Solid state electrolytes are crucial for enhancement of the safety and practicability of batteries. NASICON (Na Super Ionic Conductor) is proposed as replacement for β″-Al₂O₃ [1], because of its excellent structural strength and conductivity which is attributed to its structure wherein Na migrates in 3D voids providing ionic conductivity of 0.2 S·cm^-1 (300 °C). Furthermore, the processing temperature is relatively low (~700 °C), and high Na abundance makes it cheaper than Li-ion batteries.

The NASICON phase with formula Na_1+xZr₂Si_xP_3-xO₁₂ [0≤x≤3] is found to be conductive and is a part of the complex Na₂O-SiO₂-P₂O₅-ZrO₂ system. For a successful material design, thermodynamic properties (phase equilibria, heat capacity, stability, etc.) for all sub­systems must be known, and for prediction of the properties, a reliable and consistent database is imperative. To accomplish these milestones, a thermodynamic database of the quaternary oxide system including the NASICON phase is being developed using Calculation of Phase Diagrams (CALPHAD) method.

As part of this development, underlying binary systems such as Na₂O-ZrO₂ and ternary systems such as ZrO₂-SiO₂-P₂O₅ are studied. After preliminary results, the phase diagram construction is in progress with DTA/TG, XRD, and the experimental data along with literature values after critical analysis will be utilized for generation of Gibbs energies for all phases in the system. Using the thermodynamic dataset obtained, phase equilibria are predicted in agreement with the experimental data. The current state of work will be presented.

References:

1. B. Goodenough: “Fast Na⁺-ion transport in skeleton structures”, Mater. Res. Bull. 11(2) (1976) 203-220.
2. Warhus: “Thermodynamics of NASICON (Na_1+xZr₂Si_xP_3-xO₁₂)”, J. Solid State Chem. 72(1) (1988) 113-125.