APTES Functionalized Cellulose Nanofiber Gel‐Polymer Electrolytes for Lithium and Sodium‐Ion Batteries

ABSTRACT Cellulose nanofibers (CNFs) are emerging as sustainable polymer matrices for gel‐polymer electrolytes; however, their ion‐transport properties are limited by surface‐chemistry‐derived interactions. In this work, we demonstrate that a single, time‐controlled APTES functionalization step provides an effective and scalable route to enhance the ionic transport characteristics of CNF‐based gel–polymer electrolytes. The pristine and functionalized CNF membranes are imbibed with PF 6 − ‐based carbonate liquid electrolytes for both lithium (Li + )‐ and sodium (Na + ) systems. By optimizing the functionalization duration, the ionic conductivity increases from ≈0.07 to 0.60 mS cm −1 (Li + ) and from ≈0.1 to 2.02 mS cm −1 (Na + ), accompanied by an increased apparent cation transference number, while preserving the fibrous morphology, mechanical integrity, and thermal stability. In LiFePO 4 || Li half‐cells, the optimized A‐CNF electrolyte delivers stable rate capability and long‐term cycling beyond 1000 cycles, with reversible non‐monotonic capacity evolution associated with polarization relaxation, as supported by overpotential analysis and d Q /d V measurements. The A‐CNF electrolyte also enables efficient Na + transport in Na 3 V 2 (PO 4 ) 3 || Na half‐cells, showing minimal capacity loss (<5%) and narrow voltage hysteresis from 0.1C to 1C. These results establish single‐step APTES‐modified CNFs as a versatile and sustainable gel–polymer electrolyte platform for both lithium‐ and sodium‐ion batteries.