Study On The Improvement Of Oxygen Loss And The Mechanism Of Charge Compensation In Lithium-rich Manganese-based Cathode Materials

The design of high energy density,high power,and long-life lithium-ion batteries is still a current hotspot for research and development.Lithium-rich manganese-based cathode oxides are considered to be the most promising candidate materials for new-generation lithium-ion battery cathodes because of their high specific capacity.However,it still has some defects,such as large initial irreversible capacity loss,slow dynamic mechanics,voltage hysteresis and severe voltage drop,which lead to the deterioration of energy efficiency,power density and cycle performance,which limits its practical application.In view of the above problems,this thesis designs and optimizes the structure of Li1.2Ni0.13Co0.13Mn0.54O2 material,which restricts the migration of lattice oxygen during the cycling process through dual ion doping,while promoting Li+diffusion.Furthermore,the content of oxygen vacancies in the lithium-rich manganese-based cathode oxide was further adjusted to enhance the reversibility of the oxygen anion redox reaction,improve the cycle stability of the electrodes,and explore the mechanism of oxygen anion charge compensation.Finally,by introducing Pt nanoparticles with high catalytic activity,the oxygen-anion redox reaction activity was enhanced during the cycling process,and the conductivity of the electrode material was improved.(1)A Ce,Sn co-doped lithium-rich caode material with stable structure was designed and prepared.Through the synergistic effect of Ce and Sn,the crystal structure is effectively stabilized,the activation energy of lithium is reduced and the polarization in the redox reaction is reduced.At the same time,Ce and Sn elements diffuse into the bulk phase during the co-lithiation process,increasing the interplanar spacing,promoting Li+ migration,and significantly increasing the diffusion rate of Li+.(2)In order to reduce the initial reversible capacity loss of the electrode,the CexSn1-xO2-? composite oxide nano-modification layer with rich oxygen vacancies was synthesized by a mild solvothermal method instead of the traditional wet chemical deposition method,while the intermediate layer of spinel phase was formed between the nanoparticles and the bulk material.This special surface modification strategy significantly reduced the initial irreversible capacity loss of the electrode.The initial coulombic efficiency of the electrode was as high as 92.77%,and the irreversible capacity loss was only 24.4 mAh g-1.In view of this remarkable improvement,the mechanism of enhancing the electrochemical performance of Li-rich materials with CexSn1-xO2-6 modified electrode was proposed,and the relationship between the microstructure and performance of the materials was further explored.(3)In order to improve the cycling stability of the electrode,a surface-functionalized LiCeO2 thin film coating was synthesized for the first time.The optimized LiCeO2 coating has high Li+ conductivity and rich oxygen vacancies.This electrode exhibits excellent high rate performance and cycle stability.In view of this significant improvement,this paper proposes a lattice oxygen migration path of the LiCeO2 modified electrode during cycling to better understand the oxygen loss mechanism of lithium-rich catode materials.For LiCeO2 modified electrode materials,firstly,the rich oxygen vacancies of the LiCeO2 coating can reduce the release of oxygen in the bulk structure and enhance the reversibility of the lattice oxygen.Secondly,the higher Li+conductivity can reduce the charge transfer resistance(Rct)between the particles and the electrolyte,and accelerate the relithiation kinetics.Finally,a thin and uniform surface coating acts as a good protective barrier to prevent irreversible oxidation of the lattice oxygen.In addition,the constant current intermittent titration technique(GITT)was used to study the intercalation/extraction kinetics of Li+in the cycle.(4)Aiming at the problem of poor electrical conductivity of lithium-rich manganese-based cathode materials,this paper first introduced Pt nanoparticles with good catalytic activity into the materials,which significantly improved the loss of lattice oxygen during cycling.It was revealed that the evolution of the lattice structure after the lattice oxygen redox reaction is relatively stable in the lithium-rich oxide with Pt nanoparticles,which is in stark contrast to the apparently deformed crystal structure in the lithium-rich oxide without Pt.Our results highlight the role of Pt nanoparticles in enhancing the redox reaction of lattice oxygen and stabilizing the crystal structure.The activity of anions redox chemistry has been greatly improved,which opens up the field of vision for the design of high-energy-density lithium rich cathode oxides with stable structure.