Abstract
Spinel LiMn2O4, LiMn2-xCrxO4 (x = 0.05 ~0.2) and LiMn2-yCryO4 (y=0.05~0.2) were prepared by microwave-induced combustion method. The effects of the heat-treatment temperature, heat-treatment time, and the amount of Co- and Cr- substitution on the crystalline structures and the electrochemical properties of the lithium manganese based cathode material were studied. These results were compared to the solid-state reaction. The XRD patterns of LiMn2O4 showed that pure spinel without discernable impurities could be obtained at 700℃.With the heat-treatment temperature increasing, the crystallization and the lattice parameter increased but the average valence of Mn decreased with the increasing in heat-treatment temperature. The pure spinel was also found in the LiMn2-xCoxO4 and LiMn2-yCryO4 at 800℃ for 8 hours in air. The lattice parameter decreased with increasing the dopant Co and Cr content but the average valence of Mn increased with the increasing amount of Co- and Cr- substitution. It could be seen that the grain size increased with increasing the heat-treatment temperature, heat-treatment time, and the amount of Co- and Cr- substitution from SEM. It also indicated that the shape of grain was round and the grain size was uniform.The CV testing showed that the intensity of the redox peaks decreased and the two pairs of the redox peaks tend to merge into one which increased the reversibility for Li+ extraction/insertion during charge/discharge as the amount of Co- and Cr- substitution increased. The initial discharge capacity of LiMn2O4, LiMn1.875Co0.125O4 and LiMn1.925Cr0.075O4 were 133mAh/g, 116mAh/g and 124mAh/g and the fading rate were 0.35%, 0.25% and 0.21% per cycle individually with 1/3C (0.1 mA) at room temperature. The capacity fading of LiMn2O4 occurred mainly in the high-voltage region. The initial discharge capacity of LiMn2O4, LiMn1.875Co0.125O4 and LiMn1.925Cr0.075O4 were 127mAh/g, 114mAh/g and 121 mAh/g and the fading rate were 0.16%, 0.11% and 0.09% per cycle individually with 1C (0.3 mA) at room temperature. The capacity fading of LiMn2O4 occurred mainly in the low-voltage region. The initial discharge capacity of LiMn2O4, LiMn1.875Co0.125O4 and LiMn1.925Cr0.075O4 were 128mAh/g, 116mAh/g and 124m Ah/g and the fading rate were 0.52%, 0.28% and 0.16% per cycle individually with 1C (0.3 mA) at 55℃. The capacity fading of LiMn2O4 occurred mainly in the high-voltage region. The chemical substitution of Co3+ and Cr3+ for Mn3+ in LiMn2O4 could decrease the capacity fading rate effectively in the high-voltage and low-voltage region to improve the electrochemical properties of LiMn2O4 electrode and increased the cell life. Finally, the synthesized powder by microwave-induced combustion with smaller particle size and narrower particle size distribution than solid-state reaction had larger initial discharge capacity and smaller capacity fading rate.