Coupling active and inactive transition metals to boost calcium storage cycle stability for Prussian blue cathodes

Due to the advantages of resource abundance and low reduction potential of calcium, calcium-ion battery (CIB) becomes one of the potential candidates for energy storage devices. Prussian blue analogues (PBAs) are promising cathode materials for CIB, but they suffer from limited capacity and poor cyclability. Herein, a new PBA cathode NiCoHCF is designed with electrochemical active Co and inactive Ni. During charge and discharge process, active Co with limited redox contributes extra reversible capacity along with Fe, while inactive Ni stabilizes the structure and elevates the activity of low-spin Fe, thus enhancing rate capability and contributing capacitance capacity. By the synergistic effect of Co and Ni, the obtained PBAs cathode exhibits much better electrochemical performance than PBAs with only Co (CoHCF) or Ni (NiHCF). In-situ X-ray diffraction proves highly reversible structure evolution of NiCoHCF during cycling. X-ray adsorption spectroscopy and theoretical calculation reveal that, the activity of Co is suppressed, while that of Fe is promoted in NiCoHCF compared with CoHCF. The design strategy opens up new opportunities for the development of CIB cathode materials.