Li Shu, Cao Zhen, Peng Yuxing, Liu Lei, Wang Yonglong, Wang Shu, Wang Ji-Qiang, Yan Tianying, Gao Xue-Ping, Song De-Ying, Shen Pan-Wen
Institute of New Energy Material Chemistry, Department of Material Chemistry, Nankai University, Tianjin 300071, China.
J Phys Chem B. 2008 May 22;112(20):6398-410. doi: 10.1021/jp710898h. Epub 2008 Apr 30.
The liquid structures of nonaqueous electrolytes composed of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and acetamide, with LiTFSI/acetamide molar ratios of 1:2, 1:4, and 1:6, were studied by molecular dynamics simulations. The simulations indicate that the Li+ cations prefer to be six-coordinate by the sulfonyl oxygen atoms of the TFSI- anions and the carbonyl oxygen atoms of the acetamide molecules, rather than by the most electronegative nitrogen atom of the TFSI- anion. Therefore, close Li+-TFSI- contact pairs exist in the system. The TFSI- anion prefers to provide only one of four possible oxygen atoms to coordinate to the same Li+ cation. Three conformations (cis, trans, and gauche) of the TFSI- anions were found to coexist in the liquid electrolyte. At high salt concentrations, the TFSI- anions mainly adopt the gauche conformation in order to provide more oxygen atoms to coordinate to different Li+ cations, while simultaneously reducing the repulsion among the Li+ cations. On the other hand, the fraction of TFSI- anions adopting the cis conformation is largest for the system with the molar ratio of 1:6, in which many clusters, mainly composed of the Li+ cations and the TFSI- anions, are immersed in the acetamide molecules. The size and charge distribution of clusters were also investigated. In the system with the molar ratio of 1:2, nearly all of the ions in the PBC (periodic boundary conditions) box aggregate into a bulky cluster that gradually disassembles into small clusters with decreasing salt concentration. The addition of acetamide molecules was found to effectively relax the liquid electrolyte structure, and the system with the molar ratio of 1:4 was found to exhibit a more homogeneous liquid structure than the other two electrolyte systems with molar ratios of 1:2 and 1:6.
通过分子动力学模拟研究了由双（三氟甲基磺酰）亚胺锂（LiTFSI）和乙酰胺组成的非水电解质的液体结构，其中LiTFSI/乙酰胺的摩尔比分别为1:2、1:4和1:6。模拟结果表明，Li⁺阳离子更倾向于由TFSI⁻阴离子的磺酰氧原子和乙酰胺分子的羰基氧原子进行六配位，而不是由TFSI⁻阴离子中电负性最强的氮原子配位。因此，体系中存在紧密的Li⁺-TFSI⁻接触对。TFSI⁻阴离子更倾向于仅提供四个可能的氧原子中的一个来与同一个Li⁺阳离子配位。发现TFSI⁻阴离子的三种构象（顺式、反式和gauche式）共存于液体电解质中。在高盐浓度下，TFSI⁻阴离子主要采用gauche构象，以便提供更多的氧原子与不同的Li⁺阳离子配位，同时减少Li⁺阳离子之间的排斥力。另一方面，对于摩尔比为1:6的体系，采用顺式构象的TFSI⁻阴离子的比例最大，其中许多主要由Li⁺阳离子和TFSI⁻阴离子组成的簇浸入乙酰胺分子中。还研究了簇的大小和电荷分布。在摩尔比为1:2的体系中，PBC（周期性边界条件）盒中的几乎所有离子聚集成一个大簇，随着盐浓度的降低，该大簇逐渐分解成小簇。发现添加乙酰胺分子可有效松弛液体电解质结构，并且发现摩尔比为1:4的体系比摩尔比为1:2和1:6的其他两种电解质体系表现出更均匀的液体结构。
