Preparation And Properties Study Of High-Capacity Silicon Suboxide Anode Materials For Lithium-Ion Batteries

Although silicon suboxide is considered to be a new generation of high-capacity anode materials for lithium-ion batteries,it still faces many obstacles on the road to commercialization.Due to the huge volume expansion during the charge/discharge process and the continuous growth of the unstable solid electrolyte interface（SEI）on the surface of the material,the rapid capacity decay of silicon suboxide anode materials in application is the major problem to be overcome.In this thesis,aiming at the application bottleneck that rapid capacity decay of silicon suboxide anode materials in charge/discharge process,a series of silicon suboxide based anode materials were prepared.The composition,structure,morphology and electrochemical performance of the prepared materials were characterized and tested,and their electrochemical mechanism was investigated.In order to find a low-cost,simple and controllable method to construct a material structure that can cope with the volume expansion of silicon suboxide,white onion was selected as the biomass material,and the biomass-derived hierarchical porous Si O_x/C anode material was obtained through activation treatment,pre-calcination and aluminothermic reduction.The hierarchical porous structure provides buffer space for the volume expansion of silicon suboxide during the charge/discharge process,thereby improving the cycle stability of the material.The research found that the hierarchical porous structure of Si O_x/C can be effectively controlled by different activation treatment temperatures.The Si O_x/C-6 finally obtained by the activation temperature of 600℃showed the best electrochemical performance.It still has a reversible specific capacity of 926.5 m Ah·g^-1 after 100 cycles at a current density of 0.1 A·g^-1.In order to improve the stability of the SEI layer,SiO_x@C was prepared by the hydrolysis of tetraethyl orthosilicate and the thermal decomposition of sucrose,and Li F was introduced during the preparation of Si O_x@C to prepare a Si O_x/Li F@C composite anode material.After the introduction of Li F,the initial Coulombic efficiency,cycle performance and rate performance of Si O_x/Li F@C are improved compared with Si O_x@C.The Si O_x/Li F-0.15@C composite anode material prepared when the mass ratio of Si O₂ to Li F is 1:0.15 has the best performance.Its initial discharge specific capacity at 0.1 A·g^-1 is 1211.6 m Ah·g^-1,initial Coulombic efficiency is 72.8%,and reversible capacity after 100 cycles is 711.0 m Ah·g^-1.By comparing the composition changes of the SEI layer after cycling between Si O_x@C and Si O_x/Li F-0.15@C,it is found that the composition of the SEI layer changed less after the introduction of Li F,indicating that the introduction of Li F is beneficial to improve the stability of the SEI layer.In order to further improve the interfacial stability of the silicon suboxide anode material,the carbon coating amount of Si O_x@C was reduced,and PTFE/MWCNT was used to secondary coating to obtain Si O_x@C@PTFE/MWCNT composite anode material.When heat treatment at 400℃,the molten PTFE is conducive to bonding with the carbon coating layer,and the F element in PTFE can diffuse and migrate to the carbon coating layer to form F-doping.The PTFE/MWCNT outer coating reduces the occurrence of material surface side reactions during the cycling and improves the interfacial stability.The synergistic effect of the PTFE/MWCNT coating and the carbon coating further improves the electrochemical performance of the material.In the process of material preparation,the amount of multi-walled carbon nanotubes was fixed,and the amount of PTFE was optimized.It was found that the performance of the material was the best when the amount of PTFE was 0.04 g.Its initial discharge specific capacity at 0.1 A·g^-1 is 1458.0 m Ah·g^-1,initial Coulombic efficiency is74.8%,and reversible specific capacity after 100 cycles is 905.4 mAh·g^-1.