Preparation And Research On Three-dimensional Graphene-wrapped Nano Silicon Composites As Anode Materials For Lithium Ion Batteries

Silicon is considered as one of the most promising anode materials for high-performance lithium-ion batteries due to its low cost,high theoretical specific capacity and low operating voltage.However,the drastic volume change（～300%）during charge-discharging process and the low intrinsic conductivity of silicon seriously affect the electrochemical properties of the silicon-based anodes.Graphene has the advantages of high specific surface area,high electrical conductivity and strong flexibility,which can participate in building various composites with multi-dimensional and multi-morphological structures.By introducing nano silicon into three-dimensional graphene,with the help of the synergistic effects between them,it can be expected to obtain anode composites with excellent electrochemical performances.In this paper,the "pea-pod" like and "highly dispersed" graphene-wrapped nano silicon composites were prepared via simple and low-cost methods.By regulating the microstructures of the composites,the synergistic effects between graphene and silicon can be fully played,so as to the endow the electrodes excellent electrochemical properties.The specific research contents of this paper are as follows:（1）A composite（P-Si@GS）with the "pea-pod" like graphene-coated silicon nanoparticles was prepared by the combination of freeze-drying and solvothermal method.In P-Si@GS,the cross-linked "pods" formed by curly thin graphene sheets constituted a conductive network in which silicon nanoparticles were tightly wrapped.The unique graphene-wrapped structure with mesoporous on its surface could effectively improve the conductivity of material,alleviate the volume effect of silicon,and provide quick transport channels for electrons and ions.Due to the above synergistic effects,P-Si@GS electrode could maintain a high reversible capacity of about 1110 m Ah/ g after 210 cycles even at a high current density of 4200 m A/g.（2）A composite（P-Si@r GO-7.5）with 3D graphene-wrapped highly dispersed porous nanosilicon was synthesized by a method combining freeze-drying and in situ magnesium-thermal reduction.In P-Si@r GO-7.5,porous silicon particles were highly dispersed and individually wrapped by graphene.The porous structure of silicon endowed the composite high specific surface area and porosity,which was conducive to reduce the diffusion path of lithium ions and alleviate the stress caused by the volume change of silicon.Meanwhile,the three-dimensional graphene-wrapped structure could improve the dispersion of the silicon particles,protect the structure stability of porous silicon,improve the conductivity of the material and ensure the electrical contact between the active materials.Due to the above synergistic effects,P-Si@r GO-7.5 electrode shown excellent electrochemical properties: at a current density of 1000 m A /g,the electrode could retain a high reversible capacity of 1123 m Ah/g after 500 cycles.In addition,the effects of phase composition and structure of the composites on their electrochemical properties were analyzed by optimizing the magnesium thermal reduction time,and the reasonable reasons for the significant increase capacity of silicon-based electrode during cycling were also discussed.