Tunable crystallinity in cellulose nanofiber-polyethylene oxide electrolyte films for stable quasi-solid-state lithium-metal batteries

Solid-state lithium-metal batteries are promising next-generation energy storage systems owing to their high energy density and enhanced safety. However, conventional polyethylene oxide (PEO)-based solid polymer electrolytes have low ionic conductivity at room temperature, poor Li+ transference numbers, and unstable electrode interfaces, which hinder their practical deployment. Herein, we report quasi-solid polymer electrolyte (quasi-SPE) films prepared by blending cellulose nanofiber (CNF) with PEO in various ratios (0.50:0-0.50:2.0), incorporating 1 M Lithium bis(trifluoromethane)sulfonimide (LiTFSI) in 1,3-Dioxolane (DOL):1,2-Dimethoxyethane (DME) (1:1 vol%) with 1 wt.% LiNO3. Structural analyses [X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier-transform infrared (FT-IR) spectroscopy, and Raman spectroscopy] revealed that moderate PEO incorporation disrupted CNF crystallinity and generated amorphous domains that enhanced polymer chain mobility and Li-ion transport, whereas excessive PEO promoted recrystallization. Within the composition range of 0.50-1.50, ionic conductivity reached ~10-4 S·cm-1 at room temperature. Notably, CP-1.00 emerged as the balanced formulation for lithium-metal battery applications, achieving a Li+ transference number of 0.46 and a wide electrochemical stability window of 4.68 V vs. Li+/Li. Symmetric Li||CP-1.00||Li cells exhibited exceptional interfacial stability, sustaining plating/stripping of around 10,000 h at 0.1 mA·cm-2 with a low overpotential of 34.0 mV. In Li||CP-1.00||Lithium iron phosphate (LFP) full cells, CP-1.00 delivered 169.2 mAh·g-1 at 0.1 A·g-1 with 91.46% capacity retention after 1,500 cycles. These findings establish CNF/PEO quasi-SPEs as scalable, high-performance electrolytes that overcome the limitations of conventional solid polymer electrolytes, thereby offering a promising pathway toward safe and durable lithium-metal batteries.