Abstract
<jats:p> Understanding the dynamic processes at solid–solid interfaces is crucial for developing durable, high–energy–density solid-state batteries (SSBs). In situ and operando characterization techniques offer unique insights into the continuous evolution of interfacial phenomena. However, direct access to these buried interfaces remains challenging. In this work, we developed a methodology that applies operando X-ray photoelectron spectroscopy (XPS) to the cross-section of SSBs in a full-cell configuration, enabling simultaneous detection of chemical changes within both the composite solid electrolyte (SE) and cathode layers under controlled pressure and temperature, thereby ensuring realistic operating conditions. We also provide comprehensive guidelines and highlight several pitfalls encountered during the method’s development. Finally, we demonstrate the application of this method to a sulfide-based Li <jats:sub>6</jats:sub> PS <jats:sub>5</jats:sub> Cl system, revealing its redox behavior in both the composite cathode and SE layers, and highlighting the influence of sample preparation, surface contamination, and measurement conditions on the XPS results. </jats:p>