NMR studies on poly(ethylene oxide)-based polymer electrolytes with different cross-linking doped with LiN(SO₂CF₃)₂. Restricted diffusion of the polymer and lithium ion and time-dependent diffusion of the anion

The reorientational and translational dynamics of the polymer, anion and cation species in polymer electrolyte systems consisting of a cross-linked poly(ethylene oxide - propylene oxide) random copolymer (poly(EO - PO)) doped with LiN(SO₂CF₃)₂ at two different cross-linking densities were studied using multinuclear NMR relaxation and pulsed field gradient spin - echo (PGSE) diffusion measurements. Analysis of the data provided exquisite molecular level insight into the individual behaviors of the species. In particular, it was found that in the presence of salt the polymer chains form hyperstructures and that the lithium ions undergo curvilinear diffusion along the chains within these hyperstructures. The anions, on the other hand, diffuse independently of both the polymer and lithium ions, but nevertheless undergo anomalous diffusion due to the effects of diffusing through the microheterogeneous polymer hyperstructure network. The motional behaviors of the three species were reasonably insensitive to the degree of cross-linking but somewhat more sensitive to the salt concentration, since the microcrystallinity of these polymer electrolytes increases with salt concentration.