Department of Energy Engineering, Hanyang University, Seoul, 04763, Republic of Korea.
Institute for Chemistry and Technology of Materials, Graz University of Technology, Graz, 8010, Austria.
Nat Commun. 2019 Mar 26;10(1):1380. doi: 10.1038/s41467-019-09399-0.
Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. The significant problem of oxidizing the solid insulating lithium peroxide can greatly be facilitated by incorporating redox mediators that shuttle electron-holes between the porous substrate and lithium peroxide. Redox mediator stability is thus key for energy efficiency, reversibility, and cycle life. However, the gradual deactivation of redox mediators during repeated cycling has not conclusively been explained. Here, we show that organic redox mediators are predominantly decomposed by singlet oxygen that forms during cycling. Their reaction with superoxide, previously assumed to mainly trigger their degradation, peroxide, and dioxygen, is orders of magnitude slower in comparison. The reduced form of the mediator is markedly more reactive towards singlet oxygen than the oxidized form, from which we derive reaction mechanisms supported by density functional theory calculations. Redox mediators must thus be designed for stability against singlet oxygen.
非水锂-氧电池在放电过程中形成过氧化锂，在充电过程中氧化它。通过引入氧化还原介体，将电子空穴在多孔基体和过氧化锂之间来回穿梭，可大大促进固体绝缘过氧化锂的氧化，这是一个显著的问题。因此，氧化还原介体的稳定性是提高能量效率、可逆性和循环寿命的关键。然而，在反复循环过程中，氧化还原介体的逐渐失活尚未得到明确解释。在这里，我们表明，有机氧化还原介体主要是由循环过程中形成的单线态氧分解的。与超氧化物、过氧化物和氧气相比，它们与超氧化物的反应速度要慢几个数量级。与氧化形式相比，介体的还原形式对单线态氧的反应性明显更高，我们由此得出了密度泛函理论计算支持的反应机制。因此，氧化还原介体必须设计为稳定对抗单线态氧。
