Synthesis Of Si/C Composites And Their Electrochemical Properties

The emergence of portable electronic products such as mobile phones and electric vehicles has led to an increase in the demand for high-capacity energy storage devices,and lithium-ion batteries have attracted much attention as new energy storage devices.Graphite is currently the most widely used negative electrode material in lithium-ion batteries due to its low cost and high stability.However,the lower theoretical specific capacity of graphite(?375 m Ah·g^-1)limits its further development.Therefore,looking for new negative electrode materials is a key link in the development of lithium-ion batteries.Among tremendous amout of materials,Si and its oxide(SiO_x)are considered to be one of the most promising anode materials for lithium-ion batteries because of their extremely high theoretical specific capacity.This thesis is based on Si(and its suboxide SiOx),from the perspective of searching for a suitable composite matrix,the structure design and synthesis methods of the material are explored,combined with the biological template-oriented carbon,and the Graphite made by the reduction of graphite phase carbon nitride and new two-dimensional material MXene,designed and synthesized several new Si(SiO_x)/C composite materials,and studied the electrochemical performance of composite materials when used as anode materials in lithium ion batteries.(1)Using cabbage as a template and tetraethylorthosilicate(TEOS)as a silicon source,the SiO_x and C composite material(BS045-SiO_x)was obtained by infiltration infiltration,high temperature carbonization and subsequent ball milling magnesium thermal reduction;and then it was compared with unreduced samples and biocarbon,the morphology,structure and electrochemical performance are studied;the influence of the content of SiO_x in the composite material on the performance of the material is explored.The research results show that:SiO_x in the composite material exists in the form of nanoparticles(4.78±1.23 nm),uniformly dispersed in the carbon matrix,and the carbon matrix in the composite material has a higher degree of graphitization than the biocarbon obtained by direct heat treatment;The material exhibits good rate performance and long-life performance as andoe material for Li⁺batteries,and still retains a capacity of 794 m Ah·g^-1 after cycling for 150 cycles at a current density of100 m A·g^-1.The carbon matrix in the composite material provides a fast-migrating cross-linked network for Li⁺and electrons,and at the same time buffers the volume expansion of the SiO_x electrode material during the cycle,and improves the stability of the material.(2)Using grinding-assisted self-infiltration method to assemble dimeric cyanamide and mesoporous silica,after heat treatment(550?)and then high-temperature magnesium thermal reduction method(700?),SiO_x/graphite composite material(SiO_x/g-Carbon)was prepared.The composition structure,morphology and electrochemical performance of the material are compared with graphite reduced to g-C₃N₄ and SiO_x/g-C₃N₄.The results show that the obtained composite material has a thin-layer graphite structure,and no obvious crystal structure of SiO_x is seen.When used as anode material for lithium-ion batteries,after cycling for 150 cycles at a current density of 100 m A·g^-1,it retains a mass specific capacity of 500 m Ah·g-1 and a volume specific capacity of 111 m Ah·cm^-3.Due to the presence of graphite matrix,the migration efficiency of Li⁺(5.15×10^-14 cm²·S^-1)is higher than that of BS045-SiO_xcomposite material(3.83×10^-14 cm²·S^-1);Cyclic voltammetry results show that the SiO_x/g-Carbon composite material exhibits a higher pseudocapacitance ratio during charging and discharging.(3)On the basis of the two previous chapters,further attempts were made to introduce TEOS and template P123 between the MXene layers and condense to form porous SiO₂,and then obtain Si/C/MXene composite materials through high-temperature magnesium thermal reduction,and explore the magnesium thermal reduction process,The influence of Si source concentration on the morphology,structure and performance of the material.The results show that the increase in the mixing uniformity of the reactants and the temperature of magnesium thermal reduction before reduction is beneficial to the deep reaction of the process and can promote the formation of crystalline Si.The resulting composite material uesd as the anode of the Li⁺battery has a high initial specific capacity,and then the capacity decays rapidly.