Tuning Binary Ionic Liquid Mixtures: Linking Alkyl Chain Length to Phase Behavior and Ionic Conductivity

The use of mixed salts to generate new composite ionic liquids (ILs) provides a facile means of readily tuning or tailoring the desired properties of ionic media. Despite this, very little information is available about how the structure of the selected ions and composition impacts the properties of salt mixtures. To explore this, six binary IL₁–IL₂ mixtures based on <i>N</i>-alkyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide salts have been characterized. The physicochemical properties (density, viscosity, and ionic conductivity) and phase behavior of these mixtures are reported. The variation of the alkyl chains lengths on the cations plays a significant role in determining both the phase behavior and the physicochemical properties of the mixtures. Notably, the “tunability” of the properties of the IL mixtures is much easier to control than is found by simply making small structural changes to the ions in a given salt

Physicochemical Properties of Binary Ionic Liquid–Aprotic Solvent Electrolyte Mixtures

The properties of mixtures of ionic liquids (ILs) with a variety of different aprotic solvents have been examined in detail. The ILs selectedbis­(trifluoromethanesulfonyl)­imide (TFSI^–) salts with <i>N</i>-methyl-<i>N</i>-pentylpyrrolidinium (PY₁₅⁺), -piperidinium (PI₁₅⁺), or -morpholinium (MO₁₅⁺) cationsenabled the investigation of how cation structure influences the mixture properties. This study includes the characterization of the thermal phase behavior of the mixtures and volatility of the solvents, density and excess molar volume, and transport properties (viscosity and conductivity). The mixtures with ethylene carbonate form a simple eutectic, whereas those with ethyl butyrate appear to form a new IL–solvent crystalline phase. Significant differences in the viscosity of the mixtures are found for different solvents, especially for the IL-rich concentrations. In contrast, only minor differences are noted for the conductivity with different solvents for the IL-rich concentrations. For the solvent-rich concentrations, however, substantial differences are noted in the conductivity, especially for the mixtures with acetonitrile

Structural Interactions within Lithium Salt Solvates: Acyclic Carbonates and Esters

Solvate crystal structures serve as useful models for the molecular-level interactions within the diverse solvates present in liquid electrolytes. Although acyclic carbonate solvents are widely used for Li-ion battery electrolytes, only three solvate crystal structures with lithium salts are known for these and related solvents. The present work, therefore, reports six lithium salt solvate structures with dimethyl and diethyl carbonate, (DMC)₂:LiPF₆, (DMC)₁:LiCF₃SO₃, (DMC)_1/4:LiBF₄, (DEC)₂:LiClO₄, (DEC)₁:LiClO₄, and (DEC)₁:LiCF₃SO₃ and four with the structurally related methyl and ethyl acetate, (MA)₂:LiClO₄, (MA)₁:LiBF₄, (EA)₁:LiClO₄, and (EA)₁:LiBF₄

Structural Interactions within Lithium Salt Solvates: Acyclic Carbonates and Esters

Solvate crystal structures serve as useful models for the molecular-level interactions within the diverse solvates present in liquid electrolytes. Although acyclic carbonate solvents are widely used for Li-ion battery electrolytes, only three solvate crystal structures with lithium salts are known for these and related solvents. The present work, therefore, reports six lithium salt solvate structures with dimethyl and diethyl carbonate, (DMC)₂:LiPF₆, (DMC)₁:LiCF₃SO₃, (DMC)_1/4:LiBF₄, (DEC)₂:LiClO₄, (DEC)₁:LiClO₄, and (DEC)₁:LiCF₃SO₃ and four with the structurally related methyl and ethyl acetate, (MA)₂:LiClO₄, (MA)₁:LiBF₄, (EA)₁:LiClO₄, and (EA)₁:LiBF₄

Influence of Solvent on Ion Aggregation and Transport in PY₁₅TFSI Ionic Liquid–Aprotic Solvent Mixtures

Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the <i>N</i>-methyl-<i>N</i>-pentylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide (PY₁₅TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY₁₅TFSI–AN mixtures was found to be significantly higher than the conductivity of the corresponding −PC and −DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the −DMC and −AN mixtures. The significantly lower conductivity of the PY₁₅TFSI–DMC mixtures, as compared to those for PY₁₅TFSI–AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the −DMC mixtures. The PY₁₅TFSI–DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation–anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL–solvent mixtures and the mixture excess properties

Diffusion Coefficients from ¹³C PGSE NMR MeasurementsFluorine-Free Ionic Liquids with the DCTA^– Anion

Pulsed-field gradient spin–echo (PGSE) NMR is a widely used method for the determination of molecular and ionic self-diffusion coefficients. The analysis has thus far been limited largely to ¹H, ⁷Li, ¹⁹F, and ³¹P nuclei. This limitation handicaps the analysis of materials without these nuclei or for which these nuclei are insufficient for complete characterization. This is demonstrated with a class of ionic liquids (or ILs) based on the nonfluorinated anion 4,5-dicarbonitrile-1,2,3-triazole (DCTA^–). It is demonstrated here that ¹³C-PGSE NMR can be used to both verify the diffusion coefficients obtained from other nuclei, as well as characterize materials that lack commonly scrutinized nuclei  all without the need for specialized NMR methods

Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes

A Raman spectroscopic evaluation of numerous crystalline solvates with lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI or LiN­(SO₂CF₃)₂) has been conducted over a wide temperature range. Four new crystalline solvate structures(PHEN)₃:LiTFSI, (2,9-DMPHEN)₂:LiTFSI, (G3)₁:LiTFSI and (2,6-DMPy)_1/2:LiTFSI with phenanthroline, 2,9-dimethyl­[1,10]­phenanthroline, triglyme, and 2,6-dimethylpyridine, respectivelyhave been determined to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI^–···Li⁺ cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the determination of ionic association interactions within electrolytes containing LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI^– anion coordination present in liquid electrolyte mixtures due to the wide range of TFSI^–···Li⁺ cation interactions possible and the overlap of the corresponding spectroscopic data signatures

Solvate Structures and Computational/Spectroscopic Characterization of Lithium Difluoro(oxalato)borate (LiDFOB) Electrolytes

Lithium difluoro­(oxalato)­borate (LiDFOB) is a relatively new salt designed for battery electrolyte usage. Limited information is currently available, however, regarding the ionic interactions of this salt (i.e., solvate formation) when it is dissolved in aprotic solvents. Vibrational spectroscopy is a particularly useful tool for identifying these interactions, but only if the vibrational bands can be correctly linked to specific forms of anion coordination. Single crystal structures of LiDFOB solvates have therefore been used to both explore the DFOB^–...Li⁺ cation coordination interactions and serve as unambiguous models for the assignment of the Raman vibrational bands. The solvate crystal structures determined include (monoglyme)₂:LiDFOB, (1,2-diethoxyethane)_3/2:LiDFOB, (acetonitrile)₃:LiDFOB, (acetonitrile)₁:LiDFOB, (dimethyl carbonate)_3/2:LiDFOB, (succinonitrile)₁:LiDFOB, (adiponitrile)₁:LiDFOB, (PMDETA)₁:LiDFOB, (CRYPT-222)_2/3:LiDFOB, and (propylene carbonate)₁:LiDFOB. DFT calculations have been incorporated to provide additional insight into the origin (i.e., vibrational modes) of the Raman vibrational bands to aid in the interpretation of the experimental analysis

Solvate Structures and Computational/Spectroscopic Characterization of LiBF₄ Electrolytes

Crystal structures have been determined for both LiBF₄ and HBF₄ solvates: (acetonitrile)₂:LiBF₄, (ethylene glycol diethyl ether)₁:LiBF₄, (diethylene glycol diethyl ether)₁:LiBF₄, (tetrahydrofuran)₁:LiBF₄, (methyl methoxyacetate)₁:LiBF₄, (succinonitrile)₁:LiBF₄, (<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentamethyldiethylenetriamine)₁:HBF₄, (<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine)_3/2:HBF₄, and (phenanthroline)₂:HBF₄. These, as well as other known LiBF₄ solvate structures, have been characterized by Raman vibrational spectroscopy to unambiguously assign the anion Raman band positions to specific forms of BF₄^–···Li⁺ cation coordination. In addition, complementary DFT calculations of BF₄^–···Li⁺ cation complexes have provided additional insight into the challenges associated with accurately interpreting the anion interactions from experimental Raman spectra. This information provides a crucial tool for the characterization of the ionic association interactions within electrolytes

Solvate Structures and Computational/Spectroscopic Characterization of LiPF₆ Electrolytes

Raman spectroscopy is a powerful method for identifying ion–ion interactions, but only if the vibrational band signatures for the anion coordination modes can be accurately deciphered. The present study characterizes the PF₆^– anion P–F Raman symmetric stretching vibrational band for evaluating the PF₆^–···Li⁺ cation interactions within LiPF₆ crystalline solvates to create a characterization tool for liquid electrolytes. To facilitate this, the crystal structures for two new solvates(G3)₁:LiPF₆ and (DEC)₂:LiPF₆ with triglyme and diethyl carbonate, respectivelyare reported. DFT calculations for Li-PF₆ solvates have been used to aid in the assignments of the spectroscopic signatures. The information obtained from this analysis provides key guidance about the ionic association information which may be obtained from a Raman spectroscopic evaluation of electrolytes containing the LiPF₆ salt and aprotic solvents. Of particular note is the overlap of the Raman bands for both solvent-separated ion pair (SSIP) and contact ion pair (CIP) coordination in which the PF₆^– anions are uncoordinated or coordinated to a single Li⁺ cation, respectively