In Situ Examination of Nanoscale Reaction Pathways in Battery Materials

In order to engineer less expensive and more energy-dense batteries, new materials that can reliably store and transport active ions must be first developed. However, these materials are known for their poor reversibility due to large morphological changes during cycling. To maximize reversibility during charge and discharge, we must be able to understand and control the electrochemical reaction mechanisms of these new electrode materials. This dissertation uses in situ experiments, primarily in situ transmission electron microscopy (TEM), to understand the nanoscale reaction pathways in various high-capacity electrode materials during reactions with Li+, Na+, and K+ ions. Upon reacting with alkali-metal ions, these electrode materials often exhibit much higher specific storage capacities than conventional Li-ion battery electrode materials. In addition, these types of materials can also be used in lower-cost sodium- and potassium-based systems. Hence, they could replace electrode materials in Li-ion batteries, which would make possible engineering batteries with higher specific energy. However, the more substantial volumetric changes that these electrode materials undergo during reaction cause a significant decrease in the capacity retention. This decrease in the capacity retention is caused by the mechanical fracture of the active material and continuous growth of the solid-electrolyte interphase (SEI) on the surface of the anode particles, which both lead to very low cyclability of these systems. If these battery systems are to be improved, it is critical to understand both how the larger Na+ and K+ ions affect the nanoscale phase transformations during these reactions and how to engineer high capacity battery materials with high coulombic efficiency and longer cycle life. As part of the research described in this dissertation, studies on the Cu2S and FeS2 active materials were conducted to examine the effect that larger alkali metal ions have on the reaction mechanisms of large-volume-change materials. Evidence obtained from extensive in situ and ex situ experiments suggests that the larger volume changes associated with the sodium/potassium reactions indicate that the different reaction pathways affect the materials behavior. This altered reaction behavior results in a more stable morphology for the overall cycling of the electrode material. In an effort to aid the engineering of a high capacity battery material with longer cycle life, a study was conducted on Sb nanocrystal electrode materials that exhibited stable electrochemical behavior. This study demonstrated that small spherical particles naturally formed uniform internal voids that were easily filled and vacated during cycling. This was found to be due to the resilient lithiated oxide layer that formed after the first lithiation and subsequently prevented shrinkage during delithiation. A chemomechanical model describing the void formation was developed; this model can serve as a tool to guide the creation of oxide or other shells that enable alloying materials to undergo voiding transformations in situ. When reacting with alkali ions of different sizes, all of these materials (Cu2S, FeS2, and Sb) exhibited counter-intuitive phase evolution and mechanical degradation behavior. The findings indicate that, thanks to their high energy density, large-volume-change materials could make possible the development of next-generation batteries, whether they be Li-ion batteries or batteries with other chemistries that undergo complex morphological changes.Ph.D

Designing Atomic Edge Structures in 2D Transition Metal Dichalcogenides for Improved Catalytic Activity

Designing new materials for functional applications depends upon our ability to understand and correlate the materials structure and chemistry to functional material properties. This is even more important for two-dimensional (2D) materials where thicknesses are on the order of a single atom to a few-atomic layers; therefore, any structural or chemical modification at these length scales can have a profound effect on modifying physical and chemical properties. 2D transition metal dichalcogenides (TMDs) such as MoS2 have emerged as a promising catalyst for the hydrogen evolution reaction with defects such as vacancies and edges being linked to high catalytic active sites as opposed to basal planes for the hydrogen evolution reaction (HER) [1]. Based on these findings it is important to develop controlled synthesis methods that will promote the formation of atomic edge structures engineered for high catalytic HER activity

Non-Fermi-liquid d-wave metal phase of strongly interacting electrons

Developing a theoretical framework for conducting electronic fluids qualitatively distinct from those described by Landau's Fermi-liquid theory is of central importance to many outstanding problems in condensed matter physics. One such problem is that, above the transition temperature and near optimal doping, high-transition-temperature copper-oxide superconductors exhibit `strange metal' behaviour that is inconsistent with being a traditional Landau Fermi liquid. Indeed, a microscopic theory of a strange-metal quantum phase could shed new light on the interesting low-temperature behaviour in the pseudogap regime and on the d-wave superconductor itself. Here we present a theory for a specific example of a strange metal---the 'd-wave metal'. Using variational wavefunctions, gauge theoretic arguments, and ultimately large-scale density matrix renormalization group calculations, we show that this remarkable quantum phase is the ground state of a reasonable microscopic Hamiltonian---the usual t-J model with electron kinetic energy $t$ and two-spin exchange $J$ supplemented with a frustrated electron `ring-exchange' term, which we here examine extensively on the square lattice two-leg ladder. These findings constitute an explicit theoretical example of a genuine non-Fermi-liquid metal existing as the ground state of a realistic model.Comment: 22 pages, 12 figures: 6 pages, 7 figures of main text + 16 pages, 5 figures of Supplementary Information; this is approximately the version published in Nature, minus various subedits in the main tex

Elucidating the Structure and Composition of Individual Bimetallic Nanoparticles in Supported Catalysts by Atom Probe Tomography

Understanding and controlling the structure and composition of nanoparticles in supported metal catalysts are crucial to improve chemical processes. For this, atom probe tomography (APT) is a unique tool, as it allows for spatially resolved three-dimensional chemical imaging of materials with sub-nanometer resolution. However, thus far APT has not been applied for mesoporous oxide-supported metal catalyst materials, due to the size and number of pores resulting in sample fracture during experiments. To overcome these issues, we developed a high-pressure resin impregnation strategy and showcased the applicability to high-porous supported Pd-Ni-based catalyst materials, which are active in CO2 hydrogenation. Within the reconstructed volume of 3 × 105 nm3, we identified over 400 Pd-Ni clusters, with compositions ranging from 0 to 16 atom % Pd and a size distribution of 2.6 ± 1.6 nm. These results illustrate that APT is capable of quantitatively assessing the size, composition, and metal distribution for a large number of nanoparticles at the sub-nm scale in industrial catalysts. Furthermore, we showcase that metal segregation occurred predominately between nanoparticles, shedding light on the mechanism of metal segregation. We envision that the presented methodology expands the capabilities of APT to investigate porous functional nanomaterials, including but not limited to solid catalysts

Spontaneous and reversible hollowing of alloy anode nanocrystals for stable battery cycling

ISSN:1748-3387ISSN:1748-339

In Situ XPS Investigation of Transformations at Crystallographically Oriented MoS₂ Interfaces

Nanoscale transition-metal dichalcogenide (TMDC) materials, such as MoS₂, exhibit promising behavior in next-generation electronics and energy-storage devices. TMDCs have a highly anisotropic crystal structure, with edge sites and basal planes exhibiting different structural, chemical, and electronic properties. In virtually all applications, two-dimensional or bulk TMDCs must be interfaced with other materials (such as electrical contacts in a transistor). The presence of edge sites vs basal planes (i.e., the crystallographic orientation of the TMDC) could influence the chemical and electronic properties of these solid-state interfaces, but such effects are not well understood. Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate how the crystallography and structure of MoS₂ influence chemical transformations at solid-state interfaces with various other materials. MoS₂ materials with controllably aligned crystal structures (horizontal vs vertical orientation of basal planes) were fabricated, and in situ XPS was carried out by sputter-depositing three different materials (Li, Ge, and Ag) onto MoS₂ within an XPS instrument while periodically collecting photoelectron spectra; these deposited materials are of interest due to their application in electronic devices or energy storage. The results showed that Li reacts readily with both crystallographic orientations of MoS₂ to form metallic Mo and Li₂S, while Ag showed very little chemical or electronic interaction with either type of MoS₂. In contrast, Ge showed significant chemical interactions with MoS₂ basal planes, but only minor chemical changes were observed when Ge contacted MoS₂ edge sites. These findings have implications for electronic transport and band alignment at these interfaces, which is of significant interest for a variety of applications

Operando Synchrotron Measurement of Strain Evolution in Individual Alloying Anode Particles within Lithium Batteries

Alloying anode materials offer high capacity for next-generation batteries, but the performance of these materials often decays rapidly with cycling because of volume changes and associated mechanical degradation or fracture. The direct measurement of crystallographic strain evolution in individual particles has not been reported, however, and this level of insight is critical for designing mechanically resilient materials. Here, we use operando X-ray diffraction to investigate strain evolution in individual germanium microparticles during electrochemical reaction with lithium. The diffraction peak was observed to shift in position and diminish in intensity during reaction because of the disappearance of the crystalline Ge phase. The compressive strain along the [111] direction was found to increase monotonically to a value of −0.21%. This finding is in agreement with a mechanical model that considers expansion and plastic deformation during reaction. This new insight into the mechanics of large-volume-change transformations in alloying anodes is important for improving the durability of high-capacity batteries

Seeded Nanowire and Microwire Growth from Lithium Alloys

Although vapor–liquid–solid (VLS) growth of nanowires from alloy seed particles is common in various semiconductor systems, related wire growth in all-metal systems is rare. Here, we report the spontaneous growth of nano- and microwires from metal seed particles during the cooling of Li-rich bulk alloys containing Au, Ag, or In. The as-grown wires feature Au-, Ag-, or In-rich metal tips and LiOH shafts; the results indicate that the wires grow as Li metal and are converted to polycrystalline LiOH during and/or after growth due to exposure to H₂O and O₂. This new process is a simple way to create nanostructures, and the findings suggest that metal nanowire growth from alloy seeds is possible in a variety of systems

Reversible Tuning of the Surface Plasmon Resonance of Indium Tin Oxide Nanocrystals by Gas-Phase Oxidation and Reduction

Heavily doped oxide nanocrystals exhibit a tunable localized surface plasmon resonance (LSPR) in the infrared, a property that is promising for applications in photonics, spectroscopy, and photochemistry. Nanocrystal carrier density and, thus, spectral response are adjustable via chemical reaction; however, the fundamental processes that govern this behavior are poorly understood. Here, we study the time dependence of the LSPR supported by indium tin oxide (ITO) nanocrystals during O₂ and N₂ annealing with <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that the LSPR red-shifts upon oxidation in O₂ and blue-shifts to its original position upon reduction in N₂. A reaction–diffusion model allows us to rationalize the underlying physicochemical processes and quantitatively connect nanocrystal redox chemistry, solid-state diffusion, carrier density, and the LSPR

Probing Electron Beam Induced Transformations on a Single-Defect Level via Automated Scanning Transmission Electron Microscopy

A robust approach for real-time analysis of the scanning transmission electron microscopy (STEM) data streams, based on ensemble learning and iterative training (ELIT) of deep convolutional neural networks, is implemented on an operational microscope, enabling the exploration of the dynamics of specific atomic configurations under electron beam irradiation via an automated experiment in STEM. Combined with beam control, this approach allows studying beam effects on selected atomic groups and chemical bonds in a fully automated mode. Here, we demonstrate atomically precise engineering of single vacancy lines in transition metal dichalcogenides and the creation and identification of topological defects in graphene. The ELIT-based approach facilitates direct on-the-fly analysis of the STEM data and engenders real-time feedback schemes for probing electron beam chemistry, atomic manipulation, and atom by atom assembly