The role of potassium ions in iron hexacyanoferrate as a cathode material for hybrid ion batteries

• The charge/discharge curves with two even plateaus are observed in the NaxKyFeFe(CN)6.
• K1.90FeFe(CN)6 shows low volumetric change during electrochemical cycling.
• The positive effect of K+ ions on electrochemical properties is identified.

The synthesis of NaxKyFe[Fe(CN)6] through a simple co-precipitation method and the application of these nanoparticles in sodium-ion batteries are presented. K1.90Fe[Fe(CN)6] with a cubic structure shows low volumetric change during electrochemical cycling. It is clarified by ex-situ X-ray diffraction and cyclic voltammetry curves that K+ influences the intercalation sites of the neighboring Na+. More significantly, this hexacyanoferrate delivers a high capacity of 137.0 mAh g−1 at a current density of 14 mA g−1 and superior cyclability with 94% capacity retention after 120 cycles, showing great promise for sodium-ion battery applications.