Revealing Local Microenvironments at Active Electrochemical Interfaces with Operando Freezing Cryogenic Electron Microscopy

Local structural and chemical heterogeneities at active electrochemical interfaces are critical to determining safety, lifetime, and energy density in batteries and other devices, but they have long been challenging to characterize at the nanoscale. Here we present <i>operando</i> freezing cryogenic electron microscopy (cryo-EM) as a new technique to preserve device interfaces in an active electrochemical state for high-resolution structural and chemical characterization. We reveal that ion-depleted microenvironments form locally in the electrolyte adjacent to the lithium deposition interface in lithium metal batteries and are correlated with heterogenous growth morphologies. Moreover, we find that these depleted environments can arise locally even under conditions for which ion depletion is not predicted at steady state; this provides a mechanistic explanation for why dangerous lithium morphologies can still propagate in such systems and lead to thermal runaway. <i>Operando </i>freezing cryo-EM thus provides a method to directly visualize nanoscale heterogeneities that arise locally at electrochemical interfaces and play a key role in device failure.