Preparation And Electrochemical Performances Of Nano-silicon Modified By Transition Metal Element

Silicon has attracted numerous attention because of its ultrahigh theoretical specific capacity,low lithiation potential,and abundant reserves.However,Silicon displays some problems,such as poor electronic conductivity,large volume expansion during cycling,and low initial coulombic efficiency（ICE）,which hindered its commercialization.To solve these problems,this paper has chosen some group-ⅥB elements such as Cr,Mo,and W and group-Ⅷelements such as Co and Ni to carry out a series of studies on silicon modification from the perspective of band structure regulation and composite design based on metal silicide.The main research conclusions are as follows:（1）First-principles calculations indicate that the doping of transition metals such as Ti and V can expand the lattice of silicon,whereas the incorporation of Cr,Mn,Fe,Co,and Ni can shrink the lattice of silicon.Moreover,the doping of these transition metals can introduce the additional energy level into the forbidden band of silicon,which reduces its bandgap to some degree.The doping of Mo elements of group-ⅥB and group-Ⅷelements such as Co,and Ni can reduce the bandgap of silicon from 0.61 e V to 0.45 e V,0.21 e V,and 0.11 e V,respectively.The calculated results suggest that the doping of Mo,Co,and Ni contributes to enhancing the intrinsic conductivity of silicon.（2）To improve the cyclic stability,electric conductivity,and ICE of Si anode material,we introduced the transition metal elements into the silicon oxide precursor uniformly through the polycondensation process of ethyl orthosilicate,which is mild and controllable;Then the silicon composites with transition metal element doping and second phase metal silicide was obtained by magnesium thermal reduction;Thus,we have developed a universal method.This method has solved the problem that the doping transition metal and metal silicide cannot disperse in silicon particles uniformly by traditional ball milling and solid-phase sintering.By the above method,group-ⅥB elements（Cr,Mo,W）and Group-Ⅷelements（Co,Ni）are introduced into the silicon,forming different forms of element doping or metal silicide in silicon material.These materials display different modification features,which improve their electrochemical performance.（3）In group-ⅥB elements,the Cr element is dispersed in silicon particles in the form of Cr Si₂ nanodots through the above method.The obtained 8%Cr Si₂/Si composite under optimized conditions exhibits the reversible capacity of 2092.3 m Ah g^-1 at a current of 0.84 A g^-1,and a reversible capacity of 1085 m Ah g^-1 can be retained after 500 cycles.The results of In situ TEM lithiation and MD simulation suggest that the good cycle performance of Cr Si₂/Si can be attributed to the mitigation of the volume expansion of Si during cycling caused by embedding Cr Si₂.Mo-doped porous nanostructured silicon（Mo-PNSi）was obtained by inducing the Mo element into the preparation process of silicon,which displays the reversible capacity of 2092.3 m Ah g^-1 at a current of 0.84 A g^-1;Even at a high rate of 6.72 A g^-1,a reversible capacity of 850 m Ah g^-1 can be maintained.The electrochemical performance of optimized Mo-PNSi is mainly attributed to the improved intrinsic conductivity of silicon by Mo doping;Moreover,the hierarchical pore structure of silicon relieves its volume variation during cycling.After introducing the W element into the preparation process of silicon,the generated highly conductive WSi₂nanodots are dispersed in the silicon particles,which form a continuous conductive network.Under the optimized conditions,the WSi₂/Si can retain a reversible capacity of 1504.8 m Ah g^-1 at 0.84 A g^-1 after 200 cycles;After 1000 cycles at 4.2 A g^-1,the capacity is 903.4 m Ah g^-1.The satisfied electrochemical performance of the WSi₂/Si composite can be ascribed to the embedded WSi₂ nanodots which enhanced the electronic conductivity and mitigated the volumetric expansion of silicon.（4）In the group-Ⅷelement,after introducing the Co element into the silicon preparation process,the silicon nanomaterials with both highly conductive Co Si₂ nanodots and Co dopants inside are obtained.Under an optimized Si/Co molar ratio of 15,the reversible capacity of obtained Co Si₂/Si composite can remain at 1077.8 m Ah g^-1 after 400 cycles at 0.84A g^-1.The good cycle performance of the Co Si₂/Si composite can be attributed to the formation of conductive Co Si₂ and Mo dopants inside the silicon particles which enhance its electroconductivity;Furthermore,the dispersed Co Si₂ has also mitigated the volume variation of silicon.When the Ni element is introduced into silicon,the formation and acid removal of Ni Si₂ in the silicon matrix not only improved the crystallinity of the silicon material but also formed porous structure and Ni dopants in the silicon material.The Optimized Ni-CPSi sample display a high initial coulombic efficiency of 83.59%;Even at a high rate of 8.4 A g^-1,a reversible capacity of 682.8 m Ah g^-1 can be retained.The improved initial coulombic efficiency of Ni-CPSi is mainly attributed to its increased crystalline which helps reduce the side reactions;In addition,Ni doping has improved the intrinsic conductivity of silicon,which contribute to enhancing its electrochemical reactivity.