Facile One‐Step Modification of Biomass Hard Carbon: Tailoring Hierarchical Microstructures for High‐Energy‐Density Sodium‐Ion Battery Anode Materials

ABSTRACT Biomass‐derived hard carbon is a leading anode candidate for sodium‐ion batteries. However, molecular‐level manipulation of carbonized precursors for further performance enhancement remains challenging. Here, we depart from conventional passive modification strategies and introduce a radical‐mediated structure‐guided synthesis approach. By leveraging polyethylene glycol as an in situ, molecular‐weight‐tunable radical source, we demonstrate that thermally generated exogenous radicals actively graft onto the developing carbon framework during pyrolysis. This “molecular surgery” enables precise, one‐step control over both graphitic microcrystal growth and closed pore architecture. Crucially, we establish a direct correlation between polyethylene glycol molecular weight, the speciation/concentration of in situ generated radicals, and the resulting sodium storage performance. Aliphatic/aromatic radicals promote carbon layer extension (La increases from 11.2 to 12.4 nm), while oxygen‐containing radicals facilitate pore network reconstruction via gas evolution. The optimized hard carbon delivers a high reversible capacity of 356 mAh g − 1 with a plateau capacity of 245.3 mAh g − 1 and an initial Coulombic efficiency of 90.11%. A full cell paired with a Na 3 V 2 (PO 4 ) 3 cathode achieves an energy density exceeding 265 Wh kg − 1 . This universally applicable, one‐step strategy provides a new design principle for rational engineering of high‐performance hard carbon anodes.