Molecular and electronic insights of mixing bis(trifluorosulfonyl)imide with bis(fluorosulfonyl)imide anions in imidazolium based ionic liquids: modified hybrid electrolytes with improved performance for Li-ion battery.

Ionic liquids (IL) can be modified into hybrid electrolytes, either with lithium salts and/or organic solvents, which can effectively reduce the viscosity and ionic transferring resistance, which thereby improves the ion transport and interfacial properties of battery electrolytes. Several IL based electrolytes with an imidazolium cation (EMI) have been investigated trying to find beneficial effect of mixing [FSI]- with [TFSI]- anions. However, a compromise would exist between thermal stability and electrochemical performance at elevated temperatures. To further advance the development of the hybrid electrolytes with improved electrochemical performance by decreasing the viscosity, it is important to understand, at a molecular level, the underlying molecular and electronic interactions which influence the viscosity and transport properties of such hybrid ILs. With this in mind, the purpose of the current study is to present detailed structural and electronic insights of imidazolium [EMI] bis(trifluorosulfonyl)imide [TFSI]/bis(fluorosulfonyl)imide [FSI] hybrid electrolytic systems, and their mixtures with ethylene carbonate (EC)/dimethyl carbonate (DMC), which are potential candidates for next generation LIB electrolytes.The nature of molecular 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.The [FSI]- anions, unlike the [TFSI]- anions, exist on top position with respect to the imidazolium rings. On the basis of a distance criteria, the [EMI]+ and [FSI]- ions show distances of rather weak hydrogen bonds. Charge transfer occurs via the lone pairs of oxygen and nitrogen atom to the σ-type anti-bonding orbital of the C–H bonds (Ylp→σ*C–H), π-type anti-bonding orbitals of C=C bonds (Ylp→π*C=C) and π-type anti-bonding orbitals of N ̶ C bonds (Ylp→π*C–N), where Y = N, O, or F. The values of the stabilization energy E(2) for [EMI][FSI] ion pair conformers are generally small (E(2)n→σ* < 2 kcal/mol) for the individual E(2)n→σ*, E(2)n→π*C=C and/or E(2)n→π*N-C interactions. The [EMI]+ cation and [FSI]- anions tend to form multiple σ* and π* interactions, but reducing the strength of the individual contributions from a potential (linear) maximum.By comparison, from the SDF study, the coordination of the N1 atom of [TFSI]− anion to the atom H1 was enhanced upon the addition of carbonate into the mixture. In sharp contrast to this, the coordination of the N1 atom of [FSI]− anion to the atom H1 was decreased upon the addition of carbonate into the mixture, suggesting that H1 has a stronger interaction with N1 of the [TFSI]- anion than [FSI]- anion. Furthermore, in the pure ionic liquid, the [TFSI]- anions occupy both the on top position and in plane position in the [EMI]+ ring. With lower carbonate contenet, the [FSI]- anions occupy regions that are not occupied by [TFSI] anions which mainly occupy the on top and in plane positions. For higher carbonate content, however, adjacent [FSI]- anions are almost exclusively located on top and below the [EMI]+ ring.