Chemists at Canada’s University of Waterloo have described a key mediation pathway that explains why sodium-oxygen batteries are more energy-efficient compared with their lithium-oxygen counterparts.

“Our new understanding brings together a lot of different, disconnected bits of a puzzle that have allowed us to assemble the full picture,” says Professor Linda Nazar. “These findings will change the way we think about non-aqueous metal-oxygen batteries.”

In a paper published in the journal Nature Chemistry, Nazar and her colleagues describe their discovery of the so-called proton phase transfer catalyst. By isolating its role in the battery’s discharge and recharge reactions, they were able to boost the battery’s capacity, and achieve a near-perfect recharge of the cell. 

Unlike the traditional solid-state battery design, a metal-oxygen battery uses a gas cathode that combines oxygen with a metal such as sodium or lithium to form a metal oxide, storing electrons in the process. Applying an electric current reverses the reaction and returns the metal to its original form.

In the case of the sodium-oxygen cell, the proton phase catalyst transfers the newly formed sodium superoxide (NaO₂) entities to solution, where they nucleate into well-defined nanocrystals to grow the discharge product as micron-sized cubes. When the battery is recharged, these NaO₂ cubes readily dissociate, with the reverse reaction facilitated once again by the proton phase catalyst.

The proton phase catalyst could theoretically work with lithium-oxygen, but the lithium superoxide (LiO₂) entities are too unstable and convert to lithium peroxide (Li₂O₂). Once Li₂O₂ forms, the catalyst cannot facilitate the reverse reaction. In order to make progress on lithium-oxygen systems, researchers need to find an additional redox mediator.

“We are investigating redox mediators as well as exploring new opportunities for sodium-oxygen batteries that this research has inspired,” said Nazar.” Lithium-oxygen and sodium-oxygen batteries have a very promising future, but their development must take into account the role of how high capacity – and reversibility – can be scientifically achieved.”                 

Source: University of Waterloo via Green Car Congress