Cation influence on heterocyclic ammonium ionic liquids: a molecular dynamics study.

Four different ionic liquids (ILs) consisting of the bis(trifluoromethanesulfonyl)imide ([NTf2]-) anion, with structurally similar systematically varying cations, are investigated herein through classical molecular dynamics. The following cations were examined: pyrrolidinium ([pyrHH]+), piperidinium ([pipHH]+), N-butyl-pyrrolidinium ([pyrH4]+) and N-butyl-N-methyl-pyrrolidinium ([pyr14]+). The focus herein is on understanding the effect of increased ring size and alkyl chain addition, resulting in three protic ILs and one aprotic IL ([pyr14][NTf2]), on the physicochemical properties of the liquids studied herein. Addition of alkyl groups to the cation appears to cause a distinct weakening of inter-ionic interactions and ordering in comparison to increasing the ring size. The influence of these structural changes, however, is clearer on the ordering of like ions than oppositely charged ions. The protic ILs exhibit important similarities in the spatial arrangement of ions on account of their strong and directed H-bonding interactions. The cation is seen to influence particular conformations of the anion which further explains the more selective ordering in the protic ILs. However, the aggregation of the butyl side chain is also seen to be an important structural determinant in [pyrH4][NTf2] and [pyr14][NTf2]. We analyze the formation of domains in order to quantitatively evaluate the microheterogeneity arising in these systems from the separation of phases according to polarities. Velocity autocorrelation functions are studied in order to characterize the stronger caging effect in the protic ILs and the weakening of the caging effect upon addition of alkyl groups to the cation, these are consistent with the coordination environment within the respective liquids. In conclusion, significant correlations between the structure and properties of these ILs are observed and quantified within this contribution.

Finding the best density functional approximation to describe interaction energies and structures of ionic liquids in molecular dynamics studies.

Ionic liquids raise interesting but complicated questions for theoretical investigations due to the fact that a number of different inter-molecular interactions, e.g., hydrogen bonding, long-range Coulomb interactions, and dispersion interactions, need to be described properly. Here, we present a detailed study on the ionic liquids ethylammonium nitrate and 1-ethyl-3-methylimidazolium acetate, in which we compare different dispersion corrected density functional approximations to accurate local coupled cluster data in static calculations on ionic liquid clusters. The efficient new composite method B97-3c is tested and has been implemented in CP2K for future studies. Furthermore, tight-binding based approaches which may be used in large scale simulations are assessed. Subsequently, ab initio as well as classical molecular dynamics simulations are conducted and structural analyses are presented in order to shed light on the different short- and long-range structural patterns depending on the method and the system size considered in the simulation. Our results indicate the presence of strong hydrogen bonds in ionic liquids as well as the aggregation of alkyl side chains due to dispersion interactions.

Water in Protic Ionic Liquid Electrolytes: From Solvent Separated Ion Pairs to Water Clusters.

The large electrochemical and cycling stability of "water-in-salt" systems have rendered promising prospective electrolytes for batteries. The impact of addition of water on the properties of ionic liquids has already been addressed in several publications. In this contribution, we focus on the changes in the state of water. Therefore, we investigated the protic ionic liquid N-butyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide with varying water content at different temperatures with the aid of molecular dynamics simulations. It is revealed that at very low concentrations, the water is well dispersed and best characterized as shared solvent molecules. At higher concentrations, the water forms larger aggregates and is increasingly approaching a bulk-like state. While the librational and rotational dynamics of the water molecules become faster with increasing concentration, the translational dynamics are found to become slower. Further, all dynamics are found to be faster if the temperature increases. The trends of these findings are well in line with the experimental measured conductivities.

Water in Protic Ionic Liquids: Properties and Use of a New Class of Electrolytes for Energy-Storage Devices.

In this work, the properties of "water-in-PIL" (PIL=protic ionic liquid) electrolytes are reported based on 1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PyrH4 TFSI). Taking advantage of experimental and theoretical investigations, it is shown that the amount of water inside the electrolyte has a dramatic effect on the viscosity, conductivity, density, cation-anion interplay, and electrochemical stability of PyrH4 TFSI. The impact of water on the properties of this ionic liquid also affects its use as an electrolyte for electrochemical double-layer capacitors (EDLCs). It is shown that the presence of water improves the transport properties of PyrH4 TFSI, with a beneficial effect on the capacitance retention of the devices in which these electrolytes are used. However, at the same time, water reduces the operative voltage of EDLCs containing this PIL as the electrolyte and, furthermore, it has a strong impact on the inactive components of these systems. To suppress this latter problem, and to realize EDLCs with high stability, the use of inactive components stable in aqueous environment appears necessary.

Molecular Dynamics Simulations of Lithium-Doped Ionic-Liquid Electrolytes.

Lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) doped ionic liquids (ILs) are investigated herein, as potential electrolytes for lithium-ion batteries, via scaled-charge molecular dynamics simulations. Four model ILs based on the [NTf2]- anion and heterocyclic ammonium cations were studied with varying concentrations, ranging from 0 to 1 M solutions, of the dissolved lithium salt. The pyrrolidinium ([pyrHH]+), piperidinium ([pipHH]+), N-butyl-pyrrolidinium ([pyrH4]+), and N-butyl- N-methyl-pyrrolidinium ([pyr14]+) cations were considered to evaluate the combined effects of increased ring size, as well as the introduction of apolar groups on the nitrogen atom of the cations, on the liquid structure properties of the electrolytes. Among the investigated ILs, [pyr14][NTf2] is the only aprotic IL allowing for a comparison of protic and aprotic ILs. The lithium coordination shell is seen to be quite different in the various IL-based systems; networks of lithium ions bridged by [NTf2]- ions have interesting consequences on the solvation shells and coordination numbers. Aggregate existence and velocity autocorrelation functions are finally evaluated in order to characterize the caging effect of [NTf2]- ions around lithium ions. In conclusion, we find that the lithium mobility and transport are directly proportional to the strength of the interionic interactions within the liquids, whereas the ease of solvation shows opposite trends.

Structural Investigations on Lithium-Doped Protic and Aprotic Ionic Liquids.

Solutions of lithium bis(trifluoromethanesulfonyl)imide (LiNTf2), in four different [NTf2]--based ionic liquids, are extensively investigated as potential electrolytes for lithium-ion batteries. Solvation of the [Li]+ ions in the ionic liquids and its impact on their physicochemical properties are studied herein with the aid of molecular dynamics simulations. The cationic components of the investigated liquids were systematically varied so as to individually evaluate effects of specific structural changes; increase in ring size, the addition of an alkyl chain and absence of an acidic proton, on the solvation and mobility of the [Li]+ cations. The studied cations also allow for a direct comparison between solutions of [Li]+ salt in protic and aprotic ionic liquids. Emphasis is laid on elucidating the interactions between the [Li]+ and [NTf2]- ions revealing slightly higher coordination numbers for the aprotic solvent, benchmarked against experimental measurements. The study suggests that the ionic liquids largely retain their structure upon salt addition, with interactions within the liquids only slightly perturbed. The rattling motion of the [Li]+ cations within cages formed by the surrounding [NTf2]- anions is examined by the analysis of [Li]+ autocorrelation functions. Overall, the solvation mechanism of [Li]+ salt, within the hydrogen-bonded network of the ionic liquids, is detailed from classical and ab initio molecular dynamics simulations.