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_1.96Cu_0.45Mn_0.55[Fe(CN)₆]_0.91·□_0.09·2.14H₂O (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.