Boosting Closed Pores and Nitrogen Content in Hard Carbon via a Pre-Oxidation Approach for Fast-Charging Sodium-Ion Batteries

Abstract

Hard carbon has been widely recognized as a practical anode candidate for sodium-ion batteries because of its low cost, appropriate working potential, and structural stability. Nevertheless, its practical application remains limited by insufficient reversible capacity and poor high-rate performance. In this work, a nitrogen-enriched hard carbon with tuned structural order is synthesized from xylose via a simple air pre-oxidation step prior to carbonization, demonstrating fast-charging sodium-ion batteries. This pre-oxidation process introduces oxygen-containing groups in the precursor, enhances intermolecular cross-linking, and suppresses excessive structural fusion during pyrolysis, resulting in an optimized disordered/graphitic balance. Notably, this treatment improves nitrogen retention and modulates pore evolution, yielding enlarged interlayer spacing, smaller microcrystalline domains, and abundant closed ultramicropores, which are highly favorable for sodium storage in the plateau region. Benefiting from the synergistic effects of structural regulation and heteroatom doping, the optimized hard carbon electrode delivers a high reversible capacity of 397.4 mAh g⁻¹ and excellent rate capability of 148.5 mAh g⁻¹ at 5 A g⁻¹. This work provides a straightforward and cost-effective strategy for engineering high-performance biomass-derived hard carbon anodes for advanced sodium-ion batteries.