Dielectric relaxation and ionic conduction in solid polymer electrolyte based on a random copolymer of ethylene carbonate and ethylene oxide

Abstract

Broadband dielectric spectroscopy (BDS) is a powerful technique for studying the dielectric and ion-conductive behavior of solid polymer electrolytes (SPE). In this study a random copolymer of ethylene carbonate (EC) and ethylene oxide (EO), P(EC/EO), was used as a polymer host for SPE, and the correlation between dielectric relaxation and ionic conduction in P(EC/EO)-LiFSI electrolytes was studied. BDS measurements revealed that the DC conductivity for P(EC/EO) electrolytes is higher than for the PEC system at all salt concentrations. This is due to the much higher a-relaxation frequency, indicating that the randomly introduced EO units can provide fast segmental motion with no formation of any stable coordination structures with Li ions. FT-IR analysis indicated that the peak fraction of C = O interacting with Li ions was small, less than 10% in the 10 mol% electrolyte, but in concentrations above 40 mol% the proportion was nearly saturated at around 60–70%. A small CO-C peak interacting with Li ions appears at 1080 cm−1, but no change in peak intensity is observed, implying no formation of stable coordination structures that reduce the glass transition temperature.