Synergistic effect of metal-halide co-doping on the electrochemical potential of C₆N₈ nanocage anodes: A DFT study

This study investigates the electrochemical performance, electrochemical performance, and structural stability of halide-doped C₆N₈ nanocages as potential anode materials for sodium-ion batteries. Utilizing density functional theory (DFT) calculations, the evolution of the framework was systematically evaluated from a simple metal-doped cage (M@C₆N₈, where M = Zn, Mg) to a complex co-doped system (M/X^-/Na⁺@C₆N₈). The results show that the undoped metal-nanocage systems exhibit limited electrochemical driving forces, whereas halide incorporation substantially modulates their electronic and thermodynamic properties. Among the investigated configurations, Zn/Cl^-/Na⁺@C₆N₈ achieves the highest theoretical cell voltage of 3.65V, while Mg/F^-/Na⁺@C₆N₈ provides a favourable voltage of approximately 2.0V. Hirshfeld population analysis reveals substantial charge redistribution between the metal centres, halide ions, Na⁺, and the C₆N₈ framework, confirming that halide incorporation strongly influences the electronic environment of the active metal centre. Frontier molecular orbital analysis further demonstrates redistribution of electron density upon metal oxidation and halide incorporation. Competitive binding calculations indicate that the metal centres remain thermodynamically anchored within the C₆N₈ framework, while Na⁺ acts primarily as the mobile charge-compensating species. Overall, the results demonstrate that metal-halide co-modification provides an effective strategy for tuning the electrochemical potential, charge transfer, and structural stability of C₆N₈-based sodium-ion battery materials.