Acetonitrile-Based Highly Concentrated Electrolytes for High-Power Organic Sodium-Ion Batteries

Sodium croconate, a high-voltage organic cathode material, can be applied to high-energy-density and cost-effective organic sodium-ion batteries (OSIBs) as an alternative to traditional lithium-ion batteries. However, organic molecular cathodes generally dissolve into the electrolyte, leading to poor cyclability. Thus, an electrolyte that can address the present limitations and further facilitate the fabrication of highly reversible OSIBs must be developed. To address this gap in the literature, in this study, we demonstrate an acetonitrile (AN)-based highly concentrated electrolyte (HCE) with sodium bis(fluorosulfonyl)imide (NaFSI). This electrolyte has an ionic conductivity of 12.1 mS cm^-1, which is higher than those of previously reported HCEs. Moreover, the developed HCE (NaFSI:AN molar ratio of 1:2.7) exhibits a high Na⁺ transference number of 0.49. A full-cell OSIB bearing this electrolyte demonstrates high-power operation with improved capacity retention. The solvent-free electrolyte with the solvation structure of the [2Na⁺-FSI^-] aggregate suppresses the dissolution of organic molecules, leading to their high performance.