Improvement Effect Of Different Coating Materials On The Electrochemical Performance Of The Lithium Nickel Cobalt Manganese Oxides Cathode Materials

To meet the requirements of high energy density power battery for high capacity and high safety lithium nickel colbat manganese oxides(NCM)materials,three series of coated LiNi0.5Co0.2Mn0.3O2 samples were prepared,and the effects of sintering temperature,coating material and coating method on the high overcharging voltage performance of the coated samples during the preparation process were studied.The key factors affecting the performance of the samples were studied by the charge-discharge test,cyclic voltammetry,AC impedance spectroscopy,X-ray diffraction,X-ray photoelectron spectroscopy and scanning electron microscopy.The main conclusions are as follows:1.The NCM samples coated with lithium-rich manganese oxide were prepared by using LiNi0.5Co0.2Mn0.3O2,MnCO3 and Li2CO3 as raw materials.The results showed that coating did not change the layered structure of the LiNi0.5Co0.2Mn0.3O2.In the series of coated samples,the sample with theoretical composition of 0.15Li1.33Mn0.47O2·0.85LiNi0.5Co0.2Mn0.3O2,which was prepared at 910℃,exhibits the best discharge capacity and cycle performance.During the continuous charge-discharge cycling at the 1C rate,the optimal sample exhibits the capacity of 178mAh/g and the coulomb efficiency of 73%in the first cycle in the voltage range of 2.5 to 4.6V;and it exhibits the capacity of 156mAh/g in the 10th cycle in the voltage range of 2.5 to 4.8V;and it exhibits the capacity of 160mAh/g in the 10th cycle in the voltage range of 2.5 to 4.95V.The capacity retention of the optimal sample is 90%after 55 cycles,while the capacity retention rate of the uncoated LiNi0.5Co0.2Mn0.3O2 sample under the same conditions is 77%.In the voltage range of 2.5 to 4.95 V,the charge-discharge performance of the optimal sample is obviously improved.2.A series of NCM samples coated with lithium nickel oxides were prepared by using LiNi0.5Co0.2Mn0.3O2,Ni(OH)2 and LiOH·H2O as raw materials.The experimental results showed that the sample with a theoretical composition of 0.05LiNiO2·0.95LiNi0.5Co0.2Mn0.3O2 has layered structure and exhibits the best electrochemical performance.During the continuous charge-discharge cycle at the 1C rate,the optimal sample exhibits the capacity retention of 92%after 55 cycles tested in the voltage range of 2.5～4.6V,.2.5～4.8V and 2.5～4.95V,respectively.The cycling performance under high overcharging voltage of the optimal sample is obviously improved.The optimal sample has excellent cycling performance even charged to high voltage at 55℃ or at-22℃.3.The composite samples were prepared by mixing graphene and 0.05LiNiO2·0.95LiNi0.5Co0.2Mn0.3O2(theoretical composition)at different mass rati0.The results showed that graphene was attached to the surface of NCM particles,forming a complete conductive network with the original sample particles.In the voltage range of 2.5～4.6V,the optimal sample prepared by mixing graphene and 0.05LiNiO2·0.95LiNi0.5Co0.2Mn0.3O2 in the mass ratio of 0.011:0.425 exhibits the best charge-discharge performance and cycling performance.And it exhibits the first cycle capacity of 173 mAh/g and the columbic efficiency of 80%.The capacity retention of the sample is 91.3%after 40 cycles.The optimal sample exhibits the first cycle capacity of 202mAh/g and 40th cycle capacity of 136 mAh/g while tested at 55℃.In the voltage range of 2.5 to 4.6V,the capacity retention of the sample is 87%after 42 cycles while tested at the 1C,2C,5C,10C,20C,50C and 100C rate,respectively.Graphene significantly improved the high rate discharge performance of the composite sample.