Surface functionalized reduced graphene oxide-sulfur composite as a cathode material for high performance lithium-sulfur batteries고성능 리튬-황 전지용 환원 그래핀 옥사이드-황 양극 물질에 관한 연구

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With extinguishing petrochemical natural resources and the environmental pollution ascribed to their use; it has become inevitable that the automobile industry should try to shift the paradigm of fueling the automobiles through rechargeable battery systems. Among several rechargeable battery systems, the burden of powering portable devices as well as automobiles is being bared mainly by the Li-ion batteries from the last few decades. However the energy density of Li-ion batteries have reached its saturation point as a result of significant efforts done by the research fraternity. The typical energy density of Li-ion battery (387 Wh/kg) is quite low for the automobiles to travel a long distance. Moreover, the high cost of cathodes for Li-ion batteries made this battery even more expensive to use. Thus once again research on finding next-generation high energy density Li batteries was embarked. Among several candidates, Lithium-Sulfur (Li-S) batteries due to its high theoretical capacity (2500 Wh/g), low cost and low toxicity of earth abundant sulfur has gained much attention since the last few decades. However, the associated problems with Li-S battery such as low electrical conductivity of sulfur, dissolution of Li polysulfides in the electrolyte and volume expansion upon cycling makes this battery system unable to deliver high discharge capacity. The polysulfide dissolution is one of the main problem in which the active material loss and polysulfide shuttle phenomenon causes severe capacity fading. To overcome polysulfide dissolution, several physical and chemical entrapment strategies has been employed. The physical entrapment couldn’t hold the polysulfides for a long time since movement of polysulfides is driven by the electric field as well as due to poor non-polar ? polar interaction between carbon hosts and lithium polysulfides. However, the chemical adsorption of polysulfides by polar oxides, functional polymer coatings and functionalized/doped carbon frameworks had worked better for maintaining longer and stable cycling in Li-S battery. But due to the insulating nature of oxides and polymers, the overall electrical conductivity of the cathode decreases. However, 0D, 1D and 2D carbon host such as carbon black, carbon nanotubes and graphene structure can provide good electrical conductivity but non-polar carbon interaction can lose the polar polysulfides during the course of cycling. Thus the best way is to use functionalized carbon framework to simultaneously obtain higher electronic conductivity as well as polysulfide entrapment through functional groups attached to the carbon host. Moreover, among carbon hosts graphene oxide has exquisite benefits of high mechanical stability as well as superior electrochemical stability thus improving the ionic and electronic transportation with decreased diffusion pathways. Thus in this study, GO was taken as an initial carbon host for sulfur infiltration to make cathode material for Li-S battery. However GO has poor electronic conductivity because of the presence of excessive oxide functional groups on its surface thus GO was chemically reduced by a non-toxic reducing agent, dopamine. Dopamine additionally added amino and hydroxyl functional groups (known to entrap polysulfides) on the rGO surface to chemically bind the polysulfides. Moreover, the dopamine reduced rGO was compared with GO (containing hydroxyl and epoxy functional groups but low electronic conductivity) and thermally reduced rGO (high electronic conductivity but no significant functional groups) based sulfur composites to emphasize that for stable performance both high electronic conductivity and polysulfide binding functional groups are required. The material characterization was done using SEM, TEM, XRD, TGA and Raman Spectroscopy. Moreover the electrochemical characterization was performed by galvanostatic charge/discharge, cyclic voltammetry and electrochemical impedance spectroscopy. Since Li polysulfides exist in different colors in the electrolyte; in-situ UV-Vis spectroscopy was performed on GO-sulfur (GOS) composite and dopamine reduced GO-sulfur composite (c-rGOS) to seek evidence of polysulfide binding with polydopamine. Previously no such reports are present on in-situ characterization of polydopamine binding effect with polysulfides. Two different in-situ UV-Vis cell assemblies were fabricated to show the change in the electrolyte as well as the separator when the cell is discharge till 1.0V. It was clearly shown that no significant polysulfide dissolution took place in the polydopamine modified c-rGOS composite whereas severe polysulfide dissolution was observed in GOS composite both in the electrolyte and on the separator. Due to the binding role of polydopamine, very stable discharge capacity of 600 mAh/g at 0.5C after 300 cycles was obtained in c-rGOS composite however for GOS, it’s dropped to 277 mAh/g after just 100 cycles. This strategy to make c-rGOs composite is also helpful in making large quantity of sulfur composite since the c-rGO synthesis involves only low temperature stirring of GO with dopamine for 24h followed by simple grinding with sulfur before heat treatment at $155 ^\circ C$.
Advisors
Kim, Do Kyungresearcher김도경researcher
Description
한국과학기술원 :신소재공학과,
Publisher
한국과학기술원
Issue Date
2016
Identifier
325007
Language
eng
Description

학위논문(석사) - 한국과학기술원 : 신소재공학과, 2016.8 ,[vii, 87 p. :]

Keywords

Li-S battery; Graphene Oxide; Polydopamine; Polysulfides; in-situ UV-Vis spectroscopy; 리튬-황 전지의; 그래핀 옥사이드; 폴리도파민; 폴리설파이드 흡착; in-situ UV-Vis 분광 분석을

URI
http://hdl.handle.net/10203/221556
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=663393&flag=dissertation
Appears in Collection
MS-Theses_Master(석사논문)
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