Understanding Chemo Mechanical Interfacial Degradation Mechanisms in all Solid-State Batteries Using XRD-CT

As the transportation sector rapidly transitions from gasoline-driven to rechargeable battery-powered vehicles, there is an urgent need to develop more energy-dense batteries, with stable operation over long-term cycling. Using a solid-state electrolyte (SSE) offers the chance to exploit the significantly higher energy density of Li metal as an anode, compared to traditional intercalation anodes. However, interfacial stresses due to the non-uniform solid-solid contact at the anode/electrolyte interface can generate voids and cracks over cycling, as well as induce lithium plating [1]. Additionally, deleterious chemical and electrochemical reactions at the interface can exacerbate the interfacial instability, leading to crack propagation, metallic Li filling the voids, and eventually catastrophic failure of the battery [2]. Thus, understanding the nature of interfacial degradation mechanisms (chemical, electrochemical and morphological) is critical to achieving stable cycling with all solid-state batteries (ASSBs).<br/>In this work, we employ synchrotron XRD-CT (X-ray diffraction computed tomography) to understand interfacial degradation in ASSBs under in-situ conditions. Specifically, we focus on the anode – SSE interface in symmetric Li | SSE | Li cells. We focus our study on the popularly used argyrodite (LPSCl) SSE. We scan the cells in the pristine, cycled and burned (cycled to failure) conditions. Using microcomputed X-ray tomography, we map and contrast the extent of cracking at the Li | LPSCl interface in Swagelok cells. Then, we scan the cells using XRD-CT to chemically map various species across the interface, including Li, LPSCl and Li₂S.<br/>After registering the datasets together, we overlay the morphological microCT volume with the XRD-CT map, to correlate the presence of deleterious interfacial products such as Li₂S. Additionally, we map the strains in the various chemical species, to correlate regions of strain heterogeneity with the size and extent of cracking, as well as the quality of interfacial contact between the SSE and Li anode in that region. We also investigate the effect of heat treatment of the SSE prior to cycling on the grain size, and correspondingly the extent of strain heterogeneities and cracking in argyrodite SSEs after moderate cycling. The non-destructive and fast nature of synchrotron X-ray characterization provides an ideal avenue to pursue <i>operando</i> characterization. Such non-destructive and multimodal characterizations are essential to understanding the interplay between various degradation modes, in turn informing strategies to mitigate them and enhance their cycling stability.<br/>[1] Paul, P. P., Chen, B. R., Langevin, S. A., Dufek, E. J., Weker, J. N., & Ko, J. S. Interfaces in all solid state Li-metal batteries: a review on instabilities, stabilization strategies, and scalability. Energy Storage Materials 2022, 45, 969-1001.<br/>[2] Hao, S.; Daemi, S. R.; Heenan, T. M. M.; Du, W.; Tan, C.; Storm, M.; Rau, C.; Brett, D. J. L.; Shearing, P. R., Tracking lithium penetration in solid electrolytes in 3D by in-situ synchrotron X-ray computed tomography. Nano Energy 2021, 82, 105744.