Insights into the Chemical Nature and Formation Mechanisms of Discharge Products in Na–O2 Batteries by Means of Operando X-ray Diffraction

Abstract

The chemical nature of discharge products in nonaqueous Na–O2 batteries attracts attention due to the differing electrochemical behavior of seemingly similar battery cells forming either sodium peroxide (Na2O2) or sodium superoxide (NaO2). The experimental control parameters for the exclusive formation of one or the other phase during discharge are still unknown. In this paper, the important role of gas phase moisture on the chemical nature of these products is demonstrated by means of operando XRD. Operando XRD is rarely reported for alkali-O2 systems but provides valuable real time information about the solid discharge products, growth/decomposition rates, and side reactions in the cell. In addition, the discharge/charge mechanisms of Na2O2·2H2O are monitored in detail evidencing important differences from the cell reactions forming NaO2 and Li2O2 and refuting previous incorrect conclusions. Moreover, the effect of the applied potential window during cycling is discussed, as it provides valuable information for the future development of Na–O2 batteries as electrochemical energy stores.