One-step synthesis of novel core-shell bimetallic hexacyanoferrate for high performance sodium-storage cathode

Recently, the design of core-shell hierarchical architecture plays an important role in improving the electrochemical performance of Prussian blue analogue cathodes (PBAs). Unfortunately, the inconvenient stepwise preparation and the strict lattice-matching requirement have restricted the development of core-shell PBAs. Herein, we demonstrate a one-step synthesis strategy to synthesize core-shell manganese hexacyanoferrate (MnFeHCF@MnFeHCF) for the first time. And the formation mechanism of the core-shell hierarchical architecture is investigated by first-principles calculations. It is found that the as-obtained MnFeHCF@MnFeHCF act out the superior intrinsic natures, which not only can obtain a larger specific surface area and lower Fe(CN)₆ vacancies but also can activate more Na-storage sites. Compared with the manganese hexacyanoferrate (MnHCF), the iron hexacyanoferrate (FeHCF), and even the traditional core-shell nickel hexacyanoferrate (FeHCF@NiHCF) prepared by a stepwise method, the MnFeHCF@MnFeHCF demonstrates a superior rate performance, which achieves a high capacity of 131 mAh g⁻¹ at 50 mA g⁻¹ and delivers a considerable discharge capacity of about 100 mAh g⁻¹ even at 1600 mA g⁻¹. Meantime, the capacity retention can reach up to nearly 80% after 500 cycles. The improved performances could be mainly originated from two aspects: on the one hand, Mn substitution is helpful to enhance the material conductivity; on the other hand, the core-shell structure with matched lattice parameters is more favorable to enhance the diffusion coefficient of sodium ions. Beside, the structural transformation of MnFeHCF@MnFeHCF upon the extraction/insertion of sodium ions is instrumental in releasing the interior stress and effectively maintaining the integrity of the crystal structure.