CARBON-SULFUR NANOCOMPOSITES FOR LITHIUM-SULFUR BATTERIES
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The increasing reliance on energy storage systems is constantly pushing research efforts to find better performing, low cost electrochemical batteries. The lithium-sulfur battery has been deemed one of the most viable candidates due to its high energy density and non-toxic, inexpensive components. In order to reach its full potential, sulfur should be incorporated into a conductive carbon host structure to ensure its electrical conductivity and cycling performance. In addition, rapid capacity fading resulting from the polysulfide shuttle mechanism should be addressed. The goal of this dissertation is to employ transmission electron microscopy (TEM) to study various nanostructured carbon materials that can serve as a cathode component in a lithium-sulfur battery.
The dissertation is divided into three topics. The first topic describes the creation of graphene/sulfur composites suitable for in-situ TEM. TEM studies on sulfur are limited due to sulfur’s ability to sublimate at the operating conditions of most conventional TEMs. Therefore, we develop a layered structure in which sulfur is enveloped between two graphene sheets to stabilize the sulfur. We report the fabrication methods and TEM analysis of these structures. The second topic is the study of sulfur which is incorporated into single-walled carbon nanotubes (SWCNTs). The inner cavity of a SWCNT provides a large electrochemical interface, good mechanical stability and the potential to retain polysulfides formed during cycling. We utilize a two-step procedure consisting of thermal oxidation and high-temperature filling to produce sulfur-filled SWCNTs. Our electrochemical testing shows a clear dependence on the cell’s performance with the thermal oxidation temperature. We conclude that 475 °C is the optimal oxidation temperature for sulfur filling and results in the most stable cycling performance. The last topic is in-situ TEM studies of multi-walled carbon nanotube-sulfur composites utilizing various solid electrolytes. We examine the implications of employing a Li2S-P2S5 solid electrolyte and compare with a Li2O solid electrolyte during in-situ TEM studies. When using a Li2S-P2S5 solid electrolyte, we are able to show the formation of a lithium-sulfide phase on the surface of MWCNT-sulfur composites and show evidence of lithium dendrite growth suppression.