Aqueous battery

Abstract

To provide a chargeable-dischargeable aqueous battery including a novel combination of an aqueous electrolytic solution containing a potassium salt of a phosphorus oxoacid and a negative electrode active material.SOLUTION: An aqueous battery includes: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; and an aqueous electrolytic solution provided between the positive electrode and the negative electrode. The negative electrode active material contains a hydrogen storage alloy. The aqueous electrolytic solution contains: a solvent containing water; and a potassium salt of a phosphorus oxoacid that is dissolved in the solvent and represented by the general formula K2+nPnO3n+1, where n is an integer equal to or greater than 1.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This application relates to aqueous batteries. [Background technology]

[0002] Regarding aqueous batteries, Patent Document 1 describes an aqueous potassium ion battery that includes an aqueous electrolyte solution containing potassium pyrophosphate. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-220294 A [Non-patent literature]

[0004] [Non-Patent Document 1] Kozaburo Ohashi et al., Analytical Chemistry, 1981, 30(11), 727-731. Summary of the Invention [Problem to be solved by the invention]

[0005] However, it is difficult to say that a practical active material system for an aqueous battery having an aqueous electrolyte solution containing a potassium salt of a phosphorus oxoacid such as pyrophosphoric acid has yet been established.

[0006] The present invention provides a rechargeable aqueous battery having a new combination of an aqueous electrolyte solution containing a potassium salt of a phosphorus oxoacid and a negative electrode active material. [Means for solving the problem]

[0007] One embodiment of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an aqueous electrolyte provided between the positive electrode and the negative electrode, the negative electrode active material including a hydrogen storage alloy, and the aqueous electrolyte includes a solvent containing water and a hydrogen storage alloy having a general formula K dissolved in the solvent. 2+n P nO 3n+1 (wherein n is an integer of 1 or more) and a potassium salt of a phosphorus oxoacid represented by the formula: Effect of the Invention

[0008] According to the present invention, it is possible to provide a chargeable and dischargeable aqueous battery having a new combination of an aqueous electrolyte solution containing a potassium salt of a phosphorus oxoacid and a negative electrode active material. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view illustrating a schematic diagram of a water-based battery 10 according to one embodiment. [Diagram 2] Fig. 2(A) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Example 1. Fig. 2(B) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Example 2. Fig. 2(C) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Example 3. [Diagram 3] Fig. 3(A) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Example 4. Fig. 3(B) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Example 5. Fig. 3(C) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 1. [Figure 4] Fig. 4(A) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 2. Fig. 4(B) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 3. Fig. 4(C) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 4. [Diagram 5] FIG. 5 is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) in Reference Example 1. [Figure 6]Fig. 6(A) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) in Example 6. Fig. 6(B) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) in Example 7. Fig. 6(C) is a graph showing the charge / discharge characteristics (upper side) and cycle characteristics (lower side) in Comparative Example 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to these embodiments. The drawings do not necessarily reflect accurate dimensions. In addition, some symbols may be omitted in the drawings. In this specification, unless otherwise specified, the notation "A to B" for numerical values ​​A and B means "A or more and B or less." In such notation, when a unit is added only to numerical value B, the unit is also applied to numerical value A. Furthermore, the words "or" and "or" mean a logical sum unless otherwise specified. In addition, element E 1 and E 2 About "E 1 and / or E 2 " is written as "E 1 Or E 2 or a combination thereof” and element E 1 , … , E N (N is an integer of 3 or more) 1 , … , E N-1 , and / or E N " is written as "E 1 , … , E N-1 , or E N ", or a combination thereof."

[0011] <Water-based battery> FIG. 1 is a cross-sectional view that illustrates a schematic diagram of an aqueous battery 10 (hereinafter, sometimes simply referred to as "battery 10") according to one embodiment. In FIG. 1, the battery 10 is shown in a simplified form, and terminals and some of the exterior materials are not shown. The battery 10 has a positive electrode layer 1, a positive electrode current collector 2 connected to the positive electrode layer 1, a negative electrode layer 3, a negative electrode current collector 4 connected to the negative electrode layer 3, and an electrolyte layer 5 disposed between the positive electrode layer 1 and the negative electrode layer 3, and an exterior material 6 that houses these. The aqueous battery 10 can also function as a secondary battery.

[0012] (Positive electrode layer) The positive electrode layer 1 is a layer containing at least a positive electrode active material. In one embodiment, the positive electrode layer 1 may further contain a conductive assistant. The positive electrode layer 1 may further contain an additive such as a binder (binding material) as necessary. The thickness of the positive electrode layer 1 is not particularly limited, but may be 0.1 μm to 1 mm, or 1 to 100 μm in one embodiment.

[0013] Regarding the positive electrode active material, one type of positive electrode active material may be used alone, or two or more types of positive electrode active materials may be used in combination. In one embodiment, the positive electrode layer 1 is a layer that converts protons (hydrogen ions H + Examples of the positive electrode active material capable of absorbing and releasing protons include nickel hydroxide, nickel oxyhydroxide, nickel oxide, manganese dioxide, molybdenum trioxide, poly(aminoanthraquinone) (PNAQ), perylene-3,4,9,10-tetracarboxylic 3,4:9,10-dianhydride (PTCDA), pyrene-4,5,9,10-tetraone (PYT), poly(diphenoxyphosphazene) (PDPZ), Prussian blue derivatives, and combinations thereof. In another embodiment, the positive electrode layer 1 contains potassium ions (K + An example of a positive electrode active material capable of absorbing and releasing potassium ions is potassium cobalt complex oxide (KCoO 2etc.), potassium nickel composite oxide (KNiO 2 etc.), potassium nickel titanium composite oxide (KNi 1 / 2 Ti 1 / 2 O 2 etc.), potassium nickel manganese composite oxide (KNi 1 / 2 Mn 1 / 2 O 2 , KNi 1 / 3 Mn 2 / 3 O 2 etc.), potassium manganese oxide (KMnO 2 , K.M.n 2 O 4 etc.), potassium iron manganese oxide (K 2 / 3 Fe 1 / 3 Mn 2 / 3 O 2 etc.), potassium nickel cobalt manganese composite oxide (KNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 etc.), potassium iron oxide (KFeO 2 etc.), potassium chromium oxide (KCrO 2 etc.), potassium iron phosphate compounds (KFePO 4 etc.), potassium manganese phosphate compounds (KMnPO 4 etc.), potassium cobalt phosphate compounds (KCoPO 4 ), Prussian blue, etc. The positive electrode active material may be a solid solution of these, or a compound of a non-stoichiometric composition. In another embodiment, the positive electrode layer 1 is made of a positive electrode active material such as potassium titanate or TiO, which has a charge / discharge potential higher than that of the negative electrode active material described below. 2 , LiTi 2 (PO 4 ) 3 , sulfur (S), etc.

[0014] The shape of the positive electrode active material may be, for example, a particulate or thin film. From the viewpoint of ion conductivity and electron conductivity, the primary particle diameter of the positive electrode active material may be 1 nm to 100 μm, or 5 nm to 30 μm, or 10 nm to 10 μm in one embodiment. The primary particles of the positive electrode active material may be aggregated to form secondary particles. From the same viewpoint, the particle diameter of the secondary particles of the positive electrode active material may be 0.1 to 500 μm, or 0.5 to 100 μm, or 1 to 20 μm in one embodiment. The content of the positive electrode active material in the positive electrode layer 1 is not particularly limited, but from the same viewpoint, it may be 20 to 99 mass%, or 40 to 99 mass%, or 60 to 97 mass%, or 70 to 95 mass% in one embodiment, based on the total amount of the positive electrode layer 1 (100 mass%).

[0015] The conductive assistant may be one type of conductive assistant alone or two or more types of conductive assistants may be used in combination. The conductive assistant may be a conductive assistant that can withstand the environment during charging and discharging of the aqueous electrolyte secondary battery. Examples of the conductive assistant that can be mixed in the positive electrode layer 1 include carbon materials such as ketjen black (KB), vapor grown carbon fiber (VGCF), acetylene black (AB), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon black, coke, and graphite. Other examples of the conductive assistant that can be mixed in the positive electrode layer 1 include metal materials that can withstand the environment during charging and discharging of the aqueous electrolyte secondary battery. The conductive assistant may be in the form of, for example, powder or fiber. The amount of the conductive assistant contained in the positive electrode layer 1 is not particularly limited, but from the viewpoint of ionic conductivity and electronic conductivity, it may be 0.1 to 50 mass%, or 0.5 to 30 mass%, or 1 to 10 mass% based on the total amount of the positive electrode layer 1 (100 mass%) in one embodiment.

[0016] As for the binder (binding agent), one type of binder may be used alone, or two or more types of binders may be used in combination. As the binder, a binder that can withstand the environment during charging and discharging of the aqueous electrolyte secondary battery can be used. The binder plays a role of binding the active material or the conductive assistant to the surface of the current collector 2 and maintaining the conductive network in the electrode. Examples of the binder include styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and the like. The amount of the binder contained in the positive electrode layer 1 is not particularly limited, but from the viewpoint of ionic conductivity, electronic conductivity, and appropriate binding of the positive electrode active material, it may be 0.1 to 50 mass%, or 0.5 to 30 mass%, or 1 to 10 mass% based on the total amount of the positive electrode layer 1 (100 mass%) in one embodiment.

[0017] (Positive electrode current collector) The positive electrode current collector 2 is a conductor connected to the positive electrode layer 1. The positive electrode current collector 2 may be made of a known metal that can be used as a positive electrode current collector for aqueous secondary batteries. Examples of such metals include metal materials containing one or more elements selected from Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. Examples of the shape of the positive electrode current collector 1 include a plate shape, a foil shape, a mesh shape, a porous shape, and the like. As another example, a member including a conductive or electrically insulating substrate (e.g., a resin film, etc.) and the above-mentioned metal material vapor-deposited or plated on the surface of the substrate may be used as the positive electrode current collector 1.

[0018] (Negative electrode layer) The negative electrode layer 3 is a layer containing at least a negative electrode active material. In one embodiment, the negative electrode layer 3 may further contain a conductive assistant. The negative electrode layer 3 may further contain an additive such as a binder, as necessary. The thickness of the negative electrode layer 3 is not particularly limited, but may be 0.1 μm to 1 mm, or 1 to 100 μm in one embodiment.

[0019] As for the negative electrode active material, one type of negative electrode active material may be used alone, or two or more types of negative electrode active materials may be used in combination. In the aqueous battery 10, the negative electrode layer 1 contains a hydrogen storage alloy as the negative electrode active material. The hydrogen storage alloy is an alloy in which the reaction between the alloy and hydrogen proceeds reversibly to form a metal hydride, and this reaction makes it possible to reversibly store and release hydrogen. Examples of hydrogen storage alloys include AB 5 Type (e.g. LaNi 5 AB type with superlattice structure (e.g. TiCo, ZrCo, etc.), AB 2 Type (e.g. ZrV 0.4 Ni 1.5 etc.), A 2 B 7 Type (e.g. La 2 Ni 7 Examples of the hydrogen storage alloys include various types of hydrogen storage alloys, such as AB, ABN ... 5 MmNi, a hydrogen storage alloy 5 Here, "Mm" means misch metal. Examples of metal elements contained in the misch metal (Mm) include La, Ce, Nd, Sm, Pr, etc. In one embodiment, the misch metal Mm may further contain alkaline earth elements such as Mg, Sr, Ca, and / or transition metal elements such as V, Cr, Fe, Cu, etc. MmNi 5Preferred examples of the rare earth alloy include MmNi in which part of nickel (Ni) is replaced by other B elements such as Co, Mn, and Al, MmNi 5-x (Co, Mn, Al) x , MmNi 5-x (Co, Mn, Al, Fe) x (where x is a real number satisfying 0 < x < 5), etc. can be mentioned. Other examples of the hydrogen storage alloy include magnesium-based hydrogen storage alloys such as Mg 2 Ni, Mg 2 Cu; titanium-based hydrogen storage alloys such as Ti-Fe, Ti-Cr, Ti-Mn, Ti-Ni, Ti-Cu; and rare earth-Mg-Ni-based alloys such as Ln 1-x Mg x Ni y-z Al z (where Ln represents one or more elements selected from rare earth elements such as La, Sc, Y, Ti, and Zr, and x, y, and z are real numbers satisfying 0.05 ≤ x ≤ 0.30, 2.8 ≤ y ≤ 3.8, and 0.05 ≤ z ≤ 0.30), etc. can be mentioned. In these hydrogen storage alloys, in order to further enhance the charge-discharge reaction activity, a surface catalyst phase such as Ni, Pd, Pt, etc. may be present on the material surface. These surface catalyst phases can be provided by conventionally known methods. Particularly in a material system such as the Ti-Cr system that easily forms a strong oxide film, it is preferable to provide the above surface catalyst phase.

[0020] Hydrogen storage alloy particles having a catalyst phase on the surface can be manufactured by conventionally known methods. Examples of the method of coating the base material particles with the surface catalyst phase include PVD (Physical Vapor Deposition) methods such as sputtering method and vacuum evaporation method; CVD (Chemical Vapor Deposition) methods such as thermal CVD method, etc. Also, for example, the provision of a Ni catalyst phase on the surface of a hydrogen storage alloy particle containing Ni can be achieved by performing an acid treatment step and a washing step in the above order. In the acid treatment step, the hydrogen storage alloy powder is subjected to an acid treatment. For example, acid at room temperature of 15°C to 25°C is prepared, and the hydrogen storage alloy powder is placed in a container containing this acid and stirred for a predetermined time. On the surface of the hydrogen storage alloy particle subjected to such acid treatment, components such as rare earth elements other than Ni, Mg, and Al are dissolved by the acid, and Ni that is difficult to dissolve in acid remains, forming a Ni-rich surface catalyst phase. Inside this surface catalyst phase, a core part of the hydrogen storage alloy having a predetermined alloy composition exists. After the above-mentioned stirring for a predetermined time is completed, water is added to the container in an amount of at least twice the amount of the acid, and the mixture is stirred again for a predetermined time. Thereafter, the mixture of acid and water in the container is left to stand until the hydrogen storage alloy powder settles. After the hydrogen storage alloy powder settles, the supernatant of the mixture is removed, thereby completing the acid treatment. Then, in the washing step, the hydrogen storage alloy particles are washed with water and / or an alkaline aqueous solution to remove the acid components. The temperature of the washing solution may be, for example, 15 to 60° C. Then, the hydrogen storage alloy powder is separated from the washing solution. For example, a Pd or Pt catalytic phase can be provided on the surface of a hydrogen storage alloy particle by adding the hydrogen storage alloy powder to an aqueous solution containing Pd ions or Pt ions and stirring the solution under reducing conditions. A reducing agent may be added to and mixed with the aqueous solution. After the reaction is complete, impurities such as salts may be removed by washing.

[0021] In one embodiment, the negative electrode active material contained in the negative electrode layer 3 is 2 B 7 type hydrogen storage alloys (e.g., La 2 Ni 7 etc.) can be preferably used.

[0022] The shape of the negative electrode active material may be, for example, a particulate or thin film. From the viewpoint of ion conductivity and electron conductivity, the primary particle diameter of the negative electrode active material may be 1 nm to 100 μm, or 5 nm to 30 μm, or 10 nm to 10 μm in one embodiment. The primary particles of the negative electrode active material may be aggregated to form secondary particles. From the same viewpoint, the particle diameter of the secondary particles of the negative electrode active material may be 0.1 to 500 μm, 0.5 to 100 μm, or 1 to 20 μm in one embodiment. The content of the negative electrode active material in the negative electrode layer 3 is not particularly limited, but from the same viewpoint, it may be 20 to 99 mass%, 40 to 99 mass%, 60 to 97 mass%, or 70 to 95 mass% in one embodiment, based on the total amount of the negative electrode layer 3 (100 mass%).

[0023] As the conductive assistant in the negative electrode layer 3, the conductive assistant described above in relation to the positive electrode layer 1 can be used in the same manner. One type of conductive assistant may be used alone, or two or more types of conductive assistants may be used in combination. The conductive assistant may be in the form of, for example, powder or fiber. The amount of the conductive assistant contained in the negative electrode layer 3 is not particularly limited, but from the viewpoint of ionic conductivity and electronic conductivity, it may be 0.1 to 50 mass%, or 0.5 to 30 mass%, or 1 to 10 mass%, based on the total amount of the negative electrode layer 3 (100 mass%) in one embodiment.

[0024] As the binder in the negative electrode layer 3, the binders described above in relation to the positive electrode layer 1 can be used in the same manner. One type of binder may be used alone, or two or more types of binders may be used in combination. The amount of the binder contained in the negative electrode layer 3 is not particularly limited, but from the viewpoints of ionic conductivity, electronic conductivity, and appropriate binding of the negative electrode active material, the amount of the binder may be 0.1 to 50 mass%, or 0.5 to 30 mass%, or 1 to 10 mass% based on the total amount of the negative electrode layer 3 (100 mass%) in one embodiment.

[0025] (Negative electrode current collector) The negative electrode current collector 4 is a conductor connected to the negative electrode layer 3. The negative electrode current collector 4 may be made of a known metal that can be used as a negative electrode current collector for an aqueous secondary battery. Examples of such metals include metal materials containing one or more elements selected from Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. Considering stability in an aqueous electrolyte, the negative electrode current collector 4 may contain one or more elements selected from Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, may contain one or more elements selected from Ti, Pb, Zn, Sn, Mg, Zr, and In, or may contain Ti. All of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In have low work functions, and it is considered that electrolysis of the aqueous electrolyte is unlikely to occur even if they come into contact with the aqueous electrolyte. Examples of the shape of the negative electrode current collector 4 include a plate, a foil, a mesh, and a porous shape. As another example, a member including a conductive or electrically insulating substrate (e.g., a resin film, etc.) and the above-mentioned metal material on which the surface of the substrate is vapor-deposited or plated may be used as the negative electrode current collector 4. In one embodiment, from the viewpoint of increasing the withstand voltage on the reduction side of the aqueous electrolyte, the negative electrode current collector 4 may include a conductive member (e.g., a foil, a mesh, a porous plate, a metal-plated resin film, etc.) including the above-mentioned metal material and a coating layer including a carbon material provided on the surface of the conductive member, and the negative electrode 3 may be disposed in contact with the coating layer. Examples of the carbon material include the various carbon materials described above as examples of the conductive assistant that can be blended in the positive electrode layer 1 and the negative electrode layer 3.

[0026] (electrolyte layer) The electrolyte layer 5 includes a separator 51 provided between the positive electrode layer 1 and the negative electrode layer 3, and an aqueous electrolyte 52 (hereinafter, sometimes referred to as "electrolyte 52" or "electrolyte") impregnated in the separator 51. The separator 51 absorbs and holds the aqueous electrolyte 52. As the separator 51, a separator that can be used in an aqueous electrolyte secondary battery (e.g., nickel-metal hydride battery, zinc-air battery, etc.) can be used. Examples of such separators include water-absorbent members such as porous sheets or nonwoven fabrics formed of hydrophilic materials (e.g., cellulose, etc.). The thickness of the separator 51 is not particularly limited, and may be, for example, 5 μm to 1 mm. The aqueous electrolyte 52 includes a solvent containing water and potassium pyrophosphate dissolved in the solvent.

[0027] The solvent of the aqueous electrolyte 52 contains water. The solvent may consist of water. That is, in one embodiment, the aqueous electrolyte 52 is represented by the general formula K 2+n P n O 3n+1 (wherein n is an integer of 1 or more). For example, the aqueous electrolyte 52 may be an aqueous solution of potassium pyrophosphate. In another embodiment, the solvent may further include one or more non-aqueous solvents (organic solvents). Examples of such non-aqueous solvents include one or more organic solvents selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. In one embodiment, the solvent composition of the aqueous electrolyte 52 (the content of water and the content of the non-aqueous solvent (the total content of the non-aqueous solvents when multiple non-aqueous solvents are present)) may be water:non-aqueous solvent=50-100% by mass:0-50% by mass, or 70-100% by mass:0-30% by mass, or 90-100% by mass:0-10% by mass, or 95-100% by mass:0-5% by mass, based on the total amount of the solvents contained in the aqueous electrolyte 52 (100% by mass).

[0028] The aqueous electrolyte solution 52 contains an electrolyte of the general formula K 2+n P n O 3n+1In the present invention, a potassium salt of a phosphorus oxoacid represented by the formula (wherein n is an integer of 1 or more) (hereinafter referred to as a "potassium P(V) oxoacid salt" and the corresponding acid radical may be referred to as a "P(V) oxoacid radical") is dissolved. The oxidation number of phosphorus in the salt is +V. In one embodiment, n may be 1, and in this case, the salt is potassium orthophosphate (K 3 PO 4 In one embodiment, n can be an integer equal to or greater than 2, and the salt is a potassium salt of a condensed phosphoric acid. In one embodiment, n=2, and the salt is potassium pyrophosphate (K 4 P 2 O 7 In one embodiment, n=3, and the salt is potassium triphosphate (K 5 P 3 O 10 ). In one embodiment, n can be, for example, 1 to 10, or 1 to 6, or 1 to 3. As the P(V) oxoacid potassium salt, only one type of P(V) oxoacid potassium salt (i.e., having a single n) may be present in the aqueous electrolyte 52, or a combination of P(V) oxoacid potassium salts having multiple different n's may be present in the aqueous electrolyte 52. In the aqueous electrolyte 52, a dissociation equilibrium of the P(V) oxoacid potassium salt and an acid dissociation equilibrium between the anions generated by complete or partial dissociation of the P(V) oxoacid potassium salt and their conjugate acids can be simultaneously established, and the chemical species generated by these equilibria are also included in the "dissolved P(V) oxoacid potassium salt." For example, when the P(V) oxoacid potassium salt is potassium pyrophosphate (n=2), the "dissolved potassium pyrophosphate" is K 4 P 2 O 7 In addition, K + , K 3 P 2 O 7 - , K 3 HP 2 O 7 , K 2 P 2 O 7 2- , K2 HP 2 O 7 - , K 2 H 2 P 2 O 7 , K.P. 2 O 7 3- , K.H.P. 2 O 7 2- , K.H. 2 P 2 O 7 - , K.H. 3 P 2 O 7 , P 2 O 7 4- , HP 2 O 7 3- , H 2 P 2 O 7 2- , H 3 P 2 O 7 - , or H 4 P 2 O 7 In the aqueous electrolyte 52, the "dissolved P(V) oxo acid potassium salt" does not necessarily mean that the P(V) oxo acid potassium salt is dissolved in water. 2+n P n O 3n+1 For example, "dissolved potassium pyrophosphate" does not necessarily mean that the solution is obtained by adding K to water. 4 P 2 O 7 For example, the water may contain a potassium ion source other than potassium pyrophosphate (e.g., K, KOH, K 2 O, C.H. 3 COOK et al.) and a pyrophosphate ion source (e.g., H 4 P 2 O 7 etc.) may be added and dissolved separately, resulting in the formation of the above ions and / or associations in water.

[0029] The amount of P(V) oxoacid potassium salt in the aqueous electrolyte 52 can be selected depending on the desired performance of the battery 10. In one embodiment, the amount of P(V) oxoacid potassium salt in the aqueous electrolyte 52 is K 2+n P n O 3n+1 (wherein n is a given value depending on the salt.) The amount of the salt may be 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. The upper limit of the content is not particularly limited, but in one embodiment, from the viewpoint of suppressing an increase in viscosity, K 2+n P n O 3n+1 (where n is a given value depending on the salt.) In terms of conversion, it may be 7.0 mol or less per 1.0 kg of water. In one embodiment, when the P(V) oxoacid potassium salt is potassium orthophosphate, the content of potassium orthophosphate in the aqueous electrolyte 52 is K 3 PO 4 In terms of K, the amount may be 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. The upper limit of the content is not particularly limited, but in one embodiment, from the viewpoint of suppressing an increase in viscosity, K 3 PO 4 This can be calculated as 7.0 mol or less per 1.0 kg of water. In one embodiment, when the P(V) oxoacid potassium salt is potassium pyrophosphate, the content of potassium pyrophosphate in the aqueous electrolyte 52 is K 4 P 2 O 7 In terms of K, the amount may be 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. The upper limit of the content is not particularly limited, but in one embodiment, from the viewpoint of suppressing an increase in viscosity, K 4 P 2 O 7 This can be calculated as 7.0 mol or less per 1.0 kg of water. In one embodiment, when the P(V) oxoacid potassium salt is potassium triphosphate, the content of potassium triphosphate in the aqueous electrolyte 52 is K 5 P 3 O 10In terms of K, the amount may be 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. The upper limit of the content is not particularly limited, but in one embodiment, from the viewpoint of suppressing an increase in viscosity, K 5 P 3 O 10 This may be equivalent to 7.0 mol or less, or 6.0 mol or less per 1.0 kg of water.

[0030] In one embodiment, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte solution, the better the cycle characteristics become, and the easier it is to obtain high performance as a secondary battery.

[0031] The content of "dissolved P(V) oxoacid potassium salt" in aqueous electrolyte 52 can be determined by measuring the contents of water, P(V) oxoacid radical, and potassium in the electrolyte; if (n+3)×P(V) oxoacid radical content (mol)≦potassium content (mol), then the P(V) oxoacid radical content (mol) is determined as the P(V) oxoacid potassium salt content (mol); if not, then the potassium content (mol) / (n+3) is determined as the P(V) oxoacid potassium salt content (mol); and then calculating the P(V) oxoacid potassium salt content (mol) per kg of water from the determined P(V) oxoacid potassium salt content (mol) and the water content (kg). When the P(V) oxoacid potassium salt is a combination of multiple salts with different n (i.e., when the P(V) oxoacid radical is a combination of multiple P(V) oxoacid radicals with different n), the total content (mol) of the multiple P(V) oxoacid radicals is used as the "P(V) oxoacid radical content (mol)" and the number average value of n across the multiple P(V) oxoacid radicals is used as n in the above inequality. For example, when the P(V) oxoacid radical is a combination of orthophosphate radical (n=1) and pyrophosphate radical (n=2) (molar ratio 1:1), the number average value of n is 1.5. For example, when the P(V) oxoacid radical is a combination of pyrophosphate radical (n=2) and triphosphate radical (n=3) (molar ratio 1:1), the number average value of n is 2.5. For example, when the P(V) oxoacid potassium salt is potassium pyrophosphate, the content of “dissolved potassium pyrophosphate” in aqueous electrolyte 52 can be determined by measuring the contents of water, pyrophosphate radical, and potassium in the electrolyte; if 4×pyrophosphate radical content (mol)≦potassium content (mol), determining the pyrophosphate radical content (mol) as the potassium pyrophosphate content (mol); if not, determining the potassium content (mol) / 4 as the potassium pyrophosphate content (mol); and calculating the potassium pyrophosphate content (mol) per kg of water from the determined potassium pyrophosphate content (mol) and water content (kg). The water content can be measured by known methods such as the drying method (e.g., differential scanning calorimetry, etc.) and the Karl-Fischer method. The P(V) oxo acid radical content can be measured by known methods such as inductively coupled plasma atomic emission spectroscopy (ICP-AES), chemiluminescence, enzyme, ion chromatography, spectrophotometric determination using molybdophosphate generation colorimetry, and spectrophotometric determination using heteropoly blue generation (Non-Patent Document 1). The potassium content can be measured by known methods such as ICP-AES.

[0032] The aqueous electrolyte 52 may contain more potassium ions than (n+3)×P(V) oxoacid radicals (mol). For example, when the potassium P(V) oxoacid is potassium pyrophosphate, the aqueous electrolyte 52 may contain more potassium ions than 4×pyrophosphate radicals (mol). As an example of such a case, water may contain a potassium ion source other than the potassium pyrophosphate source (e.g., KOH, CH) in addition to the potassium pyrophosphate source. 3 COOK, etc.) is added and dissolved.

[0033] The aqueous electrolyte 52 may contain cations other than potassium ions, such as alkali metal ions, alkaline earth metal ions, transition metal ions, and the like, other than potassium ions. The aqueous electrolyte may also contain anions other than P(V) oxo acid radicals. For example, the aqueous electrolyte 52 may contain KPF6 , K.B.F. 4 , K 2 SO 4 , KNO 3 , C.H. 3 COOK, (CF 3 SO 2 ) 2 N.K., K.C.F. 3 SO 3 , (FSO 2 ) 2 Other electrolytes such as NK may also be dissolved therein.

[0034] In one embodiment, the proportion of P(V) oxoacid radicals in the acid radicals in aqueous electrolyte solution 52 and the proportion of potassium ions in the cations in aqueous electrolyte solution 52 may be 50 to 100 mol%, or 70 to 100 mol%, or 90 to 100 mol%, or 95 to 100 mol%, or 99 to 100 mol%, based on the total amount of acid radicals in aqueous electrolyte solution 52 and the total amount of cations in aqueous electrolyte solution 52 (100 mol%), respectively.

[0035] In addition to the above-mentioned solvent and electrolyte, the aqueous electrolyte 52 may contain an acid or a hydroxide for adjusting the pH of the aqueous electrolyte, and may also contain various additives.

[0036] The pH of the aqueous electrolyte 52 at 25° C. may be, for example, 3.0 to 13.0, or 7.0 to 13.0. In one embodiment, from the viewpoint of the width of the oxidation side potential window of the aqueous electrolyte, the pH may be 13.5 or less, or 13.0 or less, or 12.7 or less, or 12.5 or less. The pH may be 3.0 or more, or 4.5 or more, or 6.0 or more, or 7.0 or more. In one embodiment, the pH may be 3.0 to 13.5, or 4.5 to 13.0, or 6.0 to 12.7, or 7.0 to 12.5.

[0037] (Exterior materials) The battery 10 is used in a state housed in an exterior material 6. For the exterior material 6, an exterior material that can be used for an aqueous electrolyte secondary battery can be used. Examples of materials that can constitute such exterior materials include metal materials such as aluminum and stainless steel, as well as resin materials such as polyphenylene sulfide resin and polyimide resin. The shape of the exterior material is not particularly limited, and can be, for example, a circle (cylinder, coin, button), a hexahedron (rectangular parallelepiped, cubic), or a bag shape, or a shape obtained by processing and deforming these shapes.

[0038] A conventional nickel-metal hydride (Ni-MH) secondary battery typically uses a strongly alkaline potassium hydroxide aqueous solution as the electrolyte, a hydrogen storage alloy (MH) as the negative electrode active material, and nickel hydroxide and / or nickel oxyhydroxide as the positive electrode active material. Usually, nickel hydroxide with low conductivity is used as the positive electrode active material in the positive electrode layer, and cobalt hydroxide with high conductivity is added to it as a conductive assistant. This is because if carbon is used as a conductive assistant in a strongly alkaline potassium hydroxide aqueous solution, the carbon will be oxidized and deteriorated. By adding cobalt hydroxide to the positive electrode, the cobalt hydroxide is oxidized during the first charge and changes to cobalt oxyhydroxide, thereby increasing the conductivity of the positive electrode. However, since the electrolyte is strongly alkaline, cobalt (and metals such as zinc and manganese from the hydrogen storage alloy) is easily eluted from the positive electrode layer, and these eluted metals may precipitate on the separator, deteriorating the output characteristics and self-discharge characteristics. In contrast, according to the aqueous battery of the present invention, since the aqueous electrolyte contains potassium pyrophosphate, the aqueous electrolyte is weakly acidic to weakly basic, or neutral to weakly basic, and yet the hydrogen storage alloy can be used as the negative electrode active material for charging and discharging. This means that the aggressiveness of the electrolyte to the negative electrode active material, the positive electrode active material, and metal materials such as the current collector can be reduced, and therefore the adverse effects caused by the dissolution of metals into the electrolyte and the adverse effects of the oxidation reaction of carbon can be reduced. In the aqueous battery of the present invention, although the electrolyte is a potassium salt, the negative electrode active material is considered to absorb and release protons. When nickel hydroxide and / or nickel oxyhydroxide are used as the positive electrode active material, the positive electrode active material also operates by absorbing and releasing protons, making it a complete proton battery. It is also possible to use a carbon material as the conductive assistant of the positive electrode layer instead of the conventional cobalt compound. It is also possible to use a metal material such as Al that is weak against strong bases instead of Ni, which has excellent corrosion resistance but is expensive, as the positive electrode current collector and / or the negative electrode current collector. EXAMPLES

[0039] The present invention will now be described in more detail with reference to examples.

[0040] (1. Preparation of Electrolyte) As electrolytes to be used for charge / discharge evaluation, 5.0 mol / kg and 1.0 mol / kg potassium pyrophosphate aqueous solutions (Examples 1 and 2), 28 mol / kg and 1.0 mol / kg potassium acetate aqueous solutions (Comparative Examples 1 and 2), 22 mol / kg potassium trifluoromethanesulfonate aqueous solution (Comparative Example 3), 21 mol / kg lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) aqueous solution (Comparative Example 4), and 6 mol / L potassium hydroxide aqueous solution (Reference Example 1) were prepared. 4 P 2 O 7 and KOH were from Nacalai Tesque, Inc., and CH 3 COOK is from Fujifilm Wakosha, CF 3 SO 3 K was purchased from Merck and LiTFSI was purchased from Tokyo Chemical Industry Co., Ltd. and used as is. For each electrolyte, the electrolyte was added to pure water and stirred so that the aqueous solution had the specified concentration. The mixture after stirring was left in a thermostatic bath at 25°C for at least one day to completely dissolve the electrolyte.

[0041] (2. Preparation of MH-coated electrodes) The electrodes used for the charge / discharge evaluation were prepared as follows. 2 Ni 7 MH) and acetylene black were weighed and mixed uniformly in a mortar. Styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and water were further added and mixed uniformly in a mortar again. The mixed mass ratio was MH:acetylene black:SBR:CMC=75:20:4:1. The ink was prepared by degassing for 1 minute at 2000 rpm using a mixer (Thinky Corporation's "Awatori Rentaro") to prepare an ink. The ink was dropped onto titanium foil fixed on a glass plate and coated using a doctor blade with a gap of 150 μm. The coating was left to stand under reduced pressure and dried naturally, and then dried overnight at 60°C using a vacuum dryer. The dried electrode was punched into a circle with a diameter of 16 mm and pressed with a load of 1 ton to densify the electrode.

[0042] (3. Charge / discharge evaluation) The charge-discharge characteristics of each electrolyte solution described in 1 above were evaluated using a cell (Interchem VM4) equipped with the MH-coated electrode described in 2 above as the working electrode, a Ni mesh as the counter electrode, and a silver-silver chloride electrode (BAS) as the reference electrode. VMP3 (Biologic) was used as the device, and the current density was ±0.1 mA / cm. 2 The cutoff condition for the oxidation current side was 0.2 V against the standard hydrogen electrode (0 V against the silver-silver chloride electrode), and the cutoff condition for the reduction current side was 300 mAh / g (this was regulated by the capacity). The charge / discharge characteristics were evaluated for 30 cycles.

[0043] (4.Results) (Example 1) Figure 2(A) shows the charge / discharge characteristics (upper part of Figure 2(A)) and cycle characteristics (lower part of Figure 2(A)) when a 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2) was used as the electrolyte. Good charge / discharge activity and high cycle stability were confirmed. (Example 2) Figure 2(B) shows the charge / discharge characteristics (upper panel of Figure 2(B)) and cycle characteristics (lower panel of Figure 2(B)) when a 1.0 mol / kg potassium pyrophosphate aqueous solution (pH: 10.6) was used as the electrolyte. Although the capacity decreased relatively quickly with increasing charge / discharge cycles, good charge / discharge activity was observed in the early stages. (Example 3) Figure 2(C) shows the charge / discharge characteristics (upper part of Figure 2(C)) and cycle characteristics (lower part of Figure 2(C)) when a 2.0 mol / kg potassium pyrophosphate aqueous solution (pH: 11.2) was used as the electrolyte. Good charge / discharge activity and high cycle stability were confirmed. (Example 4) Figure 3(A) shows the charge / discharge characteristics (upper part of Figure 3(A)) and cycle characteristics (lower part of Figure 3(A)) when a 3.0 mol / kg potassium pyrophosphate aqueous solution (pH: 11.7) was used as the electrolyte. Good charge / discharge activity and high cycle stability were confirmed. (Example 5) Figure 3(B) shows the charge / discharge characteristics (upper part of Figure 3(B)) and cycle characteristics (lower part of Figure 3(B)) when a 4.0 mol / kg potassium pyrophosphate aqueous solution (pH: 11.9) was used as the electrolyte. Good charge / discharge activity and high cycle stability were confirmed. In each example, nickel hydroxide / nickel oxyhydroxide formed on the Ni mesh surface of the positive electrode is thought to operate by absorbing and releasing protons. (Comparative Example 1) Figure 3(C) shows the charge / discharge characteristics (upper part of Figure 3(C)) and cycle characteristics (lower part of Figure 3(C)) when a 28 mol / kg aqueous potassium acetate solution was used as the electrolyte. Almost no operation as a battery was observed. (Comparative Example 2) Fig. 4(A) shows the charge / discharge characteristics (upper part of Fig. 4(A)) and cycle characteristics (lower part of Fig. 4(A)) when a 1.0 mol / kg aqueous potassium acetate solution was used as the electrolyte. Almost no oxidation current flowed. (Comparative Example 3) Figure 4(B) shows the charge / discharge characteristics (upper panel of Figure 4(B)) and cycle characteristics (lower panel of Figure 4(B)) when a 22 mol / kg aqueous solution of potassium trifluoromethanesulfonate was used as the electrolyte. An inexplicably high capacity was shown at the beginning of the cycle, but thereafter the battery rapidly deteriorated, showing characteristics that made it unusable as a battery. In this comparative example, the oxidation current may be due to the dissolution reaction of the hydrogen storage alloy. (Comparative Example 4) Figure 4(C) shows the charge / discharge characteristics (upper panel of Figure 4(C)) and cycle characteristics (lower panel of Figure 4(C)) when a 21 mol / kg LiTFSI aqueous solution was used as the electrolyte. Neither charging nor discharging was observed, and the battery was completely inactive. (Reference Example 1) Figure 5 shows the charge / discharge characteristics (upper part of Figure 5) and cycle characteristics (lower part of Figure 5) when a 6 mol / kg potassium hydroxide aqueous solution was used as the electrolyte. It was confirmed that the battery operated without any problems. This result shows that the difference between the results of the other examples and the comparative example is due to the electrolyte used. (Example 6) In FIG. 6(A), the same 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2) as in Example 1 was used as the electrolyte, while the La 2 Ni7 Instead of LaNi 5 The charge / discharge characteristics (upper part of FIG. 6(A)) and cycle characteristics (lower part of FIG. 6(A)) are shown when using AB 5 It was confirmed that the secondary battery can function even when a hydrogen storage alloy of this type is used as the negative electrode active material. However, the discharge capacity is A 2 B 7 La is a hydrogen storage alloy of the type 2 Ni 7 In addition, a relatively large number of cycles were required to activate the negative electrode. (Example 7) In FIG. 6(B), the same 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2) as in Example 1 was used as the electrolyte, while the La 2 Ni 7 Instead, Pd-coated Ti 50 Cr 50 The charge / discharge characteristics (upper panel of Fig. 6(B)) and cycle characteristics (lower panel of Fig. 6(B)) are shown for the case of using Ti powder. x Cr 1-x In this system, the Pd coating acts as a catalytic phase. It was confirmed that the secondary battery can function even when a hydrogen storage alloy other than the La-Ni alloy is used as the negative electrode active material. Comparative Example 5 In FIG. 6(C), the same 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2) as in Example 1 was used as the electrolyte, while the La 2 Ni 7 Instead, non-catalyzed Ti 50 Cr 50 The charge / discharge characteristics (upper side of FIG. 6(C)) and cycle characteristics (lower side of FIG. 6(C)) are shown for the case where the powder was used. Operation as a secondary battery was almost impossible. 50 Cr 50 This example differs from Example 7 in that the powder was not Pd-coated (catalyzed). x Cr 1-x Since the oxide film on the surface of Ti-based alloys is strong, x Cr 1-xIn order to make the alloy function as a hydrogen storage alloy, it is necessary to add a catalytic phase such as Pd. From the results of Example 7 and Comparative Example 5, it was confirmed that in the aqueous battery of the present invention, even if the alloy is made of a material other than La-Ni, if the alloy has hydrogen storage and release activity in an aqueous electrolyte (i.e., if the alloy functions as a hydrogen storage alloy), and only in that case, the alloy can be used as a negative electrode active material for charging and discharging. [Explanation of symbols]

[0044] 1: positive electrode layer, 2: positive electrode current collector, 3: negative electrode layer, 4: negative electrode current collector, 5: electrolyte layer, 51: separator, 52: aqueous electrolyte, 6: exterior material

Claims

1. a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; an aqueous electrolyte provided between the positive electrode and the negative electrode; Equipped with the negative electrode active material includes a hydrogen storage alloy, The aqueous electrolyte solution is A solvent containing water; A compound of the general formula K dissolved in the solvent. ２＋ｎ P ｎ O ３ｎ＋１ (wherein n is an integer of 1 or more), and 1. A water-based battery comprising:

2. 2. The aqueous battery of claim 1, wherein the potassium salt of a phosphorus oxoacid is potassium pyrophosphate.

3. The content of potassium pyrophosphate in the aqueous electrolyte is K ４ P ２ O ７ The aqueous battery according to claim 2 , wherein the amount of the cation exchange is 2.0 mol or more per kg of water.

4. 3. The aqueous battery according to claim 2, wherein the aqueous electrolyte has a pH of 3.0 to 13.0 at 25°C.

5. 5. The aqueous battery according to claim 1, wherein the positive electrode active material is capable of absorbing and releasing hydrogen ions.

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