Moisture-engineered reed-derived hard carbon for sodium-ion batteries

With the advantages of abundant sodium resources and reduced material costs, sodium-ion batteries (SIBs) are regarded as viable candidates for next-generation large-scale energy storage systems. Despite these merits, the development of advanced anode materials with high capacity, long-term stability, and excellent rate capability remains a key bottleneck for their practical application. Hard carbon is regarded as the most viable anode material for SIBs, while its sodium-storage capability is largely determined by precursor-derived microstructures, particularly closed pores associated with the low-voltage plateau capacity. Herein, a precursor moisture-regulation strategy is developed to tailor the microstructure of biomass-derived hard carbon. Using reed as a precursor, we demonstrate that appropriate moisture content promotes the formation of closed pores during carbonization, resulting in enhanced plateau contribution and reversible capacity. The optimized hard carbon combines enlarged interlayer spacing, abundant defects, and hierarchical porosity, affording a reversible capacity of 312.5 mAh g-1 at 0.1 A g-1 with accelerated Na+ transport kinetics. Furthermore, the assembled R-HC-20//NVP full cell delivers an energy density of 334.5 Wh kg-1. Mechanistic analysis reveals that residual water regulates carbon structural evolution through precursor swelling and vapor-mediated mild etching, which facilitate oxygen-functional-group exposure, disordered carbon stacking, and initial pore formation. In contrast, excessive structural ordering in dried precursors suppresses closed-pore development and Na+ storage. This work identifies precursor moisture content as a critical parameter for controlling the microstructure and sodium-storage properties of biomass-derived hard carbon, offering a simple and effective strategy for designing advanced SIB anodes.