Observing Interfacial Reactions in Solid-State Li-Ion Batteries with X-Ray Spectroscopies

Although lithium-ion batteries (LIBs) are a key technology for enabling the transition to renewable energy sources, they remain limited back by capacity and stability issues. A better understanding of LIB degradation processes will allow more rational design to improve LIBs in terms of energy density, safety, cost, and cycle-lifetime. However, many of the key reactive areas are buried deep within LIBs, making them difficult to access with surface-sensitive techniques. In particular, the cathode-electrolyte interface (CEI) is thought to be particularly important as a site of degradation reactions, but has hitherto been mainly acessed by <i>ex-situ </i>disassembly. Such approaches inevitably change the CEI by releasing pressure and exposing the interface as a new surface, inevitably introducing a range of changes and contamination. To improve upon this issue, we introduce a novel all-solid-state <i>operando</i> battery architecture to enable access to the CEI. This design centres on a suspended thin-film cathode made via radio-frequency magnetron sputtering. This cathode, typically layered transition-metal oxide such as LiCoO₂, is combined with a solid electrolyte such as argyrodite Li₆PS₅Cl to provides a simple interface between the pure active material of the cathode and electrolyte. Avoiding the conductive or binding additives typically used in cathodes drastically simplifies the system and the number of possible interfaces, while the thinness of the cathode (tens of nanometers) allows measurements with X-ray spectroscopic techniques that would typically be restricted to surface measurements. Synchrotron-based soft X-ray absorption may be used in fluorescence yield (FY) and total electron yield (TEY) detection modes to monitor either species either in the “bulk” of the thin film (via FY) or specifically at the CEI (via TEY). Information on the chemical species and oxidations states at these different regions may therefore be observed as a function of time and the cell’s state of charge. Throughout the process, the cell is maintained in an ultra-high vacuum environment to minimise outside influences that would typically interfere with measurements. Overall, this work provides insight and approaches toward understanding the fundamental degradation mechanisms at the CEI, paving the way for longer-lasting and stable batteries.