Correlated barrier hopping dynamics of Na+ ions in poly(vinyl alcohol) biopolymer-based solid polymer electrolytes: Electrical and structural analysis

In recent years, the use of sodium-ion based solid polymer electrolyte (SPE) in energy storage applications has been attracting attention in research. However, SPEs are still suffering from lower ionic conductivity in room temperature, and sodium-ion based SPEs are not yet on par with lithium-ion based SPEs in terms of the overall performance. Therefore, understanding the conduction mechanism of a sodium-ion based SPE is crucial to design and optimize the performance of a sodium-ion based device. Herein, partially hydrolyzed PVA-based SPE was prepared with NaPF₆. From the power law region of the plotted AC conductivity graph, the frequency exponent value, s is calculated to be s < 1. The decreasing of s value when temperature increases also suggests that the SPE system follows the Correlated Barrier Hopping (CBH) model where ions conduct through correlated hopping between sites by overcoming barrier heights or potential wells, with conductivity being dependent on both temperature and frequency. The SPE with PVA:NaPF₆ ratio of 60:40 (PVA60) exhibits the highest room temperature ionic conductivity of 3.65 × 10⁻⁵ S/cm, and has the lowest calculated activation energy of 0.149 eV. FTIR spectra confirmed the complexation of PVA and NaPF₆ at certain functional groups, and deconvolution at the 800–900 cm^-1 region was done to find out the free-ions and ion-pairs percentage. The degree of crystallinity is also determined from the XRD data where PVA60 exhibits the lowest degree of crystallinity. The TGA and LSV results are also analyzed to provide an overall insight into the thermal and electrochemical stability of the samples.