Structural Investigations on Lithium-Doped Protic and Aprotic Ionic Liquids

P Ray and T Vogl and A Balducci and B Kirchner, JOURNAL OF PHYSICAL CHEMISTRY B, 121, 5279-5292 (2017).

DOI: 10.1021/acs.jpcb.7b02636

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.

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