| Lithium ion batteries have a variety of applications ranging from portable electronic devices to electric vehicles. The most common used anode materials in lithium ion batteries are still carbonaceous materials, however, alternative anode materials with higher specific capacities are in great demand to increase the energy density of batteries. Meanwhile, safety concerns of carbonaceous materials due to their low lithiated potentials close to lithium also require searching for new anode materials. Among them, tin provides much higher theoretical capacity (994 mAh g-1, 7200 mAh cc-1) than graphite (372 mAh g-1,837mAh cc-1), and behaves a slightly higher discharge voltage (0-400 mV) than metallic lithium which could reduce safety concerns during cycling, however, pure tin presents a limited cycle life due to pulverization and delamination from copper foil current collector caused by volume expansion and contraction associated with the lithiation and delithiation. In this thesis, some work have been done to improve tin's cycling performance by tunning of its structure and morphology. The main contents are as follows:(1) A binder-free three-dimensional (3D) porous Cu6Sn5 anode was prepared for lithium ion batteries. In this novel approach, tin was deposited by electro less-plating on copper foam which was served as anode current collector as well as the source of copper for Cu6Sn5 alloy formation. With optimized post-treatment condition, Cu6Sn5 alloy with thickness of 1.2?m was formed on the surface of copper foam network.3D porous Sn-Cu6Sn5 and Cu3Sn-Cu10Sn3-Cu6Sn5 composite anodes were also prepared for comparison. Electrochemical tests showed that 3D porous Cu6Sn5 anode exhibits the best electrochemical performance in terms of specific capacitance and cycleability, which delivers a rechargeable capacity of 404 mAh g-1 over 100 cycles. The cycling performance may be further improved by employing a copper foam current collector with smaller pores and larger surface area which requires further investigation.(2) Core-shell Cu6Sn5-coated TiO2 nanotube arrays as a novel design for anode material in lithium ion batteries was prepared by electroless plating techniques. In this design, Cu6Sn5 layer was coated on the inner wall surface of TiO2 nanotubes, the hollow structure of the nanotubes was still remained although the inner diameter of the nanotubes decreased from 100 nm to 50 nm. The as-prepared Cu6Sn5-coated TiO2 nanotube arrays combines the merits of the high specific capacity of tin and the structure stability of TiO2 nanotubes, and the nanotublar structure allows both facile strain relaxation of tin and rapid mass transport, leading to greatly enhanced electrochemical performances in terms of specific capacity, cycle life and rate capability. Owing to the versatility of our morphology design, the preparation process by electroless plating techniques is also helpful for making other nanotublar composite materials and 3D batteries.(3) Cu6Sn5@CNTs hybrid composite, namely Cu6Sn5 overlaying on the exterior surface of carbon nanotubes, was prepared by electroless plating techniques. As for this material, there are several factors favorable to the improvement of cycling stability of tin:?Tubular structure of CNTs could adsorb the reaction-induced stress;?The 3-D porous structure formed by CNTs could also accommodate drastic volume variation during electrochemical reactions;?The Cu6Sn5@CNTs anode has fibrous textures that can hinder the cracking or crumbling of the electrode. However, due to the stripping of Cu6Sn5 layer from CNTs caused by reaction-induced stress, we did not obtain an ideal cycling performance.(4) Besides the research work on tin-based anode materials, some work on electrochemical capacitor has also been done. A hierarchical porous MnO2-based electrode was prepared and its electrochemical performance for electrochemical capacitors was investigated. In this work, porous MnO2 film with pore size of 2-3 nm in diameter was deposited on a three-dimensional porous current collector by cathodic electrodeposition associated with subsequent controlled heat treatment at 200?for 2 hours. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) showed that the heat treatment has a great effect on the formation of the porous structure of MnO2 layer, and the disordered porous structure was caused by dehydration during the heat treatment. Cyclic voltammetry (CV) and galvanostatic charge-discharge tests showed that both energy and power densities are enhanced due to the unique hierarchical porous structure. The electrode delivers a high specific capacitance of 385 F g-1 at a high current density of 5 A g-1 within a potential window of -0.05?0.85 V, and also exhibits excellent rate capability and electrochemical stability. |
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