Aqueous battery

Abstract

The invention relates to an aqueous battery. An aqueous battery provided with a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and an aqueous electrolyte solution provided between the positive electrode and the negative electrode, the negative electrode active material containing a hydrogen storage alloy, and the aqueous electrolyte solution containing a solvent containing water and a potassium salt of an oxyacid of phosphorus represented by general formula K2 + nPnO3n + 1 dissolved in the solvent, and n is an integer of 1 or more.

Description

Technical Field

[0001] The present application relates to aqueous batteries. Background Art

[0002] Regarding aqueous batteries, Japanese Patent Application Laid-Open No. 2019-220294 describes an aqueous potassium ion battery having an aqueous electrolyte containing potassium pyrophosphate. Summary of the invention

[0003] However, it is difficult to say that a practical active material system has been established for an aqueous battery having an aqueous electrolyte solution containing a potassium salt of an oxygen-containing acid of phosphorus such as pyrophosphate.

[0004] The present invention provides a chargeable and dischargeable aqueous battery comprising a novel combination of an aqueous electrolyte solution containing a potassium salt of an oxoacid of phosphorus and a negative electrode active material.

[0005] One embodiment of the present invention is an aqueous battery, characterized in that it comprises: 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, wherein the negative electrode active material comprises a hydrogen storage alloy, and the aqueous electrolyte comprises: a solvent containing water, and a compound of the general formula K dissolved in the solvent 2+n P n O 3n+1 The potassium salt of an oxygen-containing acid of phosphorus represented by , wherein n is an integer greater than 1.

[0006] According to the present invention, it is possible to provide a chargeable and dischargeable aqueous battery comprising a novel combination of an aqueous electrolyte solution containing a potassium salt of an oxoacid of phosphorus and a negative electrode active material. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like reference numerals represent like elements, and in which:

[0008] Figure 1 A cross-sectional view schematically illustrating an aqueous battery 10 according to one embodiment;

[0009] Figure 2A Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Example 1;

[0010] Figure 2B Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Example 2;

[0011] Figure 2C Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Example 3;

[0012] Figure 3AGraph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Example 4;

[0013] Figure 3B Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Example 5;

[0014] Figure 3C Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 1;

[0015] Figure 4A Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 2;

[0016] Figure 4B Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 3;

[0017] Figure 4C Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Comparative Example 4;

[0018] Figure 5 Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Reference Example 1;

[0019] Fig. 6A Graph showing the charge-discharge characteristics (upper side) and cycle characteristics (lower side) of Example 6;

[0020] Figure 6B is a graph showing the charge and discharge characteristics (upper side) and cycle characteristics (lower side) of Example 7; and

[0021] Figure 6C It is a graph showing the charge and discharge characteristics (upper side) and the cycle characteristics (lower side) of Comparative Example 5. DETAILED DESCRIPTION

[0022] The following describes the embodiments of the present invention with reference to the accompanying drawings. However, the present invention is not limited to these embodiments. Furthermore, the drawings do not necessarily reflect the correct dimensions. In addition, in the drawings, some of the figure marks are sometimes omitted. In this specification, unless otherwise specified, for numerical values ​​A and B, the notation "A to B" means "above A and below B". In this notation, when only the numerical value B is marked with a unit, the unit also applies to the numerical value A. In addition, the term "or" means "logical and" unless otherwise specified. In addition, for element E 1 and E 2 , "E 1 and / or E 2 " means "E 1 or E 2 , or a combination thereof". For element E1 ,…,E N (N is an integer greater than 3), "E 1 ,…,E N-1 , and / or E N "This notation means "E 1 ,…,E N-1 , or E N , or a combination thereof".

[0023] Aqueous battery

[0024] Figure 1 It is a cross-sectional view schematically illustrating an aqueous battery 10 (hereinafter sometimes simply referred to as “battery 10 ”) according to one embodiment. Figure 1 The battery 10 is simplified in the figure, and the description of the terminal, part of the outer packaging material, etc. is omitted. The battery 10 has: a positive electrode layer 1, a positive electrode collector 2 connected to the positive electrode layer 1, a negative electrode layer 3, a negative electrode collector 4 connected to the negative electrode layer 3, and an electrolyte layer 5 arranged between the positive electrode layer 1 and the negative electrode layer 3, and also has an outer packaging material 6 for accommodating these. The aqueous battery 10 can also function as a secondary battery.

[0025] Positive electrode layer

[0026] 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 additive. The positive electrode layer 1 may further contain additives such as a binder (adhesive material) as needed. The thickness of the positive electrode layer 1 is not particularly limited, and in one embodiment, it may be 0.1 μm to 1 mm, or 1 to 100 μm.

[0027] For the positive electrode active material, one positive electrode active material may be used alone, or two or more positive electrode active materials may be used in combination. In one embodiment, the positive electrode layer 1 may include a proton-absorbing and releaseable proton (hydrogen ion H + ) positive electrode active material. Examples of positive electrode active materials that can occlude and release protons include nickel hydroxide, nickel oxyhydroxide (nickel hydroxide), nickel oxide, manganese dioxide, molybdenum trioxide, poly (aminoanthraquinone) (PNAQ), perylene-3,4,9,10-tetracarboxylic acid 3,4:9,10-dianhydride (PTCDA), pyrene-4,5,9,10-tetraketone (PYT), poly (diphenoxyphosphazene) (PDPZ), Prussian blue derivatives, or combinations thereof.

[0028] In another embodiment, the positive electrode layer 1 may contain potassium ions (K + ) is a positive electrode active material. Examples of positive electrode active materials that can occlude and release potassium ions include potassium cobalt composite oxide (KCoO 2 etc.), potassium nickel composite oxide (KNiO2 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 composite oxide (KMnO 2 、KMn 2 O 4 etc.), potassium iron manganese composite 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 composite oxide (KFeO 2 etc.), potassium chromium composite oxide (KCrO 2 etc.), potassium iron phosphate compounds (KFePO 4 etc.), potassium manganese phosphate compounds (KMnPO 4 etc.), potassium cobalt phosphate (KCoPO 4 ), Prussian blue, etc. The positive electrode active material may be a solid solution of these materials or a compound of non-stoichiometric composition.

[0029] In another embodiment, the positive electrode layer 1 may include potassium titanate, TiO 2 、LiTi 2 (PO 4 ) 3 , sulfur (S), etc. are used as positive electrode active materials.

[0030] The shape of the positive electrode active material can be, for example, a particle or a film. From the viewpoint of ion conductivity and electron conductivity, the primary particle size of the positive electrode active material can 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 can aggregate (agglomerate) to form secondary particles. From the same viewpoint, the particle size of the secondary particles of the positive electrode active material can be 0.1 to 500 μm, or 0.5 to 100 μm, or 1 to 20 μm in one embodiment. There is no particular limitation on the content of the positive electrode active material in the positive electrode layer 1. From the same viewpoint, based on the total amount of the positive electrode layer 1 (100 mass%), in one embodiment, it can be 20 to 99 mass%, or 40 to 99 mass%, or 60 to 97 mass%, or 70 to 95 mass%.

[0031] For the conductive aid, one conductive aid can be used alone, or two or more conductive aids can be used in combination. As a conductive aid, a conductive aid that can withstand the environment of the aqueous electrolyte secondary battery during charging and discharging can be used. As examples of conductive aids that can be combined with the positive electrode layer 1, 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 can be listed. As other examples of conductive aids that can be combined with the positive electrode layer 1, metal materials that can withstand the environment of the aqueous electrolyte secondary battery during charging and discharging can be listed. The shape of the conductive aid can be, for example, powder or fiber. There is no particular limitation on the amount of the conductive aid contained in the positive electrode layer 1. From the viewpoint of ionic conductivity and electronic conductivity, based on the total amount of the positive electrode layer 1 (100% by mass), in one embodiment, it can be 0.1 to 50% by mass, or 0.5 to 30% by mass, or 1 to 10% by mass.

[0032] For the binder (adhesive), one binder may be used alone, or two or more binders may be used in combination. As a binder, a binder that can withstand the environment during charge and discharge of an aqueous electrolyte secondary battery can be used. The binder serves to bind the active material or conductive aid to the surface of the collector 2 and maintain the conductive network in the electrode. Examples of binders include styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), acrylonitrile butadiene rubber (ABR), butadiene rubber (BR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), etc. There is no particular limitation on the amount of binder contained in the positive electrode layer 1. From the perspective of ionic conductivity and electronic conductivity, and the proper bonding of the positive electrode active material, based on the total amount of the positive electrode layer 1 (100% by mass), in one embodiment, the amount of the binder may be 0.1 to 50% by mass, or 0.5 to 30% by mass, or 1 to 10% by mass.

[0033] Positive electrode collector

[0034] The positive electrode collector 2 is a conductor connected to the positive electrode layer 1. The positive electrode collector 2 can be composed of a known metal that can be used as a positive electrode collector of an aqueous secondary battery. As an example of such a metal, a metal material 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 can be listed. As an example of the shape of the positive electrode collector 2, a plate-like, foil-like, mesh-like, porous, etc. can be listed. As another example, a substrate (such as a resin film, etc.) having conductivity or electrical insulation and a member of the above-mentioned metal material vapor-deposited or plated on the surface of the substrate can be used as the positive electrode collector 2.

[0035] Negative electrode layer

[0036] 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 additive. The negative electrode layer 3 may further contain additives such as a binder as needed. The thickness of the negative electrode layer 3 is not particularly limited, and in one embodiment, it may be 0.1 μm to 1 mm, or 1 to 100 μm.

[0037] As for the negative electrode active material, one negative electrode active material may be used alone, or two or more negative electrode active materials may be used in combination. In the aqueous battery 10, the negative electrode layer 3 contains a hydrogen storage alloy as the negative electrode active material. The hydrogen storage alloy is an alloy that reversibly forms a metal hydride by reacting the alloy with hydrogen, and can reversibly absorb and release hydrogen by utilizing the reaction. Examples of hydrogen storage alloys include AB 5 Type (such as LaNi 5 etc.), AB type of superlattice structure (such as TiCo, ZrCo, etc.), AB 2 Type (such as ZrV 0.4 Ni 1.5 etc.), A 2 B 7 Type (such as La 2 Ni 7etc.) and other types of hydrogen storage alloys. These hydrogen storage alloys are composed of a combination of an element occupying the A site with a high affinity for hydrogen (hereinafter referred to as the "A element") and an element occupying the B site with a low affinity for hydrogen but which plays a role in reducing the activation energy of the metal hydride formation reaction and its reverse reaction and reducing the temperature required for the reaction (hereinafter referred to as the "B element"). Examples of the A element include rare earth elements (such as La, etc.), Ca, Mg, Ti, Zr, V, Nb, Pt, Pd, etc. Examples of the B element include Fe, Ni, Mn, Co, Cr, Cu, Al, etc. In one embodiment, as the A element, a mixed metal (misch metal) that is an alloy containing one or more rare earth elements can be used. Examples of such hydrogen storage alloys include AB 5 MmNi type hydrogen storage alloy 5 alloy. Among them, "Mm" means mixed metal. Examples of metal elements contained in the mixed metal (Mm) include La, Ce, Nd, Sm, Pr, etc. In one embodiment, the mixed metal Mm may further contain alkaline earth elements such as Mg, Sr, Ca, and / or transition metal elements such as V, Cr, Fe, and Cu. As MmNi 5 Preferred examples of the alloy include MmNi in which a portion of nickel (Ni) is replaced by other B elements such as Co, Mn, and Al. 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.

[0038] Other examples of hydrogen storage alloys include Mg 2 Ni、Mg 2 Cu and other magnesium hydrogen storage alloys; Ti-Fe, Ti-Cr, Ti-Mn, Ti-Ni, Ti-Cu and other titanium hydrogen storage alloys; and Ln 1-x Mg x Ni y-z Al z(Wherein, Ln represents one or more elements selected from rare earth elements such as La, Sc, Y, Ti and Zr. x, y, z are each real numbers. Satisfying 0.05≤x≤0.30, 2.8≤y≤3.8, 0.05≤z≤0.30.) and other rare earth-Mg-Ni alloys. In these hydrogen storage alloys, in order to further improve the charge and discharge reaction activity, surface catalyst phases such as Ni, Pd, Pt, etc. may exist on the surface of the material. These surface catalyst phases can be set using conventionally known methods. In particular, in the case of a material system such as the Ti-Cr system that is easy to form a strong oxide film, it is preferred to set the above-mentioned surface catalyst phase.

[0039] The hydrogen storage alloy particles having a catalyst phase on the surface can be produced by conventionally known methods. Examples of methods for coating the parent material particles with the catalyst phase on the surface include PVD (Physical Vapor Deposition) methods such as sputtering and vacuum evaporation methods, and CVD (Chemical Vapor Deposition) methods such as thermal CVD methods.

[0040] In addition, for example, a Ni catalyst phase can be provided on the surface of hydrogen storage alloy particles containing Ni by sequentially undergoing an acid treatment step and a washing step. In the acid treatment step, the hydrogen storage alloy powder is subjected to an acid treatment. For example, an acid at room temperature of 15°C to 25°C is prepared, and the hydrogen storage alloy powder is placed in a container containing the acid and stirred for a predetermined time. On the surface of the hydrogen storage alloy particles subjected to such an acid treatment, rare earth elements, Mg, Al and other components other than Ni are dissolved by the acid, and Ni that is difficult to dissolve in the acid remains, forming a Ni-rich surface catalyst phase. Inside the surface catalyst phase, there is a core of a hydrogen storage alloy having a predetermined alloy composition. After the above stirring for a predetermined time is completed, water in an amount of more than twice the amount of the acid is added to the container, and stirred again for a predetermined time. Then, the mixed solution of the acid and water in the container is allowed to stand and maintained until the hydrogen storage alloy powder settles. Then, after the hydrogen storage alloy powder settles, the supernatant of the mixed solution 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 component. The temperature of the cleaning liquid may be, for example, 15 to 60° C. Then, the hydrogen storage alloy powder is separated from the cleaning liquid.

[0041] In addition, for example, hydrogen storage alloy powder can be added to an aqueous solution containing Pd ions or Pt ions, and stirred under reducing conditions to provide a Pd or Pt catalyst phase on the surface of the hydrogen storage alloy particles. A reducing agent can also be added and mixed in the aqueous solution. After the reaction is completed, impurities such as salts can be removed by further washing.

[0042] In one embodiment, as the negative electrode active material contained in the negative electrode layer 3, A can be preferably used.2 B 7 Type of hydrogen storage alloy (such as La 2 Ni 7 wait.).

[0043] The shape of the negative electrode active material may be, for example, a particle or a film. From the viewpoint of ionic conductivity and electronic conductivity, the primary particle size 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 aggregate to form secondary particles. From the same viewpoint, in one embodiment, the particle size 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. There is no particular limitation on the content of the negative electrode active material in the negative electrode layer 3. From the same viewpoint, based on the total amount of the negative electrode layer 3 (100 mass %), in one embodiment, the content of the negative electrode active material may be 20 to 99 mass %, or 40 to 99 mass %, or 60 to 97 mass %, or 70 to 95 mass %.

[0044] As the conductive aid in the negative electrode layer 3, the conductive aid described above in connection with the positive electrode layer 1 can be used in the same manner. One conductive aid can be used alone, or two or more conductive aids can be used in combination. The shape of the conductive aid can be, for example, powder or fiber. There is no particular limitation on the amount of the conductive aid contained in the negative electrode layer 3. From the viewpoint of ionic conductivity and electronic conductivity, based on the total amount of the negative electrode layer 3 (100% by mass), in one embodiment, the amount of the conductive aid can be 0.1 to 50% by mass, or 0.5 to 30% by mass, or 1 to 10% by mass.

[0045] As the binder in the negative electrode layer 3, the binder described above in connection with the positive electrode layer 1 can be used in the same manner. One binder can be used alone, or two or more binders can be used in combination. The amount of the binder contained in the negative electrode layer 3 is not particularly limited. From the viewpoint of ion conductivity and electron conductivity, and proper bonding of the negative electrode active material, based on the total amount of the negative electrode layer 3 (100% by mass), in one embodiment, the amount of the binder can be 0.1 to 50% by mass, or 0.5 to 30% by mass, or 1 to 10% by mass.

[0046] Negative electrode collector

[0047] The negative electrode collector 4 is a conductor connected to the negative electrode layer 3. The negative electrode collector 4 can be composed of a known metal that can be used as a negative electrode collector of an aqueous secondary battery. As an example of such a metal, a metal material 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 can be listed. If the stability in the aqueous electrolyte is considered, the negative electrode collector 4 may contain one or more elements selected from Al, Ti, Pb, Zn, Sn, Mg, Zr and In. The negative electrode collector 4 may contain one or more elements selected from Ti, Pb, Zn, Sn, Mg, Zr and In. The negative electrode collector 4 may contain Ti. The work functions of Al, Ti, Pb, Zn, Sn, Mg, Zr and In are all low, and it is believed that even if they are in contact with the aqueous electrolyte, it is difficult for the electrolysis of the aqueous electrolyte to occur. As examples of the shape of the negative electrode collector 4, plate-like, foil-like, mesh-like, porous, etc. can be listed. As other examples, a substrate having conductivity or electrical insulation (such as a resin film, etc.) and a component of the above-mentioned metal material deposited or plated on the surface of the substrate can be used as the negative electrode collector 4. In one embodiment, from the perspective of improving the withstand voltage of the reduction side of the aqueous electrolyte, the negative electrode collector 4 may have a conductive component (such as a foil, a mesh, a porous plate, a metal-plated resin film, etc.) containing the above-mentioned metal material and a coating layer containing a carbon material disposed on the surface of the conductive component. The negative electrode 3 can be arranged in contact with the coating layer. As examples of carbon materials, various carbon materials described above as examples of conductive aids that can be combined in the positive electrode layer 1 and the negative electrode layer 3 can be listed.

[0048] Electrolyte layer

[0049] 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 (immersed) in the separator 51. The separator 51 absorbs and retains the aqueous electrolyte 52. As the separator 51, a separator that can be used in an aqueous electrolyte secondary battery (such as a nickel-hydrogen battery, a zinc-air battery, etc.) can be used. As examples of such a separator (separator), water-absorbent members such as porous sheets or non-woven fabrics formed of hydrophilic materials (such as cellulose, etc.) can be listed. There is no particular limitation on the thickness of the separator 51, and for example, it can be 5μm to 1mm. The aqueous electrolyte 52 contains a solvent containing water and potassium pyrophosphate dissolved in the solvent.

[0050] The solvent of the aqueous electrolyte 52 contains water. The solvent may be composed of water. That is, in one embodiment, the aqueous electrolyte 52 may be composed of the general formula K 2+n P n O 3n+1(where n is an integer greater than 1.) An aqueous solution of a potassium salt of an oxygen-containing acid of phosphorus represented by. For example, the aqueous electrolyte 52 may be an aqueous potassium pyrophosphate solution. In another embodiment, the solvent may further include one or more non-aqueous solvents (organic solvents). As examples of such non-aqueous solvents, one or more organic solvents selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds and hydrocarbons can be listed. In one embodiment, regarding 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 there are multiple non-aqueous solvents)), based on the total amount of the solvent contained in the aqueous electrolyte 52 (100% by mass), it can 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.

[0051] In the aqueous electrolyte 52, as an electrolyte, a substance having a general formula K is dissolved 2+n P n O 3n+1 (where n is an integer greater than 1.) represents a potassium salt of an oxyacid of phosphorus (hereinafter referred to as "P(V) oxyacid potassium salt", and the corresponding acid radical is sometimes referred to as "P(V) oxyacid radical".). The oxidation valence of phosphorus in the salt is +V. In one embodiment, n=1, in which case the salt is potassium orthophosphate (K 3 PO 4 In one embodiment, n may be an integer greater than 2, in which case the salt is a potassium salt of condensed phosphoric acid. In one embodiment, n=2, in which case the salt is potassium pyrophosphate (K 4 P 2 O 7 In one embodiment, n=3, in which case the salt is potassium tripolyphosphate (K 5 P 3 O 10 In one embodiment, n may be, for example, 1 to 10, or 1 to 6, or 1 to 3. As the P(V) oxyacid potassium salt, only one (i.e., having a single n) P(V) oxyacid potassium salt may be present in the aqueous electrolyte 52. A combination of multiple P(V) oxyacid potassium salts having different n may also be present in the aqueous electrolyte 52.

[0052] In the aqueous electrolyte 52, the dissociation equilibrium of the potassium salt of the P(V) oxyacid and the acid dissociation equilibrium of the anions and their conjugate acids generated by the complete or partial dissociation of the potassium salt of the P(V) oxyacid can be established at the same time, and the chemical species generated by their equilibrium are also included in the "dissolved potassium salt of the P(V) oxyacid". For example, when the potassium salt of the P(V) oxyacid is potassium pyrophosphate (n=2), the "dissolved potassium pyrophosphate" includes K4 P 2 O 7 In addition, it can be used as K + , K 3 P 2 O 7 - , K 3 HP 2 O 7 , K 2 P 2 O 7 2- , K 2 HP 2 O 7 - , K 2 H 2 P 2 O 7 , KP 2 O 7 3- , KHP 2 O 7 2- , KH 2 P 2 O 7 - , KH 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 , or their associations, are present in the electrolyte. In the aqueous electrolyte 52, the "dissolved P(V) oxoacid potassium salt" is not necessarily obtained by adding K 2+n P n O 3n+1 For example, "dissolved potassium pyrophosphate" is not necessarily obtained by adding K 4 P 2 O 7 For example, potassium ion sources other than potassium pyrophosphate (such as K, KOH, K 2 O、CH 3COOK, etc.) and a pyrophosphate ion source (e.g. H 4 P 2 O 7 As a result, the above-mentioned ions and / or aggregates can be formed in water.

[0053] The content of the potassium salt of the P(V) oxyacid in the aqueous electrolyte 52 can be selected according to the required performance of the battery 10. In one embodiment, the content of the potassium salt of the P(V) oxyacid in the aqueous electrolyte 52 is K 2+n P n O 3n+1 (wherein n depends on the salt.) In terms of conversion, it can be 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. There is no particular upper limit to the content. In one embodiment, from the viewpoint of suppressing the increase in viscosity, K 2+n P n O 3n+1 (In the formula, n depends on the salt.) In terms of conversion, the content can be 7.0 mol or less relative to 1.0 kg of water.

[0054] In one embodiment, when the potassium salt of the P(V) oxoacid is potassium orthophosphate, the content of potassium orthophosphate in the aqueous electrolyte 52 is expressed as K 3 PO 4 The upper limit of the content is not particularly limited. In one embodiment, from the viewpoint of suppressing the increase in viscosity, the K 3 PO 4 The content can be 7.0 mol or less relative to 1.0 kg of water on a conversion basis.

[0055] In one embodiment, when the potassium salt of the P(V) oxoacid is potassium pyrophosphate, the content of potassium pyrophosphate in the aqueous electrolyte 52 is expressed as K 4 P 2 O 7 The content of K is preferably 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. In one embodiment, from the viewpoint of suppressing the increase in viscosity, the content of K is preferably 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. 4 P 2 O 7 The content can be 7.0 mol or less relative to 1.0 kg of water on a conversion basis.

[0056] In one embodiment, when the potassium salt of the P(V) oxoacid is potassium tripolyphosphate, the content of potassium tripolyphosphate in the aqueous electrolyte 52 is expressed as K 5 P 3 O 10The content of K is preferably 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. In one embodiment, from the viewpoint of suppressing the increase in viscosity, the content of K is preferably 2.0 mol or more, 3.0 mol or more, or 5.0 mol or more per 1.0 kg of water. 5 P 3 O 10 The content may be 7.0 mol or less, or 6.0 mol or less, relative to 1.0 kg of water on a conversion basis.

[0057] In one embodiment, as the concentration of potassium pyrophosphate in the aqueous electrolyte increases, the cycle characteristics become better, and high performance as a secondary battery is more likely to be obtained.

[0058] The content of the "dissolved P(V) oxo acid potassium salt" in the aqueous electrolyte 52 is calculated as follows.

[0059] (1) The contents of water, P(V) oxyacid radicals, and potassium in the electrolyte were measured.

[0060] (2-1) When (n+3)×P(V) oxygen-containing acid radical content (mol) ≤ potassium content (mol), the P(V) oxygen-containing acid radical content (mol) is determined as the P(V) oxygen-containing acid potassium salt content (mol).

[0061] (2-2) When (n+3)×P(V) oxygen-containing acid radical content (mol) ≤ potassium content (mol) is not satisfied, potassium content (mol) / (n+3) is determined as P(V) oxygen-containing acid potassium salt content (mol).

[0062] (3) Based on the determined P(V) oxyacid potassium salt content (mol) and water content (kg), calculate the P(V) oxyacid potassium salt content (mol) per 1 kg of water.

[0063] When the potassium salt of P(V) oxyacid is a combination of multiple salts with n different values ​​(i.e., the P(V) oxyacid is a combination of multiple P(V) oxyacids with n different values), as "P(V) oxyacid content (mol)", the total value of the content (mol) of the multiple P(V) oxyacids is used, and as n in the above inequality, the number average value of n of the multiple P(V) oxyacids is used for calculation. For example, when the P(V) oxyacid is a combination of orthophosphate (n=1) and pyrophosphate (n=2) (molar ratio 1:1), the number average value of n is 1.5. In addition, when the P(V) oxyacid is a combination of pyrophosphate (n=2) and tripolyphosphate (n=3) (molar ratio 1:1), the number average value of n is 2.5.

[0064] For example, when the P(V) oxoacid potassium salt is potassium pyrophosphate, the calculation is performed as follows.

[0065] (1) Regarding the content of "dissolved potassium pyrophosphate" in the aqueous electrolyte 52, the contents of water, pyrophosphate, and potassium in the electrolyte were measured.

[0066] (2-1) In the case where 4×pyrophosphate content (mol) ≤ potassium content (mol), the pyrophosphate content (mol) is determined as the potassium pyrophosphate content (mol).

[0067] (2-2) When 4×pyrophosphate content (mol) ≤ potassium content (mol) is not satisfied, potassium content (mol) / 4 is determined as potassium pyrophosphate content (mol).

[0068] (3) The potassium pyrophosphate content (mol) per 1 kg of water is calculated from the determined potassium pyrophosphate content (mol) and water content (kg).

[0069] The determination of water content can use known determination methods such as drying method (such as differential calorimetry, etc.), Karl-Fischer method. The determination of P (V) oxygen-containing acid radical content can use known determination methods such as inductively coupled plasma atomic emission spectroscopy (ICP-AES), chemiluminescence, enzyme method, ion chromatography, absorptiometry quantitative method using molybdophosphoric acid generation colorimetry, absorptiometry using heteropoly blue generation (Ohashi Hirosaburo et al., Analytical Chemistry, 1981, 30 (11), 727-731.). The determination of potassium content can use known determination methods such as ICP-AES.

[0070] The aqueous electrolyte 52 may contain potassium ions more than (n+3)×P(V) oxyacid (mol). For example, when the P(V) oxyacid potassium is potassium pyrophosphate, the aqueous electrolyte 52 may contain potassium ions more than 4×pyrophosphate (mol). As an example of such a situation, a potassium pyrophosphate source and a potassium ion source other than the potassium pyrophosphate source (e.g., KOH, CH 3 COOK, etc.).

[0071] The aqueous electrolyte 52 may contain cations other than potassium ions, such as alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc. In addition, the aqueous electrolyte may contain anions other than P(V) oxyacid radicals. For example, KPF may be further dissolved in the aqueous electrolyte 52. 6 KBF 4 , K 2 SO 4 、KNO 3 , CH 3 COOK, (CF 3 SO 2 ) 2NK, KCF 3 SO 3 、(FSO 2 ) 2 NK and other electrolytes.

[0072] In one embodiment, the proportion of P(V) oxygen-containing acid radicals in the acid radicals of the aqueous electrolyte 52 and the proportion of potassium ions in the cations of the aqueous electrolyte 52 are respectively based on the total amount of acid radicals in the aqueous electrolyte 52 and the total amount of cations in the aqueous electrolyte 52 as a reference (100 mol%), and can 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%.

[0073] The aqueous electrolyte 52 may contain an acid, a hydroxide, etc. for adjusting the pH of the aqueous electrolyte in addition to the above-mentioned solvent and electrolyte, and may also contain various additives.

[0074] 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 wide 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. In addition, 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.

[0075] Outer packaging materials

[0076] The battery 10 is used in a state of being contained in the outer packaging material 6. For the outer packaging material 6, an outer packaging material that can be used in an aqueous electrolyte secondary battery can be used. As a material that can constitute such an outer packaging material, for example, in addition to metal materials such as aluminum and stainless steel, resin materials such as polyphenylene sulfide resin and polyimide resin can also be listed. In addition, there is no particular limitation on the shape of the outer packaging material, for example, it can be circular (cylindrical, coin-shaped, button-shaped), hexahedral (rectangular parallelepiped, cubic), or bag-shaped, or processed to deform them, etc.

[0077] In conventional nickel-metal hydride (Ni-MH) secondary batteries, a strongly alkaline potassium hydroxide aqueous solution is typically used as an electrolyte, a hydrogen storage alloy (MH) is used as a negative electrode active material, and nickel hydroxide and / or nickel oxyhydroxide are used as a positive electrode active material. Usually, in the positive electrode layer, nickel hydroxide with low conductivity is used as a positive electrode active material, and cobalt hydroxide with high conductivity is added thereto as a conductive aid. This is because when carbon is used as a conductive aid in a strongly alkaline potassium hydroxide aqueous solution, the carbon is oxidized and degraded. By adding cobalt hydroxide to the positive electrode, the cobalt hydroxide is oxidized during the initial charge and becomes cobalt oxyhydroxide, thereby improving the conductivity of the positive electrode. However, since the electrolyte is strongly alkaline, cobalt is easily dissolved from the positive electrode layer (as well as metals such as zinc and manganese from the hydrogen storage alloy), and these dissolved metals are precipitated on the diaphragm, which can deteriorate the output characteristics and self-discharge characteristics. According to the aqueous battery of the present invention, the aqueous electrolyte contains potassium pyrophosphate, and the liquid property of the aqueous electrolyte is weakly acidic to weakly alkaline, or neutral to weakly alkaline. By using hydrogen storage alloys for negative electrode active materials, it can be charged and discharged. This means that the aggressiveness of the electrolyte to the negative electrode active material, the positive electrode active material, and the metal materials such as the collector can be reduced, and therefore, it means that the adverse effects caused by the dissolution of the metal 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, it is believed that the negative electrode active material absorbs and releases protons. In the case of using nickel hydroxide and / or nickel oxyhydroxide as the positive electrode active material, the positive electrode active material also works (acts) by absorbing and releasing protons, thus becoming a complete proton battery (ideal proton battery). As a conductive auxiliary agent for the positive electrode layer, carbon materials can also be used instead of the previous cobalt compounds. In addition, as a positive electrode collector and / or a negative electrode collector, metal materials that are not resistant to strong alkalis such as Al can also be used instead of Ni, which has excellent corrosion resistance but is expensive.

[0078] The present invention will be described in more detail below based on examples.

[0079] 1. Preparation of Electrolyte

[0080] As electrolytes for charge and 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 purchased from Norotech Co., Ltd., CH 3The COOK was purchased from FUJIFILM Wako Pure Chemical Corporation, CF 3 SO 3 The K was purchased from Merck KGaA, and the LiTFSI was purchased from Tokyo Chemical Industry Co., Ltd. and used directly as they were. For each electrolyte, the electrolyte was added to pure water and stirred so that the aqueous solution became a specified concentration. The stirred mixture was left standing in a thermostat at 25 °C for more than 1 day to completely dissolve the electrolyte.

[0081] 2. Fabrication of MH-coated electrodes

[0082] The electrodes for charge-discharge evaluation were fabricated according to the following steps. The hydrogen storage alloy (La 2 Ni 7 , hereinafter referred to as "MH") and acetylene black were weighed and uniformly mixed in a mortar. Further, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and water were added, and they were uniformly mixed again in a mortar. The mixing mass ratio was MH:acetylene black:SBR:CMC = 75:20:4:1. Defoaming treatment was performed for 1 minute at a rotational speed of 2000 rpm using a mixer (Thinky Corporation's "Avatori Rentaro") to prepare ink. The ink was dropped onto a titanium foil fixed to a glass plate, and coating was performed using a squeegee with a gap of 150 μm. The coated film was left standing under reduced pressure, and after natural drying, it was 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 1-ton load to densify the electrode.

[0083] 3. Charge-discharge evaluation

[0084] As the working electrode, the MH-coated electrode of 2. above was used, as the counter electrode, a Ni mesh was used, and as the reference electrode, an electrode unit (cell) (VM4 manufactured by Interchem Co., Ltd.) equipped with a silver-silver chloride electrode (BAS) was used. For each electrolyte of 1. above, the charge-discharge characteristics were evaluated. As the device, VMP3 (manufactured by BioLogic) was used, and the current density was set to ±0.1 mA / cm 2 . The cut-off condition on the oxidation current side was set to 0.2 V with respect to the standard hydrogen electrode (0 V with respect to the silver-silver chloride electrode), and the cut-off condition on the reduction current side was set to 300 mAh / g (which was limited by the capacity). The charge-discharge characteristics of 30 cycles were evaluated.

[0085] 4. Results

[0086] (Example 1) In Figure 2A shows the charge-discharge characteristics ( Figure 2A upper side) and cycle characteristics ( Figure 2A when an aqueous solution of potassium pyrophosphate (pH: 12.2) at 5.0 mol / kg was used as the electrolyte.Lower side). Good charge and discharge activity and high cycle stability were confirmed.

[0087] (Example 2) Figure 2B The charge and discharge characteristics when a 1.0 mol / kg potassium pyrophosphate aqueous solution (pH: 10.6) is used as the electrolyte are shown in FIG. Figure 2B upper side) and cycle characteristics ( Figure 2B Although the capacity decreases earlier as the number of charge and discharge cycles increases, it shows good charge and discharge activity in the early stage.

[0088] (Example 3) Figure 2C The charge and discharge characteristics when a 2.0 mol / kg potassium pyrophosphate aqueous solution (pH: 11.2) is used as the electrolyte are shown in FIG. Figure 2C upper side) and cycle characteristics ( Figure 2C Lower side). Good charge and discharge activity and high cycle stability were confirmed.

[0089] (Example 4) Figure 3A The charge and discharge characteristics when a 3.0 mol / kg potassium pyrophosphate aqueous solution (pH: 11.7) is used as the electrolyte are shown in FIG. Figure 3A upper side) and cycle characteristics ( Figure 3A Lower side). Good charge and discharge activity and high cycle stability were confirmed.

[0090] (Example 5) Figure 3B The charge and discharge characteristics when a 4.0 mol / kg potassium pyrophosphate aqueous solution (pH: 11.9) is used as the electrolyte are shown in FIG. Figure 3B upper side) and cycle characteristics ( Figure 3B The lower side). Good charge and discharge activity and high cycle stability were confirmed. In each example, it is believed that the nickel hydroxide / nickel oxyhydroxide formed on the surface of the Ni mesh in the positive electrode works (operates) by occluding and releasing protons.

[0091] (Comparative Example 1) Figure 3C The charge and discharge characteristics when 28 mol / kg potassium acetate aqueous solution is used as the electrolyte are shown in ( Figure 3C upper side) and cycle characteristics ( Figure 3C The operation (movement) as a battery was hardly observed.

[0092] (Comparative Example 2) Figure 4A The charge and discharge characteristics when a 1.0 mol / kg potassium acetate aqueous solution is used as the electrolyte are shown in FIG. Figure 4A upper side) and cycle characteristics ( Figure 4A On the oxidation side, almost no current flows.

[0093] (Comparative Example 3) Figure 4BThe charge and discharge characteristics when a 22 mol / kg potassium trifluoromethanesulfonate aqueous solution is used as the electrolyte are shown in FIG. Figure 4B upper side) and cycle characteristics ( Figure 4B The lower side). It shows an inexplicably high capacity at the beginning of the cycle, but then rapidly deteriorates and shows characteristics that cannot be used as a battery. In this comparative example, the oxidation current is likely to come from the dissolution reaction of the hydrogen storage alloy.

[0094] (Comparative Example 4) Figure 4C The charge and discharge characteristics when a 21 mol / kg LiTFSI aqueous solution is used as the electrolyte are shown in ( Figure 4C upper side) and cycle characteristics ( Figure 4C Neither charging nor discharging was observed, indicating complete inactivity.

[0095] (Reference Example 1) Figure 5 The charge and discharge characteristics when a 6 mol / kg potassium hydroxide aqueous solution is used as the electrolyte are shown in FIG. Figure 5 upper side) and cycle characteristics ( Figure 5 The results show that the difference between the results of other examples and comparative examples is due to the electrolyte used.

[0096] (Example 6) Fig. 6A The electrolyte used in the embodiment 1 is the same 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2), but the negative electrode active material is replaced by La 2 Ni 7 And using LaNi 5 The charge and discharge characteristics of Fig. 6A upper side) and cycle characteristics ( Fig. 6A lower side). It was confirmed that even when using AB 5 When a type of hydrogen storage alloy is used as the negative electrode active material, it can also work as a secondary battery. However, in terms of discharge capacity, it is still better to use A 2 B 7 La 2 Ni 7 In the case of , the discharge capacity is larger. In addition, the activation of the negative electrode requires a relatively large number of cycles.

[0097] (Example 7) Figure 6B The electrolyte used in the embodiment 1 is the same 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2), but the negative electrode active material is replaced by La 2 Ni 7 Using Pd-coated Ti 50 Cr 50 Charge and discharge characteristics of powder ( Figure 6B upper side) and cycle characteristics ( Figure 6B lower side). x Cr 1-x In the La-Ni alloy, the Pd coating functions as a catalyst phase. It was confirmed that the battery can function as a secondary battery even when a hydrogen storage alloy other than La-Ni alloy is used as the negative electrode active material.

[0098] (Comparative Example 5) Figure 6C The electrolyte used in the embodiment 1 is the same 5.0 mol / kg potassium pyrophosphate aqueous solution (pH: 12.2), but the negative electrode active material is replaced by La 2 Ni 7 The Ti without catalyst coating was used. 50 Cr 50 Charge and discharge characteristics of powder ( Figure 6C upper side) and cycle characteristics ( Figure 6C The lower side). It is almost impossible to work as a secondary battery. 50 Cr 50 The powder is different from Example 7 in that it is not coated with Pd (provided with a catalyst). x Cr 1-x Since the surface oxide film of the alloy is strong, in order to make Ti x Cr 1-x The results of Example 7 and Comparative Example 5 confirm that in the aqueous battery of the present invention, even if the alloy is a material system other than La-Ni system, as long as the alloy has hydrogen storage and release activity in the aqueous electrolyte (that is, as long as the alloy functions as a hydrogen storage alloy), and only when the alloy has hydrogen storage and release activity in the aqueous electrolyte, the alloy can be used as the negative electrode active material for charging and discharging.

Claims

1. Aqueous battery, characterized in that: The invention comprises: 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 comprises a hydrogen storage alloy, The aqueous electrolyte comprises: a solvent containing water, and a substance having a general formula K dissolved in the solvent 2+n P n O 3n+1 The potassium salt of an oxygen-containing acid of phosphorus represented by , wherein n is an integer greater than 1.

2. The aqueous battery according to claim 1, wherein: The potassium salt of the oxygen-containing acid of phosphorus is potassium pyrophosphate.

3. The aqueous battery according to claim 2, wherein: The content of potassium pyrophosphate in the aqueous electrolyte is 2.0 mol or more per 1 kg of water, calculated as K4P2O7.

4. The aqueous battery according to claim 2, wherein: The pH of the aqueous electrolyte at 25° C. is 3.0 to 13.

0.

5. The aqueous battery according to any one of claims 1 to 4, wherein The positive electrode active material can occlude and release hydrogen ions.

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