UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Comprehensive Study and Fundamental Understanding of Lithium Sulfur Batteries for eVTOL(2022) Fisler, Emily; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation is an attempt to bridge the traditional isolation of electrochemistry and aeronautics, particularly rotary-wing aeronautics, where integration of power and platform is vital for the development of a viable aircraft. The dissertation topic addresses the principal barriers of electric-VTOL aircraft: energy and power. It explores a promising alternative to traditional lithium-ion technology; advanced lithium sulfur batteries are projected to supply double the energy of lithium-ion. The principal conclusions are: 1. lithium sulfur can indeed achieve double the energy of lithium-ion but only at low power settings, 2. lithium sulfur exhibits high internal resistance compared to lithium-ion which explains the loss of energy at high power, and 3. discharging lithium sulfur only halfway stabilizes the batteries. A conceptual sizing analysis was developed and verified with NASA. Key battery performance targets were determined for electric VTOL aircraft. Lithium sulfur and lithium-ion coin cells were fabricated with identical overhead for a clear and consistent comparison of specific energy and power. The characteristics measured were discharge cycles, cycle life, impedance under conditions unique to electric vertical takeoff and landing aircraft namely high C-Rates, half cycles, and high transients. Equivalent circuit models were developed and validated to predict the steady-state and transient behavior of these cells. Results show that lithium sulfur provides more than twice the specific energy of lithium-ion up to currents of almost C/2. At 1C, it is comparable. Above 1C it drops drastically and by 4C the energy vanishes almost entirely. This is traced to an order of magnitude higher impedance of these cells. The price to pay for high energy is low cycle life. However, it appears this problem can be eliminated by half cycles. The dynamic behavior of lithium sulfur is richer in comparison to lithium-ion. The response is still capacitative, hence first order, but the complex Warburg and constant phase elements have far greater influence. The behavior is harder to model as it does not fit neatly into linear equivalent circuits. The key conclusion is that lithium sulfur appears to be an attractive alternative to lithium-ion with characteristics that have significant ramifications on future electric VTOL aircraft design and infrastructure.Item Beyond Li ion: Rechargeable Metal Batteries based on Multivalent Chemistry(2017) Gao, Tao; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of advanced battery technology with lower cost and higher energy density is important since various mobile applications are becoming indispensable in our daily life. While Li chemistry has approached its theoretical limit after several decades’ increment improvement, the potential of multivalent chemistry (Mg, Al, etc.) remains unexplored. Compared to Li ion chemistry, multivalent chemistry provides many intriguing benefits in terms of lowering cost and increasing energy density. First of all, minerals containing multivalent element such as Mg, Al, and etc. are much more abundant and cheaper than Li. Second, multivalent metals (Mg, Al etc.) can be directly used as anode materials, ensuring much higher anode capacity than graphite currently used in Li-ion battery. Third, the divalent or trivalent nature of the electroactive cation (Mg2+and Al3+) also promise high capacity for intercalation cathodes because the capacity of these materials are limited by their available ion occupancy sites in the crystal structure instead of its capability to accept electrons. In this dissertation, I detailed our efforts in examining some redox chemistries and materials for the use of rechargeable batteries based on multivalent metal anodes. They include intercalation cathode (TiS2) and conversion cathode (sulfur, iodine). We studied their electrochemical redox behavior in the corresponding chemistry, the thermodynamics, kinetics as well as the reaction reversibility. The reaction mechanism is also investigated with various macroscopic and spectroscopic techniques.