Highly Crystallized Na₂CoFe(CN)₆ with Suppressed Lattice Defects as Superior Cathode Material for Sodium-Ion Batteries

Prussian blue and its analogues have received particular attention as superior cathodes for Na-ion batteries due to their potential 2-Na storage capacity (∼170 mAh g^–1) and low cost. However, most of the Prussian blue compounds obtained from the conventional synthetic routes contain large amounts of Fe­(CN)₆ vacancies and coordinated water molecules, which leads to the collapse of cyano-bridged framework and serious deterioration of their Na-storage ability. Herein, we propose a facile citrate-assisted controlled crystallization method to obtain low-defect Prussian blue lattice with greatly improved Na-storage capacity and cycling stability. As an example, the as-prepared Na₂CoFe­(CN)₆ nanocrystals demonstrate a reversible 2-Na storage reaction with a high specific capacity of 150 mAh g^–1 and a ∼ 90% capacity retention over 200 cycles, possibly serving as a low cost and high performance cathode for Na-ion batteries. In particular, the synthetic strategy described in this work may be extended to other coordination-framework materials for a wide range of energy conversion and storage applications