Evaluating densification effects on sulfide-based electrolytes to examine an application in scalable solid-state battery manufacturing

To make solid-state batteries viable for industrial applications, the resulting electrode or separator layers must fulfill specific requirements, including the mechanical stability and consistent product quality. This study systematically investigates the processability of slurry-based separators comprised of β-Li₃PS₄ (LPS) and hydrogenated nitrile butadiene rubber (HNBR) as binder for uniaxial densification. Fabrication and stack pressure, densification temperature, and layer thickness are taken into account. Key properties such as coating density, adhesion strength, hardness, reduced elastic modulus, ionic conductivity and electrochemical cell performance are evaluated. To reveal the impact of binder, the ionic conductivity was also analyzed for the pure electrolyte powder. Distribution of relaxation times (DRT) analysis was applied. Significant differences in the stress-induced strain in the electrolyte crystal lattice that is assumed to improve the charge transfer were identified for the binder-based separators compared to pure electrolyte powder. Also, the fast elastic recovery after compaction, potential binder migration, and the importance of the sample cell transfer for the interpretation of the measured ionic conductivity were analyzed. The latter is also compared to separators containing Li₆PS₅Cl (LPSCl). These findings provide fundamental insights into the densification of sulfide-based electrolytes, especially with regard to the role of the binder, which is necessary for scalable battery production.