The utilization of lithium metal as an electrode/anode material in lithium-metal batteries (LMBs) has gained considerable attention due to its exceptional theoretical energy and power density, making LMBs promising for future electrochemical energy storage. However, the reactivity of lithium presents challenges, as it undergoes chemical reactions with the electrolyte, leading to the formation of a solid electrolyte interphase (SEI). The SEI is intended to protect the lithium metal and the electrolyte from further reactions, but during cycling (plating and stripping of lithium), volume changes exert mechanical stress on the SEI, causing cracks that expose the lithium and promotes further SEI growth and increased resistance. Additionally, leading to accelerated growth of dendritic lithium which can breach the separator, leading to short circuits and eventually causing thermal runaways [1]. Therefore, it is imperative to conduct further research on the SEI, which remains inadequately investigated in current literature, particularly considering its nanoscale thickness and reactivity, requiring comprehensive analysis using cryo-electron microscopy. Cryo-EM conditions (cryogenic temperatures coupled with reduced electron dose on sample) provide a strategy for handling beam sensitive materials by reducing the reactivity of the materials while enhancing their stability thus allowing for their investigation under the electron beam in their native state up to the atomic scale [2]. To gain a detailed understanding of SEI formation and long-term operation stability we utilize a robust direct Ar-glovebox --> cryo-ultramicrotomy --> cryo-TEM workflow [3] for the preparation of sample cross-sections (e.g., cross-sections of Li metal foils and Li-SEI interfaces from battery samples) suitable for scale-bridging characterization ranging from the macroscopic to the atomic level in the HR(S)TEM. For such samples, ultra-microtomy may feature larger numbers of cross-sectional TEM samples in much shorter time at drastically reduced cost in contrast to cryo-FIB lift-out preparation. Encapsulation of the sample using optimized embedding conditions is undertaken in a glovebox prior to transfer to the microtome. The successful transfer of highly reactive samples is realized by a self-developed inert-gas/cryo-transfer shuttle compatible to our ultra-microtome. Optimized cryo-ultramicrotomy yields clean block-face cross-sections for OM/SEM as well as thin TEM-ready specimens. (S)TEM imaging of as-prepared samples shows clear delineation of the Li – SEI interface consistent with the known reactions at the anode. The presented method envisages a cost-effective technique for the preparation of sensitive lithium metal allowing the statistical and comprehensive characterization of the material and contributes to the understanding of the crucial SEI to enabling LMBs.

The authors acknowledge Julia Wellmann and Gunther Brunklaus from Helmholtz Institute Münster for providing lithium foils. Part of this work was performed at the DFG-funded Micro- and Nanoanalytics Facility (MNaF) of the University of Siegen (INST 221/131-1) utilizing its major TEM instrument FEI Talos F200X (DFG INST 221/93-1, DFG INST 221/126-1) and sample preparation equipment.

[1] Yuan et al, Materials Today 53 (2022) 173

[2] Li et al, Science 358 (2017) 506

[3] Shin et al, J. Power Sources 556 (2023) 232515