The passive layer formed on lithium in a PEO20–LiTFSI–5%PC gel polymer electrolyte after different electrochemical processes was characterized using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and electrochemical impedance spectroscopy (EIS). EIS indicates that the interface resistance of lithium electrodes increases with time after fresh lithium deposition, whereas the interfacial resistance has no change with time after lithium deposition/dissolution process. The XPS analysis as well as FTIR data show that the main compositions of the passive layer are ROCO2Li, Li2CO3, LiOH, LiX (X=F, S, N, SO2CF3) and Li oxides, mostly due to the reactions occurred between lithium and PC, LiTFSI, and trace impurities (H2O, O2), and the lithium dissolution process has no distinctive effect on the composition of passive layer. XPS depth profile of the passive film detected by XPS and sputtering experiments further demonstrates that the presence of Li2CO3/LiOH is in the outer layer and Li2O, LiF mainly in the inner part of the passive layer.