Studies on electrochemistry in sodium-oxygen batteries with various anions

Abstract

Sodium oxygen ($Na-O_2$) battery is one of a promising battery system with its high theoretical energy density. However, $Na-O_2$ battery has low cyclability because it uses reactive sodium metal as a negative electrode material. Therefore, protecting sodium metal is an essential part for a development of $Na-O_2$ battery. Solid electrolyte interphase (SEI), which forms on sodium metal when it makes contact with an electrolyte, plays an important role to govern a stability of sodium metal. In this study, we investigated the effect of SEI layer on sodium metal on the stability of battery components and its performance by differentiating the electrolyte. 0.5 M sodium hexafluorophosphate ($NaPF_6$) in diethylene glycol dimethyl ether (G2) constructed stable SEI layer containing sodium fluoride (NaF) while sodium trifluoromethanesulfonate (NaOTf) in G2 did not. Furthermore, $Na-O_2$ battery with $NaPF_6/G2$ electrolyte showed stable battery components in open circuit voltage and higher cyclability in $Na-O_2$ battery. Moreover, the stability of the discharge product was higher when $NaPF_6/G2$ electrolyte was used than when NaOTf/G2 electrolyte was used. Through the studies on the reaction among the discharge product, the electrolyte, and electrodes, we could understand the stability and the performance of $Na-O_2$ battery system.