Mechanisms of Ion Transport in Lithium Salt-Doped Zwitterionic Polymer-supported Ionogel Electrolytes

Recent experimental results have demonstrated that zwitterionic (ZI) ionogel comprised of ZI polymer-supported lithium salt-doped ionic liquid exhibits improved conductivities and lithium transference numbers than the salt-doped base ionic liquid (ILE). However, the underlying mechanisms of such observations remain unresolved. In this work, we pursued a systematic investigation to understand the impact of the ZI content and salt concentration on the structural and dynamic properties of the poly(MPC) ionogel of our model ZI ionogel, ZI poly (2-methacryloyloxyethyl phosphorylcholine) (polyMPC) supported LiTFSI/N-butyl-N-methylpyrrolidinium (BMP) TFSI base ionic liquid electrolyte. Our structural analyses show strong lithium - ZI interaction consistent with the physical network characteristic observed in the experiments. An increase in ZI content causes the ionic liquid (IL) environment to resemble a neat ionic liquid due to lithium ions partitioning into the ZI polymer phase. In contrast, an increase in salt concentration led to the IL environment resembling a salt-doped ionic liquid. The diffusivities of the mobile ions in the poly(MPC) ionogel were found to be lower than the base ILE in agreement with experiments at T$>$300 K. Analysis of ion transport mechanisms shows lithium ions within the poly(MPC) ionogel travel via a combination of structural, vehicular diffusion as well as hopping mechanism. Lastly, the conductivity trend crossover between the poly(MPC) ionogel and the base ILE was rationalized via a temperature study that showed that the base ILE ions are influenced more by the variation of temperature when compared to the poly(MPC) ions.