Solvent-driven ion transport in sodium-ion gel polymer electrolytes: comparative role of ionic liquid and glyme solvents

Abstract

The present work reports a comparative investigation of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf) an ionic liquid and tetraethylene glycol dimethyl ether (TEGDME) a glyme-based solvent, highlighting their roles in modulating physical and electrochemical properties of Na-ion-conducting gel polymer electrolytes. The flexible gel electrolyte films were fabricated by standard solution cast technique. Electrochemical impedance spectroscopy revealed the room-temperature ionic conductivities of 0.36 mS cm⁻¹ for the PVdF-HFP/NaTf/TEGDME electrolyte and 12.8 mS cm⁻¹ for the PVdF-HFP/NaTf/EMITf system. The cyclic voltammetry analysis revealed that the TEGDME-based electrolyte operates over a voltage window of 3.3 V, whereas the EMITf-based electrolyte demonstrates a broader working range of 4.2 V. Differential scanning calorimetry studies indicate that both electrolyte systems exhibit thermal stability, retaining gel phase up to 95 °C. X-ray diffraction characterization show that addition of EMITf results in a reduced degree of crystallinity for the electrolyte than those of TEGDME based electrolyte. Fourier-transform infrared spectroscopy confirmed strong interactions among PVdF-HFP, NaTf, and the solvents. These findings indicate solvent-assisted Na⁺ hopping in the TEGDME-based electrolyte, whereas EMITf induces polymer matrix amorphization, enabling diffusion-dominated ion transport and superior electrochemical performance for used in energy storage devices applications.