Ultra‐Long Cycle of Prussian Blue Analogs Achieved by Equilibrium Electrolyte for Aqueous Sodium‐Ion Batteries

Aqueous sodium-ion batteries have promising prospects in large-scale electrical energy storage, which lack of suitable cathode with high specific capacity and long cycle lifespan, unfortunately. Manganese-based Prussian blue analogs (PBAs) (KMnHCF/NaMnHCF) are ideal candidates for low-cost and high theoretical specific capacity merits. But the rapid decline hinders their application, due to side reactions caused by water imbalance. Here, an equilibrium strategy, which can balance the interstitial water supplement and water attack, is proposed. As proof of the concept, xCS (x: proportion, CS: co-solvent, such as polyethylene glycol and trimethyl phosphate) equilibrium electrolytes are introduced to solve the rapid decline. Assisting with the electrolyte, KMnHCF realizes excellent performance (10 000 cycles), which is beyond most cathode materials of sodium-ion batteries. The full batteries composed of KMnHCF cathode and NaTi2 (PO4 )3 anode also display outstanding performance (7000 cycles) and promising application prospects at low-temperature and engineering scenes. And then, the equilibrium electrolyte concept is verified by NaMn0.8 Fe0.2 HCF and NaMnHCF, proving its universality for low-cost and long-life manganese based PBAs.