Improving the Stability of an RT‐NaS Battery via In Situ Electrochemical Formation of Protective SEI on a Sulfur–Carbon Composite Cathode

Abstract The room temperature sodium sulfur battery (RT‐NaS) has attracted considerable attention as a next‐generation energy storage system due to its low cost and high specific energy and the abundance of sodium and sulfur. However, the sulfur‐based cathode in RT‐NaS always undergoes dissolution of polysulfides during the cycle, which limits the utilization of active material and the stability of the RT‐NaS. In this study, the authors report an economical and simple method to suppress the dissolution of polysulfides by forming an artificial protective solid electrolyte interphase (SEI) on the cathode surface using an in situ electrochemical treatment with a fluoroethylene carbonate (FEC) electrolyte additive. The artificially formed SEI has a thickness of ≈20 nm and contains fluoride anions that improve electrical conductivity and mechanical stability. The RT‐NaS full cell with the sulfur–carbon cathode and protective SEI exhibits a 0.1 V_Na+/Na lower electrochemical polarization for desodiation, and SEI and electrical resistances that are 23% and 45% lower, respectively, than those of untreated NaS. These improvements lead to a capacity increase of ≈120% (270 to 590 mAh g⁻¹ at the 50th cycle) and a high Coloumbic efficiency of 99.5% in the high‐rate cyclability test of the 2 C rate over 200 cycles.