| Silicon is one of the most promising anode materials for lithium ion batteries, because it has the advantages of the highest specific capacity, low charge/discharge potential plateau, abundant resources and operation safety. However, silicon itself suffers the problem of significant volume expansion and contraction during Li insertion/extraction. Besides the volume change issue, the Si inherits poor electrical conductivity, leading to poor cyclic stability. Recent work has shown that there are mainly three methods to improve the cycling performance of the silicon based anode material. The first method is reducing the size of silicon domain to the nanoscale, such as nanoparticles, nanowires and nanotubes, to decrease the degree of absolute volume change. The second method is compositing silicon with buffer material, such as carbon, metal and non-metal, to enhance the ability of electrical conductivity and suppression the growth of SEI film.The third method is designing multi-level structure to restrain the volume expansion. Ceramics are often used as buffer material for silicon based anode material byabsorbing stress. But the ceramics normally don’t have activity for lithium-ion insertion, thereby decreasing the total capacity. Furthermore, the ceramics and carbon matrix exhibit interphase separation and poor binding ability. Therefore, they cannot effectively absorb the stress generated by the volume expansion of silicon. In order to tackle the above problems, we have firstly explored different binders and the optimization of the binder content. Then we used the unsaturated polyester thermoset resins as carbon source and reaction medium, and chose the silane coupling agent as ceramic precursor as the same time. Then, a series of silicon/carbon and silicon/silicon oxycarbide/carbon nanoconposites were successful synthesized by high-temperature pyrolysis methods. The influence of the additive content on the electrochemical performance has also been discussed. It has been demonstrated that those silicon/silicon oxycarbide/carbon nanoconposite materials can alleviate the significant volume variation during Li-ion insertion/extraction and improve the electrochemical performance of silion-based anode. The main results of the thesis are given as follows:1. The results indicated that with the optimized binder content of 20%, the rechargeable capacity and the first coulombic efficiency of the pure silicon nanoparticles based anode can reach maximum. It has been found that with the magnesium alginate binder, the cyclic stability and rate performance of the Silicon based lithium-ion battery has been significantly improved, After 100 cycles at 200 mAh?g-1g, reversible capacity of around 1500 mAh?g-1 is still retained. The structures of the electrode remain keep intact after cycling。2. With high-temperature pyrolysis method, we successful synthesized the Si/C nanocomposites using unsaturated polyester thermoset resins as carbon precursor. At 200 mAh?g-1g galacanostatic charge/discharge, the sample of Si/C-0.1, Si/C-0.2, Si/C-0.4 and Si/C-0.8 Si/C nanocomposites anode materials showed the first discharge specific capacity of 493 mAh?g-1, 1395 m Ah?g-1, 1267 mAh?g-1and 1509 mAh?g-1respectively. With the increasing content of silicon in the Si/C nanocomposites, the stability of cycling performance is improved successively. After 200 cycles, the capacity retention of the Si/C nanocomposites is 93.6 %, 16 %, 22.7 % and 28.2 % respectively. Among all the samples, Si/C-0.8 has exhibited the best electrochemical properties. Cycling under the 1000 mAh?g-1, the reversible specific capacity still matained at 600 mAh?g-1.3. Based on the study of Si/C nanocomposites, we further successful synthesized the Si/SiOC/C nanocomposites as anode materials using the silane coupling agent as silicon oxycarbide ceramic precursor. The result demonstrated that the addition of ceramic nanoparticles improved the mechanical properities of composites in some degree. The reversible specific capacity of the system of Si-0.2 and Si-0.8 can be maintained at 220 mAh?g-1 and 380 mAh?g-1 under cycling at 2000 mAh?g-1. When the current density is further increased to 5000 mAh?g-1, the reversible specific capacity of the system of Si-0.2 and Si-0.8 can be still kept at 150 mAh?g-1, higher than the Si/C nanocomposites under the same condition. |
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