Mediation of the Oxygen Reduction in Non-Aqueous Lithium-Air Batteries

In recent years, non-aqueous lithium-air batteries have been intensively investigated due to their high theoretical gravimetric energy, which is up to 10 times higher than lithium-ion batteries [1-4]. The discharge reaction involves the oxygen reduction reaction (ORR), forming either superoxide or peroxide. Superoxide formation is highly detrimental, because not only would the charge delivered by the battery be halved, but most importantly, superoxide induces the degradation of most known types of organic electrolytes [5-8] and carbon-containing electrodes [9-13]. Therefore, new catalysts enhancing the 2-electron reduction of oxygen in non-aqueous electrolytes are essential for the development of lithium-air batteries. Another issue in lithium-air batteries is that the reaction product, lithium peroxide, is insoluble and insulating. As a result, lithium peroxide deposits on the electrode surface and forms a passivation layer [14], which leads to capacity fading.

We have recently introduced the concept of redox shuttles as applied to solve the problem of electrode passivation in lithium-air batteries [15]. As illustrated in the figure on the example of ethyl viologen (EtV²⁺/EtV⁺), a redox shuttle will displace the oxygen reduction reaction a short distance from the electrode surface, thus avoiding passivation.

This work continues by investigating another advantage, which is the homogeneous catalysis of the oxygen reduction reaction. By combining electrochemical and UV-visible measurements, we have shown that ethyl viologen can act as a mediator for the 2-electron reduction of oxygen. This has the remarkable advantage of decreasing the lifetime of the highly reactive superoxide, which would be beneficial for the stability of electrolyte and electrode materials against irreversible degradation. Under appropriate reaction conditions, it was shown that the extent of degradation reactions undergone by ethyl viologen is <2 %. In conclusion, ethyl viologen improves the selectivity of the oxygen reduction reaction towards the formation of lithium peroxide instead of the formation of degradation products or superoxide radical anions.

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