Stefan Freunberger at the Institute for Chemistry at the Technical University of Graz is working on reducing the effect of highly reactive singlet oxygen that is responsible for the ageing process when lithium-oxygen batteries are charged or discharged.
Freunberger is looking at the effect of singlet oxygen on redox mediators, which can be reversibly reduced or oxidised. Redox mediators play a vital role in the flow of electrons between the exterior circuit and the charge storage material in lithium-oxygen batteries, and also have a considerable impact on their performance.
"Until now it was assumed that redox mediators are deactivated by superoxides and peroxides. But our experiments have shown that this is due to the action of singlet oxygen," said Freunberger. The researchers used density functional theory calculations to demonstrate why certain classes of mediators are more resistant to singlet oxygen than others and also identified its most likely avenues of attack. These insights are driving forward the development of new, more stable redox mediators for longer lasting batteries. "The more stable the mediators, the more efficient, reversible and long-lasting the batteries become," he said.
Besides deactivating redox mediators, singlet oxygen also triggers parasitic reactions, which compromise battery life and rechargeability. So, Freunberger tried to identify a suitable quencher that transforms the singlet oxygen produced into harmless triplet oxygen.
"An enzyme called superoxide dismutase blocks the formation of singlet oxygen in living cells. In its place, I used DABCOnium - which is a salt of the organic nitrogen compound DABCO - in my experiments," said Freunberger. DABCOnium is an electrolyte additive which is much more resistant to oxidation than previously identified quenchers, and is compatible with a lithium-metal anode. In this way, for the first time Freunberger created conditions for charging lithium oxygen cells that were largely free of side reactions - in other words, without parasitic reactions.