| Li Ni0.6Co0.2Mn0.2O2(NCM622) is among the medium-high nickel class cathode materials which have a comprehensive advantage in the aspects of capacity, sintering technology and costs control compared with Ni-poor or Ni-rich materials, making it one of the most promising candidates to replace the status of Li Co O2 in the lithium-ion battery market. The presented paper studied the influences of Li/TM(TM=Ni+Co+Mn) stoichiometric ratio and sintering technology on the physical, chemical and electrochemical performances of NCM622 material. After that, Al3+ ion doping was taken on the NCM622 material to achieve a material with excellent capacity, rate and cyclic properties.The sintering condition was set as 800 ℃12 h for the study of impact of Li/TM stoichiometric ratio on the physical and chemical properties of NCM622 material, and the quasi spherical powders with a diameter of ~10 μm were obtained after that process. The elemental analysis showed that the Li/TM ratio of different NCM622 samples was close to that of their own raw material mixtures. XRD results stated that no impurity phase in those materials could be found, however the p H test of NCM622 solution indicated the formation of residual alkali on the NCM622 surface. Tap density of NCM622 can be maintained at 2.20 g cm-3 in medium Li excess situation, however the material will have a significant decrease in tap density with the further increase of Li content. The chargedischarge test showed that the sample with Li excess 12% owned the best capacity and cycle performance, and its average discharge capacity of first 10 cycles reached 157.8 m Ah g-1, with a capacity retention of 93% after 100 cycles.The excess of Li element was fixed at 12% to learn the effects of sintering temperature and time on the structural and capacity performances of NCM622. Physical test results demonstrated that the appropriate temperature(780 ℃) and time(12 h) will benefit NCM622 material in the aspects of layered structure development, cation mixing and secondary particle agglomeration. XRD refinement results indicated that sample synthesized at 780 ℃ for 12 h can be described with the formula of cbaNi Lii LioOMn CN6232.02.02 552.0048.032048.0952.0][][][++++, the c/a value of which reached 4.958, with a cation mixing of 4.80%. Electrochemical test results showed that its average discharge capacity of first 10 cycles reached 156.9 m Ah g-1, with a capacity retention of 102.9% after 100 cycles.The excess of Li element was fixed at 12% and the sintering condition was set as 780 ℃12 h to study the influence of Al3+ ion doping on the room/elevated temperature capacity and cycle stability of NCM622. SEM, EDS and XRD refinement analysis results confirmed that the method described in this paper can indeed make Al3+ ions incorporate into the bulk of NCM622 material. As displayed by the XRD refinement results, Al3+ ion doping will shrink the length of a and c axes and reduce the cation mixing degree of NCM622 crystal. NCM622 sample doped with 0.5% Al3+ ion exhibited the best rate capability and cycle stability among all samples in RT and ET test condition. However the EIS tests stated that Al3+ ion doping had rare impact on the impedance of research electrode, so the excellent rate and cycling performance of that material can be only attributed to the stability of its crystal structure and the lowest cation mixing degree it owned. |
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