A strategy that has been successfully employed to tune the ionic conductivity of Li3M(III)X6 materials is halogen substitution . Specifically for the compositional tie-line between Li3YCl6 and Li3YBr6, the Li3YBr3Cl3 composition was shown to have a 7.2 10 -3 S/cm ionic conductivity at room temperature (compared to the ~1 10 -3 S/cm of the full Li3YCl6/Li3YBr6 2 ). Based on thermodynamic phase equilibrium calculations it has been predicted that Li3YCl6 should be stable between 0.62-4.21 V vs. Li/Li + , and Li3YBr6 between 0.59-3.15 V vs. Li/Li + (ref. ). According to these calculations, the materials would decompose into the Y)X3 precursor and the halogen gas X2 at high potentials (oxidation), and to metallic Y and LiX (X = Cl,Br) at low potentials (reduction). These calculations suggest that introducing Br may reduce the high-voltage stability, leading to a tradeoff between cathode compatibility and ionic conductivity. However, it has been shown that such calculations do not always reflect the practically relevant stability window of solid electrolytes, as the (possible) formation of intermediate phases are not considered .