Li-ion and related battery technologies will be important for years to come. However, we must explore alternatives if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the metal-O₂ battery; the theoretical specific energy of typical aprotic metal-O₂batteries exceeds that of Li-ion, but many obstacles hinder realization of this technology.[1-4] Overcoming these hurdles will require an understanding of the fundamental electrochemistry at the positive electrode within aprotic metal-air batteries.[5-12]

Recently interest in the Na-O₂ battery has grown due to the relatively low polarization during cycling and the high rates.[13, 14] This is despite the lower specific energy of this battery chemistry.[15] A number of groups have shown promising results with this system.[16-18] An important area of contention is the nature of the discharge product, which, unlike the lithium system that forms Li₂O₂, may form the superoxide, peroxide or oxide. The discharge product in the metal-O₂ battery directly influences the specific capacity and ease of charge. Therefore, it is important to understand the mechanism such that this can be controlled.

We have combined a range of electrochemical, spectroscopic and microscopy methods to investigate the mechanism of electrochemical O₂ reduction. The results of these studies will be presented, along with the implications for the future of rechargeable metal-O₂ batteries.

References:

1. Bruce, P.G., et al., Li-O₂ and Li-S batteries with high energy storage. Nature Materials, 2012. 11(1): p. 19-29.

2. Lu, Y.C., et al., Lithium-oxygen batteries: bridging mechanistic understanding and battery performance. Energy & Environmental Science, 2013. 6(3): p. 750-768.

3. Girishkumar, G., et al., Lithium−air battery: promise and challenges. The Journal of Physical Chemistry Letters, 2010. 1(14): p. 2193-2203.

4. Li, F., T. Zhang, and H. Zhou, Challenges of non-aqueous Li-O₂ batteries: electrolytes, catalysts, and anodes. Energy & Environmental Science, 2013. 6(4): p. 1125-1141.

5. Adams, B.D., et al., Current density dependence of peroxide formation in the Li-O₂ battery and its effect on charge. Energy & Environmental Science, 2013. 6(6): p. 1772-1778.

6. Horstmann, B., et al., Rate-Dependent Morphology of Li2O2 Growth in Li–O2 Batteries. The Journal of Physical Chemistry Letters, 2013. 4(24): p. 4217-4222.

7. Hummelshoj, J.S., A.C. Luntz, and J.K. Norskov, Theoretical evidence for low kinetic overpotentials in Li-O2 electrochemistry. The Journal of Chemical Physics, 2013. 138(3): p. 034703-12.

8. McCloskey, B.D., et al., On the mechanism of nonaqueous Li–O₂ electrochemistry on C and its kinetic overpotentials: Some implications for Li–air batteries. The Journal of Physical Chemistry C, 2012. 116(45): p. 23897-23905.

9. Mitchell, R.R., et al., Mechanisms of Morphological Evolution of Li2O2 Particles During Electrochemical Growth. The Journal of Physical Chemistry Letters, 2013. 4(7): p. 1060–1064.

10. Trahan, M.J., et al., Studies of Li-Air Cells Utilizing Dimethyl Sulfoxide-Based Electrolyte. Journal of The Electrochemical Society, 2013. 160(2): p. A259-A267.

11. Jung, H.G., et al., A transmission electron microscopy study of the electrochemical process of lithium-oxygen cells. Nano Letters, 2012. 12(8): p. 4333-5.

12. Zhai, D., et al., Disproportionation in li-o2 batteries based on a large surface area carbon cathode. Journal of the American Chemical Society, 2013. 135(41): p. 15364-72.

13. Hartmann, P., et al., A rechargeable room-temperature sodium superoxide (NaO2) battery. Nature Materials, 2013. 12(3): p. 228-232.

14. McCloskey, B.D., J.M. Garcia, and A.C. Luntz, Chemical and Electrochemical Differences in Nonaqueous Li-O-2 and Na-O-2 Batteries. Journal of Physical Chemistry Letters, 2014. 5(7): p. 1230-1235.

15. Das, S.K., S. Lau, and L.A. Archer, Sodium-oxygen batteries: a new class of metal-air batteries. Journal of Materials Chemistry A, 2014. 2(32): p. 12623-12629.

16. Hartmann, P., et al., Discharge and Charge Reaction Paths in Sodium–Oxygen Batteries: Does NaO2Form by Direct Electrochemical Growth or by Precipitation from Solution? The Journal of Physical Chemistry C, 2015. 119(40): p. 22778-22786.

17. Xia, C., et al., The critical role of phase-transfer catalysis in aprotic sodium oxygen batteries. Nat Chem, 2015. 7(6): p. 496-501.

18. Lutz, L., et al., High Capacity Na–O2 Batteries: Key Parameters for Solution-Mediated Discharge. The Journal of Physical Chemistry C, 2016. 120(36): p. 20068-20076.