The oxidation and reduction voltages were measured by drawing a constant current of 6.5 μA/cm 2 to 0 V vs Li/Li + (reduction of the solid electrolyte) i.e. galvanostatically charging to 3.5 V versus prelithiated LTO or discharging to -0.62 V versus the In-Li alloy with a current density of 6.5 μA/cm 2 . For the determination of the electrochemical stability window, the differential capacity was calculated based on the charge and discharge profiles.