【作者】 张涛；

【导师】 吴浩青； 吴宇平；

【作者基本信息】 复旦大学 ， 物理化学， 2007， 博士

【摘要】 锂离子电池是功能材料和电化学学科在能量储存领域研发成功的应用实例之一。随着应用领域的不断拓展，对电池的容量、功率和安全性能的要求越来越高。本论文主要研究储锂活性高、比容量大的硅材料，通过材料的纳米化、复合化和薄膜化，来提高其循环性能和安全性能。本论文首先研究了采用激光气相沉积法制备的纳米多晶硅粒子的电化学性能和容量衰减机理。硅材料在充分插锂状态(Li_4.4Si)，理论容量达4200 mAh／g，远高于石墨材料的理论容量（372 mAh／g）。但在锂的插脱过程中，体积变化大，电极材料容易粉化、剥落，容量迅速下降。本文所用纳米多晶硅粒子呈球形，直径约30 nm，其首次插锂容量达到2700 mAh／g，循环性能优于微米、亚微米级硅粒子。主要原因在于纳米粒子尺寸小且分布均匀，在充放电过程中绝对体积变化较小，缓解了颗粒的粉化和剥落，电极结构稳定性有所提高。高分辨率透射电镜（HRTEM）和选区电子衍射（SAED）结果表明，随着循环次数增加，多晶硅逐渐转变为亚稳的无定形态。在纳米粒子高表面能的驱动下，粒子间发生团聚、融合。扫描电镜（SEM）观察到在若干次循环后，纳米多晶硅粒子逐渐融合成亚微米级。严重的融合现象造成电极材料的锂离子扩散系数降低，导电性下降，甚至从导电基体上脱落，导致容量的衰减。制备核壳结构的纳米复合材料可以有效地解决团聚问题。采用乳液聚合法将聚丙烯腈完全包覆在纳米硅表面，形成核壳结构纳米硅／聚丙烯腈前驱体。然后将前驱体在800℃下煅烧，聚合物发生裂解、碳化，最终得到核壳结构纳米硅／炭复合材料。该材料的首次插锂和脱锂容量分别为1750 mAh／g和1137 mAh／g，经过20次充放电循环后，可逆容量保持在594 mAh／g，约为初次可逆容量的54％，其循环性能远优于纳米硅粒子。主要原因是聚合物可以在低温下（＜1200℃，相对于石墨化温度）转变为“硬炭”，包覆在纳米硅表面形成稳定的复合结构。硬炭含有很多微孔，本身具有较高的电导率，保持了整个复合材料的电化学活性。在锂离子发生插入和脱插时，具有硬结构的壳材料可以保护内部的纳米粒子，避免纳米粒子之间的团聚，同时硬的壳材料将防止复合纳米粒子本身的团聚和融合。溶胶-凝胶法也可以用来制备核壳结构纳米复合材料。以正硅酸乙酯为前驱体，经过水解缩聚过程，形成的多聚硅酸完全包覆在纳米硅粒子表面，形成稳定的交联包覆结构，再经过高温煅烧，多聚硅酸转化为氧化硅（SiO_x）粒子，包覆在纳米硅表面形成稳定的复合材料。与核壳结构硅／炭纳米材料相似，在锂离子发生插脱时，具有稳定晶体结构的壳材料可以保护内部的纳米核，防止纳米粒子间的团聚和融合。而且，SiO_x材料本身也具有储锂活性，锂离子首次插入时将部分SiO_x还原为纳米尺寸的硅，同时生成Li₂O和锂硅酸盐，形成稳定的分散网络，能够对纳米硅粒子的体积变化起到缓冲作用。核壳结构纳米硅／SiO_x复合粒子的首次插脱锂容量分别为1072 mAh／g和827 mAh／g，经过20次充放电循环后，可逆容量保持在539 mAh／g，约为初次可逆容量的65％，表现出较好的循环稳定性。采用混合煅烧法制备天然改性石墨／纳米硅复合材料。SEM观察表明，纳米硅粒子均匀地分散在碳材料表面，形成类似于核壳的包覆结构。硅含量10％的复合材料首次插锂／脱锂容量分别达到890 mAh／g和567 mAh／g。经过20次充放电循环后可逆容量保持在547 mAh／g，约为初次可逆容量的96.4％，表现出良好的循环稳定性。原因在于高温煅烧过程能够提高纳米硅粒子和碳材料载体结合的稳定性，得到结构稳定的复合材料。在锂离子发生插入和脱插时，由于纳米粒子分散在碳材料表面，并与表面具有较强的作用力，避免了大量纳米粒子之间的团聚。同时，由于碳材料本身结构稳定，在插脱锂过程中体积变化效应小，作为载体材料能够很好地保持循环体系的稳定性，因此，这种复合材料在保持了石墨材料的良好循环性能的同时，具有高的可逆容量。与纳米化相对应，薄膜化也是提高硅负极材料循环稳定性的有效方法之一。采用真空蒸发沉积法在经过粗糙化前处理的镍箔和铜箔上分别沉积硅薄膜。沉积在镍箔上，厚度达1.8μm的硅膜，其首次插锂容量甚至达到硅的理论插锂容量4200 mAh／g，首次脱锂容量为3100 mAh／g，经过200次充放电后，容量保持在1000 mAh／g以上。与此相似，沉积在铜箔上，厚度为3.6μm的真空沉积硅膜，其首次插脱锂容量分别达到3110 mAh／g和2390 mAh／g，经过200次充放电循环后，可逆容量保持在1250 mAh／g以上。真空沉积硅膜具有较大的表面积／厚度之比，可以有效减缓由于合金化带来的体积膨胀。SEM研究表明，经过粗糙化处理的镍箔表面具有很多微孔和裂纹，而沉积在上面的硅膜则呈半球状；经过粗糙化处理的铜箔表面则有很多棱形突起，真空沉积后，硅膜呈丘陵状，这种结构为锂离子插脱过程中硅材料的体积变化提供了空间。同时，硅膜由纳米级的颗粒堆积而成，充放电过程中绝对体积变化较小。TEM和SAED研究表明，多次循环后，在非晶态的硅颗粒中出现了纳米尺寸的面心立方型硅的微晶区域，在锂离子插入／脱插过程中，晶界间的滑移可以缓冲体积变化，提高硅膜整体结构的稳定性。因此，这种真空沉积硅膜具有很高的比容量和良好的循环性能。更多还原

【Abstract】 Lithium-ion batteries are one of the great successes of the application of advanced materials and electrochemistry science in the modern energy storage devices. Nowadays, owing to the rapid advancement of electronic technologies, lithium-ion batteries are required to improve their performance on capacity, power and safety. This paper is focused on the silicon materials with high capacity, trying to improve their cycleability and safety by nano-technology, composite-technology and film-technology.The electrochemical performance and capacity-fading mechanism of the polycrystalline Si nanoparticles prepared by laser vapor deposition method were investigated in detail. Silicon has a theoretical capacity of 4200 mAh/g in its fully lithiation composition, Li_4.4Si, much higher than that of the commercialized graphite （372 mAh/g）. However, the cycling performance of silicon is poor, owing to its severe volume expansion and shrinkage during the insertion and extraction of lithium ions, which results in pulverization of Si particles and eventual loss of Li⁺ storage ability. In this work, the Si nanoparticles exhibit a spherical morphology and the average particle size is 30 nanometers. Its initial lithiation capacity reaches 2700 mAh/g, showing a better cycleability than micrometer-scale silicon particles. It can be ascribed to the small absolute volume changes occurring for nanomaterials due to their small particle size and uniform particle distribution, which mitigate the pulverization and exfoliation of the particles, improving the stability of the electrodes. HRTEM and SAED results indicate that the crystal structure of Si can be transformed into the amorphous state by lithiation/de-lithiation process, accompanying with the intense aggregation of nanoparticles to reduce there high surface energy, eventually leading to their mergence to larger particles, which is proved by SEM observation, too. The severe mergence results in the irreversible accumulation of lithium in Si, the drop of conductivity and the decrease of lithium diffraction coefficients, even the exfoliation of electrode materials, which is the main reason of capacity fading for the silicon nanoparticles.The core-shell nanocomposite concept represents an efficient solution to this problem. Using the emulsion polymerization method, the Si nanoparticles were coated by polymer to form a core-shell shaped silicon/polyacrylonitrile （PAN） precursor. The precursor was heat-treated at 800°C to carbonize the PAN to form hard carbon shell. The initial lithiation and de-lithiation of this core-shell Si/C nanocomposites are 1750 mAh/g and 1137 mAh/g, respectively. After 20 cycles, the reversible capacity retains 594 mAh/g, 54% of the initial capacity, showing better capacity retention than the pristine Si nanoparticles. It is found that the coating polyacrylonitrile can be transform into stable "hard carbon" shell under low temperature （<1200°C, relative to the graphitization temperature）. The hard carbon consists of a lot of micropores, which can definitely be passages for lithium ions, keeping the conductivity and electrochemical activity of the composites. During lithiation and de-lithiation process, the hard shell can act as a barrier to protect the inner Si core from aggregating. Moreover, it can also prevent the nanocomposites themselves from aggregation and mergence.The sol-gel method was also used to prepare core-shell Si/SiO_x nanocomposite. Using tetraethoxysilane （TEOS） as the precursor, through a process of hydrolyzation and condensation, the core-shell polysilicone/Si nanocomposite was obtained. Then, the polysilicone can be transformed into the SiO_x shell by heat-treatment. Similar with the hard carbon, the SiO_x shell, which has a stable crystal structure, can protect the inner Si core from aggregation and mergence. Especially, SiO_x can also absorb and retain a large quantity of lithium ions. During lithiation process, the partial SiO_x can be reduced to Li₂O and lithium silicates, which expand only half as much as Li-Si alloys, serving as a buffer to alleviate volume expansion of the Si nanoparticles, mitigating the destruction of the Si crystalline structure. The initial lithiation and de-lithiation of this core-shell Si/SiO_x nanocomposites are 1072 mAh/g and 827 mAh/g. After 20 cycles, the reversible capacity retains 539 mAh/g, which is 65% of the initial capacity, showing better capacity retention than the pristine Si nanoparticles.The modified natural graphite （SSG）/Si nanoparticle composite was prepared by sonicated dispersion and following heat-treatment process. SEM observation indicates that the nanometer-scale Si particles were uniformly and completely coated on the surface of SSG particles, forming an analogous core-shell structure. The initial lithium intercalation and de-intercalation capacity of the 10% SSG/Si composite are 890 mAh/g and 567 mAh/g. The 10% SSG/Si composite shows a very slow capacity fading and retains 96.4% of the original capacity after 20 cycles, respectively, showing good capacity retention. It is mainly due to the heat-treatment that increases the combination force between Si nanoparticles and SSG. This combination force can prevent the aggregation and separation of Si nanoparticles during lithiation and delithiation process, keeping the stability of the Si-coated SSG material. Simultaneously, as the matrix materials, the SSG can keep stable during cycling, and its volume change is small, which increases the stability of the electrode materials.The film-technology is also one of the efficient solutions to improve the cycleability of the silicon negative materials. Using the vacuum vapor deposition method, the Si film was deposited on the Ni and Cu foil roughed by pre-treatments. The original lithiation capacity of 1.8 jim thickness Si film on the Ni foil even reaches the theoretical capacity 4200 mAh/g of silicon, and its original de-lithiation capacity is 3100 mAh/g, retaining over 1000 mAh/g after 200 cycles. Similar with it, the initial lithiation/de-lithiation capacity of 3.6 thickness Si film on the Cu foil are 3110 mAh/g and 2390 mAh/g. Its reversible capacity retains over 1250 mAh/g after 200 cycles. The ratio of surface area to thickness of the vacuum deposited Si film is biggish, which can suppress the volume expansion accompanied with alloying. SEM observation indicates that the surface of the roughed Ni foil has many micro-holes and cracks, and the deposited Si film on it exhibits half spherical. In the case of Cu foil, its surface has many protuberances with ridges, and the deposited Si film on it shows a hill-like structure. Both of the two structures provide spaces for the volume changes. Moreover, the Si film consists of nano-scale particles, which have small volume changes during cycling. HRTEM and SAED results indicate that the nano-sized face-centered Si crystallites are formed among the amorphous Si film after several cycles, and the strain derived from volume changes can be accommodated by slippage at the nanodomain boundaries, so as to improve the stability of the Si film, leading to the good capacity retention with high capacity.更多还原