Unlike conventional intercalation batteries (e.g., Li-ion) metal–air batteries use as cathode that allows oxygen to be reduced to form solid oxide products (e.g., using sodium or lithium) on discharge, which, in principle, reversibly evolve oxygen on charge. The aprotic Li–O₂ cell system has a theoretical energy density of 3,458 Wh kg⁻¹. However, the chemistry suffers from large charge overpotentials, leading to relatively low round trip energy storage efficiencies and significant capacity fading.

The use of carbon cathodes leads to the formation of highly insulating layers of Li₂CO₃ and/or other carbonate-type species at the interface; additionally, favored electrolytes such as glymes undergo nucleophilic attack by the reactive lithium superoxide intermediate initially formed on discharge. … oxidation of the large insulating Li₂O₂ toroids formed at high discharge capacities due to partial LiO₂ solubilization leads to a high over-potential on charge, a major hurdle that may be overcome with the use of redox mediators.

By contrast, the more reversible chemistry of the Na–O₂ cell is less understood. Although its overall energy density is lower than that of Li–O₂, it operates with over 93% Coulombic efficiency. … Such chemistry offers the possibility of promising energy storage (theoretical 1.1 kWh kg⁻¹based on NaO₂) and the opportunity to elucidate the underlying mechanisms because the discharge product is stable as NaO₂, without disproportionation to Na₂O₂. Importantly, it allows us to extend this understanding to Li–O₂ cells and to answer the critical question of why the overpotential is so much lower for Na–O₂ batteries.

… There has been much speculation on the mechanism of crystal growth, and two pathways have been put forward. One suggests that NaO₂ formed at the cathode migrates to the surface of the growing nuclei via the electrolyte solution. The other is that oxygen is directly reduced at the crystalline NaO₂ surface due to the possible (albeit very low) electronic conductivity of NaO₂, also providing a mechanism of charge/oxidation. Here, we demonstrate that the first pathway is operative, and but that it is almost exclusively driven by the presence of a proton phase-transfer catalyst (PPTC), which is critical to solubilizing and transporting the superoxide.

—Xia et al.

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

—Linda Nazar

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.

In 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. 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.

—Linda Nazar

Postdoctoral research associate Chun Xia along with doctoral students Robert Black, Russel Fernandes, and Brian Adams co-authored the paper. The ecoENERGY Innovation Initiative program of Natural Resources Canada, and the Natural Sciences and Engineering Research Council (NSERC) of Canada funded the project.

Resources

• Chun Xia, Robert Black, Russel Fernandes, Brian Adams & Linda F. Nazar (2015) “The critical role of phase-transfer catalysis in aprotic sodium oxygen batteries” Nature Chemistry 7, 496–501 doi: 10.1038/nchem.2260