Preparation And Surface Modification Of LiMn₂O₄ As Cathode Material For Lithium-ion Battery

With the rapid development of portable electronic devices, electric vehicles (EV) and hybrid electric vehicle (HEV) and the crisis of energy and environmen, higher requirements for the chemical power have been proposed. Lithium-ion batteries (LIBs) have been widely applied due to its many advantages, such as high voltage, large specific capacity, long cycle life, low discharge rate, no memory effect and pollution-free to environment. Cathode material is an important part of LIBs. As a commercialized cathode material, spinel LiMn₂O₄ has many advantages such as abundant resources, cheapness, pollution-free to environment, safety and so on. It has been recognized as one of the most promising cathode material of LIBs. However, fast capacity fading, especially at high temperatures, has greatly affects the electrochemical properties of LiMn₂O₄, thus limited its practical application.In this paper, spinel LiMn₂O₄ cathode materials were successly prepared by sol-gel method. Furthermore, its surface was coated by FePO₄ with different content. The pristine and FePO₄-coated LiMn₂O₄ materials were separately characterized by X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM). The electrochemical performances of pristine and FePO₄ coated LiMn₂O₄ including galvanostatic cycling, cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS) were thoroughly investigated and compared.First, the synthesize condition was optimized to be preheated at 350℃for 6h, PH = 7, annealing at 800℃for 12 h based the sol-gel method. XRD results indicate that the diffraction peak of spinel LiMn₂O₄ is in very good correspondence with JCPDS card 88-1749. SEM and HRTEM images illustrate that the surface of spinel LiMn₂O₄ is smooth and crystal stripes is clear. Since spinel LiMn₂O₄ has a three-dimensional channel, it will be very beneficial for lithium ion diffusion in charge-discharge process, guaranteeing the normal electrochemical performances of spinel LiMn₂O₄.Secondly, different content of FePO₄ was coated on the surface of LiMn₂O₄ particle. After surface modification, the diffraction peak of (111) exhibits a minute leftward shift as the FePO₄ coating content increases, implying the lattice parameter gradually increase with the coating content increases. At the same time, the lattice constants and diffraction peak widths (FWHM, average of five strongest peaks) are further calculated from the XRD patterns. It is clear that the lattice constants and diffraction peak widths increase with the FePO₄ content. The broaden of diffraction peak could be attributed to the decrease of crystallinity in the surface coating process. HRTEM images of FePO₄ coated LiMn₂O₄ shows two distinct regions (I,II). Inner layer (I) is a well crystallized LiMn₂O₄ core. Besides, a thin layer of amorphous film (II) can be clearly observed outside the core material. It should be uniform FePO₄ coating layer.Finally, the electrochemical performances of the pristine and FePO₄-coated LiMn₂O₄ materials at room temperature and 55℃were thoroughly investigated and compared. The results show FePO₄ coating can significantly improve the electrochemical performances of LiMn₂O₄ at room temperature and 55℃. The 3 wt. % FePO₄-coated LiMn₂O₄ exhibits capacity losses of only 32% and 34% at room temperature and 55 oC after 80 cycles, much better than those of the pristine material, 55% and 72%. Cyclic voltammograms (CV) at 55 oC reveal that the improvement in cycling performances of the FePO₄-coated LiMn₂O₄ could be attributed to the stabilization of the spinel structure. We believe that the separation of FePO₄ between acitive material and electrolyte and its intereaction with SEI (Solid electrolyte interphase) film contribute to the performance enhancement.