Elevating the discharge plateau of prussian blue analogs through low-spin Fe redox induced intercalation pseudocapacitance

Low-cost Prussian blue analogs (PBAs) have attracted great attentions as a group of promising cathodes for sodium-ion batteries (SIBs) due to their tailorable and open frameworks which endue the possibility of ultrahigh Na+ diffusion kinetics. Herein, a unique discharging plateau elevation was firstly determined in the iron hexacyanoferrate. The synergy between the crystal field and ligand field stabilization energy in the OH-coordinated Fe sites lowers the activation energy barrier of low-spin Fe, thus inducing the intercalation pseudocapacitance. Furthermore, we prove that the Na+ storage mechanism of high-spin Fe redox reaction features an ion–diffusion behavior while the low-spin Fe redox reaction shows a pseudocapacitance behavior. Benefitting from the improved ionic diffusivity in intercalation pseudocapacitance, the full cell achieves an outstanding rate performance and long-term cycling stability of over 3000 cycles at 500 mA g−1. It is expected that manipulating Fe redox kinetics of the PBAs through inducing special coordinated groups could be a new pathway towards the design of practical high-voltage SIBs.