Densification of cathodes and separators with sulfide-based solid electrolyte for all-solid-state batteries

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All-solid-state batteries are promising for the next generation of lithium-ion batteries. The replacement of the liquid electrolyte with a solid material can enhance the energy and power density, as well as the battery safety. Electrolytes composed of sulfides are one interesting candidate, as they have high ionic conductivity. Promising first results have been obtained with lab-scale production. As a next step, the establishment of continuous processing to realize production on an industrial-scale is necessary. One important process step here is the densification of sulfide-based cathodes and separators, which can be realized by uniaxial pressing at lab-scale or calendering. A compression can give rise to a higher power density, as well as electrical and ionic conductivity. Moreover, mechanical properties such as adhesion strength, can be increased. The solid electrolyte cannot infiltrate the pores of the active material as the liquid electrolyte, therefore, the electrode/electrolyte contact area is even more important, and significantly affected by the densification.
In this work, different strategies for the compaction of cathode and separators for all-solid-state-batteries are presented. Moreover, the effects of the densification pressure and temperature on Li3PS4-based separators, with respect to the adhesion strength as a measure for the mechanical stability, are shown. The densification was realized by a uniaxial lab-press, with regulated temperature settings. Here, an increased embrittlement of the separator with increasing densification pressure was observed, which results in a deteriorated processability, and a higher risk for short circuit. The higher fragility may be due to a stronger interlocking of the sulfidic particles. Fortunately, higher temperature during compacting can provide more stable separators and higher adhesion strength, as important requirements for favorable processability and lower risk of short circuit. This may be due to a softening of the particles or an effect of the binder. However, for scaling up to continuous processing by calendering, shorter retention times during compacting have to be considered. For this, the positive effect of temperature must be re-examined.

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