Intensive Study on the Catalytical Behavior of N‑Methylphenothiazine as a Soluble Mediator to Oxidize the Li₂O₂ Cathode of the Li–O₂ Battery

Aprotic Li–O₂ batteries have attracted worldwide interest owing to their ultrahigh theoretical energy density. However, the practical Li–O₂ batteries still suffer from high charge overpotential and low energy efficiency resulting from the sluggish kinetics in electrochemically oxidizing the insulating lithium peroxide (Li₂O₂). Recently, dissolved redox mediators in the electrolyte have enabled the effective catalytic oxidation of Li₂O₂ at the liquid–solid interface. Here, we report that the incorporation of N-methylphenothiazine (MPT), as a redox shuttle in Li–O₂ 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–O₂ 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 Li₂O₂ 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.