Cryo-STEM and Multiscale Microscopy of Earth-Abundant Cathode Particles

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Dr. Kourkoutis pioneered the development of cryogenic focused ion beam (cryo-FIB) lift-out preparation of solid-liquid interfaces and beam-sensitive electrode components with Dr. Michael Zachman, to enable high-resolution cryogenic scanning transmission electron microscopy (cryo-STEM) imaging and spectroscopic analysis of these structures [1, 2]. The remarkable results they achieved in their preliminary work sparked our interest to move from in-situ electrochemical STEM experiments to cryogenic electron microscopy (cryo-EM) [3-6], as targeted cryo-FIB lift-outs allow interfacial structures to be extracted and characterized from battery electrodes cycled in standard coin cells typical of battery research, unlike model form factors used for in-situ STEM. Inspired by the work of Dr. Lena Kourkoutis, multiscale cryogenic microscopy methods have been pursued to collect images from battery electrodes on a millimeter-to-atomic scale. We continue to build and develop these cryo-EM methods to better understand and correlate the structures of electrochemical interfaces in macroscale battery stacks with nano- and atomic-scale structural, compositional, and strain maps from cycled battery electrode materials. In our current work, we are evaluating the degradation mechanisms of earth-abundant cathode particles. The primary degradation mechanism is the formation of a thin layer on the top of the cathode particle (cathode electrolyte interface, CEI) and how it can be ameliorated by the change of electrolyte compositions. In lithium-ion batteries, the stability of cathode particles over long-term cycling relates to the composition of the cathode particles. To move away from earth-scarce resources, such as cobalt, towards more earth-abundant battery chemistries, we must evaluate both new cathode particle compositions and its surfaces to ensure they can provide similar performance and stability metrics to more standard electrode materials. This study uses advanced cryo-EM methods to understand degradation reactions along cathode particle surfaces, as well as in their interiors, that limit their long-term cycling performance in lithium-ion batteries. For this analysis, we are using a combination of electrode-scale analysis with scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) to map the porosity and sub-particle grain sizes of the cathode particles. We can then acquire 3D datasets from the particles using slice-n-view analysis for stacking a series of cross-sectional SEM images, using a Thermo Fisher Helios 5 laser plasma focused ion beam (laser PFIB) [5]. This analysis enables statistical evaluation of the structural degradation that occurs within cathode particles (e.g., cracking, strain, or phase segregation) relative to surface events (gas evolution or solid-electrolyte interphase formation, Fig.1). At smaller length scales, we are evaluating surface reactions and layers in more detail using a cryo-FIB to lift out lamella samples from areas of interest for high-resolution structural and compositional analysis using cryo-STEM with energy dispersive x-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). This multiscale microscopy approach enables insights into the structural degradation and the site-specific probability of these events occurring on the cathode, based on the particle’s porosity or grain structure. Results of this multiscale microscopy workflow and multiscale analysis will be shared, including the methods used to prepare and transfer samples between instruments without exposure to air or moisture. These methods are critical to the accurate analysis of these cathode degradation reactions and are to the credit of Dr. Lena Kourkoutis for her groundbreaking development of cryo-FIB lift-out techniques for battery materials [7]. Oblique cross-sectional SEM image of full cell battery after cycling, with circled region indicating co-located cathode-electrolyte interphase formation and gas evolution. Scale bar = 100 µm.

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