Stabilization of Highly Conductive Lithium Argyrodites by Means of Lithium Substitution: The Case of Li 6 Fe 0.5 PS 6

All‐solid‐state‐batteries (ASSB) are promising candidates for next generation lithium batteries providing high energy density and safety levels. Their success crucially depends on the properties of the available solid electrolyte materials, ideally offering sufficient conductivities in combination with stability against decomposition over a large electrochemical window. Therefore, the search for solid electrolytes with novel compositions remains of high interest. In this work we present a strategy to stabilize highly conductive lithium argyrodites by means of lithium substitution. This is different from the previously described possibilities to obtain stable structures by either substituting phosphorous or sulfide anions in the parent compound Li 7 PS 6 . This approach offers access to a new array of solid electrolyte candidates with an argyrodite‐type crystal structure. Conductivities of up to 1.4⋅10 −4 S/cm can be obtained for Fe‐substituted materials of general composition Li 7‐2x Fe x PS 6 .