Limited effects of a redox mediator in lithium–oxygen batteries: indecomposable by-products

Abstract

Redox mediators have been studied intensively to improve the energy efficiency and cycle life of lithium–oxygen batteries by facilitating the decomposition of the discharge product (lithium peroxide), thus reducing the side reactions at a high potential during charging. Nevertheless, the cycling performance of lithium–oxygen batteries with redox mediators is unsatisfactory; this problem must be resolved for the successful application of redox mediators. In this study, we confirmed that a redox mediator cannot decompose the by-products on the cathode, leading to the passivation of the cathode surface despite the successful decomposition of lithium peroxide by the redox mediator. By schematizing the routes for by-product formation, we described the processes by which the by-products are accumulated on the cathode in the absence and presence of a redox mediator. Based on the intuitive verification of the unproven relationship between a redox mediator and the by-products, we proposed a complementary strategy to overcome the limitations of the redox mediator and prevent the accumulation of the by-products, which is essential for the improvement of lithium–oxygen batteries.