【作者】 李雯；

【导师】 许云华；

【作者基本信息】 西安建筑科技大学 ， 机械设计理论， 2015， 硕士

【摘要】 2005年,Nyten等首次报道了硅酸铁锂(Li2FeSiO4)正极材料,因其硅酸铁锂化合物(Li2FeSiO4)分子结构中有两个锂离子,故而有着较高的理论比容量(331mAh/g);硅酸铁锂(Li2FeSiO4)正极材料中的硅元素储量大,因而价格也较低;硅酸铁锂与磷酸铁锂包含有类似于磷酸铁锂体系中P-O键的Si-O共价键,使其具有优异的热稳定性和化学稳定性,故具有很好的应用前景。但硅酸铁锂存在电子电导率比较低的缺点,制约了其高倍率下的充放电性能。目前的改善方法主要有导电材料的包覆、形貌结构的优化及阴阳离子掺杂的方法提高硅酸铁锂材料的电学性能。本文将通过制备工艺的创新与改善,优化材料的晶体结构形貌,并通过导电碳黑加入的方法提高硅酸铁锂(Li2FeSiO4)正极材料离子电导率,从而改善材料的电化学性能。论文的主要工作有以下几个方面:1、目前针对硅酸铁锂材料的共沉淀工艺尚未有所报道,因共沉淀工艺制备的材料粒度较小,形貌规则统一的特点,本课题创造性的提出了一种共沉淀的工艺和方法,利用共沉淀的工艺制备出了Li2FeSiO4前驱体材料,并通过焙烧得到Li2FeSiO4材料,该工艺方法操作简单,制备出的Li2FeSiO4材料形貌均一、粒度细小均匀,与其它目前报道的方法比较有很大的优势。该工艺是通过在碱性氧化环境下共沉淀得到二氧化硅,解决了碱性氧化环境下的二价铁离子沉淀及易氧化的问题,通过锂盐和碳源的加入制备出了Li2FeSiO4前驱体材料,该工艺下制备的前驱体材料具有工艺简单,材料混合均匀一致的特点,研究了该前驱体下硅酸铁锂的合成制备工艺,研究表明得到的硅酸铁锂材料为正交晶系,(Pmn21)空间群;对于800℃条件下制备的硅酸铁锂正极复合材料的衍射峰与文献报道的结果一致,并且未有明显的杂峰,说明使用该方法成功的合成出了具有纯相的Li2FeSiO4材料,其SEM图可以看出,硅酸铁锂晶粒的形貌规则,粒度细小均匀,并具有较好的电化学性能。2、我们基于共沉淀法的制备工艺,通过不同焙烧时间(4h、6h、8h、10h)制备,研究晶体生长及形貌的形成过程。通过XRD、EDS及SEM来表征材料的结构和形貌。重点研究了该工艺体系下反应的过程和晶体的生长机理,并讨论了其晶体形貌结构和性能之间的关系。测试结果表明:在800℃下4h时材料的形貌为类球形,但材料中的硅酸锂杂峰较多,说明此时生成的硅酸锂并未完全发生反应,6h时材料的形貌为球形形貌,材料中的主要峰与Li2FeSiO4样品的主峰基本一致,但有少量杂峰出现,8h时材料的形貌为花生状,此时的材料为Li2FeSiO4纯相材料,材料的形貌规则统一,粒度均匀细小,粒径分布为500-600nm。10h时生成的硅酸铁锂材料晶粒发生了连接生长的情况,花生状的硅酸铁锂逐渐的生长成为了珊瑚状的硅酸铁锂材料。对不同焙烧时间的研究表明材料的晶体生长是基于二氧化硅的球形颗粒,首先生成了类球形的硅酸锂材料,然后再有类球形的硅酸锂材料生长为了球形的硅酸铁锂材料,球形的硅酸铁锂经过高温长时间下会发生晶体生长连接形成花生状,如果时间再进一步延长将生长为珊瑚状的硅酸铁锂材料,电化学测试结果表明:样品在焙烧条件为8h时锂离子扩散系数为:1.24×10-13,存在明显的充放电平台,首次充电容量为164.9mAh/g。首次放电容量154.2mAh/g,且在0.2C的倍率条件下仍有较高的放电容量,循环20周后容量为129.9m Ah/g,仍保持着较高的容量。3、研究了掺杂不同含量导电炭黑(碳含量2%wt、4%wt、8%wt)方法对Li2Fe SiO4进行改性研究,通过XRD、SEM来表征材料的结构与形貌,形貌为均一的纳米硅酸铁锂颗粒间隙中均匀分散着纳米级的导电炭黑,其中由于加入的纳米级的导电炭黑(20-40nm)粒度较小,存在一定的团聚现象,但整体分布较为均匀。并研究了导电炭黑的加入对电学性能的影响,研究表明:碳含量为4%wt条件下所合成的Li2FeSiO4/C具有最好的电化学性能,该方法制备的Li2FeSiO4/C晶型良好,晶粒分布均匀,颗粒尺寸在500-600nm之间,EIS的测试后表明材料的电导率为2.21×10-13m2s-1,循环50周后容量保持率在85%以上。其优异的电化学性能可以归因于在该工艺条件下材料的粒径均匀细小缩短了锂离子的扩散距离;另一方面均匀的碳包覆提高了材料的锂离子扩散系数;其多孔通道可以存储电解液更有利于锂离子的嵌入脱出。更多还原

【Abstract】 In 2005,Nyten etc. obtained the synthesis of cathode material Li2 Fe Si O4 for the first time Li2 Fe Si O4 has two lithium-ion, and theory of specific capacity(331 mah/g) is equivalent to the specific capacity of anode materials commonly used. So it has the very good application prospect. The raw resource for Li2 Fe Si O4 elements are also more common in the anode material, so the price can be low. Ferric metasilicate lithium and lithium iron phosphate containing Si-O covalent bond is similar to the P-O key contained by lithium iron phosphate system, which has excellent thermal stability and chemical stability, but iron lithium silicate had a disadvantage of low electronic conductivity, which restrict its high rate charge and discharge performance. At Present the method to improve the electrical performance of lithium iron silicate materials contain, the morphology structure optimization and doping methods to improve the electrical performance of lithium iron silicate materials. This article will state the innovation and improvement in the preparation process, optimize the structure and morphology of crystal materials, and adding the conductive carbon black to improve Li2 Fe Si O4 positive-ion conductivity, so as to improve the electrochemical properties of materials. The main work of the thesis are as follows:1.The co-precipitation process for lithium iron silicate materials has not yet been reported, the results showed that materials with rules of the sample morphology and particle unifomity. Based on the co-precipitation,it puts forward the experiment technology and method. and sample is obtained by roasting Li2 Fe Si O4 materials, the technological process is simple, the preparation of the material Li2 Fe Si O4 uniform shape and uniform particle size small, compared with other methods reported at present is a big advantage. The process that silicon dioxide is obtained by co-precipitation inalkaline oxidation environment, solved the alkaline oxidation of ferrous iron ion precipitation and the problem of easy oxidation by lithium salt, and the addition of carbon source were Li2 Fe Si O4 precursor materials, preparation of precursor materials under the process is simple in process, material mixing uniform characteristics characteristics of structure, and studied the synthesis of the precursor of lithium iron silicate preparation technology, the results have shown that the crystal structure of the lithium iron silicate materials is orthogonal crystal system, belongs to space group(Pmn21); The diffraction peak of Lithium iron silicate anode composite prepared in800 ℃ is consistent with the results reported in literatures, and no obvious impurity peak, It turns out that we successfully obtain the pure phase Li2 Fe Si O4 material by this method and Its SEM figure shows that the size of uniform particle is small. In addition the electrochemical performance is excellent.2.Based on co-precipitation preparation technology, by different roasting time(4h,6h,8h,10h) preparation, studying the formation process of crystal growth and morphology. By XRD, EDS and SEM characterization of the structure and morphology can be invested. This thesis mainly studied the process system reaction and the process of crystal growth mechanism, and discussed the crystal morphology of the relationship between structure and performance. Test results show that the 4h in 800 ℃ the morphology of materials is the class ball class, and the material of the lithium silicate impurity peak is more, which show the generated lithium silicate did not react completely, the shape of morphology of the material is spherical, of the sample is almost same with Li2 Fe Si O4 materials in the materials and Li2 Fe Si O4 the main peak of the sample, but there is a small amount of impurity peak, the morphology of 8h material is peanut shape, sample is pure Li2 Fe Si O4 material at this time Rules of the morphology of pure material, the morphology of material is unified, and the size of particle is small,which ranges form 500 nm to 600 nm.the sample of 10 h generated lithium iron silicate material in the connection occurs between grains, the growth of peanut shape of lithium iron silicate gradually became a coralloid lithium iron silicate materials.Studying materials with different roasting time of crystal growth are based on spherical silicon dioxide particles, spherical lithium silicate materials generated class firstly, then another class of spherical lithium silicate material growth for spherical iron lithiumsilicate material, spherical iron lithium silicate will occur under high temperature and long time connection form between peanut shape crystal growth, if time will have a further extension the sample will grow as coralloid lithium iron silicate material, the electrochemical test results show that the samples in the roasting conditions for 8h have a largest lithium ion diffusion concentration: 1.24×10-13,there is an obvious charge and discharge platform, for the first time charging capacity of 164.9 m Ah/g. Initial discharge capacity of 154.2 m Ah/g, and in the rate of 0.2 C conditions have higher discharge capacity, cycle capacity almost has no attenuation, after 20 cycles of 129.9m Ah/g.3.The Li2 Fe Si O4 was modified by carbon coating. Effects of Vanadium substitution at different sites on the structure of Li2 Fe Si O4/C are examined by X-ray diffraction(XRD),and scanning electron microscopy(SEM).uniform lithium iron silicate with nanoscale conductive carbon black homogeneously dispersing in the clearance.as a result of the join nanoscale conductive carbon black(20-40nm) which particle size is small, there is a reunion phenomenon, but the overall distribution is more uniform. And studying the influence of the addition of conductive carbon black on the electrical performance, research has shown that carbon content is 4%wt under the condition of synthesis of Li2 Fe Si O4/C has the best electrochemical performance, The results indicate that the Li2 Fe Si O4/C sample prepared by co-precipitation method as a lithium source exhibits a good crystallinity with little impurities. The size of Li2 Fe Si O4/C particles were approximately 500-600 nm,The electronic conductivity of Li2 Fe Si O4 was about2.21×10-13 m2s-1,50 cycles capacity retention rate is above 85%. It can be ascribed to the small size of the nanocomposite to reduce the diffusion distance of Li+ in Li2 Fe Si O4/C;the large specific surface area to increase the effective contact area between the nanocomposite Li2 Fe Si O4/C and electrolyte; carbon-decoration for improving the electronic conductivity of Li2 Fe Si O4; the texture with mesoporous can be infiltrated by the electrolyte in favor of Li+migration in the electrode.更多还原