Ion Dynamics in Nanocrystalline Li2S-LiI – on the Influence of Local Disorder on Short-Range Hopping and Long-Range Ion Transport

Abstract

The enormous interest in developing powerful Li-based batteries leads to a boost in materials research. Though Li–sulfur batteries offer very high energy densities, the nature of Li-ion dynamics in the final discharge product ${\text{Li}}_2\mathrm{S}$ has not been fully understood yet. While nanocrystalline ${\text{Li}}_2\mathrm{S}$ shows enhanced ion dynamics compared to its coarse-grained counterpart, the interaction of ${\text{Li}}_2\mathrm{S}$ with another binary such as LiI seems to be rather unexplored. Herein, an equimolar mixture of ${\text{Li}}_2\mathrm{S}$ and LiI is treated in a high-energy ball mill, and both the overall and local structural changes are studied by X-ray powder diffraction and ${}^{\mathrm{6,7}}\text{Li}$ nuclear magnetic resonance (NMR), respectively . Besides the formation of amorphous regions, evidences are found for the generation of anion-mixed sites that give rise to facile Li${}^{+}$ exchange on the 2D exchange NMR timescale. Compared to a coarse-grained reference sample, the overall (bulk) ionic conductivity of nanocrystalline ${\text{Li}}_2\mathrm{S}$-LiI increases by two orders of magnitude. Besides the anion-mixing effect, this increase benefits from nanosize effects that include the formation of defect-rich interfacial regions. NMR relaxation measurements fully support this result and reveal heterogeneous dynamics with lower activation energies for both the localized hopping processes and long-range ion transport in nm-sized ${\text{Li}}_2\mathrm{S}$-LiI.