<p>This study systematically investigates the role of glycerol concentration as a plasticizer in controlling structure–ion transport relationships in methylcellulose (MC)–dextran–sorbitol nanocomposite polymer electrolyte (NPE) films doped with lithium nitrate (LiNO₃, 38 wt%) and reinforced with titanium dioxide (TiO₂, 3 wt%). The primary objective is to quantify how glycerol-induced amorphization governs ionic conductivity and transport parameters in a fully solid biopolymer electrolyte system. The films (MC 50 wt%, dextran 30 wt%, sorbitol 20 wt%) were prepared by solution casting with glycerol content varied from 8 to 40 wt%. X-ray diffraction revealed a reduction in crystallinity from 29.21 to 17.73%, indicating enhanced amorphous structure and chain flexibility. FTIR confirmed stronger hydrogen bonding, O–H peak broadening, and effective Li⁺ coordination. Electrochemical impedance spectroscopy showed a sharp drop in bulk resistance from 2170 to 118 Ω and a &gt; 23-fold increase in DC conductivity from 1.364 to 31.831 μS cm⁻¹. Ion transport parameters also improved: relaxation time decreased (11.25 → 2.81 µs), diffusion coefficient rose (6.71 × 10^–10 → 9.59 × 10^–8 cm²s⁻¹), mobility increased (2.57 × 10^–8 → 3.67 × 10⁻6 cm²V⁻¹s⁻¹), and carrier density reached 5.41 × 10²¹ cm⁻³. By correlating glycerol content with crystallinity suppression and transport enhancement, this work defines a clear composition–property framework demonstrating glycerol’s role in enhancing amorphization, ion dissociation, and segmental motion, producing a highly conductive biopolymer electrolyte that shows potential for solid-state energy storage applications and warrants further electrochemical evaluation.</p>