Transformations at interfaces between solid-state electrolytes (SSEs) and lithium metal electrodes can lead to high impedance and capacity decay during cycling of solid-state batteries, but the links between structural/chemical/mechanical evolution of interfaces and electrochemistry are not well understood. Here, we use in situ X-ray computed tomography to reveal the evolution of mechanical damage within a Li1+x Al x Ge2–x (PO4)3 (LAGP) SSE caused by interphase growth during electrochemical cycling. The growth of an interphase with expanded volume drives fracture in this material, and the extent of fracture during cycling is found to be the primary factor causing the impedance increase, as opposed to the resistance of the interphase itself. Cracks are observed to initiate near the edge of the lithium/LAGP interface, which agrees with simulations. The chemomechanical effects of interphase growth studied here are expected to play a role in a variety of SSE materials, and this work is a step toward designing durable interfaces.

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