The demand for better batteries is surging at an ever-increasing rate. Practical realization of all solid-state batteries (ASSBs) with a Li metal anode and solid-state electrolyte (SSE) will be key to unlocking higher capacities. Currently, the primary challenge for ASSBs is their interfacial instability, which affects both safety and performance. Among SSEs, sulfide-based SSEs have the highest ionic conductivity, but a lower stability, in particular the propagation of instabilities from the anode/SSE interface into the bulk. While their deleterious effects on cell performance and safety have been explored, their precise underlying mechanisms are yet to be understood. Here we propose to use microscale, operando XRD-CT to obtain structural and morphological information on the initiation and propagation of degradation in Li|SSE|Li cells. Such an understanding of the nucleation and propagation of interfacial instability is critical in designing stabilization strategies to mitigate them.