Synthesis and characterization of inverse spinel LiNiVO4 and LiCoVO4 with impedance spectroscopy

The compounds LiNiVO4 and LiCoVO4 were prepared by the solid state reaction method at 650 deg C. The X-ray diffraction patterns of both materials reveal the formation of the inverse spinel 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 deg C. Three equivalent circuits were drawn to simulate the materials' electric behavior in each of three temperature ranges, 25 to 175 deg C, 200 to 300 deg C and 325 to 500 deg C. The elements of these equivalent circuits were assigned to three different conduction processes. The activation energy values for the conductivity processes within the grains (bulk conductivity) were found to be 0.24 and 0.29 eV for LiNiVO4 and LiCoVO4, respectively. For grain boundary conductivity two different values were calculated for LiNiVO,0.64 and 0.26 eV and one for LiCoVO,0.29 eV. In the temperature region 25-150 deg C a conductance behavior which is not consistent with pure ionic conductivity was observed, since resistance increases up to two orders of magnitude with increasing temperature. This materials' electrical behavior was attributed to a possible protonic conductivity mechanism due to the reversible moisture loss and uptake.