From Ultraslow to Fast Lithium Diffusion in the 2D Ion ConductorLi0.7TiS2Probed Directly by Stimulated-Echo NMR and Nuclear Magnetic Relaxation

Abstract

${}^7\mathrm{Li}$ stimulated-echo NMR and classical relaxation NMR techniques are jointly used for the first time for a comprehensive investigation of Li diffusion in layer-structured ${\mathrm{Li}}_{0.7}{\mathrm{TiS}}_2$. One single 2D Li diffusion process was probed over a dynamic range of almost 10 orders of magnitude. So far, this is the largest dynamic range being measured by ${}^7\mathrm{Li}$ NMR spectroscopy directly, i.e., without the help of a specific theoretical model. The jump rates obey a strict Arrhenius law, determined by an activation energy of $0.41(1)\mathrm{eV}$ and a preexponential factor of $6.3(1)\times {10}^{12}{\mathrm{s}}^{-1}$, and range between $1\times {10}^{-1}{\mathrm{s}}^{-1}$ and $7.8\times {10}^8{\mathrm{s}}^{-1}$ (148–510 K). Ultraslow Li jumps in the kHz to sub-Hz range were measured directly by recording ${}^7\mathrm{Li}$ spin-alignment correlation functions. The temperature and, in particular, the frequency dependence of the relaxation rates fully agree with results expected for 2D diffusion.