The current research investigates the electrochemical performance of plasticized nanocomposite solid polymer electrolytes derived from a polyethylene oxide (PEO)–polymethyl methacrylate (PMMA) blended system with lithium iodide (LiI) as the dopant salt and tin dioxide (SnO₂) nanoparticles as the inorganic nanofillers. Thin nanofilms of the synthesized electrolytes were prepared and progressively examined by using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), ultraviolet visible (UV-Vis) spectroscopy, and scanning electron microscopy (SEM). XRD characterization confirmed the successful establishment of the polymer electrolyte matrix and reflected a significant decrease in crystallinity upon the incorporation of nanofillers whereas crystallite size was estimated using the Debye–Scherrer equation. FT-IR spectra showed prominent molecular interactions and complexation of polymer, salt, and nanofiller components. UV–Vis spectroscopy depicts information on the optical absorption behaviour, whereas SEM micrographs shows the morphological features and homogeneity of plasticized nanocomposite solid polymer electrolytes films. The addition of SnO₂ nanofillers was shown to improve both the structural and electrochemical properties of the electrolyte system, highlighting its potential usage in solid-state batteries and other high-end electrochemical devices. These enhancements make the developed nanocomposite solid polymer electrolytes viable candidates for high-performance, flexible lithium-ion battery applications, offering a promising route toward safer and more efficient energy storage systems.