The Role of Li⁺ Ions in Polyzwitterionic Ionogels: Gelator or Mobile Charge Carrier?

Introducing zwitterionic (ZI) polymers to lithium-containing, ionic liquid-based electrolytes can change the dynamics and local environment of Li⁺ ions. While Li⁺ ions enable the formation of noncovalently cross-linked ionogels, they may also act as free charge carriers. Aiming at an understanding of these roles, we investigate Li⁺ ion coordination and transport in ionogels consisting of 1-butyl-1-methyl pyrrolidinium bis(trifluoromethyl sulfonamide) (BMP TFSI), LiTFSI, and poly(2-(methacryloyloxyethyl phosphorylcholine)) p(MPC), obtained via in situ free radical polymerization. Ionic conductivity as well as self-diffusion of both IL ions benefit from increasing p(MPC) content, while Li⁺ diffusion is reduced. Simultaneously, ⁷Li spin relaxation documents Li⁺ ion immobilization, attributed to a strong affinity of Li⁺ to the negatively charged phosphate group of p(MPC). Raman spectroscopy confirms decreasing TFSI-Li⁺ coordination with increasing p(MPC) content. Mechanical analysis reveals a drastic increase in elastic modulus, suggesting noncovalent cross-link formation. Finally, the distribution of Li⁺ on different sites, such as mobile and anion-coordinated, single chain-coordinated, or dual-chain cross-linking Li⁺ is analyzed. The results allow for an in-depth discussion of the role of Li⁺, partly acting as a cross-linker and partly as a mobile charge carrier, providing guidelines for optimizing the balance between ionic conductivity and mechanical strength.