In situ microscope and impedance study of polymer electrolytes

Simultaneous impedance measurements and optical observations of polymer electrolytes were conducted in a computer-controlled setup, which combined a dielectric analyzer, a polarizing microscope with a heating stage and a digital camera. The polymer film was placed between glasses with ITO conductive layers in a cell preserving argon atmosphere. The setup allowed automated recording of impedance spectra and microscope images consecutively at a set of stabilized temperatures between −20 and 100 °C. Measurements were repeated at a constant temperature when a drift of impedance was detected. The influence of melting and crystallization phenomena on electrical properties of high molecular weight PEO and PEO:LiTFSI system (50:1 molar ratio) was investigated. The evolution of impedance spectra was correlated with changes of polymer morphology during various heating and cooling runs. Impedance spectra were analyzed by fitting an equivalent circuit, which modeled: the ionic conductivity, the dielectric constant, the electric polarization due to inhomogeneity in the bulk of sample and the electrode polarization. The results indicate that the large decrease of ionic conductivity is related to the closing of amorphous conductivity pathways by the growing spherulites in the first stage of crystallization, and to the densification of the structure and interfacial phenomena in the second stage.