Synthesis And Performances Of Nickel-based Cathode Materials For Lithium Ion Battery

Ni-based cathode materials have been considered as one of the most promising cathode materials for Li-ion batteries because of their high capacity, abundant raw material resources and environmentally friendly. However, synthsizing stoichiometric ratio Li NiO2 is a difficult work. Meanwhile, the unstable Ni4+ ions are easily reduced when they contact directly with the electrolyte, and thus decreasing the cycle life and thrmal stability of the materials. Proper doping and coating can effectively overcome these shortcomings of Ni-based cathode materials. In this dissertation, Co doped LiNi0.8Co0.2O2 and Co-Al co-doped LiNi0.8Co0.15Al0.05O2 cathode materials have been successfully synthesized via controlled crystallization co-precipitation method. The influences of preparatory technological conditions on morphologies, structures and electrochemical properties were systematically studied. In addition, conducting polymer polyaniline coating has been used to improve the performance of LiNi0.8Co0.15Al0.05O2 cathode material. And the main works are as follows:(1) Spherical Ni0.8Co0.2(OH)2 precursor was first prepared by controlled crystallization co-precipitation method, then the precursor was mixed completely with LiOH, and the resultant product was calcined at high temperature through solid state reaction to from the Li Ni0.8Co0.2O2 cathode material. In the co-precipiation process, the precursor particles with good sphericity and well particle size distribution were obtained when the pH value is 10.8 and stirring speed is 100 rpm. Additionally, the results showed that the optimal conditions for preparating LiNi0.8Co0.2O2 were as follows: O2 flow rate was 60 m L/min, Li/M ratio was 1.06, the temperature of reaction was 750 ℃ and the time of reaction was 12 h. The as-prepared LiNi0.8Co0.2O2 delivered an initial discharge capacity of 195.3 mAh?g-1 at a rate of 0.1 C(18 mA g-1) between 3.0~4.3 V, and the capacity retention rate was 82.6 % after 50 cycles at 0.1 C.(2) Spherical LiNi0.8Co0.15Al0.05O2 cathode material was synthisized via controlled crystallization co-precipitation method with EDTA as complexing agent for Al3+ ion. The Li/M ratio, solid reaction temperature and time were studied. The optimal conditions for preparating LiNi0.8Co0.15Al0.05O2 were as follows: O2 flow rate was 60ml/min, Li/M ratio was 1.06, the temperature of reaction was 750 ℃, and the time of reaction was 12 h. The LiNi0.8Co0.15Al0.05O2 can deliver an initial discharge capacity of 193.4 mAh?g-1 at a rate of 0.1 C between 3.0~4.3 V, and the capacity retention still remained at 90.1 % after 50 cycles at 0.1 C.(3) In order to further improve the electrochemical properties, polyaniline-coated LiNi0.8Co0.15Al0.05O2 was successfully synthesized. The mass ratio of polyaniline and active material was discussed, and the result showed that 10 wt% polyaniline-coated sample showed the best electrochemical performance. It shows a discharge capacity of 195.6 mAh?g-1 for the first cycle at a rate of 0.1 C between 3.0~4.3 V. Additionally, the specific capacity still remained at 181.3 mAh?g-1 after 50 cycles at 0.1 C with capacity retention of 92.6 %. Meanwhile, it shows high rate capability with discharge capacity of 143.5 mAh?g-1 even at 2.0 C.