Solvation-Hierarchy Electrolyte for Balancing Kinetics and Stability in Sodium-Sulfur Batteries

Room-temperature sodium-sulfur batteries hold immense promise for next-generation energy storage, while hindered by the dilemma of sodium polysulfide (NaPS) shuttling and redox kinetic. Here, we propose a solvation hierarchy strategy for sodium-sulfur batteries to reconcile the intrinsic trade-off between polysulfide confinement and reaction kinetics. The solvation hierarchy strategy includes the strong solvating, moderate solvating and weak solvating electrolyte, which establishes a tailored solvation environment that sterically excludes polysulfides to mitigate shuttling, while concurrently inducing a metastable state that lowers the desolvation energy barrier, promoting rapid charge transfer. The moderate solvating sec-butyl ether (SBE) distinguished by its symmetric α-methyl steric hindrance is chosen, compared to the highly solvating 1,2-dimethoxyethane (DME) and the weakly solvating ethyl butyl ether (BEE). The designed solvation-hierarchy electrolyte breaks the trade off in kinetic and stability, enabling a capacity decay rate of 0.015% per cycle after 800 cycles at 1C. Furthermore, the system demonstrates robust stability at practical conditions of high sulfur loadings of 6.8 mg cm^{- 2} and lean electrolyte of 5.1 µL mg^{- 1} and is also validated in pouch cell configurations. These findings highlight the efficacy of solvating hierarchy to balance the stability and kinetic dilemma, offering a valuable perspective for advancing high-energy-density metal-sulfur batteries.