Suppression of Parasitic Chemistry in Li-O₂Batteries Incorporating Thianthrene-Based Proposed Redox Mediators

Abstract

In the lithium-O2 battery, redox mediators lower the charging overpotential while facilitating the oxidation of the discharge product (lithium peroxide, Li2O2) to molecular oxygen. Previous studies have shown that compounds such as 9,10-dimethylphenazine and 10-ethylphenothiazine are effective as redox mediators. Herein, we investigate the radical cation chemistry of thianthrene toward Li2O2 and lithium oxide (Li2O). Given the high oxidation potential of thianthrene (4.15 V vs. Li0/Li+) several electron-rich analogs (including 2,3,6,7-tetramethoxyselenanthrene and a phenoxytellurine) were synthesized to decrease the oxidation potential toward the oxidation potential of Li2O2. Control experiments showed, in the absence of any candidate molecules, ∼15% of the electrons were diverted to parasitic chemistries (2.3 electrons per oxygen). Surprisingly, experiments with several of the candidate molecules demonstrated that the charging of a battery (with a LiFePO4 anode) yields oxygen in a 2-electron process for most of the charging process. These 2-electron yields occurred at potentials where the thianthrene analogs could not be oxidized and thus act as redox mediators for Li2O2 oxidation. Under conditions where the mediator is oxidized to the radical cation, both the charging overpotentials and yields of oxygen were diminished. These data suggest that the thianthrene scaffold serves as a poor candidate for redox mediation but acts in a beneficial manner to suppress parasitic chemistries (e.g., parasitic reactions involving the likely formation of singlet oxygen).