Spectral deconvolution in electrophoretic NMR to investigate the migration of neutral molecules in electrolytes

Overdetermination method for accurate dynamic ion correlations in highly concentrated electrolytes

Highly concentrated battery electrolytes exhibit a low flammability as well as a high thermal and electrochemical stability, and they typically form stable solid electrolyte interphases in contact with electrode materials. The transport properties of these electrolytes in batteries are strongly influenced by correlated movements of the ions. In the case of a binary electrolyte, the transport properties can be described by three Onsager coefficients and a thermodynamic factor. In order to determine these four target quantities, at least four experimental quantities are needed. Overdetermination by measuring five or more experimental quantities is uncommon. Here we have combined electrochemical impedance spectroscopy, electrophoretic NMR measurements and concentration cell measurements for characterizing two highly concentrated sulfolane/LiFSI electrolytes. Two sets of four experimental quantities and one set of five experimental quantities were compared with regard to the uncertainties of the resulting four target quantities. We show that the methods employing only four experimental quantities either lead to large uncertainties of the Onsager coefficients or to large uncertainties of the thermodynamic factor, while only the overdetermination by five experimental quantities leads to acceptable uncertainties of all four target quantities. The results for the Onsager coefficients are discussed with regard to dynamic ion correlations and to transport limitations in battery cells.

Heterogeneous Li coordination in solvent-in-salt electrolytes enables high Li transference numbers

The transport properties and the underlying coordination structure of a ternary electrolyte consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,2-dimethoxyethane (DME), and 1,3-dioxolane (DOL) is studied over a wide concentration range, up to that of a Solvent-in-Salt (SiS) electrolyte. Among other advantages for next-generation battery applications, SiS electrolytes offer a high lithium transference number (tLi) of 0.73. We analyze the transport mechanism by electrophoretic NMR (eNMR), providing the mobilities (μi) of all species. Intriguingly, in the SiS region, the mobility of the neutral species DME exceeds the cation mobility (μDME > μLi), suggesting a heterogeneous transport mechanism, where the Li+ mobility is averaged over different species. Based on Raman spectroscopy, NMR spectroscopy and MD simulations, we derive a model for a concentration-dependent Li+ coordination environment with a heterogeneous Li+ coordination in the SiS region, where the 1st coordination shell either consists of TFSI- and DOL only, or of DME, TFSI-, and DOL. Lithium ions partially coordinated by DME migrate faster in an electric field, in contrast to lithium ions solely coordinated by anions and DOL molecules, explaining the peculiarity of the rapidly migrating neutral DME molecules. Further, DME is identified as an exclusively bidentate ligand, while TFSI- and DOL act as bridging ligands coordinating different Li+ ions. Thus, Li+ coordination heterogeneity is the basis for Li+ transport heterogeneity and for achieving very high Li+ transference numbers. In addition, an effective dynamic decoupling of Li+ and anions occurs with an Onsager coefficient σ+- ≈ 0. These results provide a deeper understanding of the very efficient lithium-ion transport in SiS electrolytes, with the potential to bring further improvements for battery applications.

Seeing the Unseen: Mg2+, Na+, and K+ Transference Numbers in Post-Li Battery Electrolytes by Electrophoretic Nuclear Magnetic Resonance

The growing demand for energy storage devices worldwide combined with limited resources for lithium attracts interest in other alkali or alkaline earth metals. In addition to conductivity, the cation transference number T+ is a decisive parameter to rank the electrolyte performance. However, the existing experimental methods for its determination suffer from various intrinsic problems. We demonstrate here a novel approach for T+ determination based on determining the total conductivity with impedance spectroscopy (IS) and the partial conductivity of the anion species, with the latter being obtained from the anion mobility by electrophoretic NMR. First, this eNMR/IS approach is validated by comparing T+ values from different methods in a Li-based solvate ionic liquid electrolyte. Then, it is applied to obtain T+ of cations with nuclei not detectable in NMR transport measurements, employing bis(trifluoromethanesulfonyl)imide (TFSI)-based metal salts. Solvate ionic liquids consisting of triethylene glycol dimethyl ether (G3) and Mg(TFSI)2 or NaTFSI yield values of TNa and TMg on the order of 0.4, similar to TLi. Furthermore, we apply the method to polymer electrolytes, again testing the concept with LiTFSI, and finally investigating NaTFSI, KTFSI, and Mg(TFSI)2 in poly(ethylene oxide). Values of TNa and TK are in the range of 0.14-0.2, similar to those of TLi, while Mg2+ shows a higher transference number (TMg = 0.3). The method is very versatile as it allows quantification of T+ for any type of cation, and moreover, it is applicable to highly concentrated electrolytes without suffering from assumptions about dissociation or from unknown interfacial resistances which impede electrochemical methods.

Evaluating Strategies to Enhance Li Transference in Salt-in-Ionic Liquid Electrolytes: Mixed Anions, Coordinating Cations, and High Salt Concentration

The increased safety of salt-in-ionic liquid electrolytes compared with established carbonate-based systems has promoted intense research in this field, but low conductivities, slow lithium transport, and unfavorable lithium anion correlations still prevent a mass market application. In particular, strong Li-anion correlations lead to dominant vehicular Li transport with the same drift direction for anions and lithium in the electric field. Here, three different strategies and their mutual interplay are evaluated, which could reduce Li-anion coordination, i.e., high salt concentration, a mixed-anion composition, as well as an ether functionalization of the organic cation. To this end, two series of highly concentrated IL-based electrolytes, based on either ethylmethylimidazolium (EMIM) or the ether-functionalized 1-methoxyethyl-1-methylpyrrolidinium (Pyr12O1) organic cation, and employing mixed bis(fluorosulfonyl)imide/bis(trifluoromethylsulfonyl)imide (FSI/TFSI) anions are investigated. Measurements of conductivities, diffusion coefficients, and electrophoretic mobilities reveal no beneficial effect due to the increased heterogeneity of the FSI/TFSI-based electrolyte matrix, generally showing improved transport properties with increasing FSI share. However, a combination of both the ether-functionalized cation and high FSI content is proven successful, as lithium mobilities are positive, and vehicular transport is overcome by structural Li transport. Our study demonstrates the decisive role of synergy of the different approaches: While the single effect of a high salt concentration, weakly lithium-coordinating anions, or organic cations with lithium-affine functional groups is too weak to prevent vehicular transport, their joint effect can overcome vehicular Li transport, leading to improved Li conduction in ionic liquids.

A volume-based description of transport in incompressible liquid electrolytes and its application to ionic liquids

Transference numbers play an important role in understanding the dynamics of electrolytes and assessing their performance in batteries. Unfortunately, these transport parameters are difficult to measure in highly concentrated liquid electrolytes such as ionic liquids. Also, the interpretation of their sign and magnitude has provoked an ongoing debate in the literature further complicated by the use of different languages. In this work, we highlight the role of the reference frame for the interpretation of transport parameters using our novel thermodynamically consistent theory for highly correlated electrolytes. We argue that local volume conservation is a key principle in incompressible liquid electrolytes and use the volume-based drift velocity as a reference. We apply our general framework to electrophoretic NMR experiments. For ionic liquid based electrolytes, we find that the results of the eNMR measurements can be best described using this volume-based description. This highlights the limitations of the widely used center-of-mass reference frame which for example forms the basis for molecular dynamics simulations - a standard tool for the theoretical calculation of transport parameters. It shows that the assumption of local momentum conservation is incorrect in those systems on the macroscopic scale.

Ion correlation and negative lithium transference in polyelectrolyte solutions

Polyelectrolyte solutions (PESs) recently have been proposed as high conductivity, high lithium transference number (t₊) electrolytes where the majority of the ionic current is carried by the electrochemically active Li-ion. While PESs are intuitively appealing because anchoring the anion to a polymer backbone selectively slows down anionic motion and therefore increases t₊, increasing the anion charge will act as a competing effect, decreasing t₊. In this work we directly measure ion mobilities in a model non-aqueous polyelectrolyte solution using electrophoretic Nuclear Magnetic Resonance Spectroscopy (eNMR) to probe these competing effects. While previous studies that rely on ideal assumptions predict that PESs will have higher t₊ than monomeric solutions, we demonstrate that below the entanglement limit, both conductivity and t₊ decrease with increasing degree of polymerization. For polyanions of 10 or more repeat units, at 0.5 m Li⁺ we directly observe Li⁺ move in the "wrong direction" in an electric field, evidence of a negative transference number due to correlated motion through ion clustering. This is the first experimental observation of negative transference in a non-aqueous polyelectrolyte solution. We also demonstrate that t₊ increases with increasing Li⁺ concentration. Using Onsager transport coefficients calculated from experimental data, and insights from previously published molecular dynamics studies we demonstrate that despite selectively slowing anion motion using polyanions, distinct anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates reduce the t₊ in non-entangled PESs. This leads us to conclude that short-chained polyelectrolyte solutions are not viable high transference number electrolytes. These results emphasize the importance of understanding the effects of ion-correlations when designing new concentrated electrolytes for improved battery performance.

Relevance of the Cation in Anion Binding of a Triazole Host: An Analysis by Electrophoretic Nuclear Magnetic Resonance

Triazole hosts allow cooperative binding of anions via hydrogen bonds, which makes them versatile systems for application in anion binding catalysis to be performed in organic solvents. The anion binding behavior of a tetratriazole host is systematically studied by employing a variety of salts, including chloride, acetate, and benzoate, as well as different cations. Classical nuclear magnetic resonance (1H NMR) titrations demonstrate a large influence of cation structures on the anion binding constant, which is attributed to poor dissociation of most salts in organic solvents and corrupts the results of classical titration techniques. We propose an approach employing electrophoretic NMR (eNMR), yielding drift velocities of each species in an electric field and thus allowing a distinction between charged and uncharged species. After the determination of the dissociation constants KD for the salts, electrophoretic mobilities are measured for all species in the host-salt system and are analyzed in a model which treats anion binding as a consecutive reaction to salt dissociation, yielding a corrected anion binding constant KA. Interestingly, dependence of KA on salt concentration occurs, which is attributed to cation aggregation with the anion-host complex. Finally, by the extrapolation to zero salt concentration, the true anion-host binding constant is obtained. Thus, the approach by eNMR allows a fully quantitative analysis of two factors that might impair classical anion binding studies, namely, an incomplete salt dissociation as well as the occurrence of larger aggregate species.

A novel approach to study catalytic reactions via electrophoretic NMR on the example of Pd/NHC-catalyzed Mizoroki-Heck cross-coupling reaction

Investigation of catalytic reactions using nuclear magnetic resonance (NMR) is a crucial task, which is often challenging to perform due to rather complex transformations at the metal center. In this work, it was shown that electrophoretic NMR can be a suitable method for studying catalytic reactions and for observing the changes in the catalyst nature. As an important example involving palladium catalysts with N-heterocyclic carbine ligands (NHCs), the breakage of the Pd-NHC bond can occur during the catalytic process. Electrophoretic NMR allows the distinction of compounds in the spectra depending on the charge, thus bringing new opportunities to mechanistic studies. Here, we present independent evidence of R-NHC product formation in the Pd-catalyzed Mizoroki-Heck reaction-the key process for catalyst change from the molecular to nano-scale type.

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.

Recent advances in NMR spectroscopy of ionic liquids

This review presents recent developments in the application of NMR spectroscopic techniques in the study of ionic liquids. NMR has been the primary tool not only for the structural characterization of ionic liquids, but also for the study of dynamics. The presence of a host of NMR active nuclei in ionic liquids permits widespread use of multinuclear NMR experiments. Chemical shifts and multinuclear coupling constants are used routinely for the structure elucidation of ionic liquids and of products formed by their covalent interactions with other materials. Also, the availability of a multitude of NMR techniques has facilitated the study of dynamical processes in them. These include the use of NOESY to study inter-ionic interactions, pulsed-field gradient techniques for probing transport properties, and relaxation measurements to elucidate rotational dynamics. This review will focus on the application of each of these techniques to investigate ionic liquids.

An R-Package for the Deconvolution and Integration of 1D NMR Data: MetaboDecon1D

NMR spectroscopy is a widely used method for the detection and quantification of metabolites in complex biological fluids. However, the large number of metabolites present in a biological sample such as urine or plasma leads to considerable signal overlap in one-dimensional NMR spectra, which in turn hampers both signal identification and quantification. As a consequence, we have developed an easy to use R-package that allows the fully automated deconvolution of overlapping signals in the underlying Lorentzian line-shapes. We show that precise integral values are computed, which are required to obtain both relative and absolute quantitative information. The algorithm is independent of any knowledge of the corresponding metabolites, which also allows the quantitative description of features of yet unknown identity.

Quantification of cation-cation, anion-anion and cation-anion correlations in Li salt/glyme mixtures by combining very-low-frequency impedance spectroscopy with diffusion and electrophoretic NMR

Directional correlations between the movements of cations and anions exert a strong influence on the charge and mass transport properties of concentrated battery electrolytes. Here, we combine, for the first time, very-low-frequency impedance spectroscopy on symmetrical Li|electrolyte|Li cells with diffusion and electrophoretic NMR in order to quantify cation-cation, anion-anion and cation-anion correlations in Li salt/tetraglyme (G4) mixtures with Li salt to G4 ratios between 1 : 1 and 1 : 2. We find that all correlations are negative, with like-ion anticorrelations (cation-cation and anion-anion) being generally stronger than cation-anion anticorrelations. In addition, we observe that like-ion anticorrelations are stronger for the heavier type of ion and that all anticorrelations become weaker with decreasing Li salt to G4 ratio. These findings are in contrast to theories considering exclusively anion-cation correlations in form of ion pairs, as the latter imply positive cation-anion correlations. We analyze in detail the influence of anticorrelations on Li+ transference numbers and on the Haven ratio. In order to rationalize our results, we derive linear response theory expressions for all ion correlations. These expressions show that the Li+ ion transport under anion-blocking conditions in a battery is governed by equilibrium center-of-mass fluctuations in the electrolytes. This suggests that in future electrolyte theories and computer simulations, more attention should be paid to equilibrium center-of-mass fluctuations.

New Interpretation of X-ray Photoelectron Spectroscopy of Imidazolium Ionic Liquid Electrolytes Based on Ionic Transport Analyses

We reported a new perspective on the correlation between the electronic structure of an ionic liquid (IL)-based electrolyte probed by X-ray photoelectron spectroscopy and the transport properties analyzed by impedance spectroscopy. We highlighted the core level chemical shifts of 1-hexyl-3-methylimidazolium (bis(trifluoromethanesulfonyl)imide) (C₁C₆ImTFSI), 1-hexyl-3-methylimidazolium bis(fluorosulfonyl)imide (C₁C₆ImFSI), and 1-hexyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (C₁C₁C₆ImTFSI) laden with LiTFSI salt and vinylene carbonate (VC) or fluoroethylene carbonate (FEC) with regard to the transport properties of cations and anions. We pointed out based on detailed binding energy shift analyses a clear effect of the anion on the local organization of Li⁺ ions. The significant peak shift in the case of C₁C₆ImTFSI laden with LiTFSI corroborates the formation of [Li(TFSI)₂]^- complexes. On the contrary, the lower amplitude of the binding energy shift of C₁C₆ImFSI for both anion- and cation-related peaks indicates that the electronic distribution around the cation and the anion is not affected when the LiTFSI salt is added, which plays a strong role in the ion dynamics (lower viscosity) of the electrolyte. The X-ray photoelectron spectroscopy (XPS) result supports the preponderant role of imidazolium ionic liquid based on FSI anion to form an electrolyte less prone to form ionic complexes. The methylation of the imidazolium cation contributes to the reduction of the interaction between the C₁C₁C₆Im cation and TFSI anion, while additives VC and FEC contribute to the change of the alkyl configuration in C₁C₆Im cation, leading to the modification of the macroscopic properties of the ILs.

Improved accuracy and precision in electrophoretic NMR experiments. Current control and sample cell design

Electrophoretic NMR has the capacity to provide unique physico-chemical information but is limited by a variety of experimental artifacts, such as thermal convection and electrolytic products in the sample. Here we present some simple modifications to the experimental hardware and protocol that, in a significant number of cases, can much improve experimental accuracy and precision. We show that one can strongly reduce artifacts in a symmetric sample cell with an appropriate feeding of current and with a porous plug suitably inserted. This latter feature requires that the electric field pulses across the sensitive volume are implemented as current-controlled pulses applied to the sample. Measurements with current-controlled pulses have the additional advantage of not requiring calibration with samples of known electrophoretic mobility.

Interaction of Low Molecular Weight Poly(diallyldimethylammonium chloride) and Sodium Dodecyl Sulfate in Low Surfactant-Polyelectrolyte Ratio, Salt-Free Solutions

Coacervation is widely used in formulations to induce a beneficial character to the formulation, but nonequilibrium effects are often manifest. Electrophoretic NMR (eNMR), pulsed-gradient spin-echo NMR (PGSE-NMR), and small-angle neutron scattering (SANS) have been used to quantify the interaction between low molecular cationic poly(diallyldimethylammonium chloride) (PDADMAC) and the anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solution as a model for the precursor state to such nonequilibrium processes. The NMR data show that, within the low surfactant concentration one-phase region, an increasing surfactant concentration leads to a reduction in the charge on the polymer and a collapse of its solution conformation, attaining minimum values coincident with the macroscopic phase separation boundary. Interpretation of the scattering data reveals how the rodlike polymer changes over the same surfactant concentration window, with no discernible fingerprint of micellar type aggregates, but rather with the emergence of disklike and lamellar structures. At the highest surfactant concentration, the emergence of a weak Bragg peak in both the polymer and surfactant scattering suggests these precursor disk and lamellar structures evolve into paracrystalline stacks which ultimately phase separate. Addition of the nonionic surfactant hexa(ethylene glycol) monododecyl ether (C₁₂E₆) to the system seems to have little effect on the PDADMAC/SDS interaction as determined by NMR, merely displacing the observed behavior to lower SDS concentrations, commensurate with the total SDS present in the system. In other words, PDADMAC causes the disruption of the mixed SDS/C₁₂E₆ micelle, leading to SDS-rich PDADAMC/surfactant complexes coexisting with C₁₂E₆-rich micelles in solution.

Solvate Cation Migration and Ion Correlations in Solvate Ionic Liquids

Lithium salt-glyme mixtures are interesting candidates as electrolytes for battery applications. Depending on the type of glyme or anion and the salt concentration, they either show ionic liquid-like behavior with stable lithium-glyme complex cations or concentrated salt solution-like behavior. Here, we apply electrophoretic NMR (eNMR) to elucidate transport mechanisms by observing the migration of the molecular species in an electric field. We investigate two solvate ionic liquids, i.e., lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and lithium tetrafluoroborate (LiBF₄), in tetraglyme (G4) at different glyme-salt molar ratios X. A field-induced migration of neutral glyme molecules is directly observed, which is due to stable solvate-Li complex formation. Transference numbers, effective charges, and ionicities are derived from electrophoretic mobilities and self-diffusion coefficients, respectively, for the nuclei ¹H, ⁷Li, and ¹⁹F. The effective charges are the highest at the equimolar mixture, X = 1, they differ strongly for lithium and anion, and they show large differences between both systems. These findings are qualitatively interpreted in a speciation model, suggesting anionic clusters and solvate cations as the species dominating charge transport. The resulting effective charges can only be explained taking into account ion-ion anticorrelations in the framework of the Onsager formalism, where anticorrelations between the solvate cation and the anionic complexes arise due to momentum conservation. The contributions to the anticorrelation are most dominant at high salt concentrations and in the system with the LiBF₄^- anion due to its lower mass and ability to form larger asymmetric clusters with Li⁺. Thus, in either system, also the lithium transference number is influenced to a different extent by ion-ion anticorrelations.