Porous Si- And Ge-based Nanostructures Anodes For Lithium Ion Batteries:Synthesis, Characterization And Application

As one of the important portable batteries, lithium ion battery has become a worldwide research hotspot. Si and Ge are considered to be the ideal candidate anode materials for the next-generation battery, because of the low discharge potential, abundance on earth and the high theoretical capacity compared to graphite. The mobility and the electronic conductivity of Ge are much faster than that of Si, but the cost is much higher at the same time. Unfortunately, both the Si-and Ge-based electrodes suffer from fast capacity fading during cycles. The main challenges for the practical application of Si and Ge anodes are the huge volume change during insertion/extraction, which leads to an unstable SEI film, electrode pulverization and subsequent loss electrical contact between the active material and current collector. Therefor, the preparation of Si and Ge with excellent electrochemical performance is a frontline problem in the world.In this paper, preparation of porous Si and Ge materials were proposed to improve the cycling performance. The electrochemical performance of porous Si-and Ge-base anodes is studied and a new method to prepare porous Si-based materials which show enhanced electrical performance is invented. In detail, we obtained the following achievement.(1) Mg2Si was synthesized via a home-built continuous preparing apparatus using Si and Mg power as sources. The electrochemical performance of Mg2Si was studied. To improve the cyclic stability, carbon-coated Mg2Si was achieved by the decomposition of C2H2 gas. The chemical vapor deposition (CVD) carbon-coating with low carbon content can distinctly enhance the cyclic stability of Mg2Si materials.(2) The termal stability of Mg2Si is studied. A simple and large-scale way is reported to convert commercial Mg2Si to porous silicon, which can act as high-performance anode materials of lithium-ion batteries. The porous structure could give more sites and accommodate large volume changes during the lithium insertion and extraction process, which leads to high capacity and good cycling stability. The factors of particle size distribution, oxygen content and the heat treatment temperature were studied systemically. It provides the basis for large-scale application in the future. The Si@C composites which show better cycling performance were also prepared by different methods.(3) We demonstrate the synthesis of three-dimensional porous SiGe microparticles via the annealing of Mg2Si/Mg2Ge composites and subsequent acid pickling. When used as anode materials of lithium-ion batteries, the as-synthesized three-dimensional porous SiGe microparticles show the enhanced cycling performance due to the three-dimensional porous structures and the addition of Ge. The content of Ge in porous SiGe microparticles can be tuned by the initial content of Mg2Si and Mg2Ge in the composite, which can optimize the performance. For further enhance the performance, the carbon layer is coated onto the surface of porous microparticles. The effect of the coating of the carbon layer and the addition of Ge on the performance of three-dimensional porous SiGe microparticles has been investigated.(4) Porous Si was prepared via high-temperature annealing process and HF acid pickling process by using SiO as the source. After detailed study of SiO in different heat treatment, it found that at low temperature, Si-SiOx composite was obtained while in the high temperature, porous Si-SiO2-SiOx was achieved. The porous Si derived from SiO shows stable cycling performance, however, the capacity is relative low since the high content of SiO2.(5) Porous GeO2 was synthesized by changing the reactin atmosphere using Mg2Ge as the source. Porous Ge was achieved using Mg2Ge as the source, while the porous GeO2 was also obtained when the annealing conditions were changed. After carbon coating, the porous GeO2@C composite shows excellent cycling performance and rate performance.