The development of advanced electrochemical energy storage systems is considered as one of the most important steps to drive the shift from fossil to renewable energies forward. Among the few alternatives to Li‑ion batteries that would exceed significantly their specific energy, Li‑oxygen and Na‑oxygen batteries have attracted significant interest. In order to fully understand their electrochemical properties and to identify potential failure mechanisms, electrochemical properties and charge carrier transport of their discharge products (e.g. Na₂O₂) need to be known in detail. Here, we investigated the overall and electronic conductivity of nanocrystalline Na₂O₂ that was prepared by mechanical treatment of coarse-grained Na₂O₂ in a high-energy ball mill. The powder obtained is expected to be rich in defects and structural disorder. Thus, we expect that the sample investigated would be at the upper limit of reachable conductivities. Impedance measurements and DC-polarization experiments show that the overall conductivity is rather poor; it is in the order of 10⁻¹³ S/cm yielding diffusion coefficients in the order of 1.8 × 10⁻²³ m²/s at approximately 300 K [1]. At room temperature the partial electronic conductivity ranges between 7.6 and 9.5 × 10⁻¹⁴ S/cm. Thus, the total conductivity in Na₂O₂ is largely influenced by electronic contributions rather than ionic conduction [2]. Furthermore, we also anticipate a strong coupling between ionic and electronic charge carrier transport. We measured the electronic conductivity at different temperatures to calculate the underlying activation energy. It turned out that electronic conductivity is governed by an activation energy as high as 0.95 eV while for overall charge transport values range from 0.85 and 1.03 eV depending on the samples exact properties and measurement conditions.

Acknowledgement:

Financial support by the Deutsche Forschungsgemeinschaft (DFG Research Unit 1277, grant no. WI3600/2-2 and 4-1 and HA6996/1-2) as well as by the Austrian Federal Ministry of Science, Research and Economy, and the Austrian National Foundation for Research, Technology and Development (CD-Laboratory of Lithium Batteries: Ageing Effects, Technology and New Materials) is greatly appreciated.

References:

[1] Dunst, A.; Sternad, M.; Wilkening, M.: Overall conductivity and NCL-type relaxation behavior in nanocrystalline sodium peroxide Na₂O₂ – Consequences for Na-oxygen batteries. Mat. Sci. Engin. B 211 (2016) 85-93.

[2] Philipp, M.; Lunghammer, S.; Hanzu, I.; Wilkening, M.: Partial electronic conductivity of nanocrystalline Na₂O₂. Mater. Res. Express 4 (2017) 075508.