Electrical and dielectric study of Na2/3Mn2/3Fe1/3O2 as a cathode active material for sodium-ion batteries

Abstract

Na_2/3Mn_2/3Fe_1/3O₂ layered oxides were synthesized using an enhanced solid-state synthesis method. Analysis via XRD confirmed the high purity of the prepared samples. Rietveld refinement of the crystal structure revealed that the synthesized materials adopt a rhombohedral system with the P63/mmc space group and P2 structure type. Calorimetric study confirms the presence of two-phase transitions at 393 K and 433 K. Vibrational investigation conducted using IR spectroscopy demonstrated the presence of an MO₆ group. The dielectric properties of the material were evaluated over a frequency range of 0.1–10⁷ Hz across temperatures ranging from 313 K to 453 K. The study of the real part of the dielectric permittivity using impedance spectroscopy suggested a high dielectric constant at low frequency and indicated the existence of both space charge and dipolar polarizations. The dielectric losses were analyzed according to the Giuntini law to extract the relaxation process. As a result, the module revealed two relaxations attributed to grain boundaries and grain relaxations. The frequency-dependent conductivity was interpreted following Jonscher's law. The variation of the power law exponent with temperature, corresponding to the grains s₁ and s₂, suggests that the conduction behavior of the P2-Na_2/3Mn_2/3Fe_1/3O₂ compound can be ascribed to the NSPT and CBH models.