Reduction-Induced Topological Phase Transition to Construct K₂Mn[Fe(CN)₆] Superstructures for High-Performance Sodium-Ion Batteries

Potassium manganese-based Prussian blue analogs (KMn-HCF) hold great potential as cathodes for sodium-ion batteries (SIBs). However, the rapid synthesis process often results in excessively small particle sizes, increasing surface area and thereby intensifying side reactions with the electrolyte, which can damage the cathode electrolyte interface (CEI) and diminish cycling stability. Herein, we designed a topological phase transition strategy to assemble small KMn-HCF particles into a 600 nm cubic superstructure. Structurally, the assembled structure in KMn-HCF significantly improves the cycling stability and durability of KMn-HCF as a SIB cathode by reducing defects, enhancing the uniformity and stability of the CEI layer, minimizing contact with the electrolyte, and reinforcing structural integrity. From a compositional perspective, KMn-HCF exhibits lower Jahn-Teller distortion, reducing overall lattice distortion while allowing larger K⁺ to act as pillars, supporting the Mn-HCF framework and preventing capacity degradation caused by structural deterioration. The robust CEI layer formed in the KMn-HCF cubic superstructure delivered stable charge-discharge voltage profiles and reversible plateaus, achieving a capacity retention of 88.4%/89.1% after 1000 cycles at current densities of 0.1 and 0.5 A g^-1, respectively.