Operando X-ray photoelectron spectroscopy, to monitor the chemical and electronic interface evolution in all-solid-state batteries

The (electro-)chemical reactivity and electronic properties across electrified solid-solid interfaces critically determine the functionality of solid-state electrochemical devices. To date, questions still arise in the scientific community due to the limitations of surface-sensitive characterization techniques, especially in post-mortem mode. Particularly, parameters such as the electrochemical stability of the solid electrolyte (SE) as well as the ionic and electronic percolation within the composite electrode are essential in order to push all-solid-state lithium-ion batteries (SSBs) towards practical applications. In this work, we highlight operando X-ray photoelectron spectroscopy (XPS) as a straightforward method to monitor the SE reduction within a composite electrode consisting of Li₄Ti₅O₁₂ (LTO) as the electrode active material, (Li₂S)₃-P₂S₅ (LPS) as the SE and vapor-grown carbon fiber as the conductive additive under real working conditions. The results clarify previous discrepancies between experimental and theoretical results regarding the anodic stability limit of LPS and also reveal the limitations of post-mortem XPS measurements that can potentially lead to misinterpretations. We identify the reduction process of LPS which starts at 1.7 V vs. Li⁺/Li (1.1 V vs. InLi_x), resolved as a two-step process accompanied by the formation of an electronically insulator Li₂S and electronically conductive Li_xP species. Furthermore, we propose in our study a fundamental physical model to underline the effect of the external cell voltage on the core levels binding energy shifts which provides a direct contactless measurement of the local surface potentials of the different particles. Thanks to that, the operando methodology resolves the LTO redox activity and the distribution of conductive and non-conductive LTO species as a function of lithiation. This allows a deeper understanding of both the lithium-ion transport through the composite electrode and the characteristic metal-insulator transition associated with LTO. Finally, we discuss how the interfacial reactivity between LPS and LTO impacts the electrochemical performance of SSBs. Our novel operando XPS development paves the way for an accurate interpretation of the core levels peaks and to the assignment of the SE byproducts leading to a better description of the interfacial reaction mechanism in SLiB.