Synthesis, optical properties and conduction mechanism study of - and -NaMnO₂ materials

Herein, NaMnO2 cathode active materials, in both alpha- and gamma-phases, were synthesized via a solid-solid method. Powder X-ray diffraction analysis revealed that these compounds crystallize in a monoclinic system with the C2/m space group and orthorhombic system with the Pnmm space group for the alpha- and gamma-phases, respectively. Raman spectroscopy showed the presence of the MnO6 octahedral group in the alpha-phase, and the MnO4 tetrahedral group in the gamma-phase. The direct band gap (E-g) was calculated to 4.96 eV and 5.14 eV for the alpha- and gamma-phases, respectively, confirming that alpha-NaMnO2 has better electronic conductivity than gamma-NaMnO2. The Urbach energy values, E-u, show that the octahedral alpha-phase is more disordered than the tetrahedral gamma-phase in the NaMnO2 material. The electrical data analysis of the impedance spectra and the imaginary part of the complex modulus showed the presence of two types of relaxations, corresponding to grain and grain boundary effects in both compounds. The thermal evolution of the relaxation time and DC conductivity followed the Arrhenius law with a change in the activation energy at around T-alpha = 363 K for the alpha-phase and T-gamma = 373 K for the gamma-phase. In addition, the frequency behaviour of the AC conductivity, sigma(ac), of these two samples was analyzed using the universal Jonscher law. Therefore, the variation in s(g) for both compounds with temperature showed a change in the conduction mechanism at around T-alpha and T-gamma for the alpha- and gamma-phases, respectively.