Dual Roles of Deep Eutectic Solvent in Polysulfide Redox and Catalysis for Intermediate‐Temperature Potassium‐Sulfur Batteries

Potassium–sulfur (K‐S) batteries hold great promise for long‐duration energy storage due to their low cost and high energy density. However, the irreversible deposition of K2S2/K2S severely hinders sulfur utilization and cycling stability. Herein, a NiS–DES interfacial regulation strategy is developed that leverages the dual functionality of a deep eutectic solvent (DES) to govern the adsorption and conversion behavior of K2S2/K2S at the catalytic interface, enabling their highly reversible transformation. Specifically, DES forms moderate electronic coupling with NiS to weaken the excessively strong adsorption of K2S and prevent catalyst deactivation. Simultaneously, strong electronic interactions between DES and K2S promote interfacial activation and conversion, thereby extending the reaction pathway and enhancing reduction depth. As a result, the intermediate‐temperature K‐S batteries deliver an initial capacity of 810 mAh g−1 with a minimal capacity decay of 0.02%/cycle over 1300 cycles at 6 mg cm−2 sulfur loading. Even under lean catholyte (4.2 µL mg[sulfur]−1) and higher sulfur loading (12 mg cm−2), they achieve 521 mAh g−1 initially, retaining stability with 0.03%/cycle decay over 500 cycles. This NiS‐30DES system achieves a cell‐level energy density exceeding 150 Wh kg−1 and a low levelized cost of storage (LCOS) of $140/MWh, demonstrating strong potential for scalable long‐duration energy storage.