20:T4e8,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 (>99.2%) at a low rate (0.2 C) for >350 cycles, and excellent cycling stability with negligible capacity losses (0.006% per cycle) at a high rate (1 C) for >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.21:T446,Dual-ion batteries are known for anion storage in the cathode coupled to cation incorporation in the anode. We flip the sequence of the anion/cation-storage chemistries of the anode and the cathode in dual-ion batteries (DIBs) by allowing the anode to take in anions and a cation-deficient cathode to host cations, thus operating as a reverse dual-ion battery (RDIB). The anion-insertion anode is a nanocomposite having ferrocene encapsulated inside a microporous carbon, and the cathode is a Zn-insertion Prussian blue, Zn₃[Fe(CN)₆]₂. This unique battery configuration benefits from the usage of a 30 m ZnCl₂ "water-in-salt" electrolyte. This electrolyte minimizes the dissolution of ferrocene; it raises the cation-insertion potential in the cathode, and it depresses the anion-insertion potential in the anode, thus widening the full cell's voltage by 0.35 V compared with a dilute ZnCl₂ electrolyte. RDIBs provide a configuration-based solution to exploit the practicality of cation-deficient cathode materials in aqueous electrolytes.

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