锂离子电池正极材料Li₂FeSiO₄的改性研究

【作者】 曹璇；

【导师】 郭华军； 张宝；

【作者基本信息】 中南大学 ， 有色金属冶金， 2009， 硕士

【摘要】 论文详细综述了锂离子电池及其正极材料的发展和研究现状。Li₂FeSiO₄具有高安全性、无毒、价格低廉、资源丰富等优点,被认为是铁基正极材料中极具发展潜力的锂离子电池正极材料。然而材料本身极低的电子电导率和离子扩散能力是制约其发展的主要因素。本文选取Li₂FeSiO₄作为研究对象,对其材料改性、合成工艺、结构表征、电化学性能及电极动力学性能进行了详细研究。采用体相掺杂的方法对Li₂FeSiO₄进行了改性研究,通过高温固相反应制备了铁位掺镍的Li₂Fe_1-xNi_xSiO₄材料。利用TG/DTA技术对Li₂Fe_1-xNi_xSiO₄的前驱体进行了表征,并通过XRD、SEM、电化学测试方法研究了掺杂量对Li₂FeSiO₄晶体结构、微观形貌和电化学性能的影响。结果表明:适当的镍掺杂不会改变Li₂FeSiO₄材料基本晶体结构,材料的充放电容量和循环性能都得到了一定的提高,其中Li₂Fe_0.7Ni_0.3SiO₄样品具有最佳的放电容量和容量保持率。在前驱体中加入适量蔗糖作为碳源和还原剂,通过铁位掺锰和表面碳包覆的复合改性方式制备了Li₂Fe_1-xMn_xSiO₄/C复合材料,并系统研究了掺锰量、焙烧温度、焙烧时间、Li/Si配比对Li₂Fe_1-xMn_xSiO₄/C物理性能及电化学性能影响。在Li/Si比2.04、600℃下反应16h合成的Li₂Fe_1-xMn_xSiO₄/C材料具有较好的电化学性能,在1.5-4.8V电压区间、以C/16倍率充放电,其首次放电比容量达149.8mAh·g^-1,循环30次此后的容量保持率为90.1%。进一步研究了优化合成条件下制备的Li₂Fe_1-xMn_xSiO₄/C材料的倍率性能以及高温条件下的循环性能。随着充放电电流倍率的提高,Li₂Fe_1-xMn_xSiO₄/C材料的循环性能变差;在高温下Li₂Fe_1-xMn_xSiO₄/C材料的电化学性能得到明显改善,在C/16倍率下,样品的放电平台平稳,循环30次后的容量保持率为94.2%。分别往前驱体中掺入不同量的蔗糖和葡萄糖,制备了不同碳源的Li₂Fe_1-xMn_xSiO₄/C复合材料。结果表明:随着碳含量的增加,Li₂Fe_1-xMn_xSiO₄/C颗粒逐渐减小,振实密度也逐渐减小。与蔗糖相比,葡萄糖作为碳源合成的材料的粒径分布更均匀,颗粒的表面形貌更规则,掺碳量为15%的样品具有较好的电化学性能,在C/16倍率下的首次放电比容量为154.7mAh·g^-1,循环30次后的容量保持率为92.2%。采用循环伏安、交流阻抗、恒电位阶跃等不同的电化学测试方法对改性前后的Li₂FeSiO₄的锂离子脱嵌动力学过程进行了研究,并提出了与之匹配的等效电路图。从动力学角度进一步阐明了材料改性前后的性能差异。更多还原

【Abstract】 The development of rechargeable lithium-ion batteries and cathode materials are reviewed in detail. With the advantages of high safety, non-toxicity, low cost and abundant recource, Li₂FeSiO₄ has been considered as a promising cathode material on lithium-ion batteries. Synthesis of materials, modification process, structure characterization, electrochemical behaviors of Li₂FeSiO₄ has been involved in this study.Li₂FeSiO₄ doped with Ni was synthesized via a solid-state reaction. The precursor was characterized by TG/DTA, and the effort of Ni content on physical structure and electrochemical performance of Li₂FeSiO₄ has been investigated by XRD, SEM and electrochemical methods. The results suggested that Ni doping has increased the charge-discharge capacity, improved the cycle performance. Li₂Fe_0.7Ni_0.3SiO₄ shows the highest discharge capacity and the best cyclying stability.Li₂Fe_1-xMn_xSiO₄/C was synthesized using sucrose as carbon source and reductant, and the effort of doping amount, sintering temperature, sintering time and Li/Si ratio on Li₂Fe_1-xMn_xSiO₄/C’s performance were systematically researched. The optimized Li₂Fe_0.9Mn_0.1SiO₄/C sample was synthesized at 600℃for 16h and a Li/Si moral ratio of 2.04. It delivered an initial capacity of 149.8mAh·g^-1 between 1.5V and 4.8V at C/16 rate and a capacity retention ratio of 90.1% after 30 cycles. Rate capability and cycle performance at high temperature of Li₂Fe_0.9Mn_0.1SiO₄/C synthesized at the optimum conditions were also studied. Cycle performance become worse with increasing the discharge rate, electrochemical performance of Li₂Fe_0.9Mn_0.1SiO₄/C at high temperature were markedly improved. It showed a flat potential plateau and the discharge capacity retention ratio remained 94.2% after 30 cycles.Li₂Fe_0.9Mn_0.1SiO₄/C with different content of sucrose and glucose were prepared. The grain size and tap density of Li₂Fe_0.9Mn_0.1SiO₄/C decreased with the increasing carbon content. Compared with Li₂Fe_0.9Mn_0.1SiO₄/C using sucrose as carbon source, that using glucose displayed smaller particles and more homogeneous distribution, the sample with 15% carbon exhibited excellent performance, with an initial discharge capacity of 154.7mAh·g^-1 and a capacity retention rate of 92.2% after 30 cycles.The lithium deintercalation-intercalation kinetics of pure Li₂FeSiO₄ and modified materials were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and potential step chronoamperometry methods, and a fitting equivalent circuit diagram was built. The results further proved the modified Li₂FeSiO₄ had better electrochemical performance than the pristine one.更多还原