High Pressure Densification Investigation of Li ₃ PS ₄ and Li ₆ PS ₅ Cl Solid Electrolytes using Combine X‐Ray Diffraction, X‐Ray Absorption Tomography and Ionic Measurement

ABSTRACT Proper densification of thiophosphate solid electrolytes is crucial for enhancing ionic transport and for preventing dendrite formation and ensuring mechanical stability. However, achieving optimal densification remains challenging due to the absence of techniques capable of monitoring structural and morphological changes in real time. We design a specific airtight high‐pressure cell and clamp, enabling the in situ investigation of amorphous (Li 3 PS 4 ) and crystalline (Li 6 PS 5 Cl) sulfide‐based solid electrolytes. Using X‐ray computed tomography and diffraction at the Psiché beamline (SOLEIL synchrotron), we studied the samples under uniaxial compression up to 3700 MPa. We correlate the evolution of structure, density, and porosity with ionic conductivity measurements obtained via electrochemical impedance spectroscopy. The results reveal two densification stages: an initial closure of intergranular porosity, followed by structural densification through elimination of intragranular porosity. The porosity in Li 3 PS 4 reduces faster than in Li 6 PS 5 Cl, reflecting its higher softness. Above 500 MPa, ionic conductivity decreases, likely due to lattice contraction and increased Li + migration barriers. Upon decompression, both materials exhibit irreversible conductivity enhancement, attributed to plastic densification and increased structural disorder. These findings highlight the critical role of microstructural control and pressing conditions in optimizing the electrochemical performance of sulfide‐based solid electrolytes.