Studies on flexible and highly stretchable sodium ion conducting blend polymer electrolytes with enhanced structural, thermal, optical, and electrochemical properties

Polymer blend electrolyte films based on polyvinyl alcohol (PVA)–polyethylene glycol (PEG) with dopant sodium nitrate (NaNO₃) salt were successfully synthesized by the standard solution casting technique. These blend electrolytes were characterized by XRD, DSC/TGA, FTIR, FE-SEM, UV–Vis, ionic conductivity, Raman spectroscopic study and electrochemical measurements to optimize their structural, thermal, optical, ionic transport and electrochemical properties. XRD and DSC studies showed that the crystalline phase of the PVA–PEG blend polymer matrix decreases significantly with the content of NaNO₃ salt, which favours highly flexible polymer backbone and hence providing high ionic conductivity of the polymer blend electrolyte films. The maximum ionic conductivity is found to be 1.53 × 10⁻⁵ S cm⁻¹ at room temperature (~ 30 °C) for the blend electrolyte film PB30. The ionic transference numbers of these polymeric blend electrolytes are estimated in the range of 0.94–0.97, which showed that ion conduction is purely ionic in nature. The Raman and FTIR spectroscopic analysis confirms the complexation of the cation of dopant salt (Na⁺) with the backbone of the blend polymer matrix via Lewis acid–base interactions. The UV–visible analysis showed that optical band gap (direct and indirect band gap), absorption edge, and refractive index of the pristine polymeric blend change significantly with the concentration of dopant NaNO₃ salt, and these effects are more noticeable at the higher loading. The electrochemical stability window of the blend polymer electrolytes is observed 3.9 V for the PB 30 films, which confirm their utility as a separator membrane in the high-performance flexible solid-state electrochemical devices.