title: Direct Evidence of Solution-Mediated Superoxide Transport and Organic Radical Formation in Sodium-Oxygen Batteries.
abstract: Advanced large-scale electrochemical energy storage requires cost-effective battery systems with high energy densities. Aprotic sodium-oxygen (Na-O2) batteries offer advantages, being comprised of low-cost elements and possessing much lower charge overpotential and higher reversibility compared to their lithium-oxygen battery cousins. Although such differences have been explained by solution-mediated superoxide transport, the underlying nature of this mechanism is not fully understood. Water has been suggested to solubilize superoxide via formation of hydroperoxyl (HO2), but direct evidence of these HO2 radical species in cells has proven elusive. Here, we use ESR spectroscopy at 210 K to identify and quantify soluble HO2 radicals in the electrolyte-cold-trapped in situ to prolong their lifetime-in a Na-O2 cell. These investigations are coupled to parallel SEM studies that image crystalline sodium superoxide (NaO2) on the carbon cathode. The superoxide radicals were spin-trapped via reaction with 5,5-dimethyl-pyrroline N-oxide at different electrochemical stages, allowing monitoring of their production and consumption during cycling. Our results conclusively demonstrate that transport of superoxide from cathode to electrolyte leads to the nucleation and growth of NaO2, which follows classical mechanisms based on the variation of superoxide content in the electrolyte and its correlation with the crystallization of cubic NaO2. The changes in superoxide content upon charge show that charge proceeds through the reverse solution process. Furthermore, we identify the carbon-centered/oxygen-centered alkyl radicals arising from attack of these solubilized HO2 species on the diglyme solvent. This is the first direct evidence of such species, which are likely responsible for electrolyte degradation.
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Chun
authors: C
authors: Xia
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Russel
authors: R
authors: Fernandes
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Franklin H
authors: FH
authors: Cho
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Niranjan
authors: N
authors: Sudhakar
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Brandon
authors: B
authors: Buonacorsi
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Sean
authors: S
authors: Walker
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Meng
authors: M
authors: Xu
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Jonathan
authors: J
authors: Baugh
authors: Department of Chemistry, §Institute for Quantum Computing, ∥Department of Physics and Astronomy, and ‡Waterloo Institute for Nanotechnology, University of Waterloo , Waterloo Ontario N2L 3G1, Canada.
authors: Linda F
authors: LF
authors: Nazar