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 Li 2 MgCl 4 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 (8 a ) site and Li ion redistribution over a new Li2 (16 c ) site in Li 1.6 Sc 0.4 Mg 0.6 Cl 4 . 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 Li 2 MgCl 4 . A LiNi 0.85 Co 0.1 Mn 0.05 O 2 ASSB using a Li 1.6 Sc 0.4 Mg 0.6 Cl 4 catholyte showed good cycling stability over 180 cycles with a 4.8 V vs Li + /Li upper cutoff potential.