Nitrogen functionalization of natural hydroxyl cellulose induces a LiF-rich interphase for lithium metal batteries

Cellulose, the most abundant and renewable biopolymer, offers a sustainable and cost-effective solution for regulating lithium electrodeposition toward safer lithium metal batteries, thanks to its high nanofibrous structure and intrinsic lithiophilic property. In this work, we introduce interface-engineered cellulose-based separators by converting intrinsic hydroxyl groups on cellulose nanofibers (CNFs) to nitrogen functionalities through a trace conducting polymer coating. Both experimental and theoretical results reveal that the nitrogen moieties disrupt the compact hydrogen bond network within hydroxyl cellulose, enabling multiple nitrogen-lithium interactions that enhance lithium ion transport. In addition to an extraordinary Li⁺ transference number of 0.86 and a high ionic conductivity of 1.1 mS cm⁻¹, the nitrogen-functionalized CNF contributes to a uniform electric field and Li⁺ concentration distribution across the lithium metal surface. This facilitates the formation of a LiF-rich solid electrolyte interface and suppresses Li dendrite growth. Consequently, Li||Li cells demonstrate stable plating/stripping cycles for approximately 3000 h at a current density of 1 mA cm⁻² with a fixed capacity of 1 mAh cm⁻², while maintaining a low overpotential of 15 mV. Our work provides valuable insights into the surface functionalization of natural biomass for advancing sustainable energy storage technologies.