Synergistic Ultramicropore-Confined and Electronic-State Modulation Strategies in Sustainable Lignin-Derived Hard Carbon for Robust Sodium-Ion Batteries

The performance of hard carbon anodes in sodium-ion batteries is restricted by competing mechanisms: excessive surfaces cause irreversible reactions lowering the initial coulombic efficiency, while insufficient active sites limit capacity. To mitigate this trade-off effect, a synergistic strategy of ultramicropore confinement and electronic-state modulation in lignin-derived hard carbon was created. Using sodium lignosulfonate, a common sulfonated polymer in paper-making waste, we developed N/S-codoped hard carbon microspheres (N-S@HDM) via preoxidation-induced cross-linking and optimized pyrolysis. Preoxidation inhibits graphitic alignment, creating an expanded interlayer spacing and a closed-pore-dominated structure (94.27% at 1,300 °C). This interconnected network (<i>f</i> _a = 0.85) enables ultramicropore confinement, thus shortening diffusion paths, boosting kinetics, and providing ample Na⁺ storage sites to reduce interfacial decomposition. Concurrent N/S doping optimizes electronic states by enhancing electron delocalization, lowers charge-transfer resistance, and generates high-density adsorption sites. The optimized N-S@HDM-1300 achieves an ultrahigh initial coulombic efficiency of 90.6% and a reversible capacity of 401.5 mAh g^-1 (0.03 A g^-1), with exceptional cyclability (95.0% retention after 500 cycles). This study pioneers a dual-regulation paradigm for biomass-derived carbon materials, coupling pore engineering and electronic optimization to advance sodium-ion battery anode design.