After validating the ability of the developed force field to predict ionic conductivity of RTILs doped with Li, Na, Mg and Zn salts, a detailed picture of the metal coordination will be presented. The Li and Na cations were coordinated by 4.7-4.8 oxygen atoms from TFSI anions, while Zn and Mg cations were coordinated by approximately six oxygen atoms from TFSI. In addition to being primarily coordinated by TFSI oxygens, Na cations are also coordinated by fluorine and nitrogen atoms from TFSI to a larger extent than other metal cations. The representative solvates from MD simulations performed at 450 K are shown in Figure 1. The cation-TFSI bidentate vs. monodentate binding motifs and the propensity of salts to form large aggregates were analyzed as a function of temperature. Ionic conductivity, viscosity and self-diffusion coefficients from MD simulations agreed well with experimental data, where available. The cation-TFSI residence times followed the order Na< Li < Mg < Zn. Much longer cation-anion residence times for Zn and Mg compared to those of Na and Li were characteristic of the much slower dissolvation for the doubly charged cations and are expected to result in higher interfacial impedance.
This research was partially supported by NASA Minority University Research and Education Project (MUREP; NASA grant NNX15AP44A and interagency agreement NND16AA29I with ARL for modeling).
Figure 1. Representative metal solvates at 450 K and MD simulation box.