TU Graz researchers identify singlet oxygen as major cause of deleterious side reactions in Li-air batteries; suggested approach to counter
21 March 2017
Researchers led by a team from Graz University of Technology (TU Graz) in Austria have shown that singlet oxygen (1O2) forms in the cathode of a Li–O2 battery during discharge and from the onset charge, and that it is responsible for a major fraction of side products that cause fast ageing in the battery. A paper on their work is published in the journal Nature Energy.
The paper also suggests an initial approach as to how the storage cell can protect itself from the reactive oxygen species. Singlet oxygen is the main cause of ageing in biological cells. To counter this, nature uses an enzyme called superoxide dismutase to eliminate superoxide as a free radical. The researchers identified a class of molecules that can provide a function for the Li-O2 battery similar to that of the enzyme superoxide dismutase in biological systems.
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.
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