Preparation And Study Of Silicon Anode Materials For Lithium Ion Batteries

With the widespread use of the electric vehicles and portable electric devices,it is a great demand to develop high-energy-density lithium ion batteries.Among the numerous anode materials,Si has been considered as one of the most promising candidate for Li-ion battery,due to its high theoretical specific capacity?4200 mAh/g?,favorable working voltage?0.02/0.5 V vs.Li/Li⁺?,abundant resources,environment friendly and good safety.However,a drastic volume expansion?>300%?commonly occurs in Si electrode during lithium ions insertion process,which causing the pulverization of Si particle,the peeling-off of electrode material from the current collector and repeated formation of the SEI?solid electrolyte interphase?.Therefore,the capacity of Si electrode decreases rapidly during the discharge and charge process.To date,numerous attempts have been made to solve the problems associated with the volume change of silicon.For example,the construction of different Si nanostructure,such as Si nanoparticles,porous Si,Si films,the preparation of Si/C composites and dispersing Si in active/inactive matrixes.Among these strategies,Si@SiOx and Si@C composites have attracted much attention.This study mainly focuses on the synthesis of Si@SiO_x and Si@C composites with designed microstructure.General preparation methods of silicon include thermal reduction,silane pyrolysis,CVD,etc.Among them,magnesiumthermic reduction has been widely used due to the facile synthesis process,easily accessible of SiO₂ raw material and easily to obtain various structures.Traditional magnesiumthermic belongs to solid-liquid reaction,because the reduction temperature is above the Mg melting point?>650??.However,the experimental results show that the reactant synthesized by this method is hard to inherit the original structure of SiO₂.Furthermore the main product of the reduced SiO₂@C composites by traditional magnesiumthermic reduction is the inactive SiC phase.In order to solve the problems and realize the microstructure design of silicon anode materials,the mainly results obtained in this thesis as follows:1.The highly ordered macro-mesoporous SiO₂ was prepared and used as the raw materials.We systematically investigated the influence of different temperature on the phase and morphology of product.The results suggested the optimum temperature of magnesiumthermic reduction is 550?.And highly ordered macro-mesoporous Si@SiO_x nanocomposites were prepared successfully by low temperature magnesiumthermic reduction.2.The reaction mechanism of low temperature magnesiumthermic reduction was investigated.The reaction rate can be easily controlled by adjusting the reaction time.We realized the continuous adjustment of O/Si ratio in Si@SiO_x composites.And the electrochemical performance of Si@SiO_x inverse opals with various O/Si ratio?x?was investigated systematically.When x=1.01 the composite exhibited the best electrochemical performance.3.SiO₂@C composites was reduced by traditional magnesiumthermic reduction,and the results of the reactant phase analysis showed that formation of SiC was prior to Si and inevitable.However the formation of by-product SiC can be avoided effectively by adding AlCl₃ molten salt in the process of magnesiumthermic reduction,and that temperature could be reduced to 200³00?.The optimal reduction temperature was 300?.4.We prepared hollow SiO₂ spheres by template method and investigated the optimal conditions for synthesis of SiO₂ sphere with uniform structure,good stability and smooth surface.Si@C hollow sphere was synthesized successfully by AlCl₃molten salt assisted magnesiumthermic reduction.The influence of carbon content on the morphology and the electrochemical performance was systematically studied and when C content was 15 wt%,the Si@C sphere showed the best electrochemical performance.