【作者】 李炳江；

【导师】 孙克宁；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2014， 博士

【摘要】 近年来，氟化铁作为一种新型的锂离子电池正极材料，以其较高的平台电位、高达237mAh g1的理论容量和低廉的价格吸引了研究人员的广泛关注。尽管具有以上的这些优点，氟化铁材料固有的低锂离子电导率和电子电导率在很大程度上限制了其倍率性能的发挥。本文针对氟化铁的特点，从构筑氟化铁电极结构的角度出发，设计合成了四种具有独特微观结构的氟化铁电极材料。借助X射线衍射、拉曼光谱、扫描电子显微镜、氮气吸脱附测试、透射电子显微镜、电池充放电测试、电化学交流阻抗谱等分析测试手段，深入全面的表征了制备材料的物理化学性质并系统研究了其电化学嵌脱锂性能。采用“绿色溶剂”[Bmim][BF4]离子液体作为氟源和石墨烯纳米片的分散剂，利用离子液体咪唑阳离子与石墨烯π电子之间的强烈作用，使石墨烯在反应体系中均匀分散，这样就避免了使用氧化石墨烯所带来的后续还原处理。整个复合过程为一步原位合成法，FeF3·0.33H2O纳米晶成核和结晶长大的过程都发生在石墨烯的表面，这种原位的复合方法保证了石墨烯和氟化铁颗粒紧密的导电接触，为其优异的倍率性能和循环性能提供了结构方面的保证。FeF3·0.33H2O/GNS复合电极材料在1C充放电倍率下，经过200次循环，放电容量依然可以保持在142mAh g1；在10C充放电倍率下循环250次，容量依然可以保持在115mAh g1。采用纳米浇铸的方法，制备了具有高速电子传输速率和发达孔道结构的FeF3·0.33H2O@CMK-3复合电极材料。FeF3·0.33H2O颗粒与高电子电导的CMK-3牢固接触，构筑了一个优越的导电网络。FeF3·0.33H2O纳米晶的生长和团聚被有序介孔碳的孔道结构有效地限制，微小的FeF3·0.33H2O纳米晶可以促进电子和Li+传输，从而显著改善材料的电化学性能。复合电极材料具有适合Li+快速传输的规则有序的孔道结构，巨大的比表面积保证了电解液与活性物质的充分接触。这些因素有机结合，相互协同作用，使FeF3·0.33H2O@CMK-3复合电极材料倍率性能得到了显著的改善。在高达50C的放电倍率下，进行100次充放电循环，复合电极材料的放电容量依然可以稳定的保持在79mAh g1左右。基于碳纳米角π电子与离子液体[Bmim][BF4]咪唑环之间的π-π相互作用，采用简单的液相合成方法，制备了具有高比表面积和发达孔结构的FeF3·0.33H2O@CNHs复合电极材料。在FeF3·0.33H2O@CNHs复合电极材料中，FeF3·0.33H2O纳米颗粒主要位于相邻的锥形碳纳米管形成的空隙位置，由于相邻锥形碳管的限域作用有效地限制了FeF3·0.33H2O纳米颗粒的长大和团聚，使得具有微小尺寸的FeF3·0.33H2O（~5nm）纳米颗粒均匀的分散于碳纳米角形成的导电网络结构中。氮气吸脱附测试的结果显示，FeF3·0.33H2O@CNHs的比表面积高达268.9m2g1，平均孔径尺寸为5.59nm，发达的孔道结构和大比表面积保证了电解液的充分浸润和锂离子的快速传输。FeF3·0.33H2O@CNHs复合电极材料在0.5C、1C、2C、5C、10C和20C的倍率下进行充放电，放电容量分别为169,157,140,131、120和106mAh g1。在高达50C的充放电倍率下，复合电极材料依然可以稳定的释放高达81mAh g1的容量。在1C的充放电倍率下，50次充放电循环后放电容量为153mAh g1。采用溶剂热合成的方法，制备了具有分等级结构的自支撑FeF3·0.33H2O花状阵列电极，通过分析不同溶剂热反应时间所得样品的微观形貌，研究FeF3·0.33H2O花状阵列的生长机理。FeF3·0.33H2O花状阵列是由10nm厚的―纳米花瓣”相互连接形成直径约为1μm的分等级花状结构。进一步借助氮气吸脱附测试对花状阵列的孔径分布进行了分析，样品在3.5nm和10-20nm这两个位置存在着明显的介孔分布。其中，3.5nm的介孔为组成“纳米花瓣”的纳米颗粒之间形成的孔道；而10-20nm的介孔则对应着相邻的“纳米花瓣”之间形成的开放孔道结构。FeF3·0.33H2O花状阵列不仅具有高速的电子传输通道，还兼具开放的花状结构、独特的等级孔道结构和大比表面积，这些独特的结构特点使FeF3·0.33H2O电极材料的电化学性能得到了显著的提高。更多还原

【Abstract】 Iron fluoride has attracted a rapidly increasing amount of attention because of its high operating potential, high theoretical capacity of237mAh g1, and relatively low cost. Despite these advantages, the application of iron trifluoride has been limited due to its intrinsic drawbacks, for instance, the slow diffusion of Li+and low electron conductivity. With regards to the features of iron fluoride, our work develops several new strategies to construct four kinds iron fluoride electrode materials with unique nanostructure. X-ray diffraction, Raman spectrum, Scanning Electron Microscopy, Nitrogen adsorption-desorption techniques, Transmission Electron Microscopy, Discharge/charge measurement and Electrochemical impedance spectroscopy are applied to characterize the physicochemical properties and electrochemical behavior of Li+insertion/extraction process of the as-obtained iron trifluoride smaples.A tactful ionic-liquid (IL)-assisted approach on in situ synthesis of iron fluoride/graphene nanosheet (GNS) hybrid nanostructures was developed. To ensure uniform dispersion and tight anchoring of the iron fluoride on graphene, we employed an IL which served not only as a green fluoride source for the crystallization of iron fluoride nanoparticles but also as a dispersant of GNSs. GNSs were chosen as the starting materials instead of graphene oxide (GO), to avoid additional reductive treatment to obtain GNSs from GO, which could easily cause destruction to the structure of iron fluoride. Thus, both the nucleation and crystallization of FeF3·0.33H2O nanoparticles occurred on the surface of GNSs. It is a typical one-pot and in situ method. Owing to the electron transfer highways created between the nanoparticles and the GNSs, the iron fluoride/GNS hybrid cathodes exhibited a remarkable improvement in both high-rate and long-term cycle performance. The iron fluoride/GNS hybrid shows an impressive rate cycling performance of142mAh g1at1C after200cycles and115mAh g1at10C after250cycles.We also reported a facile strategy for the synthesis of mesoporous FeF3·0.33H2O@CMK-3nanocomposite with high electron conductivity and well-developed pore structure by nanocasting technique, in which iron fluoride nanoparticles were confined in mesoporous CMK-3. The intimate conductive contact between the FeF3·0.33H2O nanoparticles and the carbon framework provides an expressway of electron transfer for Li+insertion/extraction. Here, the CMK-3can suppress the growth and agglomeration of FeF3·0.33H2O nanoparticles during the crystallization process. The small FeF3·0.33H2O particles confined in mesoporous carbon matrix exhibited reduced electron and Li+transport resistance, which improved the electrochemical performance of the FeF3·0.33H2O@CMK-3nanocomposite. Additionally, well-defined, continuous channels provide a large specific surface area that increases the electrolyte-electrode contact area, ensuring that the electrolyte could easily penetrate the mesoporous. By combining these outstanding qualities, the FeF3·0.33H2O@CMK-3nanocomposite demonstrated excellent ultra-high rate performance. Remarkably, even under an ultrahigh charge/discharge rate of50C, the confined FeF3·0.33H2O@CMK-3showed a stable high specific capacity of79mAh g1after100cycles.Based on the π-cation interactions between the imidazolium cation of the [Bmim][BF4] and the π-electrons of the carbon nanohorns, the carbon nanohorns (CNHs) carried FeF3·0.33H2O nanocomposites (FeF3·0.33H2O@CNHs) with large specific surface area and well-developed pore structure was presented by a facile solution-based method for the first time. In the FeF3·0.33H2O@CNHs nanocomposites, the FeF3·0.33H2O nanoparticles were mainly located in the interstitial site between horn-shaped carbon nanotubes. The growth and agglomeration of FeF3·0.33H2O nanoparticles were effectively suppressed by the carbon nanohorns. The tiny FeF3·0.33H2O nanoparticles (~5nm) were well dispersed among the conductive network. The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution of the FeF3·0.33H2O@CNHs nanocomposite were measured by nitrogen isothermal adsorption analysis. The BET surface area of the FeF3·0.33H2O@CNHs nanocomposite is as high as268.9m2g1, the BJH average pore size is5.59nm. The high BET specific surface area of the FeF3·0.33H2O@CNHs nanocomposite and the well-developed pore structure provides more reaction sites and is beneficial for electrolyte access. The discharge capacities of the nanocomposite at0.5C,1C,2C,5C,10C and20C are169,157,140,131,120and106mAh g1, respectively. The FeF3·0.33H2O@CNHs composite can delivery a stable high capacity of81mAh g1even under an ultrahigh rate of50C. Meanwhile, the nanocomposite shows a stable cycle performance of153mAh g1at1C after50cycles.At last, we presented a tactful and advanced architecture design of self-supported, binder-free3D hierarchical FeF3·0.33H2O flower-like array directly growing on Ti foil by a solvothermal approach. In order to understand the formation process of the iron fluoride, the morphologies of the samples were studied by SEM at different stages of the reaction. The probable growth mechanism of the3D microflower was explored by the time-dependent analysis. The entire structure of the3D hierarchical architectureis constructed with dozens of nanopetals. These nanopetals are approximately10nm thick and500nm wide, and connect to each other through the center to form3D hierarchical structure approximately1μm in diameter. The products were scraped off from the Ti foil and collected for nitrogen isothermal adsorption measurement to further examine the pore structure of the FeF3·0.33H2O3D microflower. The sample exhibits a unique hierarchical porous structure. The smaller pore of3.5nm is mainly contributed by the pores existing between the FeF3·0.33H2O nanoparticles. The larger pores between10 nm and20nm might stem from the open space between neighboring nanopetals. The excellent electrochemical performance of the FeF3·0.33H2O flower-like array could be definitely ascribed to the synergistic effect of charge transfer expressway, high specific surface area and porous hierarchical structure.更多还原