Low-valent multielement synergy stabilizing bimetallic Prussian blue cathode for ultra-fast and highly stable sodium-ion batteries

The rational design of Prussian blue analogue (PBA) cathodes with bimetallic reaction centers represents a cornerstone strategy for high-energy sodium-ion batteries (SIBs), yet their electrochemical performance is inherently limited by structural instability and sluggish kinetics. Herein, we propose a multielement co-doping strategy to achieve a holistic optimization of bimetallic Na₂Mn_0.5Fe_0.5[Fe(CN)₆] (MFHCF) by substituting N-coordinated sites with Mg^II, Co^II, and Ni^II. Specifically, the MgCoNi-MFHCF delivers a superior rate capability (145.9 and 85.3 mAh g⁻¹ under 0.1 and 30 C, respectively), outstanding cycling stability (83.1% capacity retention over 1000 cycles), and high energy density (304.5 Wh kg⁻¹ for the full cell). In situ/ex situ techniques and theoretical calculations reveal that the MgCoNi-MFHCF experiences a reversible tri-phase transition with mitigated volume contraction/expansion, which originates from the alleviation of the Jahn-Teller distortion. It is considered that the cation doping enhances redox reaction reversibility through stabilized transition-metal coordination environments while reducing bandgaps and lowering ionic diffusion energy barrier, leading to accelerated electrochemical kinetics. This study establishes a generalizable multielement engineering strategy for high-performance cathode materials with bimetallic reaction centers for SIBs.