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.