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(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).