Modification And Electrochemical Performance Optimization Of Layered Li-rich Manganese-based Li_1.2Ni_0.16Mn_0.56Co_0.08O₂

With the development and progress of society,lithium ion battery has penetrated into all aspects of people’s life.The Mn metal in the Li-Mn-rich based cathode material is rich in resources and environmentally friendly.Compared with lithium cobalt acid,Li-rich materials have advantages such as lower price,higher discharge specific capacity and adaptability to high working voltage environment,etc.,which have attracted the attention of the electric vehicle industry.However,there are still some problems in the large-scale use of this material.For example,in the process of continuous charge and discharge,Li-Mn-rich based cathode material has an irreversible process in the initial discharge.During continuous charge and discharge,the structure is prone to phase change and the voltage decay is rapid.These defects greatly limit the market-oriented orientation of the Li-Mn-rich based cathode materials.In this paper,the structural particularity and electrochemical defects of Li-rich materials were studied as follows:（1）Firstly,using the sol gel method for synthesis of Li_1.2Ni_0.16Mn_0.56Co_0.08O₂,the prepared materials were sintered at high temperature in an atmosphere furnace to explore the influence of different calcination temperatures on the morphology and electrochemical properties of Li-rich structures,so as to determine the optimal sintering process.When the sintering temperature is 850℃and held for 16h,the cathode material has a good layered structure.At room temperature and at a voltage of 2.0-4.8 V,the discharge specific capacity of the material is 190.62 m Ah/g for the first time,and can still remain at 174.57 m Ah/g with retention rate of 91.58%after 100 cycles at the current density of 0.5 C（1 C=250 m A/g）.（2）Preparation of different content of V elements on Li_1.2Ni_0.16Mn_0.55V_0.01Co_0.08O₂with high metal ions for doping modification,get the most suitable mixing V element cathode material Li_1.2Ni_0.16Mn_0.55V_0.01Co_0.08O₂,namely V-1%of the sample.Because the radius of V ion is slightly larger than that of Mn ion,the cell volume of the sample increases,and both c value and c/a value increase,indicating that the introduction of V ion is helpful to stabilize the structure of Li-rich-Mn based cathode materials.XPS showed that the valence of basic elements Ni and Co did not change,and more Mn⁴⁺was reduced to electrochemically active Mn³⁺due to doping of V⁵⁺.The discharge specific capacity is 244.45 m Ah/g for the first time,and can still remain at 225.67 m Ah/g with retention rate of 92.3%after 100 cycles at the current density of 0.1 C.When the doping amount is greater than 2%,XRD results show that a new phase Li₃VO₄is generated,indicating that excessive doping amount of V may affect the structure of Li-rich materials.（3）By sol gel method coated La（PO₃）₃to modified Li_1.2Ni_0.16Mn_0.56Co_0.08O₂cathode material,using water bath method and high temperature calcination to preparation of different content of the composite materials.Because the residual lithium on the surface of Li-rich-Mn based cathode materials can form carbonate with water and carbon dioxide in the air and attach to the surface of the particles,resulting in the attenuation of electrochemical performance.XRD showed that the structure of the cathode material before and after coating was lamellar,and TEM showed that La（PO₃）₃and lithium salt formed a clear and uniform protective film attached to the surface of the particles.The material with coating amount of 4%（4%La@NCM）has a capacity retention rate of 94.2%after 100cycles under the voltage range of 2.0-4.8 V at 0.1 C,which is significantly higher than that of the NCM material（91.4%）,indicating that La（PO₃）₃coating can inhibit a series of harmful effects of cathode materials in contact with electrolyte.（4）Through coprecipitation synthesis Li-Rich materials Li_1.2Ni_0.16Mn_0.56Co_0.08O₂with Al₂O₃coating on its surface.The Al₂O₃coating layer not only provides a favorable environment for the deembedding of lithium ions,but also stabilizes the median voltage to some extent.The specific discharge capacity of the coating material is still 172.7 m Ah/g and the capacity retention rate is 93.57%after 100 cycles at 0.2 C under the voltage range of 2.0-4.6 V,while the capacity retention rate is still 88.1%and the NCM was only 50.41%after 100 cycles at 1 C.This is because the Al₂O₃layer on the surface of the particles can resist the damage of HF in the process of circulation,thus better maintaining the surface morphology of the particles,making the lamellar structure of the material more stable,and restraining the structural transformation to a certain extent.