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Title:Electrodeposition and speciation processes of multivalent metals for next generation batteries
Author(s):Ta, Phuong Kim
Director of Research:Gewirth, Andrew A.
Doctoral Committee Chair(s):Gewirth, Andrew A.
Doctoral Committee Member(s):Nuzzo, Ralph G.; Murphy, Catherine J.; Sottos, Nancy R.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Rechargeable Mg batteries, rechargeable Zn batteries, Rechargeable Ca batteries, Electrodeposition and stripping, ultramicrolectrode, Raman, COMSOL simulation
Abstract:Energy demand and consumption have significantly increased over the last several decades. The U.S. Energy Information Administration (EIA) projected that the total world consumption of energy will increase 48% from 2012 to 2040 in the International Energy Outlook 2016. The world electricity alone is projected to rise to double in 2050 the amount used in 2011. Modern industrialization and civilization have forced the global energy distribution and consumption to rely on fossil fuels, which are concentrated in a small area of politically sensitive countries. Since fossil fuels take billions of years to generate, yet humans are on the course of consuming such resources in a few hundreds of years; they are considered non‒renewable. In addition, fossil fuels have harmful effects on both human health and the environment. These factors have motivated research into the generation and utilization of renewable energy over the past several decades. Electrochemical energy storage (EES) is capable of changing the ways in which humans are shifting to use alternative and renewable energy instead of relying on fossil fuels. The current battery technology that has revolutionized the wireless era for global communications and guided electric‒powered vehicles is based on the ubiquitous Li‒ion chemistry. Despite Li‒ion batteries (LIBs) leading the energy storage industry as one of the most advance technologies available for commercial use, the needs for drastic growth in energy density and current issues with LIBs motivate the research and development for post Li‒ion technology. Multivalent batteries in nonaqueous electrolytes such as Zn (5851 mAh/mL), Mg (3833 mAh/mL), and Ca (2073 mAh/mL) satisfy these mentioned requirements, offering a significant enhancement in theoretical volumetric capacity compared to that of LIBs (2062 mAh/mL). Despite numerous studies towards the developments of Zn batteries and Mg batteries over the last decade, the search for compatible nonaqueous electrolytes with a metal anode and intercalation cathodes has been a challenge. In lieu of these challenges, significant efforts have been made to gain more fundamental insights regarding electrolyte properties and behaviors. Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. Cyclic voltammograms (CVs) obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI‒) and chloride‒containing electrolyte in acetonitrile exhibit current densities which are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg‒containing electrolytes in tetrahydrofuran (THF) exhibit an inverse dependence between scan rate and current density. COMSOL‒based simulations suggest that Zn electrodeposition proceeds via a simple one‒step, two electron‒transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical‒electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro‒bridged dimer [Mg2(µ‒Cl)3·6THF]+ present in active electrodeposition solutions. Chapters 2 and 4 examine the deposition mechanisms and solvation processes of Zn and Mg metals. Chapter 5 explores the desolvation process of Mg2+ cations at the Chevrel phase (Mo6S8) cathode surface. The case of rechargeable Ca batteries is just as fascinating as those of Zn and Mg systems. Ca deposition was deemed impossible almost three decades ago due to formation of passivation layers which block the movement of Ca2+ ions to the electrode surface. It was not until early 2018 that Ca deposition at room temperature was reported, thus paving a pathway with renewed interests in studying rechargeable Ca batteries. In this thesis, electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolyte. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UME) exhibit an inverse dependence between current density and scan rate, indicative of the presence of a chemical reaction step in a chemical−electrochemical (CE) deposition mechanism. However, the magnitude of change in current density as a function of scan rate is larger at the Au UME than at the Pt UME. COMSOL simulation reveals that the chemical reaction step rate (kc) obtained at the Pt UME is ~10 times faster than that at the Au UME. Field desorption ionization mass spectrometry suggests that dehydrogenation of the borohydride anions by the metal substrate is the chemical reaction step. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to the larger kc. Raman spectroscopy and electrospray ionization mass spectrometry data show Ca2+ ions are strongly coordinated to THF and weakly interacting with BH4− anions. Electron microscopy shows the surface morphology of Ca electrodeposition are different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt. Chapter 3 discusses the deposition and speciation processes of Ca metal at the Pt and Au metal substrates.
Issue Date:2020-06-16
Rights Information:Copyright 2020 Phuong Kim Ta
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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