Synthesis Of Prussian Blue Composites And Their Sodium Storage Performance

Rapid development of clean and sustainable energies, such as wind and solar energies, calls for advanced large-scale energy storage technologies. Recently, sodium-ion batteries(SIBs) have attracted wide interests in the field of large-scale energy storage due to the advantages of abundant resource, high safety and low cost. Sodium iron hexacyanoferrate(Fe-HCF) is a potential cathode material for SIBs due to its 3D open framework and suitable channels for alkali ions, high theoretical capacity, low cost, facile synthesis and eco-friendliness. However, current researches show that Fe-HCF still have some drawbacks, such as low productivity, poor electronic conductivity, side reactions with electrolyte solution, and poor cycling stability, which hinders its practical utilization. In this work, we synthesized the high-quality Fe-HCF via a modified coprecipitation method, and employed polyprrole and NiFe-PBA to coat the Fe-HCF microcubes to form Fe-HCF@PPy composites and Fe-HCF@NiFe-PBA composites.Their sodium storage performance were systematically investigated.Firstly, high-quality and sodium-enriched Fe-HCF microcubeswere prepared with a yield of ~92% by a modified coprecipitation method, which can be scaled up easily. The Fe-HCF exhibits good sodium storage performance.Secondly, conducting polymer polypyrrole(PPy) were coated on the surface of Fe-HCF to form the Fe-HCF@PPy composites by a chemical method. The Fe-HCF@PPy composite exhibits greatly improved sodium storage performance, delivering a reversible capacity of 113.8 mA h g-1 at 25 mA g-1 and a capacity up to 75 mA h g-1 at a large current density of 3000 mA g-1. After 500 cycles at 200 mA g-1, the capacity still retains 79%, indicative of an excellent cyclability. Meanwhile, Fe-HCF@PPy/pTSNa composite with better cycling stability was prepared by employing PPy doped with p-toluenesulfonate(pTSNa) to wrap the microcubes. According to the mechanism analysis, it is concluded that Fe-HCF is apt to react with NaClO4 electrolyte during charge process over 4.1 V, leading to the dissolution and the structure collapse. PPy coated on the surface of Fe-HCF can serve as not only an electronic conductor to enhance the conductivity but also a protective layer to prevent the side reactions and structure collapse of Fe-HCF, which results in remarkably improved rate capability and cycling stability.Furthermore, we employed NiFe-PBA to coat on the surface of Fe-HCF to form the Fe-HCF@Ni Fe-PBA composites and investigated their sodium storage performance. The Fe-HCF@Ni Fe-PBA composites exhibit remarkably enhanced cycling stability and rate performance when compared with the bare Fe-HCF. According to the mechanism analysis, the stable NiFe-PBA shell not only hinders the formation of an insulating over-sodiated surface, but also suppresses the side reactions and structure collapse of Fe-HCF, leading to enhanced rate capability and cycling stability.Our work demonstrates that Fe-HCF modified by PPy or NiFe-PBA shows greatly improved sodium storage performance and exhibits a potential cathode material for SIBs.