Lessening Prussian blue analogues coordinated water with a novel strategy as cathode for Sodium-ion batteries

Abstract

Prussian blue analogues (PBAs) are a promising cathode material for sodium-ion batteries, and the iron-based Prussian blue has a higher specific capacity while being widely available and inexpensive, so it has received more widespread attention. However, Fe2+ in the aqueous phase will form the structure of [Fe(H2O)6]2+, water molecules will inevitably be introduced during the synthesis process, resulting in water molecule residues and vacancy defects, which greatly affects the cycling life of PBAs, energy density and bring safety issues. Herein, a facile “ligand pre-exchange strategy” is proposed to synthesize highly crystallized PBAs. Ethylene glycol (EG) is introduced for exchanging water molecules in [Fe(H2O)6]2+ to form a water-deficient solvated structure of [Fe(EG)x(H2O)6-x]2+, which reduces the coordinated water and vacancy defects in the Prussian blue material and forms high-quality Prussian blue crystals. Meanwhile, the formation of [Fe(EG)x(H2O)6-x]2+ was demonstrated by Fourier Transform Infrared Spectrum (FT-IR) and quantum chemical calculations using Density functional theory (DFT), proving this strategy's feasibility. The PB-EG-5 electrode prepared by this strategy has excellent sodium storage performance and fast kinetics, with a specific capacity of 91.3 mAh g-1 at 1000 mA g-1 in a half cell and capacity retention of 70% after 1000 cycles, while, the full cell also has excellent electrochemical performance. This work provides a new feasible solution for the large-scale preparation of high-quality PBAs.