Inner–Outer Sheath Synergistic Shielding of Polysulfides in Asymmetric Solvent-Based Electrolytes for Stable Sodium–Sulfur Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries are garnering interest owing to their high theoretical energy density and low cost. However, the notorious shuttle behavior of sodium polysulfides (NaPS) and uncontrollable dendrite growth lead to the poor cycle stability of RT Na-S cells. In this work, we report the use of 1,2-dimethoxypropane (DMP) and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE) as inner solvent and outer diluent, respectively, in a localized high-concentration electrolyte system. Impressively, the asymmetric DMP as the inner solvent, introduced to replace the conventional solvent 1,2-dimethoxyethane (DME), shields NaPS effectively from incorporation into the inner solvation structure due to the extra methyl groups in the molecular structure. Furthermore, the TFTFE diluent, which contains electron-withdrawing perfluoro segments (-CF₃- and -CF₂-), exhibits significantly low solvation power. Consequently, the outer sheath TFTFE diluent further minimizes NaPS dissolution, thereby enhancing the cycle stability. This inner-outer sheath synergistic effect leads to the formation of highly effective cathode-electrolyte interphase (CEI) and solid-electrolyte interphase (SEI) layers simultaneously, significantly alleviating the shuttle effect and reducing the side reactions between NaPS and sodium metal. Remarkably, the Na-S cells with the designed electrolyte present long-cycling reversibility with 530 mAh g^-1 over 600 cycles at a C/2 rate and a low capacity decay rate of 0.077% per cycle. This study provides a profound understanding of the electrolyte structure involving NaPS and offers a firm basis for the rational design of electrolytes for rechargeable metal-sulfur battery systems.