Cross-linked coal-derived hard carbon with expanded interlayers and nanopores for high-plateau sodium storage

Coal is a low-cost, carbon-rich feedstock for preparing hard carbon (HC) anodes, making coal-derived carbon attractive for sodium-ion battery applications. However, direct carbonization of coal usually involves softening, aromatization, and molecular rearrangement, which promote the ordered growth of graphitic microcrystals with limited interlayer spacing and insufficient nanopores, thereby restricting sodium-storage capacity. To address this issue, we introduce a chitosan/glutaraldehyde (CS/GA)-assisted molecular-network regulation strategy to tailor both the carbon framework and surface chemistry of coal-derived HC. CS is first immobilized on oxidized coal (OC) through hydrogen bonding and electrostatic interactions and then cross-linked with GA via a Schiff-base reaction, forming a molecular network that suppresses the ordered growth of graphitic microcrystals during carbonization. Meanwhile, chitosan serves as an endogenous nitrogen source, enabling in situ nitrogen doping. The optimized SGHC-8 (CS/GA-cross-linked coal-derived HC) exhibits expanded interlayer spacing, increased defect density, and abundant nanopores, which contribute to enhanced sodium storage, especially in the low-voltage plateau region. As an anode for sodium-ion batteries, SGHC-8 delivers a reversible capacity of 350.5 mAh g-1, an initial coulombic efficiency of 80.6%, and a plateau capacity contribution of 48.5%. Combined kinetic and structural evidence reveals that Na+ storage in SGHC-8 proceeds through sequential adsorption, interlayer insertion, and nanopore filling. This work provides a feasible strategy for converting coal into high-performance HC anodes through cross-linking-induced microstructure regulation.