Epitaxial Nickel Ferrocyanide Stabilizes Jahn-Teller Distortions of Manganese Ferrocyanide for Sodium-Ion Batteries.

Manganese-based Prussian Blue, Na Mn[Fe(CN) ] (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 Mn is oxidized to Mn . Herein, the structure is stabilized by a thin epitaxial surface layer of nickel-based Prussian Blue (Na Ni[Fe(CN) ]). The one-pot synthesis relies on a chelating agent with an unequal affinity for Mn and Ni ions, which prevents Ni from reacting until the Mn 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 , of which it retains 96 % after 500 charge-discharge cycles (vs. 37 % for MnPB). Its rate capability is also remarkable: at 4 A g (ca. 55 C) it can reversibly store 70 mA h g , which is also reflected in its diffusion coefficient of ca. 10  cm  s . 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.