Novel Workflows for Cryogenic Sample Preparation of Beam-Sensitive Materials

Electrifying our society to support the energy transition requires the deployment of both terrestrial and airborne battery electric vehicles (BEVs) for electromobility as well as grid-stabilizing battery stationary battery energy storage systems (BESS). To continue to improve the energy density and safety of lithium-ion battery (LIB), insight into microstructural evolution over the course of their operational lifetime are needed. Microstructure studies require combinations of techniques including scanning electron microscopes (SEMs) for imaging on the microscale down and related techniques such as electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) for orientation and strain mapping, (scanning) transmission electron microscopy (STEM/TEM) to reveal the material’s structure and defects on the nanoscale and Atom Probe Tomography (APT) to acquire three-dimensional compositional information. Focused ion beam (FIB) microscopes can be used to cross-section samples, to prepare electron transparent lamellae for further (S)TEM imaging [1] and nanoscale needle-shaped specimens for APT microanalysis [2-4]. However, materials for LIB anodes and cathodes are air- and moisture-sensitive; hence studying them requires using argon-containing gloveboxes, inert or vacuum transfer shuttles, and dedicated sample preparation infrastructure and workflows, or they will rapidly degrade [5]. Electron-imaging is also extremely challenging as LIB materials usually react under electron-beam illumination; the heat induced by the electron beam can melt or destroy specimens almost instantly, cause severe Li migration, and/or induce dendrite formation [6]. Therefore, cryogenic stages cooled to liquid nitrogen (LN2) temperatures must be used to maintain LIBs in their native state. Likewise, subsequent TEM/STEM studies must utilize cryogenic sample holders to prevent sample degradation [7-8], and cryogenic vacuum transfer systems must be used to perform APT successfully [9-10]. This presentation will review some recent progress in the board areas of specimen preparation and imaging specifically for air-, moisture- and beam-sensitive materials. Sample preparation methods using epoxy embedding and cryo-ultramicrotomy of LIB cathode materials with plated Li and solid-electrolyte interface (SEI) in a Ar glovebox have provided extraordinary improvements in sample field of view (FOV) and length to the mm-scale, as shown in Fig. 1 [7], [13]. Further presented is progress in the use of cryo-ultramicrotomy for large-area battery sample preparation and subsequent TEM and electron energy loss spectroscopy (EELS) characterization, providing new perspective into future evolution in wide-area specimen preparation techniques, enabling analysis of mm-scale FOVs, enabling TEM/STEM data acquisition over large areas for statistical studies and uniformity calculations previously not possible. Combinations of these techniques, along with correlative APT specimen preparation from selected regions as described above, promise to bring further insights into LIB structural and chemical property and microstructural evolution. Cryo-STEM-EELS was utilized for detailed investigation of the chemical composition and bonding states within degradation layers formed on LIB electrodes, as shown in Fig. 1. Metallic lithium was identified from its characteristic EELS signatures, while spatially resolved maps reveal its distribution together with thin surface oxide layers arising from electrolyte reactions. From the fine structures of the C ionization edges distinct spectral fingerprints are obtained and used to differentiate key components of the solid electrolyte interface (SEI)/ degradation layer, including Li2CO3, lithium ethylene dicarbonate (LEDC), residual electrolyte species such as ethyl methyl carbonate (EMC), and secondary reaction products. Phase mapping is achieved by fitting reference spectra obtained from external references and internal standards to the full EELS datasets, allowing the simultaneous visualization of Li2CO3, LEDC, LiF/LiPF₆-derived species, ionic lithium, and trapped electrolyte across the interphase. Element-specific maps derived from background-subtracted Li-K, O-K, and F-K edges further resolve the heterogeneous elemental distribution produced by coupled lithium oxidation and electrolyte reduction processes. These measurements are conducted under cryogenic conditions to suppress beam-induced damage and chemical changes thereby providing a robust mechanism for the resolution of SEI chemistry and microstructural evolution. At smaller scales, I will highlight examples from LIB materials analysis in the FIB/SEM using EBSD/TKD and APT [11-12], and these techniques’ application for different material systems, such as porous brass current collectors in Fig. 2 and electrochemically formed ternary Laves phase Cu3ZnLi2 in Fig 3. FIB-prepared cross-sectional areas reveal the underlying crystalline microstructure of LIB cathode and anode materials, which generate EBSD patterns as the SEM electron beam scans across them, which in turn reveal microstructural grain and grain boundary orientation changes. Li dendrites in solid state electrolyte lithium lanthanum zirconium oxide (LLZO) can be captured using this technique, their orientation and rotational angles captured and shown using Grain Reference Orientation Deviation (GROD) maps, which show how much the lattice distorts within an individual grain. Likewise, such dendrites can be cross-sectioned to generate 3D volumetric reconstructions, as well comparable TEM/STEM imaging [6][14]. Multiscale structural and chemical analysis of electrodes cycled organic electrolytes (Org). a) cross-sectional optical microscopy image; c, d) Org electrode: cross-sectional HAADF-STEM image with (b) associated STEM-EELS chemical maps (region of interest indicated by the dashed white rectangle in (d). (e) Reference EELS spectra at the C K, O K, and F K edges used for MLLS fitting, with color coding matching the maps in (c). Plated 100X cycled brass current collector (a) FIB-prepared lamella (b) HAADF-STEM interlayer view (c) X-direction (scan direction along X-axis) grain map (d) indexed phase distribution 4D-STEM (e) diffraction pattern Laves phase (f) diffraction pattern second site indexed as Laves phase – comparatively indexed in SI 4 (g) STEM-EDX Zn distribution (at%) with functionally equivalent APT region noted as 1 (h) STEM-EDX Cu distribution (at.%) (i) TEM overview grain view (j) Zn, Cu, and Li distribution APT 2D XZ center slice directed down from plated Li surface (k) overall XZ projection Z-composition profile with labelled phases Image and caption copyright CC-BY-4.0, taken from [11]. SEM cross-sectional views and TKD of Li-plated lamellae (a) Flat polished brass lamella, showing the deformed layer (b) GROD map of flat polished brass layer showing thin deformed layer (c) 500°C vacuum dealloyed lamella showing inconsistent Li-plating (d) GROD map of 500°C vacuum dealloyed lamella showing grains and Li-filled grain boundary (e) 800°C vacuum dealloyed lamella with thin remaining Li and empty versus Li-filled pores (f) GROD map of 800°C vacuum dealloyed lamella empty pore with bottom, scale bar in (g) GROD map of 800°C vacuum dealloyed lamella Li-filled pore Image and caption copyright CC-BY-4.0, taken from [12].