Structure Design And Fabrication Of Silicon And Phosphorus Based Nanomaterials And Their Application In Energy Storage

With the development of human society,the increasing demand for better electrochemical properties of batteries is arising globally.Lithium ion batteries(LIBs)and sodium ion batteries(SIBs)are attracting more attention in energy storage fields because of their flexible properties and high energy densities.However,the poor performance of anode materials gives more challenges for their practical applications,which should be firstly addressed by developing high performance anodes with high specific capacity and superior stable cycling property.As known,the electrochemical performance of anodes is closely related to their nanostructures.Based on this,we have designed different nanostructures of silicon and phosphorus based materials and developed a series of controllable synthetic methods to fabricate the designed nanostructures,such as solid state reactions,solvothermal methods,catalytic chemical vapor deposition methods and so on.At the same time,we have also studied their superior electrochemical performance as anodes for LIBs or SIBs.Specifically,the main research content in our paper are introduced as following:1.We designed a silicon hierarchical nanostructure that was composed of carbon coated 20 nm Si nanoparticles.To achieve such unique hierarchical nanostructure,we proposed a novel solvothermal method by using Li foil to reduce SiCl4 at 280℃ in autoclave.There are some interspace between these Si nanoparticles,which can accommodate the volume expansion of Si anode during discharge processes and thus avoid the fracture of electrodes.Meanwhile,the carbon layers on Si nanoparticles can effectively enhance the coulombic efficiency of Si anodes due to their high conductivity.Due to the merits of hierarchical nanostructure and carbon layer,the synthesized Si-20@C electrode showed excellent Li-storage performance(915.8 mA h/g at 3.6 A/g after 500 cycles and 746.2 mA h/g even at a high rate of 10.8 A/g).2.A novel Cu3Si@Si core-shell nanostructure was fabricated by using a solid state reaction,in which commercial CuCl powder reacted with Si nanoparticles at high temperature.We can control the formation of Cu3Si core in Si by tuning the reaction temperature and time,this phenomenon is similar to kirkendall effect.Cu3Si core can function as the machinery substrate that can support the volume variations of Si shell,ensure the integrality of electrode and further improve the electrochemical properties of Si anode.In addition,the doped Cu can improve the conductivity of Si based electrode,resulting in a lower electrochemical impedance and higher cycling columbic efficiency.Therefore,Cu3Si@Si core shell nanoparticles can maintain a high specific capacity of 903.6 mA h/g at 2.0 A/g even after 400 cycles as anodes for LIBs.3.We developed a novel catalytic chemical vapor deposition method by using the reaction between CuCl and Si under acetylene condition at a relatively low temperature of 400℃ to fabricate in situ formed carbon nanotube wrapped Si composites.The nascent Cu*created in the reaction between Si and CuCl acts as the superior active catalyst to in situ form carbon nanotubes on the surface of Si particles,and thus decreasing the carbon nanotubes growth temperature greatly from over 700℃ to 400℃.When the synthesized carbon nanotubes wrapped Si composites evaluated as the anode for LIBs,a high specific capacity of 1031.1 mA h/g can be collected at 1.8 A/g after 500 cycles,a stable rate capacity of 868.2 mA h/g can be reserved even at 10.0 A/g.The superior Li-storage capacity of composites can be mainly ascribed to the formed carbon nanotubes,which not only can relieve the volume change of active materials,but also can improve the conductivity of electrodes and finally improve the electrochemical performance of the composites.4.We designed a unique hollow and porous red phosphorus nanostructure that have not been reported yet.A novel wet chemical process was developed to synthesize the hollow red phosphorus nanospheres with porous shells(HPNs)by using NaN3 to reduce PCls in toluene solution at 280℃.According to the contrast experiments,we firstly proposed a gas bubble directed formation mechanism for the formation of HPNs.Meanwhile,the critical size of HPNs can be also modulated by tuning the pressure in the reaction system,which can be controlled by the amount of N2 that was released from NaN3.This hollow and porous nanostructure exhibits superior advantages when applied as the anodes for LIBs and SIBs.After fully lithiation,HPNs still can keep the integrality of hollow nanostructure without collapse,which suggests the good ability of HPNs to relieve the stress caused by volume expansion of phosphorus.Due to the above mentioned merits,HPNs showed excellent electrochemical performance when studied as anodes for both LIBs and SIBs.A highest capacity of 2142.9 and 2274.5 mA h/g can be restored for L1IBs and SIBs respectively.Even with high loading ratio of phosphorus,HPNs can still show the superior cycling performance.