Quantifying Heterogeneous Degradation Pathways and Deformation Fields in Solid State Batteries

Solid-state batteries are compelling candidates for next generation energy storage devices, promising both high energy density and improved safety, by utilizing metallic Li as the negative electrode. However, they suffer from poor cyclability and rate capability, which limits their wide application. Degradation in these devices occurs through complex mechanical, chemical and electrochemical pathways, all of which produce heterogeneous deformation fields. Therefore, isolating solid-state degradation mechanisms, and explicitly linking them to the associated deformation fields requires a multimodal characterization strategy. Here, we present a novel 3-D, in situ methodology for linking degradation to deformation in solid-state cells. X-ray imaging is used to measure the morphological degradation, and combined with X-ray diffraction to quantify (electro)chemical aspects. Finally, the heterogeneous stress fields from these various pathways are mapped in situ. This heterogeneity is shown globally, from the interface to the bulk electrolyte, as well as locally, around features such as cracks and voids. Through these analyses, it was possible to delineate the effects of solid electrolyte processing, cell assembly, and cycling on the end-of-life state of the cell. Moreover, the importance of stress mitigation in these cells is highlighted, with mean stresses around the interface and some cracks comfortably exceeding the elastic limit of Li.