【作者】 郑俊超；

【导师】 李新海； 王志兴；

【作者基本信息】 中南大学 ， 冶金物理化学， 2010， 博士

【摘要】 本文在综合评述了锂离子电池及其正极材料研究现状的基础上,以锂离子电池正极材料LiFePO4、Li3V2(PO4)3及其复合体xLiFePO4·yLi3V2(PO4)3为研究对象,较为系统的研究了材料的合成工艺、电化学性能、物理特性及电极过程动力学。采用共沉淀法合成了FePO4·xH2O,并以FePO4·xH2O为原料通过碳热还原法制备出了LiFePO4／C正极材料。详细研究了制备过程pH对FePO4·xH20的纯度、表面形貌、颗粒大小、比表面积、振实密度的影响。结果表明,pH为1.5时制备出的FePO4·xH2O中存在Fe(PO4)2(OH)2杂质相,当pH分别为2、3、4、5时合成的FePO4·xH2O均为纯相；制备出的FePO4·xH2O和LiFePO4颗粒粒径均随着pH的升高而逐渐变小。以pH=2条件下合成的FeP04·xH2O为原料制备出的LiFePO4／C复合材料电化学性能最优,该样在0.1C倍率下首次放电比容量达到145mAh.g-1,50次循环后容量保持率高达98%；并且其振实密度高达1.11g·cm-3。首次采用草酸为还原剂,在常温机械活化条件下将FePO4·xH20中的三价铁还原为二价,并制备出无定形的LiFePO4,然后低温热处理制备出晶态的LiFePO4。用该方法制备LiFePO4简单可行,且制备出的材料电化学性能优良。XPS结果表明,在常温机械活化条件下,原料中的三价铁被还原为二价；在6000C,12h下合成的LiFePO4粒子分布均匀且一次粒子直径在150nm左右；TEM结果表明：LiFePO4颗粒表面被疏松的无定形碳膜包覆,碳膜的厚度在20nm左右。材料具有高比容量、优良的大倍率充放电性能和循环性能。在放电倍率分别在0.1C和10C下首次放电比容量分别为165 mAh·g-1和95mAh·g-1,50次循环后比容量分别为163mAh·g-1和85mAh·g-1,容量保持率分别为98.8%和89.5%。首次采用常温还原-低温热处理制备出了Li3V2(PO4)3正极材料。XPS测试结果表明五价钒在常温机械活化作用下,被草酸还原为三价,此方法简单可行,并且烧结温度低,适用于制备高性能Li3V2(PO4)3。当烧结温度为650℃,烧结时间为12h时,制备出的Li3V2(PO4)3性能最为优越,且具有一种疏松多孔的结构,比表面积高达25m2·g-1；单个粒子直径为200nm左右,且被厚度在10-20nm之间的纳米碳网所包覆。该样在0.1C倍率下首次放电容量高达131.57mAh·g-1,100次循环后放电容量为123.50mAh·g-1,容量保持率为95.23%。首次利用LiFePO4和Li3V2(PO4)3各自的优点,提出用常温还原-低温热处理法制备锂离子电池复合正极材料xLiFePO4·yLi3V2(PO4)3,XRD测试结果表明复合材料中主要存在橄榄石型的LiFePO4和单斜晶系的Li3V2(PO4)3。复合比例(x:y)为5：1时,在650℃下烧结12小时制备出的复合材料5LiFePO4·Li3V2(PO4)3具有最佳的电化学性能,1C放电容量高达154.13mAh·g-1,材料具有良好的倍率性能和循环性能,10C倍率下经100次循环后放电比容量仍高达112.8mAh·g-1,容量保持率为98.6%。复合材料的性能要明显优于相同条件下制备出的单一相的LiFePO4和Li3V2(PO4)3。此外,复合制备出的5LiFePO4·Li3V2(PO4)3较直接机械混合的5LiFePO4@Li3V2(PO4)3有明显的倍率性能优势。复合电极5LiFePO4·Li3V2(PO4)3的循环伏安测试表明,CV曲线上分别出现了Li3V2(PO4)3和LiFePO4的所有特征衍射峰,没有发现其他特征峰。在相同扫描速度下,复合电极5LiFePO4·Li3V2(PO4)3的四对氧化还原峰的电势间隔要明显小于单一的LiFePO4和Li3V2(P04)3,复合材料的脱嵌锂性能要明显优于单一的LiFePO4和Li3V2(PO4)3。HRTEM结果表明：复合材料5LiFePO4·Li3V2(PO4)3表面被5nm厚的碳网包覆,各个粒子间通过“碳桥”连接,且晶体表面存在着许多缺陷；通过对复合材料5LiFePO4.Li3V2(PO4)3的复合机理进行研究发现,复合材料xLiFePO4·yLi3V2(PO4)3中存在着部分Fe/V分别相互掺杂Li3V2(PO4)3／LiFePO4,并形成固溶体；掺杂后的Fe和V分别显示出了和LiFePO4、Li3V2(PO4)3中Fe和V一样的价态,分别为+2和+3价。利用线性极化法测定了LiFePO4、Li3V2(PO4)3、直接机械混合的5LiFePO4@Li3V2(PO4)3和5LiFePO4·Li3V2(P04)3电极在不同荷电态下的交换电流密度,研究发现随着电极荷电的增加,各个电极的交换电流密度均呈变大的趋势；这四个电极在荷电50%时,其交换电流密度分别为38.64、64.95、44.77和76.18mA.g-1,其中5LiFePO4·Li3V2(PO4)3的交换电流密度最高。用CV法测定了各个电极在对应的各个氧化还原峰位的锂离子扩散系数,研究发现在氧化峰处的扩散系数均要大于在还原峰处的扩散系数；复合材料中的锂离子扩散系数要明显大于单一正极材料相同峰位的锂离子扩散系数。复合正极材料5LiFePO4·Li3V2(P04)3的嵌锂过程动力学要优于单一的正极材料LiFePO4和Li3V2(PO4)3。首次以共沉淀法制备出的FeVO4·xH2O为原料制备出了LiFePO4-Li3V2(PO4)3复合材料。XRD结果表明合成的材料是橄榄石型的LiFePO4和单斜晶系的Li3V2(PO4)3两相并存。该材料表现出了优越的倍率性能和循环稳定性,在0.1C、1C和3C倍率下的放电容量分别高达142.5、138.4和123.4mAh·g-1,循环30次后放电容量为139.1、135.5和116mAh·g-1。电池的容量保持率高达97.6%、97.9%和94.0%。更多还原

【Abstract】 In this paper, the development of lithium ion batteries and cathode materials were reviewed in detail. The aims of the present study were focus on the preparation processes, the electrochemcial performances, physic properties, and the kinetics behaviors of LiFePO4, Li3V2(PO4)3 and composite materials xLiFePO4·yLi3V2(PO4)3.FePO4·xH2O precursor was synthesized from co-precipitation under various pH value. LiFePO4/C cathode material was then synthesized by carbothermal reduction using FePO4·xH2O. The effect of the solution pH on the performance of FePO4·xH2O was characterized. The results showed that there were no impurities existed in the FePO4·xH2O synthesized on the condition that the solution pH was between 2 and 5. LiFePO4 made from FePO4·xH2O precursor synthesized under the condition that the solution pH was about 2 showed excellent electrochemical performance, it exhibited a discharge capacity of 145mAh·g-1 at 0.1C, the capacity retention is about 98% after 50 cycles, and the tap density reaches 1.11 g·cm-3.It was the first time that LiFePO4 was synthesized by heating an amorphous LiFePO4, which obtained through lithiation of FePO4·xH2O by using oxalic acid as a novel reducing agent at room temperature. The effects of reaction temperature and time on the crystal structure, morphology and electrochemical properties of products were investigated. The results show that the sample synthesized at 600℃for 12h has fine particle sizes between 100-200nm, and with homogenous sizes distribution, the particles are wraped by 20nm thickness carbon webs. The synthesized LiFePO4 composites showed excellent discharge capacity of 165 mAh·g-1 and 95 mAh·g-1 at 0.1C and IOC rate. The cell retains 98.8% and 89.5% of its initial discharge capacity after 50 cycles.It was the first time that Li3V2(PO4)3 was synthesized by a room temperature reduction-low temperature calcining. XPS results showed V5+was reduced to V3+by oxalic acide, under ball milling at room temperature. Li3V2(PO4)3 particles are about 200nm in diameter and have porous structure, and coated with nano-carbon webs, the thickness of the nano-carbon webs are about 10-20nm, the specific surface area is as large as 25m2·g-1. Li3V2(PO4)3 synthesized at 650℃for 12h showed best electrochemical performance, it exhibited a capacity of 131.57mAh·g-1 at 0.1C, the capacity retention is about 95.23% after 100 cycles.In order to use the merits of LiFePO4 and Li3V2(PO4)3, composite materias xLiFePO4·yLi3V2(PO4)3 were synthesized through room temperature reduction-low temperature calcining. XRD results showed an olivine type LiFePO4 and a monoclinic component Li3V2(PO4)3 coexisted in the composite.5LiFePO4-Li3V2(PO4)3 synthesized at 650℃for 12h showed good electrochemical performance, it exibited 154.13mAh·g-1 at 1C rate. A capacity of 112.8mAh·g-1 can be obtained at IOC rate after 100 cycles, the capacity retention was 98.6%. The electrochemical performances of the composite material 5LiFePO4·Li3V2(PO4)3 are better than single phase LiFeP4 and Li3V2(PO4)3. Besides, the performances of the synthesized 5LiFePO4·Li3V2(PO4)3 are better than that of 5LiFePO4@Li3V2(PO4)3 obtained just mechanical mixed LiFePO4 and Li3V2(PO4)3 together. CV results of 5LiFePO4·Li3V2(PO4)3 electrodes showed that the oxidation and reduction peaks appeared in the way of the superimposition of the peaks of LiFePO4 and Li3V2(PO4)3. No other peaks can be found. In the same scanning velocity, the voltage intervel of the four pairs peaks of 5LiFePO4-Li3V2(PO4)3 are obviously smaller than that of single phase LiFePO4 and Li3V2(PO4)3, the lithium deintercalation/ intercalation kinetics behaviors of 5LiFePO4·Li3V2(PO4)3 are better than that single phase LiFePO4 and Li3V2(PO4)3. The coalescence machanism for the composite cathode material 5LiFePO4-Li3V2(PO4)3 were studied, the results showed some LiFePO4 and Li3V2(PO4)3 in the composite material were doped by V and Fe, respectively, and formed a solid solution. The valence of Fe and V after doping in Li3V2(PO4)3 and LiFePO4 was+2 and+3, respectively.The kinetics behaviors of LiFePO4, Li3V2(PO4)3 and 5LiFePO4-Li3V2(PO4)3 electrodes were studied by means of linear sweep voltammetry and cyclic voltammetry. It is found that the exchange current density was increased with the charge state increasing. The exchange current density is 38.64,64.95,44.77 and 76.18mA·g-1 for LiFePO4, Li3V2(PO4)3, mechanical mixied 5LiFePO4@Li3V2(PO4)3 and 5LiFePO4·Li3V2(PO4)3, respectively. The lithium diffusion coefficient around the oxidation peaks are larger than that of reduction peaks. And the lithium diffusion coefficients in the composite material 5LiFePO4·Li3V2(PO4)3 are larger than that in single phase LiFePO4 and Li3V2(PO4)3, indicated the lithium ion diffusion rate in 5LiFePO4·Li3V2(PO4)3 are faster than single LiFePO4 and Li3V2(PO4)3.Composite material LiFePO4-Li3V2(PO4)3 was synthesized using FeVO4·xH2O obtained by co-precipitation. XRD results showed that olivine type LiFePO4 and monoclinic Li3V2(PO4)3 coexisted in the composites. The composites synthesized at 700℃for 12h exibited good rate performance and stable cycle performance, and its discharge capacity is about 142.5 at 0.1C,138.4 at 1C and 123.4mAh·g-1 at 3C with satisfactory capacity retention.更多还原