Effect of Stack Pressure on the Microstructure and Ionic Conductivity of the Slurry‐Processed Solid Electrolyte Li ₇ SiPS ₈

Abstract All‐solid‐state batteries (ASSBs) have gained much interest in recent years because they promise higher energy and power densities as well as improved safety over lithium‐ion batteries (LIBs). This is achieved by using non‐flammable solid electrolytes (SEs) together with lithium metal or high‐capacity silicon anodes. One major hurdle to overcome is the permanent intimate contact of all cell components to enable long‐term cycling stability. This study investigates the macroscopic (microstructure) and microscopic (atomistic) effects of uniaxial stack pressure on the transport properties of free‐standing, slurry‐processed tetragonal ( t ‐Li 7 SiPS 8 ) sheets, containing different solid electrolyte (SE)‐to‐binder ratios (SE:B) and particle size fractions. The results demonstrate that binder content and particle size significantly influence the morphology as evidenced by synchrotron‐radiation computed tomography (CT), pressure response, and ionic conductivity of the sheets. Notably, while compression mechanics are consistent across samples, relative densities, and ionic conductivities are more dependent on binder content than particle size. Larger particles and lower binder contents generally led to higher ionic conductivities. The study also reveals that activation volumes appear to increase with binder content, suggesting that extrinsic factors, particularly the binder, may obscure the calculation of the intrinsic activation volumes of t ‐Li 7 SiPS 8 . Thus, the obtained values for binder‐containing sheets may be considered apparent values. Contrary to expectations, repeated compression cycles led to a decreased ionic conductivity and relative density, likely due to microstructural damage and increased (apparent) activation volumes. Overall, the study serves as a reminder to the community to carefully interpret intrinsic values, such as the activation volume, and by extension the activation energy, in the increasingly popular binder‐containing SE sheet systems.