Impacts of Anion Structure on Ion Conduction of Li-ion Single-ion Conducting Ionomers

Single-ion conducting ionomers are potential candidates for energy-related applications. However, their conductivity is orders of magnitude lower than the requirement for device operation. Understanding how the chemical structure impacts ion conduction offers new perspectives on designing ionomers with higher conductivity. Here, two different random copolymers containing two types of anions, bis(fluorosulfonyl)imide (TFSI-like) anion and sulfonate anion, copolymerized with poly(ethylene glycol) methacrylate were synthesized using RAFT polymerization. Synthesized polymers with different anion groups had similar glass transitions temperatures (T_g), ion contents, molecular weights, and dispersities. Li-ion conduction was measured using dielectric relaxation spectroscopy (DRS), and X-ray scattering investigated ionomer morphology. DRS reveals orders of magnitude higher conductivity for ionomers with the imide anion than with the sulfonate anion. Ion aggregation causes the lower ionic conductivity of sulfonate lithium ionomer. Our results highlight the significant impacts of anion structure on ionic conductivity. In addition to lowering T_g, weakening the binding energy between the attached anion and the Li counterion promotes ion conduction for single-ion conducting ionomers.