Sample preparation. Cathode composites of Li2FeS2 and Li5.5PS4.5Cl1.5 were prepared using a shaker mill with a frequency of 15 Hz for five minutes. Synthesis details and quality control of the synthesized materials are given in the Supporting Information (Figure , Table and). The ZrO2 shaker mill cup (15 mL) was filled with 200 mg of a Li2FeS2 and Li5.5PS4.5Cl1.5 mixture in the targeted volumetric ratio. Composites with Li2FeS2 volume fractions of 0.32, 0.41, 0.51, 0.62 and 0.74 were prepared corresponding to weight fractions of 0.4, 0.5, 0.6, 0.7 and 0.8. Twenty 3 mm ZrO2 milling media were filled in the shaker cup for the processing. Sample handling and preparation was exclusively done under inert atmosphere in an Ar-filled glovebox.