Studies Of superoxide degradation kinetics and electrolyte management for a reversible NaO2 battery

Abstract

Applications of sodium–air batteries are hampered by low coulombic efficiency due to the high reactivity of the discharge product NaO2. Although there is intensive research to improve battery performance, there is no study pinpointing the degradation kinetics of NaO2 and the possible application direction. Here, NaO2 degradation is studied experimentally by performing discharge, idling, and charge of different intervals in a diglyme-based Na-O2 battery. The recoverable NaO2 is quantified by the coulombs produced at the NaO2 characteristic plateau of approximately 2.3 V vs Na/Na+ during charge. To corroborate the experimental data, a model is proposed with the assumption of first-order degradation kinetics of dissolved NaO2 which precipitates beyond its solubility limit. The degradation rate constant and solubility of NaO2 are hence determined to be kd = ∼0.00184 s–1 and Cmax = ∼1.5 mM at room temperature. To maximize the Coulombic efficiency by storage of NaO2 in a dry form, we demonstrate a Na-O2 battery with its electrolyte dried by vacuum during idling and refilled for charge; this shows a high coulombic efficiency similar to a fresh cell. This would potentially provide a low-cost, highly abundant, and sustainable battery system for large-scale energy storage.