Structure evolution of Na2O2 from room temperature to 500 °C

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American Chemical Society
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 O22– 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. © 2020 American Chemical Society
Crystal structure, Electric batteries, Phase transformations, Electric conductivity, Neutron diffraction, X-ray diffraction
Wang, C.-H., Gui, D.-Y., Xia, Q., Avdeev, M., Ling, C. D., & Kennedy, B. J. (2020). Structure evolution of Na2O2 from room temperature to 500 °C. Inorganic Chemistry, 59(19), 14439-14446. doi:10.1021/acs.inorgchem.0c02234