The Preparation And Electrochemical Performance Of Nano-ordered Mesoporous Si/C Lithium Ion Battery Anode Materials

The research of anode materials for lithium ion batteries has gradually shifted from traditional commercial graphite materials to Si negative electrodes.Silicon has been widely concerned with the high theoretical specific capacity?3580mAh/g,Li1₄Si₅?,but is limited to poor cyclic stability and multiple performance.In the actual charging and discharging process,the expansion of silicon negative electrode/lithium intercalation will cause the electrode pulverization and falling off to reduce the capacity,and the intrinsic conductivity of the silicon material is very low,and the performance of the battery is poor.In addition,the structure of Si is easily destroyed during charging and discharging,which will directly lead to the instability of the solid electrolyte boundary film?SEI?.In order to solve the above problems,ordered mesoporous SBA-15 molecular sieves were synthesized by hydrothermal method,and ordered mesoporous Si was prepared with the ordered mesoporous silica as precursor by low temperature magnesium heat reduction.Si@SiC@C composite nanomaterials with good cyclic performance were prepared by carbon coating process,in which the abundant pores in the Si material could effectively alleviate the volume effect in the process of silicon cycle,and greatly improve the cyclic stability of the material.In addition,the mechanism of SiO₂ reduction and the formation mechanism of Si@SiC@C core-shell structure were discussed.The main results are as follows:?1?Using P₁₂₃ as a template and TEOS as a silicon source,the size controlled SBA-15 particles can be synthesized under certain conditions.The morphology and microstructure of the ordered mesoporous SBA-15 molecular sieve are influenced by the acid concentration,agitation state,and additives and other conditions.?2?The product of magnesium reduction is controlled by kinetic factors,including heat treatment temperature,holding time and heating rate.According to the T-t-v phase change schematic diagram,the three-dimensional space is divided into three phases by the mesophase generation surface???,the phase transition surface???and the phase transition end surface???:the unreacted region?SiO₂+Mg?,the intermediate phase transition zone?Mg₂Si+Si+SiO₂?and the?Si+Mg₂SiO4?phase region.The three major thermodynamic factors affect the reaction process,and the Mg₂Si phase in the reaction process is the intermediate phase,and the Si phase varies according to the order of Mg?Mg₂Si?Mg₂Si+Si?Si+?Mg₂SiO₄?order.Proves the existence of Mg₂Si?3?SBA-15 magnesium thermal reduction process:the magnesium vapor diffused to the SBA-15 interface to generate intermediate phase,and the process of two Mg₂Si is transformed into Si phase,and the two processes are affected by the kinetic diffusion.In process three,the supply of Mg source is not sufficient,after the consumption of Mg₂Si,there are still some SiO₂ remnants.When the heating temperature is high or the reaction time is long enough,the local thermal accumulation effect causes MgO to react with the residual SiO₂ to produce Mg₂SiO₄ at the interface.There are Mg₂Si,Si,MgO,Mg₂SiO₄ and a small amount of SiO₂ in the solid magnesium thermal reduction phase.?4?Through the reduction of carbon after SBA-15,Si@SiC@C exhibits excellent cycling performance and rate characteristics.The remarkable improvement of the electrochemical performance is mainly because the carbon layer coated on the mesoporous silica surface can provide the conductive network for the electrode,and SiC is beneficial to the formation of the stable SEI film on the silicon surface,thus improving its reversible capacity and greatly improving the performance of the ratio.In this paper,a simple hydrothermal synthesis of SBA-15 ordered mesoporous silica was used as a template.A lithium ion battery anode material with good cyclic stability was prepared by low temperature magnesium thermal reduction and carbon coating.The electrochemical performance of silicon negative electrode was greatly improved.This simple and easy preparation method has improved the industrial application potential of silicon based anode materials.