Synergistic regulation of closed-pore and mesoporous structures in orange-peel-derived hard carbon via temperature-controlled carbonization and NH₄Cl activation for high-performance sodium-ion batteries

A series of sodium-ion battery anode materials are synthesized from discarded orange peels by adjusting the carbonization temperature. The hard carbon obtained at 1300 °C shows the smallest specific surface area—contributing to a high initial Coulombic efficiency—and the largest average pore size, facilitating sodium-ion diffusion. At 60 mA g−1, it delivers a reversible capacity of 377.3 mAh g−1 with an initial Coulombic efficiency of 75.2%. Further activation with NH₄Cl via a two-step hydrothermal-carbonization process improves performance. At a precursor/NH₄Cl mass ratio of 1:2, the activated material retains 80.53% capacity after 400 cycles at 60 mA g−1 and maintains 204 mAh g−1 at 400 mA g−1.This “temperature-gradient carbonization combined with NH₄Cl activation” strategy balances high capacity and fast charge–discharge kinetics in biomass-derived carbon through synergistic tuning of interlayer spacing (0.392 nm) and mesoporous structure (3.296 nm). The work offers a low-cost, eco-friendly route to convert waste biomass into practical sodium-ion battery anodes.