A novel strategy for the reduction of coordinated water in Prussian blue analogues for their application as cathode materials for sodium-ion batteries†

Abstract

Prussian blue analogues (PBAs) are promising cathode materials for sodium-ion batteries, and iron-based Prussian blue has received widespread attention owing to its high specific capacity, wide availability and cost-effectiveness. However, as Fe²⁺ in the aqueous phase forms the [Fe(H₂O)₆]²⁺ structure, water molecules get inevitably introduced during the synthesis process, resulting in water molecule residues and vacancy defects, which greatly affect the cycling life and energy densities of PBAs and pose safety issues. Herein, a new facile “ligand pre-exchange strategy” was proposed to synthesize highly crystallized PBAs. Ethylene glycol (EG) was introduced to exchange with H₂O in [Fe(H₂O)₆]²⁺ to form a water-deficient solvated structure of [Fe(EG)_x(H₂O)_6−x]²⁺, which resulted in a reduction of the amount of coordinated water and vacancy defects in the PBAs and formed high-quality PBA crystals. Meanwhile, the formation of [Fe(EG)_x(H₂O)_6−x]²⁺ was confirmed using Fourier transform infrared (FT-IR) spectroscopy and quantum chemical calculations using density functional theory (DFT), proving this strategy's feasibility. The PB-EG-5 electrode prepared by this strategy exhibited excellent sodium storage performance and fast kinetics, with a specific capacity of 91.3 mA h g⁻¹ at 1000 mA g⁻¹ in a half-cell and capacity retention of 70% after 1000 cycles. Furthermore, the full cell exhibited excellent electrochemical performance. This work provides a new feasible solution for the large-scale preparation of high-quality PBAs.