Undercoordinated Molybdenum Catalysts Enable Ultrafast Quasi-Solid Sulfur Chemistry in Sodium-Sulfur Batteries.

Room-temperature sodium─sulfur (RT Na─S) batteries face sluggish redox kinetics and severe polysulfide shuttling. Here, a quasi-solid-state redox pathway is activated via an unsaturated coordination chemistry strategy, in which unsaturated MoS2 anchored on cross-linked carbon microspheres forms a multifunctional sulfur host (S@U-MoS2/C) that combines strong polysulfide adsorption with accelerated redox kinetics. Structural and electronic analyses show unsaturated Mo sites act as Lewis acid centers for rapid, selective polysulfide conversion. In situ transmission electron microscopy with newly developed Na-ion diffusion descriptors visualize ultrafast nanoscale sodiation dynamics and quantify Na-ion transport. Consequently, the S@U-MoS2/C cathode delivers an impressive capacity of 933 mAh g- 1 after 150 cycles at 200 mA g- 1 and retains 425 mAh g- 1 after 30 000 cycles at 10 A g- 1. This work provides a mechanistic blueprint for designing high-rate, long-life Na─S batteries by coupling catalysis with structural confinement.