First-Principles Study of Ti 2 NbC 2 T 2 (T = F, O) MXene for Inhibiting Shuttle Effect in Na–S Batteries

Sodium–sulfur (Na–S) batteries offer great promise for large-scale energy storage due to their high theoretical energy density and cost-effectiveness; however, their practical performance is limited by the severe shuttle effect of soluble sodium polysulfides (Na 2 S n ). In this work, we employ density functional theory (DFT) to investigate the structural and electronic properties of Ti 2 NbC 2 T 2 (T = F, O) MXenes, a recently synthesized and experimentally verified member of the MXene family. Adsorption energy calculations show that O termination significantly enhances the binding affinity toward Na 2 S n compared to F termination. Electronic structure analyses reveal strong hybridization between Na-s and S-p orbitals of polysulfides and the Ti 2 NbC 2 T 2 surface, with low-order Na 2 S n exhibiting notable S-p band shifts that increase metallicity upon adsorption. Charge density difference and Bader charge analyses confirm substantial electron transfer from Na 2 S n to Ti 2 NbC 2 T 2, particularly for O-terminated surface, indicative of robust Na–S chemical bonding. Conversely, S 8 displays minimal charge redistribution, consistent with weak physisorption. These results highlight Ti 2 NbC 2 O 2 as an experimentally accessible and highly effective MXene host capable of strongly anchoring sodium polysulfides and suppressing their dissolution, thereby mitigating the shuttle effect in Na–S batteries. This study provides fundamental insights into MXene and polysulfide interfacial chemistry and offers a valuable design strategy for next-generation Na–S host cathode materials.