Rechargeable lithium-oxygen and sodium-oxygen cells are considered as promising systems for next-generation batteries, both scientifically and technologically [1,2]. While the desired cell reaction is clear in the case of non-aqueous Li/O₂ batteries (Li₂O₂ formation), the situation is much less clear in the case of non-aqueous Na/O₂ cells. Two discharge products, with almost equal free enthalpy of formation but different number of transferred electrons and completely different kinetics, appear to compete, i.e. sodium superoxide (NaO₂, one-electron-transfer) and sodium peroxide (Na₂O₂, two-electron-transfer) [3]. In spite of the several fundamental studies published until now, the experimental control parameters for the exclusive formation of one or the other phase during discharge are still unknown.

Moreover, under certain conditions, dihydrated sodium peroxide has also been found. The formation of this product has been ascribed to undesired reactions, such as electrolyte degradation [4], which could provide the required water source. However, this hypothesis is not consistent with the results of the group of Nazar, who reported that Na-O₂ cells cannot be discharged in absence of water, which acts as a proton source [5]. Consequently, if the proton source could be supplied by the electrolyte, Na-O₂ cells with 0 ppm water could be also discharged.

This work tries to summarize and answer the major questions that propel research on Na/O₂ cells at the moment. For this purpose, theoretical and thermodynamic considerations, together with operando XRD analyses are presented. In addition, the implications on the electrochemical performance of the different discharged products are also evaluated, shedding some light on the chemistry of these promising systems. Nevertheless, many questions remain open, and their precise answer will help to improve the understanding of further non-aqueous metal-oxygen systems and to propel the research efforts on their cell chemistry.

References

[1]. P. Hartmann et al., Nat. Mat. 2013, 12, 228-232.

[2]. B. Bergner et al. J. Am. Chem. Soc. 2014, 136, 15054-15064.

[3]. C.L Bender et al., Angew. Chem. Int. Ed. 2016, (accepted).

[4]. J. Kim et al. Phys. Chem. Chem. Phys. 2013, 15, 3623-3629.

[5]. C. Xia et al. Nat. Chem. 2015, 7, 496-501.