The redox chemistry of O₂ moieties has come into the focus of much of the forefront battery research such as metal-O₂ batteries and Li-rich layered oxides (1, 2). O₂ evolution is in either case a critical yet not fully understood phenomenon (1, 3). For example, operation of the rechargeable metal-O₂ batteries depends crucially on the reversible formation/decomposition of metal (su)peroxides at the cathode on discharge/charge. The greatest challenge facing progress arises from severe parasitic reactions that decompose the electrolyte as well as the porous electrode. These processes have serious consequences. So far these parasitic reactions have been ascribed to the reactivity of superoxide and peroxide. Yet, their reactivity cannot consistently explain the observed irreversible processes. Unsurprisingly, strategies to mitigate the irreversibilities proved only partially successful. Therefore, only better knowledge of parasitic reactions may allow them to be inhibited.

Here we discuss our recent insights into irreversible parasitic reactions caused by the highly reactive singlet oxygen (¹O₂) during cycling of non-aqueous batteries that have so far been overlooked. They account for the majority of the parasitic products on discharge and nearly all on charge in Li-O₂ and Na-O₂ cells (4-6). Moreover, singlet oxygen forms upon oxidizing Li₂CO₃ above 3.8 V vs Li/Li⁺ (7). Li₂CO₃ is a universal passivating agent in Li-ion battery cathodes and decisive in interfacial reactivity. It is also a common side product in Li-O₂ cathodes, as well as the targeted discharge product in Li-O₂/CO₂ batteries. We will further discuss work on open question with respect to 1O2 non-aqueous battery chemistries such as catalysts, electrolytes, mediators, quenchers and other additives as well as the formation mechanism and the implications for metal-O2 cells and intercalation chemistries. Awareness of the highly reactive singlet oxygen gives a rationale for future research towards achieving highly reversible cell operation in a broad range of cell chemistries.

References

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(2) McCalla, E. et al.. Science 2015, 350, 1516.

(3) Luo, K; et al. JACS 2016, 138, 11211

(4) Mahne, N., et al. Nature Energy 2017, 17036.

(5) L. Schafzahl, et al, Angew. Chem. Int. Ed., 2017, 56, 15728.

(6) N. Mahne, O. Fontaine, M. Thotiyl, M. Wilkening & S. A. Freunberger, Chem. Sci., 2017, 8, 6716.

(7) N. Mahne, S.E. Renfrew, B.D. McCloskey & S.A. Freunberger, Angew. Chem. Int. Ed., doi: 10.1002/anie.201802277.