Research On Graphene-based High Performance Electrochemical Energy Storage Materials

Since it is discovered in 2004, graphene have been known as an ideal materaisl for energy storages due to its high conductivity, specific area, wide electrochemical window and high chemical stability. Aggregation/restacking poses a great challenge for harnessing the great potential of individual graphene sheets from their bulk assemblies. Inspired by the important role of hydration in biological tissues, we demonstrate that a combination of hydration and corrugation of graphene sheets (SSG) provides a simple and highly effective approach to this key challenge. In a solvated state, chemically converted graphene sheets can remain largely separated by water when face-to-face stacked together to form a free-standing film. When used as electrodes for supercapacitors, the resultant film can offer a high capacitance and ultrahigh power at a broad range of operation rates. Its maximum energy density is comparable to that of lithium ion batteries if a suitable electrolyte is used. A combination of ease of synthesis, excellent internal surface accessibility, mechanical robustness and a thin film form makes this unique graphene assembly highly promising as a new generation of electrodes for energy storage and many other applications. Based on SSG film, we studied the electrochemical energy storage performance of graphene based materials.Firstly, SSG films can be directly used as the electrodes of supercapacitors. A specific capacitance of 215 F/g (0.108 A/g) can be obtained with the sulfuric acid as the electrolyte. SSG films and its freeze dried counterpart can reach the specific areas of 180 and 71 F/g at the charge/discharge current density of 108 A/g; SSG-based supercapacitors can even obtain 170 and 158 F/g at the ultrahigh current density of 540 and 1080 A/g. Furthermore, SSG film can attain the operational frequency of 75 Hz and the energy/power density of 5.52 Wh/kg/414 kW/kg. The SSG films can be easily exchanged with neutral aqueous solutions. With the neutral solution as the electrolyte, the applied voltage can be increased to 1.6 V without destroying the structure of SSG films. The SSG film herein shows a double layered capacitive characteristic. At a charge/discharge rate of 200 A/g, a specific capacitance of 138.1 F/g can be obtained. Furthermore, the energy density can reach as high as 17.82 Wh/kg and a superior cycling performance.Secondly, ionic liquids can be used as the electrolyte of SSG films with the applied voltage of 4 V. Its specific capacitance can reach 273.1 F/g at 1 A/g, and even 180.3 and 168.0 F/g at ultrahigh 50 and 100 A/g. The energy and average power density can be as high as 150.9 Wh/kg and 198.9 kW/kg.Thirdly, the SSG film can also serve as versatile porous matrix to allow other functional materials to be incorporated to generate new exciting nanohybrids, such as PANi@SSG film. The SSG film can provide a nanocollector and high surface area for active materials, restrict the particles size within its pores and prolong the cycling performances with its flexible but rather strong structures. When used in supercapacitors, a specific capacitance of 645.5 F/g can be obtained, furthermore, 612.2 and 600.3 F/g can also attained at ultrahigh current densities of 86.2 and 258.6 A/g. PANi@SSG films can provide the maximum energy density of 22.1 Wh/kg and the combination of energy/power density of 18.9 kW/kg and 16.3 Wh/kg with a excellent cycling life.Fourthly, Graphene oxide can serve as a superior dispersant to disperse pristine CNTs into water to form stable suspensions through supramolecular interactions. These discoveries offer a very simple, cost effective and environmentally friendly strategy to address the long-standing processablity issue of CNTs. It also allows convenient integration of the 1D CNTs with the 2D graphene to form hierarchically structured carbon nanohybrids with enhanced performances. Given that CNTs are essentially rolled graphene sheets and GO is partially oxidised graphene, this work presents an excellent example that fascinating synergetic effects could exist between different graphene derivatives. We expect that chemical and/or electrical synergies between graphene derivatives will play an important role in the future development of carbon-based materials. Additionally, this work suggests that GO should be, more accurately, viewed as an amphiphilic molecule. This means that GO, like other conventional amphiphilic surfactants, may be further used to disperse other nanostructures, which will enable solution-phase processing of many new graphene-based nanohybrids.Finally, freeze dried SSG films can be directly used as the anode of lithium ion batteries, without polymeric binders or conductive agents. The reversible specific capacity is 645.2 mAh/g and the corresponding irreversible loss is 46.7 % at the first cycle. The specific capacity can reach 484.2 and 462.1 mAh/g at the tenth and twentieth cycle. After 400 cycle, it can obtain a capacity of 305.5 mAh/g. The SnO₂/graphene sheets was hydrothermally synthesized from graphene oxide and Sn(IV) salt precursors. The tin oxide particles of SnO₂/graphene are nanocrystalline and monodispersive with the size around 5nm. The SnO₂/graphene presents a reversible discharge capacity of 854.3 mAh/g and an irreversible capacity loss of about 42.0% at the first cycle. The SnO₂/graphene exhibits excellent cycling stability compared to as-obtained SnO₂, and after 200 cycles still remains a high capacity of 567.6 mAh/g. As a simple synthesis route, the self-assembly of SnO₂/graphene sheets would be a promising method to prepare anode materials for lithium ion batteries. This method is also can be easily used to prepare other monodispersive metal oxide/graphene composites.