Compositional Engineering of Lignocellulose via Selective Delignification toward Closed-Pore-Rich Hard Carbon for High-Plateau-Capacity Sodium Storage

Hard carbon derived from lignocellulosic biomass is a promising anode candidate for sodium-ion batteries due to its low cost and renewability. However, its practical application is limited by insufficient low-voltage plateau capacity, which is closely related to the closed-pore structure. Herein, we report a green and scalable compositional engineering strategy based on alkaline sulfite pretreatment, an industrially mature pulping process, to convert waste wood into closed-pore-rich hard carbon anodes. By selectively cleaving β -O-4 linkages in lignin, this pretreatment increases the relative cellulose content and fundamentally alters the carbonization pathway, yielding a highly disordered turbostratic structure. The optimized hard carbon (HHC-20) exhibits a doubled closed-pore volume (from 0.073 to 0.178 cm 3 g –1 ) and an expanded interlayer spacing (0.385 nm). Consequently, HHC-20 delivers a high reversible capacity of 347 mAh g –1 at 20 mA g –1, with an outstanding low-voltage plateau capacity of 205 mAh g –1 and an initial Coulombic efficiency of 89.1%. In situ spectroscopic characterizations reveal a sequential sodium storage mechanism involving surface adsorption, interlayer intercalation, and pore filling. This work offers a sustainable and commercially viable pathway for designing high-performance hard carbon anodes for sodium-ion batteries.