Study On Preparation And Modification Of Lithium-rich Manganese-based Ternary Cathode Material

The layered lithium-rich manganese-based cathode material x Li₂Mn O₃·（1-x）Li MO₂has a very high discharge capacity,which has aroused the research interest of a large number of researchers.In this paper,two preparation methods of co-precipitation-hydrothermal method and sol-gel method were used to synthesize the original materials.Based on the optimal preparation process,the original materials were coated and modified to improve the voltage attenuation,poor cycle stability,low first-lap coulomb efficiency and poor rate performance of lithium-rich manganese-based cathode materials.The comprehensive performance of the material was measured by analyzing the crystal structure,morphology and electrochemical performance of the material.The specific research results were as follows:1.Through comparison,we found that under the same preparation conditions,the material prepared by the co-precipitation-hydrothermal method was superior to the material prepared by the co-precipitation method in terms of the morphology and electrochemical performance of the material.Therefore,we optimized the process of preparing raw materials by co-precipitation-hydrothermal method,and finally prepared the pristine materials with the best performance.The original material had a low degree of cation mixing and a good layered structure;the particle size distribution was uniform,the particle size was about 200 nm,and the crystal growth was good;the initial discharge specific capacity at 0.2 C rate was 216.7m Ah·g^-1,the discharge specific capacity after 50 cycles was 180.7 m Ah·g^-1,and the capacity retention rate was 83.4%;the rate performance was also superior,and the discharge specific capacity at a higher rate of 5 C was 81.3 m Ah·g^-1.2.Then,on the basis of the original materials prepared by the co-precipitation-hydrothermal method,we carried out the coating modification of four materials,including La PO₄,La F₃,Ce O₂and Ce PO₄,and prepared the materials with different coating amounts.The morphology and electrochemical properties of the material have been studied.Through comparison,it was found that the electrochemical performance of the coated materials has been improved to varying degrees,and the modified material with a coating amount of 2 wt%-La F₃had the best electrochemical performance.The XRD diffraction pattern showed that the materials before and after the coating had a highly ordered layered hexagonal structure with a high degree of crystallinity,and the morphology and particle size of the material have not changed significantly,and the average particle size was 200～300 nm.It can be seen from the TEM image that the modified material with a coating amount of 2wt%-La F₃has a thin,dense and uniform coating layer of 1～2 nm on the surface.The first discharge specific capacity of the material was 384.7 m Ah·g^-1,the coulombic efficiency was73.95%,and the discharge capacity of the coated sample was improved compared with the original material,but the first lap coulombic efficiency of the coated sample was decreased compared with the original material.This was because the coating layer La F₃was inactive materials,as the amount of coating increases,it was not conducive to the release of lithium ions during the discharge process,resulting in a decrease in its discharge specific capacity and a decrease in the coulombic efficiency;the initial discharge specific capacity at a rate of 0.2 C was 324.7 m Ah·g^-1,after 50 charge-discharge cycles,the capacity retention rate was 99.54%;the rate performance was also greatly improved compared to the original material.The discharge specific capacity was 370.2 m Ah·g^-1at a rate of 0.1 C,and the specific capacity could reach 114.3 m Ah·g^-1at a high rate of 10 C.In summary,the material with a coating amount of 2 wt%-La F₃could prevent the electrolyte from corroding the active material,delay the collapse of the layered structure,stabilize the surface structure of the material,and optimize the cycle performance and rate performance of the material.3.Secondly,we used the sol-gel method and used citric acid as the chelating agent to prepare the lithium-rich manganese-based Li_1.2Mn_0.54Co_0.13Ni_0.13O₂cathode material,and discuss the influence on the morphology and electrochemical performance of the material in detail when the p H of the solution was 7,8,and 9.It was found that the material prepared at p H 8 had the best morphology and electrochemical performance.When the p H was 8,the layered structure of the prepared material was more complete,the degree of cation mixing was lower,the particle size range of the particles was 200～300 nm,and the particle size distribution was uniform,and the dispersibility was the best;the first discharge specific capacity under 0.1 C rate was the highest,311.2 m Ah·g^-1,and the coulombic efficiency was63.1%;the discharge specific capacity at 0.2 C rate was 262.1 m Ah·g^-1,and the capacity retention rate after 50 cycles was as high as 96.07%,even at a high rate of 10 C,it also reached 92.8 m Ah·g^-1.Then we carried out the coating modification study on the original material and used La F₃and RGO to coat the surface of the material respectively,and explored the change of the morphology and electrochemical performance of the material under different coating amounts,and found optimal coating amount.Tests showed that the modified material with a coating amount of 2 wt%-La F₃has the best electrochemical performance.The material had a relatively complete layered structure,a polygonal columnar shape and a uniform particle size distribution.The particle size was about 300～400 nm,the surface of the material was covered with a uniform net-like coating layer with a thickness of about 2～3 nm.The EDS spectrum can further show that La F₃was evenly coated on the surface of the original material.The first discharge specific capacity of the 2wt%coating material was 347.7m Ah·g^-1,and the coulombic efficiency was 72.28%;and it had good cycle and rate performance.The discharge specific capacity at 0.2 C rate was 325.7 m Ah·g^-1,after 50 cycles of discharge,the capacity retention rate was 99.20%.The improvement in cycle performance was due to the stable coating that could stabilize the structure of the active material by reducing O^2-loss,delaying the collapse of the layered structure during the charge and discharge cycle,and it could also protect the material from the corrosion of the electrolyte and reduce the occurrence of side reactions;the discharge specific capacity at a high rate of 10 C was 122.8 m Ah·g^-1,and the increase in rate performance may be due to the fact that part of the F^-in the coating layer diffuses into the bulk phase of the material due to the concentration gradient,which expanded the interlayer spacing of the material,made the insertion and extraction of lithium ions easier,and improved the migration rate of lithium ions,thereby improving the rate performance of the material.In addition,RGO was used to modify the surface of the original material,and the results showed that the modified material with 2wt%-RGO coating amount had the best electrochemical performance.The morphology and particle size of all materials before and after coating did not change significantly,and the average particle size was 100～200 nm.After charging and discharging the material with a coating amount of 2 wt%-RGO for 50 cycles at a rate of 0.2 C,the specific discharge capacity of the material was 284.8 m Ah·g^-1,and the capacity retention rate was as high as 99.06%.The improvement of the electrochemical performance of the material after the graphene coating was due to the good electronic conductivity of the graphene.After coating,a good conductive network was formed in the material.On the one hand,it could reduce the charge transfer resistance of the material and increase the specific capacity of the material.On the other hand,as a coating layer,it could delay the dissolution of transition metal ions in the active material,reduce side reactions between the active material and the electrolyte,stabilize the layered structure of the material,and improve the electrochemical performance of the material.