Preparation Of Modified Spinel LiMn₂O₄by High Temperature Solid-state Reaction

With the rapid development of economic society and battery technology, lithium ion battery has been used in many fields, and the demand for it is becoming more and more urgent. The spinel LiMn2O4as a lithium-ion battery cathode material has been studied intensively because of its low cost, richness in natural sources, fast charging-discharging reactions and low toxicity and so on. However, its disadvantage is the poor cycling behavior caused by Jahn-Teller distortion, dissolution of manganese into organic electrolyte, and decomposition of electrolyte solution on the electrode. Our papers focused on the preparation of the dopant metal cations, surface coating modified so as to improve energy density and increase its cycle performance of spinel LiMn2O4materials. The structure and morphology analysis of the precursors and final products were characterized by X&ray diffraction (XRD), transmission electron, microscopy (TEM), field&emission scanning electron microscopy (FE&SEM), Energy Dispersive Spectrometer (EDS). Cyclic voltammetry and galvanostatic charge-discharge test was used on the analysis of the electrochemical performance of the as-prepared electrode material. The detailed content of the dissertation is listed as follows:1. Rare earth element La was doped to modify spinel pure LiMn2O4. This paper studied the synthesis of La doped spinel LiMn2O4by solid state reaction. We prepared a series of Li1.02LaxMn2-xO4(x=0,0.02,0.05,0.1) products, and explored the material physical and chemical propertie and stability of the space structure. Besides, the influence of the rare earth element La doped on the electrochemical properties has been discussed. At25℃and1C discharge rate, the discharge capacity of the Li/Li1.02La0.05Mn1.95electrode declined from116.5mAh g-1to100.8mAh g-1with a capacity retention of86.52after200cycles.2. A series of the different metal ion doped Li1.02M0.02Mn1.98O4(M=Mn, Ti, Al) structures were prepared and their electrochemical performance were examined. The results show that at25℃and1C discharge rate, the discharge capacity of the Li/Li1.02Al0.02Mn1.98O4electrode declined from116.8mAh g-1to106mAh g-1with a capacity retention of90.75%after205cycles.3. La2O3was used as a coating agent to modify the surface of the pure spinel LiMn2O4.5.0wt.%of La2O3was coated on the surface of LiMn2O4via ball-milling method, followed by calcination at650℃for5hours in the air. The uncoated and La2O3-coated LiMn2O4materials were physically characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy. It was observed that capacity retention of76.7%after205cycles was obtained for Li/LiMn2O4electrode at1C rate at25℃while that of the coated sample was90.1%. At55℃, the bare and coated LiMn2O4showed capacity retention of69.4%and82.6%after95cycles, respectively. Rate capability was also improved. The capacity retention of uncoated and La2O3-coated materials reduced to46.9%and80.3%at10C-rate, respectively.4. The preparation of the spherical spinel LiMn2O4. We prepared nanometer spherical MnO2precursor by coprecipitation method, followed by high temperature solid phase method to produce the spinel LiMn2O4cathode material. The structure and morphology analysis of the precursors and final products were characterized by XRD, SEM, TEM. At25℃and1C discharge rate, the discharge capacity of the Li/LiMn2O4electrode declined from105.6mAh g-1to78.7mAh g-1with a capacity retention of74.53%after500cycles.