Structure–Transport Correlation in Glycerol-Plasticized CS/PVP–NaI/Al 2 O 3 Nanocomposite Polymer Electrolytes

In this study, flexible nanocomposite polymer electrolyte films based on chitosan (CS)/polyvinylpyrrolidone (PVP), NaI, and Al2O3 nanoparticles were fabricated by solution casting, with glycerol concentration systematically varied from 9 to 40 wt%. XRD, FTIR, and EIS were employed to establish the relationship between glycerol-induced structural modification and sodium-ion transport. XRD analysis revealed a progressive decrease in crystallinity from 28.63% for lowest glycerol concentration (IKAE1) to 24.62% for highest glycerol content (IKAE5), indicating enhanced amorphization with increasing glycerol content. Concurrent FTIR changes evidenced molecular reorganization involving polymer–plasticizer, polymer–salt, and polymer–filler interactions. EIS measurements demonstrated a pronounced enhancement in DC ionic conductivity, increasing from 0.054 µS cm−1 for IKAE1 to 28.2 µS cm−1 for IKAE5, corresponding to an approximately 523-fold increase. The highest-conductivity investigated composition, IKAE5, also exhibited a substantially shorter relaxation time, decreasing from 1514.7 to 31.836 µs, while the effective charge-carrier density increased from 7.88 × 1015 to 1.99 × 1018 cm−3. These results indicate that glycerol plasticization promotes polymer-chain flexibility, amorphous-phase development, and enhanced NaI dissociation, while the fixed Al2O3 content provides possible polymer–filler interaction sites. The resulting structure–transport relationship demonstrates the potential of the CS/PVP–NaI/Al2O3–glycerol system as a promising platform for sodium-ion conducting solid polymer electrolytes.