Synthesis And Application Of Si-based Nanohybrid Li-ion Battery Anodes

Silicon?Si?is regarded to be one of the most promising anode materials for Li-ion batteries due to its ultrahigh specific capacity?4200 mAh/g?compared with the commercially used graphite anode.In addition,Si holds the advantages of abundance,evoriment-friendly and high safety.However,it suffers from the dramatic volume expansion?ca.400%?during the lithiation process,which leads to poor cyclic performance.Moever,as a semiconductor,Si has low electron conductivity.To improve the electrochemical performance of Si,downsizing the size of Si to nanoscale range,compositing with buffer matrix and constructing hierarchical structure are the commom methods.Based on these methods,this thesis has adopted compositing with metal and tiatamium dioxide?TiO₂?to accommodate the volume expansion of Si,thus improving the cyclic stability of Si-based anode.Firstly,by using?3-Mercaptopropyl?trimethoxylsilane?MPTMS?coupling agent as a cross-linker,the Si/Ag/MPTMS nanocomposite was prepared.With superior mechanical strenhth and electron conductivity,silver effectively absords the mechanical stress generated by the volume expansion of Si and improve the conductivity of the Si-based materials.Besides,the thiol functional group on one end of the MPTMS has a high affinity to bind the silver nanoparticles;the silane group on the other end can react with the hydroxyl group on the silicon surface to form Si-O-Si bond.Thus,compared with the simple physical compositing,the force by forming chemical bonds via MPTMS between Si and Ag nanoparticles is stronger,which effectively matains the structure integrity.When the mass loading is above 1 mg/cm²,the capacity of Si/Ag/MPTMS reached 517 mAh/g after 100 cycles at 200 mA/g while the capacity of Si/Ag dropped to nearly zero.In addition,the capacity of Si/Ag/MPTMS reached 1100 mAh/g at high current density of 1000 mA/g.Secondly,Si/TiO₂/C nanocomposite was prepared by combining functional solvothermal reaction and calcination at high temperature.A large amount of 7 nm TiO₂partilces are homogeneously dispered around the Si.They can absorb the mechanical stress caused by the volume change during the cyclic process.Compared with the Si/C and TiO₂/C nanocomposites,the Si/TiO₂/C has the best electrochemical performance.The synthesis condition,such as calcination temperature,the content of TiO₂ and mechanical stirring affect the morphology including formation of crystal and the size of TiO₂ nanoparticles,which are related to the electrochemical performance of Si/TiO₂/C nanocomposite.It was revealed that the Si/TiO₂/C nanocomposite with 72%TiO₂ which was prepared under the condition of stirring at the solvothermal process and calcined at 800oC has the best electrochemical performance.The initial discharge capacity was 658 mAh/g with the initial coulombic efficiency of 83%.When cycled for 80 times,the discharge capacity retained at 538 mAh/g.At the current density of 2A/g and 5 A/g,the specific capacities were 250 mAh/g and 120 mAh/g,respectively.