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Solid–liquid interfaces are important in a range of chemical, physical and biological processes^{, , –}, but are often not fully understood owing to the lack of high-resolution characterization methods that are compatible with both solid and liquid components. For example, the related processes of dendritic deposition of lithium metal and the formation of solid–electrolyte interphase layers^, are known to be key determinants of battery safety and performance in high-energy-density lithium-metal batteries. But exactly what is involved in these two processes, which occur at a solid–liquid interface, has long been debated^{, , –} because of the challenges of observing such interfaces directly. Here we adapt a technique that has enabled cryo-transmission electron microscopy (cryo-TEM) of hydrated specimens in biology—immobilization of liquids by rapid freezing, that is, vitrification. By vitrifying the liquid electrolyte we preserve it …