Preparation And Electrochemical Performance Of Scattered Si/C Anode Composite Materials With Carbon Coating

In order to develop the electronic equipment and electric vehicles,people have higher expectation with the standby time and endurance of the energy storage system and the lithium-ion batteries.Therefore,the research on high energy density electrode materials becomes an urgent problem to be solved in the field of commercial lithium-ion batteries.Due to the high theoretical lithium insertion capacity,4200 m Ah/g,which is ten times higher than that of commercial graphite anode material.However,silicon,as a semiconductor material,has low electronic conductivity and the volume expansion of silicon in the process of lithium intercalation is as high as300%,resulting in the pulverization of electrode materials and the continuous formation of SEI film.As a consequence,the cycle stability and the initial coulombic efficiency(ICE)of the electrode are poor,which confine the commercial application of silicon anode materials.Considering the good conductivity and mechanical strength of carbon materials,it is a good choose to combine carbon materials with silicon.Because carbon can not only improve the electronic conductivity,but also buffer the volume expansion of the silicon anode composite.In view of the above problems,this paper intends to use different preparation processes to mix silicon nanowires or silicon nanoparticles with carbon materials to form scattered Si/C composites.Finally,the electrochemical performance of the materials were further optimized by carbon coating modification.The effects of the content of each component and the preparation process on the electrochemical performance of the materials were investigated.The main contents and results of this paper are as follows:(1)Due to the poor conductivity and volume effect,the cycling stability of silicon nanowire anode materials is poor.In this study,Si NWs/MOF@C anode composite was prepared by mixing Si NWs,metal organic framework ZIF-67 and phenolic resin with the preparation processes of phase blending method,water bath evaporation and high temperature pyrolysis.The effects of the content of each component on the electrochemical performance of the materials were investigated.The results show that the coating of phenolic resin derived carbon and the addition of MOF derivatives can achieve the maximum coordination effect on improving the conductivity and reducing the volume effect of silicon anode when the ratio of Si NWs,ZIF-67 and phenolic resin is 3:1:3 in the Si NWs/MOF@C.And the composite exhibits excellent electrochemical performance,the reversible specific capacity can be maintained at887.5 m Ah/g at the current density of 200 m A/g after 100 cycles and a rate reversible specific capacity of 508.3 m Ah/g at the current density of 4000 m A/g.(2)By optimizing the process,P-MG/Si@C anode composite was prepared with compaction,mechanical ball milling and carbon coating.Silicon nanoparticles were loaded on the surface of microcrystalline graphite under the mechanochemical force,and coated by RF-derived carbon.Finally,P-MG/Si@C was formed with compaction and acetylene pyrolysis carbon coating.Mechanical ball milling and compaction can make poor conductive Si combine with conductive MG,and make some difference in the microstructure,which can improve the electrochemical performance of P-MG/Si@C with smaller specific surface area,pore structure parameters and charge transfer resistance.The carbon coating can not only avoid the direct contact of Si and electrolyte,but also buffer the volume expansion of Si in the process of lithium intercalation and maintain the stability of the material structure.The results showed that P-MG/Si@C exhibits excellent electrochemical performance,the reversible specific capacity can be maintained at 683.3 m Ah/g for the first cycle,659.2 m Ah/g after 100 cycles at a current density of 100 m A/g,and the capacity retention rate is up to 96.48%.In addition,at a current density of 4000 m A/g,P-MG/Si@C can maintain specific capacity at 257.4 m A/g,showing excellent rate performance.(3)In view of the poor cycle stability and low ICE of Si anode material,MG/Si@Cx-y anode composite with high ICE was prepared by coating Si and MG with sucrose and acetylene pyrolysis carbon.The carbon coating can effectively alleviate the volume expansion of Si,reduce the specific surface area of the material,and avoid the direct contact of Si and electrolyte,which can reduce the consumption of lithium ion to produce irreversible products.The results showed that the ICE of MG/Si@Cx-y is around 87%,and the ICE of MG/Si@C3-20 is even as high as 87.9%.At the same time,the effects of MG and acetylene pyrolysis carbon content on MG/Si@Cx-y were investigated.The material owns high specific capacity with maintaining a certain stability when the ratio of MG to Si is 3:1,and the acetylene injection time is 20 min,the specific capacity can be maintained at 881.6 m Ah/g when the material is cycled for 100 cycles at a current density of 100 m A/g.