High-Performance Fe-Based Prussian Blue Cathode Material for Enhancing the Activity of Low-Spin Fe by Cu Doping

Iron-based Prussian blue (FeHCF) has great application potential in the large-scale production of sodium-ion (Na⁺) batteries because of its high theoretical capacity and abundant Fe ore resources. However, the Fe(CN)₆ vacancies and crystal water seriously affect the electrochemical performance. Herein, a Cu-doped FeHCF (Cu-FeHCF) cathode material is successfully prepared directly by a coprecipitation method. After Cu doping, the monoclinic structure and the quasi-cubic morphology are retained, but the electrochemical performance is significantly improved. In addition to few Fe(CN)₆ vacancies and low crystal water, the improved performance is also related to the enhanced electrochemical activity of low-spin Fe and the stabilizing effect of Cu on the crystal structure. Moreover, Cu doping also controls the side reaction to a certain extent. As a result, after Cu doping, the initial discharge capacity is enhanced from 107.9 to 127.4 mA h g^-1 at 100 mA g^-1, especially the capacities contributed by low-spin Fe increase from 30.0, 21.7, and 16.7 mA h g^-1 to 48.8, 45.4, and 43.7 mA h g^-1 for the first three cycles, respectively. Even at 2 A g^-1, Cu-FeHCF still has a promising initial capacity of 82.3 mA h g^-1 and only a 0.047% capacity decay rate for each cycle over 500 cycles. Therefore, Cu-FeHCF shows excellent application potential in the field of Na⁺ energy storage batteries.