Opportunities and Challenges of Calendering Sulfide‐Based Separators for Solid‐State Batteries

Densification of sulfide-based separators: In this study, the impact of line load, roll temperature and roller circumferential speed is investigated on slurry-based Li₃PS₄ and Li₆PS₅Cl separators. Besides basic investigations in terms of calendering such as length expansion, tensile strength and measurement of pore size distribution, the specific ionic conductivity of Li₆PS₅Cl separators is analyzed for both uniaxial pressing and calendering revealing important hints for future research studies.

Abstract

Continuous densification procedures such as calendering are crucial for sulfide-based solid-state batteries to realize industry-relevant processing. Therefore, in this study, the impact of line load, roller circumferential speed and roll temperature on slurry-based Li₃PS₄ and Li₆PS₅Cl separators compacted by a lab-calender installed in an argon-gas-filled glovebox was investigated. While the Li₃PS₄ layers became fragile in calendered state, the tested Li₆PS₅Cl separators were more suitable for calendering due to better mechanical stability. Besides basic analysis of, for example, density, length expansion, pore size distribution and specific ionic conductivity of the Li₆PS₅Cl separators, 3D images of the structures were generated based on images obtained by synchrotron tomography. Here, all calendered separators showed particle breakage of the Li₆PS₅Cl. A slight decrease of the specific ionic conductivity with increased applied line load or pressure was observed for calendering and uniaxial pressing, respectively. However, an increase in the conductivity was obtained for an increase in the stack pressure. In addition to poorer contact with the metal current collectors at low stack pressure, it is assumed that a spring back effect after densification could negatively affect the microstructure of the separator. These results highlight that a densification of binder-based Li₆PS₅Cl separators does not necessarily result in improved ionic conductivity probably due to the individual deformation behavior of the materials used.

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