Design of Highly Conductive PILs by Simple Modification of Poly(epichlorohydrin-co-ethylene oxide) with Monosubstituted Imidazoles

Abstract

High ionic conductivity poly(ionic liquid)s (PILs) are of growing interest for their thermal and electrochemical stability, processability, and potential in safe, flexible all-solid-state electrochemical devices. While various approaches to enhance the ionic conductivity are reported, the influence of cation substituents is rarely addressed. Moreover, some of the asymmetric anions recently developed for high-conductivity ionic liquids were never tested in PILs. We report the design and synthesis of twelve novel cationic PILs prepared via quaternization of N-substituted imidazoles by commercially available poly(epichlorohydrin-co-ethylene oxide) (poly(EPCH-r-EO)) with subsequent ion metathesis. They differ by imidazolium side chain length (C₁–C₆ alkyl) and presence of heteroatoms (silyl, siloxane, and fluoroalkyl) and by anion type (bis(trifluoromethylsulfonyl)imide (TFSI), 2,2,2-trifluoromethylsulfonyl-N-cyanoamide (TFSAM), tetrafluoroborate (BF₄), trifluoro(trifluoromethyl)borate (BF₃CF₃), and tricyanofluoroborate (BF(CN)₃)). TFSI-based PILs with alkyl side chains gave lower glass transition temperatures (T_g) and higher ionic conductivities than those bearing heteroatomic substituents, with n-butyl side chains providing a conductivity of 4.7 × 10^–6 S cm^–1 at 25 °C under anhydrous conditions. This increased to 1.0 × 10^–5 and 4.5 × 10^–4 S cm^–1 at 25 and 70 °C, respectively, when the TFSI anion was replaced with BF(CN)₃. All PILs showed good electrochemical (>3.2 V vs Ag⁺/Ag) and thermal (>185 °C) stability, making them excellent candidates for solid-state electrolytes in electrochemical devices.