Structure and molecular interactions in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ionic liquid/carbonate co-solvents -– combined DFT and molecular dynamics study.

Both density functional theory (DFT) and molecular dynamics (MD) based on classical force field were used to provide both structural and electronic insight into the multifold interactions occurring in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid in the presence of ethylene carbonate and dimethyl carbonate co-solvent mixtures which are currently being targeted for applications in next-generation Li-ion battery electrolytes. In order to give a visual understanding of the molecular interactions, the structures of cations, anions, and cation - anion ion pairs were systematically studied using DFT calculations. The nature of hydrogen bond interactions in a series of ion pair conformers have been thoroughly discussed by analyzing the interaction energies, stabilization energies and natural orbital analysis of the ion pair conformers. Multiple but weak C ̶ H---O/N hydrogen bonds and anion donor π*C–N interactions have been observed. Charge transfer occurs mainly from the lone pairs of oxygen and nitrogen atom to the σ-type anti-bonding orbital of the C–H bonds and π-type anti-bonding orbitals of N ̶ C bonds. According to the MD study, the addition of carbonate co-solvents into the pure ionic liquid creates a more structured system than the pure ionic liquid. The coordination of the O/N atoms of the bis(trifluoromethylsulfonyl)imide anion to the most acidic H atom of 1-ethyl-3-methylimidazolium cation showed a marked decrease with increase in carbonate concentration indicating that the C ̶ H---O/N hydrogen bond interaction is reduced by the presence of high carbonate content. Furthermore, in the pure ionic liquid, adjacent cations are almost exclusively located on top and below the ring cation, whereas the anions mainly coordinate to the cation within the ring plane. The addition of large amount of carbonate co-solvents disturb the original near ordering which is found in the pure ionic liquid. Key words: electrolyte, Li-ion, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, ethylene carbonate, dimethyl carbonate.