Polar β-Phase PVdF-HFP-Based Freestanding and Flexible Gel Polymer Electrolyte for Better Cycling Stability in a Na Battery
Raghvendra Mishra, Shishir Kumar Singh, Himani Gupta, Rupesh K. Tiwari, Dipika Meghnani, Anupam Patel, Anurag Tiwari, Vimal K. Tiwari, Rajendra Kumar Singh
Abstract
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 frequency-dependent 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.