Effect of Zn-substitution induced structural regulation on sodium storage performance of Fe-based Prussian blue

Fe-based Prussian blue (Fe-PB) has attracted wide attention as cathode materials for sodium-ion batteries due to its open frame structure, abundant iron ore resources, and simple preparation. Nevertheless, the poor cycle performance caused by the [Fe(CN)6] defects and crystal water hinders its practical application. Herein, the Fe-PB structure is regulated by Zn-substitution, and the effect of Zn-substitution induced structural regulation on sodium storage performance of Fe-PB is systematically investigated. The density functional theory calculation results confirm that Zn-substitution can reduce the bandgap and decrease the energy barrier of Na+ ions migration. Our experiment results further confirm the Zn-substituted Fe-PB composite (FeZn-PB) has a typical monoclinic structure with higher Na content, fewer [Fe(CN)6] vacancies and lower crystal water. Moreover, Zn-substitution accelerates electrons and sodium ions migration and enhances the activity of both low-spin Fe and high-spin Fe. As a cathode material for sodium-ion batteries, the FeZn-PB electrode has a higher capacity and better cycle stability than Fe-PB. Especially, FeZn-PB delivers an initial capacity as high as 145.0 mAh g−1 with a capacity contribution of 60.5 mAh g−1 from low-spin Fe at 20 mA g−1. Even at a high current density of 1 A g−1, FeZn-PB still delivers a high initial capacity of 98.5 mAh g−1 with a very low capacity decay rate per cycle of only 0.05% over 500 cycles.