Ion Gels Prepared by in Situ Radical Polymerization of Vinyl Monomers in an Ionic Liquid and Their Characterization as Polymer Electrolytes

TLDR

In ion gels, the cation diffuses faster than the anion, mirroring the behavior of the ionic liquid. The study aimed to create polymer electrolytes with high ionic conductivity by leveraging the ionic liquid’s conductivity. The authors performed in situ free‑radical polymerization of vinyl monomers in the ionic liquid 1‑ethyl‑3‑methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) to produce novel polymer electrolytes. The resulting ion gels were self‑standing, flexible, transparent films whose glass‑transition temperature decreased with EMITFSI content, behaved as a compatible PMMA/EMITFSI binary, and exhibited VTF‑dependent ionic conductivity reaching ~10⁻² S cm⁻¹ at room temperature, with increased carrier ion concentration and higher cation transference numbers at lower EMITFSI levels.

Abstract

To realize polymer electrolytes with high ionic conductivity, we exploited the high ionic conductivity of an ionic liquid. In situ free radical polymerization of compatible vinyl monomers in a room temperature ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI), afforded a novel series of polymer electrolytes. Polymer gels obtained by the polymerization of methyl methacrylate (MMA) in EMITFSI in the presence of a small amount of a cross-linker gave self-standing, flexible, and transparent films. The glass transition temperatures of the gels, which we named "ion gels", decreased with increasing mole fraction of EMITFSI and behaved as a completely compatible binary system of poly(methyl methacrylate) (PMMA) and EMITFSI. The temperature dependence of the ionic conductivity of the ion gels followed the Vogel-Tamman-Fulcher (VTF) equation, and the ionic conductivity at ambient temperature reached a value close to 10(-2) S cm(-1). Similarly to the behavior of the ionic liquid, the cation in the ion gels diffused faster than the anion. The number of carrier ions, calculated from the Nernst-Einstein equation, was found to increase for an ion gel from the corresponding value for the ionic liquid itself. The cation transference number increased with decreasing EMITFSI concentration due to interaction between the PMMA matrix and the TFSI(-) anion, which prohibited the formation of ion clusters or associates, as was the case for the ionic liquid itself.