Multiscale Cryo Electron Microscopy Reveals Interfacial Degradation and Stabilization in Lithium Metal Battery Electrodes

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Lithium-ion (Li+) transport into and out of the electrode is highly dependent on the structure, composition, and electrolyte wetting of the solid electrolyte interphase (SEI). Typically, on cathode particles, the cathode electrolyte interphase (CEI) is around 10 nm, where the composition and thickness highly impact the impendence of Li+ transfer, while protecting against cascading parasitic reactions with the electrolyte that can evolve species, such as hydrofluoric acid, that degrade the cathode’s surface [1]. At the opposing electrode, in lithium metal batteries (LMBs), various protective coatings have been used to shield lithium anodes from continuous SEI growth, preventing/inhibiting gas evolution, and consumption of the electrolyte. Although thin atomic layer deposition coatings have been successful in early cycling [2], the layers quickly lose protective abilities as the lithium metal deforms and introduces new pathways for electrolyte contact. This work focuses on methods to stabilize the SEI or CEI in two Li+ based systems. At the cathode, the use of electrolyte additives to modify the CEI thickness and composition towards a more favorable structure that promotes Li+ transport without continuously reacting with the electrolyte upon further cycling is investigated [3]. At the anode, the use of liquid metal barrier, like an artificial SEI, on the lithium metal will conform to its surface during cycling and allow for Li+ transport [4]. Although the liquid metal layers provide extended cycling abilities for a limited range, the baseline understanding for the longer life is not well understood. In both systems, the delicate CEI surface structures and liquid metal-coated lithium metal anodes require characterization that is multiscale, site specific, and minimally damaging to the complete battery stack and buried nanoscale interfaces. Characterization of the CEI and barrier layer structures, composition, and bonding were completed using multiscale cryogenic electron microscopy. Millimeter-scale cross-sections through the coin cell batteries were made using a cryogenic stage within a Helios 5 fs-laser plasma focused ion beam (laser PFIB), where complementary energy dispersive X-ray spectroscopy was able to detect variations in the composition at electrode interfaces [5]. Microscale cross-sectioning and lamella sample preparation at the Center for Integrated Nanotechnologies enabled the use of a cryo-stage on a Scios 2 Ga-ion focused ion beam (FIB) with air-free and cryo-transfer by Leica Microsystems of battery electrodes from an Ar-filled glovebox [6]. Nanoscale mapping of composition and bonding within the CEI was completed with cryo-transfer from the glovebox to a Gatan 626 cryo-holder within the Spectra 200 scanning transmission electron microscope with electron energy loss spectroscopy [7]. Therefore, these tools enable a multiscale approach to identifying the millimeter scale features of a battery stack with visualization of degradation in electrodes such as cracks in cathode particles, gas evolution, inconsistencies in the liquid metal barrier, and SEI evolution. At the microscale, this workflow enables observation of interfacial characteristics, heterogeneity in the SEI or barrier layer, and identification of electrolyte networks to the electrode surfaces. At the nanoscale, we achieve measurement of the CEI thickness, mapping transition metals within the cathodes to identify loss of active materials, and beneficial additives that are incorporated into the CEI structure. This multiscale approach allows for statistical understanding of the primary mechanisms and degradation pathways that promote improved performance or identification of parasitic degradation of the ion transport pathways within Li+ batteries. These studies have found that degradation within Li+ coin cells can arise at many levels, overall, with degradation in the form of Li+ pathway disturbances, impendence, or disconnection due to many factors. Millimeter scale factors include the formation of trapped gas bubbles that disconnect Li inventory from the opposing electrode or liquid metal barrier layer, severance that allows for liquid electrolyte contact with the lithium metal surface, Fig. 1. Microscale factors include local ion depletion of electrolyte or lithium salt in the electrolyte, reducing the Li+ transport to the electrode surface, or continuous SEI evolution that increases the distance and pathway for Li+ to travel causing impendence rise. Nanoscale factors include CEI thickness increase, transition metal dissolution from the cathode particles, and local chemical reactions that consume the electrolyte and raise impendence across interfaces, Fig. 2. Our work has found that improvement in Li+ transfer needs to branch these length scales, to improve the cyclability at every scale to prevent one of these degradation mechanisms from dominating and destroying or slowing down the Li+ transport pathways [8]. Millimeter scale cross-sectional scanning electron microscopy image at cryogenic temperature of an intact coin cell with symmetric Li foil coated in GaIn liquid metal alloy on both sides after a single cycle, separated by polymer separator saturated with 1M lithium bis(trifluoromethanesulfonyl)imide in a mixture of 1,2-dimethoxyethane and 1,3-dioxolane electrolyte. Liquid metal coating is not conformal on the lithium metal surface as any electrolyte access will evolve SEI to further misshape the barrier coating. Liquid metal is effective in creating electronic bridges throughout the evolved SEI, reducing the capacity loss from dead lithium. Micrometer to nanoscale cryogenic high angle annular dark field scanning transmission electron microscopy imaging and compositional mapping of the CEI on lithium manganese-rich cathode particles cycled with a vinylene carbonate additive in 1.2M lithium hexafluorophosphate in ethylcarbonate:ethylmethylcarbonate (3:7 w/w). Electron energy loss spectroscopy maps identify heterogeneity in the nickel and manganese content near the cathode particle’s surface.

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