Vacancy Driven Fast Ion Conduction in Lithium Deficient Magnesium Chloride Spinel

Chloride solid electrolytes are promising catholytes for 4 V all-solid-state batteries due to their intrinsic stability and excellent compatibility with high-voltage cathodes. Until now, very few spinel chlorides have been reported to have good ionic conductivity. We report a simple (no ball-milling required) and effective synthetic approach to boost the ionic conductivity of nonconductive Li2MgCl4 by aliovalent substitution, to adjust the Li ion carrier and vacancy concentration. Reducing the Li ion content leads to Li vacancies in the original fully occupied Li1 (8a) site and Li ion redistribution over a new Li2 (16c) site in Li1.6Sc0.4Mg0.6Cl4. The face-sharing Li tetrahedra and octahedra form a continuous 3D Li ion diffusion pathway with high Li vacancy and carrier concentration, driving nearly 2 orders of magnitude conductivity rise vs Li2MgCl4. A LiNi0.85Co0.1Mn0.05O2 ASSB using a Li1.6Sc0.4Mg0.6Cl4 catholyte showed good cycling stability over 180 cycles with a 4.8 V vs Li+/Li upper cutoff potential.