Tuning Polysulfide Adsorption and Catalytic Activity via Surface Functionalization of Nb2TiN2 MXene in Na-S Batteries

Sodium-sulfur (Na-S) batteries are emerging as a promising candidate for large-scale energy storage due to the natural abundance and low cost of sodium and sulfur and their high theoretical energy density.However, the sluggish conversion kinetics of higher-order soluble polysulfides (Na₂S_n, n > 2) into lowerorder insoluble species (Na₂S₂/Na₂S) lead to severe polysulfide dissolution, insulating discharge products, and rapid capacity fading. In this study, we employ first-principles density functional theory (DFT) calculations to systematically investigate the adsorption characteristics and catalytic behavior of a novel double transition metal (DTM) nitride MXene-Nb₂TiN₂ , functionalized with sulfur (S) and oxygen (O) terminal groups (Nb₂TiN₂S₂ and Nb₂TiN₂O₂ , respectively). Our results reveal that O-functionalized Nb₂TiN₂O₂ exhibits significantly stronger adsorption of Na₂S species, which is expected to mitigate the shuttle effect and improve structural stability compared to its S-functionalized counterpart. Detailed analysis of adsorption energies and charge transfer mechanisms demonstrates that lower-order polysulfides exhibit stronger binding and greater electron transfer on the O-terminated surface. Furthermore, the calculated free energy barriers for the rate-determining step of sulfur reduction reactions are 0.55 eV for Nb₂TiN₂O₂ , 0.75 eV for Nb₂TiN₂S₂ , and 1.05 eV for the polysulfides conversion in the gas phase. These findings suggest that O-functionalization facilitates more favorable reaction kinetics by stabilizing key intermediates and lowering energy barriers compared to S-functionalization. This work provides critical insights for the rational design of advanced cathode hosts to enhance the electrochemical performance and cycle life of Na-S batteries.