Ion Clusters and Networks in Water-in-Salt Electrolytes

McEldrew, Michael; Goodwin, Zachary A. H.; Bi, Sheng; Kornyshev, Alexei A.; Bazant, Martin Z.

doi:10.1149/1945-7111/abf975

Cited by 46 publications

(80 citation statements)

References 93 publications

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“…They concluded that the majority (60%) of the transport occurred via vehicular motion, while Li + hopping was found to gain more significance with large-sized anions and increased Li + concentration. Therefore, the promotion of the hopping mechanism via anion exchange is the key for enhanced Li + mobility in concentrated electrolytes such as ILs considering the limitations of the vehicular mobility in the presence of Li + aggregates at x Li + > 0.05 (Brinkkötter et al, 2018;McEldrew et al, 2021).…”

Section: Introductionmentioning

confidence: 99%

Energetics of Li+ Coordination with Asymmetric Anions in Ionic Liquids by Density Functional Theory

et al. 2021

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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.

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Section: Introductionmentioning

confidence: 99%

Energetics of Li+ Coordination with Asymmetric Anions in Ionic Liquids by Density Functional Theory

et al. 2021

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“…Whereas, as shown in Ref. 36, the associations between lithium cations and IL anions were found to be much larger than one, and thus more important to explicitly model. However, in order to avoid entirely neglecting interactions between IL cations and anions, we model the IL cations as interacting with the open association sites of the anions via regular solution interactions [30,40].…”

Section: Theorymentioning

confidence: 71%

“…Such crystalline clusters cannot be well described with the presented theory, and would instead require a theory to describe precipitation of the solid phase. However, electrolytes containing "chaotropic" salts, such as LiTFSI, form highly branched and disordered clusters with practically no intra-cluster loops [36]. Therefore, the assumption of Cayley tree clusters is reasonable for many alkali metal salts in ILs.…”

Section: Theorymentioning

confidence: 99%

“…In this article, we aim to fill this gap by developing a thermodynamically consistent theory for ionic clustering and network formation of salt-in-ionic liquids. This theory is based on the classical theories of thermoreversible association of polymer mixtures, which some of us have recently modified and extended for applications in super-concentrated electrolytes [34][35][36]. Our theory naturally depends on only a handful of parameters, and importantly it is extended here to depend on the mole fraction of alkali metal salt that is added to an IL, which shares the same anion as the alkali metal salt.…”

Section: Introductionmentioning

confidence: 99%

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Salt-in-Ionic-Liquid Electrolytes: Ion Network Formation and Negative Effective Charges of Alkali Metal Cations

McEldrew¹,

Goodwin²,

Molinari³

et al. 2021

Preprint

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Salt-in-ionic liquid electrolytes have attracted significant attention as potential electrolytes for next generation batteries largely due to their safety enhancements over typical organic electrolytes.However, recent experimental and computational studies have shown that under certain conditions alkali cations can migrate in electric fields as if they carried a net negative effective charge. In particular, alkali cations were observed to have negative transference numbers at small mole fractions of alkali metal salt that revert to the expected net positive transference numbers at large mole fractions. Simulations have provided some insights into these observations, where the formation of asymmetric ionic clusters, as well as a percolating ion network could largely explain the anomalous transport of alkali cations. However, a thermodynamic theory that captures such phenomena has not been developed, as ionic associations were typically treated via the formation of ion pairs. The theory presented herein, based on the classical polymer theories, describes thermoreversible associations between alkali cations and anions, where the formation of large, asymmetric ionic clusters and a percolating ionic network are a natural result of the theory. Furthermore, we present several general methods to calculate the effective charge of alkali cations in ionic liquids. We note that the negative effective charge is a robust prediction with respect to the parameters of the theory, and that the formation of a percolating ionic network leads to the restoration of net positive charges of the cations at large mole fractions of alkali metal salt. Overall, we find excellent qualitative agreement between our theory and molecular simulations in terms of ionic cluster statistics and the effective charges of the alkali cations.

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“…Therefore, in such a concentrated system as ILs, the formation of clusters larger than ion pairs occurs at a non-negligible level [56][57][58][59]. The description of cluster formation for bulk concentrated electrolytes in a thermodynamically consistent manner was outlined by McEldrew et al [60][61][62][63] based on the theories of thermoreversible polymers [64][65][66][67][68][69][70][71][72]. This theory is able to predict the concentrations of all possible clusters up to a percolating ionic network of infinite size [73][74][75], referred to as the gel, with indications of this gel being observed in molecular simulations [57][58][59]61].…”

Section: Introductionmentioning

confidence: 99%

Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids

Goodwin,

McEldrew,

de Souza

et al. 2022

Preprint

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Understanding the bulk and interfacial properties of super-concentrated electrolytes, such as ionic liquids (ILs), has attracted significant attention lately for their promising applications in supercapacitors and batteries. Recently, McEldrew et al. developed a theory for reversible ion associations in bulk ILs, which accounted for the formation of all possible Cayley tree clusters and a percolating ionic network (gel). Here we adopt and develop this approach to understand the associations of ILs in the electrical double layer at electrified interfaces. With increasing charge of the electrode, the theory predicts a transition from a regime dominated by a gelled or clustered state to a regime dominated by free, crowded ions. This transition from gelation to crowding is conceptually similar to the overscreening to crowding transition.

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Ion Clusters and Networks in Water-in-Salt Electrolytes

Cited by 46 publications

References 93 publications

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

Salt-in-Ionic-Liquid Electrolytes: Ion Network Formation and Negative Effective Charges of Alkali Metal Cations

Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids

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