Electrolyte for metal-air batteries and metal-air batteries

Abstract

Electrolyte for metal-air batteries, designed for use in metal-air batteries having an anode containing at least one type of metal selected from aluminium and magnesium, wherein the electrolyte comprises an aqueous solution containing at least one type of self-discharge inhibitor selected from the group consisting of M2HPO4, M3PO4, M4P2O7, MH2PO2, M2H2P2O7, LHPO4, MLPO4, L2P2O7 and LH2P2O7, wherein M is a type of metal selected from the group consisting of Li, K, Na, Rb, Cs and Fr, and L is a type of metal selected from the group consisting of Mg, Ca, Sr, Ba and Ra, wherein the content of the self-discharge inhibitor is 0.001 mol / l or more and 0.1 mol / l or less.

Description

Technical field

[0001] The present invention relates to an electrolyte for metal-air batteries and a metal-air battery. Current state of the art

[0002] An air battery, which uses oxygen as the active material, has many advantages, such as high energy density. Well-known examples of air batteries include metal-air batteries, such as aluminum-air batteries and magnesium-air batteries.

[0003] As a technique related to such air batteries, patent literature 1 discloses, for example, an aluminum-air battery which includes a cathode (air electrode), an electrolyte, and an anode in which aluminum metal is used. Furthermore, patent literature 2, patent literature 3, and patent literature 4 disclose metal-air batteries from the prior art. Patent Literature 1: JP 2014-139878 A Patent literature 2: US 2012 / 0 293 110 A1 Patent literature 3: US 2008 / 0 096 061 A1 Patent literature 4: US 2007 / 0 099 050 A1

[0004] However, metal-air batteries, which use a metal such as aluminum or magnesium in the anode, have a problem of self-discharge. SUMMARY OF THE INVENTION

[0005] The present invention was developed in light of the above circumstances. An object of the present invention is to provide an electrolyte capable of inhibiting the self-discharge of metal-air batteries and a metal-air battery using the electrolyte.

[0006] The electrolyte for metal-air batteries according to the present invention is a metal-air battery electrolyte configured for use in metal-air batteries having an anode containing at least one type of metal selected from aluminum and magnesium, wherein the electrolyte contains an aqueous solution containing at least one type of self-discharge inhibitor selected from the group consisting of M2HPO4, M3PO4, M4P2O7, MH2PO2, M2H2P2O7, LHPO4, MLPO4, L2P2O7 and LH2P2O7, wherein M is a type of metal selected from the group consisting of Li, K, Na, Rb, Cs and Fr, and L is a type of metal selected from the group consisting of Mg, Ca, Sr, Ba and Ra, wherein the content of the self-discharge inhibitor is 0.001 mol / l or more and 0.1 mol / l or less.

[0007] In the electrolyte for metal-air batteries according to the present invention, the self-discharge inhibitor is preferably Na2H2P2O7.

[0008] In the electrolyte for metal-air batteries according to the present invention, the aqueous solution is preferably basic.

[0009] In the electrolyte for metal-air batteries according to the present invention, the aqueous solution preferably contains NaOH as an electrolyte salt.

[0010] The metal-air battery of the present invention is a metal-air battery comprising: an air electrode to which oxygen is supplied; an anode containing at least one type of metal selected from aluminum and magnesium; and an electrolyte in contact with the air electrode and the anode, wherein the electrolyte is the aforementioned electrolyte for metal-air batteries.

[0011] According to the present invention, the self-discharge of metal-air batteries can be inhibited. BRIEF DESCRIPTION OF THE DRAWINGS Fig.Figure 1 is a sectional view of a schematic embodiment of the metal-air battery according to the present invention. Fig. Figure 2 is a bar chart that compares the idle potential holding times of examples 1 to 5 and comparison example 2 with comparison example 1. Reference symbol list 10 metal-air batteries 11 Anode 12 Air electrode 13 Electrolyte 14 Separator 15 Anode current collector 16 air electrode current collector 17 Outer casings 18 Water-repellent film DETAILED DESCRIPTION OF THE INVENTION 1. Electrolyte for metal-air batteries

[0012] The electrolyte for metal-air batteries according to the present invention is a metal-air battery electrolyte configured for use in metal-air batteries having an anode containing at least one type of metal selected from aluminium and magnesium, wherein the electrolyte contains an aqueous solution containing at least one type of self-discharge inhibitor selected from the group consisting of M2HPO4, M3PO4, M4P2O7, MH2PO2, M2H2P2O7, LHPO4, MLPO4, L2P2O7 and LH2P2O7, wherein M is a type of metal selected from the group consisting of Li, K, Na, Rb, Cs and Fr, and L is a type of metal selected from the group consisting of Mg, Ca, Sr, Ba and Ra.

[0013] A metal-air battery that includes an anode containing at least one type of metal selected from aluminum and magnesium causes a strong self-discharge when the anode is brought into contact with the electrolyte; therefore, it has a problem of high loss of battery capacity.

[0014] The self-discharge reaction of a metal-air battery occurs when a potential difference between the main element (Al, Mg) of the metal contained in the anode (hereinafter also referred to as the anode metal) and impurity elements (e.g., iron) contained in the metal creates a local cell. For example, if the main element of the metal is aluminum, iron, which is an impurity metal, serves as the cathode. In the cathode, a reductive decomposition reaction of water occurs on the surface of the iron. In the anode, an oxidation reaction of the aluminum occurs (that is, an elution reaction brought about by ionization).

[0015] However, if a high-purity metal is used as the anode metal, self-discharge is less likely. The problem, however, is that this increases costs and makes practical application more difficult.

[0016] The inventor of the present invention has discovered that the self-discharge of the metal-air battery can be inhibited by adding the discharge inhibitor to the electrolyte.

[0017] According to the electrolyte of the present invention, upon discharge of the metal-air battery, the anion contained in the self-discharge inhibitor preferably adsorbs onto the surface of the impurity metal, which is contained in the anode metal such as iron, thus preventing direct contact between the impurity metal and the electrolyte. It is assumed that, as a result, the formation of a local cell is inhibited, thereby inhibiting the self-discharge of the metal-air battery.

[0018] The anion contained in the self-discharge inhibitor is at least one from the group consisting of PO2. 3- , PO4 3- and P2O7 4-The selected type of anion is preferred. Of these, P2O7 is preferred. 4- especially preferred.

[0019] The cation contained in the self-discharge inhibitor is preferably at least one type selected from the group consisting of Li, K, Na, Rb, Cs, Fr, Mg, Ca, Sr, Ba and Ra. Of these, K + and Na + more strongly preferred, and Na + is even more strongly preferred. The cation is the cation of a metal that lies lower in the electrochemical series than aluminum and magnesium. Accordingly, the cation exhibits lower reactivity with aluminum and magnesium, which serve as anode metals in the electrolyte. Therefore, it is assumed that the cation will be less likely to interfere with the specific adsorption of the anion onto the impurity (e.g., iron) contained in the anode metal, where the adsorption is aimed at inhibiting self-discharge.

[0020] The cation contained in the self-discharge inhibitor can be, in addition to the cation of the aforementioned metal, one or more hydrogen atoms. + It should be included.

[0021] Specific examples of self-discharge inhibitors include Na2HPO4, Na3PO4, Na4P2O7, NaH2PO2, Na2H2P2O7, MgHPO4, NaMgPO4, Mg2P2O7, MgHPO2, NaMgPO2, MgH2P2O7, NaMgHP2O7, Na2MgP2O7, KH2PO2, LiH2PO2, K3PO4, Li3PO4, K2HPO4, Li2HPO4, K4P2O7, Li4P2O7, K2H2P2O7, Li2H2P2O7, CaHPO4, and Ca2P2O7. Of these, Na2H2P2O7 is preferred.

[0022] The concentration of the self-discharge inhibitor in the electrolyte is 0.001 mol / l or more and 0.1 mol / l or less.

[0023] The electrolyte salt is not subject to any particular restrictions, provided it is soluble in water and can offer the desired ionic conductivity. The electrolyte salt is preferably one capable of making the electrolyte neutral or basic. From the perspective of increasing electrode reactivity, one capable of making the electrolyte basic is particularly preferred.

[0024] The electrolyte salt is preferably one containing at least one type of metal selected from the group consisting of Li, K, Na, Rb, Cs, Fr, Mg, Ca, Sr, Ba, and Ra. Specific examples of the electrolyte salt include LiCl, NaCl, KCl, MgCl₂, CaCl₂, LiOH, KOH, NaOH, RbOH, CsOH, Mg(OH)₂, Ca(OH)₂, and Sr(OH)₂. Of these, NaOH and KOH are preferred. NaOH is particularly preferred.

[0025] The concentration of the electrolyte salt is not subject to any particular restrictions. The lower limit is preferably 0.01 mol / l or more, more preferably 0.1 mol / l or more, and even more preferably 1 mol / l or more. The upper limit is preferably 20 mol / l or less, more preferably 10 mol / l or less, and even more preferably 8 mol / l or less.

[0026] If the electrolyte concentration is less than 0.01 mol / l, the solubility of the anode metal may decrease. If the electrolyte concentration is more than 20 mol / l, the self-discharge of the metal-air battery is accelerated and can reduce battery performance.

[0027] The pH value of the electrolyte is preferably 7 or more, more preferably 10 or more, and particularly preferably 14 or more. 2. Metal-air battery

[0028] The metal-air battery of the present invention is a metal-air battery comprising: an air electrode to which oxygen is supplied; an anode containing at least one type of metal selected from aluminum and magnesium; and an electrolyte in contact with the air electrode and the anode, wherein the electrolyte is the aforementioned electrolyte for metal-air batteries.

[0029] In the present invention, the metal-air battery is a battery in which a reduction reaction of oxygen, which is an active material, takes place in the air electrode; an oxidation reaction of a metal takes place in the anode; and ions are carried by the electrolyte, which is arranged between the air electrode and the anode. Examples of the type of metal-air battery include a magnesium-air primary battery and an aluminum-air primary battery.

[0030] Fig.Figure 1 is a sectional view of a schematic embodiment of the metal-air battery according to the present invention.

[0031] As in Fig. As shown in Figure 1, a metal-air battery 10 comprises an anode 11; an air electrode 12 arranged remotely from the anode 11; a separator 14 containing an electrolyte 13 located between the anode 11 and the air electrode 12; an anode current collector 15 connected to the anode 11; an air electrode current collector 16 connected to the air electrode 12; and an outer casing 17 housing these elements. The outer casing 17 is partially composed of a water-repellent film 18. By using the water-repellent film 18, and so forth, the metal-air battery 10 is constructed such that the electrolyte 13 does not escape from the outer casing 17.

[0032] The electrolyte that can be used in the metal-air battery of the present invention is not described here, since it is the same as the electrolyte described above under “1. Electrolyte for metal-air batteries”.

[0033] If required, the metal-air battery of the present invention includes a separator for isolating the air electrode and the anode from each other. From the perspective of retaining the electrolyte, the separator preferably has a porous structure. The porous structure of the separator is not subject to any particular restrictions, provided it can retain the electrolyte. Examples include a grid structure in which constituent fibers are arranged regularly, a nonwoven structure in which constituent fibers are arranged randomly, and a three-dimensional network structure with separate and interconnected holes. Conventionally known separators can be used. Specific examples include porous films made of polyethylene, polypropylene, polyethylene terephthalate, cellulose, etc., and nonwovens such as a resin nonwoven and a glass fiber nonwoven.

[0034] The thickness of the separator is not subject to any particular restrictions. For example, it is preferably in the range of 0.1 to 100 µm.

[0035] The porosity of the separator is preferably 30 to 90%, more preferably 45 to 70%. If the porosity is too low, the separator tends to impair ion diffusion. If the porosity is too high, the strength of the separator tends to decrease.

[0036] The air electrode contains at least one electrically conductive material.

[0037] The electrically conductive material is not subject to any special restrictions, provided it possesses electrical conductivity. Examples would include a carbon-containing material, an electrically conductive material of the perovskite type, an electrically conductive porous polymer, a metal body, etc.

[0038] The carbon-containing material can be porous or non-porous. Preferably, the carbon-containing material is porous. This is because it has a large specific surface area and can provide many reaction sites. Specific examples of porous carbon-containing materials include mesoporous carbon. Specific examples of non-porous carbon-containing materials include graphite, carbon black, carbon carbon, carbon nanotubes, and carbon fibers.

[0039] The metal body can be made of a known metal that is resistant to the electrolyte. More precisely, the metal body can be a metal body in which a metal layer (coating film) is formed on the surface, containing at least one type of metal selected from the group consisting of, for example, Ni, Cr, and Al, or a metal body consisting entirely of a metal material made from at least one type of metal selected from the group consisting of Ni, Cr, and Al. The shape of the metal body can be a known form, such as a metal lattice, a perforated metal foil, or a foam metal.

[0040] The content of the electrically conductive material in the air electrode is, for example, preferably 10 to 99 wt%, particularly preferably 50 to 95 wt%, if the total mass of the air electrode is determined to be 100 wt%.

[0041] The air electrode may contain a catalyst that promotes electrode reactions. The catalyst may be mounted on the electrically conductive material.

[0042] A suitable known catalyst possessing oxygen-reducing capability and usable in metal-air batteries can be employed as the catalyst. Examples include at least one type of metal selected from the group consisting of ruthenium, rhodium, palladium, and platinum; a perovskite-type oxide containing a transition metal such as cobalt, manganese, or iron; a metal-coordinated organic compound with a porphyrin or phthalocyanine structure; an inorganic ceramic such as manganese dioxide (MnO₂) or cerium dioxide (CeO₂); and a composite material made from a mixture of the above materials.

[0043] The catalyst content in the air electrode is, for example, preferably 0 to 90 wt%, particularly preferably 1 to 90 wt%, if the total mass of the air electrode is determined to be 100 wt%.

[0044] If required, the air electrode contains a binding agent to fix the electrically conductive material.

[0045] Examples of binding agents include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), etc.

[0046] The binder content in the air electrode is not subject to any particular restrictions. For example, it is preferably 1 to 40 wt%, particularly preferably 10 to 30 wt%, if the total mass of the air electrode is determined to be 100 wt%.

[0047] Examples of methods for manufacturing the air electrode include a process for mixing the air electrode materials described above (such as the electrically conductive material) and roller pressing the mixture, as well as a process for applying a slurry containing the air electrode materials described above and a solvent. Examples of solvents used to manufacture the slurry include acetone, ethanol, and N-methyl-2-pyrrolidone (NMP). Examples of methods for applying the slurry include spraying, screen printing, gravure printing, die coating, squeegee or scraper application, inkjet printing, etc.More precisely, the air electrode can be formed by applying the slurries to the air electrode current collector or carrier film described below, drying the applied slurries, and then, if necessary, roller pressing and cutting the dried slurries.

[0048] The thickness of the air electrode varies depending on the intended use of the metal-air battery, etc. For example, it is preferably in a range of 2 to 500 µm, particularly preferably in a range of 30 to 300 µm.

[0049] When required, the metal-air battery of the present invention includes an air electrode current collector that collects current from the air electrode. The air electrode current collector can have a porous or a dense structure, provided it possesses the desired electron conductivity. From the perspective of air (oxygen) diffusion, it is preferably one with a porous structure, such as a grid structure. Examples of the air electrode current collector's form include a foil, a plate, and a grid (lattice) form. The porosity of the current collector with the porous structure is not subject to any particular limitations. For example, it preferably lies in the range of 20 to 99%.

[0050] Examples of materials suitable for the air electrode current collector include metallic materials such as stainless steel, nickel, aluminum, iron, titanium, copper, gold, silver and palladium; carbon-containing materials such as carbon fiber and carbon paper; and highly electron-conducting ceramic materials such as titanium nitride.

[0051] The thickness of the air electrode current collector is not subject to any particular restrictions. For example, it is preferably 10 to 1000 µm, particularly preferably 20 to 400 µm. The outer casing described below can also function as the air electrode current collector.

[0052] The air electrode current collector may have a connection that serves as a link to the outside.

[0053] The anode contains at least one active anode material.

[0054] The active anode material could include aluminum metal, magnesium metal, an aluminum alloy, a magnesium alloy, an aluminum compound, a magnesium compound, etc. Of these, aluminum metal is preferred.

[0055] An aluminum alloy, for example, is an alloy of aluminum and a metal selected from the group consisting of vanadium, silicon, magnesium, iron, zinc, and lithium. The metal forming the aluminum alloy (that is, the metal other than aluminum) can be one or more types of metal.

[0056] Examples of aluminum compounds include aluminium(III) nitrate, aluminium(III) chloride oxide, aluminium(III) oxalate, aluminium(III) bromide and aluminium(III) iodide.

[0057] In the case that the anode is aluminum metal, the purity of the aluminum is not subject to any particular restrictions. The lower limit for the elemental ratio of aluminum contained in the aluminum metal is preferably 50% or more, more preferably 80% or more, even more preferably 95% or more, and particularly preferably 99.5% or more. Furthermore, the upper limit for the elemental ratio of aluminum contained in the aluminum metal may be 99.99% or less, or it may be 99.9% or less. The aluminum metal may also contain iron. The elemental ratio of the iron contained in the aluminum metal is not subject to any particular restrictions. It may be less than 0.01% or it may be less than 0.1%.

[0058] In the aluminium alloy, the aluminium content is preferably 50 wt% or more when the total mass of the alloy is determined to be 100 wt%.

[0059] The shape of the anode is not subject to any particular restrictions, and plate, rod, or particle shapes, etc., are all possible. From the perspective of the shape, which can easily increase the performance of the metal-air battery, a particle shape is preferred. If the anode is in a particle shape, the lower limit of the particle diameter is preferably 1 nm or more, more preferably 10 nm or more, and even more preferably 100 nm or more, and the upper limit of the particle diameter is preferably 100 mm or less, more preferably 10 mm or less, and even more preferably 1 mm or less.

[0060] In the present invention, the average particle diameter is calculated by a general method. An example of the method for calculating the average particle diameter is as follows. First, for a particle shown in a photograph taken at a suitable magnification (e.g., 50,000x to 1,000,000x) using a transmission electron microscope (hereinafter referred to as TEM) or a scanning electron microscope (hereinafter referred to as SEM), the diameter is calculated, assuming the particle is spherical. Such a calculation of the particle diameter by TEM or SEM observation is performed on 200 to 300 particles of the same type, and the average of the particles is determined as the average particle diameter.

[0061] Depending on requirements, the anode contains at least one component of the electrically conductive material and one component of the binder for fixing the active anode material. For example, if the active anode material is in plate form, the anode can be an anode containing only the active anode material. Conversely, if the active anode material is in powder (particle) form, the anode can be an anode containing the active anode material and at least one component of the electrically conductive material and one component of the binder. The type and quantity of the electrically conductive material used, the type and quantity of the binder used, etc., can be the same as those of the air electrode described above.

[0062] If required, the anode includes an anode current collector that collects current from the anode. The material for the anode current collector is not subject to any particular restrictions, provided it is electrically conductive. Examples include stainless steel, nickel, copper, and carbon. The anode current collector can be in various forms, such as foil, plate, or grid. The thickness of the anode current collector is not subject to any particular restrictions. For example, it is preferably 10 to 1000 µm, and particularly preferably 20 to 400 µm. The outer casing described below can also function as the anode current collector.

[0063] The anode current collector may have a terminal that serves as a connection to the outside.

[0064] The metal-air battery of the present invention generally has an outer casing for receiving the air electrode, the anode, the electrolyte, etc.

[0065] Examples of the outer casing's shape include a coin shape, a flat plate shape, a cylindrical shape, and a laminate or layered composite shape.

[0066] The material for the outer casing is not subject to any special restrictions, provided it is resistant to the electrolyte. Examples include a metal body containing at least one type of metal selected from the group consisting of Ni, Cr, and Al, and a resin such as polypropylene, polyethylene, or acrylic resin. If the outer casing is the metal body, it can be such that only the surface consists of the metal body, or it can be such that the entire outer casing consists of the metal body.

[0067] The outer casing can be either open to the atmosphere or hermetically sealed. An open-type casing has an opening for external oxygen (i.e., an oxygen inlet) and a structure that allows at least the air electrode to be in sufficient contact with the atmosphere. The oxygen inlet may be covered with an oxygen-permeable film, a hydrogen-repellent film, etc. A hermetically sealed battery casing may have an oxygen (air) inlet tube and an outlet tube.

[0068] The water-repellent film is not subject to any special restrictions, provided it is made of a material that prevents the electrolyte from escaping and allows air to reach the air electrode. Examples of water-repellent films include porous fluoropolymer films (such as PTFE) and water-repellent, porous cellulose.

[0069] An oxygen-containing gas is supplied to the air electrode. This oxygen-containing gas could be air, dry air, pure oxygen, etc. Preferably, the oxygen-containing gas is dry air or pure oxygen, and particularly preferably pure oxygen. EXAMPLES (Example 1)

[0070] First, an aqueous solution of 1 mol / L NaOH (manufactured by Kanto Chemical Co., Inc.) was prepared. This aqueous solution was held for 8 hours at 25°C in a thermostated bath (product name: LU-113; manufactured by: ESPEC Corp.). Then, Na₂HPO₄·12H₂O (manufactured by Aldrich) was added to the aqueous solution as a self-discharge inhibitor to achieve a concentration of 0.01 mol / L. Next, the aqueous solution was stirred for 15 minutes using an ultrasonic washing machine. Finally, the aqueous solution was held for 3 hours at 25°C in the thermostated bath, resulting in an electrolyte suitable for metal-air batteries. (Example 2)

[0071] An electrolyte for metal-air batteries was prepared in the same way as Example 1, except that Na2HPO4·12H2O was changed to Na3PO4·12H2O (manufactured by Aldrich). (Example 3)

[0072] An electrolyte for metal-air batteries was prepared in the same way as Example 1, except that Na2HPO4·12H2O was changed to Na4P2O7 (manufactured by Aldrich). (Example 4)

[0073] An electrolyte for metal-air batteries was prepared in the same way as Example 1, except that Na2HPO4·12H2O was changed to NaH2PO2·H2O (manufactured by Aldrich). (Example 5)

[0074] An electrolyte for metal-air batteries was prepared in the same way as Example 1, except that Na2HPO4·12H2O was changed to Na2H2P2O7 (manufactured by Aldrich). (Comparative example 1)

[0075] An electrolyte for metal-air batteries was prepared in the same way as Example 1, except that Na2HPO4·12H2O was not added. (Comparative example 2)

[0076] An electrolyte for metal-air batteries was prepared in the same way as Example 1, except that Na2HPO4·12H2O was changed to NaH2PO4·H2O (manufactured by Aldrich). [Evaluation of self-discharge inhibition](Electrode preparation)

[0077] A 25 mm × 25 mm × 1 mm aluminum plate with a purity of 99.5% (product name: Al2N; manufactured by: Nilaco Corporation) was prepared as a working electrode. The surface of the aluminum plate was wiped with acetone. The aluminum plate was then sandwiched between nickel meshes (product name: 20 mesh; manufactured by Nilaco Corporation), and the edges of the nickel meshes were welded together. A nickel strip (manufactured by Nilaco Corporation) was welded to this and used as a current-collecting wire.

[0078] A nickel mesh (product name: 200 mesh; manufactured by: Nilaco Corporation) cut to a size of 30 mm × 30 mm × 1 mm was prepared as a counter electrode. A nickel strip was welded to the nickel mesh and used as a current collector wire.

[0079] A Hg / HgO electrode was prepared as a reference electrode. (Production of evaluation cells)

[0080] The electrolytes used were those of Examples 1 to 5 and Comparative Examples 1 and 2 (55 ml each).

[0081] Seven cell containers were prepared (the number of containers equal to the total number of electrolytes in Examples 1 and 5 and Comparison Examples 1 and 2). The working electrode, counter electrode, and reference electrode were placed in each cell container (volume 60 ml). The electrolytes (55 ml each) were added separately to the cell containers. The cell containers were covered to prevent evaporation, thus preparing the evaluation cells. The preparation of the evaluation cells was completed within 10 minutes. (Measurement of idle potential holding time)

[0082] For each of the evaluation cells using the electrolytes of Examples 1 to 5 and Comparison Examples 1 and 2, the open-circuit potential (OCV) holding time of the aluminum electrode (working electrode) was measured. Specifically, the working and counter electrodes of each evaluation cell were connected to a potentiostat / galvanostat (product name: VMP3; manufactured by: Biologic); an open circuit was maintained for 30 hours at an ambient temperature of 25°C; and the time until the working electrode potential changed from approximately -1.3 V (vs. Hg / HgO) at the start of the measurement to -0.8 V (vs. Hg / HgO) was measured.

[0083] The open-circuit potential holding time is the period during which the self-discharge reaction occurs and the aluminum electrode is completely eluted. Accordingly, it is assumed that with increasing open-circuit potential holding time, the self-discharge rate decreases, thereby inhibiting self-discharge. The results of the open-circuit potential holding time measurements are shown in Table 1. Table 1 Self-discharge inhibitor Idle potential holding time (sec) Rate of increase (%) Example 1 Na2HPO4·12H2O 27666 113 Example 2 Na3PO4·12H2O 25118 102 Example 3 Na4P2O7 29291 119 Example 4 NaH2PO2-H2O 32670 133 Example 5 Na2H2P2O7 61807 252 Comparative example 1 - 24557 100 Comparative example 2 NaH2PO4·H2O 22235 91

[0084] As shown in Table 1, the idle potential holding times of the evaluation cells using the electrolytes of Examples 1 to 5 and Comparison Examples 1 and 2 are as follows: 27666 seconds in Example 1; 25118 seconds in Example 2; 29291 seconds in Example 3; 32670 seconds in Example 4; 61807 seconds in Example 5; 24557 seconds in Comparison Example 1; and 22235 seconds in Comparison Example 2.

[0085] Fig.Figure 2 is a bar chart that compares the idle potential holding times of examples 1 to 5 and comparison example 2 with comparison example 1.

[0086] As shown in Table 1 and Fig. As shown in Figure 2, the rates of increase of the idle potential holding times of Example 1 to 5 and Comparison Example 2 relative to Comparison Example 1 are as follows: 113% in Example 1; 102% in Example 2; 119% in Example 3; 133% in Example 4; 252% in Example 5; and 91% in Comparison Example 2.

[0087] This makes it clear that the open-circuit potential holding times of the aluminum electrodes in examples 1 to 5 are longer than in comparison example 1. Particularly in the case of example 5, where Na2H2P2O7 was used as the self-discharge inhibitor, it is clear that such a remarkable self-discharge inhibitory effect is present that the open-circuit potential holding time is 2.52 times longer than in comparison example 1.

[0088] The reason why the self-discharge inhibitory effect varies depending on the anion species (phosphoric acid species) contained in the self-discharge inhibitor is unclear. However, a change in the negative charge of oxygen coordinated to phosphorus is assumed to be the cause. For example, the negative charge of oxygen decreases with a decreasing number of oxygen coordinated to phosphorus.

[0089] In the case of the self-discharge inhibitor, in which no H + It is also assumed that the negative charge of O decreases when the self-discharge inhibitor is dissolved in the electrolyte and Na is dissolved. + not completely dissociated. It is assumed that, due to the slight decrease in the negative charge of the anion described above, the adsorption of the anion to iron was altered, thus changing the self-discharge inhibiting effect. [EDX analysis of the electrode surface]

[0090] First, 50 ml of the electrolyte from Example 5 was placed in a container. Next, an aluminum plate with a purity of 99.5% (product name: Al₂N; manufactured by: Nilaco Corporation), cut to a size of 12 mm × 12 mm × 1 mm (approximately 0.4 g), was prepared. The surface of the aluminum plate was wiped with acetone. The aluminum plate was then placed in the container. A piece of paper was placed on top of the container, and the container was loosely covered. This prevented hydrogen from remaining in the container and inhibited natural volatilization of the electrolyte. The container was then placed in a thermostatically controlled bath and held at 25°C for 3 hours. Energy-dispersive X-ray analysis (EDX) was then performed on the surface of the aluminum plate. The results are shown in Table 2. Table 2 Al electrode surface area (atomic %) Discharge product surface area (atomic %) Al 98,18 35,04 Fe 0,98 42,45 Si 0,27 2,64 P 0,26 1,78 O 0,17 11,93

[0091] As shown in Table 2, the atomic composition of the aluminum metal surface exposed on the aluminum plate surface (base) is as follows: Al is 98.18 atomic%; Fe is 0.98 atomic%; Si is 0.27 atomic%; P is 0.26 atomic%; and O is 0.17 atomic%. Meanwhile, the atomic composition of the discharge product surface exposed on the aluminum plate surface is as follows: Al is 35.04 atomic%; Fe is 42.45 atomic%; Si is 2.64 atomic%; P is 1.78 atomic%; and O is 11.93 atomic%.

[0092] Therefore, as shown in Table 2, on the aluminum metal surface (base) Fe is 0.98 atomic percent and P is 0.26 atomic percent; however, on the discharge product surface, which contained iron, an impurity, Fe is 42.45 atomic percent, which is very high. P is 1.78 atomic percent, and it has been confirmed that the abundance of P on the discharge product surface is about seven times greater than the atomic composition ratio of the aluminum metal surface (base). Since, accordingly, almost no P is present on the aluminum metal surface (base) and P is mainly present on the discharge product surface, it is assumed that the anion species (phosphoric acid species) contained in the self-discharge inhibitor more preferentially adsorptively binds to the surface of the iron, which is an impurity contained in the aluminum metal, than to the aluminum metal surface itself.

[0093] Based on the above, the use of the self-discharge inhibitor causes the anion species contained in the inhibitor to preferentially adsorb onto the surface of the iron, which is an impurity present in the aluminum metal, thus inhibiting direct contact between the iron and the electrolyte. Consequently, it is assumed that the formation of a local cell is inhibited, thereby inhibiting the self-discharge of the metal-air battery.

[0094] It is assumed that, even in the case where magnesium metal is used in the anode, the anion contained in the self-discharge inhibitor more preferentially adsorbs onto the surface of the iron, which is an impurity contained in the magnesium metal, than onto the magnesium metal surface, since magnesium is a metal that, like aluminum, lies lower than iron in the electrochemical series.

Claims

[1] Electrolyte for metal-air batteries, designed for use in metal-air batteries having an anode containing at least one type of metal selected from aluminium and magnesium, wherein the electrolyte comprises an aqueous solution containing at least one type of self-discharge inhibitor selected from the group consisting of M2HPO4, M3PO4, M4P2O7, MH2PO2, M2H2P2O7, LHPO4, MLPO4, L2P2O7 and LH2P2O7, wherein M is a type of metal selected from the group consisting of Li, K, Na, Rb, Cs and Fr, and L is a type of metal selected from the group consisting of Mg, Ca, Sr, Ba and Ra, wherein the content of the self-discharge inhibitor is 0.001 mol / l or more and 0.1 mol / l or less. [2] Electrolyte according to claim 1, wherein the self-discharge inhibitor is Na2H2P2O7. [3] Electrolyte according to claim 1 or 2, wherein the aqueous solution is basic. [4] Electrolyte according to any one of claims 1 to 3, wherein the aqueous solution contains NaOH as an electrolyte salt. [5] Metal-air battery (10), comprising: an air electrode (12) to which oxygen is supplied; an anode (11) containing at least one type of metal selected from aluminium and magnesium; and an electrolyte (13) that is in contact with the air electrode (12) and the anode (11), wherein the electrolyte (13) is the electrolyte defined by any one of claims 1 to 4.

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