Tailoring Na+ Solvation Environment and Electrode‐Electrolyte Interphases with Sn(OTf)2 Additive in Non‐flammable Phosphate Electrolytes towards Safe and Efficient Na‐S Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries are promising for low-cost and large-scale energy storage applications. However, these batteries are plagued by safety concerns due to the highly flammable nature of conventional electrolytes. Although non-flammable electrolytes eliminate the risk of fire, they often result in compromised battery performance due to poor compatibility with sodium metal anode and sulfur cathode. Herein, we develop an additive of tin trifluoromethanesulfonate (Sn(OTf)₂ ) in non-flammable phosphate electrolytes to improve the cycling stability of RT Na-S batteries via modulating the Na⁺ solvation environment and interface chemistry. The additive reduces the Na⁺ desolvation energy and enhances the electrolyte stability. Moreover, it facilitates the construction of Na-Sn alloy-based anode solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI). These interphases help to suppress the growth of Na dendrites and the dissolution/shuttling of sodium polysulfides (NaPSs), resulting in improved reversible capacity. Specifically, the Na-S battery with the designed electrolyte boosts the capacity from 322 to 906 mAh g^-1 at 0.5 A g^-1 . This study provides valuable insights for the development of safe and high-performance electrolytes in RT Na-S batteries.