Study On The Preparation And Modification Of Spinel Structure Lithium Manganese Oxide Cathode Materials Assisted By Organism

Lithium spinel manganate（Li Mn₂O₄）has attracted wide attention because of its low production cost,good rate performance and high working potential.However,the problem that spinel Li Mn₂O₄is easy to dissolve its active substances in the cycle process,which leads to poor cycle stability and decreases specific discharge capacity limits its further application.In order to solve the above problems and expand the application field of spinel Li Mn₂O₄,this thesis first tries to improve the cycle stability of the material by morphology optimization.Then,the Metal Organic Frameworks was selected as the precursor to prepare spinel Li Mn₂O₄with different crystal faces exposed,which improves the specific discharge capacity.Finally,spinel Li Mn₂O₄with the best electrochemical performance was used as the cathode and assembled with different anode to compose the full cell,and the matching of cathode and anode materials and the electrochemical performance of the full cell were investigated.In this thesis,spinel Li Mn₂O₄was used as the main research object.A series of different precursors were synthesized by hydrothermal method,and a series of spinel Li Mn₂O₄with different morphology and electrochemical performance were prepared after lithiation by high temperature solid-state method.First of all,the optimum process parameters for the preparation of pure phase spinel Li Mn₂O₄by high temperature solid state method were studied by using Mn CO₃as precursor.The precursors with spherical and rod-shaped morphology were prepared by adjusting the composition of the hydrothermal solvents,and the morphology was kept intact after high-temperature lithiation.The effect of hydrothermal solvent composition on the structure and electrochemical performance of Li Mn₂O₄was investigated and the mechanism was elucidated.XRD results show that the hydrothermal solvent composition does not affect the crystal structure of the samples,but it affects the preferential growth of the crystal surface in the samples.The spherical samples have preferred growth of（111）crystal plane and the rod samples have preferred growth of（440）crystal plane.SEM results showed that changing the composition of the hydrothermal solvents can cause changes in the microstructure of the samples.With the increase of the viscosity of hydrothermal solvents,the micro-morphology of the samples changes from amorphous to regular spherical and rod-shaped,first and then to broken.The electrochemical performance results show that the sample closest to the spherical morphology has the best cycle stability.The specific capacity of the first discharge is 131.4 m Ah·g^-1at a rate of 1 C,and the capacity retention rate was 92.1%after 300 cycles.The sample closest to the rod morphology has the most excellent rate performance with the first specific discharge capacity of 103.7 m Ah·g^-1at a rate of 10 C.Second,the spinel Li Mn₂O₄was synthesized by a Manganese-based Metal Organic Frameworks as a precursor,and the preferred growth of crystal plane and the oxygen vacancy content of the sample are controlled by changing the types of Metal Organic Frameworks.The effect of the ratio of Na OH to organic ligand on the structure and oxygen vacancy content of spinel Li Mn₂O₄and the resulting change mechanism of electrochemical performance were studied by adjusting the concentrations of Na OH solutions as hydrothermal solvents,respectively.XPS results show that with the increase of Na OH contents,the oxygen vacancy contents in the samples increase first and then decrease,and the corresponding electrochemical performance increase first and then decrease.On the basis of determining the optimal Na OH dosage,the reason of the influence of organic ligands on the structure and electrochemical performance of spinel Li Mn₂O₄were further investigated by changing the numbers of carboxyl groups on the organic ligands.The results show that the specific capacity of the prepared samples at the first discharge is 134.6 m Ah·g^-1at a rate of 1 C when the molar amount of Na OH is as three times as PTA,and the capacity retention rate is 102.82%after 500cycles.The first specific discharge capacity of the samples prepared with PMA as the organic ligand is 135.6 m Ah·g^-1at a rate of 1 C,and the capacity retention rate is103.5%after 500 cycles.Finally,in this thesis,the modified Li Mn₂O₄with the best electrochemical performance was used as the cathode,and the Manganese-based Metal Organic Frameworks,Li₄Ti₅O₁₂and commercial graphite were used as the anodes,respectively,to assemble the full cells and study the optimal working voltage range and electrochemical performance.The results show the full cell-assembled of the modified Li Mn₂O₄as the anode and the Manganese-based Metal Organic Frameworks as the cathode-exhibits the best electrochemical performance with a specific discharge capacity of 125.64 m Ah·g^-1after 100 cycles at 1 C rate.