Co-intercalation of Mg²⁺ and Na⁺ in Na_0.69Fe₂(CN)₆ as a High-Voltage Cathode for Magnesium Batteries

Thanks to the advantages of low cost and good safety, magnesium metal batteries get the limelight as substituent for lithium ion batteries. However, the energy density of state-of-the-art magnesium batteries is not high enough because of their low operating potential; thus, it is necessary to improve the energy density by developing new high-voltage cathode materials. In this study, nanosized Berlin green Fe₂(CN)₆ and Prussian blue Na_0.69Fe₂(CN)₆ are compared as high-voltage cathode materials for magnesium batteries. Interestingly, while Mg²⁺ ions cannot be intercalated in Fe₂(CN)₆, Na_0.69Fe₂(CN)₆ shows reversible intercalation and deintercalation of Mg²⁺ ions, although they have the same crystal structure except for the presence of Na⁺ ions. This phenomenon is attributed to the fact that Mg²⁺ ions are more stable in Na⁺-containing Na_0.69Fe₂(CN)₆ than in Na⁺-free Fe₂(CN)₆, indicating Na⁺ ions in Na_0.69Fe₂(CN)₆ plays a crucial role in stabilizing Mg²⁺ ions. Na_0.69Fe₂(CN)₆ delivers reversible capacity of approximately 70 mA h g^-1 at 3.0 V vs Mg/Mg²⁺ and shows stable cycle performance over 35 cycles. Therefore, Prussian blue analogues are promising structures for high-voltage cathode materials in Mg batteries. Furthermore, this co-intercalation effect suggests new avenues for the development of cathode materials in hybrid magnesium batteries that use both Mg²⁺ and Na⁺ ions as charge carriers.