Resolving adsorption mechanism of sodium polysulfides on Ti_m+1C_mO₂ MXenes for application in sodium-sulfur batteries: A first-principles study

Room-temperature sodium–sulfur batteries promise great applications in large-scale energy storage systems owing to their high energy densities and environmental friendliness. However, their unfavorable features of shuttle effect due to the dissolution of high order sodium polysulfies in electrolyte solvents and poor conductivity hinder their practical usages. Suppressing the shuttle effect and enhancing the electronic conductivity of sodium-sulfur (Na-S) batteries are crucial for attaining the desirable performance of next-generation Na-S batteries. In this study, we employ first-principles calculations to evaluate the capability of Ti_m+1C_mO₂ MXenes (m = 1, 2, 3) as anchoring material for capturing sodium polysulfide clusters in Na-S batteries. All three MXenes exhibit strong adsorption with sodium polysulfides, particularly Ti₄C₃O₂, which shows outstanding affinity and higher conductivity upon adsorption. Furthermore, we uncover two distinct adsorption scenarios for low sulfur and high sulfur content of Na₂S_x clusters depending on their chemisorption ratios. The adsorption mechanism is supported by the significant Bader charge transfer quantifying the electron transfer during the adsorption process from the sulfur atoms to the MXenes. Finally, thanks to the low energy barriers of Ti₄C₃O₂ MXenes upon adsorption of Na₂S_x clusters, we demonstrate that Ti₄C₃O₂ MXenes serve as excellent anchoring material for the future technology development of Na-S batteries.