Concentration-Gradient Prussian Blue Cathodes for Na-Ion Batteries

A concentration-gradient composition is proposed as an effective approach to solve the mechanical degradation and improve the electrochemical cyclability for cathodes of sodium-ion batteries. Concentration-gradient Na_xNi_yMn_1–yFe­(CN)₆·nH₂O particles, in which the Ni content gradually increased from the interior to the particle surface, were synthesized by a facile co-precipitation process. Using these particles as the active material in cathodes resulted in an improved electrochemical performance compared to homogeneous Na_xMnFe­(CN)₆·nH₂O, delivering a high reversible specific capacity of 110 mAh g^–1 at 0.2 C and outstanding cycling stability (93% retention after 1000 cycles at 5 C). Electron microscopy illustrated that the promising electrochemical properties obtained for the gradient composition can be attributed to its robust microstructure, which alleviated the electrochemically induced stresses and accumulated damage during sodiation/desodiation, preventing hence the fracture which was observed in the homogeneous samples. . These findings render a prospective strategy to develop high-performance electrode materials for sodium-ion batteries.