A p-phenylenediamine oligomer-mediated Li–O2 battery with an extremely low charge potential of 3.1 V

Abstract

High overpotential is one of the disadvantages of lithium–air batteries, which can cause electrolyte decomposition to damage the electrochemical performance of the battery. Redox mediators (RMs) have been extensively investigated for reducing the overpotential of Li–O₂ batteries. However, most RMs have a limited effect on overpotential reduction and can only reach about 3.5 V. Herein, we report the use of a p-phenylenediamine oligomer (pPO) as an effective dual-functional RM to significantly reduce the charge potential to 3.1 V and to enhance the discharge capacity and rate capability. During the discharge process, pPO containing a benzoquinone group, is first reduced to pPOo⁻, which adsorbs lithium ions and oxygen, and promotes the formation of lithium peroxide. During charging, pPO containing active N sites is preferentially oxidized to pPO_N⁺, which combines with lithium peroxide and promotes its decomposition. Similarly, we synthesized two other oligomers (mPO and oPO) that do not affect the overpotential due to differences in the molecular structure. Furthermore, pPO can also promote surface passivation of the lithium anode by forming a protective layer that allows facile cycling of the cells directly in air. The special molecular structure and functional groups determine the interaction between the RM and products. This study offers insights into designing bifunctional RMs to improve both the oxygen reduction and the oxygen evolution reaction activities in Li–O₂ batteries.