【作者】 刘瑞卿；

【导师】 李宁；

【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】 在能源危机和环境污染的严重挑战下，电动汽车的发展在世界各国都给予了高度的重视。SnO2负极材料在目前研发的负极材料中，由于低嵌锂电压、高嵌锂容量，环境友好等优点被视为一种极具潜力的新一代动力电池负极材料。但是SnO2负极材料存在循环寿命较差和大电流充放电性能差等问题，限制了其商业化应用。为此本文对SnO2电极的嵌锂行为进行了量化计算，可以更具体直观地理解SnO2电极发生巨大体积膨胀的内因。在计算结论的基础上对SnO2材料进行性能的改进和提高，对SnO2材料进行掺杂、制备特殊形貌的SnO2材料以及特殊形貌的SnO2基复合材料，对它们的电化学性能进行研究，分析其性能提高的影响机理，为获得高性能SnO2基材料提供有价值的指导。利用第一性原理计算方法研究了SnO2负极材料在嵌锂过程中形成的晶态LixSn的电子结构、几何结构等微观变化。结果表明，SnO2材料被还原为Sn单质后，在锡与锂合金化过程中，随着嵌锂浓度的增加，晶态锡逐渐发生相变转化为LixSn，都表现出金属导电性质。LixSn在费米能级附近的电子呈现出不同的波动值，说明不同嵌锂浓度的LixSn有不同的电子传导性，这是由于不同晶体结构所具有的混合轨道的不同所引起的。Sn、Li2Sn5、LiSn和Li22Sn5在费米能级处的N(EF)值分别对应1.01、5.18、3.71和24.26states/eV，嵌锂相具有高的导电性就可以较容易地形成稳定的SEI膜和电荷传输，从而提高循环性能。但是通过计算LixSn的几何结构可知Li22Sn5的体积膨胀系数最大(343%)。由此可见，对于提高其循环性能来说，LixSn合金化合物高的导电性和大的体积膨胀是相互制约的一对矛盾。在嵌锂行为分析的结论上，考虑到提高材料的电子传导率，研究Ni、W掺杂对SnO2材料电化学性能的影响。Ni、W掺杂后，抑制了SnO2晶粒的增大，提高了SnO2的导电性，掺杂W的效果更明显。在循环前期电极粉化较少，电极完整性保持较好的时候作用不明显；在后期当电极遭受巨大体积膨胀裂纹较多粉化较严重时候，高导电性能够更好的保持电极本身的电子传导，从而延长循环寿命。通过Mott-Schottky测试研究了电极在脱嵌锂循环过程中SnO2的状态，证实了中间产物Li2O催化分解反应的部分可逆现象。对水热法制备特殊形貌SnO2微球的工艺进行了优化，制备了实心SnO2微球、中空SnO2微球和部分核壳结构的SnO2中空微球，并对这三种形貌的SnO2材料进行电化学性能的对比研究，揭示影响SnO2电极循环性能的因素。研究发现，实心SnO2微球电极首次放电容量为842.1mA h g1，循环48次后可逆容量保持在386.5mA h g1；中空SnO2微球电极的首次放电容量为996.5mA h g1，循环65次后可逆容量保持在406.5mA h g1；部分核壳结构的SnO2中空微球电极的首次放电容量为688.8mA h g1，循环100次后可逆容量保持在374.2mA h g1。结果说明中空结构的材料对于循环性能的提升有很大优势；部分核壳结构的SnO2中空微球电极的放电容量并不高，但是循环性能最好，这应该主要归因于不完全的空心核壳结构内部的微腔缓冲了巨大的体积膨胀，同时，核壳的相互支撑加强了在脱嵌锂过程中的结构稳定性。进一步观察发现，热处理后样品的充电电压平台比未经过热处理的充电电压平台高0.05V左右，随后的放电电压却比未经过热处理的电压稍低。充电电压的增加以及放电电压的降低扩展了材料脱嵌锂的有效电压范围，便利了电极材料脱嵌锂的反应，增大了电极的脱嵌锂容量。在中空SnO2微球的基础上，设计并制备了不同的中空SnO2核壳复合材料。中空SnO2/B2O3核壳复合材料尤其是含B2.1wt%复合材料在160次充放电后容量仍保持在622.7mA h g1，3900mA g1(5C)下的容量保持在528.6mA h g1以上。循环性能的提升是因为微球表面的非活性B2O3壳层缓冲了巨大的体积变化和纳米颗粒的团聚，同时，内部的空间也一定程度上容纳了一部分体积变化；倍率性能的提升归因于B原子由于自身的电子缺陷，降低了电荷传递阻抗，意味着离子传导率的提高。中空SnO2/21wt%PPy核壳复合材料电极在100次循环后的可逆容量保持在448.4mA h g1；中空SnO2/17wt%rGO/21wt%PPy三元核壳复合材料电极在100次循环后的可逆容量保持在647.8mA h g1；在电流密度为3900mA g1(5C)时，三元复合物所展现的可逆容量仍在117.6mA h g1之上。通过CV曲线计算了三种复合材料各自的Li+的扩散系数，分别为4.5×108cm2s1，7.4×109cm2s1，1.8×10-8cm2s1，都比中空SnO2微球电极的Li+扩散系数1.2×109cm2s1大，也解释了倍率性能优异和电化学性能提升的原因，为SnO2负极材料的实用化提供了指导和借鉴。更多还原

【Abstract】 The development of electric vehicles in the world has been paid close attention to face the severe challenges of energy crisis and environmental pollution. In the current development of the anode materials, SnO2is regarded as a potential next-generation anode material of power lithium ion batteries for electric vehicles, due to its advantages of low lithium inserted potential, high capacity and environmental friendliness. However, its commercial application has been limited by its poor cycle life and high current charge-discharge performance. So quantum chemical calculations for lithium intercalation behaviors were carried out in this paper, which can be more intuitive understanding the internal of dramatic volume expansion in SnO2electrodes. Based on the calculated conclusions, doped SnO2materials, special morphology SnO2materials and special morphology SnO2-based composite materials were prepared to improve the electrochemical properties. The impact mechanisms were studied to further guide the design and synthesis of high performance SnO2-based materials.The geometric and electronic structures of crystalline LixSn formed during lithiation of SnO2anode material were investigated by first-principle calculations. SnO2material is reduced to the elemental Sn, in processes of the phase transformation of crystalline Sn to crystalline LixSn with increased concentration of lithium intercalation, and the results of quantum chemical calculation showed that crystalline LixSn exhibits metallic conductivity. With the concentration of lithium insertion increasing, the electrons near the Fermi level in DOS, N(EF), display fluctuant values, declaring that the electric conductivity is different for various Li intercalation compounds. This phenomenon results from the differences of hybrid orbital caused by diverse crystal structures. The value of N at the Fermi level (EF) of Sn, Li2Sn5, LiSn and Li22Sn5are1.01,5.18,3.71and24.26states/eV, respectively. The high conductivity of lithium intercalation phases is beneficial to the formation of stabilized Solid Electrolyte Interphase (SEI) films and electron transfer, resulting in improved cycling performance. However, the volume expansion coefficient of Li22Sn5is maximum (343%) by calculating the geometry of LixSn. Thus, high conductivity and large volume expansion of LixSn alloy compounds is a pair of conflicting constraints for improving their cycling performance.Based on the analysis conclusion of lithium intercalation behavior, taking into account the electronic conductivity, the impacts of Ni and W dopants on the electrochemical properties of Ni-doped SnO2and W-doped SnO2were researched. SnO2grains were decreased and the conductivities of SnO2were improved after doping with Ni, W, and the effect of W-doped SnO2is more obvious than that of Ni-doped SnO2. At the early stage of the cycle, the impact was less obvious when the integrity of electrode was maintained well; when the electrode has more cracks and severe pulverization due to huge volume expansion at later period of the cycle, high conductivity electrodes are better able to maintain their electronic conduction, thus extending the cycle life. The portion reversible phenomenon of intermediate Li2O catalytic decomposition reaction was confirmed by Mott-Schottky testing investigating SnO2state during lithium de-intercalation cycling.Hydrothermal synthesis process of special morphology SnO2microspheres was optimized to prepare SnO2solid microspheres, SnO2hollow microspheres and partial core-shell structure SnO2hollow microspheres. The electrochemical properties of these morphology SnO2microspheres were studied to reveal the factors that affect the cycle performance of the electrodes. The study found that the three prepared SnO2microspheres exhibit different cycle performances. The initial discharge capacity is842.1mA h g1for SnO2solid microspheres,996.5mA h g1for SnO2hollow microspheres, and688.8mA h g1for partial core-shell structure SnO2hollow microspheres. The retained specific capacity for each sample is386.5mA h g1up to48cycles for SnO2solid microspheres,406.5mA h g1up to64cycles for SnO2hollow microspheres, and374.2mA h g1up to100cycles for partial core-shell structure SnO2hollow microspheres. These results suggested that the hollow structured materials have advantages for the anode cycle performance. Partial core-shell structure SnO2hollow microspheres exhibited the most excellent cycle performance. The superior stability of partial core-shell structure SnO2hollow microspheres can be attributed to the formation of smaller SnO2hollow microspheres and incomplete core-shell structure. The interior microcavities of core-shell structured materials are capable of accommodating large volume change. Simultaneously, the mutual support between core and shell heightens structural stability during Li insertion-extraction. In addition, the charging voltage platform of sample with heat treatment is higher (about0.05V) than that without heat treatment; the discharge voltage of sample with heat treatment is lower than that without heat treatment. The increased charging voltage and reduced discharge voltage expanded effective voltage range of lithium de-intercalation, facilitated the lithium de-intercalation reaction of electrode material and increased the lithium de-intercalation capacity of the electrode.Different SnO2-based hollow core-shell structure composite materials were prepared via hydrothermal-impregnation method and hydrothermal-polymerization method based on SnO2hollow microspheres. In the case of hollow SnO2/B2O3core-shell composites, the cycle performance has been greatly improved, especially for B2.1wt%electrode, still maintaining622.7mA h g1of discharge capacity at the160th cycle; at a rate of5C (3900mA g1), the specific capacity of B2.1wt%electrode is above528.6mA h g1. Apart from the inner hollow space that is able to mitigate the enormous volume change to some degree, the much improved cycle performance can be attributed to the inactive B2O3buffer layer, accommodating the enormous volume change during continuous cycling and keeping the nanoparticles from agglomeration during the charge-discharge process. The enhanced rate performance is ascribed to the electron-deficient nature of boron, which reduced the Rct, indicating enhanced ionic conductivity in the nanocomposite. The retained specific capacity is448.4mA h g1in the100th cycle for the hollow SnO2/21wt%PPy core-shell nanocomposite anode;647.8mA h g1in the100th cycle for the hollow SnO2/17wt%rGO/21wt%PPy ternary core-shell nanocomposite anode. At a current density of3900mA g1(5C), the ternary composite material still exhibited a reversible capacity above117.6mA h g1. The diffusion coefficient of Li+ions (DLi) was calculated from a linear relationship between Ip and v1/2according to CV curves. The Li+diffusion coefficients in hollow SnO2/20wt%B2O3core-shell composites, hollow SnO2/21wt%PPy core-shell nanocomposites and hollow SnO2/17wt%rGO/21wt%PPy ternary core-shell nanocomposites were calculated to be4.5×108cm2s1,7.4×109cm2s1and1.8×10-8cm2s1, which were all larger than that of SnO2hollow microspheres (1.2×109cm2s1). These data explain the reason of excellent rate capability and electrochemical performance, providing guidance and reference for SnO2practical application as anode materials.更多还原