Crystallization and melting of PEO:LiTFSI polymer electrolytes investigated simultaneously by impedance spectroscopy and polarizing microscopy

Simultaneous impedance measurements and optical observations of polymer electrolytes were conducted in an automated experimental setup that combined an impedance dielectric analyzer, a polarizing microscope with a heating stage and a digital camera. The polymer film was placed between glasses with ITO conductive layers, forming a transparent cell mounted in a custom designed holder that preserved argon atmosphere. Films of high molecular weight poly(ethylene oxide) with dissolved lithium bis(trifluoromethanesulfone) imide(LiTFSI) of two compositions: 50:1 and 6:1 (EO:Li molar ratio) were investigated in transparent cells above room temperature and in cells with gold electrodes in temperature range between -60 and 90 deg. C. Various heating and cooling runs enabled observation of the crystallization and melting. The results indicate that the decrease of conductivity observed in impedance spectra during crystallization is related to the closing of amorphous conductivity pathways by growing spherulites. In the dilute system, composition 50:1 EO:Li, amorphous areas were still visible in the film after the growth of spherulites ceased. In the film of composition 6:1, corresponding to the polymer-salt complex, densification of the structure and interfacial phenomena caused a large drop of conductivity at the late stage of crystallization. In the dense structure of crystallized P(EO)6:LiN(CF3SO2)2 film no amorphous areas were visible. Differences in the structure have a reflection in the relative change of conductivity caused by crystallization, which decreased six times for the 50:1 composition and 500 times for the 6:1 composition