Ion Transport Mechanisms and the Significance of Glass Transition Temperature in PEO based Polymer Blend Electrolytes

Abstract

Polymer blend electrolytes have become a promising avenue to enable solid electrolytes for use in lithium metal batteries. Much of the current literature has explored the effect of glass transition temperature (T_g) on the ionic conductivity of various ion-containing polymer blends. The Vogel-Tamman-Fulcher (VTF) equation is often used to describe the temperature-dependence of the ionic conductivity of these systems, as it considers the segmental dynamics of the blend and accounts for the T_g. The reduced conductivity can be calculated using the VTF parameters and describes the ionic conductivity at a set value away from T_g. Therefore, the reduced conductivity can interpret observed differences in ionic conductivity by removing the effects of segmental motion. In this work, we fit previously reported ionic conductivity data for 14 unique systems to the VTF equation, allowing us to deconvolute the effects of T_g and ion solvation environment on the ionic conductivity. We divide the blends into three distinct groups, (1) poly(ethylene oxide) (PEO) based blends with lithium perchlorate (LiClO4), (2) PEO based blends with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and (3) PEO based blends with single-ion containing polymers. We find that the relative balance between ion solvation environment and segmental dynamics is highly dependent on salt chemistry where ion transport in blends doped with LiClO4 is highly dependent on segmental dynamics while ion solvation environment plays a larger role in those doped with LiTFSI. For blends with ion-containing polymers, a complex balance between the number of charge carriers and segmental motion exists. Overall, while segmental dynamics strongly influence ion transport, our findings reveal that the incorporation of multiple polymers into an electrolyte can enable engineering of the ion solvation environment to improve ion transport.