BASF investigating sodium-air batteries as alternative to Li-air; patent application filed with USPTO
|Discharge–charge cycles of Na–O2 cells at various current densities (i.e., the rate capability). Cutoff potentials were set to 1.8 V for discharge and 3.6 V for charge. Dotted line: E0(NaO2) = 2.27 V. Hartmann et al. Click to enlarge.|
In a paper in Nature Materials, a team of researchers from BASF SE and Justus-Liebig-Universität Gießen report on the performance of a sodium-air (sodium superoxide) cell. Their work, they suggest, demonstrates that substitution of lithium by sodium may offer an unexpected route towards rechargeable metal–air batteries. BASF SE has filed a Provisional Patent Application (US 61/615901) directed to sodium-oxygen cells as described in the paper with the US Patent and Trademark Office (USPTO).
While Li-air batteries have attracted a great deal of interest as future high-capacity systems ideal for longer-range electric vehicles, there remain a number of issues to be resolved, the authors note. (Earlier post.) Replacing lithium with sodium to build an analogous Na–O2 cell with sodium peroxide (Na2O2) as the discharge product offers the opportunity to construct a cell system with a high energy density (E0 = 2.33 V, wth = 1,605 Wh kg−1 (Na2O2). However, this system can also suffer from similar high overpotentials and low energy efficiencies when using carbonate-based sodium electrolytes.
In the search for room-temperature batteries with high energy densities, rechargeable metal–air (more precisely metal–oxygen) batteries are considered as particularly attractive owing to the simplicity of the underlying cell reaction at first glance. Atmospheric oxygen is used to form oxides during discharging, which—ideally—decompose reversibly during charging. Much work has been focused on aprotic Li–O2 cells (mostly with carbonate-based electrolytes and Li2O2 as a potential discharge product), where large overpotentials are observed and a complex cell chemistry is found. In fact, recent studies evidence that Li–O2 cells suffer from irreversible electrolyte decomposition during cycling.
...Interestingly, the reactivity of sodium and lithium towards oxygen is quite different despite their close chemical relation. Sodium can form a stable superoxide NaO2, whereas LiO2 is highly unstable and is found only as intermediate species in Li–O2 cells. Thus, in a Na–O2 cell, the formation of NaO2 (sodium superoxide) during discharge will compete with the formation of Na2O2. Moreover, even though peroxide formation is thermodynamically favored (E0 (Na2O2) = 2.33 V versus E0 (NaO2) = 2.27 V), the formation of NaO2 requires the transfer of only one electron per formula unit and will be kinetically preferred relative to the two-electron transfer towards the peroxide. Indeed, in an ether-based electrolyte we found solid NaO2 to be formed reversibly and exclusively (within the precision of our analytical methods) as a crystalline product at very low overpotentials.—Hartmann et al.
|Detail sketch of the electrode assembly and the oxygen support (left). Magnified SEM image (right). Hartmann et al., SI. Click to enlarge.|
The cell consisted of a metallic sodium anode, a glass fiber separator and a carbon-fiber gas diffusion layer (GDL) as the cathode. The electrolyte was a 0.5 M solution of sodium triflate salt (NaSO3CF3) in diethylene glycol dimethyl ether (DEGDME). The built an analogous Li–O2 cell (LiSO3CF3/DEGDME) for comparison.
We were successful in constructing a room-temperature sodium–oxygen cell with an ether-based electrolyte that achieved discharge capacities of over 300 mAh g−1 (carbon), corresponding to roughly 3.3 mAh cm−2 (electrode area). Cells could be cycled several times at current densities as high as 0.2 mA cm−2 using carbon with a specific surface area orders of magnitude smaller than in studies of Li–O2 cells.
As a major breakthrough we consider the very low overpotential of less than 200 mV during charging, which is at least a factor of 3–4 times lower than for any other Li–O2 or Na–O2 cell reported in the literature. The discharge product was unequivocally identified by XRD and Raman spectroscopy to be sodium superoxide (NaO2). The oxygen reduction reaction occurs as a single-electron transfer process (O2 + e− → O2−) and seems to be kinetically highly favored, which explains the reversibility of the cell reaction....The results demonstrate that the sodium-based cell chemistry might offer—compared with lithium-based cells—unexpected opportunities in the search for reversible energy storage devices.—Hartmann et al.
Pascal Hartmann, Conrad L. Bender, Miloš Vračar, Anna Katharina Dürr, Arnd Garsuch, Jürgen Janek & Philipp Adelhelm (2012) A rechargeable room-temperature sodium superoxide (NaO2) battery. Nature Materials doi: 10.1038/nmat3486