Exploring Anion Effects on Gel Polymer Electrolytes for Sodium-Ion Batteries: A Comparative Study of Intercalation and Alloying Anode

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In this study, gel polymer electrolytes (GPEs) were synthesized by blending two non-aqueous polymers, Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and Poly (methyl methacrylate) (PMMA), containing sodium salts (Na-X, where X = CF3SO3, ClO4, BF4, PF6), along with ethylene carbonate and diethyl carbonate (DEC) as plasticizers. Physical characterization techniques, including X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy were employed to study structural changes and ion-polymer interactions in the GPEs. Differential scanning calorimetry indicated a decrease in glass transition temperature (Tg) upon the addition of salt and plasticizer. Among the prepared GPEs, GPE-2 (PVDF-HFP: PMMA - EC: DEC - NaClO4) exhibited the highest ionic conductivity at room temperature, 1.2 × 10−3 S cm−1, and the lowest activation energy of 0.25 eV. Cyclic voltammetry was performed to assess the electrochemical stability and redox behaviour, and linear sweep voltammetry confirmed that the GPEs are stable up to 4.8 V. The suitability of GPE-2 for sodium-ion batteries was demonstrated by fabricating a sodium-ion coin cell with hard carbon as an intercalating anode and CuO-TiO2 as an alloying anode. The optimum performance was obtained with hard carbon (HC) anode, demonstrating a stable charge/discharge capacity of 190 mAh g−1 after 100 cycles.

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