All-solid-state batteries employing inorganic solid electrolytes are considered a promising next-generation energy storage system. Especially, argyrodite-type Li 6 PS 5 Cl (LPSCl) was highlighted for its good ionic conductivity and malleability. In the electrode level, achieving an efficient Li + pathway even with the maximized active material proportion and electrode mass loading is vital for a high energy density. Therefore, a smaller solid electrolyte particle is highly sought for. Herein, we demonstrated a spiral jet mill process for refining LPSCl particle size. The mean particle size (D 50 ) of LPSCl was reduced from 39.9 to 1.9 μm where the ionic conductivity was decreased from 2.0 to 0.23 mS cm −1 . The postannealing in a mild-temperature range (T < 250 °C) led to the even more decreased ionic conductivity. Although the ionic conductivity was restored when T > 300 °C, this high-temperature annealing resulted in severe particle agglomeration. Spectroscopic observations revealed that particle surfaces were damaged during pulverization and irreversible sulfur losses occurred during the mild-temperature annealing.