Ionogel Electrolytes Supported by Zwitterionic Copolymers Featuring Lithium Ion-Mediated, Noncovalent Cross-Links

Abstract

A series of ionogel electrolytes, composed of an ionic liquid/lithium salt solution supported by a linear zwitterionic (ZI) copolymer scaffold containing solely noncovalent cross-links, were created in order to study the effects of varying the ZI fraction within the copolymer as well as the lithium ion concentration on gel mechanical properties and ion transport. The copolymer scaffold was synthesized via in situ UV photopolymerization of the ZI monomer 2-methacryloyloxyethyl phosphorylcholine (MPC) together with the non-ZI comonomer 2,2,2-trifluoroethyl methacrylate (TFEMA) in varying ratios, using a fixed fraction of 25 mol % total monomers. Ionic liquid electrolyte (ILE) solutions of 0.1, 0.3, 0.5, or 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolved in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI) were employed. For ionogels created using the 1 M LiTFSI ILE, varying the MPC content from 6.3 mol % to 17 mol % in the precursor solution resulted in a substantial increase in gel elastic modulus, from 20 kPa to 11 MPa. In contrast, room temperature ionic conductivity values for the same samples remained nearly constant, varying from 0.57–0.70 mS cm–1. While the in situ photopolymerization approach remains highly versatile for ionogel formation, an alternative method is introduced here for the first time that can produce ionogels “on-demand” using a pre-made ZI copolymer/ionic liquid solution to which a Li+ ILE is subsequently added.