The Role of Solvating 12-Crown-4 Plasticizer on Dielectric Constant and Ion Conduction of Poly(ethylene oxide) Single-lon Conductors

The role of solvating plasticizer on lithium ion conduction is investigated for a poly(ethylene oxide)-based single-ion conductor, plasticized with 12 wt % 12-crown-4 (12C4). The addition of 12C4 not only increases the static dielectric constant (epsilon(s)) but also accelerates the ion rearrangement (alpha(2)) and segmental motion (alpha) compared to the host ionomer. A physical model of electrode polarization is used to estimate number density of simultaneously conducting ions and their mobility. The complex of 12C4 with lithium cation lowers the activation energy of simultaneously conducting ions, increasing the simultaneously conducting ion content by >3x. This is consistent with an initial large increase in epsilon(s), which is higher than the prediction from the Landau and Lifshitz mixing rule, reflecting that ion aggregates observed in the host ionomer are solvated by 12 wt % 12C4. Furthermore, the dissolution of the aggregates promotes ion mobility owing to an increase in polymer chain flexibility, with a reduction in glass transition temperature. The plasticization directly boosts ionic conductivity of the plasticized ionomer by similar to 5x over the whole temperature range studied, revealing that ion motion is assisted by segmental dynamics.