Influence of alkali metal ions (Na+/K+) in iron-based Prussian blue frameworks on their lithium storage properties

The synthesis of Prussian blue typically necessitates the incorporation of either K⁺ or Na⁺, the resultant Prussian blue analog Cathode materials usually contain either K⁺ or Na⁺. In this work, the lithium storage performance of iron-based Prussian blue containing either Na⁺ (named Na-FeHCF) or K⁺ (named K-FeHCF) is explored and compared in detail. The results demonstrate that Na-FeHCF significantly outperforms K-FeHCF in terms of reversible capacity, cycling stability, rate capability, and electrochemical reaction kinetics. Na-FeHCF maintains a stable reversible capacity of 84.7 mAh g^-1 after 500 cycles at 100 mA g^-1, which is two times higher than the corresponding value of the K-FeHCF (35.5 mAh g^-1). Ex-situ XRD analysis reveals that Na-FeHCF maintains excellent structure stability during discharge and charge processes, while K-FeHCF undergoes an irreversibly transformations from its original cubic phase to a rhombohedral phase during repeated discharge/charge cycles. The results reported in this work could provide a reference for the structural design and performance optimization of iron-based Prussian blue cathode materials for lithium-ion batteries.