Na₂ZrFe(PO₄)₃─A Rhombohedral NASICON-Structured Material: Synthesis, Structure and Na-Intercalation Behavior

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Development of earth-abundant cathode materials is crucial for sustainable energy storage.
• NASICON (Na Super Ionic CONductor) structured materials offer potential for sodium-ion batteries due to their 3D framework.

Purpose of the Study:

• To synthesize and characterize a novel NASICON-structured sodium iron phosphate, Na2ZrFe(PO4)3.
• To investigate its electrochemical properties for sodium-ion intercalation/deintercalation.
• To understand the structural evolution during electrochemical cycling.

Main Methods:

• Sol-gel synthesis route.
• Synchrotron X-ray diffraction (XRD) for structural analysis.
• Transmission electron microscopy (TEM) for morphology.
• X-ray absorption spectroscopy (XAS) for electronic structure.
• Operando synchrotron XRD for in-situ structural monitoring during electrochemical cycling.

Main Results:

• Na2ZrFe(PO4)3 was successfully synthesized with a rhombohedral NASICON structure.
• Reversible Na-ion insertion/extraction occurred via Fe2+/Fe3+ redox at ~2.5 V.
• The rhombohedral structure was maintained within a specific Na-content range (2-3 p.f.u.) during cycling.
• Solid-solution pathway, good cyclic stability, high Na-diffusivity, and rate capability were observed within this range.
• Rhombohedral to monoclinic phase transformation occurred beyond this range.

Conclusions:

• Na2ZrFe(PO4)3 is a promising earth-abundant cathode material for sodium-ion batteries.
• Its electrochemical performance is linked to the preservation of the NASICON structure during cycling.
• The study provides insights into designing similar low-cost materials for energy storage applications.