Controlled Synthesis Of Nanosized Si By Magnesiothermic Reduction From Diatomite And Preparation Of Silicon?carbon Anode Composite Materials

Silicon materials have been the most promising anodes due to the high theoretical specific capacity of 4200 mAh g^?1,which is much higher than the commercial graphite anode materials.Besides,silicon has a moderate working potential of⁰.4 V?vs Li/Li⁺?.At the same time,Si material is the second richest element in the crust of the earth,and it is cost?effective with abundant reserves.The main challenge of Si anode is the large volumetric changes during the lithium insertion/removal process,which causes particles fracture and pulverization and damage of solid electrolyte interface?SEI?.Therefore,Si anode would suffer from a limited cycling life.Besides,Si shows a low electronic conductivity and sluggish Li?ion transportation rate,and thereby poor rate capability.In this thesis,we will report a controlled,cost?effective route to prepare nanoscale Si by magnesiothermic reduction using diatomite as the silicon source.The obtained nanoscale Si was used to prepare Si@C compositesThe work analyzed the basic principle of the magnesiothermic reduction,explored the feasibility of process,and confirmed the process steps of synthesizing nano?silicon first and then performing silicon?carbon composites.The influence of temperature,time and reaction atmosphere on electrochemical properties of Si during magnesiothermic reduction was studied.The nanoscale Si prepared under optimized conditions can deliver a high initial specific capacity(3053 mAh g^?1 on discharge,2519mAh g^?1 on charge)with a high first coulombic efficiency?82.5%?.When using sand?milled diatomite as precursor,the obtained nanoscale Si exhibits a well?dispersed morphology and has a higher first coulombic efficiency?85.6%?.The synthesized nano?silicon particles have little agglomeration,high capacity and coulomb efficiency,which is beneficial for the subsequent preparation of silicon?carbon composites.In this thesis,the influence of silicon content,drying process and different carbon sources on silicon?carbon composite materials was studied.The nanoscale Si from sand?milled diatomite was first mixed with the artificial graphite by drying ball milling.Then,the ball?milled product,polyvinylpyrrolidone?PVP?and glucose were mixed in DI water followed by wet ball milling.The ball?milled product was spray dried to prepare precursor for Si@C material.The Si@C material was obtained by high?temperature carbonization followed.The Si@C composite demonstrates a high specific capacity with the first discharge capacity of 849 mAh g^?1 and maximum charge capacity of 769 mAh g^?1 at a current density of 100 mA g^?1.At the same time,the Si@C composite has good rate capability(587 mAh g^?1 at 500 mA g^?1),and a long cycle life(480 mAh g^?1 after 200 cycles at 500 mA g^?1).Based on the preparation of Si@C composite materials,graphene was added in the preparation process to improve the conductivity of the material.The effects of single electrolyte additive of fluoroethylene carbonate?FEC?,vinylene carbonate?VC?and diethyl pyrocarbonate?DEPC?on the electrochemical performance of Si@C composites were studied.At a current density of 100 mA g^?1,the maximum charging capacity of the Si@C composite is 606 mAh g^?1 with 5%FEC,544 mAh g^?1 with5%VC,and 490 mAh g^?1 with 5%DEPC,respectively.The effects of dual electrolyte additives were studied.When the dual electrolyte additives are 3%FEC+2%VC and3%FEC+2%DEPC,the maximum charging capacity of the Si@C composite is 500mAh g^?1?3%FEC+2%VC?and 768 mAh g^?1?3%FEC+2%DEPC?at the current density of 100 mA g^?1,respectively.When the dual electrolyte additives?3%FEC+2%DEPC?is added,the Si@C composite material has a charging capacity of 593 mAh g^?1 after 100 cycles at a current density of 100 mA g^?1 and a capacity retention rate of77.2%,which demonstrates excellent cycle performance.Besides,the Si@C composite material has a good rate performance with 684 mAh g^?1 at the current density of 500mA g^?1.