Surface Gradient Doping Enables High‐Capacity and Long‐Life Manganese‐Based Prussian Blue Cathodes for Sodium‐Ion Batteries

ABSTRACT Manganese‐based Prussian blue analogues (MnPBAs) are attractive cathodes for sodium‐ion batteries (SIBs) due to their high operating voltage, low cost, and sustainability. However, their practical deployment is limited by rapid capacity decay caused by Jahn–Teller distortion and phase‐transition‐induced lattice strain, which trigger Mn dissolution and structural collapse during cycling. Herein, we introduce a surface Cu concentration‐gradient‐doped MnPBA (Cu@MnPBA) synthesized via a two‐step coprecipitation strategy to stabilize the MnPBA framework while preserving high capacity. A ∼200 nm Cu‐gradient surface layer effectively suppresses Jahn–Teller distortion, reduces lattice strain, and inhibits Mn dissolution, while an outer ∼10 nm Cu‐rich amorphous layer enhances Na + surface diffusion and suppresses interfacial side reactions. Unlike conventional coating strategies, the gradient‐doping architecture eliminates interfacial mismatch, ensuring robust structural integrity during prolonged cycling. As a result, Cu@MnPBA delivers a high specific capacity of 143.9 mAh g −1 and retains 78% of its capacity after 500 cycles at 100 mA g −1 . When paired with a hard carbon anode in a pouch cell configuration, the system maintains 75% capacity after 3000 cycles, demonstrating exceptional durability under practical operating conditions. This work establishes gradient surface doping as an effective design principle for realizing high‐capacity, long‐life Prussian blue cathodes for SIBs.