All-solid-state batteries are a potentially safer and more energy dense alternative to the modern lithium ion battery, replacing flammable organic liquid electrolytes and enabling lithium metal anodes. While solid electrolytes have been developed with ionic conductivities far in excess of modern liquid organic electrolytes, electrolyte-electrode interfacial resistance remain an issue. In spite of their fundamental importance, the nature of these interfaces remains obscured, primarily due to the limited methods of characterization of both lithium metal and buried interfaces alike. Recently, cryogenic microscopy techniques—traditionally employed by biological field—have been employed to lithium ion batteries, elucidating the nature of solid-electrolyte interfaces and lithium metal morphologies alike. Of similar difficulty, observation of solid-solid interface dynamics is limited to very few techniques, and has only been demonstrated recently, primarily via in situ transmission electron microscopy (TEM) methodologies.

We present developments in the application of cryogenic focused ion beam (FIB) and in situ TEM to enable fundamental characterization of solid-solid interfaces for lithium ion battery applications. Use of cryogenic temperatures is shown to clearly reduce damage induced in the lithium metal, preserving the morphology and structure, as demonstrated by both three-dimensional FIB slice-and-view and the observation of crystallinity in cryogenic-TEM. This technique is further extended to polymer and sulfide electrolytes, overcoming instability during FIB milling and permitting further characterization via cryo-TEM. Such cryo-prepared samples enable lithium metal for in situ TEM cycling of metal-anode-based solid-state batteries. Further, progress in in situ characterization of solid-state interfaces by TEM will be presented, showing developments in the extraction of electrochemically active nanobatteries for in situ testing applications, providing a platform for dynamic solid-solid interface characterization.