Intensive Study on the Catalytical Behavior of N -Methylphenothiazine as a Soluble Mediator to Oxidize the Li 2 O 2 Cathode of the Li–O 2 Battery

Aprotic Li–O 2 batteries have attracted worldwide interest owing to their ultrahigh theoretical energy density. However, the practical Li–O 2 batteries still suffer from high charge overpotential and low energy efficiency resulting from the sluggish kinetics in electrochemically oxidizing the insulating lithium peroxide (Li 2 O 2 ). Recently, dissolved redox mediators in the electrolyte have enabled the effective catalytic oxidation of Li 2 O 2 at the liquid–solid interface. Here, we report that the incorporation of N -methylphenothiazine (MPT), as a redox shuttle in Li–O 2 batteries, provides a dramatic reduction in charge overpotential to 0.67 V and an improved round-trip efficiency close to 76%. Moreover, the efficacy of MPT in Li–O 2 cells was further investigated by various characterizations. On charging, MPT + cations are first generated electrochemically at the cathode surface and subsequently oxidize the solid discharge products Li 2 O 2 through a chemical reaction. Furthermore, the presence of MPT has been demonstrated to improve the cycling stability of the cells and suppress side reactions arising from carbon and electrolytes at high potentials.