An NMR and Ionic Conductivity Study of Ion Dynamics in Liquid Poly(ethylene oxide)-Based Electrolytes Doped with LiN(SO2CF3)2

Abstract

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 Danion and DLi 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.