PhD Thesis Presentation
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.
(Supervisor: Prof. Jang-Kyo Kim)