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

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.