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) .