Controllable Synthesis And Properties Of Silicon-based Anode Materials For Lithium-ion Batteries

Silicon-based anode materials become a hotspot of research on lithium-ion batteries due to its advantages of high theoretical capacity and rich resources.However,the volume expansion of silicon-based anode materials in the process of Li⁺intercalation/deintercalation will cause battery failure,poor cycle life and other problems.In addition,the poor electrical conductivity of silicon-based material is not not beneficial for ion and electron transmission.In order to solve these problems,this paper starts from the dimension design,structure and composition of silicon-based anode materials,using the strategy of nano and composite modification to improve this defect.Firstly,SiO₂nanotube anode materials were prepared by hard template method.The effects of SiO₂nanotube wall thickness and carbon coating on the structure,morphology and electrochemical properties of SiO₂nanotube anode materials were studied.Secondly,a three-layer ZnO@SiO₂@C nanorods composite with an inner layer of ZnO,an intermediate layer of SiO₂and an outer layer of carbon was designed and synthesized.The effects of ZnO and carbon layer on the structure,morphology and electrochemical performances of SiO₂nanotube anode were mainly investigated.Finally,a new strategy of synthesizing silicon nanotube sheet by thermal cracking method is put forward,which uses the difference of thermal expansion coefficient to reduce SiO₂nanotubes by magnesium thermal reduction method to synthesize Si nanotube sheet anode material with special morphology and the influence of thermal burst process on the anode structure,morphology and electrochemical performance of Si nanotube sheet was explored.The main research contents and results are as follows:（1）Using ZnO as template and TEOS as silicon source,SiO₂NTs anode was prepared by template method and the wall thickness of SiO₂NTs was controlled by the stirring time.The specific discharge capacity of SiO₂NTs-1 h can reach 344.3 m Ah g^-1after400 cycles at the current density of 1 A g^-1.Then,SiO₂NTs@C composites were prepared by using glucose as carbon source,in which SiO₂NTs@C3 still has a high specific discharge capacity of 526.3 m Ah g^-1after 500 cycles at a current density of 1A g^-1.Even at a high current density of 2 A g^-1,the first specific discharge capacity can still reach 436.4m Ah g^-1,showing excellent cycle stability and rate performance.（2）In order to fully release the lithium storage capacity of SiO₂anode material and effectively alleviate the volume expansion of SiO₂during the cycle,a three-layer rod-shaped SiO₂-based composite material with ZnO as the inner layer,SiO₂as the middle layer and carbon as the outer layer was designed and synthesized.The material was used in lithium-ion batteries.The specific discharge capacity of ZnO@SiO₂@C2 is as high as 727.1 m Ah g^-1after 200 cycles at the current density of 0.2 A g^-1.The specific discharge capacity is 629.5 m Ah g^-1after 250 cycles at the current density of 1 A g^-1and the specific discharge capacity can still reach 334.5 m Ah g^-1at the current density of 5 A g^-1.（3）In this chapter,a new strategy of synthesizing Si nanotube sheets by thermal burst caused since the difference of thermal expansion coefficient is put forward.ZnO template with larger thermal expansion coefficient makes SiO₂crack at high temperature and SiO₂reduces into Si with special morphology of nanotube sheets by magnesiothermic reduction.Si nanotube sheets effectively alleviates the volume expansion of Si during charge and discharge process.The first discharge capacity reaches 2416 m Ah g^-1at the current density is 0.2 A g^-1and the discharge specific capacity is 297.4 m Ah g^-1after 400 cycles at 1 A g^-1.After carbon coating,the specific discharge capacity of 695.9 m Ah g^-1is maintained after 400 cycles at the current density of 1 A g^-1.Even at the high current density of 5 A g^-1,the specific discharge capacity can reach 712.7 m Ah g^-1.After assembling the full battery with Li Fe PO₄,the first specific discharge capacity is 98.8 m Ah g^-1at the rate of 0.5 C and the high capacity retention rate is 81.78%after 50 cycles.