Charge Transfer Induced Highly Active Low-Spin Iron of Prussian Blue Cathode Through Annealing 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, an annealing strategy for PB is put forward and successfully activates the 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-1 during the first and fifth charge-discharge cycles, respectively, surpassing those of the pristine sample. Furthermore, comprehensive investigations involving operando57Fe Mössbauer spectroscopy, ex situ X-ray absorption spectroscopy, and density functional theory calculations unveil a charge transfer-induced spin transition phenomenon occurring during the electrochemical reaction process. This phenomenon involves the transfer of charges from HS Fe to LS Fe, resulting in a spin transition. The LS Fe species in a charge-accumulated state exhibits enhanced reactivity, leading to increased specific capacity within the high-voltage range and thereby improving the electrochemical performance of PB-325. This study provides an effective and simple strategy to activate the use of LS Fe, and deepens the research and understanding of the mechanisms of high-performance PB cathode materials for SIBs.