Microscopic Structure Of Graphene And Its Influence On The Electrochemical Performance In Lithium-Ion Batteries

Graphene is a two-dimensional sheet consisted of honeycomb network. The special structure gave graphene excellent electronic, mechanics, optical and thermal properties, and promising applications in many fields. Chemical methods are thought to be the effective way of producing graphene with high-performance and low-cost. But this method is hard to control graphene's morphology. We successfully produced graphene with different microstructures by changing the feedstock sources, and then investigated the electrochemical properties. Furthermore, in previous studies about graphene's behavior in lithium-ion battery, anode materials are prepared by tabletting. Although the results are outstanding, it just reflected the properties of aggregated graphene. To characterize the intrinsic capacity of graphene, we doped inert material into graphene layer forming single-layer graphene, and got the theoretical capacity.In this paper, graphene were prepared from natural graphite, nano-graphite and two kinds of mesocarbon microbeads with different sizes, FMGP and MGP, by oxidation, rapid expansion and ultrasonic treatment. The relationship between morphology, structure and electrochemical performance of graphene nanosheets (GNSs) as anode material for lithium-ion batteries were systematically investigated by transmission electron microscope, scanning electron microscope, X-ray diffraction, and a variety of electrochemical testing techniques such as galvanostatic charge/discharge test, cyclic voltammetry and alternatingcurrent impedance. Previous studies had found the size of graphene layers is determined by the particle size of raw material, and the coulomb efficiency of graphene is also related to the raw material. Mesocarbon microbeads with high coulomb efficiency can produce high-coulombic-efficiency graphene. Accordion-like or page-like aggregates are mainly contained in the graphene which made from spherical raw materials, and the flake-like raw materials are easy to generate floccule graphene union. The reversible capacity, coulombic efficiency, rate and cycle performance, and dynamics performance of graphene have a great dependence on the layer distance, degree of order and size of layer. In the four samples, NanoG45 exhibits a 446mAh/g reversible capacity, which is 23.3% higher than NG45. On the conversely, NG45 displays excellent cycle and rate performance with a stable coulombic efficiency upon 90% after the forth cycle. The shape and size of raw material exert the significant influences on the morphology and microstructure. All the factors influenced the electrochemical properties in common. Disperse graphene by doping nano-copper can obtain Cu/C composites. When the amount of copper added to the quality of graphene 500 times, graphene will be dispersed to single-layer. We estimate the theoretical capacity of graphene as 736mAh/g, about twice that of graphite.