Solid-state batteries are promising to improve safety and energy density compared to conventional lithium-ion batteries by replacing the liquid electrolyte with a solid one. Sulfide-based solid electrolytes (SE) offer the highest ionic conductivity in comparison to oxides and polymers as potential electrolyte materials.[1] However, the scale-up of production processes is complicated due to a high sensitivity against moisture.

An important process step is the densification of the separators to achieve high power density, improved ionic conductivity and defined mechanical properties. In this study, basic investigations for a densification of Li6PS5Cl (LPSCl) based separators by uniaxial pressing and calendering were performed. Separator slurries based on p-xylene as solvent and hydrogenated nitrile butadiene rubber (HNBR) as binder were produced using a dissolver and were coated on a substrate foil by doctor blading. Resulting densities, mechanical properties, as well as ionic conductivities were analyzed and correlated with the obtained microstructure, which was investigated by scanning electron microscopy and synchrotron tomography.

For calendering, porosity was reduced down to 11 %. However, an elastic deformation of the binder after densification and, to a lower extent, the solid electrolyte is assumed. This probably negatively affects the microstructure of the separator and hinders a general increase in the ionic conductivity. Thus, an increase of density does not necessarily result in an improved performance as it is indicated in literature, yet. Moreover, particle breakage was identified for all calendered LPSCl separators. To obtain a more detailed understanding, the densification of LPSCl was also analyzed for uniaxial pressing and different stack pressures. Here, an increase in the stack pressure results in an increase of the ionic conductivity. All in all, this work addresses arising challenges related to the densification of sulfide-based electrolyte materials, as well as the high importance of the individual deformation behavior of the materials used.

[1] A. Manthiram, X. Yu, S. Wang, Nature Reviews Materials, 2, 2017.