Epitaxial Nickel Ferrocyanide Stabilizes Jahn-Teller Distortions Of Manganese Ferrocyanide For Sodium-Ion Batteries

Manganese-based Prussian Blue, Na2-delta Mn[Fe(CN)(6)] (MnPB), is a good candidate for sodium-ion battery cathode materials due to its high capacity. However, it suffers from severe capacity decay during battery cycling due to the destabilizing Jahn-Teller distortions it undergoes as Mn2+ is oxidized to Mn3+. Herein, the structure is stabilized by a thin epitaxial surface layer of nickel-based Prussian Blue (Na2-delta Ni[Fe(CN)(6)]). The one-pot synthesis relies on a chelating agent with an unequal affinity for Mn2+ and Ni2+ ions, which prevents Ni2+ from reacting until the Mn2+ is consumed. This is a new and simpler synthesis of core-shell materials, which usually needs several steps. The material has an electrochemical capacity of 93 mA h g(-1), of which it retains 96 % after 500 charge-discharge cycles (vs. 37 % for MnPB). Its rate capability is also remarkable: at 4 A g(-1) (ca. 55 C) it can reversibly store 70 mA h g(-1), which is also reflected in its diffusion coefficient of ca. 10(-8) cm(2) s(-1). The epitaxial outer layer appears to exert an anisotropic strain on the inner layer, preventing the Jahn-Teller distortions it normally undergoes during de-sodiation.