短链多硫化物在离子液体中聚集行为的分子动力学模拟
收稿日期: 2020-07-13
修回日期: 2020-07-25
网络出版日期: 2021-07-27
基金资助
国家自然科学基金资助项目(21878295);北京市自然科学基金资助项目(2192052);前沿学科重点研究项目(QYZDB-SSW-SLH022)
Molecular dynamics simulations of short-chain lithium polysulfides clustering in ionic liquids
Received date: 2020-07-13
Revised date: 2020-07-25
Online published: 2021-07-27
离子液体因其优异的物化性质、能抑制多硫化物溶解等特点，近年来被广泛应用于锂硫电池电解液中。在电池充放电产物中，难溶性Li2S和Li2S2易聚集沉积在电极表面，影响电池性能，而目前关于其团聚行为与电解液性质的微观机理研究较少。本工作利用量化计算和分子动力学模拟分析了短链Li2S和Li2S2在离子液体中的微观结构以及形成团簇的情况。通过分析体系的微观结构发现，阳离子中主要与S作用的是侧链甲基，短链多硫化物之间Li-S作用远强于与阴离子的Li-O作用。团簇尺寸分布的结果表明，短链多硫化物在[TFSI]型离子液体中易形成多分子的大团簇，Li2S2体系比Li2S体系中的大团簇比例更高；离子液体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。
胡天媛 , 王艳磊 , 霍锋 , 何宏艳 . 短链多硫化物在离子液体中聚集行为的分子动力学模拟[J]. 过程工程学报, 2021 , 21(7) : 847 -856 . DOI: 10.12034/j.issn.1009-606X.220221
Ionic liquids have been widely used in lithium-sulfur battery electrolytes in recent years due to their excellent physicochemical properties and the ability to inhibit the dissolution of lithium polysulfides intermediates. Among those products during the battery cycling processes, insoluble Li2S and Li2S2 are inclined to aggregate and deposit on the electrode surface, affecting the battery performance. However, there are few studies on the microscopic mechanism of their clustering behaviors and electrolyte properties. In this work, the microstructure of Li2S/Li2S2 in ionic liquids and the formation of clusters were studied by DFT calculations and molecular dynamics simulations. From the optimized configurations using DFT methods, it can be seen that ionic liquids and Li2S/Li2S2 always tended to form a "cation-short chain polysulfide-anion" sandwich-like structures. By analyzing the microstructures of the molecular dynamics simulation systems, it can be found that the methyl group in side chain of cation mainly interacted with S in Li2S/Li2S2, and the Li-S interaction between short-chain polysulfides was much stronger than Li-O interaction in anions. The results of cluster size distribution showed that short-chain polysulfides were more likely to form large clusters in the [TFSI]-based ionic liquid, while the proportion of large clusters in Li2S2 system was higher than Li2S systems. Moreover, the tendency of forming large clusters increased with the concentration of Li2S/Li2S2. Additionally, stronger coordination ability of anions brought smaller proportion of large Li2S clusters. However, the configuration characteristics and interaction forms of anions-Li2S will also affected the sizes and structures of clusters. These understandings could provide theoretical guidance for future systematic studies on screening and designing ionic liquids electrolytes for lithium-sulfur batteries.
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