(670e) Mechanistic Insights into the Sodium-Oxygen Battery Cathode Electrochemistry

The nonaqueous lithium-oxygen (Li-O₂) battery has received increasing attention over the past decade, owing largely to its high theoretical specific energy density. Prior study has demonstrated that the primary Li-O₂ discharge product is produced via a two-electron oxygen reduction reaction at the cathode, though the efficiency of O₂ conversion on discharge and the evolution of O₂ on charge is less than ideal.¹ A similar system, the nonaqueous sodium-oxygen (Na-O₂) battery, has been reported as a possible alternative, where the primary discharge product is produced via a one-electron oxygen reduction reaction.² Despite its lower theoretical specific energy, Na-O₂ batteries have been reported to provide improved cyclability and energy efficiency compared to their Li counterparts.^2-4 Sodium’s natural abundance and lower cost also make large-scale implementation an attractive possibility.⁴ However, prior studies have shown that Na-O₂ battery performance can be highly sensitive to operating conditions and contaminants, leading to changes in capacity, discharge product morphology, and overall cell performance, the fundamental causes of which are not all well understood.^4-6

In this work, we present research concerning the operation of nonaqueous Na-O₂ cells while varying key parameters, with a focus on the oxygen pressure in the headspace on discharge and the current density employed on discharge and charge. We characterize the dependence of cell performance on these conditions, including the cell capacity and the discharge and charge overpotentials. In particular, we examine the causes of the “sudden death” phenomena, a decrease in potential on discharge and an increase in potential on charge that limit cell capacity and efficiency.⁷ The one-electron oxygen reduction and evolution reactions at the cathode are verified quantitatively by pressure decay/rise measurements and differential electrochemical mass spectrometry (DEMS). Further qualitative study is performed using scanning electron microscopy to examine the morphology and deposition of the discharge product. Through this study, we elucidate the limitations of Na-O₂ cell capacity and the mechanistic origins of the “sudden death” phenomena.

References
1. McCloskey, B. D.; Burke, C. M.; Nichols, J. E.; Renfrew, S. E. Chem Commun 2015, 51 (64), 12701-12715.
2. Hartmann, P.; Bender, C. L.; Vračar, M.; Dürr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P. Nat Mater 2013, 12 (3), 228-232.
3. McCloskey, B. D.; Garcia, J. M.; Luntz, A. C. J Phys Chem Lett 2014, 5 (7), 1230-1235.
4. Adelhelm, P.; Hartmann, P.; Bender, C. L.; Busche, M.; Eufinger, C.; Janek, J. Beilstein J Nanotechnol 2015, 6, 1016-1055.
5. Knudsen, K. B.; Nichols, J. E.; Vegge, T.; Luntz, A. C.; McCloskey, B. D.; Hjelm, J. J Phys Chem C 2016, 120 (20), 10799-10805.
6. Xia, C.; Black, R.; Fernandes, R.; Adams, B.; Nazar, L. F. Nat Chem 2015, 7 (6), 496-501.
7. Nichols, J. E.; McCloskey, B. D. J Phys Chem C 2017, 121 (1), 85-96.