The search for the next-generation batteries has recently focused on rechargeable metal-oxygen batteries, considered very attractive as room-temperature devices with high theoretical energy densities (comparable to gasoline) for application in electrical vehicles. These batteries form oxides during the discharging process, which then –ideally- decompose into the metal ions and oxygen when charging. Lithium-oxygen systems have been largely studied; in these batteries, lithium peroxide (Li₂O₂) is formed when discharging. Recent studies found that this compound partially reacts with the electrolyte forming solid products that are difficult to remove and hinder its decomposition. As a consequence, the porous electrode remains clogged and the battery capacity fades within a few cycles.

Sodium-oxygen batteries, first reported in 2012, can be an interesting alternative given the earth-abundancy of sodium, and especially because they often show a more attractive cycle life. Most authors report that no sodium peroxide (Na₂O₂) is formed during the discharge process; instead, sodium superoxide (NaO₂) is formed. This compound forms in only one-electron transfer step and is believed to decompose more easily upon charging.

Researchers have now analyzed the discharge products of these batteries by energy-dependent transmission X-ray microscopy at the ALBA synchrotron. This technique allows distinguishing at the nanoscale between different regions of the deposits formed, according to the quantity and the chemical state of the oxygen present.

These researchers report in the journal Nano Energy that both products (sodium peroxide and superoxide) are actually present in these deposits and decompose almost simultaneously while charging. In addition to these two oxides, a complex structure of several layers formed by electrolyte decomposition products is observed at their surface.

Fortunately, all the products are eventually removed at the end of the charging process. However, even if the complete deposit decomposition may explain the superior cycle life compared to lithium-oxygen batteries, these findings indicate that further improvements in the electrolyte formulation are required for a long-term stability.

The study is a result of the collaboration between the ALBA synchrotron, and researchers from various centers, including ICMAB, Universidad del País Vasco and CIC EnergiGUNE from Spain; and Institut Charles Gerhardt Montpellier/UMR-CNRS and Réseau sur le Stockage Electrochimique de l’Energie from France.

Reference
Imanol Landa-Medrano, Andrea Sorrentino, Lorenzo Stievano, Idoia Ruiz de Larramendi, Eva Pereiro, Luis Lezama, Teófilo Rojo, Dino Tonti. Architecture of Na-O₂ battery deposits revealed by transmission X-ray microscopy.
Nano Energy, Volume 37, July 2017, Pages 224–231.

Figure
Left: concentration profiles of the different compounds along the discharge product (cubic-shaped) of a sodium-oxygen battery.
Right: maps of oxygen distribution. The scale represents the fraction of oxygen in each of the main chemical forms found in the cubic-shaped deposits.