Structural Modulation of Cu‐Mn‐Fe Prussian Blue Analogs for Practical Sodium Ion Cylinder Cells

High‐performance, cost‐effective cathodes are essential for grid‐scale sodium‐ion batteries (SIBs). Prussian blue analogs (PBAs) have shown great potential as SIB cathodes, but achieving both high capacity and long lifespan remains challenging. In this study, a series of low‐cost ternary PBAs synthesized through structural regulation is presented to simultaneously achieve high capacity, stable cycling performance, and broad temperature adaptability. Among them, CuHCF‐3 demonstrates a specific capacity of 132.4 mAh g −1 with 73.3% capacity retention over 1000 cycles. In‐depth analyses, using in situ techniques and density functional theory calculations, reveal a highly reversible three‐phase transition (monoclinic ↔ cubic ↔ tetragonal) in Na 1.96 Cu 0.45 Mn 0.55 [Fe(CN) 6 ] 0.91 ·□ 0.09 ·2.14H 2 O (CuHCF‐3), which is driven by synergistic interactions between Mn and Cu. Mn enhances conductivity, increases the operating voltage, and introduces additional redox centers, while Cu mitigates the Jahn–Teller distortions associated with Mn and buffers volume changes during cycling. This structural synergy results in excellent temperature stability across a wide temperature range (−20 to 55 °C). 18650‐type cylindrical cells based on CuHCF‐3 with high loading density achieve 73.54% capacity retention over 850 cycles. This study offers valuable insights for designing durable, high‐capacity electrode materials for SIB energy storage applications.