【作者】 陈晗；

【导师】 韩绍昌；

【作者基本信息】 湖南大学 ， 材料学， 2007， 博士

【摘要】 LiFePO₄是一种极有发展前景的锂离子电池正极材料,因具有合成原料价格低廉、环境相容性好、循环性能和安全性能优良等优点。提高LiFePO₄的电子导电率和锂离子扩散系数是商业化的关键。在优化LiFePO₄的合成条件,探讨LiFePO₄充放电过程中结构变化及电极反应动力学过程的基础上,通过稀土元素选择性掺杂、制备LiFePO₄/导电聚合物及LiFe_1-xY_xPO₄/C复合材料等途径改善LiFePO₄的电化学性能。采用正交实验法分别对LiFePO₄的预分解和合成制度进行优化,以获得最佳的工艺条件。合成温度是影响LiFePO₄的结构和性能的关键因素。当合成温度为650℃时,试样中的微晶结构完整、晶粒尺寸适中、结晶度高;试样的粒度分布均匀;试样具有最佳的电化学性能,首次放电容量D1=127.44 mAh/g,充放电效率η1=94.50 %,平台容量和平台率分别为114.87 mAh/g,90.11 %,同时,具有最优的倍率性能和循环性能。采用XRD法分析了充放电过程中Li_1-xFePO₄的结构变化。充电过程中随x的增大,LiFePO₄的含量逐渐减小,FePO₄含量逐渐增大,当x=0.88时,充电过程结束,LiFePO₄的含量最小,FePO₄含量最大。放电过程中,则正好相反。采用EIS方法研究了充放电时LiFePO₄电极反应动力学过程。充电初期中,交换电流i 0和锂离子扩散系数DLi迅速增大。当x=0.29时, i 0和DLi增加到最大,电化学脱锂反应最容易进行。当x>0.29时,DLi稍有减小, i 0基本不变。在充电后期（x>0.71）,DLi增稍有减小, i 0迅速减小,当x=0.88时,充电过程结束。放电过程中, i 0和DLi的变化正好相反。通过原位聚合的方法制备了LiFePO₄/PAn、LiFePO₄/PPy和LiFePO₄/PTh复合材料。采用TEM和SEM对LiFePO₄与导电聚合物的结合形式进行了较系统地考察,当使用6.75 %的PAn或10.56 %的PTh时,在LiFePO₄颗粒表面形成薄且较均匀的导电聚合物包覆层,它们具有良好的电化学性能及粘结性。而PPy则以颗粒的形式附在LiFePO₄颗粒表面或颗粒之间,LiFePO₄/PPy复合材料的电化学性能比LiFePO₄/PAn、LiFePO₄/PTh差,特别是倍率放电性能。采用不同的稀土金属离子Re³⁺（Re=La、Nd、Y、Er）对LiFePO₄的Li位和Fe位进行掺杂,对Li_1-xRe_xFePO₄和LiFe_1-xRe_xPO₄微观结构及性能进行了对比。结果表明:稀土金属离子掺杂Li位时,掺杂离子相同时,随掺杂量的增加晶格参数和晶胞体积均增大;掺杂量相同时,随离子半径的增大晶格参数和晶胞体积均增大。稀土金属离子掺杂Fe位时,La和Nd使晶格参数及晶胞体积均增大;Y几乎不改变晶格参数和晶胞体积;Er则使晶格参数和晶胞体积减小。当掺杂离子和掺杂量均相同时,Li_1-xRe_xFePO₄试样的晶格参数、晶胞体积均大于LiFe_1-xRe_xPO₄试样;但LiFe_1-xRe_xPO₄试样的电子导电率高很多、锂离子扩散系数较小、电化学性能较好。Li_1-xRe_xFePO₄和LiFe_1-xRe_xPO₄微观结构和宏观性能变化规律能很好地对应,因此,采用微观结构和宏观性能变化共同确定Re³⁺选择性掺杂Li位或Fe位是可行和可靠的。采用酚醛树脂和环氧树脂为碳前驱体,通过固化、预分解及合成制备出网状结构的LiFe_1-xY_xPO₄/C复合材料。采用TEM、EDS及XPS等手段对网状结构进行分析,发现网状结构是以碳为骨架,LiFe_1-xY_xPO₄颗粒附着在碳骨架上这种结构与碳包覆颗粒结构完全不同,从而其电子导电及锂离子扩散机制也明显不同。网状结构为锂离子的扩散提供了多维通道,加速了锂离子的扩散,极大地改善了LiFePO₄的倍率放电性能。碳含量是网状结构的形成的关键因素,掺杂量对网状结构的形成影响较小,当碳含量为5 %时,FY2C5、EY1C5试样具有最佳网状结构。与LiFePO₄相比,FY2C5、EY1C5试样电子导电率和锂离子扩散系数分别提高了8和3个数量级,且具有最佳的电化学性能,在C/12时,首次放电容量分别为160.71、165.71 mAh/g;在1C时,首次放电容量分别为131.43、143.96 mAh/g。更多还原

【Abstract】 Lithium iron phosphate （LiFePO₄） is a promising candidate cathode material for lithium ion batteries, due to low cost, environment amity, excellent cycling stability and safety. The key for commercializing LiFePO₄ is to improve its electronic conductivity and lithium-ion diffusion coefficient. On the basis of optimizing the synthesis conditions and investigating the structural change and the kinetic process of the electrode reaction in the charge and discharge process, some improvements for the electrochemical performances of LiFePO₄ were made by selective doping of rare earth elements, preparing LiFePO₄ conductive polymer composites and LiFe_1-xY_xPO₄/C composites.The predecomposition and synthesis system were optimized by the orthogonal method to obtain the optimized technologcial conditions. The synthesis temperature is the key influence factor on the microstructures and performances of LiFePO₄. The crystallites in the sample synthesized at 650℃possesse perfect crystal structure, high crystalline degree and few defects. The granularity distribution is uniform. The sample has the best electrochemical performance, its initial discharge capcity （D1） is 127.44 mAh/g, its initial charge and discharge efficiency isη1=94.50 %. The plateau capacity and plateau ratio is 114.87 mAh/g and 90.11 %, respectively; it displays the best rate and cycling capability.The structural change of Li1-xFePO₄ was analyzed by XRD method in the charge and discharge process. In the charge process, the content of LiFePO₄ decreases, that of FePO₄ increases gradually with increasing the x value. When x is 0.88, the charge process is over. Meanwhile the content of LiFePO₄ is lowest, that of FePO₄ is highest. The content of LiFePO₄ and FePO₄ changes reversely in the discharge process. The electrode kinetic process in the charge and discharge process was investigated by EIS method. At the beginning of charge, the exchange current （ i 0） and lithium-ion diffusion coefficient （DLi） increases rapidly. When x is 0.29, i 0 and lithium-ion diffusion coefficient reach the maximum, the electrochemical delithium reaction occurs most. At the end of charge （x>0.71）, DLi decreases slightly and i 0 decreases abruptly. When x is 0.88, the charge process is over. DLi and i 0 change reversely in the discharge process.LiFePO₄/PAn, LiFePO₄/PPy and LiFePO₄/PTh composites were prepared by in-situ polymerization method. The combining form between LiFePO₄ and the conductive polymer was investigated systematically by TEM and SEM method when 6.75 % PAn or 10.56 % PTh was applied, the thin and uniform polymer coating is covered on the surface of LiFePO₄ particles. And they display excellent electrochemical performace and strong adhesion. PPy grains are distributed on the surface of LiFePO₄ particles or among LiFePO₄ particles. The electrochemical performaces of LiFePO₄/PPy composites is worse than that of LiFePO₄/PAn and LiFePO₄/PTh composites, especially in the aspect of rate capability.LiFePO₄ was doped on Li site or Fe site by using various rare earth ions Re3+ （Re= La, Nd, Y, Er）. The microstructures and performaces of Li_1-xRe_xFePO₄ and LiFe_1-xRe_xPO₄ were comparatively investigated. The results show that when the same Re3+ is doped on Li site, the crystal lattice parameters and cell volume increase with increasing the doping amount; when the same doping amount is applied, the crystal lattice parameters and cell volume increase with increasing the radius of doping ion. When Re3+ is doped on Fe site, La3+, Nd3+ make the crystal lattice parameters and cell volume increasing and the Y3+ makes crystal lattice parameters and cell volume nearly no changing, the crystal lattice parameters and cell volume decrease with applying Er3+. When the doping ion and doping amount are the same, the crystal lattice parameters and cell volume of Li_1-xRe_xFePO₄ are larger than those of LiFe_1-xRe_xPO₄. Compared with Li_1-xRe_xFePO₄,,LiFe_1-xRe_xPO₄ displays higher electronic conductivity, good lithium-ion diffusion coefficient and more excellent electrochemical performance. The rules of the microstructure changes for Li_1-xRe_xFePO₄ and LiFe_1-xRe_xPO₄ correspond with that of the macro-performance changes. Therefore it is feasible and reliable using the change of microstructures and macro-performance to determine the selective doping of Re3+ on Li and Fe site of LiFePO₄.Using phenolic resin and epoxy resin as carbon precursor, LiFe_1-xY_xPO₄/C composites with network structure were prepared by curing, predecomposition and synthesis. The network structure was analyzed systematically by TEM, XPS and EDS methods. It was discovered that carbon is used as framework and LiFe_1-xY_xPO₄ particles were adhered on the carbon framework. This structure is entirely different from carbon coating, the conduct mechanism of electron and the diffusion mechanism of lithium-ion in network structural composite are evidently different from carbon coating composite. The network structure provides mult-channels for lithium-ion diffusion, accelerates the diffussion of lithium-ion, improves the rate capability of LiFePO₄. The experimental results show that carbon content is the key of factor forming network structure, the doping amount has few influence on the formation of network structure. When 5% polymeric carbon was applied, FY2C5、EY1C5 samples posses the most perfect network structure. Comparing with LiFePO₄, the electronic conductivities and lithium-ion diffusion coefficient of FY2C5 and EY1C5 samples increase 8 and 3 orders, respectively. FY2C5 and EY1C5 samples exhibit the most excellent electrochemical performance, their initial discharge capacities are 160.71 mAh/g and 165.71mAh/g at C/12 rate, 131.43 mAh/g and 143.96 mAh/g at 1 C rate.更多还原