The Long-Term Stability of KO2 in K-O2 Batteries.

The rechargeable K-O2 battery is recognized as a promising energy storage solution owing to its large energy density, low overpotential, and high coulombic efficiency based on the single-electron redox chemistry of potassium superoxide. However, the reactivity and long-term stability of potassium superoxide remains ambiguous in K-O2 batteries. Parasitic reactions are explored and the use of ion chromatography to quantify trace amounts of side products is demonstrated. Both quantitative titrations and differential electrochemical mass spectrometry confirm the highly reversible single-electron transfer process, with 98 % capacity attributed to the formation and decomposition of KO2 . In contrast to the Na-O2 counterparts, remarkable shelf-life is demonstrated for K-O2 batteries owing to the thermodynamic and kinetic stability of KO2 , which prevents the spontaneous disproportionation to peroxide. This work sheds light on the reversible electrochemical process of K+ +e- +O2 ↔KO2 .