Preparation And Performance Of Si-based Nanostructured Materials For Lithium-ion Batteries

Lithium-ion secondary battery has been considered as the most promising power sources in the 21st century. The commercial lithium-ion batteries usually select graphite as the anode material. The capacity of such material is merely 372 mAh/g, which is insufficient for meeting the needs of future electronic equipment. The need for high-energy and lighter lithium-ion batteries product seeks for new large capacity anode material candidates. Silicon-based materials are promising candidates for lithium-ion battery anode electrodes for their extremely high theoretical capacity (approximately 4200 mAh/g, with the formation of Li4.4Si alloy). However, the dramatic volume swing (>300%) during alloying/de-alloying processes causes the pulverization of the electrode materials and breakdown of the electrically conductive network, which restricts the cycle performance seriously. In recent years, with the rapid development of nano-technology, the application of silicon-based nano-materials in the lithium-ion battery has progressed a lot. They are expected to be used as a new generation of high-performance anode materials instead of carbon materials. As lithium-ion battery anode electrodes, silicon based nano-materials have some unique physical and chemical properties, such as better accommodation of the lithiation and delithiation strain, larger surface area, shorter transport length, high reversible capacity and long cycle life. These properties can significantly improve specific capacity and high rate performance of lithium-ion batteries. This study is mainly devoted to the fabrication of silicon-based nanostructure (silicon nanowires and porous silicon microspheres) at low cost and high throughput, the carbon coating, and the application in lithium-ion battery. The detailed contents and results in this dissertation include:1. Large-area, high quality, highly oriented single-crystalline silicon nanowires (SiNWs) were prepared by using metal-assisted etching method of silicon in HF/H2O2/H2O solutions with Ag nanoparticles as catalyst agents at near room temperature. The influence of the etching solution composition and the etching time in the etching process was studied. Besides, mesoporous carbon was filled in the SiNWs by using the resol/surfactant mixture to fill the SiNWs. Such structure greatly enhances the conductivity as well as structure stability of SiNWs, and will be applied in lithium-ion batteries.2. Magnesiothermic reduction technology was used for converting nanostructured silica OPAL into porous silicon microspheres. The as-prepared samples possessed crystalline characteristics according to XRD analysis. SEM images showed that the silicon microspheres has an average size of about 400 nm and possessed nanoporous structure. In addition, Si/C nanocomposites were prepared by using the resol to fill the nanopores of the silicon microspheres and further carbonization. The electrochemical test indicated that the carbon coating greatly enhanced the structure stability and electrical conductivity of silicon microspheres. The Si/C nanocomposites exhibited a stable reversible capacity of 1200 mAh/g after 20 cycles.3. The nanostructured Si@C spheres with silicon cores and carbon shells were prepared by coating silicon nano-particles with resol and further carbonization at 700℃. These Si@C core-shell spheres exhibited an initial discharge specific capacity of 1600mAh/g in the potential range of 1.1-0.01 V. After 30 cycles, the capacity of the Si@C core-shell spheres anode stabilized reversibly at about 700mAh/g.