Michael J. Zachman

Designer interphases for the lithium-oxygen electrochemical cell

A demonstration of stable lithium-oxygen batteries based on high–donor number liquid electrolytes and an ionomer-protected anode. An electrochemical cell based on the reversible oxygen reduction reaction: 2Li+ + 2e− + O2 ↔ Li2O2, provides among the most energy dense platforms for portable electrical energy storage. Such Lithium-Oxygen (Li-O2) cells offer specific energies competitive with fossil fuels and are considered promising for electrified transportation. Multiple, fundamental challenges with the cathode, anode, and electrolyte have limited practical interest in Li-O2 cells because these problems lead to as many practical shortcomings, including poor rechargeability, high overpotentials, and specific energies well below theoretical expectations. We create and study in-situ formation of solid-electrolyte interphases (SEIs) based on bromide ionomers tethered to a Li anode that take advantage of three powerful processes for overcoming the most stubborn of these challenges. The ionomer SEIs are shown to protect the Li anode against parasitic reactions and also stabilize Li electrodeposition during cell recharge. Bromine species liberated during the anchoring reaction also function as redox mediators at the cathode, reducing the charge overpotential. Finally, the ionomer SEI forms a stable interphase with Li, which protects the metal in high Gutmann donor number liquid electrolytes. Such electrolytes have been reported to exhibit rare stability against nucleophilic attack by Li2O2 and other cathode reaction intermediates, but also react spontaneously with Li metal anodes. We conclude that rationally designed SEIs able to regulate transport of matter and ions at the electrolyte/anode interface provide a promising platform for addressing three major technical barriers to practical Li-O2 cells.

Designing solid-liquid interphases for sodium batteries

Secondary batteries based on earth-abundant sodium metal anodes are desirable for both stationary and portable electrical energy storage. Room-temperature sodium metal batteries are impractical today because morphological instability during recharge drives rough, dendritic electrodeposition. Chemical instability of liquid electrolytes also leads to premature cell failure as a result of parasitic reactions with the anode. Here we use joint density-functional theoretical analysis to show that the surface diffusion barrier for sodium ion transport is a sensitive function of the chemistry of solid–electrolyte interphase. In particular, we find that a sodium bromide interphase presents an exceptionally low energy barrier to ion transport, comparable to that of metallic magnesium. We evaluate this prediction by means of electrochemical measurements and direct visualization studies. These experiments reveal an approximately three-fold reduction in activation energy for ion transport at a sodium bromide interphase. Direct visualization of sodium electrodeposition confirms large improvements in stability of sodium deposition at sodium bromide-rich interphases.The chemistry at the interface between electrolyte and electrode plays a critical role in determining battery performance. Here, the authors show that a NaBr enriched solid–electrolyte interphase can lower the surface diffusion barrier for sodium ions, enabling stable electrodeposition.

Characterization of Sulfur and Nanostructured Sulfur Battery Cathodes in Electron Microscopy Without Sublimation Artifacts

Lithium sulfur (Li-S) batteries have the potential to provide higher energy storage density at lower cost than conventional lithium ion batteries. A key challenge for Li-S batteries is the loss of sulfur to the electrolyte during cycling. This loss can be mitigated by sequestering the sulfur in nanostructured carbon-sulfur composites. The nanoscale characterization of the sulfur distribution within these complex nanostructured electrodes is normally performed by electron microscopy, but sulfur sublimates and redistributes in the high-vacuum conditions of conventional electron microscopes. The resulting sublimation artifacts render characterization of sulfur in conventional electron microscopes problematic and unreliable. Here, we demonstrate two techniques, cryogenic transmission electron microscopy (cryo-TEM) and scanning electron microscopy in air (airSEM), that enable the reliable characterization of sulfur across multiple length scales by suppressing sulfur sublimation. We use cryo-TEM and airSEM to examine carbon-sulfur composites synthesized for use as Li-S battery cathodes, noting several cases where the commonly employed sulfur melt infusion method is highly inefficient at infiltrating sulfur into porous carbon hosts.

Characterizing Sulfur in TEM and STEM, with Applications to Lithium Sulfur Batteries

The lithium sulfur (Li-S) battery is a promising technology with the potential to provide greater energy density at lower cost than current lithium ion batteries. One of the main challenges to improving the performance of Li-S batteries is the dissolution and loss of sulfur to the electrolyte as the battery is cycled [1]. Recently much effort has focused on nanostructured electrodes that could sequester the sulfur and prevent its loss during battery operation. Analyzing the distribution of sulfur in these electrodes is critical for creating durable Li-S batteries with high energy density.

Revealing the Internal Structure and Local Chemistry of Nanocrystals Grown in Hydrogel with Cryo-FIB Lift-Out and Cryo-STEM

Hydrogels, three-dimensional polymeric networks with entrapped solvents, have gained increasing interest in a number of fields, including novel crystal synthesis. Compared to solution-based processes, crystal growth in hydrogels opens new routes to controlling morphology and function. Additionally, hydrogels have found applications in biomedical and biological research due to their biomimetic properties, which allow them to imitate the conditions surrounding cells. Understanding processes in hydrogels requires gaining access to their internal structures. Commonly, this requires removal of the liquid from the sample, as the hydrogel will dehydrate upon entering the vacuum of the microscope. Artifacts due to drying, however, can prevent imaging of the samples’ native structure.

Pattering liquids: A novel approach to integrate functional liquids with solid state devices (Conference Presentation)

Field-effect gating with solid dielectrics is the basis for modern electronics. Electrolyte gating, however, offers far higher polarizations. Indeed, electrolyte gating has been a breakthrough to electrically induce numerous phase transitions in solids [1,2,3]. These experiments are all done by dripping mm-size drops of the electrolytes onto the active sample. Compared to integrated circuit technology this approach seems “stone-age” to us. These drops are open to the environment, and allow only for limited purity and reproducibility. Heterostructure electronic circuits have, up to now, been comprised of solid materials only. We have opened this materials space to also include liquids. We demonstrate integrated liquid capacitors and integrated liquid field effect devices which are of equal quality or even outperform standard, bulk devices. This work will accelerate discoveries based on electrolyte gating by providing a new platform, and opens a new area to exploit liquid/solid interfaces in integrated functional devices with technological promise.

Integrated Circuits Comprising Patterned Functional Liquids

Solid-state heterostructures are the cornerstone of modern electronics. To enhance the functionality and performance of integrated circuits, the spectrum of materials used in the heterostructures is being expanded by an increasing number of compounds and elements of the periodic table. While the integration of liquids and solid-liquid interfaces into such systems would allow unique and advanced functional properties and would enable integrated nanoionic circuits, solid-state heterostructures that incorporate liquids have not been considered thus far. Here solid-state heterostructures with integrated liquids are proposed, realized, and characterized, thereby opening a vast, new phase space of materials and interfaces for integrated circuits. Devices containing tens of microscopic capacitors and field-effect transistors are fabricated by using integrated patterned NaCl aqueous solutions. This work paves the way to integrated electronic circuits that include highly integrated liquids, thus yielding a wide array of novel research and application opportunities based on microscopic solid/liquid systems.

Aberration-Corrected STEM/EELS at Cryogenic Temperatures

Today’s aberration-corrected scanning transmission electron microscopes (STEM) routinely focus highenergy electrons down to a spot smaller than 1Å in diameter to perform scattering experiments that allow us to study the atomic-scale structure of materials and devices. When combined with electron energy loss spectroscopy analysis of the inelastically scattered electrons, these narrow probes can also provide atomic-scale information about the composition and local electronic structure of bulk materials, defects and interfaces [1, 2].

Revealing the Nanoscale Structure and Chemistry of Intact Solid-Liquid Interfaces in Electrochemical Energy Storage Devices by Cryo-FIB Lift-Out and Cryo-STEM

Solid-liquid interfaces play a critical role in a range of energy capture and storage devices, but often lack high-resolution characterization with the liquids intact. For example, lithium metal batteries (LMBs) offer ten times the anode storage capacity of lithium-ion batteries, but are limited by capacity fade and safety hazards due to processes at the anode-electrolyte interface [1]. These processes, breakdown of electrolyte to form a “solid-electrolyte interphase” (SEI) layer and uneven deposition of lithium metal leading to dendrite growth, are highly interrelated [2,3]. Understanding the formation and composition of SEI layers is therefore critical to controlling these processes, and will require new techniques to resolve the structural and chemical features of nanoscale SEIs at intact interfaces taken from devices.

Cryo-FIB Milling and Lift-Out for Preparation of Specimens for Cryo-TEM

Cryo-TEM is a well-established technique for investigating the near-native structure of thin frozenhydrated specimens. Specimens too thick for direct imaging, such as whole cells or tissues, must, however, be thinned prior to analysis in the cryo-TEM. Traditionally, cryo-ultramicrotomy served this function by slicing thin sections from larger samples using a diamond blade. This physical cutting process can, however, induce sectioning artifacts [2]. In this talk, we will discuss cryo-focused ion beam (cryo-FIB) techniques, which were developed more recently as alternative thinning methods.