New insights into the effects of impurities in coal on the sodium ion storage performances of coal-derived hard carbon

Hard carbons (HCs) are one of the most promising anode materials for sodium-ion batteries. The presence of impurities in coal is a critical factor in determining the electrochemical performance of coal-derived HCs. However, the effect of different components varies during pyrolysis, and the mechanism of influence on the sodium storage performance of HCs is not clear. Herein, the effects of two representative impurities in coal on the changes in microstructure during pyrolysis and the sodium storage performance of HCs are systematically investigated, using refined pitch as a template. It is found that Al₂O₃ can promote the removal of hydroxyl, alkyl groups and the generation of aromatic ether bonds during pyrolysis, thus reducing the steric hindrance effect and delaying the removal of oxygen-containing functional groups. Therefore, the addition of Al₂O₃ promotes the formation of short-range ordered microcrystals, which is conducive to the transport of Na⁺. The resulting HCs (POPA-1300) exhibits significantly improved sodium storage performance. Meanwhile, the presence of kaolinite promotes the formation of graphite-like regions, which adversely affects the sodium storage performance of HCs. This work provides theoretical guidance for the selective removal of impurities in coal and the use of residual impurities to modulate the microstructure of HCs.