Correlating the NMR self-diffusion and relaxation measurements with ionic conductivity in polymer electrolytes composed of cross-linked poly(ethylene oxide-propylene oxide) doped with LiN(SO2CF3)2

Abstract

A solvent-free solid-polymer electrolyte based on a cross-linked poly(ethylene oxide-propylene oxide) random copolymer doped with LiN(SO2CF3)2, was studied using multinuclear NMR and ionic conductivity. The NMR spin-lattice relaxation times, T1, of the bulk polymer (1H), the lithium ion (7Li) and the anion (19F) were analyzed using single exponential analysis above the glass transition temperatures. Since the temperature dependent H1 and Li7 NMR T1 values had minima, the reorientational correlation times were obtained for the segmental motion of the CH2CH2O/CH2CH(CH3)O moiety of the bulk polymer and the hopping motion of the lithium ions correlated with the segmental motion. The spin–spin relaxation of the anion signals appeared single exponential with respect to time, whereas that of the polymer and the lithium echo signals were at least bi-exponential. Since both the spin-lattice and spin–spin relaxation of the anion indicated a single component, the self-diffusion coefficients, D, were measured using F19 pulsed-gradient spin-echo (PGSE) NMR measurements. Although the PGSE attenuation data appeared single exponential at each value of the separation between the gradient pulses, Δ, the measured D values had a Δ-dependence. Phenomenologically, the anion diffuses quicker in a shorter range and the activation energy of the shorter-time diffusion is smaller than that of the longer-time diffusion. The apparent self-diffusion coefficients became smaller for longer Δ and approached a constant when Δ was longer than 0.05 s. The mean square displacements of the anion were inconsistent with standard diffusion models including “anomalous diffusion” as found for a neutral particle diffusing in a fractal network [i.e., 〈r2(Δ)〉∝Δκ with κ<1(κ≡2/dw where dw is the random walk fractal dimension)]. The apparent diffusion coefficients of the lithium ions at Δ=0.02 s are almost independent of temperature and smaller than the corresponding diffusion coefficients of the anion. Since the activation energy of the anion determined for Δ longer than 0.05 s correlates well with those obtained from the ionic conductivity, the ion conduction in the solid-polymer medium is driven mainly by fast transfer of the anions.