Free-Standing Lignin-Derived Hard Carbon Anodes for Sustainable Potassium-Ion Batteries

<jats:p>Potassium-ion batteries (KIBs) require anode materials that combine high reversible capacity with minimal first-cycle losses associated with solid electrolyte interphase (SEI) formation. Biomass-derived hard carbons (HCs) are promising candidates; although their K-storage behavior is strongly influenced by carbon structure, surface chemistry, and pore architecture. Here, industrial LignoBoost lignin was converted into binder-free, free-standing HC nanofiber mats by electrospinning followed by carbonization at 800-1400 C (HC8-HC14), enabling evaluation of their intrinsic K-storage behavior without inactive binder or conductive additives. Temperature-dependent structural and chemical evolution was characterized using Raman spectroscopy and X-ray photoelectron spectroscopy and correlated with specific surface area (331–1328 m2g-1), electrical conductivity (2.4–14 S cm-1) and electrochemical performance. All HC mats exhibited the characteristic sloping and low-voltage plateau regions of K storage. Increasing the carbonization temperature progressively decreased the sloping capacity while enhancing plateau contribution. HC14 exhibited the best overall performance, delivering a reversible capacity of 369 mAh g-1 with an initial coulombic efficiency of 69%. This improvement is associated with increased sp2-rich conjugation and development of nanocrystalline aromatic domains and together with reduced oxygen-containing functionalities and residual Na/S species, which mitigate irreversible interfacial reactions and promote reversible K storage. These findings establish direct relationships between carbonization-induced structural and surface-chemical evolution and K-storage behavior, demonstrating lignin-derived, free-standing HC nanofiber mats as sustainable, binder-free anodes for KIBs.</jats:p>