Understanding the Importance of the Lignin–Biopolymer Ratio in Optimizing the Performance of Sustainable Biomass-Derived Electrospun Carbon Fiber Anodes in Sodium-Ion Batteries

This study highlights the critical role of the lignin-biopolymer ratio in optimizing the performance of sustainable biomass-derived carbon nanofibers (CNFs) as anode materials for sodium-ion batteries (SIBs). Lignin, a renewable polymer with high carbon content (>60%), was chosen for its abundance, low cost, and environmental benefits. However, its low molecular weight and irregular structure pose challenges for electrospinning. Unlike prior studies, we demonstrate the synthesis of electrospun lignin-derived CNFs without the requirement for petroleum-based polymer blends. Instead, poly(lactic acid) (PLA), a biobased polymer, is blended with lignin to improve processability, offering a sustainable and efficient pathway for electrode fabrication. We demonstrate that the composition of the lignin-PLA precursor blend fundamentally influences the electrospinning process, microstructure, and electrochemical performance of the resulting CNFs. A 50:50 lignin-PLA ratio was identified as optimal, minimizing phase separation during electrospinning and maximizing the surface area, as characterized by SEM and BET analyses. The optimized precursor blend produces CNFs with outstanding structural integrity and a 3D interconnected conductive network, enabling high-performance SIB anodes. The resulting CNFs exhibit excellent cycling stability, achieving a reversible capacity of 170 mAh g^-1 after 900 cycles with 86% capacity retention. Additionally, the electrodes demonstrate superior rate capability, attributed to their enhanced surface area and robust architecture. This work underscores the importance of tuning the lignin-biopolymer ratio in developing sustainable, high-performance materials for energy storage applications, paving the way for greener and more cost-effective Na-ion battery technologies.