Enhancing the electrochemical performance of semicoke-based hard carbon anode through oxidation-crosslinking strategy for low-cost sodium-ion batteries

Semicoke, a coal pyrolysis product, is a cost-effective and high-yield precursor for hard carbon used as anode in sodium-ion batteries (SIBs). However, as a thermoplastic precursor, semicoke inevitably graphitizes during high-temperature carbonization, so it is not easy to form the hard carbon structure. Herein, we propose an oxidation-crosslinking strategy to realize fusion-to-solid-state pyrolysis of semicoke. The semicoke is first preoxidized using a modified alkali-oxygen oxidation method to enrich its surface with carboxyl groups, which are localization points and the cross-linking reactions occur with citric acid to build the semicoke precursor with homogeneous and abundant -C-(O)–O- groups (up to 21 at% oxygen content). The -C-(O)–O- groups effectively prevent the rearrangement of carbon microcrystals in semicoke during carbonization, resulting in the formation of an abundant pseudographite structure with larger carbon interlayer spacing and micropores. The optimized semicoke-based hard carbon shows both a high initial Coulombic efficiency of 81% and a specific capacity of 307 mAh g⁻¹, with low-voltage plateau capacity increased to 2.5 times, compared to that of the unmodified semicoke carbon. By the combination of detailed discharge curves and in situ X-ray diffraction analysis, the plateau capacity of semicoke-based hard carbon is mainly derived from interlayer intercalation of Na⁺ ion. The proposed oxidation-crosslinking strategy can contribute to the usage of low-cost and high-performance hard carbons in advanced SIBs.