NaZr₂(PO₄)₃ - a cubic langbeinite-type sodium-ion solid conductor

Area of Science:

• Materials Science
• Solid-State Chemistry
• Electrochemistry

Background:

• Synthesizing langbeinite-type phosphates with small cations like sodium (Na+) is difficult due to NaSICON phase formation.
• High-temperature solid-state reactions often yield undesired phases, hindering the development of novel solid electrolytes.

Purpose of the Study:

• To develop a synthesis route for langbeinite-type sodium zirconium phosphate (NaZr2(PO4)3).
• To investigate the crystal structure, thermal behavior, and sodium-ion (Na+) conductivity of the synthesized material.
• To evaluate its potential as a solid electrolyte for sodium-ion batteries.

Main Methods:

• Mechanochemically activated ion-exchange reaction between ammonium zirconium phosphate (NH4Zr2(PO4)3) and sodium nitrate (NaNO3).
• X-ray diffraction (XRD) for crystal structure refinement.
• Fourier-transformed infrared spectroscopy (FTIR) for validation.
• Thermal analysis (up to 730 °C).
• Electrochemical impedance spectroscopy (EIS) for conductivity measurements.
• Density functional theory (DFT) calculations for bandgap and electrochemical stability.

Main Results:

• Successfully synthesized langbeinite-type NaZr2(PO4)3 via a mechanochemical ion-exchange route.
• NaZr2(PO4)3 exhibits stability up to 730 °C, with negative thermal expansion between 25-500 °C.
• Achieved Na+ conductivity of 1 × 10^-6 S cm^-1 at 225 °C with low electronic conductivity and a wide electrochemical stability window (1.39-4.18 V vs. Na/Na+).

Conclusions:

• Langbeinite-type NaZr2(PO4)3 is a promising candidate for solid electrolytes in sodium-ion batteries.
• The developed synthesis method overcomes challenges associated with langbeinite-type phosphate synthesis.
• Further research into langbeinite-structured phosphates could lead to advanced battery materials.