Revealing the Chemical Mechanism of NaO₂ Decomposition by In Situ Raman Imaging

Sodium–oxygen (Na–O 2 ) batteries exhibit a low charging overpotential owing to the reversible formation and decomposition of sodium superoxide (NaO 2 ) on discharge and charge cycles. However, the cycling performance of the battery system is compromised by the side reactions occurring between the reactive NaO 2 discharge product with the other components of the cell including the air electrode and the organic electrolyte. In the present study, we employ a Raman imaging technique to reveal the chemical mechanism behind the decomposition reaction of NaO 2 in the presence of diglyme-based electrolyte. Our results illustrate the formation of oxalate-based side products resulting from prolonged exposure of NaO 2 to the cell electrolyte. Moreover, we show that Na 2 O 2 ·2H 2 O is not the thermodynamically favorable side product for decomposition of NaO 2 and may only be formed under the high-energy beam used by the measuring probe. The findings of this study help to better understand the underlying chemical reaction mechanisms of Na–O 2 cells.