Research On The Preparation Of Si@C Composite Anode Material By Thermal Reduction Of Submicron SiO₂magnesium

With the sustainable development of clean energy,lithium-ion batteries become an important player in efficient energy storage devices.Compared with the current graphite anodes,silicon has a high theoretical capacity of 4200 mAh·g^-1 and it is ten times larger than the capacity of graphite anodes.Silicon anodes is considered to be one of the most promising materials for lithium-ion batteries because of its low working voltage,environmental friendliness and richness.However,the huge volume expansion and poor conductivity of silicon anodes during the charge and discharge process will seriously hamper its practical application.To solve the above problem,the nano-mesoporous Si was prepared by using industrial by-product silica fume?submicron SiO₂ particles?as a precursor,by magnesiothermic reduction and subsequent etching treatment.Encapsulation of SiO_x layer on nano-mesoporous Si surface by sol-gel method to enhance the structural stability of the materials,at the same time,the composite nanostructures of Si@SiO_x@C was prepared by using phenolic resin as carbon source and carbonized at high temperature to cover a layer of amorphous carbon on the Si@SiO_x surface.The electrochemical test results show that the composite material can be used as anode materials for lithium ion batteries and the cyclic stability of Si anode material has been improved successfully.In addition,in order to improve the degree of reduction,the mechanism of magnesiothermic reduction was studied in this paper.The results of the study are as follows:?1?The main phase of magnesiothermic reduction process are MgO,Mg₂Si,Si and Mg₂SiO₄,and their formation is affected by dynamic factors such as reaction temperature,holding time and Mg/SiO₂ molar ratio.It is found that the magnesiothermic reduction product of silica fume is Mg₂Si mesophase,and the phase is accompanied by the transformation to the crystalline Si reaction.The transition of intermediate phase Mg₂Si to Si can be promoted by increasing the temperature or prolonging the heat preservation time.The generation of the by-products Mg₂SiO₄is closely related to the supply of Mg.The higher the mole ratio of Mg/SiO₂,the more the amount of Mg₂Si generated;In contrast,the lower the Mg/SiO₂ molar ratio,the more the generated by-products Mg₂SiO₄.Therefore,it is necessary to reduce the formation of the mesophase Mg₂Si and the by-products Mg₂SiO₄ by changing the different reaction parameters.?2?Different mesoporous structure can be etched through different etching treatments for the magnesiothermic reduction products.After being corroded by HCl and HF,the solid spherical structure of the pristine silica fume is transformed into multistage mesoporous structure,including micro mesopores,mesopores,and large mesoporous macropores three level channels.After HF corrosion,the removal of residual SiO₂ can form large mesopore and further increase the bulk density of the materials.?3?The Si@SiO_x core-shell structure obtained by sol-gel method,it is coated with 1-5nm thick SiO_x on the surface of nano-mesoporous Si materials,which can effectively improve the cycle stability of silicon anode material and inhibit the capacity degradation during cyclic process.?4?The surface carbon coating of the prepared Si@SiO_x core-shell structure was carried out by high temperature pyrolysis process.A uniform amorphous carbon layer was formed on the surface of Si@SiO_x core-shell structure by adjusting the surface charge distribution by CTAB.The use of good conductivity of carbon particles to provide a conductive network between nanoscale particles,thereby improving the cycling and rate performance of materials.?5?The electrochemical performance tested in the current density of 50 m A·g^-1,Si@C composite anode material shows the charge capacity of 908.5 m Ah·g^-1,after 100 cycles the capacity remained at more than 520 mAh·g^-1.It is indicated that the Si@C composites prepared by the magnesiothermic reduction can be used as anode materials for lithium-ion batteries.