When 1:30 PM - 4:00 PM Jun 18, 2018
Where 1018 H.H. Dow
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"Models of electrode and electrolyte behavior at the Continuum Scale"

Alexander Chadwick
Ph.D. Dissertation Defense

Metal electrodes are increasingly common in our daily lives, both for helpful applications such as emerging beyond-lithium-ion energy storage technologies as well as in deleterious processes such as the corrosion of the alloys employed in our vehicles and infrastructure. When a reaction occurs at the surface of a metal electrode, it is often accompanied by a change in the morphology of the interface between that electrode and its surrounding environment, typically an ionically conductive electrolyte. However, there is a complex interplay between the reaction kinetics and transport in the electrolyte that influences the observed morphologies, such as dendrites on lithium metal anodes or corrosion pits in structural alloys. In this dissertation, the effects of ionic transport in the electrolyte and variable reaction kinetics along the electrode/electrolyte interface on the morphological evolution of the electrode are examined via continuum-scale modeling at a variety of length scales and dimensionalities. Two applications are studied: the electrodeposition and electrodissolution of metal battery anodes, and the corrosion of structural metals. Preliminary studies are also included for a reduced-order model of lithium symmetric cells and a diffuse-interface model of the mechanical response of decomposed mixed-conducting protection layers for lithium anodes.

     For the electrodeposition and electrodissolution of metal anodes, a one-dimensional model that considers electrochemistry is developed for two-electrode (i.e., a coin cell) and pseudo-three-electrode (i.e., a beaker cell) systems. The model employs existing mean-field approximations of charge transport and electrostatics in the electrolyte, but a novel, morphology-aware expression is developed to capture the coarse-grained effects of nucleation and surface morphology on the reaction kinetics. The model implementation is heavily optimized to allow high-throughput determination of the physical parameters associated with electrodes and electrolytes, such as ionic diffusivities, reaction rate constants, formal potentials, charge transfer symmetry factors, and nucleation overpotentials. The model is first demonstrated for the parameterization of the kinetic and transport properties of the Mg(BH4)2 electrolyte against an experimental cyclic voltammogram. The model is then validated by comparing the predicted voltammetric behavior to experimental results for different potential scan rates. Next, the model is extended to study dendrite formation on lithium anodes. The model results indicate that the morphological evolution due to preferential deposition and dissolution of dendrites affects the galvanostatic polarization of a lithium symmetric cell. Combined, the studies of magnesium and lithium anodes demonstrate that the proposed coarse-grained model captures key features of the morphological evolution on the anode surface without the computational cost associated with multidimensional simulations.

     The final study considers corrosion of structural metals such as stainless steel and aluminum. A multidimensional phase-field approach is coupled with a multicomponent diffusion model and a new microscopic expression for the limiting reaction kinetics to study the evolving microstructure during corrosion. Simulations are performed to examine how the interfacial electrolyte composition, electrostatic potential, and local reaction kinetics influence the evolution of morphological features such as pitting on the metal surface. The new model allows for regions of the electrode surface to experience different kinetic regimes and exhibits improved agreement against experimental data as compared to previous models.

     Overall, this thesis contributes to a quantitative understanding of the behavior of electrode/electrolyte interfaces by developing new continuum-scale models and applying them to technologically important electrochemical systems. The developed models are flexible and extensible and can be utilized to design and optimize other emerging electrochemical systems.