Flash carbonization of pre-oxidized coal pitch into hard carbon enables defect and closed-pore engineering for superior sodium storage capacity

Coal tar pitch, a low-cost industrial byproduct with high carbon yield, is a superior precursor for hard carbon. Pre-oxidation and carbonization are two core procedures for pitch-derived hard carbon, yet most previous works focus on pre-oxidation while neglecting carbonization optimization. Conventional slow carbonization using a muffle furnace lacks flexibility in tailoring interlayer spacing, intrinsic defects, and closed pores, thus limiting the improvement of electrochemical performance of hard carbon. Herein, Joule heating flash carbonization is employed to treat pre-oxidized coal tar pitch at 1300 °C. This strategy effectively immobilizes pyrolytic free radicals and promotes ester group formation, meanwhile increasing structural defects, expanding the carbon interlayer spacing, enlarging the closed pore size (from 1.69 to 1.97 nm) and increasing the closed pore volume (from 0.037 to 0.122 cm 3 g −1 ). These structural advantages collectively accelerate Na + diffusion, enrich active adsorption sites and boost pore-filling capacity. The increased surface activity also induces more electrolyte decomposition and forms a thicker solid electrolyte interphase (SEI), leading to a slight decline in initial Coulombic efficiency (ICE). The resultant HC@F delivers a reversible specific capacity of 328.4 mAh g −1 at 0.1 C with an ICE of 83.9%, and an excellent rate performance of 245.4 mAh g −1 at 4 C, while maintaining good capacity retention of 84.3% over 2500 cycles. Multiple in-situ and ex-situ characterizations verify the “adsorption-intercalation-pore filling” sodium storage mechanism. This work reveals that flash carbonization regulates pitch-based hard carbon, offering a facile thermal strategy for high-performance, low-cost anodes in practical sodium-ion batteries.