Non-aqueous battery

Abstract

To provide a technology that can suppress the elution of Al from a current collector into an aqueous electrolyte when an Al-containing current collector is used in a non-aqueous battery.SOLUTION: A non-aqueous battery includes a positive electrode, an aqueous electrolyte, and a negative electrode. One or both of the positive electrode and the negative electrode have a current collector containing Al. The current collector is in contact with the aqueous electrolyte. The aqueous electrolyte contains water and potassium polyphosphate dissolved in the water. At least one of protons, hydroxide ions, and polyphosphate ions contained in the aqueous electrolyte acts as carrier ions, and the aqueous electrolyte does not have a freezing point at a temperature of -40°C or higher.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Patent Document 1 discloses an aqueous electrolyte solution for use in aqueous batteries, which contains 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 wide reduction-side potential window of the aqueous electrolyte solution makes it easy to suppress decomposition of the aqueous electrolyte solution on the negative electrode surface 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. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-220294 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional aqueous batteries have room for improvement in terms of corrosion prevention for 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. When an Al-containing current collector is used in an aqueous battery, a new technology is needed to suppress Al leaching from the current collector into the aqueous electrolyte. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> An aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, one or both of the positive electrode and the negative electrode has a current collector containing Al; the current collector is in contact with the aqueous electrolyte; the aqueous electrolyte solution comprises water and potassium polyphosphate dissolved in the water, at least one of protons, hydroxide ions, and polyphosphate ions contained in the aqueous electrolyte acts as a carrier ion; The aqueous electrolyte does not have a freezing point at -40°C or higher. water-based battery. <Aspect 2> When the aqueous electrolyte is cooled to a temperature of 0°C to -40°C, no salt is precipitated. 2. The aqueous battery of embodiment 1. <Aspect 3> At least the positive electrode has the current collector. The aqueous battery of embodiment 1 or 2. <Aspect 4> the aqueous electrolyte solution contains the water and the potassium polyphosphate dissolved at a concentration of 4 mol or more and 7 mol or less per 1 kg of the water; The aqueous battery of any one of Aspects 1 to 3. <Aspect 5> the aqueous electrolyte has a viscosity of 40 mPa·s or more and 350 mPa·s or less at 20°C; The aqueous battery of any one of Aspects 1 to 4. <Aspect 6> The battery has a bipolar structure, and a positive electrode active material layer is formed on one surface of the current collector, and a negative electrode active material layer is formed on the other surface of the current collector. The aqueous battery of any one of embodiments 1 to 5. [Effects of the Invention]

[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. [Brief explanation of the drawings]

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

[0008] 1. Water-based battery Hereinafter, an aqueous battery according to an embodiment will be described with reference to the drawings. However, the technology of the present disclosure is not limited to the following embodiment. FIG. 1 schematically shows the configuration of an aqueous battery 100 according to an embodiment. As shown in FIG. 1, the aqueous battery 100 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 and potassium polyphosphate dissolved in the water. In the aqueous battery 100, at least one of protons, hydroxide ions, and polyphosphate ions contained in the aqueous electrolyte solution 20 functions as a carrier ion. The aqueous electrolyte solution 20 does not have a freezing point above −40° C.

[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 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, and 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, when the entire positive electrode active material layer 11 (total solid content) is taken as 100 mass%, 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 may be 100 mass% or less, or 90 mass% or less. 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 can adopt any substance that can function as a positive electrode active material of an aqueous battery having at least one of protons, hydroxide ions, and polyphosphate ions as carrier ions. The positive electrode active material has a charging and discharging potential higher than that of the negative electrode active material described later, and can be appropriately selected in consideration of the potential window of the aqueous electrolyte 20 described later. The positive electrode active material is Ni(OH)2 which is a layered nickel hydroxide compound (for example, Japanese Patent Laid-Open No. 2023-154313), or A x K y Ni 1-z M z O 2±δ ·nH2O (A is at least one of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, and Sc, M is at least one of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements, 0≦x<0.5, 0<y≦0.5, 0≦z≦0.5, 0<n≦2, and the relationship of (α·x)+y≦0.5 is satisfied, where α is the valence of the cation of A. For example, Japanese Patent Laid-Open No. 2023-132287), or manganese spinel (for example, LiMn2O4), or various hydroxides and oxides such as nickel manganese cobalt-containing composite oxides (NMC). Further, the positive electrode active material may be an oxide or polyanion containing an alkali metal element. More specifically, it may be a composite oxide of an alkali metal element and a transition metal. The composite oxide is an alkali metal cobalt composite oxide (such as AmCoO2, etc., "Am" is an alkali metal element. The same applies hereinafter), an alkali metal nickel composite oxide (such as AmNiO2, etc.), an alkali metal nickel titanium composite oxide (AmNi 1 / 2 Ti 1 / 2 O2, etc.), an alkali metal nickel manganese composite oxide (AmNi 1 / 2 Mn 1 / 2 O2, AmNi 1 / 3 Mn 2 / 3 O2, etc.), an alkali metal manganese composite oxide (such as AmMnO2, AmMn2O4, etc.), an alkali metal iron manganese composite oxide (Am 2 / 3 Fe 1 / 3 Mn 2 / 3 O2, etc.), an alkali metal nickel cobalt manganese composite oxide (AmNi 1 / 3 Co 1 / 3 Mn1 / 3 The positive electrode active material may be at least one selected from alkali metal iron composite oxides (e.g., AmFeO2), alkali metal chromium composite oxides (e.g., AmCrO2), alkali metal iron phosphate compounds (e.g., AmFePO4), alkali metal manganese phosphate compounds (e.g., AmMnPO4), and alkali metal cobalt phosphate compounds (AmCoPO4). Alternatively, the positive electrode active material may be an organic active material such as Prussian blue. Alternatively, the positive electrode active material may be at least one selected from alkali metal titanium composite oxides, TiO2, sulfur (S), and the like, which exhibit a charge / discharge potential higher than that of the negative electrode active material described below. The positive electrode active material may be one that deintercalates and inserts carrier ions by intercalation, or one that deintercalates and inserts carrier ions by a conversion reaction, an alloying reaction, or the like. One type of positive electrode active material may be used alone, or two or more types may be used in combination.

[0012] The shape of the positive electrode active material may be any shape that can function as a positive electrode active material for a battery. The positive electrode active material may be, for example, particulate. The positive electrode active material may be solid, hollow, porous, 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. The average particle diameter D50 referred to in this application is the particle diameter (median diameter) at 50% of the cumulative value in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0013] 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, and stainless steel. 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.

[0014] 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.

[0015] 1.1.2 Positive electrode current collector As shown in FIG. 1 , the positive electrode 10 may include a positive electrode current collector 12 in contact with the positive electrode active material layer 11. The positive electrode current collector 12 is in contact with an aqueous electrolyte solution 20. Any positive electrode current collector 12 capable of functioning as a positive electrode current collector for an aqueous battery can be used. When a negative electrode current collector 32 (described later) contains Al, the positive electrode current collector 12 may contain Al or may not. Furthermore, when a negative electrode current collector 32 (described later) does not contain Al, the positive electrode current collector 12 contains Al. As will be described later, Al elution from a current collector into an 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.

[0016] When the positive electrode current collector 12 contains Al, the positive electrode current collector 12 may be entirely made of Al, or at least a 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.

[0017] 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, or 1 mm or less, or 100 μm or less.

[0018] 1.2 Aqueous electrolyte The aqueous electrolyte solution 20 includes water and potassium polyphosphate 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.

[0019] 1.2.1 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. Meanwhile, there is no particular upper limit to the proportion of water in the solvent. The solvent may consist of only water (100 mol% water).

[0020] The solvent may contain a solvent other than water in addition to water, to the extent that the above-mentioned problem can be solved, for example, from the viewpoint of forming an SEI (Solid Electrolyte Interphase) 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 of the total amount of solvents constituting the electrolyte solution (100 mol%).

[0021] 1.2.2 Electrolytes An electrolyte is dissolved in the aqueous electrolyte solution 20, and the electrolyte can be dissociated into cations and anions in the aqueous electrolyte solution 20. In the aqueous electrolyte solution 20, the cations and anions may be close to each other to form an association complex.

[0022] 1.2.2.1 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 P2O7), potassium tripolyphosphate (K 5-x H x P3O 10 ), potassium tetrapolyphosphate (K 6-x Hx P4O 13 ), potassium heptapolyphosphate (K 7-x H x P5O 16 Among them, potassium polyphosphates include potassium pyrophosphate (K 4-x H x When potassium pyrophosphate (P2O7) is used, even higher performance is likely to be ensured. In the aqueous electrolyte solution 20, the "potassium polyphosphate dissolved in water" can be dissociated into potassium ions and anions. In the aqueous electrolyte solution 20, the potassium ions may be completely dissociated from the anions, or may form associations by coming close to the anions. For example, when the aqueous electrolyte solution 20 contains potassium pyrophosphate dissolved in water, the "potassium pyrophosphate dissolved in water" in the aqueous electrolyte solution 20 is + , P2O7 4- , KP2O7 3- , K2P2O7 2- , K3P2O7 - It may exist as ions such as these or as associations of these ions, and also as a transition from HO to H + Pull out the HP2O7 3- , H2P2O7 2- , H3P2O7 - Ya, H + and K. + and HK2P2O7 - , HKP2O7 2- , H2KP2O7 - In the aqueous electrolyte solution 20, the "concentration of potassium polyphosphate dissolved in water" is determined by converting the ions, associations, and the like contained in the aqueous electrolyte solution 20 into potassium polyphosphate.

[0023] In the aqueous battery 100 of the present disclosure, the components and concentrations of the aqueous electrolyte solution 20 are determined so that the aqueous electrolyte solution 20 does not have a freezing point at temperatures above −40° C. According to the inventor's new findings, in order to achieve the condition that the aqueous electrolyte solution 20 does not have a freezing point at temperatures above −40° C., the concentration of potassium polyphosphate in the aqueous electrolyte solution 20 should be higher than conventional concentrations. According to the inventor's new findings, when the freezing point of an aqueous electrolyte solution is adjusted by dissolving only potassium polyphosphate in water, the freezing point of the aqueous electrolyte solution decreases as the concentration of potassium polyphosphate relative to water increases, and when the concentration of potassium polyphosphate in the aqueous electrolyte solution reaches a certain level or higher, the freezing point essentially disappears. In other words, when the concentration of potassium polyphosphate in the aqueous electrolyte solution is above a certain level, the aqueous electrolyte solution does not have a freezing point at temperatures above −60° C. For example, when the aqueous electrolyte solution 20 contains water and the potassium polyphosphate dissolved at a concentration of 4 mol or more per kg of water, the freezing point of the aqueous electrolyte solution 20 disappears, and the aqueous electrolyte solution 20 does not have a freezing point at temperatures above −40° C. However, when other components are dissolved in the water in addition to the potassium polyphosphate in the aqueous electrolyte solution 20, the minimum concentration of potassium polyphosphate at which the freezing point of the aqueous electrolyte disappears may be lower than 4 mol / kg. In this regard, the concentration of potassium polyphosphate per kg of water in the aqueous electrolyte solution 20 may be, for example, 2.5 mol or more, 3.0 mol or more, 3.5 mol or more, or 4.0 mol or more.

[0024] As described above, in order to achieve the condition that "the aqueous electrolyte solution 20 does not have a freezing point at temperatures above -40°C," it is effective to adjust the concentration of potassium polyphosphate in the aqueous electrolyte solution 20 to a higher concentration than conventional concentrations. Specifically, this condition is easily met when the aqueous electrolyte solution 20 contains water and the potassium polyphosphate dissolved at a concentration of 4 mol or more per kg of water. While there is no particular upper limit for the concentration of potassium polyphosphate in the aqueous electrolyte solution 20, an excessively high concentration may excessively increase the viscosity of the aqueous electrolyte solution 20, potentially reducing the ionic conductivity of the aqueous electrolyte solution 20. Considering the need to achieve the condition that "the aqueous electrolyte solution 20 does not have a freezing point at temperatures above -40°C" and to maintain the viscosity of the aqueous electrolyte solution 20 within an appropriate range, the aqueous electrolyte solution 20 may contain water and the potassium polyphosphate dissolved at a concentration of 4 mol to 7 mol per kg of water. This concentration may be 6 mol or less.

[0025] The aqueous electrolyte solution 20 may contain potassium ions as cations. The entire amount of potassium ions contained in the aqueous electrolyte solution 20 does not necessarily need to be calculated as "dissolved potassium polyphosphate." That is, the aqueous electrolyte solution 20 may contain more potassium ions than can be calculated as potassium polyphosphate. For example, when producing the aqueous electrolyte solution 20, if a potassium ion source other than potassium polyphosphate (e.g., KOH, CH3COOK, K3PO4, etc.) is added to and dissolved in water along with potassium polyphosphate, the aqueous electrolyte solution 20 will contain more potassium ions than can be calculated as potassium polyphosphate. 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, alkaline earth metal ions, transition metal ions, etc. other than potassium ions.

[0026] The aqueous electrolyte solution 20 may contain polyphosphate ions (which may be present in a state bound to various cations, as described above) as anions. For example, when the aqueous electrolyte solution 20 contains pyrophosphate ions as anions, the pyrophosphate ions are represented by the formula: PO 4- In addition, KP2O7 3- , K2P2O7 2- , K3P2O7 - It may form an aggregate with potassium ions, such as HO to H + Pull out the HP2O7 3- , H2P2O7 2- , H3P2O7 - Ya, H + and K. + and HK2P2O7 - , HKP2O7 2- , H2KP2O7 - In the aqueous electrolyte solution 20, the entire polyphosphate ions contained in the aqueous electrolyte solution 20 do not have to be calculated as "dissolved potassium polyphosphate." That is, the aqueous electrolyte solution 20 may contain polyphosphate ions at a concentration greater than that which can be calculated as potassium polyphosphate. For example, when producing the aqueous electrolyte solution 20, if a polyphosphate ion source other than potassium polyphosphate (e.g., polyphosphoric acid such as H4P2O7) is added to water along with potassium polyphosphate and dissolved therein, the aqueous electrolyte solution 20 will contain polyphosphate ions at a concentration greater than that which can be calculated as potassium polyphosphate. The aqueous electrolyte solution 20 may contain other anions to the extent that the above-mentioned problems can be solved. For example, it may contain anions derived from other electrolytes, as described below.

[0027] 1.2.2.2 Other components that may be contained in aqueous electrolytes The aqueous electrolyte solution 20 may contain other electrolytes, such as at least one selected from KPF6, KBF4, K2SO4, KNO3, CH3COOK, (CF3SO2)2NK, KCF3SO3, (FSO2)2NK, K2HPO4, KH2PO4, and KPO3. 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%).

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

[0029] 1.2.3 Freezing point As described above, the aqueous electrolyte 20 does not have a freezing point 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 increases and decreases, 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 and no crystallization peak temperature (freezing point temperature) is 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 temperatures above −60°C, −80°C, −100°C, or −120°C. In the aqueous battery 100 of the present disclosure, to achieve the condition that the aqueous electrolyte solution 20 has no freezing point at temperatures above −40°C, it is effective to increase the polyphosphoric acid concentration in the aqueous electrolyte solution 20 (e.g., a high concentration of 4 mol or more per kg of water), as described above. Since the aqueous electrolyte solution 20 does not have a freezing point at −40°C, Al contained in the current collector is less likely to leach into the aqueous electrolyte solution 20, as described below. 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. That is, the aqueous battery 100 of the present disclosure operates appropriately even in cold climates.

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

[0031] 1.2.4.1 Presence or absence of salt precipitation The aqueous electrolyte solution 20 preferably does not precipitate salt when cooled from 0°C to -40°C. Since 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 dissolved in the water. According to the inventor's findings, the saturated solubility of potassium polyphosphate in water is little dependent on temperature and changes little at low temperatures below 0°C. In this regard, even if potassium polyphosphate is dissolved at a high concentration (e.g., a high concentration of 4 mol or more per kg of water) in the aqueous electrolyte solution 20 at 0°C and the aqueous electrolyte solution 20 is cooled from 0°C to -40°C, precipitation of potassium polyphosphate in the aqueous electrolyte solution 20 does not substantially occur.

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

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

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

[0035] 1.3.1 Negative electrode active material layer The negative electrode active material layer 31 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, and 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, when the entire negative electrode active material layer 31 (total solid content) is taken as 100 mass%, 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. The shape of the negative electrode active material layer 31 is not particularly limited, and may be, for example, a substantially flat sheet-like negative electrode active material layer. 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, or 2 mm or less, 1 mm or less, or 500 μm or less.

[0036] The negative electrode active material can be any material that functions as a negative electrode active material for an aqueous battery and has at least one of protons, hydroxide ions, and polyphosphate ions as a carrier ion. The negative electrode active material has a lower charge / discharge potential than the above-mentioned positive electrode active material and can be appropriately selected taking into account the potential window of the above-mentioned aqueous electrolyte solution 20. For example, the negative electrode active material may be a potassium-transition metal composite oxide; titanium oxide; metal sulfide such as Mo6S8; elemental sulfur; KTi2(PO4)3; NASICON-type compound; WO3, etc. Alternatively, 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, element A is at least one of Bi and La, and 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 be one that deintercalates and inserts carrier ions by intercalation, or one that deintercalates and inserts carrier ions by a conversion reaction, an 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.

[0037] The shape of the negative electrode active material may be any shape that allows it to 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.

[0038] 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, and stainless steel. 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.

[0039] 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.

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

[0041] When the negative electrode current collector 32 contains Al, the negative electrode current collector 32 may be entirely made of Al, or at least a 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.

[0042] 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 may be 1 mm or less or 100 μm or less.

[0043] 1.4 Other Configurations The aqueous battery 100 may be a secondary battery. The aqueous battery 100 may be an aqueous battery using protons as carrier ions (aqueous proton battery), an aqueous battery using hydroxide ions as carrier ions (aqueous hydroxide ion battery), or an aqueous battery using polyphosphate anions as carrier ions (aqueous polyphosphate anion battery). In particular, when the aqueous battery 100 is an aqueous battery using at least one or both of protons and hydroxide ions as carrier ions, and especially when the aqueous battery uses at least protons as carrier ions, it is believed that high capacity and the like are more likely to be obtained. In addition to the basic configuration described above, the aqueous battery 100 may also have other configurations, such as those described below.

[0044] 1.4.1 Separators 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, the separator 40 may be a separator having a base material made of a nonwoven fabric made of cellulose, or a separator having a base material made of a film made of a resin such as polypropylene. The thickness of the separator 40 is not particularly limited and may be, for example, 5 μm or more and 1 mm or less.

[0045] 1.4.2 Bipolar Structure As described above, in the aqueous 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 liquid-impermeable, i.e., may not allow the aqueous electrolyte solution 20 to pass from the positive electrode active material layer 11 through the current collector 50 to the negative electrode active material layer 31, and vice versa.

[0046] 1.4.3 Terminals etc. In addition to the above 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.

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

[0048] 1.5.1 Manufacturing method of aqueous electrolyte The aqueous electrolyte solution 20 can be produced, for example, by mixing water with potassium polyphosphate. Alternatively, it can be produced by mixing water with a potassium ion source and a polyphosphate ion source. The mixing means is not particularly limited, and known mixing means can be used. Simply filling a container with water and potassium polyphosphate and leaving it to stand will cause them to mix together, ultimately producing the aqueous electrolyte solution 20.

[0049] 1.5.2 Cathode Fabrication 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 application method, in addition to the doctor blade method, electrostatic application, dip coating, spray coating, and the like can also be used.

[0050] 1.5.3 Anode production 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 application method, in addition to the doctor blade method, electrostatic application, dip coating, spray coating, and other methods can also be used.

[0051] 1.5.4 Storage in a battery case, etc. The aqueous electrolyte solution 20, positive electrode 10, and 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 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 components such as terminals are attached to the laminate as needed. The laminate is housed in a battery case, and the battery case is filled with aqueous electrolyte solution 20. The laminate is immersed in the aqueous electrolyte solution 20, and the laminate and the electrolyte are sealed in the battery case to obtain an aqueous battery 100.

[0052] 1.6 Effects in aqueous batteries The aqueous battery 100 of the present disclosure can suppress the elution of Al from the current collector into the aqueous electrolyte solution 20 due to the following effects.

[0053] 1.6.1 Effects of Al in the positive electrode current collector During charging and discharging of the battery, the potential of the positive electrode becomes an oxidizing potential. As a result, the Al contained in the positive electrode current collector releases electrons and becomes prone to dissolution. 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.

[0054] In the aqueous battery 100 of the present disclosure, as described above, the aqueous electrolyte 20 does not have a freezing point above −40° C. That is, in the aqueous electrolyte 20, HO molecules cannot form a crystallization network with each other, and there are few free HO molecules that can be solidified, and there are also few free hydroxide ions. Therefore, Al eluted from the current collector into the aqueous electrolyte 20 is converted into HO and OH. - Therefore, Al dissolved into the aqueous electrolyte solution 20 is less likely to diffuse through the electrolyte, and the Al dissolved into the aqueous electrolyte solution 20 is likely to remain near the current collector. Meanwhile, as described above, potassium polyphosphate is dissolved in the aqueous electrolyte solution 20. That is, 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, Al contained in the positive electrode current collector 12 is likely to be dissolved by coordinating with anions derived from potassium polyphosphate during charging and discharging of the battery. Here, for example, aluminum polyphosphate has extremely low solubility in the aqueous electrolyte solution 20. Therefore, 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.

[0055] 1.6.2 Effects of Al in the negative electrode current collector During battery charging and discharging, 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.

[0056] In contrast, in the aqueous battery 100 of the present disclosure, as described above, the aqueous electrolyte solution 20 does not have a freezing point above −40° C. That is, the aqueous electrolyte solution 20 contains little free water that can be coagulated; in other words, there is little water that can be electrolyzed. This makes it difficult for Al contained in the negative electrode current collector 32 to leach out. Furthermore, as described above, potassium polyphosphate is dissolved in the aqueous electrolyte solution 20, meaning that 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, even if Al contained in the negative electrode current collector 32 leach out into the aqueous electrolyte solution 20 during charging and discharging, the Al quickly coordinates with the 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 further suppress the elution of Al from the negative electrode current collector 32 into the aqueous electrolyte solution 20.

[0057] 1.6.3 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 thought that metals other than these are difficult to use because they leach out, for example, at the positive electrode potential. However, because Ti and Ni are expensive, alternative technologies using less expensive 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, and uses the above-mentioned aqueous electrolyte 20, thereby suppressing the leaching of Al from the current collector into the aqueous electrolyte.

[0058] 2. Method for suppressing Al elution from the current collector of aqueous batteries into the aqueous electrolyte The technology of the present disclosure also has an aspect of being a method for suppressing Al elution from a current collector of an aqueous battery that uses at least one of protons, hydroxide ions, and polyphosphate ions as carrier ions into an aqueous electrolyte. That is, the method of the present disclosure is characterized by using an Al-containing current collector in an aqueous battery that uses at least one of protons, hydroxide ions, and polyphosphate ions as carrier ions, and by using an aqueous electrolyte that satisfies the following requirements (1) and (2). Details of the aqueous electrolyte and the battery configuration are as described above. (1) The aqueous electrolyte solution contains water and potassium pyrophosphate dissolved in the water. (2) The aqueous electrolyte does not have a freezing point above −40° C.

[0059] 3. Vehicles with aqueous batteries As described above, the aqueous battery of the present disclosure can suppress Al elution from the current collector into the aqueous electrolyte solution. This means that battery degradation is likely to be suppressed. Such an aqueous battery can be suitably used in at least one type of vehicle selected from, for example, a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (BEV). Specifically, the technology of the present disclosure also provides a vehicle having an aqueous battery, the aqueous battery comprising a positive electrode, an aqueous electrolyte solution, and a negative electrode, one or both of the positive electrode and the negative electrode comprising a current collector containing Al, the aqueous electrolyte solution comprising water and potassium polyphosphate dissolved in the water, at least one of protons, hydroxide ions, and polyphosphate ions contained in the aqueous electrolyte solution acting as carrier ions, and the aqueous electrolyte solution not having a freezing point above −60° C. Details of the aqueous electrolyte solution and the battery configuration are as described above. [Example]

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

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

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

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

[0064] 4. Evaluation results (constant current) 4.1 In the case of the above condition 1-1 and 0.5m electrolyte When an oxidizing current was applied to an electrochemical cell using a 0.5 mol / kg K4P2O7 aqueous electrolyte, the Al foil on the oxidizing electrode (positive electrode) disappeared, and the electrolyte turned into a white gel. The potential-pH diagram indicated that the standard electrode potential of Al at pH 11 in this aqueous electrolyte was approximately -2.3 V (vs. SHE). Therefore, simply immersing the Al foil in the aqueous electrolyte could lead to Al dissolution (corrosion), and the application of a small oxidizing current is likely to accelerate this dissolution. In fact, when an oxidizing current was applied to the Al foil, the surface of the foil that had been in contact with the electrolyte completely disappeared. This indicates that the protective oxide film inherent to Al completely failed, allowing dissolution to proceed. The white gel-like appearance of the electrolyte after evaluation is likely due to the eluted Al ions reacting with the P2O7 ions in the electrolyte to precipitate as Al4(P2O7)3. This suggests that when Al ions interact with P2O7 ions, a compound with extremely low solubility is produced, which precipitates as a solid. Figure 3 shows the time-potential curve when a 0.5 ml aqueous electrolyte is used and a constant current is applied to the oxidation side.

[0065] 4.2 In the case of the above condition 1-1 and 5m electrolyte When an oxidation current was applied to an electrochemical cell using a 5 mol / kg K4P2O7 aqueous electrolyte, the potential continued to rise over time, reaching the cutoff voltage of the device and halting the evaluation. It is believed that the overvoltage increased as the Al foil surface became passivated, gradually insulating the solid-liquid interface electrochemically. Observing the Al foil after evaluation, the evaluation surface maintained its metallic luster, and the aqueous electrolyte showed no change from the initial state, confirming that the condition was significantly different from that of the 0.5 m electrolyte described above. As mentioned above, it has been confirmed that when Al ions and P2O7 ions interact, a solid precipitate with extremely low solubility is formed. In this regard, in a dilute solution such as the 0.5 m electrolyte described above, once dissolved, the Al ions are converted into HO or OH. - It is thought that the P2O7 ions first dissolve and diffuse as ligands, and then gradually solidify by interacting with the P2O7 ions. As a result, solids precipitated and precipitated in the aqueous electrolyte, and Al was eluted into the electrolyte. In contrast, in the case of the 5m electrolyte, the P2O7 ion concentration in the electrolyte was very high, and the discoloration of the pH test paper immersed in the electrolyte after the evaluation of the reduction side, which will be described later, was also slow. This suggests that the P2O7 ions in the electrolyte were not soluble in water, and that the P2O7 ions in the electrolyte were not soluble in water. - It is thought that there is almost no diffusion or interaction through the electrolyte, and that the Al ions eluted from the Al foil quickly interact with the P2O7 ions before diffusing farther. As a result, a solid precipitates near the surface of the Al foil, adheres to the surface of the Al foil, and a passive film (protective film) such as Al4(P2O7)3 is formed, suppressing the continuous elution of Al. Figure 4 shows the time-potential curve when a 5 ml aqueous electrolyte is used and a constant current is applied on the oxidation side.

[0066] 4.3 In the case of the above condition 1-2 and 1m electrolyte In a typical aqueous electrolyte, when a reduction current is applied, hydrogen gas is generated, as shown in the equation below, and the pH of the electrolyte rises. Therefore, when an amphoteric metal, aluminum foil, is used as the reduction electrode (negative electrode), as the pH increases and the electrolyte becomes more alkaline, aluminum dissolves from the aluminum foil into the electrolyte, eventually resulting in the aluminum foil disappearing. In fact, when a reduction current was applied to an electrochemical cell using a 1 mol / kg K4P2O7 aqueous electrolyte, the aluminum foil on the reduction electrode (negative electrode) disappeared, and the electrolyte turned into a white gel. This is thought to be due to the dissolution of aluminum from the aluminum foil into the electrolyte and the precipitation and precipitation of Al4(P2O7)3 and other compounds in the electrolyte, similar to the case when an oxidation current was applied in a 0.5 m electrolyte. Figure 5 shows the time-potential curves for a 1 m aqueous electrolyte and a constant reduction current. 2H2O+2e- → H2+2OH -

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

[0068] 4.5 Supplementary Information Comparing the dissolution of Al foils in aqueous K4P2O7 electrolytes at various concentrations, electrochemical cells using aqueous electrolytes of 4 mol / kg or higher, which clearly increased the overpotential on the oxidation side, no perforation due to Al dissolution was observed. On the reduction side, a slight expansion of the potential window was observed with increasing concentration, but no significant increase in overpotential was observed as observed on the oxidation side. On the reduction side, perforation due to Al dissolution was observed in electrolytes of 3 mol / kg or lower, but not in electrolytes of 4 mol / kg or higher. Furthermore, on the oxidation side, Al dissolution occurs in a neutral to weakly basic solution with a pH of 11, whereas on the reduction side, Al dissolution occurs after a localized increase in pH following the hydrogen evolution reaction, resulting in different pH reaction systems at the interface between the Al foil and the electrolyte. Passive films are thought to be more soluble in the strongly basic range than in the neutral range. A dense passive film is thought to inhibit dissolution of the passive film. It is thought that a dense passive film was formed on the surface of the Al foil in an electrolyte of 4 mol / kg or more on both the oxidation and reduction sides.

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

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

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

[0072] [Table 1]

[0073] As is clear from the results shown in Table 1, only Ti foil exhibited a rapid increase in electrode resistance due to passivation, similar to Al foil. Ni and Pt foils are known to be stable even at oxidizing potentials in aqueous systems with a pH higher than acidic, and they exhibited typical current-potential relationships. In contrast, Sn, Cu, Zn, and W foils, which are known to be unstable at oxidizing potentials, showed continuous metal ion dissolution even in a 5 mol / kg K4P2O7 aqueous electrolyte, unlike Al foil. This suggests that unless the solubility of the compound between the eluting metal ion and the coordinated P2O7 is low, a passive film formed by the adhesion and deposition of this compound cannot be formed, preventing the dissolution reaction. In other words, the passive film's effect of inhibiting metal foil dissolution is particularly pronounced when using metal foils containing Al.

[0074] 7. Evaluation assuming aqueous batteries A 5 mol / kg K2P2O7 aqueous solution was prepared as the aqueous electrolyte. Furthermore, Al foil or Ti foil was prepared as the current collector. Ni(OH)2 as the positive electrode active material, acetylene black (AB) as the conductive additive, and SBR and CMC as the binders were mixed in a mass ratio of Ni(OH)2:AB:SBR:CMC = 77:20:2.5:0.5 to prepare a positive electrode active material mixture. The positive electrode active material mixture was uniformly coated on the surface of the Al foil or Ti foil using a doctor blade and dried to obtain a positive electrode for evaluation. That is, the positive electrode was a positive electrode active material layer containing the positive electrode active material, etc., formed on the surface of the Al foil or Ti foil as the positive electrode current collector. Additionally, a negative electrode active material mixture was prepared by mixing La2Ni7 as the negative electrode active material, acetylene black (AB) as the conductive additive, and SBR and CMC as the binders in a mass ratio of La2Ni7:AB:SBR:CMC = 77:20:2.5:0.5. The negative electrode active material mixture was uniformly coated on the surface of Al foil or Ti foil using a doctor blade and dried to obtain a negative electrode for evaluation. That is, the negative electrode consisted of an Al foil or Ti foil serving as a negative electrode current collector, on which a negative electrode active material layer containing the negative electrode active material was formed. Evaluation cells (SB9, manufactured by EC Frontier) were prepared using the aqueous electrolyte, positive electrode, and negative electrode described above. Charge-discharge tests were performed on each of the prepared evaluation cells, and charge-discharge curves were obtained. The results are shown in Figures 7A and 7B. Figure 7A shows the results when Al foil was used as the current collector, and Figure 7B shows the results when Ti foil was used as the current collector. In Figures 7A and 7B, the solid lines are charge curves and the dashed lines are discharge curves. As is clear from the results shown in Figures 7A and 7B, all of the evaluation cells were able to be charged and discharged. This confirms that when charging and discharging were performed after the electrodes were coated and formed, sufficient conductivity was ensured between the active material and the Al foil, and a passive film was unlikely to form between solids, such as between the active material and the Al foil. In other words, it is believed that the formation of the above-mentioned passive film does not substantially affect the performance of aqueous batteries.In the above examples, an aqueous battery in which the carrier ions are protons is exemplified. However, the carrier ions in an aqueous battery may be protons, hydroxide ions, polyphosphate ions, or a combination thereof.

[0075] 8.Summary These results indicate that when an aqueous electrolyte solution containing dissolved K2P2O7 is used as the electrolyte, a high concentration of K2P2O7 of 4 mol / kg or more enables charge and discharge of the active material while suppressing Al elution from the foil at oxidation or reduction potentials. That is, an aqueous battery having a positive electrode, an aqueous electrolyte solution, and a negative electrode, wherein one or both of the positive electrode and the negative electrode have current collectors containing Al, the current collectors are in contact with the aqueous electrolyte solution, the aqueous electrolyte solution contains water and potassium pyrophosphate dissolved at a concentration of 4 mol or more per kg of water, and at least one of protons, hydroxide ions, and polyphosphate ions in the aqueous electrolyte acts as a carrier ion, can suppress Al elution from the current collector into the aqueous electrolyte solution.

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

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

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

[0079] 8A and 8B, it can be seen that when the concentration of K4P2O7 in the aqueous electrolyte is 4 mol / kg or more, there is no crystallization peak in the DSC, i.e., the aqueous electrolyte does not have a freezing point above −40° C. Furthermore, it can be seen from the results in FIG. 9 that when the concentration of K4P2O7 in the aqueous electrolyte is around 4 mol / kg, the glass transition temperature changes specifically.

[0080] These results suggest that when the concentration of K4P2O7 in an aqueous electrolyte is 4 mol / kg or higher, the solution structure (clusters, networks, etc.) changes significantly, due to the loss of interactions between HO molecules, etc. For example, it is thought that HO molecules are surrounded by ions and aggregates derived from K4P2O7, strengthening the network derived from K4P2O7 and preventing the HO molecules from forming a crystallization network.

[0081] In other words, if it is an aqueous electrolyte in which K4P2O7 is dissolved and which does not have a freezing point above -40°C, the dissolved Al can react with the anions derived from K4P2O7, and the dissolved Al can diffuse in the solution without reacting with the H2O molecules and OH molecules. - It can be said that the network does not exist, and the above two conditions are met, namely (1) Al ions dissolved from the Al foil into the aqueous electrolyte react with anions derived from K2P2O7 (e.g., P2O7 ions) to precipitate as insoluble compounds; and (2) The insoluble compounds precipitate near the aluminum foil, adhere to the surface of the aluminum foil, and form a passive film (the dissolved aluminum dissolves in water and dissolves in water). - (Does not diffuse far from the aluminum foil via For example, even if the concentration of K4P2O7 in the aqueous electrolyte is less than 4 mol / kg, it is believed that the above conditions (1) and (2) can be satisfied by dissolving other electrolytes, additives, etc. so that "the aqueous electrolyte does not have a freezing point at -40°C or higher."

[0082] 9.3 Presence or absence of salt precipitation after cooling Each aqueous electrolyte was cooled from 0°C to -40°C, and the presence or absence of salt precipitation was confirmed by DSC. As a result, no peaks due to salt precipitation were observed, regardless of the concentration of the aqueous electrolyte.

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

[0084] 9.4 Checking viscosity The viscosity of each aqueous electrolyte solution at 20°C was confirmed. Viscosity measurements were performed using a laboratory vibration viscometer (Model VM-10A-L, VM-10A-M, SEKONIC Corporation). Figure 10 shows the relationship between the concentration and viscosity of the aqueous electrolyte solution. As shown in Figure 10, when the concentration of K4P2O7 in the aqueous electrolyte solution becomes 4 mol / kg or higher, the viscosity increases rapidly. From the viewpoint of suppressing a decrease in ionic conductivity, it is preferable that the viscosity of the aqueous electrolyte solution is low. For example, if the concentration of K4P2O7 is 7 mol / kg or less, the viscosity does not increase excessively, which is preferable.

[0085] 10. Supplementary Information 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.

[0086] 11. Summary From the above results, it can be said that when constructing an aqueous battery using at least one of protons, hydroxide ions, and polyphosphate ions as carrier ions, if the aqueous electrolyte satisfies the following requirements (A) and (B), it is possible to suppress the elution of Al from an Al-containing current collector into the aqueous electrolyte at an oxidation potential or a reduction potential. (A) The aqueous electrolyte solution has water and potassium polyphosphate dissolved in the water. (B) The aqueous electrolyte does not have a freezing point above -40°C. [Explanation of symbols]

[0087] 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, one or both of the positive electrode and the negative electrode has a current collector containing Al; the current collector is in contact with the aqueous electrolyte; the aqueous electrolyte solution comprises water and potassium polyphosphate dissolved in the water, at least one of protons, hydroxide ions, and polyphosphate ions contained in the aqueous electrolyte acts as a carrier ion; the aqueous electrolyte does not have a freezing point at −40° C. or higher; water-based battery.

2. When the aqueous electrolyte is cooled from 0°C to -40°C, no salt is precipitated. The aqueous battery of claim 1 .

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

4. the aqueous electrolyte solution contains the water and the potassium polyphosphate dissolved in the water at a concentration of 4 mol or more and 7 mol or less per 1 kg of the water; The aqueous battery of claim 1 .

5. The aqueous electrolyte has a viscosity of 40 mPa·s or more and 350 mPa·s or less at 20°C. The aqueous battery of claim 1 .

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

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