Biologists have long considered the excited oxygen molecule singlet oxygen to be the main cause of aging in cells. To counter this, nature uses an enzyme to eliminate superoxide as a free radical. Superoxide also occurs in cell respiration for energy production. Stefan Freunberger of the Graz University of Technology in Austria has found parallels of oxygen chemistry in battery systems that could lead to longer-lived non-aqueous batteries.
In “Singlet Oxygen Generation as a Major Cause for Parasitic Reactions during Cycling of Aprotic Lithium-Oxygen Batteries,” published in Nature Energy, Mr. Freunberger investigates aging processes in non-aqueous batteries, such as oxygen batteries. The energy density of these batteries is considerably higher than that of today’s lithium-ion batteries, but their fast aging still makes them unusable.
“I’ve realised for a long time that superoxide cannot be the single cause of aging, as previously assumed. Singlet oxygen is quite well known because it’s so reactive. But nobody had checked to see if it occurs in batteries. Not least because there weren’t any methods to detect it there,” writes Freunberger. He goes on to show that singlet oxygen is mainly responsible for aging in non-aqueous oxygen batteries.
Freunberger’s work provides an initial approach to how a storage cell can protect itself from the reactive oxygen species. “In essence, the battery needs the function of the enzyme superoxide dismutase. We were able to identify a class of molecules which can fulfil this function. There has to be a suitable way of getting the enzyme into the battery system – either through the electrolyte itself or by means of an additive which dissolves in the electrolyte. This is an initial approach that works but it is definitely not the optimum way. Behind this big door which we pushed open, there is a lot of work to do.”