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

Abstract

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.

Lithium Solvation and Mobility in Ionic Liquid Electrolytes with Asymmetric Sulfonyl-Cyano Anion

“…The emerging structure of the Li-solvate at 0.3 < x Li ≤ 0.7 is the coordination of two [CTSFI] anions, on average, with Li + in the first solvation shell. This is consistent with our previously computed energetically favored solvate structure of Li + with [CTFSI] using density functional theory …”

Ionogel Electrolytes Supported by Zwitterionic Copolymers Featuring Lithium Ion-Mediated, Noncovalent Cross-Links

“…This is due to the increase in ILE viscosity at higher Li + concentrations, which results in lower ion mobilities and total ionic conductivity. 33,34 For the 1 M Li + ILE ionogel samples (A−D), it can be seen that increasing the ZI MPC fraction in the scaffold (D > C > B > A; see Figure 3a) results in a slight upward trend in ionic conductivity. This is consistent with our previous finding that a ZI poly(MPC) homopolymer scaffold enabled a room temperature ionogel total ionic conductivity essentially equal to that of the 1 M ILE.…”

Electro–Oxidation of Nitroxide Radicals: Adsorption–Mediated Charge Transfer Probed Using SERS and Potentiometry

“…16 Density functional theory (DFT) is commonly employed to complement SERS in the identification of vibrational frequencies. 21,22 In the case of nitroxide radicals, DFT calculations predict peaks within 40-90 cm -1 of experimentally observed peaks. [23][24][25] In this study, electrochemical SERS (eSERS) combined with DFT calculations, and transient potentiometric techniques, were used to study the electrode-electrolyte interface and confirm the presence of surface-adsorbed species.…”