Preparation And Electrochemical Properties Of Nano Porous Germanium Carbon Composite Anode Materials

Lithium-ion batteries have many advantages,such as high energy density,good cycling performance and no memory effect,which have been widely used in the fields of electronic equipment,electric vehicles and aerospace.At present,the specific capacity of commercial graphite anode is close to the theoretical limit,it is thus urgent to develop new high-capacity anode materials.Germanium anode material has the advantages of high theoretical specific capacity(1621 m Ah g^-1),low lithium-intercalated potential and high volume energy density.However,germanium has the problems of large volume effect（230%）and poor conductivity during the charge and discharge process,which hinder its engineering application.Two kinds of porous germanium/carbon composite anode materials including germanium/three-dimensional（3D）porous carbon nanosheets and germanium-carbon hollow spheres are designed and prepared in this thesis,in view of the problems existing in germanium anode materials.By constructing 3D porous structure,reducing the particle size and increasing the specific surface area of the material,the volume effect is alleviated and the conductivity of the material is improved.The influences of preparation process on the composition,structure and properties of the materials are systematically studied,and the structure-function relationship between the microstructure and electrochemical performances of the two porous germanium/carbon composite anodes is explored.The synthesis and electrochemical properties of Ge/3-dimensional graphene-like nano sheet（Ge@GPNS）composites were studied.The GPNS matrix has abundant pore structure,which can effectively restrict the growth of Ge under high temperature reduction.The use of cobalt nitrate and melamine can form a carbon-coated layer on the surface of Ge@GPNS composite anode to alleviate the volume effect of Ge during cycling.The electrochemical tests showed that the cycle stability and rate capability of Ge@GPNS anodes were significantly improved compared with pure germanium anodes.At the rate of 0.2C,the discharge specific capacity reached 1661.67m Ah·g^-1,and the capacity retention rate was 88.33%after 100 cycles.At the rate of 2C,the capacity reaches 1075.64 m Ah·g^-1.In this paper,Ge/C hollow sphere（Ge/C＿HS）composites were designed.Based on the Si O₂microsphere hard template method,the germanium carbon symbiotic hollow structure was constructed in situ by liquid-phase reduction,heat treatment and etching processes.The effects of the synthesis process on the composition,structure and properties of the materials were systematically studied.Surfactant is the key factor to control the formation of hollow spheres.Cetyl trimethyl ammonium bromide can promote the growth of precursor on the surface of Si O₂microsphere template.Etching process is the main factor affecting the stability of hollow sphere structure.Appropriate etching process can ensure the integrity of hollow sphere while removing Si O₂template.Ge/C＿HS negative electrode has good electrochemical performance.At 0.1C discharge rate,the first discharge specific capacity of the material is 1687.25mAh·g^-1.