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.