Impedance of interface between PEO:LiTFSI polymer electrolyte and blocking electrodes

The impedance spectra of electrolytes comprising poly(ethylene oxide) (PEO) with LiN(CF3SO2)2 (LiTFSI) salt were measured for a wide range of concentration of salt in polymer electrolyte (from pure PEO to molar ratio of 1:1 EO:Li) and in various states (amorphous and semicrystalline). The measurements were performed on membranes placed between gold-plated stainless steel electrodes. Investigations of crystallization and melting with simultaneous observations in a polarizing microscope were performed between glass plates covered with film of indium tin oxide. The impedance data were analyzed by least squares fitting of equivalent circuit. Two different equivalent circuits were proposed for modeling of interfacial impedance for electrolytes with low and high concentration of salt, respectively. The decrease of the contact area due to the increase of stiffness of electrolyte and changes of electrical properties of the interfacial layer due to the presence of crystalline lamellae were identified as the major causes for changes of impedance of the interface layer taking place during crystallization.