锂离子电池锡基负极材料的制备及储能行为研究

【作者】 薛雷刚；

【导师】 余爱水；

【作者基本信息】 复旦大学 ， 物理化学， 2011， 博士

【摘要】 目前商品化锂离子电池负极材料所广泛使用的碳材料存在比容量低(372mAh g-1,837mAh cc-1)及安全性欠佳的问题。金属锡具有更高的理论比容量(994mAh g-1,7200 mAh cc-1)且无安全隐患,是一种很有发展潜力的负极材料,但由于在脱嵌锂过程中伴随着巨大的体积变化,导致活性物质粉化、脱落,从而与集流体失去电接触,容量迅速衰减。这成为制约金属锡作为锂离子电池负极材料的致命缺点。为了提高锡的循环稳定性,国内外科研工作者采用的方法一般有：①引入非活性元素,制备活性/非活性合金体系；②引入其它活性元素,制备活性/活性合金体系；③锡基负极材料的纳米化；④锡基负极材料的薄膜化；⑤锡基纳米材料的碳包覆；⑥锡基氧化物；⑦锡基材料的形貌控制,如采用多孔材料和空心核壳结构等,但单一方法很难取得显著的效果。本论文针对锡铜合金体系,综合薄膜化、纳米化、形貌控制等方法制备新型锡基负极材料,希望可以缓冲锡的体积膨胀,从而提高锡基材料的循环稳定性,开展的主要研究工作如下：(1)综合利用活性/非活性体系、薄膜材料和多孔材料的优点,制备了三维多孔的锡铜合金负极。以孔径为100～200μm的多孔铜为集流体和铜源,使用化学镀锡工艺均匀包覆厚度为0.7μm的金属锡层,通过在真空条件下150℃热处理2小时,得到了三维多孔Cu6Sn5合金负极,合金层的厚度为1.2μm。不同热处理条件下还制备了三维多孔结构的Sn-Cu6Sn5和Cu6Sn5复合电极。在0～2.0 V的电位区间,电流密度100 mA g-1的条件下进行恒流充放电测试,结果显示三维多孔Cu6Sn5负极显示出最为优异的电化学性能,循环100圈后仍保持404 mAh g-1的比容量。相比于Sn-Cu6Sn5复合负极,Cu6Sn5更好的循环稳定性归因于Cu6Sn5的形成及活性材料与集流体之间更好的结合力。而Cu6Sn5复合负极较差的循环性能则因为其较差的可逆性,这可以从其放电曲线中得到较好的解释。(2)纳米材料虽然因其更小的应力,可有效缓冲锡基材料充放电过程中巨大的体积变化,但纳米材料在充放电过程中容易发生团聚,导致循环性能变差。因此,设计并合成了一种新型的空心核壳结构Cu6Sn5@Ti02纳米管阵列负极材料。首先在60 V恒电位条件下氧化钛箔制备长度为2μm,管径为100 nm的Ti02纳米管阵列,然后通过化学镀铜和化学镀锡工艺将Cu6Sn5合金层包覆在Ti02纳米管内壁上,化学镀后Ti02纳米管的管径从100 nm减至约50 nm,但仍然保持中空的结构。这种独特的“管中管”结构有望可以缓冲锡的体积膨胀、防止锡基颗粒的电化学团聚,并且可以大大减小锂离子的扩散途径。电化学测试结果显示该材料比容量为空白Ti02纳米管阵列比容量的3倍,60次循环后的容量保持率为85%,且表现出优异的倍率性能。(3)进一步以碳纳米管为载体,利用化学镀锡和化学镀铜工艺制备了Cu6Sn5@CNTs复合粉体材料。Cu6Sn5合金层被均匀地镀覆在碳纳米管外壁。材料的首次放电容量为614 mAh g-1,首次可逆充电容量为371 mAh g-1,对应的首次库伦效率为60%,相比于纯碳纳米管有较大的提高。但循环性能却仍不理想,20次循环后容量仅为223 mAh g-1。这可能是由于Cu6Sn5合金层是包覆在碳纳米管管壁的外部,在充放电过程中由于体积变化和应力增加,容易从碳纳米管上脱落。这种外包覆方式与内包覆相比,在提高锡基材料循环稳定性方面还是有较大的差距,材料结构需进一步优化。(4)除了在锂离子电池锡基负极材料方面的研究工作,在电化学电容器方面也开展了一些工作。电化学电容器是一种适于快速充放电的储能装置,与电池相比,具有非常高的功率密度,但能量密度却非常低。通常认为这是由于活性材料较低的利用效率造成的。因此,提高电极材料的利用率是提高电容器比能量的关键。本部分工作利用阴极沉积法和后续热处理制备了孔径为2-3 nm的无序多孔结构Mn02薄膜,并将其沉积在三维多孔集流体上。TEM和XPS分析结果显示MnO2的无序多孔结构是在热处理过程中失水形成的。由于集流体的大孔结构和活性物质Mn02的介孔结构,活性物质的利用率得到较大的提高。在5 Ag-1的电流密度下材料的比容量为385 Fg-1,并表现出优异的倍率性能和循环寿命。更多还原

【Abstract】 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 1.2μm 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:①Tubular structure of CNTs could adsorb the reaction-induced stress;②The 3-D porous structure formed by CNTs could also accommodate drastic volume variation during electrochemical reactions;③The 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.更多还原