LAUSR

Abstract In this work, highly crystalline transition-metal hexacyanoferrates (MHCFs) have been synthesized in the presence of citrate, which is found not only to mediate the precipitation reaction rate but also prevent the MHCF crystals from aggregation. This method is applicable to a wide range of MHCFs with different transition metals (e.g., Co, Ni, Mn and Zn). The highly crystalline MHCFs with little Fe(CN)6 vacancies and rich alkaline ions exhibited significantly improved electrochemical performance for Na ion storage in both aqueous and organic electrolytes. The effect of transition metals in MHCFs on the Na ion storage performance has been investigated in an aqueous electrolyte. Results showed that the CoHCF nanocube sample displayed the highest electrochemical performance than other MHCFs samples synthesized in this work. This CoHCF sample exhibits a specific capacity of 85 mAh g−1 at 1 A g−1. The CoHCF sample also displays a good cycling stability with capacity retentions of 92% after 12,000 cycles in aqueous electrolyte (0.5 M Na2SO4) and 78% after 10,000 cycles in organic electrolyte (NaClO4 in PC + EC with 5% FEC).