【作者】 夏阳；

【导师】 张文魁；

【作者基本信息】 浙江工业大学 ， 材料化工， 2013， 博士

【摘要】 锂离子电池作为一种最有希望的绿色储能技术,已被广泛应用于各种电子设备和混合动力交通工具。然而,在实际应用中,锂离子电池通常存在电极材料比容量低、循环衰减快、倍率性能不佳等问题。本论文针对上述问题,以正极材料LiFePO4和单质S,负极材料NiO和MnO为研究对象,设计多级多孔微/纳复合结构模型,通过将生物模板引入电极材料制备过程,探索生物模板对电极材料结构调控的影响因素,系统研究了基于生物模板的多级多孔微/纳复合结构对电极材料的电化学性能影响。主要研究内容如下：(1)首次利用一种简便的水热合成工艺制备出纺锤形多级多孔LiFePO4材料。通过调控反应时间和pH值,研究了水热合成条件对LiFePO4晶体形貌和微结构的影响。研究结果表明：反应时间为20小时,前驱体溶液pH值为10时,可成功合成多级多孔微/纳复合结构的LiFePO4纺锤体颗粒,并提出“形核-团聚-自组装排列-熟化”生长机制。通过树脂包埋技术对LiFePO4纺锤体颗粒切片剪薄,利用透射电镜(TEM)研究了颗粒内部晶粒之间的生长取向关系和组装方式。电化学性能研究表明,这种多级多孔微/纳结构不仅具有优异的结构稳定性,可确保材料在服役过程中不会出现崩塌、脱落现象,同时其较高的比表面积,可为电极材料和电解液提供更大的接触面积,增强锂离子的扩散能力。在0.1C(1C=170mA/g)倍率下,经过50次循环,该材料放电容量仍可保持在157mAh/g以上；即使在5C高倍率条件下,其可逆容量仍高达110mAh/g以上,充分显示出多级多孔微/纳复合结构具有优异的循环稳定性和倍率性能。(2)首次将活体螺旋藻同时作为生物模板和碳源原位引入LiFePO4的水热制备过程,成功合成出螺旋形多级多孔结构LiFePO4/C复合材料。研究结果表明,利用藻类生物对金属离子具有诱导吸附特性,可将前驱体粒子均匀吸附在藻类表面,从而实现生物模板形貌和结构的精确复制。通过X射线衍射(XRD)、高分辨投射电镜(HRTEM)和拉曼光谱(Raman)等微结构表征手段发现,这种复合材料是由LiFePO4纳米晶粒有序堆积并通过外层“碳网”形成多级结构,牢牢地依附在螺旋形“碳骨架”表面构成。这种特殊结构具有多重优势：一方面可以增加LiFePO4材料的比表面积；另一方面,同时保证了复合材料具有良好的离子和电子导电性。因此,该复合材料具有较高的可逆比容量(在0.1C倍率下50次循环后,可逆容量大于140mAh/g),优异的循环稳定性能(容量保持率接近100%),以及较好的倍率性能(5C倍率下,可逆容量仍可保持在100mAh/g以上)。(3)首次将生物模板法与化学浴沉积法(CBD)相结合,成功在预碳化后花粉表面构建多级多孔NiO薄片,获得中空球形多级多孔NiO/C复合电极材料。通过与无模板合成的多孔NiO粉体对比,该复合材料显示出巨大结构优势,如比表面积高、孔径分布丰富和导电性好。电化学测试表明,基于花粉为模板的NiO/C复合电极材料首次循环产生的不可逆容量仅为284mAh/g,库伦效率高达74.5%,远优于多孔NiO负极材料。在循环和倍率测试中,经过80次倍率充放电测试后,以花粉为模板的NiO/C复合电极材料仍能给出高达618mAh/g的可逆放电比容量。如此优异的电化学性能可以归因于以下几方面原因。首先,NiO纳米薄片可有效缩短锂离子迁移距离,便于锂的快速嵌入和脱出。其次,多孔结构可以帮助电解液更好的润湿材料,提供更大的反应面积,并可调节由反复充放电引起的体积膨胀所导致的应力；第三,NiO纳米薄片直接生长在导电多孔碳基底表面可以形成牢固的连接,有利于增强NiO和碳之间的电接触,减小反应电阻。(4)通过将生物模板法和液相浸渍法结合,发展了一种简便制备中空球形MnO/C复合材料的新方法。利用XRD、TEM、HRTEM和STEM等先进材料表征手段系统研究了中空球形MnO/C复合材料的微结构,结果表明：大量MnO纳米颗粒被牢固钉扎在泡沫状多孔碳基体内形成一个具有渗透性的多级多孔外壳,并且该MnO/C复合微球还具有典型的中空结构特征。研究发现采用微绿球藻作为生物模板,其扮演了多个角色：一方面它可以通过自身生物活性,利用细胞膜的静电力作用吸附前驱体离子；另一方面,由于它本身含有机物可作为可再生“绿色”碳源,在高温分解过程中可形成多孔碳。作为锂离子电池负极,该复合材料在0.1A/g倍率下,经循环50次后,容量保持率大于94%,可逆容量高达700mAh/g。同时在3A/g高倍率下,其可逆容量高于230mAh/g。MnO/C复合材料优异的电化学性能主要来自于以下几个方面：MnO纳米颗粒可缩短锂离子的扩散路径,提高锂离子扩散迁移能力；多孔结构可使电极材料更容易被电解液润湿,增大的电化学反应面积；同时,导电碳基底不仅可以防止MnO颗粒膨胀、团聚和脱落,也能为MnO提供高速电子转移通道；而中空结构可有效缓解材料在充放电中产生的剧烈体积变化。(5)首次采用裂壶藻作为生物模板和绿色碳源合成中空多孔碳微球,并结合一种新颖的溶剂热储硫工艺,成功制备出中空多孔S/C复合电极材料。研究表明：裂壶藻通过碳化处理后,可完整保持其原有形貌和尺寸,并形成一个独特的中空多孔碳微球。该碳微球不仅具有较好的结构稳定性,同时也具有优异的电导性,因此是一种理想的储硫载体材料。与硫复合后,S/C复合电极材料的形态并未发生明显改变,但硫元素以小硫分子的形式高度均匀地分散在碳微球载体的孔隙和石墨插层中,与碳形成了较强的结合力。通过组装扣式电池发现,S/C复合材料具有优异的电化学性能。在0.1A/g电流密度下循环50次后,该复合材料的可逆容量为697.2mAh/g,容量保持率高于95%,循环稳定性突出。此外,即使在高倍率循环条件下,如5A/g, S/C复合材料仍可提供452.8mAh/g可逆放电容量,显示出较好的倍率性能。更多还原

【Abstract】 Lithium-ion batteries (LIBs) are considered as the most promising rechargeable energy storage technology, which can be applied in electronic devices and hybrid electric vehicles. However, the practical application of LIBs is still hampered by the poor electrochemical performance of the electrode materials, such as the low specific capacity, poor cycling stability and rate capability. In these researches, LiFePO4and S (cathode materials), NiO and MnO (anode materials) are selected as research objects, we designed the hierarchical porous micro-/nanostructure as an ideal model for electrode materials, and used novel strategies by using various biomaterials as both templates and carbon sources to fabricate electrode materials. We have made great efforts to understand the key factors of using biotemplates to tune the microstructure and morphology of electrode materials. Moreover, the relationships between the hierarchical porous micro-/nanostructure and electrochemical performances also have been studied. The main contents and results are as follows:1. A new kind of hierarchical spindle-like LiFePO4was successfully synthesized by a facile hydrothermal method for the first time. Reaction time and pH value played multifold roles in controlling the microstructure of LiFePO4. Spindle-like LiFePO4particles with a hierarchical porous structure can be obtained after20h reaction with pH=10. A growth model of "Nucleation-Aggregation-Self-assembly&Rearrangment-Further growth" was proposed to illustrate the formation of spindle-like LiFePO4samples. In order to investigate the growth orientations and grain boundaries of these nanocrystals, spindle-like LiFePO4sample was prepared by slicing it embedded in expoxy, and studied by transmission electron microscopy (TEM). The electrochemical performances clearly demonstrated that this hierarchical porous structure has the good structural stability, and also can increase the reaction surface between the electrolyte and the electrode materials and promote lithium ion diffusion. At a0.1C rate, the initial discharge capacity was163.57mAh/g, and it could remain157.32mAh/g after50cycles, suggesting the excellent cycling stability; Even at a5C rate, the reversible capacity was still as high as110mAh/g, implying the good rate capability.2. The live spirulina was used as both the biotemplate and carbon source for the fabrication of spiral hierarchical porous LiFePO4/C. Owing to the biological activities of algae, spirulina can adsorb and take up metal ions. Therefore, we can precisely replicate the morphology and microstructure of spirulina. The X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman results show that the obtained LiFePO4/C sample was composed of abundant LiFePO4grains connected and wrapped with "carbon networks", and attached at the surface of the spiral "carbon backbone". Such unique microstructure could enhance the reaction area, and is also beneficial to increase the electronic/ionic conductivity. As a result, spirulina-templated LiFePO4/C sample exhibited high reversible capacity (at a0.1C, the specific capacity was over140mAh/g after50cycles), excellent cycling performance (the capacity retention rate is nearly100%), good rate capability (at a5C rate, the reversible capacity was over100mAh/g).3. The facile strategy was developed for the fabrication of hierarchical hollow porous NiO/C microspheres deriving from the pollen grains by combining the biotemplating technique with a chemical bath deposition (CBD) method. The structural analysis revealed that NiO nanowalls directly grew on the surfaces of pollen grains and formed a hierarchical porous structure. Compared with porous NiO particles without biotemplates, pollen-templated NiO/C sample has a large specific surface area, multiple pore size distribution and good electronic conductivity. The initial irreversible capacity loss was only284mAh/g with a high coulombic efficiency of74.5%, which was much smaller than pure NiO sample with porous structure. After80cycles with the multiple-rate test, the pollen-templated NiO/C sample still delivered a high reversible capacity of618mAh/g. These enhanced electrochemical performances can be attributed to the following reasons. Firstly, NiO nanowalls can provide short pathways for fast lithium ions intercalation/de-intercalation. Secondly, the multiple porous structures can provide a larger surface area and allow better penetration of the electrolyte, as well as accommodate the strain induced by the volume changes during the charge/discharge cycling process. Thirdly, NiO nanowalls grew directly on the surface of the porous carbon support can form good adhesion and better electrical contact between NiO and carbon, which is beneficial for reducing charge transfer resistance.4. A facile and general approach combined the biotemplating technique with the immersion method was used to synthesize a unique microstructure of MnO/C microspheres. Morphology and microstructure of the hollow porous MnO/C microspheres have been investigated by XRD, TEM, HRTEM and STEM. The results demonstrate that MnO nanoparticles were tightly embedded into the porous carbon matrix and formed penetrative shells, suggesting a hollow feature. It was found that the biotemplates of Nannochloropsis oculata (N. oculata) played multiple roles, which acted as biologic templates to adsorb and take up metal ions via the electrostatic interaction, as well as carbon sources to form porous carbon matrices by the decomposition of carbonaceous organics in cells. As the anode materials in lithium ion batteries, N. oculata-templated MnO/C composites can not only deliver a high reversible capacity of700mAh/g at a low current density of0.1A/g, but also exhibit remarkable rate performance (over230mAh/g at3A/g). Such outstanding morphology and microstructure have the following advantages for achieving excellent electrochemical performance. Firstly, nano-sized MnO particles can effectively reduce the length of Li+solid state transport paths to give high Li+diffusion rate. Secondly, the pores in the MnO/C composite can well contact with the electrolyte, and achieve large reaction area. Thirdly, the amorphous carbon matrix can keep the structural integrity of the electrode, and provide good electronic contacts to reduce the contact resistance between MnO nanocrystals and electrolyte. Fourthly, hollow structure can enlarge the reaction surface area, and reversibly accommodate the drastic volume variation during the charge/discharge cycling process.5. Hollow porous carbon microspheres were synthesized by using Schizochytrium sp. cells as both biotempates and carbon sources. Combined with a facile and novel solvothermal methode, a new sulfur-carbon (S/C) composite was fabricated by confining sulfur in the as-prepared hollow porous carbon microspheres. The microstructure and morphology characterizations demonstrate that Schizochytrium sp. could remain its original morphology and size during the carbonization process. The as-prepared carbon microspheres have a unique hollow porous structure with a diameter of1-2μm, and possess excellent structural stability and good electronic conductivity, which can be considered as an ideal carbon support material for sulfur-carbon electrodes. After the solvothermal reaction, the elemental sulfur was highly dispersed inside the interior void space and micropores of carbon microspheres, and formed strong bondings between S and C. When evaluated as a cathode material for Li-S batteries, hollow porous S/C composites show remarkable electrochemical performances with high specific capacity, good cycling stability, and superior rate capability. In particular, S/C composite electrodes exhibit a high reversible capacity of697.2mAh/g with the capacity retention of95%after50cycles at a current density of0.1A/g. Moreover, even at a high current density of5A/g, the S/C composite still can deliver fascinating capacity of452.8mAh/g.更多还原