Axial fluorine coordination boosts the activity and durability of Fe single-atom catalysts in room-temperature Na–S batteries

Abstract

Single-atom catalysts (SACs) hold promise for addressing challenges of polysulfide shuttle and sluggish sulfur reduction reaction (SRR) in room-temperature (RT) Na–S batteries. However, their structural durability under harsh electrochemical conditions remains a critical concern. Herein, we propose an effective strategy to optimize and stabilize the active sites of Fe single atoms (Fe_SACS) by modulating the local geometries through axial fluorine (F) coordination, thus significantly alleviating the stability problems faced by conventional high-performance but deactivation-prone Fe–N–C catalysts. Density functional theory (DFT) calculations and experimental results confirm that the enhanced Fe–F interactions in the second shell layer play a key role in maintaining the structural integrity of the single atoms during synthesis and operation and effectively inhibit the agglomeration behavior of Fe atoms. The F axial coordination with the optimized electronic structure enhanced the d–p hybridization between the Fe 3d orbitals and the sulfur intermediates, which significantly promoted the SRR kinetics and catalytic durability. Through comprehensive spectroscopic investigations, we further elucidate that the sulfur species undergo quasi-solid–solid conversion pathways on Fe_SACS-FCNT@S electrodes, effectively suppressing polysulfide dissolution. This work establishes a universal paradigm for designing durable SAC systems through rational coordination engineering while providing fundamental insights into structure–stability relationships for advanced metal–sulfur batteries.