Electrochemical Oxidation of Lithium Carbonate Generates Singlet Oxygen.

Solid alkali metal carbonates are universal passivation layer components of intercalation battery materials and common side products in metal-O batteries, and are believed to form and decompose reversibly in metal-O /CO cells. In these cathodes, Li CO decomposes to CO when exposed to potentials above 3.8 V vs. Li/Li . However, O evolution, as would be expected according to the decomposition reaction 2 Li CO →4 Li +4 e +2 CO +O , is not detected. O atoms are thus unaccounted for, which was previously ascribed to unidentified parasitic reactions. Here, we show that highly reactive singlet oxygen ( O ) forms upon oxidizing Li CO in an aprotic electrolyte and therefore does not evolve as O . These results have substantial implications for the long-term cyclability of batteries: they underpin the importance of avoiding O in metal-O batteries, question the possibility of a reversible metal-O /CO battery based on a carbonate discharge product, and help explain the interfacial reactivity of transition-metal cathodes with residual Li CO .