Study On Fabrication And Electrochemical Performance Of Graphene Based Anode Composites For Lithium-Ion Batteries

With the increasing demand for the full electric vehicle and the hybrid electric vehicle,developed the second generation of energy storage devices of rechargeable lithium ion batteries(LIBs)with high capacity and power density and long cycle life has been aroused great attention.However,so far the majority of commercial LIBs anodes is the graphite which has a relatively low(372 mAhg-1)theoretical specific capacity and the worse high-current characteristics.So the application of graphite has been greatly restricted in the second generation of energy storage devices of LIBs.Transition metal oxides anodes display high theoretical capacities(>600 mAhg-1),but the anodes made by transition metal oxides have a lower electric conductivity and a large volume change during the charge-discharge process,leading to irreversible capacity loss and poor cycling stability.Graphene,as a two-dimensional carbon material with a single-layer carbon atoms structure,possess many special properties,e.g.,ultrahigh specific surface area,excellent electrical performance,brilliant mechanical properties and physical chemical stability has attracted considerable interst.Like the other carbon material,it can also absorb or discharge the lithium in the process of taking off or embedding the lithium.The microstructure and electrochemical performance of the composites were investigated by using scanning electron microscope,X-ray diffraction,transimission electron microscopy,thermal gravimetric analysis,infrared spectroscopy,X-ray photoelectron spectroscopy and electrochemical workstation.When graphene with large specific surface area and excellent physical and chemical stability is used as the conductive substrate and can be combined with transition metal oxides to construct the unique 3D framework.Grphene can improve the electrochemical performance of the composites by improving the electric conductivity of transition metal oxides and buffer the volume change caused by the Li+ insertion and extraction during the charge-discharge process.In the meantime,the transition metal oxides can also enhance the lithium storage property of composite.So the graphene-based nanocomposites LIBs anode with excellent electrochemical performcance and cycle stability can be fabraicated.The main research contents and conclusions are listed as follows:(1)The hybrid graphene-Fe2O3 paper is synthesized by employing the in situ hydrothermal method.And the hydrosol graphene can be used as the growth carrier for Fe2O3 nanoparticles.Through the support of the interbedded hydrone of graphene nanosheets and surfactant SDS,Fe2O3 nanospheres with average size of 20 nm have homogeneously distributed on the surface of graphene.More strikingly,the hybrid graphene-Fe2O3 paper exhibits outstanding electrochemical performance as an anode material for lithium-ion batteries.Even at a high current density of 4 Ag-1,a high reversible specific capacity of 495 mAhg-1 can be achieved.Moreover,at the current density of 200 mAg-1,the reversible specific capacity can reach 765 mAhg-1 and the coulombic efficiency can stably maintain about 99%after 100 cycles.The excellent electrochemical performance could be attributed to the porous three-dimensional structure and the low charge transfer resistance.(2)The nitrogen and sulfur co-doped graphene/ZnO nanocomposites as the anode of lithium-ion batteries are synthesized by one-step hydrothermal route.The thiourea can be used as the source of nitrogen and sulfur for the dopant graphene.The atomic ratios of N and S elements existed in nitrogen and sulfur co-doped graphene are 4.07%and 9.99%,and many pyridine nitrogen atoms,C-S-C covalent bond,reduced(-SH)sulfur moieties and the oxidized(-SOn-)are detected in the nitrogen and sulfur co-doped graphene.The electrochemical performance of nitrogen and sulfur co-doped graphene/ZnO nanocomposites is better than those of ZnO/Graphene and ZnO nanoparticles in potential window(0.01?3.0 V).The nitrogen and sulfur co-doped graphene/ZnO nanocomposites exhibit excellent high reversible specific capacity of 720 mAhg-1 and even at a high current density of 1600 mAg-1,the rate capacity is still as high as 420 mAhg-1.The excellent electrochemical performance can be attributed to the existence of nitrogen and sulfur in the graphene which enhance the lithium storage site of graphene.(3)The N-doped graphene/nickel oxide nanocomposites are synthesized by one-step hydrothermal route and microwave treatment.And how will the microwave treatment and the introduction of nitrogen atoms have an effect on the electrochemical performance of N-doped graphene/nickel oxide nanocomposites,which has been sysyematically studied.The results reveal that the NiO nanoparticles with an average size of 30?50 nm homogeneously anchored on the nitrogen doped graphene nano-sheets.Moreover,the electrochemical performance of the N-doped graphene/nickel oxide nanocomposites is improved due to the introduction of nitrogen atoms and microwave treatment.The N-doped graphene/nickel oxide nanocomposites further treated by microwave exhibit excellent cycling stability and shows a high reversible capacity of 645 mAhg-1 even at a super high current density of 2 Ag-1·Meanwhile,a highcapacity of 1095 mAhg-1(at a current density of 0.3 Ag-1)after 100 cycles can be obtained.However the reversible capacities of the N-doped graphene/nickel oxide nanocomposites and the graphene-NiO nanoparticles are only 930 mAhg-1 and 809 mAhg-1,respectively· The EIS results reveal that the due to the nitrogen doping,the nanocomposites display a low charge transfer resistance.