Material propertiesEffect of EMIMBF4 ionic liquid addition on the structure and ionic conductivity of LiBF4-complexed PVdF-HFP polymer electrolyte films

New polymer electrolyte films of lithium tetrafluoroborate (LiBF4)-complexed poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) embedded with different quantities of 1-ethyl-3methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid were prepared by solution casting. The prepared films were characterized using various techniques: X-ray diffraction, scanning electron microscopy, impedance spectroscopy and electrochemical measurements. The pure PVdF-HFP possessed a semi-crystalline structure and its amorphicity increased with the addition of LiBF4 salt and EMIMBF4 ionic liquid. The size and interconnection of pores in the films were enhanced by EMIMBF4. Impedance measurements indicated that the room-temperature ionic conductivity of the films increased with increasing EMIMBF4 concentration until 15 wt.%, being up to 0.202 × 10−4 S cm−1, and then decreased with further increasing EMIMBF4 concentration. In addition, the temperature-dependent ionic conductivity of the polymer electrolyte films followed an Arrhenius relation and the 15 wt.% EMIMBF4-incorporated gel polymer electrolyte film exhibited a low activation energy for ionic conduction, being about 0.28 eV. Finally, the electrochemical stability window of the 85PVdF-HFP:15LiBF4+15 EMIMBF4 gel polymer electrolyte films was evaluated as approximately 4.4 V, which is a promising value for ion battery applications.