Synthesis And Electrochemical Properties Of Ni-Co-Mn (NCM622) Ternary Cathode Materials For Lithium Ion Batteries

The ternary cathode materials Li[Ni_1-x-yCo_xMn_y]O₂ for lithium ion batteries have received extensive attention at the present stage since they display the higher capacity,better rate capability and thermal stability compared with the other commercialized cathode materials such as lithium iron phosphate LiFePO₄,lithium cobalt oxide LiCoO₂ and lithium manganese oxide spinel LiMn₂O₄.However,capacity fading is a major problem needed to be addressed,especially for the ternary materials with high nickel content.There are three common approaches to improve the cycle performance of the ternary materials,such as element doping method for stabilizing the crystal structure during extensive cycling,surface coating route for minimizing side reactions between the material and the electrolyte,and optimizing compositions and synthetic conditions for obtaining materials with high capacity and stable structure.Therefore,the ternary materials with high capacity,good cycle performance and stable crystal structure still need to be further studied.In this work,co-precipitation method was employed to synthesize spherical Ni_0.6Co_0.2Mn_0.2?OH?₂ precursors with the average particle diameter of 9¹0?m through the intermittent technology,continuous technology and semi-continuous technology,respectively.Due to the good particle consistency of the precursor prepared by intermittent technology,it was further mixed with Li₂CO₃ powder to carry out the solid state reaction.With TG/DTG,XRD,SEM and electrochemical tests,and the trials of single factor experiment,orthogonal experiment and confirmatory experiment,optimum synthetic processes were obtained.Under the condition of Li/TM=1.02 and t=875?,the cathode material displays the highest capacity?161¹63 mAh/g,1.0 C?and the best cyclic retention?93⁹4%,50th?.Futher,ternary materials Li[Ni_0.6Co_0.2Mn_0.2]O₂ with different average particle diameters were prepared by the combination of co-precipitation and solid state reaction method,respectively,and the electrochemical differences were also investigated.The experimental esults indicate that the small-sized particles?3⁴?m?display the best rate capability?134.4¹37.2 mAh/g,5.0 C?but the worst cycle performance,and large-sized particles?17¹8?m?display the best capacity retention?81.1%,100th?but the worst capacity.From the view of applications,middle-sized particles?159.1 mAh/g,1.0 C and 68.4%,100th?were applicable and practicable to large scale production.Three approaches were proposed to improve the cycle performance of middle-sized particles Li[Ni_0.6Co_0.2Mn_0.2]O₂ step by step in Chapter four,through optimizing the formation system of the coin cell CR2025,the cut-off voltage and current density.The results indicate that the Li[Ni_0.6Co_0.2Mn_0.2]O₂ displays the capacity of 171.6 mAh/g?1.0 C?with the capacity retention of 83.4%at 100^th cycle under the cut-off voltage of 4.4 V with 0.2 C ampere density,and that the core-shell material(Li[Ni_0.6Co_0.2Mn_0.2]O₂ and Li[Ni_0.5Co_0.2Mn_0.3]O₂)exhibit good capacity retention of 89.1%under the optimizing formation system,and the doped material Li[Ni_0.6Co_0.2Mn_0.2]O₂@MgO with 1%of MgO displays the best result of cycle performance?165.2 mAh/g,1.0 C and 93.9%,100th?.