Abstract Narrow electrochemical stability window (1.23 V) of aqueous electrolytes is always considered the key obstacle preventing aqueous sodium‐ion chemistry of practical energy density and cycle life. The sodium‐ion water‐in‐salt electrolyte (NaWiSE) eliminates this barrier by offering a 2.5 V window through suppressing hydrogen evolution on anode with the formation of a Na + ‐conducting solid‐electrolyte interphase (SEI) and reducing the overall electrochemical activity of water on cathode. A full aqueous Na‐ion battery constructed on Na 0.66 [Mn 0.66 Ti 0.34 ]O 2 as cathode and NaTi 2 (PO 4 ) 3 as anode exhibits superior performance at both low and high rates, as exemplified by extraordinarily high Coulombic efficiency (&gt;99.2%) at a low rate (0.2 C) for &gt;350 cycles, and excellent cycling stability with negligible capacity losses (0.006% per cycle) at a high rate (1 C) for &gt;1200 cycles. Molecular modeling reveals some key differences between Li‐ion and Na‐ion WiSE, and identifies a more pronounced ion aggregation with frequent contacts between the sodium cation and fluorine of anion in the latter as one main factor responsible for the formation of a dense SEI at lower salt concentration than its Li cousin.

“Water‐in‐Salt” Electrolyte Makes Aqueous Sodium‐Ion Battery Safe, Green, and Long‐Lasting