Partial transport characterization. Two techniques were employed to evaluate the partial conductivities of the composites. The electronic conductivity was exclusively measured by chronoamperometry in electron conducting/ion blocking conditions, created by placing the sample between stainless steel current collectors. The ionic conductivity is determined in electron-blocking but ion conducting conditions by stacking SE and In/LiIn symmetrically on both sites of the sample. The polarization experiments were performed in a range of -45 mV to 50 mV, and 1 mV to 8 mV, for the electronic and ionic conductivity, respectively. All raw data and details of the data evaluation are given in the Supporting Information. Potentiostatic impedance spectroscopy was conducted to corroborate the partial ionic conductivity of the composites. The spectra were collected in a half cell configuration of In/LiIn|Li5.5PS4.5Cl1.5|Li2FeS2/Li5.5PS4.5Cl1.5. The investigated frequency range was 7 MHz to 50 mHz, with 25 sampled frequencies per decade using an excitation amplitude of 10 mV. The response was analyzed using a Z-type transmission-line model. Simultaneous determination of the partial electronic conductivities could not be done reliably. This is, because the significantly lower resistance contributions lead to increased uncertainties during fitting. Thus, the electronic resistance where fixed to the expected value from chronoamperometry to stabilize the fitting procedure. Details of the data evaluation are given in the main text and the Supporting Information.