Solid-liquid interfaces play a key role in many processes, such as biomineralization, crystal growth, and electrochemical energy generation and storage, but often lack characterization at spatial resolutions relevant to these processes. Additionally, many techniques used currently require the sample to be washed and dried, which can significantly alter the structure and chemistry of the interface. Finally, chemically reactive materials are present at solid-liquid interfaces in many devices, such as batteries, which makes preparation, storage, and characterization of these materials in an unaltered state challenging. In the work presented here, we describe cryogenic electron microscopy techniques that allow us to study the structure and bonding of intact solid-liquid interfaces and reactive materials at high resolution. We introduce a paired cryo-focused ion beam/scanning electron microscopy technique designed to enable the native structure and elemental composition of natural and engineered interfacial layers tens of nanometers thick to be characterized on surfaces in devices. Additionally, we describe a cryo-focused ion beam technique for extracting and preparing solid-liquid interface cross sections for high-resolution scanning transmission electron microscopy with the liquids intact, including a method for precisely localizing features of interest without labels, even those below the sample surface in some cases. Finally, we combine these techniques to study electrode-electrolyte interfaces processes at the nanoscale in lithium metal batteries, which are promising candidates to replace lithium-ion batteries, but which are currently limited by processes that occur at these interfaces. We find that two types of dendritic lithium structures are present at the lithium anode-electrolyte interface, one which has an extended interphase layer on its surface, and the other which is composed of uniform lithium hydride, which has not been observed before. These lithium hydride dendrites may contribute disproportionately to the significant capacity fade observed in lithium metal batteries, and our findings allowed us to propose potential methods for inhibiting these dendrites, which initial experiments support. The development of the cryogenic techniques described here has thus not only allowed us to reveal never-before-seen features of solid-liquid interfaces at the nanoscale, but has enabled practical pathways toward improving lithium metal battery performance to be proposed.