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From the viewpoint of the cost and safety, aqueous sodium-ion batteries are attractive candidate for large-scale energy storage. Although the operating voltage range of the aqueous battery is theoretically limited to 1.23 V by the electrochemical decomposition of water, the voltage restriction is a little bit eased in real aqueous battery system by the charge/discharge overvoltage. Effect of the concentrated electrolyte on the operation voltage was studied in aqueous Na-ion battery with Na 2 MnFe(CN) 6 hexacyanoferrates cathode and NaTi 2 (PO 4 ) 3 NASICON-type anode, in order to increase the discharge voltage. According to the cyclic voltammetry, the electrochemical window of diluted 1 mol kg −1 NaClO 4 aqueous electrolyte is only 1.9 V, whereas the corresponding electrochemical window of concentrated 17 mol kg −1 NaClO 4 aqueous electrolyte is widen to 2.8 V. This wide electrochemical window of the concentrated aqueous electrolyte allows the Na 2 MnFe(CN) 6 //NaTi 2 (PO 4 ) 3 aqueous sodium-ion system to work reversibly. By contrast, the framework of Na 2 MnFe(CN) 6 cathode was destroyed by the hydroxide anion generated in diluted 1 mol kg −1 electrolyte.

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