Arrhenius and non-Arrhenius conductivities in intercalated polymer electrolytes

Abstract

Intercalated polymer electrolytes (IPEs), Cd_0.75PS₃A_0.5(PEO) [A=Li, Na, K, Cs], formed by the insertion of alkali metal ions solvated by polyethylene oxide (PEO) into the interlamellar space of the layered insulating cadmium thio-phosphate form an interesting class of materials. Their ambient temperature dc conductivities are comparable to that of the corresponding solid polymer electrolytes (SPEs) formed by dissolving alkali-metal salts in PEO. The conductivity of the intercalated materials, irrespective of the cation, exhibits a change in conduction mechanism with temperature. At low temperatures (T<225 K) the dc conductivity values are small and exhibit an Arrhenius temperature dependence. Above 225 K the dc conductivity rises sharply and in this temperature regime its temperature variation depends on the alkali-cation. The dc conductivity of the Li and Na containing IPEs follow the non-Arrhenius Vogel-Tamann-Fulcher (VTF) relation, σ_dc = σ₀T^−0.5 exp(-B/(T-T₀)), while the K and Cs IPEs follow an Arrhenius dependence. The origin of the Arrhenius and non-Arrhenius conductivity of the IPEs, Cd_0.75PS₃A_0.5(PEO) [A=Li, Na, K, Cs], have been investigated by analyzing the frequency-dependent conductivity in the dielectric and electrical modulus representations. We show that the difference in behavior is related to differences in the coupling of ionic motion and polymer segmental modes. In the Li and Na containing IPEs the motions are coupled and the conductivity exhibits a VTF temperature dependence. In the K and Cs compounds these motions are decoupled, consequently, although the mean relaxation time associated with segmental motion of the intercalated PEO exhibits a VTF dependence, ionic conductivity has an Arrhenius temperature dependence.