AC Electric Conductivity of High Pressure and High Temperature Formed NaFePO4 Glassy Nanocomposite

Olivine-like NaFePO4 glasses and nanocomposites are promising materials for cathodes in sodium batteries. Our previous study concerned the influence of high pressure on DC electric conductivity of NaFePO4 glasses. This work presents the results of AC measurements of glasses and nanocomposites from pressure treatment. Depending on the temperature range, the electronic conduction mechanism changed from variable range hopping (between Fe2+/Fe3+centers) to phonon-assisted hopping. For DC electric conductivity the portrayal via the classic Mott’s Variable Range Hopping is limited to low temperatures. Spectra of AC conductivity for glasses and pressure-formed nanocomposites fulfill Jonscher’s scaling law with power exponents ranging from 0.3 to 0.9 depending on temperature and the electron hopping mechanism. Due to the decrease in distance between hopping centers, an increase of AC conductivity around 2-3 orders of magnitude was observed after high pressure-high temperature treatment. Up to 200 K temperature dependencies of AC conductivity follow the pattern suggested by Mott's theory. Above that temperature corresponds to phonon-assisted hopping with activation energies EAC≈0.40 eV (for glass: amorphous solid) and EAC≈0.18 eV (for glass + nanocrystallites nanocomposite). Notable that they are considerably lower than values of EDC for DC conductivity. Results obtained show that high-temperature compressing, just below the glass temperature, significantly leads to a stable decrease in distance between hopping centers related to nanocrystallites in a glassy matrix. Consequently, it leads to an essential rise in electric conductivity and volumetric capacity. AC spectra of electronic conductivity are disturbed by the conduction of Na+ mobile ions, which impacts polaron conductivity and can be a limiting factor for conductivity.