Hydrothermal pretreatment enables disordered hard carbon with enlarged interlayer spacing and abundant closed pores for high-performance sodium storage

Conventional biomass-derived hard carbon anodes suffer from poor structural controllability during direct carbonization, mainly manifested in irregular morphology, ordered stacking of carbon layers, and a lack of closed pore structures, which severely limit sodium-ion diffusion kinetics and cycling stability. Here, using coconut shells as the raw material, a preparation strategy involving hydrothermal pretreatment coupled with carbonization is proposed. The obtained hard carbon exhibits a regular spherical morphology, highly disordered carbon layer structure, expanded interlayer spacing, and abundant closed pores. Electrochemical tests show that it delivers a reversible capacity of 328.8 mAh/g at 50 mA/g, maintains 200.5 mAh/g at a high current density of 1 A/g, and achieves a capacity retention as high as 93.7 % after 100 cycles. Mechanistic studies reveal that hydrothermal pretreatment, through selective removal of hemicellulose and component reconstruction, suppresses graphitic ordered stacking, promotes the formation of defects and closed pores, and modifies surface functional groups, thereby reducing the interfacial charge transfer resistance. This work provides a systematic theoretical basis for tailoring the microstructure and sodium storage performance of biomass-derived hard carbon via hydrothermal pretreatment, and opens a new pathway for designing high-performance, long-cycle-life anode materials for sodium-ion batteries.