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

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°C–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 m 2 g −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 sp 2 ‐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.