Ion-conducting polymers, such as polyelectrolytes and poly(ionic liquids) (PILs), have become an important class of materials for applications related to energy storage and soft robotics. In the case of lithium ion batteries, polymer electrolyte membranes, for the most part, are able to effectively transport ions between anodes while preventing battery failure due to shorting via lithium dendrite growth. Limiting the widespread usage of polymer electrolyte membranes is the inherently low ion conductivity, which in many cases is a result of ion aggregates that reduce the number of effective ions. An effective approach to improve the ionic conductivity by breaking up ion aggregates in ion-containing polymers is to increase the dielectric constant of the host. Zwitterions, or molecules containing covalently bonded cation and anion, are promising candidates as additives to increase the dielectric constant of the materials. Despite the high ionic conductivity observed by zwitterion addition in polymerized ionic liquids (PILs), the underlying mechanism remains misunderstood. Here, we report the change in dielectric constant for a series of sulfonate-imidazolium-based zwitterions blended with glycol solvents using dielectric relaxation spectroscopy (DRS). Adding zwitterions drastically increases the permittivity compared with that of the pure glycol solvents. The Landau-Lifshitz mixing rule describes well the measured permittivities of the blends at low concentration and was further used to extrapolate to the permittivities of pure zwitterions based on their blend behavior. The dependence of measured permittivity on zwitterion structure, composition, and temperature will be discussed in the context of ion conduction in polymeric materials. The work presented here highlights the exceptional ability of zwitterions to increase permittivity and their potential to boost conductivity of PILs by creating a more polar environment.

Understanding the relationship between permittivity and ion conduction in polymer electrolyte materials

2019 Fall National Meeting

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