Preparation And Lithium Storage Performance Of Silicon-based Composite Anode Materials For Lithium-Ion Batteries

Silicon-based anode materials for lithium-ion batteries have been paid widespread attention because of their high specific capacity(4200 mAh g-1),low voltage platform(0.4 V)and good safety performance.However,the volume change of more than 400%in the lithiation/delithiation process of the silicon material makes the structure easily collapse and detach from the current collector.A stable SEI film cannot be formed,resulting in a rapid decrease in capacity and cyclability.It is an urgent problem to be solved for silicon anode materials.Herein,core-shell structure of Si material has been designed to relieve the stress caused by the volume expansion of silicon,slow down the material breakage,maintain a stable SEI film for better electrochemical performances.In this thesis,the structure of silicon composite is designed in two aspects and the main points are summarized as follows.1.A porous core-shell structure Si@SiO2 has been synthesized by a simple fluoride ion induced continuous mild chemical etching-redeposition process.By controlling the reaction time,temperature and fluoride ion concentration,the porous SiO2 coating layer with a thickness of 3-12 nm can be precisely controlled.Experimental results show that when the silicon oxide layer thickness is 5 nm,the specific capacity of the material is 1356 mAh g-1 after 300 cycles,and the capacity retention rate is 56.1%at a current density of 2.1 A g-1,which greatly improve the electrochemical performances.2.The silicon carbon composite material has been designed and prepared.Nano silicon powder was used as the silicon source,binder PVDF was the source of pyrolytic carbon,and carbon nanotubes(CNTs)were used as the carbon skeleton.The formation of carbon matrix after carbonization of PVDF can well wrap silicon material.The compressive strength and toughness of carbon nanotubes(CNTs)significantly relieve the local stress generated by the volume effect of the silicon powder during charging and discharging,and a three-dimensional conductive network has been formed.The final as-prepared silicon carbon composite material maintains a capacity of 899 mAh g-1 after 300 cycles,and the capacity retention rate is as high as 78%.