  The lithium metal-solid-state electrolyte interface plays a critical role in
the performance of solid-state batteries. However, operando characterization of
the buried interface morphology in solid-state cells is particularly difficult
because of the lack of direct optical access. Destructive techniques that
require isolating the interface inadvertently modify the interface and cannot
be used for operando monitoring. In this work, we introduce the concept of
thermal wave sensing using modified 3-omega sensors that is attached to the
outside of the lithium metal-solid state cells to non-invasively probe the
morphology of the lithium metal-electrolyte interface. We show that the thermal
interface resistance measured by the 3-omega sensors relates directly to the
physical morphology of the interface and demonstrate that 3-omega thermal wave
sensing can be used for non-invasive operando monitoring the morphology
evolution of the lithium metal-solid state electrolyte interface.
