Structural Investigations on Lithium-Doped Protic and Aprotic Ionic Liquids

Abstract

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.

Impact of Anion Asymmetry on Local Structure and Supercooling Behavior of Water-in-Salt Electrolytes

“…We chose this temperature, as nucleation itself is a rare event typically occurring on a time scale much longer than what can be reached in a direct simulation. , Panels c–e of Figure show radial distribution functions (RDFs) between the lithium cations and anions. The oxygen atoms of the anions are closest to the cations, which corresponds to coordination mostly taking place through these atoms, as is common for the studied imide anions. − A small peak is observed in the lithium–fluorine RDF of 35m LiFSI and in the Li–F 1 RDF of 35m LiFTFSI at a distance of approximately 2 Å, which is similar to the first peak in the lithium–oxygen RDF. In contrast, the Li–F CF3 RDF for the FTFSI solution clearly shows that the fluorine atoms of the CF 3 group are on average farther from the cation than the F 1 fluorine atom bound to sulfur.…”

Boosting Rechargeable Batteries R&D by Multiscale Modeling: Myth or Reality?

“…Therefore, any simulation performed requires longer simulation times to properly describe the dynamics. As a result, FPMD studies are usually coupled with classical MD, with the former determining the structural properties and validating the FF used in the classical MD, which is then used to study the dynamics of the system. − Overall, the FPMD simulations of these systems have confirmed previous spectroscopic analysis, , showing that addition of lithium salt to the IL cause the lithium cations to be trapped in cages of 2–3 TFSI anions, which limits the mobility and decreases the overall ion conductivity …”

Hopping in High Concentration Electrolytes - Long Time Bulk and Single-Particle Signatures, Free Energy Barriers, and Structural Insights

“…We find that two Li*-ions are predominantly bridged by a BF 4 anion (Figure 4b). 25,53,54 Environments containing three or more Li*-ions connected through such bridging ligands found at 5.76 M liquid were likened to a similar chain of Li-ions in the experimental crystal structure (Figure 4b,c) It is conceivable, that such bridging ligands are the ones that a Li*-ion retains as one of its nearest neighbors when it hops+diffuses to a previously occupied Li*-ion site that had this ligand in common in its cage of neighbors. These findings indicate that an anion-or solvent-bridged network underlies the bridgingligand-mediated transport of Li*-ions in HCEs.…”