Manganese-based Prussian Blue, Na2�����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�����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.