Polycationic scaffolds for Li-ion anion exchange transport in ion gel polyelectrolytes

Abstract

Ion gel polyelectrolytes (IGPs) were prepared by photopolymerization of the synthetic cationic monomers: 1-(2-methacryloyloxy)ethyl-3-butylimidazolium bis(trifluoromethane sulfonyl)imide (IMMATFSI)/bisfluorosulfonylimide (IMMAFSI) or 1-(2-methacryloyloxy)ethyl-1-methylpyrrolidinium bisfluorosulfonylimide (PYRMAFSI)/bis(trifluoromethane sulfonyl)imide (PYRMATFSI) in the presence of bis(trifluoromethane)sulfonamide (LiTFSI) solutions at different concentrations in either 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI), 1-ethyl-3-methylimidazolium bis(trifluorosulfonyl)imide (EMITFSI) or N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PMPFSI). The resulting IGPs are thermally stable, easy–to-handle solids wherein the overall ionic conductivity ranges from about 1 to about 10 mS cm⁻¹ at 25 °C. The diffusivity of the ions Li, FSI and TFSI was studied at 25 °C by PGSE-NMR. In IGPs containing imidazolium groups both in the polycationic scaffold and the ionic liquid phase, their large Li ion diffusivities (up to 40 × 10⁻¹² m² s⁻¹, well above that of the anions) suggest the existence of an important contribution of anion exchange Li transport. To a lesser extent anion exchange may also be occurring in IGPs containing pyrrolidinium groups in the polycationic scaffold and imidazolium groups in the ionic liquid phase and to an even lesser extent in those IGPs with pyrrolidinium in both the polyelectrolyte and the ionic liquid phase. Because of the dimensional and thermal stabilities of IGPs, safety of their components, and their large Li-ion diffusivity, these types of electrolytes appear as excellent candidates for Li and Li-ion batteries.