Mechanistic Study Revealing the Role of the Br 3 − /Br 2 Redox Couple in CO 2 ‐Assisted Li–O 2 Batteries

Replacing oxygen (O 2 ) with air is a critical step in the development of lithium (Li)–air batteries. A trace amount of carbon dioxide (CO 2 ) in the air is, however, influentially involved in the O 2 chemistry, which indicates that a fundamental understanding of the effect of CO 2 is required for the design of practical cells. When up to 30% CO 2 is added to Li–O 2 cells, CO 2 acts as an O 2 − scavenger. Their chemical reactions form soluble products, CO 4 2− and C 2 O 6 2− , in the tetraglyme electrolyte solution, and enhance full capacity and cell cyclability. A critical challenge is, however, the sluggish decomposition of the coproduct Li 2 CO 3 during recharge. To lower the charging overpotential, a Br 3 − /Br 2 redox couple is incorporated and its redox behavior is investigated using spectroscopic methods. The redox shuttle of Br 3 − /Br 2 decomposes amorphous Li 2 CO 3 more efficiently than its crystalline counterpart. It is revealed that Br 2 combines with Br 3 − to form a Br 2 ···Br 3 − complex, which acts as a mobile catalyst in the electrolyte solution without swift precipitation of the nonpolar Br 2 . This comprehensive study, revealing the molecular structure and redox process of mobile catalysts, provides an insight into improving the design of redox couples toward superior cycling performance.