Solid-state lithium metal batteries have garnered significant interest in recent years due to the potential for solid-state electrolytes (SSEs) to enable a lithium metal anode by suppressing dendrite growth and eliminating hazardous liquid electrolytes. However, the development of solid-state lithium metal batteries has been limited by numerous challenges that exist at the interface between lithium and SSEs. These issues typically manifest as lithium metal penetration through the electrolyte, void formation at the interface, or interfacial decomposition to form an interphase. Here, we characterize the Li/SSE interfaces between reactive oxide and sulfide SSEs in situ and operando during the cycling of symmetric cells. Using a modified cell geometry with the oxide SSE Li_(1.4)Al_(0.4)Ge_(1.6)(PO_4)_3 (LAGP), we are able to monitor interphase growth and crack propagation throughout cycling with an optical microscope. Cycling at higher current densities reveals significantly more interphase growth in LAGP, accelerating crack growth and cell failure. Operando synchrotron X-ray tomography of symmetric cells using the sulfide SSE Li_(10)SnP_2S_(12) reveals the dynamic processes of both interphase and void formation at the interface. Segmentation of voids and interphase enables us to quantify the volume of these phases and relate these parameters to changes in electrochemistry. These results demonstrate the importance of interfacial stability from both a chemical and mechanical standpoint.
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