Achieving Ultra‐Fast and Stable Sodium‐Ion Batteries Through Deep Activation of Low‐Spin Iron in Prussian Blue

Abstract Prussian blue analogs (PBAs) are promising cathode materials for sodium‐ion batteries (SIBs) due to their high theoretical capacity, abundant iron resources, and simple synthesis. However, their practical implementation is limited by [Fe(CN)₆] vacancies and crystal water, which compromise structural stability and hinder the redox activity of low‐spin iron (Fe _LS ). Herein, a modulation strategy through activating Fe _LS site by introducing Cu ²⁺ and Zn ²⁺ in iron‐based PBA is adopted. Na₁.₅₅Cu₀.₀₅₃Zn₀.₀₆₀₈Fe₀.₈₉[Fe(CN)₆]₀.₉₄□₀.₀₆·1.80H₂O (CZ‐FeFe), is successfully synthesized using co‐precipitation. The initial capacity of CZ‐FeFe is dramatically enhanced by activating the Fe _LS redox activity (from 0.48 to 0.80 e ⁻ ), verified by quasi‐in situ magnetic characterization. Theoretical calculations show improved electron transport and ion diffusion in CZ‐FeFe. Simultaneously, the incorporation of Cu ²⁺ and Zn ²⁺ is also beneficial for reducing [Fe(CN)₆] vacancies, minimizing crystal water, and slowing the phase transition between monoclinic and cubic structure, leading to superior long‐cycling stability. As a result, CZ‐FeFe exhibits a high specific capacity of 144.7 mAh g ⁻¹ at 1 C, exceptional rate performance, and remarkable long‐term stability (77.21% capacity retention after 2500 cycles at 10 C). The full‐cell performance further confirms the activation of Fe _LS (from 0.21 to 0.52 e ⁻ ), along with improvements in rate performance and cycling stability.