【作者】 徐峙晖；

【导师】 赖琼钰；

【作者基本信息】 四川大学 ， 无机化学， 2006， 博士

【摘要】 LiFePO₄作为锂离子电池正极材料具有较高的理论比容量、平坦的放电平台、优异的循环性能、良好的结构稳定性和热稳定性、低成本以及对环境友好等优点，其研究已引起了广泛的关注。然而，由于其电子电导率低以及电极行为受扩散控制致使材料比容量较低。本论文针对存在的问题致力于提高LiFePO₄材料电子电导率、减小其充放电过程中的动力学限制研究。本论文采用了固相法、碳热还原法、溶胶—凝胶法以及微乳液法合成LiFePO₄／C复合正极材料，研究了不同的合成方法、合成条件对LiFePO₄产物粒度和形貌形成机制及其对材料电化学性能的影响。本论文还用交流阻抗技术研究了Li⁺在LiFePO₄材料中的脱嵌动力学过程，得到一些有意义的结果。全文共分六章。第一章介绍了锂离子电池及其正极材料的研究背景、LiFePO₄正极材料的研究现状，提出了拟解决的相关科学问题。第二章考虑到了反应物和未来产物结构的相似性，以减小固相反应时的离子重排能量，更有利于形成产物相核，故采用与LiFePO₄同晶系的Li₃PO₄作为反应物合成目标产物；并研究了乙炔黑用量、保护气氛种类、焙烧温度对产物中Fe²⁺含量的影响及其与LiFePO₄产物电化学性能的关系。第三章设计了半固相碳热还原合成路线，使得炭更均匀地包覆在LiFePO₄产物颗粒表面，提高了LiFePO₄材料的电子电导率，稳定了产物中Fe²⁺，得到了电化学性能优异的LiFePO₄材料；并讨论了不同碳源、固相法和半固相法对产物粒度的调控作用，并对反应机理进行了研究。第四章摸索了柠檬酸为络合剂溶胶—凝胶法合成LiFePO₄材料的最佳条件；在溶胶—凝胶过程中加入聚乙二醇（PEG），使产物颗粒团聚严重、粒径分布范围广的现象得到明显改善，并对PEG调控产物粒度的机理进行了深入的讨论。此法所获得的产物粒度小且分布窄，因而具有良好的电化学性能。第五章首次采用溴化十六烷基三甲基铵（CTAB）-正辛烷—正丁醇微乳液体系合成LiFePO₄／C复合正极材料；针对反胶束微乳液体系中水核聚并导致产物颗粒不能保持均匀大小和规则形貌的缺点，通过添加PEG4000得到克服，致使所制样品的充放电性能优于同类方法制备的LiFePO₄／C复合正极材料；本章还深入研究了PEG4000在合成过程中与产物粒度和形貌之间的关系。第六章交流阻抗技术被用来研究锂离子在LiFePO₄材料中脱嵌和嵌入动力学过程。本章计算了固相法和微乳液法制备样品的表观Li⁺扩散活化能和表观电荷转移活化能。计算表明：LiFePO₄材料表面包覆碳后电子电导率大幅提高，此时Li⁺的扩散过程不再受制于电子电导率，而是受制于材料的锂离子电导率。为此，本文提出了锂离子电导率是制约LiFePO₄材料电化学性能进一步提高的瓶颈因素之观点，同时为提高Li⁺离子电导率的后继研究指出了可行性方向。更多还原

【Abstract】 The olivine LiFePO4, which offers benefits such as a quite large theoretical capacity, flat discharge curve, good cycle stability, thermal and structural stability, inexpensive cost and environmental benignity, has been attracting much attention as a promising new cathode material for lithium-ion batteries. However, it was reported that this cathode has very low electronic conductivity and diffusion-controlled kinetics of the electrochemical process, which decrease utilization the charge-discharge capacity of active material. Therefore, efforts were devoted to the increase of electron conductivity and the decrease of dynamics limitation for lithium ions insertion/extraction in LiFePO₄ material. In this dissertation, solid-state reaction, carbothermal reduction method, sol-gel method and microemulsion method were adopted to synthesize LiFePO₄/C composite cathodic material. The effect of synthetic method and synthetic conditions on the particles size, morphology and electrochemical performance of LiFePO₄ products were reasonedly studied. AC impedance technique was used to investigate the dynamics process of the Li⁺ insertion/extraction in LiFePO₄ material and obtained some meaningful results. The thesis is divided into six chapters.In the first chapter, the current survey on the lithium ion batteries cathodic materials, especially LiFePO₄ material, was detailedly introduced and correlative scientific problems were brought forward.In the second chapter, the structural similarity between reactants and product, which can decrease the ions rearrangement energy and favor the formation of product cores, was guided to select Li₃PO₄ as reactants for the synthesis of target LiFePO₄. The effect of the acetylene black amount, the protective ambience category and the sintering temperature on the contents of Fe²⁺ and the electrochemical performance of final products was investigated carefully.In the third chapter, a half-solid-state carbothermal reduction reaction was used to prepare LiFePO₄/C. This synthetic route seem ideally suited to coating carbon on the surface of LiFePO₄ particles more homogeneously, increasing the electron conductivity of LiFePO₄ material, favoring stabilization of iron as Fe²⁺ and obtaining LiFePO₄ product with good electrochemical performance. The effects of different carbon sources, solid state method and half-solid-state method on the mechanism of reaction and particles size controlling were discussed in detail.In the fourth chapter, optimal conditions for the sol-gel synthesis of LiFePO₄ by using citric acid as chelant were investigated. Polyethylene glycol （PEG） was added in the sol-gel process to improve conglomeration of the product particles and wide distribution of particles size, and obtain LiFePO₄ material with better electrochemical performance. The influence of PEG on the mechanism of particles size and morphology controlling was detailedly discussed.In the fifth chapter, an n-octane/n-butyl/cetyltrimethyl ammonium bromide microemulsion system was firstly selected to synthesize successfully LiFePO₄/C composite. The incorporation between water cores in emulsion system, which induced the inhomogeneous particles size and size distribution of the final product, was overcome by adding PEG4000 and the improved electrode performance was obtained compared with other emulsion methods. The relationship between surfactant PEG4000 added into the microemulsion and the product particles size and morphology was discussed in detail.In the last chapter, AC impedance technique was used to study dynamics process of lithium insertion/extraction in LiFePO₄ material. The apparent activation energy of the lithium ions difiEusion and the apparent activation energy of the charge transfer were calculated for the samples synthesized by solid state method and microemulsion method, respectively. The calculation indicated that the lithium ions diffusion process was not enslaved to the electron conductivity but to the lithium ion conductivity when electron conductivity was largely increased by coating carbon on the surface of LiFePO₄ material. Therefore, lithium ions conductivity was suggested a key factor for the farther enhancement of electrochemical performance of LiFePO₄ material. At the same time, some feasible approaches were suggested increasing the lithium ions conductivity.更多还原