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Decrement in conductivity and flexibility of polymer electrolytes at higher ionic liquid concentration due to the crowding of anions.

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The stable polar gel polymer electrolyte (GPE) of sodium salt and ionic liquid (IL) incorporated in the PVdF-HFP (polyvinylidine fluoride-co-hexafluoropropylene) matrix at high loading was synthesized for improved performance of the sodium battery. The 1 M sodium bis trifluoromethylsulfonyl imide salt (S) in 1-butyl-3-methylimidazolium bis trifluoromethylsulfonyl imide IL used to prepare salt-IL (SIL) solution was mixed with PVdF-HFP at different weight percentages to prepare polar GPE films. The polar β-phase PVdF-HFP in the GPE is generated by the incorporation of the above-prepared SIL. The temperature-and frequencydependent ionic conductivities with a scaling approach and dielectric relaxation analysis have been carried out for the prepared GPEs. The 70 wt % SIL containing GPE (PSIL70) shows excellent thermal stability up to 368 °C, high ionic conductivity (1.9 × 10 −3 S cm −1 at 30 °C), and sodium-ion transference number ∼0.27 with a wide electrochemical stability window (4.2 V vs Na/Na + at 30 °C). The optimized electrolyte PSIL70 is used to fabricate coin cell (Na/ PSIL70/Na-NMC) with the prepared Na-NMC cathode and Na-metal anode. The charge−discharge result shows that the specific discharge capacity of the cell is ∼108 mA h g −1 at 0.1 C (21 mA g −1 ). The capacity retention is 94% over 200 charge−discharge cycles at 0.2 C, showing high capacity with good retention for the sodium battery.

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