Engineering Closed-Pore Hard Carbon Anodes through Biomass Molecular Grafting to Enhance Plateau Capacity in Advanced Sodium-Ion Batteries

Biomass hard carbon (BHC) anodes with high low-voltage plateau capacity (LPC) represent promising anode materials for sodium-ion batteries (SIBs). However, the achievement of a high LPC is generally closely associated with the closed-pore filling mechanism. Effectively regulating the closed-pore structure to enhance the LPC remains a significant challenge. Herein, a strategy for grafting cellulose with chitosan is presented to synthesize BHCs with excellent closed-pore structures after high-temperature treatment. The grafting process enables intercalation of chitosan molecules between the cellulose chains, thereby disrupting the crystalline structure and ultimately facilitating cross-linking and structural rearrangement during pyrolysis and polycondensation. Consequently, the resulting hard carbons exhibit suppressed graphite-like phases and a large population of closed-pore architecture. The GCHC1-2 anode exhibits a closed-pore volume of 0.26 cm³ g^-1, a LPC of 247.53 mAh g^-1, and an initial Coulombic efficiency (ICE) of 87.37%. Moreover, the GCHC1-2 material also shows remarkable cycling stability, maintaining a specific capacity of 257 mAh g^-1 after 300 cycles at a current density of 300 mA g^-1, corresponding to a capacity retention of 78.07%. Comprehensive kinetic analyses further confirm that the superior electrochemical performance of GCHC1-2 is primarily attributed to the Na⁺ storage mechanism facilitated by its closed-pore structure. This study demonstrates that the molecular-scale engineering of biomass precursors enables the rational design of high-performance hard carbon anodes for SIBs.