| dcterms.abstract | The increasing demand on renewable energy worldwide is driving the pursuit of cleaner, safer and higher-energy energy storage technologies, as the concerns about environmental pollution associated with the use of fossil fuels are becoming serious with the growth of population and economy. While solar and wind energies are intermittent, rechargeable batteries are so far the most viable option for electrical energy storage. Among the various battery systems such as lead-acid, nickel cadmium, nickel metal hydride, lithium based batteries have unmatchable combination of high energy and power density, which make them the desirable choice for electric vehicles, portable electronics and power tools. As the functionalities of the portable electronics become more sophisticated and the demand for electric vehicles and storage of electricity from renewable sources increases, other advanced battery technologies such as lithium sulfur batteries and magnesium ion batteries in addition to lithium ion batteries are being developed, where cost, energy, power, life and safety are all important parameters to be taken into accounted. In this doctorate dissertation work, electroactive materials for use in lithium ion batteries, lithium sulfur batteries, lithium primary batteries and magnesium ion batteries were investigated, and in-depth understanding of the electrochemical properties of these materials was gained. Vanadium-based compounds are favorable materials for Li ion batteries due to the possibility of multiple electron transfers per formula unit within a desirable voltage range and thus a high energy density. Among the multiple oxide materials with vanadium redox centers, Li1+nV3O8 (n=0-0.2) is especially promising because of its superior electrochemical properties including high specific energy and good rate capability. Prior research efforts have centered on the modification of preparation approaches to improve the functional capacity of this material, as preparation conditions including annealing temperature have a strong impact on the electrochemical outcomes. Understanding phase transformation and structural change accompanying de(lithiation) is of great significance for achieving excellent cyclic stability of the electrode materials. From the view point of battery applications, the focus of this dissertation work on the Li1+nV3O8 material is on probing the phase evolution, phase distribution as well as impact of morphology and interfacial structure, which are vital for the practical implementation of this material. In Chapter 1, the structural evolution of Li1.1V3O8 material during electrochemical dis(charge) processes was investigated using a combination of theoretical calculations and experimental data. Density functional theory was used to predict the intermediate structures at various lithiation states as well as the stability of major phases. In order to validate these predictions, in-situ x-ray diffraction (XRD) data was collected operando, allowing for the phase transformations to be monitored under current load and eliminating the possibilities of structural relaxation processes and environmental oxidation. Rietveld refinement was performed to fit the diffraction data with the DFT-derived structures and to analyze the fractions of major phases as a function of dis(charge). The DFT calculations identified three stable states of Li-poor ? (Li1), Li-rich ? (Li2.5) and ? (Li4) phases which were validated by the in-situ XRD result. The DFT-predicted particle shape based on surface energy of (100), (001), and (010) planes rationalized the preferential orientation of Li1.1V3O8 particles along [010] in the electrode. Furthermore, the onset and offset of the ??? transition as well as the phase fractions of ? and ? determined via in-situ XRD related well with the DFT-derived relative stability of each phase. Thus, by integrating DFT calculations with experimental work, this work provides a thorough understanding of the structural transformations in Li1.1V3O8 during electrochemical dis(charge), and contributes to further development for the investigation of lithiation mechanisms of other Li ion battery materials. In Chapter 2, a synchrotron based energy dispersive X-ray diffraction (EDXRD) technique was employed to profile the phase transitions and spatial phase distribution of a pellitized Li1.1V3O8 electrode during electrochemical (de)lithiation in-situ and operando. As annealing temperature during the preparation of the Li1.1V3O8 material has a strong influence on the morphology and crystallinity, and consequently influences the electrochemical outcomes of the material, Li1.1V3O8 materials prepared at two different temperatures, 500 and 300 °C (LVO500 and LVO300) were employed in this study. EDXRD spectra of LVO500 and LVO300 cells pre-discharged at C/18, C/40 and C/150 were measured in-situ, and phase localization and relative intensity of peaks were compared. For cells discharged at the C/18 rate, although ? and ? phases were distributed uniformly within the LVO500 electrode, they were localized on two sides of the LVO300 electrode. Discharging rates of C/40 and C/150 led to homogeneous ? phase formation in both LVO500 and LVO300 electrodes. Furthermore, the phase distribution as a function of position and (de)lithiation extent was mapped operando as a LVO500 cell was (de)lithiated. The operando data indicate that (1) the lithiation reaction initiated from the side of electrode facing the Li anode and proceeded towards the side facing steel can (2) during discharge the phase transformation from Li-poor to Li-rich ? phase and the formation of ? phase can proceed simultaneously in the electrode after the first formation of ? phase, and (3) the structural evolution occurring charging is not the reverse of that during discharge and takes place homogenously throughout the electrode. In Chapter 3, lithium vanadium oxide (Li1.1V3O8) particles synthesized at two different temperatures were coated with an amorphous lithium phosphorous oxynitride (LiPON) film for the first time, and the effects of the LiPON coating on the electrochemistry of the Li1.1V3O8 materials with different morphologies were systematically investigated by comparing uncoated Li1.1V3O8 and Li1.1V3O8 coated with LiPON of various thicknesses. Galvanostatic discharge-charge cycling revealed increased functional capacity for the LiPON-coated materials. Post-cycling electrochemical impedance spectroscopy showed that LiPON-coated Li1.1V3O8 materials developed less interfacial resistance with extended cycling, rationalized by vanadium migration into the LiPON coating seen by electron energy loss spectra. Post-mortem quantitative analysis of the anodes revealed more severe vanadium dissolution for the more irregularly shaped Li1.1V3O8 materials with less LiPON coverage. Thus, this study highlights the specific benefits and limitations of LiPON coatings for stabilizing a moderate voltage Li1.1V3O8 cathode material under extended cycling in liquid electrolyte, and describes a generally applicable approach for comprehensive characterization of a composite electroactive material which can be used to understand interfacial transport properties in other functional systems. Owing to the potential to reversibly storage high amount of electrical energy at low cost, Li-S batteries have emerged as a particularly promising energy source. However, the Development of Li-S batteries is hindered by sluggish kinetics resulting from the intrinsic poor conductivity of sulfur and capacity degradation due to solubility of intermediate lithium polysulfides (LiPS). One strategy for overcoming these issues is to use TiS2, which is a good electrical conductor and LiPS absorbant, as an additive to sulfur electrodes. In Chapter 4, from a structural perspective, we probed TiS2-S composite materials during electrochemical discharge and charge reactions in propylene-oxide based glyme (DPGDME) electrolyte using in-situ XRD, revealing the synergistic effects of TiS2 and S in the composites. TiS2 was found to function effectively as a conductive additive and improve the utilization of sulfur. Intercalation of Li+ into TiS2 takes place simultaneously with the sulfur-lithium reaction, and contributes to the total realized capacity. Our work proposed a new strategy to improve the energy density and useful capacity of Li-S batteries, and provided detailed understanding on the structural mechanisms of the composite materials. Carbon monofluoride (CFx) has a high energy density, exceeding 2000 Wh/kg, yet its application in primary lithium batteries is limited by its power capability. Multi-walled carbon nanotubes (CNT‡) are appealing additives for high-power batteries, due to their outstanding electronic transport properties, high aspect ratio necessitating low volume fraction for percolation, and high tensile strength. The work in Chapter 5 describes the current state of the art in lithium-carbon monofluoride (Li/CFx) batteries and highlights opportunities for development of high-power Li/CFx batteries via utilization of carbon nanotubes. In this work, we generated several electrode architectures using CFx/CNT combinations, and demonstrated the effectiveness of CNT in enhancing the rate capability and energy density of Li/CFx batteries. First, we investigated the resistivity of CFx combined with CNT and compared the CFx/CNT composites with conventional carbon additives. Second, we built CFx-CNT electrodes without metallic current collectors using CNT as substrates, and compared their electrochemical performance with conventional CFx electrodes using aluminum foil as a current collector. Further, we fabricated multi-layered CNT-CFx-CNT composite electrodes (sandwich electrodes) and studied the impact of the structure on the performance of the electrode. Our work demonstrates opportunities for utilization of CNT in CFx electrodes and the resultant implementation of CFx as a battery cathode in next-generation high-power batteries. Magnesium-ion batteries are attractive in part due to the high environmental abundance and low cost of magnesium metal. Anode materials other than Mg metal can provide access to new electrochemistries in non-corrosive Mg2+ electrolytes. In Chapter 6, a cyclic voltammetric method for the preparation of bismuth (Bi) based anodes was developed by systematically exploring electrodeposition using a quartz crystal microbalance. Controlled deposition of Bi on carbon nanotubes substrates could be achieved, enabling the first electrochemical investigation of bismuth-carbon nanotube (Bi-CNT) composite electrodes. Quasi-reversible Mg electrochemistry of Bi-CNT composite electrodes in non-corrosive magnesium-based electrolyte was demonstrated, with an initial delivered capacity exceeding 180 mAh/g. While the initial capacities were high, significant capacity decreases were observed with repeated cycling, indicating that additional development is warranted to further optimize this system. | |
