Uncovering the binding nature of thiocyanate in contact ion pairs with lithium ions

Ion pair formation is a fundamental molecular process that occurs in a wide variety of systems, including electrolytes, biological systems, and materials. In solution, the thiocyanate (SCN-) anion interacts with cations to form contact ion pairs (CIPs). Due to its ambidentate nature, thiocyanate can bind through either its sulfur or nitrogen atoms, depending on the solvent. This study focuses on the binding nature of thiocyanate with lithium ions as a function of the solvents using FTIR, 2D infrared spectroscopy (2DIR) spectroscopies, and theoretical calculations. The study reveals that the SCN- binding mode (S or N end) in CIPs can be identified through 2DIR spectroscopy but not by linear IR spectroscopy. Linear IR spectroscopy shows that the CN stretch frequencies are too close to one another to separate N- and S-bound CIPs. Moreover, the IR spectrum shows that the S-C stretch presents different frequencies for the salt in different solvents, but it is related to the anion speciation rather than to its binding mode. A similar trend is observed for the anion bend. 2DIR spectra show different dynamics for N-bound and S-bound thiocyanate. In particular, the frequency-frequency correlation function (FFCF) dynamics extracted from the 2DIR spectra have a single picosecond exponential decay for N-bound thiocyanate and a biexponential decay for S-bound thiocyanate, consistent with the binding mode of the anion. Finally, it is also observed that the binding mode also affects the line shape parameters, probably due to the different molecular mechanisms of the FFCF for N- and S-bound CIPs.

Transitioning from Regular Electrolytes to Solvate Ionic Liquids to High-Concentration Electrolytes: Changes in Transport Properties and Ionic Speciation

Glyme-based lithium-ion electrolytes have received considerable attention from the scientific community due to their improved safety, as well as electrochemical and thermal stability over carbonate-based electrolytes. However, these electrolytes suffer from major drawbacks such as high viscosities. To overcome the challenges that hinder their full potential, the molecular description of glyme-based lithium electrolytes in the high-concentration regime, particularly in the solvate ionic liquid (SIL) and high-concentration electrolyte (HCE) regimes, is needed. In this study, model glyme-based electrolytes based on a lithium thiocyanate and either tetraglyme (G4) or a mixture of monoglyme (G1) and diglyme (G2) were investigated as a function of the solvent-to-lithium ratio using linear and nonlinear IR spectroscopies, in combination with ab initio computations as well as electrochemical methods . The transport properties reveal enhanced ionicities in the HCE and SIL regimes ([O]/[Li] ≤ 5) compared to the regular electrolytes (REs, with [O]/[Li] > 5) in both pure (G4) and mixed (G1:G2) glymes. IR and ab initio computations relate these larger ionicities to the higher concentration of charged aggregates in the HCE and SIL electrolytes ([O]/[Li] ≤ 5). Moreover, it was observed that the use of mixed glymes appears to have a minimal effect on the transport properties of REs but exhibits deleterious effects on SILs. Overall, the results provide a molecular framework for describing the local structure of lithium glyme-based electrolytes and demonstrate the key role that the nature of glyme solvation plays in the molecular structure and consequently the macroscopic properties of the Li-glyme SILs, HCEs, and REs.

Probing the Electrode-Electrolyte Interface of Sodium/Glyme-Based Battery Electrolytes

Sodium-ion batteries (NIBs) are promising systems for large-scale energy storage solutions; yet, further enhancements are required for their commercial viability. Improving the electrochemical performance of NIBs goes beyond the chemical description of the electrolyte and electrode materials as it requires a comprehensive understanding of the underlying mechanisms that govern the interface between electrodes and electrolytes. In particular, the decomposition reactions occurring at these interfaces lead to the formation of surface films. Previous work has revealed that the solvation structure of cations in the electrolyte has a significant influence on the formation and properties of these surface films. Here, an experimentally validated molecular dynamics study is performed on a 1 M NaTFSI salt in glymes of different lengths placed between two graphite electrodes having a constant bias potential. The focus of this study is on describing the solvation environment around the sodium ions at the electrode-electrolyte interface as a function of glyme chain length and applied potential. The results of the study show that the diglyme/TFSI system presents features at the interface that significantly differ from those of the triglyme/TFSI and tetraglyme/TFSI systems. These computational predictions are successfully corroborated by the experimentally measured capacitance of these systems. In addition, the dominant solvation structures at the interface explain the electrochemical stability of the system as they are consistent with cyclic voltammetry characterization.

Lithium ion Speciation in Cyclic Solvents: Impact of Anion Charge Delocalization and Solvent Polarizability

The increasing demand for lithium batteries has triggered the search for safer and more efficient electrolytes. Insights into the atomistic description of electrolytes are critical for relating microscopic and macroscopic (physicochemical) properties. Previous studies have shown that the type of lithium salt and solvent used in the electrolyte influences its performance by dictating the speciation of the ionic components in the system. Here, we investigate the molecular origins of ion association in lithium-based electrolytes as a function of anion charge delocalization and solvent chemical identity. To this end, a family of cyano-based lithium salts in organic solvents, having a cyclic structure and containing carbonyl groups, was investigated using a combination of linear infrared spectroscopy and ab initio computations. Our results show that the formation of contact-ion pairs (CIPs) is more favorable in organic solvents containing either ester or carbonate groups and in lithium salts with an anion having low charge delocalization than in an amide/urea solvent and an anion with large charge delocalization. Ab initio computations attribute the degree of CIP formation to the energetics of the process, which is largely influenced by the chemical nature of the lithium ion solvation shell. At the molecular level, atomic charge analysis reveals that CIP formation is directly related to the ability of the solvent molecule to rearrange its electronic density upon coordination to the lithium ion. Overall, these findings emphasize the importance of local interactions in determining the nature of ion-molecule interactions and provide a molecular framework for explaining lithium ion speciation in the design of new electrolytes.

Relation between microscopic structure and macroscopic properties in polyacrylonitrile-based lithium-ion polymer gel electrolytes

Polymer gel electrolytes (PGE) have seen a renewed interest in their development because they have high ionic conductivities but low electrochemical degradation and flammability. PGEs are formed by mixing a liquid lithium-ion electrolyte with a polymer at a sufficiently large concentration to form a gel. PGEs have been extensively studied, but the direct connection between their microscopic structure and macroscopic properties remains controversial. For example, it is still unknown whether the polymer in the PGE acts as an inert, stabilizing scaffold for the electrolyte or it interacts with the ionic components. Here, a PGE composed of a prototypical lithium-carbonate electrolyte and polyacrylonitrile (PAN) is pursued at both microscopic and macroscopic levels. Specifically, this study focused on describing the microscopic and macroscopic changes in the PGE at different polymer concentrations. The results indicated that the polymer-ion and polymer-polymer interactions are strongly dependent on the concentration of the polymer and the lithium salt. In particular, the polymer interacts with itself at very high PAN concentrations (10% weight) resulting in a viscous gel. However, the conductivity and dynamics of the electrolyte liquid components are significantly less affected by the addition of the polymer. The observations are explained in terms of the PGE structure, which transitions from a polymer solution to a gel, containing a polymer matrix and disperse electrolyte, at low and high PAN concentrations, respectively. The results highlight the critical role that the polymer concentration plays in determining both the macroscopic properties of the system and the molecular structure of the PGE.

Sodium β-Diketonate Glyme Adducts as Precursors for Fluoride Phases: Synthesis, Characterization and Functional Validation

Very few sodium complexes are available as precursors for the syntheses of sodium-based nanostructured materials. Herein, the diglyme, triglyme, and tetraglyme (CH₃O(CH₂CH₂O)_nCH₃, n = 2–4) adducts of sodium hexafluoroacetylacetonate were synthesized in a single-step reaction and characterized by IR spectroscopy, ¹H, and ¹³C NMR. Single-crystal X-ray diffraction studies provide evidence of the formation of the ionic oligomeric structure [Na₄(hfa)₆]²⁻•2[Na(diglyme₂]⁺ when the diglyme is coordinated, while a mononuclear seven-coordinated complex Na(hfa)•tetraglyme is formed with the tetraglyme. Reaction with the monoglyme (CH₃OCH₂CH₂OCH₃) does not occur, and the unadducted polymeric structure [Na(hfa)]_n forms, while the triglyme gives rise to a liquid adduct, Na(hfa)•triglyme•H₂O. Thermal analysis data reveal great potentialities for their applications as precursors in metalorganic chemical vapor deposition (MOCVD) and sol-gel processes. As a proof-of-concept, the Na(hfa)•tetraglyme adduct was successfully applied to both the low-pressure MOCVD and the sol-gel/spin-coating synthesis of NaF films.

Ether-based electrolytes for sodium ion batteries

Sodium-ion batteries (SIBs) are considered to be strong candidates for large-scale energy storage with the benefits of cost-effectiveness and sodium abundance. Reliable electrolytes, as ionic conductors that regulate the electrochemical reaction behavior and the nature of the interface and electrode, are indispensable in the development of advanced SIBs with high Coulombic efficiency, stable cycling performance and high rate capability. Conventional carbonate-based electrolytes encounter numerous obstacles for their wide application in SIBs due to the formation of a dissolvable, continuous-thickening solid electrolyte interface (SEI) layer and inferior stability with electrodes. Comparatively, ether-based electrolytes (EBEs) are emerging in the secondary battery field with fascinating properties to improve the performance of batteries, especially SIBs. Their stable solvation structure enables highly reversible solvent-co-intercalation reactions and the formation of a thin and stable SEI. However, although EBEs can provide more stable cycling and rapid sodiation kinetics in electrodes, benefitting from their favorable electrolyte/electrode interactions such as chemical compatibility and good wettability, their special chemistry is still being investigated and puzzling. In this review, we provide a thorough and comprehensive overview on the developmental history, fundamental characteristics, superiorities and mechanisms of EBEs, together with their advances in other battery systems. Notably, the relation among electrolyte science, interfacial chemistry and electrochemical performance is highlighted, which is of great significance for the in-depth understanding of battery chemistry. Finally, future perspectives and potential directions are proposed to navigate the design and optimization of electrolytes and electrolyte/electrode interfaces for advanced batteries.

Structural and dynamical changes observed when transitioning from an ionic liquid to a deep eutectic solvent

The microscopic molecular structure and dynamics of a new deep eutectic solvent (DES) composed of an ionic liquid (1-hexyl-3-methylimidazolium chloride) and an amide (trifluoroacetamide) at various molar ratios were investigated using linear and non-linear infrared spectroscopy with a vibrational probe. The use of the ionic liquid allows us to investigate the changes that the system undergoes with the addition of the amide or, equivalently, the changes from an ionic liquid to a DES. Our studies revealed that the vibrational probe in the DES senses a very similar local environment irrespective of the cation chemical structure. In addition, the amide also appears to perceive the same molecular environment. The concentration dependence studies also showed that the amide changes from being isolated from other amides in the ionic liquid environment to an environment where the amide-amide interactions are favored. In the case of the vibrational probe, the addition of the amide produced significant changes in the slow dynamics associated with the making and breaking of the ionic cages but did not affect the rattling-in-cage motions perceived by it. Furthermore, the concentration dependence of slow dynamics showed two regimes which are linked to the changes in the overall structure of the solution. These observations are interpreted in the context of a nanoscopic heterogeneous environment in the DES which, according to the observed dynamical regimes, appears at very large concentrations of the amide (molar ratio of greater than 1:1) since for lower amide molar ratios, the amide appears to be not segregated from the ionic liquid. This proposed molecular picture is supported by small angle x-ray scattering experiments.

Elucidating the mechanism behind the infrared spectral features and dynamics observed in the carbonyl stretch region of organic carbonates interacting with lithium ions

Ultrafast infrared spectroscopy has become a very important tool for studying the structure and ultrafast dynamics in solution. In particular, it has been recently applied to investigate the molecular interactions and motions of lithium salts in organic carbonates. However, there has been a discrepancy in the molecular interpretation of the spectral features and dynamics derived from these spectroscopies. Hence, the mechanism behind spectral features appearing in the carbonyl stretching region was further investigated using linear and nonlinear spectroscopic tools and the co-solvent dilution strategy. Lithium perchlorate in a binary mixture of dimethyl carbonate (DMC) and tetrahydrofuran was used as part of the dilution strategy to identify the changes of the spectral features with the number of carbonates in the first solvation shell since both solvents have similar interaction energetics with the lithium ion. Experiments showed that more than one carbonate is always participating in the lithium ion solvation structures, even at the low concentration of DMC. Moreover, temperature-dependent study revealed that the exchange of the solvent molecules coordinating the lithium ion is not thermally accessible at room temperature. Furthermore, time-resolved IR experiments confirmed the presence of vibrationally coupled carbonyl stretches among coordinated DMC molecules and demonstrated that this process is significantly altered by limiting the number of carbonate molecules in the lithium ion solvation shell. Overall, the presented experimental findings strongly support the vibrational energy transfer as the mechanism behind the off-diagonal features appearing on the 2DIR spectra of solutions of lithium salt in organic carbonates.

Assessing the Location of Ionic and Molecular Solutes in a Molecularly Heterogeneous and Nonionic Deep Eutectic Solvent

Deep eutectic solvents (DES) are emerging sustainable designer solvents viewed as greener and better alternatives to ionic liquids. Nonionic DESs possess unique properties such as viscosity and hydrophobicity that make them desirable in microextraction applications such as oil-spill remediation. This work builds upon a nonionic DES, NMA–LA DES, previously designed by our group. The NMA–LA DES presents a rich nanoscopic morphology that could be used to allocate solutes of different polarities. In this work, the possibility of solvating different solutes within the nanoscopically heterogeneous molecular structure of the NMA–LA DES is investigated using ionic and molecular solutes. In particular, the localized vibrational transitions in these solutes are used as reporters of the DES molecular structure via vibrational spectroscopy. The FTIR and 2DIR data suggest that the ionic solute is confined in a polar and continuous domain formed by NMA, clearly sensing the direct effect of the change in NMA concentration. In the case of the molecular nonionic and polar solute, the data indicates that the solute resides in the interface between the polar and nonpolar domains. Finally, the results for the nonpolar and nonionic solute (W(CO)₆) are unexpected and less conclusive. Contrary to its polarity, the data suggest that the W(CO)₆ resides within the NMA polar domain of the DES, probably by inducing a domain restructuring in the solvent. However, the data are not conclusive enough to discard the possibility that the restructuring comprises not only the polar domain but also the interface. Overall, our results demonstrate that the NMA–LA DES has nanoscopic domains with affinity to particular molecular properties, such as polarity. Thus, the presented results have a direct implication to separation science.

Molecular Structure, Chemical Exchange, and Conductivity Mechanism of High Concentration LiTFSI Electrolytes

High concentration lithium electrolytes have been found to be good candidates for high energy density and high voltage lithium batteries. Recent studies have shown that limiting the free solvent molecules in the electrolytes prevents the degradation of the battery electrodes. However, the molecular level knowledge of the structure and dynamics of such an electrolyte system is limited, especially for electrolytes based on typical organic carbonates. In this article, the interactions and motions involved in lithium bis(trifluoromethanesulfonyl)imide in carbonyl-containing solvents are investigated using linear and time-resolved vibrational spectroscopies and computational methods. Our results suggest that the overall structure and the speciation of the three high concentration electrolytes are similar. However, the cyclic carbonate-based electrolyte presents an additional interaction as a result of dimer formation. Time-resolved studies reveal similar and fast dynamics for the structural motions of solvent molecules in electrolytes composed of linear molecules, while the electrolyte made of cyclic solvent molecules shows slower structural changes as a result of the dimer formation. Additionally, a picosecond time scale process is observed and assigned to the coordination and decoordination of solvent molecules from a lithium-ion solvation shell. This process of solvent exchange is found to be directly correlated to the making and breaking of structures between the lithium-ion and the anion and, consequently, to the conduction mechanism. Overall, our data show that the molecular structure of the solvent does not significantly affect the speciation and distribution of the lithium-ion solvation shells. However, the presence of dimerization between solvent molecules of two neighboring lithium-ions appears to produce a microscopic ordering that it is manifested macroscopically in properties of the electrolyte, such as its viscosity.

State-Selective Polariton to Dark State Relaxation Dynamics

The modification of vibrational dynamics is essential for controlling chemical reactions and IR photonic applications. The hybridization between cavity modes and molecular vibrational modes provides a new way to control molecular dynamics. In this work, we study the dynamics of molecular vibrational polaritons in various solvent environments. We find the dynamics of the polariton system is strongly influenced by the nature of the solvents. While the relaxation from upper polariton (UP) to dark modes is always fast (<5 ps) regardless of the medium, lower polariton (LP) in low polarity solvents shows much slower transfer (10-30 ps) into dark modes, despite the fact that the LP lifetime remains within 5 ps. This result suggests that in the latter media, the energy pumped into the LP is first transferred into intermediate states which only subsequently decay into dark modes. In contrast, in solvent environments that strongly interact with the solute, the LP population relaxes into the dense dark state manifold within a much faster time scale. We propose the intermediate state to be the high-lying excited states of dark modes, which are effectively populated by LP via, e.g., ladder-climbing. Such population in the high-lying states can be retained for tens of picoseconds, which could be pertinent to recently observed cavity-modified chemistry.