(644f) 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 compared to present state-of-the-art battery technologies. Prior study has demonstrated that the primary discharge product, lithium peroxide (Li₂O₂) is produced via a two-electron oxygen reduction reaction at the cathode.¹ Though this reaction has been shown to occur in reasonably stable nonaqueous aprotic electrolytes, the efficiency of O₂ conversion to Li₂O₂ 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 to the Li-O₂ battery.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 Sodiumâ??s natural abundance and lower cost also make large-scale implementation an attractive possibility.⁴ Nevertheless, many questions remain about the fundamental electrochemical processes occurring in a Na-O₂ cell. For example, since the Li- and Na-O₂ systems appear to be analogous, one would expect the primary discharge product in aprotic electrolytes to be sodium peroxide (Na₂O₂) produced in a two-electron battery reaction. However, in batteries employing an electrolyte with an ethereal solvent, a one-electron reaction producing sodium superoxide (NaO₂) has been widely reported.^2-6 Since Na₂O₂ formation is slightly thermodynamically favored over NaO₂, this result is particularly surprising. Furthermore, prior work suggests that NaO₂ experiences fewer parasitic side reactions than Li₂O₂, leading to favorable overpotentials on charge and a higher cycle efficiency.³ However, subsequent 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.^4-6

In this work, we present unpublished research concerning the operation of nonaqueous Na-O₂ cells while varying key parameters, including the sodium ion concentration in the electrolyte, the oxygen pressure in the headspace, and the current density employed on discharge and charge. We characterize the dependence of cell performance on these conditions, including the cell capacity, discharge and charge overpotentials, and the â??sudden deathâ? phenomenon, a precipitous decrease in potential on discharge and increase in potential on charge that limits cell capacity and efficiency. 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 â??sudden deathâ? phenomenon.

Acknowledgements:
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1106400.

References:

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