Impedance spectroscopy on polymer electrolytes containing Lithium and Magnesium ions

A detailed impedance spectroscopy study, together with thermal analysis, was carried out on polymer electrolytes composed of low molecular weight poly(ethylene oxide) 400 with added lithium or magnesium triflate salts. Glass transition temperatures were always far below room temperature. The ion conductivity exhibits a maximum at intermediate salt concentrations and is significantly higher in the case of Li triflate addition. When the ion conductivity contribution was subtracted, a prominent dielectric loss peak in the MHz region was observed. Its relaxation strength scales with ion concentration, as appropriate for an ion pair relaxation in systems above the glass transition. We present evidence that this relaxation is due to the same species that give rise to the ion conductivity. The complex impedance due to the composite ion conductivity – ion pair relaxation process was found to exhibit good agreement with the Davidson-Cole [D.V. Davidson and R.H. Cole, J. Chem. Phys., 19 (1951) 1484-1490] relaxation function. The relaxation process is characterized by a power-law exponent below 0.5, which signifies an anomalous diffusion process at high frequencies crossing over to a constant conductivity at low frequencies