【作者】 汪龙；

【导师】 陈春华； Ingo Lieberwirth；

【作者基本信息】 中国科学技术大学 ， 材料学， 2010， 博士

【摘要】 能源问题是为人类所面临的最大的挑战之一,为解决这一问题,人们不断努力发展新型能源,与此同时也对储能技术也提出了越来越高的要求。而锂离子电池自从上世纪90年代商品化以来,一直在便携式电子产品市场占有主导地位。与此同时,随着手机、数码相机、笔记本电脑等众多移动电子设备进一步向小型化方向发展,人们对体积小、能量高的新型电源的需求也越来越迫切,因此具有更高容量、更长寿命以及可以实现快速充放电的锂离子电池的新型电极材料的研究已经成为材料科学的研究热点。本论文主要利用静电喷雾沉积和静电纺丝技术,制备了一系列碳复合的薄膜或纳米纤维形态的正负极材料（包括C/Co,C/Fe3O4, C/Si, LiFePO4/C和Li3V2（PO4）3/C）,以及具有双孔径分布的多孔金属氧化物（α-Fe203）薄膜电极,并对材料的纳米结构和碳复合对电化学性能的影响进行了研究。论文的第一章中,作者简要介绍了锂离子电池相对于其它储能电池的优势及其组成和工作原理,并对常见的锂离子电池正负极材料的研究现状进行了综述,最后对碳复合的电极材料的研究现状进行了介绍。作为薄膜和纳米纤维材料的有效制备手段,静电喷雾沉积和静电纺丝技术在众多研究领域有着广泛的应用。论文第二章对这两种技术的原理与实验装置进行了说明,并对论文中使用的其他仪器和方法进行了介绍。在第三章中,我们利用静电纺丝技术制备了形貌均一的不同高分子纳米纤维,通过后续热处理得到了具有高于石墨容量的无定形碳纤维。在前驱液中加入钴的无机盐后,利用高温下碳的还原性制备了包含纳米钴粒子的碳钴复合纳米纤维。纳米钴粒子的容量增加了锂离子在碳钴界面的存储,而交流阻抗测试显示碳钴复合纳米纤维的导电性也优于同样热处理温度下得到的纳米碳纤维,因此显示出更高的可逆容量和稳定的循环性能。在第四章,我们在第三章的基础上,将电化学惰性的金属钴替换成具有电化学活性的四氧化三铁,制备了C/Fe3O4复合纳米纤维。通过XRD谱图的计算和TEM测试显示在600℃C/Fe3O4纳米纤维中的Fe304颗粒尺寸在20 nm左右,并且均匀的分散在无定形的碳纤维内部。根据转化反应机理,放电过程中锂离子不仅能够在碳纤维中存储,同时可以将复合纤维中的Fe304纳米纤维还原到Fe.生成的单质Fe可以在放电后期改善电极的电子导电性,起到与C／Co复合纤维中金属Co相同的作用。非原位的SEM检测发现C/Fe3O4复合纳米纤维经过一定循环次数后在电极会发生粉化现象,而这种电极结构的变化不仅没有造成电池容量的衰减,反而由于电极与电解液接触面积的增大,降低了电池的阻抗,使得电池在长期循环过程中表现出了容量的上升。C/Fe3O4复合纳米纤维不仅可以作为高性能锂离子电池的负极材料,同时也为制备碳复合的纳米金属氧化物材料提供了一种简便易行的手段。在第五章中,利用静电喷雾沉积技术在不同的温度下制备了具有双孔径分布的多孔α-Fe203薄膜电极。三维多孔的薄膜结构避免了放电过程中由于电极剧烈体积膨胀造成的活性物质脱落,经过15次循环之后的电极保持与循环前相同的形貌,因此电极表现出了稳定的容量保持率。沉积的α-Fe203薄膜首次不可逆容量损失小于20%,并且通过对充放电曲线积分计算发现其能量转换效率也达到65.8%,高于常见的NiO,CoO等金属氧化物55-60%的数值。我们首次利用能量平均电压（Eav）对金属氧化物负极的工作电压进行评估,结果显示α-Fe203的Eav与当前热门的动力电池负极材料Li4Ti5O12相当,而容量更高（5 C条件下容量500mAh g-1）。对电池在高低温条件下的测试显示,多孔的α-Fe203薄膜电极可以在较宽的温度窗口内工作。最后,我们研究了Li20的引入对薄膜形貌和电化学性能的影响。Li20的加入使得到的薄膜更加致密,而硝酸铁的分解产物由α-Fe203转变为Fe₃O₄,在循环过程中有利于电池的容量保持稳定,但并没有将Fe2+氧化到更高的价态。硅因为理论容量远远高于其他负极材料,因而具有诱人的应用前景,而碳硅复合的负极材料也被证明可以有效的改善硅的容量衰减。在第六章中,我们首先制备了碳硅复合的纳米复合纤维,在纳米碳纤维中复合了23 wt%的纳米硅。由于硅的引入,碳纤维的容量达到了1100 mAh g-1,而纤维状的形貌也为电极提供了足够的空间,使其能够承受循环过程中电极体积的膨胀。其次,为了提高碳硅复合材料的体积能量密度,我们率先使用溶剂残炭制备碳硅复合薄膜。通过甘油分子在静电喷雾过程中的缩聚反应,我们得到了含有聚甘油的硅薄膜,而聚甘油的均匀分散和良好的粘结作用使电池在50次循环之后仍然具有80%的容量保持率。在惰性气氛下热处理之后,聚甘油发生炭化,可以进一步提高电极的可你容量。利用有机溶剂分子的缩聚制备纳米的碳硅复合材料为我们提供了制备碳包覆／复合材料的新的手段,可以进一步推广到其他的电极材料或者应用领域。在第七章中,我们制备了两种碳复合的正极薄膜（Li3V2（PO4）3/C和LiFePO4/C）。透射电镜检测发现核桃仁状的Li3V2（PO4）3/C薄膜中的磷酸钒锂颗粒平均粒径大约在50 nm,葡萄糖热分解形成的无定形碳均匀的分散在这些纳米颗粒的周围,并且部分包覆在磷酸钒锂颗粒的表面。这种碳复合的纳米结构使Li3V2（PO4）3/C表现出了良好的大倍率性能,在24 C条件下进行放电时电池的放电平台仍然在3.0 V以上。交流阻抗测试显示Li3V2（PO4）3/C薄膜电极在充放电过程中阻抗基本保持不变,因而即使在大电流下也能保证容量的发挥。通过改变溶剂中高沸点和低沸点溶剂比例,我们可控制薄膜的形貌,从而对电极的孔隙率和电化学性能做出进一步调整。在这一章的最后,我们对制备的具有笼状和海绵状复合结构的多孔LiFePO4/C薄膜进行了表征和电化学测试。实验结果表明,薄膜中纳米尺寸LiFePO4粒子可以在大的电流密度下很好的发挥容量,而其中包含的微米尺寸的颗粒当电流密度上升时容量迅速下降。由此可见,制备粒径分布均的纳米材料是提高LiFePO4材料倍率性能的关键。在论文的最后,对论文的创新和不足之处进行了总结,并对今后可能的改进和研究方向提出了建议。更多还原

【Abstract】 Energy shortage is one of the biggest challenges on the world, and the demands of energy storage technology keep growing while a lot of efforts have been devoted to developing new energy source. Lithium ion batteries have dominated the portable electronic device market since they were firstly commercialized in 1990s. Meanwhile, as the portable electronic devices such as mobile phones, digital cameras and laptops become smaller and smaller, there is an urgent demand for new power sources with smaller volume and higher energy density. Therefore, the exploitation of new electrode materials for lithium ion batteries with high energy, longer-life and capability of fast charge/discharge becomes a research hotspot in materials science nowadays.The study in this Ph.D thesis mainly focuses on the syntheses of a series of carbon composite thin films or nanofibers as negative/positive electrode materials (C/Co,C/Fe3O4, C/Si, LiFePO4/C and Li3V2(PO4)3/C). Meanwhile, a porous metal oxide thin film electrode with a bimodal pore size distribution is also presented. The effects of nanostructure and carbon composite have been investigated in this thesis.In Chapter 1, a general introduction is given as following aspects:the advantages of lithium ion batteries compared with other batteries; the working principle and research status of some regular electrode materials. At the end of this chapter, a brief introduction of carbon composite electrode materials is presented.As effective synthesis techniques for thin film and nanofibers, electrostatic spray deposition (ESD) and electrospinning are widely employed in many research areas. In Chapter 2, there is a set up description of these two techniques and an introduction of the experimental equipments and methods used in the project of this thesis.In Chapter 3, homogenous polymer nanofibers are synthesized via electrospining, and a disordered carbon fiber with higher capacity than graphite has is obtained from subsequent thermal treatment. After an inorganic cobalt salt is added into the precursor, a C/Co composite nanofiber consists of cobalt nanoparticles is obtained. The interfacial lithium storage is enhanced by introducing cobalt nanoparticles and the A.C. impedance test reveals that the C/Co nanofiber has superior conductivity compared with the carbon nanofiber obtained at the same carbonization temperature, so higher revisable capacity and more stable cycling performance have been achieved.In Chapter 4, we synthesize a C/Fe3O4 composite nanofiber based on the work of Chaper 3, and the cobalt which is electrochemical inactive is replaced by Fe3O4 that can react with lithium. According to the calculation from XRD pattern and TEM characterization,600℃derived C/Fe3O4 composited nanofiber contains some well dispersed Fe3O4 nanoparticles (20 nm in diameter) inside the disordered carbon fiber. According to the convert reaction mechnasim, lithium ions can not only be stored in the carbon nanofiber, but also reduce Fe3O4 into Fe. The as derived Fe could enhance the electronic conductivity of the electrode during discharge, which can play the same role as cobalt does in C/Co composite nanofiber. Ex-situ SEM measurement reveals that powderization phenomenon will occur in C/Fe3O4 composite nanofbier electrode after several cycles, however, such a structural change in electrode does not cause any capacity decline. On contrary, the contact area becomes larger and the cell resistance because of the powderization, so a capacity increase is observed during the long-time cycling measurement. In general, such a C/Fe3O4 composite nanofiber not only can be employed as anode material for high performance lithium ion batteries, but also provides us a new straightforward method to prepare carbon/metal oxide nano materials.In Chapter 5, we prepare a-Fe2O3 thin film electrode at different temperatures by ESD. The 3D porous structure can effectively prevent the active material breaking off from the current collector, so the thin film exhibits same morphology as it before even after 15 cycles and stable capacity retention has been achieved. The a-Fe2O3 thin film deposited at 200℃exhibits an initial capacity loss less than 20%, while the energy conversation efficiency calculated from voltage profiles is 65.8%, which is much higher than NiO or CoO that usually has a value between 55% and 60%. We introduce a novel parameter called energy average voltage (Eav) to evaluate the working potential of metal oxides. Calculation results lead us to a conclusion that the ESD prepared a-Fe2O3 thin film has higher capacity (500 mAh g-1 under 5 C) while possessing a similar working potential to Li4Ti5O12, which is a promising anode material for high power batteries, Meanwhile, the porousα-Fe2O3 thin film can work in a wide temperature window according to the high-low temperature condition tests. In the last part of this chapter, Li2O is introduced into the film and effects on film morphology and electrochemical properties are investigated. When Li2O is introduced, the film becomes denser and the production of iron nitride decomposition changes into Fe3O4 instead ofα-Fe2O3. Nevertheless,Li2O does not oxidize Fe2+ to higher oxidation state. Silicon may have attractive prospect of applications because its theoretical capacity is much higher than those of other anode materials, and carbon composite silicon based materials have been considered to be able to effectively improve the cycling performance of silicon. In Chapter 6, we synthesize a C/Si composite nanofiber with 23 wt% silicon loaded. The composite nanofiber delivered a high capacity as 1100 mAh g-1 after the introducing of silicon. Thanks to the fibrous morphology, which provides sufficient free space, the electrode can survive from the large volume expansion caused by lithium intercalation. On the other side, in the purpose of improving its volumetric capacity density of carbon silicon composite material, we firstly synthesize C/Si composite thin film by employing the solvent as carbon precursor. Silicon thin film consists of polyglycerol which is originated from the polymerization of glycerin during ESD process is obtained, and the film exhibits a capacity retention reaches to 80% after 50 cycles due to the well dispersed poly glycerin. Due to the carbonization of poly glycerin, the revisable capacity can be improved when the film is treated under Ar atmosphere at high temperature. This carbon silicon composite with organic solvent as carbon precursor shows a new approach to synthesize carbon coated materials, and could be extensively applied in synthesis of other electrode materials.In Chapter 7, we synthesize two kinds of carbon composite cathode thin film electrode (Li3V2(PO4)3/C and LiFePO4/C). It is observed from the TEM pictures that there are lots of Li3V2(PO4)3 nanoparticles well dispersed in a carbon matrix generated from glucose decomposition in the walnut-like Li3V2(PO4)3/C thin film, and parts of the particles are coated by the carbon. Such a carbon composite nanastructured Li3V2(PO4)3/C thin film exhibits excellent rate capability, and all of the discharge capacity is above 3.0 V even under a current density of 24 C. A.C. impedance spectroscopy studies reveal that there is no obvious resistance change during charge/discharge process of Li3V2(PO4)3/C thin film, therefore, most of its capacity can be delivered under high current densities. The morphology of the thin film can be controllable tuned by changing the ration of solvent with different boiling point in the precursor, so that the porosity and electrochemical performance of the film can be adjusted. In the last part of this chapter, characterization and electrochemical measurements are conducted on a porous LiFePO4/C thin film with cage/sponge-like morphology. The results indicate that the nanoparticles consisted in the film could deliver their capacity well while the particles in micrometer size show rapidly capacity decline as the current density increases. The results lead to a conclusion that the nanoparticles with narrow particle distribution should be the key factor to improve the rate capability of LiFePO4 based materials.Finally, an overview on the achievements and deficiency of this thesis is presented at the end of the thesis (Chapter 8). Some prospects and suggestions in improvement and possible research directions are also pointed out.更多还原