1. Field of the Invention
The present invention relates to an alkali metal air battery.
2. Description of the Related Art
Hitherto, it has been known that alkali metal air batterys include negative electrodes capable of occluding and discharging lithium ions, positive electrodes using oxygen in air as positive-electrode active material and containing redox catalysts, and ion-conducting media arranged between the negative electrodes and the positive electrodes. For example, a lithium air battery described in Non-patent Document 1 includes metallic lithium serving as a negative electrode and a mixture of lithium peroxide, porous carbon, a manganese dioxide catalyst, and a fluorocarbon resin, the mixture serving as a positive electrode. The lithium air battery has a discharge potential of 2.5 to 2.7 V, a charge potential of 4.2 to 4.4 V, a discharge capacity per carbon of about 1,000 mAh/g (at a current density of 70 mA/g per carbon). In an air battery, lithium oxide that is formed during discharging and disappears during charging has a thermodynamically calculated potential of about 3.0 V. Thus, the charging potential is significantly high.
In the case where the charging potential is significantly higher than the discharging potential, if the capacity is not changed during charge and discharge, the charge energy is much higher than the discharge energy because energy is expressed as the product of a voltage and a capacity; hence, charge and discharge efficiency is low. In this regard, in Non-patent Document 1, the charging potential is about 4.5 V when the positive electrode does not contain a manganese dioxide catalyst, whereas the charging potential is in the range of about 4.2 to 4.4 V when the positive electrode contain the manganese dioxide catalyst. The use of the manganese dioxide catalyst reduces the charging potential, improving the charge and discharge efficiency. In Non-patent Document 1, the reduction in charge potential is believed to be attributed to the formation of a reaction active species because of manganese dioxide. Patent Document 1 describes that, for example, cobalt phthalocyanine or cobalt porphyrin serving as a catalyst is supported on a surface of a positive electrode.    Patent Document 1: JP 2006-286414 A    Non-patent Document 1: J. Am. Chem. Soc. 2006, Vol. 128, pp. 1390-1393