Graphene-based electrodes for high-performance electrochemical energy storage
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Graphene, a two-dimensional honeycomb carbon layer, has drawn intensive attention as a promising electrode material for rechargeable batteries and supercapacitors due to its high electrical conductivity as well as chemical and physical stability. Recent progress of large-scale synthesis of graphene oxide (GO) from graphite has boosted more investigations for the conversion of GO to graphene. For energy storage applications, the role of graphene can be largely categorized into two groups: A) A inactive, supporting component for the build-up of composites with various active materials and B) Graphene itself as an active material for charge storage. Graphene, as a supporting component, is not involved electrochemical reactions to store charge or contribute to the amount of charge storage with very limited capacity or capacitance. Regarding this role, graphene-encapsulated submicron Si and two-dimensional functional carbon (TDFC) synthesis using graphene-template were studied in tow sub-themes. In the graphene-encapsulated submicron Si study, submicron Si particles were recovered from Si waste and utilized as a high-performance anode material for LIBs. From the versatile hydrothermal assembly, submicron Si particles were securely coated with graphene. The submicron Si/graphene composite exhibited good cycling stability, showing a capacity retention of 84% at the 100th cycles. In the study of the TDFC, ultrathin TDFC (thickness of 10~20nm) with abundant oxygen functional groups was prepared by GO-template assisted hydrothermal reaction of glucose. During the hydrothermal reaction, GO acted as a substrate for depositing hydrocarbon on its surface. Due to the presence of oxygen functional group on the surface of GO and the planar morphology of GO, the prepared 2D thin film enables more efficient utilization of the redox reactions compared to the conventional carbon sphere, showing a key approach to effectively utilize their redox-reactions. In addition, graphene actively participates in charge storage and thus graphene can be identified as an active material. It is known that an irreversible restacking of GO sheets during electrode preparation has limited the accessible surface area (ASA) to store ions, resulting in a low gravimetric capacity of ~100 mAh/g. In this section, three subtopics of graphene as the active material for charge storage were investigated; A) Crumpled graphene oxide cathode for lithium-ion battery (LIB). B) Stacking-controlled cabbage-like graphene electrodes for supercapacitor. C) Crumpled graphene anode for Sodium-ion battery. The second group is mostly associated with the restacking issue of GO and graphene. In the study of crumpled graphene oxide cathode, the crumpled graphene oxide was employed as cathode material for LIB. The crumpled graphene oxide has an aggregation-resistive characteristic and 3D ball-like morphology. The crumpled graphene oxide showed that the effective utilization of the surface redox reactions with enhanced electrochemical energy storage such as high rate-capability, indicating that the microstructure of graphene is an important parameter for the development of high-power LIBs. In the study of cabbage-like graphene, high density cabbage-like graphene (0.75 g/ cm3) was prepared by two step processes of liquid-phase pre-stacking of graphene and subsequent aerosol spray drying. Despite its high density, the cabbage-like graphene showed a high gravimetric and volumetric capacitance (177 F/g and 117 F/cm3) in aqueous supercapacitors. As a cathode for LIB, it showed a capacity of 176 mAh g-1 (1.0 mAh/cm2). This superior electrochemical performance suggests that the stacking-control approach could provide a new way to achieve both high gravimetric and high volumetric performance of graphene electrodes rather than avoiding restacking. In the study of crumpled graphene anode for sodium-ion battery (SIB), it has been shown that graphene stores sodium ions through capacitive mechanism and there are two sub-routes in the capacitive mechanism, ion adsorption in a similar way of double layer capacitance and ion adsorption on defective sites. In summary, this dissertation discusses the various roles of graphene for energy storage application, such as supercapacitor, LIB, SIB and hybrid supercapacitor. In each application, graphene can be identified as active or inactive material based on its role. The results in various applications using graphene would provide insights how graphene play a role for high performance energy storage.