Mapping The Nanoscale Structure of Solid Electrolyte Interphase in Lithium Metal Batteries via Cryogenic STEM-EELS

Lithium metal batteries (LMBs) offer uniquely high energy density potential among energy storage devices but face cycling stability and safety challenges. While the solid electrolyte interphase (SEI) plays a crucial role in inhibiting parasitic reactions and shaping Li behavior, it remains relatively poorly understood within LMBs [1,2]. This can largely be attributed to the challenges in characterizing the SEI, due to its nanometer length scale thicknesses, high radiation sensitivity, and poor accessibility for probing throughout the cycling process. Recently, cryogenic transmission electron microscopy (cryo-TEM) has enabled high resolution imaging of lithium metal interfaces, revealing SEI behavior on lithium which has been electrochemically deposited directly on to TEM grids [3]. In addition, while <i>in-situ</i> characterization of the SEI in LMB coin cells can be challenging due to limited space and access, recent work using the cryogenic focused ion beam (cryo-FIB) lift-out technique enables direct imaging of the SEI in cryogenically preserved anode-electrolyte interfaces [4]. In this work, using cryo-scanning transmission electron microscopy (cryo-STEM) and electron-energy-loss spectroscopy (EELS), we study cryogenically preserved lithium electrode in LMBs and show that we can map high resolution structural and chemical changes within the SEI itself.<br/><br/>LMBs with Li-free cathodes present additional challenges in interfacial stability and performance due to the larger polarization during dissolution, which studies have shown strongly influences dendrite formation upon subsequent deposition [5,6]. For this work, we sought to characterize the morphological and chemical features of the SEI at the Li metal anode in both initial dissolution and deposition cases to better understand the effects of this asymmetry. Utilizing the cryo-FIB lift-out technique, the Li anode-electrolyte interface was cryogenically preserved, and electron transparent cross-sections of a dendritic site for each case were produced. Spectroscopic mapping by cryo-STEM EELS shows us the elemental distribution across the Li dendrite interface, allowing us to characterize the general distribution and chemical composition of the SEI. Further, by incorporating a novel machine learning analysis technique optimized for low-dose spectroscopic analysis, we were able to characterize the structure within the SEI layer at nanometer resolution. In initial dissolution, we observed a multilayered structure of both inorganic and organic SEI components as well as the formation of LiH within the Li dendrite. In contrast, after initial deposition, we find a single uniform compact SEI layer consisting of mostly lithium, sulfur, and oxygen.<br/><br/>1. X. Shan, et al. The Journal of Physical Chemistry C, 125 (35), 19060-19080 (2021)<br/>2. H. Wu, et al. Advanced Energy Materials, 11(5) (2020)<br/>3. Y. Li, et al. Science, 358(6362), 506-510 (2017)<br/>4. M. J. Zachman, et al. Nature, 560, 345-349 (2018)<br/>5. J. Kasemchainan, et al. Nature Materials, 18, 1105–1111 (2019)<br/>6. S. Lang, et al. PNAS, 120(7) (2023)