金属氧化物基纳米结构的自组装合成及其电化学性能研究

发布时间：2018-01-19 11:27

本文关键词： TiO_2 ZnO 铁氧化物自组装合成原位碳化自组装协同效应锂离子电池　出处：《浙江大学》2015年博士论文　论文类型：学位论文

【摘要】：自组装合成技术是指通过调控纳米基元的自组装行为,在纳米结构合成过程采用二次组装、原位碳化等技术制备多级微纳结构或多元复合材料。自组装结构不仅兼具微纳尺度或多元材料各自的优点,同时还会产生协同效应从而表现出更加优异的性能。采用自组装合成技术能够在分子水平上调控材料的微观结构、组成及形貌,对于开发具有特殊功能的纳米材料具有明显的优势。金属氧化物因其比容量高、储量丰富、绿色环保等优点成为锂离子电池的新型热门负极材料之一,其储锂机制主要基于嵌入-脱出、合金化-去合金化和氧化-还原三类反应,本文以分属三类储锂机制的典型氧化物TiO2、ZnO和铁氧化物(Fe2O3和Fe3O4)为研究对象,针对电极材料自身的缺点,开发了几种新的自组装合成技术,设计和制备相应的分级微纳结构或复合材料,以解决其比容量低、循环寿命短、倍率性能差等问题,并对表现出的性能优势与其结构特征内在关系进行了初步探讨。论文的研究工作主要分为以下几个方面：1.针对属于嵌入脱出机制的TiO2电极比容量较低的缺点,采用环已烷回流体系,开发了一种低温环保的二次自组装法,成功获得多孔结构的TiO2亚微米球,其孔结构具有赝电容效应,有助于电极材料比容量的提高。以TiO2纳米棒为结构基元,采用十八烯和环已烷两个体系,探讨纳米棒的自组装过程、结构及其性能。十八烯体系：TiO2纳米棒在合成过程中发生一次组装并形成丝状和蜂窝状两种结构；在随后的低温静置过程中发生二次组装,丝状和蜂窝状结构作为组装基元,不同纳米棒浓度的溶液在二次组装得到不同的结构,分别呈现向列相、球晶相、层状相溶致液晶行为；环已烷体系：TiO2纳米棒溶解在环已烷溶液中并一定温度下冷凝回流,纳米棒会发生自组装行为,形成具有介孔结构的TiO2亚微米球,将其表面的有机物进行原位碳化,得到TiO2/C复合结构。电化学测试表明与未组装的TiO2/C纳米棒相比,经过二次组装的TiO2/C亚微米球具有更高的比容量和循环稳定性,在1C电流密度下循环50次后其容量为151mAhg-1,接近二氧化钛的理论容量,在高达10C的密度下,容量依然可以保持90.1mAhg-1。2.针对属于合金化机制的ZnO电极体积膨胀较大的缺点,采用原位碳化组装法成功制备出一种碳网络负载正四面体状ZnO纳米晶复合结构,表现出优异的储锂性能。实验表明碳化温度是影响产物结构和性能的关键条件,碳化温度为500℃时形成厚度适中、稳定性高的碳网络,ZnO晶体为尺寸均一的正四面体,其尺寸分布为150-200 nm,所有的ZnO晶体都嵌入在碳网络中,优于碳化温度为400℃或700℃得到的结构。电化学测试表明,在电流密度为110.7 mAg-1时,该电极材料经300次循环后比容量可达518 mAhg-1,当电流密度升至1000mAg-1时,其可逆容量依然保持375.7 mAhg-1。通过系统分析和对比,其良好的储锂性能可能来源于以下两点：首先是ZnO晶体形状和尺寸分布均一；其次就是碳网络结构的稳定作用,因为碳化得到的无定形碳网络结构较为稳定,在充放电过程中ZnO发生体积膨胀后不会发生结构塌陷,保证复合结构的完整性,有助于其循环性能的提高。3.采用简单的回流首先合成含铁的前驱体,然后通过原位碳化制备多种成分的γ-Fe2O3复合材料,该方法简单环保,并可以实现大批量的生产。实验表明碳化温度是影响产物成分的关键因素,前驱体在500或600℃下碳化得到γ-Fe2O3/C结构,当碳化温度为700℃时,部分Fe3+被碳还原成金属态,得到γ-Fe2O3/Fe/C复合结构。实验表明成分为γ-Fe2O3/Fe/C的样品作为锂离子电池负极材料,在电流密度为200 mAg-1循环215次后比容量保持为1163.1 mAhg-1,500 mAg-1电流密度下循环500次其比容量高达1404.3 mAhg-1,而且在高电流密度(4000 mAg-1)下依然保持较高的比容量(640 mAhg-"1).复合材料中的FeO和碳网络可以有效缓解充放电过程中γ-Fe2O3的体积膨胀并显著改善电极的导电性,因此相比单纯的γ-Fe2O3/C结构,多种成分复合更有益于其电化学性能的提升。4.针对属于氧化还原机制的Fe3O4电极的体积膨胀的缺点,开发了基于协同自组装效应的一步水热法,成功制备出一种三维Fe3O4/G复合网络结构。采用乙二醇-水混合体系中,以FeCl3·6H2O为铁源,油酸钠作表面活性剂和反应助剂,通过氧化石墨烯的引入,实现氧化石墨烯和四氧化三铁的同时自组装,这两个自组装过程互相影响和促进,产生自组装协同效应。实验表明协同效应的存在使氧化石墨烯自组装形成凝胶的临界浓度大大降低,在浓度为0.13 mg/mL时也可以形成复合凝胶(其他体系一般要求大于1 mg/mL),同时氧化石墨烯自组装的存在也可以促进Fe3O4晶粒尺寸分布均一并阻止其团聚。电化学测试表明Fe3O4/G复合网络在500mAg-1的电流密度下,500次循环后其可逆比容量为1164 mAhg-1,容量保持率高达99.4%；此外,Fe3O4/G也表现出较好的倍率性能,在2000 mAg-1电流密度下其容量依然保持510 mAhg-1。
[Abstract]:Self assembled synthesis technology is through the regulation of nanometer size self-assembly, using two nanometer structure assembled in the synthesis process, carbonization in situ preparation technology of multistage micro nano structure or multiple composite materials. The self-assembled structure not only has the advantages of micro nano scale or multiple materials respectively, also can produce synergistic effect and performance a more excellent performance. The self-assembly synthesis technology to microstructure control of materials at the molecular level, composition and morphology, the obvious advantage of nano materials with special functions for the development of a metal oxide. Due to its high specific capacity, abundant reserves, green environmental protection has become one of the hottest new anode materials for lithium ion batteries and the lithium storage mechanism is mainly based on insertion extraction, alloying and dealloying and redox reaction of three kinds of the typical oxygen, belonging to three categories of lithium storage material T mechanism IO2, ZnO and iron oxides (Fe2O3 and Fe3O4) as the research object, aiming at the shortcomings of electrode materials, developed several new self-assembly synthesis technology, hierarchical design and preparation of micro nano structure or composite materials, in order to solve the low specific capacity, short life cycle, and poor rate performance problems. The internal relations of performance advantage and exhibits structural features were discussed. The research work of this thesis is mainly divided into the following aspects: 1. according to the TiO2 electrode insertion and extraction mechanism belongs to the specific capacity is relatively low, using cyclohexane back fluid system, the development of a low temperature environment of the two successful self assembly method. The porous structure of TiO2 submicrospheres, the pore structure has a pseudo capacitance effect, help to improve specific capacity of electrode materials. The TiO2 nanorods structure element, the two system eighteen ene and cyclohexane, investigate the nanorods from The assembly process, structure and properties. The eighteen ene system: TiO2 nanorods in the synthesis process, once assembled and formed two kinds of filaments and honeycomb structure; in the subsequent cold static occurred two times in the process of assembling, filamentous and cellular structure as the assembly element, solution of different concentrations of nano rods have different structure in two once assembled, respectively, showed nematic phase, spherulitic phase, lamellar lyotropic liquid crystal behavior; cyclohexane system: reflux condensation of TiO2 nanorods dissolved in Cyclohexane Solution and a certain temperature will occur self-assembly of nanorods, forming a mesoporous structure of TiO2 submicrospheres, the organic matter and the surface of the in situ carbonization, TiO2/C composite structure. Electrochemical tests show that compared with non assembled TiO2/C nanorods, after two assembled TiO2/C submicrospheres has higher specific capacity and cyclic stability in 1C Flow density after 50 cycles the capacity of 151mAhg-1, close to the theoretical capacity of titanium dioxide, the density of up to 10C, the capacity can still maintain the 90.1mAhg-1.2. for the ZnO electrode volume belongs to the alloying mechanism of expansion drawbacks, assembling method was successfully prepared by a network load is ZnO tetrahedronal carbon nano composite structure by in situ carbonization, showing excellent lithium storage performance. The experimental results show that the carbonization temperature is a key condition to influence the structure and properties of the product, the moderate thickness of carbide formation temperature is 500 degrees centigrade, the stability of the high carbon network, ZnO crystal is tetrahedral with uniform size and the size distribution of the 150-200 nm, all ZnO crystals are embedded in the carbon network, the structure is better than that by carbonization temperature to 400 degrees or 700 degrees Celsius. The electrochemical tests show that when the current density was 110.7 mAg-1, the electrode material after 300 cycles the specific capacity Up to 518 mAhg-1, when the current density to 1000mAg-1, its reversible capacity remains 375.7 mAhg-1. through systematic analysis and comparison, the good lithium storage performance from the following two points: first, the ZnO crystal shape and uniform size; the second is the carbon network structure stabilizing effect, because the amorphous carbon carbide network structure more stable, the occurrence of collapse of the structure not volume expansion of ZnO in the process of charge and discharge, ensure the integrity of the composite structure, contribute to the performance of the cycle to improve the.3. first synthesis of precursor containing iron by simple reflux, gamma -Fe2O3 composite prepared by in situ carbonization and many components, the method is simple and environmental protection, and can realize batch production. Experiments show that the carbonization temperature is a key factor affecting the composition of products, precursor carbonization get gamma -Fe2O3/C structure in 500 or 600 DEG c, When the carbonization temperature is 700 degrees centigrade, part of reducing Fe3+ by carbon into metal state and get gamma -Fe2O3/Fe/C composite structure. The experimental results show that the composition of gamma -Fe2O3/Fe/C samples as the anode materials for lithium ion batteries, at a current density of 200 mAg-1 after 215 cycles the capacity retention was 1163.1 mAhg-1500 mAg-1 than the current density of 500 cycles than the capacity of its up to 1404.3 mAhg-1, and in the high current density (4000 mAg-1) to maintain high specific capacity (640 mAhg- 1). The composite of FeO and carbon network can effectively alleviate the electric charge and discharge process of gamma -Fe2O3 volume expansion and significantly improve the electrode, compared with the pure gamma -Fe2O3/C structure. Fe3O4 composite electrode of various components is more beneficial to the electrochemical performance of.4. to enhance the redox mechanism belongs to the expansion of the disadvantages, develop a cooperative self-assembly effect of one-step hydrothermal method based on a Work to prepare a three-dimensional network structure. The Fe3O4/G composite using glycol water mixed system, using FeCl3 6H2O as the source of iron, sodium oleate as surface active agent and reaction agent, through the introduction of graphene oxide, the graphene oxide and Fe3O4 and self-assembly, the two self-assembly process of each other the influence and promotion, to produce self-assembled synergistic effect. The experimental results show that the synergistic effect of the critical concentration of graphene oxide gel self-assembly is greatly reduced, at the concentration of 0.13 mg/mL can also form a composite gel (other systems generally require more than 1 mg/mL), while the graphene oxide self-assembly there can also promote Fe3O4 grain uniform size and prevent agglomeration. Electrochemical tests show that Fe3O4/G composite network in the current density of 500mAg-1, after 500 cycles, the reversible capacity is 1164 mAhg-1, the capacity retention rate of 9 9.4%, in addition, Fe3O4/G also showed good multiplier performance, and its capacity remained 510 mAhg-1. at 2000 mAg-1 current density.

【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O611.2;TB383.1

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