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 Mn²⁺ is oxidized to Mn³⁺ . 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 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 cm² 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.