Zwitterionic Copolymer-Supported Ionogel Electrolytes Featuring a Sodium Salt/Ionic Liquid Solution
Huan Qin, Matthew J. Panzer
Abstract
A series of zwitterionic (ZI) copolymers, including both binary, fully-ZI and ternary, partially-ZI copolymer variations, have been successfully employed as scaffolds to support mechanically robust ionic liquid-based gel (ionogel) electrolytes containing sodium bis(trifluoromethylsulfonyl)imide salt. All of the synthesized ionogels, which were optically transparent and freestanding without any covalent cross-links, contained 15 mol % copolymer and displayed room-temperature ionic conductivities above 1 mS cm–1 while also exhibiting high compressive elastic modulus values (0.7–11 MPa). The activation energy of ionic conductivity of an ionogel with a fully-ZI copolymer scaffold (18 kJ mol–1) was found to be noticeably lower than that of the ionic liquid electrolyte solution itself (23 kJ mol–1), highlighting the positive role that ZI moieties may play in aiding ion transport. NMR chemical shift analysis revealed favorable interactions between sodium cations and both types of ZI motifs employed here (sulfobetaine and phosphorylcholine). In addition, the sodium-ion transference number value of a ternary copolymer-supported ionogel electrolyte was determined to be larger compared to that of the ionic liquid solution (0.19 vs 0.10, respectively).