Impact of Cation and Counterion Chemical Structure on Ion Transport and Morphology in Cyclopropenium-Based Polymerized Ionic Liquids*

Polymerized ionic liquids are being explored as promising polymer electrolytes. Optimizing the electrochemical benefits of ionic liquids with the enhanced mechanical stability of a polymer backbone has the potential to create a mechanically robust, non-volatile electrolyte for batteries or fuel cells. This study investigates the structure and conductivity of trisaminocyclopropenium (TAC)-based monomeric and polymerized ionic liquids (mono-ILs and poly-ILs). Differential scanning calorimetry, X-ray scattering, and broadband dielectric spectroscopy are used to measure the local morphological and ion transport properties in the poly-ILs and mono-ILs. The poly-IL systems examined include polystyrene (PS)-TAC with different counterions and cation-modified chemistries of PS-TAC with chlorine counterions. While the mono-ILs have higher conductivity at most temperatures (1-2 orders of magnitude at 50°C), decoupling of the ions is apparent in the poly-IL systems below the glass transition, which leads to higher conductivity in some of the poly-IL systems below ambient temperature. Changing the functional groups of the TAC cations results in large changes to the glass transition, allowing for optimization of the poly-IL systems at different temperatures.