Alkaline-Earth Metals: A Green Strategy for Stabilizing Sodium-Rich Fe-Based Prussian Blue

Prussian blue analogues (PBAs), cathode materials for sodium-ion batteries, have the advantages of high theoretical specific capacity, high energy density, and low cost, especially sodium-rich Fe-based PBAs (FeHCF). However, the structural instability associated with the high sodium content caused a rapid decrease in the cycle life of FeHCF. In this research, a series of low-cost sodium-rich FeHCFs were synthesized by a green alkaline-earth metal substitution strategy, and different Mg and Ca substitutions both enhanced the electrochemical stability of FeHCF. Among them, Mg/Ca-cosubstituted FeHCF (FeHCF-MgCa) has enhanced electrochemical performance with a specific capacity and an energy density of 113.76 mAh/g and 355.46 Wh/kg, respectively, after 100 cycles at 8.5 mA/g. Various in situ and ex situ tests revealed the structural stability of FeHCF-MgCa superior to FeHCF. Notably, this alkaline-earth metal substitution strategy provides valuable insights into the synthesis and commercial application of high energy density, low cost, and green PBAs.