Unsaturation degree of Fe single atom site manipulates polysulfide behavior in sodium-sulfur batteries

Sodium | |sulfur batteries hold great promise for grid-scale energy storage, yet their performance is hindered by the shuttling and sluggish redox of sulfur species. Herein, we report a strategic design of sulfur hosts modified with coordinatively unsaturated iron single-atom (Fe‒N_x) for sodium | |sulfur batteries. Utilizing theoretical calculations, geometric descriptor γ (l_Na‒S/l_Fe‒N) and electronic descriptor φ (e_g /t_2g) simultaneously correlated with the unsaturation degree of Fe‒N_x site are proposed. A negative correlation between γ and the adsorption strength of sodium polysulfides, along with a positive correlation between φ and the decomposition capability of Na₂S are established. The Fe‒N₁ sites, with the minimum γ and maximum φ values, are identified as the optimal functional species for optimizing polysulfides behaviors. Sodium | |sulfur batteries utilizing Fe‒N₁ /S positive electrodes deliver improved sulfur utilization (81.4% at 167.5 mA g^‒1), sustained rate performance (1003.0 mAh g^‒1 at 1675 mA g^‒1), and stable cycling (83.5% retention over 450 cycles at 3350 mA g^‒1). Moreover, Fe‒N₁/S positive electrodes enable sodium | |sulfur pouch cells to deliver a sulfur utilization of 77.4% (1296.9 mAh g^‒1) at 0.1 A g^‒1. Our work offers a strategy for designing high-activity, fast redox sulfur positive electrodes and validates the practical potential of sodium | |sulfur batteries.