Study On Lithium Storage Performance Of Coaxial Structure Of CNT@Si@SiC Composites

With the continuous innovation of portable electronic device and the rapid development in transportation field leading by pure electric and hybrid vehicles,the need for large-scale energy storage is growing.Therefore,it is imperative to develop lithium-ion batteries with high energy density,long cycle life and higher safety.As a key component of lithium ion battery,the negative electrode material can largely determine the capacity and cycle performance of the battery.Currently,Silicon is considered to have the highest lithium storage capacity（4200 mAh/g）,which makes it the most promising alloy anode material.However,due to the serious volume effect in the delithiation / lithium insertion process during charging / discharging and its poor conductive efficiency such as fast capacity decay and unsatisfied rate performance,silicon cannot be put into commercial application in the field of lithium ion batteries.Therefore,designing and constructing a negative electrode material capable of effectively buffering the volume effect of the silicon-based anode while improving its conductivity has become an important research area for the development of silicon anodes.In this study,carbon nanotubes with excellent mechanical properties and electrical conductivity are used as axes to study silicon-based anode materials with coaxial structures to improve their capacity and stability as anodes for lithium-ion batteries.In this study the activated CNTs were used as templates,on the surface of which a layer of silica（obtained by the hydrolysis of TEOS）was coated through sol-gel method.Three coaxial-structure Si/C negative electrode materials with ideal performance including CNT@Si,CNT@Si@C,CNT@Si@SiC were prepared by magnesium thermal reduction and chemical vapor deposition（CVD）.The composition and structure of the material were characterized by X-ray diffraction（X-ray Diffraction）,scanning electron microscopy（Scanning Electron Microscopy）,transmission electron microscopy（Transmission Electron Microscopy）,Raman spectroscopy（Raman）,thermogravimetric analysis（TG-DTA）,and specific surface and pore analysis.The performance was tested using cyclic voltammetry（Cyclic Voltammogram）,constant current charge and discharge,alternating current impedance（Electrochemical Impedance Spectra）and other electrochemical tests.The details are as follows:1.The carbon nanotubes were activated and decontaminated by concentrated nitric acid.The CNT@Si composites with coaxial structure were prepared by sol-gel method and magnesia reduction.In order to investigate whether the capacity and stability of the silicon negative electrode were improved,silicon nanoparticles were prepared in the same way.The CNT@Si composite material with the coaxial structure and the elemental silicon material were tested under charge/discharged process at a current density of 200 m A/g.The test,although the elemental silicon material had a very high initial discharge capacity,exhibited a large capacity reduction,which decreased from a discharge amount of 2014.7 mAh/g to 311.7 mAh/g after only 10 cycles.While CNT@Si had the first coulombic efficiency of only 48 %,it still discharged 681.3 mAh/g after 50 cycles.2.The CNT@Si@C composite was obtained by depositing a layer of amorphous carbon on the surface of CNT@Si using chemical vapor deposition（CVD）method.The volume expansion during the charge/discharge process of silicon is bidirectionally suppressed.When tested under the current density of 200 m A/g,the first coulombic efficiency is 60.17 wt%.After nearly 100 cycles,the specific discharge capacity can still reach 743 mAh/g.The electrochemical performance of CNT@Si@C can be further improved compared to CNT@Si.3.Coaxial CNT@Si@SiC composites were prepared by first performing carbon deposition on the coaxial CNT@SiO₂ and then performing magnesium thermal reduction.At a current density of 200 m A/g,the first coulombic efficiency reached 53 %.After nearly 100 cycles,the specific discharge capacity is still as high as 700 mAh/g.At high current densities（2 A/g,4 A/g,8 A/g）,CNT@Si@SiC has significantly better stability and capacity than CNT@Si and CNT@Si@C.The research results show that using hollow carbon nanotubes with good mechanical toughness and electrical conductivity as the axis and dense,high-rigidity SiC as the shell can effectively relieve the volume stress of silicon charge and discharge,thereby improving the specific capacity and power of silicon-based anode characteristic.