WO2012023018A1

WO2012023018A1 - Air electrode for metal-air battery, and metal-air battery including the air electrode

Info

Publication number: WO2012023018A1
Application number: PCT/IB2011/001851
Authority: WO
Inventors: Fuminori Mizuno
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2010-08-17
Filing date: 2011-08-12
Publication date: 2012-02-23
Also published as: JP5115603B2; JP2012043568A

Abstract

An air electrode for a metal-air battery includes an air electrode catalyst that contains a metal nitride or metal oxynitride.

Description

AIR ELECTRODE FOR METAL- AIR BATTERY, AND METAL- AIR BATTERY INCLUDING THE AIR ELECTRODE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an air electrode for a metal-air battery which, when incorporated in a metal-air battery, can improve the discharge capacity thereof, and a metal-air battery that includes the air electrode.

2. Description of the Related Art

[0002] A metal-air battery is a rechargeable battery that uses a pure metal or metal compound as a negative-electrode active material and oxygen as a positive-electrode active material. A metal-air battery can theoretically have a higher capacity than a secondary battery which uses a solid positive-electrode active material because oxygen as the positive-electrode active material can be obtained from air there is therefore no need to encapsulate a positive-electrode active material in the battery.

[0003] In a lithium air battery as a type of metal-air battery, the reaction that is represented by formula (I) proceeds at the negative-electrode during discharge.

2Li→ 2Li⁺ + 2e^" (I)

The electrons generated as shown in formula (I) reach the air electrode through an external circuit after working in an external load. The lithium ions (Li⁺) generated as shown in formula (I) are migrated in the electrolyte that is interposed between the negative-electrode and air electrode from the negative-electrode side to the air electrode side by electroendosmosis.

[0004] During discharge, the reactions that are represented by formulae (II) and (III) proceed at the air electrode.

2Li⁺ + 0₂ + 2e^"→ Li₂0₂ (II)

2Li⁺ + l/20₂ + 2e→ Li₂0 (III) The generated lithium peroxide (Li₂0₂) and lithium oxide (Li₂0) are accumulated in the air electrode as solid. During charge, the reverse reaction of the reaction that is represented by formula (I) proceeds at the negative-electrode and the reverse reactions of the reactions that are represented by formulae (II) and (III) proceed at the air electrode, and metal lithium is regenerated at the negative-electrode, which allows the metal-air battery to be charged again.

[0005] Studies on electrode catalysts for metal-air batteries are actively undertaken for reduction in costs and improvement in battery performance such as energy density. Japanese Patent Application Publication No. 2009-283381 (JP-A-2009-283381) discloses a lithium-air secondary battery that includes a positive electrode of a gas diffusion type oxygen electrode that is primarily composed of carbon, a negative-electrode of a material capable of occluding and releasing metal lithium or lithium ions, and an electrolytic medium of a non-aqueous electrolyte which is interposed between the positive electrode and the negative-electrode, characterized in that the positive electrode contains an Fe-based oxide La_1-xA_xFe_1-yB_y0₃ that has a perovskite structure as an electrode catalyst.

[0006] JP-A-2009-283381 discloses the use of an Fe-based oxide that has a perovskite structure in the air electrode. However, the lithium-air secondary battery that is disclosed in JP-A-2009-283381 uses a carbonate-type solvent which is unstable to oxygen radicals as described in JP-A-2009-283381. It is, therefore, inferred that, in the lithium-air secondary battery, a level of discharge capacity is merely produced by a decomposition reaction of the carbonate-type solvent as a side reaction and the oxygen reduction reactions that are represented by formulae (II) and (III) do not occur. When the present inventor conducted a test in the same manner using an electrolytic solution solvent which is stable to oxygen radicals and an oxide that has a perovskite structure as an air electrode catalyst in an example which is described later (Comparative Example 3), it was found that the oxygen reduction reactions that are represented by formulae (II) and (III) do occur but a sufficient discharge capacity cannot be obtained.

SUMMARY OF THE INVENTION [0007] The present invention provides an air electrode for a metal-air battery which, when incorporated in a metal-air battery, can improve the discharge capacity thereof, and a metal-air battery that includes the air electrode.

[0008] A first aspect of the present invention relates to an air electrode for a metal-air battery which includes an air electrode catalyst that contains a metal nitride or metal oxynitride.

[0009] In the air electrode for a metal-air battery according to this aspect, the metal nitride may be a compound that is selected from a group that consists of titanium nitride, zirconium nitride, tantalum nitride, hafnium nitride, vanadium nitride, niobium nitride, chromium nitride and germanium nitride.

[0010] In the air electrode for a metal-air battery according to this aspect, the metal nitride may be titanium nitride.

[0011] A second aspect of the present invention relates to a metal-air battery which includes an air electrode that contains a metal nitride or metal oxynitride, a negative-electrode, and an electrolyte that is interposed between the air electrode and the negative-electrode.

[0012] According to the present invention, because the use of a metal nitride or metal oxynitride, which has electron conductivity, which is unlikely to undergo surface oxidation by 0₂ and which has N-sites in its crystal, as an air electrode catalyst enables an air electrode reaction to proceed not only at a triphasic interface that is formed by the air electrode catalyst, electrolyte, conductive material or the like but also at the interface between the air electrode catalyst and electrolyte in the metal-air battery in which the air electrode catalyst is incorporated, a high discharge capacity can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: FIG. 1 is a cross-sectional view of a metal-air battery according to an embodiment of the present invention taken along the laminated direction thereof which schematically illustrates an example of the layer structure of the metal-air battery. DETAILED DESCRIPTION OF EMBODIMENTS

[0014] An air electrode for a metal-air battery according to an embodiment of the present invention is an air electrode for a metal-air battery which contains at least an air electrode catalyst and is characterized in that the air electrode catalyst contains a metal nitride or metal oxynitride.

[0015] Metal nitrides, such as titanium nitride (TiN) and metal oxynitrides, are less likely to undergo surface oxidation than metals and noble metals, and have electron conductivity contrary to other ceramics. The present inventor found that the N-sites (atomic sites of nitrogen) in the crystal surface of metal nitrides and metal oxynitrides are especially effective as oxygen reduction sites and that metal nitrides and metal oxynitrides are therefore useful as air electrode catalysts for metal-air batteries, and has accomplished the present invention. Because incorporation of an air electrode according to an embodiment of the present invention that uses a metal nitride or metal oxynitride, which has electron conductivity, which is unlikely to undergo surface oxidation by 0₂ and which has N-sites in its crystal as an air electrode catalyst in a metal-air battery, enables an air electrode reaction to proceed not only at a triphasic interface that is formed by the air electrode catalyst, electrolyte, conductive material or the like but also at the interface between the air electrode catalyst and electrolyte in the metal-air battery, precipitation of a metal oxide such as Li₂0₂ as a discharge product is promoted and a high discharge capacity can be achieved.

[0016] The metal nitrides usable in the embodiment of the present invention include both transition metal nitrides and typical metal nitrides. Specifically, the metal nitride for use in the embodiment of the present invention may be selected from a group that consists of titanium nitride (TiN), zirconium nitride (ZrN), tantalum nitride (TaN), hafnium nitride (HfN), vanadium nitride (VN), niobium nitride (NbN), chromium nitride (Cr₂N) and germanium nitride (GeN). The metal oxynitride for use in the embodiment of the present invention may be selected from a group that consists of transition metal oxynitrides, especially tantalum oxynitride (TaON) and zirconium oxynitride (ZrO_xN_y).

[0017] The air electrode for a metal-air battery according to this embodiment may have an air electrode layer, and usually has an air electrode current collector, and an air electrode lead that is connected to the air electrode current collector as well.

[0018] The air electrode layer in the air electrode for a metal-air battery according to this embodiment contains metal nitride or metal oxynitride as described above as an air electrode catalyst. The air electrode layer may optionally contain a binder and/or a conductive material.

[0019] The metal nitrides and metal oxynitrides as listed above may be used singly or in combination with another oxygen reduction catalyst as the air electrode catalyst. Examples of the other oxygen reduction catalyst include platinum group metals such as nickel, palladium and platinum; perovskite oxides that contain a transition metal such as cobalt, manganese or iron; inorganic compounds that contain an oxide of a noble metal such as ruthenium, iridium or palladium; organic metal coordination compounds that have a porphyrin skeleton or phthalocyanine skeleton; inorganic ceramics such as manganese dioxide (Mn0₂) and cerium oxide (Ce0₂); and composite materials that are obtained by mixing these materials. The content of the air electrode catalyst in the air electrode layer is preferably 1% by mass to 90% by mass, more preferably 5% by mass to 50% by mass, based on the total mass of the air electrode layer as 100% by mass for the following reasons. When the content of the air electrode catalyst is too low, only insufficient catalytic function may be achieved. When the content of the air electrode catalyst is too high, the relatively low conductive material content may lead to a decrease of reactive sites, resulting in a decrease in battery capacity. For smoother electrode reaction, the catalyst may be supported on a conductive material, which is described later.

[0020] The conductive material for use in the air electrode layer is not specifically limited as long as it has electrical conductivity. For example, a carbon material, perovskite conductive material, porous conductive polymer, or porous metal material may be used. In particular, the carbon material may or may not be porous, but is preferably porous in this embodiment because the specific surface area is sufficiently large to provide many reactive sites. Specifically, mesoporous carbon, for example, may be used as the porous carbon material. As the non-porous carbon material, a high-specific surface area carbon material, such as graphite, acetylene black, carbon nanotubes, carbon fibers or activated carbon, may be used. The content of the conductive material in the air electrode layer is preferably 10% by mass to 99% by mass, more preferably 50% by mass to 95% by mass, based on the total mass of the air electrode layer as 100% by mass for the following reasons. When the content of the conductive material is too low, the reactive sites may decrease, causing a decrease in battery capacity. When the content of the conductive material is too high, the relative catalyst content may be too low to achieve a sufficient catalytic function.

[0021] The air electrode layer must contain at least an air electrode catalyst and a conductive material, and may optionally contain a binder that fixes the conductive material. As the binder, a rubber type resin, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE) or styrene-butadiene rubber (SBR rubber), may be used. The content of the binder in the air electrode layer, which is not specifically limited, is preferably 40% by mass or lower, more preferably 1% by mass to 20% by mass, based on the total mass of the air electrode layer as 100% by mass.

[0022] The air electrode layer is formed by applying an air electrode material that contains at least the air electrode catalyst and optionally a conductive material and a binder to a support, such as an air electrode current collector, which is described later. In preparation of the air electrode material, a solvent may be used. As the solvent for use in the preparation of the air electrode material, a solvent with a boiling point of 200° C or lower, such as acetone, N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP), may be used.

[0023] The thickness of the air electrode layer, which may differ depending on the usage of the metal-air battery, is in the range of 2 μηι to 500 μπι, preferably in the range of 5 μηι to 300 μπι.

[0024] The air electrode current collector in the air electrode for a metal-air battery according to this embodiment collects current from the air electrode layer. The material for the air electrode current collector is not specifically limited as long as it has electrical conductivity. For example, stainless, nickel, aluminum, iron, titanium or carbon may be used. As the air electrode current collector, a foil-type current collector, plate-type current collector, porous current collector, fibrous current collector, unwoven fabric current collector or mesh (grid) type current collector may be used. Above all, a carbon paper or mesh-type current collector may be used in this embodiment for high current collection efficiency. When a mesh-type current collector is used, a mesh-type air electrode current collector is usually placed in the air electrode layer. In addition, the air electrode for a metal-air battery according to this embodiment may have an additional air electrode current collector (for example, a foil-type current collector) that collects the electric charge that is collected by a mesh-type air electrode current collector. In this embodiment, the battery case, which is described later, may have the function of the air electrode current collector. The air electrode current collector has a thickness in the range of 10 μηι to 1000 μηι, preferably in the range of 20 μιη to 400 μιτι.

[0025] The metal-air battery according to this embodiment includes at least an air electrode, a negative-electrode, and an electrolyte that is interposed between the air electrode and the negative-electrode, and is characterized in that the air electrode is the above-mentioned air electrode for a metal-air battery.

[0026] FIG. 1 is a cross-sectional view of a metal-air battery according to this embodiment taken along the laminated direction thereof which schematically illustrates the layer structure of the metal-air battery. It should be noted that the metal-air battery according to this embodiment is not necessarily limited only to this example. A metal-air battery 100 has an air electrode 6 that includes an air electrode layer 2 and an air electrode current collector 4, a negative-electrode 7 that includes a negative-electrode active material layer 3 and a negative-electrode current collector 5, and an electrolyte 1 that is interposed between the air electrode 6 and the negative-electrode 7. The air electrode in the metal-air battery according to this embodiment has been described above. In the following, the other constituent elements of the metal-air battery according to this embodiment, the negative-electrode, electrolyte, separator and battery case, are described in detail.

[0027] The negative-electrode in the metal-air battery according to this embodiment may have a negative-electrode layer that contains a negative-electrode active material, and usually has a negative-electrode current collector and a negative-electrode lead that is connected to the negative-electrode current collector as well.

[0028] The negative-electrode layer in the metal-air battery according to this embodiment contains a negative-electrode active material that contains metal or alloy material. Specific examples of the metal or alloy material for the negative-electrode active material include alkaline metals such as lithium, sodium and potassium; Group 2 elements such as magnesium and calcium; Group 13 elements such as aluminum; transition metals such as zinc and iron; and alloy materials and compounds that contain such a metal. As an alloy that has a lithium element, a lithium-aluminum alloy, lithium-tin alloy, lithium-lead alloy or lithium-silicon alloy, for example, may be used. As a metal oxide that has a lithium element, a lithium titanium oxide, for example, may be used. As a metal nitride that contains a lithium element, a lithium cobalt nitride, lithium iron nitride or lithium manganese nitride, for example, may be used. Lithium that is coated with a solid electrolyte may be used as the negative-electrode layer.

[0029] The negative-electrode layer may contain only the negative-electrode active material, or may contain at least one of a conductive material and a binder in addition to the negative-electrode active material. For example, when the negative-electrode active material is in the form of a foil, the negative-electrode layer may contain only the negative-electrode active material. On the other hand, when the negative-electrode active material is powdery, the negative-electrode layer may contain the negative-electrode active material and a binder. The conductive material and binder are the same as those that have been described in connection with the air electrode, and therefore their description is omitted here. [0030] The material for the negative-electrode current collector in the metal-air battery according to this embodiment is not specifically limited as long as it has electrical conductivity. For example, copper, stainless, nickel or carbon may be used. The negative-electrode current collector may be in the form of a foil, plate or mesh (grid), for example. In this embodiment, the battery case, which is described later, may have the function of the negative-electrode current collector.

[0031] The electrolyte in the metal-air battery according to this embodiment is held between the air electrode layer and the negative-electrode layer, and has a function of exchanging metal ions between the air electrode layer and the negative-electrode layer. As the electrolyte, an aqueous electrolyte or non-aqueous electrolyte may be used.

[0032] As the non-aqueous electrolyte, a non-aqueous electrolytic solution or non-aqueous gel electrolyte may be used. The type of the non-aqueous electrolytic solution may be appropriately selected based on the type of the metal ions to be conducted. For example, a non-aqueous electrolytic solution for a lithium air battery usually contains a lithium salt and a non-aqueous solvent. Examples of usable lithium salts include inorganic lithium salts such as LiPF₆, LiBF₄, L1CIO₄ and LiAsF₆; and organic lithium salts such as LiCF₃S0₃, LiN(S0₂CF₃)₂(Li-TFSI), LiN(S0₂C₂F₅)₂ and LiC(S0₂CF3)₃. Examples of usable non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), ethyl carbonate, butylene carbonate, γ-butyrolactone, sulfolane, acetonitrile (AcN), dimethoxym ethane, 1,2-dimethoxyethane (DME), 1,3-dimethoxypropane, diethyl ether, tetraethylene glycol dimethyl ether (TEGDME), tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide (DMSO) and mixtures thereof. A solvent with high oxygen solubility may be used as the non-aqueous solvent because the dissolved oxygen can be used for the reaction efficiently. The concentration of the lithium salt in the non-aqueous electrolytic solution is in the range of 0.5 mol/L to 3 mol/L, for example. In this embodiment, a low-volatile liquid such as an ionic liquid, e.g., N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13TFSI), N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P14TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium

bis(trifluoromethanesulfonyl)imide (DEMETFSI), or

Ν,Ν,Ν-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPATFSI), may be used as the non-aqueous electrolytic solution or non-aqueous solvent. Among the non-aqueous solvents, the use of an electrolytic solution solvent which is stable to oxygen radicals is more preferred to promote the oxygen reduction reaction that is represented by the formula (II) or (III). Examples of the non-aqueous solvent include acetonitrile (AcN), 1,2-dimethoxyethane (DME), dimethylsulfoxide (DMSO), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13TFSI), and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P14TFSI).

[0033] A non-aqueous gel electrolyte that is usable in this embodiment is usually prepared by adding a polymer to a non-aqueous electrolytic solution to form a gel. For example, a non-aqueous gel electrolyte for a lithium air battery may be obtained by adding a polymer, such as polyethylene oxide (PEG), polyacrylnitrile (PAN) or polymethylmethacrylate (PMMA), to a non-aqueous electrolytic solution as described above to form a gel. In this embodiment, a LiTFSI(LiN(CF₃S0₂)₂)-PEO type non-aqueous gel electrolyte may be used.

[0034] An aqueous electrolytic solution obtained by dissolving a lithium salt in water is used in a lithium air battery among air batteries. As the lithium salt, a lithium salt, such as LiOH, LiCl, L1NO₃ or CH₃C0₂Li, may be used.

[0035] A solid electrolyte may be additionally mixed in the aqueous electrolyte or non-aqueous electrolyte. As the solid electrolyte, an Li-La-Ti-0 type solid electrolyte, for example, may be used.

[0036] The battery according to this embodiment may have a separator between the air electrode and the negative-electrode. As the separator, a porous film of polyethylene or polypropylene; or an unwoven fabric, such as a resin unwoven fabric or glass fiber unwoven fabric, may be used. When impregnated with the electrolyte, these materials usable as the separator may be used as the support for electrolyte.

[0037] The air battery according to this embodiment usually has a battery case that accommodates the air electrode, negative-electrode, electrolyte and so on. Specifically, the battery case may be in the form of a coin, flat plate, cylinder or laminate. The battery case may be open to the atmosphere or sealed. The open type battery case is a battery case that has a structure which allows at least the air electrode layer to contact the atmosphere sufficiently. On the other hand, when the battery case is a sealed battery case, the sealed battery case may have gas (air) introducing and discharging pipes. In this case, the gas to be introduced and discharged preferably has a high oxygen concentration, and more preferably is pure oxygen. The oxygen concentration may be increased during discharge and decreased during charge.

[0038] An example of the procedure to fabricate a metal-air battery according to this aspect is described below.

[0039] [Example 1] First, titanium nitride (TiN), a carbon black with a specific surface area of 60 m²/g, and PTFE were prepared as the air electrode catalyst, conductive material and binder, respectively. The conductive material, air electrode catalyst and binder were mixed at a ratio of 80% by mass: 10% by mass: 10% by mass to prepare an air electrode material.

[0040] As the current collectors, SUS304 meshes were prepared. One of the current collectors was coated with the air electrode material to prepare an air electrode. A metal lithium was bonded to another current collector to prepare a negative-electrode. Lithium bis(trifluoromethanesuIfonyl)imide was dissolved in

N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide such that the concentration of lithium bis(trifluoromethanesulfonyl)imide was 0.32 mol/kg to prepare an electrolytic solution. A polypropylene unwoven fabric was impregnated with the electrolytic solution to prepare an electrolyte layer. The electrolyte layer was interposed between the air electrode and the negative-electrode such that a laminate of current collector-metal lithium-electrolyte layer-air electrode material layer-current collector was obtained, whereby a metal-air battery of Example 1 was fabricated. All of the above procedure was carried out in a glove box under a nitrogen atmosphere.

[0041] [Comparative Example 1] A metal-air battery of Comparative Example 2 was fabricated in the same manner as in Example 1 except that, in the air electrode material preparation step in Example 1, an air electrode material was prepared by mixing the conductive material and binder at a ratio of 90% by mass: 10% by mass without using the air electrode catalyst.

[0042] [Comparative Example 2] A metal-air battery of Comparative Example 1 was fabricated in the same manner as in Example 1 except that, in the air electrode material preparation step in Example 1, manganese dioxide (Mn0₂), which is generally used in related arts, was used as an air electrode catalyst instead of TiN.

[0043] [Comparative Example 3] A metal-air battery of Comparative Example 1 was fabricated in the same manner as in Example 1 except that, in the air electrode material preparation step in Example 1, Lao_.6Sro.₄Co0₃, which is a perovskite oxide of the same type as the perovskite oxide that is described in JP-A-2009-283381, was used as an air electrode catalyst instead of TiN.

[0044] [Comparative Example 4] A metal-air battery of Comparative Example 4 was fabricated in the same manner as in Example 1 except that, in the air electrode material preparation step in Example 1, silver (Ag) was used as an air electrode catalyst instead of TiN.

[0045] The charge-discharge measurement on the metal-air batteries is described below. Constant current charge-discharge measurement was performed on the metal-air batteries of Example 1 and Comparative Examples 1 to 4 at 0.02 mA/cm² and 60°C to obtain their initial discharge capacity per unit mass of electrode. It should be noted that the discharge capacity of the metal-air battery of Comparative Example 2 was calculated by subtracting its lithium intercalation capacity, the capacity at the time when it was discharged under an Ar atmosphere, from the total capacity under an 0₂ atmosphere in view of the difference in lithium intercalation capacity between TiN and Mn0₂. The initial discharge capacities of the metal-air batteries of Example 1 and Comparative Examples 1 to 4 are summarized in Table 1 below. [Table 1]

[0046] The metal-air battery of Comparative Example 1, in which no air electrode catalyst was used, had an initial discharge capacity of 116 mAh/g, which was the lowest among the metal-air batteries of Example 1 and Comparative Examples 1 to 4. The initial discharge capacity of the metal-air battery of Comparative Example 2 was a higher than that of the metal-air battery of Comparative Example 1 due to the addition of the manganese dioxide catalyst but still as low as 134 mAh/g. The metal-air battery of Comparative Example 3, in which Lao.₆Sr₀.₄Co0₃ was used as the air electrode catalyst, had an initial discharge capacity of 145 mAh/g. The metal-air battery of Comparative Example 4, in which silver was used as the air electrode catalyst, had an initial discharge capacity of 146 mAh/g, which was comparable to that of the metal-air battery of Comparative Example 3 but lower than that of the metal-air battery of Example 1. This is believed to be because the silver surface is easily oxidized.

[0047] In contrast to the metal-air batteries of Comparative Examples 1 to 4, the metal-air battery of Example 1, in which TiN was used as the air electrode catalyst, had an initial discharge capacity of 158 mAh/g. It is believed that such a high discharge capacity was achieved because precipitation of a metal oxide such as Li₂0₂ as a discharge product was promoted as the use of TiN, which has electron conductivity, is unlikely to undergo surface oxidation by 0₂ and has N-sites in its crystal, as the air electrode catalyst enabled the air electrode reaction to proceed not only at a triphasic interface that was formed by the air electrode catalyst, electrolyte, conductive material or the like but also at the interface between the air electrode catalyst and electrolyte in the metal-air battery.

[0048] While some embodiments of the invention have been illustrated above, it is to be understood that the invention is not limited to details of the illustrated embodiments, but may be embodied with various changes, modifications or improvements, which may occur to those skilled in the art, without departing from the scope of the invention.

Claims

1. An air electrode for a metal-air battery, characterized by comprising an air electrode catalyst that contains a metal nitride or metal oxynitride.

2. The air electrode for a metal-air battery according to claim 1,

wherein the metal nitride is a compound that is selected from a group that consists of titanium nitride, zirconium nitride, tantalum nitride, hafnium nitride, vanadium nitride, niobium nitride, chromium nitride and germanium nitride.

3. The air electrode for a metal-air battery according to claim 1,

wherein the metal nitride is titanium nitride.

4. The air electrode for a metal-air battery according to claim 1,

wherein the air electrode have an air electrode layer, and the content of the air electrode catalyst in the air electrode layer is 1% by mass to 90% by mass, based on the total mass of the air electrode layer as 100 % by mass.

5. The air electrode for a metal-air battery according to claim 4,

wherein the content of the air electrode catalyst in the air electrode layer is 5% by mass to 50% by mass, based on the total mass of the air electrode layer as 100% by mass.

6. A metal-air battery characterized by comprising:

an air electrode according to any one of claims 1 to 5,

a negative-electrode, and

an electrolyte that is interposed between the air electrode and the negative-electrode.