Abstract
The lithium ion diffusion coefficient of a 93% Li β-alumina single crystal was measured for the first time using pulsed field gradient (PFG) NMR spectroscopy with two different crystal orientations. The diffusion coefficient was found to be 1.2 × 10(-11) m(2)/s in the direction perpendicular to the c axis at room temperature. The Li ion diffusion coefficient along the c axis direction was found to be very small (6.4 × 10(-13) m(2)/s at 333 K), which suggests that the macroscopic diffusion of the Li ion in the β-alumina crystal is mainly two-dimensional. The diffusion coefficient for the same sample was also estimated using NMR line narrowing data and impedance measurements. The impedance data show reasonable agreement with PFG-NMR data, while the line narrowing measurements provided a lower value for the diffusion coefficient. Line narrowing measurements also provided a relatively low value for the activation energy and pre-exponential factor. The temperature dependent diffusion coefficient was obtained in the temperature range 297-333 K by PFG-NMR, from which the activation energy for diffusion of the Li ion was estimated. The activation energy obtained by PFG-NMR was smaller than that obtained by impedance measurements, which suggests that thermally activated defect formation energy exists for 93% Li β-alumina single crystals. The diffusion time dependence of the diffusion coefficient was observed for the Li ion in the 93% Li β-alumina single crystal by means of PFG-NMR experiments. Motion of Li ion in fractal dimension might be a possible explanation for the observed diffusion time dependence of the diffusion coefficient in the 93% Li β-alumina system.
Highlights
Lithium ion conducting electrolyte materials have attracted considerable interest because of their potential use in electrochemical devices such as batteries and sensors.1–3 It is well known that solid electrolytes are fundamentally safer than their liquid counterparts
We report the direct measurement of the diffusion coefficient of Li ions for the first time in a β-alumina single crystal at room temperature using pulsed field gradient (PFG)-NMR, and a comparative discussion is presented in connection with results derived from other experimental techniques such as impedance
Because in β-alumina single crystals, monovalent cations possess 2D conductivity along the ab plane, the anisotropic diffusion coefficient of the Li ion was measured with the crystallographic c axis of the single crystal alternately oriented both perpendicular and parallel to the gradient field
Summary
Lithium ion conducting electrolyte materials have attracted considerable interest because of their potential use in electrochemical devices such as batteries and sensors. It is well known that solid electrolytes are fundamentally safer than their liquid counterparts. The line narrowing process provides a low estimate of the frequency factor and the activation energy for 7Li in β-alumina as discussed by Villa et al.. Walstedt et al reported on the microscopic co-ionic conductivity of 70% and 90% Li β-alumina single crystals by T1 relaxation measurements.. Walstedt et al reported on the microscopic co-ionic conductivity of 70% and 90% Li β-alumina single crystals by T1 relaxation measurements.8 Using both NMR and Raman spectroscopy, Villa et al provided various details concerning the microscopic motion of Li ions in the β-alumina crystal.. We report the direct measurement of the diffusion coefficient of Li ions for the first time in a β-alumina single crystal at room temperature using PFG-NMR, and a comparative discussion is presented in connection with results derived from other experimental techniques such as impedance
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