Oxygen Pressure Influences Spatial NaO2 Deposition and the Sudden Death Mechanism in Na–O2 Batteries
2018
Nichols, Jessica E. | Knudsen, Kristian B. | McCloskey, Bryan D.
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.
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