Air electrode for metal-air battery
Disclosed is an air electrode for a metal-air battery, the air electrode including a separator composed of a hydroxide-ion-conductive inorganic solid electrolyte being a dense ceramic material, and an air electrode layer disposed on the separator and containing an air electrode catalyst, an electron-conductive material, and a hydroxide-ion-conductive material, or containing an air electrode catalyst also serving as an electron-conductive material and a hydroxide-ion-conductive material. According to the present invention, the reaction resistance of the air electrode including the dense ceramic separator can be significantly reduced in a metal-air battery while ensuring the desired characteristics of the dense ceramic separator.
1. An air electrode for a metal-air battery, the air electrode comprising:
a separator comprising a hydroxide-ion-conductive inorganic solid electrolyte being a dense ceramic material; and
an air electrode layer disposed on the separator and comprising an air electrode catalyst, an electron-conductive material, and a hydroxide-ion-conductive material, or comprising an air electrode catalyst, which also serves as an electron-conductive material, and a hydroxide-ion-conductive material,
wherein the amount by volume of the hydroxide-ion-conductive material contained in the air electrode layer increases stepwise or gradually from the outer surface of the air electrode layer toward the interface between the air electrode layer and the separator, and
wherein the separator includes a first hydroxide-ion-conductive material and the air electrode layer includes a second hydroxide-ion-conductive material having a different composition from the first hydroxide-ion-conductive material.
2. The air electrode according to claim 1 , wherein the amount by volume of the hydroxide-ion-conductive material in the vicinity of the interface between the air electrode layer and the separator is 1.2 times or more the amount by volume of the hydroxide-ion-conductive material in the vicinity of the outer surface of the air electrode layer.
3. The air electrode according to claim 1 , wherein the air electrode layer comprises a first air electrode sublayer having a relatively high content of the hydroxide-ion-conductive material and a second air electrode sublayer having a relatively low content of the hydroxide-ion-conductive material such that the first air electrode sublayer is in contact with the separator and the second air electrode sublayer is exposed to external air.
4. The air electrode according to claim 1 , wherein the air electrode layer has a thickness of 1 to 50 μm.
5. The air electrode according to claim 1 , wherein the air electrode layer has a thickness of 5 to 50 μm.
6. The air electrode according to claim 1 , wherein the hydroxide-ion-conductive inorganic solid electrolyte has a relative density of 88% or more.
7. The air electrode according to claim 1 , wherein the hydroxide-ion-conductive inorganic solid electrolyte is a layered double hydroxide densified by a hydrothermal process.
8. The air electrode according to claim 1 , wherein the hydroxide-ion-conductive inorganic solid electrolyte comprises a layered double hydroxide having a fundamental composition represented by the following formula:
M 2+ 1−x M 3+ x (OH) 2 A n− x/n .mH 2 O
where M 2+ represents at least one divalent cation, M 3+ represents at least one trivalent cation, A n− represents an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4, and m is any real number.
9. The air electrode according to claim 8 , wherein M 2+ comprises Mg 2+ , M 3+ comprises Al 3+ , and A n− comprises CO 3 2− .
10. The air electrode according to claim 1 , wherein the hydroxide-ion-conductive material comprises a layered double hydroxide having a fundamental composition represented by the following formula:
M 2+ 1−x M 3+ x (OH) 2 A n− x/n .mH 2 O
where M 2+ represents at least one divalent cation, M 3+ represents at least one trivalent cation, A 2+ represents an n-valent anion, n is an integer of 1 or more, x is 0.1 to 0.4,and m is any real number.
11. The air electrode according to claim 10 , wherein M 2+ comprises Ni 2+ , M 3+ comprises Fe 3+ , and A n− comprises NO 3− and/or CO 3 2− .
12. The air electrode according to claim 1 , wherein the hydroxide-ion-conductive material comprises a polymer material having hydroxide ion conductivity.
13. A metal-air battery comprising the air electrode according to claim 1 , a metal negative electrode, and an electrolytic solution, wherein the electrolytic solution is separated from the air electrode layer by the separator of the air electrode.