| Low carbon economy and clean renewable energy now become significant topics over the world, due to the increasing demands for energy, decreasing amount of non-renewable energy, such as petrol, coal and natural gas, as well as environmental problems. With the economic development and the improvement of living standards, there is a growing emphasis on the development and utilization of water, wind, solar and other renewable energy. Comparing with the traditional energy, these new forms of energy have many advantages. However, they are subject to the climate and weather, and their output energy peaks do not match the demand. In addition, energy supply often shows big fluctuations. As a result, energy storage devices with high performance are urgently required to use energy more efficiently.Among all the energy storage devices, lithium ion batteries exhibit much more advantages, such as high voltage, high energy density, etc., thus they have been quite popularly used in portable devices since their birth, and now they are nominated as new energy source for vehicles. In order to expand the application of lithium-ion batteries in portable electronic devices and hybrid electric vehicles, we need to improve the cycling performance, rate capability and safety performance of lithium-ion batteries. Graphene, as a two-dimensional macromolecular sheet of carbon atoms with a honeycomb structure, has excellent electronic conductivity and mechanical properties, and may be the ideal conductive additive for hybrid nanostructured electrodes. Other advantages of graphene Include high surface area (theoretical value of2630m2/g) for improved interfacial contact and the potential for low manufacturing costs. Its excellent conductive could enhance the kinect properties and rate performance of electrode materials. Meanwhile, the unique two-dimensional structure of the graphene can anchor wrap and weave the nanomaterials, forming a hybrid random3D structure. This kind of loose structure could buffer the volume expansion and avoid the agglomerate during the cycling. Furthermore it may absorb the electrolyte, offering the special ion channels between the different nanoparticles to further improve its conductivity.A series of graphene composites have been synthesized. The preparation, characterization and electrochemical performance of varies nanostructured electrode materials are discussed.1. To reduce the reaction time, electrical energy consumption, and cost,LiFePO4//graphene has been synthesized by a rapid, one-pot, hydrothermal autoclave microwave method within15min. The carbon coated LiFePO4/C nanoparticles, around200nm in size, are thoroughly wrapped by crumpled micrometer-size graphene sheets. In this kind of structure, the bridging graphene nanosheets can form an effective conducting network and provide interconnected open pores that favor electrolyte absorption and reduce the diffusion path of the lithium ions. The cyclic voltammograms, charge/discharge profiles, and AC impedance measurements indicates that the kinetics of the LiFePO4/C/graphene is better than that of LiFePO4/C. LiFePO4/C/graphene composite exhibits a discharge capacity of165mAh g-1at0.1C and88mAh g-1at10C, respectively.To overcome the problems of vanadium dissolution and the higher charge transfer resistance that results from it, VO2/graphene composites have been synthesized by an in-situ hydro thermal process directly from graphene oxide and V2O5. Carbon dispersed in the electrode material can provide a pathway for electron transport, resulting in improvement of the electronic conductivity. Graphene woven VO2nanoribbons prevent the agglomeration of VO2nanoribbons, meanwhile graphene and the VO2nanoribbons together form a porous network in the random hybrid composite that can be filled with electrolyte, resulting in superior performance and enhanced reversible capacity in comparison with the pure VO2. Electrochemical tests show that the VO2/graphene omposite features high discharge capacity (380mAh g-1) and99%capacity retention after50cycles. The electrochemical impedance spectra indicate that the VO2/graphene composite electrode has very low resistance, only67%of that of pure VO2, indicating the enhancement of electronic conductivity.Li3VO4/graphene composites have been synthesized by a novel rapid one-step in situ hydrothermal method. It reveals a unique morphology in which homogenous hollow structure Li3VO4nanocubes anchored in porous graphene microsheets. The hollow structure could relax the stress of Li+insertion/desertion; increase the surface area of the materials, provide extra space for the storage of lithium ions and reduced effective diffusion distance for lithium ions which result in the promoted capacity, rate capability and cycling performance. Furthermore this graphene-wrapped nano-architecture ensures not only intimate contact between the liquid electrolyte and the active Li3VO4nanoparticles, but also high electronic conductivity for both facile mass transfer and facile charge transfer. Li3VO4/graphene exhibits a discharge capacity of403mAh g-1after50cycles, which is117mAh g-1higher than that of Li3VO4. Besides, it shows an outstanding rate performance. A highly capacity of317mAh g-1was obtained at the highest current density of2C, which is and144mAh g-1higher than that of Li3VO4. Furthermore, the EIS test indicates that Li3VO4/graphene shows the much better charge transfer resistance and lithium diffusion coefficient. |
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