Effect of Concentrated Electrolyte on Aqueous Sodium-ion Battery with Sodium Manganese Hexacyanoferrate Cathode

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.