Exploring the impact of lithium halide-based redox mediators in suppressing CO2 evolution in Li–O2 cells

Abstract

The realization of lithium–oxygen (Li–O₂) batteries has been impeded by parasitic reactions that cause cell component degradation, often accompanied by the release of CO₂ gas during oxidation reactions. The use of halide-based redox mediators (RMs) like LiBr and LiI has been proposed as a strategy to reduce overpotentials during oxygen evolution reactions and thus suppress the subsequent evolution of CO₂. However, there is a scarcity of research examining the effectiveness of these RMs in the direct mitigation of parasitic reactions. In this study, we investigated the evolution of CO₂ during the oxidation processes using an online electrochemical mass spectrometer. The results show that cells without RMs exhibited high overpotentials and significant CO₂ evolution from the first charging cycle. In contrast, the addition of 50 mM LiI to the electrolyte resulted in a delay in CO₂ evolution, observed only after several cycles. Notably, no CO₂ evolution was observed in cells containing 50 mM LiBr in 0.5 M LiTFSI during the cell cycling. Our findings demonstrate that while the mechanism of halide-based RMs may be similar, their chemical properties and electrochemical behaviour can greatly influence their ability to effectively mediate the oxygen evolution process.