Imaging the Solid-Electrolyte Interphase in Lithium Metal Batteries with Cryogenic Electron and Ion Microscopy

The solid-electrolyte interphase (SEI) controls the transport of ions and electrons during battery operation. Thus, understanding its structure and chemistry is necessary to understand battery function. However, there are significant challenges in characterizing the SEI: it is nanoscale in dimension, heterogeneous in composition, and comprised of organic and inorganics species. The first two points suggest that electron and ion microscopy methods are needed, as they have the appropriate spatial resolution. However, organic materials present significant challenges to these approaches, as these materials are usually susceptible to irradiation damage.<br/><br/>Cryogenic transmission electron microscopy (cryo-TEM) has recently been used to observe lithium metals and their SEIs without electron beam damage. Most cryo-TEM research has probed lithium by electrochemically growing lithium on copper grids or extracting it from current collectors. These approaches necessitate removing liquid electrolytes from active materials by drying and washing before being characterized. However, it is unclear whether the structure and chemistry of the SEIs remain intact during such processes.<br/><br/>Vitrification is one way to observe the SEI: this is done by rapidly freezing liquid electrolyte media. However, as the vitreous liquid electrolytes that cover the active materials are often too thick, it is difficult to probe nanoscale assemblies of SEIs at high resolution through subsequent cryo-(S)TEM imaging. It is, therefore, crucial to develop a strategy to control the thickness of vitrified samples to obtain the desired electron transparency.<br/><br/>Cryogenic focused ion beam microscopy methods (cryo-FIB) allow thickness control through the cryogenic lift-out process; samples can be lifted out from specific sample areas and subsequently attached to TEM support grids for further thinning. Using the cryogenic lift-out process to characterize SEIs in energy storage materials has two advantages: (i) the thickness of the sample of interest can be carefully controlled through FIB milling and (ii) the configuration of battery structures can be preserved and selectively extracted without losing spatial information surrounding the region of interest.<br/><br/>In this work, we have used lithium and its SEI as a model material to identify how ion and electron beam damage can occur and how cryo-FIB can mitigate them. We also demonstrate the potential of cryo-TEM analysis for beam-sensitive battery materials using cryo-FIB.<br/>In this talk, we will answer the following three critical questions:<br/>1. We will answer how lithium gets damaged by ion beam milling at cryogenic temperatures.<br/>2. We will discuss ion beam damage to SEIs.<br/>3. We will finally show the structural alteration of SEIs before and after liquid electrolytes are removed, demonstrating the possibility of high-resolution imaging of nanoscale features in SEIs with vitrified electrolytes.