Synthesis Of Porous Silicon Matrix Composites And Graphene/Silicon Composites And Their Applications In Lithium Batteries

Recently,With the industrialization process of human society,with the popularity of portable electronic products and the rapid development of electric vehicles,the research of lithium-ion secondary batteries with high energy density and high rate performance has attracted wide attention.As a typical alloy anode material,silicon has the highest theoretical specific capacity(4200 mAh·g-1)among the known lithium-ion battery anodes,and it is considered as the next ideal candidate material for the anode.However,due to the large volume expansion effect during lithium charging and low conductivity,the cycle stability and rate performance of silicon-based lithium-ion batteries are poor,which limits the commercial application of silicon-based negative lithium-ion batteries.In order to solve the above problems,porous silicon and graphene coating are two technical approaches.The former can provide enough internal buffer space for the volume expansion of silicon,thus significantly improving the cyclic stability.The latter depends on the excellent conductivity of graphene,which can significantly improve the material's rate performance,and its network structure can also bear the expansion of silicon and inhibit the pulverization and shedding.Based on the research progress of porous silicon powder preparation and graphene coating growth at home and abroad,this paper focuses on the simplicity and cost reduction of the preparation method,the optimization of the preparation process and the improvement of the electrochemical performance of the battery.PSi/C and Si/SiOx/G were prepared by Cu-Ag MACE,CVD,high energy ball-milling method and high temperature thermal oxidation method.The innovative achievements of this paper are as follows:(1)It is innovatively proposed to apply copper-silver diatomic MACE method to the preparation of porous silicon powder.The advantage of this process lies in the synergistic mechanism of Cu and Ag dual atoms,which can not only reduce the use of Ag,thus reducing the cost,but also make up for the shortcomings of uneven structure and inadequate hole depth of Cu auxiliary corrosion.It was found that the morphology of porous silicon was significantly affected by the changes of reaction temperature and hydrogen peroxide concentration.After determining the appropriate parameters,the particle size was further reduced by high-energy ball milling,and silicon powder was coated with amorphous carbon by CVD method to synthesis PSi/C composites.Due to the porosity of silicon and silicon-carbon coating,the material has high specific capacity and high cycle stability.(2)It is innovatively proposed to synthesize Si/SiOx/G composites by first growing nano-scale oxide layer on the surface of silicon powder by high temperature thermal oxidation method,then using CH4 as carbon source and using CVD method to grow graphene on the surface of silicon dioxide layer.The principle of this process is that CH4 can reduce SiO2 to SiOx at high temperature,which provides a catalytic site for graphene growth.And as a mild oxidant,OH-promotes graphene formation from graphite carbon and inhibits the formation of SiC.The SiOx in the middle layer of the composite can effectively inhibit the volume expansion of Si and improve the cyclic properties of the material.The graphene in the outer layer can effectively improve the conductivity and the rate performance of the composite.The results show that after 300 cycles at 1 A/g current density,the specific capacity of the composite is still above 1380 mAh g-1,showing excellent cyclic stability and rate performance.