Preparation And Modification Of Cathode Material LiNi_0.6Co_0.2Mn_0.2O₂ For Lithium Ion Batteries Under High Cut-off Voltage

By increasing the cut-off voltage of the Ni-rich material LiNi_0.6Co_0.2Mn_0.2O₂,its energy density can be effectively increased to meet the needs of the market.Although NCM622exhibits high specific capacity under high cut-off voltage,the rapid decline in capacity and voltage attenuation caused by the instability of the electrode material interface and structural degradation during battery operation limit its large-scale commercialization.Herein,this paper mainly uses the method of multi-element synergistic modification to improve the electrochemical performance of the Ni-rich material NCM622 under high cut-off voltage,and the effect is mainly reflected in two points:one is to increase the cycle life of the material;the other is to increase the rate performance of the material.The NCM622 in this paper is prepared by simple carbonate co-precipitation to prepare the precursor,then mix it with lithium carbonate,and then calcined at high temperature.The precursor is pre-coated with yttrium nitrate and ammonium fluoride by wet method and then calcined with lithium carbonate to achieve the co-doping effect of Y³⁺and F^-;nano-zirconia is added during the process of mixing with lithium carbonate and Zr⁴⁺doping is introduced during high-temperature calcination;the precursor is coated with lanthanum carbonate,and nano-Zr O₂is added during the process of mixing with lithium carbonate,and then calcined at high temperature.Finally,the effect of La-Zr co-modification and synergistic modification is realized,that is,La³⁺and Zr⁴⁺are uniformly doped in the particle body and the outer surface layer tightly integrates a La₂Ni_0.5Li_0.5O₄ coating layer.The specific research work is as follows:（1）Pre-coating the precursor with the"one-step method"to obtain the effect of co-doping modification with Y³⁺and F^-in one step.The first discharge specific capacity of the modified YF-NCM sample under a high cut-off voltage of 4.6 V and a current density of 2 C is 175.71m Ah g^-1,and the capacity retention can reach 92.81%after 100 cycles.At a high current density of 10 C,YF-NCM exhibits a high specific capacity of 135.61 m Ah g^-1,while the unmodified sample has only 93.81 m Ah g^-1 under the same conditions.（2）Zr doping is an effective method to improve the structural stability of Ni-rich ternary materials.A simple"solid phase method"was used to add a nano-zirconium source in the process of mixing with lithium carbonate and then calcined at a high temperature to introduce Zr⁴⁺doping.Among them,the Zr-2 sample modified with 1.5%Zr showed the best electrochemical performance.The capacity retention is 75.59%after 200 cycles under a high cut-off voltage of 4.6 V and a low current density of 2 C,which was 61.85%for the unmodified pristine sample;69.78%after 300 cycles at a high current density of 5 C,and pristine sample is only 35.73%.（3）Use a method different from the traditional method of coating a layer of lanthanum carbonate on the surface of the precursor,and then nano-zirconia is added during the lithium compounding process.After high temperature calcination,the effect of uniform La³⁺and Zr⁴⁺and La₂Ni_0.5Li_0.5O₄ coating modification is obtained.Tested under a high cut-off voltage of 4.6V and 5 C,the 500-cycle capacity retention of the unmodified P-NCM sample is only 23.28%,and under the same conditions the LZ-NCM sample that was modified by La-Zr co-modification was 61.51%.LZ-NCM has a specific capacity as high as 139.60 m Ah g^-1 at a high current density of 10 C,while the discharge specific capacity of the unmodified sample is only92.99 m Ah g^-1 under the same conditions.