Preparation And Modification Of Spherical LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Materials Via One-pot Method With Cerium,Tungsten And Molybdenum

The cathode material LiNi_0.5Co_0.2Mn_0.3O₂（NCM523）,with the advantages of lithium cobaltate,lithium nickelate,and lithium manganate,i.e.,high capacity,low cost,and high safety,is favored by researchers.However,there are still some problems for NCM523 material,e.g.,poor specific capacity,rate and safety performance under high temperature and high-rate conditions.In this study,Li[Ni_0.5Co_0.2Mn_0.3]_1-xM_xO₂ cathode materials were prepared and modified by a one-pot method using Li₂CO₃,Ce（NO₃）₃·6H₂O,（NH₄）₁₀H₂（W₂O₇）₆·xH₂O and（NH₄）₆Mo₇O₂₄·4H₂O as the lithium source and doping elements source,respectively.The purpose is to improve the safety of the material by doping high-valence metal and reducing the content of Ni³⁺in NCM523.Scanning electron microscope（SEM）,inductively coupled plasma emission spectroscopy（ICP）,X-ray photoelectron spectrometer（XPS）,X-ray diffractometer（XRD）and constant current charge and discharge detection methods were applied to achieve the comprehensive evaluation of the structure,morphology,valence of elements and electrochemical performance of the modified materials.The main results of the paper are as follows:（1）NCM523 materials were modified with different Ce doping amount,the results indicate that the electrochemical performance of Ce modified cathode material had not been improved;even under the optimal doping amount（0.3%）,the comprehensive electrochemical performance of the modified cathode material was inferior to that of NCM523.XRD and XPS analyses of show that:the cell parameters of Ce modified materials have no significant change;meanwhile,the ratio of Ni²⁺/Ni³⁺is close to that of unmodified materials,and Ce mainly exists primarily as+4 valence.It indicates that Ce is not successfully doped into the lattice of NCM523 material,and the charge balance ability Ce⁴⁺is insufficient,resulting in the decrease of electrochemical performance of modified cathode materials.（2）The rate performance of W doped cathode materials was improved.The material with a W content of 0.6 mol%showed the best electrochemical performance,the capacity retention rate was up to 96.95%after 100 cycles in the voltage range of2.8～4.3 V,at 1C current density.Meanwhile,the discharge specific capacity of the material was 167.9～111.3 mAh`g<sup>-1</sup> at the current density of 0.2～10 C;and its reversible specific capacity can reach 166.9 mAh`g^-1 when the current density was back to 0.2 C.In NCM-W_0.6%,W exists mainly as W⁺⁶,and the Ni²⁺/Ni³⁺ratio increases significantly,indicating that appropriate amount of bulk doped W⁶⁺can enhance the stability of the layered structure of the cathode material,and reduce the impedance and charge transfer impedance of the solid electrolyte interfacial film,which is beneficial for the modified material to show a good electrochemical performance.（3）The cycle stability and rate performance of MCN523 material can be improved via Mo doping.NCM-Mo_0.9%,with a Mo doping amount of 0.9%,showed the best electrochemical performance,its first discharge specific capacity was 154.4 mAh`g<sup>-1</sup> in the voltage range of 2.8～4.3 V at 1C current density and the capacity retention rate of NCM-Mo<sub>0.9%</sub>was up to 93.72%after 100 cycles.The discharge specific capacity of NCM-Mo<sub>0.9%</sub>was 169.5～125.4 mAh`g^-1 at the current density of 0.2～10 C;and its reversible specific capacity can reach 167.1 mAh`g^-1 with the current density returned to 0.2C.The doping of Mo can significantly reduce the impedance of the solid electrolyte boundary film,and reduce the amount of Ni³⁺in the cathode material to a certain extent.This indicates that the appropriate amount of bulk doped Mo⁶⁺ can enhance the stability of the layered structure of the cathode material and increase the diffusion coefficient of the lithium ion;thereby,the Mo doped cathode material shows better electrochemical performance.