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<sub>2</sub>MnFe(CN)<sub>6</sub> hexacyanoferrates cathode and NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> NASICON-type anode, in order to increase the discharge voltage. According to the cyclic voltammetry, the electrochemical window of diluted 1 mol kg<sup>−1</sup> NaClO<sub>4</sub> aqueous electrolyte is only 1.9 V, whereas the corresponding electrochemical window of concentrated 17 mol kg<sup>−1</sup> NaClO<sub>4</sub> aqueous electrolyte is widen to 2.8 V. This wide electrochemical window of the concentrated aqueous electrolyte allows the Na<sub>2</sub>MnFe(CN)<sub>6</sub>//NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> aqueous sodium-ion system to work reversibly. By contrast, the framework of Na<sub>2</sub>MnFe(CN)<sub>6</sub> cathode was destroyed by the hydroxide anion generated in diluted 1 mol kg<sup>−1</sup> electrolyte.
Nakamoto, K., Sakamoto, R., Ito, M., Kitajou, A., & Okada, S. (2017). Effect of Concentrated Electrolyte on Aqueous Sodium-ion Battery with Sodium Manganese Hexacyanoferrate Cathode. Electrochemistry, 85(4), 179-185. https://doi.org/10.5796/electrochemistry.85.179