Comparative thermo-electrochemical study of lignin- and starch-derived carbon electrodes modified with Zn(TFSI)2 and ionic liquids for lithium-ion battery applications

In this study, a comprehensive comparison of carbon electrodes derived from kraft lignin (LK) and starch-modified via hydrothermal treatment and ionic liquids, including Zn(TFSI)₂-is presented. The samples include nitrogen‑carbonized lignin (LKN₂), hydrothermally modified lignin (LKN₂H), and lignin treated with [C₄im][HSO₄] (LK-IL 1) or [C₄C₁im][MeSO₃] (LK-IL 2), in addition to hydrothermally modified starch (starchN₂H). Electrochemical impedance spectroscopy (EIS), Bode phase analysis, and temperature-dependent thermodynamic modelling reveal how precursor origin and surface chemistry influence lithium diffusion, activation energy, and capacitive performance. Among the tested materials, LKN₂H demonstrates the most balanced performance, combining low activation energy with stable diffusion across various temperature ranges. Our findings confirm that Zn(TFSI)₂ treatment introduces functional groups and Zn²⁺ species that significantly enhance lithium-ion transport, particularly in lignin-derived carbons. This strategy, which is rarely applied in biomass-based systems, offers a promising route to improve electrochemical performance through surface-level ionic engineering, complementing precursor-driven structural design. The effects of ionic liquids and precursor type are critically evaluated and contextualized with respect to existing literature.