Mechanistic Insights into Ion Transport in Polymerized Ionic Liquids

Abstract

Polymerized ionic liquids (polyILs) continue to be of great interest as electrolytes in energy conversion and storage devices. However, challenges in enhancing the ionic conductivity to ~10-3 S/cm still hinder the application of these materials. Therefore, it is urgent to improve the conductivity in these electrolytes, which strongly relies on advances in molecular-level understanding of ion transport mechanisms governing charge transport. To this end, ionic correlations and the mechanisms of ion transport in the polyILs and their corresponding ILs have been systematically studied by altering the molecular chemistry of the cation and the anion using classical molecular dynamics simulations.We first hypothesized that immobilization of one of the ions in the polymer may lead to a significant increase in negative correlations in the dynamics of the mobile ions. To verify this hypothesis, we used a polymethylmethacrylate (PMMA) based backbone with attached imidazolium cations and mobile anions, including TFSI−, PF6 −, BF4 −, and Br−. The increase in temperature enhances both intrachain and interchain ion hopping while reducing the rattling of mobile ions. The analysis of ionic correlations in both ILs and polyILs systems at various temperatures revealed that the inverse Heaven ratio was not temperature dependent in either of the systems. We further investigated the effect of side-chain flexibility of polyILs on ionic correlations by changing the side-chain length. The higher flexibility enhances local mobility of the imidazolium cations, leading to an increasing number of associating cations from more polymer chains to the anions. Another key factor in tuning ionic correlations in concentrated ionic systems is electrostatic interactions, whose role was investigated by changing the dielectric constant of the simulated systems. We believe these molecular-level insights will advance the rational design of novel polyILs for electrochemical applications.