Structure evolution of Na2O2 from room temperature to 500 oC

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 oC 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 oC and the crystal structure of β-Na2O2 is determined from neutron diffraction data at 500 oC. α-Na2O2 adapts a hexagonal 𝑃6#2𝑚 (No. 189) structure and β-Na2O2 adapts a tetragonal I41/acd (No. 142) structure. The thermal expansion coefficients of α-Na2O2 is a = 2.98×10–5 K–1, c = 2.89×10–5 K–1 and V = 8.96×10–5 K–1 up to 400 oC and a ~10% volume expansion occurs during the phase transition from α- Na2O2 to β-Na2O2 due to the re-alignment/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). Impedance analysis from room temperature to 400 oC revealed a significant enhancement of conductivity at T ≥ 275 oC. α-Na2O2 shows higher conductivity (~10 times at T ≤ 275 oC and ~3 times at T > 275 oC) in O2 compared to in Ar. We confirmed, by dielectric analysis, that this enhanced conductivity is dominated by ionic conduction.