Ab Initio Modeling of the Electron Transfer Reaction Rate at the Electrode-Electrolyte Interface in Lithium-Air Batteries

Lithium-air batteries are very promising energy storage systems for meeting current demands in electric vehicles. However, the performance of these batteries is highly dependent on the electrochemical stability and physicochemical properties of the electrolyte such as ionic conductivity, vapor pressure, static and optical dielectric constant, and ability to dissolve oxygen and lithium peroxide. Room temperature ionic liquids, which have high electrical conductivity, wide electrochemical stability window and also low vapor pressure, are considered potential electrolytes for these batteries. Moreover, since the physicochemical and electrochemical properties of ionic liquids are dependent on the structure of their constitutive cations and anions, it is possible to tune these properties by choosing from various combinations of cations and anions. One of the important factors on the performance of lithium-air batteries is the local current density. The current density on each electrode can be obtained by calculating the rate constant of the electron transfer reactions at the surface of the electrode. In lithium-air batteries, the oxidation of pure lithium metal into lithium ions happens at the anode. In this study, Marcus theory formulation was used to calculate the rate constant of the electron transfer reaction in the anode side using the respective thermodynamics data. The Nelsen’s four-point method of separating oxidants and reductants was used to evaluate the inner-sphere reorganization energy. In addition, the Conductor-like Screening Model (COSMO) which is an approach to dielectric screening in solvents has been implemented to investigate the effect of solvent on these reaction rates. All calculations were done using Density Functional Theory (DFT) at B3LYP level of theory with a high level 6-311++G** basis set which is a Valence Triple Zeta basis set with polarization and diffuse on all atoms (VTZPD) that gives excellent reproducibility of energies. Using this methodology, the electron transfer rate constant for the oxidation of lithium in the anode side was calculated in an ionic liquids electrolyte. Our results present a novel approach for choosing the most appropriate electrolyte(s) that results in enhanced current densities in these batteries.