Recently, metal–air batteries, such as lithium–air and zinc–air systems, have been studied extensively as potential candidates for ultra-high energy density storage devices because of their exceptionally high capacities. Here, we report such an electrochemical system based on sodium, which is abundant and inexpensive. Two types of sodium–oxygen batteries were introduced and studied, i.e. with carbonate and non-carbonate electrolytes. Both types could deliver specific capacities (2800 and 6000 mA h g−1) comparable to that of lithium–oxygen batteries but with slightly lower discharge voltages (2.3 V and 2.0 V). The reaction mechanisms of sodium–oxygen batteries in carbonate and non-carbonate electrolytes were investigated and compared with those of lithium–oxygen batteries.