【作者】 王强；

【导师】 李景虹；

【作者基本信息】 中国科学技术大学 ， 分析化学， 2007， 博士

【摘要】 为了满足世界范围内对能源转化和储备日益增长的需求，目前大量的研究工作直接与寻找新的材料概念以及多样化的合成方法相联系。在电极材料方面的突破是下一代锂离子电池成功开发的关键正逐步被广泛接受，这必将推动锂离子电池技术在便携式电子设备、清洁能源储备、混合动力汽车等领域的加速发展。本论文综述了锂离子电池负极材料的研究进展。针对无机类富勒烯结构、锡基复合氧化物和过渡金属硫（氧）化物的充放电循环及倍率性能较差的特点，本论文采用低温水热合成法制备了一系列具有新型结构的负极材料。利用XRD、SEM、TEM和XPS等技术对这些材料的微观结构和形貌进行了分析，采用恒电流充放电、循环伏安（CV）和电化学阻抗谱（EIS）等技术测试其电化学性能。本论文主要开展了以下几个方面的研究：1．MoS₂和WS₂类富勒烯结构基于转换反应过程展示了相当高的嵌脱锂活性，然而怎样确保快速而可逆的转换反应却是一个无法回避的问题。为了获益于这种转换反应所提供的高嵌锂容量并克服其固有的低动力学特性，通过利用碳纳米管众多优异的物理、化学和机械性质，本论文采取了将MoS₂与碳纳米管同轴复合的概念。MoS₂／CNT同轴纳米管因其具有特殊的异质结构，优势互补，产生了协同效应进而导致增强的电化学性能。基于获得的实验结果，MoS₂／CNT同轴纳米管在保持MoS₂高容量（接近400 mAh／g）的同时，还具有很好的循环稳定性（每循环衰减低于0.8 mAh／g）和改进的倍率性能。这种优异的性能表明了碳纳米管载体在增强电子电导率，维持热力学／动力学稳定性和降低电荷转移阻力方面所起的关键作用。2．对锂离子电池而言，锡基复合氧化物是最具吸引力和最具竞争力的电极材料，然而合金化过程中严重的体积效应对电极电极造成了剧烈的机械损伤，导致电极材料逐渐粉化，合金结构被破坏，从而引发循环性能下降。针对锡基复合氧化物的体积效率，通过使用独特的具有纳米级特点的自组装微球来实现良好的倍率和循环性能将是一个有效的途径。在Sn_1.0P_1.17O_4.72自组装微球中，相互连接的纳米片构筑单元采取了“松散”的堆积方式，这不仅有助于缓冲合金化过程中巨大的体积变化引发的机械应力，提高其循环稳定性，而且还能最大程度的发挥纳米尺度效应，促进倍率性能的改进。在倍率为1C时，该自组装微球表现出了较好的容量性能，可逆容量在20个循环后还保持在514 mAh／g，放电容量保持率在87％以上。而且，这种材料还具有比较良好的倍率性能，6C放电的可逆容量保持率在72％以上。此外，本论文还对TiO₂自组装微球的嵌锂电化学性能进行了表征，为具有新型纳米结构的TiO₂材料作为安全锂离子电池负极材料的可能应用提供了实验依据。3．虽然将转换反应电极材料引入锂离子电池体系看起来非常吸引人，然而真正的应用则被巨大的技术困难所困扰，譬如材料相对较低的电子导电率或者电极设计过程不恰当的配置方式。为此，本论文着手设计可以将集流体（泡沫镍）和过渡金属硫化物（Ni₃S₂）有效地结合起来从而能够利用转换反应来获得高容量的方法。此外，为了增强这种不含碳添加剂的自支撑电极的电化学转换过程的动力学性能，本论文采用了具有纳米结构的薄膜，使得电极在嵌脱锂时的界面反应位置多，扩散路径短。电化学测试结果表明在倍率为2C时，Ni₃S₂纳米薄膜电极几乎取得了稳定可逆的容量（400 mAh／g），并且在20次循环后容量保持率超过81％。值得注意的是该电极即使在16C的高倍率条件下还可以保持超过43％的可逆容量，这几乎是过渡金属硫化物在此条件下能达到的最高水平。4．考虑到TiO₂一维纳米材料工作电位适中、循环寿命长、可进行快速充放电而无安全隐患等特性，本论文研究了其作为一种有机系混合超级电容器负极材料的可行性。本论文首次提出以碳纳米管为正极，TiO₂纳米线为负极组装混合超级电容器，并考察了这种新型储能设备在0到2.8V电位窗口范围内的电化学性能。基于活性材料的总重量，在10C的倍率条件下，混合超级电容器的比能量为12.5 Wh／kg，两倍于碳基超级电容器的比能量，同时也维持了良好的循环稳定性。电化学性能的改善得益于以较大比表面积和适当孔径分布的碳纳米管为正极，在电极／电解液界面处发生静电吸附存贮大量电荷，促使TiO₂纳米线电极可以在较为宽的电位窗口内通畅地进行对应的法拉第反应，维持其优异的电容性能，这将在一定程度上改善了混合超级电容器的功率密度和能量密度，尤其在较大的电流密度下更有优势。更多还原

【Abstract】 In response to the increasing demands for energy conversion and storage worldwide, considerable efforts in this aspect have to be directly connected with a challenging search for new materials concepts as well as multidisciplinary approaches. It is generally accepted that breakthroughs in materials hold the key to new generations of lithium-ion batteries, not only for applications in portable electronics but especially for clean energy storage and use in hybrid electric vehicles. In this dissertation, we have reviewed the development in research of negative electrode materials of lithium-ion batteries. Pursuing these efforts towards improving the rate and cycling capability of inorganic fullerene-like materials, tin-based composite oxides and transition-metal sulphides （oxides）, we have explored the feasibility of by using low-temperature hydrothermal techniques to design electrode materials with novel architectures, which were characterized by XRD, TEM, SEM and XPS measurements. Their electrochemical performances have been evaluated by means of various techniques including galvanostatic method, cyclic voltammetry （CV） and electrochemical impedance spectroscopy （EIS）. The main content is the following:1. Inorganic fullerene-like materials such as WS₂ and MoS₂ have demonstrated the surprising large reactivity versus lithium based on a conversion process reaction. However, such excitement needs to be tempered because ensuring rapid and reversible conversion reactions is an unavoidable issue. To benefit from the large capacity gain advantages offered by the conversion reactions and to overcome poor kinetics, a coaxial MoS₂/CNT architecture is reported. By taking advantage of carbon nanotubes, the nanometer-sized multifunctional heterostructures offer enhanced properties through the cooperative contribution of each component. On the basis of our results, the coaxial MoS₂/CNT architecture has been confirmed to demonstrate highly reversible capacity （approaching 400 mAh/g） and excellent cyclability （less than 0.8 mAh/g per cycle）, improved rate capability. These favorable results suggest that the key role of carbon nanotubes in increasing electric conductivity, maintaining thermodynamical/kinetical stability and decreasing charge-transfer resistance of the MoS₂.2. For lithium-ion batteries, tin-based composite oxides are among the most appealing and competitive candidates for new types of electrode materials. Unfortunately, the enormous volume changes generated during the alloying processes cause critical mechanical damage to the electrode, resulting in cracking and crumbling of the electrode and a remarked loss of capacity. Such problems might be addressed with the self-assembled microspheres with exciting nanosize effects. Accordingly, achieving the outstanding rate and cycling capability become possible. The porosity obtained via nanoplate assemblies within the Sn_1.0P_1.17O_4.72 microspheres not only help accommodate the mechanical stress resulting from the large volume change during alloying to maintain the structure integrity, but also maximize the rate capability benefits of shorter diffusion pathways at the nanoscale. For such microspheres, the reversible capacity was maintained at 514 mAh/g after 20 cycles and the discharging capacity retention was more than 87% at a 1C rate. Moreover, more than 70% of the reversible capacity could be delivered even at a 6 C rate. In addition, the electrochemical reactions toward lithium involved in TiO₂ self-assembled microspheres were investigated, and superior characteristics associated with unique three-dimensional architectures have been observed. We believe that this material could be employed as an anode material for next-generation high-safety lithium-ion batteries.3. Although the introduction of the conversion electrode materials into lithium-ion batteries might seem quite attractive, implementation is beset with formidable technical difficulties such as intrinsic poor electronic conductivity of the materials or to inadequate means of formulating/configuring the electrodes. In anticipation of such needs, we embarked on a study of electrode designs that could efficiently marry the current collectors with transition-metal sulphides so as to utilize the capacity gains associated with conversion reactions. Furthermore, to favor the electrochemical conversion performances of such carbon-free self-supported configuration and fight kinetic limitations, we significantly decrease the diffusion length while preserving an effective electronic and/or ionic percolation. A preliminary result shows that the electrode is found to achieve sustained reversible capacities （400 mAh/g） and more than 81 % of the reversible capacity was retained after 20 cycles at a 2C rate. Strikingly, the electrode can sustain high rates and it turns out that more than 43% of the reversible capacity could be delivered even at a 16C rate, which is the largest level specific capacity for transition metal sulphides at the high rates.4. Taking into account the proper potential window, high specific capacity, extended cycle life, fast rate capability and low safety concerns, one-dimensional nanostructured TiO₂ seems to be extremely attractive and should be a promising candidate for nonaqueous hybrid supercapacitor application. We for the first time fabricated a hybrid supercapacitor which consisted of a carbon nanotube cathode and a TiO₂ nanowire anode, and presented the preliminary results for such an energy storage device operating over a wide range of 0-2.8 V. Based on the total weight of both active materials, such hybrid supercapacitor delivered an energy density of 12.5 Wh/kg at 10 C rate, twice higher than the values of the carbon nanotube-based supercapacitor, while maintaining desirable cycling stability. All these profit from using the carbon nanotubes with large surface and proper pore distribution as the cathode, which ensure the TiO₂ nanowire anode precede with the faradic reaction in the applied potential range. The hybrid supercapacitor exhibits improved power and energy performances by the increased potential window, particularly in the larger current density.更多还原