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Graduate Defense: Corey Efaw
August 3 @ 9:00 am - 11:00 am MDT
Title: How to Recognize and Reduce Interfacial Phenomena That Prevent the Advancement of Clean Energy
Program: Doctor of Philosophy in Materials Science and Engineering
Advisor: Dr. Michael F. Hurley, Co-chair, Materials Science and Engineering, and Dr. Eric J. Dufek, Materials Science and Engineering
Committee Members: Dr. Paul H. Davis, Materials Science and Engineering, and Dr. Hui (Claire) Xiong, Materials Science and Engineering
Nuclear energy and electrochemical energy storage, such as batteries, are key parts to the clean energy transition of critical infrastructure. This work aims to define, monitor, and modify interfacial layers that would improve the utility of materials in harsh environments seen in nuclear and energy storage applications. First, the studying of zirconium alloys, which is used as nuclear cladding, was done to better understand the degradation mechanisms within an extreme environment. High-resolution characterization techniques were used to correlate corrosion mechanisms to equivalent circuit models from novel in-core electrochemical impedance spectroscopy sensors. Advancement in this sensor technology could provide further insight and monitoring of the complex degradation mechanisms in a harsh nuclear core environment. A novel method was developed to spatially map Raman spectral features throughout the oxide cross-section, revealing a direct correlation between tetragonal zirconia phase and compressive stress, thus supporting the theory of a stress-induced breakaway phenomenon. Additionally, a comparison of interface- and relaxed-tetragonal phase revealed a difference in stabilization mechanisms, where relaxed-tetragonal phase is stabilized solely from sub-stoichiometric contributions. Coupling Raman mapping with elemental analysis via energy dispersive X-ray spectroscopy and scanning Kelvin probe force microscopy led to a distinction of secondary-phase particles and their nobility relative to surrounding zirconium oxide and metal. Lastly, a p-n junction at the tetragonal/monoclinic zirconia interface was observed, supporting the theory that the tetragonal layer at the metal/oxide interface provides an additional barrier to an otherwise diffusion-limited oxidation mechanism.
Other interfacial studies were conducted on next-generation battery anodes. High-capacity lithium, deemed the “Holy Grail” of battery materials, undergoes unstable interactions in most, if not all, environments. In a cell, this causes poor cycle life and/or possible safety concerns via dendritic-driven short circuiting. Novel development of lithium-metal batteries was accomplished firstly with a facile design of a closed-host, porous/dense bi-layer interfacial structure formed on lithium through a two-step ex-situ/in-situ process, only made possible with an electrolyte additive included in the cell. This design prevented dendrite growth, improved interfacial flexibility and ionic conduction when compared to a traditional LiF coating, reduced volume fluctuations, and prevented extensive parasitic reactions. External temperature was also examined to understand how the content and morphology of an interface effects cell stability. Operating temperature has a profound effect on the formation mechanisms of an in-situ formed interface on lithium metal, where varying temperatures improve cycle aging in different electrolyte compositions. It was found that changing from a low- to high-concentration electrolyte (LCE and HCE) and novel localized high-concentration electrolytes (LHCE) had a profound effect in solvation structures, where an increasing salt aggregation and salt-reduced interface is rendered in LHCE more so than HCE and LCE electrolytes. Temperature also played an effect, where a similar correlation between increased anion-coordination led to improved cell stability. Lastly, the development of anode-free batteries, where lithium is stored in the cathode side, could be used in certain applications with easier manufacturing requirements and reduction of excess lithium. The works presented here were done in effort to better understand and control interfacial mechanisms in both nuclear energy and energy storage fields.