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储能型磷酸铁锂材料制备及性能研究
李程峰
学位类型硕士
导师康雪雅
2011-05-31
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业材料物理与化学
关键词Lifepo4 正极材料 螯合辅助法 电化学 掺杂
摘要磷酸铁锂(LiFePO4)作为近年来锂离子电池正极材料中的后起之秀,由于具有低成本、高安全性、高温性能好、环境友好等优点,因而成为研究人员及锂电池企业广泛关注的热点。目前在储能应用中,橄榄石型LiFePO4正极材料需要解决的几个问题分别是:高倍率性能、低温性能以及振实密度等。本论文着眼于LiFePO4材料的高倍率性能和低温性能,尝试通过合成工艺的优化、金属离子掺杂等方法来使LiFePO4材料的性能得到部分提高。同时,利用SEM、TEM、循环伏安法、交流阻抗法等测试手段对所合成的材料进行了表征,并对其电极动力学性能进行了较系统的研究。 本论文通过在固相球磨体系中加入EDTA、草酸、柠檬酸等螯合剂在一定程度上提高了磷酸铁锂材料的性能及其循环稳定性。螯合剂的加入一方面促进了原料颗粒的分散,使得各原料混合更为均匀;另一方面螯合剂在随后的高温处理过程中分解为无定形碳包覆在颗粒表面抑制了颗粒的长大同时也提高了材料的导电性从而达到提高材料性能的目的。所合成的LiFePO4材料在常温下的充放电容量稳定在159.3 mAh g-1 (0.5 C)、150.5 mAh g-1 (1 C)、138.7 mAh g-1 (2 C)、129.4 mAh g-1 (5 C)和 108.2 mAh g-1 (10 C),50次充放电循环后容量基本没有衰减,表现出了比较优秀的电化学性能。 在改进合成工艺的基础上,本文也对磷酸铁锂离子掺杂改性进行了较系统的研究。通过Mn2+、V5+的掺杂制备了掺杂改性的橄榄石型LiFePO4正极材料,并从产物的结构和电化学性能方面考察了掺杂离子的改性效果。结果表明:Mn2+的掺杂可以有效的改善材料在充放电过程中的动力学性能,从而提高材料的高倍率性能;同时利用恒电流充放电法对不同温度下掺杂的LiMn0.05Fe0.95PO4材料进行了测试。采用电化学阻抗谱(EIS)研究了不同温度下LiMn0.05Fe0.95PO4的电化学阻抗性能,结果表明随着温度的降低,材料与电解液界面的电荷转移电阻显著增加成为影响LiMn0.05Fe0.95PO4低温性能的主要因素。为进一步的提高LiFePO4正极材料的低温性能,探讨了V5+的掺入对磷酸铁锂材料性能的影响。结果表明,V5+的掺入显著的提高了材料的高倍率性能及低温性能,在常温下以10C的倍率充放电时,其容量可以达到120 mAh g-1,-20°C的环境下0.5C充放电容量可达到105mAh g-1。
其他摘要Olivine LiFePO4 has been considered as the most promising cathode candidate for the next generation large scale lithium-ion batteries, because of its inherent merits including low toxicity, low cost, long cycle ability and high safety. Consequently, olivine LiFePO4 has been continuously attracting attention from academic research and industrial research fields. But in the practical field of LiFePO4 for energy storage, there are still several questions need to be overcame, which are high-rate performances, low-temperature performances and tap density. We attempted to improve the high-rate and low-temperature performances by optimizing the process of synthesis and doping with metal ions. Several techniques (such as SEM, TEM, CV, EIS etc.) have been used to characterize the synthesized composites, and the kinetic properties of the materials are carefully investigated. In this paper, the performances of LiFePO4/C composites are improved by the addition of chelate agents (such as EDTA, citric acid and oxalic acid) in the ball-mill system. The chelate agents could not only accelerate the dispersal of raw materials and the uniformity of the system, but also decompose to amorphous carbon distributed between particles or coated on the surface of particles. The carbon coating could improve the performances of LiFePO4 composites by preventing the growth of the crystals in the sinter procedure and increasing the electronic conductivity. The charge/discharge capacities of the prepared composites are 159.3 mAh g-1 (0.5 C), 150.5 mAh g-1 (1 C), 138.7 mAh g-1 (2 C), 129.4 mAh g-1 (5 C) and 108.2 mAh g-1 (10 C). On the basis of the synthesis optimization, a systematic research has been done about doping with different metal ions. The structure and performances of Mn2+ or V5+ doped LiFePO4 are studied. The high-rate performances of LiFePO4 could be enhanced effectively by doping with Mn2+, which is the result of the improved kinetic property during charge and discharge. But while the temperature decreasing, the performances of LiMnxFe1-xPO4(x= 0.05, 0.1)do not have any obvious enhancement. To clarify this phenomenon, EIS is used to characterize the impedance property of LiMn0.05Fe0.95PO4 at different temperature. The results show that the charge transfer resistance (Rct) increase largely as the temperature decreasing, and become the key factor of the poor performances at low temperature. In order to improve the low temperature performances, V5+ doped LiFePO4 composites are studied. The results show that the performances of V5+ doped LiFePO4 are enhanced largely, the charge/dischange capacity is 120 mAh g-1 at 10 C rate, and a capacity of 105 mAh g-1 could be still obtained at 0.5 C when the temperature decrease to -20 °C.
文献类型学位论文
条目标识符http://ir.xjipc.cas.cn/handle/365002/4430
专题材料物理与化学研究室
作者单位中国科学院新疆理化技术研究所
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李程峰. 储能型磷酸铁锂材料制备及性能研究[D]. 北京. 中国科学院研究生院,2011.
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