Comprehensive study of Na_2-xMnFe(CN)₆·yH₂O cathodes with cube morphology: Structure, valence state and electrochemical properties

In this paper, we have synthesized cube-like Na_2-xMnFe(CN)₆·yH₂O cathodes (C-Na_2-xMnHFC) by controlling the nucleation rate during precipitation reaction. Meantime, we have systematically studied structure, chemical composition, and valence state of cube sample, as well as its electrochemical performances. Compared with rapidly crystallized Na_2-xMnFe(CN)₆·yH₂O (R-Na_2-xMnHFC), the C-Na_2-xMnHFC exhibits a lower contents of Fe(CN)₆ defects and crystal water, higher crystalline and Na⁺ content. Besides, it delivers better electrochemical properties, which could obtain a high specific capacity of 133 mAh g^{− 1} and capacity retention rate of 80% after 100 cycles at 150 mA g^{− 1}. And even when the current density increases to 300 mA g^{− 1}, the capacity retention rate can still remain at 80%. Moreover, the discharge capacity reaches about 92% when the current density returns back to 15 mA g^{− 1} from 750 mA g^{− 1}, revealing its better stability at various rates. These improvements in electrochemical performance are attributed to the positive influences of the cube-like structure, which could stabilize the structure upon cycling, increase the contact area between the electrodes and electrolytes, and enhances the Na⁺ ions diffusion coefficient.