Resolving Heterogeneity in Battery Interphases By Cryogenic Electron Microscopy

Abstract

The stability of lithium batteries is tied to the physicochemical properties of the solid-electrolyte interphase (SEI), a surface passivation layer on the battery anode. However, owing to the difficulty in characterizing this sensitive interphase with nanoscale resolution, the distribution of SEI components at the nanoscale is poorly understood. Here, we use cryogenic scanning transmission electron microscopy (cryo-STEM) and electron energy loss spectroscopy (EELS) to map the spatial distribution of sensitive SEI components across the metallic Li anode. We reveal that LiF, an inorganic SEI component widely believed to play an important role in battery passivation, is absent within the compact SEI film (~15 nm) in direct contact with the active material. Instead, LiF particles (100-400 nm) are found to precipitate across the electrode surface independently from compact SEI formation. Based on these observations, we conclude that LiF cannot be a dominant contribution to anode passivation nor does it influence Li+ transport across the compact SEI film. Using cryo-STEM and EELS, we refine the traditional view of the SEI structure derived from ensemble-averaged characterizations and nuance the role of SEI components on battery performance.