Reed-based hard carbon spheres with controlled interlayer spacing: Citric acid pretreatment strategy enhances the performance of sodium-ion battery anodes

The practical application of biomass-derived hard carbon faces dual challenges: widespread issues with insufficient electrochemical performance and stability, coupled with the high energy consumption and severe pollution associated with traditional synthesis methods. This study proposes a green strategy to prepare hard carbon anode materials with tunable interlayer spacing through citric acid-assisted hydrothermal pretreatment combined with low-temperature carbonization of reed. This process utilizes citric acid to catalyze the hydrolysis, aromatization, and self-assembly of hemicellulose and lignin, efficiently removing impurities while forming spherical carbon composite precursors supported by reed fibers. The resulting material, CA-R-900, exhibits moderate interlayer spacing, high graphitization, abundant defect structures, and a mesoporous-dominated pore system. These synergistic properties enhance sodium storage and transport efficiency. Electrochemical performance shows CA-R-900 achieves a reversible capacity of 390.4 mA h g⁻¹ at 0.1 A g⁻¹ with an initial coulombic efficiency of 77.8 %. After 200 cycles, capacity retention reaches 98.5 %. Even at 3.0 A g⁻¹ , it maintains a capacity of 269.4 mA h g⁻¹ , demonstrating outstanding rate performance and stability. Density functional theory calculations theoretically reveal the intrinsic relationship between interlayer spacing and adsorption energy. This study pioneers an efficient and environmentally friendly approach for preparing high-performance biomass-derived hard carbon anodes. • CA-R-900 was synthesized via a simple hydrothermal method combined with low-temperature carbonization. • The reagents used are environmentally friendly organic reagents free from contamination. • The process requires no acid or alkali soaking or rinsing, only deionized water washing. • The material achieves a reversible specific capacity of 390.4 mA h g −1 at 0.1 A g −1 and an ICE of 77.8 %.