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 lower-order insoluble species (Na₂S₂/Na₂S) lead to severe polysulfide dissolution, insulating discharge products, and rapid capacity fading. MXenes, a type of 2D transition metal carbides and nitrides, are a viable choice for cathode catalysts for Na-S batteries because of their high electrical conductivity and greater affinity towards polysulfides. Hence, 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_n 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 higher electron transfer on the O-terminated surface. Furthermore, the calculated free energy barriers for the rate-determining step of S reduction reactions are significantly lower on the catalytic surfaces (0.55 eV for Nb₂TiN₂O₂ and 0.75 eV for Nb₂TiN₂S₂), than the barriers for the polysulfides conversion in the gas phase (1.05 eV). 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.