【作者】 李冰；

【导师】 王殿龙；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2007， 硕士

【摘要】 橄榄石型LiFePO₄理论比容量高、价格低廉、环境友好、循环性能优良、安全性能突出,被认为是最具开发和应用潜力的新一代锂离子电池正极材料。由于LiFePO₄的电子导电率低和锂离子扩散速度慢,导致初始容量损失和倍率放电性能差;又因其堆积密度低,导致体积能量密度小。这两点影响了LiFePO₄材料的实用化。本文以制备球形LiFePO₄为研究目的,对其合成与改性做了详细研究。采用液相沉淀-高温煅烧法合成球形LiFePO₄,研究了合成条件对前躯体NH₄FePO₄·H₂O、产物LiFePO₄结构、形貌及电化学性能的影响,通过X射线衍射（X-Ray Diffraction）分析了合成前躯体及产物的结构晶型,扫描电镜（Scanning Electron Microscope）观察了材料的形貌,恒流充放电测试研究了合成材料的比容量和循环性能,用循环伏安（Cyclic Voltammogram）和电化学阻抗谱（Electrochemical Impedance Spectroscope）简单分析了材料的电化学反应机理。研究结果表明,采用FeSO₄·7H₂O、H3PO4和NH3·H₂O为原料合成的前躯体为NH₄FePO₄·H₂O,其晶体结构与目标产物LiFePO₄的晶体结构具有相似性,在高温固相反应之前就达到“结构预置”的效果,反应过程中晶体结构变化较小,反应易于进行。掺碳对振实密度有很大影响,随着含碳量的增加,振实密度逐渐减小,掺碳5 mass%的LiFePO₄/C振实密度为1.28 g·cm^-3。采用氟化锂为锂源,蔗糖为碳源,碳含量10 mass%,550℃烧结12 h制备的LiFePO₄材料放电容量保持在120 mAh·g^-1 （0.1 C）左右。镍掺杂材料放电容量保持在110 mAh·g^-1 （0.1 C）左右,通过镍掺杂,材料的比容量和循环性能并没有得到有效改善。对LiFePO₄进行掺碳处理后Rct变小,交换电流密度j°变大,加速了Li⁺的脱嵌,电极反应速率增大,有利于电化学反应的进行,从而提高材料在实际充放电过程中的容量。更多还原

【Abstract】 Olivine-type LiFePO₄ is considered as the most promising candidate for the next generation cathode materials of lithium ion batteries because it has high theoretical specific capacity, is cheap and environmentally friendly, also has good cycling characteristics and excellent safety. However, it’s poor electronic conductivity, which leads to initial capacity loss and poor rate capability. And it’s low pile density, which leads to low volumetric specific capacity. That prevent LiFePO₄ to be put into commercially used.In this work, we aim at preparing spherical LiFePO₄. Used liquid deposit-high temperature sinter method to synthesis spherical LiFePO₄. Researched the influence of synthesis condition to NH₄FePO₄·H₂O precursor and LiFePO₄ product in structure, pattern and electrochemical performance. In addition, X-Ray Diffraction （XRD） was used to analyze the crystal structure of precursor and product, Scanning Electron Microscope （SEM） was used to observe the superficial morphology, charge-discharge test was used to study the specific capacity and cyclic properties, Cyclic Voltammogram （CV） and Electrochemical Impedance Spectroscope （EIS） was used to discuss the electrochemical reactive mechanism.The results of the study show that use FeSO₄·7H₂O, H3PO4 and NH3·H₂O as materials to prepare NH₄FePO₄·H₂O precursor, the structure of the precursor and LiFePO₄ are similarity, it is an effect of“structure preset”before sinter. Carbon doping can influence the tap density greatly, with the increase of carbon content the tap density decrease. The tap density is 1.28 g·cm^-3, when doped with 5 mass% carbon content. Using LiF as lithium source, sucrose as carbon source, carbon content is 10 mass%, 550℃sinter for 12 h to prepare LiFePO₄, the sample has a discharge specific capacity of 120 mAh·g^-1 （0.1 C）. The discharge specific capacity of nickel doping sample is about 110 mAh·g^-1 （0.1 C）, the discharge specific capacity and the cycle performance aren’t change efficiently by nickel doping. After carbon doping, Rct decrease, j°increase, it accelerates the extraction of Li⁺, so the electrode reaction speed is accelerated, it is advantage to the electrochemical reaction, also enhance the real capacity.更多还原