The Role of Carbon on the Discharge Product and Cell Performance of the Sodium Superoxide (NaO2) Battery
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  determine the reaction route. One important aim of our current study therefore was to explain the discrepancy reported in literature.  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. 
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.
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