Density Functional Theory Based Study of the Electron Transfer Reaction at the Lithium Metal Anode in a Lithium–Air Battery with Ionic Liquid Electrolytes

Room temperature ionic liquids, which have unique properties such as a relatively wide electrochemical stability window and negligible vapor pressure, are promising candidates as electrolytes for developing lithium−air batteries with enhanced performance. The local current density, a crucial parameter in determining the performance of lithium−air batteries, is directly proportional to the rate constant of the electron transfer reaction at the surface of the anode that involves the oxidation of pure lithium metal into lithium ion (Li). The electrochemical properties of ionic liquid based electrolytes, which can be molecularly tailored on the basis of the structure of their constituent cations and anions, play a crucial role in determining the reaction rate at the anode. In this paper, we present a novel approach, based on Marcus theory, to evaluate the effect of varying length of the alkyl side chain of model imidazolium based cations on the rates of electron transfer reaction at the anode. Density functional theory was employed for calculating the necessary free energies for intermediate reactions. Our results indicate that the magnitude of the Gibbs free energy of the overall reaction decreases linearly with the inverse of the static dielectric constant of the ionic liquid, which in turn corresponds with an increase in the length of the alkyl side chain of the ionic liquid cation. Nelsen’s four-point method was employed to evaluate the inner sphere reorganization energy. The total reorganization energy decreases with increase in the length of the alkyl side chain. Finally, the rate constants for the anodic electron transfer reaction were calculated in the presence of varying ionic liquid based electrolytes. The overall rate constant for electron transfer increases with increase in the static dielectric constant. The presented results provide important insight into identification of appropriate ionic liquid electrolytes to obtain enhanced current densities in lithium−air batteries.