Why charging Li–air batteries with current low-voltage mediators is slow and singlet oxygen does not explain degradation

Ahn, Sunyhik; Zor, Ceren; Yang, Sixie; Lagnoni, Marco; Dewar, Daniel; Nimmo, Tammy; Chau, Chloe; Jenkins, Max; Kibler, Alexander J.; Pateman, Alexander; Rees, Gregory J.; Gao, Xiangwen; Adamson, Paul; Grobert, Nicole; Bertei, Antonio; Johnson, Lee R.; Bruce, Peter G.

Abstract

Although Li–air rechargeable batteries offer higher energy densities than lithium-ion batteries, the insulating Li2O2 formed during discharge hinders rapid, efficient re-charging. Redox mediators are used to facilitate Li2O2 oxidation; however, fast kinetics at a low charging voltage are necessary for practical applications and are yet to be achieved. We investigate the mechanism of Li2O2 oxidation by redox mediators. The rate-limiting step is the outer-sphere one-electron oxidation of Li2O2 to LiO2, which follows Marcus theory. The second step is dominated by LiO2 disproportionation, forming mostly triplet-state O2. The yield of singlet-state O2 depends on the redox potential of the mediator in a way that does not correlate with electrolyte degradation, in contrast to earlier views. Our mechanistic understanding explains why current low-voltage mediators (<+3.3 V) fail to deliver high rates (the maximum rate is at +3.74 V) and suggests important mediator design strategies to deliver sufficiently high rates for fast charging at potentials closer to the thermodynamic potential of Li2O2 oxidation (+2.96 V).

Citation

Ahn, S., Zor, C., Yang, S., Lagnoni, M., Dewar, D., Nimmo, T., …Bruce, P. G. (2023). Why charging Li–air batteries with current low-voltage mediators is slow and singlet oxygen does not explain degradation. Nature Chemistry, 15(7), 1022–1029. https://doi.org/10.1038/s41557-023-01203-3

• Journal Article Type: Article
• Acceptance Date: Apr 14, 2023
• Online Publication Date: Jun 1, 2023
• Publication Date: 2023-07
• Deposit Date: Jun 7, 2023
• Publicly Available Date: Dec 2, 2023
• Journal: Nature Chemistry
• Print ISSN: 1755-4330
• Electronic ISSN: 1755-4349
• Publisher: Springer Science and Business Media LLC
• Peer Reviewed: Peer Reviewed
• Volume: 15
• Issue: 7
• Pages: 1022–1029
• DOI: https://doi.org/10.1038/s41557-023-01203-3
• Keywords: General Chemical Engineering, General Chemistry
• Public URL: https://nottingham-repository.worktribe.com/output/21378331
• Publisher URL: https://www.nature.com/articles/s41557-023-01203-3