Graphene-based Materials As Anodes For High Performance Lithium-ion Batteries

Lithium-ion batteries (LIBs) have been widely used in portable electronic devices due totheir high energy density, long cycle life and excellent safety. In recent years, as thedevelopment of the electric vehicles, the power density and energy density of LIBs haveneeded to be further improved. Anode material is one of the most decisive factors toimproving the electrochemical performance for LIBs. Howerver, graphite, the mostcommonly commercial anode material, has the low theoretical specific capacity (372mAhg-1), restricting the improvement of LIBs. So it is very important for meeting the applicationof LIBs in electrical equipment to design and prepare the high-performance anode materials.Graphene, as a novel carbon material, exhibits excellent electrochemical performance due toits superior electronic conductivity, large specific surface areas and intriguing mechanicalproperties. Thus, graphene-based materials were chosen as research subjects to improve thespecific capacity, cycling stability and rate performance for LIBs. The research contents ofthis paper are as follows:(1) Ultrathin-shell graphene hollow spheres have been designed and synthesized by asimple template assisted method without surfactant. It is found that the obtained graphenehollow spheres have a high surface area (248.3m2g-1), ultrathin porous shells (5nm) and aninterconnected structure. It should be pointed out that there are micropores and mesopores(1.2nm and3.2nm) in the shells. More strikingly, the as-prepared graphene hollow spheresexhibit outstanding electrochemical performance as an anode material for lithium-ionbatteries. Even at a high current density of5000mA g-1, a high reversible specific capacity of249.3mAh g-1can be achieved. Furthermore, after100cycles, about97.1%of the specificcapacity is maintained at a high current density of1000mA g-1. The excellent electrochemicalproperties could be attributed to the attractive structure advantages of the graphene hollowspheres including the high surface area, ultrathin porous shells and an interconnectedstructure.(2) Porous graphene nanosheets are synthesized by a novel facile method involvingfreeze-drying technology and thermal reduction. The graphene nanosheets with high specificsurface area (358.3m2g-1) and increased interlayer distance (0.385nm) are successfully obtained through the freeze-drying process. A high initial reversible capacity of1132.9mAhg-1for the graphene nanosheets is obtained at a current density of100mA g-1. Moreimportantly, even after300cycles at a high current density of1000mA g-1, a stable specificcapacity of556.9mAh g-1can be achieved, suggesting the graphene nanosheets exhibitsuperior cycle stability. The fascinating electrochemical performance could be ascribed to thestable porous structure, the high specific surface area, the increased layer distance between thegraphene nanosheets and the low charge transfer resistance.(3) A new facile approach is proposed to synthesize high-level nitrogen-doped graphenesheets by thermal annealing pristine graphene sheets and low-cost industrial materialmelamine. The nitrogen-doping level is as high as7.04atom%and many pyridine nitrogenatoms are detected in the nitrogen-doped graphene sheets. The high-level nitrogen-dopedgraphene sheets exhibit a superhigh initial reversible capacity of1123mAh g-1at a currentdensity of50mA g-1. More significantly, even at an extremely high current density of20A g-1,highly stable capacity of about241mAh g-1could still be obtained， which is twice than thatof the pristine graphene. Such an electrochemical performance is superior to those previouslyreported nitrogen-doped graphene sheets. The excellent electrochemical performance can beattributed to the two-dimensional structure, disordered surface morphology, highnitrogen-doping level, and the existence of pyridinic nitrogen atoms.(4) TiO2/graphene nanocomposite was first synthesized by a facile gas/liquid interfacereaction. And the storage lithium performance was studied in the different potential window(0.01-3.0V and1.0-3.0V). The results indicate that TiO2crystal is the typical anatase phaseand TiO2nanoparticles (ca.10nm in mean grain size) were successfully deposited onto thegraphene sheets. The electrochemical performance of TiO2/graphene nanocomposite is betterthan that of TiO2nanoparticles in the different potential window. Moreover, A high specificcharge capacity of499mAh g-1was obtained at a current density of100mA g-1in an enlargedpotential window of0.01-3.0V. More strikingly, the TiO2/graphene nanocomposite exhibitsexcellent rate capability between0.01and3.0V, even at a high current density of3000mA g-1,the specific charge capacity was still as high as150mAh g-1. The high specific chargecapacities can be attributed to the facts that graphene possesses high electronic conductivity,and the lithium storage performance of graphene is delivered between0.01and3.0V. (5) TiO2/nitrogen-doped graphene nanocomposite was synthesized by a facile gas/liquidinterface reaction. How will the introduction of nitrogen atoms have an effect onmicrostructure and electrochemical properties of graphene and TiO2nanoparticles, which hasbeen systematically studied. The results indicate that the TiO2nanoparticles (8~13nm in size)were homogenously anchored on the nitrogen-doped graphene sheets due to the introductionof nitrogen atoms. TiO2nanoparticles have a wide size distribution of10~15nm and areloaded on the graphene sheets. What’s more, the electrochemical performance ofTiO2/nitrogen-doped graphene nanocomposite was enhanced by doping nitrogen atoms.TiO2/nitrogen-doped graphene nanocomposite exhibits excellent cycling stability and showshigh capacity of136mAh g-1(at a current density of1000mA g-1) after80cycles. Moreimportantly, a high reversible capacity of109mAh g-1can still be obtained even at a superhigh current density of5000mA g-1. However, the reversible capacities of the TiO2/graphenenanocomposite and the bare TiO2nanoparticles are only90and23mAh g-1, respectively. TheEIS results indicate that the electrical conductivity and charge transfer resistance of thenanocomposite decrease due to the nitrogen doping.