With the growing presence of electric vehicles, there is an increasing drive to produce low-weight, high-capacity batteries that operate at room temperature. Significant attention has focused on lithium-air batteries, which reduce weight and volume by using atmospheric oxygen in place of an onboard reactant, and can thus rival the energy density of gasoline. Current lithium-air systems however suffer from several limitations, including complex chemistries and irreversible electrolyte decomposition during cycling. Addressing this problem, P. Hartmann of Justus Liebig University Giessen, Germany, A.K. Dürr of BASF, and their colleagues have demonstrated a reversibly charging/discharging sodium-air battery that may provide an alternative path to a rechargeable metal-air battery. Their results are reported in a letter published online December 2, 2012 in Nature Materials (DOI: 10.1038/NMAT3486).

A central issue for the performance of metal-air batteries is their ability to reversibly form and decompose a discharge product on the cathode. Lithium-air batteries reversibly form lithium peroxide (Li₂O₂) as a discharge product, although this currently requires nontrivial cathode materials (nanoporous gold) and a dimethyl sulfoxide-based electrolyte. Sodium-air batteries offer an alternative metal-air system, where previous work has focused on devices that formed sodium peroxide (Na₂O₂) as the discharge product. In common with the Li-based systems, these also suffered from large overpotentials and electrolyte decomposition.

To overcome these problems, Hartmann and his team developed a sodium-air system based on a sodium metal anode, glass microfiber separators, and a binder-free, carbon-fiber gas diffusion layer (GDL) cathode. The electrolyte was a 0.5 M solution of sodium triflate salt (NaSO₃CF₃) in anhydrous diethylene glycol dimethyl ether (DEGDME). No catalyst was used in the device, and the cathode surface area was relatively low compared to analogous lithium-air batteries (<1 m²/g versus ~100 m²/g).

The researchers were able to charge/discharge this system several times, with relatively low overpotentials (<200 mV) and relatively high current densities (0.5 mA/cm²). Of particular note, the researchers found that the discharge product was highly pure crystalline sodium superoxide (NaO₂), which is not thermodynamically favored relative to Na₂O₂. They hypothesize that this is due to the fact that NaO₂ requires the transfer of only one electron per formula unit (rather than two for Na₂O₂), making it kinetically favored.

The good performance of this battery suggests that sodium-air systems could display very attractive capabilities when optimized using techniques already explored in lithium-air systems. Combined with the simplicity of the cell reaction and the low cost of sodium, a sodium-based approach may therefore enable practical metal-air batteries to be realized.