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

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 Cu²⁺ and Zn²⁺ 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 Cu²⁺ and Zn²⁺ 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.