Highly Ordered Eutectic Mesostructures via Template-Directed Solidification within Thermally Engineered Templates.

Template-directed self-assembly of solidifying eutectics results in emergence of unique microstructures due to diffusion constraints and thermal gradients imposed by the template. Here, the importance of selecting the template material based on its conductivity to control heat transfer between the template and the solidifying eutectic, and thus the thermal gradients near the solidification front, is demonstrated. Simulations elucidate the relationship between the thermal properties of the eutectic and template and the resultant microstructure.

Relative Kinetics of Solid-State Reactions: The Role of Architecture in Controlling Reactivity.

Countless inorganic materials are prepared via high temperature solid-state reaction of mixtures of reagents powders. Understanding and controlling the phenomena that limit these solid-state reactions is crucial to designing reactions for new materials synthesis. Here, focusing on topotactic ion-exchange between NaFeO and LiBr as a model reaction, we manipulate the mesoscale reaction architecture and transport pathways by changing the packing and interfacial contact between reagent particles.

Enabling the electrochemical simulation of Li-ion battery electrodes with anisotropic tortuosity in COMSOL Multiphysics.

Li-ion battery electrodes, such as widely used graphite anodes, may have anisotropic tortuosity due to the non-equiaxed shape of the active material particles and the post-casting calendaring process. Such anisotropy can be ignored in conventional electrodes because all the macroscopic ion transport occurs along the electrode thickness, making the ion transport effectively one dimensional. However, the anisotropy becomes important to consider with three-dimensional architectures, such as those generated by laser-patterning.

Grain boundary formation through particle detachment during coarsening of nanoporous metals.

Grain boundary formation during coarsening of nanoporous gold (NPG) is investigated wherein a nanocrystalline structure can form by particles detaching and reattaching to the structure. MicroLaue and electron backscatter diffraction measurements demonstrate that an in-grain orientation spread develops as NPG is coarsened. The volume fraction of the NPG sample is near the limit of bicontinuity, at which simulations predict that a bicontinuous structure begins to fragment into independent particles during coarsening.

Simulation of the Electrochemical Impedance in a Three-Dimensional, Complex Microstructure of Solid Oxide Fuel Cell Cathode and Its Application in the Microstructure Characterization.

Electrochemical impedance spectroscopy (EIS) is a powerful technique for material characterization and diagnosis of the solid oxide fuel cells (SOFC) as it enables separation of different phenomena such as bulk diffusion and surface reaction that occur simultaneously in the SOFC. In this work, we simulate the electrochemical impedance in an experimentally determined, three-dimensional (3D) microstructure of a mixed ion-electron conducting (MIEC) SOFC cathode. We determine the impedance response by solving the mass conservation equation in the cathode under the conditions of an AC load across the cathode's thickness and surface reaction at the pore/solid interface.

A thermal-gradient approach to variable-temperature measurements resolved in space.

Temperature is a ubiquitous environmental variable used to explore materials structure, properties and reactivity. This article reports a new paradigm for variable-temperature measurements that varies the temperature continuously across a sample such that temperature is measured as a function of sample position and not time. The gradient approach offers advantages over conventional variable-temperature studies, in which temperature is scanned during a series measurement, in that it improves the efficiency with which a series of temperatures can be probed and it allows the sample evolution at multiple temperatures to be measured in parallel to resolve kinetic and thermodynamic effects.

Archimedean lattices emerge in template-directed eutectic solidification.

Template-directed assembly has been shown to yield a broad diversity of highly ordered mesostructures, which in a few cases exhibit symmetries not present in the native material. However, this technique has not yet been applied to eutectic materials, which underpin many modern technologies ranging from high-performance turbine blades to solder alloys. Here we use directional solidification of a simple AgCl-KCl lamellar eutectic material within a pillar template to show that interactions of the material with the template lead to the emergence of a set of microstructures that are distinct from the eutectic's native lamellar structure and the template's hexagonal lattice structure.

Quantifying Reaction and Rate Heterogeneity in Battery Electrodes in 3D through Operando X-ray Diffraction Computed Tomography.

In composite battery electrode architectures, local limitations in ionic and electronic transport can result in nonuniform energy storage reactions. Understanding such reaction heterogeneity is important to optimizing battery performance, including rate capability and mitigating degradation and failure. Here, we use spatially resolved X-ray diffraction computed tomography to map the reaction in a composite electrode based on the LiFePO active material as it undergoes charge and discharge.

Thermodynamic Overpotentials and Nucleation Rates for Electrodeposition on Metal Anodes.

Rechargeable batteries employing metal negative electrodes (i.e., anodes) are attractive next-generation energy storage devices because of their greater theoretical energy densities compared to intercalation-based anodes.

Self-Similarity and the Dynamics of Coarsening in Materials.

Two-phase mixtures, from metallic alloys to islands on surfaces, undergo coarsening wherein the total interfacial area of the system decreases with time. Theory predicts that during coarsening the average size-scale of a two-phase mixture increases with time as t when the two-phase mixture is self-similar, or time independent when scaled by a time-dependent length. Here, we explain why this temporal power law is so robustly observed even when the microstructure is not self-similar.

Localized concentration reversal of lithium during intercalation into nanoparticles.

Nanoparticulate electrodes, such as Li FePO, have unique advantages over their microparticulate counterparts for the applications in Li-ion batteries because of the shortened diffusion path and access to nonequilibrium routes for fast Li incorporation, thus radically boosting power density of the electrodes. However, how Li intercalation occurs locally in a single nanoparticle of such materials remains unresolved because real-time observation at such a fine scale is still lacking. We report visualization of local Li intercalation via solid-solution transformation in individual Li FePO nanoparticles, enabled by probing sub-angstrom changes in the lattice spacing in situ.

Charge attachment induced transport - bulk and grain boundary diffusion of potassium in PrMnO.

The transport of potassium through praseodymium-manganese oxide (PrMnO; PMO) has been investigated by means of the charge attachment induced transport (CAIT) technique. To this end, potassium ions have been attached to the front side of a 250 nm thick sample of PMO. The majority of the potassium ions become neutralized at the surface of the PMO, while some of the potassium ions diffuse through.

High-Operating-Temperature Direct Ink Writing of Mesoscale Eutectic Architectures.

High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 µm, enabling the manipulation of light in the visible and infrared range.

Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy.

Enabling ultra-high energy density rechargeable Li batteries would have widespread impact on society. However the critical challenges of Li metal anodes (most notably cycle life and safety) remain unsolved. This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting.

Electrochemical Stability Window of Imidazolium-Based Ionic Liquids as Electrolytes for Lithium Batteries.

This paper presents the computational assessment of the electrochemical stability of a series of alkyl methylimidazolium-based ionic liquids for their use as lithium battery electrolytes. The oxidation and reduction potentials of the constituent cation and anion of each ionic liquid with respect to a Li(+)/Li reference electrode were calculated using density functional theory following the method of thermodynamic cycles, and the electrochemical stability windows (ESW)s of these ionic liquids were obtained. The effect of varying the length of alkyl side chains of the methylimidazolium-based cations on the redox potentials and ESWs was investigated.

Template-Directed Directionally Solidified 3D Mesostructured AgCl-KCl Eutectic Photonic Crystals.

3D mesostructured AgCl-KCl photonic crystals emerge from colloidal templating of eutectic solidification. Solvent removal of the KCl phase results in a mesostructured AgCl inverse opal. The 3D-template-induced confinement leads to the emergence of a complex microstructure.

Model for anodic film growth on aluminum with coupled bulk transport and interfacial reactions.

Films grown through the anodic oxidation of metal substrates are promising for applications ranging from solar cells to medical devices, but the underlying mechanisms of anodic growth are not fully understood. To provide a better understanding of these mechanisms, we present a new 1D model for the anodization of aluminum. In this model, a thin space charge region at the oxide/electrolyte interface couples the bulk ionic transport and the interfacial reactions.

Tracking lithium transport and electrochemical reactions in nanoparticles.

Expectations for the next generation of lithium batteries include greater energy and power densities along with a substantial increase in both calendar and cycle life. Developing new materials to meet these goals requires a better understanding of how electrodes function by tracking physical and chemical changes of active components in a working electrode. Here we develop a new, simple in-situ electrochemical cell for the transmission electron microscope and use it to track lithium transport and conversion in FeF(2) nanoparticles by nanoscale imaging, diffraction and spectroscopy.

Coarsening of bicontinuous structures via nonconserved and conserved dynamics.

Coarsening subsequent to phase separations occurs in many two-phase mixtures. While unique scaled particle size distributions have been determined for highly asymmetric mixtures in which spherical particles form in a matrix, it is not known if a unique scaled structure exists for symmetric mixtures, which yield bicontinuous structures having intricately interpenetrating phase domains. Using large-scale simulations, we have established that unique scaled microstructures exist in symmetric mixtures evolving via nonconserved and conserved dynamics.

Three-dimensional reconstruction of a solid-oxide fuel-cell anode.

The drive towards increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel-cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces.

Diffuse-charge dynamics in electrochemical systems.

The response of a model microelectrochemical system to a time-dependent applied voltage is analyzed. The article begins with a fresh historical review including electrochemistry, colloidal science, and microfluidics. The model problem consists of a symmetric binary electrolyte between parallel-plate blocking electrodes, which suddenly apply a voltage.