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锂硫电池由于其远高于传统锂离子电池的质量能量密度、活性组分硫在自然界中储量丰富且环境友好等优势而具有广阔的发展前景，但充放电中间产物多硫化物在正负极的“穿梭效应”严重限制了其实际应用。离子液体具有宽的电化学窗口、良好的化学稳定性以及弱的路易斯酸碱性，能够抑制多硫化物的溶解，因而被广泛应用在锂硫电池电解液中。然而，离子液体与不同多硫化物之间的微观结构以及阴阳离子对多硫化物“穿梭效应”的抑制机理，目前仍缺乏全面的认识。本文构建了离子液体与多硫化物的模拟体系，利用分子动力学模拟和量化计算相结合的方法，分别对长链多硫化物在离子液体基电解液中的扩散性质以及短链多硫化物在离子液体中的聚集行为展开了研究。主要的研究总结如下： （1）基于量化计算和已有的力场参数建立了离子液体和多硫化物的力场。利用DFT方法优化离子液体和多硫化物的结构，并通过限制性静电势方法拟合得到原子电荷。阳离子的键长、键角、二面角、键能等参数来源于已有的OPLS-AA力场或文献中相似类型的替代；多硫化物的键长参数则取自文献值，键角和键能等参数分别由DFT方法优化结构和收集伸缩振动频率得到。通过调整不同的LJ参数，并用分子动力学模拟的方法对相应的体系进行验证，最终确定了Li和S的范德华和静电作用参数；离子液体与多硫化物的模拟密度值与实验值的偏差小于5%，证明力场参数是合理的。该力场综合考虑了长链和短链多硫化物不同的结构性质，具有一定的普适性。 （2）采用分子动力学模拟和量化计算系统地研究了长链Li2S8在离子液体电解液中的微观结构与扩散性质。DFT计算Li2S8与单个阴/阳离子/离子对的作用能的结果表明，阴离子与Li2S8作用的能力遵循[OTf]- > [TFSI]-，阳离子与Li2S8的作用能力则符合[P13]+ > [PP13]+，离子对相互作用能却是[PP13]型离子液体 > [P13]型离子液体，观察最优构型发现这是因为两种体系中分别形成了“double Li-O bridging”和“single Li-O bridging”的结构；进一步通过分子动力学模拟分析了纯离子液体-Li2S8体系的径向分布函数、空间分布函数以及配位数，发现阳离子中主要是侧链甲基基团与S82-中的S作用，[OTf]型离子液体中Li-O作用强于[TFSI]型离子液体；接着研究了离子液体基电解液体系中Li+和S82-的扩散情况，结果显示Li+主要与溶剂DME配位，Li+在[OTf]型离子液体中表现出较高的自扩散系数也更早脱离溶剂化壳层，同时Li+-O[OTf]- 的停留时间要比Li+-O[TFSI]-的短。综合微观结构与扩散性质，我们对扩散机理提出了合理假设，认为与Li+结合能力更强的[OTf]-在Li+扩散过程中二者的配位结构更容易破坏，加快了Li+在溶剂化壳层之间的交换速率，有利于Li+的扩散。 （3）结合量化计算和分子动力学模拟的结果分析了短链Li2S和Li2S2在离子液体中的微观结构以及形成团簇的情况。DFT方法优化的构型可以看出离子液体与Li2S或Li2S2总是倾向于形成“阳离子-短链多硫化物-阴离子”的“三明治”结构；分子动力学模拟分析体系的微观结构时发现，阳离子中主要与S作用的是侧链的甲基基团，短链多硫化物之间Li-S作用远强于阴离子中O和Li的作用，推测会造成短链多硫化物的聚集现象；不同短链多硫化物浓度下，团簇尺寸结构的结果表明，随着短链多硫化物浓度增大，体系中单体数目减少，大团簇比例增加。短链多硫化物在[TFSI]型离子液体中更容易形成多分子的大团簇，在[P13]型离子液体中形成的团簇平均尺寸比[PP13]型离子液体大，而Li2S2体系比Li2S体系中的大团簇比例更高；当使用不同配位能力的离子液体阴离子时，Li2S团簇尺寸结构与阴离子配位能力总体上呈负相关的趋势，即阴离子配位能力越强，形成大的Li2S团簇比例越少，但阴离子的构型特点和作用形式也会对团簇的尺寸结构造成影响。;The lithium-sulfur batteries have a great prospect of application due to their much higher energy density than traditional lithium-ion batteries, abundant sulfur in nature and environmental friendliness. However, the parasitic lithium polysulfides shuttle phenomenon during charge and discharge processes, severely hinders the commercialization of lithium-sulfur batteries. Ionic liquids have wide electrochemical windows, good chemical stability as well as weak lewis acidity and baiscity, which have been found to suppress the lithium polysulfides solubility. However, there is a lack of understanding of the interaction microstructures between ionic liquids and different polysulfides and the effects of anions and cations on how to suppress the shuttle behaviors. In this paper, simulation systems consist of ionic liquids and polysulfides were constructed. The diffusion properties of long-chain polysulfide in ionic liquid-based electrolytes and the aggregation behaviors of short-chain polysulfides in ionic liquids were studied using molecular dynamics simulations and quantum chemistry calculations. The main research is summarized as following: (1) The force fields of ionic liquids and polysulfides were constructed based on the quantum chemistry calculations and existing force field parameters. The structures of ionic liquids and polysulfides were optimized by DFT methods, and the atomic charges were obtained by the Restricted Electrostatic Potential method. The parameters such as bond lengths, angles, dihedrals and force constants in cations were derived from existing OPLS-AA force field or substitutions from references; Additionally, parameters for equilibrium bond lengths of polysulfide were obtained from references, and parameters of angles and force constants were calculated by optimizing structures and collecting vibration frequency respectively. Besides, the van der Waals and electrostatic parameters of Li and S were determined by adjusting different LJ parameters and verifying properties with molecular dynamics simulations; The deviations between the simulated densities and experimental values of ionic liquids and polysulfide are less than 5%, which indicate that our force fields are reasonable. Furthermore, the force field takes different structural properties of both long-chain and short-chain polysulfides into account, thus has a certain universality. (2) The microstructures and diffusion properties of long-chain Li2S8 in ionic liquid electrolytes were studied systematically using molecular dynamics simulations and quantum chemistry calculations. The interaction energy results show that the interaction of [OTf]- and Li2S8 is stronger than that of [TFSI]- with Li2S8, while the interaction energy between [P13]+ and Li2S8 is higher than that between [PP13]+ and Li2S8. The sequence of the interactions of ion pairs is [PP13]-based ionic liquid > [P13]-based ionic liquid, which can be attributed to the “double Li-O bridging” and “single Li-O bridging” structures in two systems; Furthermore, the radial distribution functions, spatial distribution functions and coordination number of the pure ionic liquid-Li2S8 systems were analyzed using molecular dynamics simulations. It was found that S atoms in S82- mainly interact with the methyl group in side chain of cations and the Li-O interaction in the [OTf]-based ionic liquid is stronger than that in the [TFSI]-based ionic liquid; Then, the diffusion properties of Li+ and S82- in ionic liquid-based electrolyte systems were studied. The results show that Li+ is mainly coordinated with solvent DME. Li+ shows a higher self-diffusion coefficient and leaves the solvated shell earlier in [OTf]-based systems, also the residence time of Li+-O[OTf]- is shorter than that of Li+-O[TFSI]- . From analysis of the microstructures and diffusion properties, we put forward a reasonable hypothesis on the diffusion mechanism, and conclude that the coordination structures of [OTf]- with stronger association with Li+ are more likely to be destroyed in the process of Li+ diffusion, which speeds up the exchange rate of Li+ through the solvated shells and is beneficial to the diffusion of Li+. (3) The microstructures of short-chain Li2S and Li2S2 in ionic liquids and the formation of clusters were analyzed by quantum chemistry calculations and molecular dynamics simulations. From the optimized configurations using DFT methods, it can be seen that ionic liquids and Li2S/Li2S2 always tend to form a "cation-short chain polysulfide-anion" sandwich-like structures; From microstructural analysis by molecular dynamics simulations, it was found that the methyl group in side chain of cations mainly interacts with S in Li2S/Li2S2, and the Li-S interaction in short-chain polysulfides is much stronger than Li-O interaction in anions, which will cause the aggregation of short-chain polysulfides; The results of cluster size distribution under different concentrations of short-chain polysulfides show that the number of monomers in systems decreases and the proportion of large clusters increases with the increase of concentration. Short-chain polysulfides are more likely to form large clusters in [TFSI]-based ionic liquids, and the average size of clusters in [P13]-based ionic liquid is larger than that in [PP13]-based ionic liquid, while the proportion of large clusters in Li2S2 system are higher than Li2S systems. Moreover, the size and structure of Li2S clusters can be generally correlated with the coordination ability of anions. In other words, stronger coordination ability of anions will bring smaller proportion of large Li2S clusters. However, the configuration characteristics and interaction forms of anions-Li2S will also affect the sizes and structures of clusters.
|胡天媛. 离子液体与多硫化物作用机制的分子动力学模拟研究[D]. 中国科学院大学,2020.|
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