CO2 and ambient air in metal-oxygen batteries: steps towards reality

Metal-air batteries, especially lithium and sodium air technologies, have attracted significant research attention in the past decade. The high theoretical specific energy (3500 Wh kg(-1) for Li-O-2 and 1600 Wh kg(-1) for Na-O-2) and moderate equilibrium potential (2.96 V for Li-O-2 and 2.3 V for Na-O-2) make these chemistries attractive energy storage platforms for transportation, autonomous aircraft, and emergent robotics technologies. The term metal-air battery, however, hardly describes the cell design under most active investigation by researchers; in most studies, O-2 is used in place of air as the active material in the battery cathode. This change, designed to eliminate the formation of electrochemically stable metal hydroxide and metal carbonate discharge products when CO2 and moisture present in ambient air react with metal ions in the cathode, introduces significant new complications for practical metal-air battery design and operation that largely defeat the competitive advantages of this storage technology. Recent work has shown that when a mixture of O-2 and CO2 is used as the active material in the cathode, it is possible to recharge a metal-O-2/CO2 cell provided steps are taken to prevent electrolyte decomposition during recharge. In this highlight, we critically review the literature on metal-O-2/CO2 cells, focusing on how the presence of CO2 in the active cathode material changes electrochemistry at the cathode and rechargeability of the cells. We also assess the progress and future prospects for metal-air battery technologies involving ambient air as the cathode gas.