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

Journal article

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).

Authors: Ahn, S; Zor, C; Yang, S; Lagnoni, M; Dewar, D

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Springer Nature
• Journal: Nature Chemistry
• Volume: 15
• Issue: 7
• Pages: 1022-1029
• Publication date: 2023-06-01
• Acceptance date: 2023-04-14
• DOI: 10.1038/s41557-023-01203-3
• EISSN: 1755-4349
• ISSN: 1755-4330
• Pmid: 37264102
• Language: English
• Keywords: affordable and clean energy; chemical sciences; physical chemistry