<p>In this study, a solid polymer electrolyte (SPE) was synthesized using the solution casting method. Lithium nitrate (LiNO₃) as the ion source and glycerol as a plasticizer were added in varying concentrations (9, 18, 27, and 36% by weight) to methylcellulose (MC) as the host polymer. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and electrical impedance spectroscopy (EIS) were used to investigate the effect of glycerol on the morphology, chemical structure, and ionic conductivity of the polymer electrolytes. According to the XRD results, the addition of glycerol, acting as a plasticizer, reduced the hump characteristic of the amorphous structure. Furthermore, glycerol enhanced ionic conductivity by increasing polymer chain mobility, improving ion dissociation, and potentially creating transient pathways for ion transport. FTIR spectroscopy confirmed the interaction between the polymer and dissolved salt, indicating the formation of polymer-salt complexes and showing an increase in the intensity of peaks associated with the hydroxyl (–OH) group as the glycerol concentration increased. EIS analysis demonstrated that DC conductivity rose from 2.9 µS/cm to 7.28 µS/cm with increasing glycerol content. Additionally, the frequency-dependent dielectric parameters, including dielectric constant (ε′) and dielectric loss (ε″), showed higher values at low frequencies, with both increasing as glycerol concentration increased, indicating that glycerol enhances ion conductivity and polarization in the polymer electrolyte. The electrical modulus analysis revealed that polarization relaxation decreased with higher glycerol concentrations, while conductivity increased at high frequencies. Glycerol significantly enhances the flexibility, amorphous nature, and ion mobility of MC-based polymer electrolytes, making them suitable for advanced applications in electrochemical devices.</p>