Gradient substitution: An intrinsic strategy towards high performance sodium storage in Prussian blue-based cathodes
Benefiting from the abundance of sodium, rechargeable sodium ion batteries (SIBs) are preferred over lithium ion batteries (LIBs) in large-scale storage of electrical energy. Low-cost Prussian blue (PB) is a promising cathode because of its ease of synthesis and rigid open framework. Nevertheless, the less activated low spin Fe in FeC 6 octahedron lowers the average potential and the overall capacity, while the lattice vacancies and side reactions between low spin Fe and the electrolyte shortens PB's cycling life. Herein, PB with a controllable gradient nickel substitution, which features a high-Ni-content outer layer and low-Ni-content inner layer in a single grain, is demonstrated for high performance sodium storage. The high-Ni-content outer layer efficiently prevents side reactions, ensuring excellent cycling stability, while a small amount of nickel ions in the inner layer efficiently activates the low spin Fe for achieving high capacity. Taking the above synergistic advantages, our gradient-PB exhibits a high reversible capacity of 114 mA h g -1 at 100 mA g -1 and 84 mA h g -1 at 1 A g -1 with very stable cycling for up to 1000 cycles. Gradient substitution in PB analogues provides a novel strategy for exploring high performance PB-based cathodes for SIBs.