MBenes Monolayers as Shuttle-Effect Suppressors for High-Performance Sodium-Sulfur Batteries

Room-temperature sodium-sulfur batteries (RT-NSBs) are promising candidates for large-scale energy storage owing to their high theoretical energy density and the natural abundance of both sodium and sulfur. However, their practical performance is severely limited by the polysulfide shuttle effect, the intrinsically poor electrical conductivity of the sulfur cathode, and sluggish electrochemical redox kinetics. In this study, we explore two-dimensional transition metal borides (MBenes, Mo₂B₂T_x, T_x = F, H, O) as cathode anchoring materials to mitigate these challenges. Using density functional theory (DFT) simulations with van der Waals corrections, we systematically investigate the interactions between sodium polysulfides (Na_₂S_ₙ, n = 1-8) and Mo₂B₂T_x monolayers, along with their electronic properties and key electrochemical parameters. Among the studied systems, Mo2B2O2 exhibits the strongest adsorption of Na_₂S_ₙ via Na-O bonds, with binding energies ranging from -2.36 to -5.84 eV, far exceeding typical electrolyte solvation energies (-0.20 to -0.90 eV), thereby effectively suppressing polysulfide dissolution and migration. This enhanced binding stems from the high surface polarity of Mo2B2O2 and its strong electrostatic affinity toward Na_₂S_ₙ species. Bader charge analysis confirms significant charge transfer from Na_₂S_ₙ to the Mo₂B₂T_x surface, improving the electronic conductivity of the adsorbed system and facilitating rapid charge transport. Furthermore, Mo₂B₂T_x lowers the energy barriers for the redox conversion of polysulfide intermediates, the decomposition of Na_₂S, and Na^⁺ ion diffusion, collectively promoting electrochemical reversibility and rate capability. These findings highlight Mo₂B₂T_x as highly promising anchoring platforms for suppressing the shuttle effect and advancing the performance of RT-NSBs.