Preparation Of Modified Silicon/Germanium-based Composites And Study On Their Lithium Storage Properties

Lithium ion batteries(LIBs)have dominated the markets of portable electronic devices and electric vehicles in recent decades due to their high output voltage,high energy density,and long cycle life.The traditional graphite anode has a theoretical capacity of only 372 m A h g^-1,which cannot meet the needs of long-life equipment.Therefore,it is of great significance to find anode materials with high theoretical capacity.Silicon has a very high theoretical capacity(4200 m A h g^-1),which has aroused widespread concern in scientific research and business circles.Although the theoretical capacity of germanium is low(1624 m A h g^-1),its inherent high conductivity(10⁴times higher than silicon)and lithium ion diffusion rate(400 times higher than silicon)make it a promising anode material.However,both silicon and germanium undergo severe volume expansion during the alloying/dealloying process,resulting in rapid decline in battery performance.Researchers have tried to modify it in a variety of ways.Among them,carbon materials are widely used to synthesize silicon/germanium-based carbon composite materials due to their advantages of good conductivity and small volume expansion.In this paper,silicon/germanium-based carbon composites with different structures were prepared as anode materials of LIBs by electrospinning and liquid phase stripping methods,and their electrochemical properties were studied in detail.The specific works are as follows:(1)In this chapter,the ultrafine germanium nanoparticles are evenly encapsulated in nitrogen-doped porous carbon nanofibers(Ge-NMCFs)by electrospinning method.The composite material has a hierarchical porous structure,which not only facilitates the transmission of ions and electrons,but also effectively relieves the volume expansion of germanium nanoparticles.In addition,nitrogen doping can effectively improve the overall conductivity of the material.Therefore,at a current density of 500 m A g^-1,the initial discharge capacity of the Ge-NMCFs nanocomposite is 1146.7 m A h g^-1,there is still 600.9 m A h g^-1after 500 cycles,with an average capacity decay rate per cycle is only 0.09%.For the other three contrast materials,including ultrafine germanium nanoparticles are evenly encapsulated in nitrogen-doped non-porous carbon nanofibers(Ge-NCFs),nitrogen-doped porous carbon nanofibers(NMCFs)and pure germanium particles(Ge).Comparing the electrochemical performance,it is found that the Ge-NMCFs electrode has the best electrochemical performance.(2)In this chapter,hydrochloric acid is used to delaminate the bulk of calcium silicide to synthesize two-dimensional nanosheets of siloxene.Then,chitosan was used as the nitrogen-doped carbon source to prepare the siloxene@nitrogen-doped carbon composite material(Si OX@C-N).The two-dimensional nanosheet structure can effectively reduce the volume expansion of the silicon-based material.The nitrogen-doped carbon material as a coating layer can significantly improve the conductivity of the composite material and can further alleviate the volume expansion of the silicon-based material.When the current density is 500 m A g^-1,the first-cycle discharge capacity of the Si OX@C-N electrode is 2120.8 m A h g^-1,and there are still 1114 m A h g^-1after 100 cycles.As a comparison,the capacity of pure siloxene two-dimensional nanosheets decays rapidly,from 2735.8 m A h g^-1in the first cycle to 213.5 m A h g^-1after 100 cycles,and the average capacity decay rate per cycle is as high as 9%.In addition,the Si OX@C-N electrode exhibits better rate performance,with a capacity of 595.9 m A h g^-1at a current density of 5000 m A g^-1,while the capacity of pure siloxene was only 106.9 m A h g^-1.