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锂离子电池正极材料LiFePO4/C的制备与性能研究
华宁
学位类型博士
导师吐尔迪·吾买尔 ; 康雪雅
2010-06-04
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业微电子学与固体电子学
关键词锂离子电池 Lifepo4 正极材料 碳热还原法 电化学特性
摘要磷酸铁锂(LiFePO4)作为新型锂离子电池正极材料具有高安全性、低成本、高温性能好、环境友好等优点,因而成为目前电池届竞相研究与开发的热点。本论文以磷酸铁锂作为研究目标,对其合成工艺、结构表征、材料改性、电化学性能以及电极动力学性能等方面进行了较系统的研究。 利用各种测试等方法研究了橄榄石型LiFePO4正极材料的合成工艺。通过分析合成过程中烧结温度、烧结时间、碳含量等工艺参数对电化学性能的影响进而确定了合成磷酸铁锂最佳的工艺条件。 研究了不同类型的碳源对LiFePO4正极材料表面包覆性能的影响。研究表明,不同碳源对合成纯相的橄榄石型LiFePO4的结构没有影响,但却影响LiFePO4颗粒的形貌和颗粒大小。从表面包覆改性的本质上说,导电剂的加入,一方面提高了LiFePO4活性颗粒间以及LiFePO4活性颗粒与电解液间的电接触。另一方面,这种均匀分散导电剂的加入将有助于抑制颗粒长大,这对于锂离子在LiFePO4及之间的有效脱/嵌起到极其重要的作用。 本文对磷酸铁锂正极材料的离子掺杂改性进行了较系统的研究。通过Zn2+和V5+的掺杂以及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。(Zn2+和V5+的掺杂)研究表明,通过掺杂能够改善LiMxFe1-xPO4正极材料的电化学性能。研究发现,采用离子半径与Fe2+或Li+相近,具有更高价态的金属离子掺杂能够增加初始放电比容量和在高倍率下提高循环寿命。 采用固相液相结合的方法,制备了LiFePO4材料。该方法具有热处理时间短,能源消耗低,合成时间短的优点。实验表明利用液固相法制备得到的LiFePO4颗粒形貌规则,粒径细小而均匀。充放电性能测试表明,该方法制备的材料提高了锂离子在LiFePO4材料中充放电的效率,改善材料的循环可逆性。LiFePO4/C的放电容量明显提高,其循环性能也有了很大改善,表现出很好的电极反应可逆性。结果表明,利用液固相相结合的方法合成LiFePO4研究是一个值得关注的课题。 采用循环伏安法研究LiMxFe1-xPO4/C电极的可逆性,并计算了锂离子在材料中的平均扩散系数。结果表明通过掺杂V离子后的电极具有较好的脱、嵌锂可逆性,氧化、还原过程中Li+的固相平均扩散系数DLi数量级在10-13cm2/s。提出了反应模型,并且利用交流阻抗测试了LiMxFe1-xPO4/C电极材料。结果表明LiMxFe1-xPO4/C为正极材料的电池在同样的频率下的电化学反应阻抗(Rct)随掺杂量的改变而变化,Rct的变小使材料在进行脱嵌埋反应时的交换电流密度i0变大,有利于电极进行大电流充放电,从而改善LiFePO4的高倍率充放电性能。
其他摘要Lithium iron phosphate, LiFePO4, has recently attracted significant interest because of its low hygroscopicity, low cost and environmentally friendly components. The aim of the present study were to focus on the preparation processes, the modification of materials, the structural characterization, the electrochemical properties, and the kinetics behaviors of the olivine lithium iron phosphate as cathode materials for rechargeable lithium batteries. Effects of the preparation conditions, such as precursor materials, calcination temperature and time, carbon content particle size and electrochemical properties of LiFePO4 were studied by using TG-DSC、XRD、SEM、TEM、LSD、CV、EIS and galvanostatic charge-discharge tests to optimize LiFePO4 production process. The effects of different carbon source on the performance of LiFePO4/C were systematically investigated. The results demonstrate that olivine-structural single-phase pure crystalline formed by the introduction of carbon. Different carbon sources can affect configuration and particle size. Organic carbon source have better peroformance such as sucrose and PEG. The introduction of carbon will greatly improve the electrochemical applications of LiFePO4 as it increases contact surface between the particles and electrolyte, due to the essence of the coating. Moreover, the amorphous structure of carbon would prohibit the particles to grow during heating due to the well-distributed powders. It is important for the effective insertion/deinsertion of Li+. The alien cation doping for LiFePO4 was systematically investigated to modify the electrochemical performance. From the Zn2+ and V5+ doping, the effects of cation doping were discussed from the aspects of structure and electrochemical performance. The results demonstrate doping could improve the electrochemical characterization such as LiZn0.02Fe0.08PO4/C and LiV0.03Fe0.97PO4/C samples. The initial discharge specific capacities of the samples were 137 mAh/g and 140 mAh/g, respectively. The capacity retain were 95% and 98% at 100th cycle. From the results, that is the supervalent cations, which have the similar radius with Li+, have some influences on the electrochemical perfprmance: both increase of the initial capacity and the suppression of the cycle fading at high rate. Attempts were made to synthesis the LiFePO4 using liquid-solid phase reaction. The main advantages of this method were simple, short-time heat-treatment, saving energy and short-time synthesis. The results show the particles have regular configuration and well-distributed size. By the modification, the discharge capacity of LiFePO4/C was improved immensely, the same as cycle performance. The results demonstrate that it is worthiness for the LiFePO4 production and modification by the liquid-solid phase reaction. The Cyclic Voltammetry (CV) studys the reversibility of LiMxFe1-xPO4/C electrode and reckons the Li+ diffusion coefficient in the materials. The results demonstrate that LiMxFe1-xPO4/C (M=V ion) is good for quasi reversible and Li+ diffusion coefficients in oxidation, reduction process were on the order of 10-13 cm2/s. Electrochemical Impedance Spectroscopy (EIS) also was used to study the LiMxFe1-xPO4/C electrode. The results demonstrate that the variation of Rct is distinct with the doping content at the same frequency. In the charge process, the Rct decreaeing, that of i0 increases gradually, is good for charge-discharge at high current, so improve the electrochemical performance at high rates.
文献类型学位论文
条目标识符http://ir.xjipc.cas.cn/handle/365002/4452
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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华宁. 锂离子电池正极材料LiFePO4/C的制备与性能研究[D]. 北京. 中国科学院研究生院,2010.
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