Controlled synthesis of reduced graphene oxide-based metal compound hybrids for lithium-ion batteries application
Date of Issue2012
School of Materials Science and Engineering
Lithium ion battery, as an effective electrochemical energy storage device, has attracted much interest recently. Advanced preparation processes of electrode materials have been developed with controlled size, morphology and composition. In order to enhance the capacity, stability and rate performance, the electrode materials will generally need to be able to (1) accommodate the strain generated during the lithium insertion/extraction and prevent significant structural collapse; (2) possess large surface area and high electrode/electrolyte contact area; (3) remain nanostructures to offer short Li ion diffusion path; (4) exhibit high electrical conductivity and also (5) maintain stable electrical contact with the current collector. Hence, the design of advanced electrode materials is of prime importance and remains as a key challenge that needs to be addressed and constitute one of the motivations behind this study. In this work, we aim to develop scalable, low-cost and environmentally friendly approaches to prepare electrode materials with advanced electrochemical properties. The chemical and physical mechanisms of these electrode materials were then investigated. Firstly, to identify the ideal carbon supports for active nanoparticles (NPs), we prepared SnO2 NPs with different carbonaceous materials, e.g. amorphous carbon, carbon nanotubes (CNT) and reduced graphene oxide (rGO). With the similar SnO2 mass loading, the electrochemical test showed that SnO2/rGO electrode delivered the highest reversible capacities, which are mainly due to its large surface area and high electrical conductivity. In addition, the mass loading of SnO2 was optimized in this part of the work. However, the SnO2 NPs tended to agglomerate during the charge-discharge process, and hence affected the long-term cyclability. Therefore, to achieve high capacity and cyclability at high current density, the agglomeration of SnO2 NPs should be prevented. Amorphous Fe2O3 NPs were introduced into SnO2/rGO composite to form ternary phase SnO2-Fe2O3/rGO nanocomposite with tailored weight ratio of three components. Electrochemical test demonstrated that the SnO2-Fe2O3/rGO electrode showed good capacity and cycling stability, especially at high current densities. The amorphous Fe2O3 NPs are believed to effectively prevent the agglomeration of SnO2. Based on above results, it indicated that MO/rGO nanocomposite structure could be excellent electrode materials. Then, we further investigated the effects of grain size, structure, surface area, and phases on their electrochemical performance. Cobalt oxides nanowalls arrays on rGO nanosheets were fabricated. The results showed structural-process-dependent performances, where smaller grain size and higher surface area can lead to higher capacity and better cycling stability. However, the lithiation process of many metal oxides involves both the insertion and conversion processes, which involves structural change of the active materials and affects the fast Li diffusion in the electrodes as well as the high-C-rate performance. In order to achieve better electrochemical performance at high C rates, we prepared organic molecular, ferrocene, decorated rGO nanohybrids. In this case, the ferrocene greatly improves the specific capacity while the rGO serves as the light-weight flexible platform to anchor the ferrocene molecules to prevent them from dissolving into the aprotic electrolyte. These ferrocene/rGO nanohybrids showed excellent high-C-rate performances. Based on all the above work, the strategy on preparation of nanocomposites for electrode materials of Li ion battery is proposed.