Iron-Vanadium Incorporated Ferrocyanides as Potential Cathode Materials for Application in Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are potential replacements for lithium-ion batteries owing to their comparable energy density and the abundance of sodium. However, the low potential and low stability of their cathode materials have prevented their commercialization. Prussian blue analogs are ideal cathode materials for SIBs owing to the numerous diffusion channels in their 3D structure and their high potential vs. Na/Na⁺. In this study, we fabricated various Fe-V-incorporated hexacyanoferrates, which are Prussian blue analogs, via a one-step synthesis. These compounds changed their colors from blue to green to yellow with increasing amounts of incorporated V ions. The X-ray photoelectron spectroscopy spectrum revealed that V³⁺ was oxidized to V⁴⁺ in the cubic Prussian blue structure, which enhanced the electrochemical stability and increased the voltage platform. The vanadium ferrocyanide Prussian blue (VFPB1) electrode, which contains V⁴⁺ and Fe²⁺ in the Prussian blue structure, showed Na insertion/extraction potential of 3.26/3.65 V vs. Na/Na⁺. The cycling test revealed a stable capacity of ~70 mAh g^-1 at a rate of 50 mA g^-1 and a capacity retention of 82.5% after 100 cycles. We believe that this Fe-V-incorporated Prussian green cathode material is a promising candidate for stable and high-voltage cathodes for SIBs.

Keywords: Prussian blue; Prussian green; high-voltage cathode; sodium-ion batteries; vanadium.

Figure 1

(a) X-ray diffraction patterns of VPB, VFPB1, VFPB2, and FPB samples; transmission electron microscope images of (b) VPB, (c) VFPB1, (d) VFPB2, and (e) FPB; (f) high-resolution image of VFPB1 samples; inset: selected area electron diffraction pattern.

Figure 2

High-resolution X-ray photoelectron spectra of (a) Fe and (b) V; (c) photographs of Prussian blue-green sample powders on weighing paper; and (d) atomic percentage of V, Fe, C, N, K(Na), and Cl in VPB, VFPB1, VFPB2, and FPB.

Figure 3

Cyclic voltammetry (CV) profiles of the first three cycles of (a) VPB, (b) VFPB1, (c) VFPB2, and (d) FPB cathodes.

Figure 4

Initial voltage profiles of (a) VPB, (b) VFPB1, (c) VFPB2, and (d) FPB cathodes.

Figure 5

Cycling performances of (a) VPB, (b) VFPB1, (c) VFPB2, and (d) FPB cathodes at 50 mA g⁻¹ over 100 cycles.

Figure 6

DCP plots during charge/discharge process at various cycles for (a) VPB, (b) VFPB1, (c) VFPB2, and (d) FPB cathodes.

Figure 7

(a) CV curves; (b) fitted lines of i/v^1/2 vs. v^1/2 at different scan-rate voltages (v) from 0.1–0.5 mV s⁻¹; (c) capacitance- and diffusion-controlled contribution ratios of the currents of VFPB1 cathodes; and (d) Nyquist plots for impedance measurement of FPB, VPB, VFPB1, and VFPB2 cathodes with inset of equivalent circuit.