Oxygen Pressure Influences Spatial NaO2 Deposition and the Sudden Death Mechanism in Na–O2 Batteries

Abstract

Over the past decade, metal–O2 batteries have been intensely studied as potential high energy density alternatives to current state-of-the-art Li-ion batteries. Of these, nonaqueous Na–O2 batteries offer high stability, improved full-cycle efficiency, and lower overpotentials, particularly on charge, when compared to the higher-energy-density Li–O2 system. However, Na–O2 batteries exhibit sudden and large overpotential increases or “sudden deaths” on discharge, substantially limiting the achievable capacity. In this work, we examine the influence of O2 pressure effects in Na–O2 batteries and the mechanism of sudden death at different O2 pressures and current density regimes. We observe that at a given current density, there exists a transition between failure mechanisms with O2 pressure as a result of different phenomena related to the deposition of the solid discharge product, sodium superoxide (NaO2). Cells operated at a lower O2 pressure are more susceptible to failure due to surface passivation resulting from thin NaO2 film coverage, whereas cells operated at a higher O2 pressure achieve higher capacities but are increasingly subject to failure due to pore clogging from substantial solid NaO2 deposition. We associate the transition between these failure mechanisms with a combination of electron and mass transfer effects, leading to dramatic differences in the spatial deposition of NaO2 through the cathode.