Synthesis Of Porous Graphene Nanomesh And Graphene-based Composites As Anode Materials For Lithium Ion Batteries

In recent years, sustainable energy resources with no pollutions have attracted more and more attentions. As one kind of such devices, lithium ion batteries ?LIBs?have been widely used in portable electronic devices, such as mobile phones, laptops and cameras, due to their high energy density, long cycle life and non-memory effect.At the same time, LIBs have shown broad applications in electric vehicles. Making LIBs with high energy/power density becomes a hotpot in researchers. The property of electrode material is one of the important factors that affect the performance of LIBs. In terms of anode materials, the current commercial graphite only has a theoretical specific capacity of 372 mAh g^-1, which couldn't satisfy the actual demand of electric vehicles.Therefore, the design and preparation of high-performance anode materials of LIBs has become a significantly important theme.As a kind of new carbon material, graphene possesses a large specific surface area,high electrical conductivity and other excellent properties. It has great applications in many fields, such as energy storage, material science and electrochemistry. Therefore,graphene-based materials are chosen as research subjects to improve the energy/power density and cycling stability of electrode materials for LIBs. The research contents of this paper are as follows:?1? Porous graphene nanomesh ?PGN? with a large specific surface area (1620 m2 g^-1)has been prepared by Chemical Vapor Deposition ?CVD? method. The porous structure and high conductivity of PGN facilitate the transfer and reaction of lithium ions and electrons. Thus, the PGN exhibits excellent cycling and rate performance. The specific charge capacity of PGN increases from 823 to 1247 mAh g^-1?increases up to 34 %?after 50 cycles at the current density of 150 mA g^-1; when the current density increases to 1000 mA g^-1, the specific charge capacity of PGN is still as high as 455 mAh g^-1.?2? Carbon nanotubes-porous graphene nanomesh ?CNT-PGN? composites with three-dimensional porous structure has been prepared by CVD method. The existing of PGN effectively suppresses the agglomeration of CNTs; while the composites show super-paramagnetism due to the existence of Fe3C nanoparticles, making the composite as magnetic drug carriers. Compared to PGN, CNT-PGN composites have more abundant pore structure. However, due to the presence of CNT, the specific surface area of CNT-PGN composites is smaller than PGN. So they couldn't provide active sites to storage more lithium ions, retarding the lithium-storage properties compare to PGN.?3? PGN exhibits excellent electrochemical performance. However its large specific surface area leads to large irreversible capacity in first charge-discharge cycle. A C03O4-PGN composite was prepared by liquid deposition method. Because C03O4 has high theoretical specific capacity (890 mAh g^-1) and Co3O4 nanoparticles can effectively prevent the agglomeration of PGN. Additionally, Co3O4 nanoparticles can be embedded into the pore structure of the PGN to reduce the specific surface area of the composite material, effectively reducing the irreversible capacity. Besides, PGN can not only accommodate the volume change of Co3O4 nanoparticles during charge-discharge processes, the three-dimensional porous structure of PGN can also accelerate the lithium ions and electrons transport. Thus, Co3O4-PGN composite exhibits high charge specific capacity based on both mass and volume. The charge specific capacity of the 70%Co-PGN composite ?mass fraction of Co3O4 in composite is 70 %? increases from 1336 to 1439 mAh g^-1 after 50 cycles at the current density of 150 mA g^-1; when the current density increases to 1000 mA g^-1, the charge specific capacity of 70%Co-PGN composite is still as high as 1072 mAh g^-1. At the same time, the 70%Co-PGN composite exhibits high charge specific capacity based on volume (at the current density of 50 and 1000 mA g^-1, the specific volume storage of the 70%Co-PGN composite is 1993 and 1678 mAh cm-3, respectively, far higher than that of the PGN).?4? Fe₂O₃-PGN composite with core@void@shell structure was prepared by liquid deposition method. Different to Co3O4-PGN composite, decoration of ultrasmall Fe₂O₃ nanoparticles can significantly change the surface condition of PGN, enhancing the electrochemical properties of the composite. The significant capacity enhancement of the 10%Fe-NMG composite ?mass fraction of Fe₂O₃ in composite is 10 %? is attributed to the positive synergistic effect between PGN and Fe₂O₃ nanoparticles due to the catalytic activity of nanoparticles for decomposition of the solid electrolyte interface film. Consequently, the Fe₂O₃-PGN composite shows excellent cycling and rate performance. The charge specific capacity of the 10%Fe-PGN composite increases from 1289 to 1567 mAh g^-1?increases up to 38 %? after 50 cycles at the current density of 150 mA g^-1; when the current density increases to 1000 mA g^-1, after 100 cycles, the charge specific capacity of the 10%Fe-PGN composite increases from 814 to 883 mAh g^-1.