Cell Chemistry of Sodium–Oxygen Batteries with Various Nonaqueous Electrolytes

Abstract

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)-, <i>N</i>-methyl-<i>N</i>-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 (p<i>K</i>_a) of these electrolyte solvents reveals that the TEGDME has the relatively large value of p<i>K</i>_a, which correlates with good stability of the electrolyte and high round-trip energy efficiency of the battery