Microscopic dynamics of lithium diffusion in single crystal of the solid-state electrolyte La${}_{2/3-x}$Li${}_{3x}$TiO${}_3$ ($x=0.13$) studied by quasielastic neutron scattering

Abstract

Quasielastic neutron scattering (QENS) measurements combined with first principles based moleculardynamics calculations were conducted to study the dynamics of Li${}^{+}$ ions in a solid-state electrolyte La${}_{2/3-x}$Li${}_{3x}$TiO${}_3$ (LLTO) with $x=0.13$. By using a large ${}^7$Li-enriched single crystal sample, a QENS signal was clearly observed along the three principal axes [110], [111], and [001] at a temperature ($T$) of 600 K. Wave vector dependence of the linewidth of the QENS signal along each direction was explained well using the Chudley-Elliot model for jumps between the A sites of the perovskite lattice through the bottleneck square, which was also supported by molecular dynamics calculations. At $T=600$ K, the estimated self-diffusion coefficient of Li${}^{+}$ ($D_{Li}$) in the $ab$ plane [$D_{Li}^{ab}=(6.8\pm 0.5)\times 10^{-6}$ cm${}^2$/s] was slightly larger than that along the $c$ axis [$D_{Li}^c=(4.4\pm 0.3)\times 10^{-6}$ cm${}^2$/s], suggesting quasi-isotropic diffusion, that is, the three-dimensional diffusion of Li${}^{+}$ ions. The decrease in $D_{Li}$ with decreasing $T$ was reasonably explained by a thermal activation process with the activation energy determined from ionic-conductivity measurements. Furthermore, the estimated values of the self-diffusion coefficient of Li${}^{+}$ ions are comparable to those in the sulfide-based Li${}^{+}$ ion conductor, Li${}_7$P${}_3$S${}_{11}$, although its ionic conductivity is 10 times larger than that for LLTO. The obtained microscopic information on Li${}^{+}$ diffusion in LLTO clarifies how to understand the Li conduction mechanism in LLTO and Li${}_7$P${}_3$S${}_{11}$ in a unified manner and can provide a way to increase the Li${}^{+}$ ionic conductivity in oxide-based solid electrolytes.

7 pages, 6 figures, 2 tables with supplemental material (3 pages, 4 figures)