Research On Preparation By High Temperature Solid Phase And Coating Modification Of Lithium-rich Manganese-based Cathode Materials

Lithium-ion batteries are called green power because of their high operating voltage,high energy density and long cycle life,which have been applied for power source in computers,communication equipment and portable electronic equipment,and have also emerged in industrial applications such as transportation systems and energy storage.Cathode material is the most important factor in the overall battery for the highest cost ratio and the greatest impact on performance.Therefore,the development and utilization of cathode material with excellent performance have been an important part of the study of chemical power supply technology.The lithium-rich manganese-based cathode material that can provide higher specific capacity is considered as a promising high-performance cathode material.However,the irreversible and charactermatic oxygen loss in the first charge leads to larger capacity loss,the lower ion diffusion coefficient leads to poorer rate performance,and the irreversible phase change and voltage attenuation in the cyclic process of this type of material are constraining its large-scale application.Therefore,in order to solve these key problems and improve the energy storage performance,the preparation and modification of lithium-rich manganese-based cathode materials were studied in this paper by high-temperature solid-phase method which is easy to industrialize,and the research results is as follows:Lithium-rich manganese-based cathode materials with different ratios of cobalt and nickel were prepared by high-temperature solid-state reaction.The effects of changes in the ratios of cobalt and nickel on the structure,morphology and electrochemical performance were studied.The results show that with the increase of cobalt content,the layered structure of the material tends to be obvious,the degree of crystal order and agglomeration gradually increase.The high cobalt content will promote the removal of lithium ions and the generation of oxygen vacancies during the first charge,which enhance the spinel phase transition kinetics,and aggravate the voltage and capacity decay.When the ratio of cobalt and nickel is 1:1,the material exhibits the best cyclic stability,with the highest specific capacity and capacity retention rate after 50 cycles.In order to improve the preparation process of traditional high-temperature solid-phase method:The MnO₂ intermediates were prepared by precipitation reaction,in which organic polyvinylpyrrolidone?PVP?and ethylene glycol?EG?as dispersants and particle size modifiers,and hydrothermal reaction,respectively.Then micro/nano-structured lithium-rich manganese-based cathode material was prepared with MnO₂ intermediates as manganese source.Inheriting the advantage of simple process for high-temperature solid-phase method,the disadvantages of serious agglomeration and difficult control of the morphology for the prepared materials are improved.With the use of primary nanoparticles with uniform particle size,the transmission path of lithium ions is significantly shortened,and rapid de-intercalation is significantly enhanced.In addition,the irreversible phase change is relieved and the structural stability is effectively improved due to the framework action of micron secondary structures.The material prepared by organic-assisted shows the best rate performance?110 mAh·g^-11 at 10 C?,and the material prepared by hydrothermal-assisted shows the best cycle stability?capacity retention rate is 94.1%after 50 cycles?.The electrochemically active material MnO₂ is used to modify the lithium-rich manganese-based cathode material.The change of electrochemical reaction before and after the modification is studied to explain the mechanism of modification for MnO₂ coating.In addition to the reversible de-intercalation of lithium ions in the MnO₂ coating,the spinel composite phase formed by the migration and diffusion of the lithium and nickel elements in the layered phase during the coating heat treatment also reversibly participates in the electrochemical reaction,which significantly reduces the first irreversible capacity loss.Meanwhile,The MnO₂ coating not only reduces the side reaction between the electrode and the electrolyte,but the spinel phase formed inside provides diffusion channel that facilitates the rapid exchange of lithium ions and effectively improves the charge transfer efficiency.The coating materials exhibit significantly improved rate performance and cycle stability.