Synthesis of halide SSEs: For the ampoule synthesis of Li3YBrxCl6-x, stoichiometric mass of the precursors (x ≤ 3: LiCl (>99%, Sigma-Aldrich), LiBr (>99.99%, Sigma-Aldrich), YCl3(99.99%, Sigma-Aldrich); x＞3: LiCl, LiBr, YBr3(99.9% REO)) were filled in quartz ampoules in an argon-filled glovebox. Then the ampoules were evacuated and refilled to 200 mbar Argon, flame-sealed and placed in a Muffle furnace. The powder mixtures were heated to 650 °C (above their melting point) in 4 hours and held at that temperature for 24 hours, then cooled down to room temperature over 24 hours. The resulting products were pulverized using pestle and mortar until a fine powder was obtained. The mechanochemical synthesis of Li3YBrxCl6-x with x=3 was carried out by ball milling of precursors (YCl3 and LiBr in a molar ratio of 1:3). Each 45-ml ZrO2 ball mill jar (Fritsch) was filled with a total amount of 3 g of material with 20 grams of Φ3mm and 60 grams of Φ10mm ZrO2 balls as milling media and tightly closed in the glovebox. The milling was conducted for 288 cycles of 5 minutes (each followed by 5 minutes rest) with a rotation speed of 500 rpm. After every 2 hours, the mixtures were homogenized in the argon filled box by scraping off the material pressed to the wall. A small part of the resulting material was taken out every 4 hours for analysis. The as-prepared products were put in the furnace afterwards for a sequential crystallization. The annealing is performed in quartz ampoules in the same way as described above, at a temperature of 200-600 °C for 5 hours with a heating rate of 2 °C per minute. The ampoules were naturally cooled down to room temperature after annealing.