Morphology Design,Preparation And Electrochemical Properties Of Silicon-based Anode Materials

The theoretical specific capacity of silicon anode is as high as 4200 m Ah g^-1,and it has low lithium intercalation potential,which makes lithium-ion batteries have relatively high voltage platform.At the same time,silicon widely exists in rocks,gravel and dust.It is considered as the main anode materials to replace the traditional graphite anode material and to build the next generation of high-performance lithium-ion batteries.However,silicon anode material has some shortcomings,for example,obvious volume expansion during the charging and discharging process,which will lead to the increased polarization,poor cycle performance and the risk of battery failure.Moreover,silicon is a semiconductor and poor conductivity,which has become an obstacle for the large-scale industrialization of silicon-based anode materials.Therefore,the research on silicon-based anode materials has become the focus on many scientific research institutions,anode material manufacturers and battery manufacturers.In this paper,the strategy of optimizing the particle structure of silicon carbon anode material and buffering the volume expansion of silicon particles in the process of charging and discharging is put forward.In addition,the conductivity of silicon carbon composite material can be improved by generating or adding conductive medium.The main innovative achievements in this paper are as follow:（1）To buffer the volume expansion of silicon-carbon composites.silicon/graphite/pitch carbon/amorphous carbon（Si@C/Gr@C）with secondary particle structure was designed and prepared.Si@C/Gr@C composite was synthesized from pitch,graphite and silicon by multiple chemical vapor deposition（CVD）and isostatic pressing.The results of half cell test show that the discharge capacity of the composite is 1295 m Ah g^-1,and the initial coulombic efficiency（ICE）is 91.2%.In addition,the capacity retention rate of Si@C/Gr@C is 90%after 100 cycles at 0.5 A g^-1.Therefore,this stategy can both improve the conductivity of the composite,and alleviate the volume expansion of silicon particles during the charge and discharge process,and thus improving the structural stability of the material and the cycle performance of the battery.（2）In order to further improve the conductivity of silicon-carbon composite materials,carbon nanotubes are added to the composite materials.Graphite/silicon/carbon nanotubes/carbon（Si/CNTs/Gr/C）composites with kiwi structure are designed and prepared.Firstly,silicon nanoparticles and carbon nanotubes are uniformly combined by electrostatic self-assembly to form a cross internal conductive network structure.Then,Si/CNTs and graphite are combined to form functional structure by ball milling and spray drying.Graphite can stabilize the structure of composite materials and form skeleton support to increase the initial coulombic efficiency of negative materials.The outer pyrolytic carbon can isolate the corrosion of electrolyte,further improve the structural stability of the composite,and thereby greatly increase the diffusion rate of lithium ion on the surface of the composite.The results of electrochemical characterization show that the composite Si/CNTs/Gr/C has high mass specific capacity and excellent cycle stability.Under the condition of 0.1 A g^-1,the Si/CNTs/Gr/C electrode has a high initial discharge capacity of 1450.6 m Ah g^-1 and the ICE of 76.8%;The capacity of Si/CNTs/Gr/C electrode was maintained at 670.3 m Ah g^-1 after 100 cycles at 0.2 A g^-1.（3）To improve the conductivity of the material and the cycle performance of the silicon-carbon composite material.Spherical graphite/nano silicon/graphene/carbon composites（Gr/Si/r GO/C）are prepared by spray drying and electrostatic self-assembly process.Graphite can be used as the carrier and matrix material for improving the conductivity of nano silicon.Graphene oxide is the key factor for the composite to have good conductive network and low ohmic resistance during cycling process.The introduction of carbon coating can prevent the direct contact between nano silicon and electrolyte,and strengthen the combination of Gr,GO and Si.The electrochemical properties show that the composite material Gr/Si/r GO/C has excellent cyclic stability.At the current density of 200 m A g^-1,the composite material Gr/Si/r GO/C has a high discharge capacity of 1212 m Ah g^-1,ICE is 80.4%,and the capacity retention ratio is81.7%after 100 cycles.（4）In order to reduce the polarization of the composite material,improve the conductivity of the material,and alleviate the volume expansion of the material.Porous silicon/graphene/carbon composites（Si/r GO/C）are designed and synthesized by electrostatic self-assembly and freeze-drying techniques.Using nano silicon,phenolic resin and graphene oxide as raw materials,nano silicon is granulated to form a porous structure by charge self-assembly process,and then is evenly compounded with graphene oxide.It has been found that a layer of nano carbon film is coated on the surface of the porous silicon graphene oxide composite by CVD process.Therefore,the structure design of the porous silicon carbon composite can effectively improve the cycling performance of the material,and the introduction of graphene into porous silicon can reduce the polarization effect during the charge discharge process.In addition,a dense carbon coating on the surface of the precursor can effectively avoid the direct contact between the nano silicon and the electrolyte,and thus avoiding the iterative growth of the SEI film during the cycle.The electrochemical performance characterization shows that the discharge specific capacity remains 1465 m Ah g^-1 and capacity retention ratio is 81.3%after 100 cycles at 0.5 A g^-1.In addition,after 800cycles,the discharge capacity of Si/r GO/C is 446.4 m Ah g^-1,and the capacity retention rate is still higher than 80%.