Hybrid gel polymer electrolyte based on 1-methyl-1-Propylpyrrolidinium Bis(Trifluoromethanesulfonyl) imide for flexible and shape-variant lithium secondary batteries

Lithium ion conducting polymer electrolytes with broad electrochemical stability, good mechanical strength, high thermal stability, and easy processability are necessary for all-solid-state and shape-variant lithium secondary batteries. Hybrid gel polymer electrolytes incorporating an ionic liquid have been attracting attention for application in solid-state lithium secondary batteries owing to their superior thermal properties compared to conventional electrolyte systems. In this study, a variety of polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), and 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PMPyrrTFSI) are prepared, and an in-depth study of their composition dependence and electrical properties is conducted to develop the optimum composition. The composition dependent ionic conductivity of the polymer electrolyte increases with increasing LiTFSI and PMPyrrTFSI and reaches a maximum value of 6.93 × 10⁻⁴ S cm⁻¹ at room temperature (25 °C) when the polymer electrolyte contains 30 wt% LiTFSI and 60 wt% PMPyrrTFSI. In addition, the optimized gel polymer electrolytes consisting of PVdF-HFP/LiTFSI/PMPyrrTFSI (70/30/60 by weight, i.e., 70PVdF-HFP/30LiTFSI/60PMPyrrTFSI) look transparent and exhibit high mechanical stability and excellent thermal stability up to 420 °C. Finally, the lithium iron phosphate (LiFePO₄)/lithium metal solid-state cells coupled with the optimized gel polymer electrolyte are prepared, and their discharge characteristics are studied. The 70PVdF-HFP/30LiTFSI/60PMPyrrTFSI based solid-state cell delivered a maximum discharge capacity of 151 mAh g⁻¹ at room temperature with a good rate capability and cycling performance.