Study On Preparation And Modification Of Li-rich Manganese-based Cathode Li_1.2Mn_0.54Ni_0.13CO_0.13O₂

Lithium-rich manganese-based cathode materials are considered to be one of the most promising lithium-ion battery anode materials due to their high specific capacity and energy density,and abundant raw materials.However,due to the relatively low ionic conductivity and poor structural stability of lithium-rich manganese-based cathode materials,the discharge specific capacity and average discharge voltage of such cathode materials gradually decay during the cycling process,which severely restricts lithium Practical applications in batteries.In this thesis,the best precursor materials are prepared from the synthesis conditions,and then the problem of structural phase transition in the electrochemical cycle is studied by doping and coating,so that the electrochemical performance of lithium-rich manganese-based cathode materials is improved.To provide the necessary theoretical basis and technical approach to promote its commercial development.The main research contents are as follows:(1)Drying temperature is an important factor affecting the sol-gel reaction process.When the drying temperature is low,the synthesized precursor particles are small and irregular in shape;when the drying temperature is too high,synthesized precursor particles are also small,but the distribution is uneven and the agglomeration phenomenon is serious.Only precursors synthesized at the proper temperature had better morphology and uniform distribution.Aiming at such problems,lithium-rich manganese-based cathode materials Li1.2Mn0.54 Ni0.13Co0.13O2 with drying temperatures of 120?,140?,and 160? were prepared by the sol-gel method.The effects of drying temperature on the performance were systematically studied.The crystal structure,morphology and electrochemical properties of each material were checked,and the data obtained were characterized and analyzed.The results show that the performance of the obtained material is best when the drying temperature is 140?.(2)Lithium-rich manganese-based cathode materials have low Coulomb efficiency for the first time due to the irreversible process of oxygen release.Owing to the strong binding capacity of Al+and O,the release of O2 during the first cycle can be suppressed.Therefore,doping Al+ can also effectively improve the first Coulomb efficiency of lithium-rich manganese-based cathode materials.Al is utilized to replace part of the Li element in Li1.2Mn0.54Ni0.33Co0.13O2,and the content of Al is controlled to produce Al-containing materials with different ratios.The X-ray powder diffraction,scanning electron microscope,and transmission electron microscope were used to characterize and observe the Al-doped materials.It was found that the Al-doped materials not only had good crystal structure and layered structure,but also improved structural stability.Electrochemical results show that the Al-doped material Li1.16Mn0.54Ni0.13Co0.13Al0.04O2 has the most excellent electrochemical performance.In the voltage range of 0.1 C and 2.0-4.8 V,the initial discharge specific capacity at room temperature is 271 mAh/g.Even at high rates of 0.5 C and 2 C,the discharge specific capacities of the materials reached 185 mAh/g and 156.1 mAh/g,and the capacity retention rates after 200 cycles were 85.2%and 87.3%,respectively.(3)As the surface structure of the material undergoes a phase change as the cycle progresses,the medium voltage and stability of the material continue to decline.The use of a transition metal compound TiO2 to form a discontinuous coating on the surface of a lithium-rich manganese-based material can effectively reduce the reaction between the active material and the electrolyte,and suppress the disappearance of oxygen vacancies at the end of the first charge.At the same time,some of the transition metal ions will also enter the crystal lattice of the parent material during the annealing process to play a role in stabilizing the structure,thereby improving the stability of the material during cycling.First,a lithium-rich cathode material Li1.2Mn0.54Ni0.13Co0.13O2 was obtained by a sol-gel method,and then the TiO2 nanoparticles were successfully coated on the sample by a wet chemical process.Compared with the original sample,the electrochemical performance of the TiO2-1%sample has been significantly improved.The first-time Coulomb efficiency and discharge specific capacity have improved significantly.First-time Coulomb efficiency has improved from 75.9%to 80.8%at 0.1 C,and the initial discharge specific capacity has reached 276.5 mAh/g.At 2 C,after 200 cycles,the capacity retention rate of coated TiO2-1%was 87.8%.It's good initial Coulomb efficiency,cycle stability and rate capability can be attributed to the ability of the TiO2 coating to suppress the loss of oxygen vacancies at the end of the initial charge,as well as its good structural stability.