锂离子电池电极材料改性机理的第一性原理研究

发布时间：2018-07-14 21:47

【摘要】：本论文以密度泛函理论为基础,采用第一性原理计算方法,探索了两种锂离子电池电极材料的改性机理,并为实验结果分析提供理论依据。其主要的内容包括:Li_4Ti_5O_(12)负极材料中本征缺陷和锌杂质的稳定性研究以及这些点缺陷对锂离子扩散的影响;α-Mn O_2·x H2O正极材料中锂离子嵌入特性和扩散动力学性能研究。对于Li_4Ti_5O_(12)负极材料,本论文采用缺陷形成能的计算公式,系统地分析其所有本征缺陷的稳定性,并研究这些本征缺陷对Li_4Ti_5O_(12)的几何结构、电子结构和锂离子扩散的影响。结果表明在富氧的条件下,VTi和LiTi的缺陷形成能是最低的,这意味着它们能够在钛酸锂晶体中大量存在;在缺氧的条件下制备钛酸锂样品的时候,Tii和TiLi就成为钛酸锂晶体中的主要缺陷,并形成n-型掺杂。由于氧离子与钛离子之间的强相互作用,本征缺陷会对钛酸锂晶体产生结构畸变,从而阻碍锂离子的扩散。对于锌掺杂钛酸锂,在富氧的条件下,晶体中会形成双电子受主缺陷ZnTi;但是在缺氧的条件下,ZnLi和Zni缺陷比ZnTi变得更稳定,而且锌离子更倾向于取代8a的锂离子位置,这从根本上证实了锌掺杂钛酸锂提高循环性能、降低容量的实验结果。另外,本论文利用第一性原理方法,深入地探索正极材料α-Mn O_2·x H2O中锂离子存储和扩散的特性。结果表明:当α-Mn O_2晶体结构中不含水分子的时候,即x=0,其为反铁磁构型且具有绝缘体的性质,不利于锂离子扩散和电子传输;当锂离子嵌入α-Mn O_2的浓度等于6.25 at%的时候,锂离子倾向于均匀分布且呈现出铁磁性;当浓度增大到12.5 at%的时候,相邻通道中的锂离子通过相互作用是整个体系呈现出反铁磁性;当水分子进入到α-Mn O_2的晶体结构中后,水分子通过范德华力聚集在同一通道中,既能够稳定α-Mn O_2的结构,也能够提高其电子电导率和促进锂离子的扩散,揭示了水分子对α-Mn O_2的改性机理。
[Abstract]:Based on density functional theory (DFT), the modification mechanism of two kinds of lithium ion battery electrode materials is explored by using first-principle calculation method, and the theoretical basis is provided for the analysis of experimental results. The main contents include the study on the stability of intrinsic defects and zinc impurities and the effect of these defects on the diffusion of lithium ions, and the study on the intercalation characteristics and diffusion kinetics of lithium ions in 伪 -Mn O _ 2x H _ 2O cathode materials. For Li4Ti5O12 negative electrode materials, the stability of all intrinsic defects is systematically analyzed by using the formula of the formation energy of defects, and the effects of these defects on the geometric structure, electronic structure and lithium ion diffusion of Li4Ti5O12 are studied. The results show that the formation energy of VTi and LiTi defects is the lowest under the condition of oxygen enrichment, which means that they can exist in large quantities in lithium titanate crystals. In the preparation of lithium titanate samples under anoxic conditions, Tii and TiLi become the main defects in lithium titanate crystals and form n-type doping. Due to the strong interaction between oxygen ions and titanium ions, intrinsic defects will distort the structure of lithium titanate crystals, thus hindering the diffusion of lithium ions. For zinc doped lithium titanate, a double electron acceptor defect ZnTi3 is formed in the crystal under oxygen-enriched conditions, but the ZnLi and Zni defects become more stable than ZnTi in anoxic condition, and zinc ions tend to replace the lithium ion position for 8 years. This basically confirms the experimental results of zinc doped lithium titanate to improve the cycling performance and reduce the capacity. In addition, the characteristics of lithium ion storage and diffusion in the cathode material 伪 -Mn O 2O 2 x H 2O are investigated by the first principle method in this paper. The results show that when there is no water molecule in the crystal structure of 伪 -Mn O _ 2O _ 2, that is, XN _ 0, which is an antiferromagnetic configuration and has the properties of insulator, it is not conducive to lithium ion diffusion and electron transport, and when the concentration of lithium ion embedded in 伪 -Mn _ 2O _ 2 is 6.25 at%, Lithium ions tend to uniformly distribute and exhibit ferromagnetism; when the concentration increases to 12.5 at%, the lithium ions in the adjacent channels exhibit antiferromagnetism through interaction; when the water molecules enter the crystal structure of 伪 -Mn O 2, the lithium ions in the adjacent channels exhibit antiferromagnetism, and when the water molecules enter the crystal structure of 伪 -Mn O 2, Water molecules gathered in the same channel through van der Waals force can not only stabilize the structure of 伪 -Mn O _ 2, but also increase its electronic conductivity and promote the diffusion of lithium ion. The mechanism of modification of 伪 -Mn O _ 2 by water molecules is revealed.
【学位授予单位】：清华大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TM912

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