Preparation And Modification Of Lithium-rich Manganese-based Cathode Materials For Lithium-ion Batteries

In recent years,the market share of lithium-ion batteries in 3C electronic products,electric vehicles,energy storage,and other fields have been rising further,and the weakness of its energy density has become increasingly prominent.As a result,high-capacity lithium-rich manganese-based cathode materials（LLOs,>250 m Ah/g）,such as Li_1.2Mn_0.6Ni_0.2O₂,have become a hot spot in the research field of the lithium-ion battery,and are considered as one of the most promising cathode materials by many researchers both domestic and overseas.However,serious problems,such as low initial Coulombic efficiency,fast capacity and voltage decay,and poor rate performance,have become obstacles on the road to its industrialization.These problems are usually caused by its complex electrochemical mechanism and unstable structure.Therefore,it is particularly important to explore the preparation process and modification method of these materials.In this thesis,Li_1.2Mn_0.6Ni_0.2O₂ cathode material was prepared by molten salt method.The sintering temperature,sintering time and doping strategy were optimized and screened,and the related modification mechanism was investigated emphatically.（1）The samples under different sintering time and temperature were obtained by molten salt method,and the doping strategy of dual elements were explored.The results show that the sample calcined at 830°C for 12 h possesses the highest specific discharge capacity of236.77 m Ah/g.In addition,sintering temperature has a great impact on the electrochemical performance of the material.When the sample is sintered at 850°C for 12 h,a better electrochemical performance can be obtained.The initial discharge capacity is 204.04m Ah/g,and the capacity retention rate after 100 cycles is 87.97%which is much higher than69.81%for the sample calcined at 830°C.And the doped element also has obvious effects on the performance of Li_1.2Mn_0.6Ni_0.2O₂ cathode materials.The sample with F and Mo dual-doping has better cycle performance with a capacity retention of after 79.89%after 200cycles.It is much higher than 54.02%of the sample with Y and F dual-doping.However,the discharge capacity of this F-Mo doped sample is still low,and the high capacity characteristics of Li_1.2Mn_0.6Ni_0.2O₂ cathode materials is not fully exerted.（2）The F-Mo dual-doped method was further carried out to improve the capacity performance of Li_1.2Mn_0.6Ni_0.2O₂ cathode materials and explore the related performance improvement mechanism in this thesis.Characterization techniques such as XRD refinement,XPS,and TEM confirm that Mo ions are successfully doped into the lattice and occupies part of Li sites,forming a spinel-like structure.This will enhance the structural stability and accelerate the diffusion rate of Li⁺.In addition,the introduction of F element promotes the activation of the Li₂Mn O₃ phase,which effectively improves the discharge specific capacity of the material.Compared with the Pristine sample,the capacity contribution of the Li₂Mn O₃phase in the 5F3M sample is increased by 37 m Ah/g.Due to this synergistic effect of F and Mo elements,the 5F3M sample still maintains a discharge specific capacity of 190.98m Ah/g after 100 cycles,which is much higher than the 165.29 m Ah/g of the Pristine sample,indicating an excellent cycling stability.At the same time,the test results of GITT and EIS also show that the introduction of F and Mo elements reduces the diffusion resistance of the electrode,and makes the material have a larger lithium-ion diffusion coefficient,thereby significantly improving the diffusion kinetics of lithium ions.Even at a rate of 2 C,the5F3M sample still maintains a discharge specific capacity of about 170 m Ah/g,while the Pristine sample only delivers a capacity of 144 m Ah/g,suggesting an improved rate performance.These results indicate that this doping strategy effectively improves the capacity,cycle,and rate performance of the Li_1.2Mn_0.6Ni_0.2O₂ cathode materials.