The Preparation,Characteration And Lithium Storage Of Si-based Material

Accompanied by rapid economic and social development, energy as the basic driving force for modern society shows of great importance. The massive consume of the traditional fossil fuels has posed serious problems of both environmental pollution and ecological destruction. Energy and environmental issues have been the two important problems that human society must pay attention to and have to solve. Development of new energy sources and energy storage become the focus for the world. Because of its abundant, clean and non-polluting, the develop of solar cells is a top priority of the strategy. On the other hand, electricity can not be directly stored. In the development of storage batteries, new high-capacity, high power and long-life lithium-ion batteries become today's focus. Si thin films and Si nano-structured materials have become the dominant materials of the next-generation thin-film photovoltaic cells. At the same time, Si nano-materials are research focus for the high energy density, high-power lithium-ion battery anodes.In this thesis, the crystallized silicon films and Si, Si1-xGex nanorod arrays were prepared at low temperature by plasma enhanced chemical vapor deposition (PECVD) method. The influences of growth conditions on microstructure, morphology and the opto-electronics properties of the samples were investigated carefully. The electrochemical properties of the Si based nanorod arrays as anode were evaluated by galvanostatic battery tastings for Li ion batteries. It was showed that the electrochemical performance of Si1-xGex anodes can be improved by regulating the Ge content in the Si1-xGex. The main contents of the paper and the corresponding results are summarized as follows:1. The microstructures and properties of the prepared crystallized silicon filmsThe crystallized silicon films were deposited on glass substrates by the conventional radio frequency (13.56MHz) and very high frequency (81.36MHz) PECVD method. Silane diluted in H2or H2+Ar was used as the source gas for preparation of crystallized Si films by conventional PECVD. The results showed that the addition of Ar in dilution gases efficiently improves the deposition rate and crystallinity due to an enhanced dissociation of the source gas as well as the energy of deexcitation of Ar*released within the growth zone. Meanwhile, excess Ar dilution will lead to the polymerization and also a bad passivation of the hydrogen on the dangling bonds which causes the increase of the optical gap and defect states in the μc-Si films. The crystallized Si films can be deposited under a lower H2dilution by the VHF-PECVD method. It is found that the crystallized Si films showed better optical properties due to the better compact and the lower defect density by using very high excitation frequency of81.36MHz.2. The preparation and antireflection of the Si nanorod arraysSi nanorod arrays were prepared by the VHF-PECVD on the electrodeposited Ni nanocone arrays. By adjusting the preparation conditions of the Ni nanocone arrays, the morphology and the density of the Si nanorods can be controlled. It is found that the well-separated nc-Si:H nanorods provide excellent low reflectance over a wide wavelength range of300-1100nm due to enhanced light scattering and appropriate modulation of the effective refractive index between air and nc-Si:H nanorod structures. The novel nc-Si:H nanorod arrays may be suitable for low-cost solar cells devices and other applications thanks to the excellent antireflection.3. The electrochemical performance of Si, Ge and Si1-xGex nanorod structuresSi, Ge and Si1-xGex nanorod structures were prepared by the VHF-PECVD on the electrodeposited Ni nanocone arrays. The obtained Si, Ge and Si1-xGex samples were directly assembled against Li metal into half cells of CR-2032with Si, Ge and Si1-xGex acted as active materials, respectively. The electrochemical performance was evaluated by the galvanostatic battery testing. It is found that the Si nanorod anode shows a high reversible capacity of3400mAh/g at C/5rate. A capacity of3249mAh/g at C/5rate is attained with retention of95%after30cycles. The result is a great improvement compared to the Si film anode. But the capacity of Si nanorod shows a drastical decrease after30cycles. Compared to Si anode, Ge anode shows a stable capacity even at C/2rate for50cycles with a capacity retention of100%, but the capacity is only460mAh/g. Herein, Si1-xGex nanorod electrodes were applied for Li-ion batteries. It is found that the capacity retention is greatly improved with increasing Ge content of the Si1-xGex electrode. For Sio.71Geo.29nanorod electrodes, The charge capacity is above1000mAh/g even after65cycles at0.5A/g. The reversible charge capacity is about1500mAh/g even at high rate of4A/g. The capacity of Sio.71Geo.29is still as high as1100mAh/g after50cycles. The binder-free Si1-xGex nanorod electrodes could be further improved for a promising candidate for high-capacity and high-power Li-ion batteries.