复合材料在锂离子电池和储氢体系的应用

【作者】 付丽君；

【导师】 吴宇平；

【作者基本信息】 复旦大学 ， 物理化学， 2010， 博士

【摘要】 随着社会的发展,人类对于能源需求量增多,石油、煤炭、天然气等不可再生能源逐渐减少,环境问题日益严重,使能源和环境成为备受关注的两大社会问题。发展低碳经济和清洁可再生能源成为各国非常重视的课题,这也对储能装置的各项指标提出了新的要求。燃料电池是研究比较多的储能装置之一,也是汽车动力的热点之一,但实现燃料电池在电动汽车的大量产业化还需时日。这其中,氢气的储存和运输是实现氢能源实用化关键的一环。锂离了电池因为其环境友好、输出电压高、高能量密度等优点,已经成为便携式电了产品可再充电源的主要选择对象,近年来,锂离子电池又成为汽车动力的热点之一。发展电动汽车,以电代油,是保证我国能源安全的战略措施,有着重要的战略意义。锂离子电池的发展,即要进一步提高锂离子电池的安全性能、循环寿命、能量密度和大倍率性能,关键在于材料创新。对于锂离子电池电极材料,目前还没有一种材料能够完全满足EV对锂离子电池提出的要求,除了寻找新材料、材料纳米化之外,复合材料的应用也是提高锂离子电池材料的一个重要途径。本论文通过对SSG石墨/VGCF、核壳结构及多孔结构TiO2/C和Ru/Li2O四个复合体系的研究,讨论了复合物对锂离子电池电极材料电化学性能提高的作用,并把复合物的概念扩及储氢领域。本文利用XRD、SEM、TEM、FTIR、XPS等方法对样品进行表征,采用CV、EIS、恒电流充放电等测试方法进行电化学性能表征,采用TDS方法对复合物的储氢性能进行表征。具体内容如下：“复合物”的概念很早就被用于锂离子电池的电极中,锂离子电池电极包括活性材料、导电剂和粘结剂,以用来保证电极的导电性及结构稳定性。论文第三章探讨了SSG石墨和导电剂VGCF (Vapor Growth Carbon Fiber)的复合均匀程度对石墨电极的电化学性能影响,并对其机理进行了分析。通过对电极制备工艺的控制,得到了SSG/VGCF复合均匀程度不同的两个电极,复合均匀、不均匀的电极分别用WF和PF电极来表示。电化学测试结果表明,WF电极首次可逆容量高,表现出良好的循环性能,而PF石墨电极,首次容量低,循环性能较差。交流阻抗研究发现,WF电极,膜电阻和电荷传质电阻几乎不随循环进行而增大,但对于PF电极,膜电阻和电荷传质电阻随着循环进行有了明显增大。对两个电极充放电后的电极进行SEM、XPS、EDX、显微拉曼分析,发现循环后两个电极表面均出现含有Li2CO3的沉积物,该沉积物由电解液分解形成,是SEI膜的组成成分,而WF电极沉积物明显少于PF电极。经过机理讨论,得出SSG/VGCF的复合均匀程度影响石墨电极的电子导电性和电极的电势分布,进而影响SEI膜的形成。对于复合程度不均匀的电极,其电势分布不一,造成了SEI膜在前期循环中形成不完善,无法抑制后续循环的电解液分解,导致持续的不可逆锂消耗和电荷损失,库仑效率低,循环性能变差。TiO2资源丰富、价格低廉、环境友好,近年来也成为锂离子电池的研究热点。但是其导电率较差,<10-10S/cm,提高导电率是对其进行改性的重要方法之一。论文第四章内容通过乳液聚合法,得到了核壳结构的TiO2-PAN复合物,再经过惰性气氛下煅烧,得到了TiO2-C核壳复合物。CV结果表示两个样品的嵌锂脱锂电位分别为1.7,2.0 V vs. Li/Li+, TiO2-C核壳复合物的循环性能优于TiO2纳米材料,电荷传质阻抗较小,锂离子表观扩散速率是纳米TiO2的十倍。主要是因为经过碳包覆,可以抑制纳米粒子在充放电过程中的团聚现象,提高了循环性能；同时,碳层的包覆,有效地提高了整个电极的导电性,降低了电荷传质阻抗,有利于锂离子在电极材料的传输,该方法可以作为提高纳米电极材料的有效方法之一。论文第五章内容用聚苯乙烯作为模板,得到了多孔的二氧化钛,并运用蔗糖包覆,惰性气氛下煅烧,得到了多孔TiO2-C复合物。多孔TiO2和TiO2-C复合物均表现出良好的锐钛矿晶型。充放电测试结果表明,在低倍率下,多孔TiO2和TiO2-C复合物两电极表现出相似的电化学性能,容量和充放电曲线相似。当增大充放电电流,多孔TiO2电极的极化明显增大,充放电曲线上升/下降趋势增快,而经过碳包覆,多孔TiO2电极的极化现象得到了抑制。充放电后两电极材料的TEM观察结果表明,TiO2会随着循环发生体积膨胀,造成结构破坏,碳包覆后,有利于抑制体积膨胀,维持循环过程中结构的稳定性。但这两种材料的容量需要进一步优化提高。论文第六章内容通过电化学的方法合成了Ru/Li2O纳米复合物,对其界面储锂机理进行了研究。电化学性能测试表明,Ru/Li2O纳米复合物在0.05-1.2 V vs.Li/Li+充放,表现出电容器的性质,且有很好的倍率性能,60C(1min)充放情况下,容量可达20 mAh/g,该储锂机理成为连接锂离子电池和电容器的桥梁。为了提供更多界面储存机理的证据,在界面储锂概念的基础上,将界面储存的概念扩展至储氢性能研究上,进一步研究了纳米复合物界面储氢性质。对Ru/Li2O纳米复合物的TEM表征结果表明,在Ru/Li2O纳米复合物的表面,有一层SEI膜,为了避免SEI膜中H的影响,将Ru/Li2O纳米复合物首先进行惰性气氛下40℃煅烧,并且在TDS测试中采用D2吸附。TDS结果表明,当室温、700 mbar吸附D2之后,煅烧后的Ru/Li2O纳米复合物在高温脱附过程中能够释放出D2,说明在该体系可以储存D2。通过氢气还原法和电化学合成方法得到了纳米Ru, TDS表明经过相同的室温D2吸附,脱附过程中没有检测到D2信号。尽管目前的研究结果和文献报道表明Li2O没有储氢性质,但关于Ru/Li2O具体的界面储氢机理,还有待进一步探索。通过对上述SSG石墨/VGCF、TiO2/C核壳结构及多孔结构和Ru/Li2O四个体系的研究,可以发现复合物对锂离子电池性能影响作用主要为以下几点：(1)提高电极的电子导电性,减小极化,提高大倍率性能；(2)维持循环过程中电极结构稳定性；(3)纳米复合物界面储锂机理的发现,将体相储锂和界面储存的概念结合起来,成为连接锂离子电池和超级电容器的桥梁,这一概念可以扩展至储氢材料体系。可以预见,本文所述的复合物概念将会成为改性锂离子电极材料的重要方法之一,提高锂离子电池的性能。更多还原

【Abstract】 Low carbon economy and clean renewable energy now become significant topics over the world, due to the increasing demands for energy, decreasing amount of non-renewable energy, such as petrol, coal and natural gas, as well as environmental problems. As a result, energy storage devices with high performance are urgently required to use energy more efficiently. Among all the energy storage devices, lithium ion batteries exhibit much more advantages, such as high voltage, high energy density, etc., thus they have been quite popularly used in portable devices since their birth, and now they are nominated as new energy source for vehicles. Besides lithium ion batteries, fuel cell is another hot topic as energy source for EVs. However the storage and transportation of hydrogen is a crucial link for its application. For development of lithium ion batteries, their high performances, i.e., safety, good cycle performance, high energy and power density and high rate performance are mainly concerned. The key step for the further development is material innovation. Besides searching for new materials with high performance and preparation of nano materials, synthesis of composites is also an effective route for the achievement mentioned above.In this dissertation, the effects of composites for the improvement of lithium ion batteries are discussed with four different composites, SSG graphite/VGCF, core-shell TiO2/C, porous TiO2/C and Ru/Li2O composites. At the main time, the composite concept has been adopted to hydrogen storage system. XRD, SEM, TEM, FTIR and XPS were used for characterization, CV, EIS and discharge-charge measurements were used for investigation of electrochemical performance. TDS was used for hydrogen storage analysis. The main contents are as following:The concept of’composites’has been used in lithium ion batteries for a long time. The electrode is composed with active material, conductive additives and binder to increase the conductivity and stability. Chapter 3 is focused on how the dispersion of SSG graphite/VGCF affects electrochemical performance of graphite electrode. With the control of technology art, two kinds of graphite electrodes were obtained, with well and poor dispersion of VGCF, respectively. They are called WF (well fabricated) and PF (Poorly fabricated) electrode afterwards. It is found that WF electrode shows high reversible capacity in first cycle and better cycling performance, however, the reversible capacity and cycling performance are poor for PF electrode. The results got from the measurements of EIS, SEM, EDX, XPS and Raman reveal that the dispersion of SSG/VGCF greatly affects the electrochemical performance of graphite electrode. When the dispersion is homogeneous, homogeneous electronic conductivity dispersion is achieved for the whole electrode, thus perfect SEI film forms in the first cycles, which could prevent further electrolyte dispersion, as well as irreversible consumption of Li, and the good cycling performance is obtained. While for PF electrode, the dispersion of VGCF and electronic conductivity are bad, no perfect SEI film forms in the first cycles, and further decomposition of electrolyte occurs in the following cycles, giving rise to bad cycling performance.Chapter 4 concerns preparation of TiO2-C nanocomposites by first adopting emulsion polymerization method to form TiO2-PAN nanocomposites, followed by calcination in inert atmosphere. CV results reveal that both nano TiO2 and TiO2-C nanocomposites present redox peaks at 1.7,2.0 V vs. Li/Li+. TiO2-C nanocomposites show better cycling performance than that of nano TiO2. EIS results indicate that after carbon coating, the charge transfer resistance decreases. The apparent diffusion efficiency of TiO2-C nanocomposites is 10 folders larger than that of nano TiO2. After carbon coating, the aggregation of nanoparticles during cycling can be suppressed, and electronic conductivity is increased, thus TiO2-C nanocomposites show better electrochemical performance.The work mentioned in Chapter 4 is preparation and electrochemical performance investigation of porous TiO2 and TiO2-C by using PS template and sucrose as carbon source. The charge-discharge results reveal that at low current rate, both porous TiO2 and TiO2-C electrodes present the similar electrochemical performance, however, when increasing the charge-discharge rate, the polarization increases greatly for porous TiO2 electrode, the charge and discharge curve increases/declines faster. After carbon coating, the polarization is suppressed, giving rise to larger reversible capacity. Furthermore, TEM micrographs of both electrodes after 30 cycles at 0.5C indicate that, both materials suffer volume expansion during cycling, however, the carbon layer of TiO2-C electrode could suppress the volume expansion and keep the stability of porous structure.Chapter 6 is about interfacial storage of lithium and hydrogen in Ru/Li2O nanocomposites. The Ru/Li2O nanocomposites were prepared by electrochemical lithiation method. The electrochemical performance of Ru/Li2O nanocomposites in the potential range between 0.05 to 1.2V vs. Li/Li+ display a capacitor performance, with excellent rate performance. They deliver 120 mAh/g at 5C (1C=120 mA/g), even 20 mAh/g at 60C. The interfacial storage mechanism could provide a bridge between lithium ion batteries and capacitor. Interfacial storage of hydrogen in Ru/Li2O nanocomposites was investigated as well for more evidence of this mechanism. To avoid the interference of H from SEI film in Ru/Li2O nanocomposites, D2 was adopted instead of H2 for absorption, and the Ru/Li2O nanocomposites were first calcined to 400℃before hydrogen adsorption. TDS was used to investigate the substance desorption. The results indicate that Ru/Li2O nanocomposites after calcination can release D2 after absorption of D2 at room temperature. In comparison, the hydrogen storage of Ru nanoparticles prepared by hydrogen reduction method and further discharging to 0.8V vs. Li/Li+ was also investigated. For Ru nanoparticles, there is no HD or D2 desorption during the TDS measurement after loading D2at room temperature. Since Li2O can not be hydrogenated, the differences of the D2 desorption between Ru-Li2O nanocomposites and Ru nanoparticles may be correlated with the interfacial ’job-sharing’storage. The above results indicate that both lithium and hydrogen could be stored at the interfaces in Ru-Li2O nanocomposites reversibly. How far hydrogen is dissociated and ionized in the proper sense of the mechanism discussed, remains to be investigated.With the results from the above four systems studied, the effects of different nanocomposites as electrode materials in lithium ion batteries can be concluded as following:First, nanocomposites with carbon components could enhance the electronic conductivity, decrease the polarization and give rise to better rate performance; Second, nanocomposites with carbon coating layer enhance the structural stability. Third, interfacial storage of lithium in nanocomposites provides a bridge between lithium ion batteries and supercapacitors. Furthermore, the interfacial storage mechanism could be used in hydrogen storage, providing more evidence for the mechanism.更多还原