Solvate Ionic Liquids at Electrified Interfaces

Solvate ionic liquids (SILs) are a promising electrolyte for Li-ion batteries; thus, their behavior at electrified interfaces is crucial for the operation of these batteries. We report molecular dynamics simulation results for a prototypical SIL of lithium triglyme bis(trifluoromethanesulfonyl)imide ([Li(G3)][TFSI]) sandwiched between electrified surfaces. At negatively charged as well as neutral electrodes, the electrolyte largely maintains the characteristics of SILs in terms of the interfacial Li⁺ ions' coordination by a similar number of oxygen atoms on G3 ligands as the bulk Li⁺ ions. The persistence of the complex ions is attributed to the 1:1 Li-G3 ratio in bulk SILs and the fact that G3 molecules readily adapt to the interfacial environment by aligning themselves with the surface to ensure good solvation of the interfacial Li⁺ ions. Nevertheless, the interfacial Li⁺ ions also display changes of solvation from that in bulk SIL by deviating from the molecular plane formed by the oxygen atoms on G3 ligands as electrodes become more negatively charged. Using density functional theory along with natural bond orbital calculations, we examine the effects of such structural distortion on the properties of the complex cation. Both the frontier orbital energies of the complex cation and the donor-acceptor interactions between Li⁺ ions and G3 ligands are found to be dependent on the deviation of Li⁺ ions from the molecular plane of the G3 ligands, which suggests that the electrochemical reduction of Li⁺ ions should be facilitated by the structural distortion. These results bear important implications for the nanostructures and properties of SILs near electrified interfaces during actual operations of Li-ion batteries and serve to provide guidance toward the rational design of new SIL electrolytes.