueous electrolyte contains potassium dihydrogen phosphate together with potassium pyrophosphate at a concentration of 4 mol / kg or more, the effect of suppressing elution of Al is more pronounced.

[0075] FIG. 4 shows the cyclic voltammogram for Example 17. As shown in FIG. 4, in Example 17, the current ceases immediately after the first cycle when the potential is increased from the OCP. In the second cycle, the current ceases to flow unless the potential is swept significantly more noble than the OCP, and the shape of the CV curve changes significantly. This suggests that after a strong passive film is formed on the surface of the Al foil in the first cycle, the passive film continues to exist on the surface of the Al foil even after the application of the potential is stopped. It is believed that the significant Al elution suppression effect of Example 17 was achieved due to the low pH of the aqueous electrolyte, the mixed effect of pyrophosphate anions and phosphate anions in the aqueous electrolyte, and the increased anion concentration. Similar trends were observed in other Examples besides Example 17, particularly in Examples 11 to 20. As described above, dissolving potassium dihydrogen phosphate together with potassium pyrophosphate in the aqueous electrolyte enhanced the Al elution suppression effect and improved the stability of the Al-containing current collector.

[0076] 3. Examples 21 to 53 3.1 Preparation of aqueous electrolyte solution Potassium pyrophosphate (K ４ P ２ O ７ ) and potassium monohydrogen phosphate (K ２ HPO ４ ) were dissolved at predetermined concentrations to obtain aqueous electrolyte solutions according to Examples 21 to 53. ４ P ２ O ７ Concentration of K ４ P ２ O７ The molar ratio of additive to K ４ P ２ O ７ ), K concentration, and pH are as shown in Table 3 below.

[0077] 3.2 Electrochemical Measurement Electrochemical cells were prepared using the aqueous electrolyte solutions of Examples 21 to 53 in the same manner as in Comparative Examples 1 to 6, and were evaluated by cyclic voltammetry, with the evaluation being performed on the three levels A, B, and C. The results are shown in Table 3 below.

[0078] 3.3 Evaluation of Low-Temperature Stability For the aqueous electrolyte solutions of Examples 21 to 53, whether or not the aqueous electrolyte solutions were frozen was confirmed in the same manner as in Comparative Examples 1 to 6. The results are shown in Table 3 below.

[0079] 3.4 Evaluation results

[0080] As is clear from the results shown in Tables 1 and 3, dissolving potassium monohydrogen phosphate together with potassium pyrophosphate in the aqueous electrolyte enhances the effect of suppressing elution of Al foil. In particular, when the aqueous electrolyte contains potassium monohydrogen phosphate together with potassium pyrophosphate at a concentration of 3 mol / kg or more, and particularly when the aqueous electrolyte contains potassium monohydrogen phosphate together with potassium pyrophosphate at a concentration of 4 mol / kg or more, the effect of suppressing elution of Al is more pronounced.

[0081] FIG. 5 shows a cyclic voltammogram for Example 47. As shown in FIG. 5, in Example 47, similar to Example 17 shown in FIG. 4, the current ceases immediately after the first cycle turning point when the potential is increased from the OCP, and in the second cycle, the current ceases to flow unless the potential is swept significantly to the noble potential side relative to the OCP, resulting in a significant change in the shape of the CV curve. Similar trends were observed for Examples other than Example 47, and similar results were obtained particularly for Examples 37 to 53. As described above, dissolving potassium monohydrogen phosphate together with potassium pyrophosphate in the aqueous electrolyte enhanced the effect of suppressing Al elution and improved the stability of the Al-containing current collector.

[0082] 4. Examples 54 to 65 4.1 Preparation of aqueous electrolyte solution Potassium pyrophosphate (K ４ P ２ O ７ ) and phosphoric acid (H ３ P.O. ４ ) were dissolved at predetermined concentrations to obtain aqueous electrolyte solutions according to Examples 54 to 65. ４ P ２ O ７ Concentration of K ４ P ２ O ７ The molar ratio of additive to K ４ P ２ O ７ ), and pH are as shown in Table 4 below.

[0083] 4.2 Electrochemical Measurement Electrochemical cells were prepared in the same manner as in Comparative Examples 1 to 6 using the aqueous electrolyte solutions of Examples 54 to 65, and were evaluated by cyclic voltammetry, with the evaluation being performed on the three levels A, B, and C. The results are shown in Table 4 below.

[0084] 4.3 Evaluation of Low-Temperature Stability For the aqueous electrolyte solutions of Examples 54 to 65, whether or not the aqueous electrolyte solutions were frozen was confirmed in the same manner as in Comparative Examples 1 to 6. The results are shown in Table 4 below.

[0085] 4.4 Evaluation results

[0086] As is clear from the results shown in Tables 1 and 4, dissolving phosphoric acid together with potassium pyrophosphate in the aqueous electrolyte solution enhances the effect of suppressing the elution of Al foil, and improves the stability of the current collector containing Al.

[0087] 5. Examples 66 to 82 5.1 Preparation of aqueous electrolyte solution Potassium pyrophosphate (K ４ P ２ O ７ ) and pyrophosphate (H ４ P ２ O ７ ) were dissolved at predetermined concentrations to obtain aqueous electrolyte solutions according to Examples 66 to 82. ４ P ２ O７ Concentration of K ４ P ２ O ７ The molar ratio of additive to K ４ P ２ O ７ ), and pH are as shown in Table 5 below.

[0088] 5.2 Electrochemical Measurement Electrochemical cells were prepared using the aqueous electrolyte solutions of Examples 66 to 82 in the same manner as in Comparative Examples 1 to 6, and were evaluated by cyclic voltammetry, with the evaluation being performed on the three levels A, B, and C. The results are shown in Table 5 below.

[0089] 5.3 Evaluation of Low-Temperature Stability For the aqueous electrolyte solutions of Examples 66 to 82, whether or not the aqueous electrolyte solutions were frozen was confirmed in the same manner as in Comparative Examples 1 to 6. The results are shown in Table 5 below.

[0090] 5.4 Evaluation results

[0091] As is clear from the results shown in Tables 1 and 5, dissolving pyrophosphoric acid together with potassium pyrophosphate in the aqueous electrolyte solution enhances the effect of suppressing the elution of Al foil, and improves the stability of the current collector containing Al.

[0092] 5. Examples 83 to 91 5.1 Preparation of aqueous electrolyte solution Potassium pyrophosphate (K ４ P ２ O ７ ) and acetic acid (CH ３ COOH) was dissolved at a predetermined concentration to obtain the aqueous electrolyte solutions of Examples 83 to 91. ４ P ２ O ７ Concentration of K ４ P ２ O ７ The molar ratio of additive to K ４ P ２ O ７ ), and pH are as shown in Table 6 below.

[0093] 5.2 Electrochemical Measurement Electrochemical cells were prepared using the aqueous electrolyte solutions of Examples 83 to 91 in the same manner as in Comparative Examples 1 to 6, and were evaluated by cyclic voltammetry, with the evaluation being performed on the three levels A, B, and C. The results are shown in Table 6 below.

[0094] 5.3 Evaluation of Low-Temperature Stability For the aqueous electrolyte solutions of Examples 83 to 91, whether or not the aqueous electrolyte solutions were frozen was confirmed in the same manner as in Comparative Examples 1 to 6. The results are shown in Table 6 below.

[0095] 5.4 Evaluation results

[0096] As is clear from the results shown in Tables 1 and 6, dissolving acetic acid together with potassium pyrophosphate in the aqueous electrolyte solution enhances the effect of suppressing the elution of Al foil, and improves the stability of the current collector containing Al.

[0097] 5. Supplementary Information All of the aqueous electrolyte solutions of Examples 1 to 91 had high ionic conductivity exceeding 10 mS / cm. Furthermore, all of the aqueous electrolyte solutions of Examples 1 to 91 had a viscosity of 20 mPa·s or more and 400 mPa·s or less at 20°C. Furthermore, all of the aqueous electrolyte solutions of Examples 1 to 91 did not precipitate salt when cooled from 0°C to -40°C.

[0098] In the above examples, potassium pyrophosphate was dissolved in the aqueous electrolyte as potassium polyphosphate. However, the potassium polyphosphate dissolved in the aqueous electrolyte is not limited to potassium pyrophosphate. The inventors have confirmed that the passivation effect of the Al foil surface can be obtained and the same effect as above can be exhibited even when a potassium polyphosphate other than potassium pyrophosphate (e.g., potassium tripolyphosphate) is dissolved in the aqueous electrolyte instead of or together with potassium pyrophosphate.

[0099] In the above examples, potassium hydrogen phosphate, phosphoric acid, pyrophosphoric acid, or acetic acid was dissolved in the aqueous electrolyte as an additive. However, the additive dissolved in the aqueous electrolyte is not limited to potassium hydrogen phosphate, phosphoric acid, pyrophosphoric acid, or acetic acid. Any additive that can lower the pH of the aqueous electrolyte and does not adversely affect the electrochemical reaction can be used. For example, even if a polyphosphoric acid other than pyrophosphoric acid or a carboxylic acid other than acetic acid is added to the aqueous electrolyte, the passivation effect on the Al foil surface is obtained by a similar mechanism, and the same effect as above is thought to be exhibited.

[0100] 6. Summary From the above results, it can be said that aqueous batteries having the following configurations can suppress the elution of Al from Al-containing current collectors into the aqueous electrolyte.

[0101] (1) A battery includes a positive electrode, an aqueous electrolyte, and a negative electrode. (2) One or both of the positive electrode and the negative electrode includes a current collector containing Al. (3) The current collector is in contact with the aqueous electrolyte. (4) The aqueous electrolyte includes water, potassium polyphosphate dissolved in the water, and at least one of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and a carboxylic acid dissolved in the water.

[0102] REFERENCE SIGNS LIST 10 Positive electrode 11 Positive electrode 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 Aqueous battery

Claims

1. An aqueous battery comprising a positive electrode, an aqueous electrolyte, and a negative electrode, one or both of the positive electrode and the negative electrode having a current collector containing Al, the current collector being in contact with the aqueous electrolyte, and the aqueous electrolyte comprising: water, potassium polyphosphate dissolved in the water, and at least one of potassium hydrogen phosphate, phosphoric acid, polyphosphoric acid, and a carboxylic acid dissolved in the water.

2. The aqueous battery according to claim 1, wherein the aqueous electrolyte contains the potassium polyphosphate dissolved in a concentration of 3 mol or more per kg of water.

3. The aqueous battery according to claim 1, wherein the aqueous electrolyte contains the potassium polyphosphate dissolved in a concentration of 3 mol to 6 mol per kg of water.

4. The aqueous battery according to claim 1, wherein the aqueous electrolyte contains the potassium polyphosphate dissolved in a concentration of 4 mol or more and 5 mol or less per kg of water.

5. An aqueous battery according to any one of claims 1 to 4, wherein the aqueous electrolyte contains potassium dihydrogen phosphate dissolved in the water, and the K concentration in the aqueous electrolyte is 11.16 mol / L or more and 14.94 mol / L or less.

6. The aqueous battery according to any one of claims 1 to 5, wherein the aqueous electrolyte does not have a freezing point at -40°C or higher.

7. The aqueous battery according to any one of claims 1 to 6, wherein the aqueous electrolyte is not accompanied by precipitation of salt when cooled from 0°C to -40°C.

8. The aqueous battery according to any one of claims 1 to 7, wherein the aqueous electrolyte has a viscosity of 20 mPa·s or more and 400 mPa·s or less at 20°C.

9. The aqueous battery according to any one of claims 1 to 8, wherein the aqueous electrolyte has a pH of 3 or more and 12 or less.

10. The aqueous battery according to any one of claims 1 to 9, wherein at least the positive electrode has the current collector.

11. An aqueous battery according to any one of claims 1 to 10, having a bipolar structure, in which 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.

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