Intrinsic pre-sodiation of lignin-based ultrafine carbon fibers for sodium-ion battery anode.

Hard carbon-based sodium-ion batteries (SIBs) anodes still face a significant challenge that the oxygen-containing functional groups and inherent defects result in an irreversible natrization during the initial charging process. The pre-sodiation strategy is designed to compensate for the consumption of sodium ions, typically achieved by adding sodium ion supplements. In this study, we propose an intrinsic pre-sodiation strategy based on hardwood kraft lignin (HKL), which is extracted using sodium hydroxide (NaOH) and sodium sulfide. Self-supported HKL ultrafine carbon fiber anodes with varying sodium contents (HKL-NaX-CFs) were prepared successfully through electrospinning, stabilization, and carbonization. The effects of NaOH on HKL under the dual influences of pre-sodiation and activation was investigated systematically, including the interaction between NaOH and HKL, the thermal properties of HKL, the morphology, microcrystalline structure and electrochemical properties of HKL-based ultrafine carbon fibers (HKL-CFs). Results indicate that pre-sodiation strategy increased the layer spacing of graphite lamella, pore volume, and diffusion-controlled capacity of HKL-CFs, leading to better capacity and rate performance. Notably, the HKL-Na2-CF anode exhibited a capacity of 231.7 mAh g at 30 mA g and 68.3 mAh g at 2000 mA g. The intrinsic pre-sodiation strategy offers an inspiration for the supplement of sodium ions.