Rational Design of Electrolyte Solvation Structures for Modulating 2e⁻/4e⁻ Transfer in Sodium–Air Batteries

Abstract

Abstract In sodium–air batteries (SABs), achieving the regulation of the electron transfer number during oxygen reduction reactions (ORRs) in the same electrolyte system remains a significant challenge. In this work, a promising strategy is proposed to dynamically modulate 2e − /4e − transfer in ORRs by regulating the electrolyte structures to realize the different performances of SABs. The 4e − ORR can be realized by decreasing the electrolyte concentration. The solvation sheath of Na + at dilute concentrations consists mainly of water molecules that hinder the access of Na + to the cathode surface due to the high solvation energies indicated by theoretical calculations, thereby impeding the 2e − reaction. In contrast, excess free water can easily access the cathode surface and trigger the 4e − ORR. The solvation energies of Na + can be remarkably reduced by increasing the electrolyte concentration, forming a water‐in‐salt unit, in which the Na + mainly coordinates with the bis(fluorosulfonyl)imide anion and can be easily released from the solvation sheath. Hence, the 2e − ORR is significantly promoted and becomes the dominant reaction. The SAB based on the 2e − reaction exhibits excellent energy density (15980 Wh kg −1 ) and good cycle performance (300 times), and the 4e − reaction exhibits excellent power density (12.09 mW cm −2 ).