Hexacyanoferrate-Type Prussian Blue Analogs: Principles and Advances Toward High-Performance Sodium and Potassium Ion Batteries

Na-ion batteries (NIBs) and K-ion batteries (KIBs) are promising candidates for next-generation electric energy storage applications due to their low costs and appreciable energy/power density compared to Li-ion batteries. In the search for viable electrode materials for NIBs and KIBs, Prussian blue analogs (PBAs) with inherent rigid and open frameworks and large interstitial voids have shown an impressive ability to accommodate big alkali-metal ions without structure collapse. In particular, hexacyanoferrates (HCFs) utilizing abundant Fe(CN)₆ resources are the most interesting subgroup of PBAs, being able to deliver a specific capacity of 70–170 mAh g^‒1 and a voltage of 2.5‒3.8 V in NIBs/KIBs. In this Review, a comprehensive discussion of the HCF-type cathode materials in terms of their structural features, redox mechanisms, synthesis control, and modification strategies based on research advances over the last ten years. The methodologies and achievements in improving the material properties of HCFs including the compositional stoichiometry, crystal water, crystallinity, morphology, and electrical conductivity are outlined, with the aim to promote understanding of these materials and provide new insights into future design of PBAs for advanced rechargeable batteries.