First-principles insights into sulfur and nitrogen co-doped Ti₂CO₂ MXene as an advanced anchoring material for sodium polysulfides in sodium–sulfur batteries

Room-temperature sodium-sulfur (Na-S) batteries have attracted significant attention for large-scale energy storage due to their high energy density, environmental compatibility, and cost-effectiveness. Nevertheless, their practical application is severely hindered by the shuttle effect resulting from the dissolution of sodium polysulfides into electrolyte solvents and the intrinsically poor conductivity of sulfur cathodes. In this work, we systematically investigate, by first-principles density functional theory calculations, the effectiveness of doping sulfur (S) and co-doping nitrogen (N) and sulfur (S) atoms on Ti₂CO₂ MXene as anchoring materials for sodium polysulfides (Na₂S _<i>x</i> ). Our results indicate that both doping and co-doping significantly enhance the adsorption strength of Na₂S _<i>x</i> clusters on Ti₂CO₂ surfaces compared to pristine MXene, with the NS co-doped Ti₂CO₂ exhibiting the strongest adsorption ability, especially for high-order polysulfides (Na₂S₆, Na₂S₈). We identify distinct adsorption mechanisms based on Bader charge analysis and projected density of states calculations, revealing substantial charge transfer from the adsorbed clusters to the MXene surface. Additionally, doping with S and NS co-doping significantly enhances electronic conductivity. Our findings offer theoretical insights into the beneficial role of heteroatom doping in MXenes and highlight NS co-doped Ti₂CO₂ as a promising candidate for mitigating polysulfide shuttle effects in next-generation Na-S batteries.