To circumvent these problems, the electrochemical stability was measured as reported in Schwietert et al. . Electrochemical cells were assembled using a simultaneous reference and counter electrode (in this case a Li-intercalation (Li4Ti5O12 (LTO-C)) and alloying compound (Li-In) with constant potential for the Li-concentrations measured) and a composite of the solid electrolyte mixed with carbon black as a working electrode (see schematic Figure ). The composite was then oxidized and reduced using galvanostatic measurements at 6.5 μA/cm 2 , so The onset of the voltage plateaus corresponds to the onset voltages of the peaks in the differential capacity curves (Figure ). This onset voltage is where decomposition is initiated, and is the limiting value used for the decomposition window that is plotted in Figure . The plot shows that the oxidative stability of Li3YBrxCl6-x can be split into two groups, one with an oxidation onset at 3.0 V for 3＜x≤6 and 3.5 V for x≤3. This two groups can be distinguished in two ways, namely their crystal structure (𝑃3 ̅ 𝑚1 vs 𝐶2/𝑚), as well as the dominant anion (Cl-rich vs Br-rich).