Theoretical Investigation on the Behavior of Tetrathiafulvalene (TTF) As a Red-Ox Mediator for Charge Process of Li-Air Battery

Abstract

Li-Air battery have recently engaged much attention as a next generation energy-storage device because of its high theoretical specific energy. Under the environment of non-aqueous solvent, Li2O2 (Li peroxide) is formed as discharge product covering the porous cathode. In order to make a Li-Air battery rechargeable, this Li2O2 should be decomposed into Li+ and O2 during the charge process. When Li2O2 is directly attached to the cathode surface, oxidation of Li2O2 undergoes by giving electron to the cathode. Whereas, TEM observation implies that such a contact is lost at an early stage of charging. Once Li2O2 is detached from the cathode surface, it becomes hard for these particles to give electron to the cathode, and thus further decomposition is suppressed. This is the origin of charging overpotential and alternative decomposition of electrolyte molecules could proceed. It is known that tetrathiafulvalene (TTF) can reduce the overpotential and promote the decomposition of Li2O2 particles with the combination of DMSO solvent and nano-porous gold electrode, and improvement of the cycle stability as a Li-Air battery is drastically achieved. To clarify the role of TTF as a redox mediator from a microscopic point of view, we systematically performed first-principles molecular dynamics (MD) simulations and elucidated how electron transfer occurs in this system, regarding decomposition of Li2O2. We prepared a model system which contains a cluster of Li2O2 (four atoms model) or Li3O2 + (five atoms model; another stable form of Li2O2) with/without DMSO solvent. The excess charge given to a Li2O2 cluster is localized while that given to a Li2O2 slab is delocalized. This means that the oxidation of Li2O2 becomes local with decrease in the cluster size. Once Li2O2 is partially converted to LiO2 by such a local oxidation, the LiO2 part is spontaneously decomposed into Li+ and O2 − around the positively biased cathode, where O2 − is oxidized to O2. These above are the reason why we treat only the smallest clusters of Li peroxide, and the objective is translated to evaluation of the charge transfer between {Li2O2 or Li3O2 +} and {O2 or TTF+} in DMSO solvent or vacuum. The first-principles MD simulation was performed at 400K using a 1.7 nm cubic cell with 20-23 DMSO molecules. GGA/PBE functional was used, and we give total charge of +2 for TTF/Li3O2 and +1 for TTF/Li2O2 systems, respectively. From Bader charge analysis after 100 ns simulations, we found the charge state of TTF becomes almost neutral in both the Li2O2 and Li3O2 systems with DMSO solvent. This indicates that TTF can oxidize the two clusters. The O-O bond length, which clearly changes reflecting the difference of charge state among O2, O2 −, and O2 2-, also supported the trend of charge. In vacuum condition (without DMSO solvent), TTF becomes positive. This implies DMSO molecules promote the oxidation of Li2O2 (or Li3O2) by TTF. On the other hand, while the charge state of O2 in the Li2O2 system approaches -1, that in the Li3O2 system becomes almost neutral. This means that O2 can oxidize and decompose Li2O2, but it is not easy for O2 to decompose Li3O2 + in DMSO. Hence, we obtained the picture that O2 molecule oxidizes and decomposes most of the Li peroxide clusters during the charge process (e.g. O2 − poor environment) and TTF+ plays a critical role in the decomposition of Li3O2 suppressing overpotential and prevent their accumulation in the final stage. We will discuss this scenario in the presentation for more details.