Study On The Preparation Of High Energy Storage Density Si/SiO_x/C Lithium Battery Anode Material By Magnesium Thermal Reduction Method

With the continuous development of the new energy industry,the use of electric vehicles is becoming more and more common,which will inevitably lead to higher requirements for their endurance capabilities.Therefore,the requirements for lithium ion batteries with high energy storage density are getting higher and higher.As far as commercial lithium-ion batteries are concerned,the capacity of cathode materials is limited,and the graphite widely used in the negative electrode of the lithium battery cannot meet the demand for high energy storage density.The theoretical capacity of Si at room temperature is 3580 mAh·g^-1,which is nearly ten times that of graphite(372 mAh·g^-1),and its discharge potential is low,the element earth abundance is rich,and it is environmentally friendly.It is one of the new anode materials for lithium ion batteries with high energy storage density.However,the Si negative electrode undergoes a large phase change volume expansion?³00%?during the Li⁺deintercalation process,resulting in the breakdown of silicon particles,tissue collapse,the stripping of active material layer and collector,and the loss of electrical contact,and the cyclic stability deteriorates rapidly in the process of charging and discharging.In addition,the breaking of the fresh surface by Si particles will continue to consume the Li⁺in the electrolyte to form a new solid electrolyte interface film?SEI?,which will result in poor performance of the Si negative electrode in electrochemical service.In order to solve the above problems,researchers have designed various nanostructures to buffer the volume expansion of Si,but the corresponding cost of technology and equipment is higher,and the Nano-Design will reduce the tap density of Si,resulting in low energy storage density in the application of Si negative electrode.The low-cost process for the preparation of porous nano-Si by magnesium thermal reduction has opened up new ideas for researchers to prepare Si anode materials.The low-cost process for the preparation of porous nano-Si by magnesiothermic reduction has opened up new ideas for researchers to prepare Si anode materials.However,the yield of Si prepared by the magnesiothermic reduction method is not high,and by-products are prone to occur during the reaction,which greatly affects the application of Si as an electrode.According to the current problems of Si negative electrode,fumed silica?SiO₂nanoparticles,FS?was used as a precursor of Si to prepare nano Si by magnesiothermic reduction in this paper.Under the theoretical guidance of the preparation of nano-Si by magnesiothermic reduction,the process optimization was carried out to reduce by-products to improve the reaction degree of Si,improve the mechanism of magnesiothermic reduction and increase the yield of Si.The ball-grinding process is used to explore the structural package model to increase the degree of magnesiothermic reduction reaction while increasing the tap density of the reduced Si material.The cyclic stability of the electrode material was enhanced by exploring the coating ratio of the buffer material SiO_x.The capacity retention rate of the composite electrodes prepared by the above process is 83%after 500 cycles at current density of 1 A·g^-1,and the cycle life of the composite electrodes is effectively improved.The main results are as follows:?1?Increasing the mixing uniformity of raw materials for magnesiothermic reduction can shorten the reaction pathway of Mg and SiO₂,and make Mg participate in the conversion of SiO₂more effectively,which can significantly reduce the production of byproducts Mg₂Si and Mg₂SiO₄.Optimizing the reaction vessel can significantly reduce the escape of Mg vapor.The combination of the two methods can obviously improve the reaction degree of magnesiothermic reduction and the yield of silicon.It is concluded that the optimum technological conditions for magnesiothermic reduction are:n_SiO2:n_Mg=1:2.5,mixing in low speed ball milling and holding for 4 hours at 660oC.?2?In the process of SiO_x coating,with the increase of SiO_x coating ratio,there is a formation mechanism of fibrillation followed by film formation,while the capacity of Si/SiO_x/C anode decreases,but the cyclic stability is obviously improved,and the cyclic stability is improved at the same time of capacity loss.?3?The optimum proportion of SiO_x coating is HSiO_1.5:Si=1:5.This is because there are a large number of fiber wrapping and SiO_x coating on the surface of Si,forming a three-dimensional fiber wrapping network,which solidifies and strengthens the structure of Si/SiO_x/C composites,showing good cyclic stability with little capacity loss.?4?The packing model formed by ball milling and acid etching after Mg thermal reduction can obviously increase the tap density of Si and consequently the energy storage density of Si/SiO_x/C negative electrode.