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., Chau, C., Jenkins, M., Kibler, A. J., Pateman, A., Rees, G. J., Gao, X., Adamson, P., Grobert, N., Bertei, A., Johnson, L. R., & 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: Nature Publishing Group
• 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