Charging-State Behavior of N/O/F-Doped Carbon Anodes for K-Ion Batteries

Abstract

Potassium-ion batteries (KIBs) are emerging as cost-effective alternatives to Li- and Na-ion systems, yet their performance is governed by adsorption-driven K storage on carbon anodes and the associated charging-state transition from K+ to neutral K. Here, density functional theory was used to elucidate the charging-state potassium storage on heteroatom-doped graphene, systematically comparing N, O, and F single doping and their multi-doped combinations (N/O, O/F, N/F, and N/O/F). By integrating molecular electrostatic potential mapping, K+/K adsorption energetics, adsorption-derived operating voltages, optimized adsorption geometries, and HOMO–LUMO characteristics, we established a unified framework linking electrostatic modulation to thermodynamic balance and structural accommodation. N doping produced a locally electron-enriched potential landscape that strengthens K+ anchoring, whereas O doping most effectively reduced the energetic mismatch between K+ adsorption and neutral K storage, thereby moderating the K+→ K conversion penalty. F doping induced pronounced nonplanarity and a compliant adsorption pocket that enhances out-of-plane accommodation of both K+ and K. In multi-doped systems, N/O and O/F exhibited complementary role sharing that stabilizes adsorption and K+→ K conversion, while N/F revealed a trade-off without O. Notably, N/O/F co-doping optimized electrostatic driving force, conversion energetics, and structural compliance, providing practical design rules for advanced carbon anodes in KIBs.