Pre-oxidation, activation, and carbonization enhance the interlayer spacing and closed pores in coal-based carbon materials for improved sodium storage

Coal is an ideal precursor for sodium-ion battery anodes owing to its low cost and high carbon yield. Low-rank coal dominates China's coal reserves, but its derived hard carbon suffers from poor sodium-ion storage performance, rooted in a lack of precise regulation of microcrystalline and closed-pore structures. Herein, a novel three-step strategy (low-temperature pre-oxidation, medium-temperature NaOH activation, and high-temperature carbonization) is developed to synergistically optimize the interlayer spacing and closed-pore structure of coal-based carbon. Pre-oxidation introduces cross-linked structures, NaOH activation constructs open pores, and subsequent carbonization yields a carbon material with an interlayer spacing of 0.391 nm, structural disorder (A_D/A_G = 2.29), and closed-pore volume of 0.126 cm³ g⁻¹. By optimizing key parameters, the as-prepared anode delivers a high sodium-ion storage capacity of 312.89 mAh g⁻¹ at 30 mA g⁻¹ with an initial coulombic efficiency of 82.47%, outperforming most reported high-rank coal-derived counterparts. It also retains 67.18% capacity after 700 cycles at 150 mA g⁻¹, demonstrating good cycling stability. This work provides a feasible pathway for the high-value utilization of low-rank coal as high-performance carbon anodes.