R. Edwin García

Microstructural Modeling and Design of Rechargeable Lithium-Ion Batteries

The properties of rechargeable lithium-ion batteries are determined by the electrochemical and kinetic properties of their constituent materials as well as by their underlying microstructure. In this paper a method is developed that uses microscopic information and constitutive material properties to calculate the response of rechargeable batteries. The method is implemented in OOF ,a public domain finite element code, so it can be applied to arbitrary two-dimensional microstructures with crystallographic anisotropy. This methodology can be used as a design tool for creating improved electrode microstructures. Several geometrical two-dimensional arrangements of particles of active material are explored to improve electrode utilization, power density, and reliability of the Li yC6uLixMn2O4 battery system. The analysis suggests battery performance could be improved by controlling the transport paths to the back of the positive porous electrode, maximizing the surface area for intercalating lithium ions, and

Dislocation filtering in GaN nanostructures.

Dislocation filtering in GaN by selective area growth through a nanoporous template is examined both by transmission electron microscopy and numerical modeling. These nanorods grow epitaxially from the (0001)-oriented GaN underlayer through the approximately 100 nm thick template and naturally terminate with hexagonal pyramid-shaped caps. It is demonstrated that for a certain window of geometric parameters a threading dislocation growing within a GaN nanorod is likely to be excluded by the strong image forces of the nearby free surfaces. Approximately 3000 nanorods were examined in cross-section, including growth through 50 and 80 nm diameter pores. The very few threading dislocations not filtered by the template turn toward a free surface within the nanorod, exiting less than 50 nm past the base of the template. The potential active region for light-emitting diode devices based on these nanorods would have been entirely free of threading dislocations for all samples examined. A greater than 2 orders of magnitude reduction in threading dislocation density can be surmised from a data set of this size. A finite element-based implementation of the eigenstrain model was employed to corroborate the experimentally observed data and examine a larger range of potential nanorod geometries, providing a simple map of the different regimes of dislocation filtering for this class of GaN nanorods. These results indicate that nanostructured semiconductor materials are effective at eliminating deleterious extended defects, as necessary to enhance the optoelectronic performance and device lifetimes compared to conventional planar heterostructures.

Validity of the Bruggeman relation for porous electrodes

The ability to engineer electrode microstructures to increase power and energy densities is critical to the development of high-energy density lithium-ion batteries. Because high tortuosities in porous electrodes are linked to lower delivered energy and power densities, in this paper, we experimentally and computationally study tortuosity and consider possible approaches to decrease it. We investigate the effect of electrode processing on the tortuosity of in-house fabricated porous electrodes, using three-dimensionally reconstructed microstructures obtained by synchrotron x-ray tomography. Computer-generated electrodes are used to understand the experimental findings and assess the impact of particle size distribution and particle packing on tortuosity and reactive area density. We highlight the limitations and tradeoffs of reducing tortuosity and develop a practical set of guidelines for active material manufacture and electrode preparation.

Spatially Resolved Modeling of Microstructurally Complex Battery Architectures

Recently, batteries with interpenetrating electrode architectures have been proposed which have the potential to outperform classical designs. These electrode structures are highly percolating particle distributions with short diffusion distances. One of the main advantages with respect to classic rocking chair battery designs is the decrease of the ohmic losses and localized joule-heating, while simultaneously delivering higher power densities and specific energies closer to the theoretical ideal. In the present paper, the rocking chair lithium-ion battery architecture is taken as a point of reference to explore two variations of a three-dimensional battery concept. The first microstructure possesses a highly branched tortuous positive electrode particle distribution, and the second architecture corresponds to a topology with perfectly ordered branches of positive electrode materials. Both architectures outperform the classic rocking chair design by a factor of five for intermediate discharge rates and by 37% for high discharge rates; however, branch ordering has the advantage of improving the macroscopic discharge time by 18% for discharge rates of 4C or higher. Simulations show that the power density of a rechargeable battery can be engineered by maximizing the electrochemical driving force for intercalation while decreasing the characteristic transport distances of the material components. Additionally, the analyzed devices greatly diminish the possibility of salt precipitation during discharge due to removal of microstructure limitations in the classic design. Results also show that for the interpenetrating architectures, when lithium plating and fracture fatigue do occur, they initiate at the tips of the branches of anode material, particularly in those regions that are closer to the electrode phase of opposite polarity. Increasing branch tortuosity induces lithium depletion at the back ohmic contact of the positive electrode at high discharge rates.

Modelling Microstructures with OOF2

OOF2 is a program designed to compute the properties and local behaviour of material microstructures, starting from a two-dimensional representation, an image, of arbitrary geometrical complexity. OOF2 uses the finite element method to resolve the local behaviour of a material, and is designed to be used by materials scientists with little or no computational background. It can solve for a wide range of physical phenomena and can be easily extended. This paper is an introduction to some of its most basic and important features.

The Effect of Microstructure on the Galvanostatic Discharge of Graphite Anode Electrodes in LiCoO2-Based Rocking-Chair Rechargeable Batteries

By starting from experimentally determined cross sections of rechargeable lithium-ion batteries, the effect of microstructure on the galvanostatic discharge of a LiCoO 2 |LiC 6 cell was numerically modeled. Results demonstrate that when small graphite particles are part of a population with large particle sizes, diminished macroscopic power densities develop and limit the response of the entirety of the cell. Small particle-size populations electrochemically interact with large particle-size populations and lead to a macroscopic capacity loss, compared to cells with uniform particle size. Such capacity loss is a result of the lithium exchange between small and large anode particle-size populations, instead of the lithium exchange between electrode particles of opposite polarity. The analysis suggests that graphite particles of size smaller than the average value dominate the macroscopic electrical response of the device, for the induced localized lithium depletion leads to an increase in the polarization losses of the anode. Lithium depletion in the anode starts in the small particles, is followed by particles of complex morphology and rough surfaces, .

Virtual piezoforce microscopy of polycrystalline ferroelectric films

An innovative methodology is presented that utilizes the experimental results of electron backscattered diffraction to map the crystallographic orientation of each grain, the finite element method to simulate the local grain-grain interactions, and finally piezoforce microscopy to infer the local properties of polycrystalline ferroelectric materials by comparing the output of the numerical calculation(s) with the experimental results. The proposed combined method resolves the local hysteretic and electromechanical interactions in polycrystalline ferroelectric films, thus quantifying the effects of grain corners and boundaries on the polycrystal’s macroscopic response. For a polycrystalline lead zirconate titanate sample, a finite range of crystallographic orientations and epitaxial strains is found to enhance the out-of-plane electrical response of the film with respect to its single-crystal, stress-free counterpart. Results show that {111} oriented grains parallel to the normal of the surface of the film...

Domain switching mechanisms in polycrystalline ferroelectrics with asymmetric hysteretic behavior

A numerical method is presented to predict the effect of microstructure on the local polarization switching of bulk ferroelectric ceramics. The model shows that a built-in electromechanical field develops in a ferroelectric material as a result of the spatial coupling of the grains and the direct physical coupling between the thermomechanical and electromechanical properties of a bulk ceramic material. The built-in fields that result from the thermomechanically induced grain-grain electromechanical interactions result in the appearance of four microstructural switching mechanisms: (1) simple switching, where the c-axes of ferroelectric domains will align with the direction of the applied macroscopic electric field by starting from the core of each grain; (2) grain boundary induced switching, where the domain’s switching response will initiate at grain corners and boundaries as a result of the polarization and stress that is locally generated from the strong anisotropy of the dielectric permittivity and th...

From Process to Modules: End-to-End Modeling of CSS-Deposited CdTe Solar Cells

In this paper, we develop an end-to-end modeling framework to explore how various multiscale phenomena in solar cells translate from materials to module level. Specifically, the model captures the physics related to 1) the pressure-dependent grain growth of polycrystalline thin films (nanometers to micrometers), 2) averaging of the effects of grain-size distribution at the centimeter scale, and 3) effects of parasitic series and shunt resistance distributions on the efficiency of thin-film solar cell modules (centimeter to meter scale). As an idealized illustrative example, we consider a number of puzzling features that are associated with close space sublimated CdTe solar cells. The model explains both the increase in the grain size with deposition pressure, as well as the saturation of cell efficiency beyond a critical grain size. The analysis shows that grain geometry and grain-size distribution are unimportant for average grain sizes larger than 1 μm. The model attributes the significant efficiency loss at the module level to the series resistance and the operating point inhomogeneity caused by parasitic shunts. Overall, the model identifies opportunities for significant improvement at all length scales of thin-film solar cell technologies.

GaN nanostructure design for optimal dislocation filtering

The effect of image forces in GaN pyramidal nanorod structures is investigated to develop dislocation-free light emitting diodes (LEDs). A model based on the eigenstrain method and nonlocal stress is developed to demonstrate that the pyramidal nanorod efficiently ejects dislocations out of the structure. Two possible regimes of filtering behavior are found: (1) cap-dominated and (2) base-dominated. The cap-dominated regime is shown to be the more effective filtering mechanism. Optimal ranges of fabrication parameters that favor a dislocation-free LED are predicted and corroborated by resorting to available experimental evidence. The filtering probability is summarized as a function of practical processing parameters: the nanorod radius and height. The results suggest an optimal nanorod geometry with a radius of ∼50b (26 nm) and a height of ∼125b (65 nm), in which b is the magnitude of the Burgers vector for the GaN system studied. A filtering probability of greater than 95% is predicted for the optimal ge...