Ionic Conductivity versus Particle Size of Ball‐Milled Sulfide‐Based Solid Electrolytes: Strategy Towards Optimized Composite Cathode Performance in All‐Solid‐State Batteries

Abstract For the fabrication of high‐energy and high‐power all‐solid‐state batteries (ASSBs), easily synthesizable solid electrolytes are needed, which enable fast ion transport inside the composite cathode as well as good contacts between cathode active material and solid electrolyte particles. Regarding the latter, the size ratio of the particles inside the composite cathode has to be optimized. Here, we use a wet ball milling process for the synthesis of agyrodite‐type Li 5.5 PS 4.5 Cl 1.5 solid electrolyte particles and study the influence of milling time on particle size and ionic conductivity. With longer milling time, both the solid electrolyte particle size and the ionic conductivity decrease, which exert an opposing influence on the cathode performance. We show that a milling time of approximately 2 h leads to an optimum cathode performance, as this time is sufficient for a favorable particle size ratio, while a strong drop of the ionic conductivity of Li 5.5 PS 4.5 Cl 1.5 is avoided.