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1、河南大学硕士学位论文锂离子电池正极材料LiMn2O4的制备及其表面改性研究姓名：卿春波申请学位级别：硕士专业：凝聚态物理指导教师：张伟风;白莹2011-05I摘 要随着便携式电子设备、电动汽车(EV)和混合动力汽车(HEV)的快速发展以及能源与环境问题的日益突出，人们对化学电源提出了更高的要求。锂离子电池以其比能量大、工作电压高、循环寿命长、自放电小、无记忆效应、无污染等优点而得到广泛的应用。正极材料是锂离子电池的重要组成部分， 而尖晶石型 LiMn2O4作为正极材料具有资源丰富、价格低廉、环境污染小、安全性能好等优点，被公认为最具应用前景的锂离子电池正极材料之一。 然而， 尖晶石型 LiMn
2、2O4容量衰减较快， 特别是高温下容量保持性不佳，这大大影响了其电化学性能，限制了它的实际应用。本文采用溶胶-凝胶法成功制备了尖晶石 LiMn2O4正极材料，并采用 FePO4对其进行了表面包覆改性。同时，采用 X 射线衍射（XRD）、激光拉曼光谱（laser Ramanspectroscopy）、扫描电子显微镜（SEM）、高分辨透射电子显微镜（HRTEM）、恒电流充放电、循环伏安（CV）、交流阻抗（EIS）等检测手段，并结合多种电化学分析方法，对材料的结构、形貌以及电池的电化学性能进行了分析研究。首先，采用溶胶-凝胶法确定了样品合成的最优化条件：350预烧 6h，PH=7，800退火 12h
3、。在上述方法和条件下合成了尖晶石结构的 LiMn2O4材料。XRD 和Raman分析表明，该条件下制备的 LiMn2O4材料的衍射峰与卡片库吻合的非常好。通过 SEM和 HRTEM 可以看出尖晶石 LiMn2O4表面光滑且结晶条纹清晰均匀。鉴于尖晶石LiMn2O4具有三维离子通道，结晶良好将有利于充放电过程中锂离子的扩散。其次，采用 FePO4对尖晶石 LiMn2O4进行了表面包覆，包覆后材料的 XRD 分析显示，随着 FePO4包覆量的增加，(111)衍射峰向低衍射角方向移动且衍射峰明显宽化， 这表明 FePO4包覆后 LiMn2O4材料的晶格参数变大，并伴随着半高宽的增大。HRTEM 结果
4、显示，尖晶石 LiMn2O4经 FePO4包覆后，在表面明显形成了比较均匀的包覆层。最后，对 FePO4包覆前后的尖晶石 LiMn2O4材料进行了室温和 55下的电化学测试。 测试结果表明， FePO4表面包覆能显著改善 LiMn2O4在室温和 55下的电化学性能。其中，3 wt.% FePO4包覆的 LiMn2O4显示出最优的电化学特性：循环 80 周后，室温下的比容量仅衰减 32%，而 55下也仅衰减 34%；未包覆的 LiMn2O4在循环 80 周后室温和 55下的比容量损失分别达到 55% 和 72%。另外，我们对未包覆的和 3 wt.% FePO4II包覆的 LiMn2O4材料进行了
5、室温和 55下的 CV 测试，结果表明，FePO4包覆能有效地改善反复充放电过程中 LiMn2O4的结构稳定性。综合各种分析测试结果，我们认为，FePO4对活性材料尖晶石 LiMn2O4和电解液的物理隔绝作用和 FePO4与 SEI膜组分的相互作用是 FePO4包覆后尖晶石 LiMn2O4电化学性能得到改善的主要原因。关键词：锂离子电池，LiMn2O4，FePO4，表面包覆，电化学性能IIIABSTRACTWiththe rapid development of portable electronic devices, electric vehicles (EV) and hybrid ele
6、ctricvehicle (HEV) and the crisis of energy and environmen, higher requirements for the chemical power havebeen proposed. Lithium-ion batteries (LIBs) have been widely applied due to its many advantages, such ashigh voltage, large specific capacity, long cycle life, low discharge rate, no memory eff
7、ect andpollution-freeto environment. Cathode material is an important part of LIBs.As a commercialized cathodematerial, spinel LiMn2O4hasmany advantages such as abundant resources, cheapness, pollution-free toenvironment, safety and so on. It has been recognized as one of the most promising cathode
8、material ofLIBs. However, fast capacityfading,especiallyat hightemperatures,hasgreatlyaffectstheelectrochemical properties of LiMn2O4, thus limited its practical application.In this paper, spinel LiMn2O4cathode materials were successly prepared by sol-gel method.Furthermore, its surface was coated b
9、y FePO4with different content. The pristine and FePO4-coatedLiMn2O4materials were separately characterized by X-ray diffraction (XRD), Raman spectroscopy(Raman), scanning electron microscopy (SEM), high-resolutiontransmission electron microscopy(HRTEM). The electrochemical performances of pristinean
10、d FePO4coated LiMn2O4includinggalvanostatic cycling, cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS) werethoroughly investigated and compared.First, the synthesize condition was optimized to be preheated at 350 for 6h, PH = 7, annealing at800 for 12 h based the sol-gel method
11、. XRD results indicate that the diffraction peak of spinel LiMn2O4isinvery good correspondence with JCPDS card88-1749. SEM and HRTEM images illustrate that thesurface of spinel LiMn2O4is smooth and crystal stripes is clear. Since spinel LiMn2O4has athree-dimensional channel,it will be very beneficia
12、l for lithium ion diffusionincharge-discharge process,guaranteeing the normal electrochemical performances of spinel LiMn2O4.Secondly, different content of FePO4was coated on the surface of LiMn2O4particle. After surfacemodification, the diffraction peak of (111) exhibits a minute leftward shift as
13、the FePO4coating contentincreases, implying the lattice parameter gradually increase with the coating content increases.Atthe sametime, the lattice constants and diffraction peak widths (FWHM, average of five strongest peaks) are furtherIVcalculated from the XRD patterns. It is clear that the lattic
14、e constants and diffraction peak widths increasewith the FePO4content. The broaden of diffraction peak could be attributed to the decrease of crystallinityinthe surface coating process. HRTEM images of FePO4coated LiMn2O4shows two distinct regions (I,II).Inner layer (I) is a well crystallized LiMn2O
15、4core. Besides, a thin layer of amorphous film (II) can beclearly observed outside the corematerial. It should be uniform FePO4coating layer.Finally, the electrochemical performances of the pristine and FePO4-coated LiMn2O4materials atroom temperature and 55 were thoroughly investigated and compared
16、. The results show FePO4coatingcan significantly improve the electrochemical performances of LiMn2O4at room temperature and 55 .The 3 wt. % FePO4-coated LiMn2O4exhibits capacity losses of only 32% and 34% at room temperatureand 55oC after 80 cycles, much better than those of the pristine material, 55% and 72%. Cyclicvoltammograms (CV) at 55oC reveal that the improvementincycling performances of the FePO4-coatedLiMn2O4could be attributed to the stabilization of the spinel structure.
