Ionic liquid–assisted synthesis of N/B/F co-doped biomass-derived hard carbon for sodium-ion batteries

Biomass-derived hard carbon is currently a promising anode material for commercial sodium-ion batteries. The incorporation of heteroatoms into hard carbon materials brings about remarkable improvements in their electrochemical performance. Nevertheless, the variety of biomass precursors and the constraints of traditional doping techniques present formidable hurdles for their practical application. In this study, a simple impregnation doping and two-step carbonization process was employed to prepare N/B/F co-doped hard carbon using Metaplexis japonica tomentum as a precursor and 1-butyl-3-methylimidazole tetrafluoroborate ionic liquid as the dopant source. The doping effect was confirmed by TEM and XPS. Furthermore, Raman spectroscopy results demonstrate that pseudo-graphitic structures, with large interlayer spacing, provide sufficient diffusion channels for Na⁺ ion intercalation and pore-filling. When evaluated for sodium-ion batteries, the as-prepared NBF co-doped hard carbon exhibits a discharge capacity of 140.3 mAh g⁻¹ at 0.1 Ag⁻¹ and 84.6 mAh g⁻¹ at 1.5 Ag⁻¹, which shows a better performance than commercial hard carbon products. Furthermore, EIS and DFT results demonstrate that co-doping could provide the highest electrical conductivity and synergistic effects, which can significantly alter the adsorption properties of carbon hosts. This method leverages biomass resources and ionic liquids to achieve controlled carbonization at relatively low temperatures, thereby optimizing the pore structure and graphitization degree of the material, which opens a feasible pathway for the preparation of heteroatom-doped biomass-derived hard carbon.

Graphical abstract