Solid–liquid interfaces are important in a range of chemical, physical and biological processes1,2,3,4, but are often not fully understood owing to the lack of high-resolution characterization methods that are compatible with both solid and liquid components5. For example, the related processes of dendritic deposition of lithium metal and the formation of solid–electrolyte interphase layers6,7 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 debated8,9,10,11 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, vitrification12. By vitrifying the liquid electrolyte we preserve it and the structures at solid–liquid interfaces in lithium-metal batteries in their native state, and thus enable structural and chemical mapping of these interfaces by cryo-scanning transmission electron microscopy (cryo-STEM). We identify two dendrite types coexisting on the lithium anode, each with distinct structure and composition. One family of dendrites has an extended solid–electrolyte interphase layer, whereas the other unexpectedly consists of lithium hydride instead of lithium metal and may contribute disproportionately to loss of battery capacity. The insights into the formation of lithium dendrites that our work provides demonstrate the potential of cryogenic electron microscopy for probing nanoscale processes at intact solid–liquid interfaces in functional devices such as rechargeable batteries.
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We thank D. Muller, H. Abruña and J. Noble for discussions. M.J.Z. and L.F.K. acknowledge support by the NSF (DMR-1654596) and the Packard Foundation. Z.T., S.C. and L.A.A. acknowledge support from the Department of Energy, Advanced Research Projects Agency - Energy (ARPA-E) through award number DE-AR0000750. This work made use of the Cornell Center for Materials Research (CCMR) Shared Facilities with funding from the NSF MRSEC programme (DMR-1719875). Additional support for the FIB/SEM cryo-stage and transfer system was provided by the Kavli Institute at Cornell and the Energy Materials Center at Cornell, DOE EFRC BES (DE-SC0001086). The FEI Titan Themis 300 was acquired through NSF MRI-1429155, with additional support from Cornell University, the Weill Institute and the Kavli Institute at Cornell. This work made use of electrochemical characterization facilities in the KAUST-CU Center for Energy and Sustainability, supported by the King Abdullah University of Science and Technology (KAUST) through award number KUS-C1-018-02.
Extended data figures and tables
Extended Data Fig. 6 Example damage series profiles and initial/final spectra taken for lithium materials relevant to this study over a range of doses at which damage occurs, dark-field cryo-STEM images of various types of damage induced in a frozen organic electrolyte at different doses with corresponding spectra, and before and after images of the regions in which the EELS maps in the main text were taken.