Boosting the cycling stability of Na3VFe(PO4)3 cathodes for sodium-ion batteries by zinc oxide coating

Abstract

Three ZnO-coated Na₃VFe(PO₄)₃/C samples were prepared using a scalable two-step method. Structural analysis indicates that the lattice parameters of the bare sample remained unchanged after the coating process. Electron microscopy and Raman spectroscopy identified both the carbon conductive phase and the ZnO coating layer. Ex situ XRD and XPS measurements demonstrate the reversibility of the sodium insertion with the redox participation of both vanadium and iron in the charge transfer reaction. Galvanostatic tests demonstrate that samples coated with 1 and 3 % of ZnO maintain higher capacities at high rates than the bare one. This improvement is attributed to their lower direct current resistance and cell impedance. Further cycling tests conducted at 1C and 5C reveal that a 3 % ZnO coating provides the best capacity retention. Cyclic voltammetry indicates that the capacitive contribution increases with the percentage of ZnO, which enhances the fast exchange of Na⁺ ions at the interface. This technique also shows that coating with 3 % achieves the highest diffusion coefficients, regardless of the voltage region. The preservation of these diffusion coefficients after cycling emphasizes the benefits of the 3 % ZnO coating in preventing electrode degradation during cycling.