Hidden oxygen gas has prevented higher voltages in batteries

An important goal of battery development is to increase the energy density of these devices. This can be achieved for instance by using electrode materials that allow high voltages during charging. At the same time, there is a risk of high voltages producing oxygen gas in the electrolyte. This undesired byproduct degrades the battery and, above all, wastes part of the usable stored energy.

It is possible to avoid oxygen gas evolution in the electrolyte by using novel sodium-based cathode materials, such as Na0.75[Li0.25Mn0.75]O2. On the other hand, this type of alkali-ion-rich compound is associated with a rapid loss of battery voltage during discharge. Now, a research team from Uppsala University, the University of Oxford, and the Diamond Light Source have found the reason behind this.

“We showed that oxygen gas is also produced in the Na0.75[Li0.25Mn0.75]O2-cathode while the battery is being charged, but the gas is stored ‘invisibly’,” says Laurent Duda, associate professor at the Department of Physics and Astronomy at Uppsala University.

“In a previous study, we reported that novel alkali-rich cathode materials owe their high capacity to oxygen activity but little did we know at the time that it was forming as gas molecules. We were able to make this surprising discovery by applying an ultra-high-resolution technique called resonant inelastic X-ray scattering, in a collaboration with colleagues at the Diamond Light Source in England. Upon discovering this, we started to suspect that hidden oxygen gas was the culprit responsible for reducing the battery voltage during discharge.”

A way to prevent hidden oxygen gas

The discovery of hidden oxygen gas aroused the researchers’ curiosity about the underlying mechanism in these cathodes. Most importantly, they wanted to find out if there was a way to prevent this from happening altogether.

The picture clarified when the colleagues at the University of Oxford synthesised a new material, Na0.75[Li0.25Mn0.75]O2, which turned out to retain its voltage much better even though its composition was very similar to the previous material. The origin of the difference was then traced using computational simulations, which showed that the formation of a certain superstructure was conducive for hidden oxygen gas to arise.

“These new results are likely to have quite an impact on the development of the research field. We have raised awareness that cathodes with certain types of crystal structure are prone to develop hidden oxygen gas. Therefore, by keeping these effects in mind, the problem can hopefully be avoided when designing new materials,” says Laurent Duda.