The thermal effects and dielectric properties of a lithium-ion polymer electrolyte, namely, polyethylene glycol (PEG 1500)–lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), are studied for different molar fractions of salt in polymer by the methods of electrochemical impedance spectroscopy and differential thermal analysis. The complicated form of the “final” diffusion impedance of the PEG 1500‑LiTFSI electrolyte system may be associated with superposition of two processes that occur simultaneously in the solid electrolyte interphase (SEI) layer and at the boundary of the electric double layer (EDL) It is found that the obtained diffusion coefficients do not fit the Arrhenius model in describing the mechanism of transfer of lithium ions in the PEG polymer matrix. It is shown that as the concentration of LiTFSI in PEG 1500 increases, the time of dielectric relaxation decreases. It is assumed that in the PEG 1500–LiTFSI system, the increase in the ionic conductivity with the increase in the temperature up to 343 K proceeds due to wave fluctuations of the lithium ion and the movements of the PEG 1500 matrix.