Tuning NaO2 Cube Sizes by Controlling Na+ and Solvent Activity in Na–O2 Batteries

• Published in: Journal of physical chemistry. C Vol. 122; no. 32; pp. 18316 - 18328
• Main Authors: Tatara, Ryoichi, Leverick, Graham M, Feng, Shuting, Wan, Stefan, Terada, Shoshi, Dokko, Kaoru, Watanabe, Masayoshi, Shao-Horn, Yang
• Format: Journal Article
• Language: English
• Published: American Chemical Society 16.08.2018
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b05418

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 NaO2 and size of NaO2 cubes in Na–O2 cells. With increasing Na salt concentration, Raman spectroscopy revealed that Na+ activity increased while free DME activity decreased. NaO2 solubility and NaO2 cube 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 NaO2 in solution; both higher Na+ activity and higher free DME activity can enhance NaO2 solubility. 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.