Impact of Li-Salt Doping on the Dynamics of Imidazolium-Based Polymerized Ionic Liquids

Polymerized ionic liquids (PolyILs) are promising electrolytes for electrochemical applications due to the unique characteristics of polymers and ILs, and selective ion transport. However, low ionic conductivity limits their practical use. Doping polyanions with Li-salts improves conductivity, but Li⁺ ion transport number drops significantly due to the strong Li⁺ ion interaction with polyanions. Thus, doping polycations with Li-salts could be a good alternative approach. In this study, we synthesized different imidazolium-based polycations with short and long linkers and doped them with LiTFSI salt. The results reveal that specific molecular architecture with doping has significant impact on polymers Tg and overall dynamics. Diffusion measurements interestingly show that smaller Li⁺ ions have slower diffusion coefficients than the bigger TFSI anions even in the case of polycations. Light scattering data analysis indicates that effective activation energies E* are highly sensitive to changes in the dielectric permittivity of the matrix, even for TFSI which is a relatively large anion. The study will be used as guidance for rational design of new PolyILs with improved ionic conductivity and Li⁺ diffusivity.