A generic F-doped strategy for biomass hard carbon to achieve fast and stable kinetics in sodium/potassium-ion batteries

N/F-doped nanospheres hard carbon (CDC-F-900) was prepared via simple annealing followed by HF solvothermal reaction. F-doping is achieved by replacing –OH and –COOH groups with F− to form C-F and C-F2. Abundant micropores and defect structures exist in CDC-F-900, providing a rapid diffusion pathway for Na+/K+. Therefore, it provides the capacity of 372.2 mAh/g after 1000 cycles at 0.1 A/g in sodium-ion batteries (SIBs). Especially in potassium-ion batteries (PIBs), it can still provide an ultra-high capacity of 214.2 mAh/g at 10 A/g after 5000 cycles. This is due to the inorganics-rich SEI which possesses higher ionic conductivity and stability, resulting in excellent long-cycle stability. Density functional theory (DFT) calculations indicate that the conjugate effect of N/F leads to an appropriate adsorption energy for K+ at defects. Not only does it increase the reversible capacity by reducing irreversible K+ adsorption, but it also accelerates ion transport rate, resulting in better rate capability. The potassium storage mechanism of CDC-F-900 is elaborated in detail through in-situ Raman spectroscopy and XRD pattern. This work provides a new insight for the design and development of low-cost, source-rich, and long-life hard carbon anodes for SIBs/PIBs.