NaFeNb(PO₄)₃ as an Electrode Material for Sodium-Ion Batteries: Insights into Phase Evolution and Capacity Fading

The NaFeNb-(PO₄)₃, NFNP, material has been designed as a candidate anode material for sodium-ion batteries, as in its pristine form it combines the presence of Fe-(III) and Nb-(V)available for possible reduction upon Na insertionallowing for the formal introduction of 3 Na ions at reasonable potentials, and the robust NASICON structure with open channels for Na migration. The NFNP material has been successfully obtained by the solid-state route and fully characterized in terms of structure and transport properties by means of diffraction, XAS, and DFT analysis. Although promising, the electrochemical testing reveals that the initially satisfactory results in terms of capacity and Coulombic efficiencies fade upon cycling. The in-depth operando investigation, with the implementation of in situ XRD and XAS, unveiled a phase transition upon cycling; this involves the formation and accumulation of a low-symmetry secondary phase delivering lower capacity related to the Nb redox couples.