溶胶—凝胶法制备磷酸铁锂正极材料掺杂改性研究

发布时间：2018-05-02 08:08

本文选题：磷酸铁锂 + 溶胶-凝胶　；参考：《武汉工程大学》2015年硕士论文

【摘要】：LiFePO_4正极材料因其特有的橄榄石晶体结构而具备了能使锂离子嵌入和脱出的充放电能力,这是它能成为正极材料的理论基础,其原材料易得且价格便宜、有较高的工作电压、安全无毒、理论比容量大、绿色环保且热稳定性好,是已初步商业化应用和具有理想前景的锂离子电池正极材料。但是,同样因为LiFePO_4自身的结构和组成的特点而使其本身电子电导率很低,且锂离子在晶体间扩散能力差。这些都限制了它的应用前景,有很多方法可以提高LiFePO_4正极材料电化学性能,其中掺杂改性是的最常用且有效的途径。在本文中,首先综述了LiFePO_4正极材料的发展和主要研究方向以及石墨烯作为导电材料的应用等。具体研究了石墨烯掺杂改性LiFePO_4正极材料的掺杂量和不同掺杂阶段对其电化学性能的影响,在最优化的掺杂量和掺杂阶段下同时加入镁离子的影响等,并挑选了几种稀土金属元素中较便宜的金属离子进行掺杂的探索性实验。以进一步优化LiFePO_4的电容量、循环与倍率性能等,促进锂离子电池的商业化发展,也为LiFePO_4正极材料以及其他正积极材料的掺杂改性提供新的思路。主要研究结果和内容如下:采用溶胶-凝胶法合成了碳包覆的LiFePO_4/C正极材料,并在其前驱体合成阶段分别加入了四种不同量的镧系稀土金属元素:氧化铥(Tm2O3)、氧化钐(Sm2O3)、氧化铈(CeO2)、氧化钆(Gd2O3),对LiFePO_4/C分别进行了质量百分比分别为1%、2%、3%、5%的掺杂实验,其中掺杂1%Gd3+、1%Ce4+和2%Ce4+会使LiFePO_4/C的电化学性能提高,而掺杂1%Gd3+的效果最好,在0.1C下LiFePO_4/C的比容量可达135.7mAh/g,10C下为85.5mAh/g,其50次充放电循环后任可保持93.3%的比容量,并具有较好的晶体结构和微观形貌。此外还在其不同合成的阶段分别加入了不同质量的石墨烯,通过XRD、SEM、Raman、振实密度仪和电池测试系统等对所制备的LiFePO_4/C的结构、形貌、组成以及各种电化学性能进行了测试,结果表明:LiFePO_4/C正极材料的比电容量随着石墨烯掺杂量的增加而增加,且在二次球磨前加入石墨烯的效果最好,但从经济实用、石墨烯的使用效率、振实密度这三个角度出发,2%石墨烯掺杂量的效果最好,其首次放电比容量为151.3mAh/g(0.1C),在10C下为122.4mAh/g,在0.1C下50次充放电循环后其比容量只损失了2.13%,它的振实密度为1.17 g/cm3,单位体积容量为177.0 mAh/cm3。然而在二次球磨前加入5%石墨烯的LiFePO_4/C正极材料其首次放电比容量为159.6 mAh/g,但在0.1C下50次充放电循环后,其比容量损失了6.23%,且单位体积容量仅为150.0mAh/cm3。基于对LiFePO_4/C正极材料掺杂改性的优化机理不同,在最优化的条件下同时掺杂了已知对其电化学性能有较好改善的镁离子,在掺杂2%镁离子时,在0.1C下首次放电比容量为162.7mAh/g(理论容量的95.7%),循环50次后其比容量损失了6.23%,其单位体积容量高达195.2mAh/cm3。
[Abstract]:Due to its unique olivine crystal structure, LiFePO_4 cathode materials have the ability to charge and discharge lithium ions embedded and removed. This is the theoretical basis for LiFePO_4 cathode materials. The raw materials are easy to obtain, the price is low, and the working voltage is high. It is a promising cathode material for lithium-ion batteries, which is safe and non-toxic, has large theoretical capacity, is environmentally friendly and has good thermal stability. However, due to the structure and composition of LiFePO_4 itself, the electronic conductivity of LiFePO_4 is very low, and the diffusion ability of lithium ion between crystals is poor. There are many ways to improve the electrochemical performance of LiFePO_4 cathode materials, among which doping modification is the most common and effective way. In this paper, the development and research direction of LiFePO_4 cathode materials and the application of graphene as conductive materials are reviewed. The influences of the doping amount of graphene doped LiFePO_4 cathode materials and the different doping stages on their electrochemical properties were studied in detail, and the effect of the addition of magnesium ions at the optimal doping level and the doping stage was also studied. Some cheaper metal ions of rare earth elements were selected for the exploratory experiment. In order to further optimize the capacity, cycle and rate performance of LiFePO_4, the commercial development of lithium-ion batteries is promoted, and a new idea for doping modification of LiFePO_4 cathode materials and other positive materials is also provided. The main results and contents are as follows: carbon coated LiFePO_4/C cathode materials were synthesized by sol-gel method. Four different amounts of lanthanide rare earth elements were added in the precursor synthesis stage: thulium Tm2O3 oxide, samarium oxide Sm2O3O _ 3, cerium oxide CeO2O _ 2, gadolinium oxide Gd _ 2O _ 3O _ 3O _ 3O _ 3, respectively. The LiFePO_4/C was doped with a mass percentage of 1 ~ (2 +) ~ (35)%, respectively. The electrochemical performance of LiFePO_4/C was improved by doping 1%Gd3, 14 and 24, but the effect of doped 1%Gd3 was the best. The specific capacity of LiFePO_4/C at 0.1C was up to 85.5 mg / g at 135.7 mAh/ g / g 10C, and the specific capacity of LiFePO_4/C could be maintained at 93.3% after 50 cycles of charge and discharge. It has good crystal structure and microstructure. In addition, graphene of different quality was added in different synthesis stages. The structure, morphology, composition and electrochemical properties of LiFePO_4/C were tested by XRDX SEMN Raman, vibrator density meter and battery test system. The results show that the specific capacitance increases with the increase of graphene doping, and the effect of adding graphene before secondary ball milling is the best. However, it is economical and practical to use graphene. The effect of 2% graphene doping is the best from the three angles of vibrational density. Its initial discharge capacity is 151.3 mAh/ g ~ (-1) C ~ (-1), and it is 122.4 m / g at 10C. After 50 charge-discharge cycles at 0.1 C, the specific capacity is only 2.13%. Its vibrational density is 1.17 g / cm ~ (-3), and the unit volume capacity is 177.0 mAh路 cm ~ (3). However, the first discharge specific capacity of the LiFePO_4/C cathode material added 5% graphene before secondary ball milling is 159.6 mAh/ g, but after 50 charge-discharge cycles at 0.1C, the specific capacity is 6.23 and the unit volume capacity is only 150.0mAh/ cm ~ (-3). Based on the different mechanism of doping modification of LiFePO_4/C cathode materials, magnesium ions, which are known to improve the electrochemical performance of LiFePO_4/C cathode materials, have been doped at the same time under the optimized conditions, and when 2% magnesium ions are doped, The first discharge capacity at 0.1C is 162.7mAh / g (the theoretical capacity is 95.7g). After 50 cycles, the specific capacity is 6.23g, and its unit volume capacity is up to 195.2mAh路 cm ~ (-3).
【学位授予单位】：武汉工程大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ131.11;TM912

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