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

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 PF₆ ^--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 LiFePO₄ || 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 dQ/dV measurements. The A-CNF electrolyte also enables efficient Na⁺ transport in Na₃V₂(PO₄)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.