Fast ion conducting lithium argyrodites Li6PS5X (X = Cl, Br) are promising solid electrolytes for rechargeable all-solid-state lithium batteries. Here we realized all-solid-state lithium batteries combining Li6PS5Br with CuS as the cathode. An initial discharge capacity of 650 mAh/g was achieved, though the reversible capacity after 20 cycles was limited to be 90 mAh/g. Based on X-ray diffraction measurements at different discharge stages we identified a peculiar reaction mechanism of CuS in this all-solid-state battery design, as the Li6PS5Br electrolyte is found to reversibly participate in the cell reaction: at intermediate stages of the discharge Cu+ is incorporated in the argyrodite leading to CuyLi6-yPS5Br. The reduced ionic conductivity of CuyLi6-yPS5Br and the volume change of the electrolyte during cycling affect the cyclic performance of the batteries in addition to the volume changes of the active materials. From the first cycle onwards both processes result - as evidenced by SEM - in progressive crack formation, dissociation of the active material from Li6PS5Br and carbon and consequential capacity fading. (C) 2014 Elsevier B.V. All rights reserved.