动力电池正极材料磷酸铁锂的制备和性能研究

发布时间：2018-10-13 16:39

【摘要】：橄榄石型磷酸铁锂(LiFePO_4)作为目前最有发展前景的锂离子动力电池正极材料之一,具有高比容量、好的循环可逆性能、低的原料成本、高的安全性能和环境友好等优点,目前已经成为电池界竞相开发和研究的热点。但其低的电子导电率和锂离子扩散系数,导致了LiFePO_4材料在高倍率充放电条件下比容量衰减迅速,从而严重地阻碍了LiFePO_4的商业化应用。针对以上存在的问题,本文通过调控LiFePO_4材料的颗粒大小和晶体生长方向,以及通过碳改性等手段来改性LiFePO_4材料,最终提高其充放电性能。本文取得了如下主要结果:首先,采用实验室规格的高剪切混合器来制备前驱体,经水热结晶,成功地实现了对LiFePO_4/C材料粒度大小的调控。通过研究获得了高剪切混合器转子转速对前驱体沉淀和LiFePO_4/C材料的晶体结构、颗粒形貌、大小及其分布的影响规律,揭示了高剪切混合器辅助的水热法实现粒度可控的关键。在高剪切混合器转速为1.3×104 rpm下制备得到的LiFePO_4/C样品,其粒度减小至~220 nm,表现出了优异的电化学性能,在0.1 C和20 C倍率下,其放电比容量分别达到160.1mAh·g~(-1)和90.8 mAh·g~(-1)。其次,在高剪切混合器辅助的水热工艺基础上,继续在前驱体的混合过程中加入了非离子型表面活性剂Tween-80。研究表明,表面活性剂Tween-80分子在水热合成过程中可以降低LiFePO_4产物粒度大小,并且可以调控LiFePO_4产物沿(010)晶面生长。制备得到的LiFePO_4/C材料其颗粒粒度减小至~100 nm,I(020)/I(111)比值高达1.19,表现出了优异的电化学性能,在0.1 C和20 C倍率下,放电容量高达166.5 mAh·g~(-1)和119.6 mAh·g~(-1)。最后,采用金属有机框架MIL~(-1)00(Fe)同时作为模板和原料,制备得到了由LiFePO_4纳米颗粒嵌入多孔连续碳骨架内部的三维多孔LFP/CNWs材料。为了进一步提高该材料的导电性能,将LFP/CNWs样品同三聚氰胺(C3N3(NH2)3)混合后,经碳热还原反应制备得到了氮改性的LFP/N-CNWs材料。该氮改性的多孔碳骨架(N-CNWs)环绕在LiFePO_4纳米颗粒表面,增大了LiFePO_4颗粒同电解液之间的接触面积,加快了锂离子和电子在整个电极材料内的传导速率,提高了LiFePO_4材料的有效利用率和倍率性能。LFP/N-CNWs样品在0.1 C和20 C倍率下的放电容量分别达到161.1 mAh·g~(-1)和93.6 mAh·g~(-1)。
[Abstract]:Olivine type lithium ferric phosphate (LiFePO_4) is one of the most promising cathode materials for lithium-ion power battery. It has the advantages of high specific capacity, good recycling reversibility, low raw material cost, high safety performance and environmental friendliness. At present, battery industry has become a hot spot in the field of development and research. However, the low electron conductivity and lithium ion diffusion coefficient lead to the rapid decline of specific capacity of LiFePO_4 materials under high rate charge and discharge conditions, which seriously hinder the commercial application of LiFePO_4. In order to improve the charge-discharge performance of LiFePO_4 materials, the particle size, crystal growth direction and carbon modification were adjusted to improve the charge-discharge properties of LiFePO_4 materials. The main results obtained in this paper are as follows: firstly, the precursor is prepared by using a high shear mixer in laboratory specification, and the particle size of LiFePO_4/C material is successfully controlled by hydrothermal crystallization. The effects of rotor speed of high shear mixer on the crystal structure, morphology, size and distribution of precursor precipitates and LiFePO_4/C materials were obtained, and the key to realize particle size control by hydrothermal method assisted by high shear mixer was revealed. The LiFePO_4/C samples prepared at high shear mixer speed of 1.3 脳 10 ~ 4 rpm showed excellent electrochemical properties when the particle size was reduced to ~ 220 nm,. The specific discharge capacity of the samples reached 160.1mAh g ~ (-1) and 90.8 mAh g ~ (-1) at the rate of 0.1 C and 20 C, respectively. Secondly, on the basis of hydrothermal process assisted by high shear mixer, Nonionic surfactant Tween-80. was added to the mixing process of the precursor. The results show that the surfactant Tween-80 molecule can reduce the particle size of LiFePO_4 products during hydrothermal synthesis and can regulate the growth of LiFePO_4 products along the (010) crystal plane. The particle size of the prepared LiFePO_4/C material was reduced to ~ 100 nm,I (0 20) / I (111) as high as 1.19, showing excellent electrochemical performance. The discharge capacity reached 166.5 mAh g ~ (-1) and 119.6 mAh g ~ (-1) at the ratio of 0.1C and 20C. Finally, MIL~ (-1) 00 (Fe), a metal-organic framework, was used as a template and a raw material to prepare a three-dimensional porous LFP/CNWs material embedded into the porous continuous carbon skeleton by LiFePO_4 nanoparticles. In order to further improve the electrical conductivity of the material, the nitrogen-modified LFP/N-CNWs material was prepared by carbothermal reduction after mixing the LFP/CNWs sample with melamine (C3N3 (NH2) 3). The nitrogen modified porous carbon skeleton (N-CNWs) encircles the surface of LiFePO_4 nanoparticles, increases the contact area between LiFePO_4 particles and electrolyte, and accelerates the lithium ion and electron conduction rate in the whole electrode material. The effective utilization ratio and the performance of LiFePO_4 were improved. The discharge capacity of LFP/N-CNWs samples at the rate of 0.1 C and 20 C was 161.1 mAh g-1 and 93.6 mAh g-1, respectively.
【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ131.11;TM912

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