From Nanoscale to Microscale: Crossover in the Diffusion Dynamics within Two Pyrrolidinium-Based Ionic Liquids

Knowledge of the ion motion in room temperature ionic liquids (RTILs) is critical for their applications in a number of fields, from lithium batteries to dye-sensitized solar cells. Experiments on a limited number of RTILs have shown that on macroscopic time scales the ions typically undergo conventional, Gaussian diffusion. On shorter time scales, however, non-Gaussian behavior has been observed, similar to supercooled fluids, concentrated colloidal suspensions, and more complex systems. Here we characterize the diffusive motion of ionic liquids based on the <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium (PYR₁₄) cation and bis­(trifluoro methanesulfonyl)­imide (TFSI) or bis­(fluorosulfonyl)­imide (FSI) anions. A combination of pulsed gradient spin–echo (PGSE) NMR experiments and molecular dynamics (MD) simulations demonstrates a crossover from subdiffusive behavior to conventional Gaussian diffusion at ∼10 ns. The deconvolution of molecular displacements into a continuous spectrum of diffusivities shows that the short-time behavior is related to the effects of molecular caging. For PYR₁₄FSI, we identify the change of short-range ion–counterion associations as one possible mechanism triggering long-range displacements

Molecular Environment and Enhanced Diffusivity of Li⁺ Ions in Lithium-Salt-Doped Ionic Liquid Electrolytes

Lithium salts dissolved in ionic liquids (ILs) are interesting alternatives to the commonly used electrolytes for Li-ion batteries. In this study, the solution of Li [bis-(trifluoromethanesulfonyl)imide] (LiTFSI) in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium TFSI (PYR₁₄TFSI) ionic liquid in the 0.1:0.9 molar ratio is studied by heteronuclear NOE and NMR diffusion measurements. The main purpose is to spot on the interions organization and mobility. NOE data support the existence of strongly coordinated Li⁺ species, whereas variable temperature measurements of the self-diffusion coefficients <i>D</i> show large, selective, and unexpected enhancement of Li⁺ mobility with <i>T</i>. The measured activation energy for Li⁺ diffusion is significantly larger than those of TFSI⁻ and PYR₁₄⁺. These findings can be related to the mechanism of Li⁺ diffusion in ILs based on disruption formation of the coordination shells of Li⁺ with TFSI anions rather than on the Brownian motion of the whole Li⁺ coordinated species

Pyrrolidinium-Based Ionic Liquids Doped with Lithium Salts: How Does Li⁺ Coordination Affect Its Diffusivity?

We present the characterization of LiX-doped room-temperature ionic liquids (ILs) based on the <i>N</i>-butyl-<i>N</i>-methyl pyrrolidinium (PYR₁₄) cation with two fluorinated anions: (trifluoromethanesulfonyl)-(nonafluorobutanesulfonyl)­imide (XIM₁₄) and bis­(pentafluoroethanesulfonyl)­imide (XBETI). The new data are also compared with previous results on PYR₁₄TFSI (bis­(trifluoromethanesulfonyl)­imide). Their local organization has been investigated via NMR nuclear Overhauser effect (NOE) experiments for {¹H–¹⁹F} and {¹H–⁷Li} that give us details on PYR₁₄⁺/X^– and PYR₁₄⁺/Li⁺ contacts. We confirm the presence of [Li­(X)₂]⁻ coordinated species in all systems. The long-range, intermolecular NOEs have been detected and provide information on the ions’ organization beyond the first solvation sphere. The ionic conductivity, viscosity and self-diffusion coefficients of the ionic mixtures have also been measured. The activation energies for the diffusion of the individual ions and for the fluidity are compared with those for the pure ILs. Finally, density functional calculations on [Li­(BETI)₂]⁻, [Li­(IM₁₄)₂]⁻, and [Li­(TFSI)₂]⁻ complexes demonstrate that the minimum energy structures for all systems correspond to a tetrahedral coordination of the Li-ion by four oxygen atoms of the anions. Assuming very simple key steps for the Li⁺ diffusion process (i.e., the concerted breaking and formation of Li–O bonds or the rearrangement around a tetrahedrally coordinated Li⁺), we calculate activation barriers that agree well with the experimental results (approximately 46 kJ/mol, in all systems)

Influence of carbonate-based additives on the electrochemical performance of Si NW anodes cycled in an ionic liquid electrolyte.

Addition of electrolyte additives (ethylene or vinylene carbonate) is shown to dramatically improve the cycling stability and capacity retention (1600 mAh g-1) of Si nanowires (NWs) in a safe ionic liquid (IL) electrolyte (0.1LiTFSI-0.6PYR13FSI-0.3PYR13TFSI). We show, using postmortem SEM and TEM, a distinct difference in morphologies of the active material after cycling in the presence or absence of the additives. The difference in performance is shown by postmortem XPS analysis to arise from a notable increase in irreversible silicate formation in the absence of the carbonate additives. The composition of the solid electrolyte interphase (SEI) formed at the active material surface was further analyzed using XPS as a function of the IL components revealing that the SEI was primarily made up of N-, F-, and S-containing compounds from the degradation of the TFSI and FSI anions

Implications of Anion Structure on Physicochemical Properties of DBU-Based Protic Ionic Liquids

Protic ionic liquids (PILs) are potential candidates as electrolyte components in energy storage devices. When replacing flammable and volatile organic solvents, PILs are expected to improve the safety and performance of electrochemical devices. Considering their technical application, a challenging task is the understanding of the key factors governing their intermolecular interactions and physicochemical properties. The present work intends to investigate the effects of the structural features on the properties of a promising PIL based on the 1,8-diazabicyclo[5.4.0]undec-7-ene (DBUH+) cation and the (trifluoro­methanesulfonyl)­(nonafluoro­butanesulfonyl)imide (IM14–) anion, the latter being a remarkably large anion with an uneven distribution of the C–F pool between the two sides of the sulfonylimide moieties. For comparison purposes, the experimental investigations were extended to PILs composed of the same DBU-based cation and the trifluoro­methanesulfonate (TFO–) or bis(trifluoro­methanesulfonyl)imide (TFSI–) anion. The combined use of multiple NMR methods, thermal analyses, density, viscosity, and conductivity measurements provides a deep characterization of the PILs, unveiling peculiar behaviors in DBUH-IM14, which cannot be predicted solely on the basis of differences between aqueous pKa values of the protonated base and the acid (ΔpKa). Interestingly, the thermal and electrochemical properties of DBUH-IM14 turn out to be markedly governed by the size and asymmetric nature of the anion. This observation highlights that the structural features of the precursors are an important tool to tailor the PIL’s properties according to the specific application