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Electrochemistry


Introduction

Modern rechargeable batteries are complex ensembles of particles of electrochemically active material with high charge capacity utilization achieved through the development of optimized chemistries and particle architectures (see Figure on the left). The research performed by the group led by Prof. Edwin García focuses on the development of thermodynamic and kinetic theories, models, and algorithms to realize improved portable and stationary energy storage technology. In order to incorporate the effects of the mesoscale microstructure and tortuosity, a theoretical framework is being developed to establish processing-property relations that combine the constitutive properties of the individual phases into realistic microstructural designs. The developed framework is directly compared against experimental results. The goal is to develop accelerated design strategies, advanced architectures (microstructures), and processing operations for high power density applications.

Batts3D: Python-Based Microstructural Modeling of Electrochemical Systems

The power and energy density delivered by rechargeable lithium-ion batteries involve electronic and ionic flow, their spatial relationships to conductivity and lithium interdiffusivity, as well as the associated property-dependent interfacial contact potentials. Fundamentally, these processes depend on the structure, size, and three-dimensional spatial distribution of electrolyte, cathodic, and the anodic phases. During battery operation, stress distributions arise due to lithium concentration-induced strains and resistive heating. Both physical processes directly affect battery performance and reliability. Here, a three-dimensional microstructural numerical framework, batts3D was developed to describe the battery charge/discharge processes to account for the details of geometry, connectivity, electrochemical properties, etc., of the component phases as well as elastic and inelastic (chemical and thermal) stresses that develop during battery use. Research focused on two main thrusts: 1) Generation of an easy-to-use three dimensional model to simulate real rechargeable battery electrodes of arbitrary geometries; and 2) Isothermal chemical stresses modeling and design for improved electrode battery materials and devices. In each thrust, microstructures are characterized using imaging methods appropriate to their length scale (3D X-ray CT scans, SEM, and AFM) to obtain images where each of the phases (active material, binders, conductive additive, porosity) is resolved to enable detailed model validation and improved battery design by guiding the experiment through numerical simulation (Bharath Vijayaraghavan, Ding-Wen Chung, David Ely, and R. Edwin García).

3D Laser Structured Conical LCO Battery Architectures

laser processed microstructure
The recent experimental development of laser structuring of three-dimensional (3D) conical architectures has led to an 80x improvement in galvanostatic cyclability compared to its thin film counterpart. To a first approximation, its main advantage resides in its shorter ion diffusion pathways and more efficient electrical transport. In this project, by using LiCoO2 as the cathode chemistry, a batts3D model is being developed to investigate the effect of shape (aspect ratio) and C-rate (current density) on the intercalation performance and the associated electromechanical reliability (see Figure). Three-dimensional simulations demonstrate the effect of the topology on the surficial electrochemical heterogeneities and its impact on the long-term performance of the device (Daw Gen Lim).

Advanced, Spatially Resolved 3D Electrochemical Simulations

3D cathode microstructure
As a direct application of Batts3D, our most recent effort is on the generation of spatially-resolved computer representations of battery material microstructures by directly using 3D X-ray CT data (see Figure). The effort allows to simulate the local and macroscopic electrochemical response of real battery electrodes as a function of galvanostatic cycling (Bharath VijayaraghavanDing-Wen Chung and David Ely). Through this analysis, the effects of the discharge-rate on deleterious effects, such as dendrite formation and salt precipitation are assessed. A comparison against previously performed two-dimensional work, is also carried out to understand the reaches and limitations of low-dimensional (1D and 2D) models (Chloe HeinenHongqian Wang, Haochen Xie). The development of an accurate understanding of the basic physical microstructural mechanisms that occur during battery operation is expected to lead to improved coarse grained (Newman-based) models that will incorporate the richness of behavior of the underlying microstructure.

Microstructurally Resolved Porous Electrode Theory

Multiscale Porous Electrode TheoryCurrently existing theories and models to describe rechargeable lithium-ion batteries are based on the well-established porous electrode theory, that assumes a uniform mixture of reactive solid particles and ignores the geometrical details of the pores, particle segregation and clustering, particle roughness, and crystallographic or microstructural anisotropy. Such descriptions are currently based on average measures of the transport and equilibrium properties of the underlying phases. For example, the tortuosity and porosity of currently used electrodes is described through the well-known Bruggemann equation. Experimentally, however, it has been found that material properties can greatly deviate from the Bruggemann ideal, especially in the limit of low porosities and particle size distributions that greatly differ from perfectly spherical and monodispersed. Current theories are empirical or do not account for the possibility of including complex microstructural features, as it has been recently proposed in advanced LIB microstructure designs. The current project develops a theoretical framework consistent with existing basic porous electrode theory principles to overcome these deficiencies and by directly comparing the predicted behavior against experimentally-determined three-dimensional representations of industrially used electrode microstructures.  Specifically, we are developing a multiscale methodology that focuses on the realization of analytical descriptions that capture the average response of continuum electrochemical systems, but will include the statistically rich spatial distribution of materials and associated physical fields. Applications focuses on rechargeable battery technology and on the development of closed-form expressions that will lead to improved Newman-like descriptions (Ding-Wen ChungDavid Ely, and R. Edwin García).

Electrochemical Strain Microscopy

ESM Modeling
A focus on the basic science on batteries is on modeling existing and emerging battery characterization techniques, such as Electrochemical Strain Microscopy (ESM). Here, numerical techniques are being developed to demonstrate the effect of the relevant transport paths within polycrystalline thin film and the extent of lithium diffusion into the electrode as a function of the ESM-tip-imposed overpotential frequency. Recent results demonstrate that the crystallographic orientation of electrochemically-actuated grains has a significant impact on the entirety of the intercalation process, including effective stored charge, discharge rate, electrochemically induced stresses, and side reactions (see Figure 3). Simulations also demonstrate that continuous battery cycling results in a cumulative capacity loss as a result of successive non-reversible lithium intercalation. Results also demonstrate that ESM has the capability of inferring the local out-of-plane lithium diffusivity and the out-of-plane contributions to Vegard's tensor (Ding-Wen Chung).
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