Mechanistic Study Revealing the Role of the Br₃⁻/Br₂ Redox Couple in CO₂-Assisted Li–O₂ Batteries

Replacing oxygen (O₂) with air is a critical step in the development of lithium (Li)–air batteries. A trace amount of carbon dioxide (CO₂) in the air is, however, influentially involved in the O₂ chemistry, which indicates that a fundamental understanding of the effect of CO₂ is required for the design of practical cells. When up to 30% CO₂ is added to Li–O₂ cells, CO₂ acts as an O₂⁻ scavenger. Their chemical reactions form soluble products, CO₄²⁻ and C₂O₆²⁻, in the tetraglyme electrolyte solution, and enhance full capacity and cell cyclability. A critical challenge is, however, the sluggish decomposition of the coproduct Li₂CO₃ during recharge. To lower the charging overpotential, a Br₃⁻/Br₂ redox couple is incorporated and its redox behavior is investigated using spectroscopic methods. The redox shuttle of Br₃⁻/Br₂ decomposes amorphous Li₂CO₃ more efficiently than its crystalline counterpart. It is revealed that Br₂ combines with Br₃⁻ to form a Br₂···Br₃⁻ complex, which acts as a mobile catalyst in the electrolyte solution without swift precipitation of the nonpolar Br₂. 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.