THESIS
2018
xviii, 114, that is, xix, 114 pages : illustrations ; 30 cm
Abstract
Development of advanced energy storage systems for portable and wearable devices is highly
desirable due to the increasing demand for high performance wearable energy sources. There
are a few considerations for the design and selection of electrode materials for this application,
such as low-cost, long life, acceptable safety, high energy and power densities and
environmental benignity. Graphene is a promising material for flexible electrodes thanks to its
inherent high electrical conductivity, tailorable architecture and large surface area. The recent
breakthrough in wet spinning of liquid crystal graphene oxide (LCGO) rendered it a promising
method to prepare electrodes with outstanding mechanical and electrical properties. The
scalability of wetspun graphene fibers in terms o...[
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Development of advanced energy storage systems for portable and wearable devices is highly
desirable due to the increasing demand for high performance wearable energy sources. There
are a few considerations for the design and selection of electrode materials for this application,
such as low-cost, long life, acceptable safety, high energy and power densities and
environmental benignity. Graphene is a promising material for flexible electrodes thanks to its
inherent high electrical conductivity, tailorable architecture and large surface area. The recent
breakthrough in wet spinning of liquid crystal graphene oxide (LCGO) rendered it a promising
method to prepare electrodes with outstanding mechanical and electrical properties. The
scalability of wetspun graphene fibers in terms of morphologies, structural and chemical
properties are the important factors in utilizing these viable architectural advances in flexible
electronic devices. A porous electrode is the central component of lithium sulfur batteries (LSBs)
and lithium oxygen batteries (LOBs), and the porosity largely affects the electrochemical
performance and reactions taking place. This thesis is dedicated to synthesizing rationally
designed electrodes using nanostructured graphene which possess improved electrochemical
performance and mechanical robustness.
1D reduced graphene oxide/carbon nanotubes/sulfur (rGO/CNT/S) fibrous cathodes are
prepared by wet spinning for the first time for battery application. By virtue of liquid crystalline
behavior of high concentration GO sheets in aqueous dispersion, rGO/CNT/S composites are
rationally assembled to form flexible and conductive fibers as lithium–sulfur battery electrodes.
A prototype of rechargeable LSB consisting of a fibrous cathode, a lithium wire and
polyethylene separator is fabricated. The high stability of the electrochemical performance
under cyclic bending and excellent mechanical flexibility of the LSB highlight great potential
of graphene-based fiber assemblies in the quest for shape-compliant electrode materials. The
facile wet-spinning process and the low-cost GO derived from abundant natural graphite flakes
as the starting material make the flexible fibrous cathodes an attractive choice for designing
next-generation power storage devices, especially for wearable electronics.
Wavy and wrinkle-rich ribbon electrodes consisting of rGO, graphene crumples and sulfur
(rGO/GC/S) are assembled by fast drying coupled with a wet spinning process. The 2D/3D
hybrid electrode structure is tailored using graphene with different dimensions and functional
features leading to its exceptional mechanical robustness and electrochemical performance. The
highly conductive graphene crumples offer large surface for electrode/electrolyte contacts
while providing strong interfacial interaction with sulfur species. A shape-conformable battery
prototype comprising an rGO/GC/S cathode and a lithium anode demonstrates a stable
discharge characteristic under repeated bending/flattening cycles. The LSB prototype presents
stable discharge behavior with high mechanical robustness against an extension up to 50%. The
above findings shed new light into developing sulfur cathodes for flexible, high performance
LSBs based on rational design of graphene structures.
Nitrogen-doped graphene fiber webs (N-GFWs) consisting of interconnected graphene fibers
are prepared by one-pot wet spinning. The graphene webs are assembled with wet spun short
graphene fibers with a highly enhanced mass/electron transport through the conductive, porous
structure. The atomic structure of N-GFWs are tailored by controlling the degree of reduction
leading to different electrochemical behaviors with multifunctional capabilities, and their
potential applications in LOBs and LSBs are demonstrated. The optimized N-GFW900 shows
good electrocatalytic activity when used as a cathode for lithium oxygen batteries. The cathode
with a high areal loading of 7.5 mg cm
-2 delivers a remarkable areal capacity of 2 mAh cm
-2 at
0.2 mA cm
-2. In addition, the freestanding N-GFW700 interlayer possesses many functionally
useful characteristics, facilitating much enhanced capacities and long-term cyclic stability. The
lithium sulfur battery delivers an excellent specific discharge capacity of 605 mA h g
-1 after
200 cycles with a low degradation rate of 0.04% per cycle at 0.5C. The approach developed
here paves the way for rational design and assembly of graphene-based electrodes with tunable
weights, satisfying various requirements for energy storage applications.
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