Halide-encapsulated C₂₄ fullerenes as molecular redox hosts for alkali metals: A density functional theory study

Halide-encapsulated C₂₄ fullerenes were systematically investigated as molecular anode materials for alkali metal-ion batteries (M = Li, Na, K) using density functional theory (DFT). While pristine C₂₄ cages interact with neutral alkali atoms, they fail to stabilize the oxidized M⁺ state, resulting in unfavorable thermodynamics and negative anode potentials. In contrast, encapsulation of halide anions (F^- and Cl^-) induces charge redistribution within the carbon framework, significantly enhancing cation stabilization and converting negative voltages into positive anode potentials. Among the investigated systems, X^-@ C₂₄-46 (X = F, Cl) exhibits the best electrochemical performance, with predicted voltages up to 3.41 V for Na/F^-@ C₂₄-46 and 3.40 V for Li/Cl^-@ C₂₄-46. The predicted trends are consistent across B3LYP, CAM-B3LYP, and HSEH1PBE functionals, confirming the robustness of the results. These findings demonstrate that internal halide encapsulation is an effective strategy for activating C₂₄ fullerenes toward alkali-metal storage applications.