Study On Preparation And Coating Modfication Of High Nickel Ternary Cathode Materials

The actual discharge capacity of the layered high-nickel ternary LiNi_0.6Co_0.2Mn_0.2O₂ material can reach above 170 mAh×g^-1,which is one of the main candidate cathode materials of Lithium ion batteries for electric vehicle（EV）.This paper aims at the technical problems that high-nickel ternary materials urgently need to solve,such as poor cycling performance and poor thermal stability,and focuses on the preparation of non-nitrogen-containing complexing agent-assisted spherical ternary precursor Ni_0.6Co_0.2Mn_0.2（OH）₂,synthesisandcoatingmodificationof LiNi_0.6Co_0.2Mn_0.2O₂ materials.The characterization was carried out by using laser particle size analyzer,XRD,SEM,TG/DTA,charge and discharge test and AC impedance test,etc.The obtained innovative results are described as the following:1.Preparation process of non-nitrogen complexing agent-assisted spherical precursor Ni_0.6Co_0.2Mn_0.2（OH）₂ was studied firstly.The results show that sodium citrate is odorless,non-toxic,and easily degradable.It can be complexed with Ni²⁺,Co²⁺,and Mn²⁺at the same time to form stable ions,and therefore,it is feasible to replace ammonium hydroxide as a complexing agent to prepare spherical Ni_0.6Co_0.2Mn_0.2（OH）₂ material.Under the basic conditions of 55℃,nitrogen atmosphere,2 mol×L^-1nickel-cobalt-manganese sulfate solution,4 mol×L^-1 NaOH solution,mechanical stirring speed of 500-600 r×min^-1 and the dropping rate of 12.5mL×h^-1,the effects of the concentration of sodium citrate and the reaction pH value on the sphericity of the material were investigated.When the concentration of sodium citrate is 0.5 mol×L^-1 and the reaction pH value is11,the supersaturation of the co-precipitation system is more appropriate to control,the secondary particles grow at a uniform rate,the prepared precursor particles are dense,the sphericity is good,and the particle size distribution is narrow.The particle size is about 2-20μm,and the medium particle size D₅₀ is about 7-8μm.2.Secondly,the effect of synthesis conditions on the structure and properties of LiNi_0.6Co_0.2Mn_0.2O₂ was investigated.The results reveal that the layered structure and the degree of grain growth of the material are the main factors affecting its electrochemical properties.Moderate lithium excess and increase the partial pressure of oxygen are beneficial to reduce the mixing of cations,improve the layered structure,reduce the interface impedance,and thus improve the charge and discharge performance.The calcination temperature has a great influence on the morphology,structure and electrochemical performance of the material.Under the conditions of lithiation ratio of 1.08,air atmosphere,pre-heating at 500°C for 6 h and then calcining at 850℃ for 12 h,the prepared material shows good grain growth,small cation mixing,perfect layered structure,and good electrochemical properties.Its 0.2 C and 5 C discharge capacity reaches172.9 mAh×g^-1 and 137.3 mAh×g^-1,respectively,and the capacity retention reaches 99.4%after charge-discharged successively at 0.2 C,0.5C,1 C,2C,5 C,and 0.2 C rate for 30 cycles.3.Finally,surface Al₂O₃-coating modification of LiNi_0.6Co_0.2Mn_0.2O₂was investigated.The results indicate that the Al₂O₃-coated LiNi_0.6Co_0.2Mn_0.2O₂ can be prepared by high-temperature annealing the mixture of lithium salt and the wet or dry Al₂O₃-coated Ni_0.6Co_0.2Mn_0.2（OH）₂ precursor,and the Al₂O₃ layer of the wet coated material is more uniform.The coated Al₂O₃ should be controlled to be 1.0%of the base material,and the coating does not affect the phase structure of the base material,but can effectively inhibit the erosion of the HF and the oxidative decomposition of the electrolyte on the surface of the material,slowing the increase of the interface impedance,thereby increasing the capacity and improving the cycle performance.When the coated Al₂O₃amount is too large,a trace amount of Al³⁺may be doped into the crystal lattice of LiNi_0.6Co_0.2Mn_0.2O₂,resulting in an increase in caion-mixing and deterioration of electrochemical properties.The discharge capacity of 1.0%Al₂O₃ wet-coated LiNi_0.6Co_0.2Mn_0.2O₂ reaches 178.3 mAh×g^-1 at 0.2 C rate,and its average discharge capacities at 1 C and 5 C rate can reach 158.3mAh×g^-1 and 135.3 mAh×g^-1,respectively.Its capacity retention can reach95.6%after charge-discharged at 0.2 C for 100 cycles,showing a good application prospect.