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Title:Numerical investigation of ion transport mechanisms in electrochemical energy storage devices: Similarity correlation and exergy destruction analysis
Author(s):Nemani, Venkat Pavan
Director of Research:Smith, Kyle C.
Doctoral Committee Chair(s):Smith, Kyle C.
Doctoral Committee Member(s):Ewoldt, Randy H.; Aluru, Narayana R.; Valocchi, Albert J.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Redox Flow Battery
Li-ion Battery
Ion Transport
Numerical Methods
Marcus Hush Chidsey Kinetics
Ion Exchange Membranes
Capacity Fade
Abstract:The shift towards renewable sources of energy requires the development of electrochemical energy storage solutions due to their scalability and ability to handle intermittency. In this thesis, the ion transport mechanisms are studied by modeling redox flow batteries (RFBs) and intercalation-based Li-ion batteries. Redox flow batteries are an emerging technology which allows independent scaling of energy and power density making them suitable candidates for grid-scale energy storage. In RFBs the redox active species is dissolved in electrolytes and stored in tanks. The electrolyte is pumped through the reactor where electrochemical reactions occur. Contemporary RFB research is focused around developing newer materials. However, the fundamental mechanisms causing polarization losses and energy inefficiencies that are inherent to the RFB design, and independent of chemistry, have received lesser attention in research. In this thesis, we present a transient multi-species RFB model using homogenized Poisson-Nernst-Planck formulation with hydrodynamic dispersion in porous media for the species transport. The RFBs modeled here can use either cost-effective non-selective separators or crossover reducing ion-exchange membranes. We also introduce Marcus-Hush-Chidsey kinetic theory based on microscopic electron transfer and solvent reorganization in modeling the redox reactions for RFBs instead of the most commonly used Butler Volmer empirical model. We detail the finite volume formulation with implicit time stepping along with a logarithmic transform of the concentration fields to solve the system of equations. A detailed stability analysis is conducted using the fixed-point iteration scheme for the two kinetic models to establish convergence. For RFBs using non-selective separators, we use Damköhler numbers to classify three RFB operating regimes: redox shuttle limited, ohmic polarized, and sufficient supporting electrolyte. The sufficient supporting electrolyte regime ensures the least capacity fade due to crossover. In the case of RFBs using ion-exchange membranes, we perform comprehensive exergy destruction analysis, using the first and second laws of thermodynamics, to quantify the energy losses arising due to electron-conduction, pore-scale mass transfer, reaction kinetics, species transport, and electrolyte mixing in the tanks. Mapping of these exergy losses enables the identification of major sources of irreversibilities for designing more energy efficient RFBs. The non-dimensional nature of results presented in this study should find applicability towards designing efficient low-cost RFBs by modifying the flow conditions, reactor geometry, electrode morphology, and engineering the redox active species and the salt ions. Li-ion batteries, in contrast with RFBs, operate on the principle of intercalation of lithium ions. The strong anisotropic behavior of graphite platelets restricts the transport of lithium ions through the electrode thickness limiting the thickness. However, thick Li-ion battery electrodes could enable batteries that cost less and have higher gravimetric and volumetric energy density. With the help of a porous electrode model with anisotropic transport processes, we propose and develop a design criterion for bi-tortuous graphite electrodes with electrolyte-rich macro-pores. Macro-pores with optimal aspect ratio spaced at short intervals enable maximum enhancement in Li-ion intercalation.
Issue Date:2019-07-12
Rights Information:Copyright 2019 Venkat Nemani
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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