Flexible gel polymer electrolyte comprising high flash point solvent adiponitrile with ethylene carbonate as co-solvent for sodium-ion batteries

Due to the unique properties like high flash point, low vapor pressure and high electrochemical stability, the adiponitrile (ADN) as co-solvent with ethylene carbonate (EC) is proposed as an electrolyte solvent for sodium-ion batteries (NIBs). Here, we report a flexible thick film of a gel polymer electrolyte (GPE) containing a Na-salt (sodium triflate), dissolved in a mixture of ADN:EC, entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) for use in NIBs. The structural/morphological studies of the GPE film are performed by X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy. The electrolyte offers excellent performance characteristics with high ionic conductivity (σRT ∼ 4.6 × 10−3 S cm−1) and Na+-transport number (tNa+ ∼ 0.77), wide electrochemical potential range (∼5.0 V vs. Na/Na+) at room temperature, and sufficient thermal stability (up to ∼120 °C), making the GPE-film attractive for sodium-ion storage. The Na-stripping/plating measurements for a long duration show excellent reversibility of the electrolyte with Na-electrodes, revealing low and stable polarization. The sodium-ion half-cell (Na0.44MnO2/GPE/Na-metal) shows prominent charge–discharge cycling stability, with a discharge capacity of ∼85.5 mAh g−1 at a current rate of 0.1C, and good rate capability up to 1C. The GPE-film has also been tested in a full sodium ion-cell fabricated with sodiated bismuth powder and Na0.44MnO2 as anode and cathode materials, respectively. The full cell offers discharge capacity of ∼55.7 mAh g−1 at 0.1C with stable cycling performance. The results indicate that the GPE film has potential applications as high performance electrolyte for Na-storage.