Synthesis, Structure And Properties Of Spinel Lithium Manganese Oxides For Lithium Ion Battery

During these years, many investigations have been made on the cathode materials of lithium ion rechargeable batteries. LiCoO₂ has been commercialized for its high potential, high capacity and excellent rechargeability. LiNiO₂ has also been developed as the substitutive cathode. However, cobalt and nickel compounds have economic and environmental problems, and the preparation of stoichiometric LiNiO₂ is extremely difficult. The cubic spinel LiMn₂O₄, due to its low cost and high environmental acceptability, has grasped more attention. However, the electrochemical performances of LiMn₂O₄ spinel phase as a cathode of lithium-ion secondary batteries need to be improved. The thesis was focused on the relationship among synthesis, structure and electrochemical performance of spinel LiMn₂O₄. The main work is as follows.MnCO₃ was prepared by liquid precipitation method and the Mn₂O₃ was obtained by thermal decomposition of MnCO₃.The spinel LiMn₂O₄ cathode was prepared as cathode materials of lithium-ion batteries by the reaction of Mn₂O₃ with different lithium compounds(LiOH, LiNO₃, Li₂CO₃) using a melt-impregnation method. TG-DTA, XRD and SEM experiments were performed to investigate the synthesis of LiMn₂O₄ and decomposition of MnCO₃. Moreover, spinel LiMn₂O₄ materials synthesized from different lithium compounds vary in their electrochemical performance. Lithium hydroxide is the optimal lithium resource for synthesis of spinel LiMn₂O₄.In this paper, in-situ high temperature XRD was applied to investigate the phase transition of precursors in the process of heat treatment for the first time. It was observed that spinel LiMn₂O₄ appear when the heat-treatment temperature was below 400℃. When the temperature was below 600℃, the crystallinity of spinel LiMn₂O₄ was improved and MnO₂ phase disappeared. At the same time, a new phase(Mn₂O₃) was formed. The optimal temperature for synthesizing pure phase spinel LiMn₂O₄ was 750℃. When heat-treatment temperature is above 800℃, the cubic LiMn₂O₄ tends to be tetragonal phase.By the use of porosity of γ - MnO₂ and water solubility of LiOH, a set of methods for preparing spinel LiMn₂O₄ cathode material precursor was established. First,LiOH was dissolved in distilled water, then the Y -MnO2 was added into the LiOH solution. Ultrasonic wave oscillation and grinding were employed to guarantee that the raw materials mixed homogeneously and LiOH infiltrated into the pore space of Y -MnO2. Thus the precursors were obtained. The optimal condition to synthesize spinel LiMn2O4 is tow-stage heat treatment process. It is beneficial to modify the particle size, and ensure the crystal parameters needed for intercalation-deintercalation of lithium. In the first stage, heat treatment temperature should be higher than 500 °C, and the second stage should be around 750°C-800°C.The mechanchemical method combined with heat treatment was also used for the synthesis of highly dispersed stoichiometric spinel LiMn2O4 from LiOH and Y -MnO2. The influences of mechanochemical activation process on the structure, morphology and electrochemical performance of the products were examined. The acceleration of solid-state reaction in the course of mechanochemical activation is promoted due to close contact between reagents. In combination with subsequent heat treatment, spinel LiMn2O4 with good electrochemical performance can be prepared. It intrigues the interest that the lattice parameter of Li3vLti2O4 synthesized in this method is lower than in other methods.The effect of cobalt doping and amount of dopant on the structure and performance of spinel LiMn2O4 was studied. The structure of LiMn2-xCoxO4 materials was analyzed by Rietveld refinement. XRD patterns indicated that with the increase of Co content in the materials, the occupation of Mn3+ (or Co3+) in octahedral 16d sites increased and the crystal structure was optimized. At the same time, in the 32e (z, z, z) sites, which are occupied by oxygen, the value of z increased. As a result, the bond between O and Mn (or Co) was strengthened, and the bond between O and Li was weakened. Therefore, doping Co in spinel phase LiMn2O4 can stabilize the structure, and will be beneficial for the diffusion of Li+ ions into the materials during the charge and discharge process.In this work, the LiMn2O4 was modified by coating its surface with a thin layer of amorphous MgO and AI2O3. Obviously, coating the surface of LiMii2O4 with AI2O3 can modify the properties of its surface, which is exposed to the electrolyte solution and avoid the parasite reactions. AI2O3 can also trap the HF from electrolyte, which reacts with the LiMn2O4 and accelerate the dissolution of Mn. The same as MgO-coated LiMn2O4, the excessive AI2O3 is harmful to the capacity of the materials. The resistances of A^Os-coated LiMn2O4 increase with the increase of AI2O3. If the heat-treatment temperature is high enough for coating, the diffusion ofAl3+ from the AI2O3 into the core material will occur to form the LiMn2-xAlx04. The kinetics of Li-ion extraction and insertion from AbC^-coated LiMn2C>4 were investigated by electrochemical impedance spectroscopy at various potentials. The result indicated that AbC^-coating separates the active cathode material from direct contact with electrolyte. As a result, the charge transfer resistance across the electrode/electrolyte is divided into the resistance across the surface film/active mass interface and that between the coating and electrolyte.