The aim of the studies was to determine the effect of inorganic fillers surface modified with acidic groups on the properties of polymer gel electrolytes: matrix morphology, temperature dependence of ionic conductivity and interfacial stability. The effect of the concentration of the lithium salt on ionic conductivity was also studied. The poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF/HFP) gels were prepared according to the so-called Bellcore process. Dry membranes were immersed in a liquid electrolyte solution. Two different lithium salts (LiCF3SO3 and LiN(CF3SO2)2) in propylene carbonate (PC) were used as liquid electrolytes. The salt concentration in the solution varied from 0.05M to 1M. Changes in the ionic conductivity as a function of the type of the filler added were analyzed on the basis of impedance spectroscopy experiments. The structure and morphology were analyzed by means of DSC and scanning electron microscopy techniques. The stability of the gel electrolytes–lithium electrode interface was evaluated by impedance spectroscopy technique in Swagelok geometry against two lithium metal electrolytes over 25 days. Finally, the lithium transference number was calculated for some of the studied gels using the polarization method.