Raman spectroscopic study of ion-ion interaction and its temperature dependence in a poly(propylene-oxide)-based NaCF₃ SO ₃-polymer electrolyte

Raman scattering measurements have been carried out on poly(propylene oxide) complexed with NaCF₃ SO₃ salt of concentration O:M = 30:1 (where O:M is the PO:Na ratio) over a temperature range of 186-360 K in order to study ion-ion associations of the dopant salt and their temperature dependence. Splitting of the symmetric stretching mode of the CF ₃SO₃^- anion into a double band was observed and attributed to the existence of different environments of the anions. A two-component band analysis led to the identification of coexisting dissociated free ions and ion pairs, suggested to be in contact. Below the glass transition temperature, T_g, the intensity of the mode corresponding to the free ions was more or less constant with temperature; the amount of free ions in the glassy state was found to be about 84% of the total salt concentration. Above T_g the amount of dissociated free ions decreased rapidly with temperature in an Arrhenius-type behavior. The resulting reduction of the number of charge carriers has little influence on the conductivity, which is reported to dramatically increase with increasing temperature. It is concluded that the major factor determining the temperature dependence of the conductivity is the mobility rather than the number of charge carriers.