NMR Investigation of Ionic Liquid−LiX Mixtures: Pyrrolidinium Cations and TFSI- Anions

Toward Metal Extraction from Regolith: Theoretical Investigation of the Solvation Structure and Dynamics of Metal Ions in Ionic Liquids

Lunar and Martian regoliths, containing feldspar, pyroxene, ilmenite, olivine, and aluminite minerals, are excellent sources of metals such as aluminum, sodium, magnesium, and iron. Ionic liquids (ILs), which are excellent solvents with extremely low vapor pressure and high electrochemical stability, can be potentially leveraged for extracting metals from regolith in an extra-terrestrial environment. A critical step in the solvation process, which determines the effectiveness of the IL solvent, is the formation of solvation shells around the metal cations. To determine the rigidity and stability of the solvation shells, which has a direct implication on the extraction of metals, we performed classical molecular dynamics simulations of dilute solutions comprising individual metal ions Na+, Mg2+, and Al3+ in two distinct ILs, [mppy][TFSI] and [mppy][HSO4]. Our results indicate that the compactness of the structure is directly related to the charge density of the metal cation and the relative size and symmetry of the IL anion. Potentials of the mean force of the metal cation with the solvating IL anion indicate the presence of energy minima with barriers that increase with the surface charge density of the cation. The increasing energy barrier leads to greater residence time of metal cations in the solvation shell, which was confirmed by evaluating corresponding autocorrelation functions. Overall, our calculations provide fundamental insights into key factors that influence the solvation of metals and can be useful in the screening of ILs for digestion of metal-containing minerals in lunar and Martian regoliths.

Mechanically Robust Poly(ionic liquid) Block Copolymers as Self-Assembling Gating Materials for Single-Walled Carbon-Nanotube-Based Thin-Film Transistors

The proliferation of high-performance thin-film electronics depends on the development of highly conductive solid-state polymeric materials. We report on the synthesis and properties investigation of well-defined cationic and anionic poly(ionic liquid) AB-C type block copolymers, where the AB block was formed by random copolymerization of highly conductive anionic or cationic monomers with poly(ethylene glycol) methyl ether methacrylate, while the C block was obtained by post-polymerization of 2-phenylethyl methacrylate. The resulting ionic block copolymers were found to self-assemble into a lamellar morphology, exhibiting high ionic conductivity (up to 3.6 × 10^-6 S cm^-1 at 25 °C) and sufficient electrochemical stability (up to 3.4 V vs Ag⁺/Ag at 25 °C) as well as enhanced viscoelastic (mechanical) performance (storage modulus up to 3.8 × 10⁵ Pa). The polymers were then tested as separators in two all-solid-state electrochemical devices: parallel plate metal-insulator-metal (MIM) capacitors and thin-film transistors (TFTs). The laboratory-scale truly solid-state MIM capacitors showed the start of electrical double-layer (EDL) formation at ∼10³ Hz and high areal capacitance (up to 17.2 μF cm^-2). For solid-state TFTs, low hysteresis was observed at 10 Hz due to the completion of EDL formation and the devices were found to have low threshold voltages of -0.3 and 1.1 V for p-type and n-type operations, respectively.

Local Volume Conservation in Concentrated Electrolytes Is Governing Charge Transport in Electric Fields

While ion transport processes in concentrated electrolytes, e.g., based on ionic liquids (IL), are a subject of intense research, the role of conservation laws and reference frames is still a matter of debate. Employing electrophoretic NMR, we show that momentum conservation, a typical prerequisite in molecular dynamics (MD) simulations, is not governing ion transport. Involving density measurements to determine molar volumes of distinct ion species, we propose that conservation of local molar species volumes is the governing constraint for ion transport. The experimentally quantified net volume flux is found to be zero, implying a nonzero local momentum flux, as tested in pure ILs and IL-based electrolytes for a broad variety of concentrations and chemical compositions. This constraint is consistent with incompressibility, but not with a local application of momentum conservation. The constraint affects the calculation of transference numbers as well as comparisons of MD results to experimental findings.

Spectroscopic analysis focusing on ionic liquid/metal electrode and organic semiconductor interfaces in an electrochemical environment

The solid-liquid interface forms an electric double layer that enables the function of electronic devices and, thus, represents an important area of electrochemical research. Because ionic liquids (ILs) are becoming prominent candidates for new high-performing electrolytes, their interface with solid substrates (e.g., metal electrodes or organic semiconductors) attracts substantial attention. An example of improvement achieved using ILs as electrolytes is a decrease in the operating voltage of transistors from >10 V in traditional SiO₂-gated transistors to <1 V in IL-gated electronic double-layer organic field-effect devices. This perspective discusses the investigation of poorly accessible IL/substrate interfaces using both attenuated total reflectance ultraviolet (ATR-UV) spectroscopy and a newly developed electrochemical setup combined with ATR-UV (EC-ATR-UV), which allows analysis of the interfacial area under the application of varying electric potential. The recent EC-ATR-UV applications in interfacial analytical chemistry are overviewed and compared to other spectroscopic methods described in the recent literature. Lastly, the supplementation of experimental data with theoretical calculations (e.g., quantum chemical calculations and molecular dynamics simulations) is also addressed.

MAS-NMR of [Pyr13][Tf2N] and [Pyr16][Tf2N] Ionic Liquids Confined to Carbon Black: Insights and Pitfalls

Electrolytes based on ionic liquids (IL) are promising candidates to replace traditional liquid electrolytes in electrochemical systems, particularly in combination with carbon-based porous electrodes. Insight into the dynamics of such systems is imperative for tailoring electrochemical performance. In this work, 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide and 1-Hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide were studied in a carbon black (CB) host using spectrally resolved Carr-Purcell-Meiboom-Gill (CPMG) and 13-interval Pulsed Field Gradient Stimulated Echo (PFGSTE) Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). Data were processed using a sensitivity weighted Laplace inversion algorithm without non-negativity constraint. Previously found relations between the alkyl length and the aggregation behavior of pyrrolidinium-based cations were confirmed and characterized in more detail. For the IL in CB, a different aggregation behavior was found compared to the neat IL, adding the surface of a porous electrode as an additional parameter for the optimization of IL-based electrolytes. Finally, the suitability of MAS was assessed and critically discussed for investigations of this class of samples.

Understanding Structural and Transport Properties of Dissolved Li2 S8 in Ionic Liquid Electrolytes through Molecular Dynamics Simulations

Lithium-sulfur batteries with high energy density are considered as one of the most promising future energy storage devices. However, the parasitic lithium polysulfides shuttle phenomenon severely hinders the commercialization of such batteries. Ionic liquids have been found to suppress the lithium polysulfides solubility, diminishing the shuttle effect effectively. Herein, we performed classical molecular dynamics simulations to explore the microscopic mechanism and transport behaviors of typical Li₂ S₈ species in ionic liquids and ionic liquid-based electrolyte systems. We found that the trifluoromethanesulfonate anions ([OTf]^- ) exhibit higher coordination strength with lithium ions compared with bis(trifluoromethanesulfonyl)imide anions ([TFSI]^- ) in static microstructures. However, the dynamical characteristics indicate that the presence of the [OTf]^- anions in ionic liquid electrolytes bring faster Li⁺ exchange rate and easier dissociation of Li⁺ solvation structures. Our simulation models offer a significant guidance to future studies on designing ionic liquid electrolytes for lithium-sulfur batteries.

Adding salt to expand voltage window of humid ionic liquids

Humid hydrophobic ionic liquids-widely used as electrolytes-have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of the water molecules are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li⁺, having significantly lowered activity. The Li⁺-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enlarged electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from electrochemical performance degradation.

Study of the Active Carbon from Used Coffee Grounds as the Active Material for a High-Temperature Stable Supercapacitor with Ionic-Liquid Electrolyte

This study reveals a simple approach to recycle wasted coffee grounds into highly valuable carbon material with superior electrochemical performance. Activated carbon prepared from wasted coffee grounds has been formed via hydrothermal acidic hydrolysis followed by a KOH chemical activation at 800 ∘C. To understand the electrochemical properties of the sample, a set of characterization tools has been utilized: N₂ and CO₂ adsorption–desorption isotherms, thermal gravimetric analysis, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy. The specific surface area obtained from a Brunner–Emmett–Teller (BET) analysis reached 2906±19m2g−1. Prepared sample (designated as ACG-800KOH) was tested as electrode material in an electric double layer capacitor (EDLC) device with ionic liquid PYR13-TFSI as an electrolyte. The EDLC test was conducted at temperatures ranging from 20 to 120 ∘C. The specific material capacitance reached 178 Fg−1 measured at 20 ∘C and 50 A g−1 and was in the range 182 to 285 Fg−1 at the 20 to 120 ∘C temperature range.

Magnetic Resonance Imaging and Molecular Dynamics Characterization of Ionic Liquid in Poly(ethylene oxide)-Based Polymer Electrolytes

Ternary systems consisting of polymers, lithium salts, and ionic liquids (ILs) are promising materials for the development of next-generation lithium batteries. The ternary systems combine the advantages of polymer-salt and IL-salt systems, thus providing media with high ionic conductivity and solid-like mechanical properties. In this work, we apply nuclear magnetic resonance 1H microimaging [magnetic resonance imaging (MRI)] techniques and molecular dynamics (MD) simulations to study the translational and rotational dynamics of the N-butyl-N-methylpyrrolidinium (PYR14) cation in poly(ethylene oxide) (PEO) matrices containing the lithium bis(trifluoromethanesulfonyl) imide salt (LiTFSI) and the PYR14TFSI IL. The analysis of diffusion-weighted images in PEO/LiTFSI/PYR14TFSI samples with varying mole ratios (10:1:x, with x = 1, 2, 3, and 4) shows, in a wide range of temperatures, a spatially heterogeneous distribution of PYR14 diffusion coefficients. Their weight-averaged values increase with IL content but remain well below the values estimated for the neat IL. The analysis of T2 (spin-spin relaxation) parametric images shows that the PEO matrix significantly hinders PYR14 rotational freedom, which is only partially restored by increasing the IL content. The MD simulations, performed on IL-filled cavities within the PEO matrix, reveal that PYR14 diffusion is mainly affected by Li/TFSI coordination within the IL phase. In agreement with MRI experiments, increasing the IL content increases the PYR14 diffusion coefficients. Finally, the analysis of MD trajectories suggests that Li diffusion mostly develops within the IL phase, although a fraction of Li cations is strongly coordinated by PEO oxygen atoms.

Attenuated total reflectance far-ultraviolet and deep-ultraviolet spectroscopy analysis of the electronic structure of a dicyanamide-based ionic liquid with Li+

Elucidating the unique electronic structure of ionic liquid molecules around Li⁺ using electronic absorption spectroscopy, theoretical calculations, and chemometric analyses.

A More Sustainable and Cheaper One-Pot Route for the Synthesis of Hydrophobic Ionic Liquids for Electrolyte Applications

An innovative one-pot synthetic process that uses water as the only processing solvent was used to obtain ionic liquids (ILs) in a yield of approximately 95 mol % and purity greater than 99.3 wt % (<2 ppm each of lithium, bromide and moisture) in a processing time of 1 h. Since no heating is needed for carrying out the reaction and no purification through sorbents is required, energy, time and chemicals can be saved to minimize waste production. The physicochemical and electrochemical validation, including tests in batteries, reported herein shows that the above-mentioned ILs have properties analogous to those of ILs prepared by standard reported procedures and show high performance without any further purification step through sorbents. These characteristics, in combination with low cost, easy execution and scale-up, sustainability and versatility, make the one-pot process even more appealing, especially for industrial-scale applications.

Dynamics of [Pyr13][Tf2N] ionic liquid confined to carbon black

The intrinsic ionic nature of room temperature ionic liquids (RTILs) bears the potential to replace classical aqueous electrolytes in electrochemical applications, for example in metal-air batteries. For a systematic adjustment of RTIL properties in porous cathodes, the ionic arrangement under confinement is of prime importance. Using spectrally resolved pulsed gradient stimulated echo nuclear magnetic resonance (PGSTE-NMR) and spin-lattice NMR relaxation time (T₁) distributions, the dynamics of 1-methyl-1-propylpyrrolidiniumbis(trifluoromethylsulfonyl)imide ([Pyr₁₃][Tf₂N]) confined to carbon black were investigated. A considerable dependence of the [PYR₁₃] mobility on the loading fraction of the carbon black pore space was found. There is evidence for a preferential layering of the RTIL adjacent to the carbon surface and a dependence of the ionic configuration on the local structure of the carbon surface. The inversion efficiency of inversion-recovery T₁ data indicates a quasi-stationary layer at the carbon surface with solid-like properties, where the bulk-like properties of the RTIL are adopted as the distance to the surface increases. From the NMR diffusion data an intermediate layer between the quasi-stationary and the bulk-like RTIL is evident. This layer shows a particularly strong pore space loading dependence. While it has an anisotropic, two-dimensional mobility with reduced diffusion perpendicular to the surface at any loading, when it interfaces a gas phase at low loading its mobility is higher than bulk diffusion by up to an order of magnitude and chemical exchange with other layers is low. This layer appears to be of particular importance for the ion exchange between RTIL environments with different spacing from the carbon surface and hence crucial for the overall dynamics of RTILs in the investigated porous environment.

Safer electrolyte components for rechargeable batteries

Among the electrochemical energy storage systems, rechargeable lithium batteries are considered very promising candidates for the next generation power sources because of their high gravimetric and volumetric energy density with respect to other cell chemistries. The lithium-ion battery technology is based on the use of electrode materials able to reversibly intercalate lithium cations, which are continuously transferred between two host structures (negative and positive electrodes) during the charge and discharge processes. Commercial lithium-ion batteries commonly use liquid electrolytes based on suitable lithium salts (solute) and organic compounds (solvents). The latter, volatile and flammable, represent serious concerns for the safety of the electrochemical devices, this so far preventing their large diffusion in applications as automotive, storage from renewable sources, smart grids.

One of the most appealing approaches is the partial or total replacement of the organic solvents with safer, less hazardous, electrolyte components. Here, a concise survey of ones of the most investigated types of alternative electrolyte components, proposed for safer and more reliable rechargeable lithium batteries, is reported.

Graphical Abstract:

Diffusion weighted magnetic resonance imaging for temperature measurements in catalyst supports with an axial gas flow

In situ thermometry of catalytic gas phase reactions allows to determine temperature profiles in catalyst beds. Diffusion weighted MRI is proposed as an alternative method for temperature measurements using capillaries filled with different liquids.

Ionic Liquid-Based Electrolyte Membranes for Medium-High Temperature Lithium Polymer Batteries

Li⁺-conducting polyethylene oxide-based membranes incorporating N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide are used as electrolyte separators for all-solid-state lithium polymer batteries operating at medium-high temperatures. The incorporation of the ionic liquid remarkably improves the thermal, ion-transport and interfacial properties of the polymer electrolyte, which, in combination with the wide electrochemical stability even at medium-high temperatures, allows high current rates without any appreciable lithium anode degradation. Battery tests carried out at 80 &deg;C have shown excellent cycling performance and capacity retention, even at high rates, which are never tackled by ionic liquid-free polymer electrolytes. No dendrite growth onto the lithium metal anode was observed.

A review of NMR methods used in the study of the structure and dynamics of ionic liquids

Recently, NMR spectroscopy has been emerging out as a powerful tool to study the structure and dynamics of ionic liquids (ILs) and ILs-Li⁺ salt mixtures. This mini-review primarily focuses on the applications of various NMR spectroscopic techniques such as self-diffusion measurements, NMR relaxometry, two-dimensional NMR, and other novel NMR approaches to study the structure and dynamics of ILs and its mixtures with lithium salts. Copyright © 2017 John Wiley & Sons, Ltd.

Revealing the formation and electrochemical properties of bis(trifluoromethanesulfonyl)imide intercalated graphite with first-principles calculations

The formation energies of TFSI–C_n GICs, electrode voltages, and activation barriers for TFSI migration are obtained with first-principles calculations.

Solvation of Al3+ cations in bulk and confined protic ionic liquids: a computational study

Nanostructured solvation of Al³⁺ in an EAN ionic liquid, forming octahedral complexes with nitrate anions.

Molecular dynamics simulations of the structure of mixtures of protic ionic liquids and monovalent and divalent salts at the electrochemical interface

Hydrogen bonded protic ionic liquids improve the transport of electrochemically relevant cations to charged walls relative to aprotic ones.

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 N-butyl-N-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 dynamics simulation of the structure and interfacial free energy barriers of mixtures of ionic liquids and divalent salts near a graphene wall

A molecular dynamics study of graphene-confined mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]) with Mg[BF₄]₂ is reported.

The importance of transport property studies for battery electrolytes: revisiting the transport properties of lithium–N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide mixtures

All three ion–ion interactions contribute to transport properties in {Li[FSI]–[Pyr₁₃][FSI]} mixtures. Tracer diffusion coefficients of LI⁺ in [Pyr₁₃][FSI] are predicted.

Facilitated Ion Transport in Smectic Ordered Ionic Liquid Crystals

A novel ionic mixture of an imidazolium-based room-temperature ionic liquid containing ethylene-oxide-functionalized phosphite anions is fabricated, which, when doped with lithium salt, self-assembles into a smectic-ordered ionic liquid crystal through Coulombic interactions between the ion species. Interestingly, the smectic order in the ionic-liquid-crystal ionogel facilitates ionic transport.

NMR spectroscopy in ionic liquds

Today, NMR spectroscopy is the most important analytical tool for synthetically working chemists. This review describes the development of NMR spectroscopic methods for use in ionic liquid media and the state-of-the art in terms of routine analytics as well as modern advanced techniques.

The influence of cations on lithium ion coordination and transport in ionic liquid electrolytes: a MD simulation study

The dynamical and structural properties in two ionic liquid electrolytes (ILEs) based on [emim][TFSI] and [pyr₁₃][TFSI] were compared as a function of LiTFSI salt concentrations using atomistic molecular dynamics (MD) simulations.

Ab Initio Simulations and Electronic Structure of Lithium-Doped Ionic Liquids: Structure, Transport, and Electrochemical Stability

Density functional theory (DFT), density functional theory molecular dynamics (DFT-MD), and classical molecular dynamics using polarizable force fields (PFF-MD) are employed to evaluate the influence of Li(+) on the structure, transport, and electrochemical stability of three potential ionic liquid electrolytes: N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]). We characterize the Li(+) solvation shell through DFT computations of [Li(Anion)n]((n-1)-) clusters, DFT-MD simulations of isolated Li(+) in small ionic liquid systems, and PFF-MD simulations with high Li-doping levels in large ionic liquid systems. At low levels of Li-salt doping, highly stable solvation shells having two to three anions are seen in both [pyr14][TFSI] and [pyr13][FSI], whereas solvation shells with four anions dominate in [EMIM][BF4]. At higher levels of doping, we find the formation of complex Li-network structures that increase the frequency of four anion-coordinated solvation shells. A comparison of computational and experimental Raman spectra for a wide range of [Li(Anion)n]((n-1)-) clusters shows that our proposed structures are consistent with experiment. We then compute the ion diffusion coefficients and find measures from small-cell DFT-MD simulations to be the correct order of magnitude, but influenced by small system size and short simulation length. Correcting for these errors with complementary PFF-MD simulations, we find DFT-MD measures to be in close agreement with experiment. Finally, we compute electrochemical windows from DFT computations on isolated ions, interacting cation/anion pairs, and liquid-phase systems with Li-doping. For the molecular-level computations, we generally find the difference between ionization energy and electron affinity from isolated ions and interacting cation/anion pairs to provide upper and lower bounds, respectively, to experiment. In the liquid phase, we find the difference between the lowest unoccupied and highest occupied electronic levels in pure and hybrid functionals to provide lower and upper bounds, respectively, to experiment. Li-doping in the liquid-phase systems results in electrochemical windows little changed from the neat systems.

Safer Electrolytes for Lithium-Ion Batteries: State of the Art and Perspectives

Lithium-ion batteries are becoming increasingly important for electrifying the modern transportation system and, thus, hold the promise to enable sustainable mobility in the future. However, their large-scale application is hindered by severe safety concerns when the cells are exposed to mechanical, thermal, or electrical abuse conditions. These safety issues are intrinsically related to their superior energy density, combined with the (present) utilization of highly volatile and flammable organic-solvent-based electrolytes. Herein, state-of-the-art electrolyte systems and potential alternatives are briefly surveyed, with a particular focus on their (inherent) safety characteristics. The challenges, which so far prevent the widespread replacement of organic carbonate-based electrolytes with LiPF6 as the conducting salt, are also reviewed herein. Starting from rather "facile" electrolyte modifications by (partially) replacing the organic solvent or lithium salt and/or the addition of functional electrolyte additives, conceptually new electrolyte systems, including ionic liquids, solvent-free, and/or gelled polymer-based electrolytes, as well as solid-state electrolytes, are also considered. Indeed, the opportunities for designing new electrolytes appear to be almost infinite, which certainly complicates strict classification of such systems and a fundamental understanding of their properties. Nevertheless, these innumerable opportunities also provide a great chance of developing highly functionalized, new electrolyte systems, which may overcome the afore-mentioned safety concerns, while also offering enhanced mechanical, thermal, physicochemical, and electrochemical performance.