Influence of Cationic Poly(ionic liquid) Architecture on the Ion Dynamics in Polymer Gel Electrolytes

The influence of the structure of cationic poly(ionic liquid)s (PIL) on the physical properties of ternary gel electrolytes composed of PIL, Pyr14TFSA ionic liquid, and LiTFSA salt is investigated. The chosen PIL vary in the position of the cationic group (backbone versus side chain); spacer length and nature (alkyl or ether); and backbone rigidity. Ion gels containing PIL are analyzed in terms of thermal behavior, viscosity, ionic conductivity, ion diffusion (multinuclear Pulsed Field Gradient NMR), and local lithium ion dynamics (7Li spin–lattice relaxation rates). The sequence of glass transition temperatures and conductivities for neat polymers correlates with expectations based on their structural variations. However, a different sequence is observed for ion gels, as here the physical properties are primarily influenced by the occurrence of a micrometer scale heterogeneity of the system. For gels based on PIL with spacer, homogeneous electrolytes with lower conductivity were obtained, whereas for PIL without spacer, the ionic conductivity of the gel was higher due to the formation of polymer- and IL/Li salt-enriched domains, respectively. In the latter case, high ion mobilities in a continuous IL/Li salt-enriched phase cause an increase in the overall gel conductivity. This enables a material selection for different types of application, where either high conductivity or homogeneity over a large temperature range is necessary.