Fabrication And Electrochemical Performance Investigation Of Si/C Composite Anode For Li-Ion Battery

Silicon(Si)materials are considered to be one of the most promising nextgeneration anode materials of Li-ion batteries because of their high theoretical specific capacity,low delithiation potential,abundance,and environmental friendliness.However,their application is limited by the problems of poor conductivity and huge volume expansion effect which will cause rapid capacity decline and battery failure.As a result,safety issues emerge ultimately.To this end,a Si-C composite strategy is adopted to tackle these problems aiming to utilize the Si efficiently as Si-C composite anode for Li-ion batteries.The main progresses are summarized as follows:1.Using polyaniline as carbon resource,a carbon shell is formed on the Si@SiO2 surface after polymerization and carbonization(Si@SiO2@C).Then,the template(SiO2)is etched away by HF to produce a hollow core-shell structured silicon-carbon composite material(Si@Hollow@C).The weight ratio of Si,C,N is 3:5.86:1.14.This unique structure plays a dual role in accommodating the volume expansion as buffer,as well as stabilizing the solid electrolyte interphase(SEI)and enhancing the electronic conductivity upon lithiation.As a result,the Si@Hollow@C anode(the weight ratio of active material,acetylene black&polyvinylidene fluoride(PVDF)is 7:1.5:1.5 hereafter.)delivers excellent electrochemical performance.At 0.05 C(1 C=1.295 A/g)rate,the discharge specific capacity is about 1207.40 mAh/g with the Coulombic efficiency of 62.38%in the formation cycle,and the reversible discharge specific capacity after is finally stabilized at 787.19 mAh/g with Coulombic efficiency of 96.05%in the subsequent cycles.With regard to Si@C composite,the electrochemical performance of Si@Hollow@C anode is remarkably better,which further implies the importance of hollow core-shell structure in improving the electrochemical performance of Si anode for Li-ion battery.2.Moreover,a certain proportion of graphene oxide(GO)is added through liquid dispersion upon the preparation of above Si@C composite materials.The silicon multi-carbon mixed material(Si@Graphene/C,the weight ratio of Si,Carbon shell(containing N)&Graphene is 3:7:1.5)is obtained after pyrolysis.In this composite,graphene and carbon shells are interconnected with each other to form a better three-dimensional conductive network on Si surface.The flexible graphene also plays a role in buffering the volume expansion of Si anode to some extent.Therefore,the material demonstrates good electrochemical performance,with a specific discharge capacity of 1125.80 mAh/g at 0.05 C(1C=1.174 A/g)rate and a Coulombic efficiency of 60.92%in the formation cycle.In the following cycles,the discharge specific capacity is maintained at 704.47 mAh/g with Coulombic efficiency of 96.39%.3.Next,using ball milling method,the Si/graphite material(the weight ratio of Si&Graphite is 1:10)is prepared by directly mixing nano-silicon with graphite powder.The Si/graphite anode delivers a reversible capacity of 579.64 mAh/g at 0.05 C(1 C=0.664 A/g)with good capacity retention.Then,the Si materials are coated by glucose via hydrothermal method.After high temperature carbonization,the Si@C-glc composite(the weight ratio of Si&C is 3:7)is prepared.Electrochemical performance tests indicate that its cycle performance is similar to previous Si@C composite,but with even faster capacity decay.At 0.05 C(1 C=1.337 A/g)after 50 cycles,the discharge capacity is only about 200.90 mAh/g.The capacity decrease can be mainly ascribed to the volume expansion of Si anode upon lithiation,which,in turn,proves the importance of the hollow core-shell structure to the enhanced performance of Si anode for Li-ion batteries.