Preparation Of Silicon/carbon Composite As Anode Materials For Lithium-ion Batteries

Due to the natural abundance of reserves,a high theoretical specific capacity,and a high operating safety,Si anode materials have become a research hotspot in lithium-ion batteries.However,it is still difficult to meet the stringent requirements in practical applications because of several problems,including its intrinsically low electrical conductivity,a large volume change upon lithiation/delithiation,a poor cycling stability,and a sluggish reaction kinetics.To address the above mentioned challenges,we have rationally designed two kinds of silicon/carbon composites and further studied their lithium storage properties.The main results can be concluded as follows:(1)SiO2 was initially coated on the surface of functionized carbon nano tubes(CNTs)via a sol-gel method using tetraethyl orthosilicate(TEOS)as precursor,then derived to CNTs/Si composites through a magnesiothermic reduction method,and finally coated by amorphous carbon utilizing resorcinol as the carbon source to obtain CNTs/Si/C composite materials.Particularly,the carbon nanotubes with a good mechanical strength would provide a robust and conductive framework to facilitate the migrations of electrons,while the suitable carbon coating layer could efficiently accommodate the huge volume variation of Si anode,leading to an improved structural integrity and a desirable cycling stability.Importantly,the critical effect of carbon coating layers on the electrochemical performances of CNTs/Si/C anode materials were also revealed.Among different samples,the CNTs/Si/C-7.5 anode material exhibited a best electrochemical performance with a large reversible capacity of 967 mAh g-1 at a current density of 100 mA g-1 and a highly reversible capacity of 710 mAh g-1 after 150 cycles.Additionally,the capacity retention can reach 74.7%after 800 cycles at a current density of 500 mA g-1.(2)Zeolitic Imidazolate Framework(ZIF)-67/Si was firstly obtained by adding nano-silicon powders in the chemical preparation of ZIF-67,while the carbon nano tubes were in-situ grown on the surface of ZIF-67/Si composites via a chemical vapor deposition(CVD)method.Consequently,the porous C/Si/CNTs composites were obtained after an acid etching treatment.Intriguingly,the carbon framework derived from ZIF-67 and in-situ grown CNTs would not only promise a rapid electron transport,but also relieve the notorious volume expansion/shrinkage of silicon upon cycling and suppress the Si electrode pulverization,which could effectively alleviate the lithium storage capability of Si anode material.The rationally designed C/Si/CNTs composite material would display an ultrahigh discharge capacity of 1818 mAh g-1 at a current density of 100 mA g-1 and maintain a large discharge capacity of 1179 mAh g-1 after 150 cycles.After 200 cycles at 500 mA g-1,it can still retain a remarkable discharge capacity of 969 mAh g-1,while an excellent rate capability of 835 mAh g-1 was further achieved at a current density of 4000 mA g-1.Therefore,C/Si/CNTs composites shows the improved cycle stability and rate capability.