NMR Studies of Nanoscale Organization and Dynamics in Polymer Electrolytes

Abstract

Multinuclear (i.e., ⁷Li, ¹⁹F, and ¹H) NMR relaxation and pulsed field gradient spin-echo (PGSE) NMR translational diffusion measurements have been used to study the reorientational and translational dynamics of the polymeric, anionic, and cationic species in a polymer electrolyte system composed of high-molecular-weight comb-branched polyethers and their precursor macromonomers of cross-linked random copolymers, with and without LiN(SO₂CF₃)₂ (LiTFSI) doping. The macromonomers are derivatives of glycerol bonded to ethylene oxide-co-propylene oxide (m(EO-PO)) and are viscous liquids with a molecular weight of approximately 8000. The results were consistent with a picture of the lithium ions undergoing local motions near the polymer chains, whereas the anions diffuse through a slowly fluctuating three-dimensional porous polymer matrix. Four years later, the macromonomer electrolyte samples were re-measured to investigate the effects of long-term aging. The NMR data revealed that the electrolyte has undergone significant structural relaxation. The findings shed light on the evolving molecular architectures that influence conductivity and help to explain the non-ideal conductivity behaviour.