Hard carbon (HC) is a promising anode material for sodium-ion batteries (SIBs) owing to its high sodium storage capacity, low cost, and tailorable pore structure. Closed pores are a fundamental structural feature of hard carbon, and their density plays a critical role in determining the HC material's low-voltage plateau capacity. However, conventional approaches to construct closed pores often rely on complex processes, leading to high production costs of high-performance HC. In this study, a low-cost preoxidation strategy achieves bidirectional microstructure modification of HC derived from carboxymethyl chitosan (CS) and pitch. The high-temperature carbonization causes CS to generate a porous carbon framework, while pitch pyrolyzes into small molecules that act as an in situ carbon source to fill slit pores within the CS matrix. Notably, this single-step process simultaneously accomplishes pore formation and filling, expands the interlayer spacing, and reconfigures its open pores into closed pores. Consequently, the optimized HC exhibits significantly improved sodium storage performance. The reversible capacity increases from 90.0 to 352 mAh g-1, the initial Coulombic efficiency rises from 64.2% to 84%, and excellent rate capability and cycling stability are also achieved. In situ Raman spectroscopy confirms that the sodium storage mechanism of HC follows an "adsorption-intercalation/pore filling" mechanism. This work provides a novel self-activation and self-filling strategy to fabricate high-capacity hard carbon anodes with well-tailored closed pores without any activation reagents for sodium-ion batteries.
Sawut et al. (2026) studied this question.