A fluoroethylene carbonate-based electrolyte with 1 M sodium bis(fluorosulfonyl)imide enables high-performance sodium metal electrodes

Abstract

Extensive academic and industrial efforts have been dedicated to developing battery-based energy storage technologies with high energy density, low cost, long cycle life, high energy efficiency, and ease of deployment in daily life. In particular, for large-scale electrical energy storage, attention has shifted to sodium (Na) metal batteries owing to the highest specific capacity and the lowest redox potential of metallic Na and the natural abundance of Na resources. Nevertheless, the exceedingly high electrochemical and chemical reactivity of Na metal electrodes toward organic liquid electrolytes and severe Na dendrite formation limit their commercialization. Since most Na deposition occurs at the interface between the anode (or the current collector) and the electrolyte, the discovery of a stable electrolyte is essential for utilizing Na metal anodes in practical applications. In this study, we demonstrate that fluorination solvent containing electrolyte dramatically enhances the reversibility of Na plating and stripping reactions in Na/Cu cells, realizes dense Na deposition, and leads to improved cycling stability at high current densities. By examining the detailed mechanism of the novel electrolyte system, we have found that the interfacial layer contains NaF, which has a high shear modulus and enhances the mechanical integrity of the interlayer by the attractive interaction between the F- ions and Na+ ions of ionic compounds such as Na2CO3 and sodium alkylcarbonates (NaO2CO-R-), resulting in the formation of mechanically strong and ion-permeable interlayers that suppress uncontrolled Na metal plating. The discovery of this work represents a step forward in the electrolyte design of Na metal anodes.