Electrodeposited Na₂VO_x[Fe(CN)₆] films As a Cathode Material for Aqueous Na-Ion Batteries

The so-called Prussian blue analogues (PBAs) are spotlighted as promising cathode materials for aqueous Na-ion batteries regarding their good performance for the application in future large-scale energy storage systems. In this work, we demonstrate that one of the PBA representatives, namely Na₂VO_x[Fe(CN)₆] thin films (VHCFs), is a promising cathode material for aqueous Na-ion batteries with very positive intercalation/deintercalation potentials, which might likely designate a new benchmark in the field. To maximize the material utilization, we have formed VHCF thin films on model current collectors from aqueous solutions. The resulting films demonstrated a very positive half-charge potential (ΔE_1/2 ≈ 0.91 V vs Ag/AgCl reference electrode) in acidic media with a specific capacity of ∼80 mAh g^-1 recorded at high C-rates (30 C) in 1 M LiNO₃, 3 M NaNO₃ and 3 M KNO₃ electrolytes in the presence of 3.6 M H₂SO₄. It is also shown that well-known solvation effects related to the nature of the alkali metal cations during intercalation and deintercalation are surprisingly not pronounced in the case of VHCFs.