Li-O2 Cell with LiI(3-hydroxypropionitrile)2 as a Redox Mediator: Insight into the Working Mechanism of I- during Charge in Anhydrous Systems.

Redox mediators (RMs) have been widely applied to reduce the charge overpotential of nonaqueous lithium-oxygen (Li-O2) batteries. Among the reported RMs, LiI is under hot debate with lots of controversial reports. However, there is a limited understanding of the charge mechanism of I- in anhydrous Li-O2 batteries. Here, we study the chemical reactivity between the oxidized state of I- and Li2O2. We confirm that the Li2O2 particles could be chemically oxidized by I2 rather than I3- species. Furthermore, our work demonstrates that the generated I- from Li2O2 oxidation would combine with I2 to give I3- species, hindering further oxidation of Li2O2 by I2. To improve the working efficiency of I- RMs, we introduce a compound LiI(3-hydroxypropionitrile)2 (LiI(HPN)2) with a high binding ability of I-. Compared with LiI, the cell that contained LiI(HPN)2 shows a significantly increased amount of I2 species during charge and enhanced Li2O2 oxidation efficiency under the same working conditions.