Silicon And Germanium Based Nanomaterials:Synthesis And Research As Anode For Lithium-ion Batteries

Graphite is the commercial anode material of lithium-ion (Li-ion) batteries, however, their limited gravimetric capacity (372mAh g-1) has prompted intensive research for alternative anode materials with large capacity at low potentials due to the rapid technological evolution of portable electronic devices, power tools and electric vehicles. silicon (Si) and germanium (Ge) exhibit high specific capacity and energy density, which are considered as the potential alternative anode materials for Li-ion batteries. However, the large volume changes (>300%) during the lithium insertion and extraction brings cracking and crumbling of the electrode, which results in the capacity fades and poor cycling life of the Si and Ge anode. It is believed we can be effectively improved the cycling performance by the synthesis of Si and Ge nano/mirco-structures or composites because the nano/micro-structures and the second phase in the composites can buffer the volume change during the charge/discharge process. However, much effort should be devoted to further improve the charge/discharge performance of the Si and Ge anode.In this dissertation, several Si and Ge-based nano/micro-structures have been synthesized. For example, Si nanowires and porous Si particles were synthesized by the etching of Si wafer and powder, respectively. SiGe porous nanorod arrays were synthesized by using ZnO nanorod arrays as the template. NixSiy-SiGe core-shell nanoarrays were synthesized via a chemical vapor deposition and co-sputtering method. The porous Ge particles were synthesized by thermal decomposition of Mg2Ge and acid washing. The above-mentioned materials were tested as anode materials of lithium ion batteries. The main innovative results are listed as follows:(1) Si nanowires and porous Si particles were massively synthesized by electroless etching of Si wafers and metallurgical Si powder. These two Si materials show the porous structures, thus can buffer the volume change during the charge/discharge process and lead to the improved cycling performance compared to bulk Si materials. Moreover, Si nanowires show the better performance than porous Si particles because of the1D nanostructures.(2) SiGe nanorod arrays were synthesized by using ZnO nanorod arrays as sacrificial templates via a co-sputtering method. When used as anode materials of Li-ion batteries, the SiGe nanorod arrays show the better cycling performance and higher reversible capacity than corresponding SiGe film and Si porous nanord arrays. It is believed that nanorod array structures can buffer the volume change and enhance the adhension between the nanorods and the current collector thus can show the enhanced performance. Moreover, the addition of Ge can further enhance the conductivity and improve the mobility of lithium ions, which can further enhance the charge/discharge performance.(3) NixSiy-SiGe core-shell nanowire arrays were synthesized by chemical vapor deposition and subsequent co-sputtering methods. When used as anode material of Li-ion batteries, the core-shell nanowire arrays show the better cycling performance and higher capacity than corresponding SiGe film and NixSiy-Si core-shell nanowire arrays. We believe that the core-shell nanowires own the good conductivity and good adhension with the current collector. Moreover, the addition of Ge can enhance the conductivity and improve the mobility of lithium ions, which can further enhance the charge/discharge performance.(4) Ge porous particles were synthesized by the thermal decomposition of Mg2Ge and subsequent acid washing. After the carbon coating, the as-synthesized Ge@C porous particles exhibit improved cycling performance than bulk Ge, bare Ge porous particles and carboneous materias. The porous structures and the carbon layer can buffer the volume change during the charge/discharge process, which may be responsible for the enhanced performance.