Revisiting the open-framework zinc hexacyanoferrate: The role of ternary electrolyte and sodium-ion intercalation mechanism

Non-flammable rechargeable aqueous sodium-ion batteries (RASB) made from natural abundant resources offer promising opportunities in large-scale energy storage, yet the low energy density as well as low voltage output and the limited cycle life hinder their practical applications. Here, we develop a high-voltage RASB based on rhombohedral zinc hexacyanoferrate as cathode materials, carbon-coated NaTi2(PO4)(3) as anode materials and ternary NaClO4-H2O-polyethylene glycol (Na-H2O-PEG) as electrolyte to overcome these drawbacks. Such an RASB can deliver a high voltage output of 1.6 V with a specific energy density of 59 Wh kg(-1) based on the total mass of active electrode materials. In addition, it possesses an excellent rate capability as an ultra-capacitor (2.7 kW kg(-1)). The capacity retention more than 91% is obtained after 100 cycles. Finally, a reversible phase transformation between rhombohedral Zn-3[Fe(CN)(6)](2) and rhombohedral Na2Zn3[Fe(CN)(6)](2) that are accompanied by the insertion/extraction of sodium ion in zinc hexacyanoferrate is unveiled.