【作者】 罗改霞；

【导师】 赵纪军；

【作者基本信息】 大连理工大学 ， 凝聚态物理， 2013， 博士

【摘要】 便携式电子产品和全球范围电动汽车的普及刺激了能源存储设备(如电池和超级电容器)向更高功率密度和能量密度的发展。电极是锂离子电池和超级电容器的核心部件,而电极材料是决定电池和电容器综合性能优劣的关键因素。因此,开发新一代高性能电极材料对锂离子电池和超级电容器的研究和应用具有重大意义。首先,对于锂离子电池正极材料,表征离子输运性质的重要参数是化学扩散系数,而锂在正极材料中嵌入或脱出时,通常伴随着晶相变化。正极材料的嵌锂化合物是锂离子电池中锂离子的临时储存容器,为获得较高的单体电池电压,倾向于选择高电势的嵌锂化合物。因此,本论文基于随机固溶体模型,采用第一性原理方法研究锂离子电池正极材料层状过渡金属氧化物LiMnxCoyNi1-x-yO2,并讨论了Mn、Co和Ni组分在晶格结构、电极电势和Li扩散行为等方面的协同效应。通过分析Li在不同局域环境下的扩散活化能预言随机固溶体LiMnxCoyNi1-x-yO2的真实环境对离子导电能力的影响,有助于挑选LiCo02为基础的多组分锂过渡金属氧化物的最佳组合,为今后的理论和实验研究提供有意义的参考数据。锂离子硫电池因其能量密度高和原材料丰富等优点而备受关注。尽管实验上Li2S基正极材料已经被广泛研究,但理论上关于Li2S基材料的Li存储行为的研究几乎是空白的。特别是,过渡金属掺杂影响Li2S材料性能的微观机制还需探讨。我们将借助第一性原理方法系统地探索过渡金属(TM=Fe, Co, Ni, Cu)掺杂对Li2S的锂嵌入/脱出行为和电极电位的影响。其次,寻找合适的负极材料,使得锂离子电池具有足够高的储锂量和很好的锂嵌入/脱出可逆性,以保证电池的高电压、大容量和长循环寿命的要求。目前商业化的锂离子负极材料主要为石墨材料,研究碳负极材料石墨的性质具有重要的实际意义。为此,我们采用密度泛函理论模拟研究在原子尺度下不同石墨层间距中的Li与Li,Li与C之间的相互作用,以及其对储锂量和嵌入能影响的微观机制；在石墨层中掺杂不同浓度的B元素,及其与Li的相互作用,并探讨对储锂量变化的影响。最后,与传统的锂离子电池相比,超级电容器具有长寿命、高功率密度的特点,但能量密度较低。为改善超级电容器体系的能量密度,我们通过第一性原理计算结合非平衡格林函数方法,从原子尺度上研究了不同尺寸孔洞缺陷和N掺杂石墨烯基超级电容器电极材料,计算了其热力学稳定性、力学性能、扩散行为和输运性质,重点考察了孔洞缺陷的存在是否会降低石墨烯电极的热力学稳定性和电导率?嵌入孔洞的石墨烯层片是否仍能保持完美的力学性能?最佳有利于离子扩散的孔洞尺寸和形状如何?N掺杂石墨烯体系中,哪种C-N键合类型能增强电催化活性?依据对上述关键问题的系统研究,旨在通过引入适当的孔洞提高超级电容器的综合性能。更多还原

【Abstract】 The popularity of portable energy storage devices and electric vehicle stimulates the development of higher power and energy density for energy storage devices such as lithium ion batteries and supercapacitors. The electrode is the core component of lithium battery and supercapacitor. Thus, the electrode material is the key factor of overall performance quality. Therefore, exploitation of high performance electrode material has great significance for the research and application for lithium battery and supercapacitor.First, the chemical diffusion coefficient is an important parameter of ion transport properties for the cathode material of Li ion battery. Li insertion/extraction in cathode material usually accompany with phase transformation of host crystal. Cathode intercalation compound is a temporary storage of lithium in battery. It is desirable to select the intercalation compound with higher potential in order to obtain higher voltage. Hence, the crystal structures, reversible potentials and activation energies of LiMnxCoyNi1-x-yO2solid solutions are studied by means of density functional theory (DFT) calculations within generalized gradient approximation (GGA) and projector-augmented-wave (PAW) method. The general trends for the synergistic effects of TM ions are discussed. By analyzing Li diffusion energy barrier in various local circumstances of the multi-component solid solutions of lithium TM oxides, we predict that the real environment of LiMnxCoyNi1-x-yO2solid solutions affect the ability of ion conduction. These may help optimize compositions in future experiments.Lithium sulfur batteries have attracted much attention due to the high theoretical specific capacity as well as abundance of raw materials. However, to the best of our knowledge, there was no theoretical study on the Li storage behavior of Li2S materials, though Li2S-based materials have been intensively investigated in experiments. In particular, the microscopic mechanism for the effect of transition metal doping on the performance of Li2S remains puzzling. These facts motivate us to perform DFT calculations on the effects of transition metal (TM=Fe, Co, Ni, Cu) doping on lithium extraction/insertion behavior and the electrode potential of Li2S.Next, seeking appropriate anode materials is crucial for lithium ion batteries with sufficient lithium storage and excellent lithium reversibility to meet the demand of high voltage, large capacity and long cycle life. Nowadays, graphite is a commercially used anode material for lithium batteries. Therefore, the study of anode graphite materials has very practical significance. Using DFT calculation we have systematically investigated the atomic structures, electronic properties and saturation Li capacity in graphite materials with different interlayer spacing and different contents of substitutional boron dopants. Oue results not only provide valuable insights into the microscopic mechanism of lithium-ion batteries but also help design new anode materials with improved performance.Finally, compared to the traditional lithium ion battery, supercapacitor has high power density and long cycle life, but lower energy density. In order to improve the energy density of supercapacitor, we investigate the graphene sheets as supercapacitor electrode material with different geometries of hole defects and/or nitrogen doping at the atomistic scale by first-principles methods and non-equilibrium Green’s function technique. We mainly examine the following critical issues:(1) Will these holes as structural defects reduce the thermodynamic stability and electrical conductivity of the graphene electrode?(2) Will the graphene sheets incorporated with holes still retain their excellent mechanical properties?(3) What is the optimal shape and size of these holes for ion diffusion?(4) Which kind of C-N bonding configurations is responsible for the enhanced electrocatalytic activity of the N-doped graphene material? For revealing these issues, we calculate the formation energies, mechanical properties, diffusion behaviors and electrical conductance, aiming to enhance the overall performance of the supercapacitors by appropriate incorporation of holes.更多还原