Regulating Polysulfide Conversion on Fe-N x Single-Atom Catalysts Supported on Graphitic Carbon Nitride Revealed by Semi-In Situ XPS of Li–S Batteries

Lithium–sulfur batteries are limited by the polysulfide shuttle effect and sluggish conversion kinetics, both strongly influenced by the catalytic interface that mediates the polysulfide redox process. This work centers on the controlled synthesis of single-atom catalysts to establish how the local coordination environment dictates sulfur redox kinetics. We show that engineering the support prior to metal incorporation, rather than the metal site alone, is an effective and reproducible lever for tuning Fe-Nx single-atom catalysis on urea-derived graphitic carbon nitride. Thermal reduction of the support before Fe loading reshapes the resulting Fe-Nx coordination environment, yielding atomically dispersed, coordinatively unsaturated sites with mixed Fe2+/Fe3+ valence and no detectable aggregation, as established by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Extended X-ray absorption fine structure analysis reveals lower average Fe-N coordination numbers on the reduced support. Density functional theory further demonstrates the catalytic role of atomically dispersed Fe sites by predicting stronger polysulfide adsorption and lower conversion barriers relative to the metal-free gCN support. To resolve how these sites act during operation, we track sulfur speciation across extended cycling using semi-in situ sulfur 2p X-ray photoelectron spectroscopy, which reveals pronounced enrichment of reduced sulfur species and suppression of oxidized intermediates, direct spectroscopic evidence of accelerated polysulfide conversion. The reduced-support catalysts also show improved durability (∼71–75 vs ∼48% capacity retention for the bare support). Together, these results establish thermal reduction as a controllable design principle for single-atom catalysts in lithium–sulfur batteries and semi-in situ X-ray photoelectron spectroscopy as a powerful probe of sulfur redox mechanisms.