Epitaxial Nickel Ferrocyanide Stabilizes Jahn–Teller Distortions of Manganese Ferrocyanide for Sodium‐Ion Batteries

Abstract Manganese‐based Prussian Blue, Na 2−δ 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 Mn 2+ is oxidized to Mn 3+ . Herein, the structure is stabilized by a thin epitaxial surface layer of nickel‐based Prussian Blue (Na 2−δ Ni[Fe(CN) 6 ]). The one‐pot synthesis relies on a chelating agent with an unequal affinity for Mn 2+ and Ni 2+ ions, which prevents Ni 2+ from reacting until the Mn 2+ 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.