Self-assembly Synthesis And Electrochemical Performance Of Silicon-carbon As Anode Materials For Lithium-ion Batteries

In recent years,the replacement of fuel vehicles by electric vehicles has became an irresistible trend due to the depletion of fossil fuels and the gradual deterioration of air quality.The traditional graphite material,as a typical commercial lithium ion battery anode material,has been unable to meet the requirements of the high energy and high power density of electric vehicles.Carbon-coated silicon composites are gradually occupying the market and research direction of lithium-ion batteries as a new promising anode material.However,if silicon materials want to completely replace graphite materials,two defects need to be solved.Firstly,there is a huge volume effect of silicon in charge and discharge process,secondly,conductivity of silicon itself is too low.In this regard,from the two major problems of solving silicon-based materials,this paper proposes the use of electrostatic self-assembly method to uniformly cover Nafion resin on the surface of silicon or silica particles,supplemented by magnesiothermic reduction,followed by high-temperature calcina-tion for carbonization,amorphous carbon layer coated silicon-based composite structure was prepared.The relationship between material structure and electroche-mical performance of Si/C composites with different silicon content was analyzed.The relationship between material structure and electrochemical properties of Si/C and SiO₂/C composites with the same Si content was also analyzed.The main research contents and conclusions are shown as following:（1）Self-assembly preparation of silicon-carbon composites with different silicon contentsThe prepared silicon-carbon composites was characterized by XRD patterns and Raman spectra showed silicon was amorphous silicon and carbon was amorphous carbon.Carbon and silicon were mainly in the form of carbon-carbon single bonds and zero-valent silicon by XPS spectra.The microscopic morphology of the silicon-carbon composite was analyzed which showed the particle size of amorphous silicon was mostly around 70 nm.It was found that the amorphous carbon can better coat the surface of the silicon particles and relieve the volume effect of silicon during the charge and discharge process.The amorphous carbon coated which can also realize the rapid transmission of electrons and ions,which provides a guarantee for the better electrochemical performance of silicon-carbon composites.The electrochemical performance of the silicon-carbon composites was tested which showed that the 10%silicon content was excellent,which the capacity of 408.5mAhg^-1 can be maintained after 70 cycles of charge and discharge at a current density of 200 mAg^-1,and the capacity of 398 mAhg^-1after a charge-discharge cycle at a high current density,it showed good cycle performance and rate capability.（2）Comparison of Si/C and SiO₂/C composites with the same silicon contentThe prepared silicon-carbon composites were characterized by XRD pattern which showed carbon was amorphous carbon and silica is amorphous SiO₂;the silicon in the Si/C composite material was mainly in the form of zero-valent silicon and the silicon in the SiO₂/C composite is mainly in the form of+4 valence by XPS spectra.The microscopic morphology of the composites were analyzed by transmission electron microscopy which showed amorphous carbon is uniformly distributed on the surface of silicon or silicon dioxide and tightly bound together,it can increase the overall structural stability and effectively inhibited the huge volume effect of silicon.The electrochemical properties of the SiO₂/C composites were tested which showed the composite with 10%Si content maintained a capacity of 303.2mAhg^-1 after 70 cycles of charge and discharge at a current density of 200 mAg^-1,and the capacity of 288 mAhg^-11 after charge-discharge cycles at a high current density.The amorphous carbon-coated Si/C and SiO₂/C composites,although the amorphous carbon was added to the SiO₂/C composite which the conductivity was increased,but the improvement of conductivity was not obvious for the electrical conductivity of silica was even worse than that of silicon.Secondly,the carbon content was reduced due to the intervention of oxygen,the coating effect was not very good and the particles size was large which leads to poor electrochemical performance.The electrochemical performance was not as good as that of the same silicon Si/C composites.