Single-atom ruthenium on nitrogen-doped carbon for catalytic polysulfide conversion in lithium-sulfur batteries

Lithium, 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.

Single-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.

Ru-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.

Atomically 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.