TECHNICAL FIELD

The present invention relates to a metal-air battery and a method for producing an air electrode thereof.

BACKGROUND ART

Conventionally, alkaline batteries and zinc-carbon batteries have widely been used as disposable primary batteries. Also, along with the evolution of IoT (Internet of Things) in recent years, the development of scattering sensors, which are installed at each and every place in the natural world such as soil and forest is advancing, and small sized high performance coin type lithium primary batteries corresponding to a variety of applications such as these sensors are being widespread.

However, the disposable batteries generally used at present are often constituted by a rare metal such as lithium, nickel, manganese and cobalt, and thus have a problem of the depletion of resources. In addition, a strong alkali such as an aqueous sodium hydroxide solution or an organic electrolytic solution is used as the electrolytic solution, and this makes the final disposal difficult. Moreover, the presently used disposable batteries may pose a risk of affecting the surrounding environment depending on the environment for use such as a case in which they are used as the driving source for sensors to be buried in soil.

In order to solve the problems as mentioned above, mention may be made of a metal-air battery as a potential candidate for a battery with a low environmental load. In the metal-air battery, oxygen and water are used as the cathode active materials and a metal such as magnesium, iron, aluminum and zinc is used for the anode, and therefore, its impact on soil contamination or the like, as well as the influence on the ecosystem, is low. In addition, these materials are abundant from the viewpoint of resources, and are inexpensive compared to rare metals. In particular, a zinc air battery, which uses zinc as the anode, is commercially available as the driving source for hearing aids or the like, and the research and development of a metal-air battery in which magnesium is used as the anode is also in progress as a battery with a low environmental load (see Non-Patent Literature 1 and Non-Patent Literature 2).

CITATION LIST

Non-Patent Literature

Non-Patent Literature 1: Y. Xue et al., “Template-directed fabrication of porous gas diffusion layer for magnesium air batteries”, Journal of Power Sources, vpl. 297, pp. 202-207, 2015.Non-Patent Literature 2: N. Wang et al., “Discharge behaviour of Mg—Al—Pb and Mg—Al—Pb—In alloys as anodes for Mg-air battery”, Electrochimica Acta, vol. 149, pp. 193-205, 2014.

SUMMARY OF THE INVENTION

Technical Problem

However, in Non-Patent Literature 1, a fluororesin is used as a binder for the air electrode, and also in Non-Patent Literature 2, a metal containing lead or indium is used for the anode. Accordingly, these batteries are constituted by materials that may pose a risk of affecting the natural environment, such as soil contamination.

As a battery for solving the problems as described above, mention may be made of the above-mentioned metal-air battery, but while they can eliminate environmental problems by not using environmentally hazardous substances such as rare metals, there is a problem that the battery performance is decreased when the battery is constituted without using rare metals or the like.

For example, with respect to a neutral electrolyte with a low environmental load, when a chloride salt such as sodium chloride and potassium chloride is used as the main electrolyte, the anode is corroded by the chloride ion and the utilization efficiency of the anode is decreased, thereby presenting a problem that the corrosion of the anode needs to be suppressed. Furthermore, the pH of the electrolytic solution is elevated along with the progression of the battery reaction and the passive film is likely to be formed on the anode, and therefore, the reaction gradually becomes unlikely to occur and the discharge capacity of the battery itself becomes small. Accordingly, there has been a problem that the passivation needs to be suppressed.

The present invention has been made to solve the problems as described above, and an object thereof is to improve the performance of a metal-air battery.

Means for Solving the Problem

A metal-air battery according to claim1comprises: an air electrode constituted by a co-continuous body having a three dimensional network structure in which a plurality of nanostructures is integrated through a noncovalent bond; an anode; and an electrolyte disposed between the air electrode and the anode and constituted by a salt containing no chloride ion.

With respect to a metal-air battery according to claim2, in the metal-air battery according to claim1, the electrolyte is constituted by one or more of an acetate, a carbonate, a citrate, a phosphate, HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid), a pyrophosphate and a metaphosphate.

With respect to a metal-air battery according to claim3, in the metal-air battery according to claim1or2, the electrolyte is an electrolytic solution or a solid electrolyte in a range from weak acidity to neutrality.

With respect to a metal-air battery according to claim4, in the metal-air battery according to any one of claims1to3, the anode is constituted by one or more of magnesium, aluminum, calcium, iron and zinc.

With respect to a metal-air battery according to claim5, in the metal-air battery according to any one of claims1to4, the nanostructure of the air electrode is a nanosheet constituted by at least one of carbon, iron oxide, manganese oxide, zinc oxide, molybdenum oxide and molybdenum sulfide, or a nanofiber constituted by at least one of carbon, iron oxide, manganese oxide, zinc oxide, molybdenum oxide, molybdenum sulfide and cellulose, and the air electrode supports a catalyst constituted by at least one metal selected from iron, manganese, zinc, copper and molybdenum, or an oxide of at least one metal selected from calcium, iron, manganese, zinc, copper and molybdenum.

In the metal-air batteries described above, the electrolyte may be constituted by an aqueous solution of any of acetic acid, sodium acetate, magnesium acetate, potassium acetate, calcium acetate, carbonic acid, sodium carbonate, magnesium carbonate, potassium carbonate, calcium carbonate, citric acid, sodium citrate, magnesium citrate, potassium citrate, calcium citrate, phosphoric acid, HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid), sodium pyrophosphate and sodium metaphosphate or of a mixture thereof.

In the metal-air batteries described above, the batteries may comprise a housing sealing the inside of the cell except for the air electrode and the housing may be constituted by a naturally degradable material.

In a method for producing the air electrode of the metal-air battery according to any one of claims1to5, a method for producing an air electrode according to claim6comprises: a gel production step of allowing a bacterium to produce a gel in which nanofibers made of iron oxide or manganese oxide are dispersed; a freezing step of freezing the gel; and a drying step of drying the frozen body of the gel.

In a method for producing the air electrode of the metal-air battery according to any one of claims1to5, a method for producing an air electrode according to claim7comprises: a gel production step of allowing a bacterium to produce a gel in which nanofibers made of cellulose are dispersed; a freezing step of freezing the gel; a drying step of drying the frozen body of the gel; and a carbonization step of carbonizing the co-continuous body obtained by the drying, by heating in a gas atmosphere in which the cellulose is not burned.

Effects of the Invention

As described above, according to the present invention, the air electrode is constituted by a co-continuous body having a three dimensional network structure in which a plurality of nanostructures is integrated through a noncovalent bond and the electrolyte disposed between the air electrode and the anode is constituted by a salt containing no chloride ion, and therefore, efficient utilization of electrons and suppression of passivation and self corrosion of the anode can be achieved, thereby obtaining an excellent effect that the battery performance of the metal-air battery can be improved. In addition, since the air electrode and the electrolyte are constituted by materials with a low environmental load, a metal-air battery that does not need to be recovered can be provided. In particular, since a chloride salt is not used for the electrolyte, an effect that the corrosion of the anode can be suppressed can be obtained. For example, by using a weak acidic salt or a salt with a buffering capacity for the electrolyte, an excellent effect that the pH rise in the electrolytic solution caused by the battery reaction can be suppressed and the discharge capacity can be greatly increased can be obtained.

