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 1.23V 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 Na2MnFe(CN)6 hexacyanoferrates cathode and NaTi2(PO4)3 NASICON-type anode, in order to increase the discharge voltage. According to the cyclic voltammetry, the electrochemical window of diluted 1molkg-1 NaClO4 aqueous electrolyte is only 1.9V, whereas the corresponding electrochemical window of concentrated 17molkg-1 NaClO4 aqueous electrolyte is widen to 2.8V. This wide electrochemical window of the concentrated aqueous electrolyte allows the Na2MnFe(CN)6//NaTi2(PO4)3 aqueous sodium-ion system to work reversibly. By contrast, the framework of Na2MnFe(CN)6 cathode was destroyed by the hydroxide anion generated in diluted 1molkg-1 electrolyte.
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