Biomass-derived hard carbon from sorghum distilled residues as a high-performance anode for sodium-ion batteries

In this study, hard carbon derived from sorghum distilled residues (SDR), synthesized at different sintering temperature, was firstly demonstrated as anode materials for Na ion batteries. We focused on crystal structure, chemical composition, surface morphologies as well as electrochemical performance and Na⁺ storage mechanism of as-synthesized hard carbon. With sintering temperature increased, these samples exhibited a higher degree of graphitization and fewer defects, leading to significant improvements in the initial coulombic efficiency (ICE) and reversible capacity. The sample sintered at 1400 °C (SDR14) showed a high reversible capacity of 295 mAh g⁻¹ at a current density of 0.1 A g⁻¹ with an ICE of 84.3 %. The superior performance of SDR14 was attributed to the synergistic effects of moderate graphitization, suitable interlayer spacing, closed-pore structures, and optimized pore distribution, which collectively enhanced Na ⁺ intercalation and extended the plateau capacity. Galvanostatic intermittent titration technique (GITT) tests revealed that sintering temperature significantly influenced Na ⁺ storage behavior. The in-situ XRD measurements further confirmed the structural stability of SDR14 during charge and discharge processes. Condition-optimized SDR14 electrode exhibited a long cycle life up to 200 cycles at 0.2 A g⁻¹, and Na₃V₂(PO₄)₃|SDR14 full-cells demonstrated superior rate performance and high energy density of 197.7 Wh/(kg of active material only) at a power density of 21.1 W/kg, showing its great potential as a high-performance anode material for Na ion batteries.