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Ionic liquid trimethylsulfonium bis(trifluoromethylsulfonyl)imide (M3STFSI) was synthesized to obtain solid polymer electrolyte (SPE) membranes by mixing it with polyethylene oxide and sodium perchlorate (NaClO4). Nine membranes were prepared with varying concentrations of high, medium, and low polymer content. X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy techniques were used to characterize both the products and the raw materials. Ionic conductivity was determined using electrochemical impedance spectroscopy (EIS), and redox phenomena were studied by cyclic voltammetry. To calculate the total ion transference number and Na+ ion transference number, chronoamperometry (CA) with EIS was employed. Additionally, the electrochemical stability in SPE (M1) was measured through linear sweep voltammetry (LSV). The NaFe(SO4)2 compound, known as Eldfellite, was utilized as a cathode in Na-ion battery tests. A Na||M1||NaFe(SO4)2 cell was prepared to determine the battery performance via galvanostatic charge/discharge profiles. Thus, the theoretical specific capacity of Eldfellite for Na+ insertion is 99 mAh g−1, and after 70 cycles at a C/10 rate vs. Na0 metallic as an anode, the specific discharge capacity was 75 mAh g−1, with a coulombic efficiency above 99%. [ABSTRACT FROM AUTHOR]