【作者】 马俊；

【导师】 康飞宇；

【作者基本信息】 清华大学 ， 材料科学与工程， 2010， 博士

【摘要】 作为下一代锂离子电池正极材料,磷酸铁锂具有许多优点,如廉价,环境相容性好、理论容量高、使用寿命长、热稳定性好、安全性好等。但是,其自身低的电子电导率和室温下低的锂离子扩散速率严重影响了它的电化学性能,并极大限制了该电极材料在动力电池领域的大规模应用。本论文针对纳米磷酸铁锂正极材料的溶胶-凝胶法制备、掺杂和表面改性进行了较为系统的研究改良现有溶胶-凝胶法的不足,首次以廉价、无毒的无机化合物FeCl₂·4H₂O、H₃PO₄和Li₂CO₃为原料合成了具有纳米尺度的、良好电化学性能的LiFePO₄/C复合正极材料,并研究了煅烧温度、保温时间及残碳量对其电化学性能的影响。研究表明,650℃下保温15 h,残碳量为4.5 wt.%的样品具有最佳的电化学性能,10 C时的放电比容量仍能保持在108 mAh/g左右。首次引入高价Sn4+离子,系统地研究了不同掺杂量（0-7 mol.%）对纳米晶LiFePO₄/C复合正极材料物理化学和电化学性能的影响,研究了高价Sn⁴⁺掺杂的电荷补偿机制,并得出Sn的掺杂属于混合价态掺杂。基于粒径大小和掺杂浓度的不同,各样品表现出了不同程度的赝电容效应。研究表明,当掺杂量为3 mol.%时,材料具有最佳的电化学性能,10 C时的放电比容量为128 mAh/g。系统研究了钒的添加量（0-13 mol.%）对LiFePO₄/C复合正极材料物理化学和电化学性能的影响,建立了相的组成和钒添加量的二元相图,并研究了高价V⁴⁺掺杂的电荷补偿机制。根据添加量的不同,钒将以不同的形态（V4+的掺杂、VO₂（B）的包覆和Li₃V₂（PO₄）₃的复合）存在于样品中。研究表明,钒的掺杂有利于细化晶粒、提高电导率和锂离子扩散系数,因此电化学性能,特别是倍率性能得到了有效的改善;纳米VO₂（B）的包覆改善了纳米颗粒的界面性能,并基于赝电容效应材料显示了最高的能量密度和功率密度;Li₃V₂（PO₄）₃的存在不利于主相锂离子扩散速率的提高,但是有利于磷酸铁锂基正极材料的电导率和倍率性能的改善。初步研究了掺锑纳米氧化锡、氧化锌包覆对磷酸铁锂正极材料电化学性能的影响。研究表明,纳米氧化锡、氧化锌的表面改性有利于磷酸铁锂电导率的提高和电化学性能的改善。更多还原

【Abstract】 As a promising cathode material for next-generation lithium ion battery, lithium iron phosphate exhibits many appealing features, such as low cost, environmental benign, suitable potential plateau, high theoretical capacity, long cycle life, ideal thermal stability, etc. However, its intrinsic electrical conductivity and lithium-ion diffusion velocity are rather poor, which seriously undermines the kinetics of LiFePO4 and thus greatly limits the large-scale application in the field of power battery. In this work, the sol-gel synthesis, doping and surface modifications of LiFePO4 were investigated systematically. The main research work and conclusions are given as follows:An innovative inorganic-based sol-gel route to synthesize nanostructured LiFePO₄/C cathode material with excellent electrochemical performance was introduced. The cheap, environmental friendly inorganic compounds （FeCl₂·4H₂O, H₃PO₄ and Li₂CO₃） were used as raw materials. The influences of the sintering temperature, holding time and residual carbon content on the electrochemical performance of LiFePO₄/C were investigated. The optimized sintering temperature and holding time were 650℃and 15 h, respectively. And the sample with 4.5 wt.% residual carbon exhibited excellent electrochemical performance, at 10 C, its discharge specific capacity was about 108 mAh/g.The supervalent Sn⁴⁺ was firstly introduced as a dopant. The effects of doping amount on the physicochemical and electrochemical performances of nanocrystalline LiFePO4/C were systemically investigated. The charge compensation mechanism of Sn⁴⁺ was studied. It was found that the doping of Sn was a mixed-valence doping. On the basis of the nanosized effect and doping concentration, samples showed pseudocapacitive behavior. When the doping amount was about 3 mol.%, the sample showed excellent electrochemical performance. At 10 C, the discharge specific capacity was about 128 mAh/g.The effects of adding amount of vanadium on the physicochemical and electrochemical properties were investigated in details. The concentration-composition phase diagram was constructed. The increasing adding of vanadium induces 1, the V⁴⁺ substituted for Fe （0 < x≤0.07） within the solid solubility; 2, beyond the solid-solution limit, the excess vanadium formed VO₂（B） coated on the surface of the V-doped LiFePO₄; 3, the excess vanadium formed the secondary phase Li₃V₂（PO₄）₃ （x≥0.11） coexisting with the V-doped LiFePO4. The V⁴⁺ doping contributes to induce the lattice distortion, refine the particle size, increase the electrical conductivity and thus greatly improve the electrochemical performance, especially the rate capability. The surface modification of nano-sized VO₂（B） is helpful to increase the electrical conductivity greatly. Due to the nanosized effect, the sample shows a high energy and power density. The secondary phase Li₃V₂（PO₄） is favorable to increase the electrical conductivity. It plays a paramount role in improving the rate capability of the LiFePO₄-based cathode, although it is not good for the lithium ion transport within the main phase.The electrochemical performances of LiFePO₄ cathode material by nanosized Sb-doped SnO₂ and ZnO coatings were preliminary examined. It was found that the coatings were beneficial to increase the electrical conductivity and enhance the electrochemical performance of LiFePO₄ cathode material.更多还原