85 巻 (2017) 4 号 p. 179-185
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 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 1 mol kg−1 NaClO4 aqueous electrolyte is only 1.9 V, whereas the corresponding electrochemical window of concentrated 17 mol kg−1 NaClO4 aqueous electrolyte is widen to 2.8 V. 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 1 mol kg−1 electrolyte.