Tailoring the Sodium-Ion Storage Performance of Lignin-Based Ultrafine Carbon Fibers through Controlled Thermal Treatment

The preoxidation and carbonization processes play a critical role in determining the performance of hardwood Kraft lignin-derived carbon fibers. Elucidating the relationship between heat-treatment parameters and the resulting material characteristics remains a key objective in ongoing research. In this study, HKL-based ultrafine carbon fibers (HKL-CFs) were fabricated through electrospinning, followed by preoxidation and carbonization, and subsequently evaluated as anode materials for sodium-ion batteries. The structural evolution and thermal behavior of HKL fibers (HKLs) under various preoxidation conditions were systematically investigated. Additionally, the influence of carbonization temperature on the microstructure and sodium ion storage performance of HKL-CFs was examined, alongside the associated sodium ion storage mechanism. The results indicate that a slow heating rate to the target preoxidation temperature is more favorable for the stabilization of HKLs compared to isothermal holding processes. Although high-temperature carbonization facilitates the development of graphitic microcrystalline structures, it concurrently introduces additional defects. Among the samples, HKL-1000-CF, which exhibits an optimal balance between graphitization degree and defect density, delivered the best electrochemical performance, with an initial discharge specific capacity of 356.5 mAh g–1, as well as exceptional rate capability and cycling stability. This work offers valuable insights into the stabilization of HKLs and elucidates the process-structure–property relationships in HKL-CFs, thereby offering a theoretical basis for further optimization.