Synthesis And Lithium Storage Properties Of High Performance Silicon/Carbon Anode Materials

Silicon is considered as an ideal anode material for the next generation of lithium ion batteries?LIBs?due to its high theoretical capacity,low Li-insertion/extraction potential and natural abundance.However,Si suffers from large volume changes?300%?and poor electrical conductivity,resulting in the poor cycle stability.Reducing the size of Si to the nanoscale,introducing pores and combining with carbon matrix are effective ways to improve its electrochemical property.So far,magnesiothermic reduction,metal assisted chemical etching?MACE?,chemical vapor deposition?CVD?,etc.,are widely used to prepare silicon-based materials.However,it remains several challenges in these methods,such as uncontrollable reaction,dependence on highly corrosive HF,high cost,use of toxic gas sources and the like,which have seriously hindered their commercialization.In this dissertation,new strategies for synthesizing silicon/graphene?Si/r GO?and silicon/carbon nanotubes/porous carbon?Si/CNTs/C?materials have been proposed to solve the aforementioned drawbacks.And the relationship between their structures and electrochemical properties has been studied.The main contents are as follows:?1?In view of the high enthalpy change and uncontrollable reaction of magnesiothermic reduction,a strategy for preparing Si/r GO composite by magnesium hydride?Mg H₂?thermal reduction was developed.This method has lower reaction temperature and less heat accumulation effect as compared to magnesio-thermic reduction,can efficiently convert the nano-Si O₂ loaded on r GO surface into fine Si nanoparticles at 500 ^oC,and hardly affects the state of r GO,showing the excellent ability to preserve morphology characteristics.Compared with the counterpart obtained by magnesiothermic reduction,this Si/r GO composite material has obviously improved electrochemical performance,realizing the synergistic effect of nano-Si and r GO.It exhibits a reversible capacity of 894 m A h g^-1 over 100 cycles at 0.2 A g^-1 and delivers a superior rate capability with 513 m A h g^-1 at 5 A g^-1.?2?Si/CNTs/C composite was synthesized by one-step thermal reaction at 650 ^oC using Mg₂Si and Li₂CO₃ as silicon and carbon sources,respectively.The process can realize the synchronous generation of Si,CNTs and porous carbon without catalyst.Microsturcture characterization shows that the obtained material has large specific surface area with 40 wt%silicon content,carbon and CNTs mainly grown on the Si surface have low graphitization degree.In the study of its reaction mechanism,it is found that the formation of Li₄Si O₄ is inevitable,which will cause the loss of silicon in the product.According to the morphology evolution of reactants,it is speculated that the nucleation process of CNTs is catalized by active Mg and the extension of CNTs is driven by structural recombination of amorphous carbon.As an anode for LIBs,Si/CNTs/C composite has high capacity(702 m A h g^-1 reversible capacity after 300cycles at 0.2 A g^-1),excellent rate performance(420 m A h g^-1 at 5 A g^-1 and long cycle life?over 1500 cycles?,the coulombic efficiency is maintained at 99%.