In this paper we report a polymer lithium cell using a PVdF-based, gel-type electrolyte formed in-situ during cell assembly. The gel electrolyte formation is monitored prior to cell assembly by electrochemical impedance spectroscopy and by Raman spectroscopy in order to determine the characteristics of the lithium salt diffusion into the gel-membrane. The results show an efficient gel formation and a fast lithium salt diffusion, this finally resulting in an optimized behaviour in a lithium cell using a high voltage spinel-type cathode. We believe that the results here reported may contribute to the enhancement of the safety of lithium batteries.