Insights into Electrochemical Oxidation of NaO₂ in Na–O₂ Batteries via Rotating Ring Disk and Spectroscopic Measurements

O 2 reduction in aprotic Na–O 2 batteries results in the formation of NaO 2, which can be oxidized at small overpotentials (<200 mV) on charge. In this study, we investigated the NaO 2 oxidation mechanism using rotating ring disk electrode (RRDE) measurements of Na−O 2 reaction products and by tracking the morphological evolution of the NaO 2 discharge product at different states of charge using scanning electron microscopy (SEM). The results show that negligible soluble species are formed during NaO 2 oxidation, and that the oxidation occurs predominantly via charge transfer at the interface between NaO 2 and carbon electrode fibers rather than uniformly from all NaO 2 surfaces. X-ray absorption near edge structure (XANES), and X-ray photoelectron spectroscopy (XPS) measurements show that the band gap of NaO 2 is smaller than that of Li 2 O 2 formed in Li–O 2 batteries, in which charging overpotentials are much higher (∼1000 mV). These results emphasize the importance of discharge product electronic structure for rationalizing metal–air battery mechanisms and performance.