Substitution of Iron for Vanadium in Phosphate Fluoride Positive Electrode Materials for Na-Ion Batteries

The phosphate fluoride ${Na}_3{V}_2{\left({PO}_4\right)}_2{F}_3$ (NVPF) is an excellent positive electrode material for Na-ion batteries. It has already been researched extensively and can deliver a high specific energy and especially impressive power capabilities, which make it suitable for application in power tools. However, concerns exist about the widespread adoption of vanadium-based cathodes at a large scale. The phosphate fluoride framework can accommodate other metal ions, including the less expensive and abundant iron. However, the resulting compound performs poorly as an electrode material in Na-ion batteries. In this work, a substitutional series replacing vanadium with iron according to ${Na}_3{V}_{2-x}{Fe}_x{\left({PO}_4\right)}_2{F}_3$ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0) is successfully synthesized through a solid–state reaction. The crystal structure of all samples is investigated using high-resolution synchrotron X-ray diffraction (sXRD), showing that all of them crystallize in the orthorhombic Amam space group, with the difference between a and b unit cell parameters, however, decreasing with increasing Fe content. Neutron diffraction is used to reveal the distribution of vanadium and iron in the transition metal sites, while Mössbauer and Raman spectroscopy confirm the presence of high-spin Fe^III, together with trivalent vanadium ions. Computational results based on density functional theory provide further insights on the voltage range of the (de)sodiation reaction of the x = 0.0, 1.0, and 2.0 compounds. The investigated materials are tested electrochemically in Na half cells. When the lower cutoff voltage allows for Fe reduction, all materials show significant reversible capacities in excess of ∼90 mA h g^–1 (with the exception of the pure Fe compound). In the same voltage window as NVPF, the x = 0.2 sample is particularly promising, even showing a slightly higher reversible capacity of ∼110 mA h g^–1 while keeping a voltage profile close to the one of the pure vanadium sample, indicating that at least 10% of the vanadium can be replaced by iron without significantly affecting the electrochemical performance.