Electrochemical Oxidation of Lithium Carbonate Generates Singlet Oxygen

Abstract Solid alkali metal carbonates are universal passivation layer components of intercalation battery materials and common side products in metal‐O 2 batteries, and are believed to form and decompose reversibly in metal‐O 2 /CO 2 cells. In these cathodes, Li 2 CO 3 decomposes to CO 2 when exposed to potentials above 3.8 V vs. Li/Li + . However, O 2 evolution, as would be expected according to the decomposition reaction 2 Li 2 CO 3 →4 Li + +4 e − +2 CO 2 +O 2 , 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 ( 1 O 2 ) forms upon oxidizing Li 2 CO 3 in an aprotic electrolyte and therefore does not evolve as O 2 . These results have substantial implications for the long‐term cyclability of batteries: they underpin the importance of avoiding 1 O 2 in metal‐O 2 batteries, question the possibility of a reversible metal‐O 2 /CO 2 battery based on a carbonate discharge product, and help explain the interfacial reactivity of transition‐metal cathodes with residual Li 2 CO 3 .