Polysulfide Mitigation at the Electrode-Electrolyte Interface: Experiments in Rechargeable Lithium Sulfur Batteries
In the field of energy storage technology, the lithium sulfur battery is intensely studied in interest of its great theoretical gravimetric capacity (1672 Ah kg-1) and gravimetric density (2600 Wh kg-1 ). The theoretical performance values satisfy viability thresholds for petroleum-free electric vehicles and other emerging technologies. However, this elusive technology remains in the research sector due to a wealth of challenges resulting from its complex chalcogenide electrochemistry. The most infamous challenge remains the polysulfide redox shuttle, a phenomenon in which lithium polysulfide intermediates are produced as elemental sulfur S 8 is reduced to lithium sulfide Li2S during the discharge cycle. Because higher order polysulfides are soluble in organic electrolyte, battery cycling can result in dissolution of the cathode, dendrite formation upon the lithium metal anode, and passivation of electrode surfaces. These problems can ultimately cause rapid capacity fade and unstable Coulombic efficiency. As lithium sulfur battery research enters its 3rd decade, it is becoming increasingly clear that solutions will be holistic or synergistic; that is, addressing the aforementioned issues by suppressing their source in the polysulfide redox shuttle rather than isolated symptoms of the underlying mechanism. This thesis serves as a summary of research performed to study polysulfide suppression and mitigation through electrode material synthesis, electrolyte design, and in situ characterization. Synthesis techniques include solid state pyrolysis, autogenic sythesis, and ultrasound sonochemistry. Material characterization techniques include isothermal nitrogen sorption; scanning, transmission, and scanning transmission electron microscopy; thermogravimetric analysis; energy dispersive X-ray spectroscopy; organic elemental analysis; X-ray diffraction; and Raman spectroscopy. Electrochemical characterization includes galvanostatic battery cycling, differential potentiometric analysis, and electrochemical impedance spectroscopy. Altogether, this research demonstrates the challenges of polysulfide degradation are not sufficiently addressed by symptomatic approaches. Synthesis pathways for carbon sulfur cathodes that encourage homogeneous sulfur distribution (i.e., autogenic or sonochemical synthesis) improve specific capacity across extended cycling, but show excessive polysulfide production at slow cycling rates. In combination with fluorinated electrolyte, carbon sulfur cathode morphology improves Coulombic efficiency at cycling rates between C/20 – 2 C but at the cost of gravimetric capacity. Synchrotron tomography characterization, developed for Advanced Photon Source Beamline 6–BM-A, evidences that fluorinated electrolytes may also effectively suppress dendrite formation on lithium metal anodes. This suggests more holistic and optimized techniques, or their combinations, may lead to effective polysulfide suppression and successful commercialization of the lithium sulfur battery. Supplementary research explores broader impact of synthesized carbon applications in lithium sulfur batteries. Pyrolysis synthesis processes are evaluated for health and environment impacts using optical by-product sizing and life cycle analysis, respectively. In the context of pyrolytic synthesis of carbon microsheets, micro and nano-sized carbonaceous particulate by-products released during synthesis must be collected to minimize health exposure risks. The environmental impact of this synthesis process is a function of mode of oxygen deficiency, that is, whether pyrolytic atmosphere is facilitated by vacuum or inert gas stream. Finally, submicron carbon spheres, a carbon morphology produced by pyrolysis of sonochemically-synthesized polymer spheres, demonstrate gravimetric capacity which is a strong function of microstructure (i.e., pore distribution, crystallite size, structural disorder). In turn, microstructural properties are determined by synthesis temperature, a dimension of synthesis pathway.
Pol, Purdue University.
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