Design of Highly Conductive PILs by Simple Modification\nof Poly(epichlorohydrin-<i>co</i>-ethylene oxide) with Monosubstituted\nImidazoles

High ionic conductivity\npoly(ionic liquid)s (PILs) are of growing\ninterest for their thermal and electrochemical stability, processability,\nand potential in safe, flexible all-solid-state electrochemical devices.\nWhile various approaches to enhance the ionic conductivity are reported,\nthe influence of cation substituents is rarely addressed. Moreover,\nsome of the asymmetric anions recently developed for high-conductivity\nionic liquids were never tested in PILs. We report the design and\nsynthesis of twelve novel cationic PILs prepared via quaternization\nof N-substituted imidazoles by commercially available poly(epichlorohydrin-<i>co</i>-ethylene oxide) (poly(EPCH-<i>r</i>-EO)) with\nsubsequent ion metathesis. They differ by imidazolium side chain length\n(C<sub>1</sub>–C<sub>6</sub> alkyl) and presence of heteroatoms\n(silyl, siloxane, and fluoroalkyl) and by anion type (bis(trifluoromethylsulfonyl)imide\n(TFSI), 2,2,2-trifluoromethylsulfonyl-<i>N</i>-cyanoamide\n(TFSAM), tetrafluoroborate (BF<sub>4</sub>), trifluoro(trifluoromethyl)borate\n(BF<sub>3</sub>CF<sub>3</sub>), and tricyanofluoroborate (BF(CN)<sub>3</sub>)). TFSI-based PILs with alkyl side chains gave lower glass\ntransition temperatures (<i>T</i><sub>g</sub>) and higher\nionic conductivities than those bearing heteroatomic substituents,\nwith <i>n</i>-butyl side chains providing a conductivity\nof 4.7 × 10<sup>–6</sup> S cm<sup>–1</sup> at 25\n°C under anhydrous conditions. This increased to 1.0 × 10<sup>–5</sup> and 4.5 × 10<sup>–4</sup> S cm<sup>–1</sup> at 25 and 70 °C, respectively, when the TFSI anion was replaced\nwith BF(CN)<sub>3</sub>. All PILs showed good electrochemical (>3.2\nV vs Ag<sup>+</sup>/Ag) and thermal (>185 °C) stability, making\nthem excellent candidates for solid-state electrolytes in electrochemical\ndevices.