BackgroundLithium, sulfur batteries are promising next-generation energy, storage systems, but their practical performance is limited by lithium polysulfide (LiPS) shuttling, sluggish sulfur redox kinetics, insulating Li2S formation, and performance decay under high-sulfur-loading and lean-electrolyte conditions.MethodsSingle-atom Ru–NC was synthesized through precursor coordination, pyrolysis, acid washing, and secondary annealing, followed by sulfur loading through melt diffusion to obtain S/Ru–NC. The materials were characterized by microscopy, XRD, Raman spectroscopy, nitrogen sorption, XPS, ICP-OES, XANES, WT-EXAFS, and EXAFS fitting. LiPS adsorption, symmetric-cell redox kinetics, Li2S nucleation/decomposition, standard-loading Li–S cell performance, high-loading lean-electrolyte cell behavior, and DFT adsorption analyses were evaluated. Minimum-energy paths were further examined by climbing-image nudged elastic band calculations, and cycled high-loading cells were characterized by SEM, separator XPS, and cathode TEM.ResultsRu–NC retained a high surface area before sulfur loading and exhibited atomically dispersed Ru–N4 coordination with a Ru content of 0.42 wt%. S/Ru–NC contained 69.4 wt% sulfur. Compared with NC, Ru–NC increased Li2S6 adsorption efficiency from 31.6% to 78.4%, increased adsorption capacity from 0.79 to 1.96 mmol g−1, reduced peak-potential separation from 0.77 to 0.58 V, and lowered interfacial charge-transfer resistance from 58.7 to 31.4 Ω. Ru–NC also improved Li2S nucleation and decomposition, increasing Li2S deposition capacity from 156.8 to 286.4 mAh g−1 and decreasing decomposition overpotential from 232 to 148 mV. In standard-loading cells, S/Ru–NC delivered 1,276.4 mAh g−1 at 0.1 C and retained 612.4 mAh g−1 after 500 cycles at 1.0 C. Under high-loading lean-electrolyte conditions, S/Ru–NC achieved 4.84 mAh cm−2 initially and retained 3.56 mAh cm−2 after 100 cycles. DFT results confirmed stronger adsorption of sulfur species and greater charge transfer on Ru–NC. In high-loading cells cycled at 1.0 C, S/Ru–NC retained 566.3 ± 24.9 mAh g-1 (2.55 ± 0.12 mAh cm−2) after 50 cycles. The calculated rate-determining Li2S2-to-Li2S barrier decreased from 1.32 eV on NC to 0.68 eV on Ru–NC, while the reverse Li2S oxidation barrier decreased from 1.21 to 0.64 eV.ConclusionAtomically dispersed Ru–N4 sites on nitrogen-doped carbon effectively couple LiPS adsorption with catalytic redox conversion, thereby improving Li2S reaction kinetics and Li–S cell performance under both standard and practical operating conditions. The matched reduction and oxidation barriers show that stronger binding does not create an irreversible Li2S trap.