Singlet Oxygen during Cycling of the Aprotic Sodium-O2 Battery

Operando Monitoring of the Solution-Mediated Discharge and Charge Processes in a Na-O2 Battery Using Liquid-Electrochemical Transmission Electron Microscopy

Although in sodium-oxygen (Na-O₂) batteries show promise as high-energy storage systems, this technology is still the subject of intense fundamental research, owing to the complex reaction by which it operates. To understand the formation mechanism of the discharge product, sodium superoxide (NaO₂), advanced experimental tools must be developed. Here we present for the first time the use of a Na-O₂ microbattery using a liquid aprotic electrolyte coupled with fast imaging transmission electron microscopy to visualize, in real time, the mechanism of NaO₂ nucleation/growth. We observe that the formation of NaO₂ cubes during reduction occurs by a solution-mediated nucleation process. Furthermore, we unambiguously demonstrate that the subsequent oxidation of NaO₂ of which little is known also proceeds via a solution mechanism. We also provide insight into the cell electrochemistry via the visualization of an outer shell of parasitic reaction product, formed through chemical reaction at the interface between the growing NaO₂ cubes and the electrolyte, and suggest that this process is responsible for the poor cyclability of Na-O₂ batteries. The assessment of the discharge-charge mechanistic in Na-O₂ batteries through operando electrochemical transmission electron microscopy visualization should facilitate the development of this battery technology.

Singlet Oxygen during Cycling of the Aprotic Sodium-O2 Battery

Aprotic sodium–O₂ batteries require the reversible formation/dissolution of sodium superoxide (NaO₂) on cycling. Poor cycle life has been associated with parasitic chemistry caused by the reactivity of electrolyte and electrode with NaO₂, a strong nucleophile and base. Its reactivity can, however, not consistently explain the side reactions and irreversibility. Herein we show that singlet oxygen (¹O₂) forms at all stages of cycling and that it is a main driver for parasitic chemistry. It was detected in‐ and ex‐situ via a ¹O₂ trap that selectively and rapidly forms a stable adduct with ¹O₂. The ¹O₂ formation mechanism involves proton‐mediated superoxide disproportionation on discharge, rest, and charge below ca. 3.3 V, and direct electrochemical ¹O₂ evolution above ca. 3.3 V. Trace water, which is needed for high capacities also drives parasitic chemistry. Controlling the highly reactive singlet oxygen is thus crucial for achieving highly reversible cell operation.