Preparation And Modification Of Nickel-cobalt-manganese Ternary Materials

LiNi_0.5Co_0.2Mn_0.3O₂(NCM523)is considered as one of the most promising lithium-ion battery cathode materials because of the advantages of high energy density,reasonable price and environmental friendliness.However,the poor cycle performance and rate performance during charge and discharge have prevented its large-scale application.In this paper,NCM523 cathode material was prepared by the hydroxide co-precipitation method.In order to build a stable interface between cathode and electrolyte,WO₃,WS₂ and nitrogen-doped carbon(N-C)material were coated on the surface of NCM523,respectively,to suppress the interface reaction and relieve the dissolution of transition metal ions,thereby improving the cycle stability and rate performance.X-ray diffraction,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,charge and discharge tests,cyclic voltammetry,and AC impedance were used to investigate the structure,micromorphology,and electrochemical performance of the materials.The main research contents are as follows:1.NCM523@WO₃ composites were prepared by coating WO₃ on NCM523 after the sintering process of hydrate ammonium meta-tungstate.The results showed that WO₃ served as an effective protective layer to mitigate interface reactions.During high temperature sintering,a small amount of W⁶⁺were doped into NCM523,which played a role in stabilizing the material structure.When the amount of WO₃ was 3 wt%,the initial discharge capacities were 84.0,171.7,159.2,136.5 and 97.5 m Ah·g^-1 at 0.2,0.5,1,2 and 5 C,respectively.Compared to NCM523,it had increased by 2.2%,3.4%,4.0%,9.6%and 16.9%.The capacity retention was 70.0%after 100 cycles at 5 C,which was 10.3%higher than NCM523.2.WS₂ was coated on the surface of NCM523 by wet chemical method and high temperature sintering.The results showed that the coating layer effectively reduced the interface reaction and provided stable channels for Li-ion transmission.When the amount of WS₂ was 2 wt%,the materials showed the best electrochemical performances.The first discharge specific capacities reached 184.4,169.5,160.8,133.3 and 91.1 m Ah·g^-1 at 0.2,0.5,1,2 and 5 C,respectively.The capacity retention kept 64.6%after 100 cycles at 5 C and showed better cycling performance than NCM523.3.NCM523@N-C cathode materials were prepared by adding N-C materials in ball milling process.The results showed that the coating of N-C materials improved the electronic conductivity of NCM523 and inhibited the interface reaction between cathode and electrolyte.When the amount of N-C materials was 4 wt%,the materials possessed an initial discharge capacities of 189.4,176.8,164.6,139.7 and 99.7 m Ah·g^-1 corresponding to 0.2,0.5,1,2 and5 C,respectively,which increased 5.0%,6.5%,7.5%,10.9%and 19.5%compared to the pristine NCM523.Its capacity retention reached 87.2%after 100 cycles at 5 C,showing excellent cycle performance.