Fast Li Ion Dynamics in the Mechanosynthesized Nanostructured Form of the Solid Electrolyte Li3YBr6

Abstract

The Y-halides Li3YBr6 and Li3YCl6 have recently gained considerable attention as they might be used as ceramic electrolytes in all-solid-state batteries. Such materials need to show sufficiently high ionic conductivities at room temperature. A thorough investigation of the relationship between ion dynamics and morphology, defect structure, and size effects is, however, indispensable if we want to understand the driving forces behind Li ion hopping processes in these ternary compounds. Li3YBr6 can be prepared by conventional solid-state synthesis routes. Nanostructured Li3YBr6 is, on the other hand, directly available by mechanosynthesis under ambient conditions. The present study is aimed at shedding light on the question of whether (metastable) mechanosynthesized Li3YBr6 might serve as a sustainable alternative to annealed Li3YBr6. For this purpose, we studied the impact of structural disorder on ionic transport by combining mechanosynthesis with soft-annealing steps to prepare Li3YBr6 in two different morphologies. While structural details were revealed by X-ray powder diffraction and by high-resolution 6Li and 79Br magic angle spinning nuclear magnetic resonance (NMR) spectroscopy, broadband impedance measurements in conjunction with time-domain 7Li NMR relaxation measurements helped us to characterize Li+ dynamics over a wide temperature range. Interestingly, for Li3YBr6, annealed at 823 K, we observed a discontinuity in conductivity at temperatures slightly below 273 K, which is almost missing for nano-Li3YBr6. This feature is, however, prominently seen in NMR spectroscopy for both samples and is attributed to a change of the Li sublattice in Li3YBr6 Although a bit lower in ionic conductivity, the nonannealed samples, even if obtained after a short milling period of only 1 h, shows encouraging dynamic parameters (0.44 mS cm–1, Ea = 0.34 eV) that are comparable to those of the sample annealed at high temperatures (1.52 mS cm–1, Ea = 0.28 eV). 7Li nuclear magnetic relaxation, being solely sensitive to Li+ hopping processes on shorter length scales, revealed highly comparable Li+ self-diffusion coefficients on the order of 10–12 m2 s–1, which we extracted directly from purely diffusion-controlled 7Li NMR rate peaks. Spin-lock 7Li NMR reveals a change from uncorrelated to correlated dynamics at temperatures as low as 220 K.