Spectroscopic investigations of solvent assisted Li-ion transport decoupled from polymer in a gel polymer electrolyte

Abstract

We present here a gel polymer electrolyte, where the Li+-ion transport is completely decoupled from the polymer host solvation and dynamics. A free-standing gel polymer electrolyte with a high volume content (nearly 60%) of xM LiTFSI in G4 (tetraglyme) ( x = 1–7; Li+:G4 = 0.2–1.5) liquid electrolyte confined inside the PAN (polyacrylonitrile)-PEGMEMA [poly (ethylene glycol) methyl ether methacrylate oligomer] based polymer matrix is synthesized using a one-pot free radical polymerization process. For LiTFSI concentrations, x = 1–7 (Li+:G4 = 0.2–1.5), Raman and vibrational spectroscopies reveal that like in the liquid electrolyte, the designed gel polymer electrolytes (GPEs) also show direct coordination of Li+-ions with the tetraglyme leading to the formation of [Li(G4)]+. Coupled with the spectroscopic studies, impedance and nuclear magnetic resonance investigations also show that the ion transport is independent of the polymer segmental motion and is governed by the solvated species {[Li(G4)]+}, very similar to the scenario in ionic liquids. As a result, the magnitude of ionic conductivity and activation energies of the gel polymer electrolyte are very similar to that of the liquid electrolyte. The Li+-ion transport number for the GPE varied from 0.44 ( x = 1) to 0.5 ( x = 7) with the maximum being 0.52 at x = 5.