Swelling Behavior and Ionic Conductivity for Polymerized Ionic Liquid Gels

Polymerized ionic liquid gels (PIL gels) are a novel class of functional materials that combine high ionic conductivity with suppressed fluidity, making them promising candidates for polymer-based electrolytes in semisolid-state batteries. However, how the network structure affects the ion transport in PIL gels remains unclear. In this study, we evaluated the network structure, swelling behavior, and ion transport processes of an imidazolium-based PIL gel using swelling analysis, rheology, and dielectric relaxation spectroscopy. Our results shown that the growth of the rubbery-plateau modulus with increasing cross-linking density and the degree of swelling in various solvents were explained well by the Phantom Network model. By estimating the functionality at the cross-linking point, we found a nonuniform network structure with defects in the tested PIL gels. We also demonstrate that the Flory–Rehner equation for polyelectrolyte gels can be used to determine both the polymer–solvent interaction parameter and the degree of counterion dissociation. In addition, it was found that the temperature dependence of ionic conductivity in PIL gels swollen with ionic liquids (ILs) was identical to that of analogous PIL/IL blends without cross-linking, indicating that the cross-linked structure does not hinder ion transport in PIL gels. One possible reason for this is the use of an ionic cross-linker structurally similar to the PIL main chain. These findings provide a clear guideline for designing high-performance PIL gels, highlighting their potential as electrolyte materials for secondary batteries with minimal liquid leakage.