Ion-exchange synthesis of low-water Prussian blue analogs for enhanced sodium storage

Iron hexacyanoferrate (FeHCF) is a promising cathode material for sodium-ion batteries (SIBs) due to its high theoretical capacity and low cost. Nevertheless, water in FeHCF is likely to take up Na+ sites leading to the reductions in capacity and rate capability. Herein, an ion-exchange method is proposed to synthesize low-water potassium-sodium mixed iron hexacyanoferrate (KNaFeHCF). The ion-exchange method can preserve the lattice structure with low vacancies and K+ with larger ionic radii can reduce the water content in FeHCF and improve Na+ reaction kinetics. Compared with the NaFeHCF synthesized by co-precipitation method, the water content of optimal sample KNaFeHCF-12 h can be decreased by 21.2%. The sample exhibits excellent electrochemical performance, with a discharge capacity of 130.33 at 0.1 and 99.49 mAh g−1 at 30 C. With a full-cell configuration with a hard carbon anode, the discharge capacity reaches 115.3 mAh g−1 at 0.1 C. This study demonstrates a viable method for producing Prussian blue cathode materials with low water content, high specific capacity, and exceptional cycling stability.