Investigations On Synthesis And Modification Of Nickel-rich LiNi_0.8Co_0.1Mn_0.1O₂ Ternary Cathode Materials

Ni-rich LiNi0.8Co0.1Mn0.1O2(NCM811 for short)ternary layered cathode materials have been widely used in the production of lithium ion batteries due to their high specific capacity and low cost.The increase of nickel can increase the specific discharge capacity,but the Li/Ni mixing is intensified,which will lead to serious irreversible phase transition in the process of charge and discharge.In addition,the extraction and insertion of lithium ions in the process of charge and discharge cause the shrinkage and expansion of the material volume,thus causing residual stress in the positive electrode.With the increase of the number of cycles,the positive electrode will have micro-crack initiation and local fracture,which leads to the degradation of battery performance.In order to solve this problem,NCM811 was prepared by solvothermal method under different reaction conditions,and then NCM811 modified with different contents of Zr was prepared by wet chemical method.Finally,the properties of NCM811 cathode materials before and after modification were tested and analyzed.The crystal structure,surface morphology and surface chemical states were characterized by XRD,SEM,HR-TEM,EDS-Mapping and XPS.The electrochemical properties are analyzed with constant current charging-discharging,CV and EIS.At the same time,the stress and strain in the process of charge and discharge are measured by a stress-strain meter.The particle morphology of cathode material before and after charge and discharge was observed by SEM.The degradation mechanism of NCM811 material was analyzed by stress-strain curve and morphology change.The specific work completed in this paper is as follows:(1)The performance of NCM811 cathode material prepared by solvothermal method were analyzed.Ni(NO3)2·H2O,Co(NO3)2·6H2O and Mn(NO3)2 aqueous solutions were dissolved in anhydrous ethanol at a molar ratio of 8:1:1 to form a transparent solution.The solution was placed in a reaction kettle and reacted at different temperatures.3[Ni0.8Co0.1Mn0.1(OH)2]·2H2O precursor material was obtained by drying,cooling,pumping and washing.Then,the precursor material and LiOH·H2O were uniformly mixed at the molar ratio of 1:1.05 and calcined at different temperatures to obtain NCM811 cathode material.First,the crystal structure of NCM811 cathode material was analyzed by XRD,and the crystal structure parameters of the cathode material were obtained.Then,the button lithium-ion battery was manufactured.The electrochemical performance of the button battery was analyzed by constant current charge-discharge,CV and EIS.At the rate of 0.5C and the voltage range of 2.5-4.3 V,the material has an initial discharge specific capacity of 128.1mAh/g,a coulombic efficiency of 95.25%in the first cycle,and a capacity retention rate of 82.06%after 100 cycles.The specific discharge capacity can also reach 86.7 mAh/g at a high rate of 2C and a voltage range of 2.5-4.3 V.In the process of charge and discharge,the stress and strain curves of the cathode materials are obtained by measuring the stress and strain values of the cathode materials with a stress-strain meter.After the charge and discharge,the crystal structure and morphology of the cathode material were observed by SEM,and the microstructure evolution diagram of the material was obtained.Finally,the morphology,crystal structure and performance of the samples under different synthesis temperatures were compared,and the optimal synthesis temperature was 200℃.(2)The performance of Zr-modified LiNi0.8Co0.1Mn0.102 cathode materials prepared by wet chemical method were analyzed.Zr(NO3)4·5H2O and LiCH3COO·2H2O were weighed in diferent proportions and dissolved into anhydrous ethanol,and then mixed with LiNi0.8Co0.1Mn0.1O2 cathode material dispersed by ultrasonic after magnetic stirring.After magnetic stirring and heating,dried powder was obtained.The dried powder was put into a tube furnace for annealing to obtain Zr-modified LiNi0.8Co0.1Mn0.1O2 ternary layered cathode material.A layer of Li2ZrO3 ionic conductor coating with a thickness of 5-15 μm was formed on the surface of the secondary particle of cathode material.In addition,a small amount of Zr doped into the crystal structure of NCM811 during annealing.Then,the crystal structure,surface morphology and surface chemical state of the cathode material were characterized and analyzed by XRD,SEM,HR-TEM,EDS-mapping and XPS to obtain the microscopic information related to the degradation of the cathode material performance,and then the button lithium ion battery was prepared.The electrochemical performance of the button cell was tested by constant current charge-discharge,CV and EIS.The initial discharge specific capacity is 161.3 mAh/g at the rate of 0.5C and the voltage range of 2.5-4.3 V.The coulomb efficiency of the first cycle is up to 98.6%,and the capacity retention rate is 82.2%after 100 cycles.The specific discharge capacity is 139.3 mAh/g at 2C rate and 2.5-4.3 V voltage range.In the process of charge and discharge,the stress and strain curves of the cathode materials are obtained by measuring the stress and strain values of the cathode materials with a stress-strain meter.After charge and discharge,SEM was used to observe the morphology of the cathode material,and the microstructure evolution diagram of the material was obtained.Finally,the optimal coating amount of Li2ZrO3 was 1 wt.%.