Structure Evolution of Na₂O₂ from Room Temperature to 500 °C

Area of Science:

• Materials Science
• Solid-state Chemistry
• Electrochemistry

Background:

• Sodium peroxide (Na₂O₂) is a key discharge product in sodium-air batteries.
• Understanding its structural and conductive properties is vital for battery performance.

Purpose of the Study:

• To investigate the structural evolution of Na₂O₂ from room temperature to 500 °C.
• To characterize the phase transition and determine the crystal structure of the high-temperature phase.
• To analyze the temperature-dependent conductivity and its relation to structural changes.

Main Methods:

• Variable-temperature neutron and synchrotron X-ray powder diffraction.
• Density Functional Theory (DFT) calculations for electronic band structure.
• Impedance and dielectric analysis for conductivity measurements.

Main Results:

• A phase transition from hexagonal α-Na₂O₂ to tetragonal β-Na₂O₂ occurs above 400 °C.
• The crystal structure of β-Na₂O₂ was determined as tetragonal I4₁/acd.
• Significant enhancement in ionic conductivity was observed above 275 °C, with α-Na₂O₂ exhibiting higher conductivity in O₂.

Conclusions:

• The structural phase transition in Na₂O₂ involves a ~10% volume expansion.
• Both α- and β-Na₂O₂ are electronic insulators.
• Enhanced ionic conductivity at higher temperatures is attributed to structural factors and is dominant over electronic conduction.