| Lithium-ion batteries have many significant advantages and are widely applied in small electronic products.It has shown good application prospects in electric vehicle,satellite,aerospace and other fields.Currently,the carbon materials are widely used as commercial lithium-ion batteries anode materials with low specific capacity g-1(372 mAh).So,the research and development of novel anode materials and the optimization for the existing battery technology are the basis to improve the performance of lithium-ion batteries.Sn-based material is one of the most promising non-carbon materials because of its high theoretical capacity(994 mAh g-1,which is about 2.7 times that of the carbon material),low stable potential plateau,low-cost,abundant resources,non-toxic and no pollution.But its huge volume change during charging/discharging cycle processes causes serious pulverization effect and loses contact between electrolyte and electrode to present poor electrochemicalperformance,which seriously hinders its further application.In order to solve pulverization effect,it is an effective way to prepare nano Sn and Sn-based alloy compounds.However,nano Sn is not sufficient to satisfy the practical demands.The research indicates that the composite of Sn-based anode and carbon material is considered to be one of the most effective methods.Currently,carbon materials include amorphous carbon,graphene,carbon fibers and carbon nanotubes.Carbon materials can act as the barrier to provide a certain pore space to accommodate the volume expansion/contraction of the Sn-based material and to hinder the agglomeration of metal particles,while carbon can also improve its electrical conductivity.Coaxial carbon nanofcber/metal composite materials own high conductivity,good flexibility,high specific surface area and good adhesion and the coaxial structure can inhibit the volume effect of Sn-based shell and improve its electrical conductivity,and thus improve the cycle stability.At present,the methods of preparing coaxial composite materials include hydrothermal method,chemical deposition and electrodeposition.Electrodeposition is a way of controllable,simple and efficient to prepare material.However,there are few reports on the preparation and electrochemical performance study of coaxial carbon nanofiber/Sn-based alloy composites using electrodeposition.In this work,carbon nanofiber flexible thin films with high performance were prepared by electrospinning,and then the coaxial carbon nanofiber/Sn-based composite was prepared by electrodepositing Sn and Sn-based alloy onto the surface of carbon nanofibers.The structure and electrochemical performance of the composite films were studied.The main contents are as follows:(1)Preparation and electrochemical performance of coaxial carbon nanofiber/Sn composite.Firstly,polyacrylonitrile nano fiber thin film was prepared by electrospinning,and then carbon nanofiber thin film matrix was obtained via the processes of preoxidation at low temperature and carbonization at high temperature.Secondly,the coaxial carbon nanofiber/Sn composite is prepared by electrodepositing Sn shell onto the surface of carbon nanofibers.Finally,the structure and electrochemical performance of the coaxial carbon nanofiber/Sn composite are characterized.The results of SEM and TEM show that Sn shell with the thickness of 20-30 nm can be uniformly deposited on the surface of carbon nanofibers.The coaxial carbon fiber/Sn interacts with each other to form a three-dimensional(3D)network structure.The electrochemical performance shows that the capacity decays rapidly before 20 cycles and the discharge specific capacity tend to be stable after 20 cycles.The discharge capacity of 2nd and 100th cycle is 1115 mAh g-1 and 514 mAh g-1,respectively,and the capacity attenuation rate is about 0.55%/cycle.(2)Preparation and electrochemical performance of coaxial carbon nanofiber/Sn/Co composite.In order to inhibit the volume effect and further improve the electrochemical performance of Sn anode,the Sn/Co alloy was deposited on the surface of carbon nanofibers using electrodeposition to form coaxial carbon nanofiber/Sn/Co alloy composite(CNFs/active/inactive alloy).The inactive material Co can be used as a frame to buffer volume expansion of Sn anode.The analysis results of SEM,EDS and TEM show that Sn/Co alloy shell with the thickness of 80-120 nm is uniformly deposited on the surface of carbon nanofibers,which forms 3D network structure of the coaxial carbon nanofiber/Sn/Co alloy,and the weight percentage of Sn,Co and C in the material is 69.1%,22.3%and 8.6%,respectively.The results of charge discharge testing indicate that the discharge specific capacity of coaxial carbon nanofiber/Sn/Co alloy composite at 2nd and 100th cycle is 1260 mAh g-1 and 702 mAh g-1 respectively,and the capacity attenuation rate is about 0.45%/cycle,which presents excellent electrochemical performance.(3)Preparation and electrochemical performance of coaxial carbon nanofiber/Sn/Sb composite.The inactive component Co in the coaxial carbon nanofiber/Sn/Co alloy composite can lead to the capacity loss of electrode.The composition alloy of Sn with the active component can avoid and reduce the loss of capacity.After changing the alloy composition,Sn/Sb alloy was deposited on the surface of carbon nanofibers using electrodeposition to form a caxial carbon nanofiber/Sn/Sb alloy composite(CNFs/active/active alloy).The analysis results of SEM,EDS,TEM and XRD show that Sn/Sb alloy shell deposits on the surface of carbon nanofibers,which forms 3D network structure of the coaxial carbon nanofiber/Sn/Sb alloy.The thickness of the deposition layer is about 60-80 nm,and the weight percentage of Sn,Sb and C in the material is 28.3%,58.5%,and 13.2%,respectively.The electrochemical performance tests show that the coaxial carbon nanofiber/Sn/Sb alloy composite has relatively high reversible specific capacity(?661 mAh g-1)and high coulombic efficiency(>97%)after 100 charge/discharge cycles.The problems of poor cycle performance,large volume change and fast capacity attenuation for Sn materials are improved. |
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