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

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).