Chemists discover key reaction mechanism behind the highly touted sodium-oxygen battery

Researchers from the Waterloo Institute for Nanotechnology, led by Professor Linda Nazar who holds the Canada Research Chair in Solid State Energy Materials, have described a key mediation pathway that explains why sodium-oxygen batteries are more energy efficient compared with their lithium-oxygen counterparts.

Understanding how sodium-oxygen batteries work has implications for developing the more powerful lithium-oxygen battery, which is has been seen as the holy grail of electrochemical energy storage.

Their results appear in the journal Nature Chemistry.

"Our new understanding brings together a lot of different, disconnected bits of a puzzle that have allowed us to assemble the full picture," says Nazar, a Chemistry professor in the Faculty of Science. "These findings will change the way we think about non-aqueous metal-oxygen batteries."

Sodium-oxygen batteries are considered by many to be a particularly promising metal-oxygen battery combination. Although less energy dense than lithium-oxygen cells, they can be recharged with more than 93 per cent efficiency and are cheap enough for large-scale electrical grid storage.

The key lies in Nazar's group discovery of the so-called proton phase transfer catalyst. By isolating its role in the battery's discharge and recharge reactions, Nazar and colleagues were not only able to boost the battery's capacity, they achieved a near-perfect recharge of the cell. When the researchers eliminated the catalyst from the system, they found the battery no longer worked.

Unlike the traditional solid-state battery design, a metal-oxygen battery uses a gas cathode that takes oxygen and combines it 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 reverts 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. The dimensions of the initially formed NaO₂ are critical; theoretical calculations from a group at MIT has separately shown that NaO₂ is energetically preferred over sodium peroxide, Na₂O₂ at the nanoscale. When the battery is recharged, these NaO₂ cubes readily dissociate, with the reverse reaction facilitated once again by the proton phase catalyst.

Chemistry says that the proton phase catalyst could work similarly with lithium-oxygen. However, the lithium superoxide (LiO2) entities are too unstable and convert immediately to lithium peroxide (Li₂O₂). Once Li₂O₂ forms, the catalyst cannot facilitate the reverse reaction, as the forward and reverse reactions are no longer the same. So, in order to achieve progress on lithium-oxygen systems, researchers need to find an additional redox mediator to charge the cell efficiently.

"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."