Structure–Property Relationships for Polyether-Based Electrolytes in the High-Dielectric-Constant Regime

Abstract

Establishing general structure–property relationships for polymer electrolytes is crucial to enable design of improved materials to advance solid-state energy storage. We report the relationship between dielectric constant, glass transition temperature, and ionic conductivity for polyether-based electrolytes with dielectric constants of the polyether host within the range 7–35 at 60 °C. The ionic conductivities of the polyether and lithium bis(trifluoromethylsulfonyl)imide mixtures ranged from 10–7 to 10–3 S/cm. In this higher-dielectric-constant regime, here defined as a polymer with a dielectric constant greater than that of poly(ethylene oxide) (ca. 9.0), the glass transition temperature increased with dielectric constant while ionic conductivity decreased. These results complement a recent report on the low-dielectric-constant regime, where the ionic conductivity was limited by the dielectric constant and ion dissociation. In the high-dielectric-constant regime explored here, segmental dynamics are slowed due to stronger polymer–polymer and polymer–ion interactions, resulting in decreased ionic conductivity and associated increase in neat polymer glass transition temperature. The disparate chemical structures of the polymers of this study, along with the results of past coarse-grained molecular dynamics simulations, support the generality of these conclusions and speak to the difficulty of identifying a single molecular characteristic leading to the design of high-conductivity polymer electrolytes. Widely used poly(ethylene oxide) represents a near-optimal balance between the low- and high-dielectric-constant regimes. To improve upon the ionic conductivity limitations of polymer electrolytes, single-component polymer hosts are unlikely to resolve the trade-off between the need for ion dissociation while retaining rapid segmental dynamics.