The effect of the liquid aprotic electrolyte composition on the performance of Li batteries with the polyimide (PI) cathode is studied. The electrolyte systems containing 1 М LiN(CF3SO2)2 (LiTFSI) in the dioxolane/dimethoxyethane mixture (2 : 1) and 1 М LiPF6 in ethylene carbonate/dimethyl carbonate mixture (1 : 1) are considered. The Li//PI cells are investigated by the methods of cyclic voltammetry (CV), staircase potential electrochemical impedance spectroscopy, and galvanostatic cycling. The electrochemical impedance studies of Li//Li cells show that the resistance of the solid electrolyte layer at the lithium electrode interface with the LiPF6 electrolyte is one-order-of-magnitude higher as compared with the layer formed at its interface with the LiTFSI electrolyte. The scanning electron microscopic (SEM) data confirm that such layer forms on the PI cathode too. The CV results demonstrate that the electrolyte composition strongly affects the reversibility of redox processes. By cycling the prototypes of Li//PI batteries it is shown that the battery prototype with LiTFSI electrolyte exhibits the better stability of discharge capacity, which allows considering this electrolyte composition as promising for the Li-polyimide system.