The effect of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) ionic liquids on the properties of the polymer electrolyte based on polyethylene glycol diacrylate (PEG-DA) and LiBF4 salt was studied. Research was carried out by spray-mass spectroscopy, differential scanning calorimetry, and electrochemical impedance spectroscopy technique in the temperature range from -40 to 120 °C.