Electrochemical Properties Of Carbon Nanofiber/carbon Nanotube/Ni Multilayer Anode Materials

Nanostructured materials are considered as candidate materials for the next generation of lithium-ion batteries due to their high specific surface area,fast electrochemical kinetics and good contact with electrolyte.Carbon nanofibers are nano scale carbon materials,which are widely used in the field of energy storage due to their large specific surface area,excellent mechanical properties and conductivity.Electrospinning is the best way to prepare carbon nanofibers because of its simple operation and low cost.However,it has been reported that the direct application of carbon nanofibers to anode materials for lithium-ion batteries will result in low speed capability and low coulomb efficiency.In order to solve this problem,polyacrylonitrile was used as spinning solution to prepare carbon nanofibers with excellent structure and morphology by electrospinning,preoxidation and carbonization.A layer of Ni was sputtered on the surface of carbon nanofibers by magnetron sputtering as catalyst,and then carbon nanotubes were grown on the surface of carbon nanofibers by chemical vapor deposition(CVD)method,and carbon nanofibers/carbon nanotubes/Ni multilayer composite with high specific surface area was prepared,which was applied to anode materials of lithium-ion battery.This paper mainly includes the following research contents:(1)Carbon nanofibers with high graphitization degree were prepared by adjusting the technological parameters of electrospinning,preoxidation and carbonization.Carbon nanofibers were applied to anode materials of lithium-ion battery and tested their electrochemical performance.Carbon nanofibers electrodes show good cycling performance(the specific discharge capacity of cycling 100 cycles under the current density of 100m A g^-1 is 244.82m Ah g^-1)and conductivity(the charge transfer resistance is 87?).However,there are also some obvious disadvantages,for example,the first cycle coulomb efficiency is very low and the irreversible capacity loss is very large;the discharge specific capacity is very low,and especially,the discharge specific capacity is lower in high current density.Therefore,the electrochemical performance of carbon nanofibers electrodes need to be improved.(2)By controlling the catalyst and the growth time,the carbon nanotubes with the best morphology and structure were grown on the surface of carbon nanofibers by CVD method.The specific surface area of carbon nanofibers is about 12.36m² g^-1,while the specific surface area of carbon nanofibers/carbon nanotubes/Ni multilayer composite material is increased obviously to 340.26m² g^-1.The increase of specific surface area is conducive to the rapid diffusion of lithium ion,providing potential for future application in lithium-ion battery anodes.(3)Carbon nanofibers/carbon nanotubes/Ni multilayer composite with high specific surface area were prepared by a novel CVD in-situ growth method,which were applied to the unsupported anode electrode of lithium-ion battery and showed good electrochemical cycling performance(the specific discharge capacity was480.82m Ah g^-1 after 100 cycles at current density of 100m A g^-1,and specific discharge capacity is 260.88m Ah g^-1 at current density of 1000m A g^-1 after 1000cycles),rate performance(262.06m Ah g^-1 at the current density of 1000m A g^-1)and the conductivity(the charge transfer resistance is 68?).The tight connection between carbon nanofibers and carbon nanotubes provides a variety of transport pathways for electrons and ions.Moreover,the three-dimensional network structure with gap and ultra-high specific surface area is conducive to the rapid diffusion of lithium ions and alleviates the volume expansion in the process of lithium ion insertion/extraction.