新型沉淀法制备高性能磷酸铁锂正极材料研究

发布时间：2018-04-18 10:57

  本文选题：锂离子电池 + 磷酸铁锂　； 参考：《华南理工大学》2011年硕士论文

【摘要】：橄榄石结构的磷酸铁锂(LiFePO_4)以其比容量高、价格低廉、热稳定性好和环境友好等优点被认为是一种最具有广阔应用前景的锂离子电池正极材料。目前工业制备LiFePO_4所用的高温固相法存在能耗高、难以得到高纯产物、产物性能不稳定等缺点。近年来,湿化学方法制备磷酸铁锂以其能耗低、结构均匀稳定、容易实现掺杂改性等重要优点而越来越受到重视。 本文设计和提出了一种湿化学制备磷酸铁锂的新方法-混合沉淀法,该方法以(NH_4)_2Fe(SO_4)_2·6H_2O、LiOH·H_2O、H_3PO_4、(NH_4)_3PO_4·3H_2O为原料,先分别制得磷酸铁和磷酸锂的沉淀,经过陈化和除去部分母液后,将两种沉淀充分混合,继续陈化,经过过滤干燥,可得到磷酸铁锂的前驱体。该方法有效地解决了传统的沉淀法及共沉淀法存在的锂盐同步沉淀困难、收率低、以及材料均匀性差等问题,可制备出纯相的高性能LiFePO_4/C材料,产物的电化学性能远高于共沉淀法制备的样品和工业样品。本文考察了各种制备条件对于材料性能的影响,运用恒电流充放电测试、循环伏安测试以及交流阻抗测试对产物的电化学性能进行了分析,并采用XRD、SEM等表征手段对产物的结构和形貌进行了表征分析。 考察了原料中锂铁比、陈化温度、焙烧温度、液相中乙醇的添加量对产物性能的影响。研究结构表明:锂铁比为1.0和1.1时能够得到纯相的LiFePO_4 ,锂过多和过少都会产生杂质相。锂铁比为1.1的样品在0.1 C的放电比容量最高,为151.7 mAh·g~(-1) ,但是在2 C放电倍率下仅为109.2 mAh·g~(-1),不及锂铁比1.0的样品的119.0 mAh·g~(-1)。所以锂铁比为1.0的样品在大倍率下具有较好的电化学性能。50℃是比较适宜的陈化温度,陈化温度为50℃的样品在不同倍率下的放电比容量都较高,在0.1 C和2 C倍率下分别为141.0 mAh·g~(-1)和125.7 mAh·g~(-1)。高温焙烧温度对产物的性能影响比较显著,在考察范围内,随着焙烧温度的升高,产物的结晶度越高,放电比容量越大。其中750℃焙烧的样品的电化学性能最好,在0.1 C和2 C的放电比容量分别达到了155.9 mAh·g~(-1)和118.9 mAh·g~(-1)。 本文研究了不同碳源及碳含量对于LiFePO_4性能的影响。以聚乙烯醇(PVA)为包覆碳源具有非常好的改性效果,碳含量为8 wt.%的样品在0.1 C和2 C的放电比容量分别为154.3 mAh·g~(-1)和134.5 mAh·g~(-1)。研究结果表明:采用葡萄糖包覆的样品也具有非常好的电化学性能,当碳含量为10 wt.%时,其0.1 C放电的比容量可达148.1 mAh·g~(-1)。 实验发现:碳含量少的样品在小倍率放电时的电化学性能与碳含量多的样品基本一样,但在大倍率放电时比容量衰减非常明显,如:碳含量为3 wt.%的样品在2 C倍率时的容量仅为72.4 mAh·g~(-1),而碳含量为12 wt.%左右的样品在2 C倍率下的放电比容量可高达125 mAh·g~(-1)。
[Abstract]:Olivine structure LiFePO4 (LiFePO4) is considered to be the most promising cathode material for lithium-ion batteries due to its high specific capacity, low price, good thermal stability and environmental friendliness.At present, the high temperature solid state method used in the industrial preparation of LiFePO_4 has the disadvantages of high energy consumption, difficulty in obtaining high purity products and unstable properties of the products.In recent years, more and more attention has been paid to the preparation of lithium iron phosphate by wet chemical method because of its advantages of low energy consumption, uniform structure stability and easy to achieve doping modification.A new wet chemical method for the preparation of lithium iron phosphate, mixed precipitation method, has been designed and proposed in this paper. The method is based on a new method, which is composed of a new method, which is composed of a mixture precipitation method. The method is based on the method of NH / NH _ 4 / S _ 2FeO _ 4 / S / S / T / T _ 6H _ 2OH _ 2O _ 4 / T _ 2H _ 2O _ 2H _ 2O _ 4 / s _ 4 3H_2O as raw material. The precipitation of Fe _ 2O _ 4 and Li _ 2PO _ 4 is prepared respectively after aging and removing part of the mother liquor.The precursor of lithium iron phosphate can be obtained by mixing the two precipitates and aging them through filtration and drying.This method can effectively solve the problems of simultaneous precipitation of lithium salt by traditional precipitation method and co-precipitation method, such as low yield and poor homogeneity of materials, and can be used to prepare pure phase high performance LiFePO_4/C materials.The electrochemical properties of the products are much higher than those prepared by coprecipitation and industrial samples.In this paper, the effects of various preparation conditions on the properties of the materials were investigated. The electrochemical properties of the products were analyzed by constant current charge-discharge test, cyclic voltammetry test and AC impedance test.The structure and morphology of the products were characterized by XRD SEM.The effects of the ratio of lithium to iron, aging temperature, calcination temperature and the amount of ethanol in liquid phase on the properties of the product were investigated.The results show that pure phase LiFePO_4 can be obtained when the ratio of lithium to iron is 1. 0 and 1. 1. The impurity phase can be produced by excessive lithium and too little lithium.The discharge capacity of the sample with Li / Fe ratio 1.1 is the highest at 0.1 C, which is 151.7 mAh / g ~ (-1), but at 2 C discharge rate is only 109.2 mAh / g ~ (-1), which is less than that of the sample with Li / Fe ratio of 151.7 mAh / g ~ (-1) ~ (-1).Therefore, the sample with Li / Fe ratio 1.0 has better electrochemical performance at large ratio. 50 鈩，

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