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.