Oxalate-Assisted Kinetic Synthesis of High-Sodium, Low-Defect Mn/Fe Prussian Blue Analogues for Enhanced Performance in Sodium-Ion Batteries

Iron-based Prussian blue analogues (PBAs) are promising cathode materials for sodium-ion batteries owing to their low cost and high theoretical capacity. However, their practical capacity is often hampered by a high lattice water content and structural defects. While the use of complexing agents and elemental substitution can mitigate these issues, the subsequent difficulty in recovering these complexing agents raises costs and hinders industrial scalability. To address this challenge, we present a novel oxalate-assisted kinetic synthesis route. This method leverages the slow dissolution of metal oxalate precipitates to gradually release Fe2+ and Mn2+ ions, which then coordinate with ferrocyanide ions to form PBAs. This controlled release kinetics eliminates the need for extraneous complexing agents, enabling the preparation of high-sodium-content, low-defect PBAs. By optimizing the Mn/Fe ratio, we found that the sample with 20% Mn doping (PBA@20% Mn) delivers an excellent specific capacity of 115.5 mAh g–1 at 0.2 C and retains 85.6% of its initial capacity after 500 cycles at 5 C. Ex-situ XRD analysis reveals highly reversible (de)sodiation processes with no significant phase transitions, accounting for the superior cycling stability. Furthermore, the oxalate in the mother liquor can be filtered and reused to produce high-quality PBAs, thereby demonstrating a consistent closed-loop process. This work not only provides a strategy for synthesizing high-performance PBAs but also proposes a green, cost-effective pathway for their industrial production.