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.