Li-air batteries and degradation
Although Li-air rechargeable batteries offer higher energy densities than lithium-ion batteries, the insulating Li2O2 oxidation formed during discharge hinders rapid and 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.
In the paper 'Why charging Li-air batteries with current low voltage mediators is slow and singlet oxygen does not explain degradation' (published in Nature Chemistry) Dr Sunyhik Ahn, Dr Ceren Zor, Dr Sixie Yang and their colleagues and collaborators investigated the mechanism of Li2O2 oxidation by redox mediators. The rate-limiting step was the outer-sphere one-electron oxidation of Li2O2 to LiO2 which follows Marcus theory. The second step was dominated by LiO2 disproportionation, which formed mostly triplet-state O2.
The yield of singlet-state O2 depended on the redox potential of the mediator in a way that did not correlate with electrolyte degradation, in contrast to earlier views. The researchers' mechanistic understanding explained why the current low-voltage mediators (<+3.3V) failed to deliver high rates (the maximum rate was at +3.74 V) and suggested important mediator design strategies to deliver sufficiently high rates for fast charging at potentials closer to the thermodynamic potential of Li2O2oxidation (+2.96 V).
