Toward understanding the anomalous Li diffusion in inorganic solid electrolytes by studying a single-crystal garnet of LLZO-Ta by pulsed-gradient spin-echo nuclear magnetic resonance spectroscopy.

Abstract

Li diffusion was observed by 7Li nuclear magnetic resonance (NMR) spectroscopy in three single-crystal samples of LLZO-Ta (Li6.5La3Zr1.5Ta0.5O12) grown by the floating zone melting method as well as a crushed sample in this study. Previously, the pulsed-gradient spin-echo 7Li NMR method was applied to Li+ diffusion measurements in inorganic solid electrolyte powder samples. Anomalous Li+ diffusion behaviors were observed such as dependence of the observing time (Δ) and pulsed-field-gradient strength (g), and the diffusive-diffraction patterns in short Δ in the echo-attenuation plots. In the powder samples, it is uncertain that the Li ions diffuse in the bulk within grain, across grains, or both. To date, the origins of the anomalous Li+ diffusion have not yet been clearly understood. From models of atomic-level lithium pathways, the micrometer-space diffusion channels are assumed to be narrow with curvatures. In contrast to the powder samples, a single crystal is supposed to be uniform without grain boundaries and the Li ions in single-crystal samples can diffuse in the bulk with negligible effects from the surface. The single-crystal samples are expected to give us proper answers. We found that the 7Li echo-attenuation plots of the single-crystal samples showed anomalous phenomena in dependence on Δ and g with much reduced manners. We found that the phenomena are inherent characteristics of Li+ diffusion in inorganic solid electrolytes. From the aspects of Li+ carrier numbers, the fast divergent Li+ diffusion constants, observed at short Δ with small g, contribute importantly to the electrochemical high ionic conduction measured by impedance spectroscopy.