Boosting sodium storage performance of hard carbons by regulating oxygen functionalities of the cross-linked asphalt precursor

Pre-oxidation treatment is a key process to produce carbon anodes for sodium ion batteries from pitch because it governs the orientation of carbon layers during subsequent carbonization. However, the relationship between the oxygen functionalities in the oxidized pitch and the evolution of carbon structure during carbonization remains unclear. Herein, cross-linked asphalts with differentiated oxygen functionalities such as C-O and C(O)-O (ester group and anhydride), are synthesized to explore their effects on the evolution of carbon structure. The planar cross-linked structures constructed by thermally stable C-O fail to prevent the sliding of carbon layer, leading the formation of short-range order carbon structures with lager lateral size. In contrast, the three-dimensional cross -linked structures constructed by C(O)-O effectively prevent the rearrangement of asphalt at early pyrolysis. Owing to the gas generated by C(O)-O removal during subsequent carbonization, the as-formed more disorder carbon structures with small microcrystalline sizes can generate more closed pores and ultra-micropores, which enhances the low-voltage plateau capacity from 96.06 to 132.2 mAh g-1. The insights for the differentiated oxygen functionalities in regulating the carbon structural conversion pave the way for further development of pitch-based hard carbons with higher performance and their practical applications in SIBs.