Impacts of Carbon Mesopores on Superoxide Degradation in the Cathode of a Na-O2 Battery

Abstract

Sodium-air batteries have attracted widespread attention due to the abundance of Na, the high energy density of NaO2, and high voltage efficiency arising from the low operation overpotentials. In this work, we focus on the impacts of mesopores in carbon cathodes and how they affect the electrochemical performance of the Na-O2 battery. Contrary to the general perception of enhanced electrochemical performance resulting from the porous carbon cathode, the presence of mesopores in the carbon cathode apparently accelerates the degradation of NaO2, while nonporous carbon demonstrates the best performance. Charge profiles of various carbon cathodes are examined. The mesoporous carbon cathode shows a very short low potential charge plateau (corresponds to the oxidation of NaO2) and proceeds quickly to two high potential charge plateaus due to the side reactions of NaO2 confined in mesopores. Theoretical density functional theory calculations indicate that solvent has a stronger interaction with the surface of small NaO2 clusters than that of bulk NaO2. The stronger interaction results in a higher possibility of inducing NaO2 degradation. Thus, the high surface area provided by the mesopores is not necessarily advantageous but has detrimental effects on the performance of cathodes in the Na-O2 batteries. This work shows how to minimize degradation and avoid degradation products being formed or stored in the carbon cathodes.