Curvature-Gated Li/Na–S Redox Chemistry on C ₆₀ : Stage-Selective Sulfur Regulation and Divergent Terminal Sulfide Kinetics

Understanding how curved molecular carbon interfaces regulate sulfur redox remains a central challenge for the rational design of advanced Li-S and Na-S batteries. Here, we present a systematic theoretical study of sulfur species adsorbed on C₆₀ and establish a mechanistic framework for curvature-gated sulfur redox. The calculated adsorption energies reveal pronounced chain-length-dependent stabilization on C₆₀, with terminal short-chain sulfides being most strongly bound, including C₆₀-Li₂S and C₆₀-Na₂S. Gibbs free-energy analysis further shows that both Li-S and Na-S pathways share a common thermodynamic bottleneck at the M₂S₄ → M₂S₂ conversion step, with uphill free-energy changes of +0.76 eV and +0.83 eV, respectively. Electronic-structure analyses demonstrate that this selective stabilization is governed by interfacial polarization rather than geometric confinement alone and evolves from weak contact for neutral sulfur to predominantly electrostatic interaction for intermediate-chain sulfides and a hybrid covalent-electrostatic mode for terminal sulfides. At the orbital level, both C₆₀-Li₂S and C₆₀-Na₂S exhibit a common motif consisting of a fullerene-centered low-lying acceptor manifold and mixed occupied interfacial frontier states. Most importantly, although Na₂S binds more strongly to C₆₀ than Li₂S, its final Na-S bond-cleavage barrier (1.05 eV) is markedly lower than the corresponding Li-S bond-cleavage barrier (1.48 eV), showing that Na₂S is kinetically more labile than Li₂S on the same curved carbon interface. These results identify C₆₀ as a curvature-gated reaction interface that reshapes sulfur redox through stage-selective thermodynamic bias, polarization-governed stabilization, and metal-dependent terminal conversion kinetics.