Vyacheslav Volkov

Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes

Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF(2): M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF(2)) while others are not (e.g., CuF(2)). In this study, we investigated the conversion reaction of binary metal fluorides, FeF(2) and CuF(2), using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level structure of individual particles and map the phase distribution in the initial and fully lithiated electrodes. Both FeF(2) and CuF(2) react with lithium via a direct conversion process with no intercalation step, but there are differences in the conversion process and final phase distribution. During the reaction of Li(+) with FeF(2), small metallic iron nanoparticles (<5 nm in diameter) nucleate in close proximity to the converted LiF phase, as a result of the low diffusivity of iron. The iron nanoparticles are interconnected and form a bicontinuous network, which provides a pathway for local electron transport through the insulating LiF phase. In addition, the massive interface formed between nanoscale solid phases provides a pathway for ionic transport during the conversion process. These results offer the first experimental evidence explaining the origins of the high lithium reversibility in FeF(2). In contrast to FeF(2), no continuous Cu network was observed in the lithiated CuF(2); rather, the converted Cu segregates to large particles (5-12 nm in diameter) during the first discharge, which may be partially responsible for the lack of reversibility in the CuF(2) electrode.

Ferroelectric order in individual nanometre-scale crystals

Ferroelectricity in finite-dimensional systems continues to arouse interest, motivated by predictions of vortex polarization states and the utility of ferroelectric nanomaterials in memory devices, actuators and other applications. Critical to these areas of research are the nanoscale polarization structure and scaling limit of ferroelectric order, which are determined here in individual nanocrystals comprising a single ferroelectric domain. Maps of ferroelectric structural distortions obtained from aberration-corrected transmission electron microscopy, combined with holographic polarization imaging, indicate the persistence of a linearly ordered and monodomain polarization state at nanometre dimensions. Room-temperature polarization switching is demonstrated down to ~5u2009nm dimensions. Ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions. This work points the way to multi-Tbit/in(2) memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.

Chemical Distribution and Bonding of Lithium in Intercalated Graphite: Identification with Optimized Electron Energy Loss Spectroscopy

Direct mapping of the lithium spatial distribution and the chemical state provides critical information on structure-correlated lithium transport in electrode materials for lithium batteries. Nevertheless, probing lithium, the lightest solid element in the periodic table, poses an extreme challenge with traditional X-ray or electron scattering techniques due to its weak scattering power and vulnerability to radiation damage. Here, we report nanoscale maps of the lithium spatial distribution in electrochemically lithiated graphite using electron energy loss spectroscopy in the transmission electron microscope under optimized experimental conditions. The electronic structure of the discharged graphite was obtained from the near-edge fine structure of the Li and C K-edges and ab initio calculations. A 2.7 eV chemical shift of the Li K-edge, along with changes in the density of states, reveals the ionic nature of the intercalated lithium with significant charge transfer to the graphene sheets. Direct mapping of lithium in graphite revealed nanoscale inhomogeneities (nonstoichiometric regions), which are correlated with local phase separation and structural disorder (i.e., lattice distortion and dislocations) as observed by high-resolution transmission electron microscopy. The surface solid-electrolyte interphase (SEI) layer was also imaged and determined to have a thickness of 10-50 nm, covering both edge and basal planes with LiF as its primary inorganic component. The Li K-edge spectroscopy and mapping, combined with electron microscopy-based structural analysis provide a comprehensive view of the structure-correlated lithium intercalation in graphite and of the formation of the SEI layer.

Ordered bilayer ruthenium–platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts

Fabricating subnanometre-thick core-shell nanocatalysts is effective for obtaining high surface area of an active metal with tunable properties. The key to fully realize the potential of this approach is a reliable synthesis method to produce atomically ordered core-shell nanoparticles. Here we report new insights on eliminating lattice defects in core-shell syntheses and opportunities opened for achieving superior catalytic performance. Ordered structural transition from ruthenium hcp to platinum fcc stacking sequence at the core-shell interface is achieved via a green synthesis method, and is verified by X-ray diffraction and electron microscopic techniques coupled with density functional theory calculations. The single crystalline Ru cores with well-defined Pt bilayer shells resolve the dilemma in using a dissolution-prone metal, such as ruthenium, for alleviating the deactivating effect of carbon monoxide, opening the door for commercialization of low-temperature fuel cells that can use inexpensive reformates (H2 with CO impurity) as the fuel.

Deterministic phase unwrapping in the presence of noise.

We present a new Fourier-based exact solution for deterministic phase unwrapping from experimental maps of wrapped phase in the presence of noise and phase vortices. This single-step approach has superior performance for images with high phase gradients or insufficient digital sampling approaching 2pi/pixel and therefore performs as a fast and practical solution for the phase-unwrapping problem for experimental applications in applied optics, physics, and medicine.

Ferroelectric Switching Dynamics of Topological Vortex Domains in a Hexagonal Manganite

Field-induced switching of ferroelectric domains with a topological vortex configuration is studied by atomic imaging and electrical biasing in an electron microscope, revealing the role of topological defects on the topologically-guided change of domain-wall pairs in a hexagonal manganite.

Ambient synthesis, characterization, and electrochemical activity of LiFePO4 nanomaterials derived from iron phosphate intermediates

LiFePO4 materials have become increasingly popular as a cathode material due to the many benefits they possess including thermal stability, durability, low cost, and long life span. Nevertheless, to broaden the general appeal of this material for practical electrochemical applications, it would be useful to develop a relatively mild, reasonably simple synthesis method of this cathode material. Herein, we describe a generalizable, 2-step methodology of sustainably synthesizing LiFePO4 by incorporating a template-based, ambient, surfactantless, seedless, U-tube protocol in order to generate size and morphologically tailored, crystalline, phase-pure nanowires. The purity, composition, crystallinity, and intrinsic quality of these wires were systematically assessed using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), selected area electron diffraction (SAED), energy dispersive analysis of X-rays (EDAX), and high-resolution synchrotron XRD. From these techniques, we were able to determine that there is an absence of any obvious defects present in our wires, supporting the viability of our synthetic approach. Electrochemical analysis was also employed to assess their electrochemical activity. Although our nanowires do not contain any noticeable impurities, we attribute their less than optimal electrochemical rigor to differences in the chemical bonding between our LiFePO4 nanowires and their bulk-like counterparts. Specifically, we demonstrate for the first time experimentally that the Fe-O3 chemical bond plays an important role in determining the overall conductivity of the material, an assertion which is further supported by recent “first-principles” calculations. Nonetheless, our ambient, solution-based synthesis technique is capable of generating highly crystalline and phase-pure energy-storage-relevant nanowires that can be tailored so as to fabricate different sized materials of reproducible, reliable morphology.

Magnetic versus Optical Solutions for Transport-of-Intensity Equation in Lorentz Phase Microscopy of Patterned Magnetic Nano-Arrays

Phase retrieval from simple intensity measurements based on the transport-of-intensity equation (TIE) opens a new dimension of scientific research along with the well developed electron holography for better understanding of non-trivial physics of various functional, especially magnetic, materials on a wide length scale. The TIE approach developed originally for light optics by Teaque [1] was found to be promising for studying magnetic structures [2] at nanoscale with electron optics. The direct relation of TIE-formalism to TEM image formation theory was proved by De Graef [2: Ch.2)] and examined by Barty et al. [2: Ch.5]. However, several questions remained unanswered. In current work we re-examine a key moment of the optical TIE solution, which has proved to work well in the light optics when the vector field I∇φ is replaced by the gradient of some function ∇ψ with I and ∇φ being measured the intensity and gradient of the retrieved object phase (i.e. vector field I∇φ must be “potential” since rot(I∇φ)= rot(∇ψ)= 0). Basically, this assumption may be a source of significant errors when TIE-formalism is applied to quantitative magnetic imaging, as follows from our independent derivation and analysis of the so-called magnetic transport-of-intensity equation (MTIE) [3] based on the Aharonov-Bohm (AB) solution for the phase shift derived from the Shrodinger equation. This equation makes a quantitative relation of defocused image contrast to micro-magnetic parameters of materials such as local magnetization and nanoscale currents. Recently, a practical fast Fourier solution for more general situation of non-potential field I∇φ was suggested in [4]. To illustrate the difference in optical (by TIE) and magnetic (by MTIE) solutions applied to general continuity equation (CE), we consider a simple example in Fig.1. The defocused Fresnel images (Fig.1 a-b) generated from known amplitude and phase (Fig.1 c) by using a standard Fresnel integral are further used to retrieve the phase by optical TIE [5] (Fig.1 e,g) and magnetic MTIE [3, 4] (Fig.1 f,h) solutions. It is clear that optical solution fails in accurate retrieval of the “phase pyramid” (Fig.1 c) associated with closure vortex structure of the squared Permalloy element of size 1.4 μm characterized by the product Bt = 1.2 T x 10 nm. A similar situation happens with the TIE solution when MTIE/TIE concepts are used for retrieval of the object phase information (Fig.2 a) from simple intensity measurements of the defocused Fresnel images of Permalloy elements. The subsequent procedure for separation of magnetostatic (Fig.2 b) and electrostatic (Fig.2 c) potentials is described in [4]. Fig. 2d shows the difference map of magnetic and optical solutions for experimental defocused Fresnel images (def = 174 μm). The most significant deviations (Fig.2 e) of the TIE-solution from magnetic one are observed at the sharp edges of permalloy elements of 400 nm in size and 5-10 nm thick. From direct MTIE-phase reconstruction we obtained the following results: (a) electrostatic phase shift of 0.8 ± 0.1 rad (in excellent agreement with independent holographic data [6]): V⋅t =122.6 V nm (24.5V⋅ 5 nm); (b) the phase gradients due to AB-magnetostatic potential suggest the product (B⋅t)max ≅ 11 ± 1 T⋅nm. This value is consistent with a requirement of independence of reconstructed phase from the experimental defocus [7].

8. Magnetic structure and magnetic imaging of RE2Fe14B (RE=Nd, Pr) permanent magnets

Publisher Summary This chapter reviews the magnetic structures observed in the RE 2 Fe 14 B (RE=Nd, Pr) system using various transmission electron microscopes (TEM) magnetic imaging techniques. It presents the studies of die-upset Nd-based permanent magnets conducted mainly at Brookhaven National Laboratory in 1993–1999. Investigations on Nd–Fe–B sintered magnets and single crystals as well as on Pr–Fe–B die-upset magnets are presented in the chapter. The chapter reviews the microstructure, including grain alignment and secondary phases of the materials and grain boundary structure and composition of the intergranular phase. The domain structure, such as the width of domains and domain walls, and the domain wall energy is described in the chapter. The Monte Carlo simulation of the effects of demagnetization fields is presented in the chapter. In situ experiments on the dynamic behavior of domain reorientation as a function of temperature, pinning, and grain boundary nucleation related to coercivity under various fields are described in the chapter.