Uni-axial densification of slurry-casted Li₆PS₅Cl tapes for solid-state pouch cells: The role of particle size distribution and densification pressure

Solid-state batteries are transformative solutions for electric vehicles, offering superior energy density and safety. Sulfide-based electrolytes like Li₆PS₅Cl (LPSCl) combine high ionic conductivity and mechanical adaptability, but challenges remain in scaling up high-performance separator tapes due to particle size distribution (PSD) and processing constraints. This study investigates the uni-axial densification of slurry-casted LPSCl tapes, focusing on PSD refinement and compaction pressure. Wet milling has been identified to effectively reduce PSD to submicron levels while preserving structural integrity and near-pristine conductivity. A critical pressure threshold (~350 MPa) for tape-casted LPSCl slurries (2.5% hydrated poly(acrylonitrile-co-butadiene)) was identified, where ionic conductivity peaks due to particle fusion and the formation of conductive networks. However, open porosity (~30%), particularly along the densification direction, and surface irregularities persist. These structural issues have significant implications for battery performance. For example, surface roughness and interfacial voids lead to localized current focusing, with current densities exceeding applied values by over 20 times. Percolating porosity accelerates dendritic failure modes, undermining stability and limiting cycling rates. This work underscores the need for optimized powder processing and densification techniques to enhance scalability and performance, advancing LPSCl-based separators for the practical adoption of solid-state batteries in electric vehicles and other high-energy applications.