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

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.