Structure evolution of Na2O2 from room temperature to 500 °C

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Date
2020-09-21
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Publisher
American Chemical Society
Abstract
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
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Keywords
Crystal structure, Electric batteries, Phase transformations, Electric conductivity, Neutron diffraction, X-ray diffraction
Citation
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
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