THESIS
2012
xxvi, 212 p. : ill. ; 30 cm
Abstract
Graphene is a new two dimensional carbon material, well known for its superior
electrical conductivity, high surface area of over 2600 m
2/g, ultrathin thickness,
structure flexibility and good mechanical properties. Nowadays, graphene has been
used in fabrication of anode and cathode together with other materials for the lithium
batteries. Elemental sulfur is one of the most promising cathode materials for the next
generation lithium batteries, especially for the high-energy, high-power batteries that
will be applied in powering hybrid electric vehicles and in storing electricity from
renewable sources, due to its highest specific capacity, abundant resources, and low
toxicity. In this thesis, we have synthesized graphene based composite materials
through the development of an...[
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Graphene is a new two dimensional carbon material, well known for its superior
electrical conductivity, high surface area of over 2600 m
2/g, ultrathin thickness,
structure flexibility and good mechanical properties. Nowadays, graphene has been
used in fabrication of anode and cathode together with other materials for the lithium
batteries. Elemental sulfur is one of the most promising cathode materials for the next
generation lithium batteries, especially for the high-energy, high-power batteries that
will be applied in powering hybrid electric vehicles and in storing electricity from
renewable sources, due to its highest specific capacity, abundant resources, and low
toxicity. In this thesis, we have synthesized graphene based composite materials
through the development of an aqueous one-pot wet chemical method, extensively
characterized the synthesized composites and explored them as cathode materials of
lithium-sulfur battery. The main results of this thesis research are summarized as
follows:
1. Sulfur-reduced graphene oxide composite (SGC) materials with uniformly
dispersed sulfur on the reduced graphene oxide sheets are prepared by developing
a simple aqueous one-pot synthesis method, in which the formation of the composite is achieved through simultaneous oxidation of sulfide and reduction of
graphene oxide. The sulfur contents in the SGC, determined by thermogravimetry
and elementary analysis, have been adjusted in the range from 20.9% to 72.5%.
(Chapter 3)
2. We have tested the electrochemical performances of the as-synthesized SGC
materials as cathode of Li-S battery after optimizing electrolyte and voltage
window. Under the electrochemical condition, we have found that the
SGC-63.6%S can deliver a reversible capacity as high as 804 mAh/g after 80
cycles of charge-discharge at a current density of 312 mA/g (ca. 0.185 C), and 440
mAh/g after 500 cycles at 1250 mA/g (ca. 0.75 C). We have further shown that the
SGC cathodes with the sulfur content around 60% in the SGC can achieve a
maximal sulfur utilization. (Chapter 4)
3. It was shown that the commonly used heat treatment cannot improve the
electrochemical performance of the SGC cathode because after heat treatment
some sulfur initially dispersed uniformly on the reduced graphene oxide sheets
aggregates into large particles, resulting in a severe polysulfides shuttle effect. On
the other hand, the heat treatment study has confirmed once again the uniform
dispersion of sulfur on the reduced graphene oxide in the SGC materials during
the one pot synthesis. (Chapter 5)
4. In order to improve further the electric transportation property of the composite
materials, we have synthesized multiwall carbon nanotubes-sulfur-graphene
composite by developing an appropriate synthesis method. In this method, the
surface of MWCNTs is modified at first with –OH groups, and then the –OH is
replace by –SH via adding the Na
2S to grow sulfur on the MWCNTs’ surface. The
result demonstrated that the sulfur-MWCNTs composite can improve the
coulombic efficiency of Li-S battery. In the case of MWCNTs-S-rGO with 30%
MWCNTs and 10% reduced graphene oxide, the cathode can deliver a reversible
capacity of 500 mAh/g after 200 full cycles with a low coulombic efficiency of
108%. (Chapter 6)
5. To improve the stability of the composite and further improve the performance of
Li-S battery, we have used PANI polymers to coat the CGS materials. It was
found that the conducting polymer can improve the cycling performance and
columbic efficiency of SGC cathode with lower sulfur content. The SGC-37.0%S
coated with 5.3% PANI can deliver a reversible capacity of 500 mAh/g after 400
full cycles with a coulombic efficiency of 105%. (Chapter 7)
KEYWORDS: Sulfur, Graphene, Composite, Li-S battery
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