Preparation And Electrochemical Properties Of Si/SiO_x/C Disordered Mesoporous Composite Nanostructures

The theoretical specific capacity of the conventional graphite anode for lithium-ion battery is 372mAh·g-1.With the rapid development of power vehicles,large charging stations and other fields,the conventional lithium-ion battery has been unable to meet the needs of the industry.Silicon is one of the most promising anode material for lithium-ion battery for its theoretical lithium storage capacity is nearly 10 times that of the conventional graphite anode.Furthermore,silicon is abundant in the earth’s crust and friendly to the environment.However,in the process of charging and discharging,the serious volume expansion(>300%)during lithiation brings about particles pulverization and losing electrical contact between the active materials and the current collector,thus capacity rapidly degrades.Meanwhile,the unstable solid electrolyte interface(SEI)film caused by structural destruction will constantly consume Li+ from the electrolyte.To solve the problem and simultaneously improve the packing density,a silicon nano composite with disordered hierarchical mesoporous atchietctures was prepared via magnesiothermic reduction and subsequent acid corrosion by using white carbon black(SiO2)as a precursor.Then SiO_x coating and carbon coating were employed to enhancing the stability and electrical conductivity of the electrode.According to the electrochemical analysis,the cyclic performance of the composite is successfully improved by this architecture.Besides,in order to obtain a larger Si yield and avoid the emergence of by-products,the mechanism of magnesiothermic reduction is analyzed and discussed in this dissertation.The main results are as follows.(1)The products of magnesiothermic reduction are MgO,Mg2Si,Si and Mg2SiO4 by-product.Their formation is controlled by kinetic factors,including heat treatment temperature,heat preservation time and the concentration of Mg in the mixture.With the increase of heat treatment temperature or heat preservation time,the order of magnesiothermic reduction is Mg→Mg2Si→Mg2Si+Si→Si.The higher the concentration of Mg,the more the Mg2Si residual,and all SiO2 will be reduced to Si.The generation of Mg2SiO4 needs to overcome large barrier,and requires uneven mixing of Mg and SiO2.Consequently,the opimun process conditions are heat preserving at 660℃ for 4h,and a mass ratio of 1:1 for Mg to SiO2.(2)The mesoporous arthitecture can be assembled by acid corroding of the magnesiothermic reduction products.After acid corroding,the original chains-like architecture of white carbon black transforms to hierarchically disordered mesoporous acchitecture,including micro-narrow mesoporous(1-5nm),medium mesoporous(5-20nm)to macro-mesoporous and macroporous(20-100nm),which originating from the inner pore of the SiO2 primary particles,particle aggregation/stacking,acid-etching of primary particles,and particle agglomeration/restacking in solution,respectively.Meanwhile,the packing density of Si particals is increased as a result of the skeleton collapse and the particles reorganization caused by acid corrosion.(3)Electrolyte is difficult to penetrate immediately into pores due to the excess carbon coating sealing micro-narrow mesoporous.Once the electrolyte penetrates,the capacity of the active materials is activated.After 100 cycle of charging/discharging,the electrode exhibites a phenomenon of capacity not fading but slowly rising.In detail,the 100th cycle capacity even increased by 25%compared to the third cycle,endowing electrode with extraordinary cycling performance.(4)SiO_x has good mechanical properties.The coating of SiO_x can buffer the volume expansion of Si,enhance the stability of electrode material(SEI film is stable)and hinder the decay of electrode capacity.In addition,the more the coating amount of SiO_x,the lower the trend of capacity degradation,the more obvious the phenomenon of slow activation.