Influence of ionic interactions on lithium diffusion properties in ionic liquid-based gel polymer electrolytes

Abstract

In this study, we characterize a ternary gel polymer electrolyte (GPE) composed of bisphenol A ethoxylate dimethacrylate (BEMA) and N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₃TFSI) ionic liquid and we particularly study the influence of salt content and polymer network composition. The effect of salt concentration in liquid solutions (1-x)Pyr₁₃TFSI-xLiTFSI (molar ratio R = x/(1-x)) is finely characterized by Arrhenius measurements, PFG-NMR and Raman spectroscopy. They highlight ions interactions evolution with the salt content. For lower salt concentrations, ions pairs with bidentate coordinations [Li(TFSI)₂]^- are preferentially formed. A salt content rise leads to the formation of ionic aggregates where several Li⁺ are bound by TFSI⁻ to form [Li_z(TFSI)_y]^(z-y) large clusters. These ionic arrangements changes modify the mean charge of the complexes involving Li. It leads to better cycling performances with higher salt concentration contrary to the ionic conductivity evolution. When the binary solution is added to the GPE, a dual transport mechanism, by diffusion in the liquid phase and hops assisted by segmental motion, is confirmed. The Li transference number is improved with success by enhancing this segmental motion with the addition of a linear polymer (poly(ethylene glycol) methyl ether methacrylate, me-PEGMA). Cycling with 70% of theoretical capacity and improved high current ability is achievable at 25 °C with a dual me-PEGMA/BEMA polymer network.