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

ABSTRACT 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.