  The solid electrolyte interphase (SEI) is regarded as the most complex but
the least understood constituent in secondary batteries using liquid and solid
electrolytes. The nanostructures of SEIs were recently reported to be equally
important to the chemistry of SEIs for stabilizing Li metal in liquid
electrolyte. However, the dearth of such knowledge in all-solid-state battery
(ASSB) has hindered a complete understanding of how certain solid-state
electrolytes, such as LiPON, manifest exemplary stability against Li metal.
Characterizing such solid-solid interfaces is difficult due to the buried,
highly reactive, and beam-sensitive nature of the constituents within. By
employing cryogenic electron microscopy (cryo-EM), the interphase between Li
metal and LiPON is successfully preserved and probed, revealing a multilayer
mosaic SEI structure with concentration gradients of nitrogen and phosphorous,
materializing as crystallites within an amorphous matrix. This unique SEI
nanostructure is less than 80 nm and is shown stable and free of any organic
lithium containing species or lithium fluoride components, in contrast to SEIs
often found in state-of-the-art organic liquid electrolytes. Our findings
reveal insights on the nanostructures and chemistry of such SEIs as a key
component in lithium metal batteries to stabilize Li metal anode.
