Electrochemical characterization. Half-cells were constructed using 12 mg Li2FeS2/Li5.5PS4.5Cl1.5 cathode composites (≈73-85 μm), 80 mg of Li5.5PS4.5Cl1.5 separator and a In/LiIn anode (In-foil towards separator, Li-foil towards current collector) in a press cell setup (10 mm diameter) with stainless steel current collectors. The separator and composite are densified at a pressure of 370 MPa before application of the anode. The relative densities of the composite layer are in the range of 85±3% independent of composition. The anode is prepared by first placing Infoil (ChemPur, 0.1 mm, 99.999%) on the separator, followed by Li foil that has been freshly prepared from a Li rod (abcr GmbH, 99.8%). For the evaluation of thicker electrodes, the cathode mass was increased to 18 and 24 mg. All cells were cycled at a controlled temperature of 298 K with an applied pressure of 40 MPa and with a rate of 0.134 C (assuming theoretical capacity 300 mAh•g -1 ). The rate capability was characterized by cycling the cells for five cycles at step-wise increasing current densities starting from 0.134 C (≈0.24 to 0.98 mA•cm -2 ) going to a maximum applied current density of 5.1 mA•cm -2 . An overview of all applied current densities in terms of C-rate and the approximated cathode thicknesses is given in Table of the Supporting Information.