The Role of Carbon on the Discharge Product and Cell Performance of the Sodium Superoxide (NaO2) Battery

Batteries based on the cell reaction between alkali metals and atmospheric oxygen are highly attractive energy stores due to their superior theoretical energy densities. However, despite continuous progress fundamental challenges in further developing these cell systems remain. Understanding the oxygen electrode reaction and improving cycle life while at the same time maximizing the practical energy capacity are some of the most important issues that have to be tackled. Here, we focus on the aprotic sodium-oxygen battery and the role of the carbon electrode on product formation and cell performance (cycle life and capacity)[1-2].

Two different discharge product have recently been reported in literature for the sodium-oxygen battery, one leading to either sodium peroxide or hydrated sodium peroxide and the other to sodium superoxide. [3-8] The thermodynamic driving force is quite similar for both cases and it remains unclear whether only kinetics or also nano-effects [9] determine the reaction route. One important aim of our current study therefore was to explain the discrepancy reported in literature. [1] We investigated whether the electrode material, the structure or the local current density have any influence on the product stoichiometry or the cell performance. Therefore, we tested several types of carbon materials with a broad range in properties. As important finding, we always found phase-pure NaO₂ as discharge product irrespective of the type of carbon used. But at the same time, the achievable capacities range from 300 to values as high as 4000 mAh∙g(C)^-1. We further studied the impact of electrolyte composition on the cell chemistry and found significant differences depending on the type of solvent used. At last, we demonstrate that the cycle life of Na/O₂ cells can be largely improved by using carbon nanotubes based electrodes yielding capacities of 231 mAh∙g(C)^-1 / 0.64 mAh∙cm^-2 for at least 140 cycles. [2]

Acknowledgment: The research was supported by the BASF scientific network of electrochemistry and batteries. C.L. Bender is grateful to Fonds der chemischen Industrie (FCI) for a scholarship.

Literature:
[1] C.L. Bender, Adv. Energy Mater., 2014, doi: 10.1002/aenm.201301863. [2] C.L. Bender, Energy Technol., 2015, doi: 10.1002/ente.201402208 [3] Q. Sun, Electrochem. Commun., 2012, 16, 22–25. [4] W. Liu, Chem. Commun., 2013, 49,1951-1953. [5] J. Kim, Phys. Chem. Chem. Phys., 2013, 15, 3623-3629. [6] Z. Jian, J. Power Sources, 2014, 251, 466-469. [7] P. Hartmann, Nat. Mater. 2013, 12, 228-232. [8] N. Zhao,  Phys. Chem. Chem. Phys., 2014, 16,15646-15652. [9] S. Kang, Nano Lett., 2014, 14, 1016- 1020.