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 ELECTROCHEMISTRY OF PRECISION NANOSTRUCTURES FOR HIGH PERFORMANCE ENERGY STORAGE DEVICES(2020) Kim, Nam; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the increase in the demand for high performing energy storage devices, the energy storage community has explored ways to improve Li-ion battery chemistry. Previous research has demonstrated that nanostructuring of Li-ion electrodes enables significant improvements in their power and energy densities. However, a systematic study is needed to quantify the impact of specific structural properties on the electrochemical behavior of the nanostructured electrode and to develop a guideline for high performance energy storage devices. In the first study of this dissertation, we investigate the impact of pore diameter, dynamic conductivity and interconnected structures on the electrochemistry of V2O5, cathode material for Li ion batteries. We determined that there were positive and negative effects of the interconnected structure depending on the material properties. When V2O5’s electronic conductivity increased with the degree of lithiation, a higher power density was measured with more interconnections. When the material’s electronic conductivity decreased with lithiation, a lower power density was measured with more interconnections. In the second study, we employ microfabrication techniques and atomic layer deposition to fabricate well defined nanochannels to study the effect of electrolyte nanoconfinement on the electrochemistry of anatase TiO2. Surprisingly, nanoconfinement resulted in high energy and power densities when compared to the bulk material. Simulations showed that the improvement in the electrode behavior was due to the negative surface charges of TiO2 which resulted high local concentration of Li ions within the nanochannel and minimal loss in the driving potential was observed at the stern layer. In the third study, we fabricate a platform for high performance 3D solid state batteries on a Si wafer to study the effect of high aspect ratio nanostructures on the electrochemical behavior of thin film solid state batteries. The V2O5 electrode in 3D scaffold showed 113 times higher capacity than the planar electrode at 2μA/cm2 and 1333 times higher capacity at 0.5mA/cm2. These studies can help to understand key structural parameters for improved Li-ion batteries, and the test platforms we developed in these studies can be applied to increase understanding of structural impacts on other ion battery chemistries as well.Item Solid electrolyte interphases on graphite anodes and sulfur cathodes(2018) Wang, Luning; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes chemical compositions and fundamental mechanisms of formation of solid electrolyte interphase (SEI) layers on a graphite anode and a microporous carbon/sulfur cathode. Direct analysis on the authentic SEIs by solution NMR spectroscopy is combined with investigations on model chemical systems. We uncovered the presence of highly similar SEI components (i.e., lithium ethylene mono-carbonate, LEMC and lithium methyl carbonate, LMC) on the two types of electrodes, but the components are generated from completely different chemical routes. The solution and solid-state properties of the compounds are fully characterized. Single crystalline X-ray diffraction (XRD) studies on the two compounds reveal layered packings and structural disorders in the lattice. Electrochemical impedance spectroscopy (EIS) measurements indicate high Li+ ionic conductivity (> 10-6 S/cm) of LEMC. Remarkable difference is observed regarding the chemical compositions and formation routes of the SEIs on the identical sulfur cathodes in Li-S vs. Na-S batteries. The formation of SEIs is found closely associated with fundamental issues such as ion solvation structures, cation coordination, etc. Operando studies on solid-liquid interface in an electrochemical cell carried out by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) visualize ionic structures in an electrical double layer (EDL) in a Au/sulfide aqueous system. We observe monolayer adsorption of bisulfide (HS-) on Au and spatial distributions of HS- in liquid/gas interface.