Singlet Oxygen during Cycling of the Aprotic Sodium-O₂ Battery

Aprotic sodium-O₂ batteries require the reversible formation/dissolution of sodium superoxide (NaO₂ ) on cycling. Poor cycle life has been associated with parasitic chemistry caused by the reactivity of electrolyte and electrode with NaO₂ , a strong nucleophile and base. Its reactivity can, however, not consistently explain the side reactions and irreversibility. Herein we show that singlet oxygen (¹ O₂ ) forms at all stages of cycling and that it is a main driver for parasitic chemistry. It was detected in- and ex-situ via a ¹ O₂ trap that selectively and rapidly forms a stable adduct with ¹ O₂ . The ¹ O₂ formation mechanism involves proton-mediated superoxide disproportionation on discharge, rest, and charge below ca. 3.3 V, and direct electrochemical ¹ O₂ evolution above ca. 3.3 V. Trace water, which is needed for high capacities also drives parasitic chemistry. Controlling the highly reactive singlet oxygen is thus crucial for achieving highly reversible cell operation.