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.
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.
Solutions of lithium bis(trifluoromethanesulfonyl)imide (LiNTf), in four different [NTf]-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 [NTf] 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 [NTf] 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.
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.
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