【作者】 钟美娥；

【导师】 周震涛；

【作者基本信息】 华南理工大学 ， 材料物理与化学， 2009， 博士

【摘要】 在已知的锂插层化合物中,橄榄石型LiFePO₄因其低成本、对环境友好、长循环寿命、良好的热稳定性和高的安全性能等优点而被认为是锂离子动力电池的最佳阴极材料之一。然而,有三个方面的缺陷阻碍了LiFePO₄的商业化进程。其一是它的电子导电率和离子传导率较低,导致其初始放电比容量较低、倍率性能较差;其二是其Fe²⁺易被氧化,造成其合成困难,且在合成过程中常采用价格较昂贵的Fe²⁺化合物为铁源,增加了材料的制备成本;其三是它的振实密度较低,导致其体积比能量较低。目前,在改善LiFePO₄的电子导电率和离子传导率方面已经取得了较大的进展,但是其制备成本较高和振实密度较低的问题仍然有待解决。本文采用价廉的Fe³⁺化合物为铁源,通过操作步骤简单、易于进行工业化生产的固相-碳热还原法来制备LiFePO₄/C复合材料。并利用X-射线衍射（XRD）、拉曼光谱、X-光电子能谱（XPS）、扫描电镜（SEM）、激光粒度分析、恒流充放电、循环伏安（CV）和交流阻抗谱（EIS）等分析测试技术对LiFePO₄/C复合材料的结构和电化学性能进行了系统的研究,还测试了所制复合材料的振实密度。主要研究工作的结果结论如下:1)为了降低LiFePO₄材料的制备成本,本文选择了价格低廉的Fe³⁺化合物为铁源、通过固相-碳热还原法来合成LiFePO₄。研究了三种碳热还原反应体系（即Fe₂O₃+蔗糖、Fe₃O₄+蔗糖以及柠檬酸铁+Fe₂O₃三种碳热还原反应体系）的热反应行为,并考察了三价铁源的种类对所制复合材料结构与性能的影响。热重-差热测试结果表明,柠檬酸铁（FeC₆H₅O₇·5H₂O）、Fe₂O₃和Fe₃O₄被还原并形成LiFePO₄晶体的温度依次升高,分别为470℃、505℃和525℃。SEM和恒流充放电测试结果发现,由Fe₂O₃和柠檬酸铁的混合物为铁源所制复合材料的晶粒尺寸最小,电化学性能最好。2)研究了烧结温度（600⁸00℃）、烧结时间（8³6h）及柠檬酸铁的添加量（10wt.% 30wt.%）等制备条件对以Fe₂O₃和柠檬酸铁的混合物为铁源、通过固相-碳热还原法所制LiFePO₄/C复合材料结构与电化学性能的影响。实验结果表明,所制材料在较宽的颗粒尺寸范围内呈多峰分布,通过改变合成条件即可提高材料的振实密度和改善其电化学性能。升高烧结高温和延长烧结时间可使LiFePO₄的晶体生长完好,结晶度提高,振实密度增大,但温度过高却会导致LiFePO₄颗粒长大,电性能变差。随着柠檬酸铁添加量的增加LiFePO₄/C材料的振实密度和放电比容量呈先增加、达到最大值后又降低的趋势。在本文的实验条件下,以700℃、24h和20wt.%柠檬酸铁添加量的制备条件可使所制LiFePO₄/C材料具有最高的放电比容量和较高的振实密度。在此最佳制备条件下得到的LiFePO₄/C材料的振实密度为1.40g·cm^-3,以0.1C、0.2C、0.5C和1.0C倍率充放电时,其首次放电比容量分别为135 mAh·g^-1、129 mAh·g^-1、126 mAh·g^-1和110 mAh·g^-1。3)为了改善上述高密度LiFePO₄/C复合材料的电导率,本文还采用金属离子对其进行锂位掺杂改性。研究结果表明,采用金属离子掺杂的方法不会改变复合材料的橄榄石型晶体结构,但可以降低其颗粒尺寸、改善其电导率、提高其振实密度,从而改善其电化学性能。在锂位掺杂磷钨酸[H₃PO₄·12（WO₃）·H₂O]所制Li_0.99W_0.01FePO₄/C复合材料具有最小的晶粒尺寸、最佳的电性能和较高的振实密度。其平均颗粒尺寸(d_0.5)从掺杂前的0.31nm降低至0.17nm;振实密度从掺杂前的1.40g·cm^-3提高至1.50g·cm^-3;当放电倍率分别为0.2C、0.5 C、1.0 C和1.5 C时,其首次放电比容量分别为146 mAh·g^-1、133 mAh·g^-1、130 mAh·g^-1和125 mAh·g^-1。4)考察了新型无机三价铁化合物——FePO₄·xH₂O的结构形态对固相-碳热还原法所制LiFePO₄/C复合材料结构和性能的影响,发现以三斜结构无水FePO₄为无机铁源所制LiFePO₄/C复合材料的电化学性能优于以不完善结晶体结构水合FePO₄·2H₂O为无机铁源所制的复合材料。并对以三斜结构的无水FePO₄为无机铁源制备LiFePO₄/C复合材料的合成工艺条件进行了探索。研究结果表明,在650℃、24h和35wt.%柠檬酸铁添加量的条件下所制的LiFePO₄/C材料具有最佳的电化学性能,以0.2C、0.5 C、1.0C倍率进行充放电时,其首次放电比容量分别为138 mAh·g^-1、128 mAh·g^-1和116 mAh·g^-1;以1.0 C充放电倍率充放电循环25次后其容量保持率达99.1 %。更多还原

【Abstract】 Among the well-known Li-insertion compounds, the olivine-type LiFePO₄ is considered as one of the most promising cathode materials for rechargeable Li-ion batteries because of its low cost, low toxicity, better thermal stability and excellent safety. However, there are three drawbacks preventing LiFePO₄ to be put into commercially used. One is its low electronic and ionic conductivity, which leads to the poor rate capability. The second problem is that the synthesis of LiFePO₄ is not easy because of the +2 oxidation state of iron in the compound, and the production cost is high as expensive divalent iron precursor compounds have to be used as the starting material to synthesize LiFePO₄. Another disadvantage is the low tap-density, which results in a low volumetric specific capacity. Tremendous efforts have so far been devoted to improve the electronic conductivity of LiFePO₄ and several effective ways have been proposed. Nevertheless, the problem concerning the high production cost and low tap-density of this material remains to be solved. In this paper, LiFePO₄/C composite cathode materials have been synthesized by solid state– carbothermal reduction method using cheap Fe³⁺ compounds as iron precursors. The micro-structures, morphologies and electrochemical performances of these composites were investigated by XRD, SEM, X-ray photoelectron spectrometry （XPS）, laser diffraction and scattering measurements, Raman spectra, galvanostatic charge-discharge, cyclic voltammetry （CV） and electrochemical impedance spectra （EIS）. The tap-density of LiFePO₄/C composite materials was also tested. The main results and conclusions were listed as following:1) In order to reduce the cost of LiFePO₄, carbothermal reduction approach was employed to synthesize the LiFePO₄ by using cheap Fe³⁺ compounds as iron precursors. The thermal reaction behavior of the starting materials composed by various Fe³⁺ compounds was studied, and the effect of different Fe³⁺ sources on the structure and properties of LiFePO₄/C has been discussed in detail. TG-DSC results showed that the temperature related to the formation of LiFePO₄ crystal by citrate ferric, Fe₂O₃ and Fe3O₄ was 470℃, 505℃and 525℃, respectively. SEM and galvanostatic charge-discharge results indicated that the material synthesized with Fe₂O₃ and citrate ferric as iron precursors had small particle size and superior electrochemical properties.2) LiFePO₄/C composite was synthesized by solid state– carbothermal reduction method using Fe₂O₃ and citrate ferric as iron precursors. The effects of synthetic conditions such as sintering temperature, sintering time, and citrate ferric additive amount on the physico-electrochemical properties of LiFePO₄/C have been studied. It was found that the LiFePO₄/C composites showed multi-peaks distribution in broad particle size range, and the tap-density and electrochemical performance of LiFePO₄ could be improved by varying the synthetic processes. Increasing the sintering temperature and extending sintering time resulted in higher crystallinity and tap-density, but in a larger particle size. In the range of 600⁸00℃, 700℃is the optimum synthetic temperature for the LiFePO₄/C with small particle sizes, high tap-density and perfect crystal. Increasing citrate ferric amount from 10wt.% to 30wt.%, the tap-density and electrochemical properties of LiFePO₄/C firstly enhance then decrease. In the present case, LiFePO₄/C synthesized at 700℃for 24h with 20wt.% citrate ferric shows best electrochemical performances and high tap-density. Under this condition, the obtained LiFePO₄/C has a tap-density of 1.40g·cm^-3 and shows an initial discharge capacity of 135 mAh·g^-1, 129 mAh·g^-1, 126 mAh·g^-1 and 110 mAh·g^-1 at 0.1C, 0.2C, 0.5C and 1.0C, respectively.3) The influences of metal ion doping modification on the structure and electrochemical properties of Li-site doped Li1-xMxFePO₄/C composite were investigated. It was found that the metal ion doping method could greatly improve the tap-density and electronic conductivity, but decrease the particle size. Among the Li-site doped composites, the Li0.99W0.01FePO₄/C synthesized by phosphotungstic acid [H₃PO₄·12（WO₃）·H₂O] shows the best electrochemical performances. The average particle size and tap-density of Li_0.99W_0.01FePO₄/C and undoped LiFePO₄/C are 0.17nm and 1.50g·cm^-3, and 0.31nm and 1.40g·cm^-3, respectively. At current densities of 0.2C, 0.5C, 1.0C and 1.5C, the Li0.99W0.01FePO₄/C composite materials have initial discharge specific capacity of 146 mAh·g^-1, 133 mAh·g^-1, 130 mAh·g^-1 and 125 mAh·g^-1, respectively.4) The effects of FePO₄·xH₂O phase structure on the structures and properties of LiFePO₄/C synthesized by solid state– carbothermal reduction method have been analyzed. It was found that the material synthesized with trigonal anhydrous FePO₄ as inorganic Fe³⁺ precursor had superior electrochemical properties to that prepared with incomplete crystal hydrous FePO₄·2H₂O as inorganic Fe³⁺ precursor. The synthetic processes of LiFePO₄/C using trigonal anhydrous FePO₄ as inorganic Fe³⁺ precursor was explored. The results show that LiFePO₄/C composite prepared at 650℃for 24h with 35wt.% citrate ferric exhibits good electrochemical performances. At current densities of 0.2C, 0.5C and 1.0C, the composite materials have initial discharge specific capacity of 138 mAh·g^-1, 128 mAh·g^-1 and 116 mAh·g^-1, respectively. After 25 cycles, the composite cathode retains 99.1 % of the first cycle discharge capacity at 1.0C.更多还原