Sodium iron hexacyanoferrate (Fe-HCF) is regarded as a potential cathode material for sodium-ion batteries (SIBs) due to its high specific capacity, low cost, facile synthesis and environmentally friendly. However, Fe-HCF always suffers from poor electronic conductivity, low crystallinity and side reactions with electrolyte, leading to poor rate performance, low coulombic efficiency and deterioration of cycling stability. Herein, we report the green and facile synthesis to encapsulate Fe-HCF microcubes with potassium nickel hexacyanoferrate (Ni-HCF) and PPy, separately, the surface modified Fe-HCF can exhibit excellent electrochemical performance as cathode in SIBs (Fig. 1).

Firstly, the Fe-HCF@PPy composite exhibits a discharge capacity of 113.0 mA h g^-1 at a current density of 25 mA g^-1 and 75 mA h g^-1 at 3000 mA g^-1. Greatly improved cycling stability is attained with 79% capacity retention over 500 cycles at 200 mA g^-1. The superior rate capability and excellent cyclability can be ascribed to the effects from PPy as both electronic conductor to enhance the conductivity and protective layer to prevent the side reactions.

Secondly, the core-shell Fe-HCF@Ni-HCF composite delivers a reversible capacity of 79.7 mAh g^-1 at 200 mA g^-1 after 800 cycles and a high coulombic efficiency of 99.3 %. In addition, Fe-HCF@Ni-HCF exhibits excellent rate performance, retaining 60 mAh g^-1 at 2000 mA g^-1. The results show that Fe-HCF@Ni-HCF integrates the advantages of both Fe-HCF and Ni-HCF, making it electrochemically stable as cathode material for SIBs.