Synergistic Cross-Linking Strategy for Closed-Pore Structure Construction of Nitrogen-Doped Hard Carbon for Enhanced Sodium Storage

Abstract The sodium storage performance of hard carbon anodes is dependent on the precise modulation of their closed-pore architecture and surface chemistry. Herein, we propose a synergistic cross-linking strategy to synthesize nitrogen-doped hard carbon enriched with uniform closed-pore structure using polyvinyl chloride (PVC) and cellulose. During the solvothermal stage, urea not only serves as a nitrogen source but also facilitates the construction of a cross-linking network alongside ZnCl2-catalyzed dechlorination and depolymerization. Upon carbonization, the robust framework confines the in situ pore formation to a nanoscale domain with average diameter of ∼1.26 nm and preserves an expanded interlayer spacing of 0.401 nm. The optimized PZNC anode delivers a high reversible capacity of 359 mAh g–1 at 0.2 C and excellent cycling stability for 99% retention after 400 cycles at 1 C, maintaining 209 mAh g–1 after 1200 cycles at 5 C. The expanded interlayer spacing reduces Na+ diffusion resistance and the closed-pore structure enhance the low-voltage plateau capacity, enabling an efficient three-stage Na+ storage mechanism. This work provides a new pathway for designing advanced hard carbon anodes through integrating precise closed-pore engineering with chemical doping.