Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item ELECTROLYTE AND INTERPHASE DESIGN FOR HIGH-ENERGY AND LONG-LIFE LITHIUM/SULFURIZED POLYACRYLONITRILE (Li/SPAN) BATTERIES(2024) Phan, An Le Bao; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lithium/sulfurized polyacrylonitrile (Li/SPAN) recently emerged as a promising battery chemistry with theoretical energy density beyond traditional lithium-ion batteries, attributed to the high specific capacities of Li and SPAN. Compared to traditional sulfur-based cathodes, SPAN demonstrated superior sulfur activity/utilization and no polysulfide dissolution issue. Compared to batteries based on layered oxide cathodes, Li/SPAN shows two significant advantages: (1) high theoretical energy density (> 1000 Wh kg-1, compared to around 750 Wh kg-1 of Li/LiNi0.8Mn0.1Co0.1O2) and (2) transition-metal-free nature, which eliminates the shortcomings associated with transition metals, such as high cost, low abundance, uneven distribution on the earth and potential toxicity. The success of Li/SPAN chemistry with those two critical advantages would not only relief the range and cost anxiety persistently associated with electric vehicle (EV) applications, but also have great implications for the general energy storage market. However, current Li/SPAN batteries still fall far behind their true potential in terms of both energy density and cycle life. This dissertation aims to provide new insights into bridging the theory-practice gap of Li/SPAN batteries by appropriate interphase and comprehensive electrolyte designs. First, the effect of Li/SPAN cell design on energy density and cycle life was discussed using relevant in-house developed models. The concept of “sensitivity factor” was established and used to quantitatively analyze the influence of input parameters. It was found that the electrolyte, rather than SPAN and Li electrodes, represents the bottleneck in Li/SPAN development, which explains our motivation to focus on electrolyte study. Another remarkable finding is that although not well-perceived, electrolyte density has a great impact on Li/SPAN cell-level energy density. Second, design principles to achieve good electrode-electrolyte compatibility were explored. Novel approaches to promote the formation of more protective, inorganic-rich interphases (SEI or CEI) were proposed and validated with proper experiments, including electrochemical tests, material characterizations (such as SEM, XPS, NMR, IR, Raman), and their correlations. Finally, based on the principles discussed in previous chapters, we developed a new electrolyte that simultaneously offers good electrochemical performance (Li CE > 99.4%, Li-SPAN full-cells > 200 cycles), decent ionic conductivity (1.3 mS cm-1), low density (1.04 g mL-1), good processability (higher vapor pressure than conventional carbonates, b.p. > 140 °C), and good safety. Outlook and perspective will also be presented. Beyond Li/SPAN, we believe that our findings regarding cell design as well as electrolyte solvation structure, interphases chemistry, and their implications on electrochemical performance are also meaningful for the development of other high-energy battery chemistries.Item INTERFACE AND STRUCTURES IN LITHIUM-GARNET QUASI-SOLID-STATE BATTERIES(2024) Gritton, Jack Evans; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A confluence of adoption of the internet of things, mobile electronics, electric vehicles, and shift towards adoption of intermittent green energy sources has led to a need for rapid improvement in battery technology in metrics ranging from rate capability and energy density to safety. While significant strides have been made through traditional liquid-based lithium-ion batteries, these oft-conflicting demands require fundamental shifts in battery chemistry, especially enabling safe incorporation of lithium metal anodes. Given their high conductivity, non-flammability, wide electrochemical stability window, and stability to lithium metal, lithium-stuffed garnets of the family LLZO provide one of the most promising alternative electrolytes to replace traditional flammable electrolytes. Two of the largest factors holding back these ceramic electrolytes are interfacial compatibilities and the interplay between processing and electrolyte mass. While drastic improvements have been made in the interface between garnet and lithium metal to improve rate capability, similar jumps in full cells have not been observed for rate and capacity. Using a varied cathode loading and a combination of EIS and DRT, we showed that garnet-catholyte interface was the main contributor to resistance in quasi-solid-state batteries of reasonable cathode loadings utilizing Pyr14TFSI based catholyte. Two methods were then used to improve this interface: modification of the garnet structure interfacing with catholyte, and modification of catholyte composition. Through the use of these methods, rate capabilities and capacity were drastically improved from the baseline system, both at elevated and room temperature. In addition to reducing interfacial resistance, cell polarization can be reduced through using thinner electrolytes. Given its higher mass density and lower conductivity in comparison to liquid electrolytes, garnet has historically had to rely on its greater stability to higher energy density electrodes to maintain competitive energy densities or utilize thin-film procedures that reduce mass but result in orders of magnitude lower conductivity than bulk produced garnets. To balance conductivity, ease of processing, and cell mass, a new combination of bulk-derived processing has been developed that allows for thin free-standing cubic garnet and thin, flexible, porous garnet. Cells using these new thin garnets achieved high cycling rates, and significant capacities.Item In-situ investigation of lithium dendrite growth and its interactions with a polymer separator in a lithium metal cell(2023) Kong, Lingxi; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lithium dendrites are metallic structures that initiate and grow inside a lithium battery duringcharging. Lithium dendrite growth can negatively affect battery cycle life and safety. Observing the dendrite growth process and revealing its interaction with other components is necessary to improve battery safety. This study uses a transparent optical cell to directly observe the dendrite growth process, explore the lithium dendrite growth modes under various current densities, evaluate the interactions between the dendrite and separator, and explore the effect of electrolyte additives on dendrite growth behavior. The dendrite growth under different current densities showed the transition of dendrite morphologies from a dense structure to a porous structure. The examination of the dendrite-separator interaction regions showed that dendrites can deform and penetrate the separator. We show that additives can enhance the uniformity of lithium dendrite distribution compared with the dendrite formed in the electrolyte without additives.Item ASSESSING THE IMPACT OF ELECTROCHEMICAL-MECHANICAL COUPLING ON CURRENT DISTRIBUTION AND DENDRITE PREVENTION IN SOLID-STATE ALKALI METAL BATTERIES(2023) Carmona, Eric Alvaro; Albertus, Paul; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The relationship between mechanical stress states and interfacial electrochemical thermodynamics of Li metal/Li6.5La3Zr1.5Ta0.5O12 and Na metal/Na-β”-Al2O3 systems are examined in two experimental configurations with an applied uniaxial load; the solid electrolytes were pellets and the metal electrodes high-aspect-ratio electrodes. Our experimental results demonstrate that (1) the change in equilibrium potential at the metal/electrolyte interface, when stress is applied to the metal electrode, is linearly proportional to the molar volume of the metal electrode, and (2) the mechanical stress in the electrolyte has negligible effect on the equilibrium potential for an experimental setup in which the electrolyte is stressed and the electrode is left unstressed. Solid mechanics modeling of a metal electrode on a solid electrolyte pellet indicates that pressure and normal stress are within ~0.5 MPa of each other for the high aspect ratio (~1:100 thickness:diameter in our study) Li metal electrodes under loads that exceed yield conditions. To assess the effect of electrochemical-mechanical coupling on current distributions at Li/single-ion conducting solid ceramic electrolyte interfaces containing a parameterized interfacial geometric asperity, we develop a coupled electrochemical-mechanical model and carefully distinguish between the thermodynamic and kinetic effects of interfacial mechanics on the current distribution. We find that with an elastic-perfectly plastic model for Li metal, and experimentally relevant mechanical initial and boundary conditions, the stress variations along the interface for experimentally relevant stack pressures and interfacial geometries are small (e.g., <1 MPa), resulting in a small or negligible influence of the interfacial mechanical state on the interfacial current distribution for both plating and stripping. However, we find that the current distribution is sensitive to interface geometry, with sharper (i.e., smaller tip radius of curvature) asperities experiencing greater current focusing. In addition, the effect on the current distribution of an identically sized lithium peak vs. valley geometry is not the same. These interfacial geometry effects may lead to void formation on both stripping and plating and at both Li peaks and valleys. This work advances the quantitative understanding of alkali metal dendrite formation within incipient cracks and their subsequent growth, and pore formation upon stripping, both situations where properly accounting for the impact of mechanical state on the equilibrium potential can be of critical importance for calculating the current distribution. The presence of high-curvature interface geometry asperities provides an additional perspective on the superior cycling performance of flat, film-based separators (e.g., sputtered LiPON) versus particle-based separators (e.g., polycrystalline LLZO) in some conditions.Item An Experimental and Analytical Investigation of Hydrogen Fuel Cells for Electric Vertical Take-Off and Landing (eVTOL) Aircraft(2019) Ng, Wanyi; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this thesis is a comprehensive investigation of hydrogen fuel cells for electric vertical take-off and landing (eVTOL) aircraft. The primary drawback of battery powered eVTOL aircraft is their poor range and endurance with practical payloads. This work uses simulation and hardware testing to examine the potential of hydrogen fuel cells to overcome this drawback. The thesis develops steady state and transient models of fuel cells and batteries, and validates the models experimentally. An equivalent circuit network model was able to capture the waveforms and magnitudes of voltage as a function of current. Temperature and humidity corrections were also included. Examination of the results revealed that the transient behavior of batteries and fuel stacks are significant primarily shortly after startup of the fuel stack and at the limiting ranges of high and low power; for a nominal operating power and barring faults, steady state models were adequate. This work then demonstrates fuel cell and battery power sharing in regulated and unregulated parallel configurations. It details the development of a regulated architecture, which controls power sharing, to achieve a reduction in power plant weight. Finally, the thesis outlines weight models of motors, batteries, and fuel cells needed for eVTOL sizing, and carries out sizing analysis for on-demand urban air taxi missions of three different distances -- 50, 75, and 150~mi of cruise and 5~min total hover time. This revealed that for ranges within 75 mi, a light weight (5000-6000~lb gross weight) all-electric tilting proprotor configuration achieves a practical payload (500~lb or more) with current levels of battery specific energy (150~Wh/kg) if high burst C-rate batteries are available (4-10~C for 2.5~min). Either a battery-only or battery-fuel cell (B-FC) hybrid power plant is ideal depending on the range of the mission: For inter-city ranges (beyond approximately 50~mi), the mission is impossible with batteries alone, and fuel cells are a key enabling technology; a VTOL aircraft with a B-FC hybrid powerplant, an aircraft with 6200~lb gross take-off weight, 10~lb/ft$^2$ disk loading, and 10~C batteries, could be sized to carry a payload of 500~lb for a range of 75~mi. For this inter-city range, the research priority centers of fuel cells, as they appear to far surpass future projections of Li-ion battery energy levels based on performance numbers (at a component level), high weight fraction of hydrogen storage due to the short duration of eVTOL missions, and lack of a compressor due to low-altitude missions, with the added benefit of ease of re-fueling. However, for an intra-city mission (within approximately 50~mi), the B-FC combination provides no advantage over a battery-only powerplant; a VTOL aircraft with a battery-only powerplant with the same weight and disk loading as before, and 4~C batteries, can carry a payload of 800~lb for a range of 50~mi. For this mission range, improving battery energy density is the priority.Item DEVELOPMENT OF LI+ AND NA+ CONDUCTING CERAMICS AND CERAMIC STRUCTURES FOR USE IN SOLID STATE BATTERIES(2016) Hitz, Gregory; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A solid state lithium metal battery based on a lithium garnet material was developed, constructed and tested. Specifically, a porous-dense-porous trilayer structure was fabricated by tape casting, a roll-to-roll technique conducive to high volume manufacturing. The high density and thin center layer (< 20 μm) effectively blocks dendrites even over hundreds of cycles. The microstructured porous layers, serving as electrode supports, are demonstrated to increase the interfacial surface area available to the electrodes and increase cathode loading. Reproducibility of flat, well sintered ceramics was achieved with consistent powderbed lattice parameter and ball milling of powderbed. Together, the resistance of the LLCZN trilayer was measured at an average of 7.6 ohm-cm2 in a symmetric lithium cell, significantly lower than any other reported literature results. Building on these results, a full cell with a lithium metal anode, LLCZN trilayer electrolyte, and LiCoO2 cathode was cycled 100 cycles without decay and an average ASR of 117 ohm-cm2. After cycling, the cell was held at open circuit for 24 hours without any voltage fade, demonstrating the absence of a dendrite or short-circuit of any type. Cost calculations guided the optimization of a trilayer structure predicted that resulting cells will be highly competitive in the marketplace as intrinsically safe lithium batteries with energy densities greater than 300 Wh/kg and 1000 Wh/L for under $100/kWh. Also in the pursuit of solid state batteries, an improved Na+ superionic conductor (NASICON) composition, Na3Zr2Si2PO12, was developed with a conductivity of 1.9x10-3 S/cm. New super-lithiated lithium garnet compositions, Li7.06La3Zr1.94Y0.06O12 and Li7.16La3Zr1.84Y0.16O12, were developed and studied revealing insights about the mechanisms of conductivity in lithium garnets.Item Energy Management of a Battery-Ultracapacitor Hybrid Energy Storage System in Electric Vehicles(2016) Shen, Junyi; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electric vehicle (EV) batteries tend to have accelerated degradation due to high peak power and harsh charging/discharging cycles during acceleration and deceleration periods, particularly in urban driving conditions. An oversized energy storage system (ESS) can meet the high power demands; however, it suffers from increased size, volume and cost. In order to reduce the overall ESS size and extend battery cycle life, a battery-ultracapacitor (UC) hybrid energy storage system (HESS) has been considered as an alternative solution. In this work, we investigate the optimized configuration, design, and energy management of a battery-UC HESS. One of the major challenges in a HESS is to design an energy management controller for real-time implementation that can yield good power split performance. We present the methodologies and solutions to this problem in a battery-UC HESS with a DC-DC converter interfacing with the UC and the battery. In particular, a multi-objective optimization problem is formulated to optimize the power split in order to prolong the battery lifetime and to reduce the HESS power losses. This optimization problem is numerically solved for standard drive cycle datasets using Dynamic Programming (DP). Trained using the DP optimal results, an effective real-time implementation of the optimal power split is realized based on Neural Network (NN). This proposed online energy management controller is applied to a midsize EV model with a 360V/34kWh battery pack and a 270V/203Wh UC pack. The proposed online energy management controller effectively splits the load demand with high power efficiency and also effectively reduces the battery peak current. More importantly, a 38V-385Wh battery and a 16V-2.06Wh UC HESS hardware prototype and a real-time experiment platform has been developed. The real-time experiment results have successfully validated the real-time implementation feasibility and effectiveness of the real-time controller design for the battery-UC HESS. A battery State-of-Health (SoH) estimation model is developed as a performance metric to evaluate the battery cycle life extension effect. It is estimated that the proposed online energy management controller can extend the battery cycle life by over 60%.Item Atomic Layer Deposition of Ru and RuO2: New Process Development, Fabrication of Heterostructured Nanoelectrodes, and Applications in Energy Storage(2013) Gregorczyk, Keith E.; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to fabricate heterostructured nanomaterials with each layer of the structure having some specific function, i.e. energy storage, charge collection, etc., has recently attracted great interest. Of the techniques capable of this type of process, atomic layer deposition (ALD) remains unique due to its monolayer thickness control, extreme conformality, and wide variety of available materials. This work aims at using ALD to fabricate fully integrated heterostructured nanomaterials. To that end, two ALD processes, using a new and novel precursor, bis(2,6,6-trimethyl-cyclohexadienyl)ruthenium, were developed for Ru and RuO2 showing stable growth rates of 0.5 Å/cycle and 0.4 Å/cycle respectively. Both process are discussed and compared to similar processes reported in the literature. The Ru process is shown to have significantly lower nucleation while the RuO2 is the first fully characterized ALD process known. Using the fully developed RuO2 ALD process, thin film batteries were fabricated and tested in standard coin cell configurations. These cells showed high first cycle gravimetric capacities of ~1400 mAh/g, which significantly degraded after ~40 cycles. Rate performance was also studied and showed a decrease in 1st cycle capacity as a function of increased rate. These results represent the first reports of any RuO2 battery studied beyond 3 cycles. To understand the degradation mechanisms witnessed in the thin film studies in-situ TEM experiments were conducted. Single crystal RuO2 nanowires were grown using a vapor transport method. These nanowires were cycled inside a TEM using Li2O as an electrolyte and showed a ~95% volume expansion after lithiation, ~26% of which was irreversible. Furthermore, a chemical irreversibility was also witnessed, where the reaction products Ru and Li2O remain even after full delithiation. With these mechanisms in mind heterostructured nanowires were fabricated in an attempt to improve the cycling performance. Core/shell TiN/RuO2 and MWCNT/RuO2 structures were fabricating using the ALD process developed in this work. While the TiN/RuO2 structures did not show improved cycling performance due to connection issues, the MWCNT/RuO2 structure showed a stable areal capacity of ~600 μAh/cm2 after ~20 cycles and were easily cycled 100 times.Item A Platform Towards In Situ Stress/Strain Measurement in Lithium Ion Battery Electrodes(2012) Baron, Sergio Daniel; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates the design, fabrication and testing of a platform for in situ stress/strain measurement in lithium ion battery electrodes. The platform - consisting of a Microelectromechanical System (MEMS) chip containing an electrochemical cavity and an optical sensing element, a custom electrochemical package and an experimental setup - was successfully developed. Silicon was used as an active electrode material, and a thin-film electrochemical stack was conceived and tested. Finally, multiple experiments showed correlation between the active material volume change inside the battery and a signal change in the optical sensing element. The experimental results, combined with the MEMS implementation of the sensing element provide a promising way to evaluate electrochemical reaction-induced stress monitoring in a simple and compact fashion, while experiments are carried out in situ.Item DEGRADATION ANALYSIS AND HEALTH MONITERING OF LITHIUM ION BATTERIES(2011) Williard, Nicholas Dane; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Degradation and health monitoring of lithium-ion batteries is explored through life-cycle testing and failure analysis. Test samples comprised of four different battery types from two different manufactures underwent aging and charge/discharge cycling using a variety of load profiles including constant current discharge, pulsed discharge, and varying depths of discharge. Data from in situ monitoring of several parameters including current, voltage, temperature and internal resistance, was analyzed in order to find the best features that could be used to track and characterize battery performance degradation. Degraded samples were disassembled according to a newly developed disassembly methodology that considered the effects of the environment on post-disassembly failure analysis results. Several different failure analysis methods were used in order to gain an understanding of how degradation mechanisms propagate from a materials stand point. Battery electrodes were investigated to observe changes in their chemical and mechanical structures.