Patent Description:
If an aqueous electrolyte solution contains a potassium salt at a high concentration, various advantageous effects such as an expansion of a potential window can be exerted. Meanwhile, according to the inventors' new knowledge, an aqueous electrolyte solution, if contains a potassium salt at a high concentration, is deteriorated in wettability of the aqueous electrolyte solution to a separator and thus is difficult to penetrate into the separator.

The present application discloses, as the aforementioned solution to problem, a plurality of aspects below.

An aqueous battery including a positive electrode, an aqueous electrolyte solution, a separator, and a negative electrode, wherein.

The aqueous battery according to aspect <NUM>, wherein.

The aqueous battery according to aspect <NUM>, wherein
the potassium salt includes a pyrophosphoric acid salt.

The aqueous battery according to any one of aspects <NUM> to <NUM>, wherein.

The aqueous battery according to aspect <NUM>, wherein
the affinity substance includes magnesium phosphate.

The aqueous battery according to aspect <NUM> or <NUM>, wherein
the affinity substance includes a phosphoric acid ester.

The aqueous battery according to any one of aspects <NUM> to <NUM>, wherein
the aqueous electrolyte solution includes the potassium salt dissolved at a concentration of <NUM> mol or more and <NUM> mol or less per <NUM> of the water.

The aqueous battery according to any one of aspects <NUM> to <NUM>, wherein
a pH of the aqueous electrolyte solution is <NUM> or more and <NUM> or less.

The aqueous battery according to any one of aspects <NUM> to <NUM>, wherein
the aqueous electrolyte solution does not involve salt precipitation when cooled from <NUM> to - <NUM>.

The aqueous battery according to any one of aspects <NUM> to <NUM>, wherein
a viscosity at <NUM> of the aqueous electrolyte solution is <NUM> mPa·s or more and <NUM> mPa·s or less.

In the aqueous battery of the present disclosure, wettability of the aqueous electrolyte solution to the separator is improved, and the aqueous electrolyte solution appropriately penetrates into the separator. As a result, an ionic conduction path between the positive electrode and the negative electrode is appropriately kept, and battery performance is enhanced.

The aqueous battery according to the embodiment will be described below with reference to the drawings, but the technology of the present disclosure is not limited to the following embodiments. <FIG> schematically shows a structure of an aqueous battery <NUM> according to one embodiment. As shown in <FIG>, the aqueous battery <NUM> includes a positive electrode <NUM>, an aqueous electrolyte solution <NUM>, a separator <NUM>, and a negative electrode <NUM>. The aqueous electrolyte solution <NUM> contains water and a potassium salt dissolved in the water. The aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher. At least one element constituting the separator <NUM> is the same in type as at least one element constituting the anion of the potassium salt.

It is possible to use, as the positive electrode <NUM>, any known positive electrode for an aqueous battery. As shown in <FIG>, the positive electrode <NUM> may include a positive electrode active material layer <NUM> and a positive electrode current collector <NUM>.

The positive electrode active material layer <NUM> contains a positive electrode active material. The positive electrode active material layer <NUM> can be impregnated with the aqueous electrolyte solution <NUM>. The positive electrode active material layer <NUM> may contain, in addition to the positive electrode active material, various additives and/or the like such as a conductive aid and/or a binder. The content of each component in the positive electrode active material layer <NUM> may be appropriately determined according to the objective battery performance. For example, the content of the positive electrode active material may be <NUM>% by mass or more, <NUM>% by mass or more, <NUM>% by mass or more, or <NUM>% by mass or more, and may be <NUM>% by mass or less or <NUM>% by mass or less, based on <NUM>% by mass of the entire positive electrode active material layer <NUM> (total solid content). The shape of the positive electrode active material layer <NUM> is not particularly limited, and the positive electrode active material layer may be, for example, a sheet-shaped positive electrode active material layer having an approximately flat surface. The thickness of the positive electrode active material layer <NUM> is not particularly limited, and may be, for example, <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, or <NUM> or less.

Any substance capable of functioning as the positive electrode active material of the aqueous battery can be used as the positive electrode active material. The charge-discharge potential of the positive electrode active material is higher than that of the below-mentioned negative electrode active material. The type of the positive electrode active material can be appropriately selected in consideration of the type of carrier ions and the potential window and the like of the aqueous electrolyte solution <NUM>. The positive electrode active material may be one which inserts and deinserts carrier ions by intercalation, or which inserts and deinserts carrier ions by a conversion reaction or an alloying reaction. The "carrier ion" may be, for example, a proton (H+), a hydroxide ion (OH-), a potassium ion (K+), or another cation or anion. Only one positive electrode active material may be used alone, or two or more thereof may be used in combination.

The positive electrode active material may be hydroxides or oxides such as Ni(OH)<NUM> being a layered nickel hydroxide compound (e.g., <CIT>), AxKyNi<NUM>-zMzO<NUM>±δ-nH<NUM>O (wherein A is at least one of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba and Sc, M is at least one of transition metal group elements, 2A group elements, 3A group elements, 2B group elements and 3B group elements, <NUM>≤x<<NUM>, <NUM><y≤<NUM>, <NUM>≤z≤<NUM>, <NUM><n≤<NUM> and (α×x)+y≤<NUM> are satisfied, α is a valence of the cation of element A, e.g., <CIT>), a manganese spinel (for example, LiMn<NUM>O<NUM>), or a nickel-manganese-cobalt-containing composite oxide (NMC). The positive electrode active material may contain a K element. Specific examples thereof include oxides and polyanions containing the K element. More specifically, the positive electrode active material may be a composite oxide of potassium and transition metal. Such composite oxide may be at least one selected from potassium cobalt composite oxide (KCoO<NUM>, etc.), potassium nickel composite oxide (KNiO<NUM>, etc.), potassium nickel titanium composite oxide (KNi<NUM>/<NUM>Ti<NUM>/<NUM>O<NUM>, etc.), potassium nickel manganese composite oxide (KNi<NUM>/<NUM>Mn<NUM>/<NUM>O<NUM>, KNi<NUM>/<NUM>Mn<NUM>/<NUM>O<NUM>, etc.), potassium manganese composite oxide (KMnO<NUM>, KMn<NUM>O<NUM>, etc.), potassium iron manganese composite oxide (K<NUM>/<NUM>Fe<NUM>/<NUM>Mn<NUM>/<NUM>O<NUM>, etc.), potassium nickel cobalt manganese composite oxide (KNi<NUM>/<NUM>Co<NUM>/<NUM>Mn<NUM>/<NUM>O<NUM>, etc.), potassium iron composite oxide (KFeO<NUM>, etc.), potassium chromium composite oxide (KCrO<NUM>, etc.), potassium iron phosphate compound (KFePO<NUM>, etc.), potassium manganese phosphate compound (KMnPO<NUM>, etc.), and potassium cobalt phosphate compound (KCoPO<NUM>). Even if the K site in the composite oxide is composed of other alkali metal elements, there is a possibility that carrier ions can be inserted and deinserted by ion exchange or the like. 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 potassium titanate, TiO<NUM> and sulfur (S) which show a noble charge-discharge potential compared to the below-mentioned negative electrode active material.

The shape of the positive electrode active material may be any shape capable of functioning as the positive electrode active material of the battery. For example, the positive electrode active material may be in the form of particles. The positive electrode active material may be a solid material, a hollow material, a material with voids, or a porous material. The positive electrode active material may be a primary particle, or a secondary particle obtained by agglomeration of a plurality of primary particles. The mean particle diameter D50 of the positive electrode active material may be, for example, <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less. The mean particle diameter D50 in the present application is the particle diameter (median diameter) at <NUM>% of the integrated value in the volume-based particle diameter distribution determined by the laser diffraction and scattering method.

Examples of the conductive aid which can be included in the positive electrode active material layer <NUM> include carbon materials such as vapor-phase-grown carbon fiber (VGCF), acetylene black (AB), Ketjen black (KB), carbon nanotube (CNT) and carbon nanofiber (CNF); and metal materials such as nickel, titanium, aluminum, stainless steel and the like which are slightly insoluble (poorly soluble) in an aqueous electrolyte solution. The conductive aid may be, for example, in the form of particles or fibers, and the size thereof is not particularly limited. Only one conductive aid may be used alone, or two or more thereof may be used in combination.

Examples of the binder which can be included in the positive electrode active material layer <NUM> 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 and the like. Only one binder may be used alone, or two or more thereof may be used in combination.

As shown in <FIG>, the positive electrode <NUM> may include a positive electrode current collector <NUM> in contact with the positive electrode active material layer <NUM>. The positive electrode current collector <NUM> may be in contact with the aqueous electrolyte solution <NUM>. It is possible to use, as the positive electrode current collector <NUM>, any material capable of functioning as the positive electrode current collector of the aqueous battery. The positive electrode current collector <NUM> may be in the form of foil, plate, mesh, perforated metal and foam. The positive electrode current collector <NUM> may be formed of a metal foil or a metal mesh. In particular, the metal foil is excellent in ease of handling or the like. The positive electrode current collector <NUM> may be composed of a plurality of foils. Examples of the metal material constituting the positive electrode current collector <NUM> 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 positive electrode current collector <NUM> preferably contains Al. In the aqueous battery <NUM>, even when the positive electrode current collector <NUM> contains Al, the Al hardly elutes into the aqueous electrolyte solution <NUM>. The positive electrode current collector <NUM> may be one in which the above metal is plated or vapor-deposited on a metal foil or a base material. When the positive electrode current collector <NUM> is composed of a plurality of metal foils, the positive electrode current collector may have some layer between the plurality of metal foils. The thickness of the positive electrode current collector <NUM> is not particularly limited, and may be, for example, <NUM> or more or <NUM> or more, and may be <NUM> or less or <NUM> or less.

The aqueous electrolyte solution <NUM> contains water and a potassium salt dissolved in the water. The aqueous electrolyte solution <NUM> may be held between the positive electrode <NUM> and the negative electrode <NUM> by the separator <NUM>. The aqueous electrolyte solution <NUM> may be in contact with the above-mentioned positive electrode current collector <NUM>, may be included in the above-mentioned positive electrode active material layer <NUM>, may be in contact with the below-mentioned negative electrode current collector <NUM>, or may be included in the below-mentioned negative electrode active material layer <NUM>.

The aqueous electrolyte solution <NUM> contains water as a solvent. The solvent contains water as a main component. Namely, water accounts for <NUM> mol% or more and <NUM> mol% or less of the total amount of solvent constituting the aqueous electrolyte solution <NUM> (<NUM> mol%). Water may account for <NUM> mol% or more, <NUM> mol% or more, or <NUM> mol% or more of the total amount of the solvent. Meanwhile, the upper limit of the proportion of water in the solvent is not particularly limited. The solvent may be composed only of water (<NUM> mol% water).

The aqueous electrolyte solution <NUM> may contain, in addition to water, solvents other than water, for example, from the viewpoint of forming solid electrolyte interphase (SEI) on the surface of the active material. Examples of solvents other than water include one or more organic solvents selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds and hydrocarbons. The solvents other than water may account for <NUM> mol% or less, <NUM> mol% or less, <NUM> mol% or less, or <NUM> mol% or less of the total amount of solvents constituting the aqueous electrolyte solution <NUM> (<NUM> mol%).

The aqueous electrolyte solution <NUM> contains a potassium salt dissolved in water. The potassium salt may be any salt capable of being dissolved in water. For example, the potassium salt may be at least one selected from KPF<NUM>, KBF<NUM>, K<NUM>SO<NUM>, KNO<NUM>, CH<NUM>COOK, (CF<NUM>SO<NUM>)<NUM>NK, KCF<NUM>SO<NUM>, (FSO<NUM>)<NUM>NK, K<NUM>HPO<NUM>, KH<NUM>PO<NUM>, KPO<NUM>, K<NUM>P<NUM>O<NUM>, K<NUM>P<NUM>O<NUM>, K<NUM>P<NUM>O<NUM>, K<NUM>P<NUM>O<NUM>, (KPO<NUM>)n, and the like. According to the inventors' new findings, the performance of the aqueous battery <NUM> is further easily enhanced when the potassium salt includes a phosphoric acid salt or an acetic acid salt, particularly, a phosphoric acid salt, further particularly a polyphosphoric acid salt, in particular, a pyrophosphoric acid salt (potassium pyrophosphate, K<NUM>P<NUM>O<NUM>).

The potassium salt in the aqueous electrolyte solution <NUM> may dissociate into potassium ions and anions. In the aqueous electrolyte solution <NUM>, potassium ions may completely dissociate from anions, or may form aggregates (associations), for example, in close proximity to anions. For example, when the aqueous electrolyte solution <NUM> contains potassium pyrophosphate dissolved in water, "potassium pyrophosphate dissolved in water" in the aqueous electrolyte solution <NUM> may be present as ions such as K+, P<NUM>O<NUM><NUM>-, KP<NUM>O<NUM><NUM>-, K<NUM>P<NUM>O<NUM><NUM>-, and K<NUM>P<NUM>O<NUM>-, or aggregates (associations) of these ions.

In the aqueous battery <NUM>, the components and concentration in the aqueous electrolyte solution <NUM> are determined so that "aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher". According to the inventors' new findings, in order to achieve the conditions that "aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher", the concentration of the potassium salt in the aqueous electrolyte solution <NUM> should be higher than ever before. For example, according to the inventors' new knowledge, when the freezing point is adjusted by dissolving only the potassium salt in water in the aqueous electrolyte solution <NUM>, the freezing point of the aqueous electrolyte solution <NUM> decreases as the concentration of the potassium salt in water increases, and when the concentration of the potassium salt in the aqueous electrolyte solution <NUM> reaches a certain level or more, the freezing point substantially disappears. Namely, when the concentration of the potassium salt in the aqueous electrolyte solution <NUM> is in a certain level or more, the aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher. For example, when the aqueous electrolyte solution <NUM> contains the potassium salt dissolved at a concentration of <NUM> mol or more per <NUM> of water, aqueous electrolyte solution <NUM> easily has no freezing point at -<NUM> or higher. However, when a plurality of kinds of the potassium salts is dissolved in the aqueous electrolyte solution <NUM> or when other components are dissolved together with the potassium salt in water, the minimum concentration of the potassium salt at which the freezing point of the aqueous electrolyte solution disappears may be less than <NUM> mol. In this respect, the concentration of the potassium salt per <NUM> of water in the aqueous electrolyte solution <NUM> may be, for example, <NUM> mol or more, <NUM> mol or more, <NUM> mol or more, or <NUM> mol or more. The upper limit of the concentration is not particularly limited. For example, the concentration of the potassium salt per <NUM> of water may be <NUM> mol or less or <NUM> mol or less.

As mentioned above, in the aqueous electrolyte solution <NUM>, the freezing point of the aqueous electrolyte solution decreases as the concentration of the potassium salt in water increases, and when the concentration of the potassium salt in the aqueous electrolyte solution reaches a certain level or more, the freezing point substantially disappears. According to the inventors' new findings, as the concentration of the potassium salt in the aqueous electrolyte solution <NUM> is higher, the potential window of the aqueous electrolyte solution <NUM> more easily expands. As the concentration is higher, elution of the current collector into the aqueous electrolyte solution <NUM> is more easily suppressed. Meanwhile, as the concentration is lower, the viscosity of the aqueous electrolyte solution <NUM> is smaller. In consideration of these, the aqueous electrolyte solution <NUM> may be, for example, one containing the potassium salt dissolved at a concentration of <NUM> mol or more and <NUM> mol or less, <NUM> mol or more and <NUM> mol or less, <NUM> mol or more and <NUM> mol or less, or <NUM> mol or more and <NUM> mol or less per <NUM> of the water.

In one embodiment, the aqueous electrolyte solution <NUM> may contain water and potassium pyrophosphate dissolved at a certain concentration or more per <NUM> of the water. The concentration of the potassium pyrophosphate may be, for example, <NUM> mol or more, <NUM> mol or more, <NUM> mol or more, or <NUM> mol or more per <NUM> of the water. The upper limit of the concentration of the potassium pyrophosphate in the aqueous electrolyte solution <NUM> is not particularly limited, and if the concentration is excessively high, the viscosity of the aqueous electrolyte solution <NUM> may increase excessively. In this respect, the concentration of the potassium pyrophosphate may be <NUM> mol or less or <NUM> mol or less per <NUM> of the water. According to the inventors' new knowledge, when the aqueous electrolyte solution <NUM> contains water and the potassium pyrophosphate dissolved at a concentration of <NUM> mol or more per <NUM> of the water, not only the aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher, but also the performance of the aqueous battery <NUM> is further enhanced. When the aqueous electrolyte solution <NUM> contains the potassium pyrophosphate dissolved in the water, not only the potassium pyrophosphate, but also other electrolytes may be contained. In this case, the potassium pyrophosphate may account for <NUM> mol% or more and <NUM> mol% or less, <NUM> mol% or more and <NUM> mol% or less, or <NUM> mol% or more and <NUM> mol% or less, and other electrolytes may account for <NUM> mol% or more and <NUM> mol% or less, <NUM> mol% or more and <NUM> mol% or less, or <NUM> mol% or more and <NUM> mol% or less, of the total amount of electrolytes (<NUM> mol%) dissolved in the aqueous electrolyte solution <NUM>.

According to the inventors' new findings, also when the aqueous electrolyte solution <NUM> contains water and potassium ions, and the molar ratio (H<NUM>O/K) of the water to the potassium ions is <NUM> or less and the aqueous electrolyte solution <NUM> contains the water and potassium pyrophosphate dissolved in the water, the aqueous electrolyte solution <NUM> easily has no freezing point at -<NUM> or higher. Here, "potassium ions" are those at least partially derived from the potassium pyrophosphate. As mentioned above, "potassium ions" may form aggregates (associations), for example, in close proximity to anions. The molar ratio of the water to the potassium ions in the aqueous electrolyte solution <NUM> may be easily specified by measuring the amount of the water and the amount of K dissolved and present in the water.

Ions, aggregates (associations), and the like contained in the aqueous electrolyte solution <NUM> can be converted into the potassium salt, thereby specifying "concentration of potassium salt dissolved in water". Here, the aqueous electrolyte solution <NUM> may contain more potassium ions than the concentration which can be converted as the potassium salt. In other words, it is not necessary for the entire potassium ions contained in the aqueous electrolyte solution <NUM> to be converted as "dissolved potassium salt". For example, when a potassium compound (for example, KOH) other than the potassium salt is dissolved together with the potassium salt in the aqueous electrolyte solution <NUM>, the aqueous electrolyte solution <NUM> may contain more potassium ions than the concentration which can be converted as the potassium salt. The aqueous electrolyte solution <NUM> may contain more anions than the concentration which can be converted as the potassium salt. In other words, it is not necessary for the entire anions contained in the aqueous electrolyte solution <NUM> to be converted as "dissolved potassium salt". For example, when the aqueous electrolyte solution <NUM> contains pyrophosphate ions (it may be P<NUM>O<NUM><NUM>- and it may be present in a state of being linked to cations as mentioned above, such as KP<NUM>O<NUM><NUM>-, K<NUM>P<NUM>O<NUM><NUM>-, K<NUM>P<NUM>O<NUM>-, etc.) as anions and also pyrophosphate ion sources (for example, H<NUM>P<NUM>O<NUM>, etc.) other than K<NUM>P<NUM>O<NUM> are dissolved together with K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution <NUM>, the aqueous electrolyte solution <NUM> may contain more pyrophosphate ions than the concentration which can be converted as potassium pyrophosphate. Meanwhile, also when potassium ion sources (for example, KOH) other than the potassium salt and anion sources (for example, H<NUM>P<NUM>O<NUM>) other than the potassium salt are dissolved in water and dissociate into potassium ions and anions in the aqueous electrolyte solution <NUM>, the potassium ions and anions are converted as "potassium salt dissolved in water".

The aqueous electrolyte solution <NUM> may contain a plurality of kinds of cations and a plurality of kinds of anions. For example, the aqueous electrolyte solution <NUM> may include not only potassium ions protons and/or hydroxide ions as carrier ions and phosphate ions as anions, but also other cations and anions. For example, alkali metal ions other than potassium ions, and alkaline earth metal ions, transition metal ions, and the like may be contained. The aqueous electrolyte solution <NUM> may contain an acid, a hydroxide and the like for pH adjustment. The aqueous electrolyte solution <NUM> may contain various additives.

The pH of the aqueous electrolyte solution <NUM> is not particularly limited. However, if the pH is too high, the potential window on the oxidation side of the aqueous electrolyte solution <NUM> may be narrow. In this respect, the pH of the aqueous electrolyte solution <NUM> may be <NUM> or more and <NUM> or less. The pH may be <NUM> or more, <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, or <NUM> or less.

As mentioned above, the aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher. This means that H<NUM>O molecules cannot form a network of crystallization with each other in the aqueous electrolyte solution <NUM>, namely, few freezable free H<NUM>O molecules are present in the aqueous electrolyte solution <NUM>. When the aqueous electrolyte solution <NUM>, which contains few freezable free H<NUM>O molecules, is used in the aqueous battery <NUM>, elution of the current collector into the aqueous electrolyte solution <NUM> can be suppressed. When the aqueous electrolyte solution <NUM> has no freezing point at -<NUM>, it is also possible to use the aqueous battery <NUM> even at extremely low temperature. Namely, the aqueous battery <NUM> appropriately operates even in cold district. The aqueous electrolyte solution <NUM> may have no freezing point at -<NUM> or higher, no freezing point at -<NUM> or higher, or no freezing point at -<NUM> or higher. In order to achieve the conditions that "aqueous electrolyte solution <NUM> has no freezing point - <NUM> or higher", it is effective to, as mentioned above, allow the concentration of the potassium salt in the aqueous electrolyte solution <NUM> to be a high concentration (for example, dissolve the potassium salt (in particular, potassium pyrophosphate) at a high concentration of <NUM> mol or more per <NUM> of water, etc.). As far as the present inventor has confirmed, the same effect can be ensured even when the aqueous electrolyte solution <NUM> has no freezing point at - <NUM> or higher.

The presence or absence of "freezing point" of the aqueous electrolyte solution <NUM> is confirmed by differential scanning calorimetry (DSC). Note that the DSC sweep rate is set at <NUM>/min for both descending temperature and ascending temperature, and the sweep range is set as follows: temperature descending to -<NUM> from room temperature, followed by temperature ascending to <NUM>. The atmosphere in DSC is an atmosphere of inert gas such as Ar, and the pressure is equal to the atmospheric pressure. However, since a sealed aluminum container is used for the evaluation, the atmosphere inside the container is the sealed atmosphere under atmospheric pressure. If the crystallization peak temperature (freezing point temperature) is not confirmed at - <NUM> or higher when the aqueous electrolyte solution is measured under the above conditions, the aqueous electrolyte solution is considered to have "no freezing point at -<NUM> or higher".

It is preferable that the aqueous electrolyte solution <NUM> does not involve salt precipitation when cooled from <NUM> to -<NUM>. Thus, when the aqueous electrolyte solution <NUM> does not involve salt precipitation due to temperature change, it becomes possible to perform stable ionic conduction even at low temperature. For example, the aqueous battery <NUM> can be used even at extremely low temperature in cold district. The aqueous electrolyte solution <NUM> may include water and potassium pyrophosphate dissolved in the water, as mentioned above. Here, according to the inventors' new findings, the saturation solubility of potassium pyrophosphate in water has low temperature dependence and scarcely changes at low temperature of <NUM> or lower. In this respect, even if potassium pyrophosphate is dissolved in the aqueous electrolyte solution <NUM> at <NUM> at high concentration (for example, high concentration of <NUM> mol or more per <NUM> of water) and the aqueous electrolyte solution <NUM> is cooled from <NUM> to -<NUM>, precipitation of potassium pyrophosphate does not substantially occur in the aqueous electrolyte solution <NUM>. Herein, "presence or absence of precipitation of salt" in the case of cooling from <NUM> to -<NUM> can be determined by, for example, DSC. Namely, when differential heat due to salt precipitation or crystallization is not exhibited in cooling of the aqueous electrolyte solution from <NUM> to -<NUM>, the aqueous electrolyte solution is considered to "not involve salt precipitation when cooled from <NUM> to -<NUM>".

If the viscosity of the aqueous electrolyte solution <NUM> is too high, the ionic conductivity of the aqueous electrolyte solution <NUM> may deteriorate. Meanwhile, if an electrolyte is dissolved at high concentration in the aqueous electrolyte solution <NUM>, the aqueous electrolyte solution <NUM> may have a certain level or more of the viscosity. In this respect, the aqueous electrolyte solution <NUM> may have a viscosity of <NUM> mPa·s or more and <NUM> mPa·s or less at <NUM>. The viscosity may be <NUM> mPa·s or more, <NUM> mPa·s or more, <NUM> mPa·s or more, or <NUM> mPa·s or more, and may be <NUM> mPa·s or less, <NUM> mPa·s or less, <NUM> mPa·s or less, <NUM> mPa·s or less, or <NUM> mPa·s or less.

The aqueous battery <NUM> includes a separator <NUM>. The separator <NUM> is provided between the positive electrode <NUM> and the negative electrode <NUM>. The separator <NUM> retains the above-mentioned aqueous electrolyte solution <NUM>.

As mentioned above, the aqueous electrolyte solution <NUM> has no freezing point at -<NUM> or higher. This means that H<NUM>O molecules cannot form a network of crystallization with each other in the aqueous electrolyte solution <NUM>, namely, few freezable free H<NUM>O molecules are present in the aqueous electrolyte solution <NUM>. More specifically, it is considered that an anion network structure called Water-in-salt is formed in the aqueous electrolyte solution <NUM> and a network of H<NUM>O molecules disappears. According to the inventors' new findings, the affinity with the separator, of the aqueous electrolyte solution <NUM> having no freezing point at -<NUM> or higher, is largely different from that of a conventional aqueous electrolyte solution having a freezing point at -<NUM> or higher. Specifically, the aqueous electrolyte solution <NUM> is poor in wettability to a conventional separator including a nonwoven fabric or a porous film, and hardly penetrates into such a conventional separator. Namely, when an aqueous battery is formed with such a conventional separator together with the aqueous electrolyte solution <NUM>, no ionic conduction path between a positive electrode and a negative electrode may be appropriately formed.

In order to solve the above problems, the separator <NUM> used in the aqueous battery <NUM> of the present disclosure is one high in affinity with the aqueous electrolyte solution <NUM>. As mentioned above, it is considered that an anion network structure called Water-in-salt is formed in the aqueous electrolyte solution <NUM>. Therefore, it is important for increasing the affinity of the separator <NUM> with the aqueous electrolyte solution <NUM> that the separator <NUM> has "anionphilicity". Specifically, it is important for at least one element constituting the separator <NUM> to be the same in type as at least one element constituting the anion of the potassium salt dissolved in the aqueous electrolyte solution <NUM>. In other words, element(s) constituting the separator <NUM> include(s) the same type of element as at least one element constituting the anion of the potassium salt dissolved in the aqueous electrolyte solution <NUM>. The separator <NUM> high in affinity with the anion of the potassium salt dissolved in the aqueous electrolyte solution <NUM> is thus used in the aqueous battery <NUM>, and therefore the wettability of the aqueous electrolyte solution <NUM> to the separator <NUM> is improved, the aqueous electrolyte solution <NUM> can be allowed to appropriately penetrate into the separator <NUM>, and the ionic conduction path between the positive electrode <NUM> and the negative electrode <NUM> is appropriately easily ensured.

"Element which is the same in type as at least one element constituting the anion of the potassium salt" (hereinafter, sometimes referred to as "element X") among the element(s) constituting the separator <NUM> may be present in the surface (front surface) of the separator <NUM>, may be present in the inside of the separator <NUM>, or may be present in both the surface and the inside. The element X may be present in only one face of the separator <NUM> (for example, facing the positive electrode <NUM>), may be present in only other face of the separator <NUM> (for example, facing the negative electrode <NUM>), or may be present in both one face and other face thereof. In particular, when the element X is present in both one face and other face of the separator <NUM>, much higher performance is easily obtained.

The type of the element X depends on the type of the anion of the potassium salt. For example, the element X may be at least one element among P, S, and O. Alternatively, the element X may be at least one element other than H and O. Alternatively, the element X may be at least one element other than H and O, and one of or both H and O, in addition to the element X, may be contained as element(s) constituting the separator.

For example, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains the above-mentioned phosphoric acid salt (preferably pyrophosphoric acid salt), one of or both P and O may be contained as element(s) constituting the separator <NUM>. Specifically, the separator <NUM> may be one having a phosphorus-containing group or an oxygen-containing group, or may be, for example, one having a phosphoric acid group or a hydroxyl group. In one embodiment, the separator <NUM> has, for example, a phosphorus-containing group. The phosphoric acid group as the phosphorus-containing group may have, for example, hydrogen substituted by other substituent, as in, for example, an alkylphosphoric acid ester group. The phosphoric acid group may be a polyphosphoric acid group or a pyrophosphoric acid group.

Alternatively, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a sulfuric acid salt, one of or both S and O may be contained as element(s) constituting the separator <NUM>. Specifically, the separator <NUM> may be one having a sulfur-containing group or an oxygen-containing group, or may be, for example, one having at least one functional group selected from a sulfo group, a sulfonyl group, a sulfuric acid group and a hydroxyl group. In one embodiment, the separator <NUM> has, for example, a sulfur-containing group. The sulfur-containing group may have, for example, hydrogen substituted by other substituent, as in, for example, an alkylsulfuric acid ester group. Alternatively, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> includes a carboxylic acid salt such as an acetic acid salt, the separator <NUM> may contain C, H and O. Specifically, the separator <NUM> may contain a carboxyl group.

The element X in the separator <NUM> may be contained in a substance attached to a base material constituting the separator <NUM>. In other words, the separator <NUM> may include a base material and an affinity substance attached to the base material, and at least one element constituting the affinity substance may be the same in type as at least one element constituting the anion of the potassium salt. The affinity substance can also be said to be a substance relatively enhancing the wettability of the aqueous electrolyte solution <NUM> to the separator <NUM> (relatively decreasing the contact angle of the aqueous electrolyte solution <NUM> to the separator <NUM>). In other words, the affinity of the affinity substance with the aqueous electrolyte solution <NUM> is higher than the affinity of the base material with the aqueous electrolyte solution <NUM>.

When the separator <NUM> includes a base material and an affinity substance attached to the base material, the base material may be, for example, a nonwoven fabric or a porous film. The nonwoven fabric may include, for example, at least one of cellulose, a resin, and glass. In one embodiment, the nonwoven fabric includes cellulose. The porous film may be one not only containing at least one polymer compound selected from polyethylene (PE), polypropylene (PP), polyester, polyamide, and the like, but also having a plurality of voids. In one embodiment, the porous film includes one of or both polyethylene and polypropylene. The base material may have a monolayer structure or may have a multilayer structure. Examples of the base material of the multilayer structure can include a base material of a two-layer structure of PE/PP, or a base material of a three-layer structure of PP/PE/PP or PE/PP/PE. The base material may be any one as long as it has a shape capable of forming the separator <NUM>, and may be, for example, one having a thickness of <NUM> or more and <NUM> or less. The base material may be one adopted as a separator in a conventional aqueous battery. In other words, the separator <NUM> may be one in which the affinity substance is attached to a conventional separator.

The affinity substance is a substance having affinity with the aqueous electrolyte solution <NUM>. Specifically, at least one element constituting the affinity substance is the same in type as at least one element constituting the anion of the potassium salt. The element(s) constituting the affinity substance depend(s) on the type of the anion of the potassium salt.

For example, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a phosphoric acid salt, the affinity substance may contain one of or both P and O. Specifically, the affinity substance may be one having a phosphorus-containing group or an oxygen-containing group, or may be, for example, one having a phosphoric acid group or a hydroxyl group. In one embodiment, the affinity substance has, for example, a phosphorus-containing group. The phosphoric acid group may have, for example, some hydrogen substituted by other substituent, as in, for example, an alkylphosphoric acid ester group. The phosphoric acid group may be a polyphosphoric acid group (for example, pyrophosphoric acid group).

Alternatively, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a sulfuric acid salt, the affinity substance may contain S. Specifically, the affinity substance may be one having a sulfur-containing group or an oxygen-containing group, or may be, for example, one having at least one functional group selected from a sulfo group, a sulfonyl group, a sulfuric acid group and a hydroxyl group. In one embodiment, the affinity substance has, for example, a sulfur-containing group. The sulfur-containing group may have, for example, some hydrogen substituted by other substituent, as in, for example, an alkylsulfuric acid ester group. Alternatively, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> includes a carboxylic acid salt, the affinity substance may contain a carboxyl group.

The affinity substance may be any one as long as it can keep the state of being attached to the base material in the aqueous battery <NUM>, and may be an inorganic substance or may be an organic substance.

For example, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a phosphoric acid salt, the affinity substance may contain an inorganic compound having P as a constituent element. For example, the affinity substance may contain a salt (phosphoric acid salt of alkaline earth metal) of at least one alkaline earth metal selected from magnesium, calcium, strontium and barium, and phosphoric acid. The phosphoric acid salt of alkaline earth metal may be a polyphosphoric acid salt, or may be a pyrophosphoric acid salt. According to the inventors' new findings, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a phosphoric acid salt and the affinity substance contains magnesium phosphate, particularly magnesium polyphosphate, further particularly magnesium pyrophosphate, the affinity of the separator <NUM> with the aqueous electrolyte solution <NUM> can be further appropriately enhanced.

Alternatively, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a phosphoric acid salt, the affinity substance may contain an organic compound having P as a constituent element. For example, the affinity substance may be an organic phosphoric acid ester. The organic phosphoric acid ester may be an alkylphosphoric acid ester. The alkylphosphoric acid ester may be a monoalkylphosphoric acid ester, may be a dialkylphosphoric acid ester, or may be a mixture thereof. According to the inventors' new findings, when the potassium salt dissolved in the aqueous electrolyte solution <NUM> contains a phosphoric acid salt and the affinity substance contains a phosphoric acid ester, particularly an alkylphosphoric acid ester, further particularly an alkylphosphoric acid ester having <NUM> to <NUM> carbon atoms, the affinity of the separator <NUM> with the aqueous electrolyte solution <NUM> can be further appropriately enhanced. The alkyl group constituting the alkylphosphoric acid ester may be linear or branched. For example, when the affinity substance contains <NUM>-ethylhexyl phosphate, the affinity of the separator <NUM> with the aqueous electrolyte solution <NUM> is further easily increased.

The affinity substance, for example, may be dispersed and present in one of or both the front surface and the inside of the base material, or may be present on the front surface of the base material, in a layered manner. In particular, when the affinity substance is present in both the front and rear surfaces of the base material, more excellent performance is easily exhibited. The amount of the affinity substance attached to the base material is not particularly limited, and may be any amount as long as voids are kept at a level allowing for penetration of the aqueous electrolyte solution <NUM> into the separator <NUM>. The method for attaching the affinity substance to the base material is not particularly limited. For example, the affinity substance may be attached to the base material by bringing the affinity substance into contact with the base material. More specifically, the affinity substance may be attached to the base material by coating the base material with a slurry or solution containing the affinity substance or immersing the base material in a slurry or solution containing the affinity substance, to bring the affinity substance into contact with the base material, and then drying them.

The separator <NUM> is not limited to one including the base material and the affinity substance. The entire separator <NUM> may be constituted from the affinity substance. It is, however, noted that when the separator <NUM> includes the base material and the affinity substance, not only the backbone of the separator <NUM> as a whole is easily kept by the base material and the separator <NUM> is excellent in ease of handling, but also a conventionally known separator may be used as the base material and thus the cost is low.

It is possible to use, as the negative electrode <NUM>, any known negative electrode for an aqueous battery. As shown in <FIG>, the negative electrode <NUM> may include a negative electrode active material layer <NUM> and a negative electrode current collector <NUM>.

The negative electrode active material layer <NUM> contains a negative electrode active material. The negative electrode active material layer <NUM> can be impregnated with the aqueous electrolyte solution <NUM>. The negative electrode active material layer <NUM> may contain, in addition to the negative electrode active material, various additives such as a conductive aid and/or a binder. The content of each component in the negative electrode active material layer <NUM> may be appropriately determined according to the objective battery performance. For example, the content of the negative electrode active material may be <NUM>% by mass or more, <NUM>% by mass or more, <NUM>% by mass or more, or <NUM>% by mass or more, and may be <NUM>% by mass or less or <NUM>% by mass or less, based on <NUM>% by mass of the entire negative electrode active material layer <NUM> (entire solid content). The shape of the negative electrode active material layer <NUM> is not particularly limited, and may be, for example, a sheet-shaped negative electrode active material layer having an approximately flat surface. The thickness of the negative electrode active material layer <NUM> is not particularly limited, and may be, for example, <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, or <NUM> or less.

It is possible to use, as the negative electrode active material, any substance capable of functioning as the negative electrode active material of the aqueous battery. The negative electrode active material has a lower charge-discharge potential than that of the above-mentioned positive electrode active material, and can be appropriately selected in consideration of the carrier ions and the potential window of the above-mentioned aqueous electrolyte solution <NUM>. For example, the negative electrode active material may be potassium-transition metal composite oxide; titanium oxide; metal sulfides such as Mo<NUM>S<NUM>; elemental sulfur; KTi<NUM>(PO<NUM>)<NUM>; NASICON-type compounds; WO<NUM>, and the like. The negative electrode active material may be a hydrogen storage alloy. The negative electrode active material may be an inorganic compound having a crystalline structure belonging to space group I23. The inorganic compound having a crystalline structure belonging to space group I23 may contain, for example, an element A and an element M and O. Here, the element A is at least one of Bi and La, the element M is at least one of Bi, Mn, Fe, Co and Ni, both the element A and the element M may be Bi. The negative electrode active material may be one which inserts and deinserts carrier ions by intercalation, or which inserts and deinserts carrier ions by a conversion reaction or an alloying reaction. Only one negative electrode active material may be used alone, or two or more thereof may be used in combination.

The shape of the negative electrode active material may be any shape capable of functioning as the negative electrode active material of the battery. For example, the negative electrode active material may be in the form of particles. The negative electrode active material may be a solid material, a hollow material, a material with voids, or a porous material. The negative electrode active material may be a primary particle, or a secondary particle obtained by agglomeration of a plurality of primary particles. The mean particle diameter D50 of the negative electrode active material may be, for example, <NUM> or more, <NUM> or more, or <NUM> or more, and may be <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less.

Examples of the conductive aid which can be contained in the negative electrode active material layer <NUM> include carbon materials such as vapor-phase-grown carbon fiber (VGCF), acetylene black (AB), Ketjen black (KB), carbon nanotube (CNT) and carbon nanofiber (CNF); and metal materials such as nickel, titanium, aluminum, stainless steel and the like which are slightly insoluble (poorly soluble) in an electrolyte solution. The conductive aid may be, for example, in the form of particles or fibers, and the size thereof is not particularly limited. Only one conductive aid may be used alone, or two or more thereof may be used in combination.

Examples of the binder which can be contained in the negative electrode active material layer <NUM> 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 and the like. Only one binder may be used alone, or two or more thereof may be used in combination.

As shown in <FIG>, the negative electrode <NUM> may include a negative electrode current collector <NUM> in contact with the negative electrode active material layer <NUM>. The negative electrode current collector <NUM> may be in contact with the aqueous electrolyte solution <NUM>. It is possible to use, as the negative electrode current collector <NUM>, any material capable of functioning as the negative electrode current collector of the aqueous battery. The negative electrode current collector <NUM> may be in the form of foil, plate, mesh, perforated metal and foam. The negative electrode current collector <NUM> may be formed of a metal foil or a metal mesh. In particular, the metal foil is excellent in ease of handling or the like. The negative electrode current collector <NUM> may be composed of a plurality of foils. Examples of the metal material constituting the negative electrode current collector <NUM> 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 <NUM> preferably contains at least one selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr and In, and preferably contains Al. All of Al, Ti, Pb, Zn, Sn, Mg, Zr and In have low work function, and even if the negative electrode current collector <NUM> comes into contact with the aqueous electrolyte solution <NUM> on the reduction potential, it is considered to be difficult for the aqueous electrolyte solution <NUM> to undergo electrolysis. The negative electrode current collector <NUM> may be one in which the above metal is plated or vapor-deposited on a metal foil or a base material. When the negative electrode current collector <NUM> is composed of a plurality of metal foils, the negative electrode current collector may have some layer between the plurality of metal foils. The thickness of the negative electrode current collector <NUM> is not particularly limited, and may be, for example, <NUM> or more or <NUM> or more, and may be <NUM> or less or <NUM> or less.

As mentioned above, in the aqueous battery <NUM>, both the positive electrode current collector <NUM> and negative electrode current collector <NUM> may contain Al. In this respect, in the aqueous battery <NUM>, a current collector containing Al may be used as a bipolar current collector which serves as both the positive electrode current collector <NUM> and the negative electrode current collector <NUM>. Namely, the positive electrode <NUM> and the negative electrode <NUM> may share one current collector. <FIG> shows an example of a bipolar structure. As shown in <FIG>, the aqueous battery <NUM> may have a bipolar structure, and a positive electrode active material layer <NUM> may be formed on one side of an Al-containing current collector <NUM> (bipolar current collector which functions as both the positive electrode current collector <NUM> and negative electrode current collector <NUM>) and a negative electrode active material layer <NUM> may be formed on the other side of the current collector <NUM>. In this case, the Al-containing current collector <NUM> may not have liquid permeability, that is, the aqueous electrolyte solution <NUM> may not permeate from the positive electrode active material layer <NUM> to the negative electrode active material layer <NUM> via the current collector <NUM> and vice versa.

The aqueous battery <NUM> may include, for example, a known structure as that of a battery, in addition to the above structure. The aqueous battery <NUM> may include, in addition to the above structure, a terminal, a battery case, and the like. The aqueous battery <NUM> may be a primary battery or may be a secondary battery. The aqueous battery <NUM> may be an aqueous proton battery with protons and/or hydroxide ions as carrier ions, or may be an aqueous potassium ion battery with potassium ions as carrier ions. Other structures are obvious to those skilled in the art who refer to the present application, and are therefore explanation will be omitted here.

The method for producing the aqueous battery <NUM> of the present disclosure is not particularly limited, and is, for example, as follows.

As mentioned above, the aqueous battery <NUM> of the present disclosure includes an aqueous electrolyte solution <NUM> having no freezing point at -<NUM> or higher, and thus elution of a current collector into the aqueous electrolyte solution <NUM> is easily suppressed and the aqueous battery can be appropriately operated even at extremely low temperature in cold district. The aqueous battery <NUM> of the present disclosure includes a separator <NUM> high in affinity with the aqueous electrolyte solution <NUM>, and thus the aqueous electrolyte solution <NUM> appropriately penetrates into the separator <NUM> and an ionic conduction path between a positive electrode <NUM> and a negative electrode <NUM> is appropriately easily ensured.

The technology of the present disclosure also has an aspect as an ionic conductor including an aqueous electrolyte solution and a separator. Namely, the ionic conductor of the present disclosure has an aqueous electrolyte solution and a separator retaining the aqueous electrolyte solution, the aqueous electrolyte solution contains water and a potassium salt dissolved in the water, the aqueous electrolyte solution has no freezing point at -<NUM> or higher, and the element constituting the separator contains an element which is the same in type as at least one of elements constituting the anion of the potassium salt. The details of the separator and the aqueous electrolyte solution are as mentioned above.

The technology of the present disclosure also has an aspect as a method for both suppressing elution of metal from the current collector into the aqueous electrolyte solution and ensuring an ionic conduction path between positive and negative electrodes in the aqueous battery. Namely, the method of the present disclosure is characterized by using an aqueous electrolyte solution which satisfies the following requirements (<NUM>) and (<NUM>) and using a separator which satisfies the following requirement (<NUM>), in the aqueous battery. The details of the aqueous electrolyte solution and the separator, and other structures of the battery are as mentioned above.

As mentioned above, according to the technology of the present disclosure, an aqueous battery is obtained in which both suppression of elution of metal from the current collector into the aqueous electrolyte solution and securement of an ionic conduction path between positive and negative electrodes are achieved. Such an aqueous battery can be suitably used, for example, in at least one vehicle selected from a hybrid electric vehicle (HEV), a plug in hybrid electric vehicle (PHEV) and a battery electric vehicle (BEV). Namely, the technology of the present disclosure also has an aspect as a vehicle having an aqueous battery, wherein the aqueous battery includes a positive electrode, an aqueous electrolyte solution, a separator and a negative electrode, the aqueous electrolyte solution contains water and a potassium salt dissolved in the water, the aqueous electrolyte solution has no freezing point at -<NUM> or higher, and at least one element constituting the separator is the same in type as at least one element constituting the anion of the potassium salt. The detail of the aqueous battery is as mentioned above.

Hereinafter, the technology of the present disclosure will be described in more detail by way of Examples, but the technology of the present disclosure is not limited to the following Examples.

An aqueous battery using an aqueous electrolyte solution is needed to be suppressed in elution of a metal current collector into the aqueous electrolyte solution. An aqueous electrolyte solution allowing for suppression of elution of Al in the case of an Al foil as a metal current collector has been examined as below. The Al foil is merely one example of the current collector. Even when other current collector than the Al foil is used, various effects described below, by the aqueous electrolyte solution, can be expected.

K<NUM>P<NUM>O<NUM> was dissolved in <NUM> of pure water at a predetermined concentration (<NUM> to <NUM> mol) to obtain an aqueous electrolyte solution for evaluation.

In an electrochemical cell (VM4, manufactured by EC FRONTIER CO. , LTD), an Al foil was used for a working electrode, Pt mesh was used for a counter electrode, Ag/AgCl was used for a reference electrode, and the above aqueous electrolyte solution was used as an electrolyte solution.

A constant current was applied to an electrochemical cell at <NUM> for <NUM> minutes under the following conditions (<NUM>-<NUM>) or (<NUM>-<NUM>). A constant electric potential was applied to the electrochemical cell at <NUM> for <NUM> minutes under the following conditions (<NUM>-<NUM>) or (<NUM>-<NUM>). Tests were carried out for each of current and potential on the oxidation side, and current and potential on the reduction side.

(<NUM>-<NUM>) Constant current: <NUM>-<NUM>-<NUM>-<NUM>-<NUM> mA/cm<NUM>, for <NUM> minutes each.

(<NUM>-<NUM>) Constant voltage: <NUM>→<NUM>→<NUM>→<NUM>→<NUM> V (vs. Ag/AgCl), for <NUM> minutes each.

(<NUM>-<NUM>) Constant current: -<NUM>→-<NUM>→-<NUM>→-<NUM>→-<NUM> mA/cm<NUM>, for <NUM> minutes each.

(<NUM>-<NUM>) Constant voltage: -<NUM>→-<NUM>→-<NUM>→-<NUM>→-<NUM> V (vs. Ag/AgCl), for <NUM> minutes each.

When an oxidation current was applied in an electrochemical cell using a <NUM> mol/kg K<NUM>P<NUM>O<NUM> aqueous electrolyte solution, an Al foil of an electrode (positive electrode) on the oxidation side disappeared and the electrolyte solution was transformed into a white gel. A potential-pH diagram shows that a standard electrode potential of Al in the aqueous electrolyte solution at pH <NUM> is about -<NUM> V (vs. SHE), so that there is a possibility that Al is eluted (corroded) just by immersing the Al foil in the aqueous electrolyte solution, and it is considered that elution is accelerated by application of a small oxidation current. In fact, as a result of application of the oxidation current using the Al foil, the evaluation surface in contact with the electrolyte solution disappeared completely, which indicates that the protective layer of Al by its original oxide film did not function at all and elution progressed. Main cause of the white gel-like appearance of the electrolyte solution after the evaluation is that eluted Al ions interacted with P<NUM>O<NUM> ions in the electrolyte solution to precipitate as Al<NUM>(P<NUM>O<NUM>)<NUM>. This suggested that when Al ions interact with P<NUM>O<NUM> ions, a compound with very low solubility is produced and precipitated as a solid. <FIG> shows a time-potential curve when a <NUM> aqueous electrolyte solution is used and a constant current on the oxidation side is applied.

When an oxidation current was applied in an electrochemical cell using a <NUM> mol/kg K<NUM>P<NUM>O<NUM> aqueous electrolyte solution, the potential continued to increase with time and reached the cut voltage of the device, causing the evaluation to be interrupted. It is considered that the overvoltage increased due to the passivation of the Al foil surface and gradual electrochemical insulation of the solid-liquid interface. Looking at the Al foil after evaluation, the evaluated surface retained its metallic luster and the aqueous electrolyte solution showed no change at all from the initial state, thus confirming that the state was very different from that in the case of the <NUM> electrolyte solution. As mentioned above, it has been confirmed that when Al and P<NUM>O<NUM> ions interact with each other, solid precipitates with very low solubility are generated. In this respect, in the dilute solution such as the above <NUM> electrolyte solution, it is considered that the dissolved Al ions are once dissolved and diffused using H<NUM>O and OH- as ligands, and then gradually solidified by interacting with P<NUM>O<NUM> ions, leading to precipitation of solids in the aqueous electrolyte solution. As a result, the solid is deposited and precipitated in the aqueous electrolyte solution, and elution of Al into the electrolyte solution has progressed. In contrast, in the case of the <NUM> electrolyte solution, the concentration of P<NUM>O<NUM> ions in the electrolyte solution was very high, and the discoloration of pH test paper immersed in the electrolyte solution after the below-mentioned evaluation on the reduction side was also slow. Therefore, it is considered that the diffusion action via H<NUM>O and OH- is scarcely exerted, and Al ions eluted from the Al foil promptly interact with P<NUM>O<NUM> ions before diffusing far away. Therefore, it is considered that the solid is precipitated near the surface of the Al foil, and the solid adheres to the surface of the Al foil to form a passive film (protective film) such as Al<NUM>(P<NUM>O<NUM>)<NUM>, leading to suppression of continuous elution of Al. <FIG> shows a time-potential curve when a <NUM> aqueous electrolyte solution was used and a constant current on the oxidation side was applied.

In an ordinary aqueous electrolyte solution, when a reducing current flows, hydrogen gas is generated and the pH of the electrolyte solution increases, as shown in the following formula. Therefore, when an Al foil made of amphoteric metal is used in the electrode (negative electrode) on the reduction side, as the electrolyte solution becomes a strong base with an increase in pH, Al is eluted from the Al foil into the electrolyte solution and thus the Al foil eventually disappears. In fact, when a reducing current was applied in an electrochemical cell using a <NUM> mol/kg K<NUM>P<NUM>O<NUM> aqueous electrolyte solution, the Al foil on the electrode (negative electrode) on the reduction side disappeared and the electrolyte solution was transformed into a white gel-like state. This is considered to be due to elution of Al from the Al foil into the electrolyte solution and precipitation of Al<NUM>(P<NUM>O<NUM>)<NUM> in the electrolyte solution, similar to the case where the oxidation current is applied in the <NUM> electrolyte solution. <FIG> shows a time-potential curve when a <NUM> aqueous electrolyte solution was used and a constant current on the reduction side was applied.

In the <NUM> mol/kg K<NUM>P<NUM>O<NUM> aqueous electrolyte solution, Al was not eluted after application of the electric current and retained its original shape, and no change was observed in the electrolyte solution. When a pH test paper was immersed in the electrolyte solution after evaluation, the color did not change immediately after immersion, but gradually changed to purple after a few minutes, indicating pH <NUM>. This suggests that when the electrolyte of the aqueous electrolyte solution is mainly composed of K<NUM>P<NUM>O<NUM>, the electrolyte solution differs from ordinary aqueous solutions in elementary properties, and the diffusion of H<NUM>O and OH- in the solution is very slow, and that Al foil is not eluted even in the presence of such a strong base because a passive film such as Al<NUM>(P<NUM>O<NUM>)<NUM> was formed on the surface of the Al foil, similar to the evaluation results on the oxidation side mentioned above. <FIG> shows a time-potential curve when a <NUM> aqueous electrolyte solution is used and a constant current is applied on the reduction side.

Comparing the presence or absence of elution of the Al foil into the K<NUM>P<NUM>O<NUM> aqueous electrolyte solution at each concentration, pitting due to elution of Al was not observed in the electrochemical cell using the aqueous electrolyte solution at a concentration of <NUM> mol/kg or more, where the overvoltage clearly increased on the oxidation side. For the reduction side, slight expansion of the potential (expansion of the potential window) was observed in response to an increase in concentration, but extreme increase in overvoltage did not occur as in the oxidation side. For the reduction side, pitting due to elution of Al was observed in the electrolyte solution at a concentration of a <NUM> mol/kg or less, but not observed in the electrolyte solution at a concentration of a <NUM> mol/kg or more. On the oxidation side, the Al elution reaction occurred in a neutral to weakly basic solution of pH <NUM>, whereas, on the reduction side, the Al elution reaction occurred after a local increase in pH after the hydrogen generation reaction, resulting in a reaction system with a different pH at the interface between the Al foil and the electrolyte solution. The passive film is considered to have higher solubility in the strong base region than in the neutral region, but if the passive film is dense, dissolution of the passive film is considered to be suppressed. In the electrolyte solution at a concentration of <NUM> mol/kg or more on both the oxidation and reduction sides, it is also considered that a dense passive film is formed on the surface of the Al foil.

In each electrochemical cell, when a constant voltage on the oxidation side was applied, the current increased linearly as the applied potential was increased in the electrochemical cell using a low-concentration aqueous electrolyte solution at a concentration of <NUM> mol/kg or less, whereas, in the electrochemical cell using a high-concentration aqueous electrolyte solution of <NUM> mol/kg or more, the current increased linearly as the applied voltage was increased. In the electrochemical cells with high-concentration aqueous electrolyte solution at a concentration of <NUM> mol/kg or more, the current saturated at <NUM> V (vs. Ag/AgCl). This indicates that the current changes in accordance with the normal Ohm's law on the low concentration side, whereas a resistive layer (the passive film mentioned above) is formed on the surface of the Al foil on the high concentration side.

In each electrochemical cell, when a constant voltage on the reduction side was applied, the current value increased as the applied potential became lower, but the limit of evaluation was - <NUM> V (vs. Ag/AgCl) in the electrochemical cell using an electrolyte solution at a concentration of less than <NUM> mol/kg. In the electrochemical cell using an electrolyte solution at a concentration of <NUM> mol/kg, the limit of evaluation was -<NUM> V (vs. Ag/AgCl), however, despite the current value of <NUM> mA/cm<NUM>, the surface of the Al foil was only slightly discolored, and no pitting or alteration due to corrosion could be observed. Unlike the oxidation side, no extreme increase in resistance on the Al foil surface was observed on the reduction side, suggesting that the passive film does not become thicker. However, when a high concentration electrolyte solution (for example, high concentration electrolyte solution at a concentration of <NUM> mol/kg or more) was used, it is considered that a passive film sufficient to suppress elution of Al ions was formed in the equilibrium state during potential application, because P<NUM>O<NUM> ions were present in sufficient activity during elution of Al ions.

A <NUM> mol/kg K<NUM>P<NUM>O<NUM> aqueous solution was prepared as the aqueous electrolyte solution. An Al foil was prepared as a positive electrode current collector. Spinel-type Li-Mn oxide (LiMn<NUM>O<NUM>, a part of Li can be replaced by K by ion exchange during charging and discharging) as a positive electrode active material, acetylene black as a conductive aid, and PVDF and CMC as binders were mixed with each other in a mass ratio of <NUM>:<NUM>:<NUM>:<NUM> to fabricate a positive electrode active material mixture. Using a doctor blade, the positive electrode active material mixture was uniformly applied to the surface of the Al foil and then dried to obtain a positive electrode for evaluation. Namely, the positive electrode is that in which a positive electrode active material layer containing a positive electrode active material and etc. is formed on the surface of the Al foil as a positive electrode current collector. In an electrochemical cell (VM4, manufactured by EC FRONTIER CO. , LTD), the above positive electrode was used as the working electrode, the above aqueous electrolyte solution was used as the electrolyte solution, Pt mesh was used as the counter electrode, and Ag/AgCl was used as the reference electrode to fabricate an evaluation cell. The evaluation cell thus formed was charged and discharged at <NUM> mA for <NUM> hour for charging and at -<NUM> mA for discharging under the conditions of a cut potential of -<NUM> V vs. Ag/AgCl, thus obtaining a charge-discharge curve. The results are shown in <FIG>. As shown in <FIG>, it can be confirmed that sufficient conductivity is ensured between the active material and the Al foil when charging/discharging is performed after coating and forming the electrode, and that a passive film is difficult to form between the solid and the solid, such as between the active material and the Al foil. Namely, in the aqueous battery, it is considered that the adverse effect due to formation of the passive film is not substantially exerted.

As is apparent from above results, when an aqueous electrolyte solution in which K<NUM>P<NUM>O<NUM> is dissolved is used as the electrolyte, it is possible to perform charging and discharge of the active material while suppressing elution of a current collector at the oxidation and reduction potentials if the concentration of K<NUM>P<NUM>O<NUM> is a high concentration equal to or more a certain concentration.

In the above, an examination was made of the case where only K<NUM>P<NUM>O<NUM> is dissolved in pure water as an electrolyte in an aqueous electrolyte solution. As a result, it has been found that when the concentration of K<NUM>P<NUM>O<NUM> is a high concentration of <NUM> mol/kg or more, elution of a current collector at the oxidation and reduction potentials is suppressed by the formation of a passive film on the surface of the current collector. However, such a concentration of "<NUM> mol/kg" is not necessarily the minimum concentration required to suppress elution of the current collector. For example, if other electrolyte and/or any additive are/is dissolved together with K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution, there is a possibility that elution of the current collector at the oxidation and reduction potentials can be suppressed even if the concentration of K<NUM>P<NUM>O<NUM> is less than <NUM> mol/kg. Namely, if two conditions:.

can be satisfied, elution of the current collector at the oxidation and reduction potentials is considered to be suppressed even if the concentration of K<NUM>P<NUM>O<NUM> is less than <NUM> mol/kg in the aqueous electrolyte solution. The inventors have investigated the difference between the physical properties of an aqueous electrolyte solution in which elution of the current collector can be suppressed by satisfaction of the conditions (<NUM>) and (<NUM>) and the physical properties of an aqueous electrolyte solution in which elution of the current collector cannot be suppressed due to no satisfaction of the conditions (<NUM>) and (<NUM>), by various tests.

The crystallization peak temperature (freezing point temperature) and glass transition temperature of each aqueous electrolyte solution were confirmed by differential scanning calorimetry (DSC). Note that the DSC sweep rate was set at <NUM>/min for both descending temperature and ascending temperature, and the sweep range was set as follows: temperature descending to -<NUM> from room temperature, followed by temperature ascending to <NUM>. The atmosphere in DSC was an atmosphere of Ar, and the pressure was equal to the atmospheric pressure. However, since a sealed aluminum container was used for the evaluation, the atmosphere inside the container was the sealed atmosphere under atmospheric pressure. <FIG> shows the relationship between the concentration and the crystallization peak temperature (freezing point) of an aqueous electrolyte solution. <FIG> shows the relationship between the concentration and the crystallization peak intensity of an aqueous electrolyte solution. <FIG> shows the relationship between the concentration and the glass transition temperature of an aqueous electrolyte solution.

As is apparent from the results shown in <FIG>, when the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is <NUM> mol/kg or more, there is no crystallization peak in DSC, that is, the aqueous electrolyte solution has no freezing point at -<NUM> or higher. As is apparent from the results shown in <FIG>, the glass transition temperature changes specifically when the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is around <NUM> mol/kg.

These results suggest that when the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is <NUM> mol/kg or more, the solution structure (clusters, networks, etc.) is significantly changed due to the disappearance of interactions between H<NUM>O molecules. For example, it is thought that H<NUM>O molecules are surrounded by ions and aggregates (associations) derived from K<NUM>P<NUM>O<NUM>, and the network (in particular, network derived from anions) derived from K<NUM>P<NUM>O<NUM> becomes stronger, and H<NUM>O molecules can no longer form a crystallization network with each other.

In other words, if "aqueous electrolyte solution in which a potassium salt is dissolved has no freezing point at -<NUM> or higher", it can be said that the eluted metal from the current collector can react with anions derived from the potassium salt, and the network of H<NUM>O molecules and OH- required for the eluted metal to diffuse in the solution is absent, and that the above two conditions, that is:.

can be satisfied. For example, even if the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is less than <NUM> mol/kg, the above conditions (<NUM>) and (<NUM>) can be satisfied by dissolving other electrolytes and additives so that "the aqueous electrolyte solution has no freezing point at - <NUM> or higher", the conditions (<NUM>) and (<NUM>) above are considered to be satisfied.

Each aqueous electrolyte solution was cooled from <NUM> to -<NUM>, and the presence or absence of salt precipitation was checked by DSC. As a result, no peaks originating from salt precipitation were observed in any of the aqueous electrolyte solution concentrations.

Among conventional highly concentrated aqueous electrolyte solutions, there are those in which the amount of dissolvable electrolyte varies greatly depending on temperature, and salts are easily precipitated by temperature changes. For example, a highly concentrated lithium imide salt solution known as an aqueous electrolyte solution for aqueous lithium ion batteries, and a highly concentrated sodium imide salt solution known as an aqueous electrolyte solution for aqueous sodium ion batteries, precipitate salt when the temperature drops from room temperature by several degrees, and there is concern that the precipitated salt may inhibit battery reaction. In contrast, the aqueous electrolyte solution for aqueous batteries in this example can be used in various environments as a stable electrolyte solution that does not freeze at extremely low temperatures and does not cause salt precipitation, as mentioned above.

The viscosity of each aqueous electrolyte solution at <NUM> was confirmed. The viscosity was measured using a laboratory vibration viscometer (MODEL VM-10A-L, VM-10A-M, SEKONIC). <FIG> shows the relationship between the concentration and viscosity of the aqueous electrolyte solution. As shown in <FIG>, viscosity increases rapidly when the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is <NUM> mol/kg or more. For example, from the viewpoint of suppressing deterioration of the ionic conductivity, the smaller the viscosity of the aqueous electrolyte solution, the better. For example, if the concentration of K<NUM>P<NUM>O<NUM> is <NUM> mol/kg or less, the viscosity does not increase excessively and is suitable.

From the above results, it can be said that when an aqueous battery is formed, it is possible to suppress elution of metal from the current collector into the aqueous electrolyte solution at the oxidation and reduction potentials if the aqueous electrolyte solution meets the following requirements (A) and (B).

It is necessary for constituting an aqueous battery with the aqueous electrolyte solution to examine a separator to be combined with the aqueous electrolyte solution. The inventors have observed the presence or absence of penetration of the aqueous electrolyte solution (aqueous solution of K<NUM>P<NUM>O<NUM>) into a cellulose-based nonwoven fabric as a common separator for aqueous batteries in dropping of the aqueous electrolyte solution into the cellulose-based nonwoven fabric. <FIG> shows the relationship between the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution and the contact angle of the aqueous electrolyte solution on the surface of the cellulose-based nonwoven fabric. The contact angle shown in <FIG> is the contact angle after a lapse of <NUM> minutes from dropping of the aqueous electrolyte solution onto the surface of the cellulose-based nonwoven fabric. As shown in <FIG>, it can be seen that the aqueous electrolyte solution does not penetrate into the cellulose-based nonwoven fabric and the aqueous electrolyte solution is repelled on the surface of the cellulose-based nonwoven fabric when the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is <NUM> mol/kg or more.

As mentioned above, when the concentration of K<NUM>P<NUM>O<NUM> in the aqueous electrolyte solution is <NUM> mol/kg or more, the aqueous electrolyte solution has no freezing point at -<NUM> or higher. Namely, freezable free H<NUM>O molecules are decreased. More specifically, it is considered that an anion network structure called Water-in-salt is formed in the aqueous electrolyte solution and a network of H<NUM>O molecules disappears. Namely, it can be said that the aqueous electrolyte solution having no freezing point at -<NUM> or higher is largely different from an aqueous electrolyte solution having a freezing point at -<NUM> or higher in terms of properties and is largely different therefrom in terms of affinity with a separator. As shown in <FIG>, it is difficult to allow the aqueous electrolyte solution having no freezing point at -<NUM> or higher to appropriately penetrate into a conventionally common separator for aqueous batteries.

It is considered that an increase in affinity of a separator with a network structure of anions contained in an aqueous electrolyte solution is important for allowing the aqueous electrolyte solution to penetrate into the separator. According to this consideration, various substances were each attached onto a conventional separator and then the wettability of the aqueous electrolyte solution to the separator was observed. The aqueous electrolyte solution here used was an aqueous solution containing K<NUM>P<NUM>O<NUM> at a concentration of <NUM> mol/kg.

A cellulose-based nonwoven fabric for aqueous batteries was prepared as a separator according to Comparative Example <NUM>. The aqueous electrolyte solution was dropped onto the surface of the separator, and the contact angle was measured based on the shape of a droplet on the surface of the separator, with a contact angle meter. The contact angle was measured both immediately after dropping (within <NUM> seconds) and after a lapse of <NUM> minutes from dropping. The state of the droplet after a lapse of <NUM> minutes from dropping is shown in <FIG>.

A separator according to Comparative Example <NUM> was obtained by coating the cellulose-based nonwoven fabric according to Comparative Example <NUM>, with a strontium fluoride (SrF<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>. The state of the droplet after a lapse of <NUM> minutes from dropping is shown in <FIG>.

A separator according to Example <NUM> was obtained by coating the cellulose-based nonwoven fabric according to Comparative Example <NUM>, with a magnesium pyrophosphate (Mg<NUM>P<NUM>O<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>. The state of the droplet after a lapse of <NUM> minutes from dropping is shown in <FIG>.

A separator according to Example <NUM> was obtained by coating the cellulose-based nonwoven fabric according to Comparative Example <NUM>, with a magnesium hydroxide (Mg(OH)<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A separator according to Example <NUM> was obtained by coating the cellulose-based nonwoven fabric according to Comparative Example <NUM>, with a titanium oxide (TiO<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A separator according to Example <NUM> was obtained by coating the cellulose-based nonwoven fabric according to Comparative Example <NUM>, with a zirconium hydroxide (Zr(OH)<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A phosphoric acid ester-based surfactant (manufactured by Tokyo Chemical Industry Co. , mixture of phosphoric acid <NUM>-ethylhexyl monoester and diester) was added to ethanol at a concentration of <NUM>% by weight, thereby providing a treatment liquid. The cellulose-based nonwoven fabric according to Comparative Example <NUM> was brought into contact with the resulting treatment liquid and then dried to allow the surfactant to be attached to the nonwoven fabric, thereby providing a separator according to Example <NUM>. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A porous film made of polypropylene was prepared as a separator according to Comparative Example <NUM>. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>. The state of the droplet after a lapse of <NUM> minutes from dropping is shown in <FIG>.

A fluorine-based surfactant was added to ethanol at a concentration of <NUM>% by weight, thereby providing a treatment liquid. The same porous film made of polypropylene as that in Comparative Example <NUM> was immersed in the resulting treatment liquid for <NUM> minutes. After immersion, washing with ultrapure water and then air drying were carried out, and a separator according to Comparative Example <NUM> was obtained. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>. The state of the droplet after a lapse of <NUM> minutes from dropping is shown in <FIG>.

A separator according to Example <NUM> was obtained by treating a porous film made of polypropylene in the same manner as in Comparative Example <NUM> except that a phosphoric acid ester-based surfactant (manufactured by Tokyo Chemical Industry Co. , a mixture of phosphoric acid <NUM>-ethylhexyl monoester and diester) was added to ethanol at a concentration of <NUM>% by weight, thereby providing a treatment liquid. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>. The state of the droplet after a lapse of <NUM> minutes from dropping is shown in <FIG>.

A separator according to Example <NUM> was obtained by coating the porous film made of polypropylene according to Comparative Example <NUM>, with a magnesium pyrophosphate (Mg<NUM>P<NUM>O<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A separator according to Example <NUM> was obtained by coating the porous film made of polypropylene according to Comparative Example <NUM>, with a calcium pyrophosphate (Ca<NUM>P<NUM>O<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A separator according to Example <NUM> was obtained by coating the porous film made of polypropylene according to Comparative Example <NUM>, with a strontium titanate (SrTiO<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

A separator according to Example <NUM> was obtained by coating the porous film made of polypropylene according to Comparative Example <NUM>, with a calcium sulfate (CaSO<NUM>) powder. The aqueous electrolyte solution was dropped onto the surface of the separator and the contact angle of the droplet was measured in the same manner as in Comparative Example <NUM>.

Table <NUM> below shows the measurement results of the contact angle of the aqueous electrolyte solution on the separator surface in each of Examples and Comparative Examples above.

It can be said from the results shown in Table <NUM>, <FIG> and <FIG> that a separator satisfying the following requirement (C) can allow the aqueous electrolyte solution to easily penetrate into the inside of the separator. (C) At least one element constituting the separator is the same in type as at least one element constituting the anion of the potassium salt dissolved in the aqueous electrolyte solution.

The inventors have carried out the same experiment with various treatment substances other than those in Examples and Comparative Examples. As a result, it has been found that when the following requirement (D-<NUM>) is satisfied, the aqueous electrolyte solution can be allowed to gradually penetrate into the inside of the separator. Namely, it can be said that the aqueous electrolyte solution can be allowed to penetrate into the separator also in Comparative Example <NUM> above, although a long time is taken. Namely, the treatment substance in Comparative Example <NUM> can be used as long as time requirements in aqueous battery production are small. (D-<NUM>) The contact angle θ<NUM> of a droplet after a lapse of <NUM> minutes from dropping of the aqueous electrolyte solution onto the separator is less than <NUM>° and the ratio θ<NUM>/θ<NUM> of the contact angle θ<NUM> after a lapse of <NUM> minutes to the contact angle θ<NUM> immediately after dropping (within <NUM> seconds) is <NUM> or less.

It can be here said that the separator satisfying the above-mentioned requirement (C) allows the following requirement (D-<NUM>) to be easily satisfied and allows the aqueous electrolyte solution to penetrate into the separator in a short time. (D-<NUM>) The contact angle θ<NUM> after a lapse of <NUM> minutes from dropping of the aqueous electrolyte solution onto the separator is <NUM>°.

Properties of the respective surfaces of the separator according to Comparative Example <NUM> and the separator according to Example <NUM> were observed with SEM. The results are shown in <FIG>. As shown in <FIG>, the Mg<NUM>P<NUM>O<NUM> powder was deposited on the surface coated with Mg<NUM>P<NUM>O<NUM>, of the separator according to Example <NUM>. It was found that fine voids through which the aqueous electrolyte solution was permeable were also present in the surface coated with Mg<NUM>P<NUM>O<NUM>, of the separator according to Example <NUM>.

A Ni foil was used for each of a working electrode and a counter electrode, an aqueous solution of <NUM>, <NUM> or <NUM> mol/kg K<NUM>P<NUM>O<NUM> was used as an electrolyte solution, and any of the following (I) to (III) was used as a separator in an electrochemical cell (SB8, manufactured by EC FRONTIER CO.

The resistance of the electrochemical cell was measured at a temperature of <NUM> in a voltage range of ±<NUM> mV and a frequency range of <NUM>-<NUM>. The same experiment was carried out with the changes in type of the separator and in concentration of the aqueous electrolyte solution. Table <NUM> below shows the type of the separator, the concentration of the aqueous electrolyte solution, and the resistance value of the electrochemical cell.

It can be said from the foregoing results that when an aqueous electrolyte solution satisfying the following requirements (A) and (B) is used in an aqueous battery, a separator satisfying the following requirement (C) is combined to improve the wettability of the aqueous electrolyte solution to the separator, allow the aqueous electrolyte solution to appropriately penetrate into the inside of the separator, and ensure an ionic conduction path between a positive electrode and a negative electrode.

Claim 1:
An aqueous battery comprising a positive electrode, an aqueous electrolyte solution, a separator and a negative electrode, wherein
the aqueous electrolyte solution contains water and a potassium salt dissolved in the water, the aqueous electrolyte solution has no freezing point at -<NUM> or higher, and
at least one element constituting the separator is the same in type as at least one element constituting an anion of the potassium salt.