Electrochemical Na⁺ storage properties of SnO₂/graphene anodes in carbonate-based and ionic liquid electrolytes

Electrolyte formulations are vital for the proper functioning of Na-ion batteries. A systematic study is conducted to optimize the electrolyte for a SnO₂/graphene anode, which is prepared via a supercritical-CO₂-assisted synthesis method. The effects of a propylene carbonate (PC) and ethylene carbonate (EC) solvent combination and a fluoroethylene carbonate (FEC) additive on the electrode charge-discharge properties are examined. Incorporations of EC and FEC promote the formation of a more robust solid electrolyte interphase layer, improving the cyclic stability of the electrode compared to that found for an electrode in a PC-only electrolyte containing 1 M NaClO₄ salt. Nevertheless, at 60 °C, an N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid electrolyte clearly outperforms the PC/EC/FEC carbonate-based electrolyte in terms of electrode capacity, rate capability, and cyclic stability. This work indicates that the electrode sodiation/desodiation performance remarkably depends on the electrolyte composition, which should be optimized for various application demands and operation temperatures of batteries.