Lithium ion micrometer diffusion in a garnet-type cubic Li₇La₃Zr₂O₁₂ (LLZO) studied using ⁷Li NMR spectroscopy

Mobile lithium ions in a cubic garnet Li₇La₃Zr₂O₁₂ (Al-stabilized) were studied using ⁷Li NMR spectroscopy for membrane and powder samples, the latter of which was ground from the membrane. Lithium diffusion in a micrometer space was measured using the pulsed-gradient spin-echo ⁷Li NMR method between 70 and 130 °C. When the observation time (Δ) was shorter than 20 ms, the echo attenuation showed diffusive diffraction patterns, indicating that the Li⁺ diffusing space is not free but restricted. For longer Δ, the values of apparent diffusion constant (D_apparent) became gradually smaller to approach an equilibrated value (close to a tracer diffusion constant). In addition, the D_apparent depends on the pulse field gradient strength (g) and became smaller as g became larger. These experimental results suggest that the lithium ions diffuse through Li⁺ pathways surrounded by stationary anions and lithium ions, and are affected by collisions and diffractions. One-dimensional profiles of the membrane sample of thickness 0.5 mm were observed from 65 to 110 °C and the area intensity, as well as the lithium occurrence near the surface, increased with the increase in temperature. The temperature-dependent area intensity showed a correspondence to the number of Li⁺ carrier ions estimated from the ionic conductivity and the equilibrated diffusion constant through the Nernst-Einstein relationship.