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

## TL;DR

This paper studies NaFeNb(PO4)3 as a potential anode material for sodium-ion batteries and finds that its capacity fades due to a phase transition during cycling.

## Contribution

The study provides new insights into the phase evolution and capacity fading mechanisms of NaFeNb(PO4)3 in sodium-ion batteries.

## Key findings

- NFNP shows initial good capacity and Coulombic efficiency but fades upon cycling.
- In situ XRD and XAS reveal a phase transition forming a low-symmetry secondary phase.
- The secondary phase reduces capacity due to lower Nb redox activity.

## Abstract

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.