For the first time, TU Graz researchers are discovering means of suppressing singlet oxygen in lithium-oxygen batteries in order to extend their lifespan.
Since 2012, Stefan Freunberger at the Institute of Chemical Technologies of Materials of Graz University of Technology has been working on the development of a new battery generation that is more efficient, more durable and more cost-effective to produce than current models. Freunberger sees great potential in lithium-oxygen batteries. In the course of his research, the scientist discovered in 2017 parallels between cell aging in living things and in batteries . In both cases, highly reactive singlet oxygen is responsible for the aging process. This is produced during the discharging and charging process in lithium-oxygen batteries and in recent years has formed a nucleus in Freunberger’s research. In the renowned journals Nature Communicationsand Angewandte Chemie shows the Graz researchers for the first time ways to minimize the negative effects of singlet oxygen.
Stable redox mediators as the key to energy efficiency
In Nature Communications , Freunberger, in collaboration with researchers from Korea and the US, describes the influence of singlet oxygen on so-called redox mediators, which can be reversibly reduced and oxidized. These mediators are essential for the flow of electrons between the external circuit and the charge storage material in oxygen batteries and significantly determine their performance. The principle of the mediators is also copied from nature, where they fulfill a variety of functions in living cells, such as stimulation conduction and energy generation. “So far it has been assumed that redox mediators are deactivated by superoxide and peroxide. However, our research shows that singlet oxygen is responsible for this, “says Freunberger.
Using so-called density functional theory calculations, the scientists were able to explain why some classes of mediators are more resistant to singlet oxygen than others. They also identified the most likely modes of singlet oxygen attack. This knowledge helps to develop new, stable redox mediators. “The more stable mediators are, the more efficient, more reversible and more durable the batteries are,” explains Freunberger.
DABCOnium effectively protects against singlet oxygen
In addition to deactivating redox mediators, singlet oxygen is also responsible for those parasitic chemical reactions that reduce the life and charge of batteries. Freunberger was therefore looking for an ideal extinguisher, the resulting singlet oxygen in harmless triplet oxygen, as it occurs in the air converts – and got suggestions from biology: “In the living cell prevents an enzyme called superoxide dismutase the formation of singleton Oxygen. I used DABCOnium – a specific salt of the organic nitrogen compound DABCO – in my experiments. “This is an electrolyte additive that is much more stable to oxidation than previously known quenchers and is compatible with lithium metal on the negative electrode.
However, singlet oxygen is not only problematic in oxygen batteries, but also in recent developments of lithium-ion batteries, as Freunberger could show last year. Extinguishers are therefore also relevant for them. Freunberger has published details of this singlet oxygen extinguisher in the journal Angewandte Chemie .
Combination of mediator and extinguisher forms ideal condition
In a next step, Freunberger now wants to merge the results and develop a new mediator class. On the one hand, this should be particularly resistant to attacks by singlet oxygen, but it can also extinguish it itself – ie combat it efficiently. This would dramatically extend the life of lithium-oxygen batteries and maximize energy efficiency.