【作者】 邹启凡；

【导师】 苏玉长；

【作者基本信息】 中南大学 ， 材料物理与化学， 2005， 博士

【摘要】 近年来,人们对锂离子二次电池正极材料进行了大量的研究。LiCoO₂材料由于高电压、高容量和循环性能优异,目前是商业应用的主要材料。LiNiO₂也被作为一种有潜力的正极材料被广泛研究。但是钴和镍价格高而且存在环境污染的问题,并且化学计量的LiNiO₂合成困难。而立方尖晶石结构的LiMn₂O₄材料由于具有价格低廉、无毒的优点而倍受关注。但LiMn₂O₄作为锂离子电池正极材料,还需要改善其电化学性能。 采用液相沉淀法制备了MnCO₃,MnCO₃经热分解后制备Mn₂O₃,然后以Mn₂O₃为锰源,分别与氢氧化锂、硝酸锂、碳酸锂反应,采用熔融浸渍法制备尖晶石LiMn₂O₄。通过正交实验,考察了合成条件对锰酸锂结构的影响大小。借助XRD等分析手段,发现锂源对制备的材料的结构和相组成影响最大。以氢氧化锂为锂源能够获得杂质含量低,结晶完整的锰酸锂。电化学性能测试也表明以氢氧化锂为锂源制备的尖晶石锰酸锂的比容量和循环性能优异。 首次采用原位高温X射线衍射技术分析热处理制备锰酸锂过程中的相变,确定了在450℃开始形成尖晶石锰酸锂,在650-700℃之间出现Mn₂O₃杂相,到750℃时能获得较纯净的尖晶石结构LiMn₂O₄。但是当热处理温度高于800℃时,锰酸锂会发生从立方相向四方相的转变,使结晶有序度降低,性能下降。 利用γ-MnO₂的多孔性和LiOH的水溶性,发展出液相浸注法,通过水溶液混合、超声波振荡等手段制备前驱体,随后进行热处理,获得尖晶石LiMn₂O₄。从形核和长大的角度看,尖晶石结构LiMn₂O₄宜采用两段热处理工艺合成,第一段热处理不宜低于500℃,第二段热处理工艺以750～800℃为佳。 采用机械活化与高温热处理结合的方法,以LiOH和γ-MnO₂为原料制备尖晶石LiMn₂O₄材料。XRD结果显示,经过机械活化后的化合物是水合锂化二氧化锰。活化后的原料,由于机械能以晶体缺陷的形式储存在原料中,反应物活性增大,在低温下进行热处理,就能获得较纯净的尖晶石LiMn₂O₄,但是产物的结晶性差,有必要提高热处理温度,促进晶体长大和结晶完善。 熔融浸渍法制备尖晶石LiMn₂O₄,产物中含有的杂相较多。液相浸注法制更多还原

【Abstract】 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.更多还原