Preparation Of Novel 3D Carbon Nanocomposites By Chemical Vapor Deposition And Their Applications In Lithium-Sulfur Batteries

The single low-dimensional carbon nanomaterials produce agglomeration effect due to van der Waals forces,greatly reducing their characteristics.Therefore,some researchers have studied the combination of carbon nanostructured allotrope and synthesized low-dimensional carbon nanostructured materials into 3D carbon nanocomposites.These composites exhibit better physical and chemical properties than their monomers and have attracted wide attention in the fields of energy storage,sensors,catalyst carriers and so on.In this paper,two carbon nanocomposites,carbon nanotubes/carbon nanofibers（CNTs/CNFs）and graphene sheets/carbon nanotubes（GS/CNTs）,were fabricated using chemical vapor deposition（CVD）methods.both were prepared using one-step CVD methods,physically characterized by a series of characterization methods,and applied to lithium-sulfur（Li-S）batteries to explore their electrochemical properties.At last,considering the synthesis methods and structural characteristics of these two composites,the synthesis of vertical graphene/carbon nanotubes/carbon nanofibers（VG/CNTs/CNFs）was explored.The specific content can be divided into the following parts:1.Imidazole was used as carbon source,and nickel foam（NF）was used as a catalyst and growth substrate to synthesize the CNTs/CNFs composite at 800°C for 30min under a mixture atmosphere of H₂/Ar.The CNFs formed a conductive network on the NF,while the CNTs grew perpendicular to the surface of the CNFs.At the same time,Ni nanoparticles were present at the end of the CNTs.Resulting in a special composite called Ni-nanocarbons-coated nickel foam（Ni-NCs-coated NF）.Due to the excellent pore structure of NF,the electrical conductivity of CNTs/CNFs,and the catalytic effect of nickel nanoparticles to lithium polysulfides（Li PSs）,it was used as the cathode current collector in Li-S batteries.As a matter of fact,the Li-S battery based on the Ni-NCs-coated NF current collector delivered an initial discharge capacity as high as 1472 m Ah g^-1 at 0.1 C and an outstanding high rate capability at 3 C(802 m Ah g^-1).Additionally,low decay rates of 0.067%and 0.08%at 0.2 C（300 cycles）and 0.5C（500 cycles）,respectively,were achieved.Overall,the Ni-NCs-coated NF collector holds great promise for the application in high-performance Li-S batteries.2.GS/CNTS arrays were synthesized on vermiculite using radio frequency plasma-enhanced chemical vapor deposition（RF-PECVD）with methane as the carbon source and Fe/Mo as the catalyst at 700°C.At the same time,the effects of different RF power（100 W,200 W,300 W,400 W）and growing times（15 min,30 min,50 min）on the morphology and quality of the GS/CNTs were systematically studied through controlled variables,and a possible growth mechanism was proposed.Specifically,the GS grew perpendicular to the CNT walls,resulting in better dispersion and a significantly increased specific surface area of the CNTs.More tips and edges appeared on the wall,leading to more conductive paths and improved conductivity.Considering the particularity of the structure,GS/CNTs were used as the cathode conductive additive for Li-S batteries.Compared to ordinary CNTs with smooth walls,a lower decay rate of 0.073%was obtained after 350 cycles at 0.5 C while the ordinary CNTs had a decay rate of 0.084%.In the rate performance test,it delivered a higher capacity(714 m Ah g^-1)than the CNTs cathode at 2 C.Demonstrating that the GS/CNTs composite had better electrical conductivity than the CNTs monomers.3.VG was grown on the surface of various substrates（nickel foam,silicon wafer and mica plate）at 600°C by RF-PECVD with methane as carbon source.It was verified that the growth of VG was not selective to the growth substrate.Subsequently,VG/CNTs/CNFs were successfully prepared on NF coated with CNTs/CNFs using the same conditions,which widened the road for synthesizing new carbon nanocomposites.