| Lithium-rich manganese based cathode materials for lithium ion batteries have been widely concerned due to its good cycling performance, low cost and environment friendly, but the poor low-temperature performance has still been its defect. If the low-temperature performance could be improved, it will promote the application of lithium-rich manganese based cathode materials in the fields of aerospace and the growing industry of electric vehicles, etc. In this thesis, Li1.07Ni0.4Mn0.53O2 cathode material was prepared by hydroxide co-precipitation method combined with high temperature solid-state method, then improved its electrochemical performance by doping and coating modification, and the properties of the prepared materials were characterized by X-ray diffraction, scanning electron microscopy and electrochemical impedance spectroscopy, etc.Firstly, in the preparation process of Li1.07Ni0.4Mn0.53O2 cathode material, effects of different lithium contents, calcining temperatures and heating times on the properties of Li1.07Ni0.4Mn0.53O2 cathode material were discussed. The results show that Li1.07Ni0.4Mn0.53O2 cathode material synthesized at 950℃ for 16 h with 8% extra lithium content exhibits higher crystallinity, complete layered structure, appropriate particle size and the optimum electrochemical performance. When cycled at 25℃ with 0.5C in the voltage range of 2.75 V ~ 4.2V, the Li1.07Ni0.4Mn0.53O2 delivers an initial discharge specific capacity of 127.0m Ah/g with capacity retention of 98.7% after 100 cycles and 96.4% after 300 cycles, which exhibits excellent cycling stability. However, the Li1.07Ni0.4Mn0.53O2 shows poor low-temperature performance, the discharge capacity obtained at-20℃ with 0.2C is only about 36.7% of the discharge capacity obtained at 25℃ with 0.2C.Secondly, Al-doped Li1.07(Ni0.4Mn0.53)1-xAlxO2 cathode material was prepared. The results show that the initial discharge specific capacity has been reduced by doping appropriate amount of Al, but the cycling stability of the Al-doped cathode material has been improved. Besides, the particle size and the charge transfer resistance have been decreased with Al-doped, and the low-temperature performance of Li1.07(Ni0.4Mn0.53)1-xAlxO2 has been improved. The discharge capacity obtained at-20℃ with 0.2C is about 41.1% of the discharge capacity obtained at 25℃ with 0.2C when x=0.3.Thirdly, Li1.07Ni0.4Mn0.53O2 was coated with different contents of Al2O3. The results show that the discharge specific capacity, cycling stability and rate capability of the cathode material have been improved by coating appropriate amount of Al2O3. But the discharge specific capacity will be decreased obviously and the cycling stability will be deteriorated with large amount of Al2O3-coated. Furthermore, the charge transfer resistance has been decreased with appropriate amount of Al2O3-coated, and the low-temperature performance has been improved. The discharge capacity obtained at-20℃ with 0.2C is about 42% of the discharge capacity obtained at 25℃ with 0.2C when 0.7 wt% Al2O3 has been coated.Finally, Co-doped Li1.07Ni0.4-x/2CoxMn0.53-x/2O2 cathode material was prepared. The results show that the layered structure has been completed, the particle size has been decreased, the discharge specific capacity and rate capability have been improved with Co-doped. As the contents of Co-doped increase, the discharge specific capacity has been on the increase, the charge transfer resistance has been decreased gradually and the discharge efficiency of low-temperature performance has been improved. When cycled at 25℃ with 0.5C in the voltage range of 2.75V~4.2V, the initial discharge capacity is 142 m Ah/g and achieves a capacity retention of 98.5% after 100 cycles with x=0.1, the discharge capacity of this sample obtained at-20℃ with 0.2C is nearly 66.1% of the discharge capacity obtained at 25℃ with 0.2C.
|
PDF Full Text Request