Lithium Argyrodite as Solid Electrolyte and Cathode Precursor for Solid‐State Batteries with Long Cycle Life.

All‐solid‐state batteries with conversion‐type cathodes promise to exceed the performance of lithium‐ion batteries due to their high theoretical specific energy and potential safety. However, the reported performance of solid‐state batteries is still unsatisfactory due to poor electronic and ionic conduction in the composite cathodes. Here, in situ formation of active material as well as highly effective ion‐ and electron‐conducting paths via electrochemical decomposition of Li6PS5Cl0.5Br0.5 (LPSCB)/multiwalled carbon nanotube mixtures during cycling is reported. Effectively, the LPSCB electrolyte forms a multiphase conversion‐type cathode by partial decomposition during the first discharge. Comprehensive characterization, especially operando pressure monitoring, reveals a co‐redox process of two redox‐active elements during cycling. The monolithic LPSCB‐based cell shows stable cycling over 1000 cycles with a very high capacity retention of 94% at high current density (0.885 mA cm−2, ≈0.7 C) at room temperature and a high areal capacity of 12.56 mAh cm−2 is achieved. [ABSTRACT FROM AUTHOR]