Elucidating the Impact of Sodium Salt Concentration on the Cathode–Electrolyte Interface of Na–Air Batteries

A promising approach to improve the specific capacity and cyclability in a Na–O2 cell using a pyrrolidinium-based ionic liquid electrolyte in a half-cell has been explored in this work. Increasing the concentration of sodium salt in an ionic liquid electrolyte produces a significant enhancement in the discharge capacity of up to 10 times, a reduction in the overpotential and an increase in the long-term cyclability. Additionally, a distinct discharge morphology is also observed, which is demonstrated to be result of a different oxygen reduction reaction mechanisms. These improvements are likely due to the solvation of Na+ in the electrolyte mixtures containing different Na+ concentrations; the coordination of Na+ by the anion of the ionic liquid dictates the discharge product morphology. At low concentrations, Na+ is strongly coordinated to the anion of the ionic liquid, and this also can have an effect on its mobility; however, at high Na+ concentration, this interaction is weakened and favors mass transport before product deposition. It therefore appears that the concentrated electrolyte strategy is a useful route to enhance the performance of Na–O2 batteries. Interestingly, when using a pressurized Swagelok-type cell, the discharge product presents a cubic morphology, which is typical of NaO2. This is the first work where this characteristic morphology appears when using an ionic liquid, opening new venues for future research.