Steven N. Ehrlich

A Size-Dependent Sodium Storage Mechanism in Li4Ti5O12 Investigated by a Novel Characterization Technique Combining in Situ X-ray Diffraction and Chemical Sodiation

A novel characterization technique using the combination of chemical sodiation and synchrotron based in situ X-ray diffraction (XRD) has been detailed illustrated. The power of this novel technique was demonstrated in elucidating the structure evolution of Li4Ti5O12 upon sodium insertion. The sodium insertion behavior into Li4Ti5O12 is strongly size dependent. A solid solution reaction behavior in a wide range has been revealed during sodium insertion into the nanosized Li4Ti5O12 (~44 nm), which is quite different from the well-known two-phase reaction of Li4Ti5O12/Li7Ti5O12 system during lithium insertion, and also has not been fully addressed in the literature so far. On the basis of this in situ experiment, the apparent Na(+) ion diffusion coefficient (DNa+) of Li4Ti5O12 was estimated in the magnitude of 10(-16) cm(2) s(-1), close to the values estimated by electrochemical method, but 5 order of magnitudes smaller than the Li(+) ion diffusion coefficient (D(Li+) ~10(-11) cm(2) s(-1)), indicating a sluggish Na(+) ion diffusion kinetics in Li4Ti5O12 comparing with that of Li(+) ion. Nanosizing the Li4Ti5O12 will be critical to make it a suitable anode material for sodium-ion batteries. The application of this novel in situ chemical sodiation method reported in this work provides a facile way and a new opportunity for in situ structure investigations of various sodium-ion battery materials and other systems.

High rate delithiation behaviour of LiFePO4 studied by quick X-ray absorption spectroscopy

A novel in situ time-resolved synchrotron X-ray absorption spectroscopy (XAS) was introduced for the dynamic studies during fast chemical and electrochemical delithiation of LiFePO(4). The lithium diffusion in LiFePO(4) and the reaction mechanisms for both processes were investigated. This approach opens new opportunities for dynamic studies of various energy storage systems.

Observation of surface layering in a nonmetallic liquid.

Oscillatory density profiles (layers) have previously been observed at the free surfaces of liquid metals but not in other isotropic liquids. We have used x-ray reflectivity to study a molecular liquid, tetrakis(2-ethylhexoxy)silane. When cooled to T/Tc approximately 0.25 (well above the freezing point for this liquid), density oscillations appear at the surface. Lateral order within the layers is liquidlike. Our results confirm theoretical predictions that a surface-layered state will appear even in dielectric liquids at sufficiently low temperatures, if not preempted by freezing.

A novel growth mode of alkane films on a SiO2 surface

Abstract Synchrotron X-ray specular scattering measurements confirm microscopically a structural model recently inferred by very-high-resolution ellipsometry of a solid dotriacontane ( n -C 32 H 66 or C32) film formed by adsorption from solution onto a SiO 2 surface. Sequentially, one or two layers adsorb on the SiO 2 surface with the long-axis of the C32 molecules oriented parallel to the interface followed by a C32 monolayer with the long-axis perpendicular to it. Finally, preferentially oriented bulk particles nucleate having two different crystal structures. This growth model differs from that found previously for shorter alkanes deposited from the vapor phase onto solid surfaces.

The Molecular Pathway to ZIF-7 Microrods Revealed by In Situ Time-Resolved Small- and Wide-Angle X-Ray Scattering, Quick-Scanning Extended X-Ray Absorption Spectroscopy, and DFT Calculations

We present an in situ small- and wide-angle X-ray scattering (SAXS/WAXS) and quick-scanning extended X-ray absorption fine-structure (QEXAFS) spectroscopy study on the crystallization of the metal-organic framework ZIF-7. In combination with DFT calculations, the self-assembly and growth of ZIF-7 microrods together with the chemical function of the crystal growth modulator (diethylamine) are revealed at all relevant length scales, from the atomic to the full crystal size.

Synthesis and Characterization of Palladium–Platinum Core–Shell Electrocatalysts for Oxygen Reduction

Core–shell materials can be employed in low-temperature fuel cell technology as an active electrocatalyst to enable a decreased loading of Pt while still providing sufficient current density. Traditional synthesis of a core–shell material, however, is very sensitive to batch size, with larger batches often exhibiting much lower activity than small-scale batches. To understand this phenomenon for a Pt monolayer deposited on a carbon-supported Pd material, a synchrotron-based X-ray diffraction technique that utilized the ability of Pd to absorb hydrogen to form a Pd hydride structure was developed. The formation of an interstitial hydride causes a large increase in the Pd lattice constant, while the Pt lattice remains unchanged. This can allow deconvolution of the X-ray diffraction reflections of Pt ML/Pd/C into contributions from Pd and Pt. Results of this technique along with synthesis conditions that result in improved electrocatalytic materials for larger batch sizes are discussed.

Nanoscale observation of delayering in alkane films

Tapping-mode Atomic Force Microscopy and synchrotron X-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point Tb in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above Tb and to a solid 3D phase on cooling below Tb. An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.

Synchrotron x-ray scattering of ZnO nanorods: Periodic ordering and lattice size

We demonstrate that synchrotron x-ray powder diffraction (XRD) is a powerful technique for studying the structure and self-organization of zinc-oxide nanostructures. Zinc-oxide nanorods were prepared by a solution-growth method that resulted in uniform nanorods with 2-nm diameter and lengths in the range 10-50 nm. These nanorods were structurally characterized by a combination of small-angle and wide-angle synchrotron XRD and transmission electron microscopy (TEM). Small-angle XRD and TEM were used to investigate nanorod self-assembly and the influence of surfactant/precursor ratio on self-assembly. Wide-angle XRD was used to study the evolution of nanorod growth as a function of synthesis time and surfactant/precursor ratio.

Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by in Situ X-ray Absorption Fingerprints

Zero-strain electrodes, such as spinel lithium titanate (Li4/3Ti5/3O4), are appealing for application in batteries due to their negligible volume change and extraordinary stability upon repeated charge/discharge cycles. On the other hand, this same property makes it challenging to probe their structural changes during the electrochemical reaction. Herein, we report in situ studies of lithiation-driven structural transformations in Li4/3Ti5/3O4 via a combination of X-ray absorption spectroscopy and ab initio calculations. Based on excellent agreement between computational and experimental spectra of Ti K-edge, we identified key spectral features as fingerprints for quantitative assessment of structural evolution at different length scales. Results from this study indicate that, despite the small variation in the crystal lattice during lithiation, pronounced structural transformations occur in Li4/3Ti5/3O4, both locally and globally, giving rise to a multi-stage kinetic process involving mixed quasi-solid solution/macroscopic two-phase transformations over a wide range of Li concentrations. This work highlights the unique capability of combining in situ core-level spectroscopy and first-principles calculations for probing Li-ion intercalation in zero-strain electrodes, which is crucial to designing high-performance electrode materials for long-life batteries.

Sagittal focusing inducing energy structure in medium to high energy resolution x-ray monochromators

Sagittal focusing is a widely used geometry to horizontally focus X-rays in synchrotron beamlines. Usually, two types of devices are used: toroidal mirrors combined with flat X-ray monochromators or flat mirrors combined with sagittal focusing monochromators. The former option seems to be better since the stresses caused by the bending of the second crystal in a double crystal monochromator (DCM) are avoided. In this work we explore the effect of sagittal focusing in medium to high energy resolution X-ray monochromators. Our studies are based on a theoretical explanation of experimental data acquired in two different experiments performed at the XRD2 beamline at Laboratorio Nacional de Luz Sincrotron (LNLS / Brazilian Synchrotron) and X18A at the National Synchrotron Light Source (NSLS / BNL / USA). The first experiment was carried out with a flat meridional bendable mirror followed by a DCM with the second crystal being sagitally bent. The second experiment was carried out with a flat Si 111 DCM followed by a toroidal, but meridional bendable mirror. Both setups are completed by a Si 551 4-bounce monochromator to scan the energy spectrum around 13.7 keV and 13.9 keV, respectively. The results show that the sagittal curvature induces an energy structure. Theoretical studies made by analyzing the reflecting angles in the mirror surface joined with the dynamical theory of X-ray diffraction in the 4-bounce X-ray monochromators confirm these results, which are also explored by the DuMond diagram.