Mn²⁺ Gradient-Release Strategy-Assisted Synthesis of Highly Crystalline Mn-Based Prussian Blue Analogue Cathodes for...

Mn-based Prussian blue analogues (MnPBAs) have emerged as attractive cathodes for aqueous sodium-ion batteries (ASIBs) due to their high theoretical capacities with an open 3D framework and cost-effective precursors. Nevertheless, framework defects ([Fe(CN)₆]^4- vacancies), together with lattice water, severely undermine the structural integrity, thereby promoting irreversible structural evolution upon cycling and accelerating capacity fade. Herein, a metal-ion "gradient release" strategy is employed to avoid the locally high ion concentration caused by instantaneous release, thereby slowing the crystallization rate of the material during coprecipitation and enabling the successful synthesis of highly crystalline MnPBAs. This strategy is based on the partial dissociation of weak-acid metal salts: Mn²⁺ is gradually released from manganese acetate (Mn(Ac)₂) via dissociation/hydrolysis-coordination equilibria and citrate complexation-decomplexation and subsequently incorporates into the [Fe(CN)₆]^4- framework. The moderated crystallization process suppresses defect formation and limits the incorporation of crystalline water into the framework, thus enhancing the electrochemical performance. Consequently, the optimized MnPBAs deliver a high specific capacity of 143.5 mAh g^-1 at 100 mA g^-1 and still retain 71.1 mAh g^-1 at 1500 mA g^-1, demonstrating excellent rate capability. Moreover, a capacity retention of 88.4% is achieved after 100 cycles at 750 mA g^-1. Also, the full cell MnPBAs//NaTi₂(PO₄)₃ delivers an operating voltage of ∼1.9 V and retains 78.8% of its capacity over 120 cycles at 750 mA g^-1. This work provides a simple and scalable pathway to improving the structural quality and electrochemical durability of MnPBA cathodes for ASIBs.