Insights into Electrochemical Oxidation of NaO2in Na-O2Batteries via Rotating Ring Disk and Spectroscopic Measurements.
O2reduction in aprotic Na-O2batteries results in the formation of NaO2, which can be oxidized at small overpotentials (<200 mV) on charge. In this study, we investigated the NaO2oxidation mechanism using rotating ring disk electrode (RRDE) measurements of Na-O2reaction products and by tracking the morphological evolution of the NaO2discharge product at different states of charge using scanning electron microscopy (SEM). The results show that negligible soluble species are formed during NaO2oxidation, and that the oxidation occurs predominantly via charge transfer at the interface between NaO2and carbon electrode fibers rather than uniformly from all NaO2surfaces. X-ray absorption near edge structure (XANES), and X-ray photoelectron spectroscopy (XPS) measurements show that the band gap of NaO2is smaller than that of Li2O2formed in Li-O2batteries, in which charging overpotentials are much higher (∼1000 mV). These results emphasize the importance of discharge product electronic structure for rationalizing metal-air battery mechanisms and performance.
Research Laboratory of Electronics, ‡Electrochemical Energy Laboratory, §Department of Mechanical Engineering, ∥Department of Chemistry, and ⊥Department of Materials Science & Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
Robert
R
Morasch
Research Laboratory of Electronics, ‡Electrochemical Energy Laboratory, §Department of Mechanical Engineering, ∥Department of Chemistry, and ⊥Department of Materials Science & Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
David G
DG
Kwabi
Research Laboratory of Electronics, ‡Electrochemical Energy Laboratory, §Department of Mechanical Engineering, ∥Department of Chemistry, and ⊥Department of Materials Science & Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
Michal
M
Tulodziecki
Research Laboratory of Electronics, ‡Electrochemical Energy Laboratory, §Department of Mechanical Engineering, ∥Department of Chemistry, and ⊥Department of Materials Science & Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
Marcel
http://orcid.org/0000-0003-2820-7006
M
Risch
Research Laboratory of Electronics, ‡Electrochemical Energy Laboratory, §Department of Mechanical Engineering, ∥Department of Chemistry, and ⊥Department of Materials Science & Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
Shiyu
S
Zhang
Research Laboratory of Electronics, ‡Electrochemical Energy Laboratory, §Department of Mechanical Engineering, ∥Department of Chemistry, and ⊥Department of Materials Science & Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
Yang
Y
Shao-Horn
X-ray absorption; X-ray photoelectron spectroscopy; direct oxidation; oxygen evolution; sodium superoxide; sodium−oxygen batteries