Unraveling the Multifunctional Mechanism of Fluoroethylene Carbonate in Enhancing High-Performance Room-Temperature Sodium-Sulfur Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries has attracted growing attentions in large-scale energy storage technology, while the serious shuttle effect and interface side reaction limit its practical application. Despite fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive or co-solvent to facilitate the optimization of electrode-electrolyte interphase in RT Na-S batteries, its crucial influence and mechanism have not been clearly understood. Herein, we deeply reveal the two-steps cathode-electrolyte interphase (CEI) formation by using FEC as the exclusive electrolyte solvent. The results demonstrate that FEC participates in both rapid nucleophilic reaction and electrochemical decomposition on cathode, which can effectively in situ construct a unique "chocolate-cookie" shaped CEI consisted of NaF-rich double layers. This CEI eliminates the shuttle effect and ensures the solid-solid conversion of sulfur. Along with the stable F-rich solid-electrolyte interphase (SEI) formed on the Na anode, the RT Na-S battery delivers an impressive performance (456 mAh g after 1000 cycles at 0.5 C) with almost 100 % Coulombic efficiency. This significantly simplifies electrolyte design and provides valuable insights into the application of FEC in practical Na-S batteries.