Growth and Studies of Li (Mn, Co) Oxides for Battery Electrodes
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Mat. Res. Soc. Symp. Proc. Vol. 606 0 2000 Materials Research Society
(C 3HB0
2 ),
2-ethylhexanoic acid (C 8H 16 0 2 ), and acetic acid solvents were purchased from Fisher
Scientific. For a particular ratio of the metals, the required amounts of the precursors were separately mixed in methoxy ethenol and 2-ethylhexanoic acid, and boiled. These solutions were mixed hot and refluxed to form a homogeneous solution. The final precursor solution was cooled and stored in sealed container. The part of this solution was slowly evaporated on a hot plate to dryness and then annealed at various temperatures to get the powder. The resulting powders were annealed in a box furnace at different temperatures to study the evolution of conversion for LiMn 20 4 and LiCoO 2. Other portion of the solution was used for thin films by spin coating at 1500 rms. Films need much less time for the transformation to stoichiometric material. 3. RESULTS AND DISCUSSION 3.1 Xray Diffraction Studies After the synthesis by the solution route, powder samples of LiCoO 2 and LiMn 20 4 were analyzed by x-ray diffraction (CuKa-radiation). Figure 1 shows the x-ray diffraction pattern for the transformation of LiCoO 2 annealed for three hours in a box furnace at different temperatures. iJ-
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Figure 1. X-ray diffraction of LiCoO 2 for the samples annealed at different temperatures The x-ray diffraction of thin film deposited by spin coating on quartz substrate, and annealed film is also shown in Figure 1. The evolution of x-ray diffraction patterns of LiMn 20 4 powder and film are shown in Figure 2. All the diffraction peaks are labeled in the Figure I and Figure 2, which coincide with LiCoO 2 and LiMn2 0 4 . As revealed by these x-ray diffraction patterns of Figures 1 and 2, a complete transformation takes place at the annealing temperature - 6500 C. Evidently, the solid state reaction process needs much higher process temperature and a long annealing time. 3.2 Raman Spectroscopic Characterization Raman spectroscopy is very sensitive tool to analyze the homogeneity, composition, and the required phases in the material through the symmetry allowed phonon modes. Raman spectra also indicates the structural phase transition, local order, disorder due to impurities, vacancies or
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doping. Two powder samples of LiMn 20 4 and LiCoO 2, prepared by solution method and annealed at temperature of 6000 C and 7000 C, were chosen for Raman studies, and their Raman phase shift spectra are shown in Figure 3 and Figure 4, respectively. These samples were grinded in an agate crucible for each annealing step to maintain good mixing.
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Figure 2. X-ray diffraction of LiMn20 4 powders by solution route, and annealed at different
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Figure 3 and Figure 4. Room temperature Raman spectra of LiMn 20 4 and LiCoO 2 samples
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The materials synthesized here are neither doped nor inhomogeneous in composition. Therefore, it is expected that structural phase disorder could domi
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