Synthesis of ROS-Resistant Redox Mediators for Improved Lithium Oxygen Battery Performance

Abstract

The quest for high-performance and long-lasting lithium-oxygen batteries (LOBs) continues to drive research towards innovative strategies to overcome critical challenges. One major bottleneck hampering their practical application is the sluggish oxygen reduction reaction (ORR) during charging, leading to high overpotentials. Redox mediators (RMs) have emerged as an attractive approach to address this issue by facilitating both ORR and the corresponding oxygen evolution reaction (OER) during discharge. However, a major limitation of current RMs lies in their vulnerability to degradation by the highly reactive singlet oxygen (1O2) species generated during battery operation. This degradation significantly diminishes their catalytic activity and hinders the overall performance and longevity of LOBs. In this study, we explore the potential of a novel class of RMs with an emphasis on structural design for enhanced durability. We introduce 7,7'-bi-7-azabicyclo[2.2.1]heptane (BAC), a unique RM featuring N-N interconnected aza-bicyclic moieties. The rationale behind this design lies in the potential of the bicyclic structure to offer superior resistance against oxidative degradation caused by 1O2, compared to traditional non-bicyclic RMs. To evaluate the efficacy and robustness of BAC, we compare its performance with that of conventional N-N non-bicyclic RMs. Our findings reveal a stark contrast between the two classes of RMs in their response to 1O2 exposure. Non-bicyclic RMs show a significant decline in their ability to facilitate OER after exposure, indicating a loss of catalytic activity due to degradation. Conversely, BAC demonstrates exceptional stability, maintaining consistent O2 profiles during the charging process. This remarkable performance signifies the superior 1O2 resistance of BAC and its potential to deliver sustained catalytic activity in LOBs. To further elucidate the underlying mechanism of BAC's superior performance, we employ theoretical calculations. These calculations suggest that the bicyclic structure of BAC offers a crucial advantage by strategically shielding the C-H bonds adjacent to the nitrogen atoms. These bonds are particularly susceptible to oxidative attack by 1O2, and the bicyclic framework effectively prevents their cleavage, which is a key degradation pathway observed in non-bicyclic RMs._x000D_ <br>In conclusion, this study presents BAC, a novel and highly stable RM with a unique bicyclic structure. Experimental and theoretical evidence strongly support the exceptional 1O2 resistance of BAC, paving the way for a new generation of robust RMs that can significantly enhance the performance and lifespan of LOBs.