Preparation And Electrochemical Properties Of Si-based Composites For Lithium Ion Batteries

The theoretical lithium storage capacity of silicon is up to 4200 m Ah g^-1 and the discharge potential is as low as 0.2 V（vs.Li⁺/Li）,In addition,it also has the advantages of high natural reserve,low cost and environmental friendliness.It is one of the anode materials for lithium ion battery with great development prospect.When silicon is used in lithium-ion batteries,it is found that the volume expansion effect of silicon is huge,and the volume change rate is as high as 400%,resulting in serious cracks in the material,electrode pulverization,large consumption of active sites for lithium storage and rapid capacity decay.In addition,the low conductivity of silicon itself leads to the high internal resistance of the battery and the consumption of a large amount of electrolyte.The above problems seriously hinder the practical application of silicon.In recent years,it has been found that SiO₂ has a theoretical capacity of up to 1965 m Ah g^-1,and also has the advantages of high reserve content,low price and high safety similar to silicon,which has attracted widespread attention.However,when SiO₂ is applied to lithium-ion batteries,it is found that the volume of SiO₂changes irreversibly during the process of lithium-ion disembedding.Due to the strong Si-O bond,the electron conductivity of SiO₂ is poor.The above problems all lead to the poor electrochemical performance and low initial Coulomb efficiency of SiO₂.Aiming at the above problems,this paper designed and successfully prepared Si and SiO₂ matrix composites with different structural morphology,and studied the morphology characterization and lithium storage electrochemical performance of the synthesized materials.The main research contents and results are as follows:1.Using SiO₂/C spheres as precursors,resorcinol and formaldehyde as carbon sources,SiO₂/Co/C hollow spheres were successfully prepared by a simple in-situ reduction method.The material shows excellent electrochemical performance,which is mainly due to the following two points:（1）The unique structure of three-dimensional nano-layered hollow spheres shortens the diffusion path of lithium ions and alleviates the volume expansion effect.（2）The addition of superfine Co nanoparticles and carbon matrix improves the conductivity of SiO₂ and catalyzes the fracture of Si-O and Li-O bonds,thus improving the electrochemical activity of SiO₂.2.A series of SiO₂-M-acetylene black composites（M=Fe,Ni and Cu）were prepared by a simple one-step method-high energy mechanical grinding.The composite of amorphous SiO₂ nanoparticles with acetylene black and M can not only improve the conductivity of SiO₂,but also prevent the agglomeration and crushing phenomenon of SiO₂ in the process of charge and discharge.In addition,the transition metal M can catalyze the breaking of Li-O and Si-O bonds,which can improve the charging and discharging capacity of the material and the reversibility of lithium ion disintercalation.Therefore,SiO₂-M-acetylene black composites have high initial coulomb efficiency,high rate performance and high electrochemical cycle stability.3.With hollow SiO₂ as the precursor,Si@C composite material with double hollow structure was prepared by magnesium thermal reduction,CVD carbon coating and etching.In terms of structure,the double-layer hollow structure is more beneficial to alleviate the volumetric expansion effect of silicon than the single hollow structure.From the perspective of carbon layer modification,the introduction of carbon can increase the electrical conductivity of the composite and further alleviate the volume expansion.Therefore,the electrochemical properties of Si@C composites are much better than that of Si.