Multieffect Preoxidation Strategy to Convert Bituminous Coal into Hard Carbon for Enhancing Sodium Storage Performance

Preoxidation is an effective strategy to inhibit the graphitization of coals during carbonization. However, the single effect of the traditional preoxidation strategy could barely increase surface-active sites, hindering further enhancement of sodium storage. Herein, a multieffect preoxidation strategy was proposed to suppress structural rearrangement and create abundant surface-active sites. Mg(NO₃)₂·6H₂O helps to introduce oxygen-containing functional groups into bituminous coal at 450 °C, which acted as a cross-linking agent to inhibit the rearrangement of carbon layers and promote structural cross-linking during the subsequent thermal carbonization process. Besides, the residue solid decomposition product MgO would react with carbon to create surface-active sites. The obtained coal-based hard carbon contained more pseudographitic domains and sodium storage active sites. The optimized sample could deliver an excellent capacity of 287.1 mAh g^-1 at 20 mA g^-1, as well as remarkable cycling stability of capacity retention of 96.1% after 200 cycles at 50 mA g^-1, and notable capacity retention of 88.9% after 1000 cycles at 300 mA g^-1. This work provides an effective and practical strategy to convert low-cost bituminous coal into advanced hard carbon anodes for sodium-ion batteries (SIBs).