【作者】 沈超；

【导师】 张宝；

【作者基本信息】 中南大学 ， 环境工程， 2011， 硕士

【摘要】 锂离子电池正极材料LiFePO4和Li2FeSi04均具有原料丰富、无毒、热稳定性强、循环性能好、理论容量高等优点,被认为是极具发展潜力的锂离子电池正极材料。本文以LiFePO4和Li2FeSi04为研究对象,对其合成工艺和材料改性进行了系统研究。前驱体FePO4·χH2O对合成LiFePO4结构与电化学性能有较大影响,通过液相沉淀法制备得到了纯相的FePO4·χH2O,详细探讨了反应物浓度对前驱体FePO4·χH2O及LiFePO4性能的影响。在既定条件下,当反应物浓度为1.0mol·L-1时合成的FePO4·2H2O结构与性能较好,以此为前驱体合成的LiFePO4电化学性能较优。针对Li2FeSiO4正极材料电子电导率低和离子扩散能力差的缺点,采用柠檬酸配合法合成Li2FeSiO4正极材料,对加入柠檬酸与未加入柠檬酸两种方法合成的Li2FeSiO4样品进行了比较,结果表明柠檬酸配合法合成的样品结晶度更完整,颗粒分布均匀,电化学性能较好,样品在C/16倍率下的首次放电比容量为135.4 mAh·g-1,30次循环后容量衰减率为15.7%；同时研究了不同配合比对Li2FeSiO4样品物理及电化学性能的影响,结果表明,配合比k=1.0时材料的物理及电化学性能最佳。研究发现,单一碳源掺杂对Li2FeSiO4/C电化学性能改进效果有限,探索了复合有机碳源对Li2FeSiO4/C的电化学性能的影响。研究表明,掺入35%柠檬酸+5%葡萄糖+25%均苯四甲酸酐的复合有机碳源对Li2FeSiO4/C材料的电化学性能改进效果显著；同时较系统研究了烧结温度和烧结时间对复合碳源改性后的材料的物相结构、微观形貌和电化学性能的影响。研究表明最佳合成条件：烧结温度为700℃,烧结时间为20h。在最佳合成条件下制备的Li2FeSiO4/C(F3)材料的电化学性能较好,样品在C/16倍率下的首次放电比容量为157.5 mAh·g-1,经30次循环后比容量保持率为92.06%。采用循环伏安法(CV)和交流阻抗图谱法(EIS)对Li2FeSiO4/C(F3)的电极界面锂脱/嵌动力学行为作进一步研究,测试结果表明掺入复合有机碳源进行表面改性后电极极化减小,可逆性和电化学性能显著提高。更多还原

【Abstract】 The development of rechargeable lithium-ion batteries and cathode materials are reviewed in detail. With the advantages of abundant recource, non-toxicity, high safety, low cost, LiFePO4 and Li2FeSi04 have been considered as a promising cathode material on lithium-ion batteries. Synthesis of materials, modification process, structure characterization, electrochemical behaviors of LiFePO4 and Li2FeSiO4 has been involved in this study.Based on the disadvantages of unstable properties of FePO4·χH2O which is the raw materials of LiFePO4, the pure FePO4·χH2O was prepared by liquid precipitation method. The effect of reactants’ concentration on the physical performance and electrochemical behavior of FePO4·χH2O precorsor and LiFePO4 was studied. The results showed that LiFePO4 synthesised by homogeneous FePO4·2H2O precorsor had the best electrochemical properties when the concentration was 1.0 mol·L-1.Based on the disadvantages of poor ion diffusion and low electronic conductivity of the pure phase Li2FeSi04, the Li2FeSi04 was prepared by sol-gel technique using citric acid as a complexing agent. Comparison research on performance of two samples which prepared by different ways by using citric acid or not have been studied, the result the sample by using citric acid with better crystallinity, uniform particles and excellent electrochemical properties. The initial discharge capacity of Li2FeSi04 synthesized is 135.4 mAh·g-1 at C/16 rate and the capacity fading is only 15.7% after 30 cycles. The effect of complex ratio on the physical structure and electrochemical properties of Li2FeSi04 was researched. The results indicated that the electrochemical properties of Li2FeSiO4 increased then decreased with the changing of complex ratio and the best sample obtained when the complex ratio is 1.0.According to preliminary studies by our group, adding only a kind of organic carbon would improve the electrochemical properties of Li2FeSi04 limited, therefore, the influence of surface modification by combined organic carbon coating on the properties of Li2FeSiO4/C was investigated. It was found that the Li2FeSi04/C by adding 35% citric acid+5%glucose+5%pyromellitic anhydride as carbon source has the outstanding electrochemical properties.Meanwhile, the effect of synthesis conditions including sintering the temperature and time on the physical performance and electrochemical behavior of Li2FeSiO4/C was studied and the synthesis conditions were optimized. The results show that the sintering temperature and time play an important role in the crystal structure and morphology of the material. The optimized sintering temperature and time were 700℃and 20h. The initial discharge capacity of Li2FeSiO4/C(F3) synthesized on the optimized condition is 157.5 mAh·g-1 at C/16 rate and the capacity retention is 92.06% after 30 cycles.The lithium deintercalation-intercalation kinetics of Li2FeSiO4/C(F3) was investigated by cyclic voltammetry and electrochemical impedance spectroscopy methods. The results further proved that the modified Li2FeSi04/C with the decreasing of electrode polarization and the improvement of reversibility and electrochemical properties.更多还原