An NMR and ionic conductivity study of ion dynamics in liquid poly(ethylene oxide)-based electrolytes doped with LiN(SO₂CF₃)₂

NMR and ionic conduction measurements have been performed for two liquid-state high-molecular-weight comb-branched polyethers, which are 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 (ethylene oxide)-co-(2-(2-methoxyethoxy)ethyl glycidyl ether) (m(EO-GE)) with molecular weights of about 8 000 and 10 000, respectively. The dynamics of the lithium ion, anion, and the macromonomers were characterized by ⁷Li, ¹⁹F, and ¹H NMR spin-lattice relaxation time (T₁) and self-diffusion coefficient (D) measurements. Because the temperature dependence of the ¹H and ⁷Li NMR T₁ exhibited minima, the reorientational correlation times were able to be calculated. Above 278 K, the segmental motions in the neat liquid-state macromonomers are faster than those in the cross-linked state, and they become almost the same at lower temperatures and are slowest in m(EO-PO). When doped with LiTFSI the segmental motions in the liquid electrolytes slowed to values similar to those in the cross-linked polymer. The translational diffusion coefficients were in the order (fastest to slowest) of anions > lithium ions > macromonomers. The diffusion of the ions correlated well with the macromonomer diffusion. The ionic conductivity of doped m(EO-PO) was higher than that of doped poly(EO-PO), and comparison of the measured ionic conductivity with estimates of the ionic conductivity calculated from D_anion and D_Li indicates high ion dissociation in the macromonomer electrolytes. The results are 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.