The Study Of The Structural Design And Electrochemical Performance Of Si-Based Carbon Composites For Lithium Ion Batteries

The development of electric vehicle urgently needs the breakthrough progress of lithium ion battery.The high theoretical capacity of silicon-based anode materials is highly expected to be used in lithium ion battery.In recent years,a certain amount of silicon has been introduced into graphite anode materials in order to match with high-capacity cathode electrode materials.However,the low silicon content in the commercialized silicon-carbon anode materials leads to its specific capacity still lower than 500 mAh g^-1,while high content silicon-carbon composites still suffer from huge volume expansion.In order to solve the problem,silicon-carbon composites with yolk-shell structure and pomegranate-like silica/carbon composites were proposed in this thesis to alleviate the volume expansion of silicon.Meanwhile,the electrolyte additive is also proposed to be introduced into the electrolyte system to promote the formation of a stable solid electrolyte interface.Scanning electron microscope,transmission electron microscope,x-ray diffraction and nitrogen adsorption-desorption test were used to investigate the morphology,structure and porosity of the composites.The effects of yolk-shell structure design,nitrogen-doped carbon content and porosity on the electrochemical properties of silicon-based carbon composites were studied by constant current charge/discharge test.The contents of the study are as follows:?1?Si/Void/C composite materialsThe spherical nano-Si with the diameter of 100 nm was used as the raw material.Tetraethyl silicate was hydrolyzed on the surface of silicon nanoparticles by sol-gel method under alkaline conditions.The obtained silica layer was used as a template for hollow layer.Then the glucose derived carbon layer was coated on the surface of Si/SiO₂ through hydrothermal method.Finally,the Si/Void/C composite materials were obtained by etching the SiO₂ layer.In this chapter,the construction of hollow structure and the effect of electrolyte additives on the electrochemical properties of Si/Void/C composites were studied.The results showed that the design of yolk-shell structure and the use of electrolyte additives greatly improve the electrochemical performance of silicon nanoparticles.The first Columbic efficiency can reach more than 82%,and the specific capacity still remain 1594.5 mAh g^-1 after 200 cycles at the current rate of 0.2?2?Si@Void@NC composite materialsThe silicon powder with the diameter of 50 nm was used as raw material,and the silica layer was also coated on the surface of silicon nanoparticles by hydrolyzed the tetraethyl silicate as a sacrifice template.And then a layer of dopamine derived nitrogen-doped carbon layer was further coated to improve the conductivity of silicon nanoparticles.Finally,Si@Void@NC composites were obtained by etching SiO₂ layer.In this chapter,the effects of the constructed hollow structures and different synthetic methods on the electrochemical performance were studied.At last,the results show that the composites synthesized by simple self-polymerization method exhibits higher specific capacity and better conductivity compared with that of the hydrothermal method.The specific capacity of Si@Void@NC composite can reach 2331.4 mAh g^-1after 50 cycles at 0.2 C with an improved initial Coulombic efficiency of 79.29%.?3?SiO₂@MNPC composite materialsThe biomass material-egg was choosen as carbon source,and added with water and ethanol to make a uniformly solution.Then adjusted the pH value with concentrated ammonia and then added tetraethyl silicate dropwisely to directly hydrolyze in the egg solution to form silica.Finally,after freeze-drying,high-temperature calcination,and selective eaching,SiO₂@MNPC composites can be obtained.In this chapter,the effects of nitrogen and phosphorus doping and porosity of carbon shell on the electrochemical properties of composites were investigated.The results showed that the three dimensional SiO₂@NPC composites with pomegranate-like structure can increase the porosity and enhance their electrochemical properties after selective etching.The specific capacity of the SiO₂@MNPC composite still reach 373 mAh g^-1 after 1000cycles at the current density of 500 mA g^-1.