Structure Evolution of Na₂O₂ from Room Temperature to 500 °C

Na₂O₂ is one of the possible discharge products from sodium-air batteries. Here, we report the evolution of the structure of Na₂O₂ from room temperature to 500 °C using variable-temperature neutron and synchrotron X-ray powder diffraction. A phase transition from α-Na₂O₂ to β-Na₂O₂ is observed in the neutron diffraction measurements above 400 °C, and the crystal structure of β-Na₂O₂ is determined from neutron diffraction data at 500 °C. α-Na₂O₂ adapts a hexagonal P62m (no. 189) structure, and β-Na₂O₂ adapts a tetragonal I4₁/acd (no. 142) structure. The thermal expansion coefficients of α-Na₂O₂ are a = 2.98(1) × 10^-5 K^-1, c = 2.89(1) × 10^-5 K^-1, and V = 8.96(1) × 10^-5 K^-1 up to 400 °C, and a ∼10% volume expansion occurs during the phase transition from α-Na₂O₂ to β-Na₂O₂ due to the realignment/rotation of O₂^2- groups. Both phases are electronic insulators according to DFT calculations with band gaps (both indirect) of 1.75 eV (α-Na₂O₂) and 2.56 eV (β-Na₂O₂). An impedance analysis from room temperature to 400 °C revealed a significant enhancement of the conductivity at T ≥ 275 °C. α-Na₂O₂ shows a higher conductivity (∼10 times at T ≤ 275 °C and ∼3 times at T > 275 °C) in O₂ compared to in Ar. We confirmed, by dielectric analysis, that this enhanced conductivity is dominated by ionic conduction.