Research On The Preparation Of Porous Carbon/silicon Anode Materials For Lithium-ion Batteries Based On Metal Precursors

Silicon has the highest theoretical specific capacity(4200 mAh g-1)and lower lithium ion intercalation/de-intercalation potential,so it is an ideal anode material for lithium-ion battery.However,silicon anodes have not been commercialized on a large scale owing to the following technical challenges.On the one hand,pure silicon has a huge volume expansion/shrinkage(over 300%)during the Li insertion/extraction process,which leads to dramatic crack of the silicon particle and pulverization from the current collector.On the other hand,the unstable solid electrolyte interphase(SEI)film will continuously form and fracture during the electrochemical process during the electrochemical process,resulting in impedance rapid increase and fast capacity decline.In addition,the conductivity of silicon is poor and can not withstand the charge and discharge at high current density.Therefore,aiming at the above problems,suppressing the volume change of the silicon,promoting the formation of stabled SEI film on silicon and improving the conductivity of the silicon were developed to solve the problems of poor cycle performance of the silicon anode materials.In light of these views,in this paper,metal compounds are introduced to synthsize silicon based composites,including surface coating metal compounds on silicon to fabricate silicon@graphene composite and utilize metal-organic frameworks to fabricate silicon@carbon and mesoporous silicon to solve those problems of silicon.First,core-shell Si@Graphene composite was prepared through in-situ catalytic growth of multi-layer graphene coated silicon by metal precursor modified silicon.Instead of mechanically dispersing nano-silicon particles into graphene to prepare Si/Graphene composites,the about 10?15 nm Ni layer is uniformly coated on the surface of nano-silicon by chemical deposition,which is believed to play as an excellent catalytic substrate to catalize the multi-layer graphene growth with a thickness of 2?5 nm.These graphene can be coated uniformly on the surface of nano-silicon to obtain the three-dimensional core-shell structure.This structure realizes the effective connection between silicon and graphene,and keeps graphene intimate contact with nano-silicon.Thus,the mechanical strength and flexibility of graphene can be fully utilized and thevolume expansion of pure silicon in the process of lithium insertion/extraction can be alleviated.Moreover,the high conductivity of graphene also improves the electron transport ability of Si electrode.In addition,coating with graphene can promote the formation of stable SEI film on silicon by protecting silicon from being directly exposed to electrolyte.The study shows that core-shell Si@Graphene composite displayed good cycle performance and high rate performance,with reversible capacity of 1909 mAh g"1 after 100 cycles at 0.2 A g-1 and reversible capacity of 975 mAh g-1 even at the current density of 5 A g-1.Furthermore,two different methods were employed to improve the electrochemical properties of silicon-based anode materials by utilizing metal-organic frameworks ZIF-67.First,Si@PC composites are fabricated by in-situ assembly of ZIF-67 on the surface of nano-silicon and then carbonization at high temperature.It was found that the addition of anionic sodium lauryl sulfate was benefit for the in-situ assembly of ZIF-67 on the surface of nano-silicon(recorded as Si@SPC).However,some nano Si is still dispersed on the surface of ZIF-67 particles.The specific capacity of the Si@SPC composites is only 401 mAh·g-1,which indicates some drawbacks about the method.Second,the mesoporous Si(M-Si)of about 70 nm were prepared by hollow SiO2 spheres using ZIF-67 as template according magnesium thermal reduction.The prepared M-Si exhibits large specific surface area(147.28 m2·g-1)and large number of wide channels(18 nm),which can promote the penetration of the electrolyte and thus significantly shorten the ion diffusion pathway.At the same time,the void can effectively accommodate the volume expansion of Si and thus greatly improve its electrochemical performance.The M-Si presented a specific capacity as high as 1018 mAh·g-1 at 0.2 A·g-1 after 180 cycles.