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-O₂ batteries, and are believed to form and decompose reversibly in metal-O₂/CO₂ cells. In these cathodes, Li₂CO₃ decomposes to CO₂ when exposed to potentials above 3.8 V vs. Li/Li⁺. However, O₂ evolution, as would be expected according to the decomposition reaction 2 Li₂CO₃→4 Li⁺+4 e⁻+2 CO₂+O₂, 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 (¹O₂) forms upon oxidizing Li₂CO₃ in an aprotic electrolyte and therefore does not evolve as O₂. These results have substantial implications for the long-term cyclability of batteries: they underpin the importance of avoiding ¹O₂ in metal-O₂ batteries, question the possibility of a reversible metal-O₂/CO₂ battery based on a carbonate discharge product, and help explain the interfacial reactivity of transition-metal cathodes with residual Li₂CO₃.