Singlet oxygen is the lowest (first) excited state of normal gaseous oxygen (triplet ground state dioxygen) and is highly reactive.

Rechargeable non-aqueous metal–O₂ (air) batteries have attracted immense interest because of their high theoretical specific energy and potentially better sustainability and cost in comparison to current lithium-ion batteries. … Practical realization, however, still faces many challenges. Perhaps the most significant obstacle arises from severe parasitic reactions during cycling. These reactions decompose the electrolyte as well as the porous electrode (typically carbon with binder), and cause poor rechargeability, high charging voltages, low efficiency, build-up of parasitic reaction products, and early cell death within a few cycles.

… Chemical oxidation of alkaline peroxides in non-aqueous media is known to generate singlet oxygen … Based on the reversible potential of Li₂O₂ formation and the energy difference between triplet and singlet oxygen, the formation of ¹O₂ in the Li–O₂ cell has been hypothesized to be possible at charging potentials exceeding 3.5 to 3.9 V versus Li/Li⁺. Only recently ¹O₂ was reported to form in small quantities between 3.55 and 3.75 V. Overall, the hitherto known processes cannot consistently explain the observed irreversibilities. Only better knowledge of parasitic reactions may allow them to be inhibited so that progress towards fully reversible cell operation can continue.

Here we show that ¹O₂ forms in the Li–O₂ cathode during discharge and from the onset charge, and that it is responsible for a major fraction of the side products in the investigated system with ether electrolyte. The lower abundance on discharge and higher abundance on charge can consistently explain the typically observed deviations of the e⁻/O₂ ratio from the ideal value of two. The origin of the ¹O₂ on charge appears to be superoxide and peroxide. The presence of trace water enhances the formation during both discharge and charge. We also show that ¹O₂ traps and quenchers as electrolyte additives can significantly reduce the amount of side products associated with ¹O₂.

—Mahne et al.

Trapping removes ¹O₂ in a chemical reaction; quenching deactivates it, for example, via a temporary charge transfer complex. Trapping is irreversible; physical quenching is preferred because neither quencher nor O₂ is consumed.

The researchers chose DMA (9,10-dimethylanthracene) as the trap and 1,4-diazabicyclo[2.2.2]octane (DABCO) as the quencher.

Corresponding authors Stefan Freunberger said that the approach of using molecular traps or quenchers works as an initial approach, but “is definitely not the optimum way.”

The level of ¹O₂ abundance makes traps less likely to be effective for long-term cycling since they will be consumed rapidly. Physical quenchers are preferred since they are not consumed. Future work should therefore focus on finding quenchers that are entirely compatible with the cell environment, with the electrochemical potential window, compatibility, and stability against superoxide and peroxide being the most prominent requirements. Equally it needs to be compatible with anodes such as possibly protected Li metal.

—Mahne et al.

The work was produced with the help of colleagues from several institutes of TU Graz, acib competence centre, Université Montpellier, the University of Southampton and the French research network RS2E.

This project is anchored in the Field of Expertise “Advanced Materials Science”, one of five research foci of TU Graz.

Resources

• Nika Mahne, Bettina Schafzahl, Christian Leypold, Mario Leypold, Sandra Grumm, Anita Leitgeb, Gernot A. Strohmeier, Martin Wilkening, Olivier Fontaine, Denis Kramer, Christian Slugovc, Sergey M. Borisov & Stefan A. Freunberger (2017) “Singlet oxygen generation as a major cause for parasitic reactions during cycling of aprotic lithium–oxygen batteries” Nature Energy 2, Article number: 17036 doi: 10.1038/nenergy.2017.36