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

Abstract

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. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim