7Li NMR diffusion studies in micrometre-space for perovskite-type Li0.33La0.55TiO3 (LLTO) influenced by grain boundaries

Abstract Perovskite-type oxide solid electrolyte Li 0.33 La 0.55 TiO 3 (LLTO) samples are known to have high ionic conductivities and grain-boundary resistances from AC impedance spectroscopy. In addition, the existence of domain structures has been shown by transmission electron microscopy and other techniques. In this study Li + migration in tetragonal and cubic samples ( t - and c -LLTO) was studied in micrometre space by pulsed-gradient spin-echo (PGSE) NMR from the room temperature up to 100 °C. The grain-boundary effects appeared clearly in the 7 Li echo attenuation plots, which were not linear and included at least two components. Previously, we reported that the Li + diffusion in inorganic solid electrolytes (garnets, NASICON, sulfides) distributes widely in time and space, illustrated by the dependences of the measuring parameters such as observation time (Δ) and pulsed-field gradient (PFG) strength ( g ). In addition to the complexities of parameter dependent Li + diffusion, grain-boundary disturbance effects were observed for Li + diffusion phenomena in LLTO samples. We indicated that a unique D Li value could be estimated from the linear echo attenuation plot at convergent measuring conditions with long Δ and large g for garnets and NASICON. The LLTO samples showed curved echo attenuation plots at the convergent measuring conditions, which were analysed by two components to give two Li + diffusion constants ( D Li-fast and D Li-slow ). These values are consistent with the tracer diffusion constants measured at higher temperatures (>150 °C). For c -LLTO, D Li-fast and D Li-slow showed good correspondences with the bulk ionic conductivity (σ bulk ) and grain-boundary ionic conductivity (σ gb ), respectively. The Li carrier numbers estimated from D Li-fast and σ bulk for c - and t -LLTO showed similar values, whereas those estimated from D Li-slow and σ gb for c -LLTO were smaller by approximately one-order of magnitude.