Site-Specific Preparation of Intact Solid-Liquid Interfaces by Label-Free In Situ Localization and Cryo-Focused Ion Beam Lift-Out

Cryo-Electron Microscopy Reveals Na Infiltration into Separator Pore Free-Volume as a Degradation Mechanism in Na Anode:Liquid Electrolyte Electrochemical Cells

Batteries utilizing a sodium (Na) metal anode with a liquid electrolyte are promising for affordable large-scale energy storage. However, a deep understanding of the intrinsic degradation mechanisms is limited by challenges in accessing the buried interfaces. Here, cryogenic electron microscopy of intact electrode:separator:electrode stacks is performed and degradation and failure of symmetric Na||Na coin cells occurs through the infiltration of Na metal through the pores of the separator rather than by mechanical puncturing by dendrites is revealed. It is shown the interior structure of the cell (electrode:separator:electrode) must be preserved and deconstructing the cell into different layers for characterization results in artifacts. In intact cell stacks, minimal liquid is found between the electrodes and separator, leading to intimate electrode:separator interfaces. After electrochemical cycling, Na infiltrates into the pore free-volume, growing through the separator to create electrical shorts and degradation. The Na infiltration occurs at interfacial regions devoid of solid-electrolyte interphase (SEI), revealing SEI plays an important role in preventing Na from growing into the separator by being a physical barrier that the plated Na cannot penetrate. These results shed new light on the fundamental failure mechanisms in Na batteries and demonstrate the importance of preserving the cell structure and buried interfaces.

Understanding Ion-Beam Damage to Air-Sensitive Lithium Metal With Cryogenic Electron and Ion Microscopy

It is essential to understand the nanoscale structure and chemistry of energy storage materials due to their profound impact on battery performance. However, it is often challenging to characterize them at high resolution, as they are often fundamentally altered by sample preparation methods. Here, we use the cryogenic lift-out technique in a plasma-focused ion beam (PFIB)/scanning electron microscope (SEM) to prepare air-sensitive lithium metal to understand ion-beam damage during sample preparation. Through the use of cryogenic transmission electron microscopy, we find that lithium was not damaged by ion-beam milling although lithium oxide shells form in the PFIB/SEM chamber, as evidenced by diffraction information from cryogenic lift-out lithium lamellae prepared at two different thicknesses (130 and 225 nm). Cryogenic energy loss spectroscopy further confirms that lithium was oxidized during the process of sample preparation. The Ellingham diagram suggests that lithium can react with trace oxygen gas in the FIB/SEM chamber at cryogenic temperatures, and we show that liquid oxygen does not contribute to the oxidation of lithium process. Our results suggest the importance of understanding how cryogenic lift-out sample preparation has an impact on the high-resolution characterization of reactive battery materials.

Recent advances and current trends in cryo-electron microscopy

All steps of cryogenic electron-microscopy (cryo-EM) workflows have rapidly evolved over the last decade. Advances in both single-particle analysis (SPA) cryo-EM and cryo-electron tomography (cryo-ET) have facilitated the determination of high-resolution biomolecular structures that are not tractable with other methods. However, challenges remain. For SPA, these include improved resolution in an additional dimension: time. For cryo-ET, these include accessing difficult-to-image areas of a cell and finding rare molecules. Finally, there is a need for automated and faster workflows, as many projects are limited by throughput. Here, we review current developments in SPA cryo-EM and cryo-ET that push these boundaries. Collectively, these advances are poised to propel our spatial and temporal understanding of macromolecular processes.

Cryo-FIB for TEM investigation of soft matter and beam sensitive energy materials

Primarily driven by structural biology, the rapid advances in cryogenic electron microscopy techniques are now being adopted and applied by materials scientists. Samples that inherently have electron transparency can be rapidly frozen (vitrified) in amorphous ice and imaged directly on a cryogenic transmission electron microscopy (cryo-TEM), however this is not the case for many important materials systems, which can consist of layered structures, embedded architectures, or be contained within a device. Cryogenic focused ion beam (cryo-FIB) lift-out procedures have recently been developed to extract intact regions and interfaces of interest, that can then be thinned to electron transparency and transferred to the cryo-TEM for characterization. Several detailed studies have been reported demonstrating the cryo-FIB lift-out procedure, however due to its relative infancy in materials science improvements are still required to ensure the technique becomes more accessible and routinely successful. Here, we review recent results on the preparation of cryo-TEM lamellae using cryo-FIB and show that the technique is broadly applicable to a range of soft matter and beam sensitive energy materials. We then present a tutorial that can guide the materials scientist through the cryo-FIB lift-out process, highlighting recent methodological advances that address the most common failure points of the technique, such as needle attachment, lift-out and transfer, and final thinning.

Molecular Dynamics Simulation and Cryo-Electron Microscopy Investigation of AOT Surfactant Structure at the Hydrated Mica Surface

Structural properties of the anionic surfactant dioctyl sodium sulfosuccinate (AOT or Aerosol-OT) adsorbed on the mica surface were investigated by molecular dynamics simulation, including the effect of surface loading in the presence of monovalent and divalent cations. The simulations confirmed recent neutron reflectivity experiments that revealed the binding of anionic surfactant to the negatively charged surface via adsorbed cations. At low loading, cylindrical micelles formed on the surface, with sulfate head groups bound to the surface by water molecules or adsorbed cations. Cation bridging was observed in the presence of weakly hydrating monovalent cations, while sulfate groups interacted with strongly hydrating divalent cations through water bridges. The adsorbed micelle structure was confirmed experimentally with cryogenic electronic microscopy, which revealed micelles approximately 2 nm in diameter at the basal surface. At higher AOT loading, the simulations reveal adsorbed bilayers with similar surface binding mechanisms. Adsorbed micelles were slightly thicker (2.2–3.0 nm) than the corresponding bilayers (2.0–2.4 nm). Upon heating the low loading systems from 300 K to 350 K, the adsorbed micelles transformed to a more planar configuration resembling bilayers. The driving force for this transition is an increase in the number of sulfate head groups interacting directly with adsorbed cations.

Challenges and triumphs in cryo-electron tomography

Cryo-electron tomography has stepped fully into the spotlight. Enthusiasm is high. Fortunately for us, this is an exciting time to be a cryotomographer, but there is still a way to go before declaring victory. Despite its potential, cryo-electron tomography possesses many inherent challenges. How do we image through thick cell samples, and possibly even tissue? How do we identify a protein of interest amidst the noisy, crowded environment of the cytoplasm? How do we target specific moments of a dynamic cellular process for tomographic imaging? In this review, we cover the history of cryo-electron tomography and how it came to be, roughly speaking, as well as the many approaches that have been developed to overcome its intrinsic limitations.

Organo-organic and organo-mineral interfaces in soil at the nanometer scale

The capacity of soil as a carbon (C) sink is mediated by interactions between organic matter and mineral phases. However, previously proposed layered accumulation of organic matter within aggregate organo-mineral microstructures has not yet been confirmed by direct visualization at the necessary nanometer-scale spatial resolution. Here, we identify disordered micrometer-size organic phases rather than previously reported ordered gradients in C functional groups. Using cryo-electron microscopy with electron energy loss spectroscopy (EELS), we show organo-organic interfaces in contrast to exclusively organo-mineral interfaces. Single-digit nanometer-size layers of C forms were detected at the organo-organic interface, showing alkyl C and nitrogen (N) enrichment (by 4 and 7%, respectively). At the organo-mineral interface, 88% (72-92%) and 33% (16-53%) enrichment of N and oxidized C, respectively, indicate different stabilization processes than at organo-organic interfaces. However, N enrichment at both interface types points towards the importance of N-rich residues for greater C sequestration.

Investigating the Structure of Neurotoxic Protein Aggregates Inside Cells

Neurodegenerative diseases affect the lives of millions of people across the world, being particularly prevalent in the aging population. Despite huge research efforts, conclusive insights into the disease mechanisms are still lacking. Therefore, therapeutic strategies are limited to symptomatic treatments. A common histopathological hallmark of many neurodegenerative diseases is the presence of large pathognomonic protein aggregates, but their role in the disease pathology is unclear and subject to controversy. Here, we discuss imaging methods allowing investigation of these structures within their cellular environment: conventional electron microscopy (EM), super-resolution light microscopy (SR-LM), and cryo-electron tomography (cryo-ET). Multidisciplinary approaches are key for understanding neurodegenerative diseases and may contribute to the development of effective treatments. For simplicity, we focus on huntingtin aggregates, characteristic of Huntington's disease.

Cryo-FIB-lift-out: practically impossible to practical reality

In this paper, we explore the development of the Cryo-Lift-Out (cryo-LO) technique as preparation tool for cryogenic transmission electron microscopy (cryo-TEM). What started in early work defying what was considered 'practically impossible' has developed into state-of-the-art practical reality. This paper presents the key hardware, basic principles and key considerations for the practical usage of cryogenic Lift-Out for those new to the field. Detailed protocols and in-depth description of considerations and points for further development are presented. The authors have attempted to formalise everything known about the technique gathered together from their expertise gained in the development of this approach. LAY DESCRIPTION: A major challenge in electron microscopy is the production of suitable samples from hydrated biological and soft-matter materials for subnanometre resolution imaging in a cryo-Transmission Electron Microscope (TEM). A well-known solution for room temperature materials is called (in situ) Lift-Out. It uses a fine needle that picks up a tiny section called a lamella. Lamellae are made by a Focused Ion Beam (FIB). In this paper, we seek to set out the beginnings of Lift-Out sample preparation conducted under cryogenic conditions and the development of this approach as applied to frozen, hydrated biological and soft-matter samples. We discuss the required basic hardware and provide a thorough description of developed protocols. We aim at those new to the field of cryo-Lift-Out to fully educate them in the finer points of hardware setup and practical considerations when attempting to perform cryo-Lift-Out and to demonstrate what has been achieved thus far. We also discuss areas of further improvement and talking points for the future direction of this promising sample preparation technique.

Analyzing Energy Materials by Cryogenic Electron Microscopy

Safe and high-energy-density rechargeable batteries are increasingly indispensable in the pursuit of a wireless and fossil-free society. Advancements in present battery technologies and the investigation of next-generation batteries highly depend on the ever-deepening fundamental understanding and the rational designs of working electrodes, electrolytes, and interfaces. However, accurately analyzing energy materials and interfaces is severely hindered by their intrinsic limitations of air and electron-beam sensitivity, which restrains the research of energy materials in a low-efficiency trial-and-error paradigm. The emergence of cryogenic electron microscopy (cryo-EM) has enabled the nondestructive characterization of air- and electron-beam sensitive energy materials in the microscale and nanoscale, and even at atomic resolutions, affording closer insights into the primary chemistry and physics of working batteries. Herein, the development of cryo-EM and the applications in detecting energy materials are reviewed and analyzed from its overwhelming advantages in disclosing the underlying mystery of energy materials. Critical sample preparation methods as the precondition for cryo-EM are compared, which strongly affect the characterization accuracy. Furthermore, new developments in the analysis of energy materials, especially bulk electrodes and interfaces in lithium metal batteries, are presented according to different functions of cryo-EM. Finally, future directions of cryo-EM for analyzing energy materials are prospected.

In situ Microfluidic Cryofixation for Cryo Focused Ion Beam Milling and Cryo Electron Tomography

We present a microfluidic platform for studying structure-function relationships at the cellular level by connecting video rate live cell imaging with in situ microfluidic cryofixation and cryo-electron tomography of near natively preserved, unstained specimens. Correlative light and electron microscopy (CLEM) has been limited by the time required to transfer live cells from the light microscope to dedicated cryofixation instruments, such as a plunge freezer or high-pressure freezer. We recently demonstrated a microfluidic based approach that enables sample cryofixation directly in the light microscope with millisecond time resolution, a speed improvement of up to three orders of magnitude. Here we show that this cryofixation method can be combined with cryo-electron tomography (cryo-ET) by using Focused Ion Beam milling at cryogenic temperatures (cryo-FIB) to prepare frozen hydrated electron transparent sections. To make cryo-FIB sectioning of rapidly frozen microfluidic channels achievable, we developed a sacrificial layer technique to fabricate microfluidic devices with a PDMS bottom wall <5 µm thick. We demonstrate the complete workflow by rapidly cryo-freezing Caenorhabditis elegans roundworms L1 larvae during live imaging in the light microscope, followed by cryo-FIB milling and lift out to produce thin, electron transparent sections for cryo-ET imaging. Cryo-ET analysis of initial results show that the structural preservation of the cryofixed C. elegans was suitable for high resolution cryo-ET work. The combination of cryofixation during live imaging enabled by microfluidic cryofixation with the molecular resolution capabilities of cryo-ET offers an exciting avenue to further advance space-time correlative light and electron microscopy (st-CLEM) for investigation of biological processes at high resolution in four dimensions.

Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.

Early-stage NiCrMo oxidation revealed by cryo-transmission electron microscopy

Hydroxide formation at the surface of corroded alloys is critical for understanding early-stage oxidation of many corrosion-resistant alloys. Many hydroxides are unstable in an ambient environment and are electron-beam sensitive, limiting the use of conventionally-prepared specimens for transmission electron microscopy characterization of these alloy-water interfaces. In order to avoid sample dehydration, NiCrMo alloys corroded in a Cl^--containing electrolyte solution were cryo-immobilized by plunge freezing. A cryo-focused ion beam microscope was used to thin the sample to electron transparency, while preserving the alloy-water interface, and the sample was then cryo-transferred to a transmission electron microscope for imaging and diffraction. The presence of rocksalt Ni_1-xCr_2x/3O and β-Ni_1-xCr_2x/3(OH)₂ phases and their orientational relationship to the underlying alloy were observed with electron diffraction, confirming the preservation of the surface structure through the fully-cryogenic sample preparation and analysis.

Cryo-STEM mapping of solid-liquid interfaces and dendrites in lithium-metal batteries

Solid-liquid interfaces are important in a range of chemical, physical and biological processes^1-4, but are often not fully understood owing to the lack of high-resolution characterization methods that are compatible with both solid and liquid components⁵. For example, the related processes of dendritic deposition of lithium metal and the formation of solid-electrolyte interphase layers^6,7 are known to be key determinants of battery safety and performance in high-energy-density lithium-metal batteries. But exactly what is involved in these two processes, which occur at a solid-liquid interface, has long been debated^8-11 because of the challenges of observing such interfaces directly. Here we adapt a technique that has enabled cryo-transmission electron microscopy (cryo-TEM) of hydrated specimens in biology-immobilization of liquids by rapid freezing, that is, vitrification¹². By vitrifying the liquid electrolyte we preserve it and the structures at solid-liquid interfaces in lithium-metal batteries in their native state, and thus enable structural and chemical mapping of these interfaces by cryo-scanning transmission electron microscopy (cryo-STEM). We identify two dendrite types coexisting on the lithium anode, each with distinct structure and composition. One family of dendrites has an extended solid-electrolyte interphase layer, whereas the other unexpectedly consists of lithium hydride instead of lithium metal and may contribute disproportionately to loss of battery capacity. The insights into the formation of lithium dendrites that our work provides demonstrate the potential of cryogenic electron microscopy for probing nanoscale processes at intact solid-liquid interfaces in functional devices such as rechargeable batteries.

A method for site-specific and cryogenic specimen fabrication of liquid/solid interfaces for atom probe tomography

A site-specific, cryogenic, focused ion beam (FIB) method is presented for the preparation of atom probe tomography (APT) specimens from a frozen liquid/solid interface. As a practical example, the interface between water and a corroded boroaluminosilicate glass has been characterized by APT for the first time. The water/glass interface is preserved throughout specimen preparation by plunge freezing the corroding glass particles with the corrosion solution into slush nitrogen. Site-specific specimen preparation is enabled through a new approach to extract and mount a small volume of material using a cryogenically cooled FIB stage and micromanipulator. The prepared APT specimens are subsequently transferred from the FIB to APT under cryogenic and high-vacuum conditions using a novel FIB/APT transfer shuttle and home-built environmental transfer hub attached to the APT system. Particular focus is given to the technical methods for specimen fabrication under cryogenic conditions. Persistent challenges are discussed in addition to future opportunities for this new specimen preparation method.