Synthesis Of Lithium Nickel Cobalt Aluminum Oxides Cathode Materials And Their Performance

Lithium nickel cobalt aluminium（NCA）has attracted wide attention for its high energy density.To further improve the NCA performance in the rate performance and cycle stability,this dissertation synthesized a series of NCA cathode materials with various compositions to study the effect on the NCA performance of the composition,stored time and coated agents.The phase structure and electrochemical performance of the samples were studied by using X-ray powder diffraction,charge-discharge test,cyclic voltammetry,AC impedance test,Fourier transform infrared spectroscopy,scanning electron microscopy and X-ray energy spectrum.The main results are as follows:1.A series of NCA samples with various theoretical compositions were prepared by using the start materials of NiSO₄·6H₂O,CoSO₄·7H₂O,polymerization oxychloride aluminum and Li OH·H₂O.The results show that the sample with the theoretical composition of Li_1.00Ni_0.82Co_0.15Al_0.03O₂ has the best electrochemical performance in all as-prepared samples,which exhibits the discharge capacity of 155mAh·g^-1 in the 1^st cycle,139mAh·g^-1 in the 40^th cycle and the capacity fading rate of 10.3%in 40 cycles while discharged at the 1C rate.And it exhibits the discharge capacity of 164mAh·g^-1 in the 1^stcycle,142mAh·g^-1 in the 40^th cycle and the capacity fading rate of 13.4%in 40 cycles at55℃.It also exhibits the discharge capacity of 162mAh·g^-1（0.1C）,152mAh·g^-1（1C）,147mAh·g^-1（2C）,138mAh·g^-1（5C）,130mAh·g^-1（10C）,respectively,while it charge-discharged continuously.2.The button cells that assembled from Li_1.00Ni_0.82Co_0.15Al_0.03O₂ cathode material were charge-discharged and stored to prepare the stored cells.The results show that a passive film composed of Li₂CO₃ and a little ROCO₂Li formed on the surface of the stored samples.The samples（fresh cell,cell cycled and stored for 20 days,cell cycled and stored for 2 months,and cell cycled and stored for 3 months）exhibit the discharge capacity of 155,132,72 and 48mAh·g^-1 in the 1^st cycle,141,123,65 and 44mAh·g^-1in the 40^th cycle and the capacity fading rate in the 40 cycles of 9%,6.8%,9.7%and 8.3%at1C rate,respectively.Above results show that the discharge performance decreased obviously after stored.3.A series of cathode mixtures were prepared by mixing Li_1.00Ni_0.82Co_0.15Al_0.03O₂and graphene with various weight ratio.The results show that the cathode mixture prepared in the mass fraction of Li_1.00Ni_0.82Co_0.15Al_0.03O₂ is 95%has the best electrochemical performance among all samples,and it exhibits the discharge capacity of 185mAh·g^-1（1C for 5 cycles）,171mAh·g^-1（2C for 5 cycles）,155mAh·g^-1（5C for 5cycles）,144mAh·g^-1（10C for 5 cycles）in the continuous charge-discharge cycles.It also exhibits the discharge capacity of 205mAh·g^-1 in the 1^st cycle,173mAh·g^-1 in the 40^thcycle and the capacity fading ratio of 15.6%in 40 cycles while charge-discharged at 55℃.The results show that the coated graphene can significantly improve the high rate performance of NCA cathode materials.