Regulating Solvent‐Separated Ion Pairs to Control Polysulfide Redox for Fast and Stable Room‐Temperature Na‐S Batteries

The solvent-separated ion pairs in local high-concentration electrolytes (LHCE) have a profound influence on rechargeable sulfur-based batteries yet are always ignored. Herein, tailored solvent-separated ion pairs are engineered by tuning the diluent ratio in LHCE to boost sulfur redox kinetics in room-temperature sodium-sulfur (Na-S) batteries. The tailored solvent-separated ion pairs allow the sparingly dissolved polysulfides to enhance localized solid-liquid-solid transformation on reactive interfaces, so the LHCE decorated with the solvent-separated ion pairs preserves the global quasi-solid-state mechanism of sulfur while modulating its intrinsic sulfur redox kinetics. Consequently, Na-S batteries achieve excellent cyclability (fading rate of 0.017% per cycle over 2400 cycles at 1.0C) and outstanding rate performance (564 mA h g^- ¹ at 2.0C), and the pouch cells can deliver an ultrahigh capacity of 912 mA h g^- ¹ at 0.1C. This work highlights the critical role of SSIPs in enabling fast and stable sulfur chemistry for durable and fast-charging Na-S batteries.