Investigating the Impact of Copper and Zinc Doping in High-Entropy Prussian Blue Analogues for Na-Ion Batteries: From Material Analysis to Device Fabrication

Abstract

High-entropy Prussian blue analogues (HE-PBAs) show great promise as active materials in Na-ion batteries, particularly due to their multimetallic synergism that enhances electrochemical performance. This study explores two HE-PBAs: Na₂Mn_0.2Fe_0.2Co_0.2Ni_0.2Cu_0.2Fe(CN)₆ (HE-PBA-1) and Na₂Mn_0.2Fe_0.2Co_0.2Ni_0.2Zn_0.2Fe(CN)₆ (HE-PBA-2). Both crystallize in monoclinic (P2₁/n) symmetry, but HE-PBA-1, with Cu, exhibits a lower bandgap, lower Na-ion diffusion barrier, higher [Fe(CN)₆] vacancy, and smaller particle size compared to HE-PBA-2 with Zn. These factors result in higher power capability for HE-PBA-1 due to its enhanced electronic conductivity and Na-ion diffusivity. Additionally, its higher [Fe(CN)₆] vacancy and smaller particle size offer more electrochemical active sites, improving energy characteristics. A Na-ion full cell with HE-PBA-1 as the positive electrode and a mixed-metallic sodium–copper–iron oxide (NaCuFe-Oxide) as the negative electrode in a hydrogel electrolyte is assembled. It achieves a specific capacity of 94 mAh g⁻¹ at 100 mA g⁻¹, an energy density of 70 Wh kg⁻¹ at 74 W kg⁻¹, a power density of 375 W kg⁻¹ at 57 Wh kg⁻¹, and excellent durability with 89% capacity retention over 500 cycles at 200 mA g⁻¹ within a 0–2 V window. A 5 V/3 mAh prototype device is tested with a solar charging module to evaluate its real-life feasibility.