层状—尖晶石复合结构富锂正极材料的制备、改性及其性能研究

发布时间：2018-05-21 11:23

本文选题：富锂正极材料 + 层状-尖晶石复合结构　；参考：《湘潭大学》2016年硕士论文

【摘要】：富锂正极材料作为目前锂离子电池正极材料的主流发展方向之一,具有放电比容量高、成本低廉、环境友好等特点,从其首次被报道至今,一直受到广泛关注。然而,材料本身存在着的循环稳定性差、首次库伦效率低、倍率性能差、尤其是电压平台衰减快等缺点一直都是研究人员关注的重点。本论文的主要目的是为富锂正极材料的改性研究提供一种新的思路、在电池的综合性能方面做出改进,因此,设计并制备了一种具有层状-尖晶石复合结构的富锂正极材料。论文从制备方法、结构特点、工艺条件的影响及电化学性能的改性几个方面对该复合结构富锂正极材料进行了系统的研究。论文的具体研究内容如下:1.采用溶剂热法和后续梯度煅烧法制备了具有层状-尖晶石复合结构的富锂正极材料。该复合材料内部同时具有层状的Li2MnO3相和尖晶石型Li4Mn5O12相。尖晶石型的Li4Mn5O12相结构内部具有3D孔径,能够为Li离子的传输提供更加自由的空间;而层状Li2MnO3相的存在则保证了材料具有较高的比容量。通过研究证明,该复合结构中两相的存在可以相得益彰,既保证材料具有较高的实际比容量,同时又改善了材料的循环性能和结构稳定性。在2.0-4.6 V的电压范围内,首次制备的层状-尖晶石复合结构富锂正极材料在0.1 C下的放电比容量为289.6 mAh/g,在10 C下的放电比容量为122.8 mAh/g;同时,在0.5 C条件下,该层状-尖晶石复合结构富锂正极材料循环200次后的容量保持率达到了88%。2.基于前面的研究,对层状-尖晶石复合结构正极材料制备方法中的实验条件进行对比与分析,以确定影响复合结构中两相组成比例的实验条件及其影响结果,同时通过对具有不同组成的样品其电化学性能的比较,确定了具有最优电化学性能的相组成比例。研究结果表明,后续高温固相反应过程中的煅烧温度和时间共同影响着材料的结构组成:(1)当煅烧温度在750℃时,结构中主要生成的是尖晶石型Li4Mn5O12相,当煅烧温度在800℃时,结构中则主要生成层状Li2MnO3相;(2)当在750℃下的煅烧时间超过10 h时,材料中才开始出现尖晶石型Li4Mn5O12相,与此同时,随着在800℃下煅烧时间的增加,结构中已生成的尖晶石相会逐渐转化成层状相。3.为了解决层状-尖晶石复合结构富锂正极材料导电性差的问题,在前面的研究基础之上,设计并制备了聚苯胺包覆的层状-尖晶石复合结构富锂正极材料。研究表明,聚苯胺的包覆在不改变材料内部复合结构的基础上,有效地提高了该正极材料的导电性,大大降低了材料在循环过程中的阻抗;同时,该聚苯胺包覆的复合结构富锂正极材料在0.1 C,2.0-4.6 V条件下的首次放电比容量为302.9 mAh/g,首次库伦效率为83.5%,10 C倍率下的放电比容量为146.2 mAh/g,0.5C条件下循环200次后的容量保持率可以达到92.4%。4.为了进一步改善富锂正极材料的电化学性能,将前面几章中采用的溶剂热法运用到其他富锂正极材料上,制备出粒径均一、分散性好的微球,粒子直径控制在1μm左右。并在此基础上,采用Na掺杂和多孔结构的双向设计对正极材料进行改性。研究结果表明,富锂正极材料的粒子直径控制在1μm,同时在材料内部进行Na掺杂,利用Na离子半径和过渡金属离子半径之间的差距,可以扩大Li层之间的晶格间距,从而增加结构的稳定性;此外,正极微球中的多孔结构可以在保证材料具有良好的振实密度和结构稳定性的同时,增加电极与电解液之间的接触面积,同时为Li离子的存储提供更多的空位。最终制备的Na掺杂富锂多孔正极微球,其在0.1 C,2.0-4.6 V条件下的首次放电比容量为305.3mAh/g,在0.5 C、5 C、10 C下的放电比容量分别为252.3 mAh/g、200.8 mAh/g、177.2 mAh/g。
[Abstract]:As one of the mainstream development direction of lithium ion battery cathode materials, lithium rich cathode materials have been widely concerned because of their high discharge specific capacity, low cost and friendly environment. However, the circulation stability of the material itself is poor, the efficiency of the first Kulun is low, and the performance is poor, especially the electricity. The main purpose of this paper is to provide a new way of thinking for the study of the modification of lithium rich cathode materials and to improve the comprehensive performance of the batteries. Therefore, a kind of lithium rich cathode material with layered spinel Shi Fuhe structure is designed and prepared. The preparation methods, the structural characteristics, the influence of the technological conditions and the modification of the electrochemical properties are systematically studied. The specific contents of this paper are as follows: 1. the lithium rich cathode material with lamellar crystal composite structure was prepared by the solvent thermal method and the follow up calcining method. The Li2MnO3 phase and spinel type Li4Mn5O12 phase in the material are at the same time. The Li4Mn5O12 phase structure of the spinel type has a 3D aperture, which can provide more free space for the transmission of Li ions, while the existence of the layered Li2MnO3 phase ensures a higher specific capacity of the material. Through the study, the existence of two phases in the composite structure has been proved. It can not only guarantee the material with higher actual specific capacity, but also improve the cyclic performance and structural stability of the material. In the range of 2.0-4.6 V voltage, the discharge specific capacity of the first prepared layered spinel composite cathode material under 0.1 C is 289.6 mAh/g, and the discharge specific capacity under 10 C is 122.8 mAh At the same time, under the condition of 0.5 C, the capacity retention rate of the lamellar spinel composite cathode material after 200 cycles reached the previous study. The experimental conditions of the preparation method of the layered spinel composite positive material were compared and analyzed to determine the experiment on the proportion of the two phase composition in the composite structure. The results show that the phase composition with the optimal electrochemical performance is determined by comparison of the electrochemical properties of the samples with different composition. The results show that the calcining temperature and time in the subsequent high temperature solid state reaction affect the structure composition of the material together: (1) when the calcining temperature is at 750. The main formation of the spinel Li4Mn5O12 phase is the Li4Mn5O12 phase. When the calcining temperature is at 800 C, the structure is mainly formed in the layered Li2MnO3 phase. (2) the spinel Li4Mn5O12 phase begins to appear when the calcining time is over 10 h at 750 C. At the same time, with the increase of calcining time at 800 C, the spinel in the structure has been formed. In order to solve the problem of poor conductivity of the lithium rich cathode material in layered spinel composite structure, the phase.3. has been designed and prepared on the basis of the previous research. The study shows that the coating of polyaniline is not changed in the composite structure of the material. On the basis, the conductivity of the cathode material is effectively improved and the impedance of the material in the cycle process is greatly reduced. At the same time, the first discharge specific capacity of the polyaniline coated composite lithium cathode material at 0.1 C and 2.0-4.6 V is 302.9 mAh/g, the first Kulun efficiency is 83.5%, and the discharge specific capacity under the 10 C ratio is 146.2 mAh. /g, 0.5C under the condition of 200 cycles, the capacity retention rate can reach 92.4%.4. in order to further the electrochemical performance of the Gai Shanfu lithium cathode material. The solvent thermal method used in the previous chapters is applied to other lithium rich cathode materials to prepare the microspheres with homogeneous particle size and good dispersion, and the particle diameter is controlled at about 1 m. The positive electrode materials are modified by Na doping and porous structure. The results show that the particle diameter of the lithium rich cathode material is controlled at 1 u m and Na doping in the material. The gap between the Na ion radius and the transition metal ion radius can be used to enlarge the lattice spacing between the Li layers, thus increasing the structure. In addition, the porous structure in the positive microspheres can increase the contact area between the electrode and the electrolyte while ensuring that the material has good vibrational density and structural stability, and provides more vacancy for the storage of Li ions. The final preparation of Na doped lithium rich porous cathode microspheres is under the condition of 0.1 C and 2.0-4.6 V. The first discharge specific capacity is 305.3mAh/g, and the discharge capacity at 0.5 C, 5 C, and 10 C is 252.3 mAh/g, 200.8 mAh/g, 177.2 mAh/g., respectively.
【学位授予单位】：湘潭大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TM912

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