Iron Phosphide Exsolution for Enhanced Electrochemical Performance of Fe-doped Na₃V₂(PO₄)₃ Cathodes in Sodium-Ion Batteries

NASICON-type Na 3 V 2 (PO 4 ) 3 is a promising polyanionic cathode material for sodium-ion batteries due to its robust 3D framework. While Fe-doping has been previously employed to enhance its conductivity, this study systematically investigates the role of exsolved iron phosphide particles in improving the electrochemical performance of Na 3 Fe x V 2– x (PO 4 ) 3 (NFVP- x ) in half-cell and symmetric full-cell configurations. NFVP- x powders are synthesized via a hydrothermal method, followed by calcination at 800 °C under a 10 vol% H 2 /N 2 atmosphere. Phase analysis confirms the formation of a single NASICON phase at low Fe-doping levels, with the FeP impurity appearing at x = 0.1 and increasing with Fe content. Rietveld analysis shows unit-cell expansion at low Fe-doping levels, followed by contraction beyond x = 0.1 due to FeP exsolution. The conductive FeP particles connect the nanosized primary particles within the agglomerates formed during synthesis, thereby improving charge transfer. The optimized NFVP-0.15 composition exhibits superior electrochemical performance in half-cells, delivering 90 mAh/g at 10C with minimal 0.01% capacity decay per cycle over 1000 galvanostatic charge–discharge cycles. Furthermore, the symmetrical cells utilizing NFVP-0.15 achieve 73 mAh/g at 1C, retaining 85% capacity after 200 cycles. These findings highlight FeP exsolution as a key strategy for enhancing the stability and rate capability of sodium-ion batteries.