Positive Electrode Reaction of Lithium–Oxygen Batteries with NO3–/Br– Redox Mediator under High Areal Capacity and Lean Electrolyte Conditions

Lithium oxygen batteries (LOBs) have attracted considerable research interest as promising candidates for next-generation rechargeable batteries. However, their cell-level performance remains unsatisfactory, and the reaction efficiency must be improved further for practical implementation of this technology. Although the combination of LiNO3 and LiBr was recognized as an effective redox mediator for improving performance of LOBs, details of the reaction mechanism remain unknown. In this study, several ex situ and in situ techniques were used to comprehensively analyze the complex chemical reactions at the positive electrode of a LOB containing LiNO3 and LiBr under high areal capacity and lean electrolyte conditions. Our analysis revealed a synergetic effect of the Br–/NO3– redox mediator on suppressing side reactions, such as the decomposition of the electrolyte and redox mediator itself. In particular, the formation of bromoform and other Br-containing organic compounds such as 1-bromo-2-methoxyethane and 1-bromo-2,2-methoxyethoxyethane was confirmed, suggesting complex side reactions involving Br–. We believe that the methodology presented herein for the comprehensive analysis will provide an accurate understanding of the complex chemical reactions in LOBs.