Morphology and Electrochemical Properties of Li1.05Al0.1Mn1.85O4-zFz as a Cathode Material for Lithium Secondary Batteries

초록

The spinel lithium manganese oxide (LiMn2O4, Fd3m) has many favorable properties as an active material of positive electrode such as low cost, low toxicity and environmental friendless. However, a poor electrochemical performance at high temperatures prevents its wider use as a cathode material for lithium secondary batteries. To overcome this problem, many research groups have studied cation substituted LiMn2-xMxO4 (M=Al, Mg, Ni, Co, Cr, etc) and some of them showed improved electrochemical performance at high temperatue. Also, Sun et al. and Amatucci et al. reported anion doped spinels which exhibit stable cyclability at high temperatures. The improved electrochemical performance of the twofold substituted LiAlyMn2-yO4-zFz was attributed to the combining positive effects which comes from the retention of small lattice parameters by Al substitution and reduced average Mn oxidation state by F substitution. In this work, we studied the structural and electrochemical properties of Li1.05Al0.1Mn1.85O4-zFz in the 4V region. Li1.05Al0.1Mn1.85O4-zFz powders were prepared by solid-state reaction method. Stoichiometric amounts of LiOH, Mn3O4, Al(OH)3, and LiF were mixed and preheated at 470℃, 530℃ for 5h in oxygen, and then heated 850℃ for 20h in air. The resulting powders were characterized with X-ray diffraction, SEM, and electrochemical methods. The SEM images in Figure1 show that F substituted Li1.05Al0.1Mn1.85O4-zFz has distinct octahedral morphology and larger particle size compared with the F-free Li1.05Al0.1Mn1.85O4. Figure2 shows the cyclabilities of the Li/Li1.05Al0.1Mn1.85O4-zFz (z=0, 0.05) cells at various current densities at 55℃. This result shows that F substituted Li1.05Al0.1Mn1.85O3.95F0.05 has more stable structure which lead to the improved cycling performances especially at high current rates.