Rechargeable metal–air batteries are very attractive because they offer very high theoretical energy densities with the potential to reduce the use of oil and thus to address the key problems of global warming. The concept of lithium–air batteries has existed for almost two decades. We recently proposed a novel and low-cost sodium–air battery system operating at above the melting point of sodium. This paper studies the composition of the SEI formed on freshly deposited sodium metal and the faradaic efficiency of the sodium deposition/dissolution processes.