Institute for Chemistry and Technology of Materials, Graz University of Technology, Graz, Austria.
Institute of Solid State Physics, Graz University of Technology, Graz, Austria.
Nat Chem. 2021 May;13(5):465-471. doi: 10.1038/s41557-021-00643-z. Epub 2021 Mar 15.
Aprotic alkali metal-O batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (O). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect O formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress O formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li drive O evolution from superoxide oxidation, while disproportionation always generates some O. We find that O suppression requires oxidation to be faster than the generation of O from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.
非质子碱金属-O 电池的化学性质高效发生面临两大障碍，即形成的碱超氧化物/过氧化物的绝缘性质和由高反应性单线态氧 (O) 引起的寄生反应。人们认识到氧化还原介体对于提高可充电性是至关重要的。然而，它们如何影响 O 的形成尚不清楚，这阻碍了改进它们的策略。在这里，我们阐明了介体过氧化物和超氧化物氧化的机理，从而解释了氧化还原介体如何增强或抑制 O 的形成。我们表明，充电首先从过氧化物氧化到超氧化物中间体开始，并且相对于 Li/Li 的氧化还原电位高于~3.5 V 会导致 O 从超氧化物氧化中释放出来，而歧化反应总是会产生一些 O。我们发现，抑制 O 需要氧化速度快于歧化反应生成 O 的速度。随着马库斯倒位区行为的驱动力增大，氧化速率减小，从而确定了最大速率区域。
