Spin-state-mediated mitigation of Jahn-Teller distortion in Prussian blue analogs enabling high-performance...

Prussian blue analogues (PBAs) have emerged as highly promising cathode materials for sodium-ion batteries (SIBs). However, their practical application is significantly hindered by marked capacity decay during cycling, which is predominantly attributed to the Jahn-Teller (J-T) distortions of redox-active metal sites. Despite extensive efforts to enhance the reversibility and cyclability of PBA cathodes, the role of the electronic configuration and spin state of transition metal atoms in alleviating structural instability, particularly through electronic modulation of J-T distortions, has remained relatively unexplored. In this work, we strategically performed the transposition of Co and Fe within the cyanide-bridged framework to investigate this relationship. The resulting CoFe-PBA (-Co-N ≡ C-Fe-) and FeCo-PBA (-Fe-N ≡ C-Co-) configurations exhibit contrasting electrochemical stability, underscoring the critical influence of metal coordination environments. Notably, CoFe-PBA demonstrates superior cyclability, retaining 84.9 % of its capacity after 500 cycles at 25 mAh·g^-1, compared to just 35.1 % for FeCo-PBA. Operando Raman spectroscopy reveals that during CoFe-PBA sodiation, cyano ligand-mediated electron redistribution occurs within the cyanide-bridged framework. This process stabilizes the lattice through cooperative metal-ligand orbital interactions. Our work establishes a direct correlation between the transition of the electronic structure of transition metals and electrode stability, offering fundamental insights for the design of high-performance PBA cathodes. These findings highlight electronic structure engineering as a pivotal strategy for advancing SIB technologies.