Charge transfer induced highly active low-spin iron of Prussian blue cathode through calcination strategy for high performance Sodium-ion batteries

Prussian blue (PB) and its analogues have garnered considerable attention due to their spacious open framework, substantial specific capacity, facile synthesis protocol and cost-effectiveness as cathode materials for sodium-ion batteries (SIBs). Nonetheless, the incomplete electrochemical reaction of low-spin (LS) Fe often results in suboptimal practical specific capacity and diminished specific energy. In this study, a calcination strategy is put forward and successfully activates LS-Fe centers within PB, which exhibits a prolonged high-voltage plateau when employed as a cathode material for SIBs. Notably, PB with LS-Fe activated (denoted as PB-325) contributes with a specific capacity of 65 and 62 mAh/g during the first and fifth charge–discharge cycles, respectively, surpassing those of the pristine material. Comprehensive investigations involving operando 57Fe Mössbauer spectroscopy, ex situ soft X-ray absorption spectroscopy, and density functional theory calculations unveil a charge transfer-induced spin transition phenomenon occurring during the electrochemical reaction process, which involves transfer of charges from HS-Fe to LS-Fe. The intrinsic narrower bandgap and charge-accumulated LS-Fe species both contribute to enhanced reactivity, especially in LS-Fe site, thus leading to increased specific capacity within the high-voltage range and improving the performance of PB-325. This study provides an effective and simple strategy to activate LS-Fe, and deepens the understanding of the mechanisms of high-performance PB cathodes for SIBs.