Influence of crystallization on dielectric properties of PEO:LiTFSI polymer electrolyte

Impedance spectra of the PEO8:LiN(CF3SO2)2 electrolyte were measured in the frequency range from 0.01 Hz to 10 MHz at temperatures between −65 °C and 90 °C in various heating and cooling runs. Polarizing microscope observations confirmed that a rapidly cooled electrolyte remained amorphous below −15 °C. Crystallization during slow cooling took place at +15 °C. Presence of the crystalline phase caused significant reduction of the strength of β relaxation and to a lesser degree the strength of α relaxation. The α relaxation time in the amorphous phase of semicrystalline electrolyte is considerably shorter than that in the entirely amorphous sample. Ionic conductivity fell more than 10 times during crystallization. Conductivity of the semicrystalline electrolyte decreased upon cooling much slower than in the case of the amorphous electrolyte. At temperature about −35 °C conductivity became equal in both states of the electrolyte. The temperature dependence of conductivity and frequency of α relaxation indicated that the glass transition in the semicrystalline electrolyte occurred at a temperature about 10 °C lower than in the amorphous sample. This finding was confirmed by the results of DSC study. Lower glass transition temperature of the amorphous regions is a consequence of depletion of the salt, which is accumulated in the lamellae of the PEO6:LiN(CF3SO2)2 crystalline complex.