Mechanistic Study Revealing the Role of the Br-3(-)/Br-2 Redox Couple in CO2-Assisted Li-O-2 Batteries

Abstract

Replacing oxygen (O-2) 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 O-2 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-O-2 cells, CO2 acts as an O-2(-) 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 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 Li2CO3 more efficiently than its crystalline counterpart. It is revealed that Br-2 combines with Br-3(-) to form a Br-2 center dot center dot center dot 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.