Manipulating Surface Chemistry on the Microarchitecture of Coal‐Based Hard Carbon for Improved Sodium Storage

The aromatic nature of coal results in highly graphitized hard carbon (HC), which significantly impacts its sodium storage performance. Constructing oxygen-containing functional groups (OFGs) can effectively enhance sodium storage performance, but the mechanistic role of OFGs in governing the surface chemical evolution of coal-based HC remains poorly understood. Herein, OFGs are introduced into coal molecules through various pre-oxidation methods. Comprehensive in situ/ex situ testing elucidated that different OFGs have different effects on the intramolecular rearrangement of coal. Compared with C═O, -OH, and C─O─C groups, the carboxyl can inhibit decarboxylation during pyrolysis, raising the upper limit of the temperature window for intramolecular carbon rearrangement from 500 to 600 °C. This effect reduces intermolecular condensation efficiency during carbonization, thereby suppressing soft carbon formation. The strategy concurrently enlarges graphite-like interlayer spacing and creates closed pores, ultimately enhancing the sodium storage capacity of coal-based HC. The optimized HC shows enhanced capacity (308 mAh g^-1) with a 1.4 times increase in low-voltage plateau capacity compared to the unmodified HC. This work elucidates the structure-function relationship between specific OFGs and carbonization behavior, develops a practical strategy to modulate coal's molecular rearrangement via targeted surface chemistry, and contributes to achieving low-cost, high-performance HC in advanced SIBs.