Lithium-Ion Diffusion in Perovskite-Type Solid Electrolyte Lithium Lanthanum Titanate Revealed by Pulsed-Field Gradient Nuclear Magnetic Resonance

Abstract

Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) is a powerful tool for measuring the diffusion coefficient of lithium in solid electrolytes. In this study, the temperature dependence of the lithium diffusion coefficient in polycrystalline Li0.29La0.57TiO3 (LLTO) is determined by carefully designed experiments. The microstructure of LLTO consists of randomly oriented grains and 90° domains. The echo decay of the PFG is not linear, as predicted by the Stejskal–Tanner equation, but is curvilinear. The decay curves are analyzed with theoretical equations for randomly oriented crystals to provide evidence of two-dimensional diffusion of Li+ ions in LLTO. The NMR and conductivity diffusion coefficients agree well with each other over a wide temperature range of 273–723 K and exhibit non-Arrhenius behavior. Since the timescale of the PFG-NMR diffusion coefficient corresponds to the bulk ionic conductivity, the origin of non-Arrhenius behavior is not the number of carriers but rather the change in the mobility of Li+ ions in LLTO. Specifically, the activation energy of local Li+-ion hopping decreases at temperatures above 450 K. Herein, the results are discussed in comparison with other experimental data and molecular dynamics simulations.