A high-rate and high-efficiency molten-salt sodium–oxygen battery

Abstract

Alkali metal–oxygen batteries can provide greater specific energy than Li-ion batteries but often suffer from low power density, cycleability, and energy efficiency due to the sluggish kinetics of the oxygen electrode and parasitic reactions at both the oxygen and alkali metal electrodes. In this study, we demonstrated a molten-salt Na–O₂ battery operating at 443 K with high areal energy (33 mW h cm⁻²_geo) and power densities (19 mW cm⁻²_geo), with high energy efficiency (∼90% at 5 mA cm⁻²_geo), and stable cycling (400 cycles with no capacity loss). Raman, pressure tracking and titration measurements were used to show that the dominant discharge product is Na₂O₂. Moreover, the redox activity of nitrate anions in the molten salt was found to be critical to enable the formation of Na₂O₂. Through ¹⁸O-labeling experiments as well as discharging Na–Ar cells, we demonstrated that the discharge reaction occurs via the electrochemical reduction of NaNO₃ to Na₂O and NaNO₂, where chemical reactions with O₂ lead to the formation of Na₂O₂ from Na₂O, and the regeneration of NaNO₃ from NaNO₂.