Effect of <i>Melaleuca viminalis</i> Extract as a Natural Dopant on Ionic Transport, Dielectric Response, Optical Behavior, and Optoelectrical Performance of MC:PEO-LiNO ₃ Polymer Electrolytes

Solid polymer electrolytes based on methyl cellulose/polyethylene oxide (MC:PEO) blends are promising for electrochemical and optoelectronic applications; however, balancing ionic transport and optical performance remains challenging. This work investigates the effect of Melaleuca viminalis extract on the structural, electrical, dielectric, and optoelectronic properties of MC:PEO–LiNO3 polymer electrolytes. Films containing 15–25 wt.% LiNO3 and 0–30 wt.% extract were prepared by solution casting and characterized using XRD, FTIR, EIS, and UV–Vis spectroscopy. XRD confirmed semi-crystalline structures with crystallinity ranging from 28.13% to 41.33%, where 10 wt.% extract enhanced amorphization, while higher loading promoted partial recrystallization. FTIR analysis revealed intermolecular interactions involving O–H, C–O, and C=O groups and modifications in Li+ coordination. EIS results showed optimum conductivity at 10 wt.% extract, reaching 2.97 μS cm−1 (IKZ6) compared to 2.31 μS cm−1 for the undoped sample, whereas excessive extract increased bulk resistance up to 10,405 Ω and suppressed ion transport. Optical analysis demonstrated tunable bandgap behavior (4.95–2.90 eV), high transparency (88–99%), and ε′ > ε″, indicating stable dielectric characteristics. These findings demonstrate the concentration-dependent role of Melaleuca viminalis extract in tuning multifunctional polymer electrolyte properties.