Co-intercalation of Mg2+ and Na+ in Na0.69Fe2(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 Na0.69Fe2(CN)6 are compared as high-voltage cathode materials for magnesium batteries. Interestingly, while Mg2+ ions cannot be intercalated in Fe2(CN)6, Na0.69Fe2(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 Na0.69Fe2(CN)6 than in Na+-free Fe2(CN)6, indicating Na+ ions in Na0.69Fe2(CN)6 plays a crucial role in stabilizing Mg2+ ions. Na0.69Fe2(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.