Tuning NaO2Cube Sizes by Controlling Na+and Solvent Activity in Na–O2Batteries

Understanding the kinetics of electrochemical oxygen reduction reaction (ORR) and controlling the chemistry, morphology, and size of discharge products are critical to realize reversible operation of metal–air batteries. Here we show that increasing Na+activity and free DME (not coordinated to Na+) activity in the solution increases the solubility of NaO2and size of NaO2cubes in Na–O2cells. With increasing Na salt concentration, Raman spectroscopy revealed that Na+activity increased while free DME activity decreased. NaO2solubility and NaO2cube size were found to exhibit a maximum at a medium concentration of Na+, which was accompanied by the highest full discharge capacity. This trend was attributed to two competing effects that stabilize NaO2in solution; both higher Na+activity and higher free DME activity can enhance NaO2solubility. These results highlight immense opportunities in the design of discharge/charge characteristics such as reaction product sizes and discharge capacity through the manipulation of the chemical physics of electrolytes as well as the solvation of reaction intermediates in the electrolytes.