Non-aqueous Metal–Oxygen Batteries: Past, Present, and Future

A metal-oxygen battery (sometimes referred to as a ‘metal-air’ battery) is a cell chemistry in which one of the reactants is gaseous oxygen, O2. Oxygen enters the cell typically in the positive electrode—perhaps after being separated from an inflow of air—and dissolves in the electrolyte. The negative electrode is typically a metal monolith or foil. Upon discharge, metal cations present in the electrolyte react with dissolved oxygen and electrons from the electrode to form a metal-oxide or metal-hydroxide discharge product. In some chemistries the discharge product remains dissolved in the electrolyte; in other systems it precipitates out of solution, forming a solid phase that grows in size as discharge proceeds. In secondary metal-oxygen batteries the recharge process proceeds via the decomposition of the discharge phase back to O2 and dissolved metal cations. In light of the processes associated with discharge and charging, reversible metal-oxygen batteries with solid discharge products are often referred to as precipitation-dissolution systems, a category that also includes lithium–sulfur batteries. The interest in metal-oxygen chemistries follows from their very high theoretical energy densities. Figure 1 summarizes the gravimetric and volumetric energy densities for several metal-oxygen couples, and compares these to the theoretical energy density of a conventional lithium-ion battery. On the basis of these energy densities, it is clear that many metal-oxygen systems hold promise for surpassing the state-of-the-art Li-ion system. Achieving this goal, however, remains a significant challenge when factors beyond energy density are accounted for: cycle life, round-trip efficiency, and cost