【作者】 张占奎；

【导师】 梁辉； 季惠明；

【作者基本信息】 天津大学 ， 材料学， 2010， 硕士

【摘要】 橄榄石型LiFeP0₄是一种新型的锂离子电池正极材料，具有价格低廉、循环性能好、充放电平台高、绿色环保、使用安全等优点，在未来的电池行业中具有充分的竞争优势。由于LiFeP0₄低的本征电子电导率和锂离子迁移率，导致其在高倍率大电流放电时性能较差，制约了其在动力电池方面的应用。本文采用水热法合成LiFeP0₄前驱粉体，研究了水热反应的机理并确定水热合成的工艺，制备具有短锂离子扩散通道的细微粉体。为解决电子电导率低的问题，通过水热前和水热后两种碳包覆工艺进行包覆碳操作，同时在后续的热处理工作中对不同的热处理工艺和不同的碳源选择进行了研究，以探索包覆碳时期、热处理工艺、碳源类型对与LiFeP0₄材料的影响。结果表明，水热反应中采用Fes04：H3P04：Li0H=1：1：3为原料，柠檬酸做络台剂，抗坏血酸作为还原剂组成反应体系。前期配液中溶液中形成LiFeP0₄晶核和大量的无定形态Fe 3（P04）2和Li3P04沉淀。水热反应体系中，1．4h内溶液中的Fe²⁺、Li⁺、P0₄^3-逐渐进入LiFeP0₄晶格中形成形貌规整、物相纯净的菱形颗粒粉体，继续反应颗粒晶体出现异常生长，颗粒均一性遭到一定影响。并最终确定以Fe²⁺计前驱液浓度为O.3mol／I，水热条件为180℃、8h的条件下制备的LiFeP0₄样品性能较好，粉体晶相纯净、形貌均一、粒径细小、晶界清晰，振实密度达到1.05g/ml。进行了水热前和水热后碳包覆工艺对比。将葡萄糖分别在水热反应前和水热反应后加入反应体系，水热前渗包覆工艺制备的样品颗粒细小均一，平均粒径为O.3gm，振实密度较高，进行热处理后电化学性能测试显示水热前碳包覆制备的样品O 1c倍率下首次充放电比容量分别为127.1mAh/g、102.6 mAh／g，10次循环后放电容量为93.14％，显示了较高的工艺优势。比较了微波热处理和氮气热处理两种热处理工艺以及不同碳源类型在热处理工艺中的影响。微波热处理工艺效率高，在较短的时间内即能碳解有机物形成包覆，微波处理后晶粒没有出现增长现象，一定程度上控制了晶粒的大小，但不足之处为有机物的碳解程度稍低，且循环性能稍微下降。氮气热处理的有机物碳解完全，但热处理后晶粒有增长现象，且工艺周期长、效率低。在两种不同的热处理工艺中，葡萄糖作为碳源所制备的LiFeP0₄／c性能比以尿素为碳源所制备的样品高，用葡萄糖作为碳源经过氮气热处理所制备样品在O.1c倍率下的首次充放电比容量分别为122.7 mAh/g、104.5 mAh／g，充放电效率为85.2％，10次循环后容量基本无衰减。更多还原

【Abstract】 Olivine-structured LiFePO₄, as a promising cathode material for lithium-ion battery, is considered as the most competitive material for rechargeable lithium-ion battery in the future due to its merits such as low cost, good electrochemical performance and security, etc. However, the low conductivity of LiFePO₄ and poor lithium diffusion limit the high rate capacity in lithium cells. Hydrothermal methods were used to prepare LiFePO₄ and the mechanism of the reaction were explored. The research on when the carbon precursor should be added was carried out. The effects of heat treatment, selection of different carbon resources were disscussed in the heating process.LiFePO₄ was synthesized by hydrothermal method using FeSO₄, H₃PO₄, LiOH as raw materials. Citric acid, as the complexing agent, and ascorbic acid as the reducing agent supply a stable environment to Fe in the process of reaction. Little LiFePO₄ crystal nucleus and lots of amorphous Fe₃（PO₄）₂ and Li₃PO₄ were produced when prepared the mixed fluid. Fe²⁺ Li⁺ PO₄^3- enter in the crystal lattice to synthesized regular LiFePO₄ crystal in the earlier 4 hours. Some lattice variance were bring in in the next synthetic time. The optimal systhesis process was using water as solvent, FeSO₄ : H₃PO₄ : LiOH=1: 1:3（mol） as raw materials, to hydrothermal react at 180°C for 6h. The concentration of the liquid compound was 0.3mol/l. The sample was prism-like particles, and average size was 0.6μm, tap density was 0.97g/ml.Glucose as carbon sources was added into the compound before hydrothermal. The particle size was smaller to be 0.3μm, and the crystal structure was more regular. The sample displayed better electrochemical performance after heat treatment. The specific charge-discharge capacity respectively were 127. 1mAh/g, 102.6 mAh/g at the first cycle at 0.1C. After 10 cycles, the sample keep 93.14% of the capacity.Microwave method and atmosphere method were designed in the heat treatment process. The efficiency of microwave heat treatment is high which can decompose organic carbon such as glucose in few minutes so as not to increase the particle size. The shortage of microwave method was it can’t decompose the carbon completely. The atmosphere heat treatment method which was capable of decomposing the organic carbon completely, however, will increase the particle size, and this method has a low efficiency. Samples with glucose as carbon source, heated by these two methods, have a better performance than that with urea as carbon source. LiFePO₄-C samples synthesized through hydrothermal method with glucose as carbon sources were tested followed by microwave heat treatment, and the samples displayed charge-discharge specific capacity of 122.7mAh/g and 104.5mAh/g and had no reduction basiclly.更多还原