Electrochemically Generated KO2 as a Phase-Transfer Mediator for Na-O-2 Batteries

Abstract

Superoxide-based Na-O-2 batteries have attracted much attention as promising alternatives to peroxide-based Li-O-2 batteries because of their small polarization for oxygen evolution reactions. However, the limited solubility of their discharge product, NaO2, leads to the surface-confined mechanism at high current densities, resulting in the poor energy density of Na-O-2 batteries. In this connection, a few protic phase-transfer catalysts, such as water and benzoic acid, have been examined to improve reversible capacity because they promote the solution-mediated mechanism. Herein, KO2, which is electrochemically generated from potassium trifluoromethanesulfonate dissolved in electrolytes during discharge, is introduced as a phase-transfer mediator for Na-O-2 batteries. The reaction mechanism of Na-O-2 batteries containing a KO2 mediator is clarified through ex situ XRD, cross-sectional SEM, and ICP analyses. KO2 plays the role of a phase-transfer mediator because the desolvation rate of KO2 is slower than that of NaO2. As a result, Na-O-2 batteries with KO2 show the solution-mediated mechanism rather than the surface-confined mechanism, thus delivering a high reversible capacity of approximately 6 mAh cm(-2). In addition, since KO2 is chemically and electrochemically more stable than previous protic phase-transfer mediators, Na-O-2 cells with KO2 show stable cycle performance, such as negligible capacity fading over 25 cycles.