Journal
NANO LETTERS
Volume 14, Issue 2, Pages 1016-1020Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nl404557w
Keywords
Size-dependent stability; Na-air batteries; NaO2; Na2O2; sodium superoxide; sodium peroxide
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Funding
- Department of Energy (DOE)'s Basic Energy Sciences program [EDCBEE]
- DOE-Chicago [DE-FG02-96ER45571]
- US China Clean Energy Research Center-Clean Vehicle Consortium of DOE [DE-PI0000012]
- TeraGrid by the Texas Advanced Computing Center (TACC) [TG-DMR970008S]
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The thermodynamic stability of materials can depend on particle size due to the competition between surface and bulk energy. In this Letter, we show that, while sodium peroxide (Na2O2) is the stable bulk phase of Na in an oxygen environment at standard conditions, sodium superoxide (NaO2) is considerably more stable at the nanoscale. As a consequence, the superoxide requires a much lower nucleation energy than the peroxide, explaining why it can be observed as the discharge product in some Na-O-2 batteries. As the superoxide can be recharged (decomposed) at much lower overpotentials than the peroxide, these findings are important to create highly reversible Na-O-2 batteries. We derive the specific electrochemical conditions to nucleate and retain Na-superoxides and comment on the importance of considering the nanophase thermodynamics when optimizing an electrochemical system.
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