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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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Transport properties of imidazolium based ionic liquid electrolytes from molecular dynamics simulations
Ionic liquids (ILs) are promising electrolytes for high-performance Li-ion batteries (LIBs), which can significantly improve the safety and energy storage capacity. Although extensive experimental and computational studies have reported, further exploration is needed to understand the properties of IL systems, their microscopic structures and dynamics, and the behavior of Li ions in ILs. We report here results of molecular dynamics simulations as a function of electric field for Li diffusion in two IL systems, [EMIM][TFSI] and [BMIM][TFSI] doped with various concentrations of LiTFSI. We find that the migration of each individual Li ion depends largely on its micro-environment, leading to differences by factors of up to 100 in the diffusivity. The structural and dynamical properties indicate that Li diffusion is affected significantly by the coordination and interaction with the oxygen species in the TFSI anions. Moreover, the IL cations also contribute to the Li diffusion mechanism by attenuating the Li–TFSI interaction.
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- Award ID(s):
- 1805022
- PAR ID:
- 10275140
- Date Published:
- Journal Name:
- Electrochemical Science Advances
- ISSN:
- 2698-5977
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Computational simulations of ionic liquid solutions have become a useful tool to investigate various physical, chemical and catalytic properties of systems involving these solvents. Classical molecular dynamics and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations of IL systems have provided significant insights at the atomic level. Here, we present a review of the development and application of the multipolar and polarizable force field AMOEBA for ionic liquid systems, termed AMOEBA–IL. The parametrization approach for AMOEBA–IL relies on the reproduction of total quantum mechanical (QM) intermolecular interaction energies and QM energy decomposition analysis. This approach has been used to develop parameters for imidazolium– and pyrrolidinium–based ILs coupled with various inorganic anions. AMOEBA–IL has been used to investigate and predict the properties of a variety of systems including neat ILs and IL mixtures, water exchange reactions on lanthanide ions in IL mixtures, IL–based liquid–liquid extraction, and effects of ILs on an aniline protection reaction.more » « less
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Gardas, Ramesh L. (Ed.)The solvation structure and transport properties of Li+ in ionic liquid (IL) electrolytes based on n-methyl-n-butylpyrrolidinium cyano(trifluoromethanesulfonyl)imide [PYR14][CTFSI] and [Li][CTFSI] (0 ≤ xLi ≤ 0.7) were studied by Raman and Nuclear Magnetic Resonance (NMR) diffusometry, and molecular dynamics (MD) simulations. At xLi < 0.3, Li+ coordination is dominated by the cyano group. As xLi is increased, free cyano-sites become limited, resulting in increased coordination via the sulfonyl group. The 1:1 mixture of the symmetric anions bis(trifluoromethanesulfonyl)imide ([TFSI]) and dicyanamide ([DCA]) results in similar physical properties as the IL with [CTFSI]. However, anion asymmetry is shown to increase Li-salt solubility and promote Li+ transference. The lifetimes of Li+-cyano coordination for [CTFSI] are calculated to be shorter than those for [DCA], indicating that the competition from the sulfonyl group weakens its solvation with Li+. This resulted in higher Li+ transference for the electrolyte with [CTFSI]. In relation to the utility of these electrolytes in energy storage, the Li–LiFePO4 half cells assembled with IL electrolyte (xLi = 0.3, 0.5, and 0.7) demonstrated a nominal capacity of 140 mAh/g at 0.1C rate and 90 °C where the cell with xLi = 0.7 IL electrolyte demonstrated 61% capacity retention after 100 cycles and superior rate capability owing to increased electrochemical stability.more » « less
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While ionic liquids (ILs) have attracted much attention as potential next-generation lubricant additives, their implementation in oil formulations has been hindered by their limited solubility in hydrocarbon fluids and corrosivity. Here, we encapsulate an oil-insoluble IL that has been studied in lubrication science, namely 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([HMIM][TFSI]), within poly(ethylene glycol dimethacrylate-buytl methacrylate copolymer) (poly(EGDM-c-BMA)) microshells using a mini-emulsion polymerization process. The synthesized poly(EGDM-c-BMA)-encapsulated [HMIM][TFSI] microparticles are shown to be dispersible in a non-polar, synthetic oil (i.e., poly-α-olefin). Tribological experiments indicated that the microcapsules act as an additive reservoir that reduces friction by releasing the encapsulated IL at the sliding interface following the mechanical rupture of the polymer shell. X-ray photoelectron spectroscopy (XPS) measurements provided evidence that [HMIM][TFSI] does not tribochemically react on steel surfaces to create a reaction layer, thus suggesting that this IL reduces friction by generating a solid-like, layered structure upon nanoconfinement at sliding asperities, as proposed by previous nanoscale studies. The results of this work do not only provide new insights into the lubrication mechanism of ILs when used as additives in base oils in general, but also establish a new, broadly-applicable framework based on polymer encapsulation for utilizing ILs or other compounds with limited solubility as additives for oil formulations.more » « less
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Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL ( i.e. [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL–mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/elsa.202100007
