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Contributed by Christopher C. Cummins, October 20, 2021 (submitted for review on September 14, 2020; reviewed by Larry Curtiss and Stefan A. Freunberger)

Lithium superoxide (LiO2) is an important intermediate of lithium-air batteries, which is a promising next-generation energy storage platform. The conductivity, stability and reactivity of LiO2 are believed to play a vital role in the cycle performance of lithium-air batteries. We proved that physical encapsulation of Li2O2 with appropriate redox active molecules may be a feasible strategy to obtain and stabilize LiO2 at room temperature while protecting solvents and electrolytes from harmful reactions from LiO2. Encapsulation with redox media will not hinder the interface electron and lithium ion transmission, and provides researchers with a model system that can summarize the charging process of lithium-air batteries.

Lithium peroxide is a key storage material for lithium-air batteries. Understanding the redox characteristics of this salt is essential to improve the performance of such batteries. After exposure to p-benzoquinone (p-C6H4O2) vapor, lithium peroxide appears dark blue. This blue powder can be formally described as [Li2O2]0.3 · [LiO2]0.7 · {Li[p–C6H4O2]}0.7, although the spectral characterization shows that the structure is more subtle. Infrared, Raman, electron paramagnetic resonance, diffuse reflection ultraviolet-visible light and X-ray absorption spectroscopy indicate that a lithium salt of benzoquinone radical anion is formed on the surface of lithium peroxide, indicating that electrons and lithium ions occur in lithium peroxide Transfer. Solid state. As a result, obligate lithium superoxide is formed and encapsulated in a Li[p–C6H4O2] shell with Li2O2 as the core. Lithium superoxide has been proposed as a key intermediate in the charge/discharge cycle of lithium-air batteries, but it has not been isolated due to its instability. The results reported in this article provide a snapshot of the solid-state lithium peroxide/superoxide chemistry, with redox mediation.

↵1 Current address: Department of Chemistry and Biochemistry, Ohio State University, Columbus, Ohio 43210.

Author contributions: MN, SZ, DGN and CCC design research; MN and SZ conducted research; MN, SZ, KSP and XF contributed new reagents/analysis tools; MN, SZ, KSP, DGN, CCC analysis data; MN , DGN and CCC wrote this paper.

Reviewers: LC, Argonne National Laboratory; and SAF, Austrian Institute of Science and Technology.

The author declares no competing interests.

This article contains online support information https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2019392118/-/DCSupplemental.

All research data is included in the article and/or SI appendix.

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