Conductivity and dielectric properties of polymer electrolytes PEO:LiN(CF3SO2)2 near glass transition

Results of measurements of PEO:LiN(CF₃SO₂)₂ polymer electrolytes of composition between 12:1 and 1.5:1 EO:Li, performed by impedance spectroscopy, impedance spectroscopy simultaneous with microscope observation, differential scanning calorimetry and X-ray diffraction were analyzed focusing on electrical properties of semicrystalline and rapidly cooled amorphous samples. In the loss spectra measured at low temperature, the occurrence of two dielectric relaxations was evidenced: local (β) and segmental (α). For each of the investigated electrolytes, the temperature dependence of the ionic conductivity and the frequency of α relaxation could be described by the VTF function with the same value of parameter T₀, which indicated close coupling of both phenomena. The local (β) relaxation exhibited an Arrhenius type temperature dependence. With increasing amount of salt, two effects were observed for amorphous samples: an increase of the glass transition temperature T_g affecting the α relaxation and changes of structure of PEO:LiN(CF₃SO₂)₂ complexes reflected in shift of the β relaxation frequency. Crystallization caused decrease of both the ionic conductivity and the strength of dielectric relaxations. The presence of crystalline phase was also reflected in a shift of the T_g of amorphous phase remaining in the system with respect to the T_g of amorphous electrolyte obtained by rapid cooling.