Li-O-2 battery is regarded as one of the most promising energy storage systems for future applications. However, its energy efficiency is greatly undermined by the large overpotentials of the discharge (formation of Li2O2) and charge (oxidation of Li2O2) reactions. The parasitic reactions of electrolyte and carbon electrode induced by the high charging potential cause the decay of capacity and limit the battery life. Here, a K-O-2 battery is report that uses K+ ions to capture O-2(-) to form the thermodynamically stable KO2 product. This allows for the battery to operate through the one-electron redox process of O-2/O-2(-). Our studies confirm the formation and removal of KO2 in the battery cycle test. Furthermore, without the use of catalysts, the battery shows a low discharge/charge potential gap of less than 50 mV at a modest current density, which is the lowest one that has ever been reported in metal-oxygen batteries.