Study On Improvement Of Ternary Nickel-rich Cathode Materials For Li-ion Batteries

In recent years,with the rapid development of lithium-ion batteries in the fields of electric vehicles,electrochemical energy storage and aerospace,people have put forward higher requirements for the life span and safety performance of the battery system.Nickel-rich ternary cathode materials have attracted much attention because of their high energy density and low cost,Li Ni_0.6Co_0.2Mn_0.2O₂materials stand out among nickel-rich materials because of their balanced comprehensive energy density,thermal stability and capacity retention,but there are still some shortcomings in the cycle process,such as structural degradation,interface side effects and serious capacity attenuation at high voltage.In this paper,Li Ni_0.6Co_0.2Mn_0.2O₂ materials were prepared by sol-gel,rheological phase and co-precipitation methods respectively,the process parameters were optimized and the optimum synthesis conditions were determined.Then the materials were modified by coating and doping,and the improvement mechanism was also analyzed.The results are as follows:Li Ni_0.6Co_0.2Mn_0.2O₂ materials were prepared by sol-gel,rheological phase and co-precipitation method,respectively.The results show that the materials achieved by co-precipitation method has compact particles,perfect crystal structure and high layered order.The initial capacity under the condition of 3.0-4.6 V and 0.1 C was 191.7 m Ah·g^-1,and the capacity retention rate was 78.36%after100 cycles at 0.5 C.Li Ni_0.6Co_0.2Mn_0.2O₂ materials were prepared by co-precipitation method,and the effects of process parameters such as p H value,calcination temperature and lithium content on the materials were systematically explored.It is found when the p H is 11.2,the calcination temperature is 830℃and lithium excess 6%,the material has a better layered hexagonalα-Na Fe O₂ structure,and the spherical particles were uniformly distributed.Under the condition of 3.0-4.6 V and 0.1 C,the first discharge capacity reached 202.6 m Ah·g^-1,which coulomb efficiency increased to 83.21%,and the capacity retention was 82.25%after 100 cycles at 0.5 C.In order to improve the cycling performance and thermal stability of high nickel materials under high cut-off voltage,Mo O₃ and Li BO₂ were used to modify Li Ni_0.6Co_0.2Mn_0.2O₂ materials.The Mo O₃coating effectively improves the interface contact and slows down the structural collapse caused by electrolyte erosion.The initial capacity was 197.9 m Ah·g^-1 at 0.1 C,and the first coulomb efficiency reached 86.10%.In particular,the capacity retention rate was 86.13%after 100 cycles at a high temperature of 55℃and 0.5 C,the thermal stability of the material is significantly improved,which is due to the excellent chemical stability of Mo O₃coating.In addition,Li BO₂ coated cathode materials with a coating thickness of about 8 nm were prepared by wet process.It was found that part of B³⁺doped into the crystal lattice led to a slight increase in Ni²⁺content,which improved the structural stability of the materials.At 3.0-4.6 V and 0.5 C,the capacity of 178.2 m Ah·g^-1 is still available after100 cycles,with a retention rate of 93.53%.This is due to that the Li BO₂ coating effectively suppresses the parasitic reaction between the solid-liquid interface and isolates the erosion of HF in the electrolyte,in addition,due to the excellent ionic conductivity of Li BO₂,the degree of electrode polarization is reduced,and the diffusion rate of Li⁺is significantly improved.Na-Al and Nb-Cl co-doping were applied to the Li Ni_0.6Co_0.2Mn_0.2O₂ materials respectively.The results show that Na⁺with larger radius as a pillar ion expands the distance between lithium layers,which is beneficial to the deintercalation and intercalation of lithium ions,while the introduction of Al³⁺improves the stability of the crystal structure,slows down the oxygen loss during the cycle,and the growth rate of charge transfer resistance decreases obviously.When the doping amount of Na-Al was 1%,the capacity retention was 92.32%after 100 cycles at the condition of 25℃and 0.5 C,and the reversible capacity of 150.3 m Ah·g^-1 can be maintained even at 5 C.Due to the electrostatic repulsion,Nb⁵⁺hinders the migration of Ni²⁺and effectively reduces the mixed arrangement of Li⁺/Ni²⁺.In addition,Cl doping can reduce the oxidation reactivity of cathode materials to electrolyte and stabilize the Mn O₆ octahedral structure in the framework through strong ion bonds,inhibiting the irreversible structure transformation process.After 100 cycles at 0.5 C,the discharge capacity was169.9 m Ah·g^-1,the corresponding retention rate was 90.80%,the synergistic effect of co-doping effectively improves the electrochemical performance of the material.