Preparation And Modification Of Nickel-rich Ternary Cathode Materials For Lithium Ion Batteries

In recent years,the rapid development of lithium ion batteries,especially the industrialization trend of new energy vehicles,has driven the development of electrode materials with high energy density,safety and low cost.Among all the cathode materials,nickel-rich layered materials have got widely attention and research.However,it also has some disadvantages,such as large initial irreversible capacity,poor cycling performance under high voltage,low capacity at large rate,which limit its application of a large-scale.In this paper,the precursor of nickel-rich ternary material was prepared by hydrothermal method,combined with high temperature sintering process to prepare nickel-rich ternary cathode materials,and the materials were modified by lithium titanate coating and niobium doping respectively.The main research results are as follows:（1）lithium titanate coated nickel-rich cathode was prepared by hydrothermal method combined with high temperature solid phase method,XRD analysis results show that the small amount of lithium titanate coating have not affect the the crystal structure of cathode,the SEM analysis show that the coating layer have no influence on the overall appearance,but the surface of materials looks smooth after coating.XPS analysis show that the absorbed oxygen on the surface of the coated samples decreased significantly,benefit from the inhibited reaction of the residual alkali with air and water on coating layer.The LTO@NCM（2.7 wt%）possess the best electrochemical performance.Between 2.8-4.6V at 0.1C,the capacity retention of the lithium titanate coated sample is 91%,while that of the pristine is 86%.CV and EIS show that the polarization of the coated samples was small and the impedance decreased obviously.（2）Nb-doped nickel-rich layered cathode are prepared by hydrothermal combined high temperature solid phase method,XRD results show that there was no impurity peak after doping,suggesting that small amount of niobium did not affect the crystal structure of material,SEM images demonstrate that whole second particle size remained,while the primary particles look more uniform,XPS results show that niobium elements exist in the form of mixed valence state.Electrochemical tests show the enhanced capacity retention of Nb-doped sample at 1C and 5C was 10.7%and 17%higher than that of pristine sample after 100 cycles.Nb-doped samples remains 155.3 mAhg^-1 at 5C,while that is 67.9 mAhg^-1 of pristine one.In CV and EIS,the Nb-doped sample possesses decreased polarization and smaller charge transfer impedance.After cycling,the XRD and SEM images show that the crystal structure of Nb-doped samples remained after cycling,while the pristine sample shows pulverization and cracks,indicating that Nb-doping played a positive role in improving the electrochemical properties of the nickel-rich layered ternary materials.（3）The Ni（OH）₂ are prepared by hydrothermal preparation of different shape and specific surface area of the flower-like microspheres,and used as template precursor to synthesis nickel-rich ternary material.The XRD and SEM show that the nickel-rich cathode synthesis by NF1（the precursor with larger specific surface area）shows low cation mixing,wider interslab of lithium layer,which are benefit to the lithium ion diffusion.The precursor with a large specific surface area is more likely to be oxidized during the sintering process,so the ratio of Ni³⁺/Ni²⁺is higher and the cation mixing is smaller,which can be proved by the XPS test results.The capacity retention rate of NCM-NF1 was 98%at 0.2C after 60 cycles,and the discharge specific capacity remained 170.9 mAhg^-1 when the discharge rate reached 5C.The CV results show that the NCM-NF1 has a smaller polarization phenomenon and dynamic performance,and the EIS shows the smaller SEI film impedance and charge transfer impedance of NCM-NF1.