Design,Preparation And Properties Of High Capacity Silicon-Based Anode Materials For Lithium Ion Batteries

Lithium-ion batteries（LIBs）are widely used in electric vehicles and electronic products because of its high energy density,long life and light weight.The energy density of LIBs is closely related to the electrode materials.As a commercial anode material,graphite has basically reached the theoretical specific capacity.In order to further improve the capacity of LIBs,it is urgent to develop high-performance anode materials.Si materials are widely concerned for its high theoretical capacity,high safety and abundant reserves in the earth’s crust.However,the Si material expands greatly during the charging process,which affects the cycling performance and coulomb efficiency（CE）,and seriously restricts the industrial application of the Si-based materials.The research shows that the nanalization of Si materials and the coating modification of Si materials using others materials can improve the electrochemical performance,improving the cycling performance and the CE of Si-based materials.In this thesis,different types of Si-based anode materials were obtained by selecting appropriate modified phase,adopting relatively simple preparation and synthesis methods.The Si nanoparticles can be fully dispersed,fixed and coated to control the expansion space of materials and improve the conductivity.The electrochemical properties of Si-based anode materials were improved gradually.At the same time,the preparation of porous nano-Si particles was also carried out with the abundant coal-based kaolin,which provided a reference for the preparation of nanometer Si materials with low cost and controllable.The specific research contents and results are as follows:（1）The nitrogen-rich silicon/carbon（Si/C）composite was prepared by gel method,freeze drying and carbonization method using high nitrogen-rich gelatin as raw material.Si nanoparticles were evenly dispersed and fixed in the gel through the process control using the special gel properties of gelatin.Then the fluffy 3D aerogel was obtained by freeze-drying,and nitrogen-rich Si/C composite was prepared through carbonizing aerogel.Si nanoparticles were fully dispersed in composite materials compared with the Si/C anode materials obtained by ordinary heating and drying.The doping of nitrogen can effectively improve the lithium storage and conductivity of the material.The discharge specific capacity is 528.1 m Ah g^-1at the current density of 0.1 A g^-1after 100cycles,and the CE is 99.7%.（2）A 3D framework Si@NC/rGO composite was facilely prepared by a novel polymer network method by using GO,Si nanoparticles,polymer monomers（acrylic amide）,and networking agents（N,N′-methylene bisacrylamide）as the raw materials.After the crosslinked polymerization,Si nanoparticles were embedded into the 3D carbon framework.The Si nanoparticles were encapsulated by 3D structure,which can effectively buffer the volume expansion of Si in the process of charge and discharge.Meanwhile,nitrogen-doped carbon framework and highly conductive graphene also play a positive role in improving the conductivity of composites.The Si@NC/rGO composite exhibits a large reversible specific capacity,good cycle stability,and excellent rate performance as an anode electrode material for LIBs.The specific capacity is maintained at 945.8 and 528.5 m Ah g^-1at 0.1 and 2 A g^-1after the100 cycles,respectively.The CE is 99.7%and 99.8%,respectively.（3）The hierarchical porous of Si@TiO₂@C composite was designed and fabricated by solvothermal and carbon-encapsulated process.The Si nanoparticles were inserted into the TiO₂layer by solvent thermal method,and then the hierarchical Si@TiO₂@C composite was obtained by coating with carbon.The special layered porous structure and the carbon layer can effectively buffer the expansion of Si during the charging and discharging process to improve the cycling performance of Si-based materials.At the same time,the special pore size structure of porous TiO₂also facilitates the rapid diffusion of Li⁺in the electrode,thus improving the rate performance.The composite exhibits an excellent electrochemistry performance about 967.4 and 531.5 m Ah g^-1after 100 cycles at 0.1 and 2.0 A g^-1,respectively.The CE is 98.6%and99.2%,respectively.（4）The porous Si nanoparticles were prepared using high abundant coal-based kaolin as raw materials,through ball grinding,calcination,selective leaching and magnesium thermal reduction processes.The porous Si nanoparticles have excellent electrochemical properties without any modification,due to its unique porous nanostructures to buffer the volume expansion of Si.The discharge capacity remains 546 m Ah g^-1at the current density of 0.1 A g^-1after 100 cycles with CE of 98.8%.Initial CE can be up to 80.0%.The preparation of porous Si nanoparticles with high reserves of raw materials is simple and controllable,which provides a new idea for the preparation of porous Si nanoparticles.