| Layered LiNi1/3Co1/3Mn1/3O2 cathode material, with the advantages of low cost, high capacity, and stable structure, combines the merits of LiCoO2,LiNiO2 and LiMnO2 cathode materials. Therefore, it is considered to be one of the most promising cathode materials for lithium ion batteries. However, there are some problems concerning LiNi1/3Co1/3Mn1/3O2 cathode material in the high charge cut-off voltage and high current rate, such as excessive capacity fading and poor cycle performance. Based on this situation, we have synthesized LiNi1/3Co1/3Mn1/3O2, LiNi1/3Co1/3Mn1/3O2-xBrx, AlCl3 coated LiNi1/3Co1/3Mn1/3O2, and AlBr3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials modified by ion-doping or surface coating via a sol-gel method, and further performed physical characterization and examination of electrochemical properties for these as-prepared cathode materials. The main contents and results of this study are as follows:Firstly, the sol-gel process of preparing LiNi1/3Co1/3Mn1/3O2 cathode materials has been optimized, and the efficiency of the preparing LiNi1/3Co1/3Mn1/3O2 product is improved. At the same time, the series of LiNi1/3Co1/3Mn1/3O2 cathode materials at different calcination temperatures for different calcination time is synthesized. As a result, LiNi1/3Co1/3Mn1/3O2 cathode material calcined at 850 ℃ for 16 h has a complete layered structure of α-NaFeO2. Its primary particle size is about 0.25 μm, mean volume diameter is about 45 μm, and D（0.50） is about 30.84 μm. At current density of 20 mA g- 1（1 C = 200 mA g- 1） and voltage range of 2.5-4.6 V, the first discharge capacity is 199.9 mAh g–1, and it remains 143.6 mAh g–1 after 80 cycles.Secondly, the series of LiNi1/3Co1/3Mn1/3O2-xBrx cathode materials with different amounts of ion doping has been synthesized. Among these LiNi1/3Co1/3Mn1/3O2-xBrx cathode materials, LiNi1/3Co1/3Mn1/3O1.85Br0.15 cathode material demonstrates the optimal electrochemical performances. The initial discharge capacities of LiNi1/3Co1/3Mn1/3O2-xBrx（x=0, 0.15） cathode materials are 199.9 and 193.9 mAh g–1, respectively at the current rate of 20 mA g–1 and in the voltage range of 2.5-4.6 V, and the discharge capacity of LiNi1/3Co1/3Mn1/3O2-xBrx（x=0, 0.15） cathode materials are 135.1 and 144.6 mAh g–1, respectively, after 100 cycles. When the current density is increased to 200 mA g-1, the LiNi1/3Co1/3Mn1/3O1.85Br0.15 cathode materials delivers the 1st discharge capacity 182.9 mAh g–1 and the 100 th discharge capacity of 141.7 mAh g–1, indicating that the rate capability and cycle performance of LiNi1/3Co1/3Mn1/3O1.85Br0.15 cathode material have been improved.Thirdly, the series of AlCl3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials with different amounts of coating layer has been synthesized. Among these AlCl3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials, 1 wt% AlCl3 coated LiNi1/3Co1/3Mn1/3O2 manifests the optimal electrochemical performances. The initial discharge capacity and coulombic efficiency of 1 wt% AlCl3 coated LiNi1/3Co1/3Mn1/3O2 is 147.0 mAh g-1 and 79.9% at the current rate of 400 mA g–1 and in the voltage range of 2.5-4.6 V. Compared to the 1 wt% AlCl3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials, the initial discharge capacity and coulombic efficiency of LiNi1/3Co1/3Mn1/3O2 is 163.2 mAh g-1 and 87.7%, slightly higher than the 1 wt% AlCl3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials. However, the discharge capacity and coulombic efficiency of 1 wt% AlCl3 coated LiNi1/3Co1/3Mn1/3O2 is 132.1 mAh g-1 and 89.8% after 100 cycles. And, the discharge capacity and coulombic efficiency of LiNi1/3Co1/3Mn1/3O2 is 48.6 mAh g-1 and 29.7% after 100 cycles, indicating that the rate capability and cycle performance of 1 wt% AlCl3 coated LiNi1/3Co1/3Mn1/3O2 materials have been improved.Lastly, the series of AlBr3 coated LiNi1/3Co1/3Mn1/3O2 materials with different amounts of coating layer has been synthesized. Among these AlBr3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials, 2 wt% AlBr3 coated LiNi1/3Co1/3Mn1/3O2 proposes the optimal electrochemical performances. The initial discharge capacity of 2 wt% AlBr3 coated LiNi1/3Co1/3Mn1/3O2 and LiNi1/3Co1/3Mn1/3O2 is 107.4 and 153.5 mAh g-1, respectively, at the charge/discharge current rate of 1000/2000 mA g-1 and in the voltage range of 2.5-4.6 V, and their specific energies are 568.5,342.3 Wh kg-1, respectively. However, the discharge capacities of 2 wt% AlBr3 coated LiNi1/3Co1/3Mn1/3O2 and LiNi1/3Co1/3Mn1/3O2 remain 70.1 and 1.4 mAh g-1 after 500 cycles. And, their specific energies are 223.3 and 4.7 Wh kg-1, respectively, after 500 cycles, indicating that the rate capability and cycle performance of 2 wt% AlBr3 coated LiNi1/3Co1/3Mn1/3O2 cathode materials have been improved. |
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