Molecular Dynamics Analysis of Charge Transport in Ionic-Liquid Electrolytes Containing Added Salt with Mono, Di, and Trivalent Metal Cations

Among many other applications, room-temperature ionic liquids (ILs) are used as electrolytes for storage and energy-conversion devices. In this work, we investigate, at the microscopic level, the structural and dynamical properties of 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide [C₄PYR]⁺[Tf₂N]⁻ IL-based electrolytes for metal-ion batteries. We carried out molecular dynamics simulations of electrolytes mainly composed of [C₄PYR]⁺[Tf₂N]⁻ IL with the addition of Mⁿ⁺-[Tf₂N]⁻ metal salts (M=Li⁺, Na⁺, Ni²⁺, Zn²⁺, Co²⁺, Cd²⁺, and Al³⁺, n=1, 2, and 3) dissolved in the IL. The addition of low salt concentrations lowers the charge transport and conductivity of the electrolytes. This effect is due to the strong interaction of the metal cations with the [Tf₂N]⁻ anions, which allows for molecular aggregation between them. We analyze how the conformation of the [Tf₂N]⁻ anions surrounding the metal cations determine the charge-transport properties of the electrolyte. We found two main conformations based on the size and charge of the metal cation: monodentate and bidentate (number of oxygen atoms of the anion pointing to the metal atoms). The microscopic local structure of the Mⁿ⁺-[Tf₂N]⁻ aggregates influences the microscopic charge transport as well as the macroscopic conductivity of the total electrolyte.