Study On Preparation And Modification Of Lithium-rich Manganese-based Cathode Materials

The lithium-rich manganese-based cathode material is one of the most promising cathode materials for the next generation of lithium-ion batteries,because of its unique anion/cation redox charge compensation mechanism and an ultra-high capacity(>250 m Ah g^-1).At the same time,it has the advantages of wide operating voltage window,friendly environment and low cost,which have attracted the attention of the world.However,in order to accelerate the commercialization of lithium-rich manganese-based cathode materials,it is necessary to overcome the inherent defects of the materials.The main disadvantages include large first irreversible capacity loss,excessive voltage decay,poor cycle stability and poor rate performance.This dissertation mainly aims at improving the electrochemical performance of lithium-rich manganese-based cathode materials,and conducts a series of researches on the adjustment of element composition,surface structure design and surface treatment of materials.The main research contents are as follows:?1?Firstly,spherical carbonate precursor with different proportions of transition metal elements(Mn_0.60Ni_xCo_0.40-xCO₃)was synthesized by solvothermal method,and then the corresponding lithium-rich manganese-based cathode material is formed after calcining at high temperature.The target materials were characterized by various test methods and the influence of different transition metal elements ratios on the electrochemical performances of the materials was analyzed,laying an experimental basis for subsequent modification research.?2?A lithium-rich cathode material Li_1.40Mn_0.60Ni_0.20Co_0.20O_2.4 was prepared by the solvothermal method and subsequent high-temperature solid phase method.We put forward a significant strategy to combine the advantages of the formation of spinel phase,surface coating and surface treatment via a one-step route to enhance the electrochemical properties of materials.The results show that a unique heterostructure?with spinel@NPPy?naphthalene sulfonic acid-doped polypyrrole?shell?is formed on the surface of the material.The initial coulombic efficiency of all processed materials exceeded 90%.Among them,the material with NPPy coating content of 3%has the best performance,with 200 cycles at 2.0?4.6 V and 0.5 C current density,and the capacity retention rate is 89%,while the raw material is only 69%,and its voltage decay is also significantly suppressed.At the same time,at a large rate of 10 C,the discharge specific capacity of the material is still 165 m Ah g^-1,while the discharge specific capacity of the raw material is only 141 m Ah g^-1.?3?Based on the crystalline phase structure of the material surface,a lithium-rich manganese-based cathode material with a larger particle size(Li_1.40Mn_0.60Ni_0.35Co_0.05O₂₊?)was first prepared by co-precipitation process and high temperature solid state method.Then,the surface of the material was pretreated with0.1 mol L^-1 HNO₃ solutions,and then the material structure was stabilized by annealing at different temperatures.This process results in the formation of different types of spinel structures on the surface of the material and the activation of Li₂Mn O₃component.Since the spinel structure has a unique 3D Li⁺diffusion channel,it can accelerate the Li⁺diffusion rate,thus increasing the rate performance and reducing the first irreversible capacity loss of the materials.The results show that Li₄Mn₅O₁₂spinel structure is formed on the surface of the annealed material at 400?,which has the best rate performance and the highest first discharge specific capacity.