Understanding the Mechanical Dynamics of Lithium-Metal Battery Formation Protocols Using Operando Spatially-Resolved Ultrasound

Formation, the initial cycles of an SEI-forming battery system, is well understood to affect the cycling behavior of the system. This is particularly crucial for anode-free lithium metal batteries due to the significant effect of Li plating morphology and SEI on long-term cyclability. Low-cost, operando tools that spatially map heterogeneities are important for the study and validation of formation protocols. Here, we use spatially-resolved ultrasound transmission and reflection during formation to study variations in gas formation and morphology of Li metal anode-free cells as a function of temperature, pressure, and current density. These non-invasive results are validated with ex situ optical and SEM imaging, and XPS is used to determine the SEI speciation. Formed cells underwent increased-rate cycling coupled with one-dimensional acoustics to understand the effect of formation protocol on mechanical and electrochemical cycling performance.