【作者】 崔妍；

【导师】 郭瑞松；

【作者基本信息】 天津大学 ， 材料学， 2010， 博士

【摘要】 本文采用三种方法来提高LiFePO₄电子电导率与锂离子扩散速率,减小LiFePO₄颗粒粒径,以达到改善LiFePO₄正极材料电化学性能（特别是高倍率性能）的目的。第一是金属离子掺杂与碳包覆;第二是金属氧化物与碳共同包覆;第三是采用冰冻干燥法制备LiFePO₄/C纳米前驱体。主要考察了不同金属离子、不同碳源、不同金属氧化物以及不同合成方法对LiFePO₄正极材料循环性能、电化学动力参数如电荷转移电阻Rct、交换电流密度i0、扩散系数D和电导率σ的影响。利用XRD、SEM、HRTEM对材料的相组成和微观形貌进行了分析,采用恒流充放电技术、交流阻抗技术与循环伏安法测试其电化学性能。利用微波法合成了LiFePO₄/(C+La³⁺)与LiFePO₄/(C+Ti⁴⁺)复合正极材料,结果表明C包覆与金属离子掺杂确实是提高材料电导率的一种效果显著的方法,尤其是掺杂与Li⁺和Fe²⁺半径相近的金属离子时,所得到的材料的晶格畸变小,结构稳定,电化学性能特别是在高倍率放电时材料的循环性能有明显改善。以草酸为碳源,采用溶胶凝胶法二次合成技术在管式气氛炉中600℃保温8 h所得的LiFePO₄/C正极材料的电化学性能最优,在0.2 C倍率下循环10次后材料的放电比容量仍能达到129.0 mAh·g^-1。采用悬浮混合法与共沉淀法分别制备了La_0.7Sr_0.3MnO₃包覆LiFePO₄/C材料与CuO包覆LiFePO₄/C材料。由高分辨透射电镜照片可以看出,进一步包覆CuO或La_0.7Sr_0.3MnO₃后,在磷酸亚铁锂颗粒表面形成了一层连续的、完整的导电纳米层。包覆La_0.7Sr_0.3MnO₃的LiFePO₄/C材料,在连续充放电35个循环后,0.5C放电倍率下放电比容量达到134.3 mAh·g^-1。CuO包覆的LiFePO₄/C材料,1C放电时,材料的放电比容量最高达到125.0 mAhg^-1,20次循环后仍维持在123.0 mAhg^-1,容量损失率仅为1.6%。以冰冻干燥法合成的LiFePO₄/C正极材料为基体,进一步包覆了La_0.7Sr_0.3MnO₃或ZnO。结果表明,对于含量为2% La_0.7Sr_0.3MnO₃包覆的材料,在0.5C和1C的放电倍率下的最高可逆放电比容量分别为143.4 mAh/g和133.6 mAh/g。对于含量为2%ZnO包覆的材料,所得材料的电化学性能最为优异,在1C与2C的放电制度下其放电比容量分别为130.7 mAh g^-1和122.6 mAh g^-1且循环性能良好,55个放电循环后再进行三次循环伏安测试,所得三次循环伏安曲线几乎是重合的,说明了此样品在循环过程中几乎没有极化现象的产生。更多还原

【Abstract】 Three methods were adopted in this work to improve electrochemical properties and to reduce particle size of LiFePO₄ cathode materials based on its low electronic conductivity and lithium-ion diffusion rate. The first was metallic ion doping and carbon coating LiFePO₄; The second is metal oxide and carbon co-coating LiFePO₄; The last is freeze-drying to synthesize the nanometer-sized precursor of LiFePO₄/C. Different metallic ions, carbon sources, metal oxides and synthesis methods are studied, focusing on their influences on charge-discharge property, cycle ability, electrochemical kinetics parameters such as charge transfer resistance, exchange current density i0, diffusion coefficient D and conductivityσ. The microstructure and morphologies of these composites were investigated by XRD, SEM and HRTEM. The electrochemical performances were evaluated by galvanostatic charge-discharge, impedance spectroscopy and cyclic voltammogram.LiFePO₄/(C+La³⁺) and LiFePO₄/(C+Ti⁴⁺) cathode materials were synthesized by microwave heating. The results indicated that the metallic ion doping and carbon coating could greatly enhance the electronic conductivity of LiFePO₄, especially when the radius of doping ion is similar to the ones of Li⁺ and Fe²⁺. The reason is that the distortion of crystalline lattice is relatively small and the structure is stable, accordingly, the electrochemical performances, particularly the high rate cycle performances, were obviously improved.The carbon-coated LiFePO₄ composite was synthesized via sol-gel method and two times firing procedure with keeping 8 h at 600℃, using oxalic acid as carbon source. It was demonstrated that the highest discharge specific capacity of 129.0 mAh·g-1 after 10 cycles at 0.2C rate was obtained for this composite cathode, with the capacity fading being only 1.07%. La_0.7Sr_0.3MnO₃+C co-coated LiFePO₄ and CuO+C co-coated LiFePO₄ were gained by suspension mixing process and precipitation method, respectively. Nanolayer structured La0.7Sr0.3MnO3 or CuO together with the amorphous carbon layer formed an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high resolution transmission electron microscopy. La_0.7Sr_0.3MnO₃+C co-coated LiFePO₄ delivered a high discharge specific capacity of 134.3 mAh·g-1 after 35 cycles at 0.5C rate. CuO+C co-coated LiFePO₄ showed a discharge specific capacity of 125.0 mAhg-1 at 1C, and the capacity remained to be 123.0 mAhg-1 after 20 cycles. The capacity fading is only 1.6%.LiFePO₄/C composite synthesized by freeze-drying method was used as a matrix. Then it was further coated with La0.7Sr0.3MnO3 or ZnO by suspension mixing process or precipitation method, respectively. The results indicated that the particle sizes of co-coated composite were about 50 nm. When the content of La0.7Sr0.3MnO3 was 2%, it delivered the highest discharge capacity of 143.4 mAh/g and 133.6 mAh/g at 0.5C and 1C, respectively. When the content of ZnO was 2%, the materials showed the best electrochemical properties. The discharge capacity reached 130.7 mAh g-1 and 122.6 mAh g-1 at the rates of 1C and 2C, respectively. The cycle performances were also very good. The three cyclic voltammogram curves were almost overlapping after 55 discharge cycles, demonstrating that the continuous integrated conductive layer between LiFePO₄ particles resulted in negligible polarization during cycling.更多还原