Influence of Structural Imperfection on Electrochemical Behavior of Prussian Blue Cathode Materials for Sodium Ion Batteries

Prussian blue compounds (PBs) have been intensely studied as promising cathode materials for sodium ion batteries due to their rigid open framework which benefits Na+ insertion/extraction. However, the existence of Fe(CN)6 vacancies occupied by coordinating H2O in the framework has great influence on the electrochemical performance. In this work, we prepared PB compounds using simple solution precipitation method at 0°C, 40°C and 80°C, respectively. The obtained Na1.73Fe[Fe(CN)6]0.98□0.02 (PB0), Na1.27Fe[Fe(CN)6]0.94□0.06 (PB40) and Na1.12Fe[Fe(CN)6]0.92□0.08 (PB80) samples were investigated to clarify the effects of Fe(CN)6 vacancies and the coordinating H2O molecules on the electrochemical behavior. CV tests indicated that coordinating H2O could be gradually extracted and attend side reaction at near 4.0V, which led to impedance growth and caused capacity decrease during cycling. Dehydrated PB samples showed increased reversible capacities. However, the cycling stability of the PBs mainly depended on the perfection of the lattice structure. Low defect structure Na1.73Fe[Fe(CN)6]0.98□0.02 exhibited superior cycling stability with initial reversible capacity of 105 mAh g−1 (100 mA g−1) and intact cubic phase lattice structure during long term cycling. Dehydrated Na1.73Fe[Fe(CN)6]0.98□0.02 showed further improved reversible capacities of 123 mAh g−1 (100 mA g−1) with 73% capacity retention after 200 cycles.