Li7GeS5Br-An Argyrodite Li-Ion Conductor Prepared by Mechanochemical Synthesis.

In recent years, the search for glassy and ceramic Li+ superionic conductors has received significant attention, mainly due to the renaissance of interest in all-solid-state batteries. Here, we report the mechanochemical synthesis of metastable Li7GeS5Br, which is, to the best of our knowledge, the first compound of the Li2S-GeS2-LiBr system. Applying combined synchrotron X-ray diffraction and neutron powder diffraction, we show Li7GeS5Br to crystallize in the F4̅3m space group and to be isostructural with argyrodite-type Li6PS5Br, but with a distinct difference in the S2-/Br- site disorder (and improved anodic stability). Electrochemical impedance spectroscopy indicates an electrical (ionic) conductivity of 0.63 mS cm-1 at 298 K, with an activation energy for conduction of 0.43 eV. This is supported by temperature-dependent 7Li pulsed-field gradient-nuclear magnetic resonance spectroscopy measurements. Overall, the results demonstrate that novel (metastable) argyrodite-type solid electrolytes can be prepared via mechanochemistry that are not accessible by conventional solid-state synthesis routes.