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.

Hierarchical Porous Carbon Spheres for High‐Performance Na–O2 Batteries

“…This is consistent with the cycling behavior of Na-O 2 batteries in Figure 4, as the Coulombic efficiency is slightly less than 100%. [38,39] In summary, hierarchical porous oxygen electrodes made of PCSs were synthesized and used as cathode material to realize an aprotic Na-O 2 battery with high capacity, high rate capability, and long cycle life. [37] In an attempt to completely decompose NaO 2 , the electrode potential was hold at 3.0 V for prolonged time (typically 10 min) after the charging voltage increased to the anodic cutoff potential, and a complete disappearance of the XRD pattern of NaO 2 was observed, as shown in Figure 5e.…”

Alkali‐Oxygen Batteries Based on Reversible Superoxide Chemistry

“…This idea is also supported by experimental measurements of other superoxide phases such as potassium, rubidium, and cesium . One explanation for the low overpotentials observed with NaO 2 could be solution‐meditated growth . Xia et al.…”

“…[23] One explanation for the low overpotentials observed with NaO 2 could be solution-meditated growth. [24][25][26] Xia et al studied the influence of water content in the electrolyte. [27] Dry electrolytes exhibited very low capacities compared with electrolytes with 10 ppm of water.R otating ring disc electrode (RRDE)m easurements also confirm an increase in the soluble superoxide species.…”

Electrocatalysis of Rechargeable Non‐Lithium Metal–Air Batteries

“…During the typical electrochemical process of Na–air battery, the sodium metal is oxidized and sodium ions migrate across the organic/organic–aqueous electrolyte. After oxygen dissolving in the regions of electrolyte near cathode, superoxide species (O 2 − ) are generated, where sodium superoxide is then formed as the discharge product . The schematic diagram of a typical Na–air battery is shown in Figure A.…”