Fabrication of pitch-derived hard carbon via bromination-assisted pyrolysis strategy for sodium-ion batteries

Pitch-derived hard carbon materials are considered one of the most promising anodes for sodium-ion batteries (SIBs) due to their low cost and high carbon yields. However, they are largely limited by inferior specific capacity and rate capability, resulting from the easy graphitization of the pitch precursor during high-temperature carbonization. Herein, a simple bromination modification coupled with a pyrolysis strategy was proposed to fabricate pitch-derived hard carbon anodes (BHC-x), aiming to engineer their microstructure and optimize the electrochemical performance of SIBs. The detailed experimental investigations demonstrated that the brominated pitch precursor obtained via hydrothermal treatment underwent cross-linking polycondensation in the presence of bromine species at 350 °C, thereby forming hard carbon with a unique disordered and closed structure during the high-temperature pyrolysis process. As a result, the optimized BHC-1500, with rich defect sites and a suitable interlayer spacing of 0.358 nm, exhibited a high reversible capacity of 250.0 mA h g^-1 at 0.1 A g^-1 with an initial coulombic efficiency (ICE) of 79.8%. It still maintained 150.0 mA h g^-1 at 5 A g^-1, outperforming unbrominated counterparts (152.1 mA h g^-1 at 0.1 A g^-1). This work proposes new insights into the optimization of pitch-based hard carbon anodes for high-performance SIBs.