Highly durable and stable sodium superoxide in concentrated electrolytes for sodium-oxygen batteries

Abstract

Rechargeable sodium-oxygen batteries have attracted considerable interest as promising candidates for next-generation batteries owing to their large energy density, high energy efficiency, and potential cost-effectiveness. The intrinsic stability of the discharge product, sodium superoxide, results in highly reversible solution-based electrochemistry in sodium-oxygen batteries, leading to their higher energy efficiency and reversibility compared with their lithium counterparts. However, recent studies have shown that extended storage of sodium superoxide in ethereal electrolytes induces dissolution of the sodium superoxide and undesirable chemical reactions with the electrolytes, resulting in significant degradation of the cell reversibility. In this study, the use of high-concentration electrolytes is explored to obstruct the dissolution of sodium superoxide and to suppress parasitic reactions during storage of sodium-oxygen batteries. Unique solvated structures are identified with the elimination of free solvents through systematic investigations using controlled electrolyte concentrations. Time-resolved ex situ characterizations reveal that sodium superoxide stored in concentrated electrolyte exhibits far greater stability and prolonged lifetime than that stored in conventional electrolytes. It is finally demonstrated that the highly durable sodium superoxide in the concentrated electrolyte succeeds in preserving both the high energy efficiency and oxygen reversibility of sodium-oxygen batteries even with long rest periods every cycle.