For the preparation of the solid-state electrolyte Na3PS4 interlayer a stoichiometric mixture of Na2S (Thermo scientific, 99%) and P2S5 (Sigma Aldrich, 99%) was prepared by ball milling with a molar ratio of 3:1. The mixture was transferred into quartz ampoules in an Ar-filled glovebox and then sealed with an acetylene torch under vacuum, followed by a heat treatment at 500 °C for 48 h in a furnace. After annealing, the powder was naturally cooled down and hand-ground for further characterization and cell fabrication.