Chemical Inhibition Method to Synthesize Highly Crystalline Prussian Blue Analogs for Sodium-Ion Battery Cathodes

The nucleation rate plays a critical role in the synthesis of Prussian blue analogs. Rapid precipitation may lead to a large number of vacancies and a large amount of interstitial water in the material, resulting in poor electrochemical performance in batteries. Hence, sodium citrate is used to compete with [Fe(CN)₆]^4- to slow down the coordination rates of Ni²⁺ and Mn²⁺ ions with ferrous cyanide ions. The feasibility of the experiment is also confirmed by theoretical analysis. Benefiting from stable crystal structure and the removal of interstitial water, the as-prepared Na₂Ni_xMn_yFe(CN)₆ sample exhibits a high reversible capacity of 150 mA h g^-1. In addition, a high rate performance of 77 mA h g^-1 is achieved at a current density of 1600 mA g^-1. Most noteworthy, the Coulombic efficiency and specific capacity gradually increase in the first few cycles, which can be ascribed to the formation of a passivation layer on the surface of the electrode. Continuous testing in an electrolyte solution of 1 M NaPF₆ dissolved in sulfone reveals that the presence of a passivation film is very important for the stability of the electrode.