A critical discussion on the analysis of buried interfaces in Li solid-state batteries. Ex situ and in situ/operando studies

Interfacial electro-chemo-mechanical phenomena determine the performance of Li solid-state batteries (SSBs), and thus the study of these processes is key to constructing more efficient and stable systems. In this regard, the analysis of interphases, including their evolution during cycling, is probably the most challenging aspect in the field of SSBs, as interfaces in these cells are inherently buried. In this perspective, a critical discussion on the various methodologies currently employed to gain access to buried interfaces and obtain reliable structural and chemical information is provided. Since this aspect is commonly overlooked in the literature, a particular focus has been placed on rigorous procedures allowing reproducible and unambiguous results to be achieved. Simultaneously, the valuable structural, electronic and chemical data collected on interphases by using these robust methods are presented. It is also shown that the development of strategies to probe buried interfaces has paved the way for progress in attractive in situ and operando measurements. On the other hand, new experiments carried out in our laboratory indicate that ion etching for depth profiling analysis, which is widely used to study solid electrolyte interphases (SEIs) and cathode electrolyte interphases (CEIs) in liquid- and solid-state Li batteries, is not suitable for elucidating the distribution of chemical components in composite materials present in SSBs due to the sensitivity of certain compounds like Li salts to ion bombardment. Finally, our discussion can be extended to other important materials and nanostructures presenting buried interfaces such as layered films with applications in photovoltaics and core-shell nanoparticles.