Joshua D. Sugar

Intercalation Pathway in Many-Particle LiFePO4 Electrode Revealed by Nanoscale State-of-Charge Mapping

The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.

Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes

Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode lithium iron phosphate (LiFePO4; LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes.

Nanoporous Pd alloys with compositionally tunable hydrogen storage properties prepared by nanoparticle consolidation

Nanoporous palladium and palladium alloys are expected to have improved mass transport rates and cycle life compared to bulk materials for energy storage and other applications due to high ratios of surface area to metal volume. Preparation of such materials with high thermal stability and well-controlled metal composition, however, remains a challenge. This work describes a scalable, bottom-up technique for preparing nanoporous palladium alloys based on partial consolidation of dendrimer-encapsulated nanoparticles (DEN). Destabilization of a colloidal suspension of DEN and purification yields high surface area material (60–80 m2 g−1) with a broad pore size distribution centered between 20 and 50 nm. This approach allows for precise tuning of product composition through adjustment of the composition of the precursor DEN. Nanoporous Pd0.9Rh0.1 alloys with uniform composition or with Rh enrichment at pore walls and grain boundaries have been prepared and these structures have been confirmed with high-spatial resolution, aberration corrected quantitative STEM-EDS. Compared to bulk alloys of the same nominal composition, the nanoporous bimetallics show much faster hydrogen uptake kinetics, and store hydrogen at much lower pressure. Pore structure remains intact to temperatures above 300 °C, suggesting that these materials will have long lifetimes at the temperatures used for hydrogen storage applications.

Influence of nanostructuring and heterogeneous nucleation on the thermoelectric figure of merit in AgSbTe2

In some cases, nanoscale microstructures improve thermoelectric efficiency, but this phenomenon has rarely been studied systematically for precipitates in bulk materials. We quantified the influence of nanostructuring on the thermoelectric figure of merit (zT) by embedding Sb2Te3 inclusions, from nanometer to micron sizes, in an Sb-rich AgSbTe2 matrix through solid-state precipitation. Nucleation/growth and coarsening regimes of precipitate formation had a clear effect on transport properties, which could be understood using the effective medium theory of a two-phase composite. The majority of precipitates nucleated heterogeneously at grain boundaries and at planar defects found in the matrix phase, forming a complex interconnected network. This heterogeneous nucleation causes the precipitate/matrix system to follow effective medium theory even at small precipitate sizes, thus lowering the figure of merit. Therefore, heterogeneous nucleation is a major obstacle to efficiency improvement using nanoscale pr...

Atomic-Layer Electroless Deposition: A Scalable Approach to Surface-Modified Metal Powders

Palladium has a number of important applications in energy and catalysis in which there is evidence that surface modification leads to enhanced properties. A strategy for preparing such materials is needed that combines the properties of (i) scalability (especially on high-surface-area substrates, e.g. powders); (ii) uniform deposition, even on substrates with complex, three-dimensional features; and (iii) low-temperature processing conditions that preserve nanopores and other nanostructures. Presented herein is a method that exhibits these properties and makes use of benign reagents without the use of specialized equipment. By exposing Pd powder to dilute hydrogen in nitrogen gas, sacrificial surface PdH is formed along with a controlled amount of dilute interstitial hydride. The lattice expansion that occurs in Pd under higher H2 partial pressures is avoided. Once the flow of reagent gas is terminated, addition of metal salts facilitates controlled, electroless deposition of an overlayer of subnanometer thickness. This process can be cycled to create thicker layers. The approach is carried out under ambient processing conditions, which is an advantage over some forms of atomic layer deposition. The hydride-mediated reaction is electroless in that it has no need for connection to an external source of electrical current and is thus amenable to deposition on high-surface-area substrates having rich, nanoscale topography as well as on insulator-supported catalyst particles. STEM-EDS measurements show that conformal Rh and Pt surface layers can be formed on Pd powder with this method. A growth model based on energy-resolved XPS depth profiling of Rh-modified Pd powder is in general agreement. After two cycles, deposits are consistent with 70-80% coverage and a surface layer with a thickness from 4 to 8 Å.

Obstacles to applications of nanostructured thermoelectric alloys

A major theme in thermoelectric research is based on controlling the formation of nanostructures that occur naturally in bulk intermetallic alloys through various types of thermodynamic phase transformation processes (He et al., 2013). The question of how such nanostructures form and why they lead to a high thermoelectric figure of merit (zT) are scientifically interesting and worthy of attention. However, as we discuss in this opinion, any processing route based on thermodynamic phase transformations alone will be difficult to implement in thermoelectric applications where thermal stability and reliability are important. Attention should also be focused on overcoming these limitations through advanced post-processing techniques.

Enhanced Kinetics of Electrochemical Hydrogen Uptake and Release by Palladium Powders Modified by Electrochemical Atomic Layer Deposition

Electrochemical atomic layer deposition (E-ALD) is a method for the formation of nanofilms of materials, one atomic layer at a time. It uses the galvanic exchange of a less noble metal, deposited using underpotential deposition (UPD), to produce an atomic layer of a more noble element by reduction of its ions. This process is referred to as surface limited redox replacement and can be repeated in a cycle to grow thicker deposits. It was previously performed on nanoparticles and planar substrates. In the present report, E-ALD is applied for coating a submicron-sized powder substrate, making use of a new flow cell design. E-ALD is used to coat a Pd powder substrate with different thicknesses of Rh by exchanging it for Cu UPD. Cyclic voltammetry and X-ray photoelectron spectroscopy indicate an increasing Rh coverage with increasing numbers of deposition cycles performed, in a manner consistent with the atomic layer deposition (ALD) mechanism. Cyclic voltammetry also indicated increased kinetics of H sorption and desorption in and out of the Pd powder with Rh present, relative to unmodified Pd.

A Free Matlab Script for Spatial Drift Correction

Introduction One of the simplest operations to perform on frames of video data is a translation to align features in adjacent frames. The vast quantities of data generated in video output from an in situ transmission electron microscopy (TEM) experiment, for example, require that data processing be performed without operator intervention. Then the operator has more time to enjoy the finer things in life. We have automated the process of feature alignment in the script we describe here. In order to quantitatively measure or track changing features during a reaction, it is necessary that the features of interest do not move from frame to frame of video data. However, this can be difficult to control when performing an in situ experiment, especially a heating experiment. An example of the measured drift that occurs while heating a sample in situ is shown in Figure 1. At temperatures ranging between room temperature and 1000°C, the sample can drift by as many as tens of micrometers depending on the heating technology used. Even a drift of a few μm, however, in the case of a microfabricated heater [1–3], is enough to move the area of interest out of the field of view at intermediate (20–80 kX) TEM magnifications. Drift can occur in these experiments for a variety of reasons. These include thermal expansion of the sample and/or sample holder during heating, pressureinduced deformation of environmental cells, or motion of the sample itself as a result of environmental stimulus or interaction with the electron beam. Although strategies are available to minimize drift, it is difficult to reduce to zero. Ideally, one would predict the sample motion and adjust the sample position during an experiment so that each frame is aligned with its previous one. The approach presented here could be applied to such a real-time feedback scheme, but this would require computer actuation of the sample position, which involves communication between our drift-correction software and the microscope stage software. Because of the several manufacturers of microscope platforms and software controls, it is not possible to easily design a drift-correction package that works for all hardware and software combinations. Therefore, a post-processing step that aligns the video frames after completion of the experiment is desirable. Our goal was to develop a simple script that is freely available and can automatically correct spatial drift in video files for a variety of formats. While the Matlab platform is not free, it is common, and the script is simple enough that it could be ported to other platforms that can encode and decode image and video files. This article describes the Matlab script and how to use it.

Powder X-ray diffraction of Metastudtite, (UO 2 )O 2 (H 2 O) 2

There has been some confusion in the published literature concerning the structure of Metastudtite (UO 2 )O 2 (H 2 O) 2 where differing unit cells and space groups have been cited for this compound. Owing to the absence of a refined structure for Metastudtite, Weck et al . (2012) have documented a first-principles study of Metastudtite using density functional theory (DFT). Their model presents the structure of Metastudtite as an orthorhombic (space group Pnma ) structure with lattice parameters of a = 8.45, b = 8.72, and c = 6.75 A. A Powder Diffraction File (PDF) database entry has been allocated for this hypothetical Metastudtite phase based on the DFT modeling (see 01-081-9033) and aforementioned Dalton Trans. manuscript. We have obtained phase pure powder X-ray diffraction data for Metastudtite and have confirmed the model of Weck et al . via Rietveld refinement (see Figure 1 ). Structural refinement of this powder diffraction dataset has yielded updated refined parameters. The new cell has been determined as a = 8.411(1), b = 8.744(1), and c = 6.505(1) A; cell volume = 478.39 A3. There are only subtle differences between the refined structure and that of the first-principles model derived from DFT. Notably, the b -axis is significantly contracted in the final refinement as compared with DFT. There were also subtle changes to the U1, O1, and O3 atom positions. Tabulated powder diffraction data (ds and Is) for the Metastudtite have been derived from the refined model and these new values can serve to augment the PDF entry 01-081-9033 with a more updated entry based on observed X-ray powder diffraction data.

Site Specific Preparation of Powders for High-Resolution Analytical Electron Microscopy Using a Ga+ Focused Ion Beam

Preparation of powders for high-resolution microscopy presents specific challenges not present in bulk materials. Nanoscale powders (diameters <~100 nm) can be directly deposited on to a TEM grid with drop casting and are usually thin enough for electron transparency in a 200 kV instrument or above. Microscale powders (diameters ~1 um), on the other hand, must be thinned to electron transparency for high-resolution spatial and chemical analysis.