Ionic Conductivity Study on Polycrystalline LiFeVO_4

摘要

The compound LiFeVO_4 was prepared by solid state reaction at 570°C for 8h. The X-ray diffraction pattern revealed an orthorhombic crystal structure. Thermogravimetric measurements revealed a reversible mechanism which is attributed to moisture loss and uptake. Impedance spectroscopy measurements were carried out at temperatures from 25 to 500°C at 25°C steps. Equivalent circuits were drawn to fit the impedance measurement results at each temperature step. The elements of the equivalent circuits were assigned to three different conductivity processes, i.e., to bulk, grain boundary and protonic conductivity. The protonic conductivity was attributed to the moisture uptake revealed in the thermogravimetric measurements. Arrhenius plots were drawn for the bulk and grain boundary conductivity processes and the activation energies were calculated. The activation energy of the bulk conductivity process was calculated to be 0.3 eV over the temperature range from 175 to 500°C and the activation energy of the grain boundary conductivity process was calculated to be 0.5 eV from 300 to 500°C and 0.15 eV from 175 to 275°C. An explanation of the existence of these two grain boundary activation energies is given based on easy paths theory. The log-log plot of conductivity versus temperature was drawn at temperatures 200, 250, 300, 400, 450 and 500°C and the hopping rates were determined at each of the above temperatures.