Influence of Sodium Tungstate on Dielectric and Electrochemical Properties of PVA/NaCMC Polymer Nanocomposites for Energy Storage Applications

The electrolyte is an essential element of modern energy storage systems, guiding ion migration between electrodes. Sodium carboxymethyl cellulose (NaCMC) has emerged as a promising green alternative for electrolyte materials. The poly(vinyl alcohol)/NaCMC polymer network has gained popularity as a prominent polymer blend. PVA/NaCMC polymer blends loaded with sodium tungstate salt were prepared as PVA/NaCMC/Na₂WO₄ polymer blend nanocomposite electrolytes via a solution casting technique. FTIR analysis reveal shifts in band assignments related to O—H and C—O groups, suggesting Na⁺ interactions with polar functional groups in PVA and NaCMC, promoting salt dissociation and anion immobilization for efficient cation mobility. Tungstate anions, on the other hand, act as a pivotal nanofiller component that optimizes the polymer blend's microstructure and properties beyond mere ion supply. Tungstate anions disrupt the semi-crystalline nature of the PVA/NaCMC polymer blend, as confirmed by XRD patterns showing reduced crystallinity with increasing Na₂WO₄ salt concentration, which increases amorphous domains and free volume for enhanced ion pathways. This leads to improved thermal stability and electrochemical stability. Sodium ions, derived from both NaCMC and the dissociated Na₂WO₄ salt, serve as the primary mobile charge carriers responsible for ion transport. They facilitate conductivity through a hopping mechanism, where sodium ions migrate between coordination sites in the polymer matrix, particularly in amorphous regions, under an applied electric field. This is evidenced by the observed ionic conductivity of 5 × 10⁻⁶ S cm⁻¹ was recorded at room temperature for the PVA/NaCMC blend containing 10 wt% sodium tungstate salt, and rose to 4.4 × 10⁻⁴ S cm⁻¹ at 80°C. The temperature dependence of the conductivity followed Arrhenius behavior. An equivalent electric circuit model was used to interpret the EIS data. The dielectric properties were investigated by examining AC conductance spectra, dielectric constants (ε′ and ε″), electric moduli (M′ and M″), and loss tangents. The dielectric permittivity increased in the low-frequency region owing to electrode polarization effects. The maximum of the loss tangents shifted with increasing temperature, accompanied by an increase in peak height at high frequencies. Sodium-tungstate-based polymer blend nanocomposite electrolytes exhibited an enhanced electrochemical stability window (2.57 V), a higher transference number (0.973), and improved ionic conductivity, making them suitable for energy storage device applications.