锰氧化物的制备、表征及电化学应用研究
发布时间：2019-04-20 11:00
【摘要】：开发高性能的空气电极催化剂可提高电化学性能并降低电池成本,具有重要的研究价值。本论文主要从合成高比表面积的超细纳米粒子以及特殊晶面暴露纳米材料的可控合成着手以提高锰氧化物的电催化活性。采用改进的共沉淀法与水热法制备锰氧化物催化剂,同时,为进一步拓展所制备Mn304八面体的应用领域,以其为前驱体合成了LiMn2O4锂离子电池正极材料。论文主要研究结果如下： 首次将MnOx/CeO2固溶体用于电催化反应,采用改进的共沉淀法制备了系列MnOx/CeO2固溶体超细纳米粒子,研究了Mn/Ce配比、煅烧温度对电催化活性的影响。研究发现将MnO2与Ce02形成固溶体后作为氧还原催化剂,使得O2在催化活性位点(即Mn4+所处位置)的移动性得到显著提高,导致更多吸附到空缺位的氧分子被活化,促进了大电流下氧还原过程的进行。另外Mn4+进入Ce02晶格能抑制Ce02晶体的生长,从而得到高比表面积的MnOx/CeO2固溶体,显著增加了氧还原活性位点,进一步促进了大电流放电下对O2催化能力。 以KMnO4为原材料,N,N-二甲基甲酰胺(DMF)作还原剂和结构导向剂,不使用任何表面活性剂或模板剂,采用水热法合成了形貌规则,尺寸均一的Mn304八面体,研究了不同DMF加入量及水热温度对所得产物形貌的影响。研究发现在该水热体系中,决定Mn304形貌的主要因素为溶剂组成和水热温度,通过采用Time-dependent形貌演化的研究方法发现八面体Mn304的形成是伴随着自组装机制和Ostwald成熟机制的晶体生长过程。电化学测量结果发现具有截顶八面体形貌的Mn304具有最优的电催化性能。结合物相表征结果可推断具有高能级晶面暴露可能导致截顶八面体Mn304上02越容易得到电子被还原,从而具有比普通Mn304纳米粒子更高催化活性。 以Mn304八面体为前驱体,通过固相锂化反应合成了LiMn2O4八面体作为锂离子电池正极材料,与采用溶胶凝胶法合成的普通纳米粒子进行了性能对比研究。研究发现采用化学锂化氧化物前驱体的方法可以很好控制目标材料的八面体形貌,从而大大提高材料的电化学性能。在1C倍率下,LiMn2O4八面体材料首次放电容量为122.7mAhg-1,200次循环后的容量保持率为84.8%。与LiMn2O4内米粒子对比八面体LiMn2O4电化学性能得到明显提高的原因可能如下：(1)该材料在充放电过程中能形成更加稳定的SEI膜,从而抑制充放电过程中Mn的溶解,提高循环性能；(2)合成的八面体具有良好的结晶度和单晶结构。
[Abstract]:The development of high performance air electrode catalysts can improve the electrochemical performance and reduce the battery cost, which has important research value. In order to improve the electrocatalytic activity of manganese oxides, the synthesis of ultrafine nanoparticles with high specific surface area and the controllable synthesis of nano-materials exposed to special crystal planes were studied in this thesis. Manganese oxide catalysts were prepared by improved coprecipitation and hydrothermal method. In order to further expand the application field of Mn304 octahedron, LiMn2O4 cathode materials for lithium-ion batteries were synthesized by using them as precursors. The main results are as follows: for the first time, MnOx/CeO2 solid solution was used in electrocatalytic reaction. A series of MnOx/CeO2 solid solution ultrafine nanoparticles were prepared by improved coprecipitation method, and the ratio of Mn/Ce was studied. Effect of calcination temperature on electrocatalytic activity. It was found that when MnO2 and Ce02 were used as catalyst for oxygen reduction, the mobility of O2 at the catalytic active site (i.e., the position of Mn4) was significantly increased, resulting in the activation of more oxygen molecules adsorbed to the vacancy site. It promotes the process of oxygen reduction under high current. In addition, the addition of Mn4 into the Ce02 lattice can inhibit the growth of Ce02 crystal, and thus obtain MnOx/CeO2 solid solution with high specific surface area, which significantly increases the oxygen reduction activity site and further promotes the catalytic activity of O2 under high current discharge. Mn 304 octahedron with regular morphology and uniform size was synthesized by hydrothermal method using KMnO4 as raw material, N, N-dimethylformamide (DMF) as reducing agent and structure directing agent, without using any surfactant or template. The effects of different amount of DMF and hydrothermal temperature on the morphology of the products were studied. It is found that the main factors determining the morphology of Mn304 in this hydrothermal system are solvent composition and hydrothermal temperature. It is found that the formation of octahedral Mn304 is accompanied by self-assembly mechanism and Ostwald ripening mechanism by means of Time-dependent morphology evolution. Electrochemical measurements show that Mn304 with octahedral morphology has the best electrocatalytic performance. It can be concluded from the phase characterization that the high-level surface exposure may lead to the reduction of electrons on the octahedral Mn304 with a truncated octahedral surface, which leads to a higher catalytic activity than that of ordinary Mn304 nanoparticles. LiMn2O4 octahedron was synthesized as cathode material of lithium ion battery by lithium reaction using Mn304 octahedron as precursor. The properties of LiMn2O4 octahedron were compared with that of ordinary nano-particles synthesized by sol-gel method. It is found that the octahedral morphology of the target material can be well controlled by the method of chemical lithium oxide precursor, which greatly improves the electrochemical performance of the material. The first discharge capacity of limn _ 2O _ 4 octahedral material is 84.8% after the first cycle of 122.7mAhg-1200 at 1C rate, and the initial discharge capacity of limn _ 2O _ 4 octahedral material is 84.8%. The reasons why the electrochemical properties of octahedral LiMn2O4 can be obviously improved compared with the octahedral LiMn2O4 particles in LiMn2O4 may be as follows: (1) the material can form a more stable SEI film during charge-discharge process, thus inhibiting the dissolution of Mn during the charge-discharge process and improving the cycle performance; (2) the octahedron has good crystallinity and single crystal structure.
【学位授予单位】：中南大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：O643.36;TM912
本文编号：2461545
[Abstract]:The development of high performance air electrode catalysts can improve the electrochemical performance and reduce the battery cost, which has important research value. In order to improve the electrocatalytic activity of manganese oxides, the synthesis of ultrafine nanoparticles with high specific surface area and the controllable synthesis of nano-materials exposed to special crystal planes were studied in this thesis. Manganese oxide catalysts were prepared by improved coprecipitation and hydrothermal method. In order to further expand the application field of Mn304 octahedron, LiMn2O4 cathode materials for lithium-ion batteries were synthesized by using them as precursors. The main results are as follows: for the first time, MnOx/CeO2 solid solution was used in electrocatalytic reaction. A series of MnOx/CeO2 solid solution ultrafine nanoparticles were prepared by improved coprecipitation method, and the ratio of Mn/Ce was studied. Effect of calcination temperature on electrocatalytic activity. It was found that when MnO2 and Ce02 were used as catalyst for oxygen reduction, the mobility of O2 at the catalytic active site (i.e., the position of Mn4) was significantly increased, resulting in the activation of more oxygen molecules adsorbed to the vacancy site. It promotes the process of oxygen reduction under high current. In addition, the addition of Mn4 into the Ce02 lattice can inhibit the growth of Ce02 crystal, and thus obtain MnOx/CeO2 solid solution with high specific surface area, which significantly increases the oxygen reduction activity site and further promotes the catalytic activity of O2 under high current discharge. Mn 304 octahedron with regular morphology and uniform size was synthesized by hydrothermal method using KMnO4 as raw material, N, N-dimethylformamide (DMF) as reducing agent and structure directing agent, without using any surfactant or template. The effects of different amount of DMF and hydrothermal temperature on the morphology of the products were studied. It is found that the main factors determining the morphology of Mn304 in this hydrothermal system are solvent composition and hydrothermal temperature. It is found that the formation of octahedral Mn304 is accompanied by self-assembly mechanism and Ostwald ripening mechanism by means of Time-dependent morphology evolution. Electrochemical measurements show that Mn304 with octahedral morphology has the best electrocatalytic performance. It can be concluded from the phase characterization that the high-level surface exposure may lead to the reduction of electrons on the octahedral Mn304 with a truncated octahedral surface, which leads to a higher catalytic activity than that of ordinary Mn304 nanoparticles. LiMn2O4 octahedron was synthesized as cathode material of lithium ion battery by lithium reaction using Mn304 octahedron as precursor. The properties of LiMn2O4 octahedron were compared with that of ordinary nano-particles synthesized by sol-gel method. It is found that the octahedral morphology of the target material can be well controlled by the method of chemical lithium oxide precursor, which greatly improves the electrochemical performance of the material. The first discharge capacity of limn _ 2O _ 4 octahedral material is 84.8% after the first cycle of 122.7mAhg-1200 at 1C rate, and the initial discharge capacity of limn _ 2O _ 4 octahedral material is 84.8%. The reasons why the electrochemical properties of octahedral LiMn2O4 can be obviously improved compared with the octahedral LiMn2O4 particles in LiMn2O4 may be as follows: (1) the material can form a more stable SEI film during charge-discharge process, thus inhibiting the dissolution of Mn during the charge-discharge process and improving the cycle performance; (2) the octahedron has good crystallinity and single crystal structure.
【学位授予单位】：中南大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：O643.36;TM912
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