Synthesis Of Graphene-Based Composite Materials And Their Applications In Energy Storage Devices

Graphene, a two-dimensional (2D) flat monolayer of sp2hybridized carbon bonded in a hexagonal lattice, has attracted extensive attention worldwide since its extraordinary physical and chemical properties were discovered. As it has ultrahigh theoretical specific surface area (SSA,2675m2/g), good electronic conductivity and good mechanical property, graphene is considered to be an ideal electrode material, which has great application potential in energy storage fields such as supercapacitors and lithium ion batteries. Consequently, on the basis of this outstanding material, in this paper we prepared several kinds of graphene-based composite materials by doping or coating graphene on other electrochemical active materials, which have exhibited prior electrochemical performance in supercapacitors or lithium ion batteries.1. Synthesis of graphene-based three dimensional (3D) porous carbon materials and their application in supercapacitors. In this work, graphene-based3D porous carbon materials were prepared from various biomass materials with a few percents of graphene oxide (～4wt%) by a simple and green process including hydrothermal reaction and activation. The composites have exhibited ultrahigh SSA up to3355m2/g and good conductivity. These outstanding properties are attributed to the following reasons:1) The insertion of single-layer graphene sheets can prevent the agglomeration of pure biomass during the hydrothermal reaction, and at the meantime3D porous structure was formed due to the cross-linking, which is in favor of forming more meso/micropores in the later activation process, thus the SSA can be greatly enhanced and the pore size distribution can be shifted from micropore side to mesopore side.2) Graphene has good conductivity, which can form a good conductive network in the composites and thus improve the conductiviy of the composites. Then various structural and morphology analyses were done to conclude that the materials consist of mainly single-layered curved and defected graphene sheets in the size of a few of nanometers. Electrochemical performance was tested by fabricating supercapacitor devices using the composites as electrode active materials. The results show that the materials exhibit excellent performance for supercapacitor in organic electrolyte and ionic liquid systems with highest specific capacitance up to216F/g and energy density up to92Wh/kg, as well as good cycle stability.2. Design and performance research on the electrochemical hybrid supercapacitor based on graphene-based composites as electrode materials. As is known to all, in the novel superior energy storage devices, supercapacitors have high power density and fast charge-discharge rate, but the energy density is low; lithium ion batteries, on the contrary, exhibit high energy density but low power density. Therefore, in this work, a supercapacitor-battery hybrid energy storage device, hybrid supercapacitor, based on graphene-based composites, has been designed and fabricated, which is expected to combine the advantages of supercapacitors and lithium ion batteries, possessing both high power density and high energy density. First, a well-structured Fe3O4nanoparticle/graphene composite was prepared as the negative electrode material by a facile approach including solvothermal reaction and an easy heat treatment. The as-prepared Fe3O4/G exhibits high reversible specific capacity exceeding1000mAh/g at the current density of90mA/g, as well as excellent rate performance and cycle stability. Then, a graphene-based3D porous carbon material with high SSA and good conductivity was used as the positive electrode material, and a hybrid supercapacitor was fabricated by matching the masses of the positive and negative electrode. Thereby, by employing these two graphene-enhanced materials, the hybrid supercapacitor was fabricated with an improved ultrahigh energy density of147Wh/kg and power density of2587W/kg in lithium ion electrolyte, so far the highest value of the reported hybrid supercapacitors. Besides, the packaged energy density of the hybrid supercapacitor is comparable to that of lead acid batteries and nickel-metal hydride batteries, and the power density also reaches that of symmetric supercapacitors, indicating that the hybrid supercapacitor could be a very promising novel energy storage system for fast and efficient energy storage in the future.3. Design and performance research on internal tandem supercapacitor based on3D graphene-based porous carbon material. In this work, to further improve the working voltage and energy density of supercapacitor, we designed an internal tandem structure in supercapacitor using the well performed3D graphene-based porous carbon material as electrode material, which is composed of two pairs of electrodes in one single cell in series. The internal tandem supercapacitor achieves a much higher working voltage of7V as well as a significantly improved energy density of36.3Wh kgcell-1(increased by33%, compared with the single supercapcitor of27.2Wh/kg), which is also about7times of that of the state-of-art commercial supercapacitors. Besides, it can also maintain good rate performance and cycle stability. Flexible internal tandem device was also designed and fabricated and demonstrated similar excellent performance, which therefore shows the universality of the internal tandem architecture design.4. Synthesis of graphene/MoS2/polyaniline composite and its application for lithium ion batteries. In this work, we synthesized graphene/MoS2/polyaniline composite by in-situ hydrothermal reaction. By analyzing its morphology and structure, we found that the composite has a loose and disordered structure, due to the introduction of graphene and polyaniline. Attibuted to the synergistic effect of graphene, polyaniline and MoS2, the composite has exhibited excellent electrochemical performances as anode material for lithium ion batteries, including high specific capacity (-1000mAh/g),great rate performance and good cycle stability. The results indicate that the graphene-modified MoS2material has very important application for lithium ion batteries.