Feasibility of achieving two-electron K–O2 batteries

A deep understanding of the oxygen (O2) reduction and evolution mechanisms is crucial for understanding metal–O2 batteries. It has become evident that the instability of superoxide in the presence of lithium (Li) ions and sodium (Na) ions is the root cause for the poor reversibility and energy efficiency of Li–O2 and Na–O2 batteries. A straightforward yet elegant method is stabilizing superoxide with the larger potassium (K) ions. Superoxide-based K–O2 batteries, invented by our group in 2013, are operated based on one-electron redox of O2/potassium superoxide (KO2) and have high energy efficiencies without any electrocatalysts. Nevertheless, limiting the anionic redox to O2/superoxide affects the capacity output. Therefore, it is attractive to explore the possibility of beyond KO2 in the K–O2 batteries, especially if the use of catalysts can still be avoided. In this research, solid KO2 was used as the condensed O2 source and pre-dissolved in the dimethyl sulfoxide (DMSO)-based electrolyte. It is encouraging to observe two sets of reversible peaks during the three-electrode cyclic voltammetry scan under an argon atmosphere. One pair of peaks is attributed to the KO2/potassium peroxide (K2O2) interconversion. Such redox has superb reversibility and a small overpotential of 239 mV in the absence of explicit electrocatalysts. Notably, it is further revealed that K2O2 reacts with gaseous O2. Therefore, a gas-open system with an O2 supply is unfavorable for realizing the reversible KO2/K2O2 redox, and a closed cell system with a KO2 supply as the starting active material is suggested instead.