Topotactic Epitaxy Self-Assembly of Potassium Manganese Hexacyanoferrate Superstructures for Highly Reversible Sodium-Ion Batteries

The cycle stability and voltage retention of a Na₂Mn[Fe(CN)₆] (NMF) cathode for sodium-ion batteries (SIBs) has been impeded by the huge distortion from NaMn^II[Fe^III(CN)₆] to Mn^III[Fe^III(CN)₆] caused by the Jahn-Teller (JT) effect of Mn³⁺. Herein, we propose a topotactic epitaxy process to generate K₂Mn[Fe(CN)₆] (KMF) submicron octahedra and assemble them into octahedral superstructures (OSs) by tuning the kinetics of topotactic transformation. As the SIB cathode, the self-assembly behavior of KMF improves the structural stability and decreases the contact area with the electrolyte, thereby inhibiting the transition metal in the KMF cathode from dissolving in the electrolyte. More importantly, the KMF partly transforms into NMF with Na⁺ de/intercalation, and the existing KMF acts as a stabilizer to disrupt the long-range JT order of NMF, thereby suppressing the overall JT distortion. As a result, the electrochemical performances of KMF cathodes outperform NMF with a highly reversible phase transition and outstanding cycling performance, and 80% capacity retention after 1500/1300 cycles at 0.1/0.5 A g^-1. This work not only promotes creative synthetic methodologies but also promotes to explore the relationship between Jahn-Teller structural deformation and cycle stability.