Visualizing The Oxidation Mechanism And Morphological Evolution Of The Cubic-Shaped Superoxide Discharge Product In Na-Air Batteries

Sodium-air (Na-O-2) batteries have recently developed as a high theoretical energy density energy storage and conversion system. In particular, Na-O-2 batteries with superoxide as the discharge product have a very high round-trip energy efficiency over lithium-air batteries due to their significantly reduced charging overpotential. However, Na-O-2 batteries yet suffer from limited cycling lives because of the formation and incomplete removal of side products during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, while the mechanism of these processes is still not fully understood. Herein, a detailed investigation on tracking the decomposition pathway of cubic-shaped micrometer-sized NaO2 discharge products in Na-O-2 batteries with carbon-based air electrodes is reported. A detailed electrochemical charging mechanism is revealed during the charging process. The evolution of the chemical compositions of the discharge/side products in air electrode during charging is also verified by synchrotron-based X-ray absorption spectroscopy experiments. The formation of these intermediate phases other than NaO2 during the charging process results in high overpotentials. These new findings can contribute to a better understanding and the rational design of future Na-O-2 batteries.