Energetics of Li+ Coordination with Asymmetric Anions in Ionic Liquids by Density Functional Theory
Energetics of Li+ Coordination with Asymmetric Anions in Ionic Liquids by Density Functional Theory
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DOI：
10.3389/fenrg.2021.725010
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发表时间：
2021-10
期刊：
影响因子：
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通讯作者：
Drace Penley;Stephen P. Vicchio;Rachel B. Getman;B. Gurkan
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文献类型：
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作者：
Drace Penley;Stephen P. Vicchio;Rachel B. Getman;B. Gurkan
利用密度泛函理论 (DFT) 研究了离子液体 (IL) 电解质中锂溶剂化物的能量学、配位和拉曼振动。与双(三氟甲磺酰基)亚胺([TFSI])、双(氟磺酰基)亚胺([FSI])和二氰胺([DCA])的对称类似物相比，检查了Li+与氰基(三氟甲磺酰基)亚胺([CTFSI])和(氟磺酰基)(三氟甲磺酰基)亚胺([FTFSI])的不对称阴离子的配位。可用于描述 Li+ 溶剂化强度的解离能是根据各个组分和 Li 溶剂化物的能量计算得出的。发现仅存在 Li+-O 配位的 Li+-[FTFSI]、Li+-[TFSI] 和 Li+-[FSI] 的计算解离能相似。 [TFSI] 阴离子一侧的氟化增加不对称性和阴离子尺寸不会导致解离能的显着差异。另一方面，对于[CTFSI]，Li+-O和Li+-N配位都存在，并且Li溶剂化物比单独的[DCA]、单独的[TFSI]或[DCA]/[TFSI]阴离子的1:1混合物的溶剂化具有更小的解离能。这一发现表明，不对称阴离子可以削弱 Li+ 溶剂化作用，不对称阴离子通过焓效应促进竞争性配位环境。在 (Li[CTFSI] n )−(n−1) 的可能的 Li 溶剂化物中，其中 n = 1、2、3 或 4，(Li[CTFSI]2)−1 被发现是最稳定的，具有单齿和双齿成键的可能性。基于这项研究，我们假设不对称阴离子的部分溶剂化和弱溶剂化能量可能会增加离子液体电解质中锂溶剂化物的结构异质性和波动。这些效应可能会进一步促进与锂离子电池相关的浓缩多组分离子液体电解质中的Li+跳跃传输机制。
The energetics, coordination, and Raman vibrations of Li solvates in ionic liquid (IL) electrolytes are studied with density functional theory (DFT). Li+ coordination with asymmetric anions of cyano(trifluoromethanesulfonyl)imide ([CTFSI]) and (fluorosulfonyl)(trifluoro-methanesulfonyl)imide ([FTFSI]) is examined in contrast to their symmetric analogs of bis(trifluoromethanesulfonyl)imide ([TFSI]), bis(fluorosulfonyl)imide ([FSI]), and dicyanamide ([DCA]). The dissociation energies that can be used to describe the solvation strength of Li+ are calculated on the basis of the energetics of the individual components and the Li solvate. The calculated dissociation energies are found to be similar for Li+-[FTFSI], Li+-[TFSI], and Li+-[FSI] where only Li+-O coordination exists. Increase in asymmetry and anion size by fluorination on one side of the [TFSI] anion does not result in significant differences in the dissociation energies. On the other hand, with [CTFSI], both Li+-O and Li+-N coordination are present, and the Li solvate has smaller dissociation energy than the solvation by [DCA] alone, [TFSI] alone, or a 1:1 mixture of [DCA]/[TFSI] anions. This finding suggests that the Li+ solvation can be weakened by asymmetric anions that promote competing coordination environments through enthalpic effects. Among the possible Li solvates of (Li[CTFSI] n )−(n−1), where n = 1, 2, 3, or 4, (Li[CTFSI]2)−1 is found to be the most stable with both monodentate and bidentate bonding possibilities. Based on this study, we hypothesize that the partial solvation and weakened solvation energetics by asymmetric anions may increase structural heterogeneity and fluctuations in Li solvates in IL electrolytes. These effects may further promote the Li+ hopping transport mechanism in concentrated and multicomponent IL electrolytes that is relevant to Li-ion batteries.