Investigation On Synthesis And Modification Of LiNi_0.6Co_0.2Mn_0.2O₂ For Lithium-ion Batteries

The high-nickel cathode material LiNi_0.6Co_0.2Mn_0.2O₂ has great potential in the field of high-capacity demand such as power cars,energy storage and so on because of its high specific capacity and low cost.However,poor cyclic stability has become a major barrier that high-nickel cathode materials are used in practice.Therefore,in this thesis,the properties of materials are supposed to be improved by optimizing the preparation process and modification measures.Then,the conditions of co-precipitation and sintering,and the content of Al₂O₃ coated on the surface of LiNi_0.6Co_0.2Mn_0.2O₂ were explored in depth.In this paper,the spherical precursor Ni_0.6Co_0.2Mn_0.2CO₃ with homogeneous composition and porous morphology was successfully prepared by carbonate coprecipitation method.The synthetic conditions of pH,ammonia concentration,stirring rate,concentration of M²⁺and reaction temperature were studied in detail.The prepared powders were investigated by using SEM,Size and Tap density.Finally,the process optimization conditions of carbonate co-precipitation method were determined.What’s more,LiNi_0.6Co_0.2Mn_0.2O₂ was prepared by solid-state sintering.Firstly,MCO₃-50 was supposed to a research object,and the effects of one-step sintering method and two-step sintering method on morphology,structure,specific surface area and electrochemical properties were investigated.The results display that the cathode material LiNi_0.6Co_0.2Mn_0.2O₂ obtained by one-step sintering method has a higher initial discharge capacity of 171 mAh·g^-1 at 0.1 C and initial coulomb efficiency of 88%.Moreover,the capacity retention and rate capability of the cathode material obtained by the one-step sintering method are also excellent.On this basis,the precursors of MCO₃-50 and MCO₃-55 were calcined at 750,800,850,900℃for 12 h respectively in order to explore the effects of sintering temperature on morphology,structure,specific surface area and electrochemical properties.The results show that NCM622-55-750 has the highest initial discharge capacity of 196 mAh·g^-1,capacity retention（79%after 100 cycles）and rate performance.Therefore,the optimum sintering temperature was 750℃about the optimal precursor MCO₃-55.Finally,LiNi_0.6Co_0.2Mn_0.2O₂ was coated with Al₂O₃ by a simple and efficient dry-coating method,and the coating amount of Al₂O₃ with the ratios of 0,0.5,1.0,2.0%was investigated thoroughly.The results of SEM,XRD and EDS show that Al₂O₃ is successfully coated on the surface of LiNi_0.6Co_0.2Mn_0.2O₂ uniformly.After the coating,the morphology,particle size and crystal structure of LiNi_0.6Co_0.2Mn_0.2O₂ have indistinct change.Particularly,when the coating amount of Al₂O₃ is 1.0%,the electrochemical performance is the most excellent because of the highest capacity retention（96%after 30cycles）.Compared to uncoated material,that is,pure LiNi_0.6Co_0.2Mn_0.2O₂,the cycle performance is improved significantly,and rate capacity is also improved.Therefore,the optimal coating amount of Al₂O₃ is 1.0%.