Cell Chemistry of Sodium–Oxygen Batteries with Various Nonaqueous Electrolytes

Development of the nonaqueous Na–O2 battery with a high electrical energy efficiency requires the electrolyte stable against attack of highly oxidative species such as nucleophilic anion O2 •–. A combined evaluation method was used to investigate the Na–O2 cell chemistry with various solvents, including ethylene carbonate/propylene carbonate (EC/PC)-, N-methyl-N-propylpiperidinium bis­(trifluoromethansulfonyl) imide (PP13TFSI)-, and tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes. It is found that the TEGDME-based electrolytes have the best stability with the predominant yield of NaO2 upon discharge and the largest electrical energy efficiency (approaching 90%). Both EC/PC- and PP13TFSI-based electrolytes severely decompose during discharge, forming a large amount of side products. Analysis of the acid dissociation constant (pK a) of these electrolyte solvents reveals that the TEGDME has the relatively large value of pK a, which correlates with good stability of the electrolyte and high round-trip energy efficiency of the battery.